The Pennsylvania State University

The Graduate School

College of Engineering

INVESTIGATION OF PROCESS PARAMETER OPTIMIZATION OF

LASER-FIRED BACK CONTACT SILICON SOLAR CELLS

A Thesis in

Engineering Science

by

Brennan Lawrence DeCesar

© 2010 Brennan Lawrence DeCesar

Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Science

May 2010

The thesis of Brennan Lawrence DeCesar was reviewed and approved* by the following:

Edward W. Reutzel Head of Laser System Engineering and Integration Applied Research Laboratory Thesis Co-Advisor

S. Ashok Professor of Engineering Science Thesis Co-Advisor

Michael T. Lanagan Professor of Engineering Science and Mechanics Associate Director of Materials Research Institute

Judith A. Todd P.B. Breneman Department Head Chair Head of Department of Engineering Science and Mechanics

*Signatures are on file in the Graduate School.

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ABSTRACT

The integration of novel laser processing techniques in solar cell production promises enhancements in both cell performance and manufacturability. The viability of laser processing and its potential for improvement have been shown in recent research. This thesis focuses on original experiments utilizing a single mode fiber laser, a Q-switched second harmonic Nd:YAG laser, and a Q-switched third harmonic laser, and it includes information obtained from previous studies using an excimer laser. The effects of variations in laser conditions and parameters—such as wavelength, power, energy, pulse duration, pulse frequency, temporal pulse shape, and focus—are studied based on the electrical and mechanical effects that they induce on the sample wafers.

Well-understood laser-materials interactions are considered in regard to the laser- fired contacts themselves and to their effects on the wafer materials. Sample wafers include base float-zone silicon and Mono2™ silicon substrates with minor alterations in overall cell structure. This thesis is a contribution to a larger research project whose purpose is to discover the processing parameters for multiple laser systems that will optimize the balance between processing efficiency and solar cell performance.

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TABLE OF CONTENTS

LIST OF FIGURES (v) LIST OF TABLES (x) LIST OF EQUATIONS (xi) ACKNOWLEDGEMENTS (xii)

Chapter 1 INTRODUCTION ...... 1

Chapter 2 LITERATURE REVIEW ...... 4

Laser-Fired Contacts ...... 4 Background of Laser Science ...... 12 Nd:YAG Laser ...... 13 Fiber Laser ...... 16 Excimer Laser ...... 17 Carbon Dioxide (CO2) Laser ...... 18 Laser Processing in the Photovoltaic Field ...... 21 Laser Cutting ...... 21 Laser Scribing ...... 22 Laser Drilling ...... 26 Laser Doping ...... 28 Temporal Pulse Shaping ...... 30 Laser Alloying ...... 38 Physical Phenomena Governing Laser-Fired Contact Formation ...... 40 Melting, Vaporization, and Fluid Flow ...... 42 Mass Ejection and Ablation ...... 50

Chapter 3 EXPERIMENTAL PROCEDURE ...... 60

Design of Experiments ...... 60 IPG Photonics Ytterbium Single Mode Fiber 1070 nm Laser ...... 74 Coherent AVIA Q-Switched Frequency-Tripled 355 nm Laser ...... 105 Quantel Brilliant Q-Switched Nd:YAG 532 nm Laser ...... 114 Lambda Physik Excimer KrF 248 nm Laser ...... 118 Laser-Fired Contact Array and Re-Metallization Experiments ...... 121 Laser-Fired Contact Carrier Lifetime Experiments ...... 127

Chapter 4 RESULTS AND DISCUSSION ...... 139

Chapter 5 CONCLUSIONS ...... 145

Chapter 6 FUTURE WORK ...... 147

REFERENCES ...... 148

Appendix TOOLS ...... 156

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LIST OF FIGURES

Figure 2-1: Cross-sectional diagram of generic laser-fired back contact silicon solar cell...... 4

Figure 2-2: Cross-sectional diagram of LFC on back side solar cell structure (2)...... 6

Figure 2-3: Efficiencies of LFC cells with varying-thickness Al layers (6)...... 7

Figure 2-4: Efficiencies of LFC cells with varying-thickness Al and Ag layers (6). .. 7

Figure 2-5: Comparison of conventional solar cell production process to potential LFC process (5)...... 10

Figure 2-6: Transitions in a three-level laser (11)...... 13

Figure 2-7: Diagram of typical Nd:YAG laser components (11)...... 14

Figure 2-8: Diagram of typical CO2 laser components (11)...... 19

Figure 2-9: Gated pulses from a CO2 laser (11)...... 20

Figure 2-10: Buried Contact Solar Cell (15)...... 23

Figure 2-11: Double-Sided Buried Contact Solar Cell (15)...... 24

Figure 2-12: Interdigitated Backside Buried Contact Solar Cell (17)...... 25

Figure 2-13: Rear surface electrode pattern of Interdigitated Backside Buried Contact Solar Cell (17)...... 25

Figure 2-14: Diagram of Emitter Wrap-Through (EWT) solar cell (19)...... 26

Figure 2-15: Profile comparison of temporal pulse shapes of a 22 ns pulse with 10 ns rise time (left) and a 3.5 ns pulse with 2 ns rise time (right) (29)...... 31

Figure 2-16: Comparison of laser-fired spots from unsliced (top) and sliced (bottom) temporal pulse shapes (29)...... 32

Figure 2-17: Rectangular/Gaussian and Falling/Rising Triangle temporal pulse shapes (31)...... 33

Figure 2-18: Ramp-down pulse shapes; schematic designs (left) and measured waveforms (right) (31)...... 34

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Figure 2-19: Top-hat temporal pulse shapes with nanosecond peaks at the beginning (top), middle (center), and end (bottom) of the pulse (28)...... 35

Figure 2-20: Temporal pulse shapes (33)...... 36

Figure 2-21: Examples of temporal pulse shapes – Ramp-down (left), Ramp-up (center), and enhanced spike (right) (34)...... 38

Figure 2-22: Schematic of a general laser-material interaction (36)...... 41

Figure 2-23: Diagrams of Gaussian laser beam profiles...... 43

Figure 2-24: Example of a temperature gradient resulting from laser processing (38)...... 43

Figure 2-25: Physical model of melt removed from laser-material interaction zone (45)...... 46

Figure 2-26: Effect of recoil pressure on melt pool geometry during laser processing (46)...... 47

Figure 2-27: Effect of laser irradiance and time on surface tension and recoil forces (46)...... 48

Figure 2-28: Cross-sectional views of ablation craters formed at 2.6x109 W/cm2 (top) and 5.1x1010 W/cm2 (bottom) (36)...... 54

Figure 2-29: Crater volume as a function of laser irradiance (increasing pulse energy) (36)...... 55

Figure 2-30: SEM cross-section images of holes drilled in silicon (48)...... 56

Figure 2-31: SEM cross-section images of crater bottoms (48)...... 57

Figure 2-32: Simulation results of mass removal contributions (48)...... 58

Figure 3-1: Cross-sectional diagrams of W1A and W1B test sample structures...... 62

Figure 3-2: Cross-sectional diagram of W0A and W0B test sample structures...... 63

Figure 3-3: Comparison of multicrystalline silicon and Mono2™ silicon (59)...... 64

Figure 3-4: Mask used for aluminum metallization of W1A and W1B...... 67

Figure 3-5: Completed test wafer (W1A)...... 67

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Figure 3-6: 7x7 LFC parameter development matrix (W1B); shown next to dime for scale...... 68

Figure 3-7: Resistance measurements probe station...... 70

Figure 3-8: Resistance measurements probe station with processed wafer on grounded platen...... 70

Figure 3-9: Current-voltage plot for LFC displaying ohmic contact...... 71

Figure 3-10: Current-voltage plot for LFC displaying non-ohmic contact...... 72

Figure 3-11: ZEMAX beam analysis simulation...... 75

Figure 3-12: IPG fiber laser optics train diagram...... 76

Figure 3-13: Focusing tests for IPG laser using silicon wafer...... 77

Figure 3-14: Comparison of LFCs fired out-of-focus away from the laser source (A), out-of-focus toward the laser source (B), and at focus (within the Rayleigh length)...... 78

Figure 3-15: Comparison of electrical resistances of LFCs fired at positions A, B, and C, corresponding to Figure 3-14...... 79

Figure 3-16: Plot of resistance as a function of power for Experiment 017...... 81

Figure 3-17: SEM images of LFCs from Experiment 017...... 82

Figure 3-18: LFC processed at 49 W and 640 µs with resistance of 32 Ω...... 83

Figure 3-19: LFC processed at 29 W and 40 µs with resistance of 400 Ω...... 84

Figure 3-20: LFC processed at 80 W and 150 µs with resistance of 92 Ω...... 85

Figure 3-21: LFC processed at 60 W and 350 µs with resistance of 91 Ω...... 85

Figure 3-22: Plot of resistance as a function of power for Experiment 018...... 86

Figure 3-23: SEM images of LFCs from Experiment 018...... 88

Figure 3-24: LFC processed at 85 W and 100 µs with resistance of 35 Ω...... 89

Figure 3-25: Plots of resistance as a function of power for Experiment 019 (W1A and W1B)...... 90

Figure 3-26: SEM images of LFCs from Experiment 019 (W1A and W1B)...... 93 vii

Figure 3-27: Diameter measurements used for spot size calculations...... 94

Figure 3-28: LFC perimeter as a function of power for Experiments 017, 018, 019...... 96

Figure 3-29: LFC resistance as a function of perimeter for Experiments 017, 018, 019...... 97

Figure 3-30: LFC resistance x area as a function of power for Experiments 017, 018, 019...... 98

Figure 3-31: LFC resistance as a function of area for Experiments 017, 018, 019. .... 99

Figure 3-32: EDS material compositions at specified positions on an LFC...... 101

Figure 3-33: Pulse ―peaks‖ and pulse ―bodies‖ used for temporal pulse shaping experiments...... 102

Figure 3-34: Images and resistances of temporal pulse shapes used for LFC processing...... 103

Figure 3-35: Power/energy curve as a function of frequency for AVIA laser...... 105

Figure 3-36: SEM image of four LFCs used for resistance measurements...... 107

Figure 3-37: Plot of resistance as a function of number of shots for W1B...... 108

Figure 3-38: Plot of resistance as a function of laser energy for W1B...... 109

Figure 3-39: SEM images of LFCs for W1A...... 111

Figure 3-40: SEM images of LFCs for W1B...... 112

Figure 3-41: LFC fired for 2 shots at 5 kHz (235 µJ)...... 113

Figure 3-42: LFC fired for 10 shots at 5 kHz (235 µJ)...... 113

Figure 3-43: Resistance values plotted by test number and number of shots for W1A and W1B...... 115

Figure 3-44: SEM images of LFCs fired on W1A and W1B...... 116

Figure 3-45: 1 Shot fired on W1B...... 117

Figure 3-46: 4 Shots fired on W1B...... 117

Figure 3-47: Optical microscope image of excimer laser-fired LFC...... 118 viii

Figure 3-48: Resistance as a function of laser energy...... 119

Figure 3-49: Resistance as a function of laser spot area...... 120

Figure 3-50: Laser spot area as a function of resistance...... 120

Figure 3-51: 18x18 LFC array (left) and 9x9 LFC array (right)...... 121

Figure 3-52: Cross-sectional diagram of test wafer used for lifetime experiments. .... 128

Figure 3-53: Laser parameters and their locations on lifetime test wafer...... 129

Figure 3-54: Wafer-10 lifetime map before e-beam Al deposition...... 130

Figure 3-55: Wafer-10 lifetime map after Al deposition and annealing at 275°C for 5 min...... 130

Figure 3-56: Wafer-10 lifetime map after laser processing and aluminum etching. ... 131

Figure 3-57: Wafer-10 lifetime map after annealing at 275°C for 10 min...... 132

Figure 3-58: Wafer-10 lifetime map after annealing at 350°C for 10 min...... 132

Figure 3-59: Wafer-11 lifetime map before e-beam Al deposition...... 133

Figure 3-60: Wafer-11 after laser processing, Al removal, and annealing at 275°C for 10 minutes...... 134

Figure 3-61: Wafer-11 after laser processing, Al removal, and annealing at 375°C for 10 minutes...... 134

Figure 3-62: Lifetime as a function of resistance for IPG, AVIA, and Quantel lasers...... 136

Figure 3-63: Lifetime as a function of resistance for IPG fiber laser...... 137

Figure 3-64: Lifetime as a function of resistance for AVIA laser...... 137

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LIST OF TABLES

Table 3-1: Laser system parameters...... 65

Table 3-2: Resistance values for Experiment 017...... 81

Table 3-3: Resistance values for Experiment 018...... 87

Table 3-4: Resistance values for Experiment 019, W1A...... 91

Table 3-5: Resistance values for Experiment 019, W1B...... 91

Table 3-6: Resistance values for W1B...... 106

Table 3-7: Averaged resistance values for W1A...... 106

Table 3-8: Resistance values for W1A and W1B...... 114

Table 3-9: Resistance values for 9x9 LFC arrays fired with IPG fiber laser, before and after re-metallization...... 123

Table 3-10: Resistance values for 18x18 LFC arrays fired with IPG fiber laser...... 124

Table 3-11: Resistance values for 9x9 LFC arrays fired with the AVIA laser, before and after re-metallization...... 124

Table 3-12: Resistance values for 9x9 LFC arrays fired with the Quantel laser, before and after re-metallization...... 124

Table 3-13: Lifetime map measurements based on laser processing parameters for Wafer-11...... 135

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LIST OF EQUATIONS

Equation 2-1: Upper limit for open circuit voltage (25) (26)...... 29

Equation 2-2: Peclet number (38)...... 44

Equation 2-3: Ratio of Reynolds number (Ma) to Grashof number (Gr) (38)...... 45

Equation 2-4: Recoil pressure due to laser firing on a surface (45) (49) (51)...... 47

Equation 2-5: Rate of homogeneous nucleation (53)...... 52

Equation 3-1: Resistance as a function of resistivity, conductor thickness, and cross-sectional area...... 72

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ACKNOWLEDGEMENTS

I would like to thank Ted Reutzel for his unparalleled ability and sincerity as an advisor and mentor and for his willingness to persistently spend incredible amounts of time helping others to succeed in their efforts. Thank you to S. Ashok for introducing me to this project and for providing new experimental ideas and theories; to Mike Lanagan for his contribution in helping me to write a more complete thesis; to Joe Flemish, Todd

Palmer, and Suzanne Mohney for their continual efforts in answering questions, explaining concepts, and providing ideas; to Ken Meinert, Steve Brown, Ed Good, and

Jay Tressler for their help in working with equipment, acquiring materials and tools, providing technical support, and taking time away from their work to provide assistance; to Zhenyan Hua, Brittany Hedrick, Ben Hall, and Brian Downey for working long hours with me on numerous occasions to try to complete experiments and to solve problems; to

Lian Zou and Dave Carlson of BP Solar for their time, support, and leadership in this project; to Tony Hoult of IPG Photonics for providing equipment and advice in support of our work.

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Chapter 1

INTRODUCTION

The need for alternatives to current energy sources based on fossil fuels has generated increased research efforts in the field of photovoltaics. These efforts have led to improvements in solar cell technology that have the potential to significantly increase cell efficiency and thus commercial viability. Silicon-based solar cells have been, and for the near future will continue to be, dominant in the photovoltaics market because of the volume of research that supports them, their relative ease of manufacture, and their low cost in comparison to other high-performance cells.

Rear-side contacted silicon solar cells have become extremely popular in the photovoltaics field, achieving efficiencies on the order of 20%. However, manufacturing efficiencies need to be improved for these cells—and for solar cells in general—to be more widely recognized as a viable means of energy production. The implementation of advanced laser processing in the photovoltaics field could provide the means for this acceptance.

The classic uses for lasers in microelectronics are cutting and welding, although specific applications in the solar industry include surface texturing, buried contact cells, annealing, and doping (1). A recent advancement is the use of laser processing to create electrical contacts on cells, which are needed so that electrical connections can be made

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through performance-enhancing passivation and high-reflectivity layers that separate the backside conduction layer and the base silicon. This technology has the capacity to replace conventional screen-printing methods—at least in part—that require more processing steps and more material, as thicker silicon wafers are needed to endure the contact from the screen-printing process—as well as the requisite high temperature firing to achieve adequate ohmic contact—without excessive warping and breakage.

The objective of this thesis is to study the laser-fired contact (LFC) process with the intent of eventually making judgments about which laser processing parameters and conditions—and which lasers—will lead to optimized solar cell viability in terms of manufacturability, cost-effectiveness, and electrical efficiency. This specific thesis will include studies of the effects of variations in laser wavelength, power, energy, pulse duration, pulse frequency, temporal pulse shape, and focus on LFC dimensions and morphologies, electrical resistances, and carrier lifetimes.

Laser processing is a thoroughly-researched and well-understood field of scientific research and engineering. Thus laser-materials interactions have been exhaustively documented, especially in regard to welding and cutting. In this thesis, knowledge of these processes is applied to the LFC technique in regard to examination of the contacts themselves and their effects on the wafers.

Processing of LFCs using different types of lasers was performed in order to obtain results from a variety of lasing techniques and conditions. These laser types include: third harmonic (frequency-tripled) Q-switched; second harmonic (frequency- doubled) Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG); ytterbium-

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doped single mode fiber; and krypton fluoride excimer. Nd:YAG lasers are very common solid-state lasers which use a solid gain medium, as opposed to a liquid or a gas.

They are known for their uses in both medical and engineering fields. Fiber lasers use rare-earth element-doped optical fibers as their gain media, and their applications are similar to those of the solid-state lasers. Excimer lasers are a type of molecular gas laser which use combinations of inert gases and halogens.

This thesis represents a portion of a larger project whose ultimate goal is to develop a complete optimized solar cell manufacturing method. Two separate, concurrent efforts are being made along with this portion of the project. One focuses on developing a deep understanding of the geometry and functional aspects of the LFC itself. The other is involved with finding a means of increasing throughput and processing efficiency by firing multiple LFCs at once using diffractive optical elements and microlens optical arrays.

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Chapter 2

LITERATURE REVIEW

Laser-Fired Contacts

Laser-fired contacts (LFCs) are researched for their potential in increased solar cell efficiencies and simplified and faster production methods which generate high cell throughput and minimal wafer losses due to breaking. The possibility of avoiding expensive chemicals and the ability to minimize and even eliminate various handling steps also contribute to their appeal. Additionally, LFCs are particularly well-suited for silicon solar cells, which constitute the majority of production photovoltaics. Figure 2-1 shows a cross-sectional diagram of a generic laser-fired back contact silicon solar cell.

Figure 2-1: Cross-sectional diagram of generic laser-fired back contact silicon solar cell.

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Part of the appeal of photovoltaics, aside from their ability to produce renewable, clean energy, is the simplicity of their design. Impinging photons of energy greater than the band gap of a silicon atom can be absorbed by the material. Electrons excited from this energy transfer move into the atom’s conduction band—leaving an oppositely charged hole in the valence band—and are able to move freely.

N-type silicon is generally made by diffusion of phosphorus atoms, while p-type silicon typically requires boron diffusion. Phosphorus diffusion creates a high concentration of electrons in the n-type silicon. When n-type and p-type silicon are put in intimate contact, diffusion due to the concentration gradient forces electrons to cross the p-n junction from the n-type silicon side to recombine with holes in the p-type side. An electric field that forms at the p-n junction from the initial charge imbalance encourages drift current, which opposes and ultimately balances electron and hole diffusion across the p-n junction. A ―depletion region‖ thus forms across the p-n junction, which is then devoid of mobile charge carriers.

If an ohmic contact is made on the n-type side of the cell, electrons that can no longer cross the depletion region at the p-n junction will follow an external circuit in order to reach the p-type side. In this way current can be collected and used to power a load. LFCs provide the requisite electrical contact between the conductive backside layer and the p-type silicon—through the dielectric passivation layers that are common on contemporary silicon solar cells—so that electrons can flow back into the cell.

A cross-sectional diagram of an LFC is shown in Figure 2-2 (2). The figure demonstrates how the LFC is used to make contact between the aluminum and the silicon through the optical reflector and passivation layers, which will be explained in this thesis. 5

Figure 2-2: Cross-sectional diagram of LFC on back side solar cell structure (2).

Conversion efficiencies over 20% have been reported for silicon-based solar cells utilizing the LFC approach on an assortment of test structures that have varying degrees of potential for mass-production (2) (3) (4) (5) (6). Contemporary production silicon solar cells typically achieve conversion efficiencies around 15%. Figure 2-3 and Figure

2-4 show conversion efficiencies of solar cells utilizing LFC processing through aluminum and aluminum and silver conductive layers, respectively (6). In the plots, the thickness of the aluminum layer does not create a significant change in efficiency.

However, the addition of silver and increases in pulse number tend to reduce efficiencies, while annealing increases efficiencies.

Many conventional silicon solar cells undergo a process called screen-printing in which a paste (typically aluminum) is applied to the surface of the cell, and the cell is annealed at high temperatures to promote diffusion of aluminum atoms into the bulk silicon (3) (5) (7). This generates an aluminum back surface field (Al-BSF) that reduces surface recombination rates at the rear side by repelling minority carriers away from rear

6

side contacts. The process is typically performed on cells that contain phosphorus-doped n-type silicon emitters and boron-doped p-type bulk silicon.

Figure 2-3: Efficiencies of LFC cells with varying-thickness Al layers (6).

Figure 2-4: Efficiencies of LFC cells with varying-thickness Al and Ag layers (6).

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An important cost-savings initiative in solar cell manufacturing is the implementation of thinner bulk silicon wafers (3) (5) (7). While using thin wafers—for example, bulk silicon of thickness ≤ 200 µm—is an excellent technique for materials efficiency, there are two major problems that must be countered. The first is a mechanical one related to the aforementioned aluminum screen-printing process: conventional solar cell manufacturing lines typically include high temperature annealing—as part of the screen-printing process in order to create diffusion—that causes wafer warping during the subsequent cooling periods (5) (7). Even minimal mechanical stresses can crack these warped wafers, and the thinner the silicon substrate, the more susceptible the wafer is to warping.

The second problem is an electrical one, and it has two parts. First, the use of thinner silicon wafers results in reduced optical absorption, especially for the longer wavelengths, which reduces the photocurrent (7) (8). Second, more of the optical generation occurs near the back of the cell, and the photogenerated minority charge carriers must diffuse further to reach the p-n junction where they are collected. This reduces the current collection efficiency. Both of these issues degrade solar cell performance.

The decreased absorption length problem has been counteracted to a large extent by introducing layers within the solar cell that promote back reflectance of sunlight (2)

(5) (7) (9). A rear side dielectric layer of, for example, silicon nitride (SiN), silicon dioxide (SiO2), amorphous silicon (a-Si), or some combination of these materials, can, depending on its thickness, act as an optical back-surface mirror that generates high internal back reflectance by returning unabsorbed photons back through the silicon and 8

producing further opportunities to create electron-hole pairs. Improvements in light trapping are necessary for high efficiencies in thin silicon photovoltaics. The dielectric layer is typically deposited using plasma enhanced chemical vapor deposition (PECVD) to thicknesses on the scale of 10 nm to 100 nm (2) (4) (5) (7). Another method of improving light trapping is surface texturing, a popular example of which is the random pyramid front surface structure (5) (9).

These coatings can also serve as rear side dielectric passivation layers to counter the diminished carrier lifetime problem. A passivation layer ties up dangling bonds that exist on the surface of a wafer. These dangling bonds are prime sites for high recombination velocities which reduce cell performance.

The minority charge carrier loss at the rear ohmic contact can also be counteracted by the creation of a back surface field—typically an Al-BSF—that is able to repel the diffusing minority charge carriers (7) (8). The BSF, which was described previously, is generated by a highly doped layer on the back of the cell. In an example of a solar cell with an n+ doped emitter, the layer would have to be p+ doped. Aluminum is one of several materials that can generate this requisite p+ doping if it is alloyed with the bulk silicon substrate. Traditionally, this alloying process has been accomplished via a high temperature annealing step, but as was mentioned previously, this often leads to undesired wafer warping and breakage.

The ability to manufacture solar cells quickly, efficiently, and with minimal resources is potentially as important as the performance of the actual cells being produced. The steps involved in a conventional, generic solar cell production process are compared to those in a potential LFC production process in Figure 2-5 (5). The number 9

of steps is greatly reduced for the LFC process. A reduction in the number of steps required to produce a finished solar cell allows fewer chances for an incomplete cell to be damaged or destroyed along the production path.

Figure 2-5: Comparison of conventional solar cell production process to potential LFC process (5).

In addition to providing potential production processes that are simpler and quicker than those currently used in manufacturing lines, LFCs also present solutions to some of the major problems with conventional solar cell processing techniques, especially in regard to thin silicon cells. Specifically, the high-temperature annealing required for aluminum diffusion to create a BSF can be avoided, thus preventing the substantial material losses from wafer breakage. LFCs are capable of producing an

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aluminum-silicon alloy—which results in a p+ doped region in the bulk silicon—that can generate the Al-BSF that repels minority charge carriers away from the back side of the cell (2) (5) (6) (7). Research has also shown the doping process to be more effective for

LFCs—due to increased dopant solubilities that can be created during laser-firing—than for conventional annealing processes (10).

Various groups have researched the LFC as a potential substitute technology for many of the currently used solar cell production methods. Numerous variations in cell structure, processing steps, and laser conditions have been studied, but there are commonalities for LFC solar cell processing. For example, 200 µm to 300 µm monocrystalline, multicrystalline, and float-zone silicon wafers have been used with notable success (2) (3) (4) (5) (6) (7). The test wafers are generally passivated with single or multiple layers of a-Si, SiN, or SiO2 to thicknesses up to 100 nm, depending on whether the goal is simply surface passivation or a combination of passivation and light reflectance or absorption.

The LFC concept has not yet gained acceptance into manufacturing facilities as a suitable approach to production solar cells. However, it is likely that with further research and continued success, the LFC approach will lead to new, viable methods of silicon solar cell production.

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Background of Laser Science

Lasers emit highly coherent electromagnetic radiation in beams of extremely low divergence (11). The main components that contribute to laser function are the gain medium, which can be a solid, liquid, gas, or plasma, and which amplifies the beam through stimulated emission; the resonator cavity, which consists of mirrors that permit light reflection, with one of the mirrors being a partially transparent output coupler that allows controlled emission of light from the cavity; and a pumping device—typically either a flash lamp or another laser.

Laser operation begins when the gain medium, which is situated inside the resonator cavity, absorbs energy from the pumping source (11). This absorption raises electrons in the gain medium to elevated quantum states. To achieve equilibrium once again, photons can be emitted through either spontaneous or stimulated emission. In the case of stimulated emission, a photon of a certain energy passes through the cavity and causes the emission of another photon of the same phase, frequency, and direction.

Population inversion occurs when the number of excited electrons exceeds the number of electrons in a lower energy state, and so the stimulated emission caused by photons passing through the medium is greater than the absorption. This is the amplification required for laser operation, and it is achieved as photons pass back and forth (resonate) through the gain medium in the resonator cavity. A diagram of this process is described in Figure 2-6 (11). The common word ―laser‖ is actually an acronym which stands for ―light amplification by stimulated emission of radiation‖ (11).

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Figure 2-6: Transitions in a three-level laser (11).

Nd:YAG Laser

Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers belong to the class of solid-state lasers. Solid-state lasers operate by doping a crystalline solid medium with ions to provide the required energy states. Neodymium is a popular dopant, and the population inversion is maintained within this medium.

Nd:YAG lasers are often used in production lines for precision welding, drilling, and contour cutting (11). Neodymium is a rare earth metallic element whose ions work well as laser media because they have sets of energy levels. Neodymium has multiple

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energy bands between 1.56 and 2.32 electron-volts (eV) which absorb light from visible to infrared regions.

The typical Nd:YAG laser is made up of a crystalline neodymium-doped yttrium aluminum garnet rod surrounded either by pump lamps of noble gases, such as xenon or krypton, or by diode lasers, which improve efficiency and optical performance. The rod is usually cooled by flowing water. A diagram of this setup is shown in Figure 2-7.

Figure 2-7: Diagram of typical Nd:YAG laser components (11).

Visible to infrared light from the lamps excites electrons in the rod, which then drop non-radiatively to 1.38 eV above the ground state. This thermalization heats the

YAG rod. 1.38 eV is a metastable (equilibrium) state in that energy loss through non- radiative transition cannot occur, and spontaneous radiative transitions are highly unlikely. Thus the neodymium atoms are able to retain this state for long periods, allowing for significant population inversion to accumulate between the 1.38 eV state and the lower states. 1064 nm is the typical (fundamental) wavelength of light emitted from

Nd:YAG lasers, and it is generated when an excited atom drops from 1.38 eV to 0.22 eV.

Thermal transitions to the YAG lattice depopulate the lower 0.22 eV state, further heating 14

the rod. Nd:YAG lasers are often operated in frequency-doubled or frequency-tripled modes, which produce laser light at 532 nm or 355 nm, respectively. Frequency manipulation is a process of nonlinear frequency conversion, and frequency tripling is usually a cascaded process achieved by first frequency-doubling the input beam and then performing sum frequency generation to reach the desired frequency, producing light in the ultraviolet range (355 nm).

Nd:YAG laser efficiency and optical performance can be greatly improved by substituting diode lasers for the xenon or krypton lamps. Diode lasers that emit 790 nm light can directly pump the lowest neodymium band at 1.56 eV, thus causing only 0.18 eV to be thermally dissipated into the YAG crystal. More efficient pumping occurs as a result, and the rod does not overheat. This is important because of the thermal gradient that develops when laser rods become heated in the center and are cooled from the outside. The gradient across the rod alters the refractive index, degrading optical performance stability and resulting in divergent output. Pumping with diode lasers can alleviate the problem to a certain extent by reducing the heat that the rod must dissipate.

This is helpful, since it is difficult to focus the beam in high-power Nd:YAG lasers that have highly divergent multimode outputs.

Nd:YAG lasers can operate either in pulsed or continuous wave modes. Pulsed modes created by pulsing the pumping source result in repetition rates that are typically below 200 Hz, and laser pulse shape and duration can be controlled through manipulation of input power to the flashlamps or diodes. Pulse durations can range from hundreds of microseconds to many milliseconds with this technique.

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Pulsing can also be performed with Q-switching. This technique requires either a mechanical or an electro-optical device within the resonator to either permit or prevent optical oscillation. An internal shutter is a simple example of a Q-switch that prevents laser action when closed and permits reflection between resonator mirrors—and thus lasing—when open. Q-switching can be performed by continuous pumping of atoms into the metastable upper laser state while the shutter is closed. Opening the shutter creates an extremely high energy, short pulse, typically of less than 10 ns, that drains the upper state. This results in a very high peak power. Material ablation is often performed using these short, high-power pulses, while welding typically requires longer, lower-power pulses or a continuous beam.

Fiber Laser

A fiber laser refers either to a laser that utilizes a doped fiber as its gain medium, or to a laser whose resonator is made of fibers (12). Some lasers with semiconductor gain media and fiber resonators are also called fiber lasers. Generally, the gain medium is a fiber that has been doped with a rare-earth ion like erbium, neodymium, ytterbium, thulium, or praseodymium. The pumping source is usually one or more laser diodes.

Forming a laser resonator with fibers requires a reflector to form a linear resonator (12). These reflectors can be ordinary dielectric mirrors, bare fiber end faces for the output couplers, or fiber Bragg gratings.

16

Some high-power fiber lasers are capable of producing several kilowatts from a single fiber because of a high surface to volume ratio and the guiding effect—avoiding thermo-optical problems under excessive heating conditions (12). Fiber lasers are also highly suitable for creating up-conversion lasers that rely on difficult transitions that require high pumping intensities. In up-conversion lasers, two or more photons transfer a laser ion to a highly excited energy level so that the laser photon energy can be larger than the energy of the pumping source.

Excimer Laser

An excimer laser, which is an abbreviation for excited dimer laser, operates utilizing a molecule composed of a halogen, such as fluorine or chlorine, and an inert gas, such as neon or xenon, that is stable only in an excited state (11). Excimer lasers—or more accurately, exciplex lasers—belong to the family of molecular gas lasers and are popular for material processing.

Since the light-emitting excimers do not have a ground state, population inversions are easily generated. Unstable ions, such as argon fluoride, are produced from the excitation of the halogens and nobles gases, and as they decompose into stable gases they release highly energetic photons, generally resulting in emission of ultraviolet light.

Excimer lasers cannot maintain continuous wave modes, so these lasers operate only in pulsed modes. Pulse durations are typically short, on the order of 10 ns, and peak power

17

is high, in the range of 100 kW. Popular molecules (and their respective laser beam wavelengths) implemented in excimer laser processing include fluorine (F2) – 157 nm; argon fluoride (ArF) – 193 nm; krypton fluoride (KrF) – 248 nm; xenon bromide (XeBr)

– 282 nm; xenon chloride (XeCl) – 308 nm; and xenon fluoride (XeF) – 351 nm (12)

(13).

Operation of excimer lasers is possible because the excimer molecule is bound in its excited state and repulsive in its ground state. While the highly inert noble gases do not usually form compounds, they can form temporarily-bound molecules with themselves (dimers) or with halogens (complexes—hence, exciplex) while in an excited state. These dimers and complexes release their excess energy through spontaneous or stimulated emission, and as they drop to their ground state, the now strongly-repulsive molecules split back into two unbound atoms. This causes the population inversion between the bound upper state and the unbound lower state and allows for laser operation.

Carbon Dioxide (CO2) Laser

CO2 lasers are widely used in materials processing both because of their relatively high output power and because of their substantial industrial history (11). A CO2 laser is a gas discharge device that uses a gas mixture of carbon dioxide, helium, nitrogen, and possibly hydrogen, water vapor, and xenon, as its gain medium (11) (12).

18

Gas discharges electrically pump the media with either direct or alternating current (20 – 50 kHz) or in the radio frequency domain (11) (12). The discharge excites nitrogen molecules into metastable vibrational levels from which they transfer their energy to the carbon dioxide molecules through collisions. Helium depopulates the lower laser level and removes heat, while hydrogen and water vapor act to reoxidize the carbon monoxide molecules formed during the discharge back into carbon dioxide. A diagram of typical CO2 laser components is shown in Figure 2-8 (11).

Figure 2-8: Diagram of typical CO2 laser components (11).

CO2 lasers typically produce 10.6 µm wavelength beams, although other wavelengths between 9 µm and 11 µm are possible. Both continuous wave and pulsed operation are possible, and these modes are controlled by the electrical discharge. Pulsed operation is generally performed using a gated mode in which the laser is driven at its normal continuous wave output, and is repeatedly turned off and on at a desired repetition rate in order to control the duty cycle (11). A temporal diagram of these gated pulses with leading edge spikes merging into common square wave shapes is shown in Figure

2-9.

19

Figure 2-9: Gated pulses from a CO2 laser (11).

20

Laser Processing in the Photovoltaic Field

There are three significant characteristics of laser light that distinguish it from light produced by ordinary sources (11). The first is that laser light is coherent, as the electromagnetic waves in a laser beam are in phase. The second is that it is nearly monochromatic, and there is a significantly smaller range of frequencies contained within a laser beam than within normal light. The third characteristic is that laser light is highly parallel, or has very low divergence, which means that the beam spreads only minimal amounts over long distances. These characteristics combine to enable the laser beam to be focused on a very small region, thus creating an extremely high power density

(irradiance) or energy density (fluence). It is because of these high irradiances or fluences that laser beams can cause such profound effects when they impinge on matter.

Laser Cutting

Lasers are often used to cut deep grooves into materials so that cleaving by mechanical pressure can be performed (1). In the photovoltaics field, this process is generally used to remove edge regions in silicon wafers. Shunts (unwanted short circuits) of the diffused emitters can be prevented by processing on the non-emitter diffused side and by limiting the depth of the laser cuts (1).

Gas-assisted laser cutting is a common manufacturing process (11). A laser is used to melt the material, and a pressurized assist gas forces the molten material away 21

from the processing site, leaving an empty cut. Cutting with CO2 or Nd:YAG lasers is a predominantly thermal process: the beam heats the material until it melts or vaporizes (11).

Laser Scribing

Lasers can be used to scribe grooves by drawing the beam across a material’s surface.

For photovoltaics, scribing is generally used to create contact grooves that are typically tens to hundreds of microns in width and in depth (1). Often the scribing process redeposits ablated material both on the surface and within the groove itself. These residual deposits can be removed with anisotropic alkaline etches (KOH or NaOH) (1).

The buried contact solar cell—a thoroughly researched solar cell design that first appeared in the late 1980s (14)—uses laser scribing to form selective emitters (1). A selective emitter increases performance by using two differently doped regions to form the emitter and the contact region. A schematic diagram of a buried contact solar cell is shown in Figure 2-10 (15). The top surface is laser-scribed to form deep, narrow channels—roughly 100 to 200 µm in depth and 20 to 25 µm in width—through the top textured dielectric and oxide layers into the bulk silicon substrate (14) (16). Heavy phosphorus diffusion is restricted to the channels in order to create a heavily doped n- type (n++) silicon region around the channels. Aluminum deposited on the rear surface forms a p-type (p+) aluminum-silicon alloyed region during an annealing step that gives rise to a back surface field. This field counteracts the loss of photogenerated minority

22

charge carriers at the rear side electrodes in a completed cell and thus reduces the photocurrent (8).

Figure 2-10: Buried Contact Solar Cell (15).

High rear surface recombination velocities degrade the electrical performance of buried contact solar cells by reducing open circuit voltages, especially when using thin substrates that prevent complete light absorption due to decreased absorption lengths (1) (8). However, short circuit current values are high, and conversion efficiencies both near and over 20% have been reported (15) (16) (17) (18).

Double sided buried contact solar cells improve the rear surface passivation of standard buried contact solar cells by replacing the deposited aluminum layer with laser- scribed channels similar to those on the front surface (1). A schematic diagram of a double sided buried contact solar cell is shown in Figure 2-11. Heavy boron diffusion within these channels creates p-type (p+) silicon regions around the channels. Despite

23

higher open circuit voltages, overall conversion efficiencies are lower than those of the standard buried contact solar cells (1). Additionally, the increased complexity of both the structure and the preparation process of these cells precludes them from being attractive designs for manufacturing.

Figure 2-11: Double-Sided Buried Contact Solar Cell (15).

Interdigitated backside buried contact solar cells represent a further advancement in the buried contact solar cell design. A cross-sectional diagram of the design is shown in Figure 2-12, while a schematic of the rear surface interdigitated electrode pattern is shown in Figure 2-13 (17). The removal of the channels from the front surface increases light-generated current, and the placement of both contacts on one side simplifies integration of the cells during module assembly (1). However, the extremely complicated processing of these cells does not make them an attractive alternative to either the standard or the double sided buried contact solar cells.

24

Figure 2-12: Interdigitated Backside Buried Contact Solar Cell (17).

Figure 2-13: Rear surface electrode pattern of Interdigitated Backside Buried Contact Solar Cell (17).

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Laser Drilling

Lasers can drill deep holes of either cylindrical or conical shape by firing into materials using high irradiance pulses (1). Solar cells generally perform better with conically- shaped holes because of the potential for reduced shading and improved plating.

Several laser-processed solar cell structures are based on the concept of laser drilling. The Emitter Wrap Through solar cell represents one of these structures (19). In this design, laser-drilled holes, or ―vias,‖ are used to wrap the front surface emitter through the cell to the rear surface, thus establishing electrical contact. A diagram of this structure is shown in Figure 2-14.

Figure 2-14: Diagram of Emitter Wrap-Through (EWT) solar cell (19).

This design differs from many conventional commercial solar cells that have both the emitter and the current-collection grid on the front surface (19). In such cells, the front side emitter improves internal collection efficiency while the front side grid reduces 26

series resistance losses. However, the screen-printed silver or aluminum lines typically used for the front side grids decrease absorption of impinging light and have poor aspect ratios (the proportionality of grid height to grid width), conductivities, and contact resistances (19).

Back contact cells in general eliminate losses to grid obscuration and low grid resistance, and they also provide simpler module assembly and packing since there are no interconnects between the front and back sides (19). Most commercial solar-grade materials have diffusion lengths that are much shorter than the thickness of the device of which they are a part, but a length much greater than this distance is necessary for efficient diffusion of photo-generated carriers from the front to the back side of the cell (19). This is a major limitation for back contact solar cells as it limits the extent of the diffusion to the rear side.

The Emitter Wrap Through design was modeled in part after a back contact

―Polka Dot‖ cell that implemented etched vias through the cell to connect a collection junction on the front surface to a current-collection junction on the back surface (20).

The Emitter Wrap Through design is also based on buried contact processes, such as those used in the standard buried contact, double sided buried contact, and interdigitated buried contact solar cell designs (19). Like the Polka Dot cell, the Emitter Wrap Through uses laser-drilled vias to connect the front side emitter to the rear side. The vias are selectively metallized for improved conductance and connected to laser-scribed grooves on the back surface.

The Emitter Wrap Through design has been shown to work well with thin silicon substrates (less than 200 µm), which makes it attractive to address for material cost 27

savings (21). Modeled efficiencies for Emitter Wrap Through cells based on multi- crystalline silicon and mono-crystalline silicon reach 18% and 21%, respectively (19).

Metallization Wrap Through and Metallization Wrap Around are two other solar cell designs that are similar to the Emitter Wrap Through cell (17). The Metallization

Wrap Through cell uses laser-drilled vias to connect the busbar from the front side collecting junction and contact grid to contacts on the back side. The Metallization Wrap

Around cell uses metallized edges to conduct current from the front side to busbars on the rear (22). Efficiencies higher than 17 % have been achieved with both cell designs.

In general, electrical performance of the Emitter Wrap Through and similar designs is characterized by high short circuit current values and low open circuit voltage values (1). Sites of high recombination rates and difficulties in uniform plating due to via geometries, along with other structure-related issues, have kept these solar cells from achieving success at the manufacturing level.

Laser Doping

Laser doping is extremely useful for its ability to incorporate dopants into silicon without high-temperature annealing that can result in wafer warping and shortened carrier lifetimes (1) (23). Low energy pulses are used in order to prevent material vaporization during processing. Large-area laser-doped emitters processed over the entire cell constitute the majority of laser-doped solar cells. The fabrication process is complicated, and electrical performance is generally low as a result of weak diffusion, poor surface

28

passivation, lack of effective anti-reflective coatings, and the introduction of damage and impurities that cause high recombination rates from laser processing (1).

Selective emitters have also been developed in which laser-doping is used to form the heavily-doped contact regions. Experiments have yielded promising results, with high shunt resistance and fill factor values. The fill factor can be increased by increasing laser pulse energy density (24). However, increased fluence (energy density) can destroy surface texturing, thus increasing surface reflectance and lowering short circuit current.

It is likely that voltage and current generated by the devices are hindered by emitter formation or passivation, and not by the quality of the doping.

An upper limit for open circuit voltage, assuming that the only source of recombination in the laser-doped emitter is emitter recombination, is given in Equation

2-1, where kT/q is the thermal voltage Vth =0.026 V at room temperature, Jsc is the short circuit current, and J0e is the current density (25) (26).

Equation 2-1: Upper limit for open circuit voltage (25) (26).

A method of creating solar cell emitters using laser doping is fully described in a patent from the University of Stuttgart (27). Full area laser-doped silicon solar cells have achieved conversion efficiencies up to 18.9% (25). The cells use lasers to create liquid state diffusion of predeposited dopant layers to create defect free p-n junctions.

29

Temporal Pulse Shaping

A major difficulty in small-scale laser-materials processing is precisely modifying highly specific parts of samples while minimizing—or avoiding altogether—damage both to the chosen part and to adjacent parts. Alterations in laser power, pulse width, pulse frequency, and pulse repetitions, among other parameter adjustments, can be made to achieve desirable results.

Temporal pulse shaping allows controlled, variable release of laser energy during the lasing process. It has an effect on both the mechanical and the thermal parts of the interaction (28). A single laser pulse can be shaped so that, instead of a nearly constant rate of energy deposition, the amplitude of output energy as a function of time can be varied based on the desired interaction of the laser beam with the material. Far more literature exists for the study of material effects and laser-induced damage based on variances in laser power, pulse width, pulse frequency, and pulse repetition than based on complicated temporal pulse shaping of the laser beam. However, some degree of control over materials interactions and defect creation has been achieved by temporally shaping the pulse of the laser. It has been shown that varying the pulse rise and fall times and the overall pulse width can have profound effects on both the actual laser spot and the surrounding area (29).

Figure 2-15 shows the comparison between a laser pulse with a 10 ns rise time and a 22 ns pulse width, and a pulse with a 2 ns rise time and a roughly 3.5 ns pulse width, both as functions of time (29).

30

Figure 2-15: Profile comparison of temporal pulse shapes of a 22 ns pulse with 10 ns rise time (left) and a 3.5 ns pulse with 2 ns rise time (right) (29).

The higher energy 22 ns pulse produced larger diameter holes in the silicon (29).

Additionally, the sample machined with the longer pulse contained a significant amount of debris expelled from the hole. Small indentations appeared in the center of the holes for both pulse types, although this observation was more pronounced and occurred at a lower energy in the 3.5 ns pulse. A final observation was that a roughly 7 µm crater formed around the holes for moderate to higher energies in the 3.5 ns pulse, while the effect did not occur for the 22 ns pulse. Figure 2-16 shows images of the laser-fired holes.

31

Figure 2-16: Comparison of laser-fired spots from unsliced (top) and sliced (bottom) temporal pulse shapes (29).

Modification of the energy balance between melting/solidification and evaporation processes, as well as conductive heat losses, during laser processing is possible through variations of the temporal pulse shape (30). Figure 2-17 shows profiles for rectangular and Gaussian temporal pulse shapes less than 1 µs in duration on the left and falling triangle and rising triangle pulses shapes on the right. If overall pulse widths and pulse energy are kept constant, maximum melt pool surface temperatures are achieved—in descending order—with the rectangular, Gaussian, falling triangle, and rising triangle pulse shapes. The greatest melt pool depths are reached with the Gaussian and falling triangle pulses.

32

Figure 2-17: Rectangular/Gaussian and Falling/Rising Triangle temporal pulse shapes (31).

Temporal pulse shaping influences melting and solidification phases during laser processing, and it has been shown to have a significant effect on weld morphology, specifically through the reduction and even elimination of porosity and solidification cracking (31). Ramp-down temporal pulse shapes greater than 1 ms in duration are shown in Figure 2-18. Over a certain range, reductions in the ramp-down gradient of laser pulse power following the main welding sector reduced or eliminated solidification cracking. However, further decreases in the gradient led to intermittent solidification cracking, most likely because of the occurrence of microsegregation to the grain boundaries of the constituent materials.

33

Figure 2-18: Ramp-down pulse shapes; schematic designs (left) and measured waveforms (right) (31).

A low-ductility brittle temperature range (BTR) near the solidus affects alloys that are at high temperatures and are within the solidification temperature range (31) (32).

Within the BTR, cracking can occur when a material is subjected to tensile strain at rates higher than the critical tensile strain rate. For normal rectangular pulses, higher cooling rates and interface velocities lead to increased strain rates as the metal cools, and this promotes the formation of defects like solidification cracks (31). Drawing out the solidification process through ramp-down pulse shaping generates lower cooling and strain rates that decrease the tendency for cracking.

Temporal pulse shaping can affect drilling efficiency (28). The pulse shape images shown in Figure 2-19 show 5 ms top-hat pulses, each with a 12 ns peak at 0 ms, 2 ms, or 5 ms. The addition of the 12 ns peak at the beginning of the pulse has no effect, as no melt has yet been produced at the interaction site. However, drilling efficiency improvements of 80% and 90% were achieved by adding the peak at 2 ms and 5 ms, respectively. 34

Figure 2-19: Top-hat temporal pulse shapes with nanosecond peaks at the beginning (top), middle (center), and end (bottom) of the pulse (28).

Three categories of temporal pulse shapes are shown in Figure 2-20 (33). Total input energy is equivalent among the three, but A-Type pulses gradually increase the energy deposition rate; B-Type pulses gradually decrease it; and C-Type pulses first increase and then decrease it. The effects of these variations on maximum surface temperature and laser transformation hardening were studied. Laser transformation hardening is a heat treatment method used to obtain a hard, wear resistant surface layer while little affecting the bulk material.

35

Figure 2-20: Temporal pulse shapes (33).

Maximum surface temperature and hardening depth increase nearly linearly with power for A-Type pulses (33). B-Type pulses attain their maximum temperature quickly, and this subsequently decreases to a stable value when the absorbed laser energy cannot compensate the heat conduction losses. C-Type pulses can be looked at as combinations of A- and B-Type. Temperatures rise with increasing power for C-Type pulses, and they continue to rise, although at a decreasing rate, once the power is reduced for the remainder of the pulse.

High-carbon content steels (>0.25%) can create an extremely hard phase called martensite within the weld and the heat-affected zone as a result of the rapid cooling (34).

These steels, along with crack-sensitive alloys like certain aluminums, benefit from ramp-down laser pulse shaping. The ramp-down pulse shape reduces weld cracks and

36

porosity by slowly reducing the energy deposition rate and thus allowing for slower cooling. A diagram of this pulse is shown on the left in Figure 2-21.

Ramp-up pulse shaping can be appropriate for materials with low melting points and reflectivity, or for materials that contain volatile contaminants, coatings, or platings

(34). Ramp-up pulse shapes slowly increase the energy deposition rate until later in the pulse when the melt pool has had sufficient time to form. A diagram of this pulse shape is shown in the center of Figure 2-21.

Enhanced-spike pulse shapes are appropriate for reflective materials with high conductivities. The initial extremely high power spike begins surface melt, and it can increase the absorption rate by a factor of 20. This allows for significant reduction in energy for the remainder of the laser pulse. This leads to more consistent coupling and reduced weld spatter.

The enhanced-spike results in the highest weld penetration and a large amount of spatter. Ramp-up and ramp-down shapes reduce the spatter, and the ramp-up has the lowest penentration depth. Ramp-down controls the cooling rate and reduces solidification cracking and porosity.

37

Figure 2-21: Examples of temporal pulse shapes – Ramp-down (left), Ramp-up (center), and enhanced spike (right) (34).

Laser Alloying

Diffusion of elements into other materials to form alloys is a useful application of laser processing. Aluminum is a popular element for creating p-n junctions within silicon substrate solar cells because it is the fastest-diffusing acceptor dopant in silicon (35).

Traditionally, annealing steps at well over 1000°C are required for thorough diffusion of aluminum into the silicon lattice. These temperatures can create substantial electrical and structural damage to wafers, and this is a major cause of decreased throughput at the production level.

Laser firing removes some of the high temperature annealing steps seen in conventional solar cell manufacturing procedures. Aluminum atoms are incorporated into the recrystallized silicon during laser firing, resulting in a p-type (p+) doped silicon 38

region (1). In a laser-fired contact solar cell design, this alloying process is used to create arrays of aluminum-silicon alloy-containing contact points on the rear surface of a wafer that give rise to local back surface fields. Point contacts and the metal-semiconductor interface are typically sites of high recombination velocities, and the back surface field acts to repel minority carriers from these areas, thus improving electrical performance.

39

Physical Phenomena Governing Laser-Fired Contact Formation

Laser irradiation of a material sets in motion complex physical phenomena that can, to an extent, be predicted and controlled based on such factors as laser irradiance and material characteristics. In a generalized interaction between a laser beam and a material, the energy of the photons from the laser beam is either reflected by surface atoms or absorbed by atoms in the material’s crystal lattice. The absorbed energy is subsequently redistributed through the lattice as vibrations, or phonons, and this gives rise to heat conduction (36). As more and more energy is absorbed, the temperature of the material rises, and certain phase change thresholds can be reached.

If energy input giving rise to heat conduction is looked at as a continuum, the first threshold beyond the solid state is the melting point, at which the solid material becomes liquid. Molten material can be removed from the processing site by means of complex fluid flow forces that arise during the laser interaction. Further energy input causes the material to overcome the boiling point, at which the liquid material begins to vaporize or leave the bulk in the form of atomic-size particles.

Continued heating can raise the temperature close to the thermodynamic critical temperature of the material. At this point, mass removal is achieved by a number of different thermal mechanisms. Figure 2-22 shows a schematic diagram of a general interaction between a laser beam and a material surface (37). The diagram makes the assumption that the boiling point of the material has been reached, and that material is

40

being removed from the processing site through both vaporization and fluid flow forces, as indicated by the blue and black arrows, respectively.

Figure 2-22: Schematic of a general laser-material interaction (36).

Multiple interrelated forces, such as vapor recoil pressure, surface tension, and buoyancy, among others, contribute to movement of the molten material and overall alterations of the processing site. As mass removal from the interaction zone of the material and the laser beam is accomplished in both liquid and vapor forms, some knowledge of heat transfer, fluid mechanics, and phase transformation physics is required to fully understand the interactions (37).

41

Melting, Vaporization, and Fluid Flow

Energy absorbed by a material gives rise to vibrations, or phonons, within the material’s lattice. If these phonons are sufficiently strong, they may cause atoms and molecules to break free from the lattice, giving rise to the less-ordered liquid matter state. Laser beam photons transfer a large amount of energy to lattice atoms in a very short time, and they are thus able to bring about phase transformations extremely rapidly.

A molten pool of liquid material, termed the melt pool, forms and propagates radially outward from the point of laser incidence on the surface as the material increases in temperature and changes phase. For Gaussian laser beam profiles, such as those shown in Figure 2-23, the center of the beam is at the highest intensity. For laser incidence on a nearly uniform material, the area of the surface directly under the center of the beam will absorb the most energy and therefore its temperature will be greater than adjacent areas. This results in temperature gradients that significantly affect movement of material in the melt pool. A visual example of a laser-induced temperature gradient is shown in Figure 2-24 (38).

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Figure 2-23: Diagrams of Gaussian laser beam profiles.

Figure 2-24: Example of a temperature gradient resulting from laser processing (38).

Both conduction and convection are important for heat transfer within the weld pool (38). Conduction transfers heat by transferring energy through vibration, while convection is the actual movement of the heated material. The relative influence of each can be determined from the value of the Peclet number, which is described in Equation

2-2, where u is the typical liquid velocity, ρ is the density, Cp is the specific heat, ΔT is

43

the change in temperature, LR is the characteristic length taken as the melt pool radius, and k is the thermal conductivity (38).

Equation 2-2: Peclet number (38).

Convection is the primary mechanism of heat transfer when the Peclet number is much greater than 1. Both the liquid velocity and the radius of the weld pool increase with laser irradiance, which corresponds to an increase in the Peclet number and thus the role of convection.

The surface tension force, also known as the Marangoni force, and the buoyancy force are important driving forces for liquid flow within the melt pool (38). The

Marangoni force occurs because of the spatial variation in density between the center and the periphery of the weld pool surface and is a result of the temperature gradient. The buoyancy force arises from the spatial variation of density within the melt pool (39). The relative extent to which each of these two forces contributes to fluid flow in the melt pool can be calculated using the formula in Equation 2-3, for which g is gravitational acceleration, β is the coefficient of thermal expansion, ΔT is the temperature difference between the peak temperature and the solidus temperature, Lb is a characteristic length for the buoyancy force in the liquid pool (approximately 1/8 the width of the weld pool), µ is the dynamic viscosity of the fluid, γ is the surface tension force, and ρ is the density (38).

44

Equation 2-3: Ratio of Reynolds number (Ma) to Grashof number (Gr) (38).

The Marangoni force is typically four to six orders of magnitude greater than the buoyancy force, and consequently the latter is often neglected in calculations for melt pool convection. For most materials, the Marangoni force decreases with an increase in temperature, and this results in a radially-outward decrease in surface tension from the center of the melt pool surface (39) (40). However, this is not always the case. Surface active trace elements, such as sulfur and oxygen in iron-based alloys, can cause the trend to be reversed so that surface tension increases at higher temperatures—the term dγ/dT changes from a negative to a positive value (41) (42) (43) (44). This leads to a reversal in the direction of weld pool melt circulation—fluid flow is driven inward from the periphery of the pool, providing greater heat transport to the bottom and thus deeper penetration. Additionally, vaporization rates tend to increase in the presence of these surface active trace elements (43).

The boiling point, or vaporization point, is the temperature at which the sum total of the equilibrium partial pressures of all the component elements equals 1 atmosphere

(38). Energy input to and beyond this point can result in cavity formation within the pool

45

and material ejection. Vaporization is the ejection of atomic-size particles from the melt pool. The peak temperature at the surface of the weld pool can exceed the boiling temperature under sufficiently high laser irradiance, and thus the vapor pressure on the weld pool surface can exceed the ambient pressure, and this excess provides a driving force that pushes the vapor away from the pool surface (39).

The vapor flux creates a recoil pressure, as shown in Figure 2-25, on the evaporating surface that is driven downward into the melt pool (37) (45) (46) (47) (48)

(49) (50). This resulting recoil pressure can become the dominant driving mechanism of melt motion within the pool.

Figure 2-25: Physical model of melt removed from laser-material interaction zone (45).

Equation 2-4 describes the recoil pressure caused by laser firing onto a surface, where B0 is the evaporation constant, Ts is the surface temperature, U is the latent heat of evaporation per atom, and k is Boltzmann’s Constant (45) (49) (51). 46

Equation 2-4: Recoil pressure due to laser firing on a surface (45) (49) (51).

The simulated effects of a 1300 W laser pulse at 2 ms (top) and 4 ms (bottom) is shown in Figure 2-26 (46). The vectors demonstrate the movement of the melt within the pool, showing the radially-outward flow of molten material from the center of the melt pool. At 4 ms, the recoil pressure has increased due to higher surface temperatures and has caused the increased deformation on the pool surface (46) (47).

Figure 2-26: Effect of recoil pressure on melt pool geometry during laser processing (46).

If the surface temperature of the pool is great enough, the recoil pressure may be sufficiently strong to overcome the surface tension force that contains the melt within the

47

pool, and melt will be ejected. The relative contributions of the surface tension and recoil forces are shown in Figure 2-27 as functions of force and time for two specified irradiances on 304 stainless steel (46). It is observed that the surface tension force levels off, while the recoil force continues to grow with time. An increase in temperature accompanies the increase in recoil force. For a higher irradiance, the point at which the recoil force equals the surface tension force occurs both in a shorter time period and at a lower value of force.

Figure 2-27: Effect of laser irradiance and time on surface tension and recoil forces (46).

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In the case of melt ejection as a result of the recoil force overcoming the surface tension force, ejection typically occurs near the periphery of the melt pool surface, as is shown in Figure 2-25 and, prior to ejection, in Figure 2-26 (45) (46).

In summary, at sufficient irradiance values, the temperature of the melt pool surface will be high enough to cause vaporization that produces a recoil pressure downward into the melt pool. If the irradiance is sufficiently high to create a large enough recoil force to overcome the surface tension force of the melt pool, molten material will be ejected from the periphery of the pool surface. This ejection will be in the form of nanometer- or micron-size particles, in contrast to the atomic-size particle ejection that is vaporization.

49

Mass Ejection and Ablation

The term ―ablation,‖ when used in regards to laser-materials processing, sometimes describes the general removal of material from the processing site as a result of the physical effects caused by the interaction between the laser beam and the material. This definition is vague because material removal can occur in the form of flowing fluid, vaporized particles, or ejected molten mass, among others. At other times, ablation depicts the explosive ejection that often occurs during high irradiance laser processing.

In this paper, ablation during laser processing will refer more specifically to the latter of the two definitions: it is the rapid ejection of greater-than-atomic-size particles, as opposed to the ejection of atomic-size particles, which is the case for vaporization.

Laser processing parameters, such as irradiance, pulse shape, and fluence, have significant effects on the ablation phenomenon, and control of these parameters can lead to control or even elimination of material ablation so that factors such as laser-fired hole width and depth and debris size and accumulation can be precisely regulated.

Generalized mass removal can be accomplished by both thermal and non-thermal mechanisms (36). Excitation of non-equilibrium electrons due to absorption of laser radiation by a material can cause broken bonds which lead to atomic-size particulate ejection from the interaction zone. Thermal mechanisms of mass removal—fluid flow, vaporization, and heterogeneous (normal) and homogeneous (explosive) boiling—occur because of the transfer of energy from excited electrons to lattice phonons, which 50

subsequently conduct heat through the material (36) (48). The heat can melt the material and cause it to reach its vaporization temperature; after this point, introducing additional energy can cause atomic-size mass removal by evaporation (36). Vaporization can occur at any pulse width (52). Micron-size particulate removal can occur due to a recoil pressure exerted on the material by the evaporating vapor. The extent to which each of these mechanisms effects material ablation can be managed by alterations in laser parameters (irradiance, pulse duration, fluence, etc.).

Heterogeneous, or normal, boiling requires that the duration of the laser pulse be sufficiently long for heterogeneous bubble nucleation to commence (52) (53). Nucleation sites can initiate on a variety of disturbances, such as gas or solid impurities or defects.

The bubbles often diffuse away, although if there is sufficient time during which the melt temperature is slightly above the boiling point, the bubbles may escape from the surface due to buoyancy effects. Heterogeneous nucleation sites occur at the melt pool surface, within the bulk of the melt pool, or at the enclosing solid-liquid interface between the pool and the surrounding material.

The normal boiling process occurs from the surface to a depth within the melt pool related to the absorption length 1/µ of the material (µ is the absorption coefficient related to the wavelength of the incident light). During normal boiling, the moving vapor bubbles that sustain the process prevent substantial temperature gradients within the melt pool, and so the temperature both at the surface and in the subsurface is either at or slightly above the boiling temperature (52). This is described by the relation δT/δx ≈ 0.

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Heterogeneous boiling is not a primary mass removal mechanism in silicon (36).

Microscopic gas inclusions that act as nucleation sites for bubble formation are rare in molten silicon, and the liquid-solid interface changes rapidly during heating and cooling of silicon.

Homogeneous boiling, often referred to as explosive boiling or phase explosion, is the most efficient thermal mechanism of ablation (53). Whereas heterogeneous boiling requires temperatures only slightly above the material’s melting point, phase explosion occurs because of homogeneous nucleation when the temperature lies sufficiently close to the thermodynamic critical temperature (53). The thermodynamic critical temperature is the maximum temperature that a liquid metal can reach (37). Beyond this point only a fluid state exists; there is no longer any distinction between gas and liquid.

By most accounts the temperature must be roughly 90% of the critical temperature, although some sources have placed this value as low as 80% of the critical temperature (36) (37) (52) (53) (54). Homogeneous nucleation results in the hot region near the melt pool surface breaking down from superheated liquid into a mixture of vapor and equilibrium liquid droplets at a rate given by Equation 2-5, for which In is the rate of the reaction, ΔGn is the free energy of formation of a stable homogeneous nucleus (a sphere of vapor within the liquid), τhn is the relevant time constant, kBT is Boltzmann’s constant times temperature, and t is time (53).

Equation 2-5: Rate of homogeneous nucleation (53).

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The condition of δT/δx ≈ 0 that applies for heterogeneous boiling also applies for phase explosion. The actual removal of the melt occurs within a few picoseconds, and the superheated ablated liquid droplets carry away a significant amount of absorbed laser energy from the melt pool both as kinetic and thermal energy (55).

Phase explosion is usually accomplished by high irradiance laser firing (36).

Indeed, irradiance is likely the most significant determining factor in the geometry of a resulting ablated crater with respect both to morphology and to overall volume. The images in Figure 2-28 show the effect on crater volume from increasing laser irradiance through increased laser energy by slightly over an order of magnitude (36). The material is pure silicon, and it has been processed using a 266 nm Nd:YAG laser. The crater increases both in depth and in width, and the near-hemispherical shape is replaced with a deep, steep-walled hole. The crater bottom becomes rough with local peaks and valleys at the higher irradiance.

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Figure 2-28: Cross-sectional views of ablation craters formed at 2.6x109 W/cm2 (top) and 5.1x1010 W/cm2 (bottom) (36).

The plot in Figure 2-29 shows the trend for increasing laser irradiance on crater volume (36). A threshold irradiance exists at roughly 2.2x1010 W/cm2 at which the crater volume abruptly increases, and micron- and larger-size particulates begin to be ejected from the processing site. Yoo et al. believe that it is at this point that phase explosion begins and takes over as the primary mechanism of mass removal.

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Figure 2-29: Crater volume as a function of laser irradiance (increasing pulse energy) (36).

The SEM images in Figure 2-30 show cross-sections of laser-drilled holes in silicon (48). It is clear that increasing laser irradiance causes a significant change in the overall volume of the craters. The smooth surface morphology in the lower-irradiance images is indicates fluid flow was the primary mechanism of melt removal. Neither vaporization nor heterogeneous or homogeneous boiling dominates mass removal at lower irradiances. However, at higher irradiances, homogeneous boiling becomes the primary method of melt removal. In addition, higher irradiances lead to increases in the amount of silicon debris left on the cavity surface, although the debris size decreases.

This is shown in Figure 2-31.

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Figure 2-30: SEM cross-section images of holes drilled in silicon (48).

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Figure 2-31: SEM cross-section images of crater bottoms (48).

Figure 2-32 shows the contributions to mass removal from fluid flow, vaporization, and boiling on n-type silicon wafers processed using a Q-switched, diode-

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pumped Nd:YAG laser operating at 1064 nm (48). Fluid flow dominates mass removal at lower irradiances and tapers off to be replaced by boiling. As explained earlier, heterogeneous boiling is not a likely mass removal mechanism in silicon, and so phase explosion dominates at higher irradiances.

Figure 2-32: Simulation results of mass removal contributions (48).

An interesting point to note is that vaporization does not play a large role in mass removal at any irradiance in silicon. Vaporization increases exponentially as a function of temperature, so it follows that the farther apart the boiling and critical temperatures of a material are, the greater the extent of vaporization near the critical temperature (48).

The boiling and critical temperatures for silicon are 3538 K and 5159 K, respectively.

The difference between the two is relatively small, and this justifies the limited contribution to mass removal by vaporization.

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Subsurface superheating was once considered a thermal mechanism of mass removal. The theory states that during laser processing, the temperature within the melt pool continues to rise until vaporization begins at the top surface (52) (56) (57) (58).

Energy removed from the surface due to the vaporization reduces the surface temperature, leaving the subsurface at a higher temperature. If the subsurface temperature is great enough to cause vaporization of material at subsurface depths, high resulting pressures may be sufficient to explosively eject the intervening material.

Evidence against this theory has since been proposed, and it is no longer considered a valid mechanism for mass removal (52) (53) (54). In a boiling mechanism, strong temperature gradients cannot exist in the moving vapor bubbles that are required for boiling to continue (52). The relation δT/δx ≈ 0 describes this requirement. As a result, the entire premise of the subsurface superheating theory—namely that the subsurface remains at a significantly higher temperature than the subsurface after energy has been removed due to vaporization—is invalid.

This is pertinent to the LFC process because of the materials interactions involved in creating the contacts. Vaporization and melting mechanisms play an important role in the creation of the LFC. An understanding of the these mechanisms can lead to a greater ability to create LFCs that meet specific electrical and morphological requirements.

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Chapter 3

EXPERIMENTAL PROCEDURE

Design of Experiments

Experiments for this thesis were performed with the intent of developing a deep understanding of the processing effects of various laser systems and parameters on LFC formation. The majority of these experiments utilized test wafers prepared jointly by BP

Solar and Penn State. These wafers are not fully functional solar cells; however, their structures are sufficiently similar to those of actual cells that experiments with them provide relevant information.

The majority of this thesis includes data from experiments that utilized test wafers that comprised a roughly 275 µm boron-doped, p-type, float-zone silicon (FZ-Si) base substrate. Two variations of these wafers were created. One contained a dual-level dielectric passivation layer containing roughly 10 nm amorphous silicon (a-Si) and

100 nm silicon dioxide (SiO2), both deposited by plasma-enhanced chemical vapor deposition (PECVD). This wafer is referred to as Wafer 1 Type A, or W1A. The other wafer structure contained only an 80 nm a-Si passivation layer and is referred to as Wafer

1 Type B, or W1B. The primary purpose of the a-Si and SiO2 layers is to close dangling bonds on the edge of the FZ-Si substrate (passivation), thus increasing carrier lifetimes.

Similar dielectric layers in some solar cells are deposited at specific thicknesses to act as

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optical back reflectors to increase light absorption. In nearly all cases, identical experiments were performed for both wafer structures.

A 0.5 µm Al layer was placed on both the top and bottom sides of the sample wafers via electron-beam deposition (e-beaming). The Al provided the conductive layers necessary both to potentially create the desired Al-Si alloy necessary to achieve an ohmic contact during laser-firing and to perform post-processing electrical characterizations.

Cross-sectional diagrams of the W1A and W1B test sample structures are shown in

Figure 3-1. A final annealing step for 10 minutes at 200ºC on a hot plate was performed to enhance electrical properties by improving passivation quality.

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Figure 3-1: Cross-sectional diagrams of W1A and W1B test sample structures.

Early experiments used another type of wafer which was designated Wafer 0

Type A or Wafer 0 Type B, or simply W0A or W0B. These wafers contained a 200 µm boron-doped p-type Mono2™ silicon layer, a front-side screen-printed aluminum layer, an

80 nm silicon nitride (SiN) dielectric passivation layer deposited with PECVD, and a backside of e-beamed aluminum either 0.5 µm or 5.0 µm in thickness. Wafers with

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0.5 µm aluminum were called W0A, and those with 5.0 µm aluminum were called W0B.

Figure 3-2 shows a cross-sectional diagram of the W0A and W0B test structure.

Figure 3-2: Cross-sectional diagram of W0A and W0B test sample structures.

Screen-printing is a very common process in solar cell manufacturing that involves depositing a conductive metal paste and forcing it through a screen onto the surface of a wafer. Mono2™ silicon is a type of silicon that was developed by BP Solar

(59) (60). It displays large volumes of monocrystallinity with minimal lattice dislocation densities. Simply, it strives to achieve monocrystalline silicon quality at the manufacturing expense of multicrystalline silicon. A comparison between a multicrystalline silicon wafer (left) and a Mono2™ silicon wafer (right) is shown in

Figure 3-3 (59). The wafers have undergone defect-etching to make the differences more pronounced. The dislocations are much more visible in the multicrystalline silicon.

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Figure 3-3: Comparison of multicrystalline silicon and Mono2™ silicon (59).

The initial experimental plan for these wafers was to create LFCs using multiple laser systems with a variety of processing parameters. These parameters are dependent on the capabilities of the individual lasers and will be described in detail later in this thesis. As previously stated, these test samples are not functional solar cells, and as such, conventional solar cell performance tests such as open-circuit voltages, short-circuit currents, and conversion efficiencies could not be measured. Instead, we attempted to determine and narrow-down optimal laser processing conditions (based on individual lasers) by studying electrical characteristics of the processed wafers and morphological and cosmetic attributes of the LFCs themselves. In this way we were able to develop both a quantitative and a qualitative understanding of the LFC process and its relationship to actual solar cell performance.

Results from four laser systems are provided in this thesis. These lasers operate at wavelengths that span the electromagnetic spectrum from ultraviolet to near-infrared

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light, and at pulse durations in the microsecond and nanosecond regimes. The parameters of the four laser systems described in this thesis are shown in Table 3-1.

Laser System Wavelength Pulse Duration

IPG Photonics Ytterbium Single 1070 nm ~20 µs – 10 ms Mode Fiber Coherent AVIA Q-Switched 355 nm 30 ns Frequency-Tripled Quantel Brilliant Q-Switched 532 nm 4 ns Nd:YAG Lambda Physik KrF Excimer 248 nm 20 ns

Table 3-1: Laser system parameters.

It is useful to experiment with various wavelengths because the absorption of each material in the test wafers changes with the wavelength of incident light. The absorption coefficient for silicon, for example, is highest at the 1070 nm wavelength of the IPG fiber laser for these experiments (13). Aluminum absorption is always lower than that of silicon.

The two regimes of pulse durations allow for study of laser-materials interactions related to LFC formation at various pulse lengths. Longer microsecond pulses may allow for adequate melting and mixing of materials, as there is a longer amount of time during which the processing site is molten, and melt-flow processes as described in the literature review of this thesis can occur. Material vaporization thresholds are often not reached, so mass removal in the form of ablation is uncommon.

Nanosecond pulses, in large part because of the high irradiances that they produce, often ablate material from the laser-material interaction zone, and thorough

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mixing of molten material is often sacrificed. For the wafer structures described in these experiments, ablation of the backside aluminum layer and mass-ejection of the underlying silicon can have significant and often detrimental effects on the electrical contacts.

Planned electrical characterizations consisted primarily of resistance and carrier lifetime measurements – two important indicators of solar cell performance. Resistance measurements provide a quantifiable means of comparing the electrical contact quality achieved by various laser conditions. Carrier lifetime measurements are intended to quantify the amount of laser-processing-induced damage and impurity introduction and their potential effects on solar cell performance. Various microscopy and machining methods were used to develop an understanding of the morphological effects of the LFC process. All of these efforts combine to produce a well-constructed and highly-detailed representation of an LFC.

Figure 3-4 shows the mask used by BP Solar to selectively metallize the test wafers with aluminum, and Figure 3-5 shows a completed Type A test wafer (W1A).

The 7x7 arrays of 1 mm x 1 mm aluminum square pads were used for parameter development studies. An image of a 7x7 LFC parameter development array is shown in

Figure 3-6 next to a dime for scale. The larger 10 mm x 10 mm aluminum square pads were used for processing 9x9 and 18x18 arrays of identical-condition LFCs.

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Figure 3-4: Mask used for aluminum metallization of W1A and W1B.

Figure 3-5: Completed test wafer (W1A). 67

Figure 3-6: 7x7 LFC parameter development matrix (W1B); shown next to dime for scale.

Parameter development experiments were performed for each laser. This generally consists of processing over large ranges of whichever parameters could be changed for the laser. For example, a 7x7 pad wafer, as shown in Figure 3-6, may contain LFCs fired with 49 different parameters: 7 different powers and 7 different pulse durations. Specific parameters that were studied for each laser are described later in this thesis.

In many cases, certain laser conditions caused cracking or complete destruction of the sample wafers. In these cases, the parameter sets were constantly narrowed in size.

Initially, resistance measurements were compared with SEM images in order to develop an understanding of the effects of specific parameters. A low resistance is desired, but not at the expense of an LFC that contains considerable damage. Initially, without lifetime measurements or any other means of quantifying ―laser damage,‖ visual

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inspections of the LFCs using an SEM were used to rate conditions based on cosmetic appearance. An LFC fired at a high power may result in a low resistance, but ablation of the underlying silicon could lead to shunts or high recombination rates, which would negatively affect final cell performance.

A small number of specific parameters were chosen to be repeated in either 9x9 or

18x18 LFC arrays on the large 10 mm x 10 mm aluminum pads. For these wafers, the resistance of the entire wafer (averaged for 81 or 324 contacts for the 9x9 or 18x18 LFC arrays, respectively) could be measured. These experiments are meant to more closely represent the processing that would occur on an actual solar cell.

Simple resistance measurements were studied to quantify laser parameters in terms of their abilities to produce electrical contacts. A multi-point probe station was used for the measurements. An image of a laser-processed wafer lying on the grounded platen is shown in Figure 3-7 and Figure 3-8. Since each 1 mm x 1mm aluminum pad had been photolithographically isolated, it was possible to measure the resistance for specific LFCs.

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Figure 3-7: Resistance measurements probe station.

Figure 3-8: Resistance measurements probe station with processed wafer on grounded platen.

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One probe sent a current through the wafer while another probe measured the voltage drop. A graph showing the current and voltage values allowed for resistance measurements to be taken. If ohmic contact is made, the curve of the current-voltage plot is linear, and its slope is the resistance, as given by Ohm’s law (V = IR). An image of the current-voltage plot for an LFC displaying ohmic contact is shown in Figure 3-9. If the curve of the plot is nonlinear, ohmic contact has not been made, as shown in Figure 3-10.

Figure 3-9: Current-voltage plot for LFC displaying ohmic contact.

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Figure 3-10: Current-voltage plot for LFC displaying non-ohmic contact.

The equation for resistance is given in Equation 3-1. The variable ρ represents the resistivity of the material. Float-zone silicon generally has a resistivity between

1 and 5 Ω cm. The variable L is the thickness of the conductor—in this experiment the aluminum is 0.5 µm—and the variable A is the cross-sectional area of the current flow.

Equation 3-1: Resistance as a function of resistivity, conductor thickness, and cross-sectional area.

The cross-sectional area of current flow is difficult to determine for LFC experiments. It is speculated that contact between the e-beamed aluminum layer and the float-zone silicon substrate is made through aluminum strands that reach across the 72

passivation layers after laser-firing. Results have shown that resistance decreases with increased LFC diameters. These results support this theory, as an LFC with a greater diameter has more area for these aluminum strands to contact the silicon.

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IPG Photonics Ytterbium Single Mode Fiber 1070 nm Laser

In August, 2009 IPG Photonics provided the Penn State Applied Research Laboratory with a 200 W ytterbium single mode fiber laser, model YLR-200-AC. The laser system had been reconfigured from a formerly 500 W laser. The fiber laser produces a 1070 nm wavelength and a nearly Gaussian beam profile. Calculated spot sizes of roughly 20 µm diameter have been verified both with beam analysis simulations and with focusing tests.

The fiber laser can produce pulses between 10 to 20 µs and 10 ms at a maximum peak power of 500 W. A useful attribute of the fiber laser is the ability to temporally shape the output pulses. This ability enabled us to study the effects of a single pulse with varying power levels. The majority of the experiments in this thesis were performed using the fiber laser.

Early experiments with the fiber laser proved that a high degree of control over the focus of the beam was necessary to produce consistent, repeatable results. An LFC that has been processed at focus looks and behaves (electrically) differently than another contact that has been processed out of focus using the same laser conditions. ZEMAX beam simulation software was used to calculate the Rayleigh range based on our optics train and laser system. The Rayleigh range is the distance over which the beam divergence is minimal enough that processing at any point within it can be considered as processing at focus. Specifically, the Rayleigh length (half the Rayleigh range) is the distance over which the radius of the beam is increased by a multiple of the radius at 74

the focal point. Figure 3-11 shows a Gaussian beam diagram and specific measurements acquired using the ZEMAX beam analysis software. The Rayleigh length based on this analysis was found to be roughly 350 µm.

Figure 3-11: ZEMAX beam analysis simulation.

Using on the results of the ZEMAX analysis, we began experiments to find the focal point based on the optics train shown in Figure 3-12. Identical-parameter laser shots were fired at recorded incremental distances that began out of focus in the direction of the laser head, passed through focus, and continued out of focus once more in the direction away from the laser. SEM images showed the morphologies of the resulting laser-fired shots, which are shown in Figure 3-13. In the figure, shots fired from 75

incremental point x30.5 mm to x30.8 mm (a 300 µm distance) are highly similar in morphology, with the exception of the misfired shot at x30.6 mm. This range of comparable shots corresponds well to the ZEMAX-calculated 350 µm Rayleigh range.

We thus chose a point at the center of this range to be considered as the focal point for the duration of the experiments with the IPG fiber laser.

Figure 3-12: IPG fiber laser optics train diagram.

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Figure 3-13: Focusing tests for IPG laser using silicon wafer.

Experimental results for out-of-focus laser processing tests are shown in Figure

3-14. Laser parameters of 55 W and 45 W for 115 µs were used. The laser beam diagram describes the beam converging after the focusing optic, the 350 µm Rayleigh range, and the beam diverging after the focal point. Position A is 500 µm outside of the focal point away from the laser source, while Position C is 500 µm outside the focal point toward the laser source. Position B is the center position within the focus.

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Figure 3-14: Comparison of LFCs fired out-of-focus away from the laser source (A), out-of-focus toward the laser source (B), and at focus (within the Rayleigh length).

SEM images in Figure 3-14 show that the LFC diameter is larger when it has been fired out-of-focus. This is sensible considering the increased beam diameter. LFCs fired at Position C appear to have been more highly affected than those fired at Position A.

The contacts are slightly larger and deeper. This is further verified with electrical resistances shown in Figure 3-15. Plot B shows resistance values taken from LFCs that have been fired at focus. At Position A, no contacts appeared below 45 W, and at

Position C, none appeared below 40 W. This means that the laser energy was reflected rather than absorbed, and thus the material irradiance threshold was not achieved. It was

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speculated that resistances measured between the top and bottom aluminum layers would be higher for contacts fired out of focus, but this has been shown to not necessarily be the case, at least for the relatively small sample size tested in this experiment. However, the experiment demonstrates the necessity of maintaining strict control over the laser focus.

Figure 3-15: Comparison of electrical resistances of LFCs fired at positions A, B, and C, corresponding to Figure 3-14.

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Parameter Development

Once control over the focal region had been achieved, a variety of laser parameters were tested in order to understand the effects of processing over a range of laser conditions.

Experiment 017 (Exp017) tested a broad range of pulse durations (40 µs to 640 µs) and a narrow range of powers (25 W to 50 W). A plot of resistance measurements as a function of laser power is shown in Figure 3-16. Specific values are given in Table 3-2. Shading is provided to reflect the relative magnitude of resistance. The 500 Ω value for 25 W and

40 µs is a fictitious value used because no LFC was fired at those conditions. It is clear that the resistance decreases with increases both in pulse duration and in laser power.

These trends match those of the morphological change in LFC diameter and depth, as shown in Figure 3-17.

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Figure 3-16: Plot of resistance as a function of power for Experiment 017.

Table 3-2: Resistance values for Experiment 017. 81

Figure 3-17: SEM images of LFCs from Experiment 017.

With increasing pulse duration and laser power (increases in total pulse energy), the size of the LFC also increases. The heat-affected zone around the contact becomes more prominent with increased laser pulse energy. The SEM images in Figure 3-18 and

Figure 3-19 demonstrate the extent to which different laser parameters affect the LFC morphology and how this further affects the electrical properties. The center portion of the contact in Figure 3-18 is roughly 100 µm in diameter. The same region in Figure

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3-19 is roughly 20 µm. Figure 3-18 was processed at 49 W for 640 µs. At a beam size of

30 µm, this equates to an irradiance of roughly 1.6 x 106 W/m2, which achieved a resistance of 32 Ω. The irradiance for the LFC shown in Figure 3-19 is roughly 1.0 x 106

W/m2 (29 W for 40 µs), and it achieved a resistance of 400 Ω. Total energies for LFCs fired in Figure 3-18 and Figure 3-19 are 31 mJ and 1 mJ, respectively. Because of the relatively long microsecond-range pulse durations, ablation effects are not observed in these LFCs.

Figure 3-18: LFC processed at 49 W and 640 µs with resistance of 32 Ω.

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Figure 3-19: LFC processed at 29 W and 40 µs with resistance of 400 Ω.

These images also demonstrate the need to consider a balance of resistances with laser damage effects. The resistance of the LFC in Figure 3-19 is far greater than that of

Figure 3-18, however the amount of laser damage caused in the latter LFC will likely lead to adverse affects, such as diminished lifetimes.

Figure 3-20 and Figure 3-21 demonstrate the ability to achieve comparable resistance values and contact morphologies while using different laser parameters. The

LFC in Figure 3-20 was processed at 80 W and 150 µs and achieved a resistance of 92 Ω.

The LFC in Figure 3-21 was processed at 60 W and 350 µs and achieved a resistance of

91 Ω. Despite the differences in processing conditions, the two LFCs are highly similar in appearance and electrical characteristics.

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Figure 3-20: LFC processed at 80 W and 150 µs with resistance of 92 Ω.

Figure 3-21: LFC processed at 60 W and 350 µs with resistance of 91 Ω.

A narrow range of pulse durations (40 µs to 100 µs) and a broad range of laser powers (25 W to 85 W) were tested in Experiment 018 (Exp018). Figure 3-22 and Table

3-3 show the LFC resistances as a function of laser power and the specific resistance

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values for each laser parameter, respectively. As before, the shading in the table is used to show relative resistance magnitudes, and the 500 Ω resistance for 25 W and 40 µs is fictitious.

Figure 3-22: Plot of resistance as a function of power for Experiment 018.

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Table 3-3: Resistance values for Experiment 018.

Trends similar to those of Experiment 017—decreasing resistance with increasing power and pulse duration—are observed. Figure 3-23 shows SEM images that demonstrate the change in LFC morphology with varying laser conditions for Experiment

018. As before, the overall size of the LFCs increase as laser power and pulse duration increase.

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Figure 3-23: SEM images of LFCs from Experiment 018.

The image in Figure 3-24 shows the morphology of an LFC processed at high laser conditions (85 W and 100 µs). The center portion of the contact is roughly 140 µm in diameter and has a prominent surrounding heat-affected zone. The image has been taken at a 60º angle from the normal to show the depth of the crater and center peak.

Again, the long pulse durations seem to avoid ablation effects.

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Figure 3-24: LFC processed at 85 W and 100 µs with resistance of 35 Ω.

Experiment 019 (Exp019) tested a mid-range of laser parameters taken from the previous two experiments. Laser power from 25 W to 55 W and pulse durations from 40

µs to 190 µs were used to fire LFCs on W1A and W1B wafers. A comparison of the resulting resistance value as functions of power is shown in Figure 3-25. Three regions are specified on each graph to indicate different regimes of laser-induced effects.

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Figure 3-25: Plots of resistance as a function of power for Experiment 019 (W1A and W1B). 90

Specific resistance values for Experiment 019 are given in Table 3-4 and Table

3-5. Resistances are lower for LFCs processed on W1B than they are for those processed on W1A. This is attributed to the additional silicon dioxide dielectric layer on W1A, as it is the only difference between the two wafer structures. It is possible that the additional layer absorbs or deflects laser energy, thereby limiting the effect of laser firing in such a way that the electrical contact produced is not as strong as it would otherwise be.

Table 3-4: Resistance values for Experiment 019, W1A.

Table 3-5: Resistance values for Experiment 019, W1B.

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SEM images of the LFCs processed on W1A and W1B are shown in Figure 3-26.

There is not a highly noticeable difference in morphology between the two wafer types.

The trends in changing size and shape that have been shown in previous experiments are also evident in Experiment 019.

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Figure 3-26: SEM images of LFCs from Experiment 019 (W1A and W1B). 93

LFC Size Comparisons

The following four series of plots—Figure 3-28, Figure 3-29, Figure 3-30, and Figure

3-31—show comparisons of LFC perimeters and areas with resistances and laser parameters for the three previously described experiments. Figure 3-27 shows the diameter measurements that were used to calculate spot areas and perimeters. The larger diameter (479 µm) is a metric representation of the heat-affected zone. Since this region is not present in all LFCs, the smaller, inner diameter (118 µm) was used for measurements.

Figure 3-27: Diameter measurements used for spot size calculations.

Figure 3-28 shows the tendency of the diameter—and thus the perimeter for round

LFCs—to increase as power and pulse duration increase. Figure 3-29 demonstrates how 94

the perimeters of the LFCs decrease as resistance values increase. Figure 3-30 shows how the product of resistance and LFC area generally increases as power increases, and

Figure 3-31 demonstrates how LFC area increases as resistance decreases, as is expected for round LFCs based on previous data.

These data tie together the relationship between the electrical resistances of the wafers with the LFC morphologies. They provide a quantifiable means to describe the physical condition of an LFC so that it can be related to other measurable attributes.

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Figure 3-28: LFC perimeter as a function of power for Experiments 017, 018, 019. 96

Figure 3-29: LFC resistance as a function of perimeter for Experiments 017, 018, 019. 97

Figure 3-30: LFC resistance x area as a function of power for Experiments 017, 018, 019. 98

Figure 3-31: LFC resistance as a function of area for Experiments 017, 018, 019. 99

Energy Dispersive X-Ray Spectroscopy (EDS)

Energy dispersive x-ray spectroscopy (EDS) was used to attempt to identify which materials were present at certain areas of the LFCs. EDS identifies and quantifies elemental composition in small areas based on characteristic x-rays that are produced when electrons impinge on a surface. A major goal of the LFC process is to produce an aluminu-silicon alloy to provide a low resistance ohmic contact between the silicon and aluminum through the passivation layer, and EDS techniques provided insight into where that alloy may have formed.

Figure 3-32 shows EDS measurements taken on an LFC fired on W1A at 65 W and 50 µs. Graph A is taken outside the contact area, and, as expected, there is a significant amount of aluminum present in the measurement. The e-beamed alumimum layer is only 0.5 µm thick, and the EDS measurement takes readings up to a few microns in depth. This explains the presence of silicon in Graph A. Graph B represents the porous area surrounding the center of the LFC. This area shows similar quantities of aluminum and silicon, with a predominance of silicon. Graph C is taken for the center region of the LFC. The graph shows a significant presence of silicon with only a trace of aluminum. Laser processing likely vaporizes the thin layer of aluminum that covers the silicon substrate. It is currently impossible to discern if the presence of aluminum in this region represents the aluminum portion of the desired aluminum-silicon alloy, or if it is simply trace amounts of aluminum remaining on the surface after processing.

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Figure 3-32: EDS material compositions at specified positions on an LFC.

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Temporal Pulse Shaping

Preliminary temporal pulse shaping experiments were conducted using the IPG fiber laser. Pulses of 30 µs durations and either 40 W or 80 W laser power were used to simulate either initial or ending ―peaks‖ in input laser energy. Beginning or trailing pulse

―bodies‖ of 35 W laser power and 50 µs, 350 µs, or 650 µs pulse durations were used to complete the temporal pulse shapes. The 30 µs peaks are shown to the left in Figure

3-33, and the 50 µs, 350 µs, and 650 µs bodies are shown to the right.

Figure 3-33: Pulse “peaks” and pulse “bodies” used for temporal pulse shaping experiments.

The intention of the initial peak is to initialize state transformation and increase absorption. Following the initial peak with a lower power pulse region could enhance diffusion and reduce sloshing of the molten material. Beginning with a lower power pulse region and ―ramping‖ to a high power peak could be used to gradually ease into the melting process to avoid the detrimental effects of vaporization and recoil pressure.

Results of the pulse shaping experiments are shown in Figure 3-34. Pulses fired with 80 W peaks achieve the lowest resistances. Trends for resistances based on total input energy are not clearly observed. 102

Figure 3-34: Images and resistances of temporal pulse shapes used for LFC processing.

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As stated, only preliminary studies have been conducted for temporal pulse shaping. However, based on the literature, the potential benefits of the technique warrant further investigation.

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Coherent AVIA Q-Switched Frequency-Tripled 355 nm Laser

The Coherent AVIA is a Q-switched laser that operates at a 355 nm wavelength, the third harmonic of the common 1064 nm. The AVIA produces 30 ns pulses at frequencies up to 100 kHz. Output laser energy is based on frequency and is described by the red curve in Figure 3-35. The maximum energy of roughly 250 µJ is achieved at 10 kHz. The focused spot size is roughly 20 µm in diameter.

Figure 3-35: Power/energy curve as a function of frequency for AVIA laser.

W1A and W1B were both tested at the laser parameters described in Table 3-6.

The frequencies and their corresponding energies are shown, along with the number of shots fired at each condition. LFCs fired with 10 shots produced non-ohmic contact at all 105

frequencies. No LFC was observed for 1 shot at 100 kHz. LFCs fired on W1A continually produced non-ohmic contacts. However, two specific laser conditions were chosen for W1A and tested multiple times to obtain average values. This data is described in Table 3-7.

Table 3-6: Resistance values for W1B.

Table 3-7: Averaged resistance values for W1A.

For all experiments, four total LFCs were fired on each aluminum pad.

Resistance measurements provided are averages of these four contacts that provide an estimate of the resistance of a single LFC. Figure 3-36 shows an SEM image of the four identical LFCs on a single aluminum pad that contributed to resistance measurements.

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Figure 3-36: SEM image of four LFCs used for resistance measurements.

Figure 3-37 shows resistance values as a function of the number of shots fired for all frequencies tested. In general, higher frequencies (lower energies) lead to higher resistances, as do increases in the number of shots fired. However, well-defined trends are not observed. High energies likely cause better aluminum diffusion. Increasing the number of shots increases the amount of ablation, which removes contact areas between the silicon and the aluminum and prevents the creation of ohmic contact. The AVIA laser did not produce LFCs that achieve consistent resistance values based on frequency or number of shots. This is evident in the somewhat ambiguous Figure 3-37.

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Figure 3-37: Plot of resistance as a function of number of shots for W1B.

Figure 3-38 shows the relationship of decreasing resistance with increasing energy and with decreasing number of shots. However, for low energy values this trend is not observed. At these conditions, there is not sufficient input energy to produce well- defined contacts, and the material thresholds for melting and diffusion are not reached.

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Figure 3-38: Plot of resistance as a function of laser energy for W1B.

Figure 3-39 and Figure 3-40 show the arrays of LFCs fired by the AVIA laser on

W1A and W1B, respectively. Frequencies (and energies) and number of shots are indicated in the figures. As frequency increases (energy decreases) the size, depth of penetration, and overall material effect of the LFC diminishes. Visible LFCs were produced at 100 kHz (10 µJ) for only 2 shots and 10 shots for W1B (images are not shown in the figures). At this low input energy, it is difficult to achieve consistent

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results, and small imperfections in the wafer could make significant differences in LFC formation. Increasing the number of shots increases the material effects of the LFC, which produces excessive ablation effects, material spatter, and deep penetration.

Figure 3-41 and Figure 3-42 show LFCs fired at 5 kHz (235 µJ) for 2 shots and 10 shots, respectively. The increased depth of penetration and ablation effects are clearly shown for both LFCs, although they are far more pronounced for the one fired at 10 shots.

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Figure 3-39: SEM images of LFCs for W1A. 111

Figure 3-40: SEM images of LFCs for W1B. 112

Figure 3-41: LFC fired for 2 shots at 5 kHz (235 µJ).

Figure 3-42: LFC fired for 10 shots at 5 kHz (235 µJ).

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Quantel Brilliant Q-Switched Nd:YAG 532 nm Laser

The Quantel Brilliant is a Q-switched Nd:YAG laser that was fitted with a second harmonic generator to produce a 532 nm wavelength laser beam. The Quantel laser produces a 4 ns pulse at an energy of 165 mJ. Multiple shots were fired for three separate tests on both W1A and W1B. The time between each shot was on the order of 1 s.

Resistance values for these tests are provided in Table 3-8. LFCs fired for 1 shot on

W1A did not produce ohmic contact.

Table 3-8: Resistance values for W1A and W1B.

Figure 3-43 shows plots of resistance values as a function of the number of shots fired for each of the three separate tests. Values for 1 shot are excluded from the plot for

W1A because the LFCs did not produce ohmic contact. The plots suggest that there is not a clearly defined or recognizable relationship between resistance and the number of

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shots fired. It is possible that initial shots marked the wafer in such a way that subsequent shots were better absorbed. It is also possible that recognizable resistance trends were not observed because of the relatively small amount of data. If there is a significant margin of error in the measurements, far more tests would be required to observe clear relationships.

Figure 3-43: Resistance values plotted by test number and number of shots for W1A and W1B.

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Figure 3-44 shows SEM images taken for the different processing conditions for

W1A and W1B. Differences between LFCs fired on the two wafers types are minimal.

As the number of shots increases, the effect on the wafer becomes more pronounced.

The heat-affected zone surrounding the center crater becomes larger, and the center crater becomes deeper and wider. Ablation affects and spattered material also increase with the number of shots fired.

Figure 3-44: SEM images of LFCs fired on W1A and W1B. 116

Figure 3-45 and Figure 3-46 show LFCs fired with 1 shot and 4 shots, respectively, on W1B. It is clear that increasing the number of shots significantly increases the depth of the hole and the amount of material that is ablated, as well as the diameter of the resulting crater.

Figure 3-45: 1 Shot fired on W1B.

Figure 3-46: 4 Shots fired on W1B.

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Lambda Physik Excimer KrF 248 nm Laser

The Lambda Physik excimer laser operates using a krypton fluoride (KrF) gas mixture that produces a 248 nm wavelength beam. The laser cavity produces a rectangular- shaped beam, and because it is focused with a conventional quartz lens, the rectangular shape is displayed to a certain extent in the LFCs on the wafer. Each spot was fired for a

20 ns pulse duration, and the pulse energy varied in increments of 5 mJ from 50 mJ to

375 mJ.

Figure 3-47: Optical microscope image of excimer laser-fired LFC.

Figure 3-48 shows the relationship between resistance and laser energy for the excimer laser. As laser energy is increased, the resistance decreases. Figure 3-49 demonstrates how resistances decrease as laser-fired spot areas increase. Figure 3-50 shows the relationship between laser energy and spot area. The appearance of groupings 118

in the data is due to the fact that not all laser energy levels were measured on the wafer.

There is a nearly linear correlation between increasing laser energy and increasing spot area, indicating that higher laser energy (and thus higher laser power, if pulse duration is kept constant) will result in larger laser-fired spots.

Resistance values for the excimer laser are lower than those of any other laser. A potential explanation for this is the fact that LFCs fired with the excimer laser are the largest produced by any laser in this research. Based on relationships discovered with the

IPG fiber laser, and on the relatively low resistances and large spot sizes achieved with the Quantel laser, the theory that larger diameter LFCs provide more area for electrical contacts to be made between the aluminum and silicon substrate is supported.

Figure 3-48: Resistance as a function of laser energy.

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Figure 3-49: Resistance as a function of laser spot area.

Figure 3-50: Laser spot area as a function of resistance.

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Laser-Fired Contact Array and Re-Metallization Experiments

In a production line, it is likely that only one specific laser parameter will be used to fire the LFCs on a solar cell. This concept of repeated parameter arrays was studied using

9x9 and 18x18 LFC arrays fired on 10 mm x 10 mm aluminum pads. Images of laser processed 9x9 and 18x18 LFC arrays are shown in Figure 3-51. Parameters were selected so that a variety of LFCs (based on morphology and resistance) could be studied in repeated arrays.

Figure 3-51: 18x18 LFC array (left) and 9x9 LFC array (right).

A 9x9 or 18x18 LFC array allowed us to take resistance measurements on an entire 10 mm x 10 mm aluminum pad, averaged for all 81 or 324 LFCs. Since contacts effectively comprise resistances in a parallel circuit, Expected Resistance values are determined by dividing previously measured resistances of a single LFC fired at a 121

specific parameter by 81 (for 9x9 arrays) or 324 (for 18x18 arrays). Measured Resistance values are the average resistances measured at various locations (corners and center of the array, measured multiple times) on the 10mm x 10 mm aluminum surface.

In some cases, multiple wafers were processed, and they are designated A and B.

Table 3-9 shows the laser parameters and resistance values for the 9x9 LFC arrays fired with the IPG fiber laser, and Table 3-10 shows the values for the 18x18 LFC arrays.

Measured resistance values are close to expected values, showing that LFC resistance scales closely with the number of LFCs; i.e. the overall resistance of an LFC array with contributions from a number of LFCs can be predicted by dividing the resistance of a single LFC fired with the same parameters by the number of LFCs in the array.

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IPG Fiber 9x9 Wafer/ Laser Expected Measured Post Experiment Parameters Resistance (Ω) Resistance (Ω) Re-Metallization (W, µs) Resistance (Ω) W1A Exp021 30, 65 386/81 = 4.77 A: 5.37

B: 5.76 B: 6.15

W1B Exp021 30, 65 254/81 = 3.14 A: 4.53

B: 3.23 B: 5.30

W1A Exp022 55, 140 151/81 = 1.86 A: 2.38

B: 2.92 B: 3.98

W1B Exp022 55, 150 97/81 = 1.20 A: 3.08

B: 1.41 B: 4.45

W1A Exp023 50, 40 202/81 = 2.49 4.41 5.63

W1B Exp023 50, 40 141/81 = 1.74 3.23 3.17

W1A Exp024 30, 215 205/81 = 2.53 3.72 4.93

W1B Exp024 30, 215 185/81 = 2.28 3.78 4.22

Table 3-9: Resistance values for 9x9 LFC arrays fired with IPG fiber laser, before and after re- metallization.

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IPG Fiber 18x18 Wafer/ Laser Expected Measured Post Experiment Parameters Resistance (Ω) Resistance (Ω) Re-Metallization (W, µs) Resistance (Ω) W1A Exp023 50, 40 202/324 = 0.62 2.02 NA

W1B Exp023 50, 40 141/324 = 0.44 3.25 NA

W1A Exp024 30, 215 205/324 = 0.63 3.74 NA

W1B Exp024 30, 215 185/324 = 0.57 2.86 NA

Table 3-10: Resistance values for 18x18 LFC arrays fired with IPG fiber laser.

AVIA 9x9 Wafer/ Laser Expected Measured Post Experiment Parameters Resistance (Ω) Resistance (Ω) Re-Metallization (kHz, µJ) Resistance (Ω) W1A 1 Shot 5, 235 10.2 16.7 16.5

W1B 1 Shot 5, 235 3.2 3.7 3.6

W1A 5 Shots 50, 50 19.0 34.1 39.8

W1B 5 Shots 50, 50 7.2 5.9 5.9

Table 3-11: Resistance values for 9x9 LFC arrays fired with the AVIA laser, before and after re- metallization.

Quantel 9x9 Wafer/ Laser Expected Measured Post Experiment Parameters Resistance (Ω) Resistance (Ω) Re-Metallization (Shots) Resistance (Ω) W1A 2 0.3 2.16 1.37

W1A 4 0.2 0.73 1.02

Table 3-12: Resistance values for 9x9 LFC arrays fired with the Quantel laser, before and after re- metallization.

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Laser parameters and resistance values for the AVIA laser and the Quantel laser are shown in Table 3-11 and Table 3-12, respectively. As with the IPG fiber laser, resistances for single LFCs scale closely with LFC arrays. These results further prove that low overall resistances can be achieved using large arrays of LFCs.

Based on SEM images and EDS measurements, it was conjectured that a significant amount of e-beamed aluminum had been ablated from the surface of the LFC.

We therefore applied a second 0.5 µm aluminum layer via e-beam deposition over the original aluminum layer and the LFCs. The samples were not cleaned prior to re- metallization to remove any oxide layers or debris. Post Re-metallization values refer to the resistance measurements taken after this process had been completed.

The theory behind this experiment is that electrical contact is made through aluminum strands that reach between the back side aluminum layer and the float-zone silicon substrate. It was speculated that resistance values would improve (decrease) if an additional aluminum layer were deposited over the LFCs.

The microsecond pulses of the IPG fiber laser tend to ablate aluminum over the contact. However, most of the original material, including some of the aluminum, is left at the processing site; there is little mass ejection of the underlying silicon. For this reason, it was thought that coating these LFCs with an extra aluminum layer would not improve resistances, and as shown in Table 3-9, resistance values increased with re- metallization.

It was expected that the re-metallization technique would work best for the LFCs fired with the nanosecond pulses of the AVIA and Quantel lasers. Nanosecond pulses

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cause a high degree of ablation, and it was assumed that re-metallization would improve electrical characteristics by recreating electrical contact between the aluminum and the silicon substrate. Results in Table 3-11 and Table 3-12 show that re-metallization is better suited for nanosecond pulses. Resistance values neither improved nor deteriorated for the AVIA laser after re-metallization. For the Quantel laser, the resistance for one sample improved and the resistance for the other deteriorated slightly.

Results for this experiment could potentially have been improved by thoroughly cleaning the wafers prior to re-metallization. A native oxide layer that forms on aluminum, in addition to debris deposited on the wafer surface during laser processing, likely diminished the quality of the e-beamed aluminum deposition and should have been removed.

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Laser-Fired Contact Carrier Lifetime Experiments

Laser parameters are relatively easily compared to resulting LFCs in terms of resistances and morphologies. However, laser processing can cause serious defects—such as inclusion of impurities, crystallographic dislocations that demonstrate unpredictable electrical properties, and thermal strain effects outside the melted regions—in silicon that affect carrier lifetimes (1). Carrier lifetimes refer to the amount of time during which photogenerated electron-hole pairs remain separated before recombination.

Traditional microelectronics are not highly affected by this, but recombination centers in solar cells reduce lifetimes and thus overall cell performance. In trying to balance resistances with LFC morphologies to determine optimal laser parameters for processing, the inclusion of carrier lifetime studies could potentially provide a third defining aspect of the process.

Laser ablation has been shown to cause thermally propagated dislocations in silicon that interact with heavily-doped phosphorous diffusion in n-type regions (1).

However, proper post-processing sodium hydroxide (NaOH) etching can prevent the dislocation formation, as can the avoidance of post-processing annealing. Laser-induced melting does not cause defect formation, but it has been shown to increase recombination (1) (61).

W1A and W1B test wafers have dielectric layers on only one side of the silicon substrate. For lifetime studies, new wafers were processed that contained 80 nm thick 127

passivation layers of PECVD amorphous silicon on both sides of the silicon substrate.

Wafers with silicon dioxide were not tested for these experiments. A cross-sectional diagram of the lifetime test wafer is shown in Figure 3-52.

Figure 3-52: Cross-sectional diagram of test wafer used for lifetime experiments.

The intent for the lifetime experiments was to process the same laser parameters from each of the three lasers that had been used for 9x9 LFC arrays and re-metallization experiments. These parameters were fired in random locations on isolated aluminum pads on the lifetime wafers, as shown in Figure 3-53. After laser processing, the aluminum was etched off using Transcene Aluminum Etchant Type D. Lifetime measurements were to be taken using microwave photoconductive decay (µ-PCD) measurements on each pad, and these values were to be compared with morphologies and overall resistances of the previously studied 9x9 LFC arrays.

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Figure 3-53: Laser parameters and their locations on lifetime test wafer.

Figure 3-54 shows a lifetime map of Wafer-10 (a lifetime test wafer) before aluminum deposition. Lifetimes are relatively uniform in the center of the wafer, but they degrade around the edges where dangling bonds increase recombination rates.

Figure 3-55 shows the lifetime map after aluminum deposition and post-deposition annealing at 275°C for 5 minutes. By the nature of the µ-PCD measurement, lifetimes could not be measured over the reflective e-beamed aluminum pads, and the lifetime measurements are extremely low in these and immediately adjacent areas. However,

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unprocessed regions—such as those on the lower left and right sides of the wafer—have clearly improved from the annealing.

Figure 3-54: Wafer-10 lifetime map before e-beam Al deposition.

Figure 3-55: Wafer-10 lifetime map after Al deposition and annealing at 275°C for 5 min.

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Laser parameters described in Figure 3-53 were fired on the wafer after the annealing step. The aluminum etchant was used to remove the aluminum pads after laser processing so that lifetime measurements could be taken over the laser processed regions.

Figure 3-56 shows the lifetime map of Wafer-10 after laser processing and aluminum etching. Lifetimes over the entire wafer degraded significantly. It is speculated that a handling step between aluminum removal (or possibly the aluminum removal itself) and the post-laser processing lifetime measurements inexplicably damaged the wafer’s electrical properties. Values between different 9x9 LFC arrays cannot be compared, as all the laser-processed regions have extremely low, indistinguishable lifetimes.

Figure 3-56: Wafer-10 lifetime map after laser processing and aluminum etching.

In an attempt to improve lifetimes, Wafer-10 was annealed first at 275°C for 10 minutes (lifetime map shown in Figure 3-57) and then at 350°C for an additional

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10 minutes (lifetime map shown in Figure 3-58). Some improvement is shown from the first annealing step, but significant lifetime recovery is displayed after the second annealing step. However, laser processed areas remain nearly indistinguishable.

Figure 3-57: Wafer-10 lifetime map after annealing at 275°C for 10 min.

Figure 3-58: Wafer-10 lifetime map after annealing at 350°C for 10 min. 132

A lifetime map of Wafer-11 before e-beamed aluminum deposition is shown in

Figure 3-59. Wafer-11 did not undergo a post-e-beamed aluminum deposition anneal.

Figure 3-59: Wafer-11 lifetime map before e-beam Al deposition.

Figure 3-60 and Figure 3-61 show Wafer-11 after laser processing, aluminum removal by etching, and annealing at 275°C for 10 minutes and 375°C for 10 minutes, respectively. Lifetime improvement is evident from the annealing step. Unlike Wafer-

10, laser-fired regions are distinguishable from one another on Wafer-11, and thus it was possible to compare lifetime measurements between laser processing parameters. These comparisons are made using a relative scale with arbitrary units, assuming 0 to be the lowest lifetimes (dark red regions on the lifetime maps) and 1200 to be the highest lifetimes (dark blue regions on the lifetime maps). Table 3-13 shows the resulting lifetime maps for the specific laser parameter used to fire each region.

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Figure 3-60: Wafer-11 after laser processing, Al removal, and annealing at 275°C for 10 minutes.

Figure 3-61: Wafer-11 after laser processing, Al removal, and annealing at 375°C for 10 minutes.

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Table 3-13: Lifetime map measurements based on laser processing parameters for Wafer-11.

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Lifetime measurements are compared to resistance values for the IPG fiber, the

AVIA, and the Quantel laser in Figure 3-62. The plot shows the relative regimes of resistance and lifetime into which LFCs fired with each laser fall. IPG fiber laser-fired

LFCs generally produced the highest resistances based on the parameters used, and lifetime values are low to moderate. The AVIA laser produced low to moderate resistances with the highest lifetimes. The Quantel laser generated the lowest resistances and relatively low lifetime values.

Figure 3-62: Lifetime as a function of resistance for IPG, AVIA, and Quantel lasers.

Figure 3-63 and Figure 3-64 show the lifetime and resistance relationships for the

IPG fiber and the AVIA laser. Both lasers show clear trends in terms of lifetime as a function of resistance, however the relationships are contradictory.

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Figure 3-63: Lifetime as a function of resistance for IPG fiber laser.

Figure 3-64: Lifetime as a function of resistance for AVIA laser.

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Figure 3-63 shows that lifetime values increase as resistances increase for LFCs fired with the IPG fiber laser. Figure 3-64 shows that lifetime values decrease as resistances increase for LFCs fired with the AVIA laser. The trend observed for the IPG fiber laser is closest to expected results: resistance values decrease as higher laser conditions (higher energy based on increased power or pulse durations) are used for LFC processing, which in turn cause lifetimes to diminish due to the increased laser-induced damage effects.

LFCs fired by these lasers are clearly different in morphology and electrical characteristics, as shown in the aforementioned experiments. As it is assumed that LFC morphology is an important factor in the way an LFC behaves electrically, it is reasonable that microsecond and nanosecond pulses fired at different wavelengths (which cause different morphologies) could result in contrasting lifetime behavior. However, more tightly controlled experiments and a deeper understanding of the LFC itself are required to develop conclusions about these results.

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Chapter 4

RESULTS AND DISCUSSION

This thesis is presented as part of a larger project that seeks to optimize a new technique for making solar cells using laser fired contacts. The goal of this particular thesis is to begin to develop an understanding of the laser-material effects that occur during LFC processing.

Experiments have been performed with four different wavelength lasers comprising two regimes of pulse durations. The IPG Photonics single mode fiber laser

(1070 nm) operates in the microsecond pulse regime. Lasers that operate in the nanosecond pulse regime include the Coherent AVIA Q-switched frequency-tripled laser

(355 nm), the Quantel Brilliant Q-switched Nd:YAG laser (532 nm), and the Lambda

Physik KrF excimer laser (248 nm).

Early experiments with the fiber laser proved that tight control over beam focus was a necessity for consistent results. Once this control had been achieved, we were able to confidently make comparisons of different laser parameters.

The microsecond pulses of the fiber laser produced LFCs with the most consistent electrical measurements. Generally, increasing the power or the pulse duration led to increases in the size and depth of the LFCs, along with decreases in resistance. This is attributed to the fact that increasing these parameters increases the amount of total input

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energy used to produce the LFC. These trends held for all experiments. Microsecond pulses tend to promote melting and mixing of the irradiated materials, whereas nanosecond pulses often lead to ablation and mass ejection. SEM images suggest that the aluminum on top of the contact had mostly vaporized, and that the silicon in the contact region had melted and recrystallized. However, most of the original material was not removed from the processing site after firing LFCs with the fiber laser.

Morphological trends held for the other two lasers as well. For the AVIA, increasing the energy and the number of shots led to larger, deeper LFCs. Increased energy also caused reductions in resistance, as was observed for the IPG fiber laser.

Significantly more material was ablated as the number of shots was increased. For LFCs fired with 5 and 10 shots, deep craters remained after firing. Each subsequent pulse ablated more material. Resistance measurements for the AVIA often showed non-ohmic characteristics. Ohmic contact likely occurs in LFCs when portions of the e-beamed aluminum on the back side are able to contact the silicon substrate through the aluminum- silicon alloy that is created during laser firing. With multiple nanosecond pulses, most of the e-beamed aluminum had been vaporized—both on and around the LFC—and thus this contact was often not produced.

For the Quantel laser, as with the AVIA, increasing the number of shots caused larger, deeper LFCs. However, resistances for these LFCs did not follow any identifiable trend. The LFCs are very large, which is partially due to the fact that a beam expander— which would have allowed for a tighter focal point— was not used in the optics train.

The central portions of the LFCs are large, ragged craters in which the aluminum has

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been vaporized and the underlying silicon has been ablated and redeposited around the crater.

From the fiber laser results, it has been shown that equivalent resistances can be achieved by using different laser parameters. In the case of the fiber laser, increased power with decreased pulse duration, or vice versa, can create LFCs with highly similar morphologies and resistances. However, the rate of energy deposition appears to be a more significant factor in LFC morphology than the total amount of input energy.

In nearly all cases, LFCs fired on W1B had lower resistances than those measured on W1A. This could potentially be due to the fact that there is an extra layer through which the laser must penetrate in order to reach the silicon. The SiO2 layer in the W1A samples may absorb impinging laser energy, reducing the amount that is able to affect the silicon. If—as speculated—ohmic contact is caused by strands of aluminum reaching over the passivation layers to reach the aluminum-silicon alloy on the contact itself, the extra passivation layer would limit this ability to some extent, as well.

Overall resistance measurements taken for 9x9 and 18x18 LFC arrays scale closely with expected resistance values based on single LFCs. In most cases, the overall measured values were only a few ohms away from expected values. This is important from a production standpoint, as these experiments most closely resemble processing that would occur in a production line.

LFC array samples were re-metallized for each wafer in an attempt to improve resistance measurements. Resistances were not expected to improve dramatically, if at all, for the fiber laser. LFCs produced with this laser did not experience significant

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ablation or mass-ejection effects, although the aluminum had been mostly vaporized on top of the contact. Re-depositing the aluminum caused resistances to increase in nearly all cases for the IPG fiber laser.

It was theorized that re-metallization would improve resistances for the nanosecond pulse lasers because of the high degree of silicon ablation that they cause. It was expected that the extra aluminum would make ohmic contact that had not existed previously. For the AVIA laser, re-metallization caused no difference in resistances, and for the Quantel laser, only one of the samples achieved a decrease in resistance, while the other increased.

It is important to note that the samples had not been etched or cleaned in any way before re-metallization. An oxide layer forms over aluminum when it is left in the air, and based on the SEM images, a significant amount of material is vaporized and redeposited on and around the LFC after laser firing. It is possible that redepositing aluminum on top of this oxide and debris would not allow ohmic contact to be made. In future experiments, the samples should be etched and cleaned to avoid these contaminations.

Carrier lifetime experiments were to be used to create a more fully developed picture of the LFC process. The intention for these experiments was to answer the question of balancing low resistance with low laser-induced damage. Preliminary lifetime experiments were conducted using 9x9 LFC arrays identical to those processed in the previously described LFC array experiments. However, the goal of comparing resistances for these 9x9 arrays to their effects on carrier lifetimes was not entirely

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fulfilled. An unexplained deterioration in overall wafer lifetime during the experimental process made the measurements for one of the lifetime wafers unreliable. Results for the remaining lifetime wafer showed opposing trends in regard to the relationship between resistance and lifetime for LFCs fired with different lasers. LFCs fired with the IPG fiber laser displayed the predicted trend of decreasing carrier lifetime with decreasing resistance. LFCs processed with the AVIA laser achieved the unexpected relationship of increased lifetimes for decreased resistance.

The small sample size processed by each laser for the lifetime experiments—and the fact that one of the two lifetime test wafers was damaged in some way during the experimental procedure—renders the reliability of the results for this study questionable.

Considering its well known effects on laser-materials interactions, the drastic difference in pulse duration—in addition to the varied wavelengths—between the IPG fiber and the

AVIA laser can be assumed to cause highly dissimilar LFCs and electrical characteristics.

Such results have been shown throughout the Experimental Procedure of this thesis.

A deeper understanding of the morphology of an LFC (an ability to distinguish specific regions of the contact and determine what contributions they offer electrically) will likely allow for more detailed explanations of the opposing lifetime-resistance relationship results. The ability to clearly identify which portion of the LFC conducts current will help to explain how the different LFC morphologies—based on the different laser parameters—affect carrier lifetimes.

Lifetime measurements proved the ability of post-processing annealing steps to enhance passivation, and thus improve carrier lifetimes. Although this is described in

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literature, this was the first instance in this research that this effect was documented.

Additionally, lifetime measurements have shown that e-beam aluminum deposition and laser firing affect lifetimes only in regions near the processing sites.

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Chapter 5

CONCLUSIONS

Variations in laser parameters—such as power, energy, pulse duration, and number of shots—have been shown to create clearly identifiable effects during laser processing.

These effects have been related to well-documented laser-materials interactions and processes in order to more thoroughly understand them. To a large extent, this understanding, in addition to a high degree of control over the laser parameter- determined effects, has made it possible to consistently and accurately achieve a desirable

LFC morphology or resistance.

Ablation effects that result from nanosecond laser pulses have been shown to lead to inconsistencies in processing and characterizations for LFCs. Microsecond pulses appear to achieve more consistent and understandable results—in terms of morphology and resistance trends—than nanosecond pulses do.

Low overall resistances are achievable using arrays of identical-parameter LFCs.

Additionally, these resistances are highly predictable and scalable based on single-LFC parameter development studies.

The SiO2 passivation layer for W1A samples caused higher single LFC resistances for nearly all cases when compared to equivalently processed LFCs on W1B

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samples. However, for identical-parameter LFC arrays, this effect was not observed because of the low overall resistance that results from the contributions of multiple LFCs.

Carrier lifetime experiments were to be used to ―round-out‖ the experimental process. The balance of resistance and laser-induced damage is not fully understood without an additional factor to quantify the extent of the detrimental electrical effects that occur during laser processing. Lifetime experiments showed that laser processing and e- beam deposition of aluminum do not significantly affect lifetimes away from the processing area. Additionally, it has been shown that annealing at relatively low temperatures can improve passivation and decrease recombination rates. For LFCs processed with the IPG fiber laser, the expected relationship of increased lifetimes with increased resistances was observed, although lifetime measurements obtained for LFCs fired with the AVIA laser contradicted these results.

A detailed understanding of the effects of laser processing on solar cells based on specific lasers and their inherent parameters has been achieved and documented in this thesis. The laser-fired contact procedure has been enhanced in regard to our ability to produce precise, consistent results. This thesis may serve as a guide and a reference for continued research and experiments in laser-fired contacts and related laser processing techniques.

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Chapter 6

FUTURE WORK

A deeper understanding of the LFC process is still required. Successful carrier lifetime experiments will provide a portion of this. In addition, a technique for polishing cross- sections of LFCs and delineating junctions in the wafer will be extremely useful for explaining the LFC process in terms of laser-materials interactions and effects. New sample structures utilizing different layering schemes and materials will be of use in further developing the LFC knowledge base. These structures will be integrated into the development of LFC models.

Results of completed solar cell performance tests would be advantageous as metrics for our project in terms of its ultimate goals. A small number of parameters could be chosen to be used for fully-functional solar cells so that important performance tests like conversion efficiencies could be performed.

The project has recently shifted toward studies with laser-fired emitters, or LFEs.

The processing is similar to that used for LFCs, although lower laser energies will likely be required. The focus has thus shifted from a process most similar to laser-welding to one more closely related to laser-doping.

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Appendix

TOOLS

Laser Processing IPG Photonics YLR-250-SM-NC Ytterbium Single Mode Fiber Laser IPG Photonics YLR-200-AC Ytterbium Single Mode Fiber Laser Coherent AVIA 355-4500 Frequency-Tripled Q-Switched Laser Quantel Brilliant B Q-Switched Nd:YAG Laser Lambda Physik AG LPX 220i Excimer Laser Lumonics JK701 Pulsed Nd:YAG Laser

Materials Characterizations SUSS MicroTec Four-Point Probe Station Keithley 4200 Semiconductor Characterization System Oxford Instruments INCA x-Sight Energy Dispersive X-ray Spectroscopy SEMILAB WT-2000 Materials Characterization Instrument FEI Quanta 2003D Focused Ion Beam Microscopy Mitutoyo Optical Microscope, Nikon lenses Philips XL 20 Scanning Electron Microscope FEI Quanta 200 Environmental Scanning Electron Microscope (EDS) Photolithography Karl SUSS MA6/BA6 Mask Aligner

156