US3015587A - Rhodium germanium film resistor - Google Patents

Description

Jan. 2, 1962 w. J. Ma DONALD 3,015,587

RHODIUM GERMANIUM FILM RESISTOR Filed Sept. 5, 1958 IN V EN TOR.

WILLIAM J. MACDONALD p xc cmwa\ mud/Z6 ATTOR N EYS United 3,tll5,537 RHQDTUM GERR EANIUM FllLli/i REfilSTtER William 3. Maclltonald, Littieton, Mass assignur to Technoiogy Instrument Corporation or Acton, a corporation of Massachusetts Filed Sept. 5, 1953, Ser. No. 75%,165 7 Claims. (Cl. 117-213) This invention relates to electrical resistor elements and more particularly to vacuum deposited film resistors.

In the past various substances have been used in making precision resistor elements the most common being metals or metal alloys, or vitreous carbon. However resistor elements of these materials become unstable when operated at high temperatures and are entirely unsatisfactory in the range 250 to 300 C. Resistors capable of stable operation in this temperature range have been made of stannous oxide coated with silicon. However they are only of limited value as the silicon coating prevents their use as variable resistors.

It is known that the noble metals are resistance stable at temperatures as high as 300 C. Few, if any other metals are stable at such temperatures but often their oxides, nitrides and hydrides are. On the other hand, the oxides, nitrides and hydrides usually produce undesirable contact noise when used as variable resistors. Resistor elements made of noble metals also have disadvantages. Their resistivity is low and they have very high temperature coefiicients of resistance. These undesirable characteristics can be minimized by alloying noble metals with less noble metals but in so doing high temperature stability is lost.

It is an object of my invention to overcome the above and analogous disadvantages. Another object of my invention is to provide new and improved resistor elements. It is a further object to provide a method of producing resistor elements which are resistance stable at temperatures of 300 C., and have high resistivity and low temperature coetficients of resistance.

When resistor elements are used as variable resistors in potentiometers special problems arise. Each time the slider passes over the surface of a variable resistor it causes a slight abrasion of the resistor element. In addition, contact between the slider and the resistor produces undesirable contact noise characteristics. Therefore, still another object of my invention is to provide a resistor element which is abrasion resistant and which produces a minimal amount of noise when used in a potentiometer.

Individually, rhodium has a low resistivity and a relatively high positive temperature coefiicient of resistance whereas germanium has almost infinite resistivity and a relatively high negative temperature coefficient of resistance. A mixture of the two has surprising and desirable characteristics. First of all, thin film may be obtained having a range of between approximately 10 to 2090 ohms per square, depending upon the ratio of rhodium to germanium. Secondly, a film comprising a mixture of the two substances may be obtained having a temperature coefiicient of resistance of between approximately 235 and 450 parts per million per C., as contrasted with pure rhodium which has a value in excess of 1000 p.p.m./ C. and germanium which has a high negative temperature coefiicient of resistance. Thirdly, rhodiumgermanium films are obtainable which are substantially resistance stable at temperatures as high as approximately 300 C.

These and other objects of my invention will best be understood and appreciated from the following description, when considered together with the accompanying drawing which is a perspective view, partly broken away, of apparatus for working the invention.

In practicing my invention a film composed of a mixture of rhodium and germanium is caused to deposit uniformly on the surface of a specially prepared substrate 13, after which the substrate with its film coating is heat treated.

After cleaning and slightly abrading the substrate material 13, it is mounted in a vacuum chamber 12 on a holder 15, which holder is in turn connected with a motor 16 in such a way as to permit the motor 16 to rotate the substrate 13 in its holder 15. Directly in front and approximately six inches away from the face of the substrate 13 is mounted a tungsten wire spiral 11 upon which rhodium has been electrolytically plated and to which germanium has been fused under a hydrogen atmosphere. This helix shaped wire ll with its rhodium and germanium load is mounted so that an electric current can be passed through it. An electrically-energized tungsten heater 14 is mounted in close relation to the substrate 13 and its holder 15.

Before beginning deposition the substrate 13 in its holder 15 is caused to rotate by means of the motor 16 and air is evacuated from the chamber 12 to a pressure level of between approximately 5 10 and 1 l0- mm. Hg. Subsequently, by means of radiation from filament heaters 14 within the chamber 12, the substrate 13 is preheated to a temperature between approximately 150 and 250 C. An electric current is then passed through the charged helix ll sufiicient to heat it above the melting points of rhodium and germanium. As the two elements melt they immediately evaporate and the gaseous rhodium and germanium atoms impinge upon the substrate surface 113 depositing as a thin film. The electric current is passed through the helix until all the rhodium and germanium have been evaporated.

Upon completion of evaporation the substrate with its film coating is cooled and exposed to the air, following which it is subjected to heat at approximately 300 C. for a suitable period, generally between to 200 hours to obtain electrical stability. The resulting heat treated rhodium germanium film is suitable for use as a precision electrical resistor element at temperatures as high as approximately 300 C. When used in a potentiometer my rhodium-germanium resistor elements are capable of a rotational life of approximately one million revolutions and generally have an equivalent noise resistance level, as measured by the apparatus disclosed in US. Patent No. 2,778,994 substantially less than 100 ohms ENR.

In practicing my invention I prefer to use clean fused quartz as a substrate material upon which the vaporized rhodium and germanium are deposited. However, I may also use clean Pyrex glass or other materials such as soft glass or an inert ceramic as the substrate material.

Regardless of the substrate composition, it is important to clean thoroughly the entire surface upon which the film is to be deposited. The same method is employed in cleaning quartz and Pyrex substrates. The surface is first polished and then abraded with an abrasive powder using an air blast. Following an acid etch which removes surface stresses and sharp surface points: created by the blasting, the surface is Washed with solvent solution. Mechanical removal of insoluble solids and a quick residue free dry thereby results. The same cleaning method is used for ceramic materials except that I omit the abrading step. Ceramic materials are too brittle and, therefore, will not abrade in the same manner as quartz or Pyrex glass. The purpose of abrading is to increase the surface area and thereby produce a greater resistance for a film of given length and breads.

The substrate 13 shown in the accompanying drawing is circular in shape so as to be suitable for use in a rotary potentiometer. However, it will be understood that my invention is not necessarily restricted to such use, and

that the shape of the substrate may be varied as desired.

The amounts of the two elements carried by the helix are determined by the film thickness desired to be produced and the ratio of the two elements carried by the helix is determined by and is in the same proportion as the ratio of the two elements in the film sought to be deposited on the substrate. I have experimented with various ratios including the following: (a) 88% rhodium, 12% germanium; (b) 70% rhodium, 30% germanium; and (c) 44% rhodium, 56% germanium. The foregoing specific ratios all have performed satisfactorily, though the 70% rhodium, 30% germanium ratio appears to be the most useful of the three for commercial purposes.

The vaporized rhodium and germanium spreads out equally in all directions from the helix. To achieve an evenly distributed film on the substrate it is of course best to position the helix directly in front of the substrate. I have found 6 inches to be a satisfactory distance between the substrate and helix but it will be apparent to those skilled in the art that this distance may be varied as part of the means for controlling the thickness of the coating.

Proper heating of the substrate is of great importance to the success of my invention. The primary purpose of preheating the substrate in a vacuum is to remove from it all moisture. It has been determined that a molecularly bonded water layer is normally present on the surface of the substrate and if it is not removed, the oxygen in the water will combine later with the germanium and cause it to oxidize. It has been found necessary to heat the substrate above 100 C. in order to remove this water layer and to assure complete removal, the substrate must be heated to a temperature of at least approximately 150. As heat in excess of approximately 250 C. tends to cause re-evaporation of the germanium after it has begun to deposit, care must be taken not to heat the substrate beyond that point. I have found 200 C. to be satisfactory in the majority of cases. In addition, the effectiveness of the subsequent stabilizing heat treatment of the substrate and film is directly dependent upon proper heating during deposition as it has been found that the resistance of the rhodium-germanium film changes while it is held at 300 C. and that the amount of this change is directly proportional to the amount of heat applied during such deposition.

Here follow four examples of my invention, the first of which I designate as my preferred embodiment.

Example I In this preferred embodiment of my invention I first polished a circular fused quartz disc to a degree comparable to a microscope slide finish and then uniformly abraded it with a 1200 mesh alumina powder using an air blast. A hydrofioric acid etch removed surface stresses created by blasting and a trichloroethylene solution washing accomplished removal of insoluble solids and produced a quick, residue free dry. The substrate was then mounted on its holder in the vacuum chamber.

A tungsten helix upon which rhodium had been electrolytically plated and germanium had been fused under a hydrogen atmosphere was prepared, the two elements being in a ratio of 70% rhodium and 30% germanium by weight. The prepared helix was mounted in the chamber six inches in front of the exposed substrate surface and the cover put over the chamber. Air was then withdrawn from the chamber to a vacuum of 1 10 mm. Hg after which rotation and heating of the substrate was begun. As the substrate in its holder revolved at 60 r.p.m., tungsten heaters adjacent to it raised its temperature to 200 C.

When the temperature of the substrate had reached 200 C. a current was passed through the loaded tungsten helix sufiicient to heat its coating above the melting points of both rhodium and germanium. Current was passed through the helix until its coating had completely evaporated; in this preferred embodiment the total amount of the two elements on the helix was sufficient to produce upon deposition, a film 200 angstroms thick. The substrate with its film coating was then cooled and exposed to the air.

At this point, after withdrawal from the vacuum chamher and before heat treatment the film had a resistance of approximately 625 ohms per square value and a temperature coefficient of resistance of +150 parts per million per degree centigrade. The film on its substrate was then heat treated. After exposure to heat of 300 C. for hours, the percentage of resistance change was +100% and had a T.C. of +280 p.p.m./ C. After an additional 600 hours of heat treating the additional percentage of resistance change was +1% and the film had a T.C. of +282 p.p.m./ C.

Example II In this example a Pyrex substrate was prepared and cleaned in the same manner as the quartz substrate of Example I. A tungsten helix was prepared in the same manner as in Example I but with a coating comprising 88% rhodium and 12% germanium. The total quantity was suflicient to produce a film 200 angstroms thick when subjected to the same vacuum evaporation process as the film in Example I. The resistance of this film upon withdrawal from the vacuum evaporation chamber and before heat treatment was approximately 490 ohms per square value and it had a T.C. of +46 p.p.m./ C. After exposure to heat of 300 C. for 100 hours, the percentage of resistance change was +.4% and T.C. was +450- p.p.m./ C. After an additional 400 hours of heat treatment the percentage of additional resistance change was +.7% and T.C. was stabilized at +450 p.p.m./ C.

Example 111 In this example a Pyrex substrate was prepared and cleaned in the same manner as the quartz substrate of Example I. A tungsten helix was prepared in the same manner as in Example I but with a coating comprising 70% rhodium and 30% germanium. The total quantity was sufircient to produce a film 200 angstroms thick when subjected to the same vacuum evaporation process as the film in Example I. The resistance of this film upon withdrawal from the vacuum evaporation chamber and before heat treatment was approximately 745 ohms per square value and it had a T.C. of 0 p.p.m./ C. After exposure to heat of 300 C. for 100 hours the percentage of resistance change was 19% and the T.C. was +270 p.p.m./ C. After an additional 400 hours of heat treatment the percentage of additional resistance change was -2% and the T.C. was stabilized at +270 p.p.m./ C.

Example IV In this example a Pyrex substrate was prepared and cleaned in the same manner as the quartz substrate of Example I. A tungsten helix was prepared in the same manner as in Example I but with a coating comprising 44% rhodium and 56% germanium. The total quantity was sufficient to produce a film 200 angstroms thick when subjected to the same vacuum evaporation process as the film in Example I. The resistance of this film upon withdrawal from the vacuum evaporation chamber and before heat treatment was approximately 10,000 ohms per square value and it had a T.C. of 100 p.p.m./ C. After exposure to heat of 300 C. for 100 hours the percentage of resistance change was 10% and the T.C. was 260 p.p.m./ C. After "an additional 400 hours of heat treatment the percentage of additional resistance change was 27% and the T.C. was stabilized at 260 p.p.m./ C.

In producing my invention the proportions of the two elements can be varied greatly by controlling the amounts of the two elements which are placed upon the tungsten helix, practical limits being set by the requirements for which the resistor element is to be used. Heat treat ment is essential in order to stabilize the film resistors and thereby make them useful for commercial and experimental purposes.

As might be expected, variations in the thickness of the film result in different resistance values for a given width and length of film. Thick coatings yield low resistance films, while thin coatings yield films of high resistance. Control of film thickness is achieved through control of the amount of the two elements loaded on the helix and to a lesser extent by the distance in the vacuum chamber between the helix and the substrate.

Rotation of the holder during evaporation is not "absolutely necessary. It is a preferred step which contributes to greater quality control, but can be omitted without harm.

Furthermore, as seen from the previous examples, different substrate materials will produce variations in both the resistance of the film upon withdrawal from vacuum and before heating and the amount of resistance change as a result of heating. I have found a quartz substrate to be most satisfactory for my own purposes.

The method described hereinabove may be employed to deposit and form films comprising molecular mixtures of substances other than rhodium and germanium. Thus, for example, I have deposited a film comprising a mixture of nickel, chromium and germanium by the same method and using the same apparatus. All three elements were fused onto the helix 11, with the nickel and chromium prior to application to the helix taking the form of Nichrome alloy.

In addition to those enumerated above certain minor variations of my invention will be apparent to those skilled in the art. Hence it is not my intention to confine the invention to the precise forms herein described but rather to limit it in terms of the appended claims.

Having thus described and disclosed my invention and the method by which it can be produced, what I claim as new and desire to secure by Letters Patent of the United States is:

1. A process for producing an electrical resistor element comprising the steps of providing as a substrate a piece of rigid electrically insulating material having a planar surface, cleaning and slightly abrading said planar surface, depositing on said planar surface a thin film comprising rhodium and germanium in predetermined relative amounts, and thereafter subjecting said substrate and film to heat treatment at an elevated temperature for an extended period of time.

2. A process as defined by claim 1 further including the step of heating said substrate to a predetermined temperature in excess of room temperature preliminary to depositing such film, said predetermined temperature being less than approximately 250 C. and more than approximately 150 C.

3. A process for producing an electrical resistor element comprising the steps of providing as a substrate a piece of rigid electrically insulating material having a planar surface, cleaning and slightly abrading said planar surface, heating said substrate to a predetermined temperature in excess of room temperature preliminary to depositing said film, said predetermined temperature being less than approximately 250 C. and more than approximately 150 C., and depositing on said planar surface a thin film comprising rhodium and germanium in predetermined relative amounts, said deposition being accomplished by means of evaporation under a vacuum of between approximately 5 10- and 1 10 said evaporation of rhodium and germanium being accomplished by passing an electric current through a spiral tungsten filament coated with rhodium and germanium, said heating and said passing of an electric current through said tungsten filament being continued until approximately all the rhodium and germanium have been evaporated from said tungsten spiral, said evaporation being followed by cooling of the substrate with its coating of rhodiumgerm-anium film and removal from the vacuum chamber.

4. A process as defined by claim 3 followed by the step of subjecting said substrate with its coating of rhodium and germanium to heat of approximately 300 C. for between approximately and 200 hours in air.

5. A process as defined by claim 3 further including the step of slowly revolving the said substrate during deposition.

6. A process as defined by claim 4 further including the step of slowly revolving said substrate during deposition.

7. A process for producing a film comprising a molecular mixture of rhodium and germanium, comprising the steps of providing a substrate having a clean planar surface, providing an electrically conductive filament, coating said filament with predetermined quantities of rhodium and germanium, placing said substrate in a vacuum chamber, preheating said substrate under vacuum to a temperature of at least approximately C. to remove all surface moisture therefrom, thereafter while said substrate is still at a temperature of 150 C. passing an electric current through said filament to heat said filament to a temperature sufficient to cause evaporation of said rhodium and germanium, discontinuing said electric current after approximately all of said rhodium and germanium have been evaporated off of said filament, and cooling said substrate prior to removal from said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,097,140 Wohrman et al. Oct. 26, 1937 2,341,219 Jones Feb. 8, 1944 2,423,476 Billings July 8, 1947 2,552,626 Fisher et a1 May 15, 1951 2,629,800 Pearsons Feb. 24,, 1953 2,778,743 Bowman Jan. 22, 1957 2,789,187 Romer Apr. 16, 1957 2,847,325 Riseman et a1 Aug. 12, 1958 2,855,493 Tierman Oct. 7, 1958

Claims (1)

1. A PROCESS FOR REDUCING AN ELECTRICAL RESISTOR ELEMENT COMPRISING THE STEPS OF PROVIDING AS A SUBSTRATE A PIECE OF RIGID ELECTRICALLY INSULATING MATERIAL HAVING A PLANAR SURFACE, CLEANING AND SLIGHTLY ABRADING SAID PLANAR SURFACE, DEPOSITING ON SAID PLANAR SURFACE A THIN FILM COMPRISING RHODIUM AND GERMANIUM IN PREDETERMINED RELATIVE AMOUNTS, AND THEREAFTER SUBJECTING SAID SUBSTRATE AND FILM TO HEAT TREATMENT AT AN ELEVATED TEMPERATURE FOR AN EXTENDED PERIOD OF TIME.

Images