GB2130795A - Electrical contacts - Google Patents

Abstract

An electrical contact comprises a barrier layer 12, e.g. of titanium or zirconium, disposed on a conductive substrate 11 e.g. of phosphor bronze. The barrier layer 12 is protected by a conductive capping layer 13 of a nitride, silicide or carbide of a transition metal, e.g. titanium or zirconium nitride. The arrangement provides a high degree of wear resistance and reliability at a significantly lower cost than a conventional gold contact. A surface coating 14 of gold or gold alloy may be included. The layers are deposited by sputtering. <IMAGE>

Description

Electrical contacts This invention relates to electrical contacts such as one employed in switches, relays and connectors.

In order to minimise wear and degradation electrical contacts are frequently made from precious metals. In relay applications, for example, where a large number of reliable switching operations must be ensured, contacts of gold and/or silver alloys may be provided. In recent years the rapidly escalating costs of precious metals have forced contact manufacturers to consider the use of alternative lower cost materials. Such materials, however, generally incur a penalty in terms of reliability and performance.

The object of the present invention is to minimise or to overcome these disadvantages.

According to the invention there is provided an electrical contact comprising a conductive base portion, a metallurgically reactive barrier layer disposed on the base portion, and a protective capping layer comprising a nitride, carbide or silicide of a transition metal.

Typically the contact may comprise a titanium or zirconium barrier layer capped with titanium nitride or zircontium nitride. In some applications a very thin precious metal contact layer may be provided on the capping layer for lower contact resistance.

The arrangement provides a stable contact material that allows a significant cost saving over conventional precious metal contacts. For example the ratio of bulk prices of gold and titanium is approximately 500:1. Furthermore, by offering improved diffusion barrier properties, there is a consequent reduction in the required material thickness. In particular this allows a significant reduction in thickness of any precious metal surface film as the presence of pinholes in the film is no longer detrimental.

The integrity of films grown by any deposition process, e.g. plating or sputtering, depends critically on the detailed microstructure of the substrate surface. This is a well-known problem area in LSI circuit metallizations: sharp steps, even of submicron size, interfere with the film growth during deposition and produce grown-in microcracks which may propagate throughout the thickness of the film. Such sharp steps are common features on all contact substrates; here the situation is aggravated even further by surface impurities and oxide inclusions. Coatings on these substrates therefore exhibit high densities or microcracks, unless thicknesses of several micrometers are used. These microcracks are extremely fast diffusion sites in the commonly used inactive barrier materials, for both metals and corrosive gases.Nickel, for instance, does not interact with copper-based substrates, and copper, as well as gold, can easily diffuse along such defects. Nickel also has inadequate resistance against corrosive gases.

We have found that the situation is entirely different with either Ti-Ti N or Zr-ZrN bi-layers.

Both titanium and zirconium form intermetallic compounds with copper, and such compounds form preferentially at the high-energy sites, i.e. the microcracks and grain boundaries. This significantly reduces the diffusion constant in the layers and effectively "plugs" those sites. This effect also reduces the energy gain in a corrosive reaction.

Furthermore titanium and zirconium metal have outstanding corrosion resistance. This is due to the formation of very stable thin oxide layers which further helps to "plug" the microcracks at the surface where they penetrate through the nitride capping layer.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a crosssectional view of a barrier layer contact.

Referring to the drawing the contact, which may comprise a relay contact, includes a base portion 11 on which is mounted a barrier layer 12 provided with a protective capping layer 13.

Typically the base portion 11 comprises a phosphorus bronze spring strip, e.g. a relay contact spring.

The barrier layer 12 comprises a metallurgically active layer of a high melting point, hard transition metal. Preferably this layer 1 2 comprises titanium, zirconium or mixtures thereof.

The active layer 12 is protected by a conductive refractory capping layer 13 which layer may comprise a nitride, carbide or silicide of a transition metal, advantageously the same metal as that used for the barrier layer. The effect of this capping layer 13 is to render the surface chemically and electrically stable, and to prevent metallurgical reactions with any subsequent precious meta! layers.

In some applications the capping layer 1 3 may be coated with a thin, e.g. 0.2 microns, surface layer 14 of gold or a gold alloy. This surface layer is considerably thinner than the coatings applied to conventional contacts as there is no necessity to provide a pin-hole free layer.

Advantageously the contact is formed by magnetron sputtering although other vacuum techniques can of course be employed. Magnetron sputtering offers extremely high deposition rates whilst maintaining the substrate temperature low enough to avoid degradation of the contact spring properties.

By using a titanium or zirconium target, a smooth transition from the metal film to its nitride is feasible by a mere change in the sputter gas composition from pure argon to a nitrogen/argon mixture. This ensures maximum adhesion and freedom from interface defects.

A preferred contact structure comprises phosphor bronze as the base layer, typically 0.25-0.5 mm in thickness on to which is deposited a layer of Titanium metal (by sputtering) typically 0.2-2 microns in thickness, with a final layer of Titanium Nitride (by sputtering) typically 0.05-0.5 microns thick.

in a typical contact fabrication sequence a phosphor bronze strip is loaded into a CVC 601 Magnetron Sputterer, the background pressure reduced to 2 x 10-5 Torr, and pure oxygen is admitted to 10-3 Torr pressure. A DC plasma is then struck for 10 minutes to clean the contact area surfaces prior to depcsition. After cleaning, the discharge is extinguished and the pressure reduced to below 5 x 10-5 Torr. Argon gas is then admitted to a pressure of 2.10-5 Torr. After sputter cleaning a Titanium target for 2 minutes, the contacts are exposed to the sputter target until the desired titanium film thickness has been deposited. Typical sputter condition are 1-10 amp, 400 volts D.C. target bias, 900 volts R.F.

substrate bias, giving a deposition rate of 1000 to 3000 Angstroms per minute. The argon flow rate into the chamber is then reduced whilst maintaining the overall pressure at 2.10-5 Torr by gradual admittance of Nitrogen gas. The transition should be accomplished smoothly over a period of approximately 1 minute. Titanium Nitride then deposited by reactive sputtering, at a rate of approximately 200 A per minute. The deposition rates of both Ti and TiN may be varied by factor of 3 depending on Argon and Nitrogen replenishment rate and target sputter current.

The passivation ("plugging") described above of the active diffusion sites in the metallurgically reactive metal film has little or no effect on the contact resistance of the finished contacts as these sites are, necessarily, parallel to the current path. The main surface remains protected by the capping nitride layer which is phased during deposition for interfacial stability.

The nitride layers are extremely resistant to corrosive attack, except for oxygen-containing environments at temperatures in excess of 10000 C. Oxidation at ordinary temperatures, however, is negligible and of little significance because of the high electrical conductance of the suboxides that might form. Even TiO for example has a resistivity of only 100 microhm cm.

Due to the combination of chemical inertness, wear resistance, low friction coefficient and good electrical conductivity, TiN and ZrN layers are very good contact materials in their own right and an attractive replacement for gold where stability of resistance is of prime interest. Typically we have measured contact resistance values between sputter deposited TiN layers ranging between 50 and 90 mP, with 10 grams contact force.

Alternatively, the Ti-TiN or Zr-ZrN system may be regarded as an ideal barrier system for the use of e.g. extremely thin gold coatings and other materials. The gold coatings do not need to be continuous and may be compared in a way to the function of mercury in mercury-wetted relays. The gold coatings may be applied in the same sputter deposition plant by a mere change of the target. A typical coating sequence may, therefore, include the stages Ti 0.5 ym, TiN 0.2 ,um, with [N2J when finishing off at the surface, to provide a titaniumrich surface as a key for an additional 0.2 ,um Au layer.

Claims (12)

Claims

1. An electrical contact comprising a conductive base portion, a metallurgically reactive barrier layer disposed on the base portion, and a protective capping layer comprising a nitride, carbide or silicide of a transition metal.

2. A contact as claimed in claim 1, wherein said barrier layer comprises titanium, zirconium or mixtures thereof.

3. A contact as claimed in claim 1 or 2, wherein said capping layer comprises titanium or zirconium nitride.

4. A contact layer as claimed in any one of claims 1 to 3, wherein said layers are vacuum sputtered layers.

5. A contact as claimed in any one of claims 1 to 4 and which includes a surface coating of a precious metal.

6. An electrical contact substantially as described herein with reference to the accompanying drawing.

7. A method of making an electrical contact, including depositing on a conductive substrate a metallurgically reactive barrier layer and a conductive capping layer overlying the barrier layer and comprising a nitride, carbide or silicide of a transition metal.

8. A method as claimed in claim 7, wherein said layers are deposited by vacuum sputtering.

9. A method as claimed in claim 8, wherein said capping layer is formed by sputtering from a metal target in the presence of gaseous nitrogen.

10. A method as claimed in claim 7, 8 or 9, wherein said capping layer is covered by a relatively thin layer of a precious metal.

11. A method of making an electrical contact substantially as described herein with reference to the accompanying drawings.

12. A contact made by a process as claimed in any one of claims 7 to 11.

1 3. A switch or relay incorporating one or more contacts as claimed in any one of claims 1 to 6 or claim 12.