Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive
Reexamination Certificate
2000-03-28
2002-06-04
Zarabian, Amir (Department: 2824)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Physical stress responsive
C438S052000, C438S238000, C204S219000, C216S002000, 43
Reexamination Certificate
active
06399410
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for anodizing silicon substrates and a method for manufacturing acceleration sensors using the anodization method.
In recent years, acceleration sensors have been used in controlling devices such as ABS (antilock brake system), airbag systems, and suspension control systems. A surface-type acceleration sensor is known as a kind of acceleration sensor. The surface-type acceleration sensor includes a silicon substrate, a displaceable mass portion formed on the upper surface of the substrate, and a deformation gauge formed on the surface of the mass portion. In recent years, anodization technologies have been used to form the mass portion. With reference to
FIGS. 9-11
, a method for manufacturing a conventional surface-type acceleration sensor
41
using anodization will now be described.
FIG. 9
shows a p-type single crystal silicon substrate
42
, which is anodized. On a predetermined area of the upper surface of the p-type single crystal silicon substrate
42
, a p
+
silicon embedded layer
43
is formed. A first epitaxial growth layer
44
, which is made of n-type silicon, is laminated on the substrate
42
. p
+
silicon diffusion layers
45
are embedded in predetermined areas of the first epitaxial growth layer
44
. A second epitaxial growth layer
46
, which is made of n-type silicon, is laminated on the first epitaxial growth layer
44
. On a predetermined area of the second epitaxial growth layer
46
, p
+
silicon diffusion layers
47
are formed and are exposed to the exterior. On the second epitaxial growth layer
46
, oxide film
48
, wiring patterns
49
, passivation film
50
, and metal protection film
51
are formed. A deformation gauge (not shown) is also formed in the second epitaxial growth layer
46
. An oxide film
52
and an electrode layer
53
are laminated in that order on the bottom surface of the substrate
42
. The electrode layer
53
is electrically connected to the substrate
42
through a connection opening
54
.
To perform anodization, the anode of a DC power supply
55
is connected to the electrode layer
53
, and the cathode is connected to a counter electrode (not shown). In this state, the substrate
42
and the counter electrode are immersed in a hydrofluoric acid solution. Then, a direct current I flows from the lower side of the substrate
42
to the upper side, which selectively becomes porous. During anodization, mainly the embedded layer
43
, the diffusion layer
45
, and the diffusion layer
47
are changed into porous silicon layer
56
(see FIG.
10
).
Then, the porous silicon layer
56
is selectively dissolved and removed by etching using alkaline etchant, which makes the substrate
42
, which includes the layers
44
,
46
, hollow. As a result, mass portions
57
are formed on the epitaxial growth layers
44
,
46
, which form the acceleration sensor
41
as shown in FIG.
11
.
However, in the conventional method, the direct current is applied to an area outside the area designated for anodization. This slows the anodization. Accordingly, a more efficient anodization method has been requested.
Also, in the conventional method, the range that becomes porous may be wider than the designated area. This increases the amount of the porous part that is removed by the etching and reduces the size of the mass portions
57
. In this way, the amount of wasteful removal increases and it is difficult to form a large mass portion
57
. Therefore, it is difficult to produce a highly sensitive surface-type acceleration sensor using the anodization method.
Further, in the conventional method, holes may be formed in the side surfaces when the porous part is expanded to the periphery of the substrate
42
. To avoid this, the size of substrate must be increased, which prevents making the acceleration sensors compact.
SUMMARY OF THE INVENTION
To solve the above problems, an objective of the present invention is to provide an anodization method for silicon substrates that efficiently makes a designated area porous.
Another objective of the present invention is to provide a method for manufacturing compact and highly sensitive surface-type acceleration sensors.
To achieve the above objectives, the present invention provides a method for anodizing a silicon substrate comprising: providing a p-type single crystal silicon substrate; forming an n-type silicon embedded layer made of n-type silicon on a predetermined area of a first surface of the p-type single crystal silicon layer, wherein an opening for permitting a current to flow is formed in the center of the n-type silicon embedded layer; forming an n-type silicon layer on the first surface of the p-type single crystal silicon substrate and on the n-type silicon embedded layer; forming a silicon diffusion layer containing a high-concentration p-type impurity on a predetermined area of the n-type silicon layer, wherein the silicon diffusion layer contacts at least the n-type silicon embedded layer in the vicinity of the interface between the p-type single crystal silicon substrate and the n-type silicon embedded layer; forming an electrode layer on a second surface, which is on the opposite side of the p-type silicon substrate from the first surface; connecting the anode of a DC power source to the electrode layer and connecting the cathode to a counter electrode, which is opposed to the p-type silicon substrate; and concentrating a current flow to an area corresponding to the opening of the n-type silicon layer in a direction from the second surface of the p-type single crystal silicon substrate toward the first surface, and advancing porosity formation in the area from the first surface toward the second surface.
In the present invention, a direct current is intensely applied to an area corresponding to an opening of the p-type single crystal silicon substrate during the anodization. Accordingly, the current is efficiently applied to the designated area, which increases the anodization speed and prevents the area outside the designated area from becoming porous.
The present invention also provides a method for manufacturing a surface-type acceleration sensor having a displaceable mass portion formed on an upper surface of a silicon substrate and a deformation gauge formed on the upper surface of the mass portion, the method comprising: providing a p-type single crystal silicon substrate; forming an n-type silicon embedded layer made of n-type silicon on a predetermined area of a first surface of the p-type single crystal silicon layer, wherein an opening for permitting a current to flow is formed in the center of the n-type silicon embedded layer; forming an n-type silicon layer on the first surface of the p-type single crystal silicon substrate and on the n-type silicon embedded layer; forming a silicon diffusion layer containing a high-concentration p-type impurity on a predetermined area of the n-type silicon layer, wherein the silicon diffusion layer contacts at least the n-type silicon embedded layer in the vicinity of the interface between the p-type single crystal silicon substrate and the n-type silicon embedded layer; forming the deformation gauge on the n-type silicon layer; forming a wiring over the n-type silicon layer; forming an electrode layer on a second surface, which is on the opposite side of the p-type silicon substrate from the first surface; connecting the anode of a DC power source to the electrode layer and connecting the cathode to a counter electrode, which is opposed to the p-type silicon substrate; concentrating a current flow to an area corresponding to the opening of the n-type silicon layer in a direction from the second surface of the p-type single crystal silicon substrate toward the first surface, and changing the area into a porous silicon layer from the first surface toward the second surface; and forming the mass portion by dissolving and removing the porous silicon layer using alkali etching.
A semiconductor device preferable for anodization for making silicon porous compri
Iwata Hitoshi
Murate Makoto
Kabushiki Kaisha Tokai Rika Denki Seisakusho
Luu Pho
Vidas,Arrett&Steinkraus PA
Zarabian Amir
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