Chemical vapor deposition of silicon onto silver surfaces

Item Type text; Thesis-Reproduction (electronic)

Authors Edgar, William Frank, 1939-

Publisher The University of Arizona.

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CHEMICAL VAPOR DEPOSITION OF SILICON

ONTO SILVER SURFACES

by

William Frank Edgar

A Thesis Submitted to the Faculty of the

DEPARTMENT OF ELECTRICAL ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 3

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission9 provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department of the Dean of the Graduate College when in his judg­ment the proposed use of the material is in the interests of scholar­ship • In all other instances, however, permission must be obtained from the author.

SIGNED: 7 ^ ^

APPROVAL BY THESIS DIRECTOR,

This thesis has been approved on the date shown below:

DateProfessor of Electrical Engineering

J . Hamilton

I

ACKNOWLEDGMENTS

The author wishes to thank Dr., Victor Ar Wells for his advice

and encouragement during the preparation of this thesis„x In addition,

I would like to thank M r , Leonard S« Raymond for his laboratory assist

ance and Mrs. Freida H« Long for her efforts in manuscript preparation

I would also like to thank Awilda, Lisa and Billy for their

patience and understanding during the last two years.

iii

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS .............................. vi

LIST OF TABLES ................................ vii

ABSTRACT ................... ix

CHAPTER

1 INTRODUCTION............................ 1

2 CHEMICAL VAPOR DEPOSITION THEORY . . . ........... . . . 4

3 LABORATORY EQUIPMENT AND MATERIALS U S E D ........... 18

3.1 Chemical Vapor Deposition S y stem..... ............. 183.2 Evaluation Equipment......................... 203.3 Materials Used ............... 21

4 CHEMICAL VAPOR DEPOSITION PROCEDURE................. . . . 22

4.1 General Remarks Concerning Procedure . . . . . . . . 224.2 Substrate Preparation . . . . . . . . . . . . . . . 224.3 Silver Surface . . . . . . . . . . 244.4 In-Process P a r a m e t e r s ....................... 284.5 Film Figure of Merit . . ............ 33

. : 4.6 Silicon Thickness Measurements . . . . . . . . . . . 374.7 Contaminants.................................. 414.8 Temperature Determination . . . . . . . 434.9 Deposition Rate Calculation ........................ 43

- 5 CHEMICAL VAPOR DEPOSITION RESULTS USING HYDROGEN . . . . 48

6 CHEMCIAL VAPOR DEPOSITION RESULTS USING H E L I U M .. 60

7 CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE-WORK . . . . . 72

7.1 Conclusions . . . . . . . . . . . . . . 72

Major Results - Hydrogen A t m o s p h e r e .......... . . . 72Major Results - Helium Atmosphere . . . . . . . . . 73

7.2 Recommendations for Future Work . . . . . . . . . . 73

iv

V

TABLE OF CONTENTS (Continued)

Page

APPENDIX A: BROOKS ROTAMETER DATA ................. 76

APPENDIX B: LIST OF SYMBOLS AND CONVERSION FACTORS............. 80

LIST OF REFERENCES . ........................................... 83

r

LIST OF ILLUSTRATIONS

Figure Page

2.1 Free Energy Function Versus Temperature for Silicon . . 11

2.2 Silver-Silicon Phase Diagram . . . . . . . . ......... 15

3.1 CVD System . . . . . ........... 19

4.1 CVD Procedure - Flow Diagram . ............. 23

4.2 Infrared Reflectance Trace Showing Heat Affectson a Silver Surface .............................. 26

4.3 Infrared Reflectance Trace Showing 9.8y Minimum . . . . 29

4.4 Wafer Position Parameters............... ............ .. 31

4.5 Infrared Temperature Monitor Trace From x-y Recorder . 32

4.6 Infrared Reflectance Traces of Reference Silver andCompleted Wafer from CVD 1 0 9 .......... 34

4.7 Schematic Diagram of Spectrophotometer ShowingScattered Rays ................................ 38

4.8 CVD Silicon Film Measurement Locations and Profile . . 40

. 4.9 Step Height Measurement P a r a m e t e r s .................... 42

4.10 Interference Diagrams .................. 46

5.1 Wafer Surfaces After CVD Runs in Hydrogen Atmosphere . 54

6.1 Wafer Surfaces After CVD Runs in Helium Atmosphere . . 64

6.2 Tilted Susceptor Configuration ................ . . . . 6 6

A.l Silane Rotameter Calibration Curve . . . . . . . . . . .77

A.2 Mainstream Helium Rotameter Calibration . . . . . . . . 78

A.3 Mainstream Hydrogen Rotameter Calibration Curve . . . . 79

vi

LIST OF TABLES

Table Page

2el Heat Capacities and Standard Heats of Formationof Silicon, Hydrogen and Silane [Hunt and Sirtl, 1970]. 7

2.2 Values of Fef(T) and H ^ g for a Temperature of 869K . . 12

2.3 Temperature, Free Energy of Reaction and EquilibriumConstant for Temperatures used in this CVD Testing . . 12

4.1 Film Figure of Merit Measurement for CVD 1 0 9 ..... 35

4.2 Microvoltmeter Readings and Corresponding SusceptorTemperatures . « • . . . ............... 44

5.1 Silane Flowrate, Susceptor Temperature MatrixHydrogen Atmosphere .......... 50

5.2 Deposition Rates, Film Figure of Merit and Susceptor Temperature for CVD Runs in Parameter Matrix -Hydrogen Atmosphere ......................... 52

5.3 Deposition Rates, Film Figure of Merit and SusceptorTemperatures with Parameters Outside the Matrix - Hydrogen Atmosphere................................ 56

5.4 Thickness Measurements for Selected Wafers -Hydrogen Atmosphere . ................................ 58

5.5 Activation Energies for CVD Runs in HydrogenParameter M a t r i x ...................................... 59

6.1 Silane Flowrate, Susceptor Temperature Matrix -Helium Atmosphere . . . . . . . . . . . . . . . . . . . 62

6.2 Deposition Rates, Film Figures of Merit and SusceptorTemperatures for CVD Runs in Parameter Matrix - Helium Atmosphere . . . . . . . . . . . . . . . . . . . . . . 63

6.3 Deposition Rates, Film Figures of Merit and Susceptor Temperatures with Parameters Outside the ParameterMatrix - Helium Atmosphere ............ 6 8

6.4 Thickness Measurements for Selected Wafers - HeliumAtmosphere . 69

. vii

LIST OF TABLES (Continued)

Table Page

6.5 Activation Energies for CVD Runs in HeliumParameter M a t r i x ............ 71

A.l Brooks Rotameters Used in CVD Testing............. 76

ABSTRACT

This work deals with a chemical vapor deposition (CVD) process

for depositing non-single crystal silicon by the pyrolysis of silane <,

The optical properties of these silicon films are of specific interest. .

Selected CVD process parameters affecting the quantity and

quality of the deposited silicon were varied over a limited range. By

in-process and post-process evaluation of the deposited silicon, opti­

mization of the process parameters and reproducibility of the silicon

layers were studied.

The CVD process and evaluation techniques are explained and

experimental results presented.

CHAPTER 1

INTRODUCTION

Chemical vapor deposition (CVD) is entering a phase of rapid

technological development * This stems from the fact that the CVD

process is more versatile than other processes such as physical vapor

deposition and electro-deposition in terms of deposition rates or

types of material that may be deposited [Cunningham and Dunn, 1967] .

A CVD process uses one or more chemical reactions to condense materi­

als and produce a solid coating of the materials on the desired sub­

strate.

One of the first uses of CVD techniques was when prehistoric

man created primitive records and drawings on the walls of caves

using pyrolytic carbon. As early as 1880, CVD coating methods were

developed by the incandescent light industry. From 1909 to 1939,

various halide-reduction and halide-decomposition processes were used

to prepare refractory metals. In 1939 the optical industry used CVD

techniques to provide films with desired optical properties such as

reflection and anti-reflection coatings. Since then CVD techniques

have been used very successfully in the electronics industry to

produce thin films for semiconductor device fabrication.

Most important CVD applications involve the unique structural

characteristics of chemically vapor-deposited materials [Blocker, 1967]

Silicon alone can run the gamut from amorphous crystal growth at low

■

temperatures to single crystal growth at higher temperatures. Present

technology allows control of the purity and physical structure of the

deposited material. Research in the CVD field is concerned with the

optimizing of process parameters for the wide range of materials

available to be deposited and the development of new processes and

applications. A novel application of CVD techniques is demonstrated

in the research being conducted on solar energy thermal converters by

the Optical Sciences Center at The University of Arizona. Using planar

layers of a reflective substance such as silver and an absorbing sub­

stance such as silicon, solar energy can be converted into heat to

provide electrical power.

The object of this research is to obtain a silicon film that

will have a very low absorption in the infrared wavelength range of 2.5

to 15 microns. This thesis, therefore, is concerned with a very limited

part of the total CVD institution. The CVD process under study used

silane gas in conjunction with hydrogen and helium gases to deposit a(

thin silicon film onto a silver coated substrate,

Two distinct phases of experimentation were considered. In

phase one the silane gas was mixed with a hydrogen mainstream gas, while

in phase two, the silane was mixed with a helium mainstream gas.

The CVD process theory will be examined in Chapter 2, as it

pertains to this experimentation. The theory will examine the physical

chemistry concepts involved in the CVD process and will present examples

of calculations used to evaluate the experimental results.

In Chapter 3 the laboratory equipment and materials used will

be described. The CVD procedure and the in-process monitoring tech­

niques used will be presented in Chapter 4 along with the measurement

techniques used. Chapters 5 and 6 discuss the results of the CVD

process in the hydrogen and helium atmospheres respectively5 and the

parameters for operation in each atmosphere.

Chapter 7 contains conclusions and recommendations for future

experimentation in this field of CVD.

Appendix A presents rotameter use and calibration data.

A list of symbols used in this thesis with their description

and units of measure is provided in Appendix B .

CHAPTER 2

CHEMICAL VAPOR DEPOSITION THEORY

The theory of chemical vapor deposition (CVD) is based on the

physical relationships encountered during the CVD chemical reaction•

By using the basic principles of chemical thermodynamics and reaction

kinetics 9 analysis of a proposed CVD reaction can be accomplished in

the initial stages of experimentation. Such analysis can serve as an

effective predictor of process feasibility and expected experimental

results. These calculations 9 based on free energies of formation9

enthalpies and entropies9 also indicate the effects of temperature^

pressure and amounts of reactants present in the process.

In order to analyze the CVD process a knowledge of the basic

thermodynamic relationships is.required. The first law of thermo­

dynamics simply states that energy may be transformed from one of many

states such as heat 9 mechanical^ electricals etc . 9 but can never be

created or destroyed. The energy is conserved. This conservation of

energy as a principle of.universal validity was mathematically stated

by Herman Von Helmholtz in 1847. An important corollary to this first

law is HessTs law which states that the total energy change in a series

of reactions depends only on the condition of the initial reactants and

final products and not on the numbers and types of the intermediate

reactions.

' ' ■ ' 4

A second, important corollary is Kirchoff1s law which states:

the rate of change of the heat of any reaction with temperature is equal

to the difference between the heat capacities, (C^), of the products

and those of the reactants. To better undertand this first law and the

two corollaries, the application of their principles to the stiochlo-

metric equation will be considered.

where a, b, and c are the numbers of moles of reactants and products,

respectively. By Hess?s law, the heat of reaction AH, commonly called

enthalpy, can be found by utilizing the heats of formation, AH^, of the

reactants and products

off by the reaction. Should AH be positive, the reaction is endother-

mic and absorbs heat.

The heat capacity (0^) at constant pressure such as in the

pyrolysis of silane is.

A chemical reaction can be represented by:

aA -> bB 4- cG (2.1)

AH = bAH£ (B) + cAHf (C) - aAHf(A) (2.2)

If the process is exothermic, AH is negative and heat is given

(2.3)

so that applying Kirchoff’s law to the reaction yields,

6

C = jrzr- = bC (B) + cC (C) - aC (A) (2.4)p di p p p

For temperatures above the standard temperature of

0 ^ is usually represented by a series

- 2C = a + a T + a0T « « .p O 1 2

so that ,

ACp = AaQ + Aa1T + Aa2T™2 . . . = (2.6)

which, upon integration, yields

AH(T) = AHq + AaQT + 0.5 Aa^ 2 - Aa2 T_ 1 . . . (2.7)

where AH^ is at standard temperature. The heat capacities for the

silane reaction used in this CVD experimentation are shown in Table

2.1.The second law of thermodynamics states that it is theoretically

possible to convert into mechanical energy all of the heat flowing from

a source at high temperature to a sink at a lower temperature. Analysis

of the Carnot thermal cycle yields the relationship between the heat

absorbed or given.up at the two temperatures.

ql q 27^ = ~ = S (2.8)1 2

298.15 C,

(2.5)

Table 2.1. Heat Capacities and Standard Heats of Formation of Silicon5 Hydrogen and Silane [Hunt and Sirtl, 1970]

SpecieAH°298 Cp(T)

KCal,mole ^ - 1Cal,mole ,T 1

H 2 (g) 0.0 6.52 + 0.78x10“3T + 5 -9 0.12x10 T

Si(s) 0.0 5.76 + 0.56x10-3T - 1.09xl05 T- 2

SiH4 Cg) 7.3 3.445 + 2.561x10-2T - 8. 78x10"6T2

where is the heat rejected into a sink at lower temperature and

is the heat absorbed from a higher temperature source T^. In

Eq. (2.8) the quantity S is called entropy.

Enthalpy (H) and entropy (S) can be related to the energy

available to perform useful work or free energy F at constant tempera­

ture and pressure by

F = H - T S (2.9)

A similar equation, shown below as Eq. (2.10), holds for differential

quantities

AF =.AH - TAS (2.10)

and, if these quantities are applied to the standard states of the

elements involved in the reactions, Eq. (2.11) results.

AF* = AH° - TAS* (2.11)

Equation (2.7) shows the temperature dependence of AH. The .

quantities,AF* and AS*, are also temperature dependent. Under the

conditions of standard atmospheric pressure, S*(T) can be represented

as

S*(T) = ■f- dT (2 .1 2 )

Hessfs law holds for free energy as well as enthalpy, or in

mathematical notation.

9

AFT ^ AFTf^products ^reactants (2.13)

If AF® 9 in the chemical reaction is positive9 net work must be put

into the system to make the chemical reaction take place, while if

AF° is negative, the chemical reaction will take place spontaneously.

If AF° is zero, no work is obtainable from the system.

The free energy of the reaction is related to the dynamic equi­

librium of the system by the relationship,

AF° = -RT An (2.14)

where R is the universal gas constant {Moore, 1962], T is the absolute

temperature and is the equilibrium constant at constant temperature.

If, in Eq.(2.1), A and C can be considered perfect real gases, then

can be related to the partial pressure of the reaction components by

[pcjc iPBibK™ = (2.15)

[PA ]a

where P is the partial pressure of component x. The equilibrium x /-

constant provides a definite relation between the concentrations of

materials. It is characteristic of the specific reaction and varies

only with changes in temperature. The equilibrium condition is called

the law of chemical equilibrium. This law states that in a system at

chemical equilibrium, the concentrations of materials which participate

10

in the reaction? must satisfy the conditions shown in Eq. (2.15).

Eq. (2o14) shows that the equilibrium constant decreases exponentially

with decreasing negative free energy of reaction. The equilibrium

constant provides an indication of the degree of completion obtainable

from a reaction. If AF° is positive, is less than one and the degree

of completion of the reaction is low. If the reaction, therefore, does

not show a high degree of completion, it may not be feasible for a

dynamic system. Consequently, the determination of AF° and are

important to the CVD process. Experience has shown that a reaction is

unlikely to be feasible if AF° is highly positive. If the AF° is

slightly positive or negative, i.e., ± 2 Kcal. per mole, the reaction

is borderline but may work. If AF* is highly negative, the reaction is

thermochemically favored and worth more detailed study [Burson and Flem­

ing, 1967].

A more convenient way of determining AF° and K can be found

by using the information available in the JANAF Thermochemical Tables

[Stull and Prophet, 1970]. These tables provide data for the quantity

-(F: - )Fef(T) = . (2.16)

The quantity in Eq. (2.16) is nearly linear over the temperature range

of interest in the silane reaction and provides a linear plot as shown

in Fig. 2.1. Consequently, AF° and for an operating temperature used

in the CVD process can be calculated. An example of the calculation is

examined for an operating temperature of 869 K, Table 2.2 contains

Temperature

(K)

11

1100

1000

900

800

700

60020 30 40

Fef(T)

Fig. 2.1 Free Energy Function Versus Temperature for Silicon

12

Table 2,2 Values of Fef(T) and H^gg for a Temperature of 869K.

SpecieFef(T) H ° 9 8

- 1 - 1 ' - 1 Cal,mole ,T Cal,mole

SiH4 54.10 7.8xl03

H 2 33.90 0

Si 6.69 0

Table 2.3 Temperature, Free Energy of Reaction and Equilibrium Constant for Temperatures used in this CVD Testing

Temperature AF̂ i 4Cal,mole •K

869 -25,410 2.457xl06

900 6-26,316 2.456x10

925 -27,039 2.440xl06

952 -27,620 2.160xl06

988 -28,547 2.060xl06

1008 -28,867 1.810xl06

1023 -29,282 l.SOOxlO6

13values for Fef(T) and H°gg at a temperature of 869 K. Since

SiH4 > Si + 2H2 (2.17)

the free energy AF̂ , of the reaction is

AF869 " AF869 <S1 + ^ - AF869 SiH4 (2'18)

this yields

AF=69 = -25,410 (2.19)

AF869jln (2.20)

£n k, = -------->410--- /n 2 1 )-1.98717 (869)

£n = 14.71 (2.22)

K? = e14”71 (2.23)

Kj = 2.457 x 10° (2.24)

The values of K̂ , at the operating temperatures encountered during this

CVD experimentation are shown in Table 2.3,

With the equilibrium conditions determined for the CVD reaction,

the rate of attainment of equilibrium or kinetics of the process will

now be considered. These are an indication of the dependence of the

reaction rates on the process variables, i.e., temperature, mass.

14It is proposed that the CVD process where silicon is deposited

on a silver substrate contains five distinct steps. They are:

1 o Diffusion of reactants to the heated substrate9

2c Adsorption of the reactant gases on the substrate,

3o Reaction on the substrate5

4„ Desorption of gaseous products from the substrate,

5 o Diffusion of desorbed products into the mainstream gas.

These are consecutive steps and if one of them is slower than the others9

it will become rate-determining. Steps 1 and 5 are usually rapid. Steps

2 and 4 are usually faster than step 3. Normally, step 3 is the rate

determining step, although there are energies associated with steps

2 and 4 which, in some reactions, may be greater than step 3. The

activation energies that will be calculated using Eq. (2.32) then, are

the apparent or overall activation energies of the reaction and include

the energies associated with absorption and desorption.

It is interesting, at this point, to look at the Ag-Si phase

diagram shown in Fig. 2.2. The melting point of pure silicon (99.8%)

is shown on the diagram to be 1410 C , while that of pure silver (99.95%)

is shown to be 961 C. The eutectic point, where crystalline silicon

and silver are in equilibrium with the melt, is 840 C. Since the range

of temperatures experienced during this CVD is from 596 C to 750 C , all

operations were performed below the eutectic point. This indicates

that solid silicon and solid silver can both exist, regardless of the

melt composition at the temperatures of interest.

Temperature

(C)

15

Overall Si Content (atomic %)20 40 80 100

1500

1400

Melt

1300

1200

Si + Melt1100

1000 -

900Ag+Melt 840 C

800100 80 60 40 20

Overall Ag Content (atomic %)

Fig. 2.2 Silver-Silicon Phase Diagram [Padnos, 1965]

16The CVD process 9 based on the pyrolysis of silane, contains

elements in the gas phase and solid phase simultaneously. Since silane

gas and solid silicon and hydrogen exist during the chemical reaction,

there must be some relationship between the concentrations of these

elementso This relationship is called a reaction rate constant. In

this CVD process a measurable parameter indicating how fast the reaction

was taking place was the deposition rate of the solid silicon• The

Arrhenius equation for the rate constant (DR) is ,

DR = A e-E/RT (2.25)

Using Eq, (2,25), where DR is the deposition rate (also reaction rate),

the activation energy at a given temperature can be calculated» Tabula­

tions of activation energies for CVD processes in the hydrogen and

helium parameter matrices can be found in Chapters 5 and 6 .

When In DR is plotted against 1/T, a straight line is expected

with a slope of -E/R. Using Eq, (2,26) to (2,32), the activation ener­

gies of the CVD experimentations were calculated. These results are

tabulated in Tables 5,5 and 6,5,

DRl = A e-E/RTl

DR 2 = A e~ E / R T 2

Jin DR 1 = Jin AC-E/RTj)

Jin DR 2 = Jin A(-E/RT2)

(2.26)

(2.27)

(2.28)

(2.29)

17

£n

where

then

DR 1 - Jin DR 2 = E/R(l/T2 - 1/Tj)- (2.30)

R(£n DR 1 - £h DR2) '(2 /T— r i/x )--- = E(Kcal,mole ) (2.31)

DR^ = deposition rate at temperature in A minute

DRg = deposition rate at temperature T2 in A minute ^

oo— — 1-3- -x ■1°- ) = E(e.v. molecule-1) (2.32)

(6 . 0 2 x 1 0 )

A Hewlett-Packard model 9100 A calculator was used to obtain, the "best

fit" to the data points*

The theory presented in this chapter constitutes only the

basic fundamentals required« Detailed research on the various aspects

of CVD is being undertaken by many individuals in an effort to more

fully understand the CVD process,

CHAPTER 3

LABORATORY EQUIPMENT AND MATERIALS USED

3ol Chemical Vapor Deposition System

The chemical vapor deposition (CVD) system consists of the gas

subsystem3 the reactor subsystem and the infrared monitoring subsystem.

The entire system is shown schematically in Fig. 3.1.

The gas subsystem provides equipments to transport and control

the gases used in the CVD process from the gas sources to the reaction

chamber. Each of the gases passed through a rotameter having cutoff

and control valves. The rotameters used in the process were Brooks

rotameters. -The rotameters were calibrated with nitrogen gas and the

flowrates were then converted to silane, hydrogen and helium flowrates

by use of the appropriate sizing factor. The sizing factor is the

ratio of the specific gravities of the nitrogen calibration gas and the

experimental gas (silane, helium, hydrogen) taken to the one-half power.

If the specific gravity of N^ is taken to be 0.967 and the specific1 /9gravity of He is taken to be 0;138, then (0.967/0.138) =2.648, the

sizing factor and the metered rate of N^ x 2.648 equals the meteredr>

rate of He. Appendix A contains calibration and descriptive information

for the rotameters used in this testing.

After passing through the rotameters, the gases mix in the gas

manifold. The equipment used for gas transport from the gas sources

18

r

I.R. Detector and

Monitor Sub-system

R.F. Induction Coilo oManifold

Gas Flow

Reactor Sub-system

O = ON-OFF valve RT = Rotameter

Silane Source Hydrogen Source Helium Source

Gas Sub-system

Fig. 3.1 CVD System

20

to the manifold is quarter inch copper tubing with the joints sealed

with teflon tape to prevent gas leakage.

The reactor subsystem has a reaction chamber which is a

circular quartz tube 38.7 cm long with an inside diameter of 70 mm.

The quartz is General Electric type 204 or equivalent. Surrounding

the center of the reaction chamber is an r.f* coil. The r .f . generator

is rated at ten kilowatts operating at a frequency of 425 kilohertz.

The water-cooled r.f. coil provides induction heating for the reaction

process. In the reaction chamber, centered in the area of the r.f.

coil is a pyrex boat support, graphite susceptor and the wafer to be

processed

The change in temperature of the susceptor due to the induction

heating and the change in the emittance of the wafer due to deposition

of silicon is monitored by the infrared detector and monitoring sub­

system. The basic component of this subsystem is a lead sulphide

infrared detector with a maximum output at 2.5 micron wavelength. The

output of the infrared detector is amplified by a tunable microvolt­

meter model IR 600 and displayed on a meter at the microvoltmeter and

simultaneously on a Hewlett Packard Model 7000 A x-y recorder and a

Hewlett Packard model 130 oscilloscope [Whitmer, 1972].

3.2 Evaluation Equipment

Spectral reflectance of the processed wafer was obtained using

a Perkin-Elmer model 137, sodium chloride, spectrophotometer. This

spectrophotometer gives a spectral reflectance in the 2.5 to 15 mireon

wavelength spectrum.

21Thickness measurements of the silicon and silver films and

microscopic examinations of the wafers were made using a Reichert

Zetopan microscope. The photographs of the wafer surface were made

using the same microscope.

The vacuum chamber used to deposit the silver film on the

silicon substrate was a conventional, oil diffusion, pumped 9 14 inch

system evacuated to 2 x 10 ^ microns. The filaments of the heating

element of the vacuum system held high purity silver. When the system

was evacuated and the filaments with the silver heated to sufficient

temperature for the silver to vaporize, the silver was deposited on the

silicon substrates.

3.3 Materials Used

The helium gas used was reactor grade helium that was passed

through a molecular sieve. The molecular sieve removes trace amounts

of impurities? i.e., H^O, hydrocarbons, etc., which might be present

in the gas. The hydrogen gas was commercial grade hydrogen which was

passed through an Engelhard DEOXO purifier. The silane was 100%

commercial grade silane.

The silicon wafers used as substrates are 1-1/2 inch in diame­

ter of various thicknesses. Since a layer of silicon dioxide was

thermally grown on the surface of the wafer, the crystalline structure

of the wafer was not important.

Standard integrated circuit processing techniques were used in

the cleaning and etching of the wafers.

/

CHAPTER 4

CHEMICAL VAPOR DEPOSITION PROCEDURE

4ol General Remarks Concerning Procedure

The chemical vapor deposition (CVD) procedure follows the flow

diagram in Fig, 4.1. The procedure is not complicated5 but care must

be exercised during the entire process to avoid wafer and processing

equipment contamination and to insure accurate in-process monitoring

of the wafer,

4 o 2 Substrate Preparation

The substrates used for the CVD experimentation were cleaned

using the pre-clean procedure as in the Solid State Engineering EE 250

Course Manual [1972]. The procedure consists basically of cleaning

the wafers using acetone, chromic solution and hydrofluoric acid etch

until the surface is hydrophobic, The wafers are then rinsed in de­

ionized water as the last step. This cleaning procedure removes

impurities present on the wafer surface and lessens the chance of

contamination of the oxidation furnace and the silicon dioxide layer.

After the wafer has been cleaned it is inserted into the

oxidation furnace for a period of 40 minutes at a furnace temperature

of 1100 C, Oxygen flowing through a bubbler causes steam to pass over

the heated wafers causing oxidation of the silicon. This process

results in a 0 .5 p layer of silicon dioxide on the wafer*s surfaces.

22

CleaningStation

OxidationFurnace

VacuumSystem

CVD Reactor P.E. 137 Reichert

(Start)SubstratePreparation

WaferOxidation

SilverDeposition

SilverThicknessMeasurement

Gas Flowrate Monitor

Temperature andEmittanceMonitor

VaporDeposition

2nd Infrared Measurement

1st Infrared Measurement

Silicon Film Thickness Measurement (Stop)

^ Spectrophotometer | Microscope

Fig. 4.1 CVD Procedure - Flow DiagramMu>

24

The wafers are then enclosed in petri dishes to lessen the chance of

contamination prior to deposition of the silver layer.

4o3 Silver Surface

After the silicon dioxide layer has been deposited on the sili­

con substrate9 a 3000 A silver surface is deposited on the wafer’s

polished side. The silver is deposited using the vacuum system de­

scribed in Chapter 3. The silver surface on the wafer is a very

important part of the CVD procedure. It is the surface upon which the

silicon from the reaction discussed in Chapter 2 is deposited. The

thickness of the silver layer? 3000 A is sufficient to provide an

opaque surface comparable to bulk silver. If the silver thickness is

less than 3000 A 5 conglomeration of the silver could result in bare

silicon dioxide lands between hillock formations. This would lessen

the reflective properties of the silver surface. With the working

temperatures of this research9 the silver doesn’t leave bare lands if

the thickness is 3000 A. The silver surface is subjected to tempera­

tures as high as 750 C in the CVD reactor. The effect of these high

temperatures on the silver is a function of the silver thickness5 the

gaseous atmosphere surrounding the wafer in the reactor and the silver

age. During the CVD experimentation? the following anomalies of the

silver surface were noted:

1. A silver surface less than 3000 A in thickness tends to give

higher film figure of merit values than a 3000 A surface all

remaining CVD parameters remaining constant.

252. A. silver surface that has been in the air more than 24 hours

prior to CVD processing tends to result in a higher film

figure of merit after deposition than one that is used when

only One to two hours old, all other CVD parameters remaining

constanto

3 o A silver surface subjected to heat in a hydrogen atmosphere

tends to deteriorate more rapidly than at the same temperature

in a helium atmosphere.

Effects of temperature on the silver surface can be seen when a wafer

is heated in a helium or hydrogen atmosphere. The change in the silver

surface after heating is visually apparent in that the silver surface

looks cloudy rather than of mirror quality. This visual change is

indicated by the reflectance values in Fig. 4,2. The reflectance of

the heat-treated silver surface is lower than its original value for

wavelengths less than 3.5 microns. This lower reflectance extended to

visible wavelengths would account for the observed surface cloudiness.

The possible causes of the above mentioned silver surface

anomalies are:

1. Formation of silver hydrides from hydrogen gas in the hydrogen

atmosphere.

2. Reaction of the silver with trace amounts of oxygen in both

the hydrogen and the helium atmospheres.

3. Interaction of the silver surface with water vapor in the

air may cause the silver to naget! or deteriorate over a

24 hour period. ,

Reflectance

(%)

100

90

80

70

60

50

40

30

20

10

0

3 4 5 6 7 8 9 10 11 12 13 14 15Wavelength (microns)

Fig. 4.2 Infrared Reflectance Trace Showing Heat Affects on a Silver Surface

S3ON

4 6 Possible hillock formation on the surface from surface

diffusion of the silver.

While it is not in the scope of this work to examine in detail

the causes of the apparent anomalies in the silver surfaces several

recent papers dealing with thin silver films may aid in the current

research.

Experimentation was under taken by Schmidt-Ihn^ Weil and Will

[1972] with 2000 A thick, single-crystal and polycrystalline films

formed by evaporation of silver onto mica and rocksalt substrates.

These films were annealed at various temperatures between 230 C and

360 Co During the annealing process, the films were observed to go

through several recrystallization stages. From an initially smooth

surface, three dimensional nuclei were formed. These nuclei grew to

isolation by consumption of the surrounding film material. The final

crystalline shape was strongly temperature dependent, so much so that

at 343 C, no resemblance to any crystalline shape could be observed.

An examination of the kinetics of hillock formation by surface

diffusion on thin silver films by Presland, Price, and Trimm [1972]

indicates that in oxygen atmospheres, surface agglomeration of thin

(1180 A) silver films occurs rapidly on annealing. The silver trans­

fers to hillocks eventually withdrawing the silver from surrounding

areas. Again, this hillock formation is a function of time, tempera­

ture, oxygen, pressure and film thickness. While the experimentation

cited in the above articles was not conducted under laboratory con­

ditions paralleling those used in this CVD process, they both indicate

that the silver surface is not a stable, passive medium when subjected

to the high temperatures found in the reaction chamber.

During experimentation, the effects of the silver surface

anomalies were minimized by insuring the silver surface was fresh for

each run (within 8 hours) and by insuring that the silver surface was

at least 3000 A in thickness» When the molecular sieve was used on the

helium mainstream gas line, a reflectance minimum at the 9.8 micron

wavelength when measuring the silicon layer, was minimized indicating

that oxygen was indeed contaminating the helium and that the sieve was

filtering out the oxygen. An example of this 9.8 micron minimum is

shown in Fig. 4.3.

4.4 In-Process Parameters

Following the deposition of the silver layer, the in-process

monitoring of the wafer begins. Samples of the silver surface are

examined using the Reichert microscope to insure that the silver sur­

face is at least 3000 A thick. The samples of the silver surface are

obtained by inserting glass microscope slides into the vacuum chamber

beside the silicon wafers. When the silver is deposited on the wafers,

it is deposited on the glass slides as well. If the silver thickness

is not 3000 A thick, the silver is taken off the wafers, the wafers

cleaned and fresh silver deposited. If the silver surface is 3000 A

thick, the Perkin-Elmer spectrophotometer is used to take a spectral

infrared measurement of the surface. A processing number (AG xxx) is

assigned both to the spectral curve and to the wafer. The infrared

Reflectance

(%)

100

20

10

14 153 4 6 8 9 137 10 11 125Wavelength (microns)

Fig. 4.3 Infrared Reflectance Trace Showing 9.8y Minimum

K>VO

measurement shows the reflectance characteristics for that particular

silver-coated wafero

After the spectral reflectance of the wafer is recorded, the

wafer is ready for CVD processing in the reactor* Care is taken to

place the wafer in the same position on the graphite susceptor for

each run, in order to eliminate a variable in the CVD process* This

positioning of the wafer, boat and susceptor inside the reactor tube

is shown in Fig, 4 ,4a and b , With the wafer positioned in the reaction

chamber, the CVD run parameters of r,f, plate voltage, mainstream and

silane gas flowrates are set. The deposition of the silicon onto the

wafer is monitored by the infrared temperature monitor* As the reaction

chamber is purged with the mainstream gas, a ten minute time line is

established on the x-y recorder for the purpose of time reference

(Fig* 4*5), As the reaction chamber is heated, the lead sulphide in­

frared detector responds to the apparent change in emittance. The

output of the infrared temperature monitor, x-y recorder is shown in

Fig, 4,5, When the CVD system has stabilized at the desired run

temperature as indicated by a predetermined reading, i*e,, 2 0 0 ]iv, the

silane is introduced into the reaction chamber. As the chemical re­

action proceeds and the silicon is deposited upon the silver surface,

the change in apparent emittance of the wafer is observed on the x-y

recorder * Runs for this experimental work .were made to the second

interference peak as shown in Fig» 4*5, Upon reaching the second

interference peak, the r.f, plate voltage is shut off and the system

is allowed to cool, A good indication of the final spectral

12

R.F. Coil

Gas Flow ReactionChamberWafer

Graphite Susceptor

Pyrex Boat

(a) Wafer Positioned in Reactor Heat Zone

(b) Wafer Positioned on Graphite Susceptor

Fig. 4.4 Wafer Position Parameters

Time Line

(a) Susceptor Heating

(b) Interferance Curve

Reactor Off

SiH. ON Ae Time Line

Fig. 4.5 Infrared Temperature Monitor Trace From x-y Recorder wCO

- . . 33reflectance can be obtained by observing the apparent emittance change

of the wafer during processing. Two indicators are Ae, the difference

between the temperature stabilization value and the first valley (T^)

and Ah, the height of the second peak. Best spectral reflectance was

obtained when Ae was as small as possible and when Ah was close to, but

less than or equal to, h^, the height of the first peak.

After the wafer has cooled for at least 15 minutes, it is

removed from the reaction chamber and given a second infrared spectral

reflectance measurement. The spectral response after CVD processing

is shown as the lower trace in Fig. 4.6.

4.5 Film Figure of Merit

In order to provide a relative indication of the quality of the

deposited silicon film, a value which shall be defined as the film

figure of merit (M^) was calculated for each completed wafer. The com­

pleted wafer consists of the substrate, the silver surface and the

deposited silicon film.

. : This calculation is based on data from infrared measurements

with the spectrophotometer. The spectrophotometer is adjusted to give

maximum scale reading using a reference silver surface, The reflec­

tance trace of this reference infrared measurement is shown as the top

trace in Fig. 4.6. A reflectance trace of the completed wafer is then

made on the same chart (shown as the lower trace in Fig. 4.6). The

reflectance values of the silver reference and the completed wafer are

recorded for ten different wavelengths. These wavelengths (shown in

Table 4.1) represent equal percentage points of thermal flux emitted by

Reflectance

(%)

100

90

80

70

60

50

40

30

20

10

0

3 4 5 6 7 8 9 10 11 12 13 14 15

Wavelength (microns)

Fig. 4.6 Infrared Reflectance Traces of Reference Silver and Completed Wafer from CVD 109

u>4>

35

Table 4.1 Film Figure of Merit Measurement for CVD 109

X -W

ThermalFlux

. %PAGX%

PsmX%

prmX%

psX%

MsX

2.500 5 98.9 71.7 93.5 75.84 0.2416

3.067 . 15 98.9 . 8 6 . 2 94.7 90.02 0.0998

3.682 25 98.9 93.7 98.8 93.70 0.0630 .

4.187 35 98.9 93.8 98.6 94.08 , 0.0592

4.802 45 98.9 93.0 97.6 94.23 0.0577

5.519 55 98.9 91.0 96.0 93.74 0.0626

6.423 65 98.9 8 6 . 8 96.0 89.42 0.1058

7.691 75 98.9 80.3 96.0 82.72 0.1728

9.811 85 98.9 89.0 99.0 88.91 0.1109

15.000 95 99.0 91.8 95.3 95.26 0.0474

T M = 1.020 u sXM = 0.102 s

36a blackbody at 800K. For exampleg from Table 4.1, it can be seen that

55% of the thermal flux emitted by a blackbody at 800K is emitted at a

wavelength less than 5.519 microns.

Using the equations shown below, M^, can be calculated for each

completed wafer.

PsmX (pAGX) ,,^ " PrmX <4"1)

where

^AGX ” reflectance of freshly evaporated mirror at X

^smX = measure< ̂ sample reflectance at Xp . = measured reference reflectance at X rmA

= sample reflectance (calculated) at X

and

MsA = 1 " psX (4.2)

1 0

I MS X - (Average) (4.3)

where M g is the film figure of merit. The wavelengths measured and the

subsequent values of for CVD 109, shown in Fig, 4.6, are tabulated

in Table 4.1.

With the silver surface acting as an opaque body

p = 1 ~ e = 1 - a (4.4)

where

p = reflectivity

e = emissivity

a = absorption

If the silicon surface and silver surface were specular,

would represent the emittance (or absorption) of the wafer at X ,

However, due to silicon nucleation and possible silver hillock forma­

tion, these surfaces are non-specular and the value of M . obtainedsXrepresents the apparent emittance (or apparent absorption) of the

wafer at X less any losses in emittance (or absorption) due to scat­

tered rays which go undetected by the spectrophotometer. This is

shown.in Fig. 4.7,

The film figure of merit is then a weighted average of the

apparent emissivity (or apparent absorption) of the silicon film

mechanically integrated over the 2.5 to 15 micron wavelength spectrum.

The lower , the lower the apparent emittance of the silicon film.

Thus, the more transparent it is in the infrared region. It is de­

sirable for this thesis to obtain silicon films with the lowest possi­

ble M o Conversely, a high M indicates an undesirable film, s s

4.6 Silicon Thickness Measurements-

The thickness of the deposited silicon can be measured in two

ways. Using the Perkin-Elmer spectrophotometer, the thickness of the

wafer for that portion of the wafer observed by the spectrophotometer

may be calculated by the following method:

38

Detector

Reference Beam LightSource

IR * Trace Measuring Beam

r” "“Detected Ray

Scattered Ray

ScatteredRay

Wafer

Fig. 4.7 Schematic Diagram of Spectrophotometer Showing Scattered Rays

39

*si ^si " ^7 (fringe order) (4.5)

where fringe order is the degree of phase shift or

. ± 7 T ± 27T9 ± Sir • o o o o

A = 2.5 microns

n . = index of refraction of sisilicon, 3.44

then

dsi 4fringe order

nsi(4.6)

Using the Reichert microscope, the thickness of any portion of

the silicon film can be measured. In order to make the measurement,

the silicon must be etched at the desired points. Then, using the

polarization interferometer after Nomarski method, the thickness can be

measured. In this experimentation, the silicon thickness was measured

at five points as shown in Fig. 4.8a. By measuring the thickness at

the five points, a thickness profile of the silicon film was obtained.

A typical profile is shown in Fig. 4.8b. Care must be taken in etching

the thin silicon film so that the etching stops at the silver surface*.

The etching solution used was a ten percent sodium hydroxide and water

solution, heated to 50 C. The wafer, covered with vacuum wax, except

for the five areas to be etched, is submerged in the solution for 15

40

C> 2

(a) Position of Steps Etched in the Deposited Silicon Film for Thickness Measurements

l.Or-

Front View

y

o.o.Left Center Right

1.0

y

o.oFront Center Rear

(b) Thickness Profile of AG 82, CVD 8 6 .Fig. 4.8 CVD Silicon Film Measurement Locations and Profile

41

minuteso Care must be taken to insure that the temperature of the

etching solution does not get so hot as to remove the wax from the

covered part of the wafer.

The resultant etched steps at the five points are then examined

using a monochromatic light at 0.5980 micron wavelength and a polarizer

attachment to the Reichert microscope. The film thickness is. obtained

from the equation shown below and the values obtained from Fig. 4.9.

xi 0.5980Film Thickness = — x — --- : (4.7)

where x^ and x̂ , are shown in Fig. 4.9.

4.7 Contaminants

A major area for concern during the CVD processing is the intro­

duction of contaminants onto the silver surface of the wafer prior to,

and during 5 the actual reaction process. The ideal situation would be

for the. silver surface to be in a vacuum instead of an air atmosphere

from the time the surface has the silver deposited upon it until the

time the reaction starts. Since this is impractical9 it is important

to handle the wafer very carefully with tweezers to avoid contamination.

Contaminants can be introduced into the reaction chamber by careless

handling procedures. Rubber gloves should be used when handling the

pyrex boat and the graphite susceptor to avoid the possibility of getting

contaminants from the hands onto these objects and subsequently intro­

ducing them into the. reaction chamber environment. Contaminants have a

Fig. 4.9 Step Height Measurement Parameters

.pronounced effect on the crystal growth characteristics of the deposited

silicon and cause defects such as growth pyramids and crystallographic

pits, .

4,8 Temperature Determination

A Chromel-Alumel thermocouple inserted into a graphite suscep­

tor was used to correlate the microvoltmeter indications and the sus­

ceptor temperature, The silver coated substrate was put on top of the

graphite and loaded into the reaction chamber in the normal CVD run

position. Temperature determinations were then made for various

microvoltmeter settings. Table 4,2 shows the microvoltmeter, tempera­

ture relationship,

4,9 Deposition Rate Calculation

The deposition rates for silicon deposited onto the silver sur­

face of the wafer were calculated using the information obtained from

the x-y recorder curves. Each CVD run produced a curve similar to that

shown in Fig, 4,5,

In order for the curve to be meaningful, an explanation of its

important aspects will be presented. To establish the time involved

in the deposition process, a ten minute time line was recorded on each

curve, After the reactor was brought up to the operating temperature

specified for a particular CVD run, the gases were introduced into the

reaction chamber and the chemical reaction started. The effect of the

silicon deposition on the curve is shown as an oscillatory waveform.

Since the temperature of the susceptor remained constant, the variation

44Table 4.2 Micro.voltmeter Readings and Corresponding

Susceptor Temperatures

MicrovoltmeterReading

pv

Temperature

K C 1 0 3/K

150 869 569 1.15

2 0 0 900 627 1 . 1 1

250 925 652 1.08

300 952 679 1.05

400 988 715 1 . 0 1

500 1008 735 0.99

600 1023 750 0.97

45

of the curve must, in some way 9 be caused by the silicon deposition.

This curve variation can be explained in terms of optical interference

phenomenon. At the time the silane is introduced into the reaction

chamber, the infrared detector is observing the photon flux emitted

from the clean silver surface. As silicon is deposited as a layer over

the silver, a phase shift due to reflection of part of the emitted

photon flux is observed. Constructive interference occurs at the peaks

of the curve and destructive interference occurs at the minimum of the

curve. This concept is shown graphically in Fig. 4.10a and b.

Using the equation

2d (4.8)

where

d = thickness of the silicon layer (pm)

X = peak wavelength of the infrared detector (2.5 pm)

n = index of refraction of silicon at 2.5 pm (3.44)

N = odd integer for constructive interference

N = even integer for destructive interference

the thickness of the silicon layer at the maximums and minimums of the

curve can be calculated. The silicon thicknesses are 1816 A and 5450 A

for the first and second maximum respectively. The silicon thickness

for the minimum is 3633 A. In explanation, the reflected portion of

the photon flux encounters a phase shift of 180 degrees due to the

difference in the index of refraction of the silicon and the silver.

46

(a) Constructive Interference

Silver Surface

Silver Surface(b) Destructive Interference

Fig. 4.10 Interference Diagrams

47For constructive interference9 then, the wave transverses a distance of

Nodd ^/4n and encounters the A/2 phase shift to emerge from the silicon

in phase. Destructive interference occurs when the wave transverses a

distance N A/4n. evenFrom the above calculated thicknesses for the minimum and maxi-

mums of the curve and the initial time line, various deposition rates

(DR) can be calculated. The deposition rate, DR1, for the first maxi­

mum equals 1816 A divided by the time from silane initiation to the

first maximum. In like manner, the DR for the first minimum, (DR2) ,

and second maximum, (DR3), can be obtained by using the respective

thicknesses and elapsed times. The DR for silicon on previously de­

posited silicon, (DR4) , was calculated by determining the differential

thickness going from the first to second maximum and dividing by the

elapsed time between the maximums.

These deposition rates are used to calculate the activation

energies. Tabulation of these deposition rates are found in Chapters

5 and 6.

CHAPTER 5

CHEMICAL VAPOR DEPOSITION RESULTS

USING HYDROGEN

This chapter presents.the results of chemical vapor deposition

(CVD) runs made using a hydrogen carrier gas• The varied parameters

of silane to hydrogen gas flowrates and susceptor temperature will be

discussed along with the reasons for the range in variance and the ex­

pected results due to the1 variance. The actual experimental results

will be discussed. These results will include visually observable data

such as surface appearance of the wafer after CVD, calculated silicon

deposition rates 9 film figures of merit and thickness gradient measure­

ments on selected wafers.

The hydrogen atmosphere for this phase of experimentation was

obtained by using hydrogen gas as the mainstream carrier gas. A

setting of 2 2 . 5 (sapphire ball) at seven pounds per square inch gauge

pressure through a Brooks rotameter results in a hydrogen flowrate of

5 liters per minute. The hydrogen flowrate of 5 liters per minute was

established during earlier CVD processing as the optimum flowrate for

the temperature range and silane quantity, under investigation. With

the hydrogen gas flowrate held constant at 5 liters per minute 9 the

susceptor temperature and the silane gas flowrate were varied. ' For

each susceptor temperature, the silane gas flowrate was varied in a

systematic way. When this area of investigation had been examined9

48

49the temperature was again changed creating a new area. The resulting

susceptor temperature9 silane gas flowrate matrix is shown in Table 5«lo

Prior to initiation.of testing within the matrix, the graphite

susceptor was etched to remove the silicon deposited during earlier work•

Etching of the susceptor was accomplished using a solution of four parts

nitric acid, one part glacial acetic acid and one-half part hydrofluor­

ic acid» After the susceptor was etched, three CVD runs were made to

coat the susceptor with a thin silicon film. The first run for suscep­

tor coating used a silane flowrate of 32 cubic centimeters (cc). per

minute and resulted in an uneven.silicon coat. The susceptor was

coated twice more at the same silane flowrate reversing the position of

the susceptor after each run. This resulted in a fairly even silicon

layer over the face of the susceptor. The silicon coat on the suscep­

tor provides a flat interface between the wafer and the susceptor and

this, in turn, provides more even heating of the wafer. This silicon

coating also eliminates a contamination source.

- As an early experiment, a CVD run was made at 750 C and a

silane flowrate of 32 cc per minute. This run, CVD 75, resulted in

a silicon deposit on the wafer of such an obviously poor quality that

no infrared measurement or further analytical tests were performed on

the wafer. This run did, however, give an indication of expected

results at other parameter values within the matrix. It indicated

that either the susceptor temperature was too high or that the silane

flowrate was too high. It was expected that within the parameter matrix,

50

Table 5.1 Silane Flowrate, Susceptor Temperature. Matrix Hydrogen Atmosphere

Silane Flowrate cc/min

Susceptor Temperature C

679 715 735 750

CVD CVD CVD6 85 8 6 87

1 2CVD CVD83 82

32* CVD84

CVD81

CVD75

*CVD 89 was conducted at SiH^ flowrate of 32 cc/min at a susceptor temperature of 652 C and is tabulated with CVD runs within the parameter matrix.

.) • • ■ • .51

there existed an optimum value for susceptor temperature and silane

flowrate that would yield the desired deposited silicon layer proper­

ties of thickness and transparency«

With the expected results in mind, testing at values within the

parameter matrix was undertaken. Table 5.1 relates general information

concerning the various CVD matrix runs. The CVD run number will be

used to.cross-reference between the various tables and figures within

this chapter.

Table 5.1 has blank areas. These areas were not investigated.

Results of testing at matrix values in adjacent areas indicated that

going to a blank area would not give desired results either due to the

silane flowrate or the susceptor temperature.

The results of experimentation within the matrix will be pre­

sented first. Subsequent testing in the hydrogen atmsophere at system

parameters different from the basic matrix will then be presented.

Within the first area of examination, the silane flowrate was

held at 6 cc per minute while the susceptor temperature was changed

as shown in Table 5.1. Runs numbered CVD-85, -8 6 , and -87 were per­

formed. Of the three runs, CVD- 8 6 had, by far, the lowest film figure

of merit (see Table 5.2). After CVD-8 6 , the wafer was shiny in

appearance with visible interference rings (circular discolorations).

Microscopic examination of the sample showed crystal nucleation.

The nucleation density was heaviest in the center of the wafer and

decreased toward the wafer's edges. Wafer samples from CVD runs 85

and 87 showed a dark brown color after deposition. Microscopic

Table 5.2 Deposition Rates, Film Figure of Merit and Susceptor Temperature for CVD Rune in Parameter Matrix - Hydrogen Atmosphere

Silane Flowrate cc/min

CVDNumber

Deposition Rate A per minute

SusceptorTemperature

Film Figure of Merit

D R 1dr 2 DR3 DR.4 C %s

6 85 50 156 430 1 0 1 715 0.810

6 8 6 . 252 257 2 0 0 182 735 0 . 1 0 2

6 87 397 430 384 371 750 0,980

1 2 82 245 276 203 187 715 0.380

1 2 83 71 69 59 56 679 0.140

32 75 3130 3947 3583 3864 750

32 81 4126 3185 2960 2593 735 0.900

32 84 749 955 789 810 679 0.840

32 89 279 249 160 132 652 0.560

LnN>

53

inspection revealed very heavy nucleation over the entire wafer result­

ing in a matted appearance, The variation in wafer surface texture can

be seen in. Fig. 5,1 for the wafers in the hydrogen atmosphere.

The second area of examination includes only two GVD runs 9 CVD

82 and 83« Of the two runs, CVD 83 was the best as shown by the lower

film figure of merit (M^), (Table 5.2). Both wafers had heavy, apparent

nucleation (Fig. 5.1). This area of testing was conducted with the

silane flowrate set at 1 2 cc per minute.

The third area of testing was conducted with the silane flowrate

- set at 32 cc per minute. In this area, four runs were made. As men­

tioned before, CVD 75 was made during the equipment instruction phase

of research and was of such obviously poor quality as to preclude fur­

ther testing. None of the three remaining runs in this area were

considered acceptable as indicated by their respective film figures of

merit (Table 5.2). CVD 81 and 84 wafers were a very dark brown in

appearance. The CVD 89 wafer had a shiny milky appearance.

A number of CVD runs were made in the hydrogen atmosphere in

addition to those contained in the parameter matrix. Earlier CVD exper­

imentation had indicated that fairly low values could be obtained if

the silane flowrate was altered during the course of the CVD run. After

the susceptor was at the desired temperature, the silane was introduced

into the reaction chamber at a flowrate of 32 cc per minute. This

„flowrate was maintained for nine minutes then increased to 64 cc per

minute for the remainder of the run. These flowrates and time values

were established in prior work.

54

CVD 86 (x90)

♦ * 1 .

Fig. 5.1 Wafer Surfaces After CVD Runs in Hydrogen Atmosphere

55

Five CVD runs were made using this procedure„ They were CVD

numbers 80, 94, 95, 99 and 100• As shown by the values in Table

5.3, CVD 80 yielded the best results.

Another parameter variation examined using the hydrogen atmos­

phere was that of introducing the silane before the reactor was turned

on. After CVD runs 80 and.83, a microscopic examination of the wafers

showed a straight line on the leading edge of the wafer perpendicular

to the direction of gas flow. It was felt that the silver surface was

too thin on these particular wafers to withstand the hydrogen turbu­

lence and the susceptor temperature involved in the particular CVD

runs o In order to eliminate this in-site etching effect, the silane

was introduced before the reactor was turned on. This procedure

resulted in providing conditions for silicon deposition at the lowest

susceptor temperature and served to provide a thin silicon protective

coating on the silver surface as the susceptor heat was increased to

operating temperature. This procedure eliminated further, apparent,

in-site etching.

Each of the CVD runs in the hydrogen atmosphere showed nuclea-

tion on the surface. ^Generally, the more dense the nucleation, the

higher the Mg of the wafer. In an effort to reduce the effects of the

nucleation, several CVD runs were made where the wafers were allowed

to cool in silane after the reactor had. been turned off. The effect

of this cooling procedure was to deposit silicon between the nuclea-

tions, negating to some extent * the detrimental effect of the nuclea­

tion on the of the wafer. The positive effect of this cooling

56

Table 5.3 Deposition Rates 3 Film Figure of Merit and Susceptor Temperatures with Parameters Outside the Matrix - Hydrogen Atmosphere ;

Note CVDNumber

Deposition Rate A per minute

SusceptorTemperature

G

Film Figure of Merit

VD R 1dr 2 dr 3 DR,4

1 80 8 6 96 81 78 715 0 . 1 0

1 94 1 0 1 1 2 1 96 93 652 0.15

1 95 295 401 432 562 715 0.43

1 99 85 1 0 2 83 82 652 0.29

1,4 1 0 0 93 1 1 1 8 6 83 596-652 0.30

1,2,3 91 80 118 116 149 . 652 0.05

1,2,3 103 8 6 128 131 176 652 0 . 1 2

NOTES t

1. Silane flowrate changes during deposition from 32 cc to 64 cc

per minute.

2. Susceptor brought up to operating temperature with silane gas ,

flowing at 32 cc per minute.

3. Wafer cooled with silane gas flowing at 32 cc per minute.

4. Susceptor temperature changed during deposition.

57procedure was apparent in the results of CVD 91 and 103» Comparative

values of CVD 91 and CVD 94 are shown in Table 5,3 o CVD 94 was neither

heated nor cooled in silane.

Thickness measurements were made using the etching technique

outlined in Chapter 4« These measurements are shown in Table 5.4.

The deposition rates (DR) for the CVD runs of the parameter

matrix in the hydrogen atmosphere are shown in Table 5.2. The activa­

tion energies for these CVD runs are shown in Table 5.5.

At a silane flowrate of 6 cc per minute, an extremely high

activation energy was encountered (4.469 ev). This is probably due to

an interaction between the hydrogen gas and silver surface. At the

low silane flowrate there are fewer silicon molecules available for

deposition than at the higher flowrates. This suggests that possibly

diffusion of reactants to the substrate becomes the reaction rate-

limiting process rather than the surface reaction at the substrate.

The variation in the activation energies may be accounted for if the

rate-limiting process changes between the five steps of the deposition

kinetic process listed previously.

The activation energy at 5450 A silicon thickness with a

silane flowrate of 6 cc was not calculated due to inconsistent data.

58

Table 5.4 Thickness Measurements Hydrogen Atmosphere

for Selected Wafers:-

CVD Deposited Silicon Film ThicknessNumber ym

Position1

Position2

Position3

Position4

Position5

83 0.5240 0.7213 0.5339 0.3289 0.5053

8 6 0.4036 0.5193 0.4271 0.3588 0.4036

94 0.3578 0.5521 0.3869 0.4624 0.3428

105 0.4012 0.6237 0.5172 0.5131 0.4240

59Table 5.5 Activation Energies for CVD Runs in Hydrogen

Parameter Matrix

SiliconThickness

A

Activation Energy ev molecule

(E)

SiH^ @ 6 cc/min SiH^ @ 1 2 cc/min SiH^ @ 32 cc/min

1816 4.469 2.671 2.052

3633 2.187 2.990 2.092

5450 2.665 2.346

3633* 2,806 2.600 2.456

*Silicon on silicon

CHAPTER 6

CHEMICAL VAPOR DEPOSITION RESULTS USING HELIUM

The second phase of chemical vapor deposition (CVD) work was

conducted in a helium atmosphere. This chapter will discuss the

parameters that were varied and present the observed results. The

parameters that were varied in this phase were the silane gas flow-

rate and the susceptor temperature. In two CVD runs, the susceptor

was tiltedo

The helium atmosphere was obtained by using helium gas as the

mainstream carrier gas. A setting of 30 (earboloy ball) on the

rotameter at seven pounds per square inch gauge pressure yields a

helium gas flowrate of nine liters per minute. This helium flowrate

provided the same silane to carrier gas ratio as used in the hydrogen

atmosphere. The helium flowrate was held constant at nine liters per

minute for part of the experimentation in the helium atmosphere. It

was also varied from this flowrate for some runs as will be discussed

later. The silane flowrates and the susceptor temperatures were

varied to form an experimentation matrix.

Prior to initiating the helium atmosphere tests, the susceptor

and the reactor tube were etched to remove deposited silicon and

impurities from their surfaces. The susceptor was then coated with a

thin silicon film by making several runs without a wafer on the sus­

ceptor and by reversing the position of the susceptor after each run.

60

61The initial parameter matrix is shown in Table 6.1. The matrix shows

the areas of testing to be discussed first.

In all the CVD runs conducted in the parameter matrix 5 a 9

minute purge of the system was conducted with helium followed by a 1

minute purge of helium and silane at the flowrates. being investigated

on that particular run. The silane was then turned off. The susceptor

was then heated to the specified temperature, the silane reintroduced

into the reaction chamber and the CVD run made to the second interfer­

ence peak as indicated on the x-y recorder. The R-F power was then

shut off and the substrate allowed to cool in the helium-silane atmos­

phere established for that particular run. The post-deposition cooling

in the helium-silane mixture was incorporated into all the CVD runs

made in the helium atmosphere. The CVD run number as shown on the

parameter matrix is used to cross-reference between the various tables

in this chapter.

In the first area of -testing, the silane gas flowrate was held

at 4 cc per minute and the susceptor temperature was varied as shown in

Table 6.1. Three CVD runs were made in this area, CVD 119, 114, and

124. Of the three runs, CVD 114 had the lowest M . As indicated insTable 6.2, the deposition rates increased with temperature within this

area.

Nucleation was apparent in the helium atmosphere, however, it

was not as heavy as in the hydrogen atmosphere. Representative pictures

of nucleation for the testing within of the helium parameter matrix can

be seen in Fig. 6.1.

62

Table 6,1 Silane FTowrate 9 Susceptor Temperature Matrix - Helium Atmosphere

Silane Flowrate; cc per minute

TemperatureC

627 652 679

4,0 CVD CVD CVD119 114 124

CM CVD CVD CVD1 2 0 1 1 1 123

13.0 CVD CVD CVD1 2 1 109 1 2 2

63

Table 6.2 Deposition Rates, Film Figures of Merit and Susceptor Temperatures for CVD Runs in Parameter Matrix - Helium Atmosphere '

Silane Flowrate cc per minute

CVDNo.

Deposition Rate A per minute

SusceptorTemperature

Film Figure of Merit

MsDRi dr 2 dr 3 DR.4 C

13.0 109 488 452 356 313 652 0 . 1 0 2

7.2 1 1 1 334 318 247 233 652 0.058

4.0 114 205 198 165 149 652 0.070

4.0 119 138 133 1 0 1 8 8 627 0.072

7.2 1 2 0 137 133 99 8 6 627 0.056

13.0 1 2 1 209 2 1 2 179 168 627 0.050

13.0 1 2 2 950 967 806 749 679 0.126

7.2 123 585 619 384 328 679 0.544

4.0 124 448 489 385 360 679 0.123

CVD 111 (x245)

CVD 124 (x225)

CVD 123 (x225)

64

Fig. 6.1 Wafer Surfaces After CVD Runs in Helium Atmosphere

The second area of testing within the parameter matrix was

conducted with the silane flowrate of 7.2 cc per minute. Three runs,

were made in this area, CVD 120, 111, and 123. Of the three runs,

CVD 120 yielded the lowest film figure of merit (M^)„

The third area of experimentation was conducted with a silane

gas flowrate of 13 cc per minute. Again three CVD runs were made at

the various susceptor temperatures. CVD 121 had the lowest Mg , not

only for this area, but for all the CVD runs made within the parameter

matrixo The M values are given in Table 6.2. s '

The physical appearance of the wafers after deposition in the

helium atmosphere was quite, different from the physical appearance of

those wafers processed in the hydrogen atmosphere. Where, in the hydro

gen atmosphere, the wafers showed heavy nucleation and visible dis­

coloration, the wafers processed in the helium atmosphere were, for the

most part, shiny in appearance. Microscopic examination showed that

while nucleation was present in the helium atmosphere, it was much

smaller and more evenly distributed over the entire surface of the

wafer.

Within the helium atmosphere, other parameter configurations

besides those in the matrix were examined in order to find the opti­

mum system configurations. Two CVD runs, Numbers 115 and 116, were

made with a tilted susceptor as shown in Fig. 6.2. A tilted susceptor

configuration was examined to determine the effect of the silane gas

flow into the reactor heat zone and the gas flow over the substrate.

The silicon starts to precipitate from the silane as the silane enters '

66

\

Tilted Boat

Susceptor

WaferGas Flow

Fig. 6.2 Tilted Susceptor Configuration

67the reactor heat zone. For a given amount of gas, then, the part of

the substrate at the rear of the heat zone may have a smaller concen­

tration of silane acting upon it. If the substrate is tilted, then,

the silane hitting all surfaces will be more nearly the same. Table

6.3 gives the run parameters for the CVD runs with tilted susceptor.

CVD 115 had a fairly low Mg . The deposition rates were comparable to

the rates of the CVD runs within the parameter matrix.

Several CVD runs were made maintaining a constant gas ratio of

silane to helium. This ratio was 0.144% silane to helium. Keeping this

percentage fixed, CVD runs were then made at varied levels of helium

and silane gas flowrates as shown in Table 6.3. CVD 133 had the lowest

as shown in Table 6.3. During this phase of experimentation, there

was clouding of the reaction chamber with a brown precipitate at the 7

liter and 5 liter helium flowrates. The quantity of precipitate,

probably SiO, increased as the helium flowrate decreased. This indi­

cates that trace oxygen is in the gas mixture either as a result of a

leak in the reactor seals or as a result of contaminants in the gas.

Placing the reactor under vacuum, however, indicated the system was

well sealed. The oxygen, therefore, most likely resulted from impur-

ities in the helium gas.

Thickness measurements were made using the etching technique

outlined in Chapter 4. These measurements for selected wafers are

shown in Table 6.4.

In the helium atmosphere, a minimum occurred in the reflec­

tance measurements at the 9.7 to 9.9 micron wavelength (Figure 4.3).

68

Table 6.3 Deposition Rates, Film Figures of Merit and Susceptor Temperatures with Parameters Outside the Parameter Matrix - Helium Atmosphere

Silane Flowrate cc per minute

CVD No.

Deposition Rate A per minute

SusceptorTemperatures

C

Film Figure of Merit

■

Notes

DR1D R 2

dr 3 dr4

4.0 115 137 140 1 2 2 116 652 0.052 1

7.2 116 123 115 97 8 8 652 0.103 1

13.0 132 390 404 337 315 627 0.063 4

1 0 . 1 133 363 369 304 282 627 0.049 2

7.2 134 648 732 636 630 627 0 . 1 0 2 3

13.0 136 309 318 262 244 627 0.063 4

1 0 . 1 137 300 264 198 172 627 0.107 . 2

7.2 138 518 543 , 461 437 627 0 . 1 0 2 3

13.0 140 90 96 59 51 596 0.375 4

1 0 . 1 141 95 83 72 64 596 0.082 2

\-Notes: . 1. Tilted Susceptor

2o Helium Flowrate 7 liters per minute

3 e Helium Flowrate 5 liters per minute

4 o Helium Flowrate 9 liters per minute

69

Table 6.4 Thickness Measurements for Selected Wafers - Helium Atmosphere

CVDNo. Position

1

Deposited Silicon Film Thickness pm

Position Position Position2 3 4

Position5

1 2 1 0.4878 0.5083 0.4873 0.3483 0.2332

1 2 2 0.4185 0.7132 0.5073 0.4634 0.6279

132 0.4176 0.6388 0.4684 0.2980 0.2980

133 0.3817 0.5820 0.4036 0.4176 0.3428 .

70

This dip is most likely caused by SiO^ inclusions in the silicon film

caused by trace amounts of oxygen in the helium gas, The magnitude of

this minimum was reduced when molecular sieve was used in the helium

gas line.

the activation energies' for the CVD runs in the helium atmos­

phere are shown in Table 6.5. The activation energies fall within — 10.48 ev molecule of each other. With the higher silane flowrates

of 7.2 cc and 13 cc per minute, the activation energies follow the same

pattern in that the highest activation energies occur near a silicon

thickness of 3633 A and the lowest activation energies are for the sili­

con on silicon depositions. This indicates that the surfaces interaction

with the depositing molecules is seemingly overcome at this thickness

and it takes less energy for the reaction to proceed.

At the slow silane flowrate of 4 cc per minute, the activation

energies do not follow the same peaking pattern. This indicates that

the rate-determining process may be changing- from the surface reaction

process to one of the other four kinetic processes, probably the dif­

fusion of the reactants to the heated substrate.

71

Table 6.5 Activation Energies for CVD Runs in Helium Parameter Matrix

SiliconThickness

A

- Activation Energy ev molecule ^

(E)

SiH. @ 4 cc min 4 SiH^ @7.2 cc/min SiH4 @ 13 cc/min

1816 1.692 2.087 2.177

3633 1.872 2 . 2 1 1 2.182

5450 1.924 1.949 2.163

3633* 2.025 1.925 2.149

*Silicon on silicon

CHAPTER 7

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK .

7 o1 Conclusions

The chemical vapor deposition (CVD) process outlined, in Chapter

4 can be used to deposit non-single crystal silicon films with low

absorption in the near infrared wavelength range (2.5 to 15 microns)

by means of the pyrolysis of silane. Strict adherence to parametric

details of the process can result in reproducible silicon surfaces.

The use of helium rather than hydrogen as the mainstream carrier gas

produces better silicon layers. The CVD process is dependent on sus­

ceptor temperature and silane to carrier gas ratios and gas flowrates.

Careful preparation and handling of the substrates during the

CVD process reduces contaminants which have a pronounced effect on the

resultant silicon layer. Silver thickness and quality must be monitored

closely in order to allow silicon layer reproducibility. Gas flow to

include accurate metering of gases and reducing contaminants such as

water vapor in the gases must be continually monitored.

Major Results - Hydrogen Atmosphere

A. Deposition rates ranging from 50 to 4,126 A per minute.

B. Heavy nucleation on deposited silicon films.

C. Relatively high film figures of merit averaging 0.41.

D. Activation energies ranging from 2.092 to 4.469 ev.

72

73Major Results - Helium Atmosphere

Ac Deposition-rates, ranging from 51 to 967 A per minute.

Bo Nucleation of deposited silicon, film observable but much

smaller than in hydrogen atmosphere.

Co Relatively low film figures.of merit averaging 0.115.

D. Activation energies from 1.692 to 2.211 ev.

Relatively little published material exists pertaining to the

thermal decomposition of silane at the temperatures encountered in this

work or with silver as the material upon which the silicon is deposited.

Thereforey the problem areas encountered and recommendations given

below may provide useful guides to future work in this field.)

7,o2 , Recommendations for Future Work

One of the parameters of CVD that is most critical in the overall

process is the amount of chemical reactants that are available in the

reaction chamber at any given moment. In order to have better control

oyer the flow of the gases involved, it is recommended that mass flow-

rate meters replace the present rotameters. This interchange would

allow lower flowrates and more precise monitoring of the quantities of

gas entering the reaction chamber.

Detailed study of the kinetics of the silver surface upon which

the silicon is deposited would provide added insight into the effect of

the reflective layer on the structure of the deposited silicon. If,

as it appears, the silver undergoes definite structural changes or

mass transfer during the CVD process, an understanding of the exact

74nature of this change would provide a base for possible corrective

actiono This corrective action may be as uncomplicated as providing

an extremely thin protective layer of some material during the initial

phase of CVD where the silver is subjected to increasing temperature.

Conversely, detailed examination may reveal that another reflective

substance, other than silver, should be considered as the reflective

base.

The reduction of contaminants on the substrate should be care­

fully explored in future experimentation. The degree to which contami­

nants can be eliminated is to a great extent a function of the labora­

tory equipments available and quality control procedures employed

during processing. Contaminant reduction could markedly change the

nature of the deposited silicon layer by reducing nucleation, thereby,

affecting the crystal growth mechanism and structure of the silicon.

The susceptor temperature monitoring system could be improved

upon by instituting -a direct thermocouple contact with the substrate

resulting in a direct temperature readout on the CVD reactor console.

This would be difficult to obtain in practice due to the physical con­

struction of the CVD reaction chamber. Such a device could be a source

of contamination which would affect the deposited silicon. Since the

silver surface is subjected to change during the susceptor temperature

increase, a reading from the infrared microvoltmeter should be taken

while the infrared detector is looking at the graphite susceptor rather

than at the silver. This could be recorded as a portion of the CVD run

data on each x-y recorder curve.

Use of the silicon etch technique to determine the thickness

of the silicon layer has the drawback of being a destructive type

measurement, The silicon is destroyed in those areas measured.

Ellipsometry techniques for measurement of silicon layers should be

investigated to allow measurement of the silicon without destroying

the deposited layero

APPENDIX A

BROOKS ROTAMETER DATA ,

Descriptive data for the Brooks rotameters used for the CVD

testing is shown in Table A.I.

Conversion curves for determining gas flowrates are shown in

Figures A.l to A.3. These curves convert directly from the nitrogen

calibration gas to tfie desired test gas by use of correct sizing

factorso

Table A.l Brooks Rotameters Used in CVD Testing

Rotameter Model Number Float

MaximumCapacity

Air

Silane R—2—15—AAA Black Glass c-i . - 151 cc m m

Helium R—2—15—C Carboloy 22.8 SCFH

Hydrogen R—2—15—C Sapphire 10.6 SCFH

76

Rotameter

Scal

e

60

Brooks Rotameter R-2—15-AAA Glass Float 6612 “ 64552

1. N 0 at 5 psig (metered gas)

10 200 30 40cc/min metered flowrate

Fig. A.l Silane Rotameter Calibration Curve

Rotameter

Scal

e

70

60 Brooks Rotameter R-2-15-C Carboloy Float 6612-64557/22

50at 7 psig (metered gas)

He at 7 psig (N x 2.648)40

30

20

10

5 10 2015 25 30cc/min x 1 0 metered flowrate

Fig. A.2 Mainstream Helium Rotameter Calibration■vj00

Rotameter

Scal

e Brooks Rotameter R-2-15-C Sapphire Float 6612-64551

at 7 psig (metered gas)40 --at

20

5 10 15 20cc/min x 1 0 metered flowrate

Fig. A.3 Mainstream Hydrogen Rotameter Calibration Curve

VO

APPENDIX B

LIST OF SYMBOLS AND CONVERSION FACTORS

Symbol Description

A Pre-exponential factor; deposition rate at

infinite temperature

A Angstrom

Ag Silver

C Temperature, Centigrade

Cp • Heat Capacity /

DR Deposition Rate (Reaction Rate Constant)

DR^ Deposition Rate at X/4 wavelength .

DRg Deposition Rate at A/2 wavelength

DRg Deposition Rate at 3A/4 wavelength

DR^ Deposition Rate of Silicon on Silicon

E Activation Energy

F Free Energy

Fef Free Energy Function

Units

A, minute

-101 A = 10 meter

Degrees

Calories,i "1 1mole ,. K

A, minute

A, minute

A, minute - 1

A, minute - 1

A, minute

Electronvolts,molecule

Calories, - 1mole

Calories, mole K ^

80

Symbol

H

He

Hg ,

h 2

4K

M s

N 2

P

R

Description

Enthalpy, Heat of Formation

Helium

Mercury

Hydrogen

Equilibrium Constant

Temperature, Kelvin

From Figure of Merit

Nitrogen

Pressure

Universal Gas Constant

Entropy

SCFH

Si

SIH4SiO

SiO,

T

cal o

cc

ev

mm

Ppsigq

Standard Cubic Feet of Air Per Hour

Silicon

Silane

Silicon Monoxide

Silicon Dioxide

Temperature as a variable

Calories

Cubic Centimeter

Electron Volt

Millimeter

Partial Pressure

Heat

Units

Calories, mole

Dimensionless

Degrees

Humber 0 1.0

lbs in ^

1.9871 calories-1 — 1 mole , K

Calories,1 -I t/“1 mole , K

f t^ hour

Degrees

-31 0 meter

mm Hg per mm Hg lbs/sq in Gauge Calories

82Symbol

A

y

yv

Description

Absorption

Emissivity

Wavelength

Microns

Microvolts

Units

Number . 0> 1.0

Number0 -> 1 o 0

Microns

ly = 1 0

meters

lyv = 1 0

volts

-6

-6

23Conversion Factors: 1 mole - 6.02 x 10 molecules

103 cal. - 2.613 x 102 2 ev

LIST OF REFERENCES

Blocker9 John M , , Jr. ’’Integrating Factors in Chemical Vapor Deposi­tion 9 ” Proceedings of the Conference on Chemical Vapor Deposition of Refractory Metals 9 Alloys and Compounds5 Edited By A* C. Schoffhouser9 American Nuclear Society9 Inc , 9

Hinsdale9 111. 1967.

Burson 9 John H., Ill 9 and J, D. Fleming) Jr. ’’Thermodynamic andKinetic Analysis of Chemical Vapor Deposition)” Proceedings of the Conference on Chemical Vapor. Deposition of Refractory Metals 9 Alloys> and Compounds 9 Edited by A. C. Schoffhouser9

American Nuclear Society9 Inc.) Hinsdale3 111., 1967.

Cunningham, J. E . 9 and W. E. Dunn. ’’Forward”, Proceedings of theConference on Chemical Vapor Deposition of Refractory Metals, Alloys and Compounds, Edited by A. C. Schoffhouser, American Nuclear Society, Inc., Hinsdale, 111., 1967.

Hunt, L. P. and Erhard Sirtl. ”A Thermodynamic Analysis of theSilicon—Hydrogen-Chlorine Vapor Deposition System,” Chemical Vapor Deposition) Second International Conference, Edited by John Mo Blocker 9 Jr. and James C. Withers, The Electrochemical Society, Inc., New York, New York, 1970.

Moore, Walter J . Physical Chemistry, 3rd Edition, Prentice-Hall, Inc.,Englewood Cliffs, New Jersey, 1962.

Padnos 9 B. N. ’’Chemical Metallurgical Properties of Silicon,”Integrated Silicon Device Technology, V, X, Research Triangle Institute, Durham, North Carolina, 1965.

Pres.land, A. E. B ., G. L. Price, and D . L. Trimm. ’’Hillock Formation By Surface Diffusion on Thin Silver Films,” Surface Science,Vol. 29, (1972).

Schmidt-Ihn, E ,, K. G . Weil, and G. Will. ’’Recrystallization and Excess Free Energy in Thin Silver Films,” Thin Solid Films, 12 (1972).

Solid State Engineering Laboratory EE 250 Course Manual, Electrical Engineering Department, The University of Arizona, 1972.

Stull, D. R. and H. Prophet. JANAF Thermochemical Tables, 2nd Edition,NSRDS-NBS37, U.S. Department of Commerce, National Bureau of Standards, July 1970.

83

Whitmer, Dennis K„ "Design .and Application of an Infrared Temperature Monitor for Use in a Horizontal Induction Heated Furnace," Master’s Thesis, Electrical Engineering Department, The University of Arizona, 1972.