Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2000-02-18
2001-05-08
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S473000, C438S516000, C438S524000
Reexamination Certificate
active
06228720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a insulated-gate semiconductor element using a silicon carbide substrate, and more particularly, to a vertical insulated-gate semiconductor element having a so-called trench structure. Furthermore, the present invention relates to a method for manufacturing such an insulated-gate semiconductor element.
2. Description of the Prior Art
In insulated-gate semiconductor elements using silicon carbide, the surface of a silicon carbide substrate is oxidized to form a silicon oxide film (oxide film), which is used as a gate insulator. Vertical silicon carbide insulated-gate semiconductor elements with a trench structure have been developed as high-power elements having high breakdown voltage and large current capacities.
FIG. 5
shows a cross section of a conventional vertical silicon carbide insulated-gate semiconductor element. This semiconductor element is made using a silicon carbide substrate as shown in FIG.
6
. For this silicon carbide substrate, an n-type epitaxial layer
102
and a p-type epitaxial layer
103
are formed by CVD on top of a conductive n
+
-type substrate
101
of silicon carbide. On the surface of this substrate, an n
+
-layer
104
is partially formed by local ion implantation and annealing. As a result, a layered structure of n
+
/p
is formed in that order from the surface in the silicon carbide substrate. To obtain the trench structure by photo-lithography and etching from the surface of the substrate, a concave portion
105
is formed in this layered structure.
A silicon oxide film (oxide film)
110
is formed by oxidation on the substrate. The oxide film
110
is etched and removed except at side surfaces
111
of the concave portion (i.e. wall surfaces of the trench structure), a bottom surface
109
of the concave portion (i.e. the bottom surface of the trench structure), and a substrate surface
106
near the concave portion. A gate electrode
112
and an insulating film
116
are formed on top of the oxide film
110
. Then, a source electrode
113
and a drain electrode
114
are formed on the two surfaces of the substrate
101
. The channel
115
, which is switched on and off by applying a voltage to the gate electrode
12
, is formed at the interface between the p-type epitaxial layer
103
and the oxide layer
110
.
This conventional technique is described, for example, in Silicon Carbide; A Review of Fundamental Questions and Applications, edited by W. J. Choyke, H. Matsunami, and G. Pensl, Akademie Verlag, 1997, Vol. II, pp. 369-388.
Silicon carbide has different oxidation speeds depending on the crystal orientation. For example, the (0001) Si-face of &agr;-SiC has the slowest oxidation speed, whereas the (000{overscore (1)}) C-face of &agr;-SiC attained by rotation for 180° has the highest oxidation speed. Therefore, when the concave portion is formed and the substrate including surfaces corresponding to several different crystal orientations is oxidized, the thicknesses of the formed oxide films will be different. When the thickness of the oxide film is not uniform throughout the trench structure, the electric field created in the oxide film depending on the voltage applied to the gate electrode also will be not uniform.
When the surface of the silicon carbide substrate is the (0001) Si-face of &agr;-SiC, an epitaxial layer with superior crystallinity can be obtained. However, when an insulated-gate semiconductor element is made using this surface, a relatively thin oxide film
110
is formed on the substrate surface
106
and the trench bottom surface
109
, and a relatively thick oxide film
110
is formed on the trench wall surfaces
111
, as shown in FIG.
5
. In this situation, the electric field applied to the oxide film on the trench bottom surface
109
is larger than the electric field applied to the channel portion
115
of the trench wall surfaces
111
. Therefore, if an oxide film of the necessary thickness to maintain the breakdown voltage is formed on the trench bottom surface, an even thicker oxide film will be formed near the channel
115
, which results in the problem that the response efficiency of the element with regard to the gate voltage is inferior.
If, on the other hand, the thickness of the oxide film
110
at the trench wall surfaces
111
is adjusted in consideration of the response time of the element, then the oxide film
110
at the trench bottom surface
109
becomes thin, and the breakdown voltage of the element decreases.
To make such an element using the (000{overscore (1)}) C-face of &agr;-SiC, a thick silicon oxide film is formed on the surface of the silicon carbide substrate and the trench bottom surface, and a thin silicon oxide film is formed on the trench wall surfaces. Such an insulated-gate semiconductor element is superior with regard to the distribution of the silicon oxide insulating film thickness, but the crystallinity of the epitaxial layer is inferior to that of an epitaxial layer formed on the (0001) Si-face of &agr;-SiC. Therefore, it cannot provide a semiconductor element with suitable properties.
Thus, with conventional insulated-gate semiconductor elements, it has been a problem to increase the breakdown voltage while maintaining good semiconductor element properties.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an insulated-gate semiconductor element with a high breakdown voltage while using a preferable surface of the silicon carbide substrate to attain superior semiconductor element properties. It is a further object of the present invention to provide a method for manufacturing an insulated-gate semiconductor element with high breakdown voltage while maintaining superior semiconductor element properties.
To achieve these objects, a method for manufacturing an insulated-gate semiconductor element comprises etching a surface of a silicon carbide substrate to form a concave portion on the surface; irradiating a particle beam from above the surface to form a defect layer in at least a bottom surface of the concave portion; heating the silicon carbide substrate in an oxidation atmosphere to form an oxide film of silicon carbide on at least a side surface of the concave portion and the bottom surface in which the defect layer has been formed; and forming a gate electrode on the oxide film.
In the region where the defect layer is formed, the oxidation speed is higher than in the region that is not subjected to defects. Consequently, this manufacturing method of the present invention improves the breakdown voltage by making the oxidation layer at the bottom surface of the concave portion thicker.
It is preferable that the surface of the silicon carbide substrate is a (111) Si-face of &bgr;-SiC or a (0001) Si-face of &agr;-SiC. An example of a (111) Si-face of &bgr;-SiC is the (111) Si-face of 3C-SiC, which is a cubic crystal. Examples of a (0001) Si-face of &agr;-SiC include the Si-faces of 4H (hexagonal crystal), 6H and of 15R-SiC (rhombohedral crystal). Using these Si-faces, epitaxial layers with superior crystallinity can be obtained. As will be explained in more detail below, with the manufacturing method of the present invention, the oxide film at the bottom surface of the concave portion can be made thicker than the oxide film at the side surfaces of the concave portion, while using these substrate surfaces.
It is preferable that the silicon carbide substrate includes a multi-layer structure of a silicon carbide substrate of a first conductivity type, a first layer of a first conductivity type formed on the silicon carbide substrate of the first conductivity type, and a second layer of a second conductivity type formed on the first layer of the first conductivity type. The first and second layers can be formed by ion implantation and are preferably epitaxial layers.
It is preferable that the concave portion is formed so that the side surface of the concave portion and the surface of the silicon carbide substrate define an angle between 80° and
Kitabatake Makoto
Takahashi Kunimasa
Uchida Masao
Uenoyama Takeshi
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Pham Long
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