GB2032174A - Electrical Contact Layers on Silicon Semiconductor Devices - Google Patents

Abstract

The formation of an unwanted oxide layer on the exposed surface of an aluminium contact of a semiconductor device is prevented by incorporating a proportion of silver in the aluminium surface region. The silver layer may be formed by alloying silver foil onto a conventional pure aluminium contact. Alternatively, sintering or vapour deposition techniques may be employed.

Description

SPECIFICATION Improvements in or Relating to Electrical Contact Layers on Silicon Semiconductor Devices This invention relates to electrical contact layers on silicon semiconductor devices.

In the field of silicon semiconductor devices, particularly high power devices comprising a single descrete device or a few integrated devices on a single silicon wafer, (for example, devices in Darlington configuration), it has been proposed that electrical connections be made to the device by sandwiching the device between metallic electrodes under compressive pressure. Since electrical contact directly to silicon has been found to be undesirable, it has been proposed to alloy or diffuse a metal onto or into the contact areas of the surface of the silicon wafer before bonding of the electrical contacts thereto.One such proposed arrangement comprises a silicon wafer formed in known manner into a semiconductor device, which may, for example, be a diode, a transistor, a thyristor, and at least one contact area which (for example, an anode or a cathode) has a thin layer of aluminium bonded thereto to present an aluminium metal surface to the contact electrode. However, as is well-known, aluminium metal readily forms a high-electricalresistance surface oxide layer, leading to high resistance joints and terminations in aluminiumcored cables with consequent over-heating when electric current crosses the oxide layer.Thus, when aluminium is used as a contact material in semiconductor devices, it can produce poor electrical connections, undesirable voltage-drops (which may even affect the semiconductive action of the device), and unwanted heating effects with consequent loss of electric power and necessary de-rating of the device.

The present invention proposes a composite electrical contact layer on a silicon semiconductor device which obviates or at least mitigates the undesired electrical effects of surface oxide on a relatively pure aluminium contact layer.

According to a first aspect of the invention, there is provided a semiconductor device having an electrode electrically connected to the surface of a semiconductor region of the device by means of an intermediate composite contact layer comprising the metals aluminium and silver, the composition of the composite layer in the vicinity of the semiconditor region being substantially wholly aluminium and in the vicinity of its opposite surface comprising at least sufficient silver to substantially prevent the formation of a surface layer of aluminium oxide.

According to a second aspect of the invention, there is provided a method of producing a semiconductor device having such an intermediate composite contact layer including the steps of depositing a layer of aluminium on the surface of the semiconductor region and subsequently depositing a layer of silver onto the exposed surface of the aluminium layer.

In order that the invention be more clearly understood and readily put into effect, preferred embodiments of the same will now be described by way of example.

Firstly, a silicon semiconductor device was fabricated conventionally as a high power diode in the form of a thin circular wafer with Cathode and Anode areas formed on the opposing major faces of the wafer. Suitably shaped and dimensioned foils of aluminium were sintered into each face, the aluminium thereby forming an electrical connection with the silicon in known manner.

Then a thin, suitably shaped and dimensioned foil of silver was applied to each aluminium layer, and sufficient heat was applied to cause the silver to migrate or diffuse into the aluminium layer, therein to form a mixture or alloy of silver and aluminium which had good surface electrical contact properties relative to substantially pure aluminium due to lack of surface oxide. Thus, when electrodes were applied under pressure to the cathode and anode contact layers, good electrical connections were made, especially in comparison to contacts made to surface-oxidised pure aluminium contact layers.

As an alternative to directly sintering the aluminium into the silicon, the aluminium could be diffused into the intended area or areas by evaporation by a known technique, followed optionally by evaporation of silver as an alternative to direct application. Alternatively, both the aluminium and silver could be evaporated practically simultaneously, the silver not at first being evaporated to avoid its being applied directly to the silicon because silver does not readily adhere to silicon, but as soon as sufficient aluminium has bonded onto or diffused into the silicon, this would form a base for simultaneous deposition of aluminium and silver to form a more-or-less homogeneous contact layer. Evaporation would be timed in either alternative to cease when a sufficient layer thickness had formed on the silicon.

Not every contact layer need be formed in accordance with the invention, since at least one conventional contact layer might be desired or necessary in some cases.

Secondly, a silicon semiconductor device fabricated in known manner as a power-switching device, i.e., a transistor or a thyristor, which, being three-terminal devices, require three separate contact layers, possibly of mutually different sizes, had non-contact-making surface areas suitably masked, and aluminium together with or followed by silver were evaporated onto the contact areas of the threeterminal device to form three good electrode contact areas in accordance with the invention. The masking was then removed as necessary, but could be left in place if no adverse effect on the electrical characteristics of the device would be caused thereby. Alternatively, the aluminium and silicon can both be applied by direct contact and suitable heating, or one metal may be evaporated and the other metal directly applied.

Claims (12)

Claims

1. A semiconductor device having an electrode electrically connected to the surface of a semiconductor region of the device by means of an intermediate composite contact layer comprising the metals aluminium and silver, the composition of the composite layer in the vicinity of the semiconductor region being substantially wholly aluminium and in the vicinity of its opposite surface comprising at least sufficient silver to substantially prevent the formation of a surface layer of aluminium oxide.

2. A semiconductor device according to Claim 1, wherein the said opposite surface of the composite conducting layer carries a further layer of silver of sufficient thickness to prevent formation of aluminium oxide on the underlying aluminium surface.

3. A semiconductor device according to either Claim 1 or 2, wherein the composite layer away from the vicinity of the semi-conductor region comprises a substantially homogeneous alloy of aluminium and silver.

4. A method of producing a semiconductor device having an intermediate composite conducting layer according to Claim 1 or 2, including the steps of depositing a layer of aluminium on the surface of the semiconductor region and subsequentially depositing a layer of silver onto the exposed surface of the aluminium layer.

SWA method according to Claim 4, wherein the aluminium layer is deposited onto the semiconductor surface by sintering of aluminium foil.

6. A method according to Claim 4 or 5, wherein the silver layer is deposited on the aluminium layer by heating silver foil, pressed onto the exposed surface of the aluminium layer, sufficiently to cause the formation of aluminium/silver alloy in at least the contact region of the dissimilar metal layers.

7. A method according to Claim 4, wherein the aluminium layer is deposited by evaporation.

8. A method according to Claim 4 or 7, wherein the silver layer is deposited by evaporation.

9. A method according to Claim 4, wherein both the aluminium layer and the silver layer are deposited by evaporation.

10. A method according to Claim 9, wherein evaporation of aluminium is commenced initially and thereafter evaporation of aluminium and silver proceeds simultaneously.

1 1. A semiconductor device substantially as herein described.

12. A method of producing a semiconductor device substantially as herein described.