FABRICATION OF A THIN FILM RESISTANCE HEATER
A Thesis Presented to
The Faculty of the Fritz J. and Dolores Russ College of Engineering and Technology
Ohio University
In Partial Fulfillment
of the Requirement for the Degree
Master of Science
by
Santhana Sathya
August, 1999 Acknowledgement
I wish to express my sincere appreciation and thanks to my program advisor, Dr. Khairul Alam, for his able guidance at every stage of this thesis work and for his continuing encouragement. I am thankful to Dr. Daniel Gulino for providing valuable resources and technical know-how that were imperative in this project. I am thankful to Dr. David Ingram for facilitating a part of this research by studying the annealed samples with Auger spectroscopy and letting me use his lab equipment. I am thankful to Mr. Lyn Bowman, Chief Engineer, SunPower Inc., Athens, Ohio for providing the industrial cooperation and plans for the micro-refrigerator. I wish to express my appreciation to NASA for the financial sponsorship of the project. I also thank Joel, Paul, and Pramod who helped me in this project at different stages. Table of Contents
Page No. Chapter 1 Introduction 1 I. 1 Micro-refrigerator 1 1.2 Hot-end for engine to drive the micro-refrigerator 1 1.3 Present Work 2
Chapter 2 Hot End for the Micro-Refrigerator 3 2.1 Stirling cycle 3 2.2 Hot-end setup 3 2.3 Implementation considerations for the hot-end 6
Chapter 3 Material Selection and Sputtering Process 3.1 Candidate materials 3.2 Deposition process selection 3.3 PVD for nichrome 3.4 Evaporation of alloys 3.5 Sputtering of alloys 3.6 Fundamental principles of sputtering 3.7 Ion-surface interactions 3.8 Magnetron sputtering
Chapter 4 Sputtering Deposition of Nichrome 19
Chapter 5 Experiments on Silicon Wafers 5.1 Stress measurement 5.2 Adhesion 5.3 Thickness measurement 5.4 Annealing 5.5 Resistance measurement 5.6 Tests on wafers
Chapter 6 Results and Analysis 6.1 Adhesion 6.2 Intrinsic stress analysis 6.3 Auger depth analysis 6.4 Auger depth analysis
Chapter 7 Conclusion 50 Appendices Appendix 3-A Mechanical properties of nichrome alloys
Appendix 3-B Composition of various nichrorne alloys
Appendix 3-C Physical properties of nichrome alloys
Appendix 3-D Ni-Cr phase diagram
Appendix 5-A Brazing alloy specification List of Figures
Page No. Figure 2- 1 Displacer effect on gas pressure
Figure 2-2 Displacer effect on piston motion
Figure 2-3 Displacer drive and ideal Stirling engine and refrigerator cycles
Figure 2-4 Schematic of a hot-end of the micro-refrigerator
Figure 3- 1 Thermal expansion of Ni-Cr vs. Temperature
Figure 3-2 Resistance measurement of NiCr strip 10
Figure 3-3 Nichrome strip - change in contact resistance with annealing cycles
Figure 3-4 Schematics of a simple sputtering system 14
Figure 3-5 Depiction of energetic particle bombardment effects on 15 surfaces and growing films
Figure 3-6 Electron motion in a magnetron 17
Figure 3-7 Effect of electric and magnetic fields on electron motion 18
Figure 4- 1 Schematic of the deposition system 20
Figure 4-2 Schematic of the vacuum chamber 2 1
Figure 4-3 Schematic of a rotary piston pump
Figure 4-4 Schematic of a cryopump 2 3
Figure 4-5 Photograph of the vacuum chamber 24
Figure 4-6 Schematic of an as-masked wafer
Figure 4-7 A solid model of the target holder Figure 4-8 Target specification
Figure 4-9 Target and substrate setup
Figure 5- 1 Ionic systems stress gauge
Figure 5-2 Outline of DekTak IIA
Figure 5-3 Graphic output for thickness of wafer
Figure 5-4 Electrical resistance measurement
Figure 5 -5 Ohmic contacts on a plain wafer
Figure 5-6 Photograph of the high vacuum furnace
Figclre 6- i Auger depth analysis of unannealed sample
Figure 6-2 Auger depth analysis of annealed sample ( I hr. at 700 deg. C)
Figure 6-3 Auger depth analysis of annealed sample (4 hr. at 700 deg. C)
Figure 6-4a Auger surface analysis of 1 hr. annealed sample showing patches on the wafer surface
Figure 6-4b Segregation of Ni and Cr atoms in the 1 hr. annealed sample
Figure 6-5a Auger surface analysis of 4 hr. annealed sample showing patches on the wafer surface
Figure 6-5b Segregation of Ni and Cr atoms in the 4 hr. annealed sample List of Tables
Page No. Table 3- 1 Properties of Candidate Materials 9
Table 4- 1 Thickness Attained with Different Deposition Times 30
Table 5-1 Wafer Treatment Data 39
Table 5-2 Stress and Resistance Measurements on Wafers 40 Chanter 1 Introduction
The Stirling cycle for refrigeration was known as early as 1873. Stirling refrigerators were commercialized in very low temperature ranges that are not well served by the Rankine cycle; e.g., cooling military infrared detectors and liquefying industrial gases. Food preservation and air-conditioning are newer applications for the Stirling cycle. In a Stirling cycle machine, a confined volume of gas is expanded at one temperature and recompressed at another with the result that heat energy is absorbed from the heat source during expansion and rejected to the environment during compression. The Stirling cycles are closed cycles with no loss of working fluid. The stages and functioning of the Stirling cycle will be explained in Chapter 2. The Stirling refrigeration technology offers the potential for higher energy efficiency than the current Ranlne technology since, the ideal Stirling refrigeration cycle is more efficient than the ideal Rankine vapor-compression cycle.
1.1 Concept of the micro-refrigerator Sun Power Inc., (Athens, OH), along with Ohio University proposed to NASA a method to develop a micro-refrigerator that works by the Stirling cycle (Bowmann, 1994). Its application would be to cool semiconductor circuits in electronic devices. Miniature fans are currently being used to serve this purpose. The micro-refrigerator is to be approximately of the order of 1 cm in diameter and 1 rnrn in thickness. The unit is to be made of silicon, with air as the worlung fluid in the chamber inside. This is accomplished by combining a number of silicon wafers in layers with the appropriate amount of material removed to form the chamber. 1.2 Hot end for the engine to drive the micro-refrigerator The design also calls for an insulation layer on top of the silicon substrate so that it serves as the heating end through which heat permeates to the working fluid of the Stirling engine. The engine drives the rnicro-refrigerator and plays the role similar to a compressor in a conventional refrigerator cycle. This thesis is the outcome of studies to establish the hot end by depositing suitable material onto the silicon substrate by means of an appropriate deposition process. It was also intended to establish a durable electrical contact to the deposited film to input energy in the form of electric current.
1.3 Present Work The Stirling worlung cycle is explained in detail in Chapter 2, which also illustrates the hot-end setup and deals with selection and deposition of the thin film material. Chapter 3 describes the theory behind magnetron sputtering process, which is used to deposit the thin film. The sputtering procedure as followed is described step-by-step in Chapter 4. Chapter 5 lists the different performance and durability tests conducted on the thin film. It also contains data observed during the test measurements conducted on the silicon wafers. The tests are described in detail in the same chapter. Analysis of the observations are made in Chapter 6. Conclusions are presented in Chapter 7. Chapter 2
Hot End for the Micro-refrigerator
2.1 Stirling cycle The Stirling thermodynamic cycle can be implemented as an engine, cooler or heat pump. The thermodynamics of free Stirling machines is shown in Figures 2-1 to 2-3. Figure 2-1 shows the effect of shuttling a volume of gas from end-to-end of a closed cylinder when the ends are kept at different temperatures. The gas is displaced from one end to another by the reciprocation of an internal part called a displacer. By definition, a displacer is a reciprocating part that has a temperature difference, but not a pressure difference, across it. The absence of a pressure difference is indicated in Figure 2-1 by the loose fit of the displacer in the cylinder.
T HIGH
T LOW
P HIGH - High Pressure; P LOW - Low Pressure; THIGH- High Temperature;
T LOW - LOWTemperature Figure 2-1 Displacer effect on gas pressure When the displacer is at the cold end of the cylinder, the pressure of the gas increases because the temperature of the gas increases. Conversely, when the displacer is at the hot end of the cylinder, the pressure of the gas, which is then at the cold end, decreases because its temperature decreases. Sinusoidal reciprocation of the displacer causes the gas pressure to vary sinusoidally.
Figure 2-2 illustrates how this variation in gas pressure can be exploited to make a piston reciprocate. By definition, a piston is a reciprocating part with a pressure difference and not a temperature difference across it. In Figure 2-2, an open cylinder contains both a displacer and a piston. The ability of the piston to support a pressure difference is indicated in Figure 2-2 by its tight fit in the cylinder. Thus, the piston confines a volume of gas in the closed end of the cylinder. When the displacer is at the cold end of its travel, the resulting rise in gas pressure pushes the piston away from the displacer. Conversely, when the displacer is at the hot end of its travel, the resulting drop in gas pressure pulls the piston back toward the displacer. Thus sinusoidal reciprocation of the displacer causes sinusoidal reciprocation of the piston.
T HIGH
T LOW
P HIGH - High Pressure; P LOW - Low Pressure; THIGH - High Temperature; T ,, - Low Temperature; P - Piston Figure 2-2 Displacer effect on piston motion Figure 2-3 illustrates how the displacer is made to reciprocate. The upper par- of Figure 2-3 shows the pressure-volume (P-V) and temperature-entropy (T-S) relations that define ideal Stirling engine (1-2-3-4) and refrigeration ( 1-2-3'-4') cycles. The lower part of Figure 2-3 shows the positions of the piston and displacer, with labels for the relevant pressures, temperatures, and heat flows at four distinctive points during an ideal engine cycle. Each ideal Stirling cycle is a repetitive sequence of four heat transfer processes - two at constant temperature and two at constant volume.
P - Pressure; V - Volume; T - Temperature; TE- Temperature at expansion end;
T, - Temperature at compression end; PM - Mean Pressure 1-2-3-4 The Stirling engine cycle; 1-2-3'-4' - The Stirling refrigeration cycle Figure 2-3 Displacer drive and ideal Stirling engine and refrigerator cycles The piston and displacer move identically in Stirling engine and refrigerator cycles. The only practical differences between them are 1) the presence or absence of a high temperature heat source and 2) the relative amounts of energy absorbed during expansion and rejected during compression in each cycle.
2.2 Hot end setup In the present work, the objective was to design a thln-film heater that will work as the hot-end of a very small Stirling engine. The design of the micro heat engine has a silicon substrate with a silicon-di-oxide layer above it and a high resistance layer on top of the silicon-di-oxide. The electrically resistive layer is to serve as the heating end through which the heat permeates to the working fluid of the Stirling pump, which in this case is air. Heat is rejected to the ambient at the other end by the heat pump. Energy is input to the hot-end through electric current that is conducted by means of ohmic contacts. A
sketch of the conceptualized hot end section is shown in Figure 2-4. + \ Eledricallv resistive film P- SiO,
Silicon
Figure 2-4 Schematic of a hot-end of the micro-refrigerator
An appropriate electrically resistive film was be chosen to produce resistive heating to the Si wafer. The candidate material must have a high electrical resistance and a coefficient of thermal expansion similar to the base Si, so that the thermal stress induced during high operating temperatures is minimum. 2.3 Implementation considerations for the hot end After the material selection, film deposition is to be achieved using a suitable deposition technique. Process selection depends on the deposition feasibility of the material and the facilities available on site. For a metal film, physical vapor deposition techniques, such as sputtering, are appropriate. This will be described in Chapter 3. In case of an inorganic material for the film, it would be possible to use chemical vapor deposition. So a trial study of the deposition feasibility of the selected material is to be made. After deciding on the appropriate deposition method, films are to be deposited on the silicon wafers. The size of the thin film to be deposited is to be determined based on the operating power and voltage of the micro-refrigerator. The power to be supplied to the hot end was specified to be 10 W at 10 V (Bowman, 1994). Silicon wafers, 2" in diameter, would be used as the deposition substrate after which the film was to be subjected to different endurance and performance tests. Tests to be conducted on the wafers included: 1. Film resistance before and after heat treatment of the wafer. 2. Stress induced on the wafer due to film deposition and stress change after heat treatment. 3. Film adhesion. 4. Electrical connectivity (metallization process), using contacts at high operating temperatures. 5. Annealing of the wafer to operating temperature cycles up to a maximum of 700°C. 6. Measurement of the film thickness. Chapter 3
- - Material Selection and Sputtering Process
3.1 Candidate Materials
To serve as the hot end of the Stirling cycle, the thin film material should have a high resistivity and a coefficient of thermal expansion (CTE) comparable to that of SiO2. AS the micro-refrigerator is to be subjected to operating temperature cycles with 700" C maximum, the CTE of the film material must be comparable to Si and Si02to avoid any physical distortion of the wafer when in use. The CTE of Si is 3.5 prnImK, and for Si02 the CTE is 8.8 pm/mK. A comparison of CTE of different available materials was made (Table 3-1). On the basis of this comparison, a standard nickel chromium alloy (20% chromium, 80% nickel) commonly known as nichrome was selected for the thin-film heater on the wafer. Nichrome has a resistivity of 1.25 p2Im and a CTE of 12.6 pm/mK. Tantalum was not a good candidate, although its CTE is also closer to that of Si and Si02, since its resistivity is low compared to that of nichrome. Figure 3-1 shows the change in expansion coefficient with temperature for nichrome.
(Source: ASM Engineering Material Handbook, 1994) Table 3-1 Properties of Candidate Materials
Material Melting Resistivity CTE Thermal Point ("C) (Mm) (Cun/mK) conductivity (WImK) Nickel 1453 0.68 - 20°C 13.3 (0-100°C) 82.9 - 100°C Nichrome 1300 - 1.250 12.6 8.9 at 670°C 1370 (78% Ni, 20% Cr) Chromium 1875 0.129 - 20°C 6.2 67 - 20°C 0.500 - 76 - 426°C 700°C 67 - 760°C 0.66 - 1000°C Tungsten 2610 0.053 - 20°C 4.6 - 25°C 155 - 20°C 130 - 700°C Copper 1085 0.01673 - 16.7 - 20°C 4.9 - 20°C 20°C 22.4 - 700°C Tantalum 2996 0.135 - 20°C 6.5 - 20°C 54.4 - 20°C 66.95 - 527°C 72.9 - 927°C 77.0 - 1327°C Molybdenum 2610 0.052 - 20°C 4.9 - 20°C 142 - 20°C 123 - 500°C Silicon 2.3 x lo8 3.5 86.28 Silicon 2200 - lo8 - lo9 4.5 13 - 1000°C Carbide 2700 Silicon-di- 1400 10'~- 12°C 8.8 (25 - 1000°C) 2.06 - 1200°C oxide 50 - 1300°C
(Source: ASM Engineering Material Reference Handbook, 1994) The ASM Engineering Material Reference Handbook (1994) and the Material Selection Handbook (1993) recommend NiCr alloy as a suitable material for electrical resistance heating: A nickel chromium alloy (#600/601) with good oxidation resistance at high temperatures can be used for furnace components and heating applications. Appendix 3-A shows a list of nichrome alloys and their mechanical properties. Appendix 3-B shows the compositions of different Ni-Cr alloys. Appendix 3-C shows the physical and electrical properties of nichrome alloys. Appendix 3-D shows the Ni-Cr phase diagram.
To check the stability of nichrome (80% Ni and 20%Cu), wire strips (0.5 rnrn thick and 5 rnrn wide) made of nichrome were heated for different intervals in an inert atmosphere (nitrogen) at 700°C. The setup used for this purpose is as shown in Figure 3-2. A dc power supply unit (50 volts max.) was used. The NiCr strip was placed in a horizontal tube into which nitrogen was passed. The tube was then heated in the furnace in cycles with 700" C maximum temperature. Two thermocouples were connected to the strip and the resistance was measured using a multimeter. A high resistance wire (made of Ni) was used in the circuit to provide potentials to the end of the nichrome strip.
nitrogen inlet thermocouples 50 V supply r NiCr *Ip
furnace (700deg C)
muftimeter
Figure 3-2 Resistance measurement for NiCr strip
Throughout the heating, nitrogen was passed into the tube to encompass the nichrome strip in an oxygen-free atmosphere. The strips showed no surface rupture and remained stable. Resistance measurements were made on this nichrome wire strip for ten temperature cycles with a peak value of 700°C. The resistance of nichrome decreases with the temperature as shown in Figure 3-3.
Figure 3-3 Nichrome strip - change in contact resistance with annealing cycles.
3.2 Deposition process selection
The two popular processes for thin film deposition are physical vapor deposition and chemical vapor deposition. Physical vapor deposition is the process for transferring atoms from a source to a substrate where film formation and growth proceed atomistically. This is achieved either by evaporation, where the atoms are removed from the surface by thermal means, or by sputtering, where the atoms are dislodged from the solid target surfaces through the impact of gaseous ions. Chemical vapor deposition is the process where atoms or molecules are introduced in the gas phase by chemical reactions and deposited on the substrate by surface reactions. The factors that distinguish PVD from CVD are as follows:
1. In PVD, solid or molten sources are required, whereas in CVD, gases are the sources of the film. 2. PVD involves physical mechanisms (evaporation or collision impact) by which source atoms enter the gas phase. 3. In PVD, a reduced pressure environment (to the order of torr), through which the depositing species are transported, is essential. 4. PVD does not involve chemical reactions in the gas phase and at the substrate surface.
3.3 PVD for nichrome
Before selecting the appropriate deposition process, a literature survey of NiCr thin film deposition precedence was made. There were inadequate references to the CVD of NiCr. Also, we possessed better and efficient in-house capabilities for PVD of the alloy. PVD technology is explained in the following paragraphs. Prior investigations have shown that nichrome can be deposited successfully with either evaporation or sputtering techniques (Makabe et al., 1993). Hence an appropriate PVD system was made available for this research.
3.4 Evaporation of alloys
Atoms in alloys are generally less tightly bound than atoms in compounds. The constituents of the alloys, therefore, evaporate nearly independently of each other and enter the vapor phase as single atoms in a manner paralleling the behavior of pure metals. When the interaction energy between A atoms and B atoms in the alloy are the same as between A-A and B-B atom pairs. no preference is shown for atomic partners since there is no significant affinity between the two types of atoms. So an even distribution of the alloy is very unlikely. A practical way to cope with this kind of fractionation is to evaporate from dual sources maintained at different temperatures.
3.5 Sputtering of alloys
In contrast to the fractionation of alloy melts during evaporation with subsequent loss of deposit stoichiometry, sputtering allows for the deposition of films having the same composition as the target source. This is the primary reason for the widespread use of sputtering to deposit metal alloy films. Though each alloy component evaporates with a different vapor pressure and sputters with a different yield, the greater disparity in vapor pressures compared with the difference in sputter yields under comparable deposition conditions explains why the film stoichiometry in sputtering is better maintained. The second, but more significant, reason is that melts homogenize readily due to rapid atomic diffusion and convection effects in the liquid phase; during sputtering, however, minimal solid-state diffusion enables the maintenance of the required altered target surface composition.
The different lunds of sputtering are:
1. DC sputtering
2. RF sputtering
3. Magnetron sputtering
4. Bias sputtering
5. Ion Beam sputtering
3.6 Fundamental Principles of Sputtering
A simple sputtering system is shown in Figure 3-4. The target i? a plate of the materials to be deposited and is connected to the negative terminal of a dc or RF power supply. Hence, it becomes the cathode. Power to the order of several kilowatts is applied to it. The substrate on which the film is to be deposited can be in any of the following states: grounded, electrically floated, biased positively or negatively, heated, cooled, or a combination of these. After evacuation of the chamber, a gas-- typically argon--is introduced and serves as the medium in which a discharge is initiated and sustained. Gas pressures usually range from a few to a hundred mtorr. After a visible glow discharge is maintained between the electrodes, a current will flow and a film then condenses on the substrate. Positive ions in the discharge strike the cathode plate and eject neutral target atoms through momentum transfer. These atoms enter and pass through the discharge region to eventually deposit on the growing film. In addition, other particles--such as secondary electrons and desorbed gases--are emitted from the target. In the electric field the negatively charged ions are accelerated toward the substrate and bombard the growing film.
-V(W INSULATION # - r GLOW DISCHARGE GLOW DISCHARGE
1 I -L-- SPUTTERING VACUUM SPUllERlNG VACUUM GAS GAS
(a> (b) Figu,-e 3-4 Schematics of a simple sputtering system (a) DC, (b) RF (Source: M.Ohring, 1992) 3.7 Ion - Surface Interactions
Critical to the analysis of the sputtering process is an understanding of what happens when ions collide with surfaces. Some of the interactions that occur are shown schematically in Figure 3-5. Each depends on the type of ion (mass, charge), the nature of surface atoms involved, and the ion energy. Many of the interactions have been capitalized upon widely in thin film processing, deposition, and characterization techniques.
Sputter yield is defined as the number of atoms or molecules ejected from a target surface per incident ion and is a measure of the efficiency of the sputtering. For the sputtering effects of a binary alloy target surface--Ni and Cr alloy in our case--let the total number of nickel atoms be n~,and chromium atoms be nc, , such that the total number is n (= n~i+ nc,).
The target concentration ratios are: CNi(= nNi/n)and Cc, (= nc,/n)
Fig 3-5 Depiction of energetic particle bombardment effects and growing films
(Source: M.Ohring, 1992) The sputter yields are SNiand Sc, such that the ratio of the sputter yield fluxes is given by
If n~,(argon being the inert gas used) sputtering gas atoms impinge on the target, the total number of atoms of Ni and Cr atoms ejected are nhCNiSNiand nA,CcrSc,. respectively.
The target surface concentration ratio is modified to
If the surface is enriched with Ni atom, (SNi > Scr), nickel begins to sputter in greater profusion. Then,
The target surface composition slowly alters to CNi/Ccr,which is the same as the original target composition. A steady-state is reached where atoms are transferred to the plasma from the bulk target. At this point, the target surface reaches a value of C'Ni/C'cr = CNiScr/CcrSNi.This state continues until the target is consumed. Conditioning the target by sputtering a few hundred layers is required to reach steady- state conditions.
3.8 Magnetron Sputtering In magnetrons, electrons are not even allowed to reach the anode but are trapped near the target, enhancing the ionizing efficiency. A magnetic field is oriented parallel to the target and perpendicular to the electric field, as shown in Figure 3-6. Fig 3-6 Electron motion in a magnetron.
(Source: M.Ohring, 1992)
The magnetic field lines first emanate normal to the target, then bend with a component parallel to the target surface, and finally return, completing the magnetic circuit. Electrons emitted from the cathode are initially accelerated towards the anode, executing a helical motion in the process; but when they encounter the region of the parallel magnetic field, they are bent back to the target.
Magnetron sputtering occurs when a magnetic field of strength, B, is superimposed on the electric field, E, between the target and the substrate. The Lorentz force, F, experienced by an electron in such a field is
where q, m and v are the electron charge, mass, and velocity, respectively. Figure 3-7 shows the three cases where the electrons are emitted.
The three cases are: a) Parallel to E and B:-where v x B vanishes. b) At an angle 8 with respect to B with velocity v:- it experiences a force of qvB sin8 in a direction perpendicular to B. The electron orbits in a circular motion with a radius r that is determined by a balance of the centrifugal force (m(v ~in@~/r) and Lorentz forces involved; i.e., r = mv sin8 / qB. The electron motion is helical, spiraling down the axis with constant velocity vcos8.
Figure 3-7 Effect of electric and magnetic fields on electron motion (Source: M.Ohring, 1992)
c) Similar to Figure 3-7(b), but the electron flow is also at an angle parallel to a uniform electric field. Corkscrew motion with constant radius occurs, but because of electron acceleration in E field, the pitch of the helix lengthens with time. Magnetic fields prolong the electron residence time in the plasma and thus enhance the probability of ion collisions. This leads to larger discharge currents and increased deposition rates.
The substrate can be biased with a negative charge so that there can be enhanced attraction of the metal atoms from the target. Normally bias voltagcs of -50 to -300 V are applied. This helps in modifying the following properties: 1. Resistivity: The resistivity of Ni and Cr are reduced to some extent.
2. Hardness and residual stress: The hardness of sputtered Cr has been shown to increase (or decrease) with magnitude of negative bias voltage. Residual stress is also similarly affected.
3. Optical reflectivity: films display a metallic luster.
4. Film morphology: The columnar morphology of Cr is totally disrupted by ion bombardment and replaced by a compact, fine-grained structure.
5. Density: increased film density has been observed in Cr.
6. Adhesion: Film adhesion is normally improved. 20
Chapter 4
Sputter Deposition of Nichrome
The PVD system used for deposition is an ultra-high vacuum-unbalanced magnetron sputtering unit. The complete deposition system is shown in Figure 4-1.
+ionization gauge
vent rotarv numr, cryo pump
,'74-,
I roughing valve 1 I rouahina valve 2 I I
power / :::? I
Argon supply
Figure 4-1 Schematic of the deposition system
The deposition system consists of the following units:
1. A vacuum chamber capable of producing lo-' torr. A schematic of this equipment is shown in Figure 4-2 This was fabricated with stainless steel and had seven access ports. The top flange (14" diameter and 3.12" high) had the port that is connected to the ionization flange. Water coolant was circulated through another port on the top flange. The side wall had ports through which the vacuum pumps are connected. The chamber was supplied with argon through another port in the wall. The target specimen is handled through another opening in the side wall, which is closed with a glass shield so that deposition can be visually observed.
Figure 4-2 Schematic of the vacuum chamber (Source: S. Lee, 1994)
2. A rotary piston pump that helps in pumping down the chamber to 1/1000 ton. This contains an eccentric and a piston. The gas is isolated fi-om the inlet after one revolution, then compressed and exhausted during the next cycle. The schematic is shown in Figure 4-3. Schematic of a rotary pump: 1. eccentric; 2. piston; 3. shaft; 4. gas ballast; 5. cooling water det; 6. optional exhaust; 7. motor; 8. exhaust; 9. oil mist separator; 10. poppet valve; 11. inlet; 12. hlnge bar; 13. casting; 14. cooling water outlet.
Fig 4-3 Schematic of a rotary piston pump (Source: M.Ohring 1992)
3. A cryopump that hrther evacuates the chamber to lo-' ton. Cryopumps are gas entrapment pumps, whlch rely on the condensation of vapor molecules on surfaces cooled below 120K. Temperamre-dependent Van der Waals forces are responsible for physically binding or sorbing gas molecules. Several kinds of surfaces are employed to condense gases. These include (1) untreated bare metal surfaces, (2) a surface cooled to 2OK containing a layer of pre-condensed gar of higher boiling point (e.g., Ar or CO2 for HI or He sorption), and (3) a micro-porous surface of very large area within molecular sieve materials, such as activated charcoal or zeolite. The latter are the working media of the common sorption pumps, which achieve forepressures of about 10" torr by surrounding a steel canister containing sorbent with a Dewar of liquid nitrogen. Cryopumps designed to achieve ultrahigh vacuum (Figure 4-4) have panels that are cooled to 20K by closed-cycle refrigerators. These cryo-surfaces cannot be directly exposed to the room temperature surfaces of the chamber because of the radiant heat load, so they are surrounded by liquid-nitrogen cooled shrouds.
LA-
Schematic of pump interior: 1. Fore -vacuum port; 2. Temperature sensor; 3. 77K shield; 4. 20 K condenser with activated charcoal; 5. Port for gage head and pressure relief valve; 6. Cold heat; 7. Compressor unit; 8. Helium supply and return lines; 9. Electrical supply cable; 10. High- vacuum flange; 11. Pump housing; 12. Temperature measuring instrument.
Fig 4-4 Schematic of a cryopump (Source: M.Ohring, 1992)
The starting point of forepressure, ultimate pressure and pumping speed of cryopumps are important characteristics. Cryopumps require an initial forepressure of about lo5 torr in order to prevent a prohibitively large thermal load on the refrigerant and the accumulation of a thick condensate on the cryopanels. In the setup used, the rotary piston pump provided forepressure to the cryopump. The ultimate pressure attained for a given gas is reached when the impingement rate on the cryosurface, maintained at temperature T, equals that on the vacuum chamber walls held at 300K. 4. A control and power supply unit, capable of supplying 1200 W. A photograph of the vacuum chamber with cryopump is shown in Figure 4-5
Figure 4-5 Photograph of the vacuum chamber
The sequence of operations to conduct deposition was as follows:
1. The shcon wafer was mounted on the substrate holder that was designed with the aid of a mask. The mask was a thin steel plate (1 mm thick) with the pattern of the deposition area cut on it so that when placed on the shcon wafer. the mask covers the wafer except for the area where the thin fiwas to be deposited. The thin fito be deposited was as shown in Fig 4-6. The mask had identical slots cut into it. The wafer was sandwiched between the mask and the wafer holder, and the mask was secured to the holder plate by screws. A solid model of the holder with the wafer on it is shown in Figure 4-7. 2. The substrate holder along with the substrate (Si wafer) was fixed to the cathode end of the vacuum chamber.
All dimensions are in mm
Figure 4-6 Schematic of an as-masked wafer Rod attachment, where the holder was fastened to the sputtering chamber
\ Threaded holes to clamp the mask onto the holder
Si Wafer \ Holder plate Nichrome thin film
U shaped metallic pipe t hroulgh wlnich water was circulated to cool the wiafer
B.1 r;j g _1 ~-., C-
Figure 4-7 The substrate holder and the Si wafer (with the thin film) The target (NiCr billet), as shown in the specifications in Figure 4-8 was mounted onto the target holder at the anode end. The target was 80% Ni and 20% Cr The substrate and target setup is shown in Figure 4-9.
0 3.000 +/-.Of0
I
SEE TAB-3-
L' L' 64 FINISH
TAB MIN I MAX A NONMAGNETIC MATERIALS -125 1.625 3 NSGN~ICMATERIALS * .062 1.225 * MAGNETIC THICKNESS DEPENDS ON PERMEABILIf Y OF TARCET MATERIAL
I W* rm USTFP INC
REV C AOD'D HCfAILIC 12-1-93 , RICHLYN ENTERPRISES ~ REV 1 REDRAW 9-11-93 - nu- THE MAK TARGET METALlC OIC f RCl
Figure 4-8 Target specification Cooling Water Power line from magne~ondrive
Fig 4-9 Target and substrate setup 3. The substrate was grounded to the vacuum chamber walls so that biased sputtering was possible. 4. Coolant water was circulated through the substrate holder to prevent the substrate f?om overheating in order to improve film adhesion. 5. The inner walls of the chamber were covered with aluminium foil to prevent coating while sputtering takes place. 6. The vacuum chamber was tightly sealed on all sides. The fasteners were tightened uniformly so that a tight seal is established. 7. The roughlng valves 1 and 2 (Figure 4-1) were kept open. The vent valve for the rotary pump was closed. The gate valve and the Ar valve were also closed. 8. The rotary pump was started and the pipes were evacuated to a pressure of 10" ton . Low pressure was now prevalent in the whole system, including the cryo pump. 9. The rotary pump was disconnected by shutting off the roughmg valve 1. Roughing Valve 2 was also shut, and the gate valve was opened by plugging in its power source. The cryopump was in action then, evacuating the chamber. 10. The pressure in the chamber was then read by an ionization gauge. Evacuation continued until the pressure in the chamber reached about lo-' tom. 11. Argon was now passed into the chamber by opening the gas cylinder valve and the mass flow meter valve. The pressure of Ar inside the chamber was adjusted to an equilibrium of 2 mtorr. That equilibrium was between the continuous evacuation by the cryo pump and the gas supply. The pressure of 2 mtorr was found to be a requirement to develop plasma that can enable deposition at the supplied voltage. This was found experimentally through repeated trials. The supply voltage was maintained in the range of 300 - 400 V. The power supply was maintained at 100 W.
12. By trials, it was found that a deposited thickness of 0.3 - 1 pm did not sustain adhesion tests. Films of the order of 1 pm peeled under the Scotch tape tests. Films with thickness greater than and equal to 0.3 pm did not withstand the stub adhesion test. This is described in Chapter 6. 13. To improve adhesion, the substrate was grounded to compensate for the repulsion caused by the localized positive charge due to glow discharge. Also, cold water was circulated through the substrate holder to cool the wafer during deposition. 14. With a power supply of 100 W, a voltage supply of approximately 300 V, and an argon pressure maintained at 8 mtorr, films of thickness of the order of 0.1 pm were achieved consistently. The deposition time was 5 minutes. Including a plasma development time of 1 minute, the process took 6 minutes. This is summarized in Table 4- 1.
Table 4-1 Thickness Attained With Different Deposition Times
Deposition time (min.) Film thickness (microns) Tendency to spall 90 3 high 45 2 high 12 0.35 high 5 0.1-0.6 negligible
The deposition time was controlled by a timer alarm. At the alarm signal, the power was shut off 15. The vacuum chamber was evacuated by shutting the gate valve and opening the rouglung valves. 16. The deposited substrate was then removed from the holder and subjected to hrther tests. Chapter 5
Experiments on the silicon wafers
An initial lot of 25 silicon wafers with oxide coatings were to be subjected to the tests mentioned in Chapter 2. The wafer and the film were to meet the performance and endurance criterion discussed in Chapter 2, in order to be used in the micro-refiigerator application.
5.1 Stress Measurement Stress is induced on the wafer during the film deposition because of a thermal or mechanical load added on the wafer. The change in stress during film deposition and during annealing had to be studied, since it would help explain the deformation that may occur on the wafer during actual operation. When a thin film is applied to a silicon wafer, it will introduce an amount of deflection to the wafer. The accurate measurement of this bow will allow the computation of the film stress. By noting deflection magnitude, the degree of the film stress can be determined. The direction of film bow indicates whether the film is under compressive or tensile stress. A stress gauge similar to that shown in Figure 5-1 was used. The operating procedure was as follows: 1. The plain wafer was kept on the knife-edged positioning block with the major flat of the wafer facing the eight o' clock positioning block. 2. The major flat of the wafer was kept fully in contact with the eight o'clock positioning block. 3. With the wafer properly positioned on the wafer support, a measurement was taken creating a baseline reading. After film deposition, a second reading was taken which gives a measure of the amount of bow introduced. Wafers under tensile stress bow towards the light sensor and, hence, show a decreased meter reading.
Figure 5-1 Ionic Systems Stress Gauge (Source: Ionic Systems Stress Gauge manual. 1990)
4. The data was then fed into the Stress software that calculates the stress induced.
5.2 Adhesion The film was required to adhere firmly to the wafer surface without any spallig. Adhesion requirements had also put a limit on the thickness of the film that can be practically deposited. Adhesion was another endurance test on the film. Since the frlm was required to be well bonded to the silicon surface and is subject to high operating temperatures, strong adhesion is imperative. A preliminary test was conducted with the aid of scotch tape. A strip of the tape was pasted on the film and peeled off If the film adhered to the tape better than to the wafer surface, adhesion was poor. This test helped in narrowing down the practical film thickness for deposition. For films of thickness higher than 1.0 pm, the quality of adhesion remained poor. Adhesion was found satisfactory at the scotch tape test level only at a film thickness close to 0.1 pm. This was fbrther verified with a more quantitative adhesion test called the Z module testing with the aid of Sebastian 5A test equipment. This method includes various breaking point testing usiig epoxy coated pull studs. The epoxy coated studs are kept in a freezer at low temperature. During the test, these studs were mounted on the wafer samples and cured for a one hour at 150°C in a hrnace to thermoset the epoxy on the stud. The wafer was then placed in the equipment and the stud was gripped and pulled until the stud breaks. It was determined that when the film thickness is at or below 0.1 pm, the studs failed with the film remaining intact. This proved that the thin films were adhesive at 0.1 pm thickness. The tests were conducted before and &er annealing. The films proved to be consistent in adhesion.
5.3 Thickness Measurement: The thickness of the films needed to be verified after deposition and also to be checked for any film material loss after annealing. Thickness of the film deposited on the wafer was measured using the DekTak IIA Surface Profile Measuring system. It has a micro-sensitive stylus that moves on a plane sensing any aberrations on the planarity of the surface. The deviations were measured digitally and recorded by a graphical output. An outline sketch of the instrument is shown in the Figure 5-2. The stylus was allowed to surf the film twice, with the wafer turned 180" during the second run. The mean reading was taken to be the thickness of the film. The graphic output of the scan program in the instrument that reads the thickness is shown in Figure 5-3. Thickness were measured for every wafer deposited. Thickness was also measured on the wafers after annealing. :US KNOB ROTARY STAGE
ROUGH
Figure 5-2 Outline sketch of DekTak IIA (Source: DekTak IIA operations manual, 1988)
HORIZ: '717uM 6,880 5,808
4,888 3,880 2,888
18 880 8
1,808 2,800
R CUR: 8 6l @ 406~4 n CUR: e A e 1,124~~ SOAN OUCTAK II Figure 5-3 Graphic output for thickness for wafer #4 5.4 Annealing The film behavior at high temperatures was to be studied in detail. The film had to withstand the regular operating temperature cycles the micro-engine was expected to undergo. Since the ambient was to be a inert atmosphere, annealing tests were preferred to be conducted in inert atmosphere or in vacuum, where the risks of contamination by oxidation were reduced. The application required the film to be under a thermal cycle with a maximum of 700 deg C in an inert environment. Annealing tests on the film were conducted by placing the film in a glass jar and flowing nitrogen through it. Oxidation was observed on the film with visible color change. This was observed for a number of trials. It was likely that oxidation was caused by the prevalent atmospheric oxygen. Therefore, annealing was in a vacuum. Under lower degrees of vacuum (of the order of 1 torr), oxidation was still observed. Annealing with the aid of a vacuum furnace also showed oxidation. At higher vacuum (of the order of torr), the oxidation was negligible, though not completely absent. These heating tests were conducted in a high vacuum furnace in the accelerator lab in the Physics Department at Ohio University. Further research was conducted to find the causes of oxidation. An Auger spectroscopic study was conducted on the oxidized portion of the wafer. Wafers that were annealed at 700°C for different heat periods were subjected to the test. It is now believed that the oxidation is due to the exodus of the oxygen atoms in the SiOz interface to the surface. This causes the oxidation of the NiCr film.
5.5 Resistance measurements Resistivity of the film is an important property to be studied. The film was intended to have a resistivity of the order of 1 pohm-m to hnction efficiently as a conducting hot-end. The consistency of the film in maintaining its resistivity after heat treatment was also studied. The circuit used was as shown in Figure 5-4. A dc power supply unit (50 volts max.) was used. A high-resistance wire (made of Ni) was used in the circuit to provide lower potentials to the ends of the high-resistance film. The multimeter readings show the resistance across the film.
50 v supply -
multirneter
Figure 5-4 Electrical resistance measurement
To supply power to the micro-refrigerator, the film acts as the metallic conducting surface. Leads were to be connected to the metal layer so that current can be passed through. A suitable binding material or a binding technique was needed for establishing these contacts. The binding was to stay intact at high temperatures, as well. Two types of Ohmic contacts were tried in the experiments: 1. Ohmic contacts with the film were established by bonding of the copper wires to the film by means of a brazing alloy paste--SC60193M. The specification sheet of the alloy is attached as shown in Appendix 5-A. The alloy paste was applied to the copper wire and film interface, and the setup heated in a hrnace for 825°C for a period of 20 minutes. Although the paste was able to thermally set and form a bond in the case of a plain Si wafer and a Cu wire interface in inert atmospheres (as shown in Figure 5-5), when used with the film, the film layer oxidized and no contacts were established. This was repeatedly observed in a number of trials. nitrogen inlet
50 V supply
L- L- resistance -furnace (700deg C)
Figure 5-5 Ohmic contacts on a plain wafer
2. The alternative Ohmic contact was made by soldering the Cu wire to the film using a regular solder. Since the melting point of the solder is 150°C, the resistance measurements were conducted below that temperature. At high voltage supply (around 3 V), the resistance of the film increases (to the order of 100 MQ) and so does the temperature. The solder melts around this time and the Ohmic contact is destroyed. Soldering was not an acceptable alternative.
5.6 Tests on the wafers For demarcation purposes, the wafers were numbered from 1 to 25. Care was taken to maintain the wafers clean from dust. The test wafers were passed through the following steps for analysis: 1. The wafer was placed on the Ionic stressgauge for the initial meter reading and reading was noted. 2. Resistance was measured on the diametrically opposed ends of the wafer by contacting film ends with the leads of the multimeter. This was to measure resistance in a relative scale between the different stages of the wafer. 3. The wafer was then moved to the site of the magnetron sputtering equipment. 4. It was then cleaned with tissue paper. Earlier, wafers were cleansed with jet gas (argon) or alcohol. But they formed films on the wafer and contributed to poor film adhesion during deposition. 5. The wafer was then deposited with the nichrome film. Wafers 1, 2, and 3 were used for trial deposition. The films deposited had poor adhesion and failed during the scotch tape test. Wafers 4-8 were used to achieve the film thickness with desirable adhesion. Wafer #8, with a film thickness of the order of 0-lpm, proved to be adhesive. The wafer was then subjected to the epoxy test for adhesion. It maintained adhesion. The wafers (except for #7) in the first lot were deposited on one side with nichrome, without any mask pattern. Film was deposited on #7 with the aid of the mask, as shown in Figure 4-7. 6. The wafers were measured for film thickness using the DekTak IIA Surface Profile measuring instrument. 7. The wafers were then read with the Ionic stress gauge. The stress induced on the wafer due to film deposition was calculated with the aid of the two readings. 8. The wafers were then measured for contact resistance, as in Step 2. 9. Wafers were then subjected to annealing to a maximum of 700°C. Wafers 6, 8, 9, 10, 11, 12, and 13 were annealed in nitrogen. The film oxidized in all cases, showing a color change to rusty brown. Wafers 14 and 15 were deposited with film at a lower power but for a longer duration of 10 minutes to check for improved adhesion. Adhesion did not change significantly. The rest of the wafers were not utilized. Since it was suspected that the oxidation during annealing might be due to the oxide coat on the silicon wafer, nitrided Si wafers were used for fbrther deposition and testing. Tables 5-1 and 5-2 show the test data collected for the wafers. Wafers with numbers less than 26 are oxide-coated, and the rest are nitride wafers. Table 5-1 Wafer Treatment Data
Wafer Cleaning Pressure Power Deposition Scotch Film Annealing Remarks (psi) (w) time tape thickness test (microns) 1 Acetone 2 100 90 Yes 3 No Failed scotch tape test 2 Acetone 2 100 45 Yes 2 No Failed scotch tape test. 3 Acetone 2 100 12 Yes 0.35 In vacuum Film also at 555 deg. failed C during annealing 4 None 2 100 5 Yes 0.2 No Failed scotch tape test. Film also failed during annealing 5 Broken 2 100 5 Yes 0.2 6 None 2 100 5 Yes 0.2 In nitrogen Low at 700 deg. failure C observed during scotch tape test 7 None 2 100 5 No 0.2 No 8 Acetone 2 100 5 Yes Wafer No and tissue broken paper 9 Air spray 2 1 00 5 Yes 0.15 No and tissue paper 10 Air spray 2 100 5 No 0.2 No and tissue paper 11 Air spray 2 100 5 No 0.1 No and tissue paper 12 Air spray 2 100 5 No 0.15 No and tissue paper 14 Air spray 8 46.5(3 11.5 Yes 0.15 No Passed and tissue lOVxl scotch paper .5A) tape test 15 Air spray 8 46.5(3 10 Yes 0. .I9 No Passed and tissue lOVxl scotch paper .5A) tape test Table 5-2 Stress and Resistance Measurements and Silicon Wafer
A - after deposition B - after annealing T - wafer in tension C - wafer in compression M - film deposition with mask * after 1 hr. anneal ** after 2 hr. anneal *** after 4 hr, anneal 11. Stressgauge readings were noted and the change in induced stress due to annealing was calculated. Wafer 36 was split to quarters and subjected to anneahg in the high vacuum furnace in the accelerator laboratory. A photograph of tl-us furnace with a pyrometer for temperature measurement is shown in Figure 5-6. A portion of it was annealed in a vacuum for 1 hr., 2 hr., and 4 hr., intervals to a maximum of 700°C. The annealing results are shown in Table 5-2. Wafer #7 was experimented for ohmic contacts with the Ag-Cu alloy paste and heated to 825°C for the paste to set. The tilm oxidized and the contacts failed.
Figure 5-6 Photograph of the high vacuum furnace Chapter 6 Results and Analysis
The following are the results and inferences fiom the tests conducted on the individual parameters. The results were also summarized earlier in Table 5-1 and Table
6.1 Adhesion As mentioned earlier, adhesion improved with thinner films and heated surfaces. Cleaning with acetone or an inert gas jet appeared to degrade adhesion. Subsequently, the wafers were cleaned by merely dusting with clean tissue. The film that was then deposited exhibited good adhesion to the substrate as demonstrated by the Sebastian Quad5 Adhesion tester. The measured adhesion was 1600 psi. Adhesion did not worsen even after annealing the tested samples. As noted, there was no evidence of spalling in the samples of film thickness less than 0.16 microns.
6.2 Intrinsic Stress Analysis Stress measurements were made both before and after annealing. In virtually all instances (for thickness below 0.16 microns), the wafers deposited with nichrome were under a small tensile stress in the range of 2 to 7x10' dynes/cm2.In all instances, after a one hour anneal, the films went fiom tension to compression, with compressive stress in the range of 0.9 to 1.3 x108 dynes/crn2. Overall, these were small stress values and indicated that the films were not under significant stress in that thickness range. Indeed, no spalling of the films had been observed during all of the handling the samples had undergone during the Auger analysis, stress analysis, and so forth. The transition from tension to compression was also logical in that, during the anneal process, the films were able to flow somewhat and relax from their tensile state. One sample--that of 0.16 micron nichrome film deposited onto a nitrided wafer--had an as-deposited stress of about 4x10~dynes/cm2 and, more interestingly, it was in compression. Very thin films are typically under tensile stress, and once a certain thickness level is exceeded, they go from being in tension to being in compression. When the films are below 0.16 microns, the atomic forces on the film act towards each other, and in that process pull the film towards the center of the wafer. This results in a tensile stress on the wafer, as detected by the stress gauge. When annealed, the tensile stress is relieved and the wafer forces turn outward, thereby subjecting the wafer to compression. It was also observed that when the film thickness was more than 0.16 microns, the wafer was under compression. In such cases, the forces in the film act outward, away from each other, and bend the wafer in a convex shape (with the film on the upper side of the wafer), thereby inducing a compressive stress on the wafer. The wafer with 0.16 micron thick film (wafer # 35) was also subjected to additional anneals beyond one hour. After annealing for an hour, the sample's stress decreased from 4 x lo8 dynes/cm2to about 1 x108 dynes/cm2. It then increased to 1.2 x 1o8 dynes/cm2at 2 hours of annealing and, finally, went to about 3 x 1o8 dynes/cm2 after four hours. A possible conclusion is that the one-hour anneal, as with the samples described above, allowed the film to relax. Further anneals gave the film additional energy to flow outward and increase the level of compressive stresses. However, all the levels of stress after annealing were smaller than the as-deposited stress, and it is very important to know that all of these stresses were small for stress in thin films. The most useful inference is that for thin films of nichrome in the 0.10 to 0.16 micron range, intrinsic stress should not be a concern.
6.3 Auger Analysis 6.3.1 Auger depth analysis A sir gle oxidized silicon wafer (wafer # 16) was deposited with about 1500 angstroms of nichrome. It was then cut into four approximately equal sized pieces. One was kept as a control and the other three were subjected to an anneal temperature of 700°C in vacuum (lo4 torr) for 1, 2, and 4 hours. The zero, one- and four- hour samples were then analyzed by Auger Electron spectroscopy (AES), and the results are shown in Figures 6- 1,6-2, and 6-3. AES analysis of the unoxidiied wafer revealed, as expected, a layer consisting of nickel and chromium in constant proportion down to the interface between the nichrome film and the silicon substrate. This is shown in Figure 6-1. At this interface point, the concentrations of Ni and Cr descend to zero, and the oxygen level begins to increase, as would be expected. Analysis of the one- hour annealed sample (Fig 6-2) showed chromium concentrated at the surface and a corresponding increase of oxygen in this same Cr rich region.
AESSp 0 KLL
200 4i0 600 900 1~~00150 i4'011 16'00 la'00 ' 2600 2200 Etch Time / seconds
Figure 6-1 Auger Depth analysis of unannealed sample. It can be seen that Ni and Cr were in constant proportion until the interface between the film and the silicon substrate. Thereafter, oxygen level increased.
It was the normal behavior expected in that Cr normally oxidizes to form CrO? and, hence, will scavenge ambient oxygen. What was interesting was that the very small amounts of water vapor andlor oxygen remaining in the system were enough to cause discernible oxidation of the nichrome surface. Analysis of the four-hour annealed sample (Fig 6-3) indicated that Ni had once again become the predominant material at the surface and that Cr and oxygen had become depleted. The only explanation for this is that the Cr02 layer was somehow lost from the surface-- probably by sublimation or flalung. The overall suggestion to come from the results is that there is no advantage in using nich~omeas the heating material because nichrome eventually becomes an essentially all nickel layer.
Fig 6-3 Auger depth analysis of annealed sample (4 hrs at 700°C). It Carl be seen that Ni is dominant in the surface of the film and chromium oxide possibly flaked or sublimated. 6.3.2 Auger depth analysis of annealed nichrome films on nitrided wafers A single nitrided wafer was deposited (wafer # 36), cut, and annealed in a fashion similar to the oxidized wafer described above. The Auger analysis was performed on this wafer, and it yielded similar results to that of the oxide wafer, but understandably showed lesser oxygen count.
6.3.3 Auger analysis across the surface During the AES analysis described above, it was observed that the surfaces of the annealed films were not uniform. During surface analysis of a one-hour annealed sample, alternating light and dark areas were observed (Figure 6-4a) with these features being about 10 to 100 microns across Performing an -4ES analysis along the line connecting several of these light and dark features revealed that these areas are alternating in levels of Cr and Ni. Ths is shown in Figure 6-4b
-- -. ---- -. I - 1 11
I i 1 i 1
Figure 6-4a Auger surface analysis for one-hour annealed sample showing patches on the wafer surface U '/data/FUM/ntcrla3 AES Ln Nl W/1-
Ofstance fro@ 1tne start / ma "Idata/FUN/nfcrla3 AES Ln Cr M/1 -
Distance from line start / 8m
Figure 6-4b Segregation of Ni and Cr atoms in the four-hour annealed sample shows that Ni is in lower concentrations at the surface and Cr atoms segregated to the surface. Analysis of a four-hour sample also showed similar results, but the percentage of atoms segregated had increased. This is shown in Figure 6-5a and Figure 6-5b.
Figure 6-5a Auger surface analysis for four-hour annealed sample showing patches on the wafer surface
f h I40
Uz \ -m 20 r3 C I 8 0.02 0.04 0.06 8.08 0.1 8.U 0.14 1.16 8.18 8.2 0t:tanca from llnr start / u - '/datL/FUM/nlcrla3 USLn Cr W2-
Y Figure 65b Segregation of Ni and Cr atoms in the 4 hr annealed sample shows that Ni concentrations increase at the surface as Chromium atoms were lost due to the flaking of chromium oxide. The AES studies show that there was a segregation of Cr and Ni atoms that disturbs the alloy concentration when annealed. This was found to happen in every case tested. Other alloys of NI and Cr, such as Alloy HX, Alloy S, Alloy W, and Alloy X (ASM Engineering Material Reference Handbook, 1994), have good resilience to high temperatures, and their resistivity and coefficient of thermal expansion were not very distinct from the 80120 Ni-Cr alloy used. One of these alloy compositions of nichrome could make a good film material without disintegration during heating. The nichrome alloy used for the tests was concluded to be unsuitable. Chapter 7 Conclusion
The Stirling cycle is an externally heated closed cycle with energy transferred from kinetic form of the moving parts to heat or useful output energy and vice-versa. Since it is reversible, the Stirling cycle can be put to use as a refrigeration cycle. A proposed Stirling cycle application is the micro-refrigerator that is made of silicon with trapped-in air as the working fluid. This micro-refrigerator is to be driven by a rnicro- engine that needs a hot-end from where heat is supplied to the working fluid. Metal alloys, such as like nichrome, are best deposited by the sputtering technique, which is one of the many physical vapor deposition processes. A method of depositing nichrome on the base silicon wafers was developed with the available magnetron sputtering equipment. Nichrome thin films were deposited on several silicon wafers. Deposition was successfbl with the film properly adhering to the substrate. The wafers were subjected to measurements of stress induced because of the thin film, tests for film adhesion, annealing in temperature cycles up to a maximum of 700°C in a neutral atmosphere, and measurements of the change in film resistance due to heat treatment. The hot-end is to receive energy by electricity. For this purpose, electrical contacts had to be established onto the thin film. These contacts were to sustain the operating temperature cycles with a high of 700°C. Contacts were provided by pasting copper wires to the thin film by a brazing alloy paste. High temperature tests were conducted on these contacts to check for durability. From all the tests conducted, it was construed that the nichrome alloy used oxidized during annealing in all environments. A segregation of Ni and Cr atoms caused the thin film surface to rupture. Hence, it was concluded that at high temperatures, the nichrome studied should not be used as a hot end for the rnicro- engine if used in the presence of air. Bibliography
1. Adibi, F.A., Petrov, I., and Greene, J.E. "Design and characterization of a compact two target ultrahigh vacuum magnetron sputtering system". J. Vac. Sci. Technology, JadFeb 1993. pp. 53.
2. ASM Handbook. Vol. 2. "Properties and selection: Nonferrous alloys and special-purpose materials". ASM Intl., 1994.
3. Berry, R.W., Hall, P.M., and Harris, M.T. Thin Film Technology. Van Nostrand Co. 1986.
4. Blocher, J.M., and Bunshah, R.F. Deposition Technologies for Thin Films and Coatings. Noyes Publications, 1982.
5. Bowman, L. "Stirling cycle for micro-refrigerator". SunPower Inc., Athens, OH, 1994.
6. Gulino, A.D. "Ion beam sputter deposited zinc telluride films". Jountal of Thin Films, Vol. 128. 1986. pp. 93.
7. Holland, L. Vacuum Deposition of Thin Films, Wiley. NY. 1956.
8. Maissel. L.I. and. Gland, R. Handbook of 'Thin Film Technology. McGraw Hill Book, NY. 1970.
9. Makabe, Y., Hirohata, Y., Yamashina, T. "Surface roughness of Mo films prepaed by sputtering". Journal of Thin Films, Vol. 229. 1993. pp. 52. 10. Metals Handbook. Vol. 1, "Properties and selection of metals". American Society of Metals. 1993.
1 1. Motojima, S., Hattori, T., and, Kurosawa, K. "Deposition and micro-hardness of Sic". Material Science, Vol. 21. 1986. pp. 44.
12. Ohring, M. The Material Science of Thin Film. Academic Press Inc., NY. 1982.
13. Vossen, J.L., and Cuomo, J.J. Thin Film Processes. Academic Press Lnc., NY. 1978.
14. Lee, S. The Design of Ultra High Vacuum System. Master of Science Thesis, Ohio Univeristy. Athens, OH. 1994. Appendix 3-A Mechanical properties of nichrome alloys
.-- i - w wur3. w- Wcirb dab mh (0.2% Yu (-1 Mh U UP8 U (1 L).S Gh ldpl HYdrar
205 29.8 90 HRB I
! I f 4 X fdulion annealed). .. ,785 114 393 52.5 43 1% 28.5 89 HRB , 1 i
(Source: ASM Engineering Material Handbook, 1994) Mechanical properties of nichrome alloys
I Iron-akkd-chnwnlum dlop (concrnud) Alloy WHT.. See Alloy 800
Alloy 825...... 690 100 310 45 JS
i I 186 n 80 HRB i
(Source: ASM Engineering Material Handbook, 1994) Mechanical properties of nichrome alloys
j Alfov 230 (a)...... ,860 121 390 57 47.7 211 30.6 92.5 HRB 1 i i
i 1 Alloy 6M...... 651 95 310 45 10 207 30 75 HRB f i I
620 90 275 a 45 ZUY 30 65-80 HRB 1 1
, I
Alloy 617 (wlutron annded . 755 110 350 51 58 211 30.6 173 HE I I i !
Alloy 625...... 930 135 517 75 42.5 nn M IWHB 1I I
(Source: ASM Engineering Material Handbook, 1994) Appendix 3-B Composition of various nichrome alloys
------Cu.rw*c WmJ 4110, 31 C. fr Ma C 51 S
Cummrmalty pun sod !ew-a14 nkluls L~ckei:(XI...... 99.0 m~n 0.3 0.4 0.35 0.15 0.35 0.01 N~ikcl'01...... 99.0 min 0.25 0.40 0.35 0.02 0.35 0.01 i 105...... 99.0 rn~n(b) 0.15 0.3 0.35 0.15 0.15 0008 / Ltckel 21 I...... 93.7 rn~n(b) 0.3 0.75 4.ZS5.25 0.20 0.15 0.015 1 .L~ckcl212...... 97.0 rnlo 0.20 0.25 1.S2.5 0.10 0.20 ... i Niikcl 2...... 99.0 m~n(b) 0.10 0.10 0.30 ... 0.10 0.W i ~uckclYO...... 99.9 min 0.01 0.05 0.003 0.02 0.005 0.W3 i Dur3n1ckcl MI . . 93.00 mtn 0.3 0.60 0.50 0.30 1.00 0.01
: h~rkcl-coppcralloys i ( 4iloy .cU)...... 63.0 rn~n(b) 3.0-34.0 2.5 0.20 0.3 0.5 0.M4 i h:lov 401...... rO.(YS.D(b) bal 0.75 2.25 0.10 0.75 0.015 1 .\lloy Rd05 ...... 63.0 rn~n(bj 3.0-34.0 2.5 2.0 0.3 0.5 0.025-0.060 1I 1 -\lluy 150...... 3.0-33.0 bal 0.4-1.0 1.O ...... 0.02 I ; :\rloy K-500 ...... 63.0 rn~n(b) 37.0-33.0 2.0 1.5 0.25 0.3 0.0 1 " POYI&..m(.r U~Y ' >i Cr Pr G M. w P(b n Al C .Ma 9 I / N~ckrl~hram~umand oukkhnwiwp-irw a- 1 Alloy 230 .... bal 20 3.0 5.0 2.0 14.0 ... . . 03 0.10 03 0.4 ,Alloy 600 ... .T.O mm(b) 14.0-17.0 6.0-10.0 .-...... 0.15 1.0 05 , .Uloy b01 .... 58.W.O 21.0-25.0 bd .-...... 1.0-1.7 0.10 1.0 0.50 Alloy 617 ... .W.5 rnm 20.0-24.0 3.0 10.0-15.0 8.0-10.0 . - . ... 0.6 0.~15 0.m.15 I 0 1.0 i ;Ulov 63 ... .%.0 mtn M.0-23.0 5.0 1.0 8.0-10.0 - - - 3.154.!5(~) 0.40 0.40 0.10 0.50 050 ;Allov 690 .... 58.0 mm 1.0-31.0 7.0-11.0 ...... 0.05 OM 0s dloy 715 ... .jO.O-5S.W) r 1.~L21.0 b.4 1.0 2.S3.30 ... 4.75-5.q~) 0.65-1.15 Od[M.BD 0.08 035 035 .Alloy X750.. . m.0 mm@) 14.d17.0 5.0-9.0 1.0 ...... 0.70-1.q~) Ls2.75 0.4C-1.a) 0.0s 1.W 03 .Alloy 751 .... 70.0 rnm(b) 14.0-17.0 5.0-9.0 ... ..- ... 0.7-1.a~) L0-2.6 - - . 0.10 1.0 0.5 i Nby .w i 7wd) .... .78.0 -W 1.O ...... 05 0.3 0.05 - - - ... 1 VC-zz ... 51.6 21 i 55 72 135 4.0 ...... 0.01 1.0 0.1 .y C-Tl6 . . bal 14S-16.5 4.0-7.0 23 15.0-17.0 3.MJ . - ...... 0.01 1.0 0.08 +l~loy (33.. ... bd 21.0-235 18.0-21.0 5.0 6.M.O 1.5 03~) - - - ... 0.015 1.0 1.0 iAby HX ....bal 5)SZl.O 17.0-20.0 05-23 8.0-10.0 0.2-1.0 - - - ...... O.OS-OOS-OIS1.0 1.0 AIovS ...... bal 145-17.0 3.0 2.0 14.0-163 1.0 ... .-. 0.10-0.9 0.02 0.30-1.0 Odao.75 ,I Aky W ..... 63.0 5.0 6.0 23 24.0 ...... -. 0.12 1.0 1.0 hllq X ...... bal 2030-2.3.00 17.0-20.0 05-25 8.k10.0 O.tI.0 . - . 0.15 030 O.M-0.15 1.0 1.0
(Source: ASM Engineering Material Handbook, 1994) Appendix 3-C Physical properties of nichrome alloys
Alloy Thermal Coefficient of Electrical resistivity conductivity thermal expansion mi2.m W1m.K pdm. K 230(a) 8.9 12.6 1250 600 14.9 13.3 1030 60 1 11.2 13.75 1190 617 13.6 11.6 1220 625 9.8 12.8 1290 690 13.5 14.06 1148 718 11.4 13.0 1250 X750 12.0 12.6 1220 75 1 9.8 11.2 1220 C-276 10.0 14.6 1300 G3 11.6 13.3 1250
(Source: ASM Engineering Material Handbook, 1994) Appendix 3-0 Ni-Cr phase diagram
Cr-Ni 274
WCIGMI LC. CCNT NICKCL
*00. I I I I I I I I I i I I i I 1 200 r I I I I I I jji I 0 j 1 I 1 ! I 0 10 20 30 40 $0 SO 10 a0 90 100 C r ATOMIC CCR CENT UICXCL Mi
(Source: ASM Engineering Material Handbook, 1994) Appendix 5-A Heraeus Inc., SMT Material Composition: SC60193 is 72% silver, 28 % copper brazing paste. It contains no activator, so it is used in an inert or reducing atmosphere. There are no residues remaining on the board after the reflow. The organics that the metal powder is dispersed in bum out very cleanly.
Properties:
Viscosity: 250 - 350 poise; Brookefield HBT, SC4- 14 spindle at 10 rpm, 25 deg. C
Percent Metal: 93+/- 1 %
Alloy: 72/28 Ag/Cu
Powder Mesh Size: -325
Activity: Nonactivated