Fabrication of a Thin Film Resistance Heater
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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.