CH718081B1 - ELASTIC ELEMENT FOR A MICROMECHANICAL SYSTEM. - Google Patents

Abstract

The invention relates to an elastic element (1) for a micromechanical system, comprising: – a core (3) of core material; – a first layer (5) provided on said core, said first layer (5) being of a first material having an intrinsic stress having a first magnitude and a first sign; – a second layer (7) provided on said first layer (5), said second layer (7) being of a second material different from said first material and having an intrinsic stress having a second magnitude and a second sign; characterized in that said second sign is opposite said first sign. The invention also relates to a method of manufacturing such an elastic element.

Description

Technical area

[0001] The present invention relates to the field of micromechanics, in particular but not exclusively watchmaking and MEMS (micro-electro-mechanical systems). In particular, it relates to an elastic element intended to be incorporated into such systems.

State of the art

[0002] Over the last decades, micromachined elastic elements have become common in micromechanical systems, particularly in MEMS devices and watchmaking. In particular, monocrystalline silicon has become a popular material for these elastic elements, whether in the form of conventional spiral springs or leaf springs, although other materials such as amorphous or polycrystalline silicon, silicon carbide , glass, quartz and metal are also used. These elastic elements are typically micro-machined, for example by etching a plate of material through a mask, by sintering in a mold or by electrodeposition (for example LIGA) in the case of the metal (which is typically nickel, copper, nickel phosphorus and the like).

[0003] However, these materials often exhibit an undesirable property which requires compensation, such as an undesirable thermoelastic response (i.e. a variation in elastic modulus as a function of temperature changes), sensitivity to moisture or chemicals, a build-up of static electrical charges, undesirable electrical properties, or it is necessary to add functionality to the elastic element, such as piezoelectric properties, electrical conductivity or electrical interconnection, color, etc.

[0004] To take the specific example of monocrystalline silicon, this material has a strongly negative thermoelastic coefficient dE/dT, that is to say that when the temperature increases, the Young's modulus of the material decreases and the elastic element becomes less rigid. In the <100> direction of the {001} plane, this variation is approximately -60 ppm/K. In the case of a watch oscillator, where the elastic element provides a restoring force to an inertial mass in order to make it oscillate around a neutral position, this can lead to significant errors in the operation of the oscillator, of the order of 155 seconds per day in a temperature range of 23°C ± 15°C.

[0005] In the case where the elastic element is used in a MEMS device, such as a sensor, to provide a restoring force to a micromechanism constructed to measure a value of a particular parameter (for example, an acceleration, force or the like), temperature variations can cause significant measurement errors by affecting the stiffness of the elastic element. In the case of a MEMS actuator, such as a steerable micromirror, these changes result in inaccuracy in the positioning of the actuator and any element affected by it (e.g., a mirror).

[0006] Document EP 1 422 436 provides a solution to this problem in the context of a watch oscillator, the elastic element being provided with an external functional layer of a material with a strongly positive thermoelastic coefficient, namely dioxide of silicon SiO2 (dE/dT approximately +215 ppm/K). The thickness of this layer can be chosen to tune the thermoelastic coefficient of the entire elastic element as desired, so as to make it substantially zero, or to obtain a desired positive or negative value in order to thermocompensate for the influence of temperature on the particular inertial mass (e.g. the balance wheel) with which it interacts as an oscillator.

[0007] Oxidation by thermal growth, that is to say the oxidation of the surface, is the main technology making it possible to form such layers for several reasons. First of all, it is efficient and can be precisely controlled. Simple to implement, it eliminates silicon surface defects and at the same time protects the silicon core by completely covering its surface and leaving any possible structural surface defects inside the oxide layer. In addition, such a process can also be used to tune the frequency because it consumes silicon, and therefore makes it possible to tune the rigidity of the elastic element if it is partially or completely removed (in the latter case, this would represent a additional step before forming the thermocompensation layer).

However, the disadvantage of such a solution is that the silicon dioxide layer presents a significant intrinsic stress when thermal growth oxidation processes or other high temperature processes are used to form this layer. These internal stresses are due to high temperatures, typically 900 °C or more, which create self-interstitial point defects in the silicon crystal, local variations in internal stresses linked to the topography of the silicon, and the accumulation of stresses. when the elastic element is cooled to room temperature. Even if the oxide layer is removed, this can sometimes create permanent deformation of the silicon.

[0009] All these residual intrinsic stresses are problematic in general, including in conventional flat spiral springs, but are particularly problematic in modern unconventional oscillators such as the so-called “Généquand” oscillator, where the topology of the elastic elements is much more complex than a simple spiral spring. Therefore, in the case of overstressed bending mechanisms, the internal stress can create additional undesirable effects such as stiffening or softening of the spring, weakening effects and even geometric deformation of the elastic elements like bending, buckling , twisting, etc. This can lead to unbalanced behavior and, in the case of oscillators, frequency variations depending on the orientation of the gravity vector and a nonlinear dependence of frequency on temperature. In the case of elastic elements of MEMS devices, the same undesirable effects are also problematic.

[0010] In addition to the growth of functional layers on the material of the core by reaction of a chemical species (such as oxygen) with said material, deposition methods for the formation of functional layers, such as the deposition of atomic layers (ALD), molecular vapor deposition (MVD), various forms of chemical vapor deposition (CVD), various forms of physical vapor deposition (PVD) or diffusion depending on the layer material (e.g. dielectric layers such as SiO2, Si3N4, metals, polymers, ceramics, etc.) can also induce undesirable stresses, depending on the material, the deposition process chosen, the temperature of the process and the heat treatments carried out. These stresses can be, depending on the case, compressive (negative) or tensile (positive) stresses.

[0011] Finally, document EP 2 337 221 discloses a spiral spring in silicon (or other) provided with two coatings, namely a first in SiO2 and a second in GeO2 deposited on the SiO2 so as to cancel not only the thermal coefficients of the first but also second order. However, this document does not teach anything regarding the aforementioned problem caused by internal constraints in SiO2 and/or silicon.

The aim of the present invention is therefore to at least partially overcome at least some of the aforementioned drawbacks of the prior art.

Disclosure of the invention

[0013] More precisely, the invention relates to an elastic element for a micromechanical system, as defined in claim 1. This elastic element comprises:

[0014] - a core of a core material, such as monocrystalline silicon, polycrystalline silicon, amorphous silicon, silicon carbide, glass, quartz or a metal;

[0015] - a first layer provided on at least part of the surface of said core, said first layer being of a first material presenting an intrinsic stress having a first magnitude and a first sign. Said first material is typically silicon dioxide, silicon nitride or a metal, to the extent that it is different from the core material;

[0016] - a second layer provided on at least part of the surface of said first layer, said second layer being of a second material different from said first material and having an intrinsic stress having a second magnitude and a second sign. Said second material may be silicon dioxide, silicon nitride or a metal, as long as it is different from the first material.

[0017] According to the invention, the second sign is opposite to said first sign. In other words, if the intrinsic stress of the first layer is positive (tension), the second material is chosen with a negative intrinsic stress (compression). Likewise, if the intrinsic stress of the first layer is negative (compression), the second material is chosen with a positive intrinsic stress (tension).

[0018] Consequently, the undesirable intrinsic stress induced by the first layer can be at least partially, if not totally, compensated by the addition of the second layer, and can even be overcompensated if desired. Indeed, the residual inherent stress throughout the elastic element can be tuned by carefully choosing the material and thickness of the third layer to a desired value, whether positive or negative. This can be achieved by choosing a suitable material and a suitable process to form the second layer.

[0019] Advantageously, said first layer is arranged on at least four adjacent exterior faces of said core (that is to say the four side faces, in other words the upper face, the lower face and the two side faces), and said second layer is disposed over substantially the entire exterior surface of said first layer. The thickness of each layer can be uniform or variable, as desired.

Typically, said core has a height of between 1 and 800 µm, a width of between 1 and 100 µm, and a length of between 0.5 and 10 mm. In the case of an elastic element for a timepiece resonator, said core may have a height of between 100 and 600 µm, a width of between 12 and 50 µm, and a length of between 200 µm and 5 mm.

Advantageously, the first layer has a thickness of between 0.1 and 4 µm, and/or said second layer has a thickness of between 0.05 and 1 µm.

The elastic element as defined above can be part of a mechanical oscillator, or of a MEMS device such as a MEMS sensor or a MEMS actuator.

[0023] The invention further relates to a method of manufacturing an elastic element, as described above, for a micromechanical system. This process includes the steps of:

[0024] a) definition of said core material and the dimensions of said core, namely its length, width and height;

[0025] b) definition of said first material and the dimensions of said first layer, namely its thickness, in order to compensate for a property of said core and/or in order to add functionality to said elastic element, as is well known in the literature ;

[0026] c) determining the effect of the inherent stress of said first layer on said elastic element;

[0027] d) definition of said second material and the dimensions of said second layer, namely its thickness, in order to at least partially compensate for the inherent stress generated by said first layer;

[0028] e) formation of said core according to its determined dimensions;

[0029] f) formation of said first layer on said core according to the determined dimensions thereof;

[0030] g) formation of said second layer on said first layer according to the determined dimensions thereof.

[0031] Consequently, an elastic element having predetermined dimensions and predetermined mechanical properties can be manufactured, each of the core, the first layer and the second layer being formed with the dimensions determined in steps a), b) and d ).

[0032] In a particular variant, said core and said first layer are formed by providing a blank of said core material and by reacting the material of said blank with a chemical species to form said first layer and to reduce the dimensions of said core by way to give it its determined dimensions. This is particularly (but not exclusively) the case when the core is made of silicon and said chemical species is oxygen, the first layer being formed by oxidation of the surface of the silicon blank, which reduces the cross section silicon while growing the first layer, until the final dimensions of the core are reached.

Advantageously, said second layer is formed by physical vapor deposition or chemical vapor deposition, which do not consume or otherwise react with the material of the first layer and therefore do not reduce its dimensions.

Brief description of the drawings

Other details of the invention will appear on reading the detailed description in relation to the figures, which illustrate: – Figure 1: a schematic sectional view of an elastic element according to the invention; – Figure 2: a schematic illustration of a test setup for testing the rigidity of an elastic element according to the invention; and – Figure 3: a graph of the stress as a function of the thickness of the second layer, the first layer and a part of the core of an elastic element according to the invention.

Realization of the invention

[0035] Figure 1 schematically illustrates a sectional view of an elastic element 1 according to the invention.

This elastic element 1 can be a spiral spring (that is to say a flat spring substantially spiral) for a timepiece oscillator, or a leaf spring of any known shape for a timepiece oscillator. watchmaking (for modern variants based on bending, see for example the EPFL thesis Flexure Pivot Oscillators for Mechanical Watches, Etienne Thalmann, Thesis Number 8802, June 11, 2020) or for a MEMS device such as a sensor (for example of the type described in S. Beeby et al, MEMS Mechanical Sensors, Artech House on Demand, 2004), an actuator, a time base (for example of the type described in Ng, E. et al. The long path from MEMS resonators 10 timing products, Proceedings of 28th IEEE International Conference on Micro Electro Mechanical Systems. 1-2 (IEEE, 2015)) or the like, including, but not limited to, the types mentioned in the introduction. In the case of a mechanical oscillator, the elastic element 1 is intended to oscillate an inertial mass in translation and/or rotation around a neutral point, whereas in a MEMS device, the elastic element 1 is intended to provide a restoring force for a sensor or actuator.

The elastic element 1 comprises a core 3 of a core material, which is typically monocrystalline silicon but can also be polycrystalline or amorphous. Other suitable materials are glass, silicon carbide, metal (typically nickel, copper, nickel phosphorus or the like), and the like. Depending on the role, the cross section of core 3 can be between 1 µm<2> and 8000 µm<2> (for example, core 3 typically has a height between 1 and 800 µm and a width between 1 and 100 µm), and depending on the material it can be produced by etching a plate of material through a mask, sintering in a mold, electroforming or the like, as is generally known in the field of micromachining and therefore does not have to be explained exhaustively here. The length of the core is typically between 0.5 mm and 10 mm. The “length” is defined as the longest dimension of the elastic element 1, the “width” is the other dimension in the plane of the elastic element 1 in which it is intended to flex, and the “height” is the dimension perpendicular to this plane.

[0038] A first layer 5 is applied to at least some of the exterior surfaces of the core 3, typically at least all the exterior surfaces illustrated in Figure 1. This first layer 5 is a functional layer intended to add a property or a function to the elastic element 1 or to compensate for an undesirable property of the core 3, is made of a first material different from said core material, and has an intrinsic stress having a first magnitude and a first sign. The first magnitude is typically between - 1 GPa (i.e. 1 GPa in compression) and +1 GPa (i.e. 1 GPa in tension), depending on the material. The first layer 5 can have a uniform or variable thickness, and can be placed on all or part of the exterior surface of the core 3.

[0039] In addition to functions such as thermocompensation, it is also possible to give the elastic element a color, an electrical conductivity, an electrical interconnectivity, a piezoelectric response or a chemical barrier or any other function.

[0040] Typically, the material of the first layer (that is to say the first material) is silicon dioxide (SiO2) formed by thermal oxidation of the surface of the silicon material which forms the core 3. In this case, before the thermal oxidation, a silicon blank is provided whose width and height are greater than those of the final core, these dimensions being reduced in a controlled and predictable manner by the thermal oxidation, as is generally known and not should not be explicitly illustrated.

However, the invention is also applicable to other materials such as Si3N4, silicon carbide, silicon nitride, polysilicon, metal (for example tungsten) or the like, and/or to other processes for forming the first layer which induces an intrinsic stress in the first layer 5, deposited by atomic layer deposition (ALD), molecular vapor deposition (MVD), various forms of chemical vapor deposition (CVD) or physical vapor deposition (PVD), or diffusion.

The first layer 5 can be formed by growth or deposition in a single step, but it is also possible to add other steps such as etching the first layer partially or completely, additional growth or deposition steps. thermal deposition is also possible if desired.

[0043] On the surface of the first layer 5 is formed a second layer 7, of a second material which has an intrinsic stress of opposite sign to that of the first material, this stress typically being between -1 GPa and +1 GPa , of sign opposite to that of the first material. The type of material, its deposition method and its thickness can be chosen so as to at least partially compensate for the intrinsic stresses in the first layer 5 and the intrinsic stresses which result in the core 3. The second layer 7 is therefore a layer stress management, because it is used to vary the overall intrinsic stresses in the elastic element 1, by compensating for the intrinsic stresses generated by the first layer 5. It should be noted that the second layer 7 can have a uniform thickness, or a variable thickness according to needs, and can be present on all or part of the first layer 5. The thickness of the second layer 7 can be chosen so as to substantially completely compensate for the intrinsic stress generated by the first layer 5, or alternatively can overcompensate or undercompensate it, depending on needs. Therefore, a certain degree of tuning of the properties of the elastic element is possible.

[0044] The material of the second layer (that is to say the second material) is typically deposited on the first layer, such that the first layer is not consumed by it, although this does not or not obligatory. For example, various known chemical vapor deposition (CVD) or physical vapor deposition (PVD) processes can be used, depending on the choice of the second material.

For example, if the first layer 5 is SiO2, the second layer 7 can be Si3N4, or another material having a positive inherent stress (traction). In the opposite case of a first layer 5 made of Si3N4, for example, the second layer 7 can be made of SiO2 or another material having an inherent negative (compressive) stress. Other material pairs may be selected based on the knowledge of those skilled in the art.

As a concrete example, we will consider the case where the core 3 is made of monocrystalline silicon, and the first layer 5 is SiO2 obtained by thermal oxidation. In this case, SiO2, which is typically used as a thermocompensation layer to modify or cancel the negative thermoelastic coefficient of silicon, presents a significant intrinsic compressive stress, of the order of 230 MPa for a first layer 5 of 1000 nm d 'thickness. As compressive stresses are defined as having a negative sign, this stress can be more concisely represented as -230 MPa.

[0047] In order to compensate for this first intrinsic constraint, the second layer 7 is made of silicon nitride (Si3N4), deposited by low pressure CVD (LPCVD) or plasma activated CVD (PECVD) as is generally known. This material has an intrinsic stress of approximately 1 GPa in tension, i.e. +1 GPa.

[0048] In order to illustrate the concept, simulations were carried out on different fully overstressed beams, that is to say beams which are rigidly fixed at both ends by clamps 9, as illustrated in Figure 2 In order to determine the apparent rigidity of each beam 1, it was made to oscillate in its plane of lowest rigidity (i.e. parallel to its width W) in each simulation, the observed frequency being used to calculate apparent stiffness using well-known beam equations. These beams have heights H and widths W typical of the elastic elements of watch oscillators. The results are as follows: A Monocrystalline silicon, L=2 mm, W=15 µm, H=500 µm 34303 258 B Like (A), with a first layer of SiO2 of 1 µm 5 without intrinsic stress (W=17 µm au total) 35085 304 C Like (A), with a first layer of SiO2 of 1 µm 5 with an intrinsic stress of -230 MPa (W=17 µm total, H=502 µm total) 17307 74 D Like (C), with a second layer 7 of 0.2 µm of Si3N4 with an intrinsic stress of +1GPa (W=17.4 µm in total, H=502.4 µm in total) 35080 314 E Like (C), with a second layer 7 of 0, 5 µm of Si3N4 with an intrinsic stress of +1GPa (W=18 µm total, H=503 µm total) 49961 666

Table 1

[0049] By comparing references B and D, it is clear that similar mechanical properties for a thermally compensated elastic element can be obtained not only with an ideal SiO2 layer, without intrinsic stress (reference B), but also with the application of a second layer 7 of Si3N4 on a conventional layer of SiO2 obtained by thermal growth, which is much simpler and more economical to produce than a thermocompensation layer of SiO2 without constraints as in reference B.

[0050] Furthermore, it emerges from the various simulations that the variation in the thickness of the first and second layers 5, 7 can be used to adjust the rigidity of the elastic element 1 to a desired value, within limits wide.

[0051] The graph in Figure 3 illustrates the stress profile modeled through the thickness of the elastic element 1 with reference E at its center line, the axis X representing the distance relative to the surface of the elastic element 1 and the Y axis representing the second modeled Piola-Kirchhoff constraint. This graph shows that the stress gradient at the transition between second layer 7 and first layer 5 is very high, with the stress increasing from +1 GPa to -230 MPa, then to zero at the transition to Si core 3. In each case, this stress transition occurs over a distance of several atomic thicknesses.

[0052] Taking into account the above, the principle of the invention having been described, a method of manufacturing an elastic element 1 on its basis will now be disclosed, with the presumption that the desired mechanical properties of the element elastic 1 have been predetermined.

First of all, the material of the core, as well as the desired final dimensions of the core 3, are determined.

[0054] We then define the first material and the dimensions of the first layer 5, in particular its thickness and the areas of the core on which it must be formed, in order to add functionality to the elastic element 1 and/or to compensate an undesirable property of core 3, such as variations due to temperature.

The second material can then be chosen, as well as the dimensions of the second layer 7, in particular its thickness and the areas of the first layer 5 on which it must be formed.

The determination of the dimensions of the core 3 and the layers 5, 7 can be carried out by modeling and/or experimental study, and can be an iterative process to obtain the desired mechanical properties for the elastic element 1, in particular its rigidity. , the way its rigidity changes with temperature, etc. Furthermore, in the case where the first layer 5 must be formed by reacting the material of the core 3 with a chemical species such as oxygen, the dimensions of a blank of material intended to form the core 3 can also be determined . The blank (not illustrated) has a width and a height determined such that, after its material has been consumed to form the first layer 5, the dimensions of the core 5 are those previously determined. In the case where the formation of the second layer 7 consumes material from the first layer 5, this is taken into account during the formation of the different layers in a similar manner.

[0057] Once all the dimensions mentioned above are determined, the core 3 is formed, then the first layer 5 on at least part of said core 3, then the second layer 7 on at least part of said first layer 5 by suitable growth or deposition methods as discussed above.

[0058] Although the invention has been described with reference to specific embodiments, variations are possible without departing from the scope of the appended claims.

Claims (14)

1. Elastic element (1) for a micromechanical system, comprising: – a core (3) of a core material; – a first layer (5) provided on said core, said first layer (5) being of a first material having an intrinsic stress having a first magnitude and a first sign; – a second layer (7) provided on said first layer (5), said second layer (7) being of a second material different from said first material and having an intrinsic stress having a second magnitude and a second sign; characterized in that said second sign is opposite said first sign. 2. Elastic element (1) according to claim 1, wherein said first layer (5) is provided on at least four adjacent exterior faces of said core (3), and said second layer (7) is provided on substantially the entire exterior surface of said first layer (5). 3. Elastic element (1) according to any one of the preceding claims, wherein said core material is one of: monocrystalline silicon, polycrystalline silicon, amorphous silicon, silicon carbide, glass, quartz or a metal. 4. Elastic element (1) according to any one of the preceding claims, wherein said first material is one of: silicon dioxide, silicon nitride or a metal. 5. Elastic element (1) according to claim 4, wherein said second material is one of: silicon dioxide, silicon nitride or a metal. 6. Elastic element (1) according to one of the preceding claims, wherein said core (3) has a height of between 1 and 800 µm, a width of between 10 and 40 µm, and a length of between 1 and 400 mm , or alternatively with a height between 100 and 600 µm, a width between 12 and 50 µm, and a length between 200 µm and 5mm. 7. Elastic element (1) according to claim 6, wherein said first layer (5) has a thickness of between 0.1 and 4 µm. 8. Elastic element (1) according to claim 6 or 7, wherein said second layer (7) has a thickness of between 0.05 and 1 µm. 9. Mechanical oscillator comprising an elastic element (1) according to any one of claims 1 to 8. 10. MEMS device comprising an elastic element (1) according to any one of the preceding claims. 11. Method for manufacturing an elastic element (1) according to any one of claims 1 to 8 for a micromechanical system, comprising the steps of: a) definition of said core material and the dimensions of said core (3); b) definition of said first material and the dimensions of said first layer (5) in order to compensate for a property of said core (3) and/or in order to add functionality to said elastic element (1); c) determining the effect of the inherent stress of said first layer (5) on said elastic element (1); d) definition of said second material and the dimensions of said second layer (7) in order to at least partially compensate for the inherent stress generated by said first layer (5); e) formation of said core (3); f) formation of said first layer (5) on said core; g) formation of said second layer (7) on said first layer (5). 12. Method according to the preceding claim, wherein said core (3) and said first layer (5) are formed by providing a blank of said core material and reacting the material of said blank with a chemical species to form said first layer (5) and to reduce the dimensions of said core (3) so as to give it its determined dimensions. 13. Method according to the preceding claim, wherein said core material is silicon and said chemical species is oxygen. 14. Method according to one of claims 11 to 13, wherein said second layer (7) is formed by physical vapor deposition or chemical vapor deposition.