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Pulsed Fiber Laser “Sumilas*1” for Micro-Processing

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Pulsed Fiber Laser “Sumilas*1” for Micro-Processing ENVIRONMENT, ENERGY & RESOURCES Pulsed Fiber Laser “SumiLas*1” for Micro-Processing Motoki KAKUI*, Shinobu TAMAoKI, Yasuomi KAneUchI, hiroshi KohdA, Yuji TAnAKA, Yasuhiro oKAMoTo, Yoshiyuki Uno and Brian BAIRd In order to meet requirements in the next generation micro-processing, we have developed a 1.06µm pulsed fiber laser employing a MOPA (master oscillator power amplifier) configuration. This laser features broad pulse width flexibility 2 (100ps to 20ns), excellent beam quality (M ≦1.3), and a wide range of pulse repetition frequencies (50kHz to 1MHz). With these attributes of the laser, the optimum pulse width for P1 processing both (a)-Si (amorphous silicon) and CIS or CuInSe (copper indium diselenide) types of solar cells was studied. As a result, it was found that the pulse width of 10 to 20ns gives the best scribing quality on (a)-Si solar cells while the pulse width around 500ps leads to excellent results for CIS samples. The scanning speeds of 2500mm/s and 5000mm/s have been achieved for (a)-Si and CIS samples, respectively. In this study, surface texturing of STAVAX, a through-hardening stainless steel for plastic molding, was also conducted. The surface roughness and water repellency on the STAVAX surface were controlled by adjusting the pulse width, number of scanning and assist gas condition. Keywords: laser, pulse, micro-processing, solar cell, texturing 1. Introduction thermal effects that lead to dross and debris, it has been believed that the pulse must be shortened. As shown in Fig. Recently, laser processing has been replacing some 2, however, a pulse width shorter than 100fs tends to de- conventional mechanical or chemical processing in various grade the processing quality because of its nonlinear ef- industries because of the following advantages(1): fects. Therefore, a pulse width ranging from 1ps to 100ps (a) non-contact processing, is considered most appropriate for micro-processing(3). (b) focus control and flexible positioning by optical guid- Q-switching*2 and mode-locking*3 methods have been ance, and well known for pulse generation(4). In general, the latter (c) reduction of environmental impact. method is used to generate the pico-second (ps) optical In micro-processing of electronic components, pulsed pulse(5)-(7). However, due to its instability and high opera- lasers have become particularly important due to the trend tional cost, the mode-locked ps laser has not become the in downsized components and diversification of materials. mainstream. Therefore, the Q-switching laser, which is Despite the widespread use in the various industries, comparatively inexpensive, is widely used in today's market the ablation phenomena caused by the laser pulse has not despite the pulse width range of several tens to hundreds yet been fully elucidated because the phenomena involve nano-seconds (ns), which results in the thermal effects in various effects such as plasma generation, evaporation, laser processing. Recently, MOPA*4 type lasers have been melting and shock waves as shown in Fig. 1(2). To avoid the attracting attention because of the excellent capability of controlling the temporal pulse shapes. However, most of the MOPA lasers now in use have the pulse width tuning Laser beam Focusing lens Damage by adjacent Nonlinear Thermal structure effects effects Sputter Surface ripples by Recast layer shock wave Debris Excellent Heat affected zone SumiLas Melted zone Micro cracks Processing quality Poor 100fs 1ps 10ps 100ps 1ns 10ns Shock wave Pulse width Fig. 2. Relationship between processing quality and pulse width(3) Fig. 1. Various effects accompanying laser ablation(2) SumiLas can cover the range between the broken lines. SEI TECHNICAL REVIEW · NUMBER 72 · APRIL 2011 · 127 range of around 10ns to several hundreds ns, where ther- (a) mal effects often negatively affect processing quality(8),(9). To address the problem, we have developed a MOPA- type pulsed fiber laser “SumiLas” by utilizing optical fiber communication technology such as high-speed pulse modulation and low-noise and high-gain optical amplifi- cation. SumiLas has a variable pulse width range from 100ps to 20ns, and its configuration is relatively simple as shown in Fig. 3. Thus, SumiLas enables high processing quality at the same level of almost as excellent as the mode-locked ps lasers(6),(7) and at a comparable price to (b) 2D Profile the Q-switching lasers. 4.7 218 3.53 163 2.35 109 Master Z Axis oscillator Output pulses Y Axis (mm) 1.18 64.4 Pre-amplifier Booster-amplifier Pulse 0 0 generator 0 1.57 3.14 4.7 X Axis (mm) Optical head Pump-LD Pump-LD Fig. 4. (a) Appearance of SumiLas and (b) example of output beam profile Control circuit USB connector Fig. 3. Schematic diagram of SumiLas 40 (a) 50kHz 70kH 30 This paper presents examples of micro-processing 100kH using SumiLas: P1 structuring on the two types of thin-film 20 solar cells, namely amorphous (a)-Si and CIS*5(10); and sur- 200kH face texturing of stainless steel(11),(12). In both cases of a-Si 10 500kHz and CIS cells, excellent processing quality has been demon- Optical power (kW) strated. While the result of P1 structuring to a CIS cell was 0 (6),(7) equivalent to that obtained by ps-lasers , it was revealed 0 10 20 30 40 50 that conventional ns-pulses were more suitable for a-Si cell Time (ns) processing. This paper also reports on the second harmonic gen- 80 (b) eration of SumiLas, which is an important factor to realize 500kHz P2 and P3 structuring on solar cells. 60 2MHz 40 20 2. Performances of SumiLas Optical power (kW) 0 Figure 4 (a) shows a photograph of SumiLas. The di- 0510 mensions (in mm) of the SumiLas are 500 (depth) x 482 Time (ns) (width) x 200 (height). It contains a specially designed pulse generator to directly modulate the seed laser diode Fig. 5. Examples of output pulse shapes under (a) standard mode and oscillating at 1060nm. Its repetition rate is normally tuned (b) short pulse mode in the range from 50k to 1MHz. Output pulses are ampli- fied by optical amplifiers which employ Yb-doped fiber, and the average output power available is 15W. An optical head box containing a collimator and isolator is usually at- mode, and (b) when the short pulse mode is adopted. As tached at the end of the delivery fiber. The output beam shown in Fig. 5 (a), the FWHM (full width of the half max- diameter is set to about 1.0mm. The beam quality is excel- imum) of the optical pulses decreases as the repetition rate lent as shown in Fig. 4 (b), and typical M2 *6 is less than 1.1. becomes lower, mainly because of the transient response Figures 5 show the examples of the pulse shapes (a) of the fiber amplifiers, while the width of the pulse base al- when the electric pulse width is set to 20 ns in the standard ways coincides with the electric pulse width, namely 20ns 128 · Pulsed Fiber Laser “SumiLas” for Micro-Processing in this case. In the short pulse mode, the FWHM is in the Sample (1) is utilized for a-Si type or CdTe type solar range of 100 to 200ps, and the width of the pulse base is cells, and the coating on it is mainly composed of SnO2. always shorter than 1ns. Sample (2) with Mo coating is used for CIS (CuInSe) type solar cells. Substrate for both samples comprises soda-lime glass. The output beam diameter has been set to 1.0mm. A 3. P1 Structuring on Solar Cells(10) beam expander was inserted after the optical head box to magnify the beam diameter 5 times. The galvano scanner Today, crystalline Si-type solar cells are most popularly “Scanlab Hurryscan II-14” and the f-theta lens “Scanlab ID used all over the world. In order to achieve the grid parity, 114286” with the focusing length of 100mm have been em- however, active R&D of the novel configuration is under ployed instead of a high-speed linear stage. The laser progress. A thin-film type solar cell structured on a glass beam is radiated on the glass-side of the samples. The panel is considered to be one of the most promising can- beam spot diameter on the coating surface has been cal- didates. In the fabrication process, P1 to P3 structuring, as culated to be 34µm. shown in Fig. 6, needs to be performed. In almost all types Both the standard mode and short-pulse mode shown of thin-film solar cells, P1 structuring can be done at the in Fig. 5 (a) and (b), respectively, have been tried for this wavelength of 1060nm, while P2 and P3 structuring re- experiment. The average power and the repetition rate quires the laser oscillating at 530nm. The area between P1 have been optimized to achieve the target scanning speed and P3 scribing lines is called a “dead zone” because this of 2500mm/s for Sample (1), and 5000mm/s for Sample area does not contribute to the power conversion, and (2). The optimized conditions are shown in Table 1, and needs to be reduced in the future. Moreover, the laser the P1 structuring results are shown in Fig. 7, Photo 1, 2, scanning speeds for P1 structuring need to be increased to and Fig. 8. Figure 7 shows the inspection result of P1 struc- higher than the current value of around 1000 to turing on Sample (1) with 20ns pulse with the optical mi- 1500mm/s in order to reduce the fabrication cost. To meet croscope and SEM*7. There has been no damage, such as these requirements simultaneously, MOPA type fiber lasers micro-crack, either on glass or on TCO coating.
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