Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2001-11-02
2003-05-27
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C438S048000, C361S280000
Reexamination Certificate
active
06568269
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an acceleration sensor and to a manufacturing method thereof.
2. Background Art
FIG. 6
is, for example, a plan view showing a part of an acceleration detector of a monolithic capacitance type disclosed in the Japanese Patent Publication (unexamined) No. 178954/1996. In this conventional art, the acceleration detector
100
contains a differential capacitor
100
A surrounded by a dotted line in the drawing. The differential capacitor
100
A includes a pair of capacitors. A first capacitor is formed between two electrodes
101
,
103
, and a second capacitor is formed between two electrodes
102
,
104
. The electrodes
101
,
102
are electrically common. The common electrodes
101
,
102
are lifted from a silicon substrate and are moving electrodes movable in response to acceleration. The other electrodes
103
,
104
are stationary fixed electrodes. These electrodes
101
,
102
,
103
and
104
are all made of a polysilicon material. When acceleration is applied to the substrate, the movable electrodes
101
,
102
being the common electrodes will move in such a manner that capacitance of the second capacitor may increase and capacitance of the first capacitor may decrease. These two capacitors are connected to a signal regulation circuit wherein the differential capacitance is converted into a corresponding voltage. By this voltage value, acceleration is detected.
It is a recent trend that smaller and cheaper acceleration sensor has been increasingly required. In the conventional acceleration sensor as described above, however, the polysilicon material deposited onto the silicon substrate by plasma CVD method must be formed approximately ten times as thick as a polysilicon membrane used in ordinary LSI. Therefore, the conventional acceleration sensor takes an extremely long period for deposition in a plasma CVD apparatus, resulting in a problem of longer construction time and increasing cost. Moreover, since distances between the moving electrodes
101
,
102
and the fixed electrodes
103
,
104
bear a direct relation to increase or decrease of capacitance of the capacitors, it is important to form these electrodes precisely at their positions. However, particularly the moving electrodes
101
,
102
are liable to be warped due to residual stress of polysilicon, whereby the fixed electrodes
103
,
104
and the moving electrodes
101
,
102
may be varied from designed values in relative positions and distances thereof. Accordingly, performance of individual acceleration sensors is not always uniformly or stably exhibited thereby eventually causing another problem of lower product reliability.
SUMMARY OF THE INVENTION
The present invention was made to solve the above-discussed problems, and has an object of providing an acceleration sensor capable of being manufactured economically as well as easily and which is highly reliable. The invention also provides a manufacturing method of the acceleration sensor.
An acceleration sensor according to the invention comprises: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member disposed over the substrate surface and joined to said movable electrode to be displaceable together with said movable electrode;
wherein said mass member includes a thin polyimide membrane provided over the substrate surface and silicon nitride membranes provided respectively on a pair of main surfaces of said thin polyimide membrane substantially parallel to the substrate surface.
By this acceleration sensor according to the invention, a shorter construction time and lower cost have been achieved as compared with the conventional construction mainly fabricated of a polysilicon membrane. Further, since the silicon nitride membrane is provided as a flattening membrane onto a pair of the main surfaces of the thin polyimide membrane substantially parallel to the substrate surface, the thin polyimide membrane is restrained from being warped. Therefore the movable electrode and the fixed electrode can be formed at precisely the same relative positions and distances as designed. As a result, instability or non-uniformity in performance of individual apparatus can be surpassed whereby a highly reliable acceleration sensor can be eventually obtained.
In the acceleration sensor according to the invention, it is preferable that each of said silicon nitride membranes is covered with a metal membrane, and that end faces between the pair of said main surfaces of said thin polyimide membrane are covered with a metal membrane.
Another acceleration sensor according to the invention comprises: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member including a thin polyimide membrane disposed over the substrate surface and joined to said movable electrode to be displaceable together with the movable electrode,;
wherein a pair of main surfaces of said thin polyimide membrane substantially parallel to the substrate surface and end faces between the pair of the main surfaces of said polyimide membrane are respectively covered with a metal membrane.
By this acceleration sensor according to the invention, a shorter construction time, lower cost and higher reliability due to restraint of the polyimide membrane from being warped, a plating process conventionally required can be omitted. As a result, there arises a further advantage of simplifying the manufacturing process.
In the acceleration sensor according to the invention, it is preferable that said metal membrane is formed by the material selected from the group consisting of tungsten (W) and titanium nitride (TiN).
By this acceleration sensor according to the invention, the metal membrane is formed by the material selected from the group consisting of tungsten (W) and titanium nitride (TiN), thereby being capable of serving as a flattening membrane for restraining the polyimide membrane from being warped and as a metal membrane necessary for use as an electrode.
A method of manufacturing an acceleration sensor according to the invention, the acceleration sensor comprising: a fixed electrode including a plurality of first rod-like patterns aligned parallel to each other on a substrate surface; a movable electrode including a plurality of second rod-like patterns aligned parallel to each other over the substrate surface so as to be opposite to each of the plurality of said first rod-like patterns with predetermined distances; and a mass member disposed over the substrate surface and joined to said movable electrode to be displaceable together with said movable electrode;
the method comprises the steps of:
forming a silicon oxide membrane on the substrate surface by plasma CVD method;
applying a first resist onto the silicon oxide membrane and patterning said first resist by photomechanical process;
removing a predetermined portion of said silicon oxide membrane by etching using said first resist as a mask;
forming a first silicon nitride membrane on the substrate surface by plasma CVD method for covering said silicone oxide membrane;
applying a polyimide material onto said first silicon nitride membrane and forming a thin polyimide membrane by setting the polyimide material at 300° C. to 370° C.;
forming a second silicon nitride membrane on said thin polyimide membrane by plasma CVD method;
applying a second resist onto said second silicon nitride membrane and patterning said second resist by photomechanical proces
Chapman John E.
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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