Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
2001-10-05
2003-05-06
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06557414
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an inertia sensor suitable for general use in an apparatus which controls the position and posture of a body in motion by detecting acceleration and angular velocity and a method of fabricating the same and more particularly, to an inertia sensor fabricated through semiconductor fabrication process and a method of fabricating the same.
The inertia sensor, especially, an acceleration sensor and/or a rotational angular velocity sensor (gyroscope or yaw rate sensor) finds their wide demand for sensors necessary for vehicle stability control, airbag and navigation systems of a car and for prevention of unintentional movement of cameras and compact video cameras. Prior arts will be described hereunder by mainly taking an angular velocity sensor, for instance.
A variety of angular velocity sensors such as a rotary type gyroscope using a rotating sphere or top and an optical fiber gyroscope using optical fibers have hitherto been developed. The rotary type gyroscope and optical fiber gyroscope are on the one hand highly precise but on the other hand are apt to be increased in size.
Under the circumstances, with the aim of reducing the apparatus size, a vibratory gyroscope devoid of rotary body and being operative to vibrate or rock a mass body has been developed and many kinds of piezoelectric type vibratory gyroscope reduced in size by mounting a piezoelectric device on a triangle pole or cylinder have been produced. In the vibratory gyroscope, however, small parts required to be precisely assembled, facing difficulties in fabrication. Further, any of the aforementioned angular velocity sensors including the vibratory gyroscope using the triangle pole or cylinder consist of a great number of separate parts, making it difficult to make the sensor portion (sensing part) integral with the associated circuit section.
Recently, to solve these problems, study and development of a compact vibratory gyroscope has been made actively by using a micromachining technique to which the silicon semiconductor fabrication process technique is applied. Through this technique, sensors can be produced at low costs and are suitable for mass production. Further it is expected that the sensor portion and the peripheral circuit section can be incorporated in one chip. The aforementioned technical trend stands with other sensors such as acceleration sensors.
Especially for vibratory gyroscopes, various types of sensors have been proposed and discussed, but till now, less studies have been made on a sensor of simple structure (with a high sensitivity) giving importance to mass production adaptability.
The basic operational principle of the angular velocity sensor fabricated by using the micromachining technique will be described by way of example of the sensor disclosed in U.S. Pat. No. 5,349,855.
The basic principle of the angular velocity sensor is that when a mass body constantly vibrating or rotating, moves in a direction along with a first axis with an angular velocity having a rotation axis parallel to a second axis which is vertical to the first axis direction, the angular velocity can be found by detecting a Colioris' force generated in a third axis direction vertical both to the first and second axes. The Colioris' force can be known by measuring an amount of displacement of the mass body. Hereinafter, the vibrating or rotating mass body is termed as a vibratory body.
The angular velocity sensor exemplified above is constructed of a vibratory body, a support structure for supporting the vibratory body, a driver for applying drive force necessary to vibrate the vibratory body and a detector for detecting the displacement due to Colioris' force. The vibratory body is spaced apart from a substrate by means of a support beam of suitable shape and is driven electrostatically by using comb teeth electrodes. The direction of vibration is parallel to the substrate. Under this condition, when the rotation is applied to the vibratory body with the rotation axis parallel to the substrate but is vertical to the vibration direction, the vibratory body is displaced by Colioris' force in a direction vertical to the substrate. This displacement is detected as a change in electrostatic capacitance by using an electrode disposed at the bottom of the vibratory body and the substrate, thereby measuring the Colioris' force.
Examples of another angular velocity sensor fabricated by using the micromachining technique in which a drive electrode and a displacement detecting electrode are provided on a plane parallel to a substrate and a vibratory body is allowed to move on the plane only are disclosed in, for example, JP-A-09-189557 and JP-A-09-119942.
Now, the following points must considered carefully.
The provision of any of the aforementioned angular velocity sensors fabricated by using the semiconductor fabrication process presupposes a so-called surface micromachining technique in which the steps of forming a film (or player) such as insulating film and a polysilicon film on a silicon wafer and patterning the film by etching are repeated. In this case, in order to separate various structures from the substrate, the surface micromachining technique further needs a process in which a layer (sacrificial layer) that is to be extinguished in a later step is formed in advance. Layers incorporating the structures is then superposed on the sacrificial layer and the sacrificial layer is removed by etching in a final step. As a result, the vibratory body taking the form of a thin film is so formed as to be slightly spaced apart from the silicon wafer, making it difficult to cause the vibratory body to vibrate sufficiently.
On the other hand, many examples of an angular velocity sensor have been known which are fabricated by using a bulk micromachining technique, according to which, in contrast to the surface micromachining technique, a silicon wafer per se is etched by means of a device capable of working the wafer at a high aspect ratio thereby producing a structure. For example, JP-A-7-120266, JP-A-5-240874 and The Institute of Electrical Engineers of Japan, E-department (T. IEE Japan); Vol. 118-E, No. 12, '98 show the examples as above. In any of these examples, electromagnetic force is used for drive means and the sensor is comprised of a worked silicon wafer, a glass substrate and a permanent magnet. In this case, the vibratory body is so worked as to have a sufficiently large volume (mass) and therefore, vibration necessary for sensing can be caused with ease.
In the sensor fabricated by the surface micromachining technique, it is easy to corporate the sensor portion with the detection and signal processing circuits of the sensor in one chip concurrently.
The surface micromachining technique, however, includes the step of etching the sacrificial layer in the course of process and so, after the vibratory body and support means are separated from the substrate in this step, cleaning is carried out during which the vibratory body tends to affix to the substrate and the fine interdigital patterns tend to affix to each other. Therefore, in order to raise the yield, special contrivance such as a freezing dry method must be employed. Further, when considering the integration with the circuit section, a high-quality protective film devoid of defects such as pinholes is required to be prepared in advance of the etching step to prevent the circuit section from being etched. As will be seen from the above, the etching step is very laborious and time-consuming and unless being assisted by new contrivance, it cannot be suited for mass production.
Further, in the sensor fabricated by the surface micromachining technique, the distance between the substrate and the vibratory body corresponds to the thickness of the sacrificial layer and is narrow, approximately amounting up to several &mgr;m. Consequently, when the sensor is operated in the atmosphere, a large viscous resistance due to air acts on the vibratory body, raising a technical problem wh
Komachiya Masahiro
Matsumoto Masahiro
Sakurai Kohei
Suzuki Seikou
Crowell & Moring LLP
Hitachi , Ltd.
Moller Richard A.
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