Measuring and testing – Fluid pressure gauge – Diaphragm
Utility Patent
1998-06-24
2001-01-02
Felber, Joseph L. (Department: 2732)
Measuring and testing
Fluid pressure gauge
Diaphragm
C361S283400, C438S053000
Utility Patent
active
06167761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pressure sensor for detecting pressure, particularly a capacitance type pressure sensor for use in controlling an automotive engine and a method for manufacturing the same.
2. Prior Art
As a conventional capacitance type pressure sensor there is known, for example, the pressure sensor disclosed in Japanese Patent Publication No. Hei 7-50789. In this known capacitance type pressure sensor, a first electrode is formed by impurity diffusion on a monocrystal silicon substrate, and a second electrode is formed in the form of a diaphragm and in opposition to the first electrode through a gap with use of a polycrystal silicon which has been rendered electrically conductive.
According to the construction of the above capacitance type pressure sensor, when pressure is applied to the diaphragm-like second electrode, the second electrode is displaced by the applied pressure, and with this displacement of the second electrode, the capacitance between the first and second electrodes changes, which change in the capacitance is detected to detect the pressure.
In the prior art described above, however, since the first electrode is formed by diffusion, the junction capacitance with the silicon substrate is very large, resulting in increase in the stray capacitance between the first and second electrodes. Therefore, the pressure detecting accuracy is low and it has so far been impossible to perform a high-accuracy pressure detection.
Besides, the second electrode comes into direct contact with air which is a medium to be measured, so if foul air with gasoline, etc. incorporated therein contacts the second electrode, there occurs a leak current, making it difficult to effect an accurate detection of capacitance. Thus, it has so far been impossible to perform the pressure detection with a high accuracy.
Further, since the diaphragm which is displaced by pressure is formed using a polycrystal silicon, it is apt to undergo a plastic deformation and hence there has been the possibility that the correlation between applied pressure and the change in capacitance may vary. This gives rise to the problem that the pressure detecting accuracy and reliability are deteriorated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a capacitance type pressure sensor capable of diminishing stray capacitance, suppressing plastic deformation of a diaphragm portion which undergoes pressure and further capable of effecting pressure detection with a high accuracy, as well as a method for manufacturing such a capacitance type pressure sensor.
(1) According to the present invention, in order to achieve the above-mentioned object, there is provided a capacitance type pressure sensor comprising a diaphragm which is a monocrystal silicon substrate and which is displaced by applied pressure, an insulating film formed on the diaphragm, a first electrode formed on the insulating film, a second electrode formed in opposition to the first electrode through a gap, and means for detecting a change in capacitance between the first and second electrodes and converting the detected capacitance change into a voltage, which capacitance change occurs due to a change in the distance between the first and second electrodes caused by the pressure applied to the diaphragm.
(2) Preferably, in the above (1), the insulating film is divided in at least two or more.
(3) Preferably, in the above (2), the first electrode is divided in at least two or more.
(4) Preferably, in the above (3), the area of the first electrode is smaller than the area of the insulating film.
(5) Preferably, in the above (1) to (4), a hollow portion is present partially between the first electrode and the silicon substrate.
(6) Preferably, in the above (1) to (5), the first electrode is formed by a polycrystal silicon.
(7) Preferably, in the above (1) to (6), the pressure sensor is made integral with a signal processing circuit for processing a signal provided from the pressure sensor, and the material of a gate electrode of a MOS transistor included in the signal processing circuit is the same as the material of the first electrode.
Between the diaphragm and the first electrode is formed an insulating film to diminish the stray capacitance generated between the first electrode and the monocrystal silicon substrate, and also between the first electrode and impurity.
If the insulating film and the first electrode are divided minutely into a plurality of regions, it becomes possible to diminish a stress strain induced by a difference in thermal expansion coefficient among the diaphragm, insulating film and first electrode. That is, in the case where the first electrode and the insulating film are divided plurally, it is possible to diminish the stress strain induced by a difference in thermal expansion coefficient among them in comparison with the case where they are not divided plurally.
Where the first electrode in the pressure sensor is formed by a polycrystal silicon, the MOS transistor gate electrode in the signal processing circuit for processing a signal provided from the pressure sensor is also formed using the polycrystal silicon and hence the pressure sensor and the signal processing circuit can be made integral with each other.
(8) According to the present invention there also is provided a method for manufacturing the capacitance type pressure sensor, comprising the steps of doping an impurity into a monocrystal silicon substrate; forming an oxide film over a predetermined portion on the impurity; forming on the oxide film a polycrystal silicon film which has been rendered electrically conductive and which serves as a first electrode and subsequent patterning; forming a sacrificial layer on at least the first electrode and subsequent patterning; forming on at least the sacrificial layer a polycrystal silicon film which has been rendered electrically conductive and which serves as a second electrode; forming an insulated polycrystal silicon film on the second electrode and subsequent patterning; removing the sacrificial layer; forming a protective film on the insulated polycrystal silicon film; and etching a predetermined portion of the monocrystal silicon substrate on the side opposite to the impurity-doped side to form a diaphragm of the impurity.
In the case where the first electrode is formed by a polycrystal silicon and the MOS transistor gate electrode used in the signal processing circuit for processing a signal provided from the pressure sensor is also formed by the polycrytal silicon, it is possible to fabricate the pressure sensor and the signal processing circuit simultaneously in one piece with each other.
(9) According to the present invention there is further provided a capacitance type pressure sensor comprising a diaphragm which is a monocrystal silicon substrate and which is displaced by applied pressure, a first electrode formed on the diaphragm and divided in at least two or more, a second electrode formed in opposition to the first electrode through a gap, and means for detecting a change in capacitance between the first and second electrodes and converting the detected capacitance change into a voltage, which capacitance change occurs due to a change in the distance between the first and second electrodes caused by the pressure applied to the diaphragm.
(10) Preferably, in the above (9), the first electrode is formed by a polycrystal silicon.
(11) Preferably, in the above (9) or (10), the pressure sensor is made integral with a signal processing circuit for processing a signal provided from the pressure sensor, and the material of a gate electrode of a MOS transistor included in the signal processing circuit is the same as the material of the first electrode.
Since the first electrode is divided into plural regions, it is possible to diminish a stress strain induced by a difference in thermal expansion coefficient between the diaphragm and the first electrode.
(12) According to the present invention there is further provided
Hanzawa Keiji
Horie Junichi
Ichikawa Norio
Kuryu Seiji
Matsumoto Masahiro
Evenson, McKeown, Edwards & Lenahan, P.L.L.C
Felber Joseph L.
Hitachi, Ltd. and Hitachi Car Engineering Co., Ltd.
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