SYNTHESIS OF FUNCTIONAL MULTILAYER COATINGS BY

PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

A dissertation submitted to the

Division of Graduate Studies and Research of the University of Cincinnati

in partial fulfillment of the requirement for the degree of

Doctor of Philosophy in the Department of Electrical and Computer Engineering and Computer Science in the College of

Engineering

2004

by

Zhigang Xiao

M. S. Physics, East China Normal University, Shanghai, China, 1992

Committee Chair: Dr. T. D. Mantei

ABSTRACT

Silicon dioxide, silicon-containing polymer, silicon nitride, metal nitride, and germanium thin films were grown by electron cyclotron resonance (ECR) microwave plasma enhanced chemical vapor deposition (PECVD), and multilayer coatings were grown for high hardness and high corrosion resistance. Silicon dioxide was grown from hexamethyldisiloxane (HMDSO), 1,3,5,7- tetramethylcyclotetrasiloxane (TOMCTS ), and octamethylcyclotetrasiloxance (OMCTS) in a oxygen plasma. The grown silicon dioxide thin films were hard and colorless. Silicon nitride was

grown from hexamethyldisiloxane (HMDSO) and tetramethylsilane (TMS) in an ammonia (NH3) plasma. The silicon nitride thin films grown from HMDSO were hard and transparent while the silicon nitride thin films grown from TMS were black and hard. Silicon-containing polymer was grown from 100% OMCTS. The polymer thin films are colorless, had relatively low hardness and very good salt-fog corrosion resistance. Titanium nitride, zirconium nitride, and chromium nitride were grown from titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2- methyl-2-butoxide and zirconium t-butoxide, and bis(ethylbenzene)chromium in an ammonia plasma. The grown titanium nitride and zirconium nitride thin films had characteristic gold coloring and high hardness while the grown chromium nitride thin films were black gray and had high hardness. Germanium thin films were grown from tetramethylgermane (TMG) in a argon plasma.

The deposited germanium films were uniform and had polished-like shining surface. X-ray photoelectron spectroscopic (XPS) analyses showed the films contained 97 % germanium atomic concentration with less than 1 % carbon, and X-ray diffraction (XRD) analyses showed the films had the crystal structure of <220>.

Hard corrosion-resistant silicon-containing multilayer coatings were grown in a high-density microwave electron cyclotron resonance discharge. The multilayer coatings consist of a relatively soft silicon-containing polymer thin film as the bottom layer and a hard silicon dioxide or silicon nitride thin film as the top layer. Silicon-containing polymer thin films were grown from 100%

OMCTS. Silicon dioxide and silicon nitride thin films were grown from OMCTS with O2 and

HMDSO with NH3, respectively. The multilayer structures combined high surface hardnesses with good corrosion resistance, surviving 1800 to 2600 hours in an ASTM B117 salt-fog corrosion test.

Multilayer coatings with a titanium nitride or zirconium nitride bottom layer and a transparent silicon-containing polymer or silicon dioxide top layer were grown in a high-density microwave electron cyclotron resonance discharge for protective or decorative coating application. The grown multilayer coatings had gold coloring and good film thickness.

ACKNOWLEDGEMENTS

My deepest appreciation goes to my advisor, Dr. T. D. Mantei. I thank him for his academic guidance, recommendations, and suggestions throughout this dissertation research. I am very grateful to him for his general financial support, encouragement, and concern throughout my pursuit of this doctoral work.

My gratitude extends to my dissertation committee members: Dr. J. T. Boyd, Dr. A. M.

Ferendeci, Dr. P. Kosel, and Dr. W. J. Van Ooij. I thank them for reviewing this thesis work and serving on my dissertation committee.

My special thanks also extend to the following individuals for their help in completing this dissertation research. I appreciate my colleague, Y. Qi, for his help and collaboration, appreciate R.

Flennniken and D. Dotson for their technical assistance, and appreciate D. Q. Zhu for her assistance to get the contact angle measurement and EIS results in this dissertation.

Finally, I thank my parents and all other family members for their love and support and thank my wife, Qunying Yuan, for her love and understanding during the completion of this thesis work.

Table of Contents

Chapter 1 Introduction and Experimental Systems

1.1 Introduction ------13

1.2 ECR Plasma Generation ------17

1.3 ECR Reactor ------19

1.4 DLI Sub-System ------24

1.5 Sample Preparation ------26

1.6 Statistical Methodology for Experimental Design ------27

1.7 Film Characterization ------30

Chapter 2 Deposition of Silicon Dioxide Coatings

2.1 Deposition Precursors ------38

2.2 Experimental Conditions ------41

2.3 Results and Discussions ------41

2.4 Summary ------43

Chapter 3 Deposition of Silicon Nitride Coatings

3.1 Deposition Precursors ------53

3.2 Experimental Conditions ------54

3.3 Results and Discussions ------54

3.4 Summary ------58

Chapter 4 Deposition of Silicon-Containing Polymer Coatings

4.1 Deposition Precursors and Experimental Conditions ------70

4.2 Results and Discussions ------70

4.3 Summary ------76

1

Chapter 5 Deposition of Silicon-Containing Multilayer Coatings

5.1 Deposition Precursors and Experimental Conditions ------93

5.2 Results and Discussions ------93

5.3 Summary ------96

Chapter 6 Deposition of Titanium Nitride, Zirconium Nitride, and Chromium

Nitride monolayer and Multilayer Coatings

6.1 Deposition Precursors ------103

6.2 Experimental Conditions ------105

6.3 Results and Discussions ------106

6.4 Summary ------110

Chapter 7 Deposition of Germanium Coatings

7.1 Deposition Precursors ------120

7.2 Experimental Conditions ------121

7.3 Results and Discussions ------121

7.4 Summary ------122

Chapter 8 Summary

8.1 Conclusions ------129

8.2 Future Work ------132

References

Appendices

2

List of Tables

Table 1.1. The cleaning process for brass and stainless steel substrates. ------27

Table 1.2. A two-level three-factor full factorial experimental design. ------29

Table 2.1. Properties of HMDSO, OMCTS, and TMCTS, where HMIS represents Hazardous Material

Information System with Healthy-Flammability-Reactivity while the level of each index

ranges from 0 to 4. ------39

Table 2.2. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited films for

varying bias and O2/HMDSO flow ratio.------44

Table 2.3. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited films for

varying bias and O2/OMCTS flow ratio. ------45

Table 2.4. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited films for

varying bias and O2/TMCTS flow ratio. ------46

Table 3.1. Properties of TMS, where HMIS represents Hazardous Material Information System with

Healthy-Flammability-Reactivity while the level of each index ranges from 0 to 4. ------53

Table 3.2. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings for

varying microwave power, NH3/HMDSO flow ratio, and pressure. ------59

Table 3.3. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings for

varying microwave power and NH3/HMDSO flow ratio. ------60

Table 3.4. Optical transmittance and color index of deposited coatings on quartz substrates for varying

microwave power and NH3/HMDSO flow ratio. ------60

Table 3.5. The lifetime of five silicon nitride coatings in an ASTM B117 salt-fog corrosion test. ------61

Table 3.6. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings from

3

TMS and NH3. ------61

Table 4.1 Growth rates, elastic modulus, and hardness of silicon-containing polymer coatings on

silicon wafers. ------77

Table 4.2 Growth rates and tumble test results of silicon-containing polymer coatings on brass

substrates. ------78

Table 4.3. Thicknesses and B117 salt-fog corrosion test lifetime of silicon-containing polymer films

for two pre-cleaning gases and three deposition pressures. ------79

Table 4.4. The composition of brass substrate surfaces after oxygen or argon plasma clean. ------79

Table 4.5. The status of four samples in a B117 salt-fog corrosion test after 1776-hours running. ------80

Table 4.6. Optical transmittance and color index for silicon-containing polymer coatings grown from

100 % OMCTS on quartz substrates. ------80

Table 4.7. Contact angle analysis of four silicon-containing polymer thin films grown at 2000 W,

100 sccm OMCT, 1.33 Pa (10 mTorr) or 4 .00 Pa (30 mTorr) with argon or oxygen plasma

pre-clean. ------81

Table 5.1. Thicknesses and salt-fog corrosion test lifetimes of multilayer coatings with a silicon-

containing polymer bottom layer, and either a silicon dioxide layer or a silicon

nitride top layer, for varying O2/OMCTS and NH3/HMDSO flow-rate ratios. ------97

Table 5.2. Tumble test results of multilayer coatings with a silicon-containing polymer bottom layer,

and either a silicon dioxide layer or a silicon nitride top layer, for varying

O2/OMCTS and NH3/HMDSO flow-rate ratios. ------97

Table 5.3. Salt-fog corrosion test and tumble test results of the multilayer coating with a silicon-

containing polymer bottom layer and two silicon dioxide layers grown with varying

O2/OMCTS flow-rate ratios. ------98

4

Table 5.4. Salt-fog corrosion test and tumble test results of multilayer coatings with a silicon-containing

polymer bottom layer and two silicon nitride layers grown with varying

NH3/HMDSO flow-rate ratios. ------98

Table 5.5. Contact angle analysis of multilayer coatings with a silicon-containing polymer bottom

layer, and either a silicon dioxide layer or a silicon nitride top layer, for varying

O2/OMCTS and NH3/HMDSO flow-rate ratios. ------99

Table 6.1. Properties of titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2-

methyl-2-butoxide and zirconium t-butoxide), and bis(ethylbenzene)chromium, where x ranges

from 0 to 4. ------104

Table 6.2. Growth rates, hardness, and atomic concentrations of deposited TiC xOyNz films grown from

titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium at microwave power of 2500

W and deposition time of 10 min for varying flow rates of NH3. ------111

Table 6.3. Growth rates, hardness, and atomic concentrations of deposited ZrCxOyNz films from

zirconium 2-methyl-2-butoxide and zirconium t-butoxide at microwave power of 2500 W and

deposition time of 10 min for varying flow rates of NH3. ------111

Table 6.4. Growth rates, hardness, and atomic concentrations of deposited CrCxOyNz films from

bis(ethylbenzene)chromium at microwave power of 2500 W and deposition time of 10 min for

varying flow rates of NH3. ------112

Table 6.5. Color coordinates of four TiC xOyNz and TiC xOyNz films grown from titanium (IV)

isopropoxide, tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide, and zirconium

t-butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time

of 10 min and color coordinates of TiN, ZrN, and 24 Karat gold from Ref. 152. ------112

Table 6.6. Salt-fog corrosion test and tumble test results of TiC xOyNz and ZrCxOyNz films grown from

5

titanium (IV) isopropoxide and zirconium 2-methyl-2-butoxide at the microwave power of

2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min. ------113

Table 6.7. Salt-fog corrosion test and tumble test results of multilayer coatings with a TiC xOyNz layer,

grown from titanium (IV) isopropoxide at the microwave power of 2500 W, NH3 flow rate of

250 sccm, and for 10 min, and a silicon dioxide layer. ------113

Table 6.8. Salt-fog corrosion test and tumble test results of multilayer coatings with a ZrCxOyNz layer,

grown from zirconium 2-methyl-2-butoxide at the microwave power of 2500 W, NH3 flow rate

of 250 sccm, and for 10 min, and a silicon dioxide layer. ------114

Table 6.9. Salt-fog corrosion test and tumble test results of multilayer coatings with a TiC xOyNz or

ZrCxOyNz bottom layer, grown from titanium (IV) isopropoxide or zirconium 2-methyl-2-

butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and for 10 min, a

silicon-containing polymer middle layer, and a silicon dioxide top layer. ------114

Table 7.1. Properties of TMG, where HMIS represents Hazardous Material Information System with

Healthy-Flammability-Reactivity while the level of each index ranges from 0 to 4. ------120

Table 7.2. Growth rates, the root mean square (RMS) surface roughness over 50 ´ 50 mm2, and atomic

concentrations of deposited coatings. ------123

Table 7.3. Hole mobility and sheet resistance of the grown germanium film at various temperatures.--123

Table 8.1. Results of monolayer and multilayer coatings grown by PECVD. ------130

6

List of Figures

Fig. 1.1. The mechanism of electron cyclotron resonance (ECR). ------18

Fig. 1.2. The schematic of the permanent magnet ECR system. ------21

Fig. 1.3. The top view of the ECR system. ------22

Fig. 1.4. The schematic of the deposition system. ------23

Fig. 1.5. Schematic of tube connections of vaporizer system. ------25

Fig. 1.6. The schematic of the direct liquid injection (DLI) sub-system. ------26

Fig. 1.7 (a). The configuration for EIS. ------37

Fig. 1.7 (b). The modeled electronic circuits for EIS. ------37

Fig. 2.1 The chemical structures of HMDSO, TMCTS and OMCTS. ------40

Fig. 2.2. Growth rate response surface for coating deposition from HMDSO and O2. ------47

Fig. 2.3. Growth rate response surface for coating deposition from OMCTS and O2. ------47

Fig. 2.4. Growth rate response surface for coating deposition from TMCTS and O2. ------48

Fig. 2.5. Carbon atomic concentration response surface for coating deposition from HMDSO and O2. --48

Fig. 2.6. Carbon atomic concentration response surface for coating deposition from OMCTS and O2. --49

Fig. 2.7. Carbon atomic concentration response surface for coating deposition from TMCTS and O2. --49

Fig. 2.8. Elastic modulus response surface for coating deposition from HMDSO and O2. ------50

Fig. 2.9. Elastic modulus response surface for coating deposition from OMCTS and O2. ------50

Fig. 2.10. Elastic modulus response surface for coating deposition from TMCTS and O2. ------51

Fig. 2.11. Hardness response surface for coating deposition from HMDSO and O2. ------51

Fig. 2.12. Hardness response surface for coating deposition from OMCTS and O2. ------52

Fig. 2.13. Hardness response surface for coating deposition from TMCTS and O2. ------52

7

Fig. 3.1 The chemical structure of TMS. ------53

Fig. 3.2. Statistical response surface for growth rates of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr). ------62

Fig. 3.3. Statistical response surface for growth rates of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr). ------62

Fig. 3.4. Statistical response surface for elastic modulus of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr). ------63

Fig. 3.5. Statistical response surface for elastic modulus of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr). ------63

Fig. 3.6. Statistical response surface for hardness of deposited coatings as a function of microwave power

and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr). ------64

Fig. 3.7. Statistical response surface for hardness of deposited coatings as a function of microwave power

and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr). ------64

Fig. 3.8. Statistical response surface for growth rates of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio. ------65

Fig. 3.9. Statistical response surface for atomic concentration of nitrogen in deposited coatings as a

function of microwave power and NH3/HMDSO flow ratio. ------65

Fig. 3.10. Statistical response surface for elastic modulus of deposited coatings as a function of

microwave power and NH3/HMDSO flow ratio. ------66

Fig. 3.11. Statistical response surface for hardness of deposited coatings as a function of microwave

power and NH3/HMDSO flow ratio. ------66

Fig. 3.12. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with 75

sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min. ------67

8

Fig. 3.13. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with 50

sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min. ------68

Fig. 3.14. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with 25

sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min. ------69

Fig. 4.1. Statistical response surface for growth rates of deposited silicon-containing polymer thin

films as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr). ------82

Fig. 4.2. Statistical response surface for growth rates of deposited silicon-containing polymer thin

films as a function of microwave power and OMCTS flow rates at 1.33 Pa (10 mTorr). ------82

Fig. 4.3. Statistical response surface for hardness of deposited silicon-containing polymer thin films

as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr). ------83

Fig. 4.4. Statistical response surface for hardness of deposited silicon-containing polymer thin films

as a function of microwave power and OMCTS flow rates at 1.33 Pa (10 mTorr). ------83

Fig. 4.5. Statistical response surface for number of breaks over a 2.54 cm ´ 2.54 cm square area in a

tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

0.27 Pa (2 mTorr). ------84

Fig. 4.6. Statistical response surface for number of breaks over a 2.54 cm ´ 2.54 cm square area in a

tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

1.33 Pa (10 mTorr). ------84

Fig. 4.7. Statistical response surface for sum of broken area over a 2.54 cm ´ 2.54 cm square area in a

tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

0.27 Pa (2 mTorr). ------85

Fig. 4.8. Statistical response surface for sum of broken area over a 2.54 cm ´ 2.54 cm square area in a

tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

9

1.33 Pa (10 mTorr). ------85

Fig. 4.9. FTIR absorbance spectra of silicon-containing polymer thin films with varying microwave

power and OMCTS flow rates at 0.27 Pa (2 mTorr). ------86

Fig. 4.10. FTIR absorbance spectra of silicon-containing polymer thin films with varying microwave

power and OMCTS flow rates at 1.33 Pa (10 mTorr). ------87

Fig. 4.11. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at 2000W,

100 sccm OMCT, 1.33 Pa (10 mTorr), and 20 min with pre-clean of oxygen plasma. ------88

Fig. 4.12. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at 2000W,

100 sccm OMCT, 4.00 Pa (30 mTorr), and 40 min with pre-clean of oxygen plasma. ------89

Fig. 4.13. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at 2000W,

100 sccm OMCT, 1.33 Pa (10 mTorr), and 20 min with pre-clean of argon plasma. ------90

Fig. 4.14. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at 2000W,

100 sccm OMCT, 4. 00 Pa (30 mTorr), and 40 min with pre-clean of argon plasma. ------91

Fig. 4.15. EIS analysis of brass substrate. ------92

Fig. 5.1. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer

grown from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, and a silicon dioxide

top layer grown from OMCT and O2 at 2000 W, 400 sccm O2, 50 sccm OMCT,

and 10 min. ------100

Fig. 5.2. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer grown

from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, and a silicon nitride top layer

grown from HMDSO and NH3 at 2500 W, 400 sccm NH3, 100 sccm HMDSO, and 10 min. --101

Fig. 5.3. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer grown

from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, a silicon dioxide middle

10

layer grown from OMCT and O2 at 2000 W, 100 sccm O2, 50 sccm OMCT, and 5 min, and a

silicon dioxide top layer grown from OMCT and O2 at 2000 W, 400 sccm O2, 50 sccm

OMCT, and 5 min. ------102

Fig. 6.1. Substrate temperatures as a function of deposition time with titanium (IV) isopropoxide and

zirconium 2-methyl-2-butoxide as precursors, microwave power of 2500 W, and NH3 flow rate

of 250 sccm. ------115

Fig. 6.2. Hardness of the titanium nitride thin films grown from titanium (IV) isopropoxide and

tetrakis(dimethylamino)titanium at microwave power of 2500 W and NH3 flow rate of 250 sccm

as a function of displacement into surface. ------115

Fig. 6.3. Hardness of the zirconium niride-like thin films grown from zirconium 2-methyl-2-butoxide and

zirconium t-butoxide at microwave power of 2500 W and NH3 flow rate of 250 sccm as a

function of displacement into surface. ------116

Fig. 6.4. Hardness of the chromium nitride thin film grown from bis(ethylbenzene)chromium at

microwave power of 2500 W and NH3 flow rate of 250 sccm and p-type <100> silicon wafer as

a function of displacement into surface. ------116

Fig. 6.5. Scanning electron micrograph of the cross section of the TiC xOyNz film grown on <100> silicon

wafer from tetrakis(dimethylamino)titanium at the microwave power of 2500 W, NH3 flow rate

of 250 sccm, and deposition time of 10 min. ------117

Fig. 6.6. Scanning electron micrograph of the cross section of the ZrCxOyNz film grown on <100> silicon

wafer from zirconium t-butoxide at the microwave power of 2500 W, NH3 flow rate of 250

sccm, and deposition time of 10 min. ------118

Fig. 6.7. Scanning electron micrograph of the cross section of the CrCxOyNz film grown on <100> silicon

wafer from bis(ethylbenzene)chromium at the microwave power of 2500 W, NH3 flow rate of

11

250 sccm, and deposition time of 10 min. ------119

Fig. 7.1 The chemical structure of TMG. ------121

Fig. 7.2. AFM micrograph of the surface of the germanium film grown on silicon wafer. ------124

Fig. 7.3a. Scanning electron micrograph of the surface of germanium film grown on silicon wafer. ----125

Fig. 7.3b. Scanning electron micrograph of the surface of germanium film grown on silicon wafer. ---126

Fig. 7.3c. Scanning electron micrograph of the surface of germanium film grown on silicon wafer.----127

Fig. 7.4. Scanning electron micrograph of the cross section of germanium film grown on silicon

wafer. ------128

Fig. 7.5. X-ray diffraction (XRD) spectrum of germanium film grown on silicon wafer. ------128

12

Chapter 1

Introduction and Experimental Systems

1.1 Introduction

The investigation of vacuum-based synthesis of hard protective coatings with high corrosion resistance and high wear resistance has become of great industrial interest in recent years. Thin films can be grown by vacuum deposition processes including physical vapor deposition (PVD) 1, 2, thermal chemical vapor deposition (CVD) 3-7, and more recently, plasma-enhanced chemical vapor deposition (PECVD) 8-10. Conventional thermal CVD processing relies on thermal energy to activate gas phase and surface reactions. Temperatures between 600oC and 900oC are usually required to achieve suitably high deposition growth rates, which makes CVD undesirable for deposition on temperature-sensitive substrates. PVD processes such as sputtering can be slow and the deposited thin films may have poor conformity. Plasma-enhanced chemical vapor deposition is an extension of conventional CVD in which gas phase plasma electron impact is substituted for thermal agitation, thereby achieving acceptably high deposition rates at lower substrate temperatures.

1.1.1 Silicon Dioxide and Silicon Nitride

PECVD has been used in recent years to grow silicon dioxide 11-20 and silicon nitride 21-32 from a wide range of silicon-containing precursors. Silicon nitride coatings are widely used for industrial applications such as dielectric thin films, microelectronic and optical device elements, and hard protective layers, because of their attractive electrical, mechanical and chemical properties.

13

33-41 Silane (SiH4) has often been used for silicon dioxide and silicon nitride deposition , because the high volatility of silane enhances deposition rates. Silane is toxic and explosive, however, and there is a demand for less-hazardous alternatives. Organosilicon precursors are safer and more easily handled, and have therefore attracted recent attention.

In this dissertation work, silicon dioxide coatings were grown from hexamethyldisiloxane

(HMDSO), octamethylcyclotetrasiloxane (OMCTS), and tetramethylcyclotetrasiloxane (TMCTS), each of which is relatively inexpensive, easily volatilized, and safer than silane 42-47.

Compounds that have internal silicon-nitrogen bonds have been used with ammonia or nitrogen to deposit silicon nitride thin films by many researchers. The compounds include hexamethyldisilazane, hexamethylcyclotrisilazane 48, 1,1,3,3,5,5-hexamethylcyclotrisilazane 49, 50, and tetrakis-(dimethylamino)silicon 51, 52. In this work, silicon nitride coatings were grown from hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with ammonia (NH3). HMDSO and

TMG are environmentally benign with a low hazard rating, which simplifies storage and use, and have high vapor pressures at ambient temperature which greatly simplifies gas delivery. HMDSO and TMG have no silicon-nitrogen bonds, however, unlike the nitrogen-containing organosilanes cited above, which means that a quantity of atomic nitrogen sufficient for the synthesis of silicon nitride coatings must be supplied by a plasma-decomposed nitrogen source. Ammonia was used in this research rather than molecular nitrogen, because ammonia can be more easily dissociated by plasma electron impact.

14

1.1.2 Polymer Coatings

Polymer coatings are good candidates for protective coatings with good adhesion and high corrosion resistance, because they are soft and pinhole-free 53-58. Plasma assisted processing has been developed by many researchers to deposit polymer coatings 59-62. In plasma assisted polymer processing, polymerization is accomplished by passing the vaporized monomer gas through a glow discharge zone prior to condensation on a substrate. In the research described here, OMCTS was used to deposit silicon-containing polymer coatings.

1.1.3 Silicon-Containing Multilayer Coatings

Plasma-grown silicon dioxide and silicon nitride films usually have good mechanical properties with high elastic modulus and hardness, but tend to be brittle, which reduces adhesion and corrosion resistance. In contrast, soft silicon-containing polymer thin films grown by plasma polymerization of organosilicon monomers are pinhole-free and adherent to a variety of substrates, and have been widely used as corrosion protective coatings, selective gas permeation membranes, hydrophobic layers, and biocompatible films 63-68. Multilayer coating structures have therefore been proposed in recent years to combine hardness and adhesion 69-74, thus providing enhanced performance not available from monolayer coatings.

In this dissertation work, silicon-containing multilayer protective coatings were grown with PECVD for hardness and corrosion resistance. The multilayer structures comprise a relatively soft silicon- containing polymer thin film directly adhering to the substrate, capped with hard silicon dioxide or silicon nitride coatings, thus combining high surface hardness with excellent adhesion and corrosion resistance. The underlying polymer layer is plasma-grown from 100% octamethycyclotetrasiloxane

15

(OMCTS), while the hard layers are either high quality silicon dioxide layers grown from OMCTS in an oxygen plasma, or silicon nitride layers grown from hexamethyldisiloxane (HMDSO) in ammonia plasma.

1.1.4 Metal Nitride Thin Films and Multilyer Coatings

Titanium nitride (TiN), zirconium nitride (ZrN), and chromium nitride (CrN) thin films can be deposited by various physical vapor deposition (PVD) 75-89 and chemical vapor deposition (CVD) techniques 100-105. Such metal nitride films have been used for a wide range of applications such as wear and corrosion protective coatings, diffusion barrier layers, and decorative coatings. These films can have high chemical stability, high hardness, and good adhesion to a variety of substrates.

In this research, hard titanium nitride, zirconium nitride, and chromium nitride thin films were grown with plasma-enhanced chemical vapor deposition (PECVD) in a high-density microwave electron cyclotron resonance (ECR) discharge. The organometallic deposition precursors were titanium (IV) isopropoxide (C12H28O4Ti) and tetrakis(dimethylamino)titanium (C8H24N4Ti), zirconium 2-methyl-2-butoxide (C20H44O4Zr) and zirconium t-butoxide(C16H36O4Zr), and bis(ethylbenzene)chromium (((C2H5)xC6H6-x)2Cr), where x = 0 – 4, with ammonia (NH3) as the reactive gas. These precursors are relatively cheap and have low hazard ratings. However, these organometallic precursors are all liquid compounds with very low vapor pressure even at temperatures of above 100 °C, making their introduction into the processing chamber very difficult.

To solve this problem, a direct liquid injection (DLI) system was developed to deliver liquid organometallic precursors into the processing chamber, by vaporizing the precursors at temperatures up to 220 °C.

16

The deposition rates for titanium nitride coatings and zirconium nitride coatings are low and do not satisfy industrial requirements for coating thickness. In this work, therefore, multilayer coatings were deposited with PECVD as protective coatings or decorative coatings with gold-like color and suitable thickness. The multilayer coatings consist of a titanium nitride or zirconium nitride layer, covered by silicon-containing polymer and (or) silicon dioxide layers.

1.1.5 Germanium Thin Films

A germanium-containing compound, tetramethylgermane ((CH3)4Ge), was tested for plasma- enhanced chemical vapor deposition (PECVD) of germanium films. Germanium films grown at moderate substrate temperatures and acceptably high deposition rates have mirror-like surfaces and low impurity levels.

106, 107 108 Germane (GeH4) , germanium tetrafluoride (GeF4) , and germanium tetrachloride (GeCl4)

109 are often used for germanium growth by thermal chemical vapor deposition (CVD) and plasma- enhanced chemical vapor deposition (PECVD). These precursors are toxic and explosive, however, and there is thus a demand for less hazardous alternative compounds. In the thesis research, tetramethylgermane (TMG) 110 was used as the precursor to grow germanium films in a high-density argon plasma. TMG has a twofold advantage: It is relatively safe and has a low hazard rating, and

TMG has a high vapor pressure at ambient temperature (~ 5 ´ 104 Pa (375 Torr ) at 27 °C) which greatly simplifies gas delivery.

1. 2 ECR Plasma Generation

17

A microwave electron cyclotron resonance (ECR) plasma reactor was used to grow various coatings in this dissertation work. ECR plasma reactors typically generate low-pressure, high-density plasma

111-117, and have therefore been used by many researchers to synthesize thin films 118-125.

ECR discharges are generally excited at microwave frequencies (e.g., 2.45 GHz), and the microwave absorption requires application of a dc magnetic field (875 Gauss at resonance) 126. Fig.

1.1 shows the ECR mechanism.

r B

r E(t) e r T E(t + ) 2

Fig. 1.1. The mechanism of electron cyclotron resonance (ECR).

r An electron moves circularly around the axial line of the magnetic field B with an electron cyclotron frequency:

18

qB v = c m where B is the magnetic field strength, q is the electron charge, and m is the electron mass. The

3 electron cyclotron frequency f c (GHz) @ 2.8´10 B(Gauss) for f c = 2.45GHz , we obtain a resonant

magnetic field Bres @ 875Gauss. In the presence of a magnetic field, the average power absorbed by an electron from a microwave electric field with a frequency v is given by 127:

æ q 2E 2u öæ 1 1 ö P = ç o ÷ç + ÷ av ç 4m ÷ç 2 2 2 2 ÷ è øèu + (v -v c ) u + (v +v c ) ø

where Eo is the applied electric field strength, v is the frequency of the applied electric field, n is

the electron-neutral collision frequency, and v c is the cyclotron frequency. Resonance occurs when

the electron gyrates at the frequencyv c =v . At resonance, the gyrating electron changes direction in its circular orbit in phase with the applied electric field. Thus, the electron is continuously accelerated until it collides with other particles. When collisions occur, energy is transferred from the accelerated electron to the particle. ECR enhances power absorption so that high-density plasma can be generated.

1.3 ECR Reactor

A microwave electron cyclotron resonance plasma reactor 128-133, shown schematically in

Fig. 1.2, was used to generate high-density plasma for the synthesis of various coatings in this dissertation work. The system comprises a main process chamber, a vacuum load-lock for sample introduction, a microwave power source, and a pumping system. The deposition chamber is a stainless-steel cylinder, 30 cm diameter by 35 cm high, with a 10 cm diameter quartz window in the top flange for microwave power introduction. The deposition substrate was placed 25 cm below the

19

lower window surface. Films were deposited with no intentional substrate heating. The process chamber was evacuated by an 1100 l/s compound turbomolecular pump backed by a mechanical pump. The chamber pressure could be controlled with a gate valve above the turbomolecular pump.

Precursor flows were controlled by an MKS 1150 heated vapor source mass flow controller while liquid organometallic precursors were controlled and delivered into the process chamber by a direct liquid injection (DLI) system. Microwave power (2.45 GHz, 0-2500 W) was introduced by a rectangular waveguide into the main chamber through the quartz window. Microwave power is absorbed in a thin disk-shaped region below the quartz window through electron cyclotron resonance. The high-density plasma is generated in the absorption region, and then diffuses down the process chamber. In contrast to conventional electromagnet ECR sources, here the axial magnetic field required for cyclotron resonance was provided by a cylindrical Nd-Fe-B permanent magnet 114. A set of 16 Nd-Fe-B permanent magnet bars was also arranged around the deposition chamber wall to form a multipolar magnetic confinement and improve downstream plasma uniformity 134. The mechanism of multipolar magnetic confinement is shown in Fig. 1.3. These multipolar magnetic bars generate a horizontal magnetic field, which reflects electrons and ions back into the plasma, thus avoiding chamber wall loses and improving the plasma uniformity. The active gas was introduced through four ports just under the input microwave window and the deposition precursor gas was introduced downstream just above the substrate. Fig. 1.4 shows the whole deposition system schematically. A type-K thermocouple in contact with the exposed substrate surface was used to measure substrate temperatures during deposition.

20

Fig. 1.2. The schematic of the permanent magnet ECR system.

21

Fig. 1.3. The top view of the ECR system.

22

Fig. 1.4. The schematic of the deposition system.

23

1.4 Direct Liquid Injection System

The organometallic precursors for metal nitride coatings are liquids with very low vapor pressures, and their introduction into a process chamber is a central problem in PECVD. Carrier gases such as argon and helium can be used to transport liquid organometallic precursors into a process chamber, but they also increase the deposition pressure and dilute the precursor, which can result in low deposition growth rates. Thermal vaporizers are therefore often used to vaporize liquid organometallic precursors for use in PECVD processes 135, 136. Direct liquid injection (DLI) systems are available commercially but are expensive and are prone to suffer from contamination, requiring laborious and expensive rebuilds. In this work, a robust and inexpensive thermal vaporizer was designed for the delivery of organometallic liquid precursors. The vaporizer has proved more resistant to contamination than some commercially available vaporizers, and is easy and inexpensive to rebuild when contamination does occur.

The vaporizer consists of three stainless steel tubes (2 m long and 3 mm diam; 1.5 m long and 6 mm diam; and 0.75 m long and 12.5 mm diam, respectively). The three tubes are joined in series, from the longest to the shortest, and are wound into a compact coil. The 2 m section is connected to an MKS micro pump while the 0.7 m section is connected to the vapor inlet of the process chamber. Figure 1.5 shows the three tubes schematically. Four flexible wire heaters are wrapped uniformly around the coiled tubes, and the coil is wrapped in aluminum foil and thermally insulating tapes to improve heating uniformity. The temperature of the vaporizer is controlled by changing the power to the four heaters, and a thermocouple is attached near the center of the coil to measure the coil temperature.

24

The direct liquid injection system consists of a micro pump, a vaporizer and an electronic controller. Fig. 1.6 shows the DLI system, in which the pump delivers precursor into the vaporizer, which vaporizes the precursor. The rate of liquid delivery, in cubic centimeters per minute, can vary from a minimum of 0.006 ccm to a maximum of 2.500 ccm. The range of the temperature in the vaporizer is from room temperature to 250 0C.

Liquid Precursor from Micro Pump 2 m Long and 3 mm Diam Tube

1.5 m Long and 6 mm Diam Tube Vaporized 0.75 m Long and 12.5 Precursor Gas mm Diam Tube to Processing Chamber

Fig. 1.5. Schematic of tube connections of vaporizer system.

25

Electronic controller

Vapor gas Vaporizer Liquid source Pump

Fig. 1.6. The schematic of the direct liquid injection (DLI) system.

1. 5 Sample Preparation

Substrates in this work were 5 cm diameter silicon wafers, 5 ´ 5 cm2 quartz slides, and 5 ´ 5 cm2 brass and stainless steel coupons. Prior to deposition, brass and stainless steel substrates were ultrasonically cleaned by using the cleaning method described in Table 1.1, where I172 represents isoprep 172 (an alkaline cleaning solution), ROW represents reverse osmosis water, VRS represents versaclean (a neutral cleaning solution), DIW represents deionized water, and RT represents room temperature. After being loaded and transferred to the processing chamber, all substrates were then subjected to an in situ oxygen or argon plasma clean cycle for 3 minutes just prior to coating deposition.

26

Table 1.1. The cleaning process for brass and stainless steel substrates.

Step7 Step1 Step 2 Step 3 Step 4 Step 5 Step 6 (Drying)

I172 VRS Chemical ROW ROW DIW DIW (3 %) (5 %)

Ultrasonics Yes No Yes No Yes No

Temperature (o F) 120 100 120 100 RT RT 200

Exposure Time (s) 180 120 120 120 60 60 180

1.6 Statistical Methodology for Experimental Design

Statistical experiment design methodology was used to efficiently explore the available parameter space, characterize the relations between deposition parameters and film properties, and determine the dominant deposition parameters and their interactions.137-140 Full factorial experiments are very useful for measurement problems in which a large number of experimental parameters are involved, and are suitable for the systems in which there are interactions between parameters.

In a general full factorial design, a fixed number of levels is selected for each of the variable factors, and then experiments are run with all possible combinations. For a two-level full factorial design, two levels (a low-value level and a high-value level) for each of the experimental variables are selected. Therefore, the total number of experimental runs for the k-factor experiment is 2 k . For example, there are 23 runs for a two-level three-factor full factorial experiment. Table 1.2 shows a

27

two-level three-factor full factorial experimental design, where A, B, and C are three factors. +1 represents the high-value level, -1 represents the low-value level, and 0 represents the center level.

Two runs for the center point are designed to test the system.

In this work, two-level, two or three-factor and three-level, two-factor full factorial experimental designs were used and multi-regression analyses were performed using StatisticaTM software to relate output responses to input variables.

28

Table 1.2. A two-level, three-factor full factorial experimental design.

Stat. Expt. Design Design: 2**(3-0) design

Standard Run A B C 1 (C) 0 0 0 2 -1 -1 -1 3 1 -1 -1 4 -1 1 -1 5 1 1 -1 6 -1 -1 1 7 1 -1 1 8 -1 1 1 9 1 1 1 10 (C) 0 0 0

29

1.7 Film Characterization

The thickness of coatings was measured with a Filmetrics thin-film measurement system in the laboratory of Dr. Altan M. Ferendeci at the University of Cincinnati and a diamond stylus Tencor P-

10 surface profiler at the Ingersoll-Rand Corporation. The Filmetrics thin-film measurement system uses an optical spectral reflectance method to measure the reflectance of perpendicular incident light from the thin film over a range of wavelengths. The reflectance R of light incident perpendicularly from material n1 to material n2 can be obtained as:

2 2 (n2 - n1) + k R = 2 2 (n2 + n1 ) + k

where n1 and n2 are the refractive indexes and k is the extinction coefficient of material 2. The reflectance can also be expressed in the following:

æ 2p ö R = A + Bcosç nd ÷ è l ø where l is the wavelength , n is the refractive index, d is the film thickness, and A and B are parameters related to n, k. The Filmetrics thin-film measurement system can measure the variation of reflectance over a range of wavelengths, and then determine n, k and d values by fitting the reflectance variation curve with the above mathematic equations. The diamond stylus Tencor P-10 surface profiler measures the thickness of films by measuring the step formed from the bottom of films to the top.

Coating compositions were measured at Kent State University with a Kratos Axis Ultra X-ray photoelectron spectrometer using a monochromatized Al Ka x-ray source operating at 300 W, or at

Evans East with a Physical Electronics 5700LSci ESCA spectrometer using a monochromatic

30

aluminum x-ray source operating at 350 W. In XPS analysis, samples are loaded and transferred into a ultra-high vacuum chamber, where the surfaces of samples are irradiated by monoenergetic soft X-rays and electrons are activated to be emitted from the sample surface, an analysis system then collects the emitted electrons according to their kinetic energy and measure the energy. Mg Ka

(1253.6 eV) and Al Ka (1486.6 eV) X-rays are often used as the X-ray source. The X-ray photons interact with atoms in the sample surface and activate electrons to be emitted by the photoelectric effect. The kinetic energy of photoelectrons can be obtained as the following:

1 mV 2 = hn - E -f 2 b

Here hn is the energy of photon, Eb is the binding energy, and f is the work function. The binding energy can then by calculated according to the equation. Each element has its unique binding energy, XPS can thus be used to identify element composition and determine the concentration of the elements in materials. Initially, the surfaces of the specimens were sputtered. The exposed near- surfaces were examined by low-resolution survey scans to determine which elements were present and the concentration of those elements observed. The quantification of the elements was accomplished by using the atomic sensitivity factors. The approximate escape depth (3l sinq) of the carbon electrons was 80Å. The analytical conditions for a Physical Electronics 5700LSci ESCA spectrometer at Evans East are:

X-ray source Monochromatic aluminum

Source power 350 watts

Analysis region 0.8 mm diameter

Exit angle 65°

Charge correction C-(C,H) in C 1s spectra at 284.8 eV

31

Charge neutralization electron flood gun

Ion sputtering 3 kV Ar+, 3.5 mm x 3.5 mm raster

Sputter rate 18 Å/min for TiN (54 Å/min for SiO 2) where the exit angle is defined as the angle between the surface plane and the electron analyzer lens.

The elastic modulus and hardness of coatings were determined by nanoindentation using a continuous stiffness measurement technique, performed with an MTS nanoindenter using a diamond tip mounted on a standard XP head. This work was performed at the Ingersoll-Rand

Corporation. In the nanoindentation hardness measurement, a diamond indenter is forced into specimen with a controlled load. Indentation hardness is defined as the applied load divided by the projected contact area between the indenter and the sample.

The optical transmittance and color index of coatings were measured by a Minolta spectrophotometer at Ingersoll-Rand. The L*a*b* color space model was used in the spectrophotometer. L* indicates lightness while a* and b* define the chromaticity plan. The higher the L* value, the lighter the color; a perfect white would have an L* = 100.0 while a perfect black would have an L* = 0.0. The + a* is the red direction, -a* is the green direction, +b* is the yellow direction, and -b* is the blue direction.

The corrosion resistance of coatings was tested by a standard ASTM B117 salt-fog corrosion test at

Ingersoll-Rand, and by electrochemical impedance spectroscopy (EIS) in the laboratory of Dr. Wim

J. Van Ooij at the University of Cincinnati. In the ASTM B117 salt-fog test, a salt-fog environment

32

was formed in a sealed tank from a 5% sodium chloride solution (Ulrich) at 35 oC, and samples were exposed in the salt fog environment. The samples were checked twice per day until visible corrosion spots appearing on the surfaces of samples. A lifetime is then defined as the time when a sample survived in the salt fog test without visible corrosion on the surface of sample.

In the electrochemical impedance spectroscopy (EIS) analysis, the applied voltages and frequencies were provided with a Standford Research Systems Model SR810 DSP Lock-In Amplifier. The corrosive electrolyte was 3 % NaCl solution. In EIS measurements, a frequency-dependent impedence, Z(f), is obtained by applying a sinusoidal alternating potential signal to the tested system in a range of frequencies. Z(f) is expressed as the following:

Z(f) = V(t) / I(t) where f is the frequency, t is the time, V(t) is the sinusoidal alternating potential signal and I(t) is the time-dependent current response.141 Figure 1.7 (a) shows the basic electrochemical circuit configuration. The coated surface of sample was underneath an electrolyte, an ac voltage was applied to the metal substrate and the electrolyte. The entire configuration can be modeled as a nested RC circuit (Figure 1.7 (b)), where CCoat and RCoat are the capacitance and resistance of the coating, CInt and RInt are the capacitance and the resistance at the interface between the substrate and coating, and RSE is the total resistance of the substrate and the electrolyte. The total impedance of the circuit can be obtained as following

1 Z = R + Tot SE 1 jwC + Coat 1 R + Int 1 jwCInt + RCoat

33

When corrosion happens in the coating layer or at the interface between the coating layer and substrate, the CCoat, RCoat, CInt, and RInt values vary, therefore the total impedance varies due to corrosion. Inversely, if the total impedance varies, it can be derived that corrosion has happened.

The variation of total impedance can be measured as a function of applied voltage frequency.

The chemical bonding of coatings was analyzed by a Fourier transform infrared (FTIR) spectroscopic measurement in the laboratory of Dr. James Boerio at the University of Cincinnati. A

FTIR spectroscope transmits infrared radiation into thin films on metal substrates, and then the symmetric and asymmetric vibration information of bonding dipoles in the films is contained in the reflected infrared ray. The bonding dipoles absorb energy from the infrared ray, vibrate and re- radiate signals, which are related to chemical bonding. The signals are decoded by a Fourier transform spectrum for chemical bonding. The requirement for sample preparation for FTIR analyses includes opaque substrates relative to infrared light (such as metal substrates) and thin films (< 1000 Å).

The wear resistance of coatings was analyzed by a tumble test at Ingersoll-Rand. The tumble machine was full of sand and small pieces of metal with sharp edges and can rotate like a rotating wheel with its axis fixed. The samples were tumbled in the tumble machine for 10 minutes.

Computer-based image analysis was then used to find the number of breaks over a 2.54 cm ´ 2.54 cm square area on coatings and the sum of all break areas on the 2.54 cm ´ 2.54 square area.

The surface energy of coatings was measured by a contact angle measurement system in the laboratory of Dr. Wim J. Van Ooij at the University of Cincinnati. Contact angle measurement is a

34

common method for surface analysis related to surface energy and tension. The contact angle describes the shape of a liquid droplet resting on a solid surface. When drawing a tangent line from the droplet to the touch of the solid surface, the contact angle is the angle between the tangent line and the solid surface. If a liquid with well-known properties is used, the resulting interfacial tension can be used to identify the nature of the solid. When a droplet of liquid rests on the surface of a solid, the shape of the droplet is determined by the balance of the interfacial liquid/solid forces, for example, when a droplet of high surface tension liquid is placed on a solid of low surface energy, the liquid surface tension will cause the droplet to form a spherical shape (lowest energy shape).

The contact angle measurement system includes a syringe to apply a droplet of liquid and a camera with image to reveal the profile of the droplet on the computer screen. Computer software calculates the tangent to the droplet shape and the contact angle and the surface energy.

The crystal structure of coatings was analyzed by a Philips X'pert Diffractometer in the Advanced

Materials Characterization Center at the University of Cincinnati. The instrument is a convenient analytical tool, and the accompanying software in a Windows environment is an industry standard.

The microstructures of coatings were measured by a Hiatachi S-4000 Field Emission Scanning

Electron Microscopy (SEM) in the Advanced Materials Characterization Center at the University of

Cincinnati. The SEM system has 20 Angstrom resolution. It is equipped with an Oxford Isis Energy

Dispersive Spectroscopy system including a light element detector and digital imaging and an

Oxford solid-state BSE detector and an Oxford Opal Backscattered Diffraction Pattern system for crystallographic analysis.

35

Atomic force microscopy (AFM) examination of coating surfaces was achieved by a Digital

Instruments Dimension 3100 atomic force microscopy (AFM) in the laboratory of Dr. Andrew

Steckl at the University of Cincinnati. AFM is a new technique that reveals three-dimensional pictures of surfaces with such high resolution that individual atoms can be imaged. The AFM determines the contours of a sample surface by using a cantilever spring to sense the force between the outermost atom on the probe and the sample surface.

The hole mobility and sheet resistance of coatings were measured by a Hall and Van Der Pauw measurement system of MMR technology in the laboratory of Dr. Andrew Steckle at the University of Cincinnati. The measurement system includes a K-20 programmable temperature controller, H-

50 Hall and Van Der Pauw controller, MPS-50 programmable power supply, M-50 benchtop electromagnet, MMR thermal stage with cryogenic refrigerator, Dewar and vacuum accessories.

The K-20 temperature controller can adjust and maintain the temperature of the MMR thermal stage over a wide temperature range. The H-50 Hall and Van Der Pauw controller provides four probe method measurements of the electrical parameters of the samples. The M-50 benchtop electromagnet provides the magnetic field over samples and the MPS-50 programmable power supply controls the magnitude of that magnetic field. The MMR's Hall System can make automatic measurements of resistivity, mobility and carrier concentration of a wide range of samples using the

Van der Pauw method over a temperature range from 80 K to 400 K.

36

Metal Substrate Interface Coating Electrolyte

- +

Fig. 1.7 (a). The configuration for EIS.

CCoat

RSE

CInt

RCoat

RInt

Fig. 1.7 (b). The modeled electronic circuits for EIS.

37

Chapter 2

Deposition of Silicon Dioxide Coatings

2.1 Deposition Precursors for Silicon Dioxide

Hexamethyldisiloxane (HMDSO), 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS), and octamethylcyclotetrasiloxane (OMCTS) were used to grow silicon dioxide coatings by PE-CVD 42,

142. Table 2.1 gives their composition, molecular weight, vapor pressure, boiling and melting points, and the health-flammability-reactivity hazard ratings, and Fig. 2.1 shows their chemical structures.

The three precursors have low hazard ratings. HMDSO has a fairly high vapor pressure at room temperature which facilitates gas delivery, while OMCTS and TMCTS were heated to 40 - 70°C to increase vapor flow.

38

Table 2.1. Properties of HMDSO, OMCTS, and TMCTS, where HMIS represents Hazardous

Material Information System with Healthy-Flammability-Reactivity while the level of each index ranges from 0 to 4.

Chemical Molecular Boiling Melting Precursor Vapor Pressure HMIS Formula Weight Point Point

7.33 ´ 103 Pa HMDSO C6H18OSi2 162.38 (55 Torr) 99-100 °C -67 °C 1-4-0 at 30 °C 1.33 ´ 102 Pa OMCTS C8H24O4Si4 296.62 (1 Torr) 175-176 °C 17.4 °C 1-2-0 at 23 °C 1.33 ´ 102 Pa TMCTS C4H16O4Si4 240.51 (1 Torr) 134-135 °C -69 °C 2-3-1 at 30 °C

39

CH3 CH3

CH3 Si O Si CH3 HMDSO

CH3 CH3

CH3

SiH O

O SiH CH3 TMCTS

SiH O

O SiH CH3 CH3

CH3 CH3

Si O CH3 O Si CH3 OMCTS CH3 Si O CH3 O Si

CH3 CH3

Fig. 2.1 The chemical structures of HMDSO, TMCTS and OMCTS.

40

2.2 Experimental Conditions

Silicon dioxide coatings were grown from HMDSO, OMCTS, or TMCTS with oxygen by PECVD.

The two experimental factors were the ratio of oxygen flow to precursor flow and the value of the applied alternating substrate bias voltage. Three-level, two-factor full factorial experimental designs were used to investigate the effects of the ratio of oxygen flow to precursor flow and substrate bias voltage on the deposition growth rates, elastic modulus, hardness, and atomic concentrations of the coatings. The HMDSO flow rate was fixed at 100 sccm while the OMCTS and TMCTS flow rates were fixed at 50 sccm. The O2 flow rates varied from 100, 250 to 400 sccm, giving O2/precursor flow ratio values of 1:1, 2.5:1, and 4:1 for HMDSO and 2:1, 5:1 and 8:1 for OMCTS and TMCTS; the substrate bias voltage varied from 0, 20, to 40 V. The microwave power was fixed at 900 W and the deposition time was five minutes. Process chamber gas pressures were 0.67 Pa (5 mTorr) – 2.0

Pa (15 mTorr) before plasma ignition, depending on the O2 and precursor flow rates.

2.3 Results and Discussions

Tables 2.2, 2.3, and 2.4 summarize the deposition conditions and the growth rates, elastic moduli, hardness, and elemental compositions of silicon dioxide thin films deposited from HMDSO,

OMCTS, and TMCTS. Figs. 2.2-2.13 show the response surfaces of deposition growth rates, atomic concentrations of carbon, elastic modulus, and hardness.

Film thicknesses were measured with a Filmetrics thin-film measurement system, whose accuracy was calibrated with a diamond stylus Tencor P-10 surface profiler. The deposition growth rates were then obtained by dividing the thicknesses by the deposition time. The deposition growth rates range from 0.32 to 1.08 mm/min. Applied bias has little effect on the growth rates, but the growth

41

rates are strongly affected by the O2/precursor ratio. Figures 2.2-2.4 show growth rate response surfaces versus the O2/precursor ratio and applied bias for HMDSO, OMCTS, and TMCTS. At low

O2/precursor ratios, the growth rates increase with increasing flow ratio. At higher flow ratios, the growth rate with OMCTS saturates while the growth rates with HMDSO and TMCTS decrease.

Coating compositions were measured with a Kratos Axis Ultra X-ray photoelectron spectrometer using a monochromatized Al Ka x-ray source operating at 300 W. The coatings consist of silicon, carbon, and oxygen. Applied substrate bias had little effect on coating composition, but the

O2/precursor flow ratio was significant. At the low O2/precursor flow ratios, the coatings have a large carbon component, up to 36% for HMDSO. As the flow ratio increased, the carbon content decreased. At the highest O2/precursor flow ratios, the deposited coatings are close to stoichiometric silicon dioxide and the carbon percentages are as low as 12% for HMDSO, 3.4% for OMCTS, and

1.4% for TMCTS. Figures 2.5-2.7 show the response surfaces of carbon atomic concentrations versus the O2/precursor ratio and applied bias for HMDSO, OMCTS, and TMCTS. The atomic concentration of carbon decreases rapidly with increasing O2/precursor ratios while the applied substrate bias has little effect on the carbon atomic concentration.

The elastic modulus and hardness of deposited coatings were determined by nanoindentation using a continuous stiffness measurement technique 143, 144, performed with an MTS nanoindenter using a diamond tip mounted on a standard XP head 145. The elastic modulus ranges from 14.7 GPa to 101

GPa, and the hardness ranges from 1.38 GPa to 8.72 GPa. The elastic moduli and hardnesses of deposited coatings increase with increasing O2/precursor flow ratios. Applied bias had little effect on the elastic modulus and hardness of coatings. Figures 2.8-2.13 show the response surfaces of

42

coating elastic modulus and hardness. The elastic modulus and hardness of coatings increases as the flow ratio increases.

2.4 Summary

Silicon dioxide coatings were grown from organosilicon precursors HMDSO, OMCTS, and

TMCTS with oxygen by PECVD. The deposited coatings were transparent and colorless without any visible cracking or buckling. Maximum growth rates of 0.7 – 1.1 mm/min, elastic modulus of

73 – 101 GPa, and hardnesses of 6 – 9 GPa were achieved. Growth rates, elastic moduli, and hardnesses generally increased with the O2/precursor flow ratio, while atomic concentrations of carbon decrease rapidly with increasing O2/precursor flow ratios. The highest growth rates were obtained with HMDSO, while the highest quality coatings were grown from TMCTS, in terms of hardness and carbon content.

43

Table 2.2. Growth rates, elastic moduli, hardnesses, and atomic concentrations of deposited films for varying bias and O2/HMDSO flow ratio.

Atomic concentration Bias O2/HMDSO Growth rate Elastic modulus Hardness (%) (V) Ratio (µm/min) (GPa) (GPa) Si C O 0 1 0.56 33.3 1.4 30.8 34.6 34.6 0 2.5 0.97 53.0 4.1 30.8 24.9 44.4 0 4 1.01 52.0 4.5 31.7 12.4 56.0 20 1 0.48 55.8 1.9 29.9 36.4 33.7 20 2.5 0.99 61.4 4.8 31.6 25.8 42.6

20 4 1.08 60.9 5.5 31.4 13.3 55.3 40 1 0.51 39.6 1.5 29.7 35.6 34.8 40 2.5 0.97 73.1 6.0 30.1 26.8 43.1 40 4 0.93 67.8 5.8 32.9 13.9 53.2

44

Table 2.3. Growth rates, elastic moduli, hardnesses, and atomic concentrations of deposited films for varying bias and O2/OMCTS flow ratio.

Atomic concentration Bias O2/OMCTS Growth rate Elastic modulus Hardness (%) (V) Ratio (µm/min) (GPa) (GPa) Si C O

0 2 0.32 35.6 1.8 31.2 30.8 38.0

0 5 0.67 14.7 2.6 32.3 15.7 52.0

0 8 0.75 89.6 6.6 31.3 3.8 64.9

20 2 0.34 50.1 2.3 33.9 32.1 34.0 20 5 0.70 51.1 3.3 30.7 15.4 53.9 20 8 0.74 83.4 6.9 31.8 3.4 64.8

40 2 0.34 46.7 2.5 30.3 27.0 42.7

40 5 0.64 60.0 3.6 30.5 15.3 54.2

40 8 0.76 98.0 7.4 31.0 3.5 65.5 20 5 0.61 60.9 2.3 42.7 19.8 37.6

45

Table 2.4. Growth rates, elastic moduli, hardnesses, and atomic concentrations of deposited films for varying bias and O2/TMCTS flow ratio.

Atomic concentration Bias O2/TMCTS Growth rate Elastic modulus Hardness (%) (V) Ratio (µm/min) (GPa) (GPa) Si C O

0 2 0.86 48.3 3.2 35.6 17.4 47.0 0 5 0.93 52.8 4.2 36.1 9.5 52.4

0 8 0.67 100.6 8.3 35.4 1.7 62.9

20 2 0.89 34.3 3.2 35.9 17.6 46.6

20 5 0.88 54.4 4.0 35.5 9.6 54.9

20 8 0.74 92.5 8.0 36.1 1.4 62.5

40 2 0.82 55.4 3.6 35.9 16.9 47.2

40 5 0.91 65.2 4.2 35.9 8.0 56.1

40 8 0.60 101.0 8.7 35.4 2.5 62.1 20 5 0.96 60.8 5.2 35.7 9.3 55.1

46

Fig. 2.2. Growth rate response surface for coating deposition from HMDSO and O2.

Fig. 2.3. Growth rate response surface for coating deposition from OMCTS and O2.

47

Fig. 2.4. Growth rate response surface for coating deposition from TMCTS and O2.

Fig. 2.5. Carbon atomic concentration response surface for coating deposition from HMDSO and

O2.

48

Fig. 2.6. Carbon atomic concentration response surface for coating deposition from OMCTS and

O2.

Fig. 2.7. Carbon atomic concentration response surface for coating deposition from TMCTS and O2.

49

Fig. 2.8. Elastic modulus response surface for coating deposition from HMDSO and O2.

Fig. 2.9. Elastic modulus response surface for coating deposition from OMCTS and O2.

50

Fig. 2.10. Elastic modulus response surface for coating deposition from TMCTS and O2.

Fig. 2.11. Hardness response surface for coating deposition from HMDSO and O2.

51

Fig. 2.12. Hardness response surface for coating deposition from OMCTS and O2.

Fig. 2.13. Hardness response surface for coating deposition from TMCTS and O2.

52

Chapter 3

Deposition of Silicon Nitride Coatings

3.1 Deposition Precursors for Silicon Nitride

Hexamethyldisiloxane (HMDSO) and tetramethylsilane (TMS) were used to grow silicon nitride coatings by PECVD.129 The properties and chemical structure of HMDSO have been shown in

Table 2.1 and Fig. 2.1. Table 3.1 gives the composition, molecular weight, vapor pressure, boiling and melting point, and the health-flammability-reactivity hazard rating of TMS; Fig. 3.1 shows the chemical structure of TMS.

Table 3.1. Properties of TMS, where HMIS represents Hazardous Material Information System with

Healthy-Flammability-Reactivity while the level of each index ranges from 0 to 4.

Chemical Molecular Boiling Melting Precursor Vapor Pressure HMIS Formula Weight Point Point

7.85 ´ 104 Pa TMS C4H12Si 88.22 (589 Torr) 22.6-22.7°C -99°C 1-4-0 @ 20°C

CH3

CH3 Si CH3

CH3

Fig. 3.1 The chemical structure of TMS.

53

3.2 Experimental Conditions for Silicon Nitride Growth

Two-level three-factor full factorial experiments were performed to investigate the effects of microwave power, NH3/HMDSO flow-rate ratio, and pressure on the deposition growth rates, elastic modulus, hardness, and atomic concentrations of the coatings. The microwave power was varied from a low value of 500 W to a center value of 750 W and a high value of 1000 W. The

HMDSO flow rate was fixed at 50 sccm and the NH3/HMDSO flow ratio was varied from a low value of 1 to a center value of 1.5 and a high value of 2. The total gas pressure prior to plasma ignition varied from a low value of 0.53 Pa (4 mTorr) to a center value of 0.93 Pa (7 mTorr) and a high value of 1.33 Pa (10 mTorr). The deposition time was fixed at 15 min. After initial screening of the two-level three-factor full factorial experiments, two-level two-factor full factorial experiments were performed to further investigate the effects of microwave power and the

NH3/HMDSO flow-rate ratio on the deposition growth rates, elastic modulus, hardness, and atomic concentrations of the coatings. The microwave power was varied from a low value of 1000 W to a center value of 1500 W and a high value of 2000 W. The HMDSO flow rate was fixed at 100 sccm while the NH3/HMDSO flow ratio varied from a low value of 1:1 to 4:1. The total gas pressure prior to plasma ignition ranged from 1.20 Pa (9 mTorr) to 2.13 Pa (16 mTorr) and the deposition time was fixed at 15 min. The deposition conditions for silicon nitride coatings from TMS and NH3 were 25 sccm TMS, 250 sccm NH3, 2500 W microwave power, 0.85 Pa (6.4 mTorr) pressure prior to plasma ignition, and for 5 min.

3.3 Results and Discussions

Tables 3.2 and 3.3 summarize the characterization of SiC xOyNz coatings grown on 5 cm diameter silicon substrates by PECVD with HMDSO and NH3. Growth rates range from 0.20 to 0.50

54

mm/min.

XPS characterization showed the coatings consist of silicon, nitrogen, carbon, and oxygen: There is always significant carbon, particularly at a low NH3/HMDSO ratio, and oxygen. The important point for what follows, however, is that the atomic concentration of nitrogen ranges up to 13.7 % at high power and high ammonia/HMDSO flow ratio.

The elastic modulus ranges from 12.5 GPa to 176 GPa, and the hardness ranges from 0.9 GPa to

12.2 GPa. The coating with maximum elastic modulus of 176 GPa and hardness of 12.2 GPa contains the maximum nitrogen atomic concentration of 13.7 %. The coatings are thus far from stoichiometric silicon nitride, but can have elastic modulus and hardness values comparable to those reported for silicon nitride deposited by thermal CVD, i.e., 130-185 GPa and 12-20 GPa, respectively 146.

Figures 3.2-3.7 show the deposition growth rates, elastic modulus, and hardness in the two-level three-factor full factorial experiments as functions of microwave power and NH3/HMDSO ratio at pressure of 0.53 Pa (4 mTorr) and 1.33 Pa (10 mTorr). The microwave power enhances the deposition growth rates and hardnesses. The growth rates decrease with increasing NH3/HMDSO ratios at high microwave power while the hardnesses increase with increasing NH3/HMDSO ratios.

The deposition growth rates, elastic modulus, hardness, and nitrogen atomic concentrations in the two-level two-factor full factorial experiments are plotted as functions of microwave power and

NH3/HMDSO ratio in Figs. 3.8-3.11. Figure 3.8 shows that the deposition growth rates increase

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strongly with increasing microwave power: Higher microwave power increases dissociation, thus providing more silicon-containing fragments and more atomic nitrogen. Conversely, the growth rates decreases with increasing NH3/HMDSO flow ratio: The coatings grown at the lowest

NH3/HMDSO ratios (1:1) have higher growth rates (0.33 - 0.5 mm/min) but also the highest atomic carbon concentrations (34 - 35%), indicating that large precursor fragments are being incorporated into the coatings, giving thicker films. Coatings grown at the highest NH3/HMDSO ratios (4:1) have lower growth rates and also the lowest carbon (13 - 24%).

Figures 3.9-3.11 show that the nitrogen atomic concentration, the elastic modulus, and the coating hardness all increase with increasing microwave power and with increasing NH3/HMDSO ratio.

The microwave power is directly responsible for gas dissociation, while higher NH3/HMDSO ratios provide a more abundant nitrogen atom source, resulting in coatings with more incorporated nitrogen, closer to silicon nitride.

To measure the coating optical transmittance and color of coatings, five thin films approximately 5 mm thick were grown on 5 x 5 cm2 quartz substrates. The optical transmittance and color index of the coatings were measured using a Minolta spectrophotometer. The spectrophotometer uses the

L*a*b* color space model 147, 148, where L* indicates lightness while a* and b* define the chromaticity plan. The higher the L* value, the lighter the color; a perfect white would have an L* =

100.0 while a perfect black would have an L* = 0.0. The + a* is the red direction, -a* is the green direction, +b* is the yellow direction, and -b* is the blue direction. Table 3.4 summarizes the deposition conditions and measured optical transmittance and color index of the coatings. The deposited coatings have high optical transmittance and low color index values.

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For corrosion testing, five films with thicknesses of about 8 mm were grown on 5 x 5 cm2 brass substrates, again using the deposition conditions in Table 3.3. The coatings were then subjected to a standard ASTM B117 salt-fog corrosion test, in which the samples were exposed to a salt fog from a 5% sodium chloride solution (Ulrich) at 35 oC. Table 3.5 shows the salt-fog corrosion testing results. Two of the five samples, which were grown at low NH3/HMDSO ratio, lasted 840 hours without any visible corrosion, while the others lasted 744 hours. Lower flow-rate ratios gave softer and more flexible coatings, and improved adherence and corrosion resistance.

Table 3.6 summarizes the characterization of SiC xOyNz coatings grown on 5 cm diameter silicon substrate by PECVD with TMS and NH3. The film thickness and dielectric constant were measured with a spectroscopic ellipsometer. The deposited film has growth rates of 13.6 nm/min, elastic modulus of 174.7 GPa, hardness of 14.0 GPa, and refractive index of 1.8 at 620 nm, with atomic concentrations of 44.3 % Si, 7.2 % C, 10.9 % O, and 37.6 % N.

Fourier transform infrared (FTIR) spectroscopic measurements were performed on three coatings grown on brass substrates from TMS and NH3 with varying TMS flow rates. Figures 3.12, 3.13 and 3.14 show the FTIR absorbance spectra. Absorption peaks can be identified as N-H bonding at about 3346 cm-1, Si-H bonding at about 2150 cm-1, Si-O bonding at about 1200 cm-1, Si-N bonding at about 860 cm-1, and Si-Si bonding at about 500 cm-1. As the composition of the films departs from stoichiometric silicon nitride due to high oxygen content in the films, the Si-N peak becomes intermixed with the Si-O peak centered on about 1100 cm-1, which is the prominent feature in the spectrum of silicon oxynitride.

57

3.4 Summary

Silicon nitride coatings were grown by PECVD in a high-density microwave ECR discharge, using

HMDSO and TMS as the deposition precursors and NH3 as the reactive gas. The coatings grown from HMDSO are colorless and transparent. Deposition rates up to 0.5 mm/min were achieved; increasing microwave power raised deposition growth rates, while increasing the NH3/HMDSO ratio lowered growth rates. Increasing microwave power and NH3/HMDSO ratio increased the atomic concentrations of nitrogen, the elastic modulus, and the hardness of coatings. The coatings with the maximum elastic modulus of 176 GPa and hardness of 12.2 GPa were grown at highest microwave power (2000 W) and highest flow-rate ratio (4:1). Samples lasted 840 hours in an

ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surfaces of brass substrates. The coating grown from TMS has a high elastic modulus and hardness and refractive index values, and is close to stoichiometric silicon nitride.

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Table 3.2. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings for varying microwave power, NH3/HMDSO flow ratio, and pressure.

Pressure Run Power Growth rate Elastic modulus Hardness Ratio (Pa) No. (W) (µm/min) (GPa) (GPa) (mTorr)

1 1.5 750 0.93 (7) 0.34 18.5 1.4

2 1 500 0.53 (4) 0.23 22.9 1.1

3 1 500 1.33 (10) 0.23 21.5 0.9 4 1 1000 0.53 (4) 0.50 19.4 2.0 5 1 1000 1.33 (10) 0.53 23.8 2.6 6 2 500 0.53 (4) 0.20 32.6 1.7 7 2 500 1.33 (10) 0.21 29.2 1.5

8 2 1000 0.53 (4) 0.46 34.3 3.3 9 2 1000 1.33 (10) 0.34 43.2 4.0 10 1.5 750 0.93 (7) 0.33 24.8 1.8

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Table 3.3. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings for varying microwave power and NH3/HMDSO flow ratio.

Atomic concentration (%) Run Power Growth rate Elastic modulus Hardness Ratio No. (W) (µm/min) (GPa) (GPa) Si C O N

1 2.5 1500 0.39 31.1 3.8 36.1 27.0 25.6 11.4

2 4 2000 0.37 176 12.2 41.6 24.1 19.7 13.7

3 4 1000 0.18 54.1 3.2 35.4 13.1 44.9 6.6

4 1 2000 0.50 21.1 2.7 35.3 34.4 23.7 6.6

5 1 1000 0.34 12.5 0.9 33.6 35.2 27.0 4.3

6 2.5 1500 0.37 23.1 2.7 36.0 28.1 25.9 10.1

Table 3.4. Optical transmittance and color index of deposited coatings on quartz substrates for varying microwave power and NH3/HMDSO flow ratio.

Color Index Ratio Power Optical Transmittance (%) (W) at 360 nm L* a* b*

2.5 1500 91.85 96.20 -0.11 0.15

4 2000 91.56 95.70 -0.20 0.17

4 1000 92.01 96.10 -0.08 0.12

1 2000 92.11 96.11 0.07 0.15

1 1000 92.40 97.51 0.02 0.13

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Table 3.5. The lifetime of five silicon nitride coatings in an ASTM B117 salt-fog corrosion test.

Ratio Power Lifetime (W) (hours)

2.5 1500 744

4 2000 744

4 1000 744

1 2000 840

1 1000 840

Table 3.6. Growth rates, elastic modulus, hardness, and atomic concentrations of deposited coatings

from TMS and NH3.

Atomic concentration Power Growth rate Elastic modulus Hardness n (%) Ratio (W) (nm/min) (GPa) (GPa) (at 620 nm) Si C O N 10 2500 13.6 174.7 14.0 1.8 44.3 7.2 10.9 37.6

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Fig. 3.2. Statistical response surface for growth rates of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr).

Fig. 3.3. Statistical response surface for growth rates of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr).

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Fig. 3.4. Statistical response surface for elastic modulus of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr).

Fig. 3.5. Statistical response surface for elastic modulus of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr).

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Fig. 3.6. Statistical response surface for hardness of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 0.53 Pa (4 mTorr).

Fig. 3.7. Statistical response surface for hardness of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio at pressure of 1.33 Pa (10 mTorr).

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Fig. 3.8. Statistical response surface for growth rates of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio.

Fig. 3.9. Statistical response surface for atomic concentration of nitrogen in deposited coatings as a function of microwave power and NH3/HMDSO flow ratio.

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Fig. 3.10. Statistical response surface for elastic modulus of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio.

Fig. 3.11. Statistical response surface for hardness of deposited coatings as a function of microwave power and NH3/HMDSO flow ratio.

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0.90 0.85

0.80 1250

0.75

0.70

0.65

0.60

0.55

0.50

0.45

Log(1/R) 0.40

0.35

0.30 490 0.25 927 0.20

0.15 3348

0.10

0.05 1670

0.00 4000 3000 2000 1000 Wavenumbers (cm-1) Fig. 3.12. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with

75 sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min.

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0.80 0.75

0.70 1226

0.65

0.60

0.55

0.50

0.45

0.40

Log(1/R) 0.35

0.30

0.25

0.20 881

0.15 493

0.10 3324 1660 1486

0.05

0.00

4000 3000 2000 1000 Wavenumbers (cm-1) Fig. 3.13. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with

50 sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min.

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0.80 0.75

0.70 1120

0.65

0.60

0.55

0.50

0.45

0.40

Log(1/R) 0.35

0.30

0.25

0.20

0.15

0.10 516 1481

0.05 3339 0.00

4000 3000 2000 1000 Wavenumbers (cm-1) Fig. 3.14. FTIR absorbance spectra of the silicon nitride coating deposited on brass substrate with

25 sccm TMS, 250 sccm NH3, 2500 W microwave power, and for 10 min.

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Chapter 4

Deposition of Silicon-Containing Polymer Coatings

4.1 Deposition Precursors and Experimental Conditions for Polymer Growth

Silicon-containing polymer coatings grown by plasma polymerization of organosilicon monomers are pinhole-free and adherent to a variety of substrates, and have high corrosion resistance.

Octamethylcyclotetrasiloxane (OMCTS) was used as the deposition precursor to grow silicon- containing polymer coatings.130 Two-level, three-factor full factorial experiments with repeated center points were performed to investigate the effects of microwave power, pressure, and OMCTS flow rate on the deposition growth rates, elastic modulus, and hardness of polymer coatings. The input parameter ranges were microwave power, 500 W to 1000 W; gas pressure prior to plasma ignition, 0.27 Pa (2 mTorr) to 1.33 Pa (10 mTorr); and OMCTS flow rate, 40 sccm to 80 sccm. The substrates were 5 cm diameter silicon wafers and 5´5 cm2 brass substrates. The deposition time was fixed at 15 minutes. For corrosion testing, two-level, two-factor full factorial experiments, where the pre-clean gas is a qualitative factor, were performed to investigate the effects of the pre-clean gas and the deposition pressure on the corrosion resistance of silicon-containing polymer coatings grown on brass substrates.

4.2 Results and Discussion

4.2.1 Polymer Deposition

Table 4.1 gives the experimental run conditions for samples grown on silicon wafers with 100%

OMCTS, together with measured growth rates, elastic moduli, and hardnesses. The deposition growth rates range from 0.07 µm/min to 0.59 µm/min. The elastic moduli range from 8.7 GPa to

63.5 GPa, and the hardness values range from 0.6 GPa to 1.5 GPa.

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Table 4.2 gives the experimental run conditions for samples grown on brass substrates with 100%

OMCTS, together with measured growth rates and tumble test results. Growth rates of silicon- containing polymer coatings grown on brass substrates range from 0.057 um/min to 0.273 um/min.

The tumble test was performed to test the wear resistance of coatings. The tumble machine was full of sand and small pieces of metal with sharp edges and can rotate like a rotating wheel with its axis fixed. The samples were tumbled in the tumble machine for 10 minutes. Computer-based image analysis was then used to find the number of breaks over a 2.54 cm ´ 2.54 cm square area on coatings and the sum of all break areas on the 2.54 cm ´ 2.54 cm square area. Number of breaks over a 2.54 cm ´ 2.54 cm square area on coatings ranges from 34 to 1158, and sum of all broken areas on the 2.54 cm ´ 2.54 cm square area ranges from 1.17 ´ 10-2 cm2 to 357.5 ´ 10-2 cm2, respectively.

Multiple regression fitted surfaces are plotted in Figures 4.1 and 4.2 for the deposition growth rates as functions of microwave power and OMCTS flow rate for pressures of 0.27 Pa (2 mTorr) and 1.33

Pa (10 mTorr). The figures show that the deposition growth rates generally increase with increasing microwave power at both pressures, i.e., with increased dissociation. At the lower pressure of 0.27

Pa (2 mTorr), growth rates increase with the OMCTS flow rate at high microwave power, and decrease with increasing OMCTS flow rate at low microwave power. At high power and low pressure, the discharge is precursor-starved, while at low power the discharge is presumably insufficiently ionized. At 1.33 Pa (10 mTorr), growth rates decrease with increasing OMCTS flow rates regardless of the microwave power.

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Figures 4.3 and 4.4 show multiple regression fitted surfaces for the coating hardnesses as functions of microwave power and OMCTS flow rate, again for pressures of 0.27 Pa (2 mTorr) and 1.33 Pa

(10 mTorr). At 0.27 Pa (2 mTorr) the hardness increases with increasing microwave power and

OMCTS flow rate, behavior typical of a precursor- and power-starved discharge with insufficient ion flux energy to the substrate; similar effects can be observed in low pressure plasma-assisted etching 115. At 1.33 Pa (10 mTorr) the hardness increases with increasing microwave power at lower OMCTS flow rates, again signaling a power-starved plasma, but decreases with power at higher flows; The latter behavior remains unclear.

Figures 4.5 and 4.6 show multiple regression fitted surface for number of breaks over a 2.54 cm ´

2.54 cm square area in a tumble test of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr) and 1.33 Pa (10 mTorr); Figs. 4.7 and 4.8 show multiple regression fitted surface for sum of all broken areas over a 2.54 cm ´2.54 cm square area in a tumble test of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr) and 1.33 Pa (10 mTorr). The number of breaks and the sum of all broken areas both decrease with increasing microwave power at high OMCTS flow rates while microwave power has little effects on number of breaks and sum of all broken areas at low OMCTS flow rates.

Fourier transform infrared (FTIR) spectroscopic measurements were performed on polymer coatings grown on brass substrates. Figures 4.7 and 4.8 show the FTIR absorbance spectra:

-1 -1 Absorption peaks can be identified as CH3 asymmetric stretching at 2963 cm to 2962 cm , SiH

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-1 -1 -1 -1 stretching at 2173 cm to 2164 cm , CH3 symmetric bending at 1263 cm to 1259 cm , SiOSi asymmetric stretching at 1131 cm-1 to 1003 cm-1, and SiC stretching at 815 cm-1 to 794 cm-1.

CH3 and SiH bond peaks in the spectra indicate that the films contain carbon and hydrogen together with silicon and oxygen. The deposited coatings are indeed characterized by the polymer characteristics owing to much CH3 bonding, Si-H bonding, and Si-CH3 bonding components involved in the coatings.

4.2.2 Effect of Plasma Cleaning Cycle

Polymer films were grown on brass substrates for corrosion resistance evaluation in an ASTM B117 salt-fog corrosion test, in which the samples were exposed to a salt fog from a 5% sodium chloride solution (Ulrich) at 35 oC. The previous work in our Laboratory suggested that the in situ plasma cleaning cycle was of particular importance for adhesion and corrosion resistance. A two-level two- factor full factorial experiment was therefore performed in which the variables were the cleaning gas (oxygen or argon) and the deposition pressure. Substrates were cleaned for 3 min in a plasma generated with 300 sccm of either oxygen or argon and 1500 W microwave power. The subsequent deposition conditions were 100% OMCTS, gas pressure prior to plasma ignition, 1.33 Pa (10 mTorr) to 6.66 Pa (50 mTorr); 100 sccm OMCTS flow rate, 2000 W microwave power, and deposition time, 20 min. Table 4.3 summarizes the deposition conditions and corrosion results.

The lifetime of the polymer coatings on brass in the ASTM B117 salt-fog corrosion test, that is, the time before visible corrosion appeared on the coated substrate surface, ranged from 23 hours to

3185 hours. An oxygen plasma pre-clean was clearly preferable to argon, as the oxygen plasma removes organic surface contamination rather than simply “graphitizing” the organics by argon ion

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bombardment 149. In addition, a lower deposition gas pressure is preferable to high pressure for corrosion resistance: A lower gas pressure raises the plasma electron temperature Te and the plasma

1/2 sheath potential Vs. This enhances the ion flux density to the substrate (~Te ) and the ion

126 bombardment energy (~Te), compacting the deposited films .

To investigate the pre-clean effects of oxygen and argon on substrates, four brass substrates were subjected to an in situ oxygen or argon plasma clean cycle for 3 minutes, and then the cleaned surfaces of the substrates were analyzed by a Kratos Axis Ultra X-ray photoelectron spectrometer using a monochromatized Al Ka x-ray source operating at 300 W. Table 4.4 summarizes the pre- clean conditions and the XPS results. The XPS results do not indicate obvious difference between oxygen and argon plasma clean. The possible reason is that the cleaned substrates were exposed to air before the XPS analysis and then reacted with oxygen and carbon in air. But in our current experimental systems, we can not do XPS analysis on cleaned substrate surfaces immediately after plasma cleaning without being exposed to air.

To prove that the above corrosion test results of the coatings mainly come from the effects of pre- clean gases and deposition pressure and the thickness of the coatings was not one of the main factors for the test results, four coatings of similar thicknesses were grown on 5 x 5 cm2 brass substrates. The samples grown in pressure of 1.33 Pa (10 mTorr) were grown for 20 min while the samples grown in the pressure of 4.0 Pa (30 mTorr) were grown for 40 min. Table 4.5 shows the coating thicknesses and status of the samples in an ASTM B117 salt-fog corrosion test after 1776 hours. The four samples have similar thicknesses of coatings (8.6 µm - 9.2 µm). The test results further indicate that oxygen plasma clean is better than argon plasma clean while low deposition

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pressure is better than high deposition pressure as indicated by the above experiment. The possible reason for the test results is the same as that explained for the above experiment.

4.2.3 Polymer Characterization

To measure the coating optical transmittance and color of coatings, three thin films were grown on

5 x 5 cm2 quartz substrates. The optical transmittance and color of the quartz substrates with coatings were measured using a Minolta spectrophotometer. Table 4.6 summarizes the deposition conditions and the optical transmittance and color index of the samples. The coatings have high optical transmittance and low color index, indicating that the coatings are high transparent clear coatings.

To measure the contact angle and surface energy of coatings, four thin films were grown on 5 x 5 cm2 brass substrates at 2000 W, 100 sccm OMCTS, 1.33 Pa (10 mTorr) and 20 min or 4.0 Pa (30 mTorr) and 40 min with oxygen or argon plasma pre-clean. Table 4.7 summarizes the contact angle and surface energy of deposited coatings and brass substrate. It shows that higher deposition pressure gives thin films with lower surface energy while pre-clean gases have little effect on surface energy.

To investigate the corrosion resistance of silicon-containing polymer thin films with electrochemical impedance spectroscopy (EIS), four thin films were grown on brass substrates at

2000 W, 100 sccm OMCTS, 1.33 Pa (10 mTorr) and 20 min or 4.0 Pa (30 mTorr) and 40 min with oxygen or argon plasma pre-clean. Figs. 4.11 to 4.15 show the results of the EIS analyses for the four samples and brass substrate. In the EIS measurement, the variation of impedances from 0 hours

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to 96 hours for the coating grown at high deposition pressure of 4.0 Pa (30 mTorr) with argon plasma pre-clean (Fig. 4.14) is obviously different from other three cases and the impedances decrease continuously from 0 hour to 96 hours. Corrosion happened in the coating grown at high deposition pressure of 4.0 Pa (30 mTorr) with argon plasma pre-clean, making the impedance to decrease.

Two sets of three silicon-containing polymer thin films were grown on brass substrates for testing of cooling and heating effects of substrates on coatings. The samples were grown at microwave power levels of 1000, 1500, and 2000 W, while the other deposition conditions for the growth of samples were fixed at 100 sccm OMCTS, 1.33 Pa (10 mTorr), and 20 min with oxygen plasma clean. To test the cooling and heating effects of substrates on coatings, one set of samples was cooled to -30 oC, while the other was heated up to 400 oC. All deposited coatings were found to adhere to substrates well without any visible cracking or stripping.

To evaluate film stress, three silicon-containing polymer thin films were grown on silicon wafers at microwave power levels of 1000 W, 1500 W and 2000 W. The other deposition conditions for the growth of samples were 100 sccm OMCTS, 1.33 Pa (10 mTorr), and 30 min. The coated surfaces of three samples were flat without any visible curving compared to an uncoated silicon wafer.

4.3 Summary

Silicon-containing polymer coatings have been grown from 100% OMCTS precursor by PECVD in a high-density microwave ECR discharge. The deposited coatings are transparent and colorless, with deposition growth rates up to 0.59 µm/min. The coatings are softer than silicon dioxide or

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silicon nitride coatings, with nanoindentation hardness value range of 0.6 GPa to 1.5 GPa. The silicon-containing polymer thin film grown at the conditions of oxygen plasma pre-cleaning and low deposition pressure lasted 3185 hours in an ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates.

Table 4.1 Growth rates, elastic modulus, and hardness of silicon-containing polymer coatings on silicon wafers.

Flow Rate Power Pressure Growth Rate Elastic Modulus Hardness Run No. (sccm) (w) (Pa)(mTorr) (mm/min) (GPa) (GPa)

1 60 750 0.80 (6) 0.563 8.691 0.933 2 80 1000 0.27 (2) 0.592 11.725 1.526

3 80 1000 1.33 (10) 0.103 32.015 0.779

4 80 500 0.27 (2) 0.063 35.331 1.006 5 80 500 1.33 (10) 0.066 63.548 1.522 6 40 1000 0.27 (2) 0.316 13.131 1.123 7 40 1000 1.33 (10) 0.402 11.918 1.154 8 40 500 0.27 (2) 0.307 10.596 0.757 9 40 500 1.33 (10) 0.257 11.054 0.630 10 60 750 0.80 (6) 0.179 25.079 1.238

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Table 4.2 Growth rates and tumble test results of silicon-containing polymer coatings on brass substrates.

Tumble Test Results Run Flow Rate Power Pressure Growth Rate Sum of Break No. (sccm) (w) (Pa)(mTorr) (mm/min) Number Of Areas Breaks (´10-2cm2) 1 60 750 0.80 (6) 0.065 67 2.20

2 80 1000 0.27 (2) 0.273 35 1.17

3 80 1000 1.33 (10) 0.063 49 1.79 4 80 500 0.27 (2) 0.057 1158 217.57 5 80 500 1.33 (10) 0.094 604 357.50

6 40 1000 0.27 (2) 0.252 178 10.86 7 40 1000 1.33 (10) 0.260 34 1.18 8 40 500 0.27 (2) 0.071 50 1.29 9 40 500 1.33 (10) 0.058 109 2.49 10 60 750 0.80 (6) 0.132 37 1.40

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Table 4.3. Thicknesses and B117 salt-fog corrosion test lifetime of silicon-containing polymer films for two pre-cleaning gases and three deposition pressures.

Deposition Pressure Deposited Films Run Pre-Clean Gas (Pa)(mTorr) No. (300 sccm, 1500 W) Thickness Lifetime (100 sccm OMCTS, 2000 W) (µm) (Hours)

1 Ar 1.33 (10) 9.4 169

2 Ar 4.0 (30) 4.5 23

3 Ar 6.66 (50) 1.3 94

4 O2 1.33 (10) 9.0 3185

5 O2 4.0 (30) 3.7 681

6 O2 6.66 (50) 1.7 169

Table 4.4. The composition of brass substrate surfaces after oxygen or argon plasma clean.

Atomic Concentration (%) Clean Gas Power (300 sccm) (W) C O Cu Zn Other

Ar 1300 24.3 40.9 22.8 10.6 1.4

Ar 1500 35.1 38.1 17.0 2.9 6.9

O2 1300 28.4 36.6 28.4 2.7 3.9

O2 1500 35.5 37.9 19.9 1.4 5.3

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Table 4.5. The status of four samples in a B117 salt-fog corrosion test after 1776-hours running.

Deposition Pressure Deposited Films Run Pre-Clean Gas (Pa)(mTorr) No. (300 sccm, 1500 W) Thickness Lifetime (100 sccm OMCTS, 2000 W) (µm) (Hours)

1 Ar 1.33 (10) 9.2 192

2 Ar 4.0 (30) 8.8 72

3 O2 1.33 (10) 9.1 >1776

4 O2 4.0 (30) 8.6 946

Table 4.6. Optical transmittance and color index for silicon-containing polymer coatings grown from 100 % OMCTS on quartz substrates.

Color Index Flow Rate Power Pressure Optical Transmittance (%) (sccm) (w) (Pa)(mTorr) at 360 nm L* a* b*

75 1500 5.33 (40) 92.05 97.28 0.06 0.13

100 2000 8.0 (60) 92.47 97.20 -0.21 0.14

50 2000 1.33 (10) 90.45 97.13 -0.01 0.22

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Table 4.7. Contact angle analysis of four silicon-containing polymer thin films grown at 2000 W,

100 sccm OMCT, 1.33 Pa (10 mTorr) or 4.0 Pa (30 mTorr) with argon or oxygen plasma pre-clean.

Contact Angle ( o ) Engergy (mJ/m2) Pre-Clean Pressure Run No. Gas (Pa)(mTorr) Water Metlylend Dispersive Polar Total

1 Ar 1.33 (10) 96.7 56.0 29.6 3.7 33.3

2 Ar 4.0 (30) 107.0 70.6 23.9 1.4 25.3

3 O2 1.33 (10) 100.6 63.6 27.9 2.6 30.5

4 O2 4.0 (30) 96.3 76.3 19.6 6.8 26.4

Substrate 85.3 55.0 29.1 8.3 37.4

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Fig. 4.1. Statistical response surface for growth rates of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr).

Fig. 4.2. Statistical response surface for growth rates of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 1.33 Pa (10 mTorr).

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Fig. 4.3. Statistical response surface for hardness of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr).

Fig. 4.4. Statistical response surface for hardness of deposited silicon-containing polymer thin films as a function of microwave power and OMCTS flow rates at 1.33 Pa (10 mTorr).

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Fig. 4.5. Statistical response surface for number of breaks over a 2.54 cm ´ 2.54 cm square area in a tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

0.27 Pa (2 mTorr).

Fig. 4.6. Statistical response surface for number of breaks over a 2.54 cm ´ 2.54 cm square area in a tumble test of the coatings as a function of power and OMCTS flow rates at 1.33 Pa (10 mTorr).

84

Fig. 4.7. Statistical response surface for sum of broken area over a 2.54 cm ´ 2.54 cm square area in a tumble test of deposited coatings as a function of microwave power and OMCTS flow rates at

0.27 Pa (2 mTorr).

Fig. 4.8. Statistical response surface for sum of broken area over a 2.54 cm ´ 2.54 cm square area in a tumble test of the coatings as a function of power and OMCTS flow rates at 1.33 Pa (10 mTorr).

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0.27 Pa (2 mTorr)

40 sccm, 500 W

40 sccm, 1000 W

80 sccm, 500 W Absorbance [a.u]

80 sccm, 1000 W

4000 3000 2000 1000 Wavenumber [cm-1]

Fig. 4.9. FTIR absorbance spectra of silicon-containing polymer thin films with varying microwave power and OMCTS flow rates at 0.27 Pa (2 mTorr).

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1.33 Pa (10 mTorr)

40 sccm, 500 W

40 sccm, 1000 W

80 sccm, 500 W Absorbance [a.u]

80 sccm, 1000 W

4000 3000 2000 1000 -1 Wavenumber [cm ]

Fig. 4.10. FTIR absorbance spectra of silicon-containing polymer thin films with varying microwave power and OMCTS flow rates at 1.33 Pa (10 mTorr).

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9 0

?: 0 hours 8 -10 à: 24 hours ? : 48 hours 7 *: 96 hours -20

6 -30

5 -40

4 -50

3 -60 Phase (Degree) Log Modulus (Ohm) 2 -70

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz)

Fig. 4.11. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at

2000W, 100 sccm OMCT, 1.33 Pa (10 mTorr), and 20 min with pre-clean of oxygen plasma.

88

10 0

?: 0 hours 9 -10 à: 24 hours ? : 48 hours 8 *: 96 hours -20

7 -30

6 -40 5 -50 4

-60 Phase (Degree) Log Modulus (Ohm) 3

-70 2

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz)

Fig. 4.12. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at

2000W, 100 sccm OMCT, 4.0 Pa (30 mTorr), and 40 min with pre-clean of oxygen plasma.

89

9 0

?: 0 hours 8 -10 à: 24 hours ? : 48 hours 7 *: 96 hours -20

6 -30

5 -40

4 -50

3 -60 Phase (Degree) Log Modulus (Ohm)

2 -70

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz) Fig. 4.13. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at

2000W, 100 sccm OMCT, 1.33 Pa (10 mTorr), and 20 min with pre-clean of argon plasma.

90

9 0

?: 0 hours 8 -10 à: 24 hours ? : 48 hours 7 *: 96 hours -20

6 -30

5 -40

4 -50

3 -60 Phase (Degree) Log Modulus (Ohm)

2 -70

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz) Fig. 4.14. EIS analysis of silicon-containing polymer thin film grown from 100 % OMCT at

2000W, 100 sccm OMCT, 4.0 Pa (30 mTorr), and 40 min with pre-clean of argon plasma.

91

4 0

?: 0 hours 3.5 à: 24 hours -10

3 -20

2.5 -30 2 -40 1.5 Phase (Degree) Log Modulus (Ohm) -50 1

0.5 -60

0 -70 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz) Fig. 4.15. EIS analysis of brass substrate.

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Chapter 5

Deposition of Silicon-Containing Multilayer Coatings

5.1 Deposition Precursors and Experimental Conditions

OMCTS and HMDSO were used as the deposition precursors to grow silicon-containing multilayer coatings by PECVD. The bottom layer of multilayer coatings was a silicon-containing polymer coating, grown from 100 % OMCTS by PECVD.130 The middle and top layers of multilayer coatings were silicon dioxide or silicon nitride coatings. Silicon dioxide coatings were grown from

OMCTS in an oxygen plasma, while silicon nitride coatings were grown from HMDSO in an ammonia plasma. Silicon-containing polymer coatings were grown at 2000 W, 100 sccm OMCT, for 10 min; silicon dioxide coatings were grown at 2000 W, 50 sccm OMCTS, with O2/OMCTS flow-rate ratios of 2:1, 5:1, or 8:1, for either 5 or 10 min; silicon nitride coatings were grown at

2500 W, 100 sccm HMDSO, with NH3/HMDSO flow-rate ratios of 1:1, 2.5:1, or 4:1, for either 5 or 10 minutes.

5.2 Results and Discussions

Table 5.1 summarizes the deposition conditions and corrosion test results for six multilayer samples. The multilayer coatings with silicon dioxide top layers lasted 1728 to 1800 hours in an

ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, almost independent of the O2/OMCTS flow-rate ratio, while the structures with silicon nitride top layers lasted 2492 to 2616 hours, again almost independent of the flow ratio.

Table 5.2 summarizes the deposition conditions and tumble test results for six multilayer coatings grown on brass substrates. The number of breaks and the sum of broken areas over a 2.54 cm ´ 2.54

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cm square area on the multilayer coatings with silicon dioxide top layers ranges from 21 to 40 and

0.74 ´ 10-2 cm2 to 1.63´ 10-2 cm2, respectively, while the number of breaks and the sum of broken areas over a 2.54 cm ´2.54 cm square area on the multilayer coatings with silicon nitride top layers ranges from 12 to 25 and 0.57 ´ 10-2 cm2 to 1.12´ 10-2 cm2, respectively. The number of breaks and the sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the multilayer coatings decrease with increasing O2/OMCTS and NH3/HMDSO flow ratios.

Two multilayer coatings with three layers were grown on brass substrates for corrosion and tumble testing. One of the coatings consisted of a polymer bottom layer, a silicon dioxide middle layer, and a silicon dioxide top layer grown with more oxygen, while the other consisted of a polymer bottom layer, a silicon nitride middle layer, and a silicon nitride top layer grown with more ammonia.

Tables 5.3 and 5.4 summarize the deposition conditions, corrosion test results, and tumble test results. The multilayer coating with silicon dioxide middle and top layers lasted 1704 hours in a

ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 18 breaks and 0.66 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test, the multilayer coating with silicon nitride middle and top layers lasted 2400 hours in a ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 9 breaks and 0.5 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test.

To measure the contact angle and surface energy of multilayer coatings, four multilayer coatings were grown on 5 x 5 cm2 brass substrates. The coatings consisted of a polymer bottom layer, and either a silicon dioxide or a silicon nitride top layer. Silicon dioxide top layers were grown at 2000

94

W, 50 sccm OMCT, and 100 or 400 sccm O2, for 10 min while silicon nitride top layers were grown at 2500 W, 100 sccm HMDSO, and 100 or 400 sccm NH3, for 10 min. Table 5.5 summarizes the contact angles and surface energies of the coatings. Higher O2/OMCTS and NH3/HMDSO flow ratios yield deposited coatings with higher surface energies.

Three multilayer coatings were grown on brass substrates for corrosion resistance testing with electrochemical impedance spectroscopy (EIS). Figure 5.1 shows the results of the EIS analysis for the multilayer coating with a polymer bottom layer grown from 100 % OMCTS at 2000 W and 100 sccm OMCTS for 10 min, and a silicon dioxide top layer grown from OMCTS and O2 at 2000 W,

400 sccm O2, and 50 sccm OMCTS, for 10 min.

Figure 5.2 shows the results of the EIS analysis for the multilayer coating with a polymer bottom layer grown from 100 % OMCTS at 2000 W and 100 sccm OMCTS for 10 min, and a silicon nitride top layer grown from HMDSO and NH3 at 2500 W, 400 sccm NH3, and 100 sccm HMDSO, for 10 min.

Figure 5.3 shows the results of the EIS analysis for the multilayer coating with a polymer bottom layer grown from 100 % OMCTS at 2000 W and 100 sccm OMCTS for 10 min, a silicon dioxide middle layer grown from OMCTS and O2 at 2000 W, 100 sccm O2, and 50 sccm OMCTS, for 5 min, and a silicon dioxide top layer grown from OMCTS and O2 at 2000 W, 400 sccm O2, and 50 sccm OMCTS, for 5 min.

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The multilayer coatings with a polymer bottom layer and a silicon dioxide or silicon nitride top layer have similar corrosion performance. The impedance at low frequency increases slightly at 96 hours compared to those at 24 hours and 48 hours, while the multilayer coating with three layers has a continuously decreasing impedance at low frequency from 24 hours to 96 hours.

5.3 Summary

Soft adherent silicon-containing polymer undercoatings were combined with harder silicon dioxide or silicon nitride middle or top layers to achieve better performance than monolayer coatings alone.

These multilayer structures have hard top surfaces and have better adhesion and corrosion resistance than monolayer silicon dioxide or silicon nitride, lasting 1700 to 2600 hours in a salt spray corrosion testing. These characteristics could make such multilayer structures attractive for protective and functional coatings.

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Table 5.1. Thicknesses and salt-fog corrosion test lifetimes of multilayer coatings with a silicon- containing polymer bottom layer, and either a silicon dioxide layer or a silicon nitride top layer, for varying O2/OMCTS and NH3/HMDSO flow-rate ratios.

Multilayer Coatings O2/OMCTS Flow Ratio NH3/HMDSO Flow Ratio for Silicon Dioxide Layer for Silicon Nitride Layer Thickness Lifetime (2000 W, 10 min) (2500 W, 10 min) (µm) (hours)

2 9.5 1800 5 9.1 1728 8 9.0 1728 1 9.6 2492 2.5 9.0 2492 4 8.1 2616

Table 5.2. Tumble test results of multilayer coatings with a silicon-containing polymer bottom layer, and either a silicon dioxide layer or a silicon nitride top layer, for varying O2/OMCTS and

NH3/HMDSO flow-rate ratios.

Tumble Test Results O2/OMCTS Flow Ratio NH3/HMDSO Flow Ratio for Silicon Dioxide Layer for Silicon Nitride Layer Sum of Break Number Of (2000 W, 10 min) (2500 W, 10 min) Areas Breaks (´10-2 cm2) 2 40 1.63 5 28 1.05 8 21 0.74 1 25 1.12 2.5 20 0.85 4 12 0.57

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Table 5.3. Salt-fog corrosion test and tumble test results of the multilayer coating with a silicon- containing polymer bottom layer and two silicon dioxide layers grown with varying O2/OMCTS flow-rate ratios.

Tumble Test Results Layer Two Layer Three Thickness Lifetime Sum of Break Number (2000 W, 5 min) (2000 W, 5 min) (mm) (hours) Areas Of Breaks (´10-2 cm2)

100 sccm O2 / 50 400 sccm O2 / 50 sccm OMCTS sccm OMCTS 9.3 1704 18 0.66

Table 5.4. Salt-fog corrosion test and tumble test results of multilayer coatings with a silicon- containing polymer bottom layer and two silicon nitride layers grown with varying NH3/HMDSO flow-rate ratios.

Tumble Test Results Layer Two Layer Three Thickness Lifetime Sum of Break (2500 W, 5 min) (2500 W, 5 min) (mm) (hours) Number Areas Of Breaks (´10-2 cm2)

100 sccm NH3 / 100 400 sccm NH3 / 100 sccm HMDSO sccm HMDSO 9.1 2400 9 0.5

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Table 5.5. Contact angle analysis of multilayer coatings with a silicon-containing polymer bottom layer, and either a silicon dioxide layer or a silicon nitride top layer, for varying O2/OMCTS and

NH3/HMDSO flow-rate ratios.

O2/OMCTS NH3/HMDSO Contact Angle ( o ) Engergy (mJ/m2) for Silicon Dioxide for Silicon Nitride Layer Layer Water Metlylend Dispersive Polar Total (2000 W, 10 min) (2500 W, 10 min) 2 92.0 60.3 27.0 5.9 32.9

8 80.7 55.6 28.6 10.5 39.1

1 92.7 56.3 29.2 5.7 34.9

4 83.6 50.6 31.1 8.5 39.6

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9 0

?: 0 hours 8 -10 à: 24 hours ? : 48 hours 7 *: 96 hours -20

6 -30

5 -40

4 -50

3 -60 Phase (Degree) Log Modulus (Ohm)

2 -70

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz) Fig. 5.1. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer

grown from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, and a silicon dioxide top layer

grown from OMCT and O2 at 2000 W, 400 sccm O2, 50 sccm OMCT, and 10 min.

100

9 0

?: 0 hours 8 à: 24 hours -10 ? : 48 hours *: 96 hours

7 -20

6 -30

5 -40

4 -50 Phase (Degree) Log Modulus (Ohm) 3 -60

2 -70

1 -80

0 -90 -2 0 2 4 6

Log Freq (Hz)

Fig. 5.2. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer

grown from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, and a silicon nitride top layer

grown from HMDSO and NH3 at 2500 W, 400 sccm NH3, 100 sccm HMDSO, and 10 min.

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9 0

?: 0 hours 8 -10 à: 24 hours ? : 48 hours 7 *: 96 hours -20

6 -30

5 -40

4 -50

3 -60 Phase (Degree) Log Modulus (Ohm)

2 -70

1 -80

0 -90 -2 -1 0 1 2 3 4 5 6

Log Freq (Hz) Fig. 5.3. EIS analysis of the multilayer coating with a silicon-containing polymer bottom layer

grown from 100 % OMCT at 2000 W, 100 sccm OMCT, and 10 min, a silicon dioxide middle layer

grown from OMCT and O2 at 2000 W, 100 sccm O2, 50 sccm OMCT, and 5 min, and a silicon

dioxide top layer grown from OMCT and O2 at 2000 W, 400 sccm O2, 50 sccm OMCT, and 5 min.

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Chapter 6

Deposition of Titanium Nitride, Zirconium Nitride, and Chromium Nitride

Monolayer and Multilayer Coatings

6.1 Deposition Precursors

Titanium nitride (TiN), zirconium nitride (ZrN), and chromium nitride (CrN) thin films have high hardness and high corrosion resistance, and are good protective coatings. Titanium nitride and zirconium nitride have gold coloring, and therefore are often used as decorative coatings. Titanium

(IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide and zirconium t-butoxide), and bis(ethylbenzene)chromium were used as the deposition precursors to grow titanium nitride, zirconium nitride, and chromium nitride coatings by PECVD with ammonia

131 (NH3) as the reactive gas, respectively. Table 6.1 gives their chemical formulas, molecular weights, vapor pressures, boiling and melting points, and the health-flammability-reactivity hazard ratings. These precursors have relatively low hazard ratings compared to other organometallic precursors. However, these organometallic precursors are all liquid compounds with very low vapor pressure, even at temperatures of above 100 °C; their introduction into the processing chamber is thus difficult. To solve this problem, a direct liquid injection (DLI) system was developed to deliver liquid organometallic precursors into the processing chamber by vaporizing them at temperatures up to 200 °C. 132

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Table 6.1. Properties of titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium

2-methyl-2-butoxide and zirconium t-butoxide), and bis(ethylbenzene)chromium, where x ranges from 0 to 4.

Vapor Molecular Boiling Melting Precursor Chemical Formula Pressure HMIS Weight Point Point (Pa) 0.12 (0.9 Titanium 58 °C Ti(OCH(CH ) ) 284.25 mTorr) 15-19 0C 2-3-1 Isopropoxide 3 2 4 at 1 mm at 50 0C Tetrakis(Dimethyl- 50 0C Ti(N(CH ) ) 224.20 NA NA 3-3-2 amino)Titanium 3 2 4 at 0.5 mm

Zirconium 2- 138 0C Zr(C H (CH ) CO) 436.79 NA NA 2-1-1 Methyl-2-Butoxide 2 5 3 2 4 at 5 mm

Zirconium T- 90 0C Zr(OC H ) 383.68 NA NA 2-3-1 Butoxide 4 9 4 at 5mm

Bis(Ethylbenzene) 140-146 0C Cr((C H ) C H ) NA NA NA 2-2-2 Chromium 2 5 x 6 6-x 2 at 1 mm

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6.2 Experimental Condition

The temperatures of the vaporizer were set at 150 oC for titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, 200 oC for zirconium 2-methyl-2-butoxide and zirconium t- butoxide, and 180 oC for bis(ethylbenzene)chromium. The flow rates of the vapor precursor gases were fixed at 5 sccm and the microwave power was fixed at 2500 W for all depositions; the flow rate of ammonia (NH3) was 80 sccm or 250 sccm. The deposition pressure ranged from 0.67 Pa (5 mTorr) to 1.20 Pa (9 mTorr) and the substrate temperatures during deposition were approximately

400 oC. Figure 6.1 shows an example of substrate temperatures as a function of deposition time with titanium (IV) isopropoxide and zirconium 2-methyl-2-butoxide as precursors, microwave power of

2500 W, and NH3 flow rate of 250 sccm. The substrate temperatures rise sharply and then reach a saturation value of about 400 °C.

6.3 Results and Discussion

Tables 6.2, 6.3, and 6.4 summarize the characterization of TiC xOyNz, ZrCxOyNz, and CrCxOyNz thin films grown on 5 cm diameter <100> silicon wafers and 5 ´ 5 cm2 stainless steel substrates by

PECVD. The deposition time was 10 min for all films. Film thicknesses and hardnesses were measured for the films grown on silicon while atomic concentrations and color indexes were measured for the films grown on stainless steel. The deposition growth rates were 14.6 to 16.4 nm/min for titanium nitride, 10.1 to 12.5 nm/min for zirconium nitride, and 18.9 to 20.4 nm/min chromium nitride.

Film compositions were measured with a Physical Electronics 5700LSci ESCA spectrometer using a monochromatic aluminum x-ray source operating at 350 W (Evans East). The XPS results show

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that films are TiC xOyNz, ZrCxOyNz, and CrCxOyNz with significant carbon and oxygen. The atomic concentration of nitrogen ranges from 7.3 to 37.9 % in TiC xOyNz, 9.6 to 31.3 % in ZrCxOyNz, and

10.7 to 15.3 % in CrCxOyNz. Increasing NH3 flow rates increase the atomic concentration of nitrogen in the films. High flow rates of NH3 provide a more abundant nitrogen atom source, resulting in films with more incorporated nitrogen.

The hardness of deposited films ranges from 19.8 to 28.0 GPa for TiC xOyNz, 16.9 to 20.9 GPa for

ZrCxOyNz, and 25.2 to 30.5 GPa for CrCxOyNz. The films with the maximum hardnesses contain the maximum nitrogen atomic concentration of 37.9 % in TiC xOyNz, 31.3 % in ZrCxOyNz, and 15.3 % in CrCxOyNz. The hardness values of these films are comparable to those reported for metal nitride deposited by thermal CVD 150-153.

Figures 6.2-6.4 show the hardness of titanium nitride, zirconium nitride, and chromium nitride thin films, and the hardness of a bare silicon wafer, as a function of displacement into the surface. The titanium nitride, zirconium nitride, and chromium nitride thin films were grown from titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide and zirconium t-butoxide, and bis(ethylbenzene)chromium at microwave power of 2500 W and NH3 flow rate of

250 sccm.

The color of deposited titanium nitride and zirconium nitride films was measured with a Minolta spectrophotometer. Table 6.5 shows the measured L*, a*, and b* values of four films grown on 5 ´ 5 cm2 stainless steel substrates from titanium (IV) isopropoxide, tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide, and zirconium t-butoxide at a microwave power of 2500 W and

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NH3 flow rate of 250 sccm. Table 6.5 also shows values for stoichiometric titanium nitride and zirconium nitride and 24 Karat gold. The L*, a*, and b* values of deposited titanium nitride and zirconium nitride films are fairly consistent with those reported for titanium nitride and zirconium nitride films deposited by PVD 147.

Figures 6.5, 6.6, and 6.7 show scanning electron micrographs of the cross section of titanium nitride, zirconium nitride, and chromium nitride films grown on 5 cm diameter <100> silicon wafers with tetrakis(dimethylamino)titanium, zirconium t-butoxide, and bis(ethylbenzene)chromium at a microwave power of 2500 W and NH3 flow rate of 250 sccm. The deposition time for the titanium nitride and zirconium nitride films was 10 min while the deposition time for the chromium nitride films was 30 min. The scanning electron micrographs show that all films have columnar microstructures.

The ASTM B117 salt-fog corrosion test and tumble test were used to evaluate titanium nitride and zirconium nitride monolayer and multilayer coatings for corrosion and abrasion. Table 6.6 shows the salt-fog corrosion test and tumble test results of TiC xOyNz and ZrCxOyNz monolayer films grown from titanium (IV) isopropoxide and zirconium 2-methyl-2-butoxide at a microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min. The TiC xOyNz sample lasted

1020 hours in the ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 8 breaks and 0.41 ´ 10-2 cm2 sum of broken areas over a

2.54 cm ´ 2.54 cm square area on the coating in the tumble test. The ZrCxOyNz sample lasted 1020 hours in the ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated

107

surface of brass substrates, and had 24 breaks and 1.26 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test.

Table 6.7 summarizes the deposition conditions and salt-fog corrosion test and tumble test results of three multilayer thin films with a TiC xOyNz bottom layer and a silicon dioxide top layer, or a polymer bottom layer and a TiC xOyNz top layer. The two samples with a TiC xOyNz bottom layer and a silicon dioxide top layer were grown at 2500 W, 250 sccm NH3, and 10 min for the TiC xOyNz bottom layer and 100 or 200 sccm O2, 50 sccm OMCT, 2000 W, and 10 min for the silicon dioxide top layer. The samples lasted 816 and 744 hours in the ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 53 and 25 breaks and

2.71 ´ 10-2 cm2 and 0.99 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test. The sample with a polymer bottom layer and a TiC xOyNz top layer was grown at 2000 W, 100 sccm OMCT, and 10 min for the polymer bottom layer and 2500 W, 250 sccm NH3, and 10 min for the TiC xOyNz top layer. The samples lasted 1104 hours in the ASTM

B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 10 breaks and 0.67 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test.

Table 6.8 summarizes the deposition conditions and salt-fog corrosion test and tumble test results of three multilayer thin films with a ZrCxOyNz bottom layer and a silicon dioxide top layer, or a polymer bottom layer and a ZrCxOyNz top layer. The two samples with a ZrCxOyNz bottom layer and a silicon dioxide top layer were grown at 2500 W, 250 sccm NH3, and 10 min for the ZrCxOyNz bottom layer and 100 or 200 sccm O2, 50 sccm OMCT, 2000 W, and 10 min for the silicon dioxide

108

top layer. The samples lasted 696 and 624 hours in the ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 57 and 31 breaks and

3.29 ´ 10-2 cm2 and 1.40 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test. The sample with a polymer bottom layer and a TiC xOyNz top layer was grown at 2000 W, 100 sccm OMCT, and 10 min for the polymer bottom layer and 2500 W, 250 sccm NH3, and 10 min for the TiC xOyNz top layer. The sample lasted 1056 hours in the ASTM

B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 17 breaks and 0.67 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test.

Multilayer thin films with a TiC xOyNz or ZrCxOyNz bottom layer, a polymer middle layer, and a silicon dioxide top layer were grown for salt-fog corrosion and tumble testing. Table 6.9 summarizes the deposition conditions and test results. The TiC xOyNz and ZrCxOyNz bottom layers were grown at 2500 W, 250 sccm NH3, and 10 min from titanium (IV) isopropoxide and zirconium

2-methyl-2-butoxide, respectively; the polymer middle layers were grown at 2000 W, 100 sccm

OMCTS, and 5 min; the silicon dioxide top layers were grown at 2000 W, 200 sccm O2, 50 sccm

OMCT, and 5 min. The sample with a TiC xOyNz bottom layer lasted 960 hours in the ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 15 breaks and 0.80 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test. The sample with a ZrCxOyNz bottom layer lasted 936 hours in the

ASTM B117 salt-fog corrosion test before visible corrosion appeared on the coated surface of brass substrates, and had 23 breaks and 1.15 ´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble test.

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6. 4 Summary

TiC xOyNz, ZrCxOyNz, and CrCxOyNz thin films have been grown by PECVD in a high- density microwave ECR discharge, using titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide and zirconium t-butoxide, and bis(ethylbenzene)chromium as the deposition precursors and NH3 as the reactive gas. The PECVD metal nitride thin films are hard and have nano-indentation hardness values of up to 28 GPa for

TiC xOyNz, 20.9 GPa for ZrCxOyNz, and 30.5 GPa for CrCxOyNz. Up to 37.9 %, 31.3 %, and 15.3 % atomic concentrations of nitrogen were achieved in TiC xOyNz, ZrCxOyNz, and CrCxOyNz films, respectively. Increasing NH3 flow rates increased the atomic concentrations of nitrogen and the hardness of coatings. The PECVD films of TiC xOyNz and ZrCxOyNz have characteristic gold coloring comparable to those of PVD titanium nitride and zirconium nitride films or even stoichiometric titanium nitride and zirconium nitride, lasted 1020 hours in an ASTM B117 salt-fog corrosion test without color change or visible corrosion. The TiC xOyNz films had 8 breaks and 1.26

´ 10-2 cm2 sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating in the tumble

-2 2 test while the ZrCxOyNz films had 24 breaks and 0.61 ´ 10 cm sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the coating. Multilayer thin films with a titanium nitride or zirconium nitride layer and additional transparent polymer layers and (or) silicon dioxide layers have been grown for the salt-fog corrosion test and tumble test. The lifetime of the multilayer thin films in the

ASTM B117 salt-fog corrosion test ranged from 624 hours to 1104 hours; the breaks and the sum of broken areas over a 2.54 cm ´ 2.54 cm square area on the multilayer coatings in the tumble test ranged from 10 to 57 and 0.67 ´ 10-2 cm2 to 3.29 ´ 10-2 cm2, respectively. The multilayer thin films have gold coloring as titanium nitride and zirconium nitride monolayer thin films.

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Table 6.2. Growth rates, hardness, and atomic concentrations of deposited TiC xOyNz films grown from titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium at microwave power of 2500

W and deposition time of 10 min for varying flow rates of NH3.

NH Growth Rate Hardness Atomic Concentration (%) Precursor 3 (sccm) (nm/min) (GPa) Ti C O N

Ti(OCH(CH3)2)4 80 16.4 19.8 27.1 17.2 48.4 7.3

Ti(OCH(CH3)2)4 250 15.2 25.2 45.8 5.3 14.5 34.4

Ti(N(CH3)2)4 80 15.7 20.0 27.0 13.9 50.4 8.7

Ti(N(CH3)2)4 250 14.6 28.0 47.6 4.5 10.0 37.9

Table 6.3. Growth rates, hardness, and atomic concentrations of deposited ZrCxOyNz films from zirconium 2-methyl-2-butoxide and zirconium t-butoxide at microwave power of 2500 W and deposition time of 10 min for varying flow rates of NH3.

NH3 Growth Rate Hardness Atomic Concentration (%) Precursor (sccm) (nm/min) (GPa) Zr C O N

Zr(C2H5(CH3)2CO)4 80 12.5 16.9 41.5 2.1 46.8 9.6

Zr(C2H5(CH3)2CO)4 250 11.1 19.3 45.8 1.1 24.3 28.8

Zr(OC4H9)4 80 11.4 17.3 42.9 3.8 37.8 15.5

Zr(OC4H9)4 250 10.1 20.9 44.7 2.0 22.0 31.3

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Table 6.4. Growth rates, hardness, and atomic concentrations of deposited CrCxOyNz films from bis(ethylbenzene)chromium at microwave power of 2500 W and deposition time of 10 min for varying flow rates of NH3.

Atomic Concentration (%) NH3 Growth Rate Hardness (sccm) (nm/min) (GPa) Cr C O N

80 20.4 25.2 30.2 25.9 33.2 10.7

250 18.9 30.5 32.5 21.1 31.1 15.3

Table 6.5. Color coordinates of four TiC xOyNz and TiC xOyNz films grown from titanium (IV) isopropoxide, tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide, and zirconium t- butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min and color coordinates of TiN, ZrN, and 24 Karat gold from Ref. 152.

Stoichiometry L* a* b*

Ti(OCH(CH3)2)4 TiC 0.10O0.27N0.63 61.1 4.2 26.6

Ti(N(CH3)2)4 TiC 0.09O0.19N0.72 74.2 3.4 35.7

Zr(C2H5(CH3)2CO)4 ZrC0.02O0.45N0.53 60.9 3.9 18.1

Zr(OC4H9)4 ZrC0.04O0.40N0.57 71.2 1.0 19.8

Ref. 152 TiN 77-78 2 – 5 33-37

Ref. 152 ZrN 86 – 89 -3 - -1 23-25

Ref. 152 Gold 88 – 91 -3.7 – 1 27-34

112

Table 6.6. Salt-fog corrosion test and tumble test results of TiC xOyNz and ZrCxOyNz films grown from titanium (IV) isopropoxide and zirconium 2-methyl-2-butoxide at the microwave power of

2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min.

Tumble Test Results Lifetime Sum of Break Number Of (hours) Areas Breaks (´10-2cm2)

Ti(OCH(CH3)2)4 1020 8 0.41

Zr(C2H5(CH3)2CO)4 1020 24 1.26

Table 6.7. Salt-fog corrosion test and tumble test results of multilayer coatings with a TiC xOyNz layer, grown from titanium (IV) isopropoxide at the microwave power of 2500 W, NH3 flow rate of

250 sccm, and for 10 min, and a silicon dioxide layer.

Tumble Test Results Lifetime Sum of Break Layer One Layer Two Number (hours) Areas Of Breaks (´10-2cm2) 100 sccm O , 50 sccm Ti(OCH(CH ) ) 2 816 53 2.71 3 2 4 OMCT, 2000 W, 10 min

200 sccm O2, 50 sccm Ti(OCH(CH3)2)4 OMCT, 2000 W, 10 min 744 25 0.99 100 sccm OMCTS, Ti(OCH(CH ) ) 1104 10 0.67 2000 W, 10 min 3 2 4

113

Table 6.8. Salt-fog corrosion test and tumble test results of multilayer coatings with a ZrCxOyNz layer, grown from zirconium 2-methyl-2-butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and for 10 min, and a silicon dioxide layer.

Tumble Test Results Lifetime Sum of Break Layer One Layer Two Number (hours) Areas Of Breaks (´10-2cm2) 100 sccm O , 50 sccm Zr(C H (CH ) CO) 2 696 57 3.29 2 5 3 2 4 OMCT, 2000 W, 10 min 200 sccm O , 50 sccm Zr(C H (CH ) CO) 2 624 31 1.39 2 5 3 2 4 OMCT, 2000 W, 10 min 100 sccm OMCTS, Zr(C H (CH ) CO) 1056 17 1.41 2000 W, 10 min 2 5 3 2 4

Table 6.9. Salt-fog corrosion test and tumble test results of multilayer coatings with a TiC xOyNz or

ZrCxOyNz bottom layer, grown from titanium (IV) isopropoxide or zirconium 2-methyl-2-butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and for 10 min, a silicon- containing polymer middle layer, and a silicon dioxide top layer.

Tumble Test Results Lifetime Sum of Break Layer One Layer Two Layer Three Number (hours) Areas Of Breaks (´10-2cm2) 200 sccm O , 100 sccm 2 50 sccm Ti(OCH(CH ) ) OMCTS, 960 15 0.78 3 2 4 OMCT, 2000 W, 5 min 2000 W, 5 min 200 sccm O , 100 sccm 2 50 sccm Zr(C H (CH ) CO) OMCTS, 936 23 1.15 2 5 3 2 4 OMCT, 2000 W, 5 min 2000 W, 5 min

114

C) o

400

300

200 : titanium (IV) isopropoxide 100 : zirconium 2-methyl-2-butoxide

Substrate Temperature ( 0 0 5 10 15 20 Time (min)

Fig. 6.1. Substrate temperatures as a function of deposition time with titanium (IV) isopropoxide

and zirconium 2-methyl-2-butoxide as precursors, microwave power of 2500 W, and NH3 flow rate

of 250 sccm.

35 30 25 20 15

10 ¦ : tetrakis(dimethethlyamino)titanium Hardness (GPa) 5 ¨: titanium (IV) isopropoxide 0 0 10 20 30 40 50 60 70 80 90 100 Displacement Into Surface (nm)

Fig. 6.2. Hardness of the titanium nitride thin films grown from titanium (IV) isopropoxide and

tetrakis(dimethylamino)titanium at microwave power of 2500 W and NH3 flow rate of 250 sccm as

a function of displacement into surface.

115

35 30 25 20 15 10 Hardness (GPa) ¦ : zirconium t-butoxide 5 ¨: zirconium 2-methyl-2-butoxide 0 0 10 20 30 40 50 60 70 80 90 100 Displacement Into Surface (nm)

Fig. 6.3. Hardness of the zirconium niride-like thin films grown from zirconium 2-methyl-2- butoxide and zirconium t-butoxide at microwave power of 2500 W and NH3 flow rate of 250 sccm as a function of displacement into surface.

35 30 25 20 15 10 Hardness (GPa) ¦ : bis(ethylbenzene)chromium 5 ¨: Si wafer 0 0 10 20 30 40 50 60 70 80 90 100 Displacement Into Surface (nm)

Fig. 6.4. Hardness of the chromium nitride thin film grown from bis(ethylbenzene)chromium at microwave power of 2500 W and NH3 flow rate of 250 sccm and p-type <100> silicon wafer as a function of displacement into surface.

116

Fig. 6.5. Scanning electron micrograph of the cross section of the TiC xOyNz film grown on <100> silicon wafer from tetrakis(dimethylamino)titanium at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min.

117

Fig. 6.6. Scanning electron micrograph of the cross section of the ZrCxOyNz film grown on <100> silicon wafer from zirconium t-butoxide at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min.

118

Fig. 6.7. Scanning electron micrograph of the cross section of the CrCxOyNz film grown on <100> silicon wafer from bis(ethylbenzene)chromium at the microwave power of 2500 W, NH3 flow rate of 250 sccm, and deposition time of 10 min.

119

Chapter 7

Deposition of Germanium Coatings

7.1 Deposition Precursors

Germanium thin films are often grown by thermal chemical vapor deposition (CVD). Germane

(GeH4), germanium tetrafluoride (GeF4), and germanium tetrachloride (GeCl4) are often used as the deposition precursors. These precursors are highly toxic and explosive and there is a demand for less-hazardous alternatives. In this thesis research, tetramethylgermane (TMG) was used as the precursor to grow germanium films in a high-density argon plasma.133 Argon plasma has high ion bombardment, which enhanced the dissociation of deposition precursor without chemical reaction with germanium. Table 7.1 gives the composition, molecular weight, vapor pressure, boiling and melting point, and the health-flammability-reactivity hazard rating of TMG, and Figure 7.1 shows the chemical structure.

Table 7.1. Properties of TMG, where HMIS represents Hazardous Material Information System with Healthy-Flammability-Reactivity while the level of each index ranges from 0 to 4.

Chemical Molecular Boiling Melting Precursor Vapor Pressure HMIS Formula Weight Point Point

5.0 ´104 Pa (375 TMG (CH ) Ge 132.75 43.4 °C -88 °C 2-4-0 3 4 Torr) @ 27°C

120

CH3

CH3 Ge CH3

CH3

Fig. 7.1 The chemical structure of TMG.

7.2 Experimental Conditions

TMG and Ar were metered by mass flow controllers with the TMG and Ar flow rates fixed at 15 sccm and 100 sccm, respectively. Other conditions were pressure, 0.67 Pa (5 mTorr); microwave power, 1500 W; and deposition time, 20 min.

7.3 Results and Discussion

The deposited germanium films were uniform and had mirror-like specularly reflecting surfaces. Figure 7.2 shows an atomic force microscopy of the film over 50 ´ 50 mm2 with the root mean square (RMS) surface roughness of 3.76 nm. Deposition growth rates were 125 nm/min, found by measuring the film thicknesses with a diamond stylus Tencor P-10 surface profiler and dividing the thickness by the deposition time. Tale 7.2 gives the growth rates and composition of the germanium films. Film composition, measured with a Physical Electronics 5700LSci ESCA spectrometer (Evans East), was 97.2% Ge, 1.6% O, 0.4% C, and 0.8% Si. The 0.8% silicon presumably came from contamination on the process chamber walls, because silicon-containing precursors had been used previously. The low carbon percentage (0.4%) indicates that the TMG

121

was decomposed by the plasma, with mainly germanium deposited on the substrates and methyl groups pumped away. Table 7.3 gives Hall effect hole mobility and Van Der Pauw sheet resistance values for three measurement temperatures. The mobility and sheet resistance at 300 K compare favorably with pre-anneal values obtained by others using an electron beam growth method.154

Figures 7.3 and 7.4 show scanning electron micrographs of the surface and cross section of a deposited germanium film on p-type <100> silicon. The surface of deposited germanium film consists of uniform nano-scale asperities. This is consistent with the surfaces of germanium thin films grown by surfactant mediated epitaxy (SME) 155, 156. Figure 7.5 shows X-ray diffraction spectrum of the deposited germanium, and it shows that the film has the crystal structure of <220>.

7.4 Summary

Nearly-pure germanium thin films with specularly reflecting surfaces can be grown by PECVD from tetramethylgermane in a high-density argon plasma, at a substrate temperature of 250°C and growth rate of 125 nm/min.

122

Table 7.2. Growth rates, the root mean square (RMS) surface roughness over 50 ´ 50 mm2, and atomic concentrations of deposited coatings.

TMG Ar Power Growth rate surface roughness (RMS) Atomic concentration (%) (sccm) (sccm) (W) (nm/min) (nm) Ge C O Si

15 100 1500 125 3.76 97.2 0.4 1.6 0.8

Table 7.3. Hole mobility and sheet resistance of the deposited germanium film at various temperatures.

Temperature Mobility Sheet Resistance (K) (cm2/Vs) (kW/cm2) 145 3.85 111.64

185 4.85 69.53 300 6.33 21.44

123

Fig. 7.2. AFM micrograph of the surface of the germanium film grown on silicon wafer.

124

Fig. 7.3a. Scanning electron micrograph of the surface of the germanium film grown on silicon wafer.

125

Fig. 7.3b. Scanning electron micrograph of the surface of the germanium film grown on silicon wafer.

126

Fig. 7.3c. Scanning electron micrograph of the surface of the germanium film grown on silicon wafer.

127

Fig. 7.4. Scanning electron micrograph of the cross section of the germanium film grown on silicon wafer.

Ge <220>

Intensity (a.u.)

25 30 35 40 45 50 55 60 2q (degree)

Fig. 7.5. X-ray diffraction (XRD) spectrum of the germanium film grown on silicon wafer.

128

Chapter 8

SUMMARY

8.1 Conclusions

Silicon dioxide, silicon-containing polymer, silicon nitride, titanium nitride, zirconium nitride, chromium nitride, germanium, and multilayer thin films were grown from environmentally benign organosilicon and organometallic precursors by electron cyclotron resonance (ECR) microwave plasma enhanced chemical vapor deposition (PECVD). The objectives of high deposition growth rates and low deposition substrate temperature have been achieved in thin film deposition. Table 8.1 summarizes the results of monolayer and multilayer thin films grown by PECVD. The deposited silicon dioxide was hard, transparent, and colorless; the silicon-containing polymer was transparent and colorless, and had high corrosion resistance in the salt-fog test; the silicon nitride had high elastic modulus and hardness. Soft but adherent silicon-containing, polymer-like undercoatings can be combined with harder silicon dioxide or silicon nitride-like top layers to achieve better performance than monolayer coatings alone. The silicon dioxide and silicon nitride coatings have high hardness, but usually have poor adhesion to the substrate and short lifetime in the salt fog test.

The silicon-containing polymer layers have long lifetime in the salt fog test (up to 3185 hours), but have low surface hardness values (~1 GPa). The multilayer structures, however, have high surface hardness values (8 GPa to 12 GPa) and have much better corrosion resistance than silicon dioxide or silicon nitride monolayer, lasting 1700 to 2600 hours in the salt fog test. These multilayer structures could be good for protective and functional coatings application. The deposited titanium nitride, zirconium nitride, and chromium nitride had excellent hardness while the titanium nitride and zirconium nitride had additional characteristic of gold coloring. These metal nitride thin films could be used as protective and decorative coatings. The titanium nitride-containing or zirconium

129

nitride-containing multilayer coatings had good thickness and gold coloring, could have better performance than titanium nitride or zirconium nitride monolayers. The deposited germanium film had 97 % Ge with a specularly reflecting surface, and the film had the crystal structure of <220>.

Table 8.1. Results of monolayer and multilayer thin films grown by PECVD.

Film Results

Silicon dioxide Growth rates of films: 0.56 – 1.08 µm/min; hardness of films: 1.4 – 6.0

grown from GPa; carbon percentage concentration of films: 36.4 – 12.4 %; color:

HMDSO with O2 colorless and transparent.

Silicon dioxide Growth rates of films: 0.32 – 0.76 µm/min; hardness of films: 1.8 – 7.4

grown from GPa; carbon percentage concentration of films: 30.8 – 3.4 %; color:

OMCTS with O2 colorless and transparent.

Silicon dioxide Growth rates of films: 0.60 – 0.96 µm/min; hardness of films: 3.2 – 8.7

grown from GPa; carbon percentage concentration of films: 17.6 – 3.4 %; color:

TMCTS with O2 colorless and transparent.

Growth rates of films: 0.18 – 0.53 µm/min; hardness of films: 0.9 – 12.2 Silicon nitride GPa; carbon percentage concentration of films: 35.2 – 13.1 %; nitride grown from percentage concentration of films: 4.3 – 13.7 %; lifetime in ASTM B117 HMDSO with NH3 salt-fog tests: 744 - 840 hours; color: colorless and transparent.

Silicon nitride Growth rates of films: 13.6 nm/min; hardness of films: 14.0 GPa; carbon

grown from TMS percentage concentration of films: 7.2 %; nitride percentage concentration

with NH3 of films: 37.6 %; color: black.

Silicon-containing Growth rates of films: 0.06 – 0.59 µm/min; hardness of films: 0.6 – 1.5

130

polymer coatings GPa; number of breaks and sum of break areas in tumble tests: 34 – 1158 grown from 100 % and 1.18 ´10-2 – 357.50 ´10-2 cm2; lifetime in ASTM B117 salt-fog tests: 23

OMCTS - 3185 hours; color: colorless and transparent.

Thickness of films: 8.1 – 9.6 µm; number of breaks and sum of break areas Silicon-containing in tumble tests: 9 – 40 and 0.50 ´10-2 – 1.63 ´10-2 cm2; lifetime in a ASTM multilayer coatings B117 salt-fog tests: 1704 - 2616 hours; color: colorless and transparent.

Growth rates of films: 14.6 – 16.4 nm/min; hardness of films: 19.8 – 28.0

GPa; carbon percentage concentration of films: 17.2 – 4.5 %; nitride

Titanium nitride percentage concentration of films: 7.3 – 37.9 %; number of breaks and sum

of break areas in tumble tests: 8 and 0.4 ´10-2 cm2; lifetime in ASTM B117

salt-fog tests: 1020 hours; color: gold coloring.

Growth rates of films: 10.1 – 12.5 nm/min; hardness of films: 16.9 – 20.9

GPa; carbon percentage concentration of films: 3.8 – 1.1 %; nitride

Zirconium nitride percentage concentration of films: 9.6 – 31.3 %; number of breaks and sum

of break areas in tumble tests: 24 and 1.26 ´10-2 cm2; lifetime in ASTM

B117 salt-fog tests: 1020 hours; color: gold coloring.

Growth rates of films: 18.9 -20.4 nm/min; hardness of films: 25.2 – 30.5

Chromium nitride GPa; carbon percentage concentration of films: 25.9 – 21.1 %; nitride

percentage concentration of films: 10.7 – 15.3 %; color: gray.

Number of breaks and sum of break areas in tumble tests: 10 – 57 and 0.67 TiN and ZrN ´10-2 – 3.29 ´10-2 cm2; lifetime in ASTM B117 salt-fog tests: 624 – 1104 multilayer coatings hours; color: gold coloring.

131

Growth rates of films: 0.125 µm/min; germanium percentage concentration

of films: 97.2 %; carbon percentage concentration of films: 0.4 %; oxygen

percentage concentration of films: 1.6 %; surface roughness (RMS): 3.76 Germanium grown nm over 50 µm ´ 50 µm; the mobility and sheet resistance: 3.85 cm2/Vs and from TMG 111.64 kW/cm2 at 145 K, 4.85 cm2/Vs and 69.53 kW/cm2 at 185 K, 6.33

cm2/Vs and 21.44 kW/cm2 at 300 K; XRD analysis: polycrystalline; color:

black.

8.2 Recommendations for Future Work

Higher deposition growth rates and lower deposition substrate temperature are always the objective of PECVD and worth further pursuing. The microwave power, precursor and active gas flow, and deposition pressure can be further varied to explore their effects on deposition growth rates and deposition substrate temperature.

Multilayer structures with more layers from polymer to hard coating are interesting, and could have better performance.

Thin film deposition on plastic substrates has been attracting extensive attention and research interest. Multilayer coating growth on plastic substrates by PECVD is a good research topic. The coatings could be used as encapsulation or barrier coating application.

132

Pulsing plasma is a promising research. It can lower deposition temperature and make complex plasma chemistry. Pulse-modified PECVD could be used for plasma polymerization and deposition of polymer coatings.

Direct liquid injection (DLI) system is good for delivering liquid precursors. We used it to deliver organometallic precursors in this dissertation research. It is also good for delivering organosilicon precursors for silicon-containing thin film deposition and could produce better thin films.

High oxygen percentage always exists in the PECVD-grown metal nitride films. Hydrogen can be used to decrease the oxygen concentration in metal nitride deposition by PECVD. Multilayer structures with more layers can be studied and grown with PECVD for metal nitride-containing multilayer coatings for better performance.

Thin film devices on plastic is currently of interest. Germanium is a good device material. PECVD can deposit germanium on plastic substrates because of low deposition substrate temperature. The research on PECVD-grown germanium could make important progress in the study of thin film devices on plastic.

133

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Appendices:

X-ray photoelectron spectroscopy (XPS) of the silicon dioxide thin film grown from HMDSO

144

X-ray photoelectron spectroscopy (XPS) of the silicon dioxide thin film grown from OMCTS

145

X-ray photoelectron spectroscopy (XPS) of the silicon dioxide thin film grown from TMCTS

146

X-ray photoelectron spectroscopy (XPS) of the silicon nitride thin film grown from HMDSO

147

4 x 10 030059100.spe 9

8

-N1s

7

6

5 -Si2s

-O1s c/s -Si2p 4

3

2 -N KLL -C1s -Ar2p -O KLL -Ar2s -C KLL -O KLL 1 -Ar LMM

0 1200 1000 800 600 400 200 Binding Energy (eV)

X-ray photoelectron spectroscopy (XPS) of the silicon nitride thin film grown from TMS

148

X-ray photoelectron spectroscopy (XPS) of the titanium nitride thin film grown from tetrakis(dimethylamino)titanium

149

4 022395130.spe x 10 15

-Zr3d

-Zr3p1

10 -Zr3p3

-N1s c/s -O1s

-Zr3s 5

-Cr2p3 -Ar2p -O KLL -N KLL -Fe2p3 -Ar2s -C1s -Zr4p

-Si2s -Si2p -Zr4s

0 1200 1000 800 600 400 200 Binding Energy (eV)

X-ray photoelectron spectroscopy (XPS) of the zirconium nitride thin film grown from zirconium 2-methyl-2-butoxide

150

4 x 10 022395130.spe 15

-Zr3d

-Zr3p1

10 -Zr3p3

-N1s

c/s -O1s

-Zr3s 5

-Cr2p3 -Ar2p -O KLL

-N KLL -Fe2p3 -Ar2s -C1s

-Zr4p

-Si2s -Si2p -Zr4s

0 1200 1000 800 600 400 200

Binding Energy (eV)

X-ray photoelectron spectroscopy (XPS) of the zirconium nitride thin film grown from zirconium t-butoxide

151

X-ray photoelectron spectroscopy (XPS) of the chromium nitride thin film grown from bis(ethylbenzene)chromium

152

5 x 10 030059130.spe 7

6 -Ge2p3

5

-Ge2p1 4

-Ge LMM c/s 3

-Ge LMM -Ge LMM 2 -Ge LMM -Ge LMM -Ge LMM

-Ar2s -Ge3p 1 -Ge3d -Ge3s

-Ar2p

0 1200 1000 800 600 400 200 Binding Energy (eV) X-ray photoelectron spectroscopy (XPS) of the germanium thin film grown from tetramethylgermane

153