Patent Description:
As known, MEMS ("Micro Electro Mechanical Systems") electronic devices such as actuators and/or sensors, are typically formed in a substrate accommodating a cavity, and comprise a movable structure, which is suspended on the cavity and constrained to a fixed region through elastic elements. The substrate, the movable structure and the elastic elements may be monolithic, generally of semiconductor material, for example silicon.

These electronic devices further comprise one or more actuation and/or detection structures, coupled to the movable structure, and an electric control circuit, integrated in the same substrate or formed in a different substrate and coupled to the actuation and/or detection structures, capable of providing electrical drive signals to the actuation structures and/or receiving electrical detection signals from the detection structures.

In detail, the actuation structures, when controlled by the respective electrical drive signals, may cause a displacement, for example a rotation, of the movable structure. Similarly, the detection structures are configured to generate respective electrical signals, which are a function of an operating condition of the movable structure, for example are a function of the rotation extent of the movable structure.

The electric control circuit and the actuation and/or detection structures are generally electrically connected to each other through conductive tracks, for example formed by a stack of conductive layers.

The conductive tracks typically extend on the fixed region, on the elastic elements, and sometimes partially on the movable structure.

In use, the displacement of the movable structure causes a deformation, for example a torsion, of the elastic elements which constrain the movable structure to the fixed region.

The deformation of the elastic elements generates mechanical stress in the elastic elements themselves and in the conductive tracks integral therewith.

If the mechanical stress has a high value, for example if it exceeds the plastic deformation threshold of the materials that form the conductive tracks, the conductive tracks may be subject to delamination and/or break. Consequently, the electrical connection between the electric control circuit and the actuation and/or detection structures may be compromised and the MEMS electronic device may be subject to malfunctions or break.

At present, MEMS devices, for example micromirrors, are known wherein the conductive tracks are formed by a specific stack of materials having a high number of metal layers designed to have a greater resistance to mechanical stress. However, this solution involves a high complexity of design and formation of the conductive tracks; in fact, for example, the number and the material of the metal layers need to be modified according to the stress resistance requirements required by the specific application.

Furthermore, the conductive tracks may be manufactured using complex chemical/physical processes capable of avoiding the formation of accidental structures such as unwanted over-etching. However, this involves the use of more complex and expensive processes, resulting in an increase in complexity and costs of production of the MEMS device. Furthermore, these precautions may prove to be insufficient to reduce the probability of break of the conductive tracks during use.

Furthermore, MEMS micromirrors are known wherein the rotation amplitude of the movable structure is voluntarily maintained within a reduced angle range, for example of few degrees, in order to reduce the mechanical stress and thus the probability of break of the conductive tracks. However, this is not achievable in specific applications wherein high rotation angles of the movable structure are desired.

Document <CIT> discloses a planar electromagnetic actuator comprising a mobile portion supported by a fixed portion through torsional bars. The mobile portion, the fixed portion and the torsional bars are formed integrally with a semiconductor substrate. The planar electromagnetic actuator further comprises a drive coil extending on the mobile portion. The drive coil is electrically connected to external connection terminals, which are arranged on the fixed portion, through wiring tracks. The wiring tracks extend suspended above the surface of the torsional bars.

Document <CIT> discloses a planar electromagnetic actuator comprising a fixed portion, a mobile portion and torsional bars, formed integrally with a semiconductor substrate. The planar electromagnetic actuator comprises electrical connection tracks that connect a drive coil, arranged on the mobile portion, with external connection terminals, arranged on the fixed portion. The electrical connection tracks are arranged lifted from the torsional bars.

The aim of the present invention is to overcome the drawbacks of the prior art.

According to the present invention, a MEMS device and a manufacturing process thereof are provided, as defined in the attached claims.

For a better understanding of the present invention, embodiments thereof are now described, purely by way of nonlimiting example, with reference to the attached drawings, wherein:.

Hereinafter, an electronic device made using MEMS ("Micro Electro Mechanical System") technology, in particular a MEMS micromirror, is described.

<FIG> show a MEMS micromirror <NUM> symmetrical with respect to a rotation axis A, parallel to a first axis X of a Cartesian reference system XYZ.

The MEMS micromirror <NUM> is formed here in a die <NUM>, which is delimited by a top surface 5A and comprises a plurality of substrates of semiconductor material, for example silicon, bonded to each other.

In this embodiment, the die <NUM> is formed by a first and by a second substrate <NUM>, <NUM>, bonded to each other through a bonding element <NUM>, for example of silicon oxide.

The first substrate <NUM> has a thickness Z<NUM> for example comprised between <NUM> and <NUM>, in particular of <NUM>, and is covered by one or more insulating layers, herein simply referred to as insulating layer <NUM>, which is delimited by the top surface 5A of the die <NUM>. The insulating layer <NUM> is thin, with a thickness for example smaller than <NUM>, and is therefore neglected hereinafter.

The second substrate <NUM> has a thickness for example comprised between <NUM> and <NUM>, in particular of <NUM>, and may be bonded to a support substrate, not shown here, on the opposite side with respect to the first substrate <NUM>.

The die <NUM> comprises a frame <NUM>, surrounding a pass-through cavity <NUM>, and a movable structure <NUM> suspended in the pass-through cavity <NUM> and coupled to the frame <NUM> through elastic elements <NUM>.

The frame <NUM> is formed by the second substrate <NUM> and by the portion of the first substrate <NUM> bonded and superimposed thereto, while the movable structure <NUM> and the elastic elements <NUM> are formed by respective portions of the first substrate <NUM> suspended in the pass-through cavity <NUM>.

In detail, the MEMS micromirror <NUM> comprises two elastic elements or springs <NUM>, each being here bar-shaped, extending along the rotation axis A. Each elastic element <NUM> has a first anchoring end, integral with the frame <NUM>, and a second anchoring end, integral with the movable structure <NUM>, and a width W<NUM>, parallel to a second axis Y of the Cartesian reference system XYZ, which is constant in the shown embodiment.

As visible in <FIG> and highlighted by a dashed line in <FIG>, the MEMS micromirror <NUM> further comprises two buried cavities <NUM>, one for each elastic element <NUM>, arranged inside the first substrate <NUM>, which extend for the whole length of the elastic elements <NUM> and thus divide each elastic element <NUM> into a first and a second portion 25A, 25B, superimposed to each other.

The first portion 25A of the elastic elements <NUM> delimits here upwardly the buried cavities <NUM> and has a thickness comprised for example between <NUM> and <NUM>, in particular of <NUM>.

The second portion 25B of the elastic elements <NUM> delimits here downwardly the buried cavities <NUM> and has a greater thickness than the first portion 25A, comprised for example between <NUM> and <NUM>, in particular of <NUM>.

In detail, each buried cavity <NUM> extends in the first substrate <NUM>, from the top surface 5A of the die <NUM>, at a depth Z<NUM>, measured along a third axis Z of the Cartesian reference system XYZ, comprised for example between <NUM> and <NUM>, in particular of <NUM> and has a height Z<NUM> for example comprised between one fiftieth and one fifth of the thickness Z<NUM> of the respective elastic element <NUM>, for example comprised between <NUM> and <NUM>, in particular of <NUM>.

In detail, here, each buried cavity <NUM> has, along the first axis X, a greater length than the respective elastic element <NUM>, so that each buried cavity <NUM> extends beyond the anchoring ends of the respective elastic element <NUM>. In other words, each buried cavity <NUM> extends partially inside the frame <NUM> and partially inside the movable structure <NUM>.

In this embodiment, each buried cavity <NUM> has, parallel to the second axis Y, an extension equal to the width W<NUM> of the respective elastic element <NUM>; in other words, in this embodiment, the buried cavities <NUM> communicate with the pass-through cavity <NUM>.

Consequently, the first and the second portions 25A, 25B of the elastic elements <NUM> are mechanically coupled to each other only through the portions of the frame <NUM> and of the movable structure <NUM> adjacent to the ends of the buried cavities <NUM>.

However, each buried cavity <NUM> may have a width smaller than the width W<NUM> and/or a length smaller than or equal to the respective elastic element <NUM>, even if this, in some embodiments, may involve a greater mechanical coupling between the first and the second portions 25A, 25B.

The MEMS micromirror <NUM> further comprises one or more actuation structures <NUM>, contact pads <NUM> and conductive tracks <NUM>. The conductive tracks <NUM> extend between, and are in direct electrical connection with, the contact pads <NUM> and the actuation structures <NUM>.

The actuation structures <NUM>, here depicted only schematically, may be for example of electromagnetic, piezoelectric or electrostatic type, are formed in a per se known manner, and are coupled to the movable structure <NUM>, so as to cause, in use, a rotation thereof around the rotation axis A.

In this embodiment, the actuation structures <NUM> are arranged on the movable structure <NUM>, on the top surface 5A of the die <NUM>.

The contact pads <NUM> and the conductive tracks <NUM> are used to electrically connect the actuation structures <NUM> to an electric circuit, not shown here, capable of providing and/or receiving electrical control signals of the MEMS micromirror <NUM>. This electric circuit may be integrated in the die <NUM> or in a separate die, electrically connected to the die <NUM>.

In detail, in the embodiment in question, the conductive tracks <NUM> are formed, as shown in <FIG>, by a multilayer. In particular, here the multilayer comprises a seed layer <NUM>, for example of copper, useful during manufacturing and arranged in contact with the top surface 5A of the die <NUM>, and a stack of conductive layers <NUM>, whose number, thickness and material may vary according to the requirements of the specific application, for example according to the desired level of stress resistance or resistance to the passage of current.

In this embodiment, the stack of conductive layers <NUM> is formed by the succession of a first metal layer 48A, for example of nickel, directly overlying the seed layer <NUM>, a second metal layer 48B, for example of gold, over the first metal layer 48A, a third metal layer 48C, for example of nickel, over the second metal layer 48B, and a fourth metal layer 48D, for example of gold, over the third metal layer 48C.

The conductive tracks <NUM> extend, at least partially, on the elastic elements <NUM>, in a position contiguous to the insulating layer <NUM> and thus integral with the first portions 25A of the elastic elements <NUM> and have a width W<NUM>, along the second axis Y, smaller than the width W<NUM> of the elastic elements <NUM> and thus than the width of the buried cavities <NUM>.

The MEMS micromirror <NUM> further comprises a reflective region <NUM>, formed by one or more materials capable of reflecting a light radiation, and arranged on the movable structure <NUM>, here on the top surface 5A of the die <NUM>.

The reflective region <NUM> may, for example, be of aluminum, if the light radiation is in the visible frequency range, or of gold, if the light radiation is in the infrared frequency range.

In use, when it is desired to rotate the movable structure <NUM>, and thus the reflective region <NUM>, for example to direct a light beam, an electrical signal is provided, by the electric control circuit, to the actuation structures <NUM>, which, in a per se known manner, cause a rotation of the movable structure <NUM> around the rotation axis A.

The rotation of the movable structure <NUM> is accompanied by a deformation, in particular a torsion, of the elastic elements <NUM> around the rotation axis A.

The torsion of the elastic elements <NUM> generates a high mechanical stress in the elastic elements <NUM>, both inside the elastic elements <NUM> and in the corresponding anchoring ends.

However, the conformation of the elastic elements <NUM>, in particular the presence of the buried cavities <NUM> which decouple the first portion 25A from the second portion 25B of the respective elastic elements <NUM>, causes the highest stress values to be distributed in the second portion 25B.

In fact, as described hereinabove, since the first and the second portions 25A, 25B are mechanically decoupled both along the first axis X and along the second axis Y, the first and the second portions 25A, 25B of each elastic element <NUM> behave, respectively, as a first and a second spring arranged in parallel.

Furthermore, the second portion 25B of each elastic element <NUM> has a greater thickness than the respective first portion 25A and thus has a greater elastic constant. Consequently, given the same deformation (torsion), the mechanical stress develops more in the second portion 25B.

Therefore, the first portion 25A, and thus also the respective conductive track <NUM> integral therewith, is subject to a lower mechanical stress. This causes the conductive tracks <NUM>, arranged above the first portion 25A, less stressed, to be in turn subject to lower stress with respect to a case wherein the elastic elements <NUM> are solid.

Furthermore, since the buried cavities <NUM> extend beyond the anchoring ends of the elastic elements <NUM>, the high stress values which generally form at the anchoring ends move inside the frame <NUM> and the movable structure <NUM> and are distributed therein, thus reducing the stress at the interface with the conductive tracks <NUM>. Furthermore, the conformation and the position of the conductive tracks <NUM> on the frame <NUM> and on the movable structure <NUM> may be designed so that they are far from the most stressed points of the frame <NUM> and of the movable structure <NUM>.

Simulations performed by the Applicant, not shown here, confirm that the described solution causes the mechanical stress in the conductive tracks <NUM> to remain below the stress threshold, for example lower than <NUM> MPa, beyond which plastic deformation of the metal layers forming the conductive tracks <NUM> occurs.

Consequently, the conductive tracks <NUM> are less subject to the risk of delamination and/or break during the rotation of the movable structure <NUM> and therefore the MEMS micromirror <NUM> has improved reliability.

This allows to design MEMS micromirrors whose movable structure <NUM> is able to rotate with a high rotation angle, for example with an angle around the rotation axis A up to ±<NUM>°, in particular of about ±<NUM>°, without the risk of delamination and/or break of the conductive tracks <NUM>.

The MEMS micromirror <NUM> may be formed from a work body <NUM> (intended to form the die <NUM>), for example a Silicon-on-Insulator (SOI) wafer, shown in <FIG>, which includes a first layer <NUM> (intended to form the second substrate <NUM>) of semiconductor material, for example monocrystalline silicon, and a structural layer <NUM> (intended to form the first substrate <NUM>), also of semiconductor material, for example monocrystalline silicon, bonded to each other through a bonding layer <NUM>, for example of silicon oxide.

The first layer <NUM> has a first and a second surface 205A, 205B and a thickness for example of <NUM>; the structural layer <NUM> has a first and a second surface 210A, 210B, and a thickness, for example comprised between <NUM> and <NUM>, in particular of <NUM>.

In detail, the first surface 205A of the first layer <NUM> is bonded to the second surface 210B of the structural layer <NUM>.

Subsequently, <FIG>, work trenches <NUM> are formed in first portions <NUM> of the structural layer <NUM>. In particular, a pair of first portions <NUM>, separated from each other by a second portion <NUM> of the structural layer <NUM>, is provided for each micromirror <NUM> to be formed. In detail, in each first portion <NUM>, a group of work trenches <NUM> is formed, which delimit a respective plurality of pillars <NUM> of semiconductor material. The work trenches <NUM> are formed for example by using known lithographic and selective etching steps from the first surface 210A of the structural layer <NUM>.

For example, the work trenches <NUM> each have a depth comprised for example between <NUM> and <NUM>, and each pillar <NUM> is arranged at a distance comprised, for example, between <NUM> and <NUM> from an adjacent pillar <NUM>.

In <FIG>, an epitaxial layer, for example of thickness comprised between <NUM> and <NUM>, is grown, through an epitaxial growth step, on the first surface 210A of the structural layer <NUM> (which thus increases in thickness; for simplicity the thickened epitaxial layer thus obtained is indicated again with <NUM>). One or more annealing steps of the work body <NUM> are then performed, for example in a reducing environment, for example in a hydrogen atmosphere, at high temperatures, for example higher than <NUM>.

As described, for example, in the European patent application <CIT>, the one or more annealing steps cause a migration of the semiconductor atoms, here silicon, which tend to move to a lower energy position. Consequently, also due to the close distance between the plurality of pillars <NUM>, the semiconductor atoms of the plurality of pillars <NUM> migrate completely, forming first work cavities <NUM>, which are intended to form the buried cavities <NUM>. The first work cavities <NUM> are delimited upwardly by a semiconductor layer, formed partially by epitaxially grown atoms and partially by migrated atoms, which forms a closing layer <NUM>, which constitutes a portion of the structural layer, here again indicated by <NUM>.

The height of the first work cavities <NUM> (and thus of the buried cavities <NUM>) and the depth in the structural layer <NUM> at which they are buried may be adjusted, during the design step, by modifying the depth of the work trenches <NUM> and the thickness of the epitaxial layer on the first surface 210A of the structural layer <NUM> and/or by performing a further epitaxial growth.

In this embodiment, as visible in <FIG>, the first work cavities <NUM> have a width W<NUM>, along the second axis Y, greater than the width W<NUM> of the buried cavities <NUM> of <FIG>, so as to facilitate the manufacturing steps of the buried cavities <NUM>.

However, the width W<NUM> of the first work cavities <NUM> may be equal to the width W<NUM> of the buried cavities <NUM> (i.e. equal to the width of the elastic elements <NUM>).

Subsequently, again with reference to <FIG>, one or more insulating layers, for simplicity indicated as insulating layer <NUM> in <FIG>, are deposited on the first surface 210A of the structural layer <NUM>.

In this embodiment, a first insulating layer 233A, for example of silicon oxide, and a second insulating layer 233B, for example of silicon nitride, are deposited on the first surface 210A of the structural layer <NUM>.

Subsequently, <FIG>, a stack of conductive layers <NUM> is formed on the second insulating layer 233B. The conductive layers <NUM> comprise here a first seed layer <NUM>, for example of copper, useful for improving the adhesion of the successive conductive layers, and a first, a second, a third and a fourth metal layer 243A, 243B, 243C, 243D, for example of nickel, gold, nickel and gold, respectively. Then, <FIG>, the stack of conductive layers <NUM> is shaped, so as to form the contact pads <NUM> and the conductive tracks <NUM>.

As visible in <FIG> in top plan view, wherein the first and the second insulating layers 233A, 233B are shown in transparency for clarity, the conductive tracks <NUM> extend, at least partially, on the pairs of first portions <NUM> of the structural layer <NUM>; i.e. the conductive tracks <NUM> are each arranged above a respective first work cavity <NUM>, herein depicted with a dashed line.

Then, <FIG>, the actuation structures <NUM> are formed in a known manner and selective etchings are performed to selectively remove the first and the second insulating layers 233A, 233B where the reflective region <NUM> is intended to be formed. The reflective region <NUM> is thus formed directly on the first surface 210A of the structural layer <NUM>, for example by depositing one or more materials capable of reflecting a light beam.

Subsequently, <FIG>, a first mask layer <NUM> is deposited on the second surface 205B of the first layer <NUM> and is lithographically shaped so to form a first opening <NUM>.

Then, <FIG>, portions of the first layer <NUM> and of the bonding layer <NUM> are removed up to the second surface 210B of the structural layer <NUM>, from the second surface 205B of the first layer <NUM>, forming a second work cavity <NUM>. For example, a series of selective etchings may be performed, using the first mask layer <NUM>.

Then, <FIG>, a second mask layer is deposited on the work body <NUM> and defined so to form a mask region (here shown in transparency) surrounding a second opening <NUM>.

The second opening <NUM> faces the structural layer <NUM>, in a position adjacent to the second portions <NUM> of the structural layer <NUM> and partially overlapping the first portions <NUM>, on the side of the conductive tracks <NUM>.

In particular, the mask region, at the conductive tracks <NUM>, has width W<NUM> along the second axis Y, so as to completely cover the conductive tracks <NUM> and have a smaller width than the width W<NUM> of the first work cavities <NUM>.

A series of selective etchings is then performed using the mask region, through the second opening <NUM>, to remove the first and the second insulating layers 233A, 233B and the structural layer <NUM>, up to the second surface 210B of the structural layer <NUM>; i.e. so to communicate with the second work cavity <NUM> and thus forming the pass-through cavity <NUM>.

In this manner, the first work cavities <NUM> communicate laterally with the pass-through cavity <NUM> and form the buried cavities <NUM>.

Furthermore, the remaining parts of the pairs of first portions <NUM> and of the second portion <NUM> of the structural layer <NUM> form, respectively, the elastic elements <NUM> and the movable structure <NUM>.

Known processing steps follow, such as, for example, thinning of the first layer <NUM>, dicing of the work body <NUM>, and packaging of the corresponding die, so as to form the MEMS micromirror <NUM>.

<FIG> show the present MEMS device according to a different embodiment. In particular, <FIG> show a MEMS micromirror <NUM> having a general structure similar to the one of the MEMS micromirror <NUM> shown in <FIG>; consequently, elements in common are provided with the same reference numbers.

In detail, also here the MEMS micromirror <NUM> is formed in the die <NUM> comprising the frame <NUM>, which delimits the pass-through cavity <NUM>, and the movable structure <NUM> suspended in the pass-through cavity <NUM> and coupled to the frame <NUM> through two elastic elements <NUM>. The MEMS micromirror <NUM> further comprises the actuation structures <NUM>, the contact pads <NUM> and the conductive tracks <NUM>.

The elastic elements <NUM> are formed in a first substrate <NUM>, similar to the first substrate <NUM> of the MEMS micromirror <NUM>; an insulating layer <NUM> (similar to the insulating layer <NUM> of <FIG>, thus formed by one or more insulating layers) extends on the first substrate <NUM> and delimits the first surface 5A of the die <NUM>. The elastic elements <NUM> are also here bar- or rod-shaped, with width W<NUM> along the second axis Y, and are constrained to the frame <NUM> and to the movable structure <NUM> through respective anchoring ends.

Also here, each elastic element <NUM> comprises a first and a second portion 125A, 125B, but these portions are contiguous, not separated by cavities.

A pair of trenches <NUM> extends, through the insulating layer <NUM>, in the first substrate <NUM>, on the sides of each conductive track <NUM>, for a portion of the thickness of the first substrate <NUM>.

In detail, each trench <NUM> is delimited downwardly by a bottom wall <NUM> and laterally by a respective first side wall 152A and by a respective second side wall 152B of the first substrate <NUM>. The bottom wall <NUM> and the side walls 152A, 152B form respective corners with respect to each other.

In particular, the depth of the trenches <NUM> is smaller than half the thickness of the first substrate <NUM>. For example, the trenches <NUM> may have a depth comprised between <NUM> and <NUM>.

Thus, the trenches <NUM> laterally delimit the first portion 125A of the elastic elements <NUM>, which therefore have a smaller thickness than the second portions 125B.

In use, when the movable structure <NUM> is actuated in rotation, the mechanical stress in the elastic elements <NUM> is more concentrated at the superficial portions of the first substrate <NUM> adjacent to the corners existing between the first side wall 152A and the bottom wall <NUM> and between the second side wall 152B and the bottom wall <NUM>. In fact, the trenches <NUM> form discontinuity regions in the elastic elements <NUM>.

The stress concentration in the superficial portions of the first substrate <NUM> adjacent to the corners, thus at a distance from the stack of conductive layers <NUM>, is verified by simulations performed by the Applicant, not shown here.

Consequently, the stress is concentrated in a zone with greater mechanical resistance, since the first substrate <NUM>, being of semiconductor material such as silicon, has high stress resistance (high plastic deformation threshold). The metal layers which form the conductive tracks <NUM> (and which have a lower plastic deformation threshold) are instead subject to a reduced stress value, for example lower than <NUM> MPa, thus they are subject to a stress that is lower than their plastic deformation threshold.

Consequently, the conductive tracks <NUM> have a lower risk of delamination and/or break, even in case of a high rotation angle of the movable structure <NUM>, for example up to ±<NUM>°, in particular of about ±<NUM>°, with respect to the rotation axis A.

The MEMS micromirror <NUM> may be formed, from the work body <NUM> of <FIG>, similarly to the MEMS micromirror <NUM>, except that the buried cavities <NUM> are not formed, but the trenches <NUM> are formed on the side of the conductive tracks <NUM>, as briefly described hereinbelow.

The first and the second insulating layers 233A, 233B, the conductive tracks <NUM>, the contact pads <NUM>, the actuation structures <NUM>, the reflective region <NUM> and the pass-through cavity <NUM> are formed similarly to the MEMS micromirror <NUM>.

In detail, <FIG>, the first and the second insulating layers 233A, 233B are deposited on the first surface 210A of the structural layer <NUM> and the conductive tracks <NUM> are formed as previously described with reference to the MEMS micromirror <NUM>. Subsequently, the trenches <NUM> are formed by selectively removing portions of the structural layer <NUM>, of the first and the second insulating layers 233A, 233B contiguous, along the second axis Y, to the conductive tracks <NUM> and identified for clarity with a dashed line in <FIG>.

Then, the elastic elements <NUM> and the movable structure <NUM> are released, as described with reference to <FIG>.

A further embodiment of the present MEMS device is described hereinafter.

In detail, <FIG> shows a MEMS micromirror <NUM>, which, similarly to the MEMS micromirror <NUM>, <NUM>, is formed in the die <NUM> comprising the frame <NUM> surrounding a pass-through cavity <NUM>, and a movable structure <NUM> suspended in the pass-through cavity <NUM> and fixed to the frame <NUM> through the elastic elements <NUM>, which are delimited by a dashed line, for clarity, in <FIG>.

The MEMS micromirror <NUM> further comprises the contact pads <NUM>, the conductive tracks <NUM>, the actuation structures <NUM> and the reflective region <NUM>, similarly to what described regarding the MEMS micromirror <NUM>, <NUM>.

The movable structure <NUM> is formed by a platform <NUM>, formed in the first substrate <NUM>, and by one or more stiffening structures, here two stiffening structures <NUM>.

Here, the stiffening structures <NUM> are formed by portions 7A of the second substrate <NUM> bonded to the platform <NUM> through portions 8A of the bonding element <NUM>. The stiffening structures <NUM> thus extend, from the platform <NUM>, through the pass-through cavity <NUM>.

The MEMS micromirror <NUM> comprises trenches <NUM> each extending in the platform <NUM>, from a bottom surface 325A thereof, up to a respective superficial portion <NUM>, on the sides of each stiffening structure <NUM>. The trenches <NUM> have a much smaller depth than the thickness of the platform <NUM>, for example comprised between <NUM> and <NUM>.

In use, the movable structure <NUM> is subject to rotation, typically at a high frequency, for example of a few kilohertz. This generates mechanical stress in the movable structure <NUM> which tends to deform; the stiffening structures <NUM>, on the other hand, oppose this deformation, causing a stress in the bonding zone of the stiffening structures <NUM> to the platform <NUM>.

Similarly to what described for the MEMS micromirror <NUM>, the trenches <NUM> represent discontinuities in the first substrate <NUM>, which cause the stress to be concentrated inside the first substrate <NUM>, in proximity to the bottom wall <NUM> of the trenches <NUM>. In this manner, the highest stress values are concentrated far from the portions 8A of the bonding element <NUM>, thus reducing the risk that the high stress causes a break of the portions 8A and thus a detachment of the stiffening structures <NUM>.

It will be clear that the MEMS micromirror <NUM> may be manufactured, from the work body <NUM> of <FIG>, in a manner similar to what described as regards the manufacturing of the MEMS micromirror <NUM>.

Finally, it is clear that modifications and variations may be made to the MEMS device <NUM>, <NUM>, <NUM> and to the manufacturing process thereof described and illustrated herein, without thereby departing from the protective scope of the present invention, as defined in the attached claims.

For example, the different described embodiments may be combined to provide further solutions.

For example, although in <FIG> and <FIG>, the elastic elements <NUM>, <NUM> have a constant width, they may also have a different shape, for example they may have a non-constant width, for example they may be dog-bone-shaped or folded, according to the stress resistance requirements required by the specific application.

For example, the MEMS micromirror <NUM>, <NUM> may be configured so to rotate also around a second rotation axis, transversal to the first rotation axis A.

Furthermore, the MEMS micromirror <NUM>, <NUM>, <NUM> may comprise other structures useful for its operation, for example detection structures such as piezoresistive sensors, in particular for detecting the rotation amplitude of the movable structure <NUM>, <NUM>, and the conductive tracks may electrically connect the contact pads to such other structures.

The present MEMS device may be different from a micromirror, for example it may be a temperature sensor, a pressure sensor, or a microvalve, comprising movable and/or deformable structures suspended on a pass-through cavity and subject, in use, to high stress and wherein the buried cavities <NUM> or the trenches <NUM>, <NUM> allow the stress to be concentrated at regions which are more resistant to stress, as described hereinabove.

For example, the buried cavities <NUM> may be formed by depositing a sacrificial layer, for example of silicon oxide, whereon semiconductor material, for example silicon, is then deposited and which may then be removed to form a cavity.

Claim 1:
A MEMS device (<NUM>) comprising:
a body (<NUM>) of semiconductor material defining a support structure (<NUM>);
a pass-through cavity (<NUM>) in the body, surrounded by the support structure;
a movable structure (<NUM>) suspended in the pass-through cavity;
an elastic structure (<NUM>) extending in the pass-through cavity between the support structure (<NUM>) and the movable structure (<NUM>), the elastic structure comprising a first portion (25A) and a second portion (25B) and being subject, in use, to mechanical stress;
a metal region (<NUM>) extending on the first portion (25A) of the elastic structure (<NUM>); and
a buried cavity (<NUM>) in the elastic structure;
the buried cavity extending between the first and the second portions of the elastic structure.