GB2110197A - Electrical contacts - Google Patents

Abstract

Electrical contacts are prepared whereby at least one metallic contact surface is chemically treated with an external reagent so as to form a thin layer of a crystalline compound containing the metal on its surface. The compounds include metal sulphides, selenides, carbides, nitrides, borides, phosphides and silicides.

Description

SPECIFICATION Apparatus including electrical contacts The invention is concerned with apparatus including electrical contacts which depend on mechanical contact for closure. A category of included apparatus may be characterized as "low current" or "nonarcing" and has traditionally relied upon gold. The category includes electronic devices such as wire and cable connectors, printed wiring boards, and integrated circuits. The unique position of gold contact surfaces in the electrical arts has been secure from the beginning. While gold is classifed as a member of the class of "noble metals", it alone is truly nonreactive with commonly encountered ambient constituents. Only by this nonreactivity has the integrity of required contact properties been maintained. With recent increases in prices, efforts to replace gold have increased. Studies have been successful generally in terms of reduction in gold rather than in eliminating. Significant cost savings have resulted from reduction in gold layer thickness by attention to deposition techniques, e.g., gold plating conditions, and also to substrate surface composition and condition. Electrical contact structures in which gold is replaced by other "noble metals" have realized some degree of success. Platinum-group metals, platinum and palladium, for example, sometimes alloyed with silver have been successfully used. However, even such materials have been found to be somewhat reactive with common ambient constituents. The indispensability of gold is illustrated by a recent development in which a palladium-silver alloy contact is coated with a very thin gold layer to improve reliability. In circumstances where arcing results upon make or break a variety of non-noble metals, notably Cu-containing alloys have found use. Here reliance may be had on arcing to "punch through" high resistance surface compounds produced by reaction with ambient constituents. A number of chemical compounds characterized by metallic or near metallic conductivity have been reported in the literature. Films of such materials have sometimes been formed by deposition of the compound, e.g., by sputtering. See for example, Proceedings of the Electrochemical Society, 80, page 216 (1979). Generally, such work has been of a fundamental research nature, although observed electrical properties have provoked comment. According to the present invention there is provided apparatus comprising mating electrical contacts at least one of which comprises a substrate and a contact layer in intimate contact with the said substrate, the said contact layer having a first mating surface wherein physical contact with the said mating surface results in electrical connection, the said contact layer comprises a firstcrystallographic phase that is produced by a procedure comprising in situ reaction using energies associated with conventional ambient to produce a chemical compound of which the said first crystallographic phase is comprised, the said reaction involving reactants including a first reactant which is an integral part of the said substrate, the said first reactant being designated substrate reactant and a second reactant which is introduced during reaction. Embodiments of the invention provide for contact surfaces which depend fortheir electrical characteristics primarily upon chemical compounds. Illustrative compounds, generally well characterized in the literature, are chemically grouped as silicides, carbides, nitrides, phosphides, borides, sulfides and selenides. Since a main objective of the embodiment is economic, it is compounds of platinum-group metals as well as precious metals (ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold) that are excluded. From the thermodynamic standpoint, like gold, appropriate compounds in accordance with the embodiments are generally at true equilibrium. Under usual ambient circumstances compounds of the embodiments do not react with ambient constituents at least on prolonged exposure so that chemical change at the free contact surface is avoided. From the electrical standpoint, contacts in accordance with the embodiments have low contact resistance. Resistivity as measured by a four point probe

For usual structures, contact resistance is generally below about 10 milliohms. This resistance level is seen after high humidity-temperature cycling under test conditions used for qualifying telephone equipment. The embodiments contemplate the formation of thin layers of compound contact material (layers of the order of 10 micrometers or less in thickness) produced by in situ reaction with substrate material. Contemplated reactions which may involve vapor phase or liquid phase reactant involve at least one constituent- usually a metallic constituent- of the substrate. It is significant that such in situ formed layers may be directly substituted for gold layers deposited, for example, by electroplating, sputtering, etc. The embodiments therefore, contemplate formation of contact layers on partially fabricated or on otherwise completed devices. Thin-film compound contacts of the invention are generally useful in nonarcing applications which have traditionally been served by gold. While the lower melting compounds, e.g., sulfides and selenides, are generally limited to such use, others are not so limited. Glossary A. Contact Compound These are the in situ formable materials characterized by bulk resistivity of 10-3 cm or less. All such compounds are generally stable to the extent that resistivity does not increase beyond such maximum value under test conditions applied to relevant gold devices. In situ formation involves at least one constituent- usually a metallic element-contained in the substrate upon which the layer is formed. Appropriate constituents do not include platinumgroup metals or other precious metals (prohibited elements are ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold). B. Nonarcing The meaning here is consistent with the usually recognized meaning in that normal operation does not result in formation of a luminous region at the contact upon make or break. Contemplated conditions under which the characteristic is satisfied are "dry circuit" conditions. Contemplated devices of the embodiments are generally nonarcing under contemplated operating conditions, as well. C. Electronic Devices These are generally small current devices such as electronic tubes, transistors, etc., as used, e.g., in communications computers, as distinguished from large current devices, such as those used for power applications. D. Small Current This is the circuit current normally passing through the compound contact layer and its mating surface resulting in circuit closure. This corresponds with an open circuit voltage of 50 millivolts or less. Use of the terminology does not require actual measurable open circuit voltage as described since contemplated devices include those in which continuous closure is required. Resulting currents are usually below 100 ma or, for many devices, below 50 ma. E. Contact Compound Layer This is the in situ formed contact compound in the form of a layer having a thickness of the order of 10 Microm or less. Contact resistance under a load of 100 grams is generally below 10 milliohms before or after appropriate testing, e.g., for some purposes, at high humidity (90 percent relative humidity air at 90 degrees Fahrenheit) and temperature cycling between -40 degrees C and +140 degrees C. F. Contact Again, the term is used in its traditional context. It contemplates a free surface of contact compound in the mechanical sense. Ordinarily open and closed contact conditions are contemplated, but use may also involve sustained closure. Pressure on closure typically does not exceed 100 grams. The front or back electrode in the usual carbon granual transmitter is illustrative of a normally closed contact. The following examples were conducted on structures of similar or identical configurations. Structures consisted of a flat compound contact contacted by a hemispherical probe. All hemispherical probes were surfaced with soft gold (99.99 weight percent). Experiments with compound contact probes yielded approximately the same contact resistance measurements. Open circuit voltages were less than 20 millivolts resulting in currents of less than 10 milliamperes upon closure. The probe structure was parabolic in cross-section with the contacting region approximately defining hemisphere having a diameter of one millimeter. Tests were conducted in normal air ambient at room temperature. Electrical measurements were made in the conventional four point manner with current and voltage contacts to the probe and the flat contact. Measurements conducted at 100 grams in each instance were reliably reproduced and fell well within the bounds required for most device designs. For comparison purposes the identical measurements conducted with a flat and dome both have 99.99 weight percent gold yielded a resistance value of 0.7 milliohms under the test conditions. Layer thickness was estimated from weight gain. Composition was determined by X-ray analysis.

Example 1 Composition-Nickel Phosphide, NiP. Preparation- Nickel was exposed to phosphorus vapor at a temperature of 440 degrees C for 2 hours thereby resulting in a compound layer of an approximate thickness of 5 Microm. Resistance-18 milliohms at 100 grams.

Example 2 Composition-Titanium Nitride, TiN. Preparation-Titanium was exposed to ammonia gas at a temperature of 1100 degrees C for a period of 1 hour resulting in a layer thickness of approximately 8 Microm. Resistance- 50 milliohms at 100 grams.

Example 3 Composition-Tantalum Nitride, TaN. Preparation-Tantal um metal was reacted in ammonia for 1 hour at a temperature of 1100 degrees C to yield a layer of a thickness of about 10 Microm. Resistance-150-200 milliohms at 100 grams.

Example 4 Composition-Titanium Carbide, TiC. Preparation-Titanium metal was exposed to acetylene for a period of 1 half hour at a temperature of 950 degrees C, yielding a layer of a thickness of about 2 Microm. Resistance-90 milliohms at 100 grams.

Example 5 Composition- Copper Selenide, CuSe. Preparation - A 90 percent copper, 10 percent nickel substrate was exposed to a 0.3 percent solution of selenium in dichlorobenzene at 170 degrees C for 10 minutes. This resulted in a coating thickness of 5 Microm. Resistance-10 milliohms at 100 grams.

Example 6 Composition -Copper Sulfide, CuS. Preparation-A80 percent copper, 20 percent cobalt substrate was exposed to sulfur vapor at 187 degrees C using a nitrogen carrier gas for 2.5 hours. This resulted in a coating thickness of 10 Microm. Resistance-3.0 and 7.8 milliohms at 100 and 15 grams, respectively. Variations on the examples included different reactant states. So, for example, the CuSe of Example 5 was reproduced by reacting a similar surface with selenium vapor using a nitrogen carrier to yield substantially identical electrical properties. The material of Example 6 was produced by an alternative technique in which reaction was with molten sulfur as well as by use of dichlorobenzene solvent. Electrical properties were substantially identical.

Example 7 Composition -Titanium Silicide, TiSi. Preparation - A titanium substrate was exposed to an atmosphere composed of 10 percent silane, 90 percent N2 gas at a temperature of 900 degrees C for a period of 1 hour. Coating thickness was approximately 3.5 Microm. Resistance-50 to 60 milliohms at 100 grams. It has been stated that contact composition in the embodiments avoids the use of gold, silver, and metals of the platinum group (Ru, Rh, Pd, Re, Os, Ir, Pt). Generally, exclusion is based on economics which, after all, is the major thrust of the invention. Functional material in the embodiments is produced by in situ reaction involving at least one reactant which is present as a substrate constituent and another reactant introduced externally. The externally introduced reactant is generally in fluid form, either vapor or liquid. Introduction may involve a carrier, for example, to introduce the external reactant in the vapor phase but permit reaction at a temperature below its vaporization temperature. One embodiment depends upon the concept of replacing a thin layer of conventional contact material - generally gold-with a layer of a chemical compound.An important teaching permits substitution of the one layer for the other and thereby minimizes or avoids device redesign. Formation of gold or gold-containing layers in prior art devices is

the compounds of the present invention is accomplished by in situ reaction. The invention does not depend upon designation of compound composition. The technical literature includes reports of compounds with measured electrical properties and known stability in some encountered ambients so, for example, titanium nitride has been studied to result in a literature reference, 1980 Proceedings of the Electrochemical Society, 316 (1979), reporting bulk resistivity values of

usual air ambient over usually prescribed temperature ranges of operation. Similar information is available for a variety of compounds including silicides of titanium, zirconium, vanadium, niobium, tantalum, iron, cobalt, e.g., TiSi2, TiSi, ZrSi, VSi2, NbSi2, TaSi2, FeSi2, CoSi2, carbides of titanium, tantalum, tungsten, molybdenum, chromium, niobium, vanadium, hafnium, zirconium, lanthanum-group metals, e.g.TiC, TaC, WC, W2C, MoC,

La2C3, Ce2C3, Nd2C3, nitrides ortitanium, zirconium, niobium, tantalum, chromium, tungsten, hafnium, e.g., tiN, ZrN, NbN, TaN, Cr2N, WN2, HfN, boridesof titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, e.g., TiB, TiB2, ZrB2, HfB2, V3B2. V3B4, NbB, NbB2, TaB, TaB2, Cr3B2, CrB2, Mo2B, MoB2 and phosphides of a variety of materials. Whereas compounds with requisite electrical properties are generally of known stability in air ambient, i.e., in 02, N2, and H2O, other considerations may well require screening. Compounds tested in the work which led to this disclosure have manifested stability in the presence of sulfur bearing ambient material, H20, SO2, S03, H2S, as well as in

In general, electrical properties of contacts produced in accordance with the invention are primarily due to the compounds noted. Reaction may involve more than two reactants, e.g., may proceed by reaction of an external reactant with two or more substrate constituents, e.g., where both yield compounds otherwise appropriate for the practice of the invention. It is expected that contacts produced by the embodiments will at least in initial stages contain little if any material in addition to the contemplated compound/s.However, structures may include unintentional material. Examples include substrate ingredients which under fabrication conditions migrate into the compound layer sometimes to the free contact surface. Resulting mixture, again consistent with prior experience, may result in improvement of the contact surface. In accordance with example 6, analysis of the free contact surface has revealed presence of 2 weight percent cobalt. Studies of the nominal composition CuS showed increased resistivity upon inclusion oftin, iron, zinc, manganese, titanium, chromium, nickel, aluminium, silicon, antimony and cobalt in solid solution (all were in the range of 1-15 weight percent). The contact-compound rich phase is desirably composed of at least 85 percent of compound/s in accordance with the inventive teaching. Second phase modifications, on the other hand, which may involve elements or compounds to the extent not dissolved in the contact compound rich phase have only a linear effect on contact resistance and may be tolerated in larger amounts. Such second phase may be unintentional or may be introduced deliberately in order ot modify physical characteristics. In general, such second phase should be present only in an amount to occupy up to 30 percent of the free surface area. To a first approximation under usual circumstances, this limit may be expressed as weight percent. Intentional modification of composition includes constituents added after formation of the compound. Examples include gold diffusion to produce a graded structure of good electrical and mechanical properties. Structures may also be graded by altering reactant composition during reaction. Appropriate procedures for forming compounds suitable forthe practice of the invention are described in the examples. In general, external reactant/s are brought into contact with substrate reactant/s in a fluid form - either vapor or liquid. Expedient processing to result in, e.g., desired layer thickness, sometimes suggests use of a carrier to permit reaction at temperatures at which external reactant might otherwise not be fluid/vapor. It has been found desirable to limit compound formation to layers of a thickness no greater than approximately 25Microm. From the electrical standpoint, compounds of the nature described usually have greater bulk resistivity than substrates on which formed so that greater thickness unnecessarily increases series resistance. In addition, compounds layers of some included compositions show cohesion values which are less than adhesion to the substrate. The observable phenomenon is an effective integrity which varies inversely as layer thickness. All compounds tested with the exception of CuS and CuSe had sufficient adhesion to permit most demanding device operation as produced by reaction with substrate composed solely of reactant. CuS and CuSe, however, showed substantially improved adhesion when prepared on substrate containing additional ingredients. Examples 5 and 6 exemplify satisfactory device requirements. cuS had improved adhesion when prepared on a substrate containing greater than 15 weight percent cobalt. Adhesion of CuSe is increased by variants of example 6, e.g. by use of substrates in which copper is alloyed with nickel. In like manner, procedural variations, e.g., variation in temperature, pressure, etc., may result in desired structures by compositional grading. Detailed discussion has been restricted largely to creation of the contact surface itself. Fabrication of the entire apparatus may place other demands so, for example, substrate composition may be chosen with a view to ductility to permit fabrication by coldworking. Minimum thickness is determined on the basis of substrate protection. Monolayers are generally inadequate. Substrate protection improves dramatically to layer thicknesses of up to about 3 micrometers. For a better understanding of the invention, reference is made to the accompanying drawings in which:- FIGS. 1A, 1 B, 2A, 2B, 3 and 4 are perspective views of nonarcing contacts incorporating in situ formed compound contact layers of embodiments. FIGS. 1A and 1 B are cross-sectional elevational views of a wire connector of a design used for telephone handset interconnection. It consists of wires 10 contained in recess 11 and molded in detail 12 to terminate in spring portions 13 provided with contacting surface 14 produced in accordance with an embodiment. Mating wires 15 are mechanically fixed in position by locking inserts 16 and 17 and terminate in spring portions 18, also surfaced with a compound contact layer of an embodiment. Upon inserting portion 1 B in 1A as shown by arrows 19, spring portions 14 and 18 are brought into mechanical and electrical contact.FIGS. 2A and 2B are sectional and elevation views depicting a circuit board connector. FIG. 2A shows a printed circuit board receptacle 20 provided with a spring contact 21 having a substrate 22 and contact layer 23 in accordance with an embodiment. In installation, circuit board 25 of FIG. 2B is inserted into recess 24 of receptacle 20. Contact is completed when compound contact 26 engages contact layer 23. Contact 26 is produced by in situ reaction with substrate 27. FIG. 3 is a sectional view of a momentary makebreak contact in use. Compound contact regions 30 and 31 produced on substrates 32 and 34 are brought into compressive contact upon distortion of element 33 from the configuration shown in solid outline to the configuration shown in phantom. FIG. 4 is an elevation view of a wire wrapped connector consisting of post 40 provided with compound contact layer 41.Wire 42 may be conventional gold plated, or surfaced with a layer in accordance with an embodiment.

Claims (23)

1. Apparatus comprising mating electrical contacts at least one of which comprises a substrate and a contact layer in intimate contact with the said substrate, the said contact layer having a first mating surface wherein physical contact with the said mating surface results in electrical connection, the said contact layer comprises a first crystallographic phase that is produced by a procedure comprising in situ reaction using energies associated with conventional ambient to produce a chemical compound of which the said first crystallographic phase is comprised, the said reaction involving reactants including a first reactant which is an integral part of the said substrate, the said first reactant being designated substrate reactant and a second reactant which is introduced during reaction.

2. Apparatus of claim 1, wherein the said first crystallographic phase comprises at least 85 percent by weight of the said chemical compound.

3. Apparatus of claim 2, wherein the said first crystallographic phase constitutes at least 50 percent of the area of the said first mating surface.

4. Apparatus of claim 3, wherein said contact layer includes a second phase which contains material introduced subsequent to the said reaction.

5. Apparatus of claim 2, wherein the said crystallographic phase constitutes substantially the entirety of the said area.

6. Apparatus of any one of claims 1,2,3,4 or 5, wherein the said chemical compound is selected from sulfides, selenides, carbides, nitrides, borides, phosphides and silicides.

7. Apparatus of claim 6, wherein the said substrate reactant is an element selected from copper, titanium, tantalum, molybdenum, chromium, niobium, vanadium, hafnium, zirconium, yttrium, lanthanum-group metals, cobalt, nickel and iron.

8. Apparatus of claim 6, wherein the average thickness ofthe said contact layer is less than 120 micrometers.

9. Apparatus of claim 8, wherein the bulk resistivity of the said contact layer is a maximum of 1 x 10-3 -cm.

10. Apparatus of claim 5, wherein the said compound is selected from sulfides, selenides, carbides, nitrides, borides, phosphides and silicides of copper, titanium, tantalum, molybdenum, chromium, niobium, vanadium, hafnium, zirconium, yttrium, lanthanum-group metals, cobalt, nickel and iron.

11. Apparatus of claim 1, wherein normal operation contemplates continued closure between the said mating surfaces.

12. Apparatus of claim 1, wherein normal operation contemplates intermittent closure between the said mating surfaces.

13. Apparatus of claim 1, wherein normal operation contemplates an electrical contact current of a maximum of 50 milliamperes.

14. Apparatus of claim 11, wherein the said current does not exceed 20 milliamperes.

15. Apparatus of claim 12, wherein the said current does not exceed 10 milliamperes.

16. Apparatus of claim 1, wherein the said second reactant is introduced in the vapor phase.

17. Apparatus of claim 15, wherein the said second reactant is above its vaporization temperature.

18. Apparatus of claim 15, wherein the said second reactant is below its vaporization temperature and is introduced with a carrier.

19. Apparatus of claim 1, wherein the said second reactant is introduced in a liquid phase.

20. Apparatus of claim 18, wherein the said second reactant is molten.

21. Apparatus of claim 18, wherein the said second reactant is in solution.

22. Apparatus of claim 1, wherein normal contemplated operation is nonarcing.

23. An electrical contact prepared in accordance with and substantially as hereinbefore described with reference to any one of the examples.