CH702353A2 - Thermocompensated resonator i.e. hairspring, for timepiece, has body comprising core with material, where body comprises two coatings allowing resonator having thermal coefficients of first and second orders to be zero - Google Patents

Abstract

The resonator i.e. hairspring (1), has a body utilized in deformation and comprising a core with a material, where the body comprises two coatings allowing the resonator having thermal coefficients of first and second orders to be zero. The core of the body includes thermoelastic coefficients of the first and second orders to be negative, where the core of the body comprises monocrystalline silicon. The body has a section in the form of a quadrilateral whose sides are entirely coated, where the two coatings comprise silicon dioxide and germanium dioxide, respectively.

Description

Field of the invention

The invention relates to a thermocompensated resonator of the sprung balance, MEMS or tuning fork type making it possible to manufacture a time or frequency base whose thermal coefficients are substantially zero at least at first and second orders.

Background of the invention

[0002] Document EP 1 422 436 discloses a hairspring formed from silicon and coated with silicon dioxide in order to make the thermal coefficient substantially zero around the COSC procedure temperatures, that is to say between +8 and +38 ° vs. Likewise, document WO 2008-043 727 discloses a MEMS resonator which has similar qualities of low drift of its Young's modulus in the same temperature range.

[0003] However, the drift of the frequency even only in the second order of the above disclosures may require complex corrections depending on the applications. For example, for electronic watches to be COSC certified, an electronic correction based on a temperature measurement must be performed.

Summary of the invention

The object of the present invention is to overcome all or part of the drawbacks mentioned above by providing a thermally compensated resonator at least at first and second orders.

[0005] To this end, the invention relates to a thermocompensated resonator comprising a body used in deformation, the core of the body comprising a first material characterized in that the body comprises at least a first and a second coatings allowing said resonator to have substantially zero first and second order thermal coefficients.

[0006] Advantageously according to the invention, the body of the resonator used in deformation has as much coating as the order of the thermal coefficient to be compensated. Thus, according to the sizes and signs of each order of the materials of the core and of each coating, a calculation of each thickness is carried out in order to compensate for each order.

[0007] In accordance with other advantageous features of the invention: the body has a third coating allowing said resonator to have substantially zero first, second and third order thermal coefficients; the core of the body has first and second order thermoelastic coefficients that are negative like monocrystalline silicon; the body has a section substantially in the form of a quadrilateral whose faces are identical in pairs or whose faces are fully coated; the first coating has positive first order and negative second order thermoelastic coefficients such as silicon dioxide; the second coating has a positive second order and positive first order thermoelastic coefficient such as germanium dioxide or has a negative first order thermoelastic coefficient; the first coating is inverted with the second coating; deposition of said coatings is preferred on the faces parallel to the neutral plane of the body in order to modify the frequency of said resonator as intensely as possible; the body is a bar wound on itself forming a balance-spring and is coupled with a flywheel or has at least two bars mounted symmetrically to form a tuning fork or is a MEMS resonator.

[0008] Finally, the invention also relates to a time or frequency base, such as for example a timepiece, characterized in that it comprises at least one resonator in accordance with one of the preceding variants.

Brief description of the drawings

[0009] Other features and advantages will emerge clearly from the description which is given below, by way of indication and in no way limiting, with reference to the accompanying drawings, in which: FIG. 1 <sep> is a general perspective representation of a balance spring; fig. 2 <sep> is a representative section of the spiral spring of FIG. 1; fig. 3 <sep> is a representation of several embodiments according to the invention; fig. 4 <sep> is a graph representing the elastic moduli of each material according to a first embodiment of the invention; fig. 5 <sep> is a graph showing the elastic moduli of each material according to a second embodiment of the invention; fig. 6 <sep> is a graph showing the absence of frequency variation of a resonator according to the invention; fig. 7 <sep> is a graph showing the first and second order variation in the thermal coefficient of a silicon sprung balance coated with silicon dioxide; fig. 8 <sep> is a graph showing the first and second order variation of the thermal coefficient of a silicon sprung balance coated with germanium dioxide; fig. 9 <sep> is a graph showing the first and second order change in the thermal coefficient of a silicon sprung balance coated with silicon dioxide and germanium dioxide.

Detailed description of the preferred embodiments

[0010] As explained above, the invention relates to a resonator which can be of the sprung balance, tuning fork or more generally a resonator of the MEMS type (abbreviation from the English terms "Micro Electro Mechanical System"). In order to provide a simpler explanation of the invention, only the application to a sprung balance is presented below. However, other resonator applications such as the above will be feasible for a person skilled in the art without undue difficulty from the teaching below.

[0011] Likewise, the explanation is given to a core of the body, in our example a balance spring, formed from monocrystalline silicon. However, the core material is not limited to monocrystalline silicon but can expand to different types of materials such as, for example, polysilicon, glass, nitride, diamond, monocrystalline quartz or metal .

[0012] In the graph in FIG. 6, we can see the frequency drift characterization for current resonators as a function of temperature. A first solid line curve entitled "Quartz-z-cut" shows the frequency drift of a 32 kHz monocrystalline quartz tuning fork made in a slightly rotated z-cut. A second curve in short dashed lines entitled "Si-SiO2" shows the frequency drift of a silicon MEMS resonator coated with silicon dioxide.

For these two curves, it can be seen that the drift is non-zero over a wide range of temperatures, in particular between -20 and 80 ° C. This frequency drift is mainly linked to the variation of the Young's modulus as a function of temperature. However, even low between 10 and 40 ° C, the frequency drift of the two examples of current manufacture may require a correction extrinsic to the resonator. This is the case, for example, with an electronic watch that contains a quartz tuning fork electronically corrected on the basis of a measurement of the temperature of the watch, in order to be COSC certified.

Thus, advantageously, the aim of the invention is to provide a resonator whose frequency drift as a function of temperature is further minimized as shown by the curve in short and long dashed lines entitled "composite" whose scale is intentionally kept identical compared to the other two curves in order to show the important difference of the drift. More precisely, the body of the resonator, according to the invention, has as many coatings as there are orders of the thermal coefficient to be compensated.

Preferably, the body of the resonator thus comprises at least two coatings and possibly a third if the second order compensation still induces an unacceptable frequency drift. However, from the compensated third order, the frequency drift becomes negligible for any resonator. Thus, according to the sizes and signs of each order of the materials of the core and of each coating, a calculation of each thickness is carried out in order to globally compensate each order.

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

or

At a constant which depends on the reference point, in ppm; T0 the reference temperature, in ° C; α the first order thermal coefficient, expressed in ppm. ° C <-> <1>; β the second order thermal coefficient, expressed in ppm. ° C <-> <2>; γ third order thermal coefficient, expressed in ppm. ° C <-> <3>.

[0017] In addition, the thermoelastic coefficient (CTE) represents the relative variation of the Young's modulus as a function of temperature. The terms "α" and "β" which are used below thus represent respectively the thermal coefficients of the first and second order, that is to say the relative variation of the frequency of the resonator as a function of temperature. The terms “α” and “β” depend on the thermoelastic coefficient of the body of the resonator and on the coefficients of expansion of the body. In addition, the terms “α” and “β” also take into account the coefficients specific to a possible separate inertia, such as, for example, the balance for a balance-spring resonator. As the oscillations of any resonator intended for a time or frequency base must be maintained, thermal dependence also includes a possible contribution from the maintenance system. Preferably, the body of the resonator is a core 3 covered with at least two coatings 4, 5.

[0018] In the example illustrated in FIGS. 1 to 3, we can see a hairspring 1 formed with its ferrule 2 of which at least the first and second orders of the thermoelastic coefficient of the body are compensated. Fig. 2 provides a cross-section of the body of the hairspring making it easier to see its quadrilateral-shaped section. The body can thus be defined by its length h, its height h and its thickness e. Fig. 3shows possible alternatives A, A ́, B, C and D without being exhaustive. Obviously, coatings 4 and 5 are not to scale with respect to the dimensions of web 3, in order to better show the locations of each part 3, 4 and 5.

In a first alternative A, only one face of the section is covered successively by the coating 4 then by the coating 5. The stacking order between the coatings 4 and 5 is not fixed, that is to say that is, coatings 4 and 5 can be interchanged. In addition, when it is the faces parallel to the neutral plane F of the bar which are covered, this modifies the frequency of said resonator with more intensity than if the deposition was carried out on those perpendicular to the bending plane F. Of course, it can also be imagined that each coating 4, 5 is present on a different face as illustrated in alternative A ́.

[0020] In a second alternative B or C, the section of the body has identical faces two by two. Thus either two parallel faces comprise the two coatings 4, 5 stacked in no particular order, that is to say that the coatings 4 and 5 can be inverted, as in example B, or each of the parallel faces have one of the coatings 4, 5 as in Example C. Of course, it can also be imagined that a coating 4 is present on two adjacent faces and that the other two faces are covered by a coating 5.

In a third alternative D, the section of the bar comprises faces which are entirely coated successively by the coating 4 and then by the coating 5. The order of stacking between the coatings 4 and 5 does not however here either more importance, that is, coatings 4 and 5 can be swapped.

As illustrated in FIG. 4, a graph showing the thermal dependence of the Young's modulus of each material can be seen to illustrate an embodiment of the invention which uses silicon, silicon dioxide, and germanium dioxide. Thus, the Young's modulus of silicon decreases with increasing temperature while the Young's modulus of the other two materials increases with increasing temperature. In addition, the growth is more marked for silicon dioxide than for germanium dioxide between the two temperature values, i.e. between -20 ° C and 80 ° C.

In fact, the thermoelastic coefficient of silicon is negative in the first and second order when the thermoelastic coefficients of the other two materials are positive in the first order. However, the second order thermoelastic coefficient is negative for silicon dioxide while it is positive for germanium dioxide.

[0024] However, this interpretation of FIG. 4focuses on the thermoelastic coefficient of materials. It is necessary to further take into account the expansion coefficients of the materials and the effect of the oscillation maintenance system to finally obtain the α, β coefficients of the change in the frequency of the resonator. To understand this final interpretation, its two coefficients are shown in Figs. 7 and 8.

[0025] Thus, for FIG. 7, the core 3 has first and second order thermoelastic coefficients which are negative, like silicon, and covered by a coating 4 which has positive first order and negative second order thermoelastic coefficients, like silicon dioxide. The coefficients of expansion of the materials are also taken into account, in particular that of the balance (18 ppm / ° C). The effect of the oscillation maintenance system is neglected here. It can also be seen in fig. 7that the orders α (solid line) and β (broken lines) do not have the same unit. We see that from a certain coating thickness, the first order α is compensated, that is to say crosses line 0, on the other hand, the second order β only decreases with respect to the material of l soul alone. We therefore understand that if the first order α can be compensated, this is not the case for the second order β.

In FIG. 8, core 3 has first and second order thermoelastic coefficients that are negative, like silicon, and covered by a coating 5 that has first and second order thermoelastic coefficients that are positive, like germanium dioxide. As for fig. 7, it can be seen in FIG. 8 that the orders α (solid line) and β (broken lines) are not the same unit. We see that from a thin coating, the second order β is compensated, that is to say crosses line 0, on the other hand, the first order α is compensated at a greater thickness. However, it is impossible for the two orders α and β to be compensated with a thickness of a single material.

This is due to the differences in magnitudes of the thermoelastic coefficients at each order of each material. Thus, if it seems illusory to find a material for the coating that is exactly “reverse” to that of the core, which would make it possible to deposit a single compensation layer, the invention proposes, for each order to be compensated, to add a coating. Each coating is therefore not intended to "directly" correct an order, but makes it possible to refine each of the compensations.

[0028] By way of example, calculations have been shown in FIG. 9. In this example, core 3 has first and second order thermoelastic coefficients that are negative, like silicon. The core 3 is covered by a first coating 4 which has positive first order and negative second order thermoelastic coefficients, such as silicon dioxide. The first coating 4 is itself covered by a second coating 5 which comprises a thermoelastic coefficient of the first and second orders which are positive, such as germanium dioxide.

As visible in FIG. 9, we see that it becomes possible by calculation to adjust each coating thickness 4, 5 so that the compensation of orders α and β converge substantially at the same final thickness, that is to say that the two curves α and β intersect line 0 at the same thickness. In our example of fig. 9, the core 3, the first coating 4 and the second coating 5 thus have respective thicknesses of about 40, 3.5 and 3.6 microns.

Thus, depending on the thickness of the desired core 3 or that of the desired final section, it is possible to offer a resonator whose thermal compensation is much improved compared to those "Quartz-z-cut" or “Si-SiO2” represented in fig. 6.

[0031] Of course, the present invention is not limited to the example illustrated but is susceptible of various variations and modifications which will be apparent to those skilled in the art. In particular, other materials for core 3 or for coverings 4, 5, etc. can be envisaged by obtaining improved thermal compensation.

For example, it is very likely that a material which will be called X (such as stabilized zirconium or hafnium oxides), which has negative first order thermoelastic coefficients (as is the case of most materials) and second order positive, can achieve thermal compensation. This example is illustrated in fig. 5. It is therefore understood that, for this type of material, the first coating α necessarily has a greater thickness compared to that of the embodiment of FIG. 4.

Claims (18)

1. thermocompensated resonator (1) comprising a body used in deformation, the core (3) of the body comprising a first material characterized in that the body comprises at least a first and a second coating (4, 5) for said resonator d have first and second order thermal coefficients (α, β) substantially zero. 2. Resonator (1) according to the preceding claim, characterized in that the body comprises a third coating for said resonator to have first, second and third order thermal coefficients (α, β, γ) substantially zero. 3. Resonator (1) according to claim 1 or 2, characterized in that the core (3) of the body has first and second order thermoelastic coefficients which are negative. 4. Resonator (1) according to the preceding claim, characterized in that the core (3) of the body comprises monocrystalline silicon. 5. Resonator (1) according to one of the preceding claims, characterized in that the body comprises a section substantially in the form of a quadrilateral whose faces are identical in pairs. 6. Resonator (1) according to one of claims 1 to 4, characterized in that the body comprises a section substantially in the form of a quadrilateral whose faces are fully coated. 7. Resonator (1) according to one of the preceding claims, characterized in that the first coating (4) comprises thermoelastic coefficients of the first order positive and the second negative order. 8. Resonator (1) according to the preceding claim, characterized in that the first coating (4) comprises silicon dioxide. 9. Resonator (1) according to claim 7 or 8, characterized in that the second coating (5) comprises a thermoelastic coefficient of the second positive order. 10. Resonator (1) according to the preceding claim, characterized in that the second coating (5) comprises a thermoelastic coefficient of the first positive order. 11. Resonator (1) according to the preceding claim, characterized in that the second coating (5) comprises germanium dioxide. 12. Resonator (1) according to claim 9, characterized in that the second coating (5) comprises a thermoelastic coefficient of the first negative order. 13. Resonator (1) according to one of claims 9 to 12, characterized in that the first coating (4) is interchanged with the second coating (5). 14. Resonator (1) according to one of the preceding claims, characterized in that the deposition of said coatings is preferred on the parallel faces relative to the neutral plane (F) of the body to modify with the greatest intensity possible frequency said resonator. 15. Resonator (1) according to one of the preceding claims, characterized in that the body is a bar wound on itself forming a hairspring and is coupled with a flywheel. 16. Resonator (1) according to one of claims 1 to 14, characterized in that the body comprises at least two bars mounted symmetrically to form a tuning fork. 17. Resonator (1) according to one of claims 1 to 14, characterized in that the body is a MEMS resonator. 18. Timepiece characterized in that it comprises at least one resonator according to one of the preceding claims.