CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Stage application of PCT international application PCT/FR2018/050597, filed on Mar. 14, 2018, which claims the priority of French Patent Application No. 17 52155, filed Mar. 16, 2017, both of which are incorporated herein by reference in their entireties.

TECHNICAL FIELD AND PRIOR ART

The invention relates to the field of microdevices, in particular that of the microdevices of the type microelectromechanical systems (MEMS) and/or nanoelectromechanical systems (NEMS) and/or micro-opto-electro-mechanical systems (MOEMS) and/or nano-opto-electro-mechanical systems (NOEMS), comprising mobile elements capable of moving under the effect of an outside action (mechanical, electric, magnetic) with respect to a fixed portion of the device. Hereinafter, the term “microdevice” is used to designate in particular a device of the MEMS and/or NEMS and/or MOEMS and/or NOEMS type.

Microdevices are generally made by the implementation of conventional techniques of collective etching of a plate of semiconductor material (for example of silicon). They are small and not costly to manufacture. The fields of uses can be highly varied, ranging from consumer goods to fields of very specific uses.

Some microdevices each comprise a plurality of mobile elements, for example such as some inertial sensors which require a plurality of mobile masses in order to carry out inertial measurements. One of the fields of use of these sensors is for example the inertial measurement of angular values such as angular position (gyroscopes) and speed of rotation (pyrometers). The performance of such an inertial sensor depends directly on the balancing of the resonator, and more particularly on the balancing among the various mobile masses of the sensor. This requires the presence of at least two mobile masses mounted symmetrically one with respect to the other in such a way that they can move in phase opposition with respect to one another, thus providing balancing of the sensor.

These mobile masses can be made side by side in the same layer of material, as is described in the documents US2014/0299947A1 and EP 2 887 013 A1. However, such a sensor of angular value comprising two mobile masses arranged side by side and made in the same layer of material has the disadvantage of engendering a dynamic unbalance resulting from the movement of the overall centre of gravity of the mobile masses at their frequency of vibration. This causes forces of reaction in the support and thus a loss of energy of the vibration. Moreover, the presence of the mobile masses side by side means, for the sensor, significant dimensions and thus a significant production cost.

Alternatively, it is also known to make, in the same layer of material, concentric mobile masses. However, the main disadvantage of the presence of at least two concentric mobile masses made in the same layer is that these two masses cannot have identical shapes. This configuration can lead to problems of balancing of these masses. Moreover, the concentric fabrication of these mobile masses means, for the sensor, significant dimensions, and thus a significant production cost. These significant dimensions are also found even when a single mobile mass is made in a single layer of material.

Another configuration involves making the two mobile masses of a sensor one above the other, each in a different layer of material. This configuration allows to prevent the problems encountered with the configurations described above in which the mobile masses are arranged side by side or concentrically. However, the fabrication of such sensors poses other problems.

Thus, the document US 2011/0300658 A1 describes the fabrication of such a sensor comprising two superimposed mobile masses, from an SOI double substrate, that is to say, comprising, above a support layer and a buried dielectric layer, two superficial layers of silicon superimposed one above the other and spaced apart from one another by another dielectric layer. The disadvantage of such a fabrication method is that the two mobile masses must be made during the same step of etching of the two superficial layers of silicon. The two mobile masses made are thus necessarily identical. Moreover, the multilayer etching that must be carried out through the silicon/dielectric/silicon stack in a single technological step is not easy to implement. This method is not therefore adapted to the fabrication of a sensor comprising two superimposed mobile masses having a different geometry.

In the documents US 2005/0091843 A1 and US 2009/0283917 A1, it is proposed to form a sensor comprising two mobile masses made in two distinct substrates. A sealing of the two substrates is then carried out in order to superimpose the two mobile masses made and obtain the sensor. With this method, the mobile masses of the sensor can be similar or not. However, a disadvantage of this method is that the sealing between the two substrates requires carrying out an alignment of the two substrates with respect to one another. This step of alignment is very sensitive and difficult to implement, and a misalignment between the two substrates can lead in particular to a misalignment of the two centres of gravity of the superimposed mobile masses, which can compromise the balancing of the sensor.

The document US 2015/0375995 A1 describes the fabrication of a first mobile element of a sensor in a substrate, then the sealing of a cover covering the first mobile element, then a thinning of the portion of the substrate used as a mechanical handle (at the rear face of the substrate), and finally the fabrication of a second mobile element in this thinned portion of the substrate from its rear face. This method allows the fabrication of mobile elements that are similar or not. However, it is necessary to carry out a sealing with alignment of the substrate and of the cover. Moreover, the alignment of the second mobile element with respect to the first mobile element poses problems since the alignment marks arranged on the rear face of the substrate and used for the alignment of the cover disappear during the thinning of the portion of the substrate used as a mechanical handle. The alignment of the second mobile element must therefore be carried out either by using infrared vision of the elements, or by carrying out the alignment with respect to the transferred cover. In both cases, this can lead to significant misalignment between the two mobile elements.

DESCRIPTION OF THE INVENTION

An aim of the present invention is to propose a microdevice, for example of the inertial sensor type, comprising at least one mobile element and not having at least a portion of the disadvantages of the microdevices of the prior art, that is to say, limiting or eliminating the effect of dynamic unbalance, which eliminates the alignment constraints encountered during the fabrication of the microdevices of the prior art, and which can be made in a compact manner with a reduced cost.

For this, the present invention proposes a microdevice comprising at least:a first layer of material;a second layer of material;a fixed portion;a first mobile element comprising at least a first portion of the first layer of material and at least a second portion of the second layer of material;first suspension means formed in the first layer of material and/or in the second layer of material;

and wherein:the first mobile element is capable of moving with respect to the fixed portion;the first and second layers of material are arranged one above the other;the first portion of the first layer of material is rigidly connected to the second portion of the second layer of material;the first mobile element is suspended from the fixed portion by the first suspension means.

This distribution of the portions of material forming the mobile element inside the various layers of material avoids leading to a significant dynamic unbalance resulting from the movement of the overall centre of gravity of the microdevice.

The features of this microdevice are compatible with a fabrication not involving the implementation of an alignment of elements intended to be sealed to one another. This microdevice can thus be made without having to implement such an alignment.

The fabrication of a mobile element in two superimposed layers further allows to reduce the bulk of this mobile element.

When the mobile element comprises various portions coming from two distinct layers electrically insulated from one another, a plurality of different electric potentials can be applied to this mobile element.

In general, the expression “layer of material” can designate a monolithic layer of material or a set of a plurality of superimposed layer.

The microdevice may further comprise:a second mobile element comprising at least a first portion of the second layer of material and/or at least a second portion of the first layer of material;second suspension means formed in the first layer of material and/or in the second layer of material;

and wherein:the first and second mobile elements may be capable of moving with respect to one another and with respect to the fixed portion;the second mobile element may be suspended from the fixed portion by the second suspension means;when the second mobile element comprises the first portion of the second layer of material and the second portion of the first layer of material, the first portion of the second layer of material may be rigidly connected to the second portion of the first layer of material.

The second portions of the first and second layers of material may correspond to simple portions of these layers, for example having a rectangular shape, or may have a more complex geometry, for example forming a frame, a plurality of studs, a grid, etc.

This microdevice may thus comprise a fixed portion from which at least two superimposed mobile elements are suspended. The distribution of the portions of material forming the mobile elements in the two layers of material may correspond to a top-to-tail positioning of the mobile elements. These mobile elements may thus be arranged in an interlocking manner with respect to one another in the microdevice.

The mobile elements may be made in layers of material superimposed one above the other. This reduces the bulk of the microdevice with respect to those in which the mobile elements are made in the same layer of material, either one next to the other or concentrically, which leads to a lesser production cost. This advantage is also found when the microdevice only comprises a single mobile element.

Moreover, such a microdevice does not have any constraints limiting the shape and the dimensions of the mobile elements which can be similar or not for each of the mobile elements. When the shape and the dimensions of the mobile elements are similar, this facilitates the balancing of the microdevice.

Advantageously, the first and second mobile elements may be capable of moving substantially in phase opposition with respect to one another, for example when the microdevice is subjected to an outside action (movement, acceleration, rotation, impact, etc.). In this case, the distribution of the portions of material forming each of the mobile elements in the two layers is very advantageous since the dynamic unbalance which could be caused by such a movement is reduced.

The first and second mobile elements may be capable of moving substantially in phase with respect to one another.

The centres of gravity of the first and second mobile elements may be aligned one above the other according to an axis substantially perpendicular to main faces of the first and second layers of material. This advantageous configuration reduces even more the effects of overall dynamic unbalance, or even eliminates them.

The first and second mobile elements may be symmetrical with respect to a centre of symmetry located between the first portions of the first and second layers of material and on the axis substantially perpendicular to the main faces of the first and second layers of material.

The microdevice may be such that:at least a first end of the first portion of the first layer of material is connected to the second portion of the second layer of material and at least a second end of the first portion of the first layer of material forms a free end of the first mobile element, andat least a first end of the first portion of the second layer of material is connected to the second portion of the first layer of material and at least a second end of the first portion of the second layer of material forms a free end of the second mobile element.

In this case, the second end of the first portion of the first layer of material may be arranged between the first end of the first portion of the first layer of material and the second portion of the first layer of material, and the second end of the first portion of the second layer of material may be arranged between the first end of the first portion of the second layer of material and the second portion of the second layer of material.

The first and second mobile elements may be arranged in an interlocking and/or concentric manner with respect to one another.

The first portion of the first layer of material may be connected to the second portion of the second layer of material by means of at least a first portion of an intermediate layer arranged between the first and second layers of material, and/or the first portion of the second layer of material may be connected to the second portion of the first layer of material by means of at least a second portion of the intermediate layer.

The microdevice may further comprise elements for detection and/or measurement of a movement of the first mobile element and/or of the second mobile element with respect to the fixed portion and/or of a movement of the first mobile element with respect to the second mobile element, and/or elements capable of moving the first mobile element and/or the second mobile element with respect to the fixed portion and/or moving the first mobile element with respect to the second mobile element.

In an advantageous configuration, the first mobile element may form a first detection element, and the second mobile element may form a second detection element, wherein the first and second detection elements may be capable of carrying out an inertial measurement of angular values to which the microdevice is subjected.

The first mobile element and/or the second mobile element may be capable of moving in translation and/or in rotation with respect to the fixed portion and/or with respect to one another.

The fixed portion may comprise portions of the first and second layers of material arranged around the first and second mobile elements and/or surrounded by the first and second mobile elements.

The fixed portion may comprise a support layer such that the first layer of material is arranged between the support layer and the second layer of material.

According to a specific embodiment, the microdevice may further comprise at least one intermediate mobile element suspended from the fixed portion and capable of moving with respect to the fixed portion, and the first and second mobile elements may be suspended from the intermediate mobile element and be capable of moving with respect to the intermediate mobile element and the fixed portion.

In this specific embodiment, the intermediate mobile element provides an absorption of the vibrations exterior to the microdevice. Thus, the mobile elements do not undergo these exterior vibrations.

Moreover, the microdevice according to this specific embodiment may be such that:the fixed portion comprises a fixed frame at least partially surrounding the intermediate mobile element, and the intermediate mobile element forms a mobile frame suspended from the fixed frame, and/orthe microdevice further comprises a first cavity, at least a portion of the walls of which is formed by the fixed portion and in which the intermediate mobile element is encapsulated, and a second cavity arranged in the first cavity, at least a portion of the walls of which is formed by the intermediate mobile element and in which the first and second mobile elements are encapsulated.

Advantageously, the second cavity may be insulated from the first cavity.

Thus, it is possible to encapsulate the first and second mobile elements in a specific atmosphere (in terms of nature of gas and/or pressure) different than that in which the intermediate mobile element is located, for example different than the ambient air in which the microdevice is located. The second cavity may be hermetically or impermeably sealed with respect to the first cavity.

The invention also relates to a method for fabricating a microdevice, comprising at least:etching of a first layer of material such that at least a first portion of the first layer of material forms a first portion of at least a first mobile element;fabrication of a second layer of material on the first layer of material;etching of the second layer of material such that at least a second portion of the second layer of material forms a second portion of the first mobile element and is rigidly connected to the first portion of the first layer of material;

these steps being implemented such that the first mobile element is suspended from a fixed portion by first suspension means formed in the first layer of material and/or in the second layer of material, the first mobile element being capable of moving with respect to the fixed portion.

Advantageously, the method may be such that:the etching of the first layer of material is implemented such that at least a second portion of the first layer of material forms a first portion of at least a second mobile element;the etching of the second layer of material is implemented such that at least a first portion of the second layer of material forms a second portion of the second mobile element and is rigidly connected to the second portion of the first layer of material;the steps of the method are implemented such that the second mobile element is suspended from the fixed portion by second suspension means formed in the first layer of material and/or in the second layer of material, the first and second mobile elements being capable of moving with respect to one another and with respect to the fixed portion.

The fabrication of the second layer of material on the first layer of material may comprise a transfer of the second layer of material onto the first layer of material such that an intermediate layer is arranged between the first and second layers of material, the method being capable of further comprising, after the etching of the second layer of material, an etching of the intermediate layer such that at least a first portion of the intermediate layer connects the first portion of the first layer of material to the second portion of the second layer of material and, when the second mobile element is made, at least a second portion of the intermediate layer connects the first portion of the second layer of material to the second portion of the first layer of material.

According to a first embodiment, a sacrificial layer may be arranged between the first layer of material and a support layer, the method being capable of further comprising, after the etching of the second layer of material, the fabrication of an access to the sacrificial layer through the support layer then an etching of a portion of the sacrificial layer via the access formed through the support layer, releasing at least the first portion of the first layer of material (and also the second portion of the first layer of material when the second mobile element is made) with respect to the support layer. In this case, the releasing of the first mobile element (and of the second mobile element when the latter is made) may be carried out from two opposite faces of the microdevice.

According to a second embodiment, a sacrificial layer may be arranged between the first layer of material and a support layer, the method being capable of further comprising, between the etching of the first layer of material and the fabrication of the second layer of material, an etching of a first portion of the sacrificial layer partially releasing at least the first portion of the first layer of material (and the second portion of the first layer of material when the second mobile element is made) with respect to the support layer, and after the etching of the second layer of material, an etching of a second portion of the sacrificial layer via openings formed through the first and second layers of material during the previous etchings of the first and second layers of material, completing the releasing of the first portion of the first layer of material (and of the second portion of the first layer of material when the second mobile element is made) with respect to the support layer. In this case, the releasing of the first mobile element, and optionally of the second mobile element, may be carried out without having to form an access to the sacrificial layer from a rear face of the support layer, and only from a front face of the microdevice.

The steps of etching of the first and second layers of material may form releasing holes passing through the first and second layers of material and which are aligned above each other. Thus, the realising of the first portion (and optionally of the second portion) of the first layer of material may be carried out by using etching agents reaching the sacrificial layer via these releasing holes.

The step of etching of the second layer of material may form additional releasing holes passing through the second layer of material. This configuration allows in particular to obtain different mobilities of the mobile elements with respect to one another, when a plurality of mobile elements are made.

