US3537891A

US3537891A - Resistor films of transition metal nitrides and method of forming

Info

Publication number: US3537891A
Application number: US670091A
Authority: US
Inventors: John R Rairden
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1967-09-25
Filing date: 1967-09-25
Publication date: 1970-11-03
Anticipated expiration: 1987-11-03

Description

Nov. 3, 1970 J. R. RAIRDEN m 3,537,891

RESISTOR FILMS OF TRANSITION METAL NITRIDES AND METHOD OF FORMING Filed Sept. 25-, 1967 3 Sheets-Sheet 1 NITROGEN FEAR/A16 G83 /6 :3'4T

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RESISTOR FILMS OF TRANSITION METAL NITRIDES AND METHOD OF FORMING 3 Sheets-Sheet 2 25, 1967 N/OD/ll/fl Nitride Fly/17$ Filed Sept Fig. 3.

Niobium Nitride Films /nven/or John A. Ra/foenfl,

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-w m 0 -w 0 m I). 5 W m mm w w 2 M 9 .5 F mw w. u n 0 .I 8 0 n w M -w 4 M 0 2 m m s m a. m a W F m m n 5 wm m M e we. mm m -0 m mm 5 M a f w H m e m m 0 l h m a m m. -M G \=C E B M W s o Em uu QEmk o m m, 0 m m Nov. 3, 1970 Filed Sept. 25, 1967 Temperature Coefficient of Q 8 l l J Res/stance (PPM/"Cl 4. R. RAYIRDEN m RESISTOR FILMS OF TRANSITION :METAL NITRIDES METHOD OF FORMING AND 5 Sheets-Sheet s F ig. 6 300v I v Zirconium Nitride Films 200- i/iii 0 '00 1050 I500 7 '00 es'oo Resistance (fl/Cl at C E /00 5 0 v F/g] 8%:- Hafnium Nitride Films E E a a $3 -2oo u? St-x300 I o 500 I000 a Resistance (.Q/a at 25 '6) Fig. 8. Titanium Nitride Films and Vanadium Nitride Films /nvent0r l I I I I I l l byaoLa ta -4' H/s Attorney- John ii. Roiraen ET,

United States Patent 01 fice 3,537,891 Patented Nov. 3, 1970 3,537,891 RESISTOR FILMS OF TRANSITION METAL NITRIDES AND METHOD OF FORMING John R. Rairden HI, Niskayuna, N.Y., assignor to General Electric Company, a corporation of New York Filed Sept. 25, 1967, Ser. No. 670,091 Int. Cl. C23c 11/08, 11/14 US. Cl. 117-215 10 Claims ABSTRACT OF THE DISCLOSURE Resistor thin films of nitrides ofthe groups IV and V transition metals are formed by the reactive evaporation of the chosen transition metal in a nitrogen atmosphere between 5 10 to torr. The substrate upon which the resistor film is deposited is preheated to a temperature of about 420 C. and the transition metal is deposited at a rate of approximately 100 A. per minute atop the substrate. Upon completion of the deposition of the nitride resistor film to the desired thickness, evaporation of the metal is discontinued and the resistor film is heat treated at a temperature of approximately 400 C. for 8 minutes in a nitrogen environment greater than 0.5 torr to produce a resistor film having a zero (p.p.m./ C.) temperature coefiicient of resistance between C. and 125 C. Tantalum and niobium nitride resistor films formed by reactive evaporation have been found to be very abrasion resistant exhibiting no change in resistance after traversal of 10,000 revolutions with a carbonaceous spring biased contact while substantial variations in resistance were noted for both nickel-chrome and zirconium nitride films traversed by the identical contact.

This invention relates to thin film resistors and in particular to thin film transition metal nitride resistors formed by reactive evaporation in a low pressure nitrogen atmosphere.

Among the desirable characteristics for a resistor thin film fabricated into printed circuitry are high resistivity, low coefficient of resistance and stability and thin films of tantalum nitride formed by the reactive sputtering of tantalum in a partial nitrogen pressure of approximately 10- torr heretofore have been known to possess these resistor characteristics. For example, tantalum nitride films produced by reactive sputtering have been known to exhibit a specific resistivity in the order of 200 to 250 micro ohm centimeters, a temperature coefiicient of resistance of approxmiately zero (p.p.m./ C.) between 25 C. and 125 C. and excellent stability upon heat treatment of the resistor film in the presence of air at temperatures in the range of 250 to 400 C.

I have discovered however that resistor thin films formed by the reactive evaporation of a transition metal in 'a low pressure nitrogen atmosphere, e.g. between 5 X10 to 10* torr, exhibit a specific resistivity substantially higher (approximately 67% for comparable tantalum nitride resistor films) than the specific resistivity obtainable by reactive sputtering. The exact cause of this phenomena is unknown. However one possible cause of the variation in resistivity between reactively sputtered re sistor films and reactively evaporated resistor films may be postulated by considering the specific resistivity of a resistor film to be a composite of the resistivity produced by lattice vibration, the resistivity due to surface function and the resistivity due to defect or interruptions in the repetitive geometry of the crystalline structure. Because the surface function resistivity is primarily a function of the thickness of the film and because the resistivity due to lattice vibration is primarily a function of the composition of the material (both of which film characteristics are assumed to be identical for comparable resistor films formed by reactive sputtering and reactive evaporation), the specific resistivity variations between comparable resistor film-s formed by the two methods would appear to result from a divergence in the repetitive crystalline structure of the two films. Thus the crystalline structure of resistor films may vary dependent upon the process employed in the reactive formation of the resistor thin films.

The resistor elements formed by the reactive evapora tion process of this invention also has been found to exhibit a fine grain size, excellent wear resistance, relative immunity to corrosion, temperature stability and high resistance.

It is therefore an object of this invention to provide a novel method of producing superior resistor films of transition metals.

It is also an object of this invention to provide resistor films of transition metal nitrides having high resistivity, fine grain size, temperature stability and superior wear resistance.

It is a further object of this invention to provide a potentiometer having a thin film resistor element exhibiting superior wear resistance.

These and other objects of this invention generally are achieved by thin film resistors having a nitride of a transition metal chosen from the group consisting of niobium, tantalum, vanadium, titanium, zirconium and hafnium positioned atop a non-conductive substrate where the transition metal nitride is formed by the reactive evaporation of the chosen transition metal in a low pressure nitrogen atmosphere between approximately 5 10- to 10 torr. Preferably these thin film resistors are formed by positioning a substrate and a transition metal selected from the group consisting of niobium, tantalum, vanadium, titanium, zirconium and hafnium within an enclosed chamber and introducing a quantity of nitrogen bearing gas into the chamber to produce a low pressure nitrogen atmosphere in a range between approximately 5X10" to 10- torr. After the substrate is heated to a temperature in excess of 30 C., the transition metal is evaporated in the nitrogen atmosphere and a thin film nitride resistor is condensed atop the substrate. To increase the resistivity and to decrease the temperature coefficient of resistance of the resistor film, the film is baked in a nitrogen atomsphere above 0.5 torr at elevated temperatures above 30 C. prior to removal of the resistor film from the nitrogen atomsphere of the enclosed chamber. By admitting air into the chamber after the post-deposition baking of the film while the resistor film is still warm, e.g. at approximately 245 C., a thin oxide stabilizing layer is formed atop the resistor film.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is an isometric section of an apparatus suitable for forming resistor thin films in accordance .with this invention,

FIG. 2 is a graph depicting the variation of resistance with nitrogen pressure for niobium nitride resistor films formed by reactive evaporation in a low pressure nitrogen atmosphere,

FIG. 3 is a graph depicting the variation of resistance with temperature coeflicient of resistance for niobium nitride films formed by reactive evaporation,

FIG. 4 is a graph depicting the variation of nitrogen pressure with temperature coefficient of resistance for tantalum nitride resistor films formed by the method of this invention,

FIG. 5 is a graph depicting the variation of temperature coeflicient of resistance with resistance for tantalum nitride resistor films deposited utilizing the apparatus of FIG. 1,

FIG. 6 is a graph depicting the variation of resistance with temperature coefficient of resistance for zirconium nitride resistor films deposited by reactive evaporation in a low pressure nitrogen atmosphere,

FIG. 7 is a graph depicting the variation of resistance with temperature coefiicient of resistance for hafnium nitride resistor films deposited in accordance with this invention, and

FIG. 8 is a graph depicting the variation of resistance with temperature coefficient of resistance for both titanium nitride and vanadium nitride resistor films deposited by reactive evaporation in a low pressure nitrogen atmosphere.

An apparatus suitable for the deposition of transition metal nitride thin films in accordance with this invention is depicted in FIG. 1 and generally includes an evaporation chamber 10 having an electron beam source 12 for the evaporation of a transition metal source 14. Suitable means, such as conduit 16, are provided for the controlled admission of a nitrogen bearing gas 18, e.g. nitrogen or ammonia, into the enclosed chamber and the electron beam evaporated transition metal reacts with the nitrogen within the enclosed chamber to deposit a transition metal nitride resistor film upon substrate 20.

Evaporation chamber 10 generally includes a water cooled stainless steel envelope 22 seated upon a circular base 24 and a gasket is provided intermediate the vertical sidewalls of the chamber and the base to seal the interior of the chamber. Approximately centrally positioned within base 24 is a circular aperture 32 which communicates the interior of the chamber with vacuum pump 34 through

lines

36 and 38 for the evacuation of the sealed chamber while an enclosed liquid nitrogen trap 40 is positioned between

evacuation lines

36 and 38 to effectively seal the chamber from contamination during operation.

A second aperture 42 within the base 24 communicates the interior of evaporation chamber 10 with a nitrogen bearing gas source 18 through conduit 16 and an automatic pressure control valve 44 is inserted within conduit 16 intermediate nitrogen bearing gas source 18 and aperture 42 to control the flow of low pressure nitrogen into the chamber prior to and during reactive evaporation. An ionization gauge 46 mounted within evaporation chamber 10 and communicated to automatic pressure control valve 44 by electrical lead 48 passing through an aperture in the base of the chamber serves to automatically govern the operation of the control valve thereby regulating the pressure of nitrogen bearing gas within the chamber to produce a nitrogen pressure between 5X10 to 10- torr during evaporation of source 14.

A water cooled crucible 50 is approximately centrally positioned upon base 24 and is supported in a slightly elevated position above the base by machine screws 52 extending through suitable apertures in a pair of vertically extending, slightly convergent magnetic pole pieces 54. Crucible 50 is interiorly bored (not shown) to allow a flowing coolant to function as a heat transfer agent for the crucible. The transition metal source 14, from which the resistor film is fabricated, is positioned within the cup 56 of the water cooled crucible at a location along the arcuate path of the electron beam produced by electron beam source 12. Transition metal source 14 can be any metal selected from the group consisting of niobium, tantalum, vanadium, titanium, zirconium or hafnium and may have the physical configuration of an ingot.

The heat source used in the preferred method of this invention to evaporate a portion of transition metal source 14 is a beam generated by electron beam source 12 which source is depicted as a transverse electron gun fixedly secured to the sidewall of crucible 50. The electron gun generally includes a cathode 60 suitably energized, e.g. with a DC potential 58 of approximately 10 kv. by electrical leads 62, for the generation of electrons and a centrally apertured anode 64 at ground potential for acceleration of the generated electrons in a generally vertical stream within evaporation chamber 10. When the electron stream generated by the electron gun passes outside the generally enclosed gun into the magnetic field produced by magnetic pole pieces 54, the stream is deflected in an arcuate direction to impinge the electrons upon transition metal source 14 within cup 56'. For the evaporation of the transition metal source, a 3.2 kw. deposition power was found to provide suflicient bombardment of the transition metal source to deposit the metal nitride at an acceptable rate of approximately 100 A. per minute upon substrate 20 situated 14 inches from transition metal source 14.

The distance from source 14 to substrate 20 preferably is fixed relative to the nitrogen gas pressure in the chamher, as controlled by pressure control valve 44, to produce one or more collisions between the evaporated transition metal in the gas phase and a nitrogen molecule before deposition of the metal upon the substrate. To effectuate this result, a nitrogen pressure of 1 10- torr generally requires a minimum source to substrate distance of 5 cm. with a tenfold decrease in nitrogen pressure, e.g. to 1 10- torr, requiring approximately a tenfold increase in source tosubstrate distance, e.g. to 50 cm.

The substrate 20 upon which the transition metal nitride resistor film will be deposited lies within a ledge of rectangular frame 68 which frame is supported in an elevated position by an angularly shaped brace 70. Substrate 20 may be any non-conductive heat resistant material, e.g. soda lime glass, quartz, mica or magnesium oxide, and the lower face of the substrate is situated in a generally confronting attitude with transition metal source 14. A tungsten wire heater 72 and a heat reflector 74 are positioned slightly above the substrate face remote from the transition metal source and a platinum/platinum-rhodium thermocouple 76 is seated along the edge of the upper face of the substrate to produce a visual indication of the substrate temperature upon temperature gauge 88. Energization of heater 72 is provided by a 40-volt, 5 ampere source of alternating current 80 connected to the heater through a switch 82 and electrical leads 78.

A mask 84 apertured to the dimension desired for the thin film resistor, e.g. 1 mm. x 10 mm, is positioned upon the upper outer extension of pivotal rod 86 and is rotatable by suitable external means (not shown) to a position in an underlying relationship with substrate 20 to control the geometry of the transition metal resistor thin film deposited upon the substrate.

In the operation of this invention, substrate 20 is cleaned in a suitable manner, for example, soda lime glass microscopic substrates preferably are cleaned by boiling the substrate in water containing a detergent, successively rinsing the substrate in cold water, hot deionized water, and isopropyl alcohol and then drying the substrate in hot vapors of isopropyl alcohol. After cleaning and drying, the substrate is positioned within frame 68 which is situated at a suitable distance, e.g. 14 inches, above a highly pure transition metal source 14 seated in cup 56 of crucible 50. Thermocouple 76 then is positioned along the upper edge of the substrate remote from source 14 to measure the substrate temperature and evaporation chamber 10 is evacuated to a low pressure of approximately 10 torr by pump 34. Upon evacuation of the evaporation chamber, the chamber is purged with nitrogen bearing gas 18 at pressures preferably in a nitrogen range of 1X10 torr to 8X10 torr for a period of approximately 5 minutes and mask 84 is positioned below the face of the substrate to permit the deposition of the resistor film upon a selected area of the substrate. Electron beam source 12 then is energized to produce an evaporation of transiton metal source 14 at a suflicient rate to deposit approximately 50 to 400 A. per minute of the metal nitride upon substrate 20 with nitrogen pressure in the evaporation chamber preferably being maintained in a range between approximately 5x10 torr to torr during the evaporation by ionization gauge 46 and pressure control valve 44. To assure superior resistive characteristics in the deposited film, the substrate preferably is preheated to and maintained at a temperature in a range between 420 C. to 465 C. during the deposition of the transition metal. When substrates more refractory than soda lime glass are employed, the substrate may advantageously be heated to temperatures in excess of 465 C. Upon completion of the deposition of the transition metal nitride resistor film to the desired thickness, electron beam source 12 is deenergized and the deposited resistor film is baked at an elevated temperature, e.g., preferably above 370 C., for a period of approximately minutes in a nitrogen atmosphere greater than 0.5 torr. After the post-deposition baking of the resistor film is completed, the resistor film is allowed to cool to about 245 C. whereupon air is admitted to chamber 10 and an oxide coating is formed upon the resistor film. The resistor film then is temperature cycled between C. and 125 C. to produce a high specific resistivity low thermal coefficient of resistance transition metal nitride resistor film.

A more complete understanding ofthe reactive evaporation method of this invention and the unique properties of the transition metal nitride resistor films formed thereby can be better exemplified by the following specific examples of the reactive evaporation of various transition metals.

EXAMPLE 1 After a cleaned soda lime glass slide substrate was positioned within rectangular frame 68 and a source of pure niobium seated within 'cup 56 of water cooled crucible 50 at a distance 14 inches from the substrate, switch 82 was closed to energize heater 72 with electric power from source 80 and the substrate temperature was raised to approximately 420 C. The closed chamber then was evacuated to approximately 5X10 torr by evacuation pump 34, whereupon pressure control valve 44 was activated by ionization gauge 46 to admit and maintain nitrogen at a pressure of8 10" torr within the chamber. After energization of electron beam source 12 at a sufficient potential to evaporate the niobium source, niobium nitride was deposited through a 1 x 10 mm. mask for a -minute period at a rate of approximately 100 A. per minute upon the soda lime glass substrate whereupon energization of electron beam source 12 was terminated. Upon completion of the deposition of the niobium nitride resistor film, nitrogen was admitted to the chamber to raise the nitrogen pressure in the chamber greater than 0.5 torr. and the resistor film was baked at a temperature of 400 C. in the relatively high pressure nitrogen atmos phere for a period of 15 minutes. The baked niobium nitride film then was permitted to cool to approximately 245 C. whereupon air was admitted to the chamber to form an oxide layer atop the warm niobium nitride resistor film.

Upon complete cooling of the resistor thin film, the slide was removed from frame 68 and indium solder dots were placed along the surface of the resistor film to permit four probe resistance measurements which measurements disclosed the specific resistivity of the sample to be 580 micro-ohm-centimeters. The resistor film stabilized upon the first thermal cycle, e.g. from 25 to 125 C., and the temperature coefiicient of resistance of the deposited film measured zero (p.p.m./ C.) between 25 C. and 125 C. subsequent to the initial thermal cycling.

When niobium nitride resistor films were deposited under conditions identical to those heretofore described in Example 1 except for the pre-heating of the substrate, the niobium nitride resistor films deposited on the unheated substrate were found to have a high negative thermal coefiicient of resistance and a very high resistance which increased markedly upon thermal cycling. Thus, to effectively produce thin niobium nitride resistor films suitable for thin film circuitry according to the method of this invention, the substrate should be heated to a temperature at least in excess of 30 C. prior to the initiation of, and during, evaporation. When a blank shield is employed to protect the substrate from the initial depositions during evaporation, substrate preheating by an external source may not be necessary and preheating of the substrate to a temperature in excess of 30 C. may be accomplished by heat generated during the evaporation process.

The importance of the post-heat treatment of the deposited niobium nitride resistor films in relatively high pressure nitrogen, e.g. a pressure greater than 500 microns of mercury, immediately after deposition was shown by the fact that 5 niobium nitride resistor films post-heat treated in the relatively high pressure nitrogen atmosphere at temperatures in excess of 380 C. exhibited resistance values in execess of 265 ohms per square and Zero (p.p.m./ C.) temperature coefficients of resistance between 25 C. and 125 C. Two films deposited under the identical conditions but heat treated and subsequently cooled in high vacuum (approximately 2 10 torr) exhibited a low resistance and a decidedly positive temperature coefiicient of resistance.

The admission of air into evaporation chamber 10 while the niobium nitride resistor film was cooling, e.g. at 245 C., after the post-heat treatment in the relatively high pressure nitrogen atmosphere was found to produce stability in the films on the first thermal cycle between 25 C. and 125 C. while resistor films completely cooled in a nitrogen atmosphere generally required a plurality of temperature cycles before stability was obtained. The rapid stabilization of niobium nitride resistor films partially cooled in air appears probably to be due to a limited oxidation on the resistor film surface and perhaps in the grain boundaries of the film.

An increase in the resistance of the deposited niobium nitride resistor films with increasing nitrogen pressure in the evaporation chamber generally was observed and is depicted in FIG. 2. It will be noted from the kinetic theory of gases that for nitrogen pressure readings in excess of approximately 1.5 l0 torr, the mean free path of the vaporized niobium atoms is less than the source to substrate distance of 14 inches. This fact in conjunction with the graph of FIG. 2 indicates that a gas phase reaction is required for the formation of high ohmic value niobium nitride resistor films. Thus for optimum resistor films, the source to substrate distance is fixed relative to the nitrogen gas pressure in the chamber to produce at least one collision between niobium atoms in the gas phase and a nitrogen molecule prior to deposition of the niobium nitride resistor film on the substrate.

The effect of deposition rate upon the resistance properties of niobium nitride resistor films formed by reactive evaporation was examined by varying the deposition rates from approximately A. per minute to approximately 1200 A. per minute. The deposition time of each reactive evaporation also was varied to produce resistor films of substantially the same thickness from each deposition. Subsequent resistance measurements of the various resistor films indicated all films to have relatively identical resistances.

A graph, illustraetd in FIG. 3, of resistance against temperature coefficient of resistance values for reactively evaporated niobium nitride resistor films indicates thick resistor films (e.g. low resistance films) to have a positive temperaure coefficient of resistance with thin resistor films (e.g. high resistance films) having a negative temperature coeflicient of resistance. This thickness effect suggests a layered gradiation of the electrical properties in the film with the portion of the film proximate the substrate having a negative temperature coeificient of resistance and the upper layers of the film having a positive temperature coefficient of resistance. This layered effect of the resistor films also was indicated by anodization of a relatively thick resistor film to gradually convert the outer layers of the film to non-conducting oxides. During the anodization process, the temperature coefficient of resistance was observed to go from positive to negative and a film with a near zero temperature coeificient of resistance was obtained upon a balancing of the positive and the negative temperature coefficient of resistance portions of the films.

X-ray diffraction of the several niobium nitride resistor films indicated that hexagonal films of Nb N having a small crystallite size, e.g. less than 1000 A., are deposited during the reactive evaporation process. A preferred orientation of (1120) planes parallel to the substrate was observed for resistor films deposited at approximately 100 A. per minute while films deposited at a faster deposition rate exhibited little or no orientation.

EXAMPLE 2 After a clean soda lime glass slide substrate was placed within frame 68 and a tantalum source 14 positioned within cup 56 of water cooled crucible 50 approximately 14 inches from the source, envelope 22 was seated upon base 24 and heater 72 was energized through switch 82 to raise the temperature of the substrate to 465 C. The pressure of the evaporation chamber then was reduced by evacuation pump 34 to approximately 5 10 torr whereupon nitrogen from source 18 was admitted to the chamber to raise the nitrogen pressure in the chamber to approximately 6X10- torr and electron beam source 12 was energized to evaporate the tantalum source at a rate sufficient to deposit approximately 250 A. per minute of tantalum nitride upon the glass substrate. Evaporation of the tantalum source was continued until a tantalum nitride resistor film 3300 A. thick was deposited atop the substrate. After completion of the resistor film deposition, nitrogen pressure in the chamber was raised above 0.5 torr and heating of the resistor film was continued at 465 C. for a period of 15 minutes before the resistor was allowed to cool to room temperature and removed from the evaporation chamber. Four indium solder contacts then were made to the tantalum nitride resistor film to permit resistance measurements by the four probe technique and the resistor film was temperature cycled between C. and 125 C. for approximately four cycles. Subsequent resistance measurements of the film indicated a zero (p.p.m./ C.) temperature coefficient of resistance be tween 25 C. and 125 C. and a specific resistivity of approximately 420 micro ohm centimeters, e.g. approximately 67% higher than the specific resistivity of tantalum nitride resistor films formed by reactive sputtering. Tantalum nitride resistor films which were not post-heat treated in the nitrogen atmosphere of the chamber were found to exhibit a positive temperature coefficient of resistance and a smaller specific resistivity.

The effect of nitrogen gas pressure upon film electrical properties was found to be slightly different than that observed for niobium films. In a pressure range between 1X10 torr to 8 10 torr, the resistance values of the film for various pressures were found to be approximately identical. However, the temperature coefficient of resistance values of the tantalum nitride films (shown in FIG. 4) systematically become more positive with increasing nitrogen pressures. As will be observed from FIG. 5, the trend of tantalum nitride resistor films deposited by reactive evaporation is for low resistance value films, e.g. relatively thick films, to have a positive temperature coefiiicent of resistance while high resistance films, e.g. thin films, have a negative temperature coefficient of resistance.

X-ray diffraction analysis of a film 3300 A. thick formed by the reactive evaporation method heretofore described revealed only hexagonal tantalum nitride to be present within the resistor. This configuration differs from tantalum nitride resistor films formed by reactive sputtering under similar conditions wherein tantalum nitride of a cubic structure is favored by sputtering in nitrogen pressures of 1 10- mm. of mercury and higher. The approximate means particle size in the tantalum nitride resistor films formed by reactive evaporation was found to be approximately 130 A. and there was some preferred orientation of (0001) planes parallel to the substrate surface upon which the resistor film was deposited. No load resistance life tests of the tantalum nitride resistor films of this invention disclosed the lower resistance films, e.g. films having a resistance less than 250 ohms/ square, to be more thermally stable than the higher resistance, e.g. thinner films. Admission of air to the evaporation chamber during the cooling cycle when the tantalum nitride resistor reached approximately 2450 C. permitted the resistor to be stabilized in a single temperature cycle.

EXAMPLE 3 After a zirconium source and a cleaned soda lime substrate were placed in evaporation chamber 10, the chamber was evacuated to approximately 5 10 torr by evacuation pump 34 whereupon nitrogen from nitrogen bearing gas source 18 was admitted to the evaporation chamber to rise the nitrogen pressure of the chamber to approximately 8 x10 torr. The substrate then was heated by alternating current source to a temperature of approximately 465 C. and evaporation of the zirconium source was initiated by energizing electron beam source 12. The evaporation of the zirconium source was continued at a rate to produce a deposition of 200 to 300 A. per minute of zirconium nitride upon the substrate which substrate was positioned 14 inches from the source. After deposition of a film approximately 7000 A. thick, evaporation of the zirconium source was terminated and the deposited zirconium nitride resistor film was post-heated at 465 C. for a period of 15 minutes in a nitrogen atomsphere greater than 0.5 torr. Upon subsequent cooling of the resistor film, four indium solder dots were deposited atop the zirconinum nitride resistor film to permit four probe resistive measurements and the resistor film was thermal cycled between 25 C. and C. to stabilizer the film. Subsequent measurement of resistance values of the film disclosed a resistance of approximately 305 ohms per square. The temperature coefiicient of resistance of the film after themal cycling measured zero (p.p.m./ C.) between 25 C. and 125 C. X-ray diffraction analysis of an approximately 7000 A. thick resistor film showed only ZrN lines to be present with a preferred 0rientation of (111) planes parallel to the substate surface upon which the resistor film was deposited.

As can be observed from the graph of FIG. 6, a zero (p.p.m./ C.) temperature coefficient of resistance between 25 C. and 125 C. was found in reactively evaporated zirconium nitride resistor films over a wide range (from 400 ohms/ sq. to 1400 ohms/ sq.) of resistance values.

9 EXAMPLE 4 After positioning a clean soda lime glass slide substrate within frame 68, a hafnium source was positioned in the cup 56 of crucible 50 and envelope 22 of evaporation chamber 10 was positioned on base 24. The substrate then was heated to a temperature of approximately 465 C. by heater 72 and the evaporation chamber was evacuated to a pressure of approximately 5X10 torr by pump 34. Upon exhausting of the air from the evaporation chamber, nitrogen from nitrogen bearing gas source 18 was admitted into the chamber and the nitrogen pressure in the chamber was raised to 8 X 10 Electron beam source '12 then was energized and a resistor film of hafnium nitride was deposited at a rate of 200 to 400 A. per minute upon the heated substrate which substrate was positioned 14 inches from the hafnium source. After deposition of a 4000 A. thick hafnium nitride film, evaporation of the hafnium source was terminated andthe film was heated at 465 C. for 15 minutes in a nitrogen atmosphere in excess of 0.5 torr. The deposited resistor film then was cooled to room temperature and temperature cycle between C. and 125 C. to stabilize the resistor film. Subsequent vmeasurements disclosed the hafnium nitride resistor to have a resistance of approximately 472 ohms per square and a'temperature coefficient of resistance of -60 p.p.m./ C. between 25 C. and 125 C. X-ray diffraction analysis of a 12,000 A. thick hafnium nitride resistor film disclosed small crystallite HfN present with a strong preferred orientation of (111) planes parallel to the substrate surface upon which surface the resistor films were deposited. As will be noted from the graph, depicted in FIG. 7, of resistance vs. temperature coeflicient of resistance for reactively evaporated hafnium nitride resistor films, no films, regardless of film thickness, were deposited having either a positive coefiicient of resistance or a resistance value less than approximately 350 ohms per square. It is supposed that the latter phenomenon results from discontinuities in the surface layer as the film becomes thicker.

EXAMPLE 5 Titanium nitride resistor films were deposited from a titanium source employing the procedure described in the previous examples. The substrate was heated to a temperature of 455 C. prior to deposition and the titanium nitride was deposited at a rate of 50 A. per minute in a nitrogen atmosphere of 8 10 torr. After deposition, the films were heated for 15 minutes at the deposition temperature in a nitrogen atomsphere greater than 0.5 torr. As can be seen from the graph of FIG. 8, the temperature coefficient of resistance values of titanium nitride resistor films deposited by reactive evaporation change rather rapidly from positive to negative as film thickness is decreased. A resistor film having a zero (p.p.m./ C.) temperature coefiicient of resistance between 25 C. and 125 C. exhibited a resistance of approximately 270 ohms per square upon measurement utilizing the four probe technique. X-ray analysis of the film indicated that random TiN of very small crystalline size, e.g. less than 1000 A., was deposited.

EXAMPLE 6 Vanadium nitride films were deposited utilizing the procedure of the previous examples. The nitrogen pressure in the chamber was maintained at 8 10 torr, the substrate was heated to 370 C. and the deposition rate of vanadium nitride upon the substrate was measured to be approximately 70 A. per minute. Subsequent to the deposition, the resistor film was baked in nitrogen for 15 minutes at a temperature in excess of 370 C. As can be seen from the graph, e.g. FIG. 8, depicting the resistance vs. temperature coefiicient of resistance for reactively evaporated vanadium nitride resistor films, the temperature coefficient of resistance of the films becomes negative very rapidly as film thickness decreases. X-ray dilfraction analysis of the deposited film indicated VN with no preferred orientation of the crystals.

One preferred utilization for the niobium nitride and tantalum nitride resistor films formed by the reactive evaporation method of this invention is in the fabrication of variable resistors of the slide potentiometer type wherein a wiper arm is traversed a desired distance along the length of an elongated resistive element. In examining the suitability of the tantalum nitride and niobium nitride resistor films for utilization in thin film potentiometers, a niobium nitride film was deposited in a split annular configuration atop a glass substrate and a spring-loaded commercial sliding potentiometer contact of carbonaceous material was revolved atop the niobium nitride film for 10,- 000 revolutions. Resistance measurements of the niobium nitride resistor film both before and after rotation of the sliding contact indicated the resistance of the film to be 2800 ohms. A tantalum nitride film then was deposited in a split annular configuration upon a glass substrate utilizing the reactive evaporation method of this invention and the spring-loaded carbonaceous contact was revolved atop the deposited tantalum nitride resistor film. No change was observed in an 8,800 ohm tantalum nitride resistor film after 10,000 revolutions.

It was interesting to note that while tantalum nitride and niobium nitride films showed superior wear resistance to the carbonaceous contact of the potentiometer, a zirconium nitride film deposited in a split annular configuration upon a glass substrate by reactive evaporation showed a variation in resistance from 2400 ohms to 4100 ohms after 10,206 revolutions. A second zirconium nitride re sistor film similarly deposited upon a glass substrate by the reactive evaporation method of this invention also exhibited a variation in resistance from 1500 ohms to 1700 ohms after the commercial sliding contact had revolved 10,000 times over the resistor film.

When a commercially utilized film consisting of nickel and 20% chrome was deposited upon the glass substrate and the spring-loaded sliding contact was revolved over the deposited film for 10,000 revolutions, a resistance variation in the deposited film from a value of 5500 ohms prior to the initiation of the revolutions of the sliding contact to an ohmic value of 8700 ohms after 10,000 revolutions was observed.

While several examples of this invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from this invention in its broader aspects; and therefore the appended claims are intended to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of forming thin film resistors comprising positioning a substrate and a transition metal selected from the group consisting of niobium, tantalum, vanadium, titanium, zirconium and hafnium within an enclosed chamber, heating said substrate to a temperature in excess of 30 C., evacuating said chamber and introducing nitrogen into said chamber to produce a nitrogen pressure between 5X10 and 10* torr, converting at least a portion of saidtransition metal to the gas phase by evapora tion of at least a portion of said transition metal and depositing a thin film transition metal nitride resistor atop said substrate at a rate between 50 and 400 A./minute.

2. A method of forming thin film resistors according to claim 1 further including baking said nitride resistor at elevated temperatures above 30 C. in a nitrogen atmosphere greater than 0.5 torr.

3. A method of forming thin film resistors comprising positioning a substrate and a transition metal selected from the group consisting of niobium, tantalum, and vanadium within an enclosed chamber, heating the substrate to a temperature in excess of 420 C., evacuating said chamber and introducing nitrogen into said chamber to produce a pressure between 5X10 and 10- torr, evaporating at least a portion of the chosen transition metal within said chamber, depositing a thin film transition metal nitride resistor atop said substrate, increasing the nitrogen atmosphere within said chamber to a pressure greater than 0.5 torr, and baking said nitride resistor within said nitrogen atmosphere.

4. A method of forming thin film resistors according to claim 3 wherein said transition metal is niobium.

5. A method of forming thin film resistors according to claim 1 wherein said transition metal is niobium.

6. A method of forming thin film resistors comprising positioning a substrate and a transition metal selected from the group consisting of niobium, tantalum, vanadium, titanium, zirconium and hafnium within an enclosed chamber, evacuating said chamber and introducing a gas selected from the group consisting of nitrogen and ammonia into said chamber to produce a low pressure nitrogen atmosphere in a range between approximately 5 10 to 10" torr, heating said substrate to a temperature in excess of 30 C., evaporating said transition metal in said nitrogen atmosphere, and condensing a thin film nitride resistor atop said substrate.

7. A thin film resistor formed by the method claim 1.

8. A method of forming thin film resistors according to claim 1 wherein said atmosphere is nitrogen and further including baking said nitride resistor at elevated temperatures above 30 C. in a nitrogen atmosphere above at least 0.5 torr.

UNITED STATES PATENTS 3,159,556 12/1964 McLean et a1. 204-56 X 3,181,209 5/1965 Smith 117107.1 X 3,242,006 3/1966 Gerstenberg 117--106 X 3,315,208 4/1967 Gerstenberg 117-106 X 3,365,692 1/1968 Sartain 338308 X OTHER REFERENCES Holland, Vacuum Deposition of Thin Films, 1956, pp. 109 to 114 relied upon.

Holland, Thin Film Microelectronics, 1965, pp. 178 to 182 relied upon.

Hass, Physics of Thin Films, vol. 1963, pp. 1, 55 and 207 to 211 relied upon.

ALFRED L. LEAVI'IT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R.