Thin films by thermal evaporation

138 Pages·05.39 MB·English

by  McAllister, Howard C

This thesis, having been approved by the
specud Faculty Committee, is accepted by
the Graduate School of the
University of Wyoming,
in partial fulfillment of the requirements
for the degree of Has ter of Science__
_________
Dean of the Graduate School.
Date August 21, 1950__
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
THIN FILMS BY THERMAL EVAPORATION
by
Howard C. M cAllister
A Thesis
Submitted to the Department
of Physics and the Graduate School
of the University of Wyoming in Partial
Fulfillm ent of Requirem ents for the Degree of
M aster of Science
T LIBRARY
□ r t;-u:
IJNJVERSiTY OF Wt'OivUNb
University of Wyoming
Laram ie, Wyoming
August, 1950
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Number: EP23704
INFORMATION TO USERS
The quality of this reproduction is dependent upon the quality of the copy
submitted. Broken or indistinct print, colored or poor quality illustrations and
photographs, print bleed-through, substandard margins, and improper
alignment can adversely affect reproduction.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if unauthorized
copyright material had to be removed, a note will indicate the deletion.
®
UMI
UMI Microform EP23704
Copyright 2007 by ProQuest LLC.
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
ProQuest LLC
789 E. Eisenhower Parkway
PO Box 1346
Ann Arbor, Ml 48106-1346
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE OF CONTENTS
Chapter Page
I. INTRODUCTION................................................................................. 1
U. DESIGN AND CONSTRUCTION OF THE
EVAPORATING UNIT..................................................................
6
General Description of the A p p aratu s............................. 10
The Vacuum S y s te m .................................................................. 13
Controlling In stru m e n ts.......................................................... 34
III. EVAPORATING TECHNIQUE . . ............................................ 38
IV. PROPERTIES OF THIN F IL M S ..................... 58
D ielectric Constant and Index of Refraction . . . . . 64
Velocity and Wave Length of R a d ia tio n ......................... 71
Thin Film s of M etals.................................................................. 73
Thin Film s of D ie le c tric s..................................................... 79
Interference F ilte r s .................................................................. 85
Polarized Light and M easurem ent of Optical
C o n sta n ts.................................................................................. 87
V. CONCLUSION......................................................................................... 93
Current Developments in Evaporating Techniques . . 93
Current Development of Thin F ilm s ................................. 98
BIBLIOGRAPHY..................................................................... 102
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF TABLES
Table Page
I. Probability of Evaporated Molecules Reaching
the S u b stra te .................................................................................. 9
II. Operating Data for Cenco Megavac Pum p. .................... 23
III. Evaporating C haracteristics of Various M etals with
Nine Different Filam ent M e ta ls ......................................... 41
IV. Satisfactory Filam ent M aterials for the Evaporation
of Various M etals ...................................................................... 46
V. Suggested Methods for Evaporation of Various M etals . 49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF ILLUSTRATIONS
Plate Page
I. Structure and Optical Properties of Silver Film s
Evaporated R apidly...................................................................... 76
II. Structure and Optical Properties of Silver Film s
Evaporated S lo w ly ...................................................................... 77
Figure
1. Reflecting interference filte r..................................................... 4
2. Essential components of therm al evaporation apparatus 5
3. Front view of evaporating u n it................................................. 11
4. Schematic diagram of vacuum s y ste m ................................ 14
5. Base p la te ............................................................................................. 16
. B rass conductor mounted in base plate................................ 17
6
7. GCB connector mounted in base p la te ................................ 17
. Wilson s e a l......................................................................................... 19
8
9. Schematic diagram of rotary oil-sealed pum p................ 21
10. C ross-section of diffusion p u m p ............................................ 26
11. Pumping speed of the MC-275 at various pressures and
heater currents .......................................................................... 30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
V
Figure Page
12. Speed vs. vacuum for glass-m etal pump type
GM-220 ...................................................................................... 32
13. Speed vs. forepressure for all-m etal diffusion
pump type MC-275 32
14. Evaporating d ev ices................................................................. 39
15. Electron m icrographs of silver films evaporated
in 2 seconds...................................................‘ ........................ 56
16. Electron m icrographs of silver films evaporated
in 20 m in u te s.......................................................................... 56
17. M etallic and dielectric plates in an electric field. . 59
18. Schematic diagram of the variation of index of
refraction with frequency for substances
transparent to visible radiation..................................... 67
19. Induction of wave motion by an oscillating charge . 71
20. A nti-reflection f ilm ................................................................. 80
21. Transm itting interference filter .....................................
8 6
22. Plane of incidence ..................................................... 89
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER I
INTRODUCTION
The process of preparing thin films by therm al evaporation has
been known since ^ but, surprisingly enough, did not begin to be­
1 9 1 2
come the laboratory tool it should until 1933 when Strong^ dem onstrated
its usefulness for the production of highly reflective m etallic coatings
for the m irrors of reflecting telescopes. Coating by therm al evapora­
tion now has developed into a routine production process in a number
of industries. As an example, many large searchlights contain re­
flectors which have a coating of aluminum protected by a film of silicon
oxide, both deposited by therm al evaporation. The use of therm al
evaporation to deposit the reflecting surface in seal-beam headlights is
becoming m ore and more common. Other articles now prepared by
this method include: aluminized plane and spherical m irrors for tele­
vision with silica protection, aluminized car reflectors on plastic base,
resistors for high frequency radio work, aluminized plastic jewelry and
mouldings with reflecting surfaces, aluminized fluorescent screens for
1. P. Pringsheim and R. Pohl, Deutsch. Phys. Gessel. , Verh. ,
14, 506 (1912), cited by J. Strong, Procedures in Experim ental
Physics, New York: Prentice-H all, Inc. , 1938. p! l .
6 8
2. J. Strong, Astrophysics J. , 83, 401 (1936).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
cathode ray tubes, and aluminized cellulose acetate sheets for decora­
tive purposes.
Film s formed by evaporation of non-m etals also have many uses.
The nature of these applications can be indicated by pointing out that
sufficiently thin films (of the order of magnitude of a wave length of
light) of all transparent bodies show brilliant colors. These may be
m ost easily observed in soap-bubbles, or in thin films of oil upon
water. These phenomena are attributed to interference taking place
between the light reflected from the front and rear surfaces of the
film.
3
This property of thin films was used by Blodgett to reduce, or
entirely elim inate, the reflection of monochromatic light from glass.
By covering the surface of a piece of glass with an appropriate film of
soaps of fatty acids, it was found possible to produce a condition such
that the light reflected from the outer surface was equal in intensity and
opposite in phase to that reflected from the glass-film surface. Thus
complete interference occurs and no light is reflected. Since no light is
reflected, more light must be transm itted. If the lenses of an optical
instrum ent are coated in such a manner, the efficiency of the instrument
is increased. Of course, soap film s were not satisfactory for, say, a
3. K. B. Blodgett, Science, 89, pp. 60-61 (1939). K. B.
Blodgett, Phys. Rev., 55, pp. 391-404 (1939).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.