GB1563473A

GB1563473A - Electrical resistors

Info

Publication number: GB1563473A
Application number: GB999078A
Authority: GB
Original assignee: Welwyn Electric Ltd
Current assignee: Welwyn Electric Ltd
Priority date: 1978-03-14
Filing date: 1978-03-14
Publication date: 1980-03-26

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRICAL RESISTORS (71) We, WELWYN ELECTRIC LIMITED, of Bedlington, Northumberland NE22 7AA a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electrical resistors and more particularly to a resistor comprising a metal film resistance element on an electrically insulating substrate and having improved performance as a result of the provision of a semiconducting metal oxide film on the surface of the metal film.

The present invention provides an electrical resistor comprising a substrate of electrically insulating material, a metal film resistance element on said substrate and a metal oxide film with controlled semiconducting properties overlying said resistance element, said metal oxide film comprising an oxide or oxides of one or more metals, different from the metal of the metal film selected from a group consisting of antimony, bismuth, cadmium, copper, indium, iron, thallium, tin and titanium, electrically conducting terminals being provided electrically connected to said resistance element.

Preferably said metal film resistance element comprises an alloy containing nickel and chromium.

The said metal oxide film should preferably have an electrical resistivity at least ten times greater than that of said metal film resistance element and such that the major proportion of electric current flow between said terminals occurs in said metal film resistance element.

The semiconducting properties of said metal oxide film may be predetermined by oxygen deficiency of the oxide or oxides of the said one or more metals therein and/or by means of a host element, selected from the said group of metals, in the said metal oxide film, which is doped with another element serving as a valency-controlling element and either selected from the said group of metals or comprising a further element, the conductivity of the metal oxide film increasing to a maximum when said host element is fully doped by said valencycontrolling element. When a said valencycontrolling element is included, this is selected in accordance with the valency of the said host element and will usually be in a higher valency state than the host element.

The semiconducting metal oxide film may also comprise free oxide or oxides of an element dispersed with the said oxygendeficient metal oxide or oxides and/or oxide or oxides of the host element doped with the valency-controlling element, said free oxide or oxides serving to increase the resistivity of the semiconducting metal oxide film to a desired value.

In one embodiment of the invention, the said semiconducting metal oxide film comprises oxygen-deficient tin oxide.

In another embodiment the said semiconducting metal oxide film comprises oxygen-deficient tin oxide with a said free oxide dispersed therewith, said free oxide preferably comprising boric oxide.

In a further embodiment, the semiconducting metal oxide film comprises tin oxide and antimony oxide, wherein tin constitutes a said host element doped with antimony which serves as a said valencycontrolling element.

In a still further embodiment, the semiconducting metal oxide film comprises a dispersion of a said free oxide with tin oxide and antimony oxide, wherein tin constitutes a said host element doped with valency-controlling antimony, said free oxide preferably comprising boric oxide and/or antimony oxide.

Preferably the semiconducting metal oxide film is formed by a high temperature vapour phase hydrolysis process. When such a process is used, although the high temperature involved therein (e.g. up to 6000 C) would by itself impart an improvement in stability to the metal film resistance element, we have found that this improvement is much less than that obtained by depositing the metal oxide film.

The following example illustrates the invention.

An electrical resistor is prepared by vacuum depositing a metal film electrical resistance element, comprising a nickelchromium alloy, onto a cylindrical ceramic substrate 2.5 mm. in diameter and 8 mm. in length. The techniques for depositing such a film by vacuum evaporation or sputtering are well known in the art.

(Although in this example the preparation of a single resistor is described, for convenience, in practice a batch of such resistors would be prepared simultaneously). By way of example, the resistance element could suitably have an electrical sheet resistivity of about 600 ohms per square.

The substrate with the nickel-chromium resistance element thereon is then heated to about 600"C and allowed to come into contact with a vapour from a solution comprising: Stannic Chloride (SnCl4-5H2O) 870 g Antimony Trichloride (SbCl3) 1200 g Boric Acid (H3BO3) 5 g Water (H2O) 270 cm3 Concentrated Hydrochloric acid (HCl) 600 cm3 Vapour phase hydrolysis of the tin, antimony and boron compounds occurs and a semiconducting metal oxide film comprising oxides of tin, antimony and boron is deposited onto the surface of the nickel-chromium resistance element, the deposition being continued until the oxide film has a resistivity at least ten times greater than that of the underlying resistance element.In the case of a nickelchromium resistance element having a resistivity of about 600 ohms per square, it is preferred to provide an overlying metal oxide film having a resistivity of about 100,000 ohms per square. In the resulting metal oxide film, a host element therein, comprising tin is doped with antimony which serves as a valency-controlling element and in addition, free oxides comprising boric oxide and antimony oxide are dispersed with the tin/antimony oxides.

Metal terminal caps, suitably comprising nickel-plated steel are pressed onto the ends of the resulting resistor body to form an interference fit and make electrical connection with the metal film resistance element through the metal oxide film. A helical groove is then cut through the oxide and metal film to produce a desired resistance value measured between the terminal caps.

Several resistors prepared in this way and of 100KB resistance value have been tested by maintaining them at a temperature of 155"C in air for 1000 hours. For comparison, samples have also been tested under the same conditions but comprising a similar nickel-chromium film deposited on a ceramic substrate and without subsequently applying the metal oxide film. These samples have been subjected to a thermal stabilising treatment in air at 3000C for 24 hours before applying terminal caps and helixing to 100KS. The resistance value of the resistors with the metal oxide film overlying the metal film has been found to change by less than 0.1 ,' during this test, while the resistance value of the resistors without the metal oxide film changed by about 0.3%.

WHAT WE CLAIM IS: I. An electrical resistor comprising a substrate of electrically insulating material, a metal film resistance element on said substrate and a metal oxide film with controlled semi-conducting properties overlying said resistance element, said metal oxide film comprising an oxide or oxides of one of more metals different from the metal of the metal film selected from a group consisting of antimony, bismuth, cadmium, copper, indium, iron, thallium, tin and titanium, electrically conducting terminals being provided electrically connected to said resistance element.

2. A resistor according to Claim 1 in which said metal film resistance element comprises an alloy containing nickel and chromium.

3. A resistor according to Claim I or 2 in which said metal oxide film has an electrical resistivity at least ten times greater than that of said metal film resistance element and such that the major proportion of electric current flow between said terminals occurs in said metal film resistance element.

4. A resistor according to Claim 1, 2 or 3 in which the semiconducting properties of said metal oxide film are predetermined by oxygen deficiency of the oxide or oxides of the said one or more metals therein and/or by means of a host element, selected from the said group of metals, in the said metal oxide film, which is doped with another element serving as a valency-controlling element and either selected from the said group of metals or comprising a further element, the conductivity of the metal oxide film increasing to a maximum when said host element is fully doped by said valencycontrolling element.

5. A resistor according to Claim 4 in which said valency-controlling element is selected in accordance with the valency of

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Claims

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improvement is much less than that obtained by depositing the metal oxide film.

The following example illustrates the invention.

An electrical resistor is prepared by vacuum depositing a metal film electrical resistance element, comprising a nickelchromium alloy, onto a cylindrical ceramic substrate 2.5 mm. in diameter and 8 mm. in length. The techniques for depositing such a film by vacuum evaporation or sputtering are well known in the art.

(Although in this example the preparation of a single resistor is described, for convenience, in practice a batch of such resistors would be prepared simultaneously). By way of example, the resistance element could suitably have an electrical sheet resistivity of about 600 ohms per square.

The substrate with the nickel-chromium resistance element thereon is then heated to about 600"C and allowed to come into contact with a vapour from a solution comprising: Stannic Chloride (SnCl4-5H2O) 870 g Antimony Trichloride (SbCl3) 1200 g Boric Acid (H3BO3) 5 g Water (H2O) 270 cm3 Concentrated Hydrochloric acid (HCl) 600 cm3 Vapour phase hydrolysis of the tin, antimony and boron compounds occurs and a semiconducting metal oxide film comprising oxides of tin, antimony and boron is deposited onto the surface of the nickel-chromium resistance element, the deposition being continued until the oxide film has a resistivity at least ten times greater than that of the underlying resistance element.In the case of a nickelchromium resistance element having a resistivity of about 600 ohms per square, it is preferred to provide an overlying metal oxide film having a resistivity of about 100,000 ohms per square. In the resulting metal oxide film, a host element therein, comprising tin is doped with antimony which serves as a valency-controlling element and in addition, free oxides comprising boric oxide and antimony oxide are dispersed with the tin/antimony oxides.

Metal terminal caps, suitably comprising nickel-plated steel are pressed onto the ends of the resulting resistor body to form an interference fit and make electrical connection with the metal film resistance element through the metal oxide film. A helical groove is then cut through the oxide and metal film to produce a desired resistance value measured between the terminal caps.

Several resistors prepared in this way and of 100KB resistance value have been tested by maintaining them at a temperature of 155"C in air for 1000 hours. For comparison, samples have also been tested under the same conditions but comprising a similar nickel-chromium film deposited on a ceramic substrate and without subsequently applying the metal oxide film. These samples have been subjected to a thermal stabilising treatment in air at 3000C for 24 hours before applying terminal caps and helixing to 100KS. The resistance value of the resistors with the metal oxide film overlying the metal film has been found to change by less than 0.1 ,' during this test, while the resistance value of the resistors without the metal oxide film changed by about 0.3%.

WHAT WE CLAIM IS: I. An electrical resistor comprising a substrate of electrically insulating material, a metal film resistance element on said substrate and a metal oxide film with controlled semi-conducting properties overlying said resistance element, said metal oxide film comprising an oxide or oxides of one of more metals different from the metal of the metal film selected from a group consisting of antimony, bismuth, cadmium, copper, indium, iron, thallium, tin and titanium, electrically conducting terminals being provided electrically connected to said resistance element.
2. A resistor according to Claim 1 in which said metal film resistance element comprises an alloy containing nickel and chromium.
3. A resistor according to Claim I or 2 in which said metal oxide film has an electrical resistivity at least ten times greater than that of said metal film resistance element and such that the major proportion of electric current flow between said terminals occurs in said metal film resistance element.
4. A resistor according to Claim 1, 2 or 3 in which the semiconducting properties of said metal oxide film are predetermined by oxygen deficiency of the oxide or oxides of the said one or more metals therein and/or by means of a host element, selected from the said group of metals, in the said metal oxide film, which is doped with another element serving as a valency-controlling element and either selected from the said group of metals or comprising a further element, the conductivity of the metal oxide film increasing to a maximum when said host element is fully doped by said valencycontrolling element.
5. A resistor according to Claim 4 in which said valency-controlling element is selected in accordance with the valency of

the said host element and is in a higher valency state than said host element.
6. A resistor according to Claim 4 or 5 in which the semi-conducting metal oxide film also comprises free oxide or oxides of an element dispersed with said oxygendeficient metal oxide or oxides and/or oxide or oxides of the host element doped with the valency-controlling element, said free oxide or oxides serving to increase the resistivity of the semiconducting metal oxide film to a desired value.
7. A resistor according to Claim 4, 5 or 6 in which said semiconducting metal oxide film comprises oxygen-deficient tin oxide.
8. A resistor according to Claim 6 in which said semiconducting metal oxide film comprises oxygen-deficient tin oxide with a said free oxide dispersed therein, said free oxide comprising boric oxide.
9. A resistor according to Claim 4, 5 or 6 in which said semi-conducting metal oxide film comprises tin oxide and antimony oxide and wherein tin constitutes a said host element doped with antimony which serves as a said valency-controlling element.
10. A resistor according to Claim 6 in which the semiconducting metal oxide film comprises a dispersion of a said free oxide with tin oxide and antimony oxide, wherein tin constitutes a said host element doped with valency-controlling antimony.
11. A resistor according to Claim 10 in which said free oxide comprises boric oxide and/or antimony oxide.
12. A resistor according to any one of the preceding Claims in which said semiconducting metal oxide film is formed by a high temperature vapour phase hydrolysis process.
13. An electrical resistor substantially as herein described with reference to the example.