Crystal resonator device

A resonator comprises a quartz resonator element (11) sealed between a pair of flat quartz housing members (12, 13) the resonator comprises an active central region 110 defined by an annular recess 111 within which vibrational energy is trapped. The crystal orientation of the housing may be aligned with that of the resonator to minimise thermal effects.

This invention relates to quartz crystal resonators and in particular to 
resonators suitable for high frequency close tolerance applications. 
BACKGROUND OF THE INVENTION 
A low loss quartz resonator typically comprises a quartz plate of suitable 
crystal orientation or cut driven via electrodes disposed on either side 
of the plate. It has been found that by thickening the electroded portion 
of the plate, either by metallising the electrodes or by providing the 
plate with a convex contour, it is possible to trap the mechanical 
vibration within the electrode region so as to provide a low loss device. 
The device can be mounted via its edge or rim without significant 
interference with the vibration of its active region. Typically the device 
is mounted at discrete points using spring clips and a silver loaded resin 
whereby electrical contact to the device is effected. 
Such a design has a number of problems. The frequency of the device is 
inversely proportional to its thickness and is thus sensitive to the 
presence of surface films such as water or organic materials. It is for 
this reason that close tolerance devices are hermetically sealed either in 
vacuum or in a dry nitrogen atmosphere. However, it has been found that 
the silver loaded region used for contacting the device is a source of 
trace organic materials which cause ageing of the device. 
Typically the trapped vibrational mode of the device is a thickness shear 
mode. However, such a mode is inevitably associated with flexural 
vibrations which are not trapped and hence reach the edge of the device. 
With conventional discrete point mounts, these vibrations are partially 
reflected back into the resonator area. At certain frequencies these 
interfere constructively to produce whole-plate resonances. These 
resonances can be close together in frequency as well as having poor 
temperature coefficients. The result is that as their frequency passes 
through the frequency of the wanted trapped resonance, they interfere with 
that resonance and cause `activity dips` with an associated frequency 
glitch. Furthermore, slight variations in the boundary conditions at the 
edge of the plate can cause large changes in the frequencies of these 
plate resonances, and hence the temperatures at which they interfere with 
the main mode. This leads to thermal hysteresis which causes problems with 
temperature compensated crystal oscillators. 
Another major problem with conventional discrete point mounting is the 
vibration sensitivity of the final device. In theory if the mount was 
completely symmetrical, and the vibration was placed at the centre of 
symmetry, then the vibration sensitivity vanishes in all three axes. 
However, in practice this is very difficult to achieve because of the 
difficulty in placing the silver loaded resin or other mounting structures 
at the precise point required. 
The object of the invention is to minimise or to overcome these 
disadvantages. 
One approach to the above problem is to mount the device in a quartz 
package. This technique is discussed in our specification No. 2202989B 
which describes and claims a crystal resonator assembly, including a 
crystal resonator, and first and second housing members mated together to 
define a cavity in which the resonator is located, wherein the housing 
members are formed from the same crystal material and hence the same 
crystal orientation as the resonator. 
Whilst this structure has proved satisfactory in operation, the manufacture 
of the housing members within which the resonator is encapsulated 
represents a significant process cost. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a quartz crystal 
resonator device, including a resonator element and first and second 
housing members, wherein the resonator element comprises a quartz plate 
having a contoured central portion surrounded by a annular recess whereby 
vibrational energy is trapped in the central portion, and wherein said 
housing members comprise each a flat quartz plate secured to the 
respective surface of the resonator element at its periphery.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to the drawing, the device comprises a quartz resonator element 
11 mounted between a pair of flat quartz housing plates 12, 13. The 
resonator element 11 comprises a quartz plate which is contoured in its 
generally central region to define a convex or lens-shaped region 110 
surrounded by an annular recess 111. The geometry, i.e. the depth and 
radius of curvature of the recess, is defined so as to ensure trapping of 
the preferred thickness-shear vibrational mode within the region 110. 
Preferably the recess and the contoured surface are symmetrical about the 
centre of the resonator plate. 
The resonator element 11 may be contoured by a radio frequency plasma 
etching process. 
The rim of the resonator element 11 is of the same thickness as the central 
region 110. This permits the use of flat sealing plates 12, 13 secured to 
the periphery of the element 11 by a hermetic seal 14 formed e.g. from a 
low melting point glass or a metal alloy. The thickness of the seal 14 
determines the separation between the plates 12, 14 and the active region 
of the resonator element 11. As the seal is wholly inorganic the problem 
of contaminant emission is eliminated. In some applications the seal may 
comprise an electrostatic bond or a diffusion bond. 
The sealing process is conducted at elevated temperatures, e.g. 400.degree. 
to 500.degree. C., and under reduced pressure or vacuum. 
The outer surface of one or both of plates 12 may provide a substrate e.g. 
for a film circuit to provide an integral oscillator package. 
Advantageously the inner surface of the plates 12, 13 and both surfaces of 
the resonator element 11 are coated with gold to effect balanced molecular 
flexes between surfaces. The gold coating of the element 11 also provides 
the driver electrodes. 
The plates 12, 13 are aligned with respect to their crystal orientation 
with the resonator element 11 so as to substantially eliminate thermal 
mismatch. The crystal cuts used for the resonator element are typically 
AT-cuts or SC-cuts, the latter being a doubly rotated cut with a number of 
advantages such as stress compensation. SC-cut devices are however more 
expensive to manufacture. The angular rotation around the X-axis 
determines the thermal expansion coefficient in the plane of the plate. As 
this angle is the same for both the SC-cut and the lower cost AT-cut, it 
is possible to use AT-cut housing plates together with an SC-cut resonator 
element. 
The packaged structure has low vibration sensitivity in the two lateral 
directions. Low sensitivity in the thickness direction is ensured by 
maintaining the contour and hence the vibrational distribution 
symmetrically about the centre of the contour of the resonator plate 11. 
Flexural modes generated by the excitation of the trapped thickness-shear 
mode will inevitably reach the edge of the resonator plate. By virtue of 
the hermetic seal around the boundary of the resonator plate, this energy 
will be coupled into the two sealing plates 12, 13. By appling a suitable 
acoustic e.g. a plastics film absorber 15 to the outside surfaces of these 
plates, these flexural waves are prevented from being reflected back into 
the resonator plate. In this way troublesome activity dips and their 
effected on thermal hysteresis are substantially eliminated. We have also 
found that roughening the outer surfaces of the plate 12 provides 
effective acoustic absorption. 
The gold applied to the inner surfaces of the sealing plates 12, 13 can be 
used to fine adjust the device to frequency. A controlled laser beam 
directed through such a plate evaporates gold from the surface so that the 
gold 16 is deposited on the surface of the resonator plate. This extra 
gold increases the mass loading on the resonator plate and hence reduce 
its frequency. 
The structure may be mounted in a plastics package containing a 
shock-absorbing foam. 
It will be appreciated that the technique described above can be employed 
in the construction of filters which comprise a number of resonators 
appropriately coupled together. It is possible to fabricate a number of 
such resonators on a single monolithic resonator plate with either 
electrical or acoustic coupling between the individual resonators. Such 
multiple resonators suitably connected can then be sealed by quartz 
plates, as described above, to make a filter. Alternatively, any number of 
single or multiple resonators can be aligned and stacked on top of one 
another using the same sealing process for the package with closure plates 
supplying the outer layers of the devices. Such a device can be connected 
either as a filter or as a number of isolated resonators for applications 
which require a number of frequencies.