CH705725A2 - Thermo-compensated ceramic resonator i.e. microelectromechanical system resonator, for use in timepiece, has body whose portion comprises electrically-conductive coatings, which change in Young's modulus as function of temperature - Google Patents

Abstract

The resonator has a body comprising a web (8) formed by ceramics material, where the ceramics material comprises glass, metallic glass and art glass ceramic. A portion of the body comprises electrically-conductive coatings (2, 4), which change in Young's modulus as a function of temperature. The ceramic material of the web enables the resonator having frequency, which is varied according to the temperature equal to zero. The coatings form a barrier against moisture. The body comprises an attachment layer between the web and the coatings.

Description

Field of the invention

[0001] The invention relates to a thermocompensated resonator of the sprung balance, tuning fork or more generally MEMS type making it possible to manufacture a time or frequency base whose thermal coefficients are substantially zero in the first order or even the second order.

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 in the frequency of the above disclosures may necessitate complex corrections depending on the applications. For example, in order for electronic quartz watches to be COSC certified, an electronic correction based on a temperature measurement must be performed.

Summary of the invention

The aim of the present invention is to alleviate all or part of the drawbacks mentioned above by providing a ceramic resonator which is thermally compensated at least to the first order.

[0005] To this end, the invention relates to a thermocompensated resonator comprising a body used in deformation, the core of the body being formed by ceramic characterized in that at least part of the body comprises at least one coating whose variations in Young's modulus as a function of temperature are of the opposite sign to those of the ceramic used for the core in order to allow said resonator to have a frequency variation as a function of temperature at least to the first order substantially nothing.

[0006] Advantageously according to the invention, the body of the resonator used in deformation may have a single coating to compensate for one or two orders. Thus, according to the sizes and signs of each order of the coating material, the calculation of the coating thickness is carried out in order to compensate for at least the first order.

[0007] In accordance with other advantageous features of the invention: the body core consists of glass, metallic glass, technical ceramic or ceramic glass; the body has a section substantially in the form of a quadrilateral, the faces of which are identical in pairs; the body has a section substantially in the form of a quadrilateral, the faces of which are fully coated; said at least one coating forms a moisture barrier; said at least one coating is electrically conductive; the body has a tie layer between the core and said at least one coating; the body is a bar wound on itself forming a hairspring and is coupled with a flywheel; the body comprises at least two bars mounted symmetrically in order to form a tuning fork; the body 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 perspective representation of part of a sprung balance resonator; - fig. 2 <sep> is a representative section of the spiral spring of FIG. 1; - fig. 3 and 4 <sep> are alternative sections of a resonator according to the invention; - fig. 5 <sep> is a general perspective representation of a tuning fork type resonator; - fig. 6 and 7 <sep> are alternative sections of a resonator according to the invention.

Detailed description of the preferred embodiments

As explained above, the invention relates to a timepiece comprising a resonator which may 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 applications to a sprung balance and a tuning fork are presented below. However, other applications for the resonator will be feasible for one skilled in the art without undue difficulty from the teaching below.

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

or: - is the relative variation in frequency (ppm or 10 <-> <6>); - At a constant which depends on the reference point (ppm); - T0 the reference temperature (° C); - α the first order thermal coefficient (ppm. ° C <-> <1>); - β the second order thermal coefficient (ppm. ° C <-> <2>); - γ the third order thermal coefficient (ppm. ° C <-> <3>).

[0012] 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 orders, 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 (forming a flywheel) for a balance-spring resonator.

[0013] As the oscillations of any resonator intended for a time or frequency base must be maintained, the thermal dependence also includes a possible contribution from the maintenance system.

[0014] The most important parameter is therefore the thermoelastic coefficient (CTE) which should not be confused with the English abbreviation CTE for "Constant of Thermal Expansion" which relates to the coefficient of expansion.

[0015] The thermoelastic coefficient (CTE) of most metals is very negative, in the order of -1000 ppm. ° C <-> <1>. It is therefore unimaginable to use them to make a hairspring. Complex alloys have therefore been developed, such as Nivarox CT, in order to respond to this problem. However, they remain difficult to control, particularly as regards their manufacture.

[0016] Advantageously, the invention relates to alternative ceramic materials for forming said resonators. A ceramic can be considered as an article having a vitrified body or not, of crystalline or partially crystalline structure, or of glass, the body of which is formed of essentially inorganic and metallic substances or not, and which is formed by a molten mass which solidifies on cooling, or which is formed and brought to maturity, at the same time or subsequently, by the action of heat.

[0017] A ceramic according to the invention therefore encompasses in particular simple glasses, metallic glasses, technical ceramics such as silicon carbide or ceramic glasses. Thus, advantageously according to the invention, the resonator formed from ceramic may comprise at least one coating whose variations in Young's modulus as a function of temperature are of opposite sign to those of the ceramic used for the core in order to allow said resonator to have a frequency variation as a function of the temperature at least at the first order substantially zero.

[0018] It is also interesting that the coating can be electrically conductive in order to prevent the movement of the body from generating electrostatic forces specific to influencing the trajectory of the body. Finally, it is also preferable that the coating offers a permeability suitable for forming a barrier against humidity such as, for example, silicon nitride.

In an example illustrated in FIGS. 1 and 2, we can see a hairspring 1 whose body 5 is formed with its ferrule 3 and whose thermal coefficients in the first order a or even in the second order /? are compensated by the use of two materials for respectively the core 8 and the coating 6. Figure 2 shows a section of the body 5 of the balance spring 1 to better see its quadrilateral-shaped section. The body 5 can thus be defined by its length l, its height h and its thickness e.

[0020] FIG. 2 shows an example where the core 8 is fully coated. Obviously, fig. 2 only presents a non-limiting example. Thus, the hairspring 1 can comprise a coating 2, 4, 6 on at least a part such as one or more faces or even the whole of the outer surface of the body 5, like the examples illustrated in FIGS. 3 and 4. For information, coatings 2, 4, 6 are not to scale with respect to the dimensions of core 8, in order to better show the locations of each part.

It is therefore understood that the body according to the invention may include in a nonlimiting manner a section substantially in the form of a quadrilateral with only one face is coated or whose faces are identical in pairs or whose faces are entirely coated identically or not.

[0022] Similarly, advantageously according to the invention, one can see a resonator 11 of the tuning fork type in FIG. 5. The body 15 of the resonator is formed of a base 13 connected to two arms 17, 19 intended to oscillate. By way of example, the tuning fork 11 used is of the inverted type, that is to say that the base 13 extends between the two arms 17, 19, of the webbed type, that is to say that the two arms 17, 19 have at their end fins 20, 22 and of the groove type (also known under the English name "groove"), that is to say that the two arms 17, 19 have grooves 24. 26. It is understood, however, that there are a multitude of possible variants of tuning forks which may, in a non-exhaustive manner, be of the inverted type and / or of the groove type and / or of the conical type and / or of the palmate type.

[0023] The tuning fork 11, advantageously according to the invention, comprises thermal coefficients of the first order α or even the second order β which are compensated by the deposition of a layer 12, 14, 16 against the core 18 of the body 15. Figs. 6 and 7 provide two non-exhaustive examples of cross section of the body 15 of the tuning fork 11 according to the plane A-A. The grooved quadrilateral shaped sections show the core 18 of the body 15 covered with at least one coating 12, 14, 16 on at least a portion such as one or more faces or even the entire outer surface of the body 15. As for the first example, the coatings 12, 14, 16 are not to scale with respect to the dimensions of the core 18, in order to better show the locations of each part.

[0024] The core 8, 18 of the resonator 1, 11 is formed by ceramic. However, there is a wide variety of ceramics. Therefore, ceramics which have low thermoelastic coefficients (CTE) and expansion (αspi) are preferred.

[0025] It is thus possible to use quartz glass also called fused quartz (in English "fused quartz"). Contrary to what the use of the word quartz suggests, it is not a crystalline material but an amorphous silica.

[0026] According to the method of manufacturing quartz glass, the value of the thermoelastic coefficient (CTE) obtained is generally low and positive, that is to say between 50 and 250 ppm. ° C <-1>. In addition, the coefficient of aspi expansion of quartz glass is approximately 0.5-ppm. ° C <-1>, which is very low. This means for the example of quartz glass that the coating 2, 4, 6, 12, 14, 16 preferably has a thermoelastic coefficient (CTE) which is negative. As explained above, such a coating can therefore comprise a metal or a metal alloy or another ceramic such as silicon carbide.

[0027] Of course, other glasses from the family of alkaline silicates, borosilicates or aluminosilicates are perfectly possible: CTE (ppm. ° C <-> <1>) <sep> Material - 209 <sep> Soda glass -35 <sep> Sibor glass -48 <sep> Borosilicate (a) + 117 <sep> Borosilicate (b ) - 18 <sep> Aluminosilicate (a) - 70 <sep> Aluminosilicate (b)

[0028] By way of example, pyrex <®> or the BF33, AF45 glasses from the company Schott® can be used: <sep> BF33 <sep> AF45 SiO2 <sep> 81 <sep> 50 B2O3 <sep> 13 <sep> 14 Al2O3 <sep> 2 <sep> 11 BaO <sep> <sep> 24 Na2O-K2O <sep> 4 <sep> (ppm. ° C <-> <1> αspi) <sep> 3.25 < sep> 4.5CTE (ppm. ° C <-1>) <sep>> 0 <sep>> 0where; - αspi is the coefficient of expansion of the material (ppm. ° C <-> <1>); - CTE is the thermoelastic coefficient of the material (ppm. ° C <-> <1>).

Photostructurable glasses as disclosed in document WO 2007/059 876 (incorporated by reference into the present patent application) are also possible. Indeed, the photolithography-type manufacturing process is very precise for the adjustment of the thermoelastic coefficient (CTE). Finally, ceramic glasses, such as zerodur, for example, are also possible.

As explained above, it is understood that the ceramics can include both positive thermoelastic coefficients (CTE) and negative first order and second order. This is why the coating (s) 2, 4, 6, 12, 14, 16, used for core 8, 18 may incidentally have both negative and positive first order and second order thermoelastic coefficients (CTEs). It is therefore understood that the resonator 1, 11 can be formed for example by a ceramic core totally or partially covered with a coating also made of ceramic.

Thus, depending on the method of depositing the coating 2, 4, 6, 12, 14, 16, it may be preferable to choose a material having good adhesion with respect to the ceramic, such as chromium or titanium. However, as an alternative, a tie layer such as chromium or titanium can also be deposited prior to the main coating 2, 4, 6, 12, 14, 16 to improve the adhesion and / or the permeability of said coating.

Finally, in the case where the core 8, 18 would include a thermoelastic coefficient (CTE) negative to the first order or even to the second order, preferably germanium oxide (GeO2), tantalum oxide ( Ta2O5) and / or oxides of zirconium or hafnium can be used.

[0033] Examples have been sought by considering a resonator at 4 Hz with a balance wheel with an inertia of 16 mg-cm <2>. The coefficient of expansion of the balance αbalinfluences the thermal dependence of the frequency of the resonator.

[0034] For the hairspring, height h and length l of the turns are fixed, only their thickness e is adjusted in order to obtain the right torque. The thickness d of the coating, assumed to cover all the faces of the turns, is adjusted so that there is thermal compensation at least at the first order α of the frequency of the resonator.

[0035] The properties of the materials used for the core or the coating of the hairspring are summarized in the following table: CTE (ppm. ° C <-1> (ppm. ° C <-1> <sep> E (Gpa) <sep>) <sep> αspi) Zerodur <sep> 90 <sep> 76 <sep> 0.0 Al <sep> 68 <sep> -430 <sep> 24.0 CVD-SiC <sep> 466 <sep> -50 <sep> 2.2 6H-SiC <sep> 220 <sep> -12 <sep> 2 , 8 SiO2 <sep> 72 <sep> 215 <sep> 0.4 Metallic Glass <sep> 117 <sep> -160 <sep> 12.0 TeO2 <sep> 51 <sep> 40,000 <sep> 15.5

A first example consists of coating with a metal (here a layer of AI) a zerodur hairspring sold by the company Schott <®> with a substantially zero coefficient of expansion.

This glass could also be coated with a layer of SiC deposited by chemical vapor deposition (also known by the English abbreviation "CVD"). CVD-SiC is a polycrystalline material reputed to be mechanically and chemically resistant. SiC also exists in crystalline form, for example hexagonal under the name of 6H-SiC. The properties of the latter differ from those of polycrystalline form. In the example below, it is compensated by SiO2.

[0038] Finally, a final example is taken from a metallic glass compensated by a layer of TeO2.

A table summarizes the different examples: Core <sep> Coating <sep> αbal (ppm. ° C '') <sep> e (µm) Zerodur <sep> Al <sep> 10 <sep> 1.15 Zerodur <sep > CVD-SiC <sep> 15 <sep> 0.79 6H-SiC <sep> SiO2 <sep> 10 <sep> 1.77 Metallic Glass <sep> TeO2 <sep> 15 <sep> 0.06

Claims (11)

1. Thermocompensated resonator (1, 11) comprising a body (5, 15) used in deformation, the core (8, 18) of the body (5, 15) being formed by ceramic characterized in that at least one part of the body (5, 15) has at least one coating (2, 4, 6, 12, 14, 16) whose variations of the Young's modulus as a function of temperature (CTE) are of opposite sign to those (CTE) ) of the ceramic used for the core (8, 18) to allow said resonator to have a frequency variation as a function of the temperature at least first order (α, β) substantially zero. 2. Resonator (1, 11) according to the preceding claim, characterized in that the core (8, 18) of the body (5, 15) comprises glass, metal glass, technical ceramic or ceramic glass. 3. Resonator (1, 11) according to claim 1 or 2, characterized in that the body (5, 15) comprises a section substantially in the form of a quadrilateral whose faces are identical in pairs. 4. Resonator (1, 11) according to claim 1 or 2, characterized in that the body (5, 15) comprises a section substantially in the form of a quadrilateral whose faces are fully coated. 5. Resonator (1, 11) according to one of the preceding claims, characterized in that said at least one coating (2, 4, 6, 12, 14, 16) forms a barrier against moisture. 6. Resonator (1, 11) according to one of the preceding claims, characterized in that said at least one coating (2, 4, 6, 12, 14, 16) is electrically conductive. 7. Resonator (1, 11) according to one of the preceding claims, characterized in that the body (1, 11) comprises a bonding layer between the core (8, 18) and said at least one coating (2 , 4, 6, 12, 14, 16). 8. Resonator (1) according to one of the preceding claims, characterized in that the body (5) is a bar wound on itself forming a hairspring and is coupled with a flywheel. 9. Resonator (11) according to one of claims 1 to 7, characterized in that the body (15) comprises at least two bars (17, 19) mounted symmetrically to form a tuning fork. 10. Resonator (1, 11) according to one of claims 1 to 7, characterized in that the body is a MEMS resonator. 11. Timepiece characterized in that it comprises at least one resonator (1, 11) according to one of the preceding claims.