Diaphragm-type semiconductor pressure sensor

Details Diaphragm-type semiconductor pressure sensor Diaphragm-type semiconductor pressure sensor

2003-05-02

2004-09-14

Oen, William (Department: 2855)

Measuring and testing

Fluid pressure gauge

Electrical

Reexamination Certificate

active

06789431

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-146500 filed on May 21, 2002.
BACKGROUND OF THE INVENTION
The present invention relates to a diaphragm-type semiconductor pressure sensor, which includes a semiconductor substrate having: active surface and back surface of (
110
) crystallographic face orientation; and a diaphragm that has been formed in the active surface by forming a recess in the back surface, and relates to a semiconductor wafer used for manufacturing the diaphragm-type semiconductor pressure sensor.
The diaphragm-type semiconductor pressure sensor includes a semiconductor substrate that has an active surface of (
110
) crystallographic face orientation and a back surface, which is opposite to the active surface, of (
110
) crystallographic face orientation. Hereafter, this type of semiconductor substrate will be referred as a (
110
) semiconductor substrate.
As shown in
FIG. 12
, a proposed diaphragm-type semiconductor pressure sensor includes a rectangular (
110
) semiconductor substrate
10
having four sides
10
a
. The (
110
) semiconductor substrate
10
includes a diaphragm
14
used for detecting a pressure. The diaphragm
14
is located at a bottom of a recess
13
, or in the active surface of the (
110
) semiconductor substrate
10
. The recess
13
has been formed by an isotropically etching a portion of a silicon substrate, from which the (
110
) semiconductor substrate has been formed, from the back surface thereof.
The diaphragm
14
includes gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
, which are piezoresistive elements. As shown in
FIG. 12
, the gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
are made up of two center gauge resistors Rc
1
, Rc
2
, which are located at the central area of the diaphragm
14
, and two side gauge resistors Rs
1
, Rs
2
, which are located at the periphery of the diaphragm
14
. The four gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
make up a bridge circuit used for detecting the pressure. When the diaphragm
14
is strained by a pressure to be detected, the resistances of the gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
vary in response to the strain of the diaphragm
14
, and the pressure is detected on the basis of the variation in the resistances.
In the manufacturing process of the proposed diaphragm-type semiconductor pressure sensor, a plurality of rectangular regions, which become sensor chips, are formed in a silicon wafer, which has an active surface of (
110
) crystallographic face orientation, a back surface, which is opposite to the active surface, of (
110
) crystallographic face orientation, and an orientation flat having a crystallographic face of (
100
) orientation. The regions are defined by forming scribe lines substantially parallel to the orientation flat and scribe lines substantially orthogonal to the orientation flat. Then, gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
are formed using semiconductor process techniques such as ion implantation and diffusion in the area of each of the regions where a diaphragm
14
is to be formed. Next, a portion of the silicon wafer is anisotropically etched from the back surface in each of the regions to form a recess
13
and simultaneously the diaphragm
14
in the active surface of the silicon wafer. With the above steps, a (
110
) semiconductor wafer is formed. Finally, the (
110
) semiconductor wafer is diced into a plurality of semiconductor pressure sensors shown in FIG.
12
.
In the semiconductor pressure sensor of
FIG. 12
, in which a (
110
) semiconductor substrate is used, the strain of the diaphragm
14
is used for detecting the pressure applied to the diaphragm
14
, as described above. Two crystallographic axes of <
110
> and <
100
> orientations exist on a crystallographic plane of (
100
) orientation. However, the piezoresistive coefficient of silicon along a crystallographic axis of <
110
> orientation is much greater, for example, about fifty times greater, than that along a crystallographic axis of <
100
> orientation. That is, the sensitivity in detecting the strain generated along a crystallographic axis of <
110
> orientation is much greater than that along a crystallographic axis of <
100
>orientation. Therefore, the gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
have been formed such that the gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
substantially extend along a crystallographic axis of <
110
> orientation in the semiconductor pressure sensor of
FIG. 12
in order to increase the sensitivity.
A crystallographic plane of (
100
) orientation includes only one crystallographic axis of <
110
> orientation, so the arrangement of the gauge resistors Rc
1
, Rc
2
, Rs
1
, Rs
2
shown in
FIG. 12
is substantially the best to gain the highest sensitivity in pressure detection. The pressure sensor of
FIG. 12
has been bonded to a sealing substrate such as a glass stand, which is not shown in the figure, at the back surface of the (
110
) semiconductor substrate
10
using anodic bonding and so on such that the recess
13
is hermetically sealed by the sealing substrate to form a pressure reference room.
Lately, there have been demands for shrinking the semiconductor pressure sensor of
FIG. 12
for the purpose of cost reduction and soon. To shrink the semiconductor pressure sensor of
FIG. 12
, the (
110
) semiconductor substrate
10
needs to be shrunk.
However, if the (
110
) semiconductor substrate
10
was shrunk with simply shrinking the diaphragm
14
without changing layout, the sensitivity in pressure detection would worsen. Even if the (
110
) semiconductor substrate
10
was shrunk without shrinking the diaphragm
14
or changing layout, the minimum width L of the contact area between the back surface of the (
110
) semiconductor substrate
10
and the sealing substrate would become narrower. That is, the frame-like portion of the (
110
) semiconductor substrate
10
, which surrounds the diaphragm
14
, needs to be narrowed.
The hermeticity of the pressure reference room is expressed using the molecular leak rate equation (1) in vacuum engineering,
Q
=(2&pgr;
V/
3)×
r
3
×(
P
1
−
P
2
)/
L
  (1)
where Q is the leak rate of the pressure reference room, r is the radius of a leak passage LP at the boundary between the back surface of the (
110
) semiconductor substrate
10
and the sealing substrate, L is the length of the leak passage LP, or the above-mentioned minimum width of the back surface, V is the average velocity of gas molecules, P
1
is the pressure outside the pressure reference room, and P
2
is the pressure in the pressure reference room. As understood from the equation (1), the leak rate Q is inversely proportionate to the length L of the leak passage LP. Therefore, if the (
110
) semiconductor substrate
10
was shrunk without shrinking the diaphragm
14
or changing layout, it would become difficult to assure the hermeticity of the pressure reference room. As a result, the reliability of the pressure sensor of
FIG. 12
would worsen.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above aspects. A first object of the present invention is to shrink a diaphragm-type semiconductor pressure sensor without shrinking the diaphragm thereof or shortening the minimum width of the back surface thereof in order to make the most of the dimensions of the sensor. A second object of the present invention is to provide a semiconductor wafer that can be used to shrink a diaphragm-type semiconductor pressure sensor in order to make the most of the dimensions of the sensor.
To achieve the first object, a diaphragm-type semiconductor pressure sensor according to the present invention includes a substantially rectangular (
110
) semiconductor substrate, which has four sides, an active surface of (
110
) crystallographic face orientation, and a back surface, which is opposite to the active surface, of (
110
) crystallographic face orientation. Each of th

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