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LfHENIICAL VAPOR DEPOSITION OF SILICON DIOXIDE THIN FILMS FOR COMPOSITE THEAAIO-OXIDATIVE DURABILIT);
A dissertation presented to the Faculty of the College of Engineering and Technology Ohio university
in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Chemical Engineering
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Sudarsan Neogi/ November, 1992
'OHIOUNIVERSITY LIBRARY ACKNOWLEDGEMENT
It was a grand experience to finish such voyage. But this pilgrimage could not have been possible without the indispensable and generous help and advice from my teachers and friends.
At the outset I would like to say to my advisor Dr. Daniel A. Gulino, that I consider myself really fortunate and honored to work with you. In every conceivable way I feel a deep sense of gratitude to you for your help, advice, encouragement and inculcating a deep sense of commitment for this work.
Next I wish to thank my wife Swati for her enthusiasm and encouragement for my work.
I am also extremely thankful' ~o Dr. David Ingram for his generous help in
RBS analysis and other useful advice . .I
Last but not the least, let me mention the invaluable help and advice I did receive from my good friends Jim, Harvey and Tom while pursuing this work. PUBLICATIONS/PRESENTATIONS
1. S. Neogi and D. A. Gulino, "Chemical Vapor Deposition of Several Silicon-based Dielectric Films", Materials and Manufacturing Processes, Vol. 3, No.3, 433, 1991.
2. S. Neogi, D. C. Ingram and D. A. Gulino, "Improved Adhesion of Silicon Dioxide Thin Film to Polyimide Composites by Ion Implantation". HITEMP Review 1991 (Advanced High temperature Engine Materials Technology Program), NASA CP 10082, 1991, .t-\ 11-1 to A 11-10.
3. S. Neogi and D. A. Gulino, "High Temperature Oxidation Resistance of Si02 Coated Polyimide Composite", AIChE Journal, Vol. 38, No.9, 1379, 1992.
4. S. Neogi and D. A. Gulino, "Silicon Dioxide Thin Films Deposited by PECVD onto Polymeric Composites for High Temperature Oxidation Resistance", presented at the Third Topical Conference on Emerging Technologies in Materials, AIChE Annual Meeting, Los Angeles. CA, November 17-22, 1991.
5. S. Neogi and D. A. Gulino, "Die thylsilane as a Precursor for the Low Temperature
Deposition of Si02 by LPCVD onto Polyimide Composites", submitted for presentation at the AIChE Annual Meeting, to be held in Miami, FL, November 1-6, 1992. TABLE OF CONTENTS
LIST OF FIGURES iv
LIST OF TABLES vii
1.0 INTRODUCTION...... 1
2.0 LITERATURE REVIEW 11
2.1 Silicon Dioxide Films 11
2.2 eVD and Pyrolytic Silicon Dioxide 11
2.3 Plasma Deposited Silicon Dioxide 17
2.4 Surface Modification by Ion Implantation 19
2.5 Modeling of Low Pressure CVD Reactors 23
2.6 Modeling of Plasma Enhanced CVD Reactors 25
2.7 Oxidative degradation of Polyimide Composite in Space 27
2.8 Thermo-oxidative Degradation of Polyimide Composites 28
3.0 RESEARCH PLAN 30
4.0 EXPERIMENTS 33
4.1 Experiment 1: Deposition of the TEOS-Si02 Thin Film onto Silicon Substrate by LPCVD Technique 33
4.1.1 Materials 33
4.1.2 Analysis of the Films 36
4.1.3 Experimental Procedure 44
4.2 Experiment 2: Deposition of the TEOS-Si02 Thin Film onto Polyimide Composite Substrate by PECVD Technique 46 11
4.2.1 Materials 46
4.2.2 Analysis of the Films 50
4.2.3 Experimental Procedure 54
4.3 Experiment 3: Deposition of the DES-SiOz Thin Film onto Silicon Substrate by LPCVD Technique 56
4.3.1 Materials 56
4.3.2 Analysis of the Film 59
4.3.3 Experimental Procedure 60
4.4 Experiment 4: Ion Implantation of the Thin Films 62
4.4.1 Ion Implanter 62
4.4.2 Experimental Procedure 62
5.0 RESULTS AND DISCUSSION 65
5.1 LPCVD Process 65
5.1.1 Film Thickness 66
5.1.2 Film Composition 68
5.2 PECVD Process 73
5.2.1 Film Composition 80
5.2.2 Refractive Index 80
5.2.3 Film Density 84
5.2.4 Etch Test 85
5.2.5 Adhesion of the Films 85
5.2.6 Stress of the Films 88
5.2.7 SEM Micrographs 88 III
5.3. LPCVD Process from DES 91
5.4 Ion Implantation 93
5.5 Mass Loss Experiments 94
6.0 CONCLUSIONS AND RECOMMENDAnONS 101
REFERENCES 104
APPENDICES 108
APPENDIX I...... 108
APPENDIX II 118
APPENDIX III 123
APPENDIX IV 136 IV
LIST OF FIGURES
Figure 1.1. Normalized energy distribution curve for an electron and the required energy for dissociation and ionization 3
Figure 1.2. Plasma-material interactions 4
Figure 1.3. Displacement of an atoms at the interface as a result of collision with incoming ions 9
Figure 1.4. Interaction of the incoming ions with the of the electronic structure of the film and substrate 10
Figure 4.1. Reaction chamber for TEOS-Si02 deposition by LPCVD 34
Figure 4.2. Schematic diagram of the LPCYD experimental system 45
Figure 4.3. Reaction chamber for TEOS-Si02 deposition by PECVD 47
Figure 4.4. Structure of PMR-15 51
Figure 4.5. Schematic diagram of the PECVD experimental system 55
Figure 4.6. Reaction chamber for DES-Si02 deposition by LPCyD 57
Figure 4.7. Schematic diagram of the LPCYD experimental system from DES 61
Figure 4.8. Schematic diagram of Ohio University Van de Graff accelerator 63
Figure 5.1. Profile of a typical Si02 Film 67
Figure 5.2. Arrhenious plot for Si02 deposition from 69 TEOS (LPCVD). v
Figure 5.3. Plot of Deposition rate vs. pressure (LPCVD) 70
Figure 5.4a. Rutherford backscattering spectrum (RBS) of
a typical Si02 (LPCVD) film 71
Figure 5.4b. Proton recoil detection (PRD) spectrum of a typical SiOz (LPCVD) film 72
Figure 5.5. FTIR spectrum of a typical SiO, (LPCVD) film 74
Figure 5.6. Arrhenius plot from Si02 deposition from TEOS (PECYD) 76
Figure 5.7. Plot of deposition of rate vs. rf power (PECVD) 77
Figure 5.8. Plot of deposition rate vs. pressure (PECVD) 79
Figure 5.9a. Rutherford backscattering structure of a typical SiOz (PECVD) Film 81
Figure 5.9b. Proton recoil detection structure of a
typical Si02 (PECVD) film 8'=
Figure 5.10. FTIR spectrum of a typical SiOz (PECVD) film S3
Figure 5.11. SEM micrograph of as received PMR-15 composite sample 89
Figure 5.12. SEM micrograph of Si02 coated (lOOOA) PNIR-15 composite sample 89
Figure 5.13. SEM micrograph of SrO, coated (800A) PMR-15 after an exposure at 390C for 300 hrs 90
Figure 5.14. SEM micrographs of Si02 coated (lOOOA) PMR-15 after an exposure at 390C for 300 hrs 90
Figure 5.15. FTIR spectrum of a typical Si02 (LPCYD) film from DES 92
Figure 5.16. Adhesion improvement by ion implantation 94 VI
Figure 5.17. Effect of exposure to ambient atmosphere at elevated
temperature on the mass of Si02 coated samples of PMR-15 composite samples 96
Figure 5.18. Percent mass loss at 300 hrs exposure to ambient atmosphere at 390C as function of Si02 film thickness 97
Figure 5.19. Effect of exposure to ambient atmosphere at elevated temperature on the mass of PMR-15 polyimide cornposue· samp1'"es twice coatedwiWIth S'O1 2 . 98
Figure 5.20. Effect of exposure to air plasma on the mass of PMR-15 polyimide composite samples 100 VII
LIST OF TABLES
Table 1.1. Plasma material interactions and their effects 5
Table 2.1. Comparisons of Si02 films from various organosilanes 16
Table 2.2. Data showing the cases where electronic energy loss causes enhancement of adhesion 21
Table 4.1. Typical beam parameters in RBS analysis 39
Table 4.2. Film thickness vs. color 42
Table 5.1. Deposition parameters of the LPCVD process 65
Table 5.2. Process conditions for TEOS-Si02 film deposition by the LPCVD process...... 66
Table 5.3. Deposition parameters of the PECVD process 73
Table 5.4. Process conditions for TEOS-Si02 film deposition by the PECVD process 78
Table 5.5. Refractive index vs. deposition temperature 84
Table 5.6. Film density vs. deposition temperature 85
Table 5.7. Peel adhesion ("Scotch tape") test results 86
Table 5.8. Scratch test conditions for a typical PECVD film 87
Table 5.9. Deposition parameters of the DES-Si02 film 91
Table 5.10. Process conditions for DES- Si02 film deposition by the LPCVD process 93 1
1.0 INTRODUCTION
The technology of thin film deposition has advanced dramatically over the past twenty years. The advancement was driven primarily by the need for new products and devices in the electronics and optical industries. Thin films are also being applied as anticorrosive coatings for turbine blades, hard coatings for machine tools, and others. Now they are also being proposed to be applied for protection from oxidation of space station components parked in low earth orbit.
Chemical vapor deposition (CYD) has emerged as a very important route for creating thin films on a variety of substrate materials. The synthesis is accomplished by a technique in which a gas or a mixture of gases interacts with the surface of a substrate at a relatively high temperature, resulting in the decomposition of some of the components of the gas and leading to the formation of a solid film or coating of a metal or compound on the substrate. The first application of CVD can probably be traced back to the prehistoric times, when the caveman used soot formed out of incomplete oxidation of firewood to paint and draw figures in the walls of the caves.
Perhaps this is the first application of pyrolytic carbon.
Fundamentally, CYD requires a source of precursor gases, a heated reactor, a vacuum system, a substrate, and a system for the disposal of exhaust gases. The precursor gases include a variety of reactive gases such as silane, methane, ammonia, chlorine, tetraethoxysilane and others. The precursor gases also include inert gases such as nitrogen, and hydrogen, as a reducing gas. Some of the precursors are in the 2 form of liquids with high vapor pressures at room temperature, such as silicon tetrachloride [SiCI4], diethylsilane [SiH2(C2Hs) 2] ' titanium tetrachloride [TiCI 4] and a host of others. These liquids are heated to temperatures of about 60C and the issuing vapor is carried into the reaction chamber by passing (bubbling) an inert carrier gas through the liquid.
The reaction chamber is heated to the desired temperature using techniques such as resistance or induction heating. Also, the substrate may be heated directly by passing an electric current through it. In such cases when the walls of the reactor are at a relatively low temperature, the techniques are called cold-wall CVD. When the walls of the reaction chamber are heated using the external heat source, the technique is called hot-wall CVD.
In the case of conventional CVD, the reaction temperature ranges from 900C to about 200Ge depending on the type of coating to be deposited. By using metal organic precursors (MOCVD), the reaction temperature can be lowered to about
SOO-800C. The reaction temperature can be further lowered by providing more energetic activation of the vapor phase reaction. The techniques in this category include plasma-enhanced CVD (PECVD) and laser-activated CVD. In both of these cases, the chemical reactions in the gas phase are activated by glow discharge
(plasma) or by focussing a laser beam into the gas mixture.
The atmospheric or low pressure thermal CVD of microelectronic films occurs typically at 600 to lOOOC, which corresponds to thermal energies below 100 meV.
To achieve the decomposition of the source gases (with binding energies 1-3 ev), the 3 thermal CVD process needs high substrate temperatures and catalytic reactions between donor molecules and substrate surfaces. As a consequence, the deposition rates for thermal CVD are low. The need to lower the processing temperature led to the development of the plasma enhanced chemical vapor deposition (PECVD) technique.
A plasma is a collection of positive, negative and neutral particles in which the density of the negatively charged particles is equal to the density of the positively charged ones. When an energetic electron strikes a neutral reaction gas molecule, it can form free radicals, ions, and excited species (responsible for the typical plasma glow). Figure 1.1 shows the normalized energy distribution curve for an electron and the required energy for dissociation and ionization.
Electron Energy Distribution /~ Dissociaticn \
I \ "" Ianization .\ :::::::>,:::::::::.::.:: .,( . \ . .
-.i'~ __
Electron Energy) E
Figure 1.1. Normalized energy distribution curve for an electron and the required energy for dissociation and ionization. 4 The electron energy required for ionization is much greater than the energy required for dissociation. Very few electrons have the necessary energy to ionize gas molecules, yet many electrons have sufficient energy to dissociate them. This is reflected in the relative concentrations of free radicals and ions. Roughly, for every
106 reactive gas molecules, 1(f will form free radicals and only one will ionize.
This plasma electron energy spectrum partially overlaps the electron-impact
Plasma
• - .t 1
Figure 1.2. Plasma-material interactions.
induced dissociation cross section maxima for many polyatomic source reactants. So, the onus of the task of feed gas dissociation is carried by the plasma electrons. As a result, the deposition temperature of the films on substrates become possible below
400C. PECVD processes also raise the deposition rates because the reactants are more directly and efficiently dissociated. Figure 1.2 shows schematically the 5 interactions occuring at a solid surface exposed to radio-frequency discharges
(plasmas). However, exposure to a glow discharge (plasma) may introduce radiation
damage and vacuum chamber impurities to the growing film.
A variety of phenomena take place on the material surface affecting the film
deposition. Primarily, the bombardment of the energetic particles (mainly positive
ions) affects the surface chemistry, the bonding structure, and the properties of the
deposited films on the substrate. All possible kinds of plasma-material interactions
are listed in Table 1.1 with a brief description of their effects.
Table 1.1. Plasma material interactions and their effects. Number Result of Interaction Effect of Deposition 1 Surface diffusion Improve step coverage, alter crystallinity and orientation. 2,3 Nucleation and Promote adsorption, film densification, growth and rearrangement of bonds. 4 Adsorption, Reaction Creation of adsorption sites, reaction promotion and enhancement of film adhesion. 5 Sputtering Reduction of deposition rate and preferential removal of film component. 6 Implantation Modification of film stress, defect creation and enhanced film adhesion. 7 Secondary electron Change of gas phase chemistry. emission 8 Surface and near Impurity incorporation, defect creation. surface damage
The fundamental difference between conventional CVD and plasma enhanced CVD
(PECVD) is that the thermodynamic principles of chemical reactions which govern 6 the former does not apply to PECVD. The dissociation of a gas in a plasma
environment is nonselective, therefore the coatings formed in PECVD reaction can
be quite different from those produced in conventional thermal CVD reactions. As
a result, by overcoming the equilibrium thermodynamic barrier, unique coatings can
be developed.
The type of chemical reactions often encountered in the CVD processes are
thermal decomposition, oxidation, reduction, hydrolysis, disproportionation and
synthesis. Most typical reactions in CVD processes are heterogeneous in nature.
The formation of a film by CVD consists of process steps beginning with diffusion
of reactant gases through the boundary layer to the substrate surface, followed by
adsorption and reaction. For each CVD process, it is important to investigate the
relative importance of independent variables such as substrate temperature, operating pressure, substrate orientation, and reactor geometry. By operating at low pressure
(less than 0.5 torr) it is possible to eliminate diffusion as the controlling step in a deposition. It has been found that using the low pressure CVD (LPCVD) process, films of highly uniform thickness can be produced by conducting the reaction in the surface reaction controlled regime.
In the present study, both low pressure CVD and plasma enhanced CVD have been used as a new way to apply protective coatings onto composite materials.
Although various CVD techniques have been extensively studied and applied for thin film deposition in the field of microelectronics, its scope for macro applications, such as preparing protective coatings for structurals, is yet to be fully explored. An 7 important advantage of the CVD technique is its ability to prepare a large variety of
coatings and to provide near conformal coverage of an irregularly shaped or rough
surface, such as might be encountered at the surface of composite materials.
The composite materials coated in the present study are polyimides reinforced
with carbon fibers. Polyimides have already found extensive application in space
exploration, and the most well known brand, Dupont Kapton, will be used as the
solar cell mount for the space station Freedom solar array. The maximum service
temperature of conventional polyimides is about 350C. At higher temperatures, in
an ambient air environment, conventional polyimides undergo oxidative degradation,
rendering them useless as structural materials at these temperatures.
Therefore, the main focus of this study is to explore new ways to improve the oxidation resistance of polymeric composites in higher temperature applications, which will extend the usefulness of modern materials.
Oxidation protection of any material by the application of a protective coating requires first and foremost that the coating itself be immune to oxidation. This has, in' general restricted the choice to materials already in their highest oxidation state, such as metal oxides, for this use.
A significant amount of work has been done in the area of oxidation resistant coatings for space applications. For example, the prevention of atomic oxygen attack on space borne solar mirrors and photovoltaic array blankets have been accomplished by the application, by a variety of techniques, of coating materials such as silicon dioxide and aluminum oxide. In general, ceramic materials such as silicon dioxide, 8 silicon carbide and silicon nitride are highly oxidation resistant even at temperatures
as high as 1300C. Ceramics have been successfully applied as oxidation resistant
coatings to metals, other ceramics and carbon-carbon composites.
In this study, a stoichiometric Si02 film was deposited onto the polyimide
composite surface from an organa-silicon precursor, tetraethylorthosilicate (TEOS)
by the PECVD technique. The TEOS contains sufficient oxygen to allow the
formation of the stoichiometric silicon dioxide by pyrolytic and plasma induced
decomposition and without an external supply of oxygen. The pyrolytic, plasma
induced decomposition of tetraethylorthosilicate (TEOS) occurs through a
heterogeneous reaction which occurs .at the substrate surface. The reaction can be
represented in an overall stoichiometric equation
Also a second organic precursor, diethylsilane (DES) was used to deposit Si02
thin films by the LPCVD process. This process involves thermal decomposition of
DES in oxygen. It has been proposed that the reaction proceeds as follows. 9 The main advantage of this process is the lower deposition temperatures (in the range of 390C to 450C) which makes it a promising alternative to the PECVD-TEOS process.
displaced film atom •
• ion beam o .....e~!'-----
substrate film
Figure 1.3. Displacement of atoms at the interface as a result of collision with incoming ions.
Another important parameter considered in this study is adhesion of the deposited film on the composite surface. In an attempt to enhance adhesion, post deposition ion-implantation was performed. As an energetic ion passes through a material, it loses energy by two separate processes. In one process, atoms in the material are displaced through direct collisions, which is referred to as nuclear energy loss (Figure 1.3).
In the other process, energy is deposited into the electronic structure of the substrate material, and this process is referred to as electronic energy loss (Figure
1.4). When an energetic particle passes through an interface, both forms of energy loss can occur, and the magnitude of each depends on the ion energy and mass and atomic number of the ion and the masses and atomic numbers of the atoms adjacent 10 to the interface. By choosing ion beams which deposit most of their energy in
electronic energy loss at the interface, it is possible to affect the chemistry of the interface so as to improve adhesion.
ion beam
substrate film
Figure 1.4. Incoming ions interact with the electronic structure of the film and substrate. 11
2.0 LITERATURE REVIEW
2.1 Silicon dioxide films
Silicon dioxide is the mostly extensively applied dielectric in thin film form,
Thermally grown Si02 has found extensive application in the silicon semiconductor
device industry. Si02 films prepared in this way have thus been so extensively
studied and characterized that they serve as yardstick against which can be measured
the characteristics and properties of Si02 films prepared by other techniques.
Thermal growth of Si02 is generally accomplished in an atmosphere of oxygen,
wet oxygen, or steam in the temperature range 900-1200C. The product is very
similar in structure and properties to fused silica. Pliskin (1965) found that for films
of thicknesses ranging from 3000-8000 ~ the refractive index is 1.4618. In another
study, Pliskin (1966) reported that, as film thicknesses increase, the refractive index
approaches that of fused silica, which is 1.4601. Other film properties, such as etch rate and IR spectrum, also approach that of pure silica.
2.2 CVD and Pyrolytic Silicon dioxide
Goldsmith and Kern (1967) prepared Si02 films in an atmospheric CVD process by controlled oxidation of silane (SiH4) at temperatures ranging from 250
500C, but most depositions reported by Pliskin (1977) took place at temperatures in the range of 400-450C. The deposition process is not fully understood. However a 12 number of investigators have reported their observations. Emeleus and Stewart
(1935, 1936) studied the reaction at low pressure and observed that oxidation of
silane proceeds by a chain branching, free radical mechanism. Under low pressure
(0.3 to 0.5 torr), Tobin et. ale (1980) reported that the reaction proceeds as follows.
(Primary Reaction)
SiH4 + 2 0z ------> SiOz + 2 H20 (Secondary Reaction)
They determined that the ratio of oxygen to silane should be maintained at
a value greater than 10. When the ratio drops to 4, an absorption band in the
spectrum of the film at a frequency of 870-880 em" was observed (Pliskin, 1977) and
was attributed to the formation of an intermediate compound Si203•
The principal low pressure chemical vapor deposition (LPCVD) processes for
deposition of Si02 are the low temperature oxidation (LTG) of SiH4 at 400C, the
high temperature oxidation (HTO) of dichlorosilane, SiH2Cl2 by N20 at 900e, and
a reaction based on pyrolytic decomposition tetraethyl orthosilicate (TEOS). The
salient features of these three methods such as reactor geometry, growth rate, step
coverage, film adhesion, and film purity were summarized by Becker et. ale (1987).
It was Jordon who first reported deposition of Si02 film by thermal decomposition of alkoxysilanes in 1961. He used a number of ethoxysilanes, such as ethylethoxysilane, vinylethoxysilane, and phenylethoxysilane, which are all liquid at 13
room temperature with boiling points ranging from 100C to 300C. All of these
materials yielded film coatings at a temperature of 700C, but he found that
organosilicon compounds which had less than three oxygen atoms per molecule did
not produce satisfactorily adhering films.
KIerer (1965) studied the mechanism of formation of Si02 films by the
thermal decomposition of ethyltriethoxysilane. He proposed that the silane
undergoes transformation to complex silicon-alkyl radicals by thermal decomposition.
These radicals then become chemisorbed on a hot surface and suffer further
decomposition to form Si02 films.
Adams and Capio (1979) reported the rate expression for TEOS-Si02
deposition in the following form
1 2 kPTEOS 1 2 1 +KPTEOS
where rSiD is the rate of deposition of TEOS- Si02 film, PTEOS is the partial pressure
of TEOS reactants, k is the surface reaction rate constant, and K is the equilibrium
constant of the reaction.
This expression indicates that the reaction is heterogeneous in nature and the
TEOS molecules dissociates on the surface. In general, they reported a deposition rate of 200-300 A/min at temperature range 700-750C with thickness uniformity better than + 1% over the deposition zone. 14
Baker et.al. (1987) studied the low pressure CVD of Si02 by pyrolysis of
TEOS at pressures below 1.0 Torr and temperatures between 650-800C. In their
graph of deposition rate as a function of deposition temperatures the nonlinear
nature of the curve above 700C showed the transition from the reaction controlled regime to the diffusion controlled regime. This also indicated that depletion of TEOS started under such reaction conditions. He also found an activation energy of 40 kcaljmol calculated from the Arrhenius plot of the temperature vs. the deposition rate, which was in agreement with previous measured data by Adams and Capio
(1979).
Baker also studied the dependence of deposition rate and thickness variation against deposition pressure ( 300 mTorr). He suggested that in this pressure range,
Si02 film growth was dependent both on the number of TEOS molecules impinging on the substrate and on the velocity of desorption of the TEOS dissociation products.
He also reported increase of the film thickness variation with increasing pressure, which was possibly due to the combined effect of reduced diffusivity and depletion of reactants. He suggested lowering of the deposition pressure and simultaneous increase in TEOS partial pressure to achieve better uniformity of the film thickness.
Rojas et.al. (1989) have deposited 200-1000 A thick TEOS-Si02 films on Si substrates in the 600-780C temperature range by LPCVD techniques. Their studies have shown that deposited films have good conformal step coverage and thickness uniformity and which can be controlled by adjusting the process pressure (0.2 - 0.7 torr). 15 Hochberg and OMeara (1989) studied new organic precursors for the LPCVD
of Si02 films below 400C from liquid sources. This was mainly done to meet the
stringent demands for advanced very large scale integration (VLSI) circuit designs.
The type of compounds studied by them were ethoxysilanes, acetoxysilanes, cyclic
siloxanes and alkylsilanes. The deposition results from alkylsilanes were quite
promising. The di, tri, and tetra alkylsilanes produced conformally coated Si02 films
in the temperature range 350-650C. Out of these they found diethylsilane (DES) gave
the best result because SiOz films were deposited from DES at a temperature as low as 350 C and at a reactor pressure 800 mTorr and Oxygen: DES flow ratio of 2:1.
Gelemt (1990) proposed two .new source compounds, TMCTS ( 2,4,6,8 tetramethyl cyclotetrasiloxanes) and DES (diethylsilane) which he found offered performance advantages over silane and even TEOS. Comparisons of as-deposited
Si02 films from various organosilanes are listed in the Table 2.1.(Galernt, 1990).
Recently Huo et ale (1991) used diethylsilane to prepare high quality SiOz films on Si substrates by the LPCVD technique at temperatures below 400 C. The deposited films achieved 83% conformality and contain less than 1 atomic % carbon concentration and a low residual stress « 109 dynes/sq. cm). The conformality of a deposited film is defined as the ratio of the film thickness deposited in the valley
(S) of the substrate runners to that deposited on the top (T). But unlike previous study reported by Hochberg and OMeara,(1989), Huo et al.(1991) reported to have used an oxygen: DES ratio of up to 12 : 1 to obtain satisfactory Si02 films below
400C. 16
Table 2.1. Comparisons of Si02 Films from Various Organosilanes.
TEOS TMCfS DES Process Temp. Range 650-750 525-650 340-475 (C) Vapor Pressure 1.5 6 200 (Torr at 20 C) Conformality 70-95 80-98 80-90 (S/T in %) 1% HF Wet Etch Rate 2.5 3 2.3 (A/min) Refractive Index 1.444 1.455 1.460 @700C @590 C @410 C Film Stress at 28 C 3-8 4-7 >4 (109 dynes/em")
A very recent work reported by Coon et al.(1992) studied the adsorption and deposition kinetics of diethylsilane on silicon surfaces. They used laser induced thermal desorption (LITD), temperature programmed desorption (TPD), and Fourier transform infrared techniques (FTIR). They suggested that diethylsilane dissociatively adsorbs on porous silicon surfaces at 300K and forms SiH and Si~H5 species. Subsequently the ethylene desorption and growth of hydrogen concentration ethyl group decomposition were consistent with a f3-hydride elimination mechanism for the Si~H5 surface species. 17 They also found a low activation barrier for this decomposition which
suggested that the Si surface may catalyze the f3-hydride elimination reaction.
Another recent work published by Paterson and Oztruk (1992) studied the
LPCVD of Si02 in a horizontal CVD furnace using DES and oxygen. They reported
a maximum deposition rate of 275Ajrnin. at a temperature ranging from 425-500C.
They also found a threshold of > 950 mTorr pressure for gas phase reaction at
deposition temperature of 450C. Rutherford Backscattering Spectroscopy (RBS)
studies confirmed that the deposited films are stoichiometric Si02 for temperature
less than 450C.
2.3 Plasma Deposited Silicon dioxide
Most of the plasma enhanced CVD (PECVD) films are produced from
organa-silicon compounds, which have a high surface mobility. The larger the mean
free path for surface migration, the better is the prospect to improve step coverage
and conformality. The most widely used compound is tetraethylorthosilicate (TEOS).
The deposition of silicon dioxide using TEOS alone at temperatures below 400C is best accomplished using a plasma environment (Adams, 1986). Others have used oxidants like ozone or oxygen with TEOS (Nur Selamoglu, et.al. 1989). Mackens and
Merkt (1988) found that the deposition rate depended critically on many parameters, such as the r.f. power, reactor geometry, and oxygen flowrate, and, since the 18 deposition kinetics for PECVD processes are difficult to model accurately, the best
deposition condition must be determined empirically.
It was also reported by Takamatsu (1986) that while using oxidants such as
ozone, the deposited silicon dioxide was found to contain higher concentrations of
silanols (SiOH) and became hygroscopic, which are certainly unwelcome features for
device reliability.
Chin and Van de Ven (1988) studied deposition of Si02 from TEOS and
oxygen in an rf plasma. They found that the profile of the deposited films over a step
depended on the oxygen : TEOS ratio, and rf power and the films obtained under
optimized conditions meet the requirement of an interlayer dielectric used for
multilevel interconnections in the Ie chips. The films, they reported, are
9 mechanically stable, have a low stress « 1.5 X 10 dynes/sq.cm) and are more
conformal than those obtained with silanes (SiH4).
Kulisch (1988) et ale investigated the suitability of TEOS as a source material
for PECVD of Si02 thin films. They reported to have achieved high quality
stoichiometric Si02 films with almost no contamination from the source TEOS
molecule. The conformal coverage of the films was also reported to be excellent
Hills et.al. (1990) published a useful report on how TEOS-Si02 emerged as
a key dielectric at AT&T. They reported that the Si02 film generated from an rf plasma in TEOS and oxygen met all the stringent mechanical and electrical quality requirements. The films made by PECVD techniques were preferred by them over
the LPCVD films because the latter technique often caused hillock formations, which 19 are prone to produce intermetal shorts. The films also showed better conformality and less tendency to develop bread loaf profiles (wavy and irregularly contoured surface) and voids than from silane (SiH4) and N20.
Another important study investigating the role of oxygen excitation in PECVD of Si02 from TEOS was reported by Raupp et al.(1992). This study reported an increase in the deposition rate with increasing rf power, increasing total pressure, and decreasing wafer temperature. A simple plasma deposition model was presented in which deposition occurs through both an ion-assisted and an oxygen atom assisted pathway. The relative contribution of these pathways were identified using limited step coverage measurements on low aspect ratio trenches.
2.4 Surface Modification by Ion Implantation
Scientists working in the domain of radiation effects in insulators, such as oxides, minerals, and ionic crystals, recognized the important role of physico-chemical processes accompanying the implantation of ions. Energetic ions are slowed down in target materials by momentum transfer, which is referred to as nuclear stopping, or by excitation of the electronic structure of the target atom, which is called electronic stopping. In the semiconductor industry, the ion implantation technique has been a standard method for doping semiconductor materials to produce
Integrated Circuits (ICs). 20 In 1969, adhesion improvement of AI films (500 A) on soda glass was reported by Collins, et ale after low dose irradiation with 120 KeV Ar ions. Gokulberger and
Kleinfelder (1972) obtained a patent for the process whereby a beam of 100 Kev Ne ions would produce adhesion of Cu films on SiD2•
In the early 1970s,British researchers began to explore the effect of ion beams on non-semiconductor materials (Dearnaley, 1980) The initial effects were directed at the study of the effect of ion-implantation on friction and microhardness in metals.
During the last ten years, the ion-implantation technique has been increasingly used for modification and improvement of surface properties for a wide variety of materials (Legg and Legg, 1989).
In this review, ion-implantation effects will be discussed in connection to their effectiveness in adhesion improvement of thin films.
The pioneering work in this field was done by Baglin et al.(1989), and they helped to strengthen the concept that during the process of irradiation, nuclear energy loss phenomena were responsible for enhancement of adhesion of ~hin films
to substrates. They irradiated 100 ACu films on AI20 3 substrates with 200KeV He + and 280 KeV Ne + and examined the change in adhesion using a Scotch tape peel test. In both of these cases, adhesion was improved, but Ne + beam produced a six fold increase in adhesion over He + beams. Baglin et al.(1987) have also found that by such irradiation only a monolayer of surface materials has been displaced and caused such an enhancement of adhesion. 21 A significant contribution in this field was made by Ingram and Pronko (1986),
who supported the concept that nuclear energy losses caused an enhancement of
adhesion. For Cu-Mo systems they have shown that Cu became more adherent in
proportion to the nuclear energy loss of the irradiated species at the surface.
However, along with the above references, a growing list of works have been
reported to point out the major role of electronic energy transfer from an irradiation
to a film substrate interface. Such works reported studies of implantation by ions
with higher energies in the range of 10-20 MeV, electrons of 2-20 KeV, gamma rays,
and photons. A number of studies suggesting the importance of electronic energy
deposition for promotion of adhesion was reported by Colligen and Kheyrandish
(1989). Some of those results were listed by them in Table 2.2 showing the
importance of electronic energy deposition for promotion of adhesion.
Table 2.2. Data Showing the Cases where Electronic Energy Loss causes Enhancement of Adhesion Film Substrate Irradiation Adhesion Test Au-Glass 2 MeV He Q-Tip Sn-GaAs 2 MeV He Q-Tip
Pt-Al203 5-10 keV Q-Tip
Au-Si02 2 MeV He Scotch Tape
Ag-Si02 20 MeV CI Scotch Tape Au-Teflon 20 MeV CI Scotch Tape Au-Si 10/21 eV photon Q-Tip 22 Another important study was done by Lin and Cole (1989), who used laser
irradiation to change the electrochemical properties of polyimide surfaces to improve
the adhesion of Cu thin films for microelectronics applications.
A significant improvement in adhesion of Au and Cu films on a variety of
substrates following irradiation by 20 MeV CI+, 100 MeV Kr+, and 5 Me V F+ ions
was due to the studies done by Tombrello (1984). All such bombardment were
reported to produce negligible nuclear energy transfer to the interface. He proposed
that the losses in electronic energy causes the formation of new bonds, and an
exchange of electrons across the interface was needed to improve adhesion.
The mobility of ions across reactive interfaces produced an intermediate layer
consisting of both the film and the substrate, called ion beam mixing ( Baglin,1987).
The extent of such diffusional mixing was reported to be closely related to the thermodynamic driving force for chemical bond formatiton. In the absence of such driving force, no evidence of more than ballistic mixing was found for non reactive systems. This observation helped to strengthen the picture that improved adhesion is a result of intermixture of film and substrate atoms to the extent of a monolayer or two, enabling the formation of locally stable chemical bonding of these atoms.
Such configuration could represent redistribution of bonds, which was energetically preferred over the original terminated substrate structure coated with as-deposited metal film (Baglin, 1987).
In another study, Pappas et al.(1991) studied the bonding of Cu and Cr films to several polyimides. The polyimides they worked with were derived from 3, 3', 4, 23 4', biphenyl tetracarboxylicacid di-anhydride-p-phenylene diamine (BPDA-PDA) and
pyromellitic dianhydride-4, 4', oxydianiline (PMDA-ODA) polyimide formed from
polyamic acid or polyamide ethyl ester precursors. The polymer surface was subjected
to ion bombardment with Ar+ and/or O2 + ion beams prior to metal deposition,
which led to a significant improvement in adhesion of metal films.
Other important work was the study made by Howarth et ale (1983), where it
was demonstrated that the adhesion of Cu to polyimide could be enhanced by
bombardment with 1.5 Mev He or 3 Mev Ne at fluences which would not cause
significant levels of displacement damage of the polymers.
It was Baglin (1987) who addressed a new possibility of dramatically and
directly tailoring the interface layers, so that starting with clean substrate materials,
a continuous bonding network across the interfacial region could be formed. The
main thrust of such modification was to restructure the interface layer to improve its fracture toughness and to lower the yield stress of the film. The concept of such interface modelling in general remains to be studied by explicit atomic scale modelling, perhaps via molecular orbital calculations of atomic structure. One such study was reported by Johnson and Pepper (1982).
2.5 Modeling of Low Pressure CVD Reactors
The development of CVD reactors and the selection of operating parameters so far have been mostly done on an empirical basis. It was only in recent years that 24 the efforts were directed to model and design the reactor systems from reaction
engineering analysis.
Two major reactor geometries have been considered and analyzed in cold wall
CVD reactors at atmospheric pressure. Cold wall means that the reactor wall was
not directly heated, but that the heating was restricted to the susceptor (substrate
holder). Eversteizn et ale (1970), Rundle (1971), and Takahashi et ale (1972)
considered horizontal reactors with a flat plate susceptor for deposition of Si by the
CVD technique. Fuji et ale (1972), Dittman (1974), and Manke and Donaghey (1977)
proposed a model for epitaxical growth of Si in a barrel reactor.
Gieske et al.(1977) and Hitchman. (1979) undertook the effort to model a LPCVD reactor. They presented experimental data, possible kinetics, and flow
regimes and attempted to correlate these with the mass transfer effects of the
reactants.
Kuiper, et al., proposed a detailed model for LPCVD growth of polycrystalline
Si. This model was restricted to isothermal conditions and they provided no comparison between experimental findings and model predictions.
Jensen and Graves (1983) presented a detailed mathematical model for the hot wall multiple disk-in-tube LPCVD reactor for the deposition of polycrystalline
Si from SiH4• Their model included the convective and diffusive mass transport and analyzed the effects on growth rates and film uniformity of variations of flow rate,
reactor temperature profiles and SiH4 concentration in the flow. 25 Typically, LPCVD processes involve several reactants, so the consideration of
the effects of multicomponent diffusion might be important, as observed by Roenigk
and Jensen (1985,1986). Thermal entrance effects and the reactions taking place in
the tube (hot-wall) before deposition could also influence the overall reactor
performance (Roenigk 1986). Another important issue in LPCVD reactor modelling
is the transition to the molecular flow regime, where the continual formulation is no
longer valid. This could become important in modelling LPCVD processes at low
pressure (less than 10 Pa.) (Meyerson et aI, 1986).
The fundamental chemistry far the LPCVD system is mostly very complex.
In many reaction systems, the determination of the mechanistic pathways are still awaiting acceptable explanation. Perhaps the simplest system for practical application for which LPCVD reactor modelling has been successful was the deposition of polycrystalline Si from SiH4 (Roenigk and Jensen,1985). However, the mechanisms growth of silicon nitride and silicon dioxide films from various precursor materials are not yet well understood. There can be so many possible reactions in a narrow operating range that it is difficult to obtain statistically meaningful rate parameters from experimental data.
2.6 Modeling of Plasma Enhanced CVD Reactors
There is a considerable body of experimental and spectroscopic information providing exciting insights into the plasma processing of CVD systems. Some of the 26 studies of note were those of Dreyfus et ale (1985) and Gottscho and Miller (1984).
They made some simplified assumption about the state of the reacting species and
their kinetics, which were important as the initial effort but fell short of simulating
the actual situation. However, only a few modelling studies have so far been published. This is perhaps due to the complexity of the process.
Reinberg (1978) first proposed the radial flow reactor geometry, which was capacitatively coupled and the substrates immersed in the glow discharge region and thus subject to ion bombardment. The basic idea of such a geometry was to level off the electron density variations against residence time distributions so that the deposition rate became uniform over the entire substrate. Unfortunately, no quantitative analysis was presented to support the qualitative design arguments.
The AMT reactor proposed by Sherman (1984) had a rotating bottom electrode to achieve uniformity of deposition rate. Roster and Engle (1981) proposed the tubular ASM reactor which was like a classical multiple wafer LPCVD reactor, except the wafers were placed parallel, rather than perpendicular to the flow.
At the present time, a satisfactory model of the PECVD reactor is yet to be done because of the complex nature of the discharge physics and also because of insufficient understanding of the underlying plasma chemistry and surface interactions. Since the plasmas used in PECVD processing are weakly ionized gases, the plasma reactor modelling problem may be divided into two subgroups. First, the discharge structure as a function of electronic parameters, such as frequency and power, must be determined, and secondly, the transport and reaction of neutral 27 species (free radicals) leading to the deposition of films on the substrate must be
studied. The non-equilibrium nature of the discharge structure makes the first
problem more intractable. However, the second problem is equivalent to those of
conventional reaction kinetics and is easier to tackle.(Jensen,1987)
2.7 Oxidative Degradation of Polyimide Composite in Space
Atomic oxygen is the primary constituent (80%) in the low earth orbit (LEO) which extends from 200-700 km range.(Leger and Visentine,1986). Neutral oxygen radicals (atomic oxygen) are produced by the dissociation of molecular oxygen by ultraviolet radiation from the sun. It has been observed that the surface materials " of the spacecraft orbiting in LEO are significantly degraded. The main cause of such degradation is due to attack by oxygen radicals (Leger,1986). Data received from shuttle flights have shown that even a short term exposure to LEO environment could have harmful effects on the surface of the spacecraft, especially if those were composed or covered with organic materials (Zimcik et al.,1985,1987). The materials such as polyimides, graphite epoxy, carbon and some metals are subject to loss of mass and surface texturing when exposed to the LEO environment. UV rays from the sun having a wavelength of 2430 A are strong enough (energy 5.0 eV) to cause oxygen molecule to dissociate into oxygen radicals (Rutledge et al.,1988). The type ofdamage observed as encountered by the organic polymeric substances were erosion
(mass loss) and surface roughening. This, in turn, led to irreversible deterioration 28 of the physical characteristics (optical, mechanical,electrical) for which the surface and structural materials were designed.
A number of studies have examined and explored the possibility of applying thin film coatings to the exposed surfaces of the polymeric materials to protect them from attack by oxygen radicals.
Klenberg-Saphieha et al.(1989) reported that coatings of silicon dioxide, silicon nitride, and silicon oxynitride deposited in a plasma environment on Kapton polyimide substrate and on epoxy resin substrate caused a significant protection of the coated surface against oxidative degradation. Wertheimer et al.(1984) also investigated silicon dioxide, silicon nitride, silicon oxynitride and organosilicon coatings deposited by microwave plasma on various polymeric substrate. The coated films as characterized by them were very thin, strongly adhering, and free from pinholes.
2.8 Thermooxidative Degradation of Polyimide Composites
Scientists at the NASA Lewis Research center are actively engaged to develop advanced high temperature engine materials. One class of materials which is a main focus of their studies is polyimide matrix composites. The use of this composite has led to a significant reduction of weight over traditional aerospace materials and also a reasonable savings in manufacturing cost has been realized. However the main obstacle to overcome the limitation of their application is their low thermo-oxidative 29 stability as pointed out by Meador (1989). According to Meador, polyimide matrix composite can only operate in atmospheric conditions for extended periods of time at temperatures from 288-316 C.
A great deal of research efforts are directed to develop new polymer matrix composite with upper use temperature approaching 425 C. 30
3.0 RESEARCH PLAN
The objective of the present research is to explore new ways to improve the
oxidation resistance of polyimide based composites in higher temperature
applications. To meet this objective, very thin (500-3000 A) film coatings of silicon
dioxide were applied to prevent oxygen contact with the polyimide surface. The plan of this research is subdivided into four main components.
(1) Design and the installation of a low pressure chemical vapor deposition
(LPCVD) and a plasma enhanced chemical vapor deposition (PECVD) system to carry out deposition of the thin dielectric films from organic precursors.
(2) Parametric studies of both the LPCVD and PECVD processes and characterization of the deposited films.
(3) Experiments to study the mass loss of the coated samples at higher temperatures and also at plasma environment.
(4) Ion implantation on the deposited film as a means to improve its adhesion.
1) Installation of the Chemical Vapor Deposition systems.
The chemical vapor deposition (CVD) technique has been chosen as the method for deposition of the protective thin films onto the composite surface. Two different reactor geometries were chosen for LPCVD and PECVD systems, and both of the reactors were equipped with powerful vacuum apparatus and associated monitoring gauges. Two organic chemicals, tetraethylorthosilicate (TEOS) and 31 diethylsilane (DES) were used as the reactant. Argon is the carrier gas while using
TEOS. DES, being a liquid with sufficiently high vapor pressure, does not need any
carrier gas, but it needs oxygen as a coreactant for stoichiometric deposition of Si020
The deposition of Si02 was carried out on both silicon and polyimide composite
samples. Sufficient care was taken to minimize contamination of the films during
deposition. One such important measure was to achieve a very low base pressure ( <
10-6 torr) prior to the deposition.
2) Parametric studies and characterization of the films.
The number of parameters which play an important role in chemical vapor
deposition were studied to find out the optimum nature of their values. For LPCVD
system, the reactor pressure, substrate temperature and carrier gas flow rate are the
independent variables, which were studied in relation to their effects on the
deposition rate and uniformity of the coatings. In the case of the PECVD system,
four independent variables, 'such as reactor pressure, substrate temperature, carrier
gas flow rate and RF power are studied against film deposition rate and other
properties of the films. The characteristics of the films studied are stoichiometry,
thickness, refractive index, density, etch rate, uniformity, stress, and adhesion of the
films to the substrate.
. (3) Mass loss experiments.
The Si02-coated polyimide samples were exposed to the temperature of 32 interest in an atmospheric condition for a certain length of time to monitor their change of masses. Some samples were also put inside a plasma asher (which acts as the simulator of the low earth orbit) to find out mass loss characteristics of the coated composites against time.
The low earth orbit (LEO) atomic oxygen environment was simulated using the plasma asher operated with ambient air as the carrier gas. One hour of exposure in this asher is equivalent to 1019 atoms/crrr' equivalent fluence as measured by
Kapton erosion from early shuttle flights. For Kapton, 26 hr of exposure in the asher produces erosion equivalent to one year exposure in space (Gulino et al.,1988).
4) Ion Implantation to improve adhesion of the films.
Coated polyimide samples were exposed to radiation by ions for different lengths of time. The whole purpose of such implantation is to improve adhesion of the deposited films by initiating a change in their electronic and/or nuclear energy levels. 33
4.0 EXPERIMENTS
As outlined in the research plan (chapter 3), four major experiments have
been performed in this research. These experiments are (1) Deposition of TEOS
Si02 thin film onto Silicon substrate by the LPCVD technique. (2) Deposition of the
TEOS-Si02 thin film onto polyimide composite substrate by the PECVD technique.
(3) Deposition of the DES-Si02 thin film onto polyimide composite substrate by the
LPCVD technique. (4) Ion-Implantation on the thin film coated polyimide composite surface to enhance adhesion of the film.
The materials used in these experiments, varIOUS analytical tools and techniques used for characterization of the thin films, and the experimental procedures undertaken are described in the following section of this chapter.
4.1 Experiment 1: Deposition of the TEOS-Si02 Thin Film onto Silicon Substrate by
LPCVD technique.
4.1.1 Materials:
(1) Reaction Chamber: The bell-jar shaped reactor shown in the figure 4.1 is made of stainless steel, 25 em diameter by 65 em height, mounted over a collar fitted with six 2.5 inch eonflat flange openings for feed-throughs. The collar is mounted over a cold water baffle trap which prevents backscreaming of oil vapors.
(2) Diffusion Pump: The diffusion pump is supplied by Bendix Inc. It has a normal pumping speed of 8.2 liters per sec. at a pressure of 10-3 torr. It is capable of evacuating the reaction chamber to a base pressure of 10-7 torr. 34
Bell jar -:
~ ~ ....- Reactant Substrate Heater Inlet
Vacuum
Figure 4.1. Reaction chamber for TEOS-Si02 deposition by LPCVD.
(3) Roughing Pump: This is a centrifugal pump used to evacuate the system from atmospheric pressure to 0.01 torr. The pump backs the diffusion pump and vents it to the atmosphere.
(4) Substrate Heater: The heater used is manufactured by Microscience Inc.
It is designed for thin film deposition application. The range of operation for the heater is up to lOOOe surface temperature and will withstand an oxygen environment.
The heater is available as a complete flange mounted assembly. It has a Conflat 35 style flange with an insertion length to suit a particular application. The dimension
of the ceramic heater module is 40 em (1), 40 cm(w), 14 em (h).
(5) Temperature controllers: The model 7200 series controller is supplied by
Beckman Industrial. It is microprocessor based, single loop, digital temperature
controller in a standard 1/4 DIN package. The controller monitors the substrate
temperature and controls the current flow to the heating element. The temperature
control action is effected by a proportional integral derivative (PID) controller with
a set point and actual temperature readouts. The temperature sensor is a K type
thermocouple (display range -200 to 1350C).
(6) Pressure Sensors:
a) Thermocouple Gauge: This type of gauge measures heat flow under
a pressure dependent condition. A constant amount of current is delivered to the
heated wire and a thermocouple is spot welded to its midpoint. With the increase
in pressure, heat flows to the walls, and the temperature of the wire decreases. A
low resistance DC microammeter is connected to the thermocouple and its scale is
calibrated in pressure units.
The 2800 digital gauge controller is used for the thermocouple units.
The range of pressure it measures is from 10-3 torr to 1.0 torr.
b) Ionization Gauge: In the ultra high vacuum region (10-3 to 10-11 torr), where the particle density becomes extremely small, this type of gauge is used.
The basic principle to measure the gas pressure is the ionization of the gas molecules and the collection of the ions and their subsequent amplification by sensitive and 36 stable circuitry. The ionization gauge is manufactured by Granville-Phillips. The
series 274 Bayard-Alpert type ionization gauge tube is used. The ion-gauge
controller, IGC Series 330, is also supplied by Granville-Phillips.
6) Flow meters: Rotameters used to measure the flow are supplied by Aalborg
Inc. The range of flow it can measure is from 200 c.c] min. to 1600 c.c] min.
7) Substrate: Silicon wafers having orientation (111) and diameter of three
inches are obtained from Monsanto.
8) Reactant: Tetraethylorthosilicate (99.999%) is used as the reactant, and is
supplied by Aldrich Chemical Company.
Physical Data of Tetraethylorthosilicate, Si(OC;Hs)4 :
Formula weight: 208.33
Boiling point: 168C
Density: 0.934 gm/cc
Vapor pressure (60C) : 6 mm Hg
9) Carrier Gas: Argon is as the carrier gas, supplied by AGA Inc.
4.1.2 Analysis of the Films.
Analysis is done to characterize the film in terms of its properties, such as composition, thickness, density, refractive index, stress, and adhesion to the substrate.
1) Composition of the Film: The composition of the films can be analyzed by various methods, such as IR spectroscopy, Rutherford Backscattering Spectroscopy,
Secondary Ion Mass Spectroscopy, Auger Spectroscopy and X-ray Photoelectron 37 Spectroscopy. In the present research, IR and Rutherford Backscattering
Spectroscopy are used to determine the stoichiometry of the film.
a) IR Spectroscopy: The term infrared spectroscopy is used to describe the measurement of the absorption of radiation as a function of wavelength in the analytically useful portion of the IR spectrum (2.5 to 50 microns). When irradiated with infrared light, a sample can transmit, scatter, or absorb the incident light. When the energy difference between two vibrational states matches with the energy of the incident light, the absorbed incident radiation usually excites the molecule into a higher vibrational state. Thus the absorption spectrum in the infrared is often used as a fingerprint of the molecular species. Traditionally, IR spectra have been produced by transmission, that is, transmitting light through the sample, measuring the light intensity at the detector and comparing it to the intensity obtained with no sample in the beam, all as a function of the infrared wavelength.
Recent advances in computerized IR spectroscopy and fast Fourier transform algorithms have introduced the implementation of a new method of IR spectroscopy.
Fourier Transform Infrared Spectroscopy (FTIR) utilizes a Michelson
Interoferometer to obtain the interferograrn; the intensities of light (I) transmitted through the interferometer and recorded as a function of the position (6) of the moving mirror. These data are fast fourier transformed into the intensity (~) of the infrared radiation as a function of wavenumber (v) by the expression
- 00 - {1(v) = i I(6)cos(21rv6)d6 ~
One of the major advantages of FfIR is that less time is necessary to obtain 38
a spectrum at a given signal/noise ratio.
b) Rutherford Backscattering Spectroscopy (RBS): RBS utilizes the elastic
collisions between incident energetic ions and the nuclei of target atoms to measure
the masses of the target atoms. Hydrogen or Helium ions are usually used and
accelerated to an energy of 0.5 to 2.0 MeV. The ion beam is deflected in a magnet
to select an ion beam of a specific energy before it is directed onto the sample to be
analyzed. For an elastic collision, the energy of an ion (mass = M 1) scattered from
a target nuclei (mass = M 2) at a scattering angle (8) is given by
2 2 2 B, = K Eo = [M1 Cose + (M 2 - M1 Sin e ) 1/ 2] 2 · Eo
where Eo is the energy of the incident ion. Since M1 and e are kept constant
for a particular measurement, the energies of the scattered ions (Es) are related to
the masses of the target atoms. Most of the incident ions penetrate the target due
to their small scattering cross section. As the ions penetrate they lose energy and
this energy loss is characterized by a stopping cross section which is dependent on
the target material, the incident ion, and its energy. For an elemental target, the
stopping cross section e, is given by
€ = (-liN) dE/dx
where N is the atomic density of the target material. These expressions can be combined with the cross section for elastic scattering to derive the distribution of atomic compositions in a sample.
c) Proton Recoil Detection (PRD): This is very similar to RBS. The basic interaction between target nucleus and the incident ion is the same elastic scattering. 39 However, in PRD, a heavy ion which is accelerated and collides with a light atom in
the surface of the target. The light atom recoils in a forward direction, and the sample is tilted so that the forward recoiling light atom can escape and be detected.
As in RBS, the kinematics of the scattering in PRD can be described by classical mechanics. It is a specialized technique and is used for the measurement of the distributions of light atoms in the near surface layers of materials. PRD can be used to detect light atoms up to atomic mass of 20 amu, but it is most useful for the very light ones, particularly for hydrogen isotopes, which are difficult to measure using other methods.
Table 4.1 shows the typical beam parameters set in the present analysis.
These parameters were used in simulating the RBS spectrum using the software
RUMP developed at Cornell University. This is then matched with the experimental spectrum.
Table 4.1: Typical Beam Parameters in RBS Analysis.
2.0 Mev He+2 Charge 10J..Lc @30 nAmp
Carr 7.443 21 Theta 0 Phi 12 (150)
Corr 1.25 Omega 2.38
In the Table, theta refers to the incident angle, omega to the detector solid angle, phi to the angle between the detector and the normal on the target surface, and 'carr' signifies the correction necessary to normalize the signal to that of the sample surface. 40 d) Ellipsometry: The technique of ellipsometry is concerned with the
measurement of changes in the state of polarization of elliptically polarized light
upon reflection from a thin transparent film on a reflecting substrate. The
ellipsometer creates an elliptically polarized monochromatic light beam and then
evaluates the light beam on reflection of a thin film. A monochromatic beam of light
(such as a laser) passes into a polarizer where it becomes plane polarized. It then
passes through a compensator which converts it into an elliptically polarized light
beam. After reflection from the substrate/thin film, it passes through an analyzer.
If it has been converted back to plane polarized when it has been reflected, then it will be possible to rotate the analyzer to find a true minimum intensity. The technique then is to adjust the polarizer until the reflected light is plane polarized.
The analyzer is rotated to determine the position corresponding to a minimum intensity. This information, along with the theoretical model of the optical process, permits the calculation of the film thickness and refractive index independently.
e) Density: From the refractive index, the film densities are calculated using the Clausius-Mossotti relationship (Rojas, 1990).
2 2 p = K(n - 1)/(n + 1) where p is the density, n is the refractive index of the film and K is a constant equal to 8.1148. The value of K is determined empirically from data obtained on thermally grown Si02•
f) Stress in the Film: When a thin film is deposited on a wafer substrate, it will introduce an amount of bow (radius of curvature) to the substrate. The accurate 41 measure of this curvature will allow the computation of the stress induced by the
film. The direction of the film bow will dictate whether the film is under
compressive or tensile stress. If a wafer has bowed away from the light sensor after
deposition, so as to signify increasing wafer to bow gap, it is under compressive
stress. A wafer which bows towards the light sensor, thus decreasing wafer to probe
gap, is under tensile stress.
The stress in the as-deposited film is determined by measuring the curvature I of the wafer before and after depositing the film. Most of the stress measurement equipment employs a laser scanning system for measuring the radius of curvature of
a wafer. In the present research, the' radius of curvature of the wafer is measured I by a different method. Before and after depositing the film, the wafer surface is profiled using a profilometer. From the profile, the radius of curvature (R) can be
measured geometrically. From the value of R, the stress (a) can be calculated using I the formula
a = (E/l-v)(h 2/6t) l/R
where E is the Youngs modulus of the substrate,v is the Poissons ratio of the
substrate, h is the thickness of the substrate, and t is the thickness of the film.
(g) Thickness of the Film and the Rate of Deposition
1) Color: The simplest technique to determine the thickness of the film is by
its color. As long as the film is not reflective and is deposited on a reflective
substrate, as it is in the case of Si02 on polished Si, the color of the film can be
correlated to its thickness. Although not very precise, such information is very 42 helpful for quick evaluation of thickness in the laboratory. Another useful aspect of
this technique is that one can make immediate judgements as to film uniformity.
Table 4.2 shows the Si02 film thicknesses vs. color.
Table 4.2: Film Thickness vs. Color.
Film Thickness (A) Color
500 tan
750 brown
1000 dark violet
1250 royal blue
1500 metallic blue
1750 yellow green
2000 light gold
2250 yellow orange
2500 orange
2750 red violet
3000 blue
2) Surface Profilometry: The thickness of the film is measured by a surface profiler manufactured by Sloan Technology. The profiler model DEKTAK lIA is a 43 microprocessor based instrument used for very accurate measurements on very small vertical features ranging in heights from less than looA to a maximum of 655,OooA.
The instrument acquires the data by moving the sample beneath a diamond-tipped
stylus. Vertical movements of the stylus are sensed by an LVDT (linear variable differential transformer), digitized, and stored in the instruments memory. Stylus movement is changed from an analog to a digital signal using an integrating A-D converter. The digitized scans are stored in the computer memory for display, manipulation, measurement and printing, all from a single scan. The stored information is displayed on the console video screen and may be manipulated to magnify specific areas of the trace. . ~ilm thicknesses are measured by placing a witness sample consisting of a silicon wafer fragment in the chamber adjacent to the substrate of interest. A portion of the witness sample is masked with a second piece of silicon wafer during deposition. Afterward, the mask was removed and the resulting step profiled with a profilometer. Direct measurement of film thickness on those composite samples subsequently studied by RBS confirmed the values obtained from the witness technique.
Film thickness uniformity was calculated from five separate measurements in in different locations on the silicon wafer witness samples. The film thickness variation across the wafer was calculated as follows.
Variation( ) = lOO(Dmax - Dmin)/(Dmax + Dmin) 44
where D max and Dmin are the maximum and minimum thickness, respectively.
Measurement of film thickness provides a means to determine the rate of deposition. This was done by dividing the film thickness by the time of deposition assuming deposition rate to be constant.
h) Wet Etch Test: The wet etch test is done with a 100 H20:1 HF (by volume) solution. It is indicative of the film quality since it is related to its density, chemical bonding, stoichiometry, and levels of impurity. In particular, etch rates are an indication of relative porosities (densities) of these films. If the deposited film is of
good quality, which means almost pure Si02 with little impurities and constituting a network of silica, densified without incorporation of OR groups, the etching rate of such a film will be lower than that obtained from less pure films.
4.1.3 Experimental procedure.
A schematic diagram of the experimental system is shown in Figure 4.2.
Deposition is performed at the specified pressure and temperature. The reaction chamber and the connecting lines are first purged with Argon for five minutes to drive off any undesirable components. The roughing pump is then turned on to evacuate the system and establish a base pressure of the order of 10..3 torr.
Attainment of such pressure usually takes about fifteen minutes. The diffusion pump is switched on and the whole chamber is allowed to reach a pressure as low as 10-7 torr. It takes about an hour to attain such a pressure.
Subsequent to achieving such low pressure, the heater is turned on with the preset Pill settings in the controller. The main purpose of this delayed start of ROTAMETER~ PRESSURE VACUUM \j GAUGE CHAMBER THERMOCOUPLE GAUGE o IONIZATION . GAUGE SUBSTRATE HEATER, I / , IJ . BUBBLER /
GATE VALVE ARGON PLATE GAS CYLINDER HEATER tr:======IL TO n======IJ VENT t FORE WATER INLET PUMP WATER OUTLET
DIFFUSION ~ PUMP
Figure 4.2. Schematic diagram of the LPCVD experimental system. ~ lI\ 46 heating is twofold: 1) rmrurmze oxidation of the substrate surface pnor to the
deposition reaction, and 2) keep contaminants from reacting with the substrate.
The gas inlet to the bubbler is then turned on and the bubbler is heated to
60C using heating tape to ensure an increased, steady reactant vapor throughput.
The stainless steel delivery tubing is also kept hot (60C) so as to prevent
condensation of the issuing vapor in the line.
While filling the reactant gas in the chamber (carried by argon), the pressure
in the chamber starts increasing. At such point, the 16 inch gate valve (between
reaction chamber and the diffusion pump) is kept only slightly open to maintain the
pressure of the chamber at around 1.0' ~orr. It sometimes takes a few minutes before
the system becomes steady and can maintain the desired pressure and temperature.
After the deposition is carried out for a specific period, the gas delivery to the
chamber is cut off and the gate valve is kept open. Simultaneously, the temperature
setting in the controller is slowly lowered. The reaction chamber is allowed to attain
a low pressure (10-6 torr), while the system starts cooling off. Finally, before opening
the bell jar, the 16 inch gate valve is shut off, permitting the diffusion pump to be kept on.
4.2 Experiment 2: Deposition ofthe TEOS-SiOzThin Film onto Polyimide Composite
Substrate by PECVD Technique
4.2.1 Materials:
(1) Reaction Chamber: Half of the reaction chamber is made of glass and the 47
Silicone Gasket RF electrode / Glass ~L Brass ~,-~
Reactant Inlet
/ React1o~ ./ I Chambe/- Substrate RF electrode I Heater Vacuum
Figure 4.3. Reaction chamber for lEOS-Si02 deposition by PECVD.
other half is made of brass. The horizontal tubular shaped reactor is shown in
Figure 4.3. These are two open ended cylinders designed to fit into each other to form a closed chamber. The dimensions of the chamber are 28 em (length) by 10 em (diameter). The chamber is sealed with a flat silicone gasket which seats on the raised tip on the half portion made of brass. The gas delivery tube is located at the back of the glass chamber and the connection to the vacuum hose is located at the other end.
(2) Radio Frequency (RF) Generator: The RF generator is a solid state crystal controlled oscillator designed to provide up to 150 watts of continuous wave 48 13.56 MHz power to the reaction chamber. Maximum power transfer to the reaction chamber is accomplished by matching the output impedance of the amplifier to the input impedance of the reaction chamber. Two toggle switches provide manual tuning control for matching the 50 ohm output impedance to the capacitive load of the reaction chamber. In the event of excessive impedance mismatch, an audio assisted alarm sounds to indicate a detuned plasma.
(3) Vacuum system: The vacuum system includes the vacuum pump, the vacuum pump hose, the vacuum valve, and the control circuitry. The vacuum valve is controlled in series with the RF generator switch to prevent the RF power from turning on unless the chamber is evacuated. The vacuum pump used in the system is manufactured by Alcatel. The model 1004 AC Chemical Series pump is designed for pumping corrosive gases by virtue of its being charged with perfluoropolyether oil. It has a pumping capacity of 4.5 M3/hr (air displacement) and can attain vacuum down to 10-3 torr range.
(4) Gas Supply System: The gas supply to the reaction chamber consists of the glass tube sealed on its end. Connections to the delivery tube are fastened with special clips to prevent the possible leakage of contaminants into the chamber.
(5) Audio Assisted Tuning: The audio assisted tuning and the protection circuit consists of a sonalert and its associated circuitry. As the impedance matching between generator and the chamber degrades, the reflected RF power increases.
When the reflected power exceeds the maximum allowable level, the sonalert is activated to indicate potential damage to the RF generator. The system must then 49 be retuned by keeping the reflected power to an acceptable minimum level which
will silence the alarm.
(6) Safety Interlocks: Three power interlocks are provided into the instrument
to prevent injury to the operating personnel.
Door interlocks: cuts off AC to the RF power supply.
Right side front: shuts of all primary power.
Left side front: Shuts of all primary power.
(7) Bubbler and the Gas Delivery system: A 250 ml conical flask with a
stoppered inlet and outlet tubing is used as the bubbler. Stainless steel tubing of
diameter 0.25 inch, with Swagelok connections are used in the delivery lines. Two
0.25 inch needle valve are used each in the inlet and outlet lines. The same
rotameter assembly as used in the previous experiment is utilized in this experiment.
(8) Substrate Heater and Controllers: The Omegalux WS series strip heater
is used. It has two terminals placed at a right angle on one end. The dimensions of
the heater are Scm (length) by Scm (width). A Minitrol Power controller is used to
control heating. It is manually adjusted from 5% to 100% of full power. This power
range corresponds to the temperature range between 60C to 450C.
(9) Temperature indicator: A thermocouple (type J) is provided with the heater to monitor the temperature. A digital thermometer supplied by Omega is used.
(10) Cooling Water system: To prevent deposition on the walls of the reacting chamber, a coiled copper tubing (diameter 0.25 inch) is brazed to the outside of the 50 portion of the chamber made out of brass. The inlet and outlet of the copper tubing
are connected by tygon tubes.
(11) Pressure Sensors: The thermocouple type gauge tube and the
corresponding indicator as described in the previous experiment are used.
(12) Carrier gas: Argon is used as the carrier gas and is supplied by AGA Inc.
(13) Reactant: Tetraetbylorthosilicate(TEOS) with 99.999% purity, supplied
by Aldrich Chemical, is used as the reactant.
(14) Substrate: Polyimide matrix and carbon fibre reinforced composite
(PMR-15) as supplied NASA Lewis Research center is used as the substrate. Along
with it, silicon wafer is also used as the substrate which serves as the witness coupon
to measure the thickness and stress of the films. The structure of the PMR-15 is
shown in Figure 4.4 .
4.2.2 Analysis of the Films.
The composition, thickness, refractive index, density and stress in the
deposited film are measured using the same procedures as described in the previous
section. The film deposited on the polyimide composite is examined under a scanning electron microscope (SEM). Also, the deposited film is subjected to peel adhesion tests to examine the adhesion of the film to the substrate.
a) Scanning Electron Microscopy: The interactions of the energetic electrons with the surface of a material are used in scanning electron microscopy. Due to such interactions, both secondary and backscattered electrons are emitted. Secondary COOMe H~O- ~INH2 MeOOC~0 ~ COOMe 2 I + + 2 3 HC-0 ~ OXCOOH 2 HOOC&--l0cooH NE MDA BDTE
Heat. Pressure -H ...CH 20 30H
o o 0 o I I I I I C" C N poe ~N/ / H 'c:©-~~:N CH~ -, c 2C-@rN 2 C I I I I o o 0 o
Figure 4.4. Structure of PMR-15.
U\ ~ 52 electrons have a very low kinetic energy « 50 ev) and are ejected from the target
atoms due to excitation by a small amount of energy from an incident electron..
Backscattered electrons are the ones from the incident beams which have been
subjected to scattering (primarily elastic) which results in the electron escaping from
the target. The energies of these electrons range from the incident beam energy
down to the very low energies of the secondary electrons. In addition to these
events, the electrons of the incident beam can give rise to characteristic X-rays. X
rays are generated when an electron excites a target atom by ejection of an electron
from the inner shell and the atom returns to the ground state by emission of
electromagnetic radiation. So the generated X-rays have energies which are characteristic of the atoms in the target. The imaging of these backscattered electrons or characteristic X-rays are done in SEM, which has a resolution limit of about 1 micron.
A simple scanning electron microscope consists of a source of electrons, an accelerating electrode, a series of electron lenses to provide a very finely focused spot of electrons on the sample, the specimen, detectors, and the electronics necessary to image the signal of interest. The signal can be a specimen current, the backscattered or secondary electrons, or characteristic X-rays. The incident electron beam is focused to a fine spot on the sample and the position of the spot on the sample is moved in a pattern called a raster. When the beam is moving, the signal from one of the detectors is displayed on a cathode ray tube, whose beam is also being moved in the same pattern (raster) as the electron beam on the sample. So the picture on 53 the cathode ray tube a presents a very magnified image of the signal from the
sample. For a detailed description of SEM one may refer to Goldstein et. al.,1981.
b) Peel Adhesion test: This is a standard method for measuring adhesion by
using Scotch tape, as recommended by ASTM. It assesses the adhesion of films to
various metallic and non-metallic substrates by applying and removing pressure
sensitive tape over the film. These methods are used to establish if the adhesion of
a coating to a substrate is at a generally adequate level.
A film area is selected which is free of blemishes and surface imperfections.
A piece of tape (0.5 inch long) is placed over the film and the place over the tape
is smoothed by finger and then is rubbed firmly by the eraser end of a pencil. The
color under the almost transparent tape is a useful indication of when good contact has been made. After about one minute of application, the tape is removed by seizing the free end and pulling it off rapidly at an angle of 180 degrees(or as close as possible). The applied area is then inspected for removal of coating from the substrate.
c) Scratch Test: This test was performed by Scratch Tester (CSR-02) developed by RHESCA, with the technology transferred from Tokyo University. A phonograph cartridge designed for this system was used as the sensing unit, which vibrates at 30 Hz at the amplitude of 80p,ffi (max.). A diamond cone, polished into a certain radius, was used as the stylus. On an inclined stage a sample was placed and the stage moved at a speed of 30J.Lm/sec in the horizontal direction. The stylus 54 was placed on the surface of the film and scratches the film surface as the cartridge
vibrates. The pressure of the stylus on the sample increases to up to 1 N by the
elastic force of the cantilever. Due to the vibration of the cartridge, when the stylus
touches the film surface, a frictional force is generated, and the stylus initiates a
relative motion against the cartridge. This relative motion is transduced as an output voltage from the cartridge. Until the film is broken, the relationship between the load of stylus and the output voltage is equal to the relationship between the load and frictional forces. When the fim is broken, the breakage is detected as distortion of the output waveform, from which the type of breakage and the critical load can be analyzed.
4.2.3 Experimental Procedure
A schematic diagram of the expermental system is shown in Figure 4.5.
The deposition of Si02 films on polyimide composite substrate is performed in a plasma enhanced chemical vapor deposition (PECVD) reactor, which consists of a process chamber with a capacitively coupled radio frequency generator(13.56 MHz).
In addition to rf power, an electrically powered substrate heater is positioned in the center of the deposition chamber. Polyimide composite samples (2 sq.cm) and silicon wafer pieces are placed horizontally on the heater. The silicon wafer serves as the witness coupon for characterization of process parameters.
Before delivering the reactant into the chamber, the system is purged with argon for five minutes. This is done to drive away any contaminants present in the ROTAMETER-.-.- \J COOLING REACfOR coa/ I 1
TC
TCGAUGE ~ HEATER C\J <, RF'" I ELEcrRICAL ELECTROI)E- CONNECTION PLATE TO VENT HEATER t ARGON GAS CYLINDER
VACCUUM PUMP
Figure 4.5. Schematic diagram of the PECVD experimental system. U\ U\ 56 chamber and to minimize unwanted side reactions. Tetraetylorthosilicate (TEOS,
liquid, 99.999%) is placed in a conical flask, which serves as the bubbler and is
heated to 60C to enhance vaporization of the liquid. The carrier gas, argon, is
introduced into the bubbler by 0.25 inch diameter stainless steel tubing. The
entrained vapor is carried to the reaction chamber through stainless steel tubing
which is heated to 60C by heating tape wrapped around it to prevent condensation
of the reactant vapor inside the tubing. The argon flow rate is monitored and
controlled by passing the gas through a rotameter installed in the line. The pressure
in the chamber is controlled by a throttle valve and is measured by a thermocouple
gauge. The deposition is carried out ,in the pressure range 0.5 to 1.0 torr and the
temperature of the substrate is maintained from 265 to 420C range. The rf power
is turned on only at such pressures when a pink white glow indicates the onset of
plasma conditions inside the reactor.
4.3 Experiment 3: Deposition of DES-Si02 Thin Film onto Polyimide Composite
Substrate by LPCVD technique
4.3.1 Materials
1) Reaction Chamber: The reaction chamber is made of quartz. It consists of an open ended tube of dimension 10 em diameter and 15 em long. It is shown in Figure 4.6. The tube is positioned around the substrate heater and is held in a fixed position by means of a an aluminum disc and a rubber o-ring which provides seal around the tube at one end. At the other end of the tube a small (5 57
Pressure Sensor
Substrate Heater Silicone Gasket "~L ,- To Vacuum Reactant Inlet
Aluminum Disk Glass
Figure 4.6. Reaction chamber for DES-Si02 deposition by LPCVD.
em long) piece of quartz tube with a slightly smaller diameter acts as an inner tube.
The edge of this inner tube is raised at one end in the shape of a flange, and a silicone gasket is placed around it to act as a vacuum seal. The quartz tube and the heater assembly are installed inside the housing used for the first experiment. The main advantage of this new chamber is its volume. Without this the belljar used as the reaction chamber is so big that it is very difficult to maintain an appreciable concentration of the TEOS inside. As a result of such dilution effect inside the reaction chamber it often becomes too cumbersome to make the deposition possible.
2) Vacuum Pump: Two vacuum pump are used to improve evacuation 58 of the chamber and a lower base pressure. The pumps are manufactured by Edwards
Inc. The outlet tubing to the vacuum pumps is attached to the aluminum disc.
3) Substrate Heater and the Temperature Controllers: The same
assembly of heater and the controller as used in the first experiment, is used.
4) Pressure Sensor: Series 275 analog readout convectron gauge
manufactured by Granville-Phillips, is used. The gauge tube is installed horizontally
so that the port is oriented vertically downward to ensure that no system condensates
or other liquids collect in the gauge tube. The transducer is a Pirani gauge providing
rapid response, good accuracy, and stable calibration. The analog indicator has a
range of pressure from 1000 torr to 5, millitorr.
5) Bubbler and the Reactant: The reactant (DES) and the vessel
(bubbler) are supplied by Schumacher. The bubbler is equipped with diaphragm
breakseals in both inlet and outlet stems. It includes teflon valves attached to each
stem. These valves are shipped in their open position. The incoming gas line from
the argon cylinder is attached to a 0.25 inch teflon valve with the help of a swagelok
fitting. The outlet line from the bubbler to the reaction chamber is attached to the
0.75 inch teflon valve. The outlet diaphragm is broken by turning the valve handle
clockwise. The inlet diaphragm is broken by turning the valve handle counterclockwise.
Diethylsilane, [SiH2(~H5)2]' 99.99% is supplied by Schumacher.
~H5)2 Physical Data of SiH2(
Formula weight: 88.2 59 Boiling point: 56 C
Density: 0.6843 gmjcc (20C)
Vapor pressure: 207 torr (20C)
Oxygen (mixture of70% oxygen and 30% nitrogen) is supplied by AGA
Gas Inc.
5) Mass Flow Controller: A mass flow controller, model UFC 9350,
manufactured by Unit Instruments Inc., is used to control the flow of DES vapor into
the reaction Chamber. The mechanism of the mass flow control action is the sensing
of the thermal flow (mass flow times specific heat of the fluid). The thermal transfer
due to the temperature difference, between the tube wall and average gas
temperature is small. As a result, the thermal transfer that occurs is due to either
a change in the tube temperature due to flow of gas which is sensed by the
transducer circuitry, or a change in the power level required to maintain a constant
tube temperature. In addition to the thermal flow sensor, an MFC has servo circuitry
and a proportional flow control valve. The flow through the sensor is kept laminar.
4.3.2 Analysis of the Films
The composition, thickness, refractive index, density and stress in the
deposited film are measured using the same procedure as described in the previous
section. The film deposited on the polyimide composite is examined under a scanning electron microscope (SEM). The deposited film is subjected to peel
adhesion test to examine the adhesion of the film to the substrate. 60
4.3.3 Experimental Procedure
A schematic diagram of the experimental system is shown in Figure 4.7.
The deposition is carried out in the reaction chamber shown in the Figure 4.6. Prior
to the deposition the reaction chamber and the delivery lines are purged with
nitrogen to drive away any undesirable constituents. Two rotary vane vacuum pumps
are used to evacuate the system. As soon as the system achieves a base pressure 0.1
torr, heater is turned on. The temperature of the substrate heater is maintained
between 350C to 390C during all runs. Diethylsilane has a high vapor pressure,(207
torr at 20e) so a smaIl amount of carrier gas is passed to initiate the flow. The flow
of DES is controlled by a mass flow controller. The oxygen flow rate is controlled
and monitored by a rotameter. The DES delivery tube is wrapped with heating tape
to reduce the possibility of condensation.
Preceding all depositions, the silicone wafers and polyimide composite samples
are cleaned with acetone. Some of the samples are exposed to air plasma for a very
brief period (one minute) to clean the surface of any impurities. The pressure in the reaction chamber is controlled by a throttle valve and is measured by a convectron gauge. The deposition is carried out in the temperature range 370C to 390C and in the pressure range of 0.5 to 1.0 torr. ROTAMETER I \J REACTOR
MFC TC CNVECfRON GAUGE INLET [><}-- HEATER
OlITLET
TO VENT t
GAS CYLINDER BUBBLER
VACUUM PUMP
Figure 4.7. Schematic diagram of the LPCVD experimental system from DES.
0\~ 62 4.4. Experiment 4: Ion-Implantation on the Thin Films
4.4.1 Ion-Implanter: An ion-implanter contains an ion source in which the ions
are created from a plasma, and a chamber to impinge the ions on the target material.
The chamber pressure is maintained around 10-6 torr. The ions are fired as a
monoenergetic beam from the source to the object in the chamber through a beam
line held under high vacuum. Biasing the source at a voltage of 3 Mev causes the
ions to be accelerated to high speeds and aimed into the chamber as a beam. This
process is line-of-sight. Because of this line-of-sight limitation, a manipulator is
required to rotate the target in the beam to implant all sides of the target. Using a
micropositioning system, small areas of the sample were exposed to a 3.0 Mev beam
2 of He + ions with a beam current 20-60 nA for various length of time (30 minutes
to 2 hours). The schematic diagram of the ion-implanter is shown in Figure 4.8. The
schematic diagram of the Tandem Van de Graaff Accelarator used in the present
study is shown in the Figure 4.8.
4.4.2 Experimental Procedure
Ohio university Van de Graaff Accelarator was used to generate the ion
beam. The ion beam entered a implantation chamber provided with multiple ports.
Specially made sample holder was used to hold and move the sample from outside of the chamber. Si02 coated polyimide samples of size 25 m.m by 10 m.m were subjected to ion implantation. Prior to project the ion beam on the sample, low pressure (10-6 torr) was established in the chamber with the help of a diffusion pump. 63
r
OhiO university Acc.lerator Loboratory
T
Figure 4.8. Schematic diagram of the Ohio University
Tandem Van de Graaff acclarator. 64 Ion Implantation was done at four points along the length of the each sample at every 5m.m distance. The dose of the ion density used was in the range of 20 nA to
50 nA. The implantation time was varied between one to four hours. 65 5.0 RESULTS AND DISCUSSION
5.1. Low Pressure Chemical Vapor Deposition (LPCVD) process.
The deposition parameter for the LPCVD process are listed in Table 5.1.
Table 5.1. Deposition parameters for the LPCVD process.
System Pressure: 400-1000 mTorr Substrate Temperature: 725 - 825 C Gas Composition: TEOS, Argon Carrier Gas Flowrate: 200-250 seem
In the present research, parametric studies were done to find out the range
of optimal deposition parameters for stoichiometric Si02 deposition on silicon wafer and polyimide substrates. After an intensive array of runs over a wide range, the reported values in Table 5.1 indicate an acceptable range of operation. The process conditions for deposition of Si02 by LPCVD are listed in Table 5.2. Of the three important parameters studied, the carrier gas flow rate has been found to have very little effect on the deposition rate and film quality over a threshold limit. The flow during the deposition was maintained in the laminar region. The Reynolds number calulated was in the range 100 to 200. The small variation of the color of the deposited film was indicative of the thickness variation which might be due to the variation of flow. 66
Table 5.2. Process condition for TEOS-Si02 film deposition by the LPCVD process.
Substrate Pressure Carrier gas Deposition Uniformity temperature (mTorr) flowrate rate variation (C) (seem) (AOjmin) (%)
725 1000 200 75 7.0
750 1000 200 100 9.2
750 800 200 80 8.2
750 600 200 65 8.0
750 400 200 51 5.6
775 1000 200 155 10.2
800 1000 200 210 11.0
825 1000 200 200 9.5
5.1.1. Film Thickness
Film thicknesses were measured by a surface profilometer as dicussed in Chapter 4.
The range of film thicknesses measured was from 800A to 2000A A typical thickness profile of a film is shown in Figure 5.1. The uniformity of such films 7 to 11%. It has been found that more uniform films were obtained with the lowering of the deposition rate. Direct measurement of film thickness on those composite samples =l: • • t1: • • • • •- •- 2,000 • • • • • • • • • • • l,S00 • • • • • • • • • ~ ~. • • --r IV-V- - -...... 1,000 ~ ..... · -_.... • .--r-r • • • • • • • ( • • 5013 I • • • • • • • • • • • --~ J • • ~3 -- ~-. • - • • •- •- 1, 600 2 ..(10i3 o 200 400 600 800 1,200 - of'j-- R (:lJR: 905 A @ 1.,052JJt'1 RA= l-t. H t'l I::LIR: 1, 067 A @ 1 J 944J-';'1 ~:L(IAt·'[IEKT ~.~~ Figure 5.1. Profile of a typical Si0 film. 0\ 2 -....J 68 subsequently studied by Rutherford Backscattering Spectroscopy (RBS) confirmeed the values from the witness technique. The maximum deposition rate obtained was 210A/minute. The deposition rate data is a key parameter which was used to see its dependence on other system variables such as reactor pressure and substrate temperature. The deposition rate was found to increase with increasing substrate temperature. This positive dependancy of the deposition rate against the temperature shows more precursor molecules become activated to take part in the reaction as the temperature increased. Figure 5.2 shows the Arrhenious plot of deposition rate against temperature. A linear regression was done and from the slope of the line the I activation energy was calculated and found to be 31.2 Kcal/rnole, The deposition rate was also found to increase with the system pressure as shown in the plot of Figure 5.3. With an increase in pressure, the partial pressure of TEOS was increased, so availability of more TEOS molecules made the deposition rate higher. 5.1.2 Film Composition The stoichiometry of the as-deposited oxides have been evaluated by Rutherford Backscattering Spectroscopy (RBS). The result showed that the oxide deposited at the temperature range 750C to BOOC were near stoichiometric with less than 5 atomic percent hydrogen as the only impurity. A RBS and PRD plot for an as-deposited film (deposition temperature 750C) were shown in Figure 5.4a and Figure 5.4b respectively. 5.8 5.6 ~ .....,(» 5.4 ro L. It c 5.2 •..p0 --en 0 5- C- Ol -0~ 4.8 -c 4.6 4.4 e 4.2 4 9.10e-04 9.40e-04 9.70e-04 11K 1.00e-03 Figure 5.2. Arrhenius plot for Si0 2 deposition from TEOS (LPCVD). 0\ \0 ~ o 1200 e 0C 750 900 e (LPCVD). temperature: pressure (mTorr) e 600 vs. rate Deposition Pressure e deposition 300 5.3. Plot of Figure I I I 1 -I o 01 80 60-' 40 20 140 120 100 c E Q) c 0 (/) 0 C- O) ~ n1 "-" a: :+:i 0 .~ Ener qy (MeV) (1.6 0.8 1.0 '1.2 1.4 1.6 1.8 -) I I CJl.. II I I I I I 50 a_ D ..- ~.~~~lf..) in Si in Si0 .r- o Si02 2 U Q) .... I'.j jl) - 0 E .\~ L ')0 o &.0- ~~-~~Y' Z \: 10 1- Si in bulk silicon o I I I I ~=-=--1 50 100 150 200 250 Cho nriel Figure 5.4a. Ruthrford backscattering spectrum of typical Si0 2 (LPCVD) film. Both the Si and 0 in the film are observed. The peak is due to an oxygen resonance. ...... ,J~ Energy (MeV) 0.6 0.8 1.0 1.2 1.4 1.6 1.8 5 II I I I Iii I 4 .- -0 - ()) >=3·- "D ()) N -- o 2 E L 0 Z 0, I T · I I 50 100 150 200 250 Channel Figure S.4b. Proton recoil detection spectrum of a typical (LPCVD) film. Si02 "-l N 73 Fourier transform infrared spectroscopy (FTIR) over the spectral range 400- 4000 ern" was used to identify the bonding characteristics in the deposited films. A typical transmittance spectrum was shown in Figure 5.5. Both the band at 1070 ern", indicative of Si-O stretching, and the one at 800 ern", indicative of Si-O bending, indicate Si02 composition. 5.2 Plasma Enhanced Chemical Vapor Deposition (PECVD) Process. The deposition parameters of the PECVD process are listed in Table 5.3. Table 5.3. Si02 Deposition Parameters of PECVD. System Pressure: 300-1000 mTorr Substrate Temperature: 265-425C Gas Composition: TEOS, Argon RF Frequency: 13.56 mHz RF Power: 0.22-0.40 W/ crrr' Carrier Gas flow rate: 180-250 seem Parametric studies were done to find the optimum range of deposition variables. The thicknesses of the deposited films were in the range sooA to 3000A. The process conditions of the deposited films are listed in the Table 5.4. The deposition rate for each run was determined by dividing the film thickness by the Q) o c: ro ~ .....- E en c: 1-ro I-- 1500 1000 500 wavenumber(cm-1) '-l "J::.. Figure S.S. FTIR spectrum of a typical Si02 (LPCVD) film. 75 time of deposition. The highest deposition rate achieved was 215 A/min. Film thickness uniformity was calculated from five separate measurements in different locations on the silicon wafer witness samples. The calculation procedure of the film uniformity was the same as described in the section 5.1. The deposition rate was found to decrease when the substrate temperature was increased. The trend is similar to that reported by Chin and Van de Ven (1988), Kulisch, et al.(1989), and Chang, et al.(1990). This behavior may indicate an adsorption controlled reaction. As the substrate temperature rises, the active species gain kinetic energy; consequently, the probability of adsorption is reduced, and that of desorption is increased. Increasing temperature may also promote surface recombination of the adsorbed precursors, thus causing the negative dependence of the deposition rate with temperature. The apparent heat of adsorption, which is the thermodynamic correspondent for the activation energy of the TEOS-SiOz film, was calculated from an Arrhenius plot (Figure 5.6) and found to be 4.1 kcal/rnole (17kJ/mole). This compares very well with an activation energy of 4.7 kcal/rnole (19 kl/mole) reported by Chin And Van de Ven (1988) and in qualitative agreement with a value of 2.6 kcal/rnole (11 kl/mole) repoted by Webb, et ale (1989). In both of these experiments , the investigators used a mixture of TEOS and oxygen as the reactant precursors. The deposition rate was found to increase with RF power (Figure 5.7). With such an increase in power, the plasma density increases with a corresponding increase in the concentration of ions and free radicals formed as a result of TEOS 5.80 5.60 ~ .....,Q) 5.40 «Sl- eo 5.20 .-:p ~ 5.00 0- m ~ 4.80- c 4.60 RF Power: 130 W 4.40 - e Deposition Pressure: 1.0 Torr 4.20 4 00 .. ~------~---r------1 1.20'e-03 1·70e-03 11K 2.20e-03 Figure 5.6. Arrhenius plot for Si0 2 deposition from TEOS (PECVD). ~ 0\ 25 o I , , 50 70 90 110 130 150 RF Power (Watts) '-J Figure 5.7. Plot of deposition rate vs. rf power (PECVD). '-J 78 dissociation. This is expected to enhance the decomposition of the gaseous TEOS species and lead to a higher deposition rate. Table 5.4. Process conditions for TEOS-Si02 film deposition by PECVD. Substrate Pressure Carrier gas Deposition Uniformity Temperature (mTorr) (Ar) flow Rate (A/min) Variation (C) rate (seem) (%) 265 1000 200 215 8.2 300 1000 200 165 24.5 300 800 220 150 6.7 300 600 .250 135 11.6 300 400 200 140 13.4 300 300 180 131 15.6 320 800 250 167 4.8 340 800 200 175 2.6 390 1000 220 123 11.4 425 1000 250 80 22.4 The process total pressure was varied from 300-1000 mTorr and the deposition rate was found to be affected slightly. This is shown in Figure 5.8. Two opposing forces interplay as the pressure is increased. As the TEOS partial pressure is increased, the deposition rate is also increased. Consequently, reduction of the effective diffusivity of the species is likely to change the reaction from the surface- ~ \0 1200 e C 0 900 (PECVD). RF Power: 130 - 135 W pressure (mTorr) e 600 vs. Pressure e Deposition temperature: 330 deposition rate of e 300 Plot 5.8. Figure - 0 0 80 60- 40 20 - 200 180- 160 140 120 - 100- c E Q) ctJ c 0 0 C- O) +-' ~ -- ~ a: -+=i -CI) 0 .~ 80 controlled regime to the diffusion-controlled one, and thus dampen the deposition rate. 5.2.1 Film Composition The stoichiometry of the films was analyzed by Rutherford Backscattering Spectroscopy (RBS) and the Si:O ratio was found to be 1:2 (to within 10%). The hydrogen content in the films was analyzed using Proton Recoil Detection (PRD). The PRD spectrum showed less than 3% hydrogen, and this was the only measurable impurity for all films deposited under the process coditions listed in Table 5.4. Both the experimental RBS spectrum and the simulated (theoretical) result for a typical film are shown in Figure 5.9a. The, PRD spectrum of a typical film is shown in Figure 5.9b. Fourier transform infrared spectroscopy (FTIR) over the spectral range 400 4000 ern" revealed the boding structure in the deposited film. Figure 5.10 shows a typical transmittance spectrum. The strong absorption band at 1070 em" (Si-O stretching) is similar in location to that of thermally grown Si02, which has been reported at 1080 ern" (Pliskin, 1977). The band at 800 Cm-1 (Si-O bending) is also indicative of Si02 (Pliskin, 1977). The FTIR spectrum of the films are includedin Appendix I. 5.2.2 Refractive Index The refractive indices (n) of the films match closely to that of thermally grown Si02 (1.46). Although a few samples were analyzed, covering the deposition E11erg j' ( ~AeV) 0.6 0.8 1.0 1.2 1.'1 1.6 1.8 6l') I i I i i I I II 5() .- -0 .~40- Si in Si02 >- o in Si02 '0 (}> . r'J ~3l) o f: I\~A_ ~ 20 _'vV\ z 10 .- Si in bulk silicon 01 I I I ~1 50 100 150 200 250 Channel Figure 5.9a. Rutherford backscatteringspectrum of a typical Sial (PECVD) film. Both the Si and 0 in the film are observed. The peak is due to an oxygen resonance. , 00~ Energy (MeV) 0.4 0.6 0.8 '1.0 '1.2 1.4 1.6 25. 'i 2.0 0- u - .-(l) >- 1.5 _.- u Q) N -- 0E 1.0·- L 0 z 0.5~' I 0.0 I I I I I 50 100 150 200 250 Channel Figure 5.9b. Proton recoil detection spectrum of a typical Si02 (PECVD) film. 00 N ~. (J) o c CO -..-i::: E CJ) c ~ CO r-'- 2000 1500 1000 500 wavenumber(cm-1) Figure S.lO. FTIR spectrum of a typical Si0 (PECVD) film. 00 2 UJ 84 temperature range of 268C to 390C (measured as the temperature of the heater surface.the temperature of the substrate surface itself averaged approximately lOC less than the heater surface), behavior similar to that seen previously (Rojas, 1989) was observed. This behaviour is an increase in refractive index with increasing temperature to a point (in the present case, 300C), followed by a decrease at still higher temperatures. The refractive index of the films analyzed were shown in Table 5.5. Table 5.5. Values for refractive index and deposition temperature. Refractive Index (n) Deposition Temperature (C) 1.4611 268 1.4672 300 1.4615 390 The ellipsometric data of refractive index of the films analyzed are shown In Appendix II. 5.2.3 Film Density The film densities (Table 5.6) were calculated from the refractive index using the Clausius- Mossotti (CM) relationships (Rojas,et al., 1989). k(n2-1) p=---- n 2 +2 85 where p is the density and k is a constant equal to 8.1148. The value for the constant is obtained empirically from data obtained on thermally grown (920C) Si02• Table 5.6. Values for film density and deposition temperature. Density (gm/crrr') Deposition Temperature(C) 2.227 268 2.252 300 2.229 390 5.2.4 Etch Test A wet eteh test was also performed on the SiOz films. Upon exposure to a 100:1 H20:HF solution, the etch rate was observed to vary between 145A and 225A per minute. This compares favorably with other reported etch studies (Huo, et al.,1991) and indicates a reasonably dense film with little or no pinholes or other bulk defects. 5.2.5 Adhesion of the Films To test the adhesion strength of the films, peel adhesion ("Scotch tape") tests (ASTM, 1984a; ASTM, 1984b) and scratch tests were performed. The results of the peel adhesion tests, as shown in Table 5.7, indicate the thinner films (less than loooA thick) were more adherent (as judged by the percentage of the film surface under the 86 tape which remained on the substrate) to the composite surface than were the thicker films (generally greater than 2000A thick), which appeared to lose adhesion to the substrate. This behavior is most likely due to increased tensile stress in the thicker films. Table 5.7. Peel adhesion ("scotch tape") test results. Film Thickness (A) Passed Test ? 500 yes 800 yes 1000 partial 1500 no 2500 no 3500 no 5000 no Scratch Test. Two films made by PECVD from TEOS were subjected to scratch test. The test conditions for film 1 are listed in Table 5.8. All these tests were done with the stage motion parallel to the grain of the substrate. The noisiness of the data plots for this sample were attributed to the substrate roughness. The critical points were chosen to denote positions where a change in slope occurred or the pattern of 87 Table 5.8. The scratch test conditions for a typical PECVD film. No. Excitation Stage Angle Stage Speed Stylus level (urn) (deg.) (um/s) Radius (urn) 1 60 3 10 15 2 60 3 10 15 3 60 3 10 100 4 60 3 10 100 5 40 3 6 100 6 40 3 6 100 ..., 7 40 .:> 6 15 8 40 3 6 15 the data plot changed. The position labelled A on test 2 denotes a corresponding position on a photo where major film deterioration occurred (Appendix III). Test 3 and 4 were at the same conditions as 1 and 2 but using a different a stylus radius and they also yielded a repeatable results. By using microscopic observation, test 5 and 6 did not seem to break the film. From such tests it was verified that major film deterioration occurred at approximately 14 mN. It seemed that using the 15 urn stylus yielded the best results on this film. All scratch test data are included in the Appendix III. 88 5.2.6. Stress of the film. The radius of curvature due to film deposition was measured geometrically from the profile of the wafer. This measurement are generally done by a "stress gauge". Because of the small size of the sample and non-availability of the stress gauge, this simple and unconventional technique was applied. Stress in films with thickness varying from 1000A to 2000A was measured. For such films, the calculated stress was found in the order of 1013 dynes/em. which was substantially higher than expected order of 1010 dynes/em. This may be attributed to the limitation of this measurement technique. A sample calculation was given in the Appendix IV. 5.2.7. SEM Micrographs. Several polyimide composite samples both coated Si02 and uncoated ones were observed under SEM to reveal morphological features of the substrate-film structure. The SEM micrographs are shown in Figure 5.11 to 5.14. By comparing as received composite samples with coated ones, the latter appeared to become somewhat smooth on its surface. SEM micrographs of a few coated samples after an exposure at 390C for 300 hours were also taken. These samples clearly showed the thermo-oxidative effect on the surface. Deep cracks are seen and the surface looked partly fused with formation of islands. 89 Figure 5.11. SEM micrograph of as received PMR-15 composite sample. Figure 5.12. SEM micrograph of Si02 coated (lOOOA) PMR-15 composite sample. 90 Figure 5.13. SEM micrographof Si02 coated (800A) PMR-15 after an exposure at 390C for 300 hrs. Figure 5.14. SEM micrograph of Si02 coated (IOOOA) PMR-15 after an exposure at 390C for 300 hrs. 91 5.3. Low pressure chemical vapor deposition (LPCVD) of DES-Si02- The deposition parameters for the process are listed in the Table 5.9. Table 5.9. DES-Si02 deposition parameters. System pressure: 1000 mTorr Substrate temperature: 390-420C Gas composition: DES, oxygen, nitrogen Ratio of DES: Oxygen = 1-2 : 2 The process conditions for deposition ofSi02 from DES by LPCVD are listed in Table 5.10. The deposited film thickness varied from sooA to iooos, At an oxygen:DES ratio less than 2, no film was deposited; instead a fine white powder was produced inside the reaction chamber. This powder was a product of gas phase reaction at low oxygen:DES ratio. FfIR analysis of the film revealed the strong primary peak at 1075 ern" (shown in Figure 5.15) which pointed out the presense of near stoichiometric Si02 composition. 93 Table 5.10. Process conditions of DES-Si02• Substrate Pressure Oxygen/DES Deposition temperature(C) (mTorr) ratio rate(A/min) 390 1000 0.5 No film 390 1000 0.75 No film 400 1000 1.0 No film 400 1000 2.0 143 420 1000 2.0 172 5.4. Ion Implantation. Ion implantation was done at four points along the length of the polyimide coated with Si02 film (thickness 5000A) sample at every 5mm distance for a period of two hour with a 2 Mev beam of He + + at a fluence of 11 nA. The scotch tape adhesion test was then performed on the implanted sample. No film was observed to peel off the sample. To determine the presense of Si on the composite sample EDAX was performed and the presense of Si was noted. Another polyimide composite sample coated with Si02 film (thickness 5500A) was subjected to peel adhesion test. It was clear that a substantial portion of the film peeled off. To be sure, further RBS analysis was done on the sample surface both 94 at the tested and the unabused regions, RBS revealed that the good amount of the film was peeled %Sma off at the tested region. I) s~coaQq 3000 A tbici: OIl !'MR.I$ pdyinUde I Ha-b ...... _ A companion of the first ~-----~~-6 sample (coated with sooaA film together) was then subjected to ion implantation. The dose of the ion t5 'SA~= density was increased from 20nA to ~ e) AltJt6...... -..adhesi..1St OIl nebt Iide. onr, ioDimpj.amed SOnA. After continuing for four . hours, Scotch tape test was done. Figure 5.16. Adhesion improvement by ion implantation. The particular region where the film was ion implanted, adherence was improved. Except for a small rounded off region, as shown in the Figure 5.16, film was peeled off by the Scotch tape test. 5.5. Mass loss experiments. Several coated samples were tested for their ability to resist oxidation by exposing them to high temperature in an ambient environment for increasing lengths of time. Uncoated samples were also tested for comparison. The masses of samples were monitored at regular intervals for up to 300 hrs at several temperatures in the range 380-400C. The results indicated that the coatings were effective in preventing 95 thermooxidative mass loss under these atmosphere and temperature conditions. The weight losses after exposure of the coated samples were all significantly lower than that of the uncoated one. This is shown in Figure 5.17. Figure 5.18 shows the observed percent mass loss after 300 hours as a function of SiOz film thicknss. There appears to be a minimum in this curve in the BOO-lOOoA range, suggesting that thinner films gave poor coverage, perhaps at the edges and corners. Thicker films were observed to spall (as a result of tensile stress, as discussed before), which would likely result in defects permitting oxidation of the substrate surface. The best results were obtained' with twice coated samples as shown in Figure 5.19. In these cases, one film of 1250-150oA was applied, the chamber brought to atmosphere, and then the deposition process repeated to apply a second coating directly over the first. A possible explanation for this behavior may be elucidated from studies done by Crowell, et al.(1990 and 1991). They investigated the fundamental mechanism of Si02 film growth from TEOS onto Si02 surfaces, and found that partial dissociation of the TEOS molecule occurs upon adsorption to the SiOz surface, leaving two-to-three ethoxy ligandsper reaction site. This adsorbed species then decomposes to form Si02 by beta-hydride elimination. They also point out the involvement of hydroxy groups in the initial reaction of TEOS with the Si02 surface. Thus, consumption of hydroxyl groups to produce adsorbed siloxane on Si02 surfaces is a possible mechanism and is likely to produce more uniform coverage for the second film. 100 98- 96 - VJ (J) «J 94 - :E o 300A eu 92-' c -- I 500A en -- * ·C 90 0 I o 800A ~ 8B- 0 +-' \J 900 A c: I 0> 86- .....u I 6 1000 A ID 84- a. + 1250 A 82 - o 2000A 80- 0 uncoated 78 0 50 100 150 200 250 300 OvenExposureTime at 3900 C (hr) Figure 5.17. Effect of exposure to ambient atmosphere at elevated temperature on the mass \0 0'. of Si02 coated samples of PMR-15 polyimide composite. \0 --...J --, 2500 • • ""-- •• • J • -_.- • o 5 o~ 20 15 10 It... (f) en 0 (IJ (IJ ctS c Q) o '- Q) ca 0 +-' .c: 0 (t) ..J :E 0... 100 95 - (I) (I) «1 ~ 90- ca c CJ) -;:: 0 .... a 85 - ... c: Q) 0 L- a> 0- 80 -1 o uncoated \1 3500 A (twice coated, 1500 A,then 2000 A) 75 -1 6. 2500 A (twice coated, 1250 A,then 1250 A) 70 I I I I I I I o 50 100 150 200 250 300 Oven ExposureTime at 3900 C (hr) Figure 5.19. Effect of exposure to ambient atmosphere at elevated temperature on the mass \0 00 of PMR-15 polyimide composite samples "twice coated" with Si020 99 As a check, polyimide composite samples were heated to 390C in a nitrogen- only atmosphere at a pressure of 0.01 Torr. A small amount of mass loss (less than 1%) was observed after 200 hours. This is likely due to loss of volatile impurities, but overall indicates that the composite is not subject to pyrolytic decomposition at this temperature. It is important to note that oxidation of the coated samples was not altogether eliminated, and this is likely due to oxidation at the edges or corners of the samples. Current efforts are directed to further improve the coating of the deposited sample so that better oxidation resistance is achieved. I To examine the effectiveness of the Si02 coated polyimide composite in the plasma environment (air plasma), coated samples along with uncoated ones were put inside an plasma asher. The mass loss occurred in the coated ones were significantly lower than that in the uncoated one. This result is shown in Figure 5.20. The coated and uncoated samples exposed in nitrogen plasma showed very little mass loss which again indicated the intrinsic stability of the composites. 100 BI"e:ti--- (d 90 I ~ ~ / UJ 80 (fJ ~ Cl1 1 + uncoated :E 0 (ij 500A c: 70 I * CD • at: o 900A ~. 0 '4- ~ 0 60 0 1200A ...... c: Q) 0 6 1600A '"'- ~- Q) 50 0.. 0 1600 A(twice) I 40 2 ~ o o 101 6.0 CONCLUSIONS AND RECOMMENDATIONS Near stoichiometric silicon dioxide films were deposited from tetraethylorthosilicate (TEOS) by LPCVD and PECVD onto silicon wafer and polyimide composite (PMR-15) substrates. A more recent organic precursor, diethylsilane (DES) was also proved effective for SrO, deposition at lower temperature by the LPCVD technique. The growth rate of the films was correlated with process parameters and was found to proceed by an adsorption controlled reaction mechanism (in the case of PECVD), which is substantiated by the observation that the deposition rate decreased as the substrate temperature was increased. At thicknesses ranging from 1000-3000 ~ the TEOS-SiOz film provided ! significant thermooxidative protection to polyimide composites at temperatures of 390-400C. Preliminary results also showed that subjecting such films to ion implantation led to an enhancement of adhesion to the substrate. Like any useful scientific and technological pursuit, this research has answered some pressing problems of today as well as highlighted the challenges for tomorrow. The finding of this work has opened broader and promising possibilities for further advancement to come. However, the singlernost achievement of this work can be stated generally as to introduce an new idea for composite protection against harsh environmental situation as may be encountered in space. The results of this research convinced and encouraged NASA scientists regarding the effectiveness of the thin SiOz film acting as a protective barrier being deposited by CVD technique. In addition to such applications, this research has also demonstrated the possibility of 102 preparing high quality Si02 film (which is being extensively used in the manufacture of microelectronics) from less painful sources like TEOS and DES at not so severe process conditions. Finally I would like to mention the fact that while this dissertation is being written and argued, some of the composite samples coated by PECVD technique in our laboratory are destined to travel in space by a shuttle flight so as to measure the shielding capability of the film against exposure to atomic oxygen fluence of about 2 x 1020 atoms/sq.cm, The salient features of the recommended work are presented as follows. (1) To complete the characterization of the LPCVD process to deposit SiOz film from DES which was left incomplete due to partial destruction of the experimental setup by an explosion. (2) To design and install a larger PECVD reactor equipped with finer electronic controls so as to coat a larger dimension substrate (3 inch diameter) to make the analysis of the film and the interface easier. (3) To study the conformality of the coatings, aluminum trenches with a very small clearance should be used as a substrate. This will help in understanding the effectiveness of the coating at the edges and corners of the composite samples. (4) To understand fully the behavior of the coated samples against thermal cycling, an arrangement has to be made to cool the sample quickly at a specified temperature. (5) Post and pre-surface treatment of the samples should be further investigated. Preliminary study indicated its usefulness in enhancing adhesion of the 103 film to the substrate. This can be done to expose the sample either to ultraviolet radiation or to ion irradiation. A test matrix may be made to quantify the dose and time of such exposures. 104 REFERENCES Adams, A C., Proc. Electrochem. Soc., 111 (1986). Adams, A C., and C. D. Capio, J. Electrochem. Soc., 126 (6), 1042 (1979). Baglin, J. E. E., Mat. Sci. & Engg. B1, 1 (1988). Baglin, J. E. E., Ion Beam Modification of Insulators, Ed. by P. Mazzoldi and G. W. Arnold, Elsevier, Amsterdam, ch. 15, 585 (1987). Baglin, J. E. E., H. J. Clark, and J. 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Rutledge, S., B. Bank, F. Difilippo, J. brady, T. Dever, and D. Hates, NASA Technical Memo. 100122, NASA Jet Propulsion Lab., California, Nov 10-11 (1986). 107 Sherman, A., Thin Solid Films, 113, 135 (1984). Takahasi, R., Y. Koza, and K. Sugarwara, J. Electrochem. Soc., 119(10), 1406 (1972)0 Tobin, P. J., J. B. Brice, and L. M. Campbell, J. Electrochem. Soc., 127, 2222 (1980). Tomborello, T.A, Proc. Mater. Res. Soc., 25 173 (1989). Wertheimer, M. R., J. E. Klemberg-Sapieha, and H. P. Schreiber, Thin solid, Films, 115, 109 (1984). Zimcik, D. G., and C. R. Maag, AIAA, J. spacecraft & Rockets, 25, 162, 1988. Zimcik, D. G., R. C. Tennyson, L. J. Kok, and C. R. Maag, Proc. Third European Symp. on Spacecraft Materials in Space Environment, The Netherlands, ESA SP-232, P-81-89, (1985). 108 APPENDIX I The FTIR spectrum of the LPCVD and PECVD films are included in this section. The specification of the films are as follows. Sample Thickness/A) Deposition Deposition Temperature(C) Pressure/m'Iorr) PE-1 800 268 1000 PE-2 1000 300 1000 PE-3 800 300 800 PE-4 800 340 600 PE-5 1000 390 1000 LP-l 800 725 1000 LP-2 1000 750 1000 LP-3 1200 750 800 LP-4 1000 775 600 109 ~ tIJ• ~ ~ ...... v ,... c, I E 0 ~ 0 E C'-) ~ 0 0 0 ,.... ~ '- a Q) ...2 (J .c t) 0.. E C'-) :::J ~ c: Q) ~ ca> ....-4. ~ < e ....i 0 ~ 0 It) ,.- souannusuarj, 110 N I IlJ ~ 0 ~. ~ 0 .,- e~ I cd 0 tI'.) 0 E ~ ,... o 0 '---' e a.. 2 ~ (,) aoUBU!WSUBJ.L 111 ("t') ~• ~ ~ .,... co 0 I ...... 0 0..e 0 E cd o VJ ..,... ~ ~ 0 a... CD a .....2 .c u Q) E 0.. ::s en c: ~ CD ~ E ...:~ ~ < ~ 0 ...... ~ 0 ~ ..,...LC) aoueu,IWSUBJ..L 112 o o LO -.i- ~• P-.c t) ~ ~ ..... 0.s· I ~ CJ'.) 0 E t+-t 0 0 0 ~ 0 E ~ .....2 ..... Q) (J Q) .c 0. CJ'.) E ~ :Jc::: ~ Q) > ~ a1 ..< ~ f ....~ ~ -0 0 ID,.... aoUBJJ!WSUBJ.l ~ c ca --:s:: E UJ c: l-m i- 1500 1000 500 wavenumber (cm-1) . Figure AI.S. FflR spectrum of sample PE-5...... UJ 114 o o ,....o / o o ,...L() eoueunusuaij, 115 ,..--- --..,. ----. 0 o LO (,J, Q.. ..J ~ 0 'y- ~ I E ~ ~ 0 E e..- O o c '--' E 0 '- 'y- Q) ...... 2 u 0 .c ~ E c.I:) :;, ~ c:: Q) t r---: > ..: ca < ~ Q.> -~= fi: 0 0 LC) 'y- aouBUlwSUBJ.L 116 ("f') I Q.. ~,.... ~ 0 0 I 0.. E 0 E c-: 0 en ~ c...... ~ ~ 0 Q) E .0 .....'--= U aoueUIWSUBJ.l 117 ~, 0 c. 0 ~ 0 ~ <0 '-r- ,... 0- I E ~ E VJ 0 ~ ~ 0 E l- ~ '- CD ~ u .c 0 0.. E VJ ::J ~ c: ..... CD G: > e-\ as ..: < 0 ~ ~ 0 ~ LO e.o= '-r- ~ aoueUIWSUBJ.l 118 APPENDIX II The index of refraction data of the samples are included in this section. The specifications of the films are as follows. Sample Thickness(A) Deposition Deposition Ternperature(C) Pre$ure(mTorr) PE-1 800 268 1000 PE-5 1000 390 1000 PE-7 1200 300 600 LP-l 725 725 1000 Index of Refraction for sample .PE-1 1.4961 1.491 6 1.4871 c .0 1.4826 -f.Jo ~1.4782 'to Q) (k: 1.4738 '+- o 1.4693 x Q) -g1.4648 1.4603 1.4558 ,.~ 1.451 3· ',,,,,,,., I',,,I' I 3000 3720 4440 51 60 5880 6600 7320 8040 8760 9480 10200 Wavelength (A) ~ ~ \0 Index of Refraction for sample PE-5 1.483 1.480 1.477 c .0+J 1.474 o ~ 't 1.471 (l) 0:: 1.468 \t- o 1.465 x Q) -g1.462 1.459 1.457 1.454' ',·,,,,,·,·,·,·, ... ' -z== , 3000 3720 4440 51 60 5880 6600 7320 8040 8760 9480 10200 Wavelength (A) ~ N o Index of Refraction for sample PE-7 1.51 6 1.51 2 1.507 c .2 1.502 4-'o ~1.497 "(l) ~ 1.492 '+- o 1.487 x Cl> -g1.482 1.477 1.472 1.467 1 ' I,I, I'==::' 2 I' I, I' I,I ,= I 3000 3720 4440 51 60 5880 6600 7320 8040 8760 948010200 Wavelength (A) ..... N~ Index of Refraction for Sample .LP-1 1.4897 1.4853 1.4808 c .21.4763...... o ~1.4718 'f Q) 0:: 1.4674 'f- o 1.4629 x (J) -g1.4584 1.4539 1.4495 1.4450' ,,,,,,,,',,,, II', 1:::7">n aI 3000 3720 4440 51 60 5880 6600 7320 8040 8760 9480 10200 Wavelength (A) ~ N N 123 APPENDIX III The scratch test results are of the coated samples are included here. The specifications of the samples are given as follows. Sample Film thicknessrA) Deposition Deposition Temperature(C) Pressure(mTorr) 507 800 350 1000 508 1000 350 1000 T1tIe: CSA-02 RHESCACoI' LTD 1992/6/7 OSU507-! WITHGRAIN 15.(/,~, Sensor No.3, 100.0 [g!mm]. r • iOO;~rum] LOAD RATE: 61.36 [mN!mm] STAGE ANGLE: 3.00 [deg] STAGE SPEED: 10 .0 [um/II] EXCITING LEVEL: 60 rum] Xi X2 /.-'- •I •I I I I o'"---- &_-4-- L --t-- L ..L _ ._ ..L _- ~ [min] Scratch DATA 30.9 [mN/m1n] Xi: 0.297 [mill] X2: 0.723 [nun] L: 0.426 [mm] We: 2.23 [gf] (21.87 [mN]) ~ N~ ~ N U\ [min] [mN!mln] rum] [dug] 30.7 --.-- , BO 1.. [mm] 3.00 A rum] 0.425 ------ LEVEL: 1992/5/7 t- • I ~ i I , I 1 - L: ANGLE: .. I)~rnl TO tOO;O •• L [mm] r • STAGE EXCITING Co [liN]) 0.787 DATA ----t--- [g/mm]. [um/e] ... RHESCA [mN/lJIlIl] X2: 10 (21.80 Xi ..-:..t..- . 100.0 SCratch 51.36 CSR-02 [mm] [gf] BOOA ----.-- No.3. SPEED: tIe: GRAIN RATE: 0.363 2.22 T1 OSU507-2 WITH Sensor LOAD STAGE Xi: We: T1tle : CSR-02 RJ£SCA Co•• LID 1992/6/7 OSU507-3 WITHGRAIN Sensor No.3. 100.0 [g/mm]. r • 100.0 rum] LOAD RATE: 51.38 [IIN/IIIID] STAGE ANGLE: 3.00 [dog] STAGE SPEED: 10.0 [um/e] EXCITING LEVEL: 60 rum] Xi X2 I •I I •I I -----~--~---__lf-----L.--.--.L-.-~ [m1n] Scratch DATA 30.9 [mN/m1n] X1 : 0 •263 [mm] X2: 0 •517 [nun] L: 0 •315 [lUI] We: 1.65 [Uf] (16. 16 [tIN]) ~ tv 0\ T1tIe: CSR-02 RHESCACo•• LTO 1992/5/7 OSU507-4 WITHGRAltl Sensor ue.a 100.0 [g/rmn]. r • 100.0 rum] LOAD RATE: 51.36 [mN/mm] STAGE ANGLE: 3.00 [dog] STAGE SPEED: 10.0 rum/a] EXCITING LEVEL: 60 rum] Xi X2 • •I I •I I I • o -----L------l" .---.---.-L-----L __._~ [min] Scratch DATA 30.9 [mN!min] Xi : 0.255 [mm] X2: 0 .594 [11II] L: 0 .339 [mm] We: 1.78 [gf] (17.41 [mN)) ~ tv "-J TltIe: CSR-02 RHESCACo•• LTO 1992/6/7 OSU507-5 WITHGRAIN Sensor No.3. 100.0 [g!mm]. r • 100.0 rum] LOAD RATE: 51.36 [1IN!mn] STAGE ANGLE: 3.00 [deg] STAGE SPEED: 6.0 [um!s] EXCITING LEVEL: 40 rum] Xi X2 o -.----·---·----...-f-----··---l---·L ..~_._-_..- .-..J..------i [min] Scratch DATA 18.6 [mN/m1n] Xi: 0.314 [mm] X2: 0.663 [mm] L: 0.349 [mm] We: 1.83 [gf] (11.92 [lIN]) ~ N ex> Title: CSR-02 RHESCACo••LTD 1992/5/7 OSU507-8 WITHGRAIN Sensor No.3, 100.0 [g/mm]. r • 100.0 rum] LOADRATE: 61.36 [mN!mm] STAGEANGLE: 3.00 [deg] STAGE SPEED: 6.0 rum/e] EXCITING LEVEL: 40 rum] Xi X2 I I .-1-....-'--J _1 t··--__. L.__ j [min] Scratch DATA 18.4 [mN!m1n] Xi: 0.303 [mm] X2: 0.494 [mil] L: 0.191 [mm] We: 1.00 [gf] ( 9.80 [mN]) ...... N\0 ~ Vol o [min] ~ [mN!m1n] rum] [dog] 18.6 40 [mm] 3.00 0.164 rum] LEVEL: 1992/5/7 L: A~tGLE: 15.0 LTD I X2 , I 1 •• [mm] ... r • STAGE EXCITING Co ! [mN]) O. 408 DATA [g,'mm]. [um/B] RHESCA [m~J/mm] 8.43 Xi X2: ( 36 6.0 100.0 Scratch 51. CSR-02 [mm] [gf] No.1. SPEED: GRAIN RATE: 0.244 0.B6 Title: : OSU507-8 WIT~I Sensor LOAD STAGE Xi: We T1tIe: CSR-02 Rf'£SCA Co•• LTO 1992/5/7 OSU507-9 WITHGRAIN Sensor No.1, 100.0 [g/mm]. r n 15.0 rum] LOAD RATE: 51.36 [lIN/11Im] STAGE ANGLf:: 3.00 [dog] STAGE SPEED: 6.0 [um/s] EXCITING LEVEL: 40 [UIn] Xl, X2 I •I I I I I A o-~ _.-_._-- .... · ....I: _. . -. " J.. - - - ...~ [min] SCratch DATA 18.6 [InN/min] Xi : 0.266 [mm] X2: 0.471 [am] L: 0 •206 [mm] Ne: 1.08 [gf] (10.55 [mN]) ~ w~ Title: CSR-02 RHESCACo.• LTD 1992/4/17 PECVD-50B1000A '1 Sensor No.3, 100.0 [g/mm]. r as 100.0 rum] LOADRATE: 68.53 [rnN/mm] STAGEANGLE: 4.00 [deg] STAGE SPEED : 10 rum/a] EXCITING LEVEL: 60 [um] Xi X2 ...... o 2 [min] Scratch DATA 41.0 [mN!min] Xi : 0.323 [mm] X2: 0.570 [mm] L: 0 •248 [mm] We: 1.73 [gf] (16.93 [mN]) ~ W tv Title: CSR-02 RHESCACo.• LTD 1992/4/17 PECVO-508 1000A '2 Sensor' No.3. 100.0 [g/mm]. r I:Z 100.0 [urn] LOADRATE: 68.53 [mN/mm] STAGEANGLE: 4.00 [deg] STAGESPEED: 10.0 rum/a] EXCITINGLEVEL: 60 rum] Xi X2 o 1 2 [min] Scratch DATA 41.2 [mN/min] X1 : 0 •277 [mm] X2: 0 .694 [mm] L: 0 .418 [mm] We : 2 •92 [g f] (28 •58 [mN]) ~ V.) V.) \. 11tIe: CSR-02 AHESCACo•• LTO 1992/5/7 OSU50B-3 WITHGRAIN Sensor No.1. 100.0 [g!mm]. r a 15.0 rum] LOAD RATE: 51.36 [mN/mlll] STAGE AN6LE: 3.00 [dog] STAGE SPEED: 6.0 [um/e] EXCITING LEVEL: 40 rum] Xi X2 , • o - ..1-----1-----1------1- ~ [min] Scratch DATA 18.6 [mN/m1n] Xi: 0.232 [mm] X2: 0.299 [nun] L: 0.067 [mm] We : 0.35 [gf] ( 3.42 [mN]) ~ w~ Tit 1e: CSR-02 RHESCA Co•• LTO 1992/5/7 OSU50B-4 Sensor No.1. 100.0 [g/mm]. r • 15.0 rum] LOAD RATE: 51.36 [ft\N/III1D] STAGE ANSLE: 3.00 [dog] STAGE SPEED: 10.0 rUm/s] EXCITING LEVEL: 60 rum] xs X2 • 0-- ..__1J._-·--_·_-i···~_·__L .__-l.._._---~[min] Scratch DATA 30.9 [mH/mIn) Xi : 0 •297 [1ftIft] X2: 0 .369 [mm] L: 0 .072 [mill] We: 0 .38 [gf] ( 3. 71 [IBN]) ~ w Ul 136 APPENDIX IV The stress (a) in the film is determined by using the following formula. E h 2 1 0= (l-v) 6t R where E (Youngs modulus) = 1.055 x 1012 dynes/sq.cm v (Poissons ratio) = 0.446 t (Thickness of the film) = 1000 A = 10-5 em h (Thickness of the substrate) = 0.037 em R (Radius of curvature) = 1.35 em By using the above data, the stress (0) is found to 3.2 x 1013 dynes/em. The radius of curvature of the sample was calculated as follows. The profiles of the sample was measured before and deposition of the film. From the chord length (A) and the central angle (8), the R was determined by using the relationship R = A/e = 0.35/.26 = 1.35