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The Pennsylvania State University The Pennsylvania State University The Graduate School College of Engineering PROCESSING OF SAPPHIRE SURFACES FOR SEMICONDUCTOR DEVICE APPLICTIONS A Thesis in Electrical Engineering by Kevin W. Kirby © 2008 Kevin W. Kirby Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2008 The thesis of Kevin W. Kirby was reviewed and approved* by the following: Jerzy Ruzyllo Professor of Electrical Engineering Thesis Adviser John D. Mitchell Professor of Electrical Engineering W. Kenneth Jenkins Professor of Electrical Engineering Head of the Department of Electrical Engineering *Signatures are on file in the Graduate School. ii ABSTRACT This thesis explores the preparation of sapphire surfaces for use in semiconductor device applications. Sapphire has shown promise in a few niche applications as a device substrate due to its insulating nature and extremely stable behavior. These properties make sapphire a suitable alternative for applications where standard silicon substrates provide inadequate performance. As the technology behind the applications involving sapphire substrates has improved, sapphire substrate surface preparation has become more of a concern. In an effort to further the development of sapphire surface processing, wet chemical cleaning treatments were explored during this study. Chemistries common to the industry were chosen including Standard Clean 1 (SC1), Standard Clean 2 (SC2), hydrofluoric acid (HF), and a 3:1 mixture of sulfuric acid (H2SO4) and phosphoric acid (H3PO4). Chemical treatments were analyzed by means of wetting angle measurements, AFM analyses, and XPS surveys. Several alkali metals and other contaminants were shown to be present on the surface of the samples tested along with a significant amount of carbon, implying organic contamination. Several treatments caused a change in surface morphology, but many treatments did not affect surface composition. Most noticeably, SC1 appeared to be an effective treatment for organic contamination on sapphire surfaces. Ultimately, these treatments could prove to be an integral part of an effective sapphire surface cleaning sequence, and these treatments should be explored in more detail. iii TABLE OF CONTENTS LIST OF TABLES............................................................................................................. vi LIST OF FIGURES .......................................................................................................... vii ACKNOWLEDGEMENTS............................................................................................... ix 1. INTRODUCTION ....................................................................................................... 1 2. BACKGROUND ......................................................................................................... 3 2.1. Sapphire Material Properties ................................................................................ 3 2.1.1. Sapphire Crystal Structure.............................................................................. 3 2.1.2. Notable Mechanical and Electrical Properties ............................................... 4 2.1.3. Diffusion Behavior of Various Elements in Sapphire.................................... 6 2.2. Applications for Sapphire in Semiconductor Electronics..................................... 9 2.2.1. Silicon on Sapphire (SOS) Technology ......................................................... 9 2.2.2. Fabrication of SOS wafers............................................................................ 15 2.2.3. RF Applications for SOS Technology.......................................................... 20 2.2.4. Other Applications for SOS Technology ..................................................... 21 2.3. Applications for Sapphire in Semiconductor Photonics..................................... 23 2.3.1. Epitaxial Deposition of GaN on a Sapphire Substrate ................................. 23 2.3.2. GaN Based Optoelctronic Devices............................................................... 28 2.3.3. ZnO Based Optoelectronic Devices ............................................................. 29 2.4. Current Methods for Sapphire Wafer Cleaning and Surface Processing............ 31 2.4.1. Wet Chemical Treatments ............................................................................ 31 2.4.2. Dry Chemical Treatments............................................................................. 35 2.4.3. Mechanical Polishing ................................................................................... 37 3. OBJECTIVES OF THIS STUDY.............................................................................. 40 4. EXPERIMENTAL PROCEDURES.......................................................................... 41 4.1. Chemical Treatments .......................................................................................... 41 iv 4.2. Measurement Techniques ................................................................................... 42 5. EXPERIMENTAL RESULTS AND DISCUSSION ................................................ 45 5.1. Wetting Angle Measurements ............................................................................ 45 5.2. Surface Roughness Measurements ..................................................................... 48 5.3. Atomic Composition Analysis............................................................................ 54 6. SUMMARY............................................................................................................... 61 7. REFERENCES .......................................................................................................... 63 v LIST OF TABLES Table 1. A selection of important electrical and mechanical properties of sapphire.......... 7 Table 2. TXRF results from two separate sources (a) and (b). Particle concentrations are given in terms of the atoms/cm2 count...................................................................... 55 Table 3. Atomic percentage of various elements on the sapphire surface following treatment. .................................................................................................................. 57 vi LIST OF FIGURES Figure 1. A graphical representation of the sapphire crystal orientations most commonly used for semiconductor applications. Source: http://americas.kyocera.com/kicc/industrial/crystal.html. .......................................... 5 Figure 2. Arrhenius plots for bulk diffusion in sapphire for Pt, Ag, Ga, Y, Cu, Co, Fe, and Cr. ............................................................................................................................. 10 Figure 3. A schematic comparison of (a) a SOS device and (b) a bulk silicon device. The potential for complete isolation and lack of doped well structures in SOS devices allow for the creation of more compact and efficient devices. Source: Reference 20. .................................................................................................................................. 12 Figure 4. A graphical depiction of the “kink” effect observed in SOI devices. Source: D. Neamen, “An Introduction to Semiconductor Devices,” New York: McGraw-Hill Higher Education, 2006. ........................................................................................... 14 Figure 5. A representation of the (a) r-plane crystal orientation and the (b) lattice mismatch between r-plane sapphire and <100> silicon. Source: Reference 20. ...... 17 Figure 6. A process flow diagram for Solid Phase Epitaxial Regrowth (SPER). Source: Reference 20. ............................................................................................................ 19 Figure 7. An illustration of the epitaxial relationship between sapphire and GaN for various crystallographic orientations. Source: Reference 45.................................... 25 Figure 8. Visual evidence of single crystal GaN island formation on GaN nucleation layers. Source: Reference 47. ................................................................................... 27 Figure 9. Representation of the <0001> sapphire orientation and lattice sites that promote an epitaxial relationship with ZnO. Source: Reference 57. ...................................... 30 Figure 10. The etch rate vs. temperature for various chemical etchants of sapphire. Source: Reference 63. ............................................................................................... 33 Figure 11. The etch rate and etch selectivity as a function of plasma inductive power at a DC bias of -600V. Source: Reference 77. ................................................................ 38 Figure 12. Wetting angle of sapphire samples (a) treated with various chemicals for a period of 10 minutes and (b) treated with HF for varying lengths of time............... 47 Figure 13. Wetting angle over time for samples treated with HF for varying lengths of time. .......................................................................................................................... 49 Figure 14. RMS roughness of wafer samples treated with various combinations of SC1, SC2, and (a) HF or (b) 3:1 H2SO4:H3PO4................................................................. 50 vii Figure 15. AFM Images of the sapphire surface after (a) no treatment, (b) treatment with HF, (c) treatment with SC1, (d) treatment with H2SO4:H3PO4. ............................... 52 Figure 16. Counts per second versus
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