Pressure sensor

Details Pressure sensor Pressure sensor

1999-07-19

2002-04-02

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Fluid pressure gauge

Diaphragm

C073S718000

Reexamination Certificate

active

06363791

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor absolute pressure sensor that detects the pressure change to be measured by the change in capacitance, and in particular to improving the electrode structure thereon, and increasing the leakage resistance of the reference pressure cavity that is indispensable to the absolute pressure sensor.
2. Description of Related Art
In a semiconductor pressure sensor that measures absolute pressure, the sealing characteristic of the reference pressure cavity
5
formed between a silicon substrate
3
having a diaphragm
1
and a silicon wall
2
and another substrate
4
bonded at the region of the silicon wall
2
of this silicon substrate
3
, as shown for example in
FIG. 4
, is important. The reason is that in the case of a semiconductor pressure sensor, the deflection of this diaphragm depends on the pressure difference produced between the externally applied pressure and the inside of the reference pressure cavity. Therefore, when a leakage from the outside into the reference pressure cavity occurs, because the pressure in the reference pressure cavity gradually changes or fluctuates, even when the pressure applied from the outside is constant, the pressure value output by the sensor varies with time. In addition, this fluctuation depends on the rate of the leakage.
As shown in
FIG. 4
, a resistor
6
is formed in the diaphragm
1
, and in the case of a semiconductor pressure sensor (referred to hereinbelow as a “piezoresistant pressure sensor”) that detects the pressure of a measured medium based on the change in the resistance of the resistor
6
, generally a hermetically sealed reference pressure cavity
5
is easily obtainable, and the leakage rate is small. The reason is that the detection circuit
7
for detecting the variation of this resistance can be formed on the surface of a silicon substrate
3
, and can be formed completely independently of the reference pressure cavity
5
. That is, normally, in a piezoresistant pressure sensor, the reference pressure cavity is sealed with a high degree of airtightness by the application of a widely used technology such as electrostatic bonding because there is no part where an electrode is brought out to the outside, etc., from within the reference pressure cavity.
In contrast, in the case of capacitance pressure sensor, because the pressure of the measured medium is detected based on the fluctuation in the capacitance between the diaphragm that deflects according to the pressure and the opposite substrate, as shown in the representative structure of
FIG. 5
, it is necessary to provide electrodes
8
,
9
respectively on the diaphragm
1
side and the facing substrate
4
side. In addition, normally, one of the electrodes
8
,
9
has a structure covered by an insulating layer
10
(the electrode
9
on the substrate side in FIG.
5
).
Generally, in a semiconductor pressure sensor, the diaphragm is has as a starting material a silicon substrate. Therefore, on the diaphragm side, several technologies are used that form an electrode for capacitance detection by providing a high conductivity by doping in high concentration boron or phosphorus in the silicon substrate. In addition, when doping technology is used, a lead that connects the electrode in the reference pressure cavity and the external electrode can be easily obtained.
In addition, in
FIG. 5
, on substrate
4
opposite the diaphragm, glass substrates are widely used. The reason is that a glass substrate can be hermetically sealed to the silicon substrate at low cost by electrostatic bonding. However, on this glass substrate, because it is not possible to apply this doping technology, on the electrode
9
on the glass substrate
4
side, conventionally thin metallic layer has come to be used.
Here, in order to detect fluctuation in the capacitance, it is necessary to extend an electrode
9
on the glass substrate
4
side positioned within the reference pressure cavity
5
to an external electrode. (Hereinbelow, this lead is referred to as a feedthrough.)
The formation cost of the feedthrough is least expensive if it is formed at the same time as the electrode
9
. Therefore, the thin metallic layer of the feedthrough comprises the same material as the electrode
9
, and usually has the same thickness. In addition, the pattern width of the feedthrough should be as narrow as possible in order to decrease the parasitic capacity of the pressure sensor as a whole. However, in contrast, in order to make the electrical resistance small, the pattern width should be as wide as possible. Thus, the balance between both the parasitic capacity and the electrical resistance determines the width of the feedthrough.
The structure formed by this feedthrough and the external electrode is shown in
FIG. 6
to FIG.
8
.
FIG. 6
is a vertical cross-section of the entire semiconductor pressure sensor,
FIG. 7
is a planar drawing showing the electrode on the glass substrate side, and
FIG. 8
is a planar drawing showing the electrode on the silicon substrate side. In these figures, reference numeral
9
is the electrode on the glass substrate side,
11
is the external electrode on the glass substrate side,
12
is the feedthrough that connects the electrode
9
on the glass substrate side and the external electrode
11
,
8
is an electrode on the silicon substrate side,
13
is an external electrode on the silicon substrate side, and
14
is a lead that connects the electrode
8
on the silicon substrate side and the external electrode
13
.
In the structure shown in
FIG. 6
to
FIG. 8
, when the silicon substrate
3
and the glass substrate
4
undergo electrostatic bonding, as shown in
FIG. 9
, a difference in level is produced on the insulating layer
10
reflecting the difference in level produced by the feedthrough
12
, and as a result, gaps
15
are made between the difference in level of the insulating layer
10
and the silicon substrate
3
. This gap
15
becomes a leakage path between the outside of the sensor and the reference pressure cavity
5
.
Here, as a means of excluding this leakage path, U.S. Pat. No. 5,528,452 by Wen K. Ko, proposes limiting the thickness of the metallic layer of the feedthrough to 0.1~0.3 &mgr;m. It is disclosed that by making the feedthrough have this thickness, leakage to the reference pressure cavity is not produced.
However, when manufacturing a sensor having the extremely low leakage rate equal to or below 1×10
−13
atm·cc/sec, even if the thickness of the metallic layer of the feedthrough is made 0.1 &mgr;m, in the structure of the conventional sensor shown in
FIG. 6
to
FIG. 8
, there is the major problem that the rate of defects due to this leakage exceeds 50%.
This phenomenon is caused by various changes in the size of the leakage path shown in
FIG. 9
due to variations in the surface roughness of the glass substrate, or variations in the thickness of the metallic layer of the feedthrough, or further, variations in the thickness of the insulating layer, etc., and when this path exceeds a certain size, the leakage rate to the reference pressure cavity exceeds 1×10
−13
atm·cc/sec.
SUMMARY OF THE INVENTION
In consideration of the above-described problem, the present invention has as an object providing a pressure sensor that prevents leakage to the reference pressure cavity from outside the sensor in a capacitor pressure sensor, and that has a structure which can decrease defects due to leakage in comparison with conventional technology.
In order to attain the above object, in a pressure sensor of the present invention that bonds the upper surface of a first substrate a second substrate having a diaphragm and a thick part at this thick part, provides a first electrode covered by insulating layer positioned opposite to the diaphragm of the second substrate on the surface of the first substrate, and at the same time provides a second electrode on the diaphragm of the second substrate, and detects the pressure fluctuatio

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