U.S. patent number 3,639,165 [Application Number 04/738,563] was granted by the patent office on 1972-02-01 for resistor thin films formed by low-pressure deposition of molybdenum and tungsten.
This patent grant is currently assigned to General Electric Company. Invention is credited to John R. Rairden, III.
United States Patent |
3,639,165 |
Rairden, III |
February 1, 1972 |
RESISTOR THIN FILMS FORMED BY LOW-PRESSURE DEPOSITION OF MOLYBDENUM
AND TUNGSTEN
Abstract
High resistivity, low temperature coefficient of resistance
films are formed by evaporating a molybdenum or tungsten source in
a low-pressure atmosphere, e.g. 5.times.10.sup.-.sup.4 torr, of a
nitrogen bearing gas, a carbon bearing gas or an inert gas and
depositing a resistor film atop a preferably unheated dielectric
substrate.
Inventors: |
Rairden, III; John R.
(Niskayuna, NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
24968525 |
Appl.
No.: |
04/738,563 |
Filed: |
June 20, 1968 |
Current U.S.
Class: |
428/433; 338/308;
427/250; 204/192.22; 428/338 |
Current CPC
Class: |
H01C
17/08 (20130101); C23C 14/24 (20130101); Y10T
428/268 (20150115) |
Current International
Class: |
H01C
17/08 (20060101); C23C 14/24 (20060101); H01C
17/075 (20060101); H01c 007/00 () |
Field of
Search: |
;117/227,16C,107
;204/192 ;338/308 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jarvis; William L.
Claims
What I claim as new and desire to secure by Letters Patent of the
United States is:
1. A method of forming high-resistivity, low-temperature
coefficient of resistance films comprising the steps of:
positioning a dielectric substrate and a source material from 9 to
125 centimeters apart within an evaporation chamber;
selecting said source material from the group consisting of
molybdenum and tungsten;
evacuating said chamber;
introducing a deposition gas into said chamber to produce a working
pressure within said chamber of from 1.times. 10.sup.-.sup.3 to
1.times. 10.sup.-.sup.4 torr;
selecting said deposition gas from the group consisting of nitrogen
bearing gases, carbon-bearing gases and inert gases;
vaporizing at least a portion of said source material; and
depositing a resistor film upon said substrate.
2. A method of forming high-resistivity, low-temperature
coefficient resistance films according to claim 1 wherein said
source is tungsten and said substrate is maintained at a
temperature less than 370.degree. C. during deposition.
3. A method of forming high-resistivity, low-temperature
coefficient resistance films according to claim 1 wherein
deposition gas is selected from the group consisting of nitrogen,
methane, ammonia, and argon.
4. A method of forming high-resistivity, low-temperature
coefficient resistance films according to claim 3 wherein said
deposition gas is nitrogen, and said source is evaporated by
electron beam evaporation.
5. A resistor film formed by the method of claim 3.
6. A method of forming high-resistivity, low-temperature
coefficient resistance films according to claim 1 wherein said
source is molybdenum and said substrate is maintained at a
temperature less than 150.degree. C. during deposition.
7. A method of forming high-resistivity low-temperature coefficient
resistance films according to claim 6 wherein said deposition gas
is nitrogen, and said source is evaporated by electron beam
evaporation.
8. A resistor film formed by the method of claim 7.
9. A method of forming high-resistivity, low-temperature
coefficient of resistance films comprising the steps of:
positioning a dielectric substrate a predetermined distance from a
source selected from the group consisting of molybdenum and
tungsten within an evacuation chamber, wherein said predetermined
distance is from 9 to 125 centimeters,
evacuating said chamber and introducing a deposition gas selected
from the group consisting of nitrogen, ammonia and methane into
said chamber to produce a pressure of from 1.times. 10.sup.-.sup.3
to 1.times. 10.sup.-.sup.4 torr to effect at least one collision
between a vaporized source molecule and a gaseous molecule prior to
deposition of vaporized material from said source upon said
substrate,
vaporizing said source, and
depositing a resistor film upon said substrate to a thickness less
than 10,000 A.
10. A method of forming high-resistivity, low-temperature
coefficient of resistance films according to claim 9 wherein said
substrate is maintained in a temperature less than approximately
50.degree. C. during said deposition.
11. A method of forming high-resistivity, low-temperature
coefficient of resistance films according to Claim 10 wherein said
source is tungsten and said film is deposited in a nitrogen
atmosphere to a thickness producing a resistance less than 5.times.
10.sup.-.sup.3 ohms per square.
12. A method of forming high-resistivity, low-temperature
coefficient of resistance films according to Claim 10 wherein said
deposition gas pressure within said chamber is set relative to said
source to substrate distance to effect a plurality of collisions
between a vaporized source molecule and a gas molecule prior to
deposition.
Description
This invention relates to a method of forming thin film resistors
and in particular to the formation of resistors by the vacuum
deposition of tungsten or molybdenum in an atmosphere of a nitrogen
bearing gas, a carbon-bearing gas or an inert gas.
For suitability in sophisticated electronic printed circuitry, thin
film resistor components generally must be characterized by a
high-resistivity, a low-temperature coefficient of resistance and
highly stable electrical properties upon aging. Among techniques
heretofore proposed for forming suitable films are high vacuum
depositions of tungsten or molybdenum employing electron beam
evaporation and/or sputtering techniques. Similarly, in my
copending U.S. application Ser. No. 675,990, filed Oct. 17, 1967,
now U.S. Pat. No. 3,504,325 and assigned to the assignee of the
present invention, there is described and claimed .beta.-tungsten
resistor films and the method of their formation by the deposition
of a vaporized tungsten source under controlled environmental
conditions of oxygen pressure and substrate temperatures. The
formation of resistor thin films by evaporation of a Group IV or V
metal in a nitrogen atmosphere generally is disclosed and claimed
in my copending U.S. application Ser. No. 670,091, filed Sept. 25,
1967 now U.S. Pat. No. 3,537,891. Notwithstanding the suitable
characteristics of these thin film resistors for utilization in
printed circuitry, commercial production of resistor films also
requires a minimum number of controlled variables during resistor
film formation to lower fabrication costs. Many presently employed
commercial methods of resistor film formation however require
sophisticated apparatus for maintaining the substrate at a precise
elevated temperature during either the deposition or the subsequent
appealing of the deposited film to obtain suitable resistor
characteristics.
It is therefore an object of this invention to provide a novel
method of forming tungsten and molybdenum resistor films having
high-resistivity, low-temperature coefficient of resistance and
good stability upon aging.
It is also an object of this invention to provide an economical
method of constructing tungsten and molybdenum resistor films
having characteristics suitable for printed circuitry.
It is a further object of this invention to provide a method of
forming tungsten and molybdenum resistor films having good
stability without post deposition aging treatments.
These and other objects of this invention generally are achieved by
positioning a dielectric substrate and a source material selected
from the group consisting of molybdenum and tungsten within an
evaporation chamber which chamber, after evacuation, is filled with
a gas selected from the group consisting of the nitrogen bearing
gases, the carbon bearing gases and the inert gases in sufficient
quantities to produce a source to substrate distance-pressure
arithmetic product greater than 35.5.times. 10.sup.-.sup.4 torr
centimeters. The source then is vaporized by any suitable
technique, e.g., electron beam evaporation, and a resistor film is
deposited upon the substrate. In general, highest resistivity films
for a fixed deposition pressure are obtained when the substrate is
unheated, with suitable tungsten films being obtainable to a
maximum substrate temperature of approximately 455.degree. C. and
suitable molybdenum films being obtainable to a maximum substrate
temperature of 155.degree. C.
The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself, together with
further objects and advantages thereof may best be understood by
reference to the following description, taken in connection with
the accompanying drawings, in which:
FIG. 1 is a schematic view of apparatus suitable for forming
resistor films in accordance with this invention,
FIG. 2 is a graph depicting the variation of resistivity with
pressure for various substrate temperatures during tungsten
depositions,
FIG. 3 is a graph depicting the variation of temperature
coefficient of resistance with resistivity for resistor films
formed by tungsten depositions,
FIG. 4 is a graph depicting the variation of temperature
coefficient of resistance with film resistance for films formed by
tungsten depositions in accordance with this invention,
FIG. 5 is a graph depicting the aging characteristics of tungsten
resistor films,
FIG. 6 is a graph depicting the variation of resistivity with
pressure for various substrate temperatures during molybdenum
depositions,
FIG. 7 is a graph depicting the variation of temperature
coefficient of resistance with resistivity for resistor films
formed by molybdenum deposition in accordance with this invention,
and
FIG. 8 is a graph depicting the variation of temperature
coefficient of resistance with resistance for films formed by
molybdenum depositions in accordance with this invention.
Apparatus suitable for forming resistor films in accordance with
this invention is depicted in FIG. 1 and generally includes an
evaporation chamber 10 having a transverse electron beam gun 11
positioned therein for evaporation of a portion of a source
material 12, e.g., a metal selected from the group consisting of
tungsten and molybdenum, positioned within cup 13 of a water cooled
crucible 14. The evaporated source then passes through the
controlled gaseous environment within the chamber and is deposited
to a thickness less than 10,000 A. upon substrate 15 to form a high
resistivity, low temperature coefficient of resistance film.
Evaporation chamber 10 is conventional in structure and generally
includes a stainless steel envelope 17 positioned atop a circular
base 18 with a suitable sealant, shown as gasket 19, being provided
between envelope 17 and base 18 to assure isolation of the
evaporation chamber interior. Evacuation of the chamber is
accomplished through an aperture 20 approximately centrally
positioned within base 18 and communicated to exhaust pump 21 by
exhaust lines 22 and 23. A liquid nitrogen trap 24 is positioned
intermediate exhaust lines 22 and 23 to prevent contamination of
the chamber by a backfeed through the exhaust lines during
evacuation of chamber 10.
A second aperture 25 within base 18 permits the admission of a
suitable deposition atmosphere, e.g., a gas such as nitrogen,
ammonia, methane, carbon monoxide, carbon dioxide, argon, or neon,
into the chamber through conduit 28 and motor driven variable leak
valve 29 to continuously maintain the gaseous pressure within the
evaporation chamber at a desired level (as will be explained
hereinafter) for the formation of high-resistivity, low-temperature
coefficient resistance films. An ionization gauge 30 positioned
within the enclosed chamber and communicated to automatic valve
controller 31 through electrical lead 32 functions to control the
operation of variable leak valve 29 and regulate the gaseous
pressure within chamber 10.
Substrate 15, upon which the resistor film is to be deposited, can
be any suitable nonconductive material, e.g., soda lime glass
quartz, mica, or aluminum oxide, and is seated within a rectangular
frame 33 positioned at the upper end of an angularly shaped
stantion 34 protruding upwardly from base 18. To permit heating
(when desired) of the substrate during deposition, a tungsten
heater coil 35 is situated in an overlying attitude relative to the
substrate and is energized by an alternating current source 37
through disconnect device 38. A heat reflector 40 shrouds tungsten
heater coil 35 to concentrate the generated heat from the coil upon
the surface of the substrate and a platinum/platinum rhodium
thermocouple 41, connected to a temperature gauge 42 through
electrical lead 43, is positioned along the edge of the substrate
face remote from the source 12 to permit visual monitoring of the
substrate temperature. A suitably apertured shield 44 is positioned
in a generally underlying attitude relative to the substrate to
limit the film deposition area to a suitable size, e.g., 1 mm. by
10 mm. (10 square), thereby permitting simplified film resistance
measurements by conventional methods such as the four-probe
technique.
The transverse electron gun 11 utilized for evaporation of source
12 generally includes a cathode 47 energized by a negative DC
potential 46 through leads 36 for the emission of electrons and a
grounded anode 48 having an oval aperture 49 through which the
generated electrons are propelled as a stream. As the generated
electron stream passes beyond anode 48, the magnetic field produced
by a pair of generally upstanding, slightly convergent pole pieces
26 deflects the electrons in an arcuate attitude to impinge the
electron stream upon source 12 thereby evaporating a portion of the
source. Energization of cathode 47 with a DC potential of -7.5
kilovolts and a generation of a 700 milliampere stream from the
electrode has been found to provide sufficient evaporation of the
source to deposit 245 A. per minute upon a substrate positioned
approximately 35.5 cm. from the source.
In the operation of the method of this invention a suitable
nonconductive substrate such as a soda lime glass substrate, after
being suitably cleaned, is seated within rectangular frame 33 and a
source material selected from the group consisting of tungsten and
molybdenum is positioned within cup 13 of water cooled crucible 14
at a suitable distance, e.g., 35.5 cm., from the substrate.
Stainless steel envelope 17 then is placed upon circular base 18
and exhaust pump 21 is operated to evacuate the chamber to a
relatively low pressure of approximately 5.times.10.sup.-.sup.5
torr. Upon evacuation of the chamber, variable leak valve 29 is
operated to purge the system for a suitable period, e.g., 10
minutes, with the gas selected to be employed during the
evaporation and the pressure in the chamber is regulated relative
to the source to substrate distance to produce a source to
substrate distance-gas pressure arithmetic produce between
35.5.times. 10.sup.-.sup.3 torr cm. to 35.5.times. 10.sup.-.sup.4
torr cm., e.g., a gaseous level between 1.times. 10.sup.-.sup.3
torr to 1.times. 10.sup.-.sup.4 torr for a 35.5 cm. source to
substrate distance. Electron beam gun 11 then is energized to
evaporate the chosen source in sufficient quantities to deposit a
resistor film at a convenient rate of, for example, 300 A. per
minute, upon a preferably unheated substrate (as will be more fully
explained hereinafter). Upon deposition of a film of a desired
resistance, e.g., to a thickness less than 10,000 A., vaporization
of the source is terminated and the film is allowed to cool in the
deposition gas atmosphere. Although either nitrogen bearing gases,
e.g., nitrogen or ammonia; carbon-bearing gases, e.g., carbon
monoxide, carbon dioxide, methane etc.; or inert gases, e.g.,
argon, neon, etc., can be employed as the deposition gas utilizing
the method of this invention, nitrogen is preferred because
resistor films deposited therein exhibit a substantially higher
resistivity than resistor films deposited under similar conditions
in other gaseous atmospheres contemplated by this invention. For
example, an approximately 900 A. tungsten film deposited upon a
room temperature substrate in an argon pressure of 8.times.
10.sup.-.sup.4 torr exhibited a resistivity of 1,040 .mu. ohm-cm.
and an 813 A. tungsten film identically deposited in an ammonia
atmosphere exhibited a resistivity of 1,028 .mu. ohm-cm. while a
slightly thicker, e.g., 1,473 A. tungsten film deposited in a
nitrogen atmosphere under otherwise identical conditions exhibited
a substantially higher resistivity of 2,290 .mu. ohm-cm.
As can be noted from the graph of FIG. 2 depicting the variation of
resistivity with pressure for resistor films formed by electron
beam evaporation of tungsten in a nitrogen atmosphere utilizing a
35.5 cm. source to substrate span, increasing deposition pressures
produce an increase in the measured resistivity of the deposited
resistor films. For example, an increase in pressure from
0.1.times. 10.sup.-.sup.3 torr to 0.8.times. 10.sup.-.sup.3 torr
nitrogen generally effected approximately a doubling in the
measured resistivity of the tungsten resistor films deposited under
otherwise identical conditions. To obtain extremely high film
resistivities, e.g., above 1,000 .mu. ohm-cm., a deposition
pressure above approximately 0.4.times. 10.sup.-.sup.3 torr is
required for the 35.5 cm. source to substrate span, e.g., a source
to substrate-pressure arithmetic product above 14.2.times.
10.sup.-.sup.3 torr cm. Because the mean free path of a molecule,
e.g., the average travel distance required for a vaporized tungsten
molecule to collide with a gaseous molecule within the evaporation
chamber, is characterized by a source to substrate-pressure
arithmetic product of at least 5.times. 10.sup.-.sup.3 torr cm.,
high-resistivity tungsten films only are obtained when one or more
collisions occur between the evaporated tungsten and gaseous
deposition atmosphere prior to the deposition of the film upon the
substrate. Although the upper limit of the pressure range employed
for forming films by the method of this invention generally is
limited primarily by the apparatus utilized for deposition, e.g.,
electron beam apparatus tends to become inoperative at pressures in
excess of about 2.times. 10.sup.-.sup.3 torr, an inordinate number
of collisions, e.g., 10 or more, between the vaporized metal and
the gas within the chamber can result in poor continuity and
adhesion of the deposited material.
While the graph of FIG. 2 shows the variation of resistivity with
pressure for a 35.5 cm. source to substrate distance, it is to be
realized that extremely high measured resistivities of the
deposited film are obtained by the controlled interaction (or
collision) of the evaporated metallic source molecules with the
deposition gas within the chamber and therefore are not limited to
the specific pressures of FIG. 2. For example, a tungsten source
evaporated in a nitrogen pressure of 4.times. 10.sup.-.sup.4 torr
utilizing a 17.71 cm. source to substrate distance and deposited at
300 A. per minute upon a glass substrate exhibited a measured
resistivity of 545 .mu. ohm-cm. (well above the bulk resistivity of
tungsten) and a temperature coefficient of resistance of 105
p.p.m./.degree. C.
When different source to substrate distances are employed for
deposition, the deposition gas pressure within the chamber is
altered to assure a desired number of collisions between the
evaporated source and the deposition gas to obtain the high
measured resistivity in the film. For example, as the mean free
path of the evaporated source molecule is decreased by an increase
in the deposition gas pressure, the source to substrate distance is
decreased to assure a similar interaction between the evaporated
source molecules and the gaseous atmosphere prior to deposition
upon the substrate. As a practical matter however, source to
substrate spans between 125 cm. and 9 cm. are required for vacuum
evaporation of high resistivity films. In general, elongated source
to substrate spans permit a more uniform coating upon a larger
substrate area. The lower deposition pressures associated with the
elongated source to substrate spans however can tend to accentuate
the presence of background gases, e.g., gases other than the
deposition gas, within the deposition chamber tending to slightly
alter the resistor film characteristics. Highest resistivity films
were obtained utilizing a source to substrate span greater than 20
cm.
Notwithstanding the desirability of a relatively high gaseous
pressure during the tungsten deposition to assure sufficient
collisions for the formation of high resistivity films, x-ray
analysis of selected high resistivity films of this invention
indicate the films to be .beta.-tungsten rather than a tungsten
nitride or a tungsten carbide. It is therefore postulated that all
the various gaseous atmospheres employed during deposition serve to
produce interstitual impurities in the deposited films to
substantially increase the resistivity in the deposited films
relative to the resistivity of bulk tungsten while the more active
gases, such as nitrogen and methane, "chemically react" with the
evaporated tungsten to form extremely high resistivity
.beta.-tungsten films.
As can also be seen from the graph of FIG. 2 the resistivity of
tungsten films deposited at a given pressure varies inversely with
the substrate temperature during deposition with highest resistance
films being formed upon substrates at room temperature. When
substrate temperatures above 455.degree. C. are employed, a
conventional .alpha.-tungsten film is deposited rather than the
.beta.-tungsten structure required for extremely high resistivity
films. Films deposited on substrates maintained at a temperature of
approximately 240.degree. C. exhibit both .beta.-tungsten and
.alpha.-tungsten characteristics. Thus to obtain high resistivity
films, the substrate should be maintained at a temperature below at
least 455.degree. C. during deposition and preferably below
240.degree. C. Because films deposited upon unheated substrates
produce the highest resistivity and generally require no regulation
of the substrate temperature thereby minimizing fabrication costs,
the formation of films upon unheated substrates is preferred.
Referring more particularly to FIG. 3, tungsten films formed by the
method of this invention can approach a resistivity of
approximately 1,100 .mu. ohm-cm. before the temperature coefficient
of resistance of the film exceeds -50 p.p.m./.degree. C. when
temperature cycled between 25.degree. C. and 125.degree. C.
Furthermore because the temperature coefficient of resistance curve
of FIG. 3 is relatively flat, films having similar temperature
coefficients of resistance can be formed over a wide range of
resistivity; e.g., resistor films having a temperature coefficient
of resistance within .+-.25 p.p.m./.degree. C. are capable of
exhibiting resistivities varying from approximately 450 .mu.
ohm-cm. to 850 .mu. ohm-cm.
Tungsten films formed by the method of this invention also are
characterized by an extremely high resistance for a generally
tolerable temperature coefficient of resistance, e.g., a
temperature coefficient of resistance less than .+-.200
p.p.m./.degree. C. As illustrated in FIG. 4, tungsten films having
resistances above 2,000 ohms per square have been obtained with a
temperature coefficient of resistance of less than -150
p.p.m./.degree. C.
The deposition rate employed in the formation of the resistor films
of this invention generally has been found not to be extremely
critical with high resistivity tungsten films being formed at
deposition rates between 50 to 1,000 A. per minute. While the
deposition rate does have some effect on the characteristics of the
films formed, e.g., lower deposition rates generally tend to
produce higher resistivity films, the effect of deposition rate on
film characteristics is minimal compared with the effect produced
by variations in either the substrate temperature and the pressure
employed during the film deposition.
Extremely thin tungsten resistor films, e.g., films having a
resistance above 1,500 ohms per square, generally exhibit an
unstable aging characteristic, as is shown in FIG. 5. Thus when
tungsten resistor films having a high resistance are desired,
protective coatings, such as silicon monoxide, should be deposited
atop the resistor film prior to the admission of atmospheric gases
into deposition chamber 10 to inhibit aging of the resistor film.
Because extremely thick tungsten films can tend to crack glass
substrates due to stresses induced in the film during deposition
and due to the difference in coefficient of thermal expansion of
the film and underlying substrate, preferably the resistor films
are deposited to a thickness less than 10,000 A.
Molybdenum films generally exhibit a resistivity which varies with
the nitrogen deposition pressure in a manner similar to that of
tungsten, e.g., a rising resistivity for increasing nitrogen
pressure, as can be seen from FIG. 6 wherein resistivity variation
with nitrogen pressure is depicted for molybdenum depositions
employing a source to substrate distance of approximately 35.5 cm.
Thus, the source to substrate deposition gas pressure arithmetic
product required for the formation of high resistivity molybdenum
films generally is in excess of 3.6.times. 10.sup.-.sup.3 torr cm.
with molybdenum films exhibiting resistivities above 350 .mu.
ohm-cm. being formed only at gaseous pressures greater than the
mean free path of the vaporized molybdenum molecule, e.g., greater
than 0.15 microns nitrogen pressure in the graph of FIG. 6.
The resistivities of molybdenum films deposited in accordance with
this invention generally vary inversely with the substrate
temperature employed during deposition with relatively
high-resistivity molybdenum films being formed upon substrates at
approximately 25.degree. C. As illustrated in FIG. 6, the
resistivity of the deposited molybdenum films drops rapidly with
increasing substrate temperature and high-resistivity films are
formed only when the substrate is maintained at a temperature of
less than 150.degree. C. during deposition. The extreme dependency
of the molybdenum resistor characteristics upon substrate
temperature was exemplified by the deposition of three molybdenum
film samples under identical conditions except for a variation in
substrate temperature. The molybdenum film deposited at room
temperature exhibited a resistivity of approximately 1,740 .mu.
ohm-cm. and a temperature coefficient of resistance of
approximately 0 p.p.m./.degree. C. while the molybdenum resistor
films deposited at temperatures of 205.degree. C. and 465.degree.
C. exhibited decreasing resistivities of 425 .mu. ohm-cm. and 27
.mu. ohm-cm., respectively.
As can be seen from the graph of FIG. 7, the temperature
coefficient of resistance of the deposited molybdenum films when
temperature cycled between 25.degree. C. and 125.degree. C.
asymptomatically approaches a level of approximately less than +50
p.p.m./.degree. C. for nitrogen pressures above 0.6 microns
utilizing a 35.5 cm. source to substrate span. Thus, molybdenum
depositions at relatively high pressure, e.g., a source to
substrate-gas pressure arithmetic product above 21.3.times.
10.sup.-.sup.3 torr centimeters, generally produce films having a
low temperature coefficient resistance, e.g., less than +50
p.p.m./.degree. C. and a high measured resistivity, e.g., above
1,000 .mu. ohm-cm. for an unheated substrate. As can be seen
however from a comparison of FIGS. 4 and 8, films formed by
molybdenum depositions generally are characterized by a slightly
lower resistance for a given temperature coefficient of resistance
than tungsten films similarly formed. For example, while tungsten
films exhibit a temperature coefficient of resistance less than
-100 p.p.m./.degree. C. up to approximately 1,100 ohms per square,
molybdenum films exhibit a temperature coefficient of resistance of
-100 p.p.m./.degree. C. only up to approximately 900 ohms per
square.
A more complete understanding of the principles of this invention
can be obtained from the following specific examples of resistor
film depositions employing various sources and deposition
gases.
EXAMPLE 1
After a soda lime glass microscopic slide was cleaned by boiling in
water containing detergent, rinsing in cold then hot deiodized
water, rinsing in isopropyl alcohol and drying in isopropyl alcohol
vapors, the substrate was placed in a stainless steel frame located
approximately 35.5 cm. from a tungsten source in an evaporator
chamber. The chamber then was evacuated to a pressure of
approximately 5.times. 10.sup.-.sup.5 torr whereupon nitrogen was
admitted into the chamber to purge the chamber for approximately 10
minutes. After purging, the chamber was sealed, the pressure in the
chamber raised to 8.times. 10.sup.-.sup.4 torr nitrogen and the
electron beam gun employed for evaporation of the tungsten source
energized with sufficient power to produce a deposition rate of
approximately 275 A. per minute upon the glass substrate.
Deposition was continued for approximately 320 seconds to produce a
resistor film having a thickness of about 1,473 A. After the
deposited film was cooled in the nitrogen atmosphere of the
deposition chamber, subsequent measurement of the resistance
characteristics of the deposited film employing the conventional
"four probe" technique indicated the resistivity of the film to be
2,290 .mu. ohm-cm. while temperature cycling of the resistor film
from 25.degree. C. to 125.degree. C. indicated the film to have a
temperature coefficient of resistance of -120 p.p.m./.degree.
C.
EXAMPLE 2
A soda lime glass substrate was cleaned as in the previous example
and placed within a stainless steel holder 35.5 cm. from a tungsten
source in a vacuum chamber. The substrate then was preheated to
295.degree. C. during the evacuation of the chamber to a pressure
of approximately 5.times. 10.sup.-.sup.5 torr whereupon the chamber
was purged with methane for 15 minutes, during which period the
substrate heating was reduced to 280.degree. C. After purging and
sealing the chamber, the methane pressure within the chamber was
set at 8.times. 10.sup.-.sup.4 torr and the electron beam gun
energized to vaporize the tungsten in sufficient quantities to
produce a deposition rate of approximately 360 A. per minute upon
the substrate. Deposition was continued for 12 minutes to produce a
resistor film of approximately 4,500 A. upon the substrate and the
substrate was maintained at 280.degree. C. for 15 minutes in the
methane atmosphere of the chamber subsequent to the deposition.
After cooling in the methane atmosphere, subsequent measurement of
the tungsten film disclosed a measured resistivity of 386 .mu.
ohm-cm. and a temperature coefficient of resistance of +78
p.p.m./.degree. C. when cycled between 25.degree. C. and
125.degree. C.
EXAMPLE 3
Deposition techniques identical to that of example 1 were employed
except for the fact that ammonia was utilized as the gaseous medium
within the chamber during deposition and the deposition was
conducted for a period of 220 seconds to produce a film of 813 A.
Subsequent resistance measurements of the film indicated the film
to have a measured resistivity of 1,028 .mu. ohm-cm. and a
temperature coefficient of resistance of -160 p.p.m./.degree. C.
when cycled between 25.degree. C. and 125.degree. C.
EXAMPLE 4
An approximately 893 A. resistor film was deposited in an 8.times.
10.sup.-.sup.4 torr argon atmosphere under conditions otherwise
identical to those of example 3. Subsequent resistance measurements
indicated the deposited film to have a measured resistivity of
1,040 .mu. ohm-cm. and a temperature coefficient of resistance of
-120 p.p.m./.degree. C. when cycled between 25.degree. C. and
125.degree. C.
EXAMPLE 5
A cleaned glass substrate was positioned within a substrate holder
of an evaporation chamber and a molybdenum source was placed within
the chamber at a span of 35.5 cm. from the substrate. After
evacuation of the chamber to a pressure of approximately 5.times.
10.sup.-.sup.5 and a 15 minute purge of the chamber with nitrogen
gas, the pressure within the chamber was set at 8.times.
10.sup.-.sup.4 torr nitrogen. Energization of the electron beam
source then was initiated at a sufficient power level to produce a
deposition rate of 265 A. per minute for approximately 3 minutes
thereby forming a film approximately 789 A. thick. Subsequent
measurements of the film characteristics indicated the film to have
a measured resistivity of 1,170 .mu. ohm-cm. and a temperature
coefficient of resistance of -30 p.p.m./.degree. C. when cycled
between 25.degree. C. and 125.degree. C.
EXAMPLE 6
A molybdenum source was placed within an evaporation chamber at a
span of 35.5 cm. from a clean glass substrate and the chamber
filled with methane to a pressure of 8.times. 10.sup.-.sup.4 torr
after an initial exhaust and a purge for approximately 10 minutes
with methane gas. Upon energization of the electron beam source, a
film was deposited at a rate of approximately 885 A. per minute for
20 seconds to form a 295 A. thick film upon the substrate.
Subsequent resistance measurements of the film indicated the film
to have a resistivity of 337 .mu. ohm-cm. and a temperature
coefficient of resistance of +130 p.p.m./.degree. C. when cycled
between 25.degree. C. and 125.degree. C. The resistance of the
deposited film measured approximately 1,070 ohms/sq.
EXAMPLE 7
This example was conducted in a manner identical to example 5
except for the employment of ammonia gas as the atmosphere within
the chamber. The molybdenum source was evaporated at a rate of 935
A. per minute for 70 seconds to produce a 1,090 A. thick film.
Subsequent measurements of the resistive characteristics of the
film indicated a measured resistivity of 2,580 .mu.ohm-cm., a
temperature coefficient of resistance of -290 p.p.m./.degree. C.
when cycled between 25.degree. C. and 125.degree. C., and a
resistance of 2,300 ohms/sq.
While the invention has been described with respect to certain
specific embodiments, it will be appreciated that many
modifications and changes may be made without departing from the
spirit of the invention. I intend, therefore, by the appended
claims, to cover all such modifications and changes as fall within
the true spirit and scope of my invention.
* * * * *