Silicon carbide (SiC), one of wide gap semiconductors, is drawing attention as a material for implementing high-frequency power devices, and heat-proof, radiation-proof devices. Researches on silicon carbide semiconductor devices such as MOSFETs have been actively conducted because an oxide film (SiO2) can be formed from silicon carbide using the same method as a method for forming an oxide film from silicon (Si). Further, SiC has a wider band gap and a larger dielectric breakdown electric field strength than those of Si. Thus, a semiconductor device employing SiC can achieve more excellent switching characteristics and a larger breakdown voltage than those of a semiconductor device employing Si, for example.
Generally, in such a SiC semiconductor device, a wire (pad) is provided to facilitate extraction of an electric signal from an electrode formed on a substrate thereof. Such a wire is employed in not only the SiC semiconductor device but also a Si semiconductor device in order to smoothly exchange electric signals with an external component.
FIG. 35 is a schematic cross sectional view showing a state between an electrode and a wire in a general SiC semiconductor device. As shown in FIG. 35, in a SiC semiconductor device 99A, a SiC semiconductor substrate 99 has one main surface having an electrode 98 provided thereon. In SiC semiconductor device 99A, carbon 97 may be deposited on an opposite main surface of electrode 98 to SiC semiconductor substrate 99. Here, electrode 98 is an ohmic electrode making ohmic contact with SiC semiconductor substrate 99. It should be noted that the term “main surface” refers to a surface having the largest area among flat surfaces constituting surfaces. Carbon 97 (C) thus deposited prevents a wire 96 from being directly in contact with electrode 98 in a region where carbon 97 exists, upon forming wire 96 on the opposite main surface of electrode 98 to SiC semiconductor substrate 99. Thus, carbon 97 deteriorates adhesion between electrode 98 and wire 96. This leads to a failure such as detachment of wire 96 from electrode 98, thereby affecting durability and electric characteristics of SiC semiconductor device 99A.
In order to solve the above-described problem, it is considered to utilize a structure in which an alloy layer of Ni and Si is formed on SiC substrate 95 as disclosed in Japanese Patent Laying-Open No. 7-99169 (hereinafter, referred to as “Patent Literature 1”), for example. FIG. 36 is a schematic cross sectional view showing the structure in which the alloy layer of Ni and Si is formed on a SiC semiconductor layer. Patent Literature 1 discloses that in an electronic device 95A shown in FIG. 36, Ni—Si alloy layer 94 is formed on SiC substrate 95 and is then subjected to thermal treatment to have a function as an ohmic electrode. Further, Patent Literature 1 discloses to form an ohmic electrode by providing thermal treatment to a stacked structure obtained by forming a Si layer on SiC substrate 95 and forming a Ni layer on the Si layer.
It is known that carbon 97 shown in FIG. 35 is formed in the following manner. That is, the thermal treatment in forming electrode 98 causes reaction between a metal constituting electrode 98 and SiC of SiC semiconductor substrate 99, thereby generating C (carbon) from SiC as a residue, which is then deposited on the surface of electrode 98. In view of this, in order to attain the ohmic contact, Patent Literature 1 discloses to form Ni—Si alloy layer 94, the alloy of metal (Ni) and Si, on the main surface of SiC semiconductor substrate 99, thereby forming electronic device 95A shown in FIG. 36. Patent Literature 1 also discloses to form electronic device 95A shown in FIG. 36 in the following alternative manner: a Si layer is formed on the main surface of SiC semiconductor substrate 99, a Ni layer is provided on the Si layer, and then thermal treatment is provided thereto.
Normally, reaction temperature for silicidation (alloying) of Si and a metal is lower than that of SiC and the metal. The reaction temperature of Si and the metal is considered to be lower due to the following reason. That is, for the silicidation reaction of SiC and the metal, bonds between Si and C in SiC need to be broken. On the other hand, for the silicidation reaction of Si and the metal, bonds do not need to be broken. Accordingly, energy required for the silicidation reaction between Si and the metal becomes smaller.
Accordingly, the reaction of Si and Ni proceeds at the upper side (upper surface side of SiC substrate 95 in FIG. 36) in the structure while increasing temperature to provide the thermal treatment. When the reaction of Si and Ni proceeds to complete the silicidation, silicidation resulting from the reaction of Si of SiC with Ni is prevented, thereby preventing C from being generated due to the reaction between Si of SiC and Ni. Hence, C is most unlikely to reach the uppermost surface of the structure (the uppermost surface of the Ni—Si alloy layer in FIG. 36). Accordingly, it is considered that substantially no C is deposited on the surface of the ohmic electrode.