Title:
METHOD OF MAKING REFRACTORY METAL CARBIDE THIN FILM RESISTORS
United States Patent 3665599
Abstract:
The resistance value of a refractory metal carbide film resistor is adjusted and stabilized at any desired resistance value up to approximately 100 times the resistance value of the originally deposited film by heat treating the metal carbide film in an oxidizing atmosphere at temperatures above the anticipated operating range of the resistor. The heat treatment increases the resistance of the applied film by converting a layer of the exposed film to a non-conductive oxide.
US Patent References:
Electrical resistor and method and apparatus for producing resistors
Pritikin - August 1958 - 2849583
Formation of firmly adherent coatings of refractory materials on metals
Kreuchen et al. - September 1961 - 3001893
Method of stabilizing the electrical resistance of a metal oxide film
Davis - October 1963 - 3108019
Method of making a multi-layer electrical circuit
Lemelson - February 1965 - 3169892
Method of depositing films of controlled specific resistivity and temperature coefficient of resistance using cathode sputtering
Mayer et al. - July 1968 - 3395089
Electrical resistor and method and apparatus for producing resistors
Pritikin - August 1958 - 2849583
Formation of firmly adherent coatings of refractory materials on metals
Kreuchen et al. - September 1961 - 3001893
Method of stabilizing the electrical resistance of a metal oxide film
Davis - October 1963 - 3108019
Method of making a multi-layer electrical circuit
Lemelson - February 1965 - 3169892
Method of depositing films of controlled specific resistivity and temperature coefficient of resistance using cathode sputtering
Mayer et al. - July 1968 - 3395089
Inventors:
Herczog, Andrew (Painted Post, NY)
Smith, Harold R. (Denver, CO)
Smith, Harold R. (Denver, CO)
Application Number:
05/032254
Publication Date:
05/30/1972
Filing Date:
04/27/1970
View Patent Images:
Export Citation:
Assignee:
Corning Glass Works (Corning, NY)
Primary Class:
Other Classes:
338/308, 427/102, 427/103
International Classes:
H01C7/22; H01C17/26; H01C17/22; H01C7/00; H01C17/00
Field of Search:
29/620,621 338/308,306,307,309 117/201,215,221,229,216
US Patent References:
Primary Examiner:
Campbell, John F.
Assistant Examiner:
Di Palma, Victor A.
Claims:
1. A method of producing thin film electrical resistors comprising the steps of
2. The method of claim 1 wherein the insulating substrate is selected from the group consisting of alkali-free glass and thermally oxidized silicon.
3. The method of claim 1 further comprising the step of applying electrically conductive terminals to the substrate prior to depositing the refractory metal carbide film.
4. The method of claim 3 wherein the electrically conductive terminals are an alloy made from aluminum, gold and chrome.
5. The method of claim 1 wherein the step of depositing said film of refractory metal carbide is by vacuum vapor deposition.
6. The method of claim 5 wherein said film of refractory metal carbide is titanium carbide.
7. The method of claim 6 further comprising the step of degassing the titanium carbide in a vacuum not less that 10-4 torr at approximately 1,000° C. prior to being deposited on said substrate.
8. The method of claim 1 wherein said converting and stabilizing steps are performed simultaneously.
9. The method of claim 8 wherein the converting and stabilizing steps are continued for at least 1 hour.
10. The method of claim 1 further comprising the step of masking portions of the film surface prior to any heat treatment to prevent refractory metal carbide below said masked surface from being converted to a refractory metal oxide.
11. The method of claim 10 further comprising the steps of patterning said mask to increase the length-to-width ratio of the masked portions of refractory metal carbide, and then converting the unmasked portions of refractory metal carbide, throughout its entire thickness, to refractory metal oxide.
12. The method of claim 1 further comprising the step of applying a layer of glass over said combination film.
13. A method of producing thin film resistors comprising the steps of
14. The method of claim 13 wherein said converting and stabilizing steps are performed simultaneously.
15. The method of claim 14 wherein the converting and stabilizing steps are continued for at least 1 hour.
16. The method of claim 13 further comprising the step of masking portions of the film surface prior to any heat treatment to prevent titanium carbide below said masked surface from being converted to titanium oxide.
17. The method of claim 16 further comprising the steps of patterning said mask to increase the length-to-width ratio of the masked portions of titanium carbide, and then completely converting the unmasked portions of titanium carbide throughout its entire thickness, to titanium oxide.
18. The method of claim 13 further comprising the step of applying a layer of glass over said combination film.
19. A method of producing thin film resistors comprising the steps of
20. The method of claim 19 further comprising the steps of
2. The method of claim 1 wherein the insulating substrate is selected from the group consisting of alkali-free glass and thermally oxidized silicon.
3. The method of claim 1 further comprising the step of applying electrically conductive terminals to the substrate prior to depositing the refractory metal carbide film.
4. The method of claim 3 wherein the electrically conductive terminals are an alloy made from aluminum, gold and chrome.
5. The method of claim 1 wherein the step of depositing said film of refractory metal carbide is by vacuum vapor deposition.
6. The method of claim 5 wherein said film of refractory metal carbide is titanium carbide.
7. The method of claim 6 further comprising the step of degassing the titanium carbide in a vacuum not less that 10-4 torr at approximately 1,000° C. prior to being deposited on said substrate.
8. The method of claim 1 wherein said converting and stabilizing steps are performed simultaneously.
9. The method of claim 8 wherein the converting and stabilizing steps are continued for at least 1 hour.
10. The method of claim 1 further comprising the step of masking portions of the film surface prior to any heat treatment to prevent refractory metal carbide below said masked surface from being converted to a refractory metal oxide.
11. The method of claim 10 further comprising the steps of patterning said mask to increase the length-to-width ratio of the masked portions of refractory metal carbide, and then converting the unmasked portions of refractory metal carbide, throughout its entire thickness, to refractory metal oxide.
12. The method of claim 1 further comprising the step of applying a layer of glass over said combination film.
13. A method of producing thin film resistors comprising the steps of
14. The method of claim 13 wherein said converting and stabilizing steps are performed simultaneously.
15. The method of claim 14 wherein the converting and stabilizing steps are continued for at least 1 hour.
16. The method of claim 13 further comprising the step of masking portions of the film surface prior to any heat treatment to prevent titanium carbide below said masked surface from being converted to titanium oxide.
17. The method of claim 16 further comprising the steps of patterning said mask to increase the length-to-width ratio of the masked portions of titanium carbide, and then completely converting the unmasked portions of titanium carbide throughout its entire thickness, to titanium oxide.
18. The method of claim 13 further comprising the step of applying a layer of glass over said combination film.
19. A method of producing thin film resistors comprising the steps of
20. The method of claim 19 further comprising the steps of
Description:
BACKGROUND OF THE INVENTION
This invention relates to the production of thin film electrical resistors and more particularly to the production and trimming of highly stable resistors with low temperature coefficients of resistance such as those used in microminiaturized circuits.
As used herein and as is well known in the art, the resistance, R, of a thin-film resistor is determined by the product of its sheet resistivity, p s times the number of squares; that is R = p s × number of squares. The number of squares, sometimes referred to as the aspect ratio, depends upon the resistor geometry, and is dimensionless; that is resistivity in ohms per square is independent of the size of the square. For example, the resistivity value of a given material is the same in ohms per square centimeter, square inch, or square mile. Therefore, with a rectangular film the number of squares is equivalent to the film length L, divided by the film width, W; the resistance then, is, R = p s × L/W. The sheet resistivity, on the other hand, is determined by the film resistivity p, divided by the film thickness t; that is, p s = p/t. Finally, the film resistivity depends upon the composition of the material which comprises the film itself.
Thin film resistors should have a low TCR (temperature coefficient of resistance), which is generally expressed in parts per million per degree centigrade. This characteristic is important since small changes in temperature will create relatively large changes in resistance if the TCR is high. Drift or instability, another important property of thin film resistors, may be defined as an irreversible change of resistance. This effect is noticeable in some types of thin film resistors at temperatures as low as 100° C. and results in a severe use limitation on such resistors. Such resistors should also possess chemical durability so that the materials which come in contact with a finished article do not react, combine or otherwise chemically deteriorate the film during long periods of exposure thereto. Many prior art thin film resistive materials are adversely affected by one or more of the heretofore mentioned properties.
Due to the manner in which thin film resistors are fabricated, it is impossible to cause them to have a desired resistance value solely by controlling the techniques by which the film is deposited. This is particularly true of those resistors used in microminiaturized circuits since such resistors consist of a layer of resistive material deposited on an insulating substrate. As indicated hereinabove, the resistance of such a thin film resistor depends upon the composition thereof as well as the physical dimensions thereof including length, width and thickness. Therefore, The resistance of the initially applied film should be adjustable to a predetermined final value, and it is preferable that the resistance should be adjustable to a wide range of permanent values. Trimming or adjusting the resistance value of thin film resistors heretofore was generally accomplished by such methods as cutting, abrading or burning to remove portions of the film or to create grooves through the film in preselected patterns to effectively increase the length-to-width ratio of the film.
It has been found that heretofore known electrical conductive compositions, although suitable for some applications, do not have all of the above-noted properties. Accordingly, it is an object of this invention to provide a thin film resistor which overcomes the heretofore noted disadvantages of prior art thin film electrical components. Another object is to provide an improved method of trimming thin film resistors.
SUMMARY OF THE INVENTION
Briefly, this invention relates to a method of producing thin film electrical resistors comprising the following steps. A 100 A. - 5,000 A. film of a refractory metal carbide is deposited on a surface of an insulating substrate. The substrate-refractory metal carbide film composite structure is heated in an oxidizing atmosphere to temperatures between 180° C. and 400° C. to adjust the sheet resistance of the deposited film by converting a layer of the refractory metal carbide to a non-conductive refractory metal oxide, and to form a combination film of conductive refractory metal carbide and non-conductive refractory metal oxide which has a higher sheet resistance than the deposited all refractory metal carbide film. The composite body is further heated in an oxidizing atmosphere to a temperature which is between 150° C. and 220° C. and which is also no greater than the temperature used for converting the layer of refractory metal carbide to refractory metal oxide to stabilize the sheet resistance of a combination film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an oblique illustration of a substrate and electrodes over which a thin film of titanium carbide has been deposited.
FIG. 2a is a sectional view taken at lines 2a-2a of FIG. 1.
FIG. 2b is a sectional view of the thin film resistor of FIG. 1 after subsequent heat treatment.
FIG. 3 is an oblique illustration of an alternate embodiment.
FIG. 4 is a sectional view of FIG. 3 after additional heat treatment.
FIG. 5 is a sectional view of a further resistor embodiment having a protective coating.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, a suitable supporting member or substrate 10 of any desired shape is formed from such materials as alkali-free glass or thermally oxidized silicon. If desired, thin metal terminals or electrodes 12 can be formed on the substrate in appropriate locations by any suitable manner well known to one skilled in the art such as vacuum deposition or sputtering. A thin layer of any conductive metal is suitable; however, an alloy of aluminum, chrome and gold is particularly suitable as terminals for this invention if the terminals are likely to be used as solder points. These thin film electrodes can serve as terminals for measuring resistance values while the resistors are being adjusted, or possibly for attaching lead wires to the finished resistor. A film 14 of a refractory metal carbide is then deposited to the desired thickness on a surface of the substrate 10 while the substrate is maintained at approximately 200° C. Any suitable process such as vacuum deposition, vacuum sputtering or the like may be used. For the purpose of this invention, the term "refractory metal" includes titanium, zirconium, vanadium, niobium, tantalum, hafnium, chromium, tungsten, molybdenum and combinations thereof. Since titanium is the preferred refractory metal for use in this invention, the following discussion will refer specifically to this metal, however, the other listed refractory metals are also suitable. Pure titanium carbide may be provided from commercial sources, or it may be prepared from titanium carbide powder sintered into suitable size pellets and degassed in a high vacuum of approximately 10 - 4 torr at approximately 1,000° C. Another method of preparing useable titanium carbide is to sinter titanium powder and carbon powder to approximately 1,500° C. in a vacuum of at least 10 - 4 torr. A particularly suitable method for depositing the film 14 is to heat with an electron beam degassed pellets of titanium carbide which are in proximity with the substrate in a vacuum of approximately 10 - 5 torr. The deposition time varies between 5-60 minutes depending upon the desired thickness. The deposition process is continued until a film thickness between 100 A. and 5,000 A. is obtained. Since the sheet resistance, p s of the titanium carbide will decrease as thickness increases, the thickness of the film deposition can be determined by monitoring the change in the resistance during deposition. The film thickness can of course be monitored or measured by other appropriate methods such as interferometry or in the case of films approximately 1,000 A. or less by optical absorption. Measuring film thickness up to 1,000 A. of titanium carbide is possible since such films are still about 5 percent transparent to visible light whereas a 1,000 A. metal film is opaque.
FIG. 2a is a sectional view of FIG. 1, wherein the film 14 is illustrated prior to heat treatment. As shown in FIG. 2b, the resistance value of the deposited film has been adjusted by converting a layer 16 of the titanium carbide film 14 to titanium oxide. Adjustment and stabilization of the sheet resistance may be accomplished in one oxidizing heat treatment operation (not less than one hour at 180° C.) or the resistor may be heat treated at any temperature between 180°-400° C. to more rapidly oxidize the titanium carbide and thereby increase the resistance to some value slightly less than the desired value. However, for purposes of efficiency and ease of control, temperatures between 200° C.-260° C. are preferred. Stabilization and final trimming or adjusting to the desired value is then accomplished by heat treatment in an oxidizing atmosphere, including air, by choosing a temperature that is below that used for rough trimming or adjusting and also a temperature that is between 150° C. and 220° C. for not less than 1 hour. Accurately controlling the deposition thickness of a film less than 100 A. is very difficult, and since only a thin layer of titanium carbide converted to titanium oxide may constitute a large percentage of the overall film thickness adjusting the resistance of such film is also difficult. Furthermore, as films approach this thickness, the TCR becomes unacceptably high, that is, greater than 800 ppm/°C. Films of thicknesses greater than 5,000 A. do not always satisfactorily adhere to the substrate and adjusting the resistance of such thick films is slower since a much larger amount of titanium carbide must be reduced to titanium oxide to obtain a noticeable change in the overall resistance.
Film or sheet resistance of the titanium carbide film can be readily increased up to 100 times the resistance of the originally applied titanium carbide film by converting the required thickness of titanium carbide to non-conductive titanium oxide. For example, if one-half the thickness of a film of titanium carbide is converted to titanium oxide the resistance value of the titanium carbide and titanium oxide combination film should be approximately twice the resistance value of the titanium carbide film. Therefore, a single selected film thickness can be used to produce resistors of widely varying values. For example, a titanium carbide film thickness of approximately 400 A. can be used to obtain resistors having resistance values around 600-900 ohms/square and TCR values around 200 parts per million or the same film thickness may be used to obtain resistors having a resistance value of approximately 50,000 ohms/square depending upon the amount of titanium carbide converted to titanium oxide. Although resistance values higher than 50,000 ohms/square can be achieved by the practice of this invention, adjusting sheet resistances to close tolerances above 50,000 ohms/square becomes very difficult, and the TCR values increase beyond 800 ppm/° C.
If the film is unnecessarily thicker than that required for the desired sheet resistivity the time required for adjusting and stabilizing may be excessive. Therefore, a film thickness should be deposited that results in a sheet resistance that approximates, but is still sufficiently less than the desired sheet resistance to allow for accurate adjusting and adequate stabilization. To prevent further oxidation and the accompanying increase in sheet resistance of resistors that may encounter high temperature operating conditions, the film of titanium oxide should be at least 50 A. in thickness or the resistive film should be covered with a layer of protective material such as glass.
Adjusting the resistance by altering the effective length-to-width ratios of the resistor by any of the suitable methods known to one skilled in the art can be used in conjunction with this invention. However, a novel method of adjusting length-to-width ratios may be accomplished by the practice of this invention by masking selected portions of film by any well known technique such as photofabrication of masks of photoresistive material. The masked film is then subjected to the previously disclosed oxidizing heat treatment. Masking the film will prevent oxidation of the titanium carbide under the masked portions. Referring to FIG. 3, unmasked portions 30 of the film can be substantially completely converted to non-conductive titanium oxide, resulting in a patterned resistive film 32 having a very large effective length-to-width ratio. After the unmasked portion 30 has been formed and masking removed, the resultant patterned film 32 may then be trimmed by converting a layer 32a into titanium oxide, as shown in FIG. 4, the remaining portion 32b of titanium carbide constituting a patterned trimmed resistor. Such patterning allows the use of films with relatively low sheet resistance values and their corresponding low TCR values.
If the oxide layer on top of the resistive film is too thin, that is, less than about 50 A., a protective layer is necessary to prevent further oxidation of the titanium carbide layer. As shown in FIG. 5 a layer 50 of glass is disposed on the thin titanium oxide layer 52. The resistive film 54 is thus protected against further oxidation which would increase the resistance of the finished resistor. The glass layer 50 may be applied to either the embodiment of FIG. 1 or that of FIG. 3.
Following is a specific example of a method of making a thin film resistor in accordance with the present invention. Thin film terminals of aluminum-gold-chrome alloy were deposited on an alkali-free glass substrate by a vacuum vapor deposition process. Titanium carbide pellets for use as the source material were prepared from titanium carbide powder by degassing and sintering the powder into pellets in a vacuum of at least 10 - 4 torr at approximately 1,000° C. A thin film of approximately 400 A. of titanium carbide was then deposited over one surface of the glass substrate and also over the predeposited aluminum-gold-chrome alloy terminals by vacuum vapor deposition. The deposition was accomplished in a vacuum of approximately 10 - 5 torr and evaporation of the source material was obtained by electron beam heating. A 400 A. film with a sheet resistance before heat treatment of approximately 700 ohms/square was deposited on the substrate. The substrate and film combination was then heated to approximately 200° C. until the sheet resistance increased to approximately 1,800 ohms/square. The temperature was then reduced to approximately 180° C. for at least 1 hour as the sheet resistance was slowly increased and stabilized at a desired value of 2,000 ohms/square. Resistors produced by this method possess a film stability of better than 1 percent of the adjusted film resistance.
This invention relates to the production of thin film electrical resistors and more particularly to the production and trimming of highly stable resistors with low temperature coefficients of resistance such as those used in microminiaturized circuits.
As used herein and as is well known in the art, the resistance, R, of a thin-film resistor is determined by the product of its sheet resistivity, p s times the number of squares; that is R = p s × number of squares. The number of squares, sometimes referred to as the aspect ratio, depends upon the resistor geometry, and is dimensionless; that is resistivity in ohms per square is independent of the size of the square. For example, the resistivity value of a given material is the same in ohms per square centimeter, square inch, or square mile. Therefore, with a rectangular film the number of squares is equivalent to the film length L, divided by the film width, W; the resistance then, is, R = p s × L/W. The sheet resistivity, on the other hand, is determined by the film resistivity p, divided by the film thickness t; that is, p s = p/t. Finally, the film resistivity depends upon the composition of the material which comprises the film itself.
Thin film resistors should have a low TCR (temperature coefficient of resistance), which is generally expressed in parts per million per degree centigrade. This characteristic is important since small changes in temperature will create relatively large changes in resistance if the TCR is high. Drift or instability, another important property of thin film resistors, may be defined as an irreversible change of resistance. This effect is noticeable in some types of thin film resistors at temperatures as low as 100° C. and results in a severe use limitation on such resistors. Such resistors should also possess chemical durability so that the materials which come in contact with a finished article do not react, combine or otherwise chemically deteriorate the film during long periods of exposure thereto. Many prior art thin film resistive materials are adversely affected by one or more of the heretofore mentioned properties.
Due to the manner in which thin film resistors are fabricated, it is impossible to cause them to have a desired resistance value solely by controlling the techniques by which the film is deposited. This is particularly true of those resistors used in microminiaturized circuits since such resistors consist of a layer of resistive material deposited on an insulating substrate. As indicated hereinabove, the resistance of such a thin film resistor depends upon the composition thereof as well as the physical dimensions thereof including length, width and thickness. Therefore, The resistance of the initially applied film should be adjustable to a predetermined final value, and it is preferable that the resistance should be adjustable to a wide range of permanent values. Trimming or adjusting the resistance value of thin film resistors heretofore was generally accomplished by such methods as cutting, abrading or burning to remove portions of the film or to create grooves through the film in preselected patterns to effectively increase the length-to-width ratio of the film.
It has been found that heretofore known electrical conductive compositions, although suitable for some applications, do not have all of the above-noted properties. Accordingly, it is an object of this invention to provide a thin film resistor which overcomes the heretofore noted disadvantages of prior art thin film electrical components. Another object is to provide an improved method of trimming thin film resistors.
SUMMARY OF THE INVENTION
Briefly, this invention relates to a method of producing thin film electrical resistors comprising the following steps. A 100 A. - 5,000 A. film of a refractory metal carbide is deposited on a surface of an insulating substrate. The substrate-refractory metal carbide film composite structure is heated in an oxidizing atmosphere to temperatures between 180° C. and 400° C. to adjust the sheet resistance of the deposited film by converting a layer of the refractory metal carbide to a non-conductive refractory metal oxide, and to form a combination film of conductive refractory metal carbide and non-conductive refractory metal oxide which has a higher sheet resistance than the deposited all refractory metal carbide film. The composite body is further heated in an oxidizing atmosphere to a temperature which is between 150° C. and 220° C. and which is also no greater than the temperature used for converting the layer of refractory metal carbide to refractory metal oxide to stabilize the sheet resistance of a combination film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an oblique illustration of a substrate and electrodes over which a thin film of titanium carbide has been deposited.
FIG. 2a is a sectional view taken at lines 2a-2a of FIG. 1.
FIG. 2b is a sectional view of the thin film resistor of FIG. 1 after subsequent heat treatment.
FIG. 3 is an oblique illustration of an alternate embodiment.
FIG. 4 is a sectional view of FIG. 3 after additional heat treatment.
FIG. 5 is a sectional view of a further resistor embodiment having a protective coating.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1, a suitable supporting member or substrate 10 of any desired shape is formed from such materials as alkali-free glass or thermally oxidized silicon. If desired, thin metal terminals or electrodes 12 can be formed on the substrate in appropriate locations by any suitable manner well known to one skilled in the art such as vacuum deposition or sputtering. A thin layer of any conductive metal is suitable; however, an alloy of aluminum, chrome and gold is particularly suitable as terminals for this invention if the terminals are likely to be used as solder points. These thin film electrodes can serve as terminals for measuring resistance values while the resistors are being adjusted, or possibly for attaching lead wires to the finished resistor. A film 14 of a refractory metal carbide is then deposited to the desired thickness on a surface of the substrate 10 while the substrate is maintained at approximately 200° C. Any suitable process such as vacuum deposition, vacuum sputtering or the like may be used. For the purpose of this invention, the term "refractory metal" includes titanium, zirconium, vanadium, niobium, tantalum, hafnium, chromium, tungsten, molybdenum and combinations thereof. Since titanium is the preferred refractory metal for use in this invention, the following discussion will refer specifically to this metal, however, the other listed refractory metals are also suitable. Pure titanium carbide may be provided from commercial sources, or it may be prepared from titanium carbide powder sintered into suitable size pellets and degassed in a high vacuum of approximately 10 - 4 torr at approximately 1,000° C. Another method of preparing useable titanium carbide is to sinter titanium powder and carbon powder to approximately 1,500° C. in a vacuum of at least 10 - 4 torr. A particularly suitable method for depositing the film 14 is to heat with an electron beam degassed pellets of titanium carbide which are in proximity with the substrate in a vacuum of approximately 10 - 5 torr. The deposition time varies between 5-60 minutes depending upon the desired thickness. The deposition process is continued until a film thickness between 100 A. and 5,000 A. is obtained. Since the sheet resistance, p s of the titanium carbide will decrease as thickness increases, the thickness of the film deposition can be determined by monitoring the change in the resistance during deposition. The film thickness can of course be monitored or measured by other appropriate methods such as interferometry or in the case of films approximately 1,000 A. or less by optical absorption. Measuring film thickness up to 1,000 A. of titanium carbide is possible since such films are still about 5 percent transparent to visible light whereas a 1,000 A. metal film is opaque.
FIG. 2a is a sectional view of FIG. 1, wherein the film 14 is illustrated prior to heat treatment. As shown in FIG. 2b, the resistance value of the deposited film has been adjusted by converting a layer 16 of the titanium carbide film 14 to titanium oxide. Adjustment and stabilization of the sheet resistance may be accomplished in one oxidizing heat treatment operation (not less than one hour at 180° C.) or the resistor may be heat treated at any temperature between 180°-400° C. to more rapidly oxidize the titanium carbide and thereby increase the resistance to some value slightly less than the desired value. However, for purposes of efficiency and ease of control, temperatures between 200° C.-260° C. are preferred. Stabilization and final trimming or adjusting to the desired value is then accomplished by heat treatment in an oxidizing atmosphere, including air, by choosing a temperature that is below that used for rough trimming or adjusting and also a temperature that is between 150° C. and 220° C. for not less than 1 hour. Accurately controlling the deposition thickness of a film less than 100 A. is very difficult, and since only a thin layer of titanium carbide converted to titanium oxide may constitute a large percentage of the overall film thickness adjusting the resistance of such film is also difficult. Furthermore, as films approach this thickness, the TCR becomes unacceptably high, that is, greater than 800 ppm/°C. Films of thicknesses greater than 5,000 A. do not always satisfactorily adhere to the substrate and adjusting the resistance of such thick films is slower since a much larger amount of titanium carbide must be reduced to titanium oxide to obtain a noticeable change in the overall resistance.
Film or sheet resistance of the titanium carbide film can be readily increased up to 100 times the resistance of the originally applied titanium carbide film by converting the required thickness of titanium carbide to non-conductive titanium oxide. For example, if one-half the thickness of a film of titanium carbide is converted to titanium oxide the resistance value of the titanium carbide and titanium oxide combination film should be approximately twice the resistance value of the titanium carbide film. Therefore, a single selected film thickness can be used to produce resistors of widely varying values. For example, a titanium carbide film thickness of approximately 400 A. can be used to obtain resistors having resistance values around 600-900 ohms/square and TCR values around 200 parts per million or the same film thickness may be used to obtain resistors having a resistance value of approximately 50,000 ohms/square depending upon the amount of titanium carbide converted to titanium oxide. Although resistance values higher than 50,000 ohms/square can be achieved by the practice of this invention, adjusting sheet resistances to close tolerances above 50,000 ohms/square becomes very difficult, and the TCR values increase beyond 800 ppm/° C.
If the film is unnecessarily thicker than that required for the desired sheet resistivity the time required for adjusting and stabilizing may be excessive. Therefore, a film thickness should be deposited that results in a sheet resistance that approximates, but is still sufficiently less than the desired sheet resistance to allow for accurate adjusting and adequate stabilization. To prevent further oxidation and the accompanying increase in sheet resistance of resistors that may encounter high temperature operating conditions, the film of titanium oxide should be at least 50 A. in thickness or the resistive film should be covered with a layer of protective material such as glass.
Adjusting the resistance by altering the effective length-to-width ratios of the resistor by any of the suitable methods known to one skilled in the art can be used in conjunction with this invention. However, a novel method of adjusting length-to-width ratios may be accomplished by the practice of this invention by masking selected portions of film by any well known technique such as photofabrication of masks of photoresistive material. The masked film is then subjected to the previously disclosed oxidizing heat treatment. Masking the film will prevent oxidation of the titanium carbide under the masked portions. Referring to FIG. 3, unmasked portions 30 of the film can be substantially completely converted to non-conductive titanium oxide, resulting in a patterned resistive film 32 having a very large effective length-to-width ratio. After the unmasked portion 30 has been formed and masking removed, the resultant patterned film 32 may then be trimmed by converting a layer 32a into titanium oxide, as shown in FIG. 4, the remaining portion 32b of titanium carbide constituting a patterned trimmed resistor. Such patterning allows the use of films with relatively low sheet resistance values and their corresponding low TCR values.
If the oxide layer on top of the resistive film is too thin, that is, less than about 50 A., a protective layer is necessary to prevent further oxidation of the titanium carbide layer. As shown in FIG. 5 a layer 50 of glass is disposed on the thin titanium oxide layer 52. The resistive film 54 is thus protected against further oxidation which would increase the resistance of the finished resistor. The glass layer 50 may be applied to either the embodiment of FIG. 1 or that of FIG. 3.
Following is a specific example of a method of making a thin film resistor in accordance with the present invention. Thin film terminals of aluminum-gold-chrome alloy were deposited on an alkali-free glass substrate by a vacuum vapor deposition process. Titanium carbide pellets for use as the source material were prepared from titanium carbide powder by degassing and sintering the powder into pellets in a vacuum of at least 10 - 4 torr at approximately 1,000° C. A thin film of approximately 400 A. of titanium carbide was then deposited over one surface of the glass substrate and also over the predeposited aluminum-gold-chrome alloy terminals by vacuum vapor deposition. The deposition was accomplished in a vacuum of approximately 10 - 5 torr and evaporation of the source material was obtained by electron beam heating. A 400 A. film with a sheet resistance before heat treatment of approximately 700 ohms/square was deposited on the substrate. The substrate and film combination was then heated to approximately 200° C. until the sheet resistance increased to approximately 1,800 ohms/square. The temperature was then reduced to approximately 180° C. for at least 1 hour as the sheet resistance was slowly increased and stabilized at a desired value of 2,000 ohms/square. Resistors produced by this method possess a film stability of better than 1 percent of the adjusted film resistance.