GB1560537A

GB1560537A - Piezoelectric microresonator

Info

Publication number: GB1560537A
Application number: GB3714476A
Authority: GB
Original assignee: Ebauches SA
Current assignee: Ebauches SA
Priority date: 1975-09-18
Filing date: 1976-09-08
Publication date: 1980-02-06
Also published as: CH1212475A4; FR2325093B1; JPS5236954A; DE2640886B2; DE2640886C3; DE2640886A1; NL7610149A; FR2325093A1; CH600423B5

Description

(54) PIEZOELECTRIC MICRORESONATOR (71) We, EBAUCHES S.A. of 1 faubourg de rHopital, CH-2001 Neuchitel, Switzerland; a Swiss body corporate do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a piezoelectric microresonator comprising a resonating part connected to suspension arms the ends of which are integral with a support member, the said resonating part and the said arms comprising thin metallic layers thereon forming excitation electrodes and connection means respectively.

The problem is to provide a resonator of very small dimensions which can be used in an electronic watch, whilst satisfying the following conditions: - Quality factor of the order of 5.104; - Not to be too fragile before mounting on a stand to permit an easy manipulation and the automation of the mounting process; - Simplifying the electric connections between the exciting electrodes and the terminals of an oscillator circuit; good resistance to shocks when the resonator is mounted on its stand; - The adjustment of the oscillating frequency must be simple.

A microresonator is already known which is in the form of a quartz crystal plate comprising perpendicular suspension arms, the assembly formed by a chemical etching from a quartz plate of small thickness of the order of 0.040 mm. Metallic electrodes (thickness 1000 A), are deposited on a face by evaporation under vacuum, for permitting the application of an electric field which creates a bending vibration mode having a resonant frequency of 100 kHz when the plate is supported at the end of the arms.

However, the quality factors obtained with such resonators are only of the order of 1000, which is at least 50 times too small.

There is also known a miniaturised tuning fork formed of two coupled quartz plates, on which metallic films are disposed by evaporation to form the electrodes. This construction leads to greatly reduced dimensioning (of the order of 5 x 1 x 0.025 mm), for relatively low frequencies of 10 to 100 kHz, and the quality factor Q is of the order of 5 x 104. This device is produced using semi-conductor manufacturing techniques, in large quantity and at a low price; 70 tuning forks may be made simultaneously on a plate of quartz. The resistance to shock is excellent when the tuning fork is soldered on its stand.

However, it is difficult to adjust the oscillating frequency of this miniature tuning fork by altering the capacitance of one of the variable capacitors of the associated oscillator circuits. Additionally, the equivalent series resistance is very high, which creates very difficult problems for the manufacture of an oscillator circuit having a very small consumption and of good stability. Finally, it is relatively difficult to automate the mounting process.

According to the present invention, a piezoelectric microresonator comprises a resonant part vibrating in a longitudinal extension-contraction principal mode and having suspension arms cooperating with a support member extending around at least part of said resonant part, the resonant part, the arms and the support member having thin metallic layers thereon which form excitation electrodes on the resonant part and connection means on the arms and the support member, said resonant part, said suspension arms and said support member having the same thickness and forming a unitary structure fashioned from a plate of piezoelectric material, said suspension arms being so arranged as to warrant mechanical decoupling between the resonant part and the support member.

The present invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a first embodiment of the invention; Figure 2 is a section on the line Il-Il of Figure 1; Figures 3 and 4 are perspective views which show particular orientations of the resonant part with respect to the axes of a quartz crystal and the disposition of the electrodes; and Figure 5 is a plan view of a second embodiment.

The resonator shown in Figures 1 and 2 is is made from a rectangular plate 1 of quartz of a thickness less than 100 x 10-6 m, cut, for example, perpendicularly to the X axis of the crystalline system. The length of the plate also forms an angle of between -5 and 5 with the Y axis of the quartz crystal.

Using a known photolithographic process, then etching, or by using a sand jet, two slots 2 a resonant part 4 and two suspension arms 5 and 6. The resonant part 4 is presented in the form of a parallelepiped-ic plate having length/width and width/thickness ratios greater than five. The suspension arms 5 and 6 are constituted by two rectilinear portions disposed perpendicularly to the longitudinal axis of the plate 1, each of them being positioned halfway along the length and on each side of the plate 1. The resonant part 4 and the arms 5 and 6 are thus mechanically decoupled from the rest of the plate 1 which acts as a support. The members 4, 5, 6, however, remain connected to the plate 1.

Metallic layers 7 and 8 are disposed on each of the faces of the resonant plate 4, on the arms 5 and 6, and on surfaces 9, 10 and 11 of the plate 1, so that the resonator may be soldered on a stand (not shown) whilst ensuring the electric connection.

Figure 3 shows the orientation of the plate 4 with respect to the X and Y axes of the quartz crystals. The longitudinal axis of the plate extends in a direction Y' forming an angle o: with the Y axis of the crystal. The application of an alternating voltage on the electrodes 7, 8 creates an electric field E, in the direction X, which causes vibration of the resonating plate 4 in a principal mode of longitudinal extension-contraction. The arms 5 and 6 are connected to the plate 4 in the region of the nodal points of the vibration. Experience shows that the width of the arm does not introduce nay disturbance if it is less than a tenth of the length of the plate 4.

Figure 4 shows a variation where the plate of quartz is cut parallel to the axis X, the normal to the plane of the plate forming an angle comprises between 5 and +5 with the optical axis Z of the quartz. The longitudinal axis of the plate 4a is parallel to the axis Y.

The electrical field is then applied in the direction X by means of two pairs of elecrtodes 13 and 14 on the two edges of each of the faces of the plate 4a. The two electrodes of each pair 13 and 14 extend (in a manner not shownO on each of the faces of corresponding arms, then on tracks of the support plate 1, which are connected together and connected to one of the terminals of an oscillator circuit. With a suitable excitation, the plate 4a vibrates likewise in a principal mode of extension-contraction.

Due to the crystalline structure of the piezoelectric material used, it may be observed that the extension-contraction vibration mode is accompanied by secondary modes. With a quartz crystal cut of X +5 , for example, an extensioncontraction of the width of the plate is observed as well as a shearing in the plane of the plate which deforms the rectangle into a parallelogram. A support impeding these deformations, would indirectly reduce the principal deformation energy along the axis 'Y', which would imply a reduction in the quality factor of the resonator.

Thus, the arms 5 and 6 must be shaped and dimensioned to enable the plate to vibrate with a minimum of disturbance.

These coupled deformations depend on the crystalline structure and are thus different for quartz, or other piezoelectric materials such as lithium tantalate, lithium niobate, in different cuts. For this reason, special geometrics must be developed such as shown in Figure 5.

The plate 15 has a narrowed central portion, from each side of which an arm extends, each of which arms forms an extension of a portion 16 or 17 parallel to the main axis of the resonant plate 15. The portions 16 or 17 and plate 15 are coupled via a rectilinear arm or by a curved arm. The parts 16 and 17 can be connected together, in which case, only a single slot need be provided to uncouple the active part of the suspension arms.

The main advantage of microresonators obtained by cutting out from thin plates of piezoelectric crystals resides in the possibility of providing suspension arms presenting a relatively complex form and adapted to the different types of deformations generated in the active part.

From the mass production point of view, one has moreover, the very great advantage of being able to produce a large number of resonators on the same thin plate, whilst using known metallisation techniques, chemical etching or other processes. Moreover, the precision of the dimensions obtained by the photolithographic techniques favour a good reproduction of the characteristcs of such a resonator.

Thanks to this great simplification, obtained in conserving the properties of stability and frequency, and electric parameters well adapted to oscillator circuits having a very small comsumption, one can choose freely the crystalline cut, as for example for the quartz, the GT cut type which gives a remarkable frequency stability in the temperature range 0 to +100us. The GT cut is described by W.P.

Mason in "A New Quartz Crystal Plate, Designated GT", Proceedings of the IRE, May 1940, p. 220-223. The active part is in the form of a rectangle the width/length ratio of which is between 0.85 and 0.9. In this case the arms have a similar geometry to that of Figure 5 and extend from the ends of the nodal axis parallel to the length of the rectangle. At a frequency of the order of 780 kHz the resonator would have dimensions of 4.9 x 4.2 x 0.4 mm with electric parameters well adapted to the manufacture of an oscillator circuit having a small consumption suitable for wrist-watches.

WHAT WE CLAIM IS:1. A piezolectric microresonator comprising a resonant part vibrating in a longitudinal extension-contraction principal mode and having suspenison arms cooperating with a support member extending around at least part of said resonant part, the resonant part, the arms and the support member having thin metallic layers thereon which form excitation electrodes on the resonant part and connection means on the arms and the support member, said resonant part, said suspension arms and said support member having the same thickness and forming a unitary structure fashioned from a plate of piezoelectric material, said suspension arms being so arranged as to warrant mechanical decoupling between the resonant part and the support member.

2. A microresonator as claimed in claim 1, in which the resonant part is a parallelepipedic plate having a length/width ratio greater than 5 and a width/thickness ratio greater than 5, the suspension arms being formed of two rectilinear segments disposed perpendicularly to the longitudinal axis of said parallelepipedic plate, at the middle of each side of the said plate and in the plane of the plate.

3. A microresonator as claimed in claim 1, in which each of the suspension arms is connected to said support member by a portion parallel to the main axis of the resonant part, the said portion being connected to the said resonant part by a rectilinear or curved portion.

4. A microresonator as claimed in claim 2 or 3, in which the suspension arms have a width which is smaller than a tenth of the length of the resonant part.

5. A microresonator as claimed in claim 1, in which the plate is cut from quartz in the form of a rectangle, the plane of the said plate being perpendicular to the X axis of the quartz crystal whilst the longitudinal axis of the said plate forms an angle of between -5" and +5 with the Y axis of the quartz crystal.

6. A microresonator as claimed in claim 1, in which the said plate is cut from quartz in the form of a rectangle, the plane of the said plate being parallel to the X axis of the quartz crystal and the normal to the said plane forming an angle of between -5" and +5 with the optical axis Z of the quartz crystal.

7. A microresonator as claimed in claim 1, in which the plate is cut from quartz with an orientation with respect to the crystalline axes system corresponding to that of the GT cut.

8. A microresonator as claimed in claim 1, 4 or 5, in which the resonant part is formed as a plate symmetrically narrowed at the centre where it is supported by the two suspension arms.

9. A microresonator as claimed in claim 1, in which the said metallic layers are deposited on each of the two faces of the said plate, on a portion of the resonant part as electrodes, on one of the surfaces of each of said arms and on surfaces of the sides of the said support member so as to enable soldering of the plate on a stand and to provide electrical connection between the said electrodes and an oscillator circuit.

10. A microresonator as claimed in any preceding claim, in which the thickness of the said plate is less than 100 x 106 m.

11. A piezoelectrci microresonator substantially as herein described with reference to and as illustrated in Figs. 1 and 2 or Fig.

3 or Fig. 4 or Fig. 5 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

adapted to oscillator circuits having a very small comsumption, one can choose freely the crystalline cut, as for example for the quartz, the GT cut type which gives a remarkable frequency stability in the temperature range 0 to +100us. The GT cut is described by W.P.

Mason in "A New Quartz Crystal Plate, Designated GT", Proceedings of the IRE, May 1940, p. 220-223. The active part is in the form of a rectangle the width/length ratio of which is between 0.85 and 0.9. In this case the arms have a similar geometry to that of Figure 5 and extend from the ends of the nodal axis parallel to the length of the rectangle. At a frequency of the order of 780 kHz the resonator would have dimensions of 4.9 x 4.2 x 0.4 mm with electric parameters well adapted to the manufacture of an oscillator circuit having a small consumption suitable for wrist-watches.

WHAT WE CLAIM IS:1. A piezolectric microresonator comprising a resonant part vibrating in a longitudinal extension-contraction principal mode and having suspenison arms cooperating with a support member extending around at least part of said resonant part, the resonant part, the arms and the support member having thin metallic layers thereon which form excitation electrodes on the resonant part and connection means on the arms and the support member, said resonant part, said suspension arms and said support member having the same thickness and forming a unitary structure fashioned from a plate of piezoelectric material, said suspension arms being so arranged as to warrant mechanical decoupling between the resonant part and the support member.
2. A microresonator as claimed in claim 1, in which the resonant part is a parallelepipedic plate having a length/width ratio greater than 5 and a width/thickness ratio greater than 5, the suspension arms being formed of two rectilinear segments disposed perpendicularly to the longitudinal axis of said parallelepipedic plate, at the middle of each side of the said plate and in the plane of the plate.
3. A microresonator as claimed in claim 1, in which each of the suspension arms is connected to said support member by a portion parallel to the main axis of the resonant part, the said portion being connected to the said resonant part by a rectilinear or curved portion.
4. A microresonator as claimed in claim 2 or 3, in which the suspension arms have a width which is smaller than a tenth of the length of the resonant part.
5. A microresonator as claimed in claim 1, in which the plate is cut from quartz in the form of a rectangle, the plane of the said plate being perpendicular to the X axis of the quartz crystal whilst the longitudinal axis of the said plate forms an angle of between -5" and +5 with the Y axis of the quartz crystal.
6. A microresonator as claimed in claim 1, in which the said plate is cut from quartz in the form of a rectangle, the plane of the said plate being parallel to the X axis of the quartz crystal and the normal to the said plane forming an angle of between -5" and +5 with the optical axis Z of the quartz crystal.
7. A microresonator as claimed in claim 1, in which the plate is cut from quartz with an orientation with respect to the crystalline axes system corresponding to that of the GT cut.
8. A microresonator as claimed in claim 1, 4 or 5, in which the resonant part is formed as a plate symmetrically narrowed at the centre where it is supported by the two suspension arms.
9. A microresonator as claimed in claim 1, in which the said metallic layers are deposited on each of the two faces of the said plate, on a portion of the resonant part as electrodes, on one of the surfaces of each of said arms and on surfaces of the sides of the said support member so as to enable soldering of the plate on a stand and to provide electrical connection between the said electrodes and an oscillator circuit.
10. A microresonator as claimed in any preceding claim, in which the thickness of the said plate is less than 100 x 106 m.
11. A piezoelectrci microresonator substantially as herein described with reference to and as illustrated in Figs. 1 and 2 or Fig.

3 or Fig. 4 or Fig. 5 of the accompanying drawings.