Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2001-03-02
2002-10-22
Nguyen, Tuan H. (Department: 2873)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S603000, C438S105000, C257S743000
Reexamination Certificate
active
06468890
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a semiconductor device with ohmic contact-connection and also to a method for the ohmic contact-connection of a semiconductor device.
The invention relates, in particular, to a semiconductor device which is composed of silicon carbide (SiC) at least in a semiconductor region which is contact-connected.
Silicon carbide in monocrystalline form is a semiconductor material having outstanding physical properties. On account of its high breakdown field strength, SiC is a semiconductor material of interest, inter alia, particularly for power electronics, even for applications in the kV range. Owing to the large energy gap, which also enables emission or detection of short-wave light in the blue or ultraviolet spectral region, SiC also constitutes a promising semiconductor material for optoelectronics.
Since the commercial availability of wafers made of monocrystalline silicon carbide, in particular of the 6H and 4H polytypes, and also the technological understanding of SiC have increased, SiC components are now also receiving more and more attention. Thus, a description has already been given e.g. of a Schottky diode, a pn diode, various transistors such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), a MESFET (Metal Semiconductor Field Effect Transistor) or a JFET (Junction Field Effect Transistor), but also an LED (Light Emitting Diode), a laser diode or a photodetector, in each case based on silicon carbide.
In each case at least one stable ohmic contact on a semiconductor region made of SiC is required for the functioning of these components. This at least one ohmic contact is situated e.g. on a back side of the wafer. In this case, the lowest possible contact resistance in conjunction with the smallest possible contact area is sought in order to avoid undesirable losses at the semiconductor-metal junction.
The review articles “
Ohmic contacts to SiC”
by G. L. Harris et al. from “
Properties of Silicon Carbide”
ed. by G. L. Harris
INSPEC,
1995, pages 231-234 and “A critical review of ohmic and rectifying contacts for silicon carbide” by L. M. Porter and R. F. Davis, Materials Science and Engineering, B34, 1995, pages 83-105 contain summaries of contact-connection methods for silicon carbide of different polytypes and conduction types. Accordingly, nickel has been used the most often hitherto as a contact material for an ohmic contact on n-conducting SiC. After the nickel material has been applied, a heat-treatment process for forming the ohmic contact is usually performed at a process temperature of above 900° C. The lowest documented contact resistance for n-conducting SiC is 1.10
−6
&OHgr;cm
2
. In this embodiment, the nickel contact is heat-treated for five minutes at 1000° C. However, the substrate which has undergone ohmic contact-connection is composed of n-conducting 6H—SiC having a high dopant concentration of 4.5·10
20
cm
−3
, which is not very practicable. The nickel contact is situated on the (000{overscore (1)}) face, i.e. on the carbon face, of the 6H—SiC substrate.
U.S. Pat. No. 3,510,733 describes an ohmic contact between a lead wire and a semiconductor region made of n-conducting silicon carbide. The lead wire is composed either of pure chromium or of 20% chromium and 80% nickel or of 15% chromium, 60% nickel and 25% iron or of stainless steel with a material portion of from 11 to 20% chromium, up to 12% nickel, up to 2% magnesium, up to 1% silicon, up to 0.3% carbon and with a main proportion of iron. The main requirement placed on the material of the lead wire and of the ohmic contact is sufficient ductility and resistance to oxidation even at a high temperature. Connecting the cross-sectional area of the lead wire to the semiconductor region results in a punctiform contact delimited, in particular, by the wire geometry, such as e.g. diameter. The lead wires used have a diameter of 0.0508 mm and 0.127 mm. The ohmic connection between the lead wire and the semiconductor region made of silicon carbide is achieved by heating above the melting point of the materials used. These temperatures lie between 1500 and 1900° C. However, the process conditions during the production of a semiconductor device made of silicon carbide often limit a maximum permissible process temperature for achieving an ohmic contact to a distinctly lower value.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a semiconductor device with ohmic contact-connection and method for the ohmic contact-connection of a semiconductor device that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which the favorable properties of silicon carbide are utilized to practical advantage and the contact-connection of n-conducting SiC is improved in comparison with the prior art.
It is also an object of this invention, to provide a method for the ohmic contact-connection of a semiconductor device that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type. In particular, the contact on the semiconductor region is intended to be heated during the heat-treatment process to a lower process temperature than in the prior art, but at least a comparable contact resistance to that in the prior art is nonetheless intended to be achievable.
With the foregoing and other objects in view there is provided, in accordance with the invention a semiconductor device with ohmic contact-connection comprising at least
a semiconductor region made of n-conducting silicon carbide,
adjoining the semiconductor region a largely homogeneous ohmic contact layer made of a material having a first component able to form a silicide at a process temperature of at most 1000° C., and a second component able to form a carbide at a process temperature of at most 1000° C.,
and a junction region extending into the semiconductor region and into the ohmic contact layer, containing a silicide formed from the first component and the silicon of the silicon carbide and a carbide formed from the second component and the carbon of the silicon carbide.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the contact-connection of a semiconductor device which comprises
applying a largely homogeneous ohmic contact layer made of a material having a first component able to form a silicide and a second component able to form a carbide to a semiconductor region made of n-conducting silicon carbide, the two material components being applied simultaneously, and
subjecting the structure comprising semiconductor region and ohmic contact layer to a heat-treatment process with heating to a process temperature of at most 1000° C.,
whereby a silicide is formed from the first material component and the silicon of the silicon carbide, and a carbide is formed from the second material component and the carbon of the silicon carbide, in a junction region extending into the semiconductor region and into the ohmic contact layer.
The invention is based on finding that it is possible to produce a stable ohmic contact to a semiconductor region made of n-conducting silicon carbide with very low contact resistance from a material comprising two material components, if one material component forms a silicide with the silicon of the silicon carbide and the other material component forms a carbide with the carbon of the silicon carbide. Consequently, a quaternary material system comprising the silicon and the carbon of the silicon carbide and also the two material components is present in the junction region extending both into the semiconductor region and into the ohmic contact layer. Consequently, after severing of the atomic bond between the silicon and the carbon of the silicon carbide at an elevated temperature, there is available for each of the two elements a new bonding partner with which a silicide and a carbide, respectively, can be formed.
This silicide and carbide formation essen
Bartsch Wolfgang
Schörner Reinhold
Stephani Dietrich
Greenberg Laurence A.
Locher Ralph E.
Nguyen Tuan H.
Siced Electronics Development GmbH & Co. KG
Stemer Werner H.
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