The present invention relates to a crystal oscillator in the shape of a tuning fork, an angular velocity sensor element, an angular velocity sensor, and a method of fabricating a tuning fork shaped crystal oscillator, and, in particular, to an angular velocity sensor element and a method of fabrication thereof.
Angular-velocity sensor elements are used in applications such as vehicle guidance systems and devices for preventing camera shake. The present applicants have already applied for a Japanese patent for an angular velocity sensor element obtained by using direct bonding to attach two tuning fork shaped crystal elements.
A perspective view of a prior-art example of an angular velocity sensor element is shown in FIG. 7.
This angular velocity sensor element is provided with a composite tuning fork shaped crystal element 1 obtained by attaching two tuning fork shaped crystal elements 1a and 1b by direct bonding, as shown in FIG. 7. The width of each of the tuning fork shaped crystal elements 1a and 1b is arranged on the X-axis of the crystalline axes (XYZ) of the crystal, with the length thereof on the Y-axis and the thickness thereof on the Z-axis. When one of these tuning fork shaped crystal elements 1a and 1b is viewed in an upright attitude, the negative direction along the X-axis is to the right-hand side and the positive direction thereof is to the left-hand side. In other words, the −X face of the crystal that is orthogonal to the −X-axis direction is taken to be the right-side surface of the tuning fork shaped crystal. The tuning fork shaped crystal elements 1a and 1b are bonded together with the ±X-axis directions thereof oriented in opposite directions.
When the composite tuning fork shaped crystal element 1 is fabricated, two Z-cut crystal wafers 2a and 2b are first bonded together directly, with the ±X-axis directions thereof oriented in opposite directions. Etching masks 3 as shown in FIG. 8 in the shape of a tuning fork are then formed on the front and rear of this directly bonded composite crystal wafer 2. These etching masks 3 are formed in such a manner that the right-hand direction (as seen when the composite tuning fork shaped crystal elements 1 that will be formed later are stood upright) is arrayed along the −X-axis direction. The composite crystal wafer 2 is then selectively etched by wet etching, to obtain a large number of the composite tuning fork shaped crystal elements 1 (which will become angular velocity sensor elements).
An electrode for driving the tuning fork in resonance is provided on each of the tines of the tuning fork of the angular velocity sensor element, in order to detect the Coriolis force imposed thereon. In this case, a surface electrode 6a on one tine of the tuning fork (on the right-hand side in the linkage diagram of FIG. 9) and a rear-surface electrode 7b on the other tine of the tuning fork (the left-hand side in FIG. 9) are connected in common and lead out to a first drive terminal D1. Similarly, a rear-surface electrode 6b on the other tine of the tuning fork leads out to a second drive terminal D2.
Electrodes 6c and 7c on the inner side surfaces of the two tines of the tuning fork are connected together, electrodes 6d and 7d on the outer side surfaces thereof are also connected together, and these connections lead out to first and second sensor terminals S1 and S2, respectively. A surface electrode 7a on the other tine of the tuning fork leads out to a monitor terminal M as shown in FIG. 9.
A schematic configurational diagram of a prior-art example of an angular velocity sensor is shown in FIG. 10. The angular velocity sensor shown in FIG. 10 is formed of an oscillation circuit 8 connected to the first and second drive terminals D1 and D2, to excite the tuning fork into resonance; charge amplifiers 9a and 9b and a differential amplifier 10 connected to the sensor terminals S1 and S2, to amplify the electrical charge due to the Coriolis force; a synchronization detection circuit 11 that detects the amount of electrical charge due to the Coriolis force from the differential amplifier 10; a low-pass filter 12 that obtains a DC voltage that is a smoothed output from the synchronization detection circuit 11 in response to angular velocity (a detected angle of rotation); a charge amplifier 9c connected to the monitor terminal M, to amplify the electrical charge due to the resonance of the tuning fork; and an automatic gain control (AGC) circuit 13 that fixes the amplitude of the tuning fork's resonance in accordance with the magnitude of the thus amplified electrical charge. Note that the synchronization frequency for the synchronization detection circuit 11 is supplied from the monitor terminal M in this case. The electrical charge caused by the Coriolis force is detected by the thus-configured angular velocity sensor, to reliably determine the angle of rotation thereof.
However, the angular velocity sensor element of this prior-art example does not have a favorable configuration from the viewpoints of improving the crystal impedance (hereinafter abbreviated to CI) when acting as a tuning fork shaped oscillator, and the breaking strength, frequency stability, and CI stability with respect to the drive power (in other words, the drive level characteristics thereof), as well as the uniformity between elements.
The problems with the angular velocity sensor element of this prior-art example are discussed below. The angular velocity sensor element of the above-described configuration is obtained by using wet etching to etch the composite crystal wafer 2 shown in FIG. 8 and thus obtain the individual composite tuning fork shaped crystal elements 1 (see FIG. 7). Since the crystal has etching anisotropy, the etching speed is different in the different crystalline axis directions. Moreover, the etching mask is formed on the composite crystal wafer 2 in this case in such a manner that the two crystal wafers 2a and 2b in this case are connected together with the X-axis directions thereof oriented in opposite directions and the right side surface of each completed tuning fork form the −X face, when the tuning fork shaped crystal elements 1a and 1b are viewed in an upright attitude. For that reason, a distinctive configuration is formed in the handle portion of the tuning fork (the lower surface portion of the groove of the tuning fork).
An exploded perspective view of the tuning fork with the tines cut away is shown in FIG. 11A, to illustrate this distinctive configuration of the handle portion of the tuning fork. As is clear from this FIG. 11A, a mountain-shaped portion 100 is created where the connective interface forms a peak in the handle portion of the tuning fork. The inner side surfaces of the root portions of tines 101 and 102 of the tuning fork are connected to the mountain-shaped portion 100. An inclined surface 100a that is the main part of this mountain-shaped portion 100 configures a surface that is called the R face of the crystal and surfaces 100b and 100c to the left and right thereof configure r faces of the crystal. This is because the etching speed of the R face is greater than that of the r faces.
A perspective view of a cutaway through the base portion of the tuning fork and the tine 101 of the tuning fork is shown in FIG. 11B. As is clear from this perspective view, a protruberance 101a caused by etching anisotropy is created in the side surface of the tine portion of the tuning fork that abuts the +X face of the crystal. The ridge line of this protruberance 101a is on the +X face of the tuning fork shaped crystal elements 1a and 1b and is created along the longitudinal direction of the tine portion of the tuning fork in the vicinity of a boundary 105 at which the tuning fork shaped crystal elements 1a and 1b are connected. This ridge line crosses the above-described mountain-shaped portion 100 at a position that is slightly offset from the ridge line of the mountain-shaped portion 100.
The above-described physical shape of the handle portion of the tuning fork obstructs the previously mentioned electrical characteristics. For example, when the left and right tines of the turning fork resonate, stresses are generated in the vertical direction in the handle portion of the tuning fork. However, since the mountain-shaped portion 100 suppresses any change in position in the vertical direction, the resonance of the tuning fork is also suppressed. In other words, the left and right tines 101 and 102 of the tuning fork are restrained mechanically by the mountain-shaped portion 100, increasing the load on the resonance of the tuning fork. This increases the CI.
In addition, since stress concentrations can easily occur in the vicinity of an intersection P between the ridge line of the protruberance 101a and the mountain-shaped portion 100 (see FIG. 11B) when the tines of the tuning fork are resonating, this could cause problems such as cracks when the electrical power applied to the tuning fork is increased. This is a cause of deterioration in the drive level characteristics.
In particular, when the amplitude level is low in an angular velocity sensor that is designed to maintain a fixed amplitude level for the tuning fork resonance by the AGC circuit provided with the monitor terminal of the above-described configuration, there is an increase in the electrical power that automatically drives the amplitude to be constant. This can easily cause damage to the tuning fork shaped crystal oscillator. (See Japanese Patent Laid-Open Publication No. 2002-188922, hereinafter referred to as Reference Document 1).