US3203830A

US3203830A - Electrical resistor

Info

Publication number: US3203830A
Application number: US154578A
Authority: US
Inventors: William J Ostrander; Carl H Schenskie
Original assignee: International Resistance Co
Current assignee: International Resistance Co
Priority date: 1961-11-24
Filing date: 1961-11-24
Publication date: 1965-08-31
Anticipated expiration: 1982-08-31

Description

Aug. 31, 1965 w. J. OSTRANDER ETAL 3,203,330

ELECTRICAL RESISTOR Filed Nov. 24, 1961 2 Sheets-Sheet 1 FIG! U 4 I I l I E z E 5 o. I I l T 3 E 5 IO 5 E E 5 5 2 I E t soo- 2 m 0 E o E 5 U S 1&1 'IOOO- E I U E l- D g la! "ISOO- 3 z I Lu I ll 1 1 4O 5O 5O 7O 3O mo 30 4O 5O 6O 7O 8O 90 I00 VOLUME PERCENT CHRQMUM VOLUME PERCENT CHROMIUM INVENTORS WILLIAM J. OSTRANDER CARL H. SCHENSKIE ATTORNEY Aug. 31, 1965 Filed Nov. 24, 1961 TEMPERATURE COEFFICIENT OF FILM THICKNESS m ANGSTROM UNITS RESISTANCE/RESISTIVITY OHM CM 'c" 30 4O 5O 6O 7O 8O 9O VOLUME PERCENT CHROMIUM W. J. OSTRANDER ETAL ELECTRICAL RES ISTOR 2 Sheets-Sheet 2 IIIIIII IIIIHI I l IIIIIII I I IIHIII I llllllli BULK HRTOMIUM I I I I lllllll FIG.6

I I llllll l I IIIIIII l I lllllll I llllllll I l I I I l SIO SOURCE CHROMIUM-SILICON OXIDE MIXTURE FIG.7

C HROMI UM Cr SOURCE I I l l I 456789IOIII2 SAMPL E NUMBER INVENTORS WILLIAM J. 05 TRANDER CARL H. SCHENSKIE ATTORNEY United States Patent 3,203,830 ELECTRICAL RESISTOR William J. Ostrander, Harrington, N.J., and Carl H. Schenskie, Philadelphia, Pa., assignors to International Resistance Company, Philadelphia, Pa. 1 Filed Nov. 24, 1961, Ser. No. 154,578

4 Claims. (Cl. 117-227) The present invention relates to an electrical resistor and the method of making the same. More particularly, the present invention relates to an electrical resistor having a relatively high resistivity and a low temperature coeficient of resistance, and the method of making the same.

With the development of more complex electronic equipment, it has been found necessary to use in such equipment electrical components, such as resistors, which are highly stable. By a highly stable resistor, it is meant a resistor Whose resistance remains constant or changes only slightly when the resistor is in use. It is well known that when a resistor is used in an electrical circuit, the resistor heats up because of the electrical current passing therethrough. Also, such resistors are subjected to changes in temperature because of the environment in which the electronic equipment is used. It is also Well known that most resistance materials change in resistance value when subjected to changes in temperature. Such a change in resistance value is known as the temperature coeificient of resistance of the material. Therefore, the temperature coefficient of resistance of a resistor is an important factor which atiects the stability of the resistor.

As stated in United States Letters Patent No. 2,847,325 to J. Riseman et 211., issued August 12, 1958 entitled Apparatus and Method for Evaporating Films in Certain Types of Electrical Components, resistors having a substantially zero temperature coefiicient of resistance can be formed with very thin films of certain metals. Although such resistors are very stable with regard to temperature, the thin metal films have only a low resistivity. Therefore, it is desirable to have a resistor which is stable with regard to temperature, but which has a relatively high resistivity of the resistance material.

It is an object of the present invention to provide a novel resistor.

It is another object of the present invention to provide a novel resistor which is stable with regard to temperature.

It is still another object of the present invention to provide a resistor which has a low temperature coeflicient of resistance, and a relatively high resistivity of the resistance material.

It is a further object of the present invention to provide a novel method for making a resistor.

It is a still further object of the present invention to provide a method for making a resistor having a low temperature coefiicient of resistance and a relatively high resistivity of the resistance material.

Other objects will appear hereinafter.

FIGURE 1 is a sectional view of the resistor of the present invention.

FIGURE 2 is a schematic view of an apparatus for making the resistor of the present invention.

FIGURE 3 is a schematic view illustrating the manner of making the resistor of the present invention.

FIGURE 4 is a graphical illustration of the variation in conductivity of the resistance material of the present invention with variations in the ratio of the components of said material.

FIGURE 5 is a graphical illustration of the variation in the temperature coefficient of resisistance of the resist- 3,203,830 Patented Aug. 31, 1965 ance material of the present invention with variations in the ratio of the components of said material.

FIGURE 6 is a graphical illustration of the variation in the ratio of temperature coefficient of resistance to resistivity of the resistance material of the present invention with variations in the ratio of the components of said material.

FIGURE 7 is a graphical illustration comparing the thickness of the resistance material film of the present invention to the thickness of a metal film.

For the purpose of illustrating the invention there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

Referring initially to FIGURE 1, the resistor of the present invention is generally designated as 10. Resistor 10 comprises a base substrate 12 of an electrical insulating material, such as a ceramic or glass, and a layer 14 of the resistance material of the present invention coated on the surface of the base substrate 12. Caps 16 of an electrically conductive metal are mounted on the ends of the base substrate 12, and are electrically connected to the resistance material layer 14. Terminal wires 18 of an electrically conductive metal are secured to the caps 16, and project therefrom. Although the base substrate 12 is shown as being circular in transverse cross-section, the base substrate may also be ilat with the resistance material layer 14 being coated on one or both surfaces thereof. Also, the resistor 10 may be provided with any type of terminals well known in the art in place of the caps 16 and terminal wires 18.

The resistance material layer 14 comprises a homogenous mixture of a metal selected from the group of chromium, molybdenum, tungsten, nickel and alloys of such metals and silicon oxide. As used herein, the term silicon oxide is meant to refer to the oxide or oxides of silicon in the resistance material layers of the present invention whether it be a mixture of the oxides of silicon or only one of the oxides. As will be explained in detail later, the resistance material layer 14 is formed on the base substrate 12 by simultaneously evaporating the metal and silicon monoxide in a vacuum, and depositing the mixture on the surface of the base substrate. It has been found that the conductivity, which is the inverse of the resistivity, and the temperature coetficient of resistance of the resistance material layer 14 of the present invention varies with a variation in the ratio of the metal to the silicon oxide in the resistance material layer. As can be seen in FIGURE 4 in which the metal is chromium, as the volume percentage of the chromium in the resistance material layer 14 increases, the conductivity of the resistance material increases very rapidly until the resistance material layer 14 contains approximately 50% chromium and 50% silicon oxide. Increasing the volume percentage of chromium in the resistance material layer further still increases the conductivity, but at a much slower rate.

Referring toFIGURE 5, it can be seen that upon increasing the volume percentage of chromium in the resistance material layer 14, the temperature coeflicient of resistance of the resistance material changes rapidly from a high negative temperature coeliicient of resistance to a zero temperature coefiicient of resistance at approximately 43% chromium. Upon increasing the volume percentage of chromium in the resistance material layer further, the temperature coeiiicient of resistance of the material increases slightly in a positive direction, and then substantially levels olf.

By comparing FIGURE 4 with FIGURE 5, it can be seen that in the range of approximately 42 to 50% chromium the resistance material of the present invention has a relatively low temperature coefficient of resistance, and a conductivity which is considerably less than the conductivity of bulk chromium so that the resistivity of the material is relatively high as compared to the resistivity of the bulk metal. Thus, with a resistance layer 14 of the present invention which contains between 42 and 50% chromium by volume, the resistor of the present invention will have a relatively high resistance and a relatively low temperature coefficient of resistance. Furthermore, as can be seen from FIGURE 4, in the range of 42 to 50% chromium in the resistance material layer 14 of the present invention, the resistivity of the resistance material layer varies greatly, approximately tenfold, so that resistors of the present invention can be formed over a large range of resistivities and still have a relatively low temperature coefiicient of resistance. Thus, the resistance layer 14 of the present invention, which is a homogeneous mixture of chromium and silicon oxide, permits the manufacture of resistors over a large range of relatively high resistance values with the temperature coefiicient of resistance of the resistors being relatively low so that the resistors are relatively stable with regard to temperature.

Although the resistance material layer 14 of the present invention which contains between 42 and 50% by volume of chromium provides a resistor having preferred characteristics, it should be understood that the resistance layer of the present invention containing more than 50% or less than 42% by volume of chromium provides resistors having desired characteristics. In the range of less than 42% by volume of chromium, there are provided resistors having very high resistivities and high temperature coefficients of resistance. Such resistors have application where there is required a resistor of high resistance and where the temperature coefficient of resistance is not an important factor, or where there is required a resistor having a very large temperature coefiicient of resistance, such as a thermistor. In the range of more than 50% by volume of chromium, there are provided resistors having temperature coeflicient of resistance which is substantially uniform and relatively low over this range and having a resistivity which is higher than that of bulk chromium.

Another novel feature of the resistance material layer of the present invention is shown in FIGURE 6 in which the ratio of the temperature coefiicient of resistance to the conductivity is plotted against the composition of the resistance material layer. As can be seen, this ratio dedecreases with an increase in the amount of chromium in the resistance material layer. As can be seen, this ratio decreases with an increase in the amount of chromium in the resistance material layer until the resistance material layer of the present invention contains approximately 60% chromium. The resistance material layer of the present invention containing more than 60% chromium has a substantially constant ratio of temperature coefficient of resistance to conductivity. The novel feature is that this ratio for the resistance material layers of the present invention containing more than 60% chromium is approximately the same as the ratio of the temperature coefficient of resistance to conductivity for bulk chromium. Thus, the resistance material layers of the present invention having more than 60% chromium have the same characteristics as bulk chromium with regard to the ratio of the temperature coeflicient of resistance to conductivity. However, as can be seen in FIGURES 4 and 5, such resistance material layers of the present invention have a relatively low temperature coefiicient of resistance and a resistivity higher than that of bulk chromium.

Still another feature of the resistance material layer of the present invention can be seen in FIGURE 7 in which the thickness of the resistance material layer of the present invention is compared with the thickness of chromium. As can be seen, the resistance material layer of the present invention is thicker than a similarly formed layer of chromium. It is well known that the thicker a resistance material layer is the more stable the layer is electrically with regard not only to temperature but to other conditions, such as oxidation, humidity, etc. Thus, the resistance material layers of the present invention are more stable than pure metal resistance layers of the same resistance.

Although the discussion of the characteristics of the resistance material layer of the present invention has been with regard to such layers in which the metal is chromium the resistance material layers of the present invention in which the metal is either molybdenum, tungsten, nickel or alloys of such metals may have similar characteristics.

As was previously stated, the resistance material layer 14 of the present invention is coated on the base substrate 12 by simultaneously evaporating the metal and silicon monoxide from separate sources in an evacuated chamber, and condensing the vapors of the metal and the silicon oxide on the base substrate 12. Although silicon monoxide is evaporated, it is believed that some or all of the silicon monoxide vapors may be oxidized, depending on the particular pressure or the type of atmosphere in the chamber. Thus, it may be possible to control the type of silicon oxide achieved in the resistance material layer by either controlling the pressure in the coating chamber or by leaking controlled quantities of certain gases, such as air or oxygen, into the chamber during the coating operation.

Thus, the resistance material layer 14 of the present invention so formed may be either a mixture of the oxides of silicon or merely a single oxide of silicon. Referring to FIGURE 2, there is schematically shown an apparatus for carrying out this method. The apparatus comprises a base plate 24 and a dome-shaped cover 22, such as a bell jar, mounted on and hermetically sealed to the base plate 20 to provide a chamber therein. An exhaust pump 24 is connected to the chamber within the cover 22 by a pipe 26 extending through the base plate Ztl. A pair of evaporating

sources

28 and 30 are mounted in spaced relation in the chamber within the cover 22. The evaporation sources 28 and 30 are of a material capable of being heated to the evaporating temperatures of the metal and the silicon monoxide respectively, but which has an evaporating temperature higher than that of the metal and silicon monoxide. For example, the evaporating source 28 for chromium may be a strip of tantalum having substantially pure chromium coated thereon, and the evaporating source 3'3 for the silicon monoxide may be a boatshaped piece of tantalum adapted to contain the silicon monoxide. The evaporating

sources

28 and 30 are electrically connected across sources of electrical current, such as the

batteries

32 and 34. Although the evaporating

sources

28 and 30 are shown as being connected to separate sources of electrical current, they may be connected to the same source of electrical current with separate rheostats being provided to control the temperature to which each of the evaporating sources is heated. The base substrate 12 is suitably supported by any means well known in the art in the chamber within the cover 22, and is positioned over and between the evaporating

sources

28 and 30.

To coat the surface of the base substrate 12 with the layer 14 of the resistance material of the present invention using chromium as the metal, the chamber within the cover 22 is evacuated to a pressure of less than 5X 1() millimeters of mercury. The current to the evaporating

sources

28 and 30 is then simultaneously turned on to simultaneously heat the evaporating sources to a temperature sufiicient to evaporate the chromium and the silicon monoxide, which is approximately 1500 C. for chromium and approximately 1100 C. for silicon monoxide. As shown in FIGURE 3, when the evaporating sources 28 and 39 are heated to the evaporating temperatures of the chromium and the silicon monoxide, the vapors of chromium and silicon monoxide diituse in all directions from the evaporating sources, as indicated by the lines radiating from the evaporating sources. In the area between the evaporating

sources

28 and 30, the vapors of the chromium and the silicon monoxide co-mingle. Thus, by placing the base substrate 12 in this area, the mixed vapors of the chromium and the silicon oxide condense on the cooler base substrate so as to provide on the surface of the base substrate the layer 14 of the resistance material of the present invention which is a homogeneous mixture of chromium and silicon oxide.

As is well known, the density of the vapors from an evaporating source decreases with the distance from the source. Thus, from a point directly over the chromium evaporating source 28 to a point directly over the silicon monoxide evaporating source 30, the ratio of the amount of chromium vapors to the amount of silicon monoxide vapors in the mixture of the vapors varies with the percentage of the chromium vapors in the mixture being greater at the point directly over the chromium evaporating source than at the point directly over the silicon monoxide evaporating source. Therefore, by properly positioning the base substrate 12 with regard to the evaporating

sources

28 and 30, a resistance material layer 14 of the present invention having any desired ratio of chromium to silicon oxide can be obtained. Thus, knowing from the graphs of FIGURES 4 and 5 the percentages of chromium and silicon oxide required in the resistance material layer 14 to obtain a resistor having a desired resistivity and temperature coefiicient of resistance, the base substrate 12 can be properly positioned with regard to the evaporating

sources

28 and 30. The proper position of the base substrate 12 with regard to the evaporating

sources

28 and 30 to obtain a resistance material layer 14 having a desired ratio of chromium to silicon oxide can be obtained either experimentally or theoretically by the following formula:

where F =volume fraction of chromium W =rate of evaporation of silicon monoxide W =rate of evaporation of chromium P =density of silicon monoxide P =density of chromium T ==thickness of film due to silicon monoxide T =thickness of film due to chromium d=distance between sources h distance between plane of substrate and plane of evaporating sources y=the distance measured along the substrate plane from a point on the substrate to the point directly over one of the evaporating sources Knowing the resistivity of the resistance material layer 14, the resistance value per unit area of the base substrate 12 will be determined by the thickness of the resistance material layer. Thus, by controlling the time that the base substrate 12 is exposed to the chromium and the silicon monoxide vapors a resistor 10 of substantially any desired resistance value can be obtained. One well known method of obtaining a resistor of a desired resistance value is to continuously measure the resistance across the base substrate 12 during the coating operation, and stopping the evaporation when the desired resistance value is reached.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

We claim:

1. An electrical resistor having a relatively high resistance and low temperature coefiicient of resistance comprising a base substrate of an electrical insulating material, and a layer of a resistance material on the surface of said base substrate, said resistance material layer consisting essentially of a homogeneous mixture of chromium and silicon oxide with at least 42% by volume of the mixture being chromium.

2. An electrical resistor having a relatively high resistance and low temperature coeflicient of resistance comprising a base substrate of an electrical insulating material, and a layer of a resistance material on the surface of said base substrate, said resistance material layer consisting essentially of a homogeneous mixture of chromium and silicon oxide with at least by volume of the mixture being chromium.

3. An electrical resistor having a relatively high resistance and low temperature coeflicient of resistance comprising a base substrate of an electrical insulating material, and a layer of a resistance material on the surface of said base substrate, said resistance material layer consisting essentially of between approximately 442 to 50% by volume of chromium and 58 to 50% by volume of silicon oxide.

4. An electrical resistor having a relatively high resistance and low temperature coeflicient of resistance comprising a base substrate of an electrical insulating material, and a layer of a resistance material on the surface of said base substrate, said resistance material layer consisting essentially of approximately 43% by volume of chromium and 57% by volume of silicon oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,808,351 10/57 Colbert et al. 117-407 X 2,852,416 9/58 McNary et a1. 1l7-227 X FOREIGN PATENTS 834,490 3/52 Germany. 882,174 7/53 Germany.

RICHARD D. NEVIUS, Primary Examiner.

Claims

1. AN ELECTRICAL RESISTOR HAVING A RELATIVELY HIGH RESISTANCE AND LOW TEMPERATURE COEFFICIENT TO RESISTANCE COMPRISING A BASE SUBSTRATE OF AN ELECTRICAL INSULATING MATERIAL, AND A LAYER OF A RESISTANCE MATERIAL ON THE SURFACE OF SAID BASE SUBSTRATE, SAID RESISTANCE MATERIAL LAYER CONSISTING ESSENTIALLY OF A HOMOGENEOUS MIXTURE OF CHROMIUM AND SILICON OXIDE WITH AT LEAST 42% BY VOLUME OF THE MIXTURE BEING CHROMIUM.