Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – On insulating substrate or layer
Reexamination Certificate
2000-09-22
2002-01-01
Elms, Richard (Department: 2824)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
On insulating substrate or layer
C438S149000, C438S156000, C438S165000, 43, C257S295000, C257S396000
Reexamination Certificate
active
06335267
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor substrate and a method of fabricating a semiconductor device and, more particularly, to a semiconductor substrate and a method of fabricating a semiconductor device which prevent particles of dust from being produced at an edge of the substrate.
2. Description of the Background Art
An SOI (silicon on insulator) device including a semiconductor element formed on an SOI substrate is superior to a bulk device in its decreased junction capacitance and improved device isolation breakdown voltage, but has inherent problems to be described below.
FIG. 40
is a sectional view of an SOI substrate
10
. The SOI substrate
10
has a triple layer structure comprising a silicon substrate
1
, a buried oxide film
2
formed in an upper major surface of the silicon substrate
1
, and a single crystalline silicon layer (referred to hereinafter as an SOI layer)
3
formed on the buried oxide film
2
. A polysilicon layer
4
is formed on edges and a lower major surface of the single crystalline silicon substrate
1
. The polysilicon layer
4
is provided to getter contaminants such as heavy metal provided during wafer fabrication steps and a transistor wafer process. A structure having such a polysilicon layer is known as a poly-back-coat structure (PBC structure).
Methods of fabricating the SOI substrate include a SIMOX (separation by implanted oxygen) method and a bonding method. The SOI substrate fabricated by the SIMOX method (SIMOX substrate) is employed as an example in the following description.
In the SIMOX method, oxygen ions are implanted into a single crystalline silicon substrate at a dose of, for example, 0.4×10
18
/cm
2
to 3×10
18
/cm
2
, and thereafter the silicon substrate is annealed at a temperature of about 1350° C. to provide the SOI structure.
FIG. 41
is a partial detailed view of an edge of the SOI substrate
10
. For purposes of explanation, the semiconductor substrate is divided into four sections: an upper major surface (on which semiconductor elements are to be formed), a central section of the upper major surface (including active regions), an edge section including a section surrounding the central section and side surfaces, and a lower major surface.
FIG. 41
shows an area X in detail in which the buried oxide film
2
and the SOI layer
3
meet the polysilicon layer
4
. As illustrated in
FIG. 41
, since the edge section has a curved surface having a great curvature, vertically directed oxygen ions are implanted in a slanting direction into the edge section, decreasing an effective implantation energy in the edge section. The result is the reduction in the thickness of the buried oxide film
2
and the SOI layer
3
in the edge section, creating a structure wherein the SOI layer
3
is prone to exfoliate.
Additionally, the step of thinning the SOI layer
3
during the fabrication of the SOI device promotes the exfoliation of the SOI layer
3
. The step of thinning the SOI layer
3
is described with reference to
FIGS. 42 and 43
.
The SOI layer
3
in the SOI substrate
10
has a suitable thickness as shown in
FIG. 42
when the substrate is fabricated. The step of thinning the SOI layer
3
is to suitably reduce the thickness of the SOI layer
3
in accordance with the specs of a desired semiconductor device, and comprises oxidizing the SO
1
layer
3
and removing the resultant oxide film to adjust the thickness of the SOI layer
3
.
FIG. 43
shows an oxide film
5
formed on the SOI layer
3
. The thickness of the oxide film
5
is generally determined based on the thickness of the SOI layer
3
in the central section of the SOI substrate
10
, that is, semiconductor element formation regions (active regions). The problems that arise herein are the reduced thickness of the SOI layer
3
in the edge section of the SOI substrate
10
as above described, and the formation of the polysilicon layer
4
in the edge section of the SOI substrate
10
. An area Y shown in
FIG. 42
is illustrated in more detail in
FIG. 44
, and an area Z shown in
FIG. 43
is illustrated in more detail in FIG.
45
.
FIG. 46
shows the edge section after the removal of the oxide film
5
.
As illustrated in
FIG. 44
, the polysilicon layer
4
is comprised of a multiplicity of single crystal grains GP. Because of individually different crystal orientations of the single crystal grains GP, the oxygen ions are implanted to different depths due to channeling, causing the buried oxide film
2
to be formed at varied depths.
Further, different oxidation rates of the polysilicon layer
4
depending on the crystal orientations of the single crystal grains GP result in different thicknesses of the oxide film
5
in accordance with the respective single crystal grains GP as shown in
FIG. 45
after the oxidation of the polysilicon layer
4
.
The reduced thickness of the SOI layer
3
in the edge section of the SOI substrate
10
might cause the oxide film
5
to be contact with the buried oxide film
2
depending on the single crystal grains GP and cause the SOI layer
3
to be completely oxidized. In such cases, part of the SOI layer
3
might be surrounded by the buried oxide film
2
and the oxide film
5
. For example, an SOI layer
30
shown in
FIG. 45
is surrounded by the oxide film
5
and the buried oxide film
2
.
When wet etching is performed on the oxide film
5
using an etchant such as hydrofluoric acid for thinning the SOI layer
3
in the SOI substrate
10
under the above described conditions, the buried oxide film
2
as well as the oxide film
5
is etched as shown in FIG.
46
. Then, the SOI layer
30
is lifted off into a particle suspended in the etchant. In some cases, the SOI layer
30
might adhere to the central section of the SOI substrate
10
. The adhesion of particles to the semiconductor element formation regions causes the formation failures of semiconductor elements and, accordingly, the decrease in fabrication yield.
As above described, the background art semiconductor substrate, particularly the SOI substrate, has the drawback that the SOI layer in the edge section of the substrate exfoliates into particles to cause the decrease in fabrication yield. The production of the particles is also a problem for semiconductor substrates other than the SOI substrate.
SUMMARY OF THE INVENTION
A first aspect of the present invention is intended for a method of fabricating a semiconductor device using a semiconductor substrate having a first major surface, a second major surface opposite from the first major surface, and a side surface, the first major surface including a central section in which active regions are to be formed and a peripheral section, the peripheral section and the side surface defining an edge section. According to the present invention, the method comprises the steps of: (a) forming a first oxide film so as to cover the central section and the edge section of the semiconductor substrate; (b) selectively forming an oxidation-resistant film on the first oxide film in the central section; (c) further oxidizing the edge section of the semiconductor substrate using the oxidation-resistant film as a mask to form a second oxide film in the edge section, the second oxide film being thicker than the first oxide film; and (d) forming semiconductor elements in the active regions.
Preferably, according to a second aspect of the present invention, in the method of the first aspect, the semiconductor substrate is an SOI substrate formed by a SIMOX technique; the semiconductor substrate comprises a buried oxide film and an SOI layer formed in a sequentially stacked relation in the entire first major surface; and the step (c) comprises the step of (c-1) forming the second oxide film so as to completely oxidize the SOI layer extending in the edge section and to oxidize part of the edge section which has not been oxidized.
Preferably, according to a third aspect of the present invention, in the method of the first aspect, the semiconductor substrate is a
Hirano Yuichi
Ipposhi Takashi
Iwamatsu Toshiaki
Maeda Shigenobu
Yamaguchi Yasuo
Elms Richard
Luu Pho
Mitsubishi Denki & Kabushiki Kaisha
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