INVESTIGATION OF DIAMOND ETCHING BY A MICROWAVE
INVESTIGATION OF DIAMOND ETCHING BY A MICROWAVEPLASMA-ASSISTED SYSTEMByDzung Tri TranA DISSERTATIONSubmitted toMichigan State Universityin partial fulfillment of the requirementsFor the degree ofDOCTOR OF PHILOSOPHYElectrical Engineering2010
ABSTRACTINVESTIGATION OF DIAMOND ETCHING BY A MICROWAVEPLASMA-ASSISTED SYSTEMByDzung Tri TranDiamond deposition technology advances have opened several potential applications fordiamond-based devices and components. Many diamond applications, such as micro-electromechanical systems (MEMS) fabrication and electronic devices, require the micro-structuring ofthe diamond and other applications, such as optical and thermal management components,require smoothing the diamond surface. Because of the high chemical inertness property ofdiamond, a key technique for micro-structuring and surface modification of diamond is plasmaassisted etching. The objective of this study is to investigate and develop processes and theassociated understanding of plasma-assisted etching of diamond for micro-structuring andsmoothing of diamond substrates.The etching of three types of chemical vapor deposition (CVD) diamond includingnanocrystalline diamond (NCD), microcrystalline diamond (MCD) and single crystal diamond(SCD) is investigated using a 2.45 GHz microwave plasma-assisted etching reactor system. Theplasma reactor has a 25 cm diameter discharge located inside a 30 cm diameter cavity applicatorand it has an independent rf bias capability for the substrate holder that facilitates ion energycontrolled reactive ion etching at low pressures. The feed gases for the etching process includemixtures of oxygen (O2), sulphur hexafluoride (SF6), and argon (Ar). The etching reactoroperation is investigated for both magnetized electron cyclotron resonance (ECR) and non-
magnetized plasma operation for the pressure ranges of 1-40 mTorr and 4-100 mTorr,respectively. The plasma characteristics are investigated using visual plasma dischargeobservations and single Langmuir probe measurements. For both ECR and non-magnetizedplasma reactor operation, a high density plasma with charge particle densities of1011 2 x1012 cm 3 is obtained.The etch rate, anisotropic etching profile, and surface roughness are measured versus inputetching reactor parameters including pressure, substrate bias, microwave power and gasmixtures. Anisotropic etching is demonstrated and the measured etching rates range from 4 - 15µm/h. A highly anisotropic etching profile is obtained at a pressure of 4 mTorr. The selectivity ofthe plasma-assisted diamond etching process is measured for various mask materials includingaluminum, gold, titanium, silicon dioxide and silicon nitride. Aluminum gave the highestselectivity with a value of 56.The use of the plasma-assisted diamond etching process is also investigated for thesmoothing or polishing of rough microcrystalline diamond (MCD) surfaces. Three plasmaassisted polishing methods investigated include the use of (1) plasma-assisted etching of MCDfilms coated with a sacrificial layer and etched with a selectivity of one, (2) photo-resist reflowon the rough MCD surface to expose the high portions of the MCD sample, and (3) microroughing of the surface by plasma-assisted etching prior to mechanical polishing. The surfaceroughness reduction rate and the final surface roughness obtained by the three techniques arestudied and comparisons are made. The plasma-assisted smoothing of MCD samples from asurface roughness of 3800 nm down to 50 nm is demonstrated.
Copyright byDzung Tri Tran2010
ACKNOWLEDGEMENTSThe author wishes to express his sincere appreciation to Professor Dr. Timothy Grotjohnfor his guidance, encouragement and support throughout the development of this research andwriting of this dissertation. I would also like to thank other members of the author’s guidancecommittee: Professor Dr. Jes Asmussen, Professor Dr. Donnie Reinhard and Professor Dr. GregM. Swain for their valuable assistant and support with the writing of this dissertation. The authorwould like to thank Dr. Thomas Schuelke, Michael Becker, Lars Haubold, David King, TraceyHock and Kagan Yaran for their encouragement and support. In addition, the author would liketo thank Professor Dr. Stanley L. Flegler, Dr. Tim Hogan and Dr. Carol Flegler for their hours ofdiscussion and/or training. The author would like to thank Dr. Ning Xi and his students: JiangboZhang and King Lai for help me with the AFM images. The author would also like to thank Mr.Brian Wright, Karl Dersch and Mrs. Roxanne Peacock for their technical support. Last the authorwould like to thank all friends and co-worker: Charlee Fansler, Shannon Demlow, Stanley Zuo,K.W. Hemawan, Jeffri J Narendra, Jing Lu, Yajun Gu, Nutthamon Suwanmonkha, ChandraRomel, Muhammad Ajimal Khan, Muhammad Farhan, Konrad Loewe, Christina Palm andMitchell Parr for providing me with their knowledge.My deepest thanks are extended to my family: Dad, Mom, Sisters and Brothers for theirloves which have brought me up to this point. I would also like to special thank my wife and sonfor their loves and support throughout my graduate study at Michigan State University.v
TABLE OF CONTENTSLIST OF TABLES . . . .xLIST OF FIGURES . . xiCHAPTER 1INTRODUCTION . . 11.1Motivation . 11.2Research Objective . 31.3Outline of Dissertation . 3CHAPTER 2LITERATURE REVIEW . 52.1Introduction . . . 52.2CVD Diamond Review . . . 52.2.1 Crystal Structure . . . . . 72.2.2 CVD Diamond Deposition . . . . 82.3Diamond Etching Chemistry. .92.3.1 Diamond Etching Mechanism . . . .92.3.2 The Diamond Etching Parameters . . . . 112.4Diamond Etching System . 142.4.1 Reactive Ion Etching. . . 142.4.2 Inductive Coupling Plasma Etching (ICP). 162.4.3 Reactive Ion Beam Etching (RIBE). 172.4.4 ECR Plasma Etching . . 182.5Literature Review of Diamond Etching . 192.6Literature Review of Diamond Smoothing. 332.7Summary . 36CHAPTER 3EXPERRIMENTAL EQUIPMENT AND METHODS . . . 373.1Introduction. 373.2Microwave Plasma-Assisted Etching Systems. 373.2.1 The Microwave Power System. 383.2.2 The Microwave Plasma Reactor. 403.2.3 The Vacuum System. 433.2.4 The Gas Delivery and Cooling System. 453.2.5 The Performance of Plasma Etcher. 463.3Other Related Instruments. 50CHAPTER 4PLASMA ETCHING THEORY . . . 544.1Introduction. 54vi
4.2Plasma Etching Fundamentals . 544.2.1 Plasma Parameters. 554.2.2 Theory of Microwave Propagation. 584.2.3 Plasma Wall Interaction. 604.2.4 Ion Kinetic . 624.2.5 Plasma Density . 664.2.6 Diffusion Process . . 674.3Plasma Etching Mechanism . 704.3.1 Sputtering. 734.3.2 Chemical Etching. . 774.3.3 Ion-Enhanced Energetic Etching. . 784.3.4 Ion-Enhanced Inhibitor Etching. . 794.4Surface Interaction of Plasma Etching. 804.4.1 Generating Etchant Species. 804.4.2 Adsorption and Desorption Process . 834.4.3 Chemical Kinetics . 854.4.4 Surface Kinetic Models . 874.4.5 A Simple Empirical Model for Diamond Etching . 90CHAPTER 5CHRACTERIZATION OF PLASMA ETCHER. . . 1005.1Introduction. .1005.2Plasma Behavior .1005.3Plasma Diagnostic using SLP . 1115.3.1 Introduction . 1115.3.2 The SLP Structure . 1125.3.3 The SLP Setup . 1145.3.4 Theory of SLP. 116d 2I5.3.5 Method to Measure the. 1212dV5.4The SLP Results. 1225.4.1 EEDF versus Pressure. 1225.4.2 Plasma Density. 1435.4.3 Electron Temperature. 1455.4.4 Compare of plasma Density between 17.8 and 30.5 cm Reactor. 1465.5Summary . 148CHAPTER 6DIAMOND ETCHING EXPERIMENTAL RESULTS . 1496.1Introduction. 1496.2The Input Parameters. 1496.2.1 Microwave Power . 1496.2.2 Pressure. 1506.2.3 Gas Flow Rate. 1506.2.4 Substrate Bias. 151vii
6.3Experimental Etching Results. 1516.3.1 Etch Rates . 1516.3.2 Anisotropic . . . 1626.3.3 Mask Selectivity . . . 1796.3.4 Surface Morphology . . . 1816.3.5 Surface Roughness . . . 1876.3.6 The Radial Uniformity . . . . 1906.4Summary . 194CHAPTER 7DIAMOND SMOOTHING . . . 1967.1Introduction. 1967.2Diamond Smoothing Mechanism. 1977.2.1 Micro-Chipping . 1987.2.2 Phase Transformation. 1997.2.3 Diffusion Carbon Atoms. 1867.2.4 Evaporation/Ablating. 2007.2.5 Sputtering . 2017.2.6 Chemical Reaction . 2017.2.7 Summary . 2027.3Techniques Used to Polish Diamond Film.2047.3.1 Mechanical Polishing . 2047.3.2 Thermal-Chemical Polishing . . 2077.3.3 CAMP Polishing. . . . . 2097.3.4 Laser Polishing . . . 2107.3.5 Dynamic Friction Polishing. . . 2117.3.6 Electrical Discharge Machining . . 2127.4MCD Planarization using Plasma Etcher . 2137.4.1 Introduction . . 2137.4.2 Photo Resist Reflow Method. . 2157.4.3 The Plasma Roughing for Mechanical Polishing. 2217.4.4 Plasma Assisted Etching with Selectivity of One. 2257.5Summary . . 233CHAPTER 8SUMMARY AND FUTURE RESEARCH. . 2358.1Summary of Findings. . . 2358.2Characterize the Plasma-Assisted Etching System. 2358.2.1 Discharge Performance . 2368.2.2 Plasma Diagnostic using SLP Probe . 2368.3Investigating the Diamond Etching Process . 2388.3.1 Etch Rate . . 2388.3.2 Anisotropic Etch . . 2398.3.3 Mask Selectivity . . 2408.3.4 Etch Surface Morphology. . . 2408.4Diamond Smoothing . 241viii
8.58.6Future Research . . . . . 242Conclusion . . . . 243BIBLIOGRAPHY. . .244ix
LIST OF TABLESTABLE 2.1: Material Properties of Diamond and Its Future Applications . .06TABLE 2.2: Literature Review of Plasma-Assisted Diamond Etching . .20TABLE 6.1: Metallization Techniques . . . 167TABLE 6.2: Mask Selectivity Comparison between Narrow and Wide. . 176TABLE 6.3: Etch Selectivity of Several of Mask Materials . . 176TABLE 6.4: Comparison of NCD Etching Rates . . . 178TABLE 6.5: The Series of Experiment Input Variables . . . . 189TABLE 6.6: The Etched Surface Smoothness Results . . . . . . 193TABLE 7.1: Multi-Layers Coating of SOG Surface Roughness on Silicon . . 231TABLE 7.2: The Selectivity of SOG versus SF6 . . . . . . . 233.x
LIST OF FIGURESFIGURE 2.1: Diamond Structure .8FIGURE 2.2: RIE Plasma System.15FIGURE 2.3: ICP Plasma System.16FIGURE 2.4: RIBE Plasma System.17FIGURE 2.5: ECR Plasma System .19FIGURE 2.6: SEM Image the Anisotropic Etching of Single Crystal Diamond .26FIGURE 2.7: SEM Image of the Single Crystal Diamond Emitter Tips Array.27FIGURE 2.8: SEM Images of Diamond Etched under Different Gas Ratio of CF4/ O2.29FIGURE 2.9: SEM Image of the Diamond Surface Etched without Mask. 30FIGURE 2.10: SEM Image of Anisotropic Etching Single Crystal Diamond . .31FIGURE 2.11: SEM Images of Diamond Surfaces Etched .33FIGURE 3.1: Block Diagram of the Microwave Power System.38FIGURE 3.2 Block Diagram of Microwave Power System:. 39FIGURE 3.3: Diagram of Microwave Plasma Reactor. 41FIGURE 3.4: Magnets Ring Configuration. 42FIGURE 3.5: The Vacuum System.43FIGURE 3.6: The Gas Delivery and Cooling System.45FIGURE 3.7: The Cavity Modes .47FIGURE 3.8: The Cavity Length Ls versus Microwave Power (pressure of 1 mTorr). . .48FIGURE 3.9: The Cavity Length Ls versus Microwave Power (pressure of 10 mTorr) .49FIGURE 3.10: The Cavity Length Ls versus Microwave Power (pressure of 10 mTorr) 50xi
FIGURE 4.1: Schematic of the Plasma Sheath Region . . . .60FIGURE 4.2: Plasma Density and Potential across a Sheath . . .61FIGURE 4.3: Diagram of Etcher Plasma with RF Biasing . . . . . . 63FIGURE 4.4: Vdc versus Vrf peak in Argon Plasma .65FIGURE 4.5: The Mechanism of Plasma Etching.72FIGURE 4.6: Sputtering Yield versus Ion Energy.75FIGURE 4.7: Typical Profile of Trenching and Mask Erosion . .76FIGURE 4.8: A Simple of Diamond Etching Model . . . . . . .92FIGURE 5.1: Plasma Shape Imaging Set Up.101FIGURE 5.2: The Plasma Shape Picture as Observed from Below.102FIGURE 5.3: The ECR Plasma Shape variation with Cavity Length.103FIGURE 5.4: The ECR Plasma Shape variation with the Microwave Power.104FIGURE 5.5: The ECR Plasma Shape variation with Short Position and Modes .104FIGURE 5.6: Plasma Shape versus Gas Mixtures.105FIGURE 5.7: Plasma Shape versus Pressures .106FIGURE 5.8: Plasma Shape versus Microwave Power for a Gas Mixture.107FIGURE 5.9: Plasma Shape versus Reflected Microwave Power .108FIGURE 5.10: Plasma Shape versus Cavity Length with a Gas Mixture of.109FIGURE 5.11: Non-ECR Plasma Shape versus Pressures TM 012 .110FIGURE 5.12: Non-ECR Plasma Shape versus Pressures TM 013 .111FIGURE 5.13: Single Langmuir Probe I-V Curve . .112FIGURE 5.14: Single Langmuir Probe Structure. . .113FIGURE 5.15: Single Langmuir Probe Set Up . . .114xii
FIGURE 5.16: The SLP Probe measurement Diagram . . .115FIGURE 5.17: Magnified View of Small Area near SLP Probe. . .116FIGURE 5.18: EEDF at Pressure of 4 mTorr (ECR) . .123FIGURE 5.19: EEDF (Log Plot) at Pressure of 4 mTorr (ECR) . .124FIGURE 5.20: EEDF at Pressure of 4 mTorr (Non-ECR) . . .125FIGURE 5.21: EEDF (Log Plot) at Pressure of 4 mTorr (Non-ECR) .126FIGURE 5.22: EEDF at Pressure of 10 mTorr (ECR) . . .127FIGURE 5.23: EEDF (Log Plot) at Pressure of 10 mTorr (ECR) . . .128FIGURE 5.24: EEDF at Pressure of 10 mTorr (Non-ECR) . . .129FIGURE 5.25: EEDF (Log Plot) at Pressure of 10 mTorr (Non-ECR) .130FIGURE 5.26: EEDF at Pressure of 15 mTorr (ECR) . . .131FIGURE 5.27: EEDF (Log Plot) at Pressure of 15 mTorr (ECR) . .132FIGURE 5.28: EEDF at Pressure of 15 mTorr (Non-ECR) . . .133FIGURE 5.29: EEDF (Log Plot) at Pressure of 15 mTorr (Non-ECR) .134FIGURE 5.30: EEDF at Pressure of 25 mTorr (ECR) . . .135FIGURE 5.31: EEDF (Log Plot) at Pressure of 25 mTorr (ECR) . . .136FIGURE 5.32: EEDF at Pressure of 25 mTorr (Non-ECR) . . .137FIGURE 5.33: EEDF (Log Plot) at Pressure of 25 mTorr (Non-ECR) .138FIGURE 5.34: EEDF at Pressure of 45 mTorr (ECR) . . .139FIGURE 5.35: EEDF (Log Plot) at Pressure of 45 mTorr (ECR) . . .140FIGURE 5.36: EEDF at Pressure of 45 mTorr (Non-ECR) . . .141FIGURE 5.37: EEDF (Log Plot) at Pressure of 45 mTorr (Non-ECR) .142FIGURE 5.38: The Plasma Density versus Pressure .145xiii
FIGURE 5.39: The Electron Temperature versus Pressure . .146FIGURE 5.40: Comparison the Plasma Density versus Pressure . .147FIGURE 6.1: Etch Rate versus Microwave Power . .152FIGURE 6.2: Etch Rate versus Pressure (ECR) . .154FIGURE 6.3: Etch Rate versus Substrate Bias . . .155FIGURE 6.4: Etch Rate versus Substrate Bias . . . . .157FIGURE 6.5: Etch Rate versus Argon (ECR) . . . .158FIGURE 6.6: Etch Rate versus SF6 . . .160FIGURE 6.7: SEM Cross Section Image to Measure the Anisotropic Angle.162FIGURE 6.8: Anisotropic Angle versus Pressure .163FIGURE 6.9: A Highly Anisotropic Angle Profile (NCD). .164FIGURE 6.10: A Highly Anisotropic Angle Profile (MCD). . . .165FIGURE 6.11: A Highly Anisotropic Angle Profile (SCD). . . .166FIGURE 6.12: The Hard Mask Patterns . . . .169FIGURE 6.13: Aluminum Mask on NCD Sample . . . .170FIGURE 6.14: Gold Mask on NCD Sample. . . . . . .170FIGURE 6.15: Ti Mask on NCD Sample. . . . . . .171FIGURE 6.16: SiO 2 Mask on NCD Sample. . . . . . .171FIGURE 6.17: Si3 N 4 Mask on NCD Sample. . . . . .172FIGURE 6.18: Dimension Used for Dektak and SEM. . . . .173FIGURE 6.19: Example of SEM Method . . . . . .175FIGURE 6.20: Mask Pattern Transferred Used for All Mask Materials .177FIGURE 6.21: The Etch Selectivity versus the DC Substrate Bias . .180xiv
FIGURE 6.22: NCD and MCD Etched Surface . . . . .182FIGURE 6.23: SEM Image of Spire Like Shape of Whiskers. . . . .183FIGURE 6.24: Comparison of Whiskers Formed on the NCD. . . . .184FIGURE 6.25: SCD Surface Etched without SF6 . . . . . .185FIGURE 6.26: SEM Image of the NCD and MCD Etched Surface. . .186FIGURE 6.27: MCD Etched Surface. . . .187FIGURE 6.28: SCD Pre-Etch and Etched Surface. . . .188FIGURE 6.29: The Radial Etching Uniformity of Diamond. . .191FIGURE 6.30: The Etch Rate versus Pressure Model . . .192FIGURE 6.31: Comparison the Etch Rate versus Pressure between Theory and Experiment.193FIGURE 7.1: Diamond Polishing Mechanism Diagram . .202.FIGURE 7.2: Mechanical Polishing Schematic . . .205FIGURE 7.3: Thermal Polishing of Diamond Schematic . .207FIGURE 7.4: The CAMP Polishing Schematic . .208FIGURE 7.5: Laser Polishing Schematic . . .210FIGURE 7.6: Schematic of Dynamic Friction Polishing.211FIGURE 7.7: Schematic of EDM Polishing.212FIGURE 7.8: The Average Surface R a Analytical Function.213FIGURE 7.9: Diagram of Photo-resist Reflow Method .216FIGURE 7.10: Smoothing MCD Process .218FIGURE 7.11: The Peak High of Crystal versus Etching Time.219FIGURE 7.12: The Surface Roughness versus Etching Time.220FIGURE 7.13: The Surface Morphology of MCD .221xv
FIGURE 7.14: Plasma Roughing of Surface for Mechanical Polishing.222FIGURE 7.15: The Roughness versus Steps .223FIGURE 7.16: The Surface Roughness versus Lapping Time .224FIGURE 7.17: The Selectivity of NCD and SiO 2 .226FIGURE 7.18: The Selectivity of one Diamond Smoothing Process .228FIGURE 7.19: Optical Images of Polishing Process by Selectivity of One Method .229FIGURE 7.20: The Surface Roughness versus Processing Cycles .230xvi
CHAPTER 1INTRODUCTION1.1 Motivation:Diamond has been identified as the best material for high frequency and high powerelectronic devices due to its excellent electrical and thermal properties [Davi, 1988]. Varioustypes of applications were also achieved with CVD diamond including sensors [Vesc, 1996], tipsfor cold cathodes [Nish, 2000], electronic devices [Tsug, 2003], MEMs fabrication [Kohn,1999], and micro-optics [Lee, 2008]. Due to advances in deposition technology, CVD diamondcan now be grown over large substrate areas for nano-crystalline diamond (NCD) and microcrystalline diamond (MCD) (150-200 mm) [King, 2008] or at high growth rates for single crystaldiamond (SCD) from 50-150 µm/h [Yan, 2002]. Those results have opened new potentialapplications as the prices of diamond-based devices may start to drop down.The key problem is how to structure or smooth the diamond film due to its chemical inertnessproperty? Etching diamond chemically (wet etch) or shaping mechanically is very hard. Most ofthe previous studies to fabricate structures on diamond have used the dry etching (plasmaetching) method [Hwan, 2004]. Thus the etching of diamond using plasma plays an importantrole in post-processing of CVD diamond.1
The development of diamond based devices requires an increased control over the etchingprocess, e.g. control of the anisotropic etching, the etching rate and the surface roughness.Microwave plasma assisted etching (Electron Cyclotron Resonance (ECR) or non-ElectronCyclotron Resonance (non-ECR) is one of the suitable techniques to etch diamond because itproduces high density discharges and the ion energy is controllable. The high density dischargesare capable of achieving high etch rates over large substrates. The ion energy control allowsmin
diamond, a key technique for micro-structuring and surface modification of diamond is plasma-assisted etching. The objective of this study is to investigate and develop processes and the associated understanding of plasma-assisted etching of diamond for micro-structuring and
Etching is a process of removing material from the substrate’s surface. In general, there are two categories that etching can be divided into dry etching, and wet etching. The focus of this section will be solely on dry etching. Wet etching, a process where the substrate is submerged
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After etching, line width and the length only for the tail along the circuit was measured to obtain etching factor. Etching factor was defined as shown in Figure 3. From the etching test results, etching factor improved when matte side surface roughness decreased. Flatter foil seemed to be better to create narrower traces.
etching is usually faster than the rates for many dry etching processes and can easily be changed by varying temperature or the concentration of active species. Wet Etch Synonyms: chemical etching, liquid etching Definition: Wet etching is a material removal process that uses liquid chemicals or etchants to remove materials from a wafer.
Plasma Etching Page 2 OUTLINE Introduction Plasma Etching Metrics – Isotropic, Anisotropic, Selectivity, Aspect Ratio, Etch Bias Plasma and Wet Etch Summary The Plasma State - Plasma composition, DC & RF Plasma Plasma Etching Processes - The principle of plasma etching, Etching Si and SiO2 with CF4
Al etching start and thus to diff erent etching depths or times (Fig. 118). The formation of hydrogen in the etching reaction is also problematic for a homogeneous etching result. The constantly produced H 2 bubbles stick to the surface and block the etching process through a sup
Central system with thermostat control Thermostat control. ONE DIAMOND TWO DIAMOND THREE DIAMOND FOUR DIAMOND FIVE DIAMOND Guestroom Amenities 22 The Diamond Rating Criteria The Diamond Rating Criteria 23 SECTION THREE Tou
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