CH710308A2 - Thermocompensated silicon resonator. - Google Patents

Abstract

The invention relates to a method for manufacturing a thermocompensated silicon resonator (15) for a mechanical clock movement or other precision instrument, comprising determining a predetermined doping level of the silicon so as to obtain a desired thermocompensation value; adding a dopant in the silicon in sufficient quantity to obtain the predetermined doping level; crystallize the doped silicon; and forming the thermocompensated resonator (15) in the crystallized doped silicon.

Description

Technical area

[0001] The present invention relates to a method of manufacturing a thermocompensated silicon resonator for a mechanical watch movement or other precision instrument. The invention also relates to a resonator obtained by the method as well as to a regulating member comprising such a resonator.

State of the art

[0002] The precision of mechanical watches depends on the stability of the natural frequency of the oscillator, typically formed by the sprung balance. When the temperature varies, the thermal expansions of the hairspring and the balance, as well as the variation of the Young's modulus of the hairspring, modify the natural frequency of this oscillating assembly, disturbing the precision of the watch.

Most of the methods proposed for compensating for these frequency variations are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the return torque exerted by the hairspring on the balance and the moment of inertia of the latter , as shown in the following relation:

where F is the natural frequency of the oscillator, C is the constant of the return torque exerted by the balance spring of the oscillator, and I is the moment of inertia of the balance of the oscillator.

[0004] For example, since the discovery of Fe-Ni-based alloys having a thermoelastic coefficient, or thermal coefficient of Young's modulus (hereinafter CTE), positive, the thermal compensation of the mechanical oscillator is obtained by adjusting the CTE of the balance-spring as a function of the thermal expansion coefficients of the balance-spring and the balance. Indeed, by expressing the torque and the inertia from the characteristics of the hairspring and the balance, then by deriving equation (1) with respect to the temperature, we obtain the thermal variation of the natural frequency (or thermal coefficient of the frequency, hereinafter CTF);

where αS is the coefficient of thermal expansion of the balance spring, and αB is the coefficient of thermal expansion of the balance.

[0005] By adjusting the self-compensation term A = 1/2 (CTE + 3αS) to the value of the thermal expansion coefficient of the balance αB, it is possible to cancel equation (2). Thus, the thermal variation of the natural frequency of the mechanical oscillator can be eliminated. In equation (2), the CTE of the hairspring is in practice much higher than its coefficient of thermal expansion, and the latter can be neglected.

[0006] Silicon resonators are increasingly used to replace crystal oscillators and as oscillators in precision instruments. For example, silicon is increasingly used in the manufacture of balance springs and other types of resonators for watch movements.

[0007] However, the CTE of silicon is strongly influenced by temperature and compensation for this effect is necessary for its use in horological applications. Indeed, the CTE of silicon is of the order of –60x10 <–> <6> / ° C and the thermal drift of a silicon spiral spring is thus about 155 seconds / day, for a temperature variation. of 23 ° C +/– 15 ° C. This makes it incompatible with watchmaking requirements of around 8 seconds / day.

[0008] In document JP 6 117 470, a spiral-shaped spring is made of monocrystalline silicon. It is dimensioned so as to have a constant return torque, to provide an electrical measuring device with great precision. However, this document is silent as to the thermal stability of the constant of the return torque of this spring. It cannot therefore be used directly as a spiral spring in a timepiece.

[0009] Document EP1 422 436 describes a spiral spring cut from a {001} monocrystalline silicon wafer. The hairspring comprises a layer of SiO2, exhibiting a CTE opposite to that of silicon and formed around the outer surface of the hairspring, in order to minimize the thermal drift of the balance-spring assembly. However, the presence of a relatively thick layer, around 6% the width of the hairspring, results in a hairspring with a dark and dull surface finish, having an unsightly appearance.

[0010] The applicant's patent CH 699 780 describes a silicon spiral spring, intended to equip a mechanical balance-sprung clockwork oscillator, of which, according to one embodiment, the outer surface is doped so as to compensate at least partially the thermal coefficient of the Young's modulus of silicon. Doping is carried out by a chemical diffusion process or by an ion implantation process using a non-metallic element such as boron, phosphorus, nitrogen, carbon, or a metallic element, or a mixture of these elements.

[0011] Document CH 700 032 mentions a silicon balance spring for a watch movement, doped so as to compensate for the effects of temperature variations over the period of the oscillator. The document mentions in particular the doping of the silicon spring with sodium, or metals such as boron or zirconium.

Brief summary of the invention

[0012] To overcome various drawbacks of the state of the art, the invention provides various technical means.

[0013] According to the invention, a method of manufacturing a thermocompensated silicon resonator for a mechanical watch movement or other precision instrument, comprising: determining a predetermined doping rate of the silicon so as to obtain a desired thermocompensation value; adding a dopant in the silicon in an amount sufficient to obtain the predetermined doping rate; crystallize the doped silicon; and forming the thermocompensated resonator in the crystallized doped silicon, for example by etching.

[0014] In one embodiment, the step of adding the dopant is performed when the silicon is in the liquid phase.

[0015] Still in one embodiment, the dopant is of type N or of type P.

[0016] The method may include one or more steps of fine-tuning the doping rate of the resonator so as to obtain the desired thermocompensation. In said one or more steps of fine adjustment of the doping rate can be carried out with an N or P type dopant.

[0017] This solution has the advantage in particular over the prior art of achieving a sufficient silicon doping rate to obtain the desired thermocompensation. As the doping is carried out homogeneously throughout the volume of the resonator, thermocompensated resonators having a higher quality factor Q and having varying shapes and thicknesses can be obtained. The higher Q factor would come from the elimination of friction at the Si – SiO2 interface.

Brief description of the figures

[0018] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: FIGS. 1a to 1c illustrate a method of manufacturing a thermocompensated silicon resonator, according to one embodiment; and fig. 2 shows CTE values of a doped silicon resonator as a function of its resistivity.

Example (s) of embodiment of the invention

[0019] According to the invention, the manufacture of a thermocompensated silicon resonator is based on the dependence of the temperature coefficient of the frequency (hereinafter CTF) of the silicon on the dopant concentration in the silicon.

[0020] According to one embodiment, a method of manufacturing a thermocompensated silicon resonator comprises the steps of: determining a predetermined doping rate of the silicon so as to obtain a desired thermocompensation value; adding a dopant in the silicon in an amount sufficient to obtain the predetermined doping rate; crystallize the doped silicon; and forming the thermocompensated resonator in the crystallized doped silicon, for example by etching.

[0021] In one embodiment, the step of adding the dopant is performed when the silicon is in the liquid phase. This allows very high doping rates to be obtained over a large thickness of silicon. Crystallized silicon can take the form of an ingot, usually cylindrical, or even a wafer.

[0022] According to one embodiment, the predetermined doping rate is estimated from the relationship between the CTF of the doped silicon and the concentration of dopant or dopants. The CTF of silicon can be determined experimentally as a function of its doping rate. The relationship between CTF and CTE is given by Equation 2, assuming that the coefficient of thermal expansion of the resonator does not vary with the doping rate (see e.g., Ashwin et. Al., "Temperature Compensation of Silicon Resonators via Degenerate Doping ”, IEEE Transactions on Electron Devices, vol. 59, no.1, 2012).

[0023] It will be noted here that the resonator may be a balance spring such as in a conventional regulating member constituted by the balance-spring assembly. However, the resonator can take other forms, including a vibrating tuning fork blade, possibly in combination with a mass element. In such a case, the terms CTF, CTE and αs from equation (2) apply for the resonator and, where applicable, the term as applies to the mass element.

The steps leading to forming the crystallized doped silicon can be carried out by a Czochralski process. In particular, the silicon is melted in a crucible, for example at a temperature around 1425 ° C. The dopant (s) are added to the molten silicon. A precisely oriented seed crystal is immersed in the molten silicon. The seed crystal is slowly rotated upwards. By precisely controlling the gradients of temperature, tensile speed and rotation, it is possible to extract a large single crystal, for example, in the form of a cylindrical ingot from the melt.

The crystallized doped silicon can then be cut in the form of wafers in which the thermocompensated resonator is machined.

[0026] Preferably, the silicon is doped with an N-type dopant which makes it possible to positively compensate the CTF, and therefore the CTE, of the silicon. For example, the doping element includes a non-metallic element such as phosphorus. The doping element can also include a metallic element such as Al, As or Sb, or a combination of at least one of the non-metallic and / or metallic elements. Alternatively, the silicon can be doped with a P-type dopant such as boron, nitrogen, Al or Ga.

After machining of the resonator, it is possible to measure the thermocompensation of the resonator thus obtained. For example, the resonant frequency of the resonator can be measured as a function of temperature (CTF).

[0028] The graph of FIG. 2 shows experimental values of CTE of the doped silicon resonator as a function of its p resistivity for an IM type dopant (squares) and for a P type dopant (diamonds). It should be noted that these values may vary depending on the type of silicon, in particular depending on the crystallographic orientation for monocrystalline silicon. CTE values were determined using the relationship between CTF and CTE given by equation (2), knowing the coefficient of thermal expansion as of the resonator. The doping rate is proportional to the resistivity p. In particular, a linear regression of the relationship between CTE values and the p resistivity of doped silicon can be used to determine the doping lethals. The resistivity p of the doped silicon of the resonator can be measured by a four point measurement.

[0029] In one embodiment, the optimum thermocompensation of the resonator is obtained for a coefficient of thermal expansion of silicon of the order of 26.53x10 <–> <6> ° C <–> <1>. The linear regression in fig. 2 gives a resistivity p value of 1.98x10 <–> <3> Ω cm allowing the predetermined doping rate to be known.

[0030] In the event that the thermocompensation is not sufficient, the manufacturing process may include a step of fine-tuning the doping rate of the resonator so as to obtain the desired thermocompensation value. In the fine adjustment step, the doping can be carried out in particular by ion implantation or diffusion from a source of solid or gaseous dopants. The fine adjustment of the doping rate, and therefore of the thermocompensation, can be carried out iteratively, by repeating the step of measuring the resonant frequency of the resonator and the step of fine adjustment of the doping rate, until 'so that the desired thermocompensation value is obtained.

[0031] The step or steps of fine adjustment of the doping rate can be carried out with an N-type or P-type dopant.

Alternatively, it is also possible, for each of the steps of fine adjustment of the doping rate, to measure the resistivity p of the doped silicon of the resonator so as to ensure that the corresponding doping rate has been obtained. to the desired thermocompensation value.

[0033] In a preferred embodiment illustrated in FIGS. 1a to 1c, the crystallized doped silicon takes the form of a layer formed on an insulator layer in a Silicon On Insulator (SOI) type configuration. In this variant, the method of manufacturing the resonator can comprise: providing a layer of a substrate 13, for example made of silicon; on the substrate 13, forming an insulating layer 12, for example of silicon oxide; on the insulating layer 12, depositing a doped silicon layer 10, as described above; on the doped silicon 10, depositing a photoresist 11; structure the photoresist 11 (fig. 1a); and machining the doped silicon layer through the openings 14 of the phoresist 11 (fig. 1b) so as to form the resonator 15.

[0034] The machining of the doped silicon layer can be carried out by a deep reactive ionic etching process so as to form the resonator on the insulating layer.

The method also comprises a step of etching the insulating layer 12 so as to free the resonator 15 from the substrate 13 (Fig. 1c).

[0036] The method of the invention can be used to manufacture a resonator for a mechanical watch movement or other precision instrument. The resonator can take the form of a spiral spring or a tuning fork type resonator.

[0037] The resonator of the invention can be used in a regulating member for a mechanical watch movement or other precision instrument.

Unlike the doping methods conventionally used for doping a silicon watch resonator, for example by ion implantation or by diffusion, the method of the invention makes it possible to achieve a silicon doping rate sufficient to obtain the desired thermocompensation. In addition, the method of the invention makes it possible to dope the silicon homogeneously throughout the volume of the resonator.

This is advantageous, in particular in the case where the resonator comprises a section which is not uniform, in particular in the case where the resonator having variable shapes and thicknesses, for example in the case of a tuning fork.

It will be recalled that the known techniques for thermocompensation of silicon using an oxide layer on the surface of the silicon results in a significant increase in the stiffness of the spring element made of silicon. With the process of the invention, thermal compensation by doping only marginally affects the stiffness of the resonator.

In comparison with the techniques based on the addition of an oxide layer to the surface of silicon, the method of the invention makes it possible to obtain a resonator having a higher quality factor Q, a lower hysteresis by reduction of acoustic losses and stresses due to the non-correspondence at the interface between the silicon layer and the insulating layer serving for thermal compensation.

Reference numbers used in figures

10 doped silicon 11 photoresist 12 insulator 13 substrate 14 opening 15 resonator

Claims (14)

A method of manufacturing a thermocompensated silicon resonator for a mechanical watch movement or other precision instrument, comprising: choose a desired thermocompensation value; determining a predetermined doping rate of the silicon from a relationship between the thermal coefficient of the frequency (CTF) of the doped silicon and the concentration of dopant or dopants so as to obtain said desired thermocompensation value; adding a dopant in the silicon in sufficient quantity to obtain the predetermined doping level; crystallize the doped silicon; and forming the thermocompensated resonator in the crystallized doped silicon. 2. The method of claim 1, wherein the step of adding the dopant is performed when the silicon is in the liquid phase. The method of claim 1 or 2, wherein the step of determining a predetermined doping rate comprises using experimental measurements of the thermocompensation of the machined resonator. The method according to one of claims 1 to 3, wherein the step of determining a predetermined doping rate comprises using a linear regression between values of the thermoelastic coefficient (CTE) of the doped silicon resonator and the resistivity (p) of the doped silicon. 5. The method according to one of claims 1 to 4, wherein the dopant is N-type or P-type. 6. The method according to one of claims 1 to 5, further comprising one or more steps of fine adjustment of the doping rate of the resonator by ion implantation or diffusion. The method according to claim 6, wherein said one or more fine-tuning steps of the doping rate is performed with an N-type or P-type dopant. 8. The method according to one of claims 1 to 7, wherein the predetermined doping rate provides for a coefficient of thermal expansion of silicon of the order of 26.53x10 <-> <6> ° C1. 9. The method according to one of claims 1 to 8, wherein the steps leading to form the crystallized doped silicon are carried out by a Czochralski process. The method according to one of claims 1 to 9, wherein the crystallized doped silicon is formed on an insulating layer in a Silicon On Insulator (SOI) configuration. 11. The method according to claim 11, wherein the step of machining the crystallized doped silicon is performed by a method of deep reactive ion etching of the doped silicon layer so as to form resonators on the insulating layer. 12. The method of claim 12 comprising a step of etching the insulating layer so as to release the resonators. 13. Resonator obtained by the process according to one of claims 1 to 13. 14. A regulating member for a mechanical horological movement or other precision instrument comprising the resonator according to claim 14.