Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
2000-12-08
2003-12-23
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C438S105000, C438S931000
Reexamination Certificate
active
06667495
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a semiconductor configuration with ohmic contact-connection, as well as to a method for contact-connecting a semiconductor configuration.
The invention relates, in particular, to a semiconductor configuration of the above-mentioned type which includes a predetermined polytype of silicon carbide at least in specific semiconductor regions, in particular semiconductor regions that are contact-connected.
Silicon carbide (SiC) in monocrystalline form is a semiconductor material having outstanding physical properties which make that semiconductor material appear to be of interest particularly for power electronics. That is the case even for applications in the kV range, inter alia due to its high breakdown field strength and its good thermal conductivity. Since the commercial availability of monocrystalline substrate wafers, especially ones made of 6H and 4H silicon carbide polytypes, has risen, silicon carbide-based power semiconductor components, such as e.g. Schottky diodes, are now also receiving more and more attention. Other silicon carbide components which are becoming increasingly widespread are pn diodes and transistors such as, for example, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
Stable ohmic contacts to semiconductor regions of different conduction types are indispensable for the functioning of those components. In that case, the lowest possible contact resistances are sought in order to minimize undesirable losses at the semiconductor-metal junction.
An overview paper entitled “
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 contains a summary of contact-connection methods for silicon carbide having different polytypes and conduction types. With regard to the contact-connection of n-conducting and p-conducting SiC, the overview paper and the cross-references cited reveal the current state of the art that is generally accepted by experts, as outlined below:
The above-mentioned overview paper only specifies methods in which silicon carbide having only a single conduction type in each case is provided with an ohmic contact.
The contact-connection of n-conducting SiC is accordingly effected through the use of a thin contact layer of a metal or through the use of a layer sequence of different materials. The contact layers are heat-treated at temperatures of between 600° C. and 1100° C. In particular transition metals such as nickel, for example, yield a very good ohmic contact after the thermal treatment on n-conducting, highly doped SiC. That is because at temperatures of around 1000° C., a metal silicide is formed from the transition metal and the silicon contained in the SiC. In comparison therewith, contact techniques for n-conducting SiC which work without a corresponding thermal treatment yield a relatively high contact resistance or a current-voltage characteristic that does not correspond to Ohm's law. Furthermore, the thermal treatment also has a positive effect on the thermal stability of the ohmic contacts being formed.
Aluminum is predominantly used for contact-connecting p-conducting SiC. Since aluminum is readily soluble in SiC and acts as an acceptor, a zone that is highly doped with aluminum can be produced in a boundary region between the aluminum-containing contact region and the semiconductor region made of SiC. In order to avoid evaporation of the aluminum, which melts at a temperature as low as 659° C., during a subsequent thermal treatment, at least one covering layer made of a material having a higher melting point, such as e.g. nickel, tungsten, titanium or tantalum, is applied on the aluminum.
A paper entitled “
Thermally stable low ohmic contacts to p
-
type
6
H
-
SiC using cobalt silicides
” by N. Lundberg, M. Östling from Solid-State Electronics, Vol. 39, No. 11, pages 1559-1565, 1996 discloses a method for contact-connecting p-conducting SiC which uses the formation of cobalt silicide (CoSi
2
). A very low contact resistance can be achieved with the method described and the contact material used.
A paper entitled “
Reduction of Ohmic Contact Resistance on n
-
Type
6
H
-
SiC by Heavy Doping
” by T. Uemoto, Japanese Journal of Applied Physics, Vol. 34, 1995, pages L7 to L9 discloses a layer structure being formed of a titanium layer having a thickness of 15 nm and an aluminum layer having a thickness of 150 nm as a possible ohmic contact both on p-conducting and on n-conducting silicon carbide. However, a good contact resistance on the n-conducting semiconductor region is attained only when the dopant concentration in the n-conducting semiconductor region is chosen to be very high. The disclosed dopant concentration of 4.5·10
20
cm
−3
is considerably above the dopant concentrations that are usually used in a silicon carbide semiconductor configuration at the present time. Such a high dopant concentration can only be produced with considerable additional outlay. Thus, during ion implantation, for example, there is the risk of the n-conducting semiconductor region being damaged.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a semiconductor configuration with ohmic contact-connection and a method for contact-connecting a semiconductor configuration, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provide improved contact-connection of n-conducting and p-conducting SiC in comparison with the prior art. In this case, the contacts on the n-conducting and p-conducting semiconductor regions in each case are intended both to have a low contact resistance and to be thermally stable. Moreover, only dopant concentrations which can be produced in a simple manner with currently available technologies are intended to be provided for the n-conducting and p-conducting SiC.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor configuration with ohmic contact-connection, comprising at least one first semiconductor region made of n-conducting silicon carbide, and at least one second semiconductor region made of p-conducting silicon carbide, the n-conducting and the p-conducting silicon carbide each having a dopant concentration of between 10
17
cm
−3
and 10
20
cm
−3
; at least one first contact region adjoining the first semiconductor region, and at least one second contact region adjoining the second semiconductor region; the first and second contact regions having an at least approximately identical material composition being practically homogeneous within the respective contact region; and the first and second contact regions formed of a material composed at least of a first and a second material component, the first material component being nickel and the second material component being aluminum.
With the objects of the invention in view, there is also provided a method for contact-connecting a semiconductor configuration, which comprises forming at least one first practically homogeneous contact region on a first semiconductor region made of n-conducting silicon carbide, and forming at least one second practically homogeneous contact region on a second semiconductor region made of p-conducting silicon carbide; applying an at least approximately identical material having a practically homogeneous material composition within each respective contact region for the first and second contact regions; providing each of the first and second semiconductor regions with a dopant concentration of between 10
17
cm
−3
and 10
20
cm
−3
; and forming the material at least of a first and a second material component, with nickel as the first material component and aluminum as the second material component.
In this case, the invention is based on the insight that, contrary to the customary procedure employed by experts, in which ohmic contact is made with n-conducting and p-conduc
Friedrichs Peter
Peters Dethard
Schörner Reinhold
Greenberg Laurence A.
Le Thao X.
Locher Ralph E.
Pham Long
SciCED Electronics Development GmbH & Co. KG
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