HK1166557B - First and second orders temperature-compensated resonator - Google Patents

Description

First and second order temperature compensated resonators

Technical Field

The present invention relates to a sprung balance, tuning fork or, more generally, a temperature-compensated resonator of the MEMS type for generating a time base or frequency, the first and second order temperature coefficients of which are substantially zero.

Background

EP patent No. 1422436 discloses a balance spring or hairspring formed of silicon and coated with silicon dioxide so that the temperature coefficient is substantially zero around the COSC (cont ô e of firm susse des chronotrees) certification process temperature (i.e., between +8 and +38 ℃). Also, the document WO 2008-043727 discloses a MEMS resonator with similar properties of low drift from its young's modulus over the same temperature range.

However, in the above disclosure, even only second order frequency drift may require complex corrections depending on the application. For example, for electronic quartz watches to be able to obtain COSC certification, electronic corrections must be performed based on temperature measurements.

Disclosure of Invention

It is an object of the present invention to overcome all or part of the above disadvantages by providing first and second order temperature compensated quartz resonators.

The invention therefore relates to a temperature-compensated resonator comprising a body used in deformation, the core of the body being cut at an angle from a quartz crystal (θ) A formed plate is formed, the quartz crystal defining first and second order temperature coefficients, characterized in that the body comprises a coating at least partially deposited on the core and having first and second order temperature-dependent changes in young's modulus of opposite sign to said first and second order temperature coefficients, respectively, of said resonator, such that said first and second order temperature coefficients are substantially zero.

Advantageously according to the invention, the resonator body used in the deformation has only one coating for compensating the two steps. Therefore, depending on the size and sign of each step of the coating material, the chamfer angle and the thickness of the coating in a single crystal quartz are calculated to compensate for the first two steps.

According to other advantageous features of the invention:

-the body comprises a substantially quadrangular-shaped section, the faces of said section being in the same pair;

-the body comprises a substantially quadrangular-shaped section, the faces of which are all coated;

-selecting the chamfer of the plate such that the first and second order temperature coefficients are negative and the coating comprises positive first and second order young's modulus changes;

-the coating comprises germanium oxide;

-selecting the cut angle of the plate such that said first and second order temperature coefficients are positive and negative, respectively, and the coating has first and second order young's modulus variations that are negative and positive, respectively;

-the coating comprises synthetic diamond;

the body is a strip wound around itself to form a balance spring or a thin spring and coupled with the inertial mass;

the body comprises at least two symmetrically mounted arms forming a tuning fork;

-the tuning fork is of inverted and/or grooved and/or tapered and/or finned type;

the body is a MEMS (micro-electro-mechanical system).

Finally, the invention also relates to a time base or frequency base, such as for example a timepiece, characterized in that it comprises at least one resonator according to any one of the preceding variants.

Drawings

Further features and advantages will become apparent from the following description, given by way of non-limiting indication, with reference to the accompanying drawings, in which:

FIGS. 1 to 4 are general perspective views of multiple types of tuning fork resonators;

FIGS. 5A, 5B, 6A and 6B are alternatives to the resonator profiles of FIGS. 1 through 4;

fig. 7 is a general perspective view of a portion of the sprung balance resonator;

fig. 8 is a representative cross-section of the balance spring of fig. 7;

FIG. 9 is a graph illustrating tuning fork first and second order temperature coefficients as a function of the tuning fork cut angle in a single crystal quartz;

fig. 10 is a graph showing first and second order temperature coefficient changes of a quartz tuning fork cut at an angle equal to 8.4 ° with respect to the Z-axis according to the thickness of the germanium oxide layer;

FIGS. 11 and 12 are schematic illustrations of cut angles relative to the crystallographic axis of a quartz crystal.

Detailed Description

As mentioned above, the invention relates to a quartz resonator, which may be of the sprung balance or tuning fork type or more generally a MEMS (micro-electromechanical system). To simplify the explanation of the invention, the only applications presented below are the balance and the tuning fork. However, other resonator applications may be implemented by those skilled in the art without any difficulty in light of the following teachings.

FIG. 9 is a graph showing the first and second order temperature coefficient drift characteristics of the present tuning fork resonator according to the cut angle along the z-axis of the quartz crystal.

Fig. 11 and 12 show spatial interpretations relative to the z-axis of a single crystal quartz. The quartz crystal has crystal axes x, y, z. The x-axis is the electrical axis and the y-axis is the mechanical axis. In the example of fig. 11 and 12, the height of the balance spring or tuning forkhThus having an orientation with respect to the crystallographic axis z which depends on the selected cut angleθ。

Of course, the corner cutθWill not be limited to a single angle relative to an axis, as rotation at multiple angles relative to multiple axes is also possible to achieve the desired technical effects within the present invention. By way of example, the final corner cutθAnd thus may be the result of a first angle phi with respect to the x-axis and a second angle theta with respect to the z-axis.

FIG. 9 shows the first order temperature coefficientαIntersecting the zero axis at cut angles of about 0 degrees and 12 degrees, as shown by the continuous line. It is therefore evident that depending on the cut angle of the single crystal quartz it is possible to "naturally" obtain a first order temperature of substantially zeroCoefficient of degreeαI.e. the resonator has a first order frequency variation that is virtually independent of temperature.

These advantageous features have been used for decades to form time bases for timers with a chamfer close to 0 degrees.

FIG. 9 also shows the second order temperature coefficientβNever intersecting the zero axis as shown by the dashed line. It is therefore apparent that even for the current cut angle close to 0 degrees, quartz is due to the second order temperature coefficientβWhile remaining sensitive to temperature changes, but to a lesser extent than for first order temperature coefficientsα。

Finally, in FIG. 9, it can be seen that the negative cut angle in the single crystal quartz symmetrically forms its first orderαAnd second orderβA resonator with a negative temperature coefficient.

Advantageously, the idea of the invention is to modify the quartz cut angle with a single coatingθThereby compensating for the first order of the quartz resonatorαAnd second orderβTemperature coefficient to obtain a resonator that is insensitive to temperature variations.

By definition, the relative frequency variation of a resonator follows the following relationship:

wherein:

-is a relative frequency change in ppm (10)^-6) Represents;

-Ais a constant dependent on a reference point, in ppm;

- ₀Tis a reference temperature in units of;

-αis a first order temperature coefficient in ppm DEG C^-1Represents;

-βis a second order temperature coefficient in ppm DEG C^-2Represents;

-γis a third order temperature coefficient in ppm DEG C^-3And (4) showing.

Also, the Coefficient of Thermal Elasticity (CTE) represents a relative change in young's modulus according to temperature. The terms used below "α"and"β"therefore denotes the first and second order temperature coefficients, respectively, i.e. the relative frequency change of the resonator as a function of temperature. Term "α"and"β"depends on the thermoelastic coefficient of the resonator body and the thermal expansion coefficient of the body. Also, the term "α"and"β"also taking into account the coefficients characteristic of any individual inertial component of the balance, such as for example a sprung balance resonator.

The thermal dependence may also include contributions from the maintenance system, since oscillations of any resonator intended for a time or frequency base must be maintained. Preferably, the resonator body is a quartz core coated with a single coating on top of the metallization, typically over at least a part or all of its outer surface and possibly if piezoelectric actuation is desired. Obviously, in this latter case, the connection pads must remain open, whatever the coating chosen.

The examples shown in fig. 1 to 4 show tuning fork variants 1, 21, 31, 41 applicable to the present invention. They are formed by a base 3, 23, 33, 43 connected to a double arm 5, 7, 25, 27, 35, 37, 45, 47, double arm 5, 7, 25, 27, 35, 37, 45, 47 intended to oscillate in respective directions B and C.

The variants of fig. 2 to 4 show that the inverted tuning forks 21, 31, 41, i.e. the bases 23, 33, 43, extend between the two arms 25, 27, 35, 37, 45, 47 in order to optimize the decoupling between the locking and active areas of the resonators 21, 31, 41 and to optimize the vibrating arm length for a given object surface. The variants of fig. 2 to 4 show that the notch-type tuning forks 21, 31, 41, i.e. the double arms 25, 27, 35, 37, 45, 47 comprise notches 24, 26, 34, 36, 44, 46 for depositing electrodes to increase the piezoelectric coupling and thus provide a small-sized resonator with excellent electrical parameters.

Furthermore, fig. 1 shows a tapered arm variant 5, 7, i.e. in which the profile gradually decreases away from the base 3, so as to better distribute the elastic stress over the length of the arm and so as to increase the coupling of the electrodes. Finally, fig. 1 and 4 show a fin-shaped tuning fork 1, 41, i.e. double arms 5, 7, 45, 47 comprising fins 2, 8, 42, 48 at their ends to increase the oscillation inertia of the arms 5, 7, 45, 47 of the resonator 1, 41 to provide the resonator with an optimal length for a given frequency. It is therefore evident that there are a plurality of possible tuning fork variants, which may be, in a non exhaustive manner, of the inverted and/or grooved type and/or tapered and/or finned type.

Advantageously, according to the invention, each tuning fork 1, 21, 31, 41 comprises first and second order temperature coefficients, compensated by the deposition of the layers 52, 54, 56, 52 ', 54', 56 'on the core 58, 58' of the tuning fork 1, 21, 31, 41. Fig. 5A, 5B, 6A and 6B present four non-exhaustive examples of cross sections of the tuning forks 1, 21, 31, 41 along the plane a-a, which more clearly show their quadrangular or H-shaped section at least partially coated with the layers 52, 54, 56, 52 ', 54 ', 56 '. Of course, to more clearly illustrate the location of each portion 52, 54, 56, 52 ', 54 ', 56 ', the size of coating 52, 54, 56, 52 ', 54 ', 56 ' relative to core 58, 58 ' is not to scale.

A study was first conducted on a tuning fork resonator 1, which tuning fork resonator 1 was cut in a single crystal quartz along a negative angle with respect to the z-axis, i.e. along negative first and second order temperature coefficients. Therefore, materials having positive first and second order coefficients of thermal elasticity CTE1, CTE2 are sought. Germanium oxide (Ge 0) has been found₂) Tantalum oxide (Ta)₂O₅) And stabilized zirconia or hafnia meet these characteristics.

Analysis was performed to find the cut angle in quartz with a single coatingθThereby compensating for the first and second order alpha and beta of the quartz resonatorTemperature coefficient. For the case of fig. 5A, i.e. the coatings 52, 54 on each side wall of the arms 5, 7 of the tuning fork 1, it is found that the first and second order temperature coefficients of the tuning fork resonator 1 converge at an angle of-8.408 degrees with respect to the z-axisθAnd the thickness of each layer 2, 4dAt 5.47 μm.

This convergence is illustrated in fig. 10, which fig. 10 clearly shows that both the first order alpha and second order beta temperature coefficients of the tuning fork 1 are for the same thickness of the layers 2, 4dAll intersecting the zero axis.

For FIG. 6A, i.e., coating 56 completely covering arms 5, 7 of tuning fork 1, it was found that the first and second order temperature coefficients of tuning fork resonator 1 converge at an angle of-8.416 degrees relative to the z-axisθAnd thickness of layer 6dIs at 4.26 μm. The following conclusions were therefore drawn: corner cutθSubstantially equal to the variation of fig. 5A, however, the desired thickness of the coating 56dMuch smaller.

In a similar explanation of the grooved tuning fork profiles shown in FIGS. 5B and 6B, the angle can also be determinedθAnd thicknessd. The situation of fig. 6B is particularly advantageous because the coating 56' at the edge of the groove increases the surface on which the compensation layer is active. It is therefore apparent that for the particular case of FIG. 6B, the thickness of the coating 56dIt will be necessary to be even smaller.

It is to be noted that, for all the variants described above, the bases 3, 23, 33, 43 do not necessarily have to be coated, although it is necessary to coat the arms 5, 7, 25, 27, 35, 37, 45, 47. In fact, it is at the stress areas where the coating 52, 54, 56, 52 ', 54 ', 56 ' must be present.

In the example shown in fig. 7 and 8, balance spring 11 can be considered as having body 15 integral with collet 13 and in which the first and second order temperature coefficients of the body are compensated. Fig. 8 presents a section of body 15 of balance spring 11, which more clearly shows the quadrangular shape thereof in section. The body 15 may thus be formed from its lengthlHeight, heighthAnd thicknesseAnd (4) limiting. Fig. 8 shows an example in which the core 18 is entirely coated in a manner similar to fig. 6A. Of course, fig. 8 shows only a non-limiting example,and with respect to tuning forks 1, 21, 31, 41, balance spring 11 may have a coating on at least part of the outer surface or on the entire outer surface of body 15.

Therefore, secondly, a study was made of a sprung balance resonator whose balance spring 11 cuts into a single crystal quartz with negative first and second order temperature coefficients α and β and with a coating material whose first and second order thermoelastic coefficients CTE1, CTE2 are positive.

Analysis was performed to find the cut angle in quartz with a single coatingθThereby compensating for the first and second order temperature coefficients of the quartz resonator.

For the case of fig. 8, i.e. coating 16 that covers body 15 of balance spring 11 entirely, it is found that the first and second order temperature coefficients of the resonator converge on a plurality of thermal expansion values of the balance:

wherein:

- α_balis the coefficient of thermal expansion of the balance, expressed in ppm. ° c-1;

-θis the tangent angle in quartz, expressed in degrees;

-dis Ge0₂The thickness of the coating, expressed in μm.

Thus, in light of the above explanation, the teachings of the present invention are not limited to a particular coating material or to a particular resonator or even to a particular deposition area of the coating. The exemplary cut with respect to the z-axis of the quartz crystal is also not limiting. Other references in the quartz crystal (such as x and y axes) are also possible, as described above, multiple rotations are possible.

It is thus clear that it is possible to compensate in an advantageous manner, according to the invention, the first and second order temperature coefficients of any quartz resonator having a single layerThe first and second order coefficients of thermal elasticity CTE1, CET2 andαandβthe sign is opposite. It must therefore be understood that alternative cut angles for single crystal quartzθ ’It is also possible to perform a compensation in which the first and second order temperature coefficients are not negative.

By way of non-limiting example, if alternative first and second order temperature coefficientsα ’Andβ ’positive and negative, respectively, it is possible to use alternative coatings whose first and second order coefficients of thermal expansion CTE1 ', CET 2' have opposite signs, i.e. negative and positive, respectively. The coating may thus be formed of synthetic diamond, which advantageously means that the resonator may remain transparent.

Claims (15)

1. A temperature compensated resonator comprising a body for use in deformation, wherein a core of the body is formed by a plate formed with cut angles in a quartz crystal defining first and second order temperature coefficients, characterized in that the body comprises a coating at least partially deposited on the core and having first and second order young's modulus variations as a function of temperature, the first and second order young's modulus variations being of opposite sign to the first and second order temperature coefficients, respectively, of the resonator such that the first and second order temperature coefficients are substantially zero.

2. The resonator according to claim 1, characterised in that said body has a substantially quadrangular-shaped cross section with equally centred faces.

3. The resonator according to claim 1, characterized in that said body comprises a substantially quadrangular-shaped section, the faces of said section being entirely coated.

4. The resonator according to claim 1, characterized in that the cut angle of the plate is chosen such that the first and second order temperature coefficients are negative.

5. A resonator according to claim 4, characterized in that the coating has a positive first and second order Young's modulus change.

6. The resonator according to claim 5, characterized in that the coating comprises germanium oxide.

7. The resonator according to claim 5, characterized in that the coating comprises tantalum oxide.

8. The resonator according to claim 1, characterized in that said cut angle of said plate is chosen such that said first and second order temperature coefficients are positive and negative, respectively.

9. The resonator according to claim 8, characterized in that said coating comprises first and second order young's modulus changes being negative and positive, respectively.

10. The resonator according to claim 9, characterized in that the coating comprises synthetic diamond.

11. The resonator according to claim 1, characterized in that said body is a strip wound around itself to form a balance spring and coupled with the inertial component.

12. The resonator according to claim 1, characterized in that said body comprises at least two symmetrically mounted arms forming a tuning fork.

13. The resonator according to claim 12, characterized in that said tuning fork is inverted and/or grooved and/or tapered and/or finned.

14. The resonator according to claim 1, characterized in that said body is a MEMS.

15. Timepiece, characterized in that it comprises at least one resonator according to any one of the preceding claims.