US 3639165 A
High resistivity, low temperature coefficient of resistance films are formed by evaporating a molybdenum or tungsten source in a low-pressure atmosphere, e.g. 5x10<->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.
Beschreibung (OCR-Text kann Fehler enthalten)
United States Patent Rairden, III Feb. 1, 1972  RESISTOR THIN FILMS FORMED [$6] References Cited LOW-PRESSURE DEPOSITION OF UNI IED STA 1 ES PA'I ENTS MOLYBDENUM AND TUNGSTEN 3,140,460 7/1964 Turkat ..338/308 1 lnvemofl J Rflrden, Nlskayunav 3,242,006 3/1966 Gerstenberg..... ...33s/30s x  Assigneez General Electric Company 3,258,413 6/1966 Pendergast ..204/l92 22 Filed; June 20, 1963 Primary Examiner-William L. Jarvis Attorney-Richard R. Brainard, Paul A. Frank, John J. Kis- [211 App]. No.: 738,563 sane, Frank L. Neuhauser, Oscar B. Waddell and Melvin M.
Goldcnberg  US. Cl ..ll7/227, 117/107, 204/192,  ABSTRACT 338 308  Int Cl "01c [7/00 High resistivity, low temperature coefficient of resistance films are formed y evaporating a molybdenum or tungsten  FueldoiSearch ..ll7/227, 106 C, 107,204/192, source in a lowmessure atmosphere 5x104 to", of a 338/308 nitrogen bearing gas, a carbon bearing gas or an inert gas and depositing a resistor film atop a preferably unheated dielectric substrate.
12 Claims, 8 Drawing Figures RES/S r/ wry (MICRO Oil/76' c/v) 6'00 1 i i i run/es TEN F/L MS DEPOS/ TED a Y REAC r/ vs EVAPOR/i r/o/v I5 '6 mmoaav PRESSURE (MICRO/VS Hg) PAIENTED FEB 1 m2 SHEET 1 BF 4 DEPOS/ TIO/V GAS Inventor: dohrv F2. Rdir-den,
AL M tor-nay.
RESISTOR THIN FILMS FORMED BY LOW-PRESSURE DEPOSITION OF MOLYBDENUM AND TUNGSTEN 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 US. Pat. No. 3,504,325 and assigned to the assignee of the present invention, there is described and claimed B-tungsten resistor films and the a 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 US. application Ser. No. 670,091, filed Sept. 25, 1967 now US. 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 355x 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 C. and suitable molybdenum films being obtainable to a maximum substrate temperature of 155 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:
HO. 1 is a schematic view of apparatus suitable for forming resistor films in accordance with this invention,
HO. 2 is a graph depicting the variation of resistivity with pressure for various substrate temperatures during tungsten depositions,
HO. 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 [7 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. g
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 pennit 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., l mm. by 10 mm. (10 square), thereby permitting simplified film resistance measurements by conventional methods such as the fourprobe technique.
The transverse electron gun ll 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 suitabledistance, e.g., 35.5 cm., from the substrate. Stainless steel envelope 17 then is placed upon circular base l8 and exhaust pump 21 is operated to evacuate the chamber to a relatively low pressure of approximately X10" torr. Upon evacuation of the chamber, variable leak valve 29 is operated to' purge the system for a suitable period, e.g., 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 10 torr cm. to 35.5 10 torr em., e.g., a gaseous level between 1X10 torr to l 10- torr for a 35.5 cm. source to substrate distance. Electron beam gun ll then is energized to evaporate the chosen source in sufficient 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 8X10 torr exhibited a resistivity of 1,040 u ohmcm. and an 813 A. tungsten film identically deposited in an ammonia atmosphere exhibited a resistivity of 1,028 u ohmcm. 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 a 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.l 10- torr to 0.8 10 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 p. ohm-cm., a deposition pressure above approximately 0.4X10' torr is required for the 35.5 cm. source to substrate span, e.g., a source to substrate-pressure arithmetic product above 14.2)(10 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 5X10 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 limitof 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 2X10 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 4X10 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 [L ohm-cm. (well above the bulk resistivity of tungsten) and a temperature coefficient of resistance of 105 p.p.m./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 fi-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 B-tu ngsten 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 tem' pcrature. When substrate temperatures above 455 C. are employed, a conventional a-tungsten film is deposited rather than the B-tungsten structure required for extremely high resistivity films. Films deposited on substrates maintained at a temperature of approximately 240 C. exhibit both B-tungsten and a-tungsten characteristics. Thus to obtain high resistivity films, the substrate should be maintained at a temperature below at least 455 C. during deposition and preferably below 240' 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 1 ohm-cm. before the temperature coefficient of resistance of the film exceeds -50 p.p.m./ C. when temperature cycled between 25 C. and 125 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./ C. are capable of exhibiting resistivities varying from approximately 450 ,u ohm-cm. to 850 .t 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./ 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 1 50 p.p.m./ 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 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 3.6Xl0 torr cm. with molybdenum films exhibiting resistivities above 350 ,uohm-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 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 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 n. ohm-cm. and a temperature coefficient of resistance of approximately 0 p.p.m./ C. while the molybdenum resistor films deposited at temperatures of 205 C. and 465 C. exhibited decreasing resistivities of 425 uohm-cm. and 27 #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 C. and C. asymptomatically approaches a level of approximately less than +50 p.p.m./ 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 2l.3 l0 torr centimeters, generally produce films having a low temperature coefficient resistance, e.g., less than +50 p.p.m./ C. and a high measured resistivity, e.g., above 1,000 zohm-cm. for an unheated substrate. Ascan 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 -l00 p.p.m./" C. up to approximately 1,100 ohms per square, molybdenum films exhibit a temperature coefficient of resistance of l00 p.p.m./ C. only up to approximately 900 ohms per square.
A more complete understanding of the principles of this in vention 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 5X10 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 l Otorr 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 ,uohm-cm. while temperature cycling of the resistor film from 25 C. to 125 C. indicated the film to have a temperature coefficient of resistance of l 20 p.p.m./ 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 C. during the evacuation of the chamber to a pressure of approximately 5X10 torr whereupon the chamber was purged with methane for 15 minutes, during which period the substrate heating was reduced to 280 C. After purging and sealing the chamber, the methane pressure within the chamber was set at 8X10 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 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 p. ohm-cm. and a temperature coefficient of resistance of +78 p.p.m./ C. when cycled between 25 C. and 125 C EXAMPLE 3 Deposition techniques identicalto 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 z ohm-cm. and a temperature coefficient of resistance of -l 60 p.p.m./ C. when cycled between 25 C. and 125 C.
EXAMPLE 4 An approximately 893 A. resistor film was deposited in an 8X10torr argon atmosphere under conditions otherwise identical to those of example 3. Subsequent resistance measurements indicated the deposited film to have a measured re-' sistivity of 1,040 p. ohm-cm. and a temperature coefficient of resistance of l20'p.p.m./C. when cycled between 25 C. and 125C.
EXAMPLE 5 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.
,Sub'sequent measurements of the film characteristics indicated the film to have a measured resistivity of 1,170 1. ohmcm. and a temperature coefficient of resistance of -30 p.p.m./ C. when cycled between 25 C. and 125 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 8X10" 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 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 ,u. ohmcm. and a temperature coefficient of resistance of +130 p.p.m./ C. when cycled between C. and 125 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 ptohm-cm, a temperature coefficient of resistance of -290 p.p.rn./ C. when cycled between 25 C. and 125 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.
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 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 1X10 to 1 X 1 Otorr; 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 tem-' perature less than 370 C. during deposition. I
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 C. during deposition.
'7. A method offorming 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 1X10 to 1X10 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 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 5X10ohms 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.