` ~2~S533 This invention relates to a high-frequency piezoelectric resonator and in particular a filter as well as to a method of fabrication of said resonakor.
U.S. patent No 3,694,677 discloses a piezo-electric resonator in which a quartz disk is providedwith a thinned central region forming a diaphragm, said thinned region being jolned to the non-thinned region by a zone which forms a step. An electrode extends from the exterior of the disk across the stepped region to the center of the thinned region. There is placed on the other face of the disk an~her electrode which also e~tends from the periphery of the disk to the center of the thinned region. Said other face can be 1at or else it can also have a stepped zone which must pass over the electrode.
It is found in practice that the fabrication of electrodes by sputtering or evaporation of a metal film-layer does not prove satisfactory. The fabrication of these elactrodes must in fact meet a number of essential requirements. In the first place, the electrodes must have an electrical resistance which is as low as possible and must also be capable of withstanding the vibrations to which the crystal is subjected. In the second place, the electrodes must not have an excessive mass in order to avoid any risk of producing considerable changes in the frequency of the crystal.
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~205533 In practice, the electrodes employed are of silver or preferably of aluminum since this is a relatively lightweight material having good electrical conductivity.
The thickness of the electrodes is of the order of approximately 1000 Angstroms, thus ma]cing it possible to obtain a resonator having a high Q factor or loaded quality factor.
If the electrodes are fabricated in accordance with the method taught by U.S. patent No 3,694,677, it is found that they have a brittle point at the level of the step. This means that, af~ter a certain period of use and sometimes even immediately after the aging treatment usually performed in order to stabilize the resonator, this latter becomes unserviceable as a result of inter-ruption of electrical continuity.
The production of a reliable piezoelectric resonator therefore makes it necessary to modify the fabrication of the electrodes in order to ensure satis-factory service life of said electrodes.
Furthermore, the present Applicant has found that any accumulation of material at the level of the stepped zone induces stresses in the thinned region of the crystal. These stresses impair the thermal drift of the crystal.
Accordingly, it is an ob~ect of the present invention to provide a high-frequency resonator having ~CI SS33 electrodes which are reliable over a long period of time without thereby affecting the properties of the resonator.
The invention also relates to a high-frequency piezoelectric resonator of the type comprising a plate or disk of quartz (or like piezoelectric material) in which a non-thinned region surrounds a thinned region forming a diaphragm. The thinned region is joined to the non-thinned region by means of at least one zone forming a step and provision is made for at least one electrode which extends across the step from the non-thinned region to the thinned region. The resonator is distinguished by the fact that said electrode is associated with a con-ductive tongue formed by a metallic film-layer deposited in the stepped zone.
Said metallic film-layer i5 preferably deposited by sputterin~ or vacuum evaporation.
The conductive tongue aforesaid can be placed beneath the electrode and can be of gold.
The tongue can have a mean thickness of the order of 1000 Angstroms.
One electrode can be of silver, One electrode can have a thickness withln the range of 500 to lOOO Angstroms.
The conductive tongue can be of greater width.
Said conductive tongue can extend from the non-thinned region to that portion of the thinned region which ,y ~ .
is adjacent to the stepped zone.
The invention further relates to a resonator made up of two elementary resonators coupled together in such a manner as to form a fil~er.
Finally, the invention is concerned with a method of fabrication of a resonator of the aforementioned type comprising a prelimlnary step of formation by ion machining of a region of reduced thickness in the central portion of a quartz plate or quartz disk, the thinned region being separated from the non-thinned region by a zone which forms a step. The invention essentially comprises the following process steps :
a) a step involving deposition of a conductive tongue which extends across the stepped zone ;
b) a step which involves mounting of the plate or disk on at least two conductive masts by means of a conductive cement ;
c) a step involving deposition of the electrodes, one electrode being so arranged as to cover the conductive tongue at least ~o a partial extent, deposition by the sputtering technique being performed in such a manner as to ensure that each conductive mast aforesaid is in contact with one electrode.
In a preferred embodiment, the method comprises an operation involving measurement of the resonator fre-quency during deposition of the electrodes in such a `" ~ZOSi533 manner as to stop the deposition process as soon as the desired value of frequency has been attained.
Other features of the invention will be more apparent upon consideration of the following description and accompanying drawings, wherein :
- Figs. 1 and 2 are respectively a vertical sectional view and a top view illustrating a resonator in accordance with the invention ;
- Fig. 3 illustrates a masking device for the formation of the conductive tongues in accordance with the invention ;
- Figs. 4 and 5 illustrate the deposition operations involved in the formation of the conductive tongues in accordance with the invention î
- Figs. 6a and 6b are two vertical sectional views at right angles to each other and showing a resonator in accordance with the invention and in which the electrodes are so arranged as to form a filter.
In Figs. l a~d 2, a piezoelectric disk comprises a non-thinned peripheral region l surrounding a thinned region 2, the junction between these two regions being constituted by a zone forming a step as a result of an ion machining process. Starting from the surface, the step is constituted by a cylindrical region 4 followed by a conical region 3. For example in the case of a disk having a thickness of 50 microns and a thickness of the 0553;~
diaphragm 2 of the order of 5 to lO microns, the cylin-drical region 4 has a depth of approximately 10 microns.
One of the electrodes 5 is placed on the flat underface of the disk and extends from the periphery of this latter to the center of the diaphragm 2 and terminates in a cir-cular central region ll. The electrode 5 has the general shape of a triangle, the base of which is located at the periphery of the disk and the vertex of which is located at the center of the diaphragm 2. This shape facilitates the electrical connection of the electrodes at ~heir periphery while adding the smallest possible quantity of material at the level of the diaphragm 2.
The other electrode 6 also extends from the periphery of the disk to the center of the diaphragm 2 and terminates in a circular central region 12 located opposite to the circular region ll. Likewise said electrode has the general shape of a triangle, the base of which is located at the periphery of the dis]c. A
conductive tongue 8 is preferably placed beneath the electrode 6 and extends from the periphery of the disk to the external portion of the diaphragm 2 which is adjacent to the stepped zone 3. Said conductive tongue 8 is a metallic layer which is deposited on the disk and has a rectangular shape when looking from above. In this embodiment, the tongue 8 is therefore of greater width than the electrode, thus increasing the area on which an lZ(J5533 electrical contact is established at the level of the stepped zone. Furthermcre, the conductive tongue 8 extends at the level of the diaphragm over a distance which is ]ust sufficient to facilitate electrical connection at the level of the step. It will be noted that the acti~e zone of the conductive tongue is constit-uted by the portion located at the level of the step or in other words the zone shown at 9, by the portion 10 which extends over a minimum distance on the diaphragm 2, and by the portion which i5 joined to the flat portion of the electrode 6.
In Fig. 3, a masking device 20 for the formation of the conductive tongues comprises a base plate 15, an intermediate plate 16 provided with a plurality of holes 17 (only one of which is illustrated~ and a top plate 18 provided with openings 19 (only one o~ which is illus-trated) corresponding to the shape of the conductive tongues to be formed on a piezoelectric disk provided with a diaphragm 2. The holes 17 have an external diameter corresponding to the di~neter of the piezoelectric disks.
The thickness of the intermediate plate 16 is substantially greater than that of the piezoelectric disks, with the result that the disks are maintained in position by clamping between the plates 15 and 18. The plates 15, 16 and 18 are held ~ogether by means of clamping screws 22 which are engaged within holes 23 and cooperate with nuts 22'.
~,'213S~33 ~8--As shown in Fig. 4, the support 20 fitted with the piezoelectric disks is mounted on a plate 21 which is inclined at an angle ~ with respect to the direction of sputtering or evaporation.
As shown in detail in Fig. 5, the portion of region 3 on which the conductive tongue i5 formed can be placed substantially at right angles to the direction of sputtering as a result of the angle of inclination afore-said. Maximum thickness in the active zone of the con-ductive tongue can thus be obtained by evaporation. The conductive tongue 8 can be of any metal which permits de-position either by sputtering or evaporation, gold being particularly suitable for this purpose. Its thickness 5for example of the order of 1000 Angstroms) is such as to permit reinforcement of the electrode 6 without adding an excessive quantity of material at the level of the stepped portion.
Each piezoelectric disk fitted with its con-ductive tongue is bonded to two conductive masts by means of a conductive cement. The electrodes 5, 6 are then deposited, each of the electrodes being in contact with one of the masts. The electrode which extends across the stepped portion covers the conductive tongue at least partially.
During deposition of the electrodes, the frequency of the resonator is measured as soon as this is il Z1~5533 permitted by the thickness of the electrodes. The addition of material at -the level of the diaphragm as a result of deposition of the electrodes has the effect of reducing the frequency of the quartz. The deposition operation is interrupted when the desired frequency of the quartz is precisely attained.
In a subsequent process step, the quartz is placed within a vacuum-tight enclosure and subjected to an aging treatment tresidence time of a few hours at a temperature of the order of 95C).
As shown in Figs. 6a and 6b, a piezoelectric filter is obtained at the level of a diaphragm 2 as a result of coupling between two individual resonators. To this end, two electrodes are placed close together on one of the faces of the diaphragm, and a grounded electrode is disposed on the opposite face, thereby ensuring that its extremity is located opposite to the gap between the two electrodes of the othex face. Thus one electrode 27 is disposed on the top face of a quartz disk, extends from the periphery of said disk to the center of the diaphragm
2, and has a circular termination 32 at the center. Two electrodes 2~ and 25 disposed in a general direction at right angles to the electrode 27 extend from the peri-phery of the underface of the piezoelectric disk to the center of the diaphragm 2. At the center, a gap 33 is left between said electrodes 24 and 25 and the width of ,. .
said gap determines the coupling between the two resonators.