US 3452314 A
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Description (OCR text may contain errors)
June 24, 1969 M. SAPOFF ETAL 3,452,314
LOW NOISE THERMISTOR ASSEMBLY AND METHOD Filed May 22. 1967 FIG. 2 315 FIG. 4
United States Patent 01 lice 3,452,3'14 Patented June 24, 1969 3,452,314 LOW NOISE THERMISTOR ASSEMBLY AND METHOD Meyer Sapotr, West Orange, and John G. Froemel, Verona, N.J., assignors to Victory Engineering Corporation, Springfield, N.J., a corporation of elaware Filed May 22, 1967, Ser. No. 640,200 Int. Cl. H01c 7/04, 7/00 U.S. Cl. 338-22 4 Claims ABSTRACT OF THE DISCLOSURE Thermistor materials are deposited on a substrate by evaporation or sputtering. One or more terminals and a resistance pattern of gold is then deposited through a mask on desired areas of the thermistor material. The thermistor assembly is then heated in a furnace at 500 degrees centigrade for fifteen minutes to partially diifuse the gold into the thermistor material. After the heating cycle, platinum wires iridium) are welded to the gold terminals. The resulting thermistors inject considerably less noise into a circuit when passing current.
Background of the invention This invention relates to a means and method for making thermistors producing almost no noise when passing current. The invention has particular relationship to thermistors which are to be used in amplifier or telephone circuits passing voice frequency currents.
Summary of the invention Therrnistors used for measuring purposes, particularly in bridges, may inject noises into the circuit without producing any harmful effects. However, when used in any circuit which is connected to a sound transducer it is important to keep the noise as low as possible. It has been found that the main source of noise is in the connection between the thermistor material and the metal electrodes which make contact with them. Plating or eavporating a metal onto a thermistor surface is always susceptible to microscopic arcing and minor circuit breaking due to insufficient pressur ebetween the two materials. It has been found that the use of gold sputtered on a terminal area and the surface of the thermistor and then treated in a furnace will either alloy or diifuse into the thermistor material and produce a bond that is substantially free of noise. The thermistor material may be evaporated onto a nonconducting substrate or it may be deposited onto the substrate by sputtering techniques in a rarified atmosphere of an inert gas.
Brief description of figures FIGURE 1 is a plan view of an area of thermistor material deposited evenly onto a nonconducting substrate.
FIGURE 2 is a plan view of one form of the invention and shows the thermistor material deposited in a resistor pattern on a substrate.
FIGURE 3 is a plan view of the device shown in FIGURE 1, with the gold terminal material deposited on the thermistor material in another pattern which will produce a low resistance between the terminals. This view also shows the terminal wires.
FIGURE 4 is a cross-sectional view showing the substrate, the thermistor material, and a gold terminal before the application of the heating cycle.
FIGURE 5 is a cross-sectional view taken along line 5-5 of FIGURE 2 and is similar to FIGURE 4 except that the gold has been alloyed into the thermistor material and a terminal wire has been welded to the gold.
Description 0] preferred embodiment Referring now to FIGURE 1, a substrate 10 is shown with an area of thermistor material 11 evaporated thereon. The deposition of the thermistor material may be done by either evaporation or sputtering. This constitutes the first step in the production of a low resistance thermistor. Next, two gold terminals 12, 13, are deposited on the thermistor material as shown in FIGURE 3. The gold is preferably deposited by sputtering through a stainless steel mask which can be positioned directly in front of the thermistor material during the sputtering process. Apparatus for performing this step is described in co-pending patent application Ser. No. 435,119, filed Feb. 25, 1965, in the names of John G. Fromel and Meyer Sapoff, for Thin Film Resistance Elements and Method, now abandoned.
The thermistor array is next placed in a furnace and heated to 500 centigrade for about fifteen minutes. This heating does not change the thermistor material because it is made up of a mixture of oxides but causes the gold to diffuse partially into the thermistor. After heating, Wires 14 and 15 are welded to terminal areas 16- and 17, and the thermistor is ready for use.
The above described thermistor is obviously a low resistance element since the distances between terminals is short and since the terminals are long. If a high resistance thermistor is desired, the type shown in FIGURE 2 is fabricated. In this case the thermistor material 20 is sputtered onto a base 10 through a mask so that the thermistor material has a long path between its end portions '21 and 22. In order to provide suitable terminals for inclusion in a circuit, gold areas 23 and 24 are deposited through a mask in a manner similar to that described above. The thermistor is next given a heat treatment as described above by heating to 500 centigrade for fifteen minutes. After the heating operation, wire terminals 14 and 15 are added.
FIGURES 4 and 5 illustrate the method of noise re duction due to heating. In FIGURE 4, the substrate 10 carries a layer of thermistor material 22 and a gold terminal 24. As is shown in the drawing, the gold rests on top of the thermistor material but there is no evidence of combination. After the heating cycle, the thermistor material 22 and gold terminal 24 have been diffused together as is indicated by the merging cross-hatching lines in FIGURE 5. This construction accounts for the lack of noise. A platinum wire 15 or any other type of wire may be welded or soldered to the gold terminal 24 and the thermistor is ready for use.
It will be obvious from the above description that many other forms of thermistor material and many other arrangements of gold terminals can be used for this process.
Having thus fully described the invention what is claimed as new and desired to be secured by Letters Patent of the United States is:
1. A thermistor comprising a non-conductive substrate, thermistor material deposited on the substrate to form a resistance, a plurality of gold terminal areas deposited on the thermistor material at predetermined positions to form terminals for the application of an electric voltage, said gold terminals diffused into the thermistor material by the application of heat after the gold has been deposited, and a terminal wire secured to each of said gold terminals for connection to an external circuit.
2. A thermistor as claimed in claim 1 wherein the thermistor material is a mixture of metal oxides.
3. A method of forming thermistors having gold diffused terminals for the reduction of electric circuit noise comprising the following steps: depositing thermistor materia1 on a non-conductive substrate, depositing gold on predetermined terminal areas to jorm circuit terminals, heating the array in a furnace of a temperature within the range of 400 to 600 degrees centigrade for a time interval in excess of 10 minutes, and finally attaching wire terminals to the gold terminals to form an electric circuit component.
4. A method according to claim 3 wherein said gold is sputtered from a target in a rarified atmosphere of inert gas through a mask to deposit only on predetermined areas.
References Cited UNITED STATES PATENTS 6/1947 Pearson 33822 8/ 1956 Eisler 338-309 10/1956 Thomsen 338-309 1/1959 Howatt 29612 12/1960 Baasch 338-22 11/1965 Girard 3-3 822 9/ 1966 Smith 29-612 Primary Examiner.
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