Patent ID: 12222492

DETAILED DESCRIPTION

FIG.2is a schematic depiction of a MEMS (MicroElectroMechanical System) device10, in particular a mirror device of the vector-scanner type, according to a first embodiment.

The microelectromechanical device10is formed in a die of semiconductor material, in particular silicon, and is provided with a tiltable structure12. The tiltable structure has a main extension in a horizontal plane XY and can be driven so as to rotate about a first rotation axis A1, which is parallel to a first horizontal axis X of the aforesaid horizontal plane XY, and about a second rotation axis A2, which is parallel to a second horizontal axis Y of the horizontal plane XY and defines, with the first horizontal axis X, the aforesaid horizontal plane XY (in the embodiment illustrated, the sides of the die are moreover parallel to the horizontal axes x, y).

As will be discussed hereinafter, the tiltable structure12may moreover be driven so as to carry out movements along a vertical axis Z, orthogonal to the horizontal plane XY.

The first rotation axis A1represents a first median axis of symmetry for the microelectromechanical device10; and the second rotation axis A2represents a second median axis of symmetry for the same microelectromechanical device10, which also has a central symmetry with respect to a center O (which moreover represents the center of the aforesaid tiltable structure12).

Basically, the microelectromechanical device10may be divided into four quadrants with respect to the aforesaid center O, having a same arrangement and configuration.

The tiltable structure12is suspended above a cavity13, obtained in the die, and has, in the embodiment illustrated, a generically circular shape in the horizontal plane XY. The tiltable structure12carries at the top a reflecting surface12′ so as to define a mirror structure.

The tiltable structure12is elastically coupled to a fixed structure14, formed in the same die. In particular, the fixed structure14defines, in the horizontal plane XY, a frame14′ (in the example having a substantially squared shape), which delimits and surrounds the aforesaid cavity13, and further comprises a supporting structure15(which will be described in greater detail hereinafter), which extends within the cavity13starting from the same frame14′.

The tiltable structure12is elastically supported above the cavity13and is connected, in each quadrant, to a respective lever element16. The lever element16extends, in the embodiment illustrated, in a diagonal direction, inclined at 45° with respect to the first and second horizontal axes x, y, starting from an edge portion of the tiltable structure12, at a first end thereof, toward a vertex portion of the frame14′, at a second end thereof. In the embodiment illustrated, the lever element16has a substantially rectangular shape in the horizontal plane XY, elongated along the aforesaid diagonal direction.

In particular, the tiltable structure12is elastically coupled to each lever element16, at its first end, by a respective elastic suspension element18, having a high rigidity with regard to movements out of the horizontal plane XY (along the orthogonal axis Z, transverse to said horizontal plane XY) and furthermore configured so as to enable a relative rotation between the same tiltable structure12and the lever element16.

In detail, each of the elastic suspension elements18comprises a first portion18aof a folded type, connected to the aforesaid edge portion of the tiltable structure12, and a second portion18b, of a torsional type, formed by a rectilinear element arranged along the diagonal direction and connected between the first portion18aand the first end of the lever element16.

Each lever element16, at its second end, is furthermore elastically coupled to the aforesaid supporting structure15, in particular by respective elastic connecting elements20, of a torsional type, having a rectilinear extension substantially transverse with respect to the aforesaid diagonal direction, on opposite sides with respect to the lever element16, to define a lever rotation axis L for the lever element16.

As will be discussed hereinafter, the lever element16is configured to rotate about the lever rotation axis L, so that its first end (coupled to the tiltable structure12) moves upward along the vertical axis Z (and consequently its second end moves downward) or downward (and consequently its second end moves upward). In a corresponding manner, the tiltable structure12moves upward or downward at its edge portion coupled to the aforesaid first end of the lever rotation element16.

In greater detail, the aforesaid supporting structure15comprises: central arms15a, which extend starting from the frame14′ toward the tiltable structure12, in the embodiment described, along the first rotation axis A1or the second rotation axis A2, at the center with respect to the cavity13, terminating at a distance from the same tiltable structure12; and connecting arms15b, having a first end elastically coupled to a respective lever element16by a respective elastic connecting element20and a second end fixed with respect to a respective central arm15a, to which it is joined at the corresponding first rotation axis A1or second rotation axis A2.

Consequently, in each quadrant of the microelectromechanical (MEMS) device10, two connecting arms15bare present, extending on opposite sides with respect to the corresponding lever element16, toward a respective central arm15a, which extends along the first and, respectively, the second rotation axis A1, A2.

The microelectromechanical device10further comprises an actuation structure22, which is elastically coupled to the lever elements16and is configured to cause the movement of the tiltable structure12, and in particular its rotation about the first rotation axis A1or the second rotation axis A2(or, as will be discussed hereinafter, its displacement along the vertical axis Z).

The actuation structure22comprises, in each quadrant in which it is possible to divide the microelectromechanical device10: a first pair of driving arms22a, arranged externally to the connecting arms15b, between the same connecting arms15band the frame14′ (being separated therefrom by portions of the aforesaid cavity13); and a second pair of driving arms22b, arranged internally to the connecting arms15b, between the same connecting arms15band a corresponding lever element16(being separated therefrom by further portions of the aforesaid cavity13). In the embodiment illustrated, the driving arms22a,22bhave a generically trapezoidal (or fin) shape in the horizontal plane XY.

Each driving arm22a,22bis suspended in cantilever fashion above the cavity13and carries, at a top surface thereof (opposite to the same cavity13) a respective piezoelectric material region23(in particular of PZT—lead zirconate titanate), having substantially the same extension in the horizontal plane XY as the driving arm22a,22b(for example, and in a way not highlighted in the figures for simplicity, the piezoelectric material regions23may have a dimension smaller by approximately 30 μm for each side than the underlying driving arm22a,22b).

In a possible implementation, the driving arms22a,22bare shaped so as to provide substantially a same piezoelectric actuation area as a result of biasing of the respective piezoelectric material regions23.

Each driving arm22a,22bhas a respective first end fixedly coupled to a corresponding connecting arm15b(and, therefore, fixed with respect to the frame14′) and a respective second end elastically coupled to the corresponding lever element16.

In detail, the driving arms22aof the first pair are elastically coupled to the corresponding lever element16, at its second end (in the proximity of the frame14′), by first elastic driving elements24, which extend externally to the elastic connecting elements20, between the same elastic connecting elements20and the frame14; and the driving arms22bof the second pair are elastically coupled to the corresponding lever element16by second elastic driving elements25, which extend internally to the elastic connecting elements20, between the same elastic connecting elements20and the tiltable structure12.

The first and second elastic driving elements24,25therefore extend on opposite sides of the elastic connecting elements20with respect to the lever rotation axis L and are each formed by a respective folded elastic element, having a high rigidity in regard to movements out of the horizontal plane XY (along the orthogonal axis Z) and being compliant to torsion (about a rotation axis parallel to their direction of extension, transverse to the corresponding lever element16).

During operation, as on the other hand will be evident from an examination ofFIG.2, biasing (for example with a positive voltage difference ΔV) of the piezoelectric material regions23carried by the driving arms22aof the first pair causes an upward displacement (along the vertical axis Z) of the respective second end elastically coupled to the corresponding lever element16; this displacement is transmitted by the first elastic driving elements24, thus causing upward displacement of the second end of the lever element16and rotation of the same lever element16about the lever rotation axis L, leading to a corresponding downward displacement of the first end and, therefore, of the coupled edge portion of the tiltable structure12.

This rotation of the lever element16is not hindered—but rather enabled—by the second elastic driving elements25and occurs, for example, with a zero biasing (for example, with a zero-voltage difference ΔV) of the piezoelectric material regions23carried by the driving arms22bof the second pair.

Likewise, biasing (for example, once again with a positive voltage difference ΔV) of the piezoelectric material regions23carried by the driving arms22bof the second pair causes an upward displacement (along the vertical axis Z) of the respective second end elastically coupled to the corresponding lever element16; this displacement is transmitted by the second elastic driving elements25, thus causing upward displacement of the first end of the lever element16and rotation of the same lever element16about the lever rotation axis L, leading to a corresponding upward displacement of the second end and, therefore, of the coupled edge portion of the tiltable structure12.

The above rotation of the lever element16is not hindered—but is rather enabled—in this case by the first elastic driving elements24and occurs, for example, with a zero biasing (for example, with a zero-voltage difference ΔV) of the piezoelectric material regions23carried by the driving arms22aof the first pair.

In a possible embodiment, the piezoelectric material regions23of the driving arms22aof the first pair in a given one of the quadrants into which the microelectromechanical device20is divided are electrically connected (in a manner not illustrated, by appropriate electrical connection elements) to the piezoelectric material regions23of the driving arms22bof the second pair in the quadrant of the microelectromechanical device10arranged symmetrically with respect to the center O. Likewise, the piezoelectric material regions23of the driving arms22bof the second pair in the given one of the quadrants into which the microelectromechanical device10is divided are electrically connected to the piezoelectric material regions23of the driving arms22aof the first pair in the quadrant of the microelectromechanical device10arranged symmetrically with respect to the center O.

As a result of the aforesaid electrical connections, four sets of driving electrodes are obtained, which are represented schematically inFIG.2with different shades of grey, from the darkest to the lightest: Set1, Set2, Set3, and Set4. Biasing of the aforesaid sets of driving electrodes allows obtainment of desired rotations of the tiltable structure12about the first and second rotation axes A1, A2; in a possible non-limiting implementation, it is possible to obtain four different rotations of the tiltable structure12and four corresponding quasi-static driving positions.

In detail: application of the voltage difference ΔV, for example positive, to the driving electrodes of the sets Set2 and Set3, with zero biasing of the driving electrodes of the sets Set1 and Set4 leads to a negative rotation about the rotation axis A1; application of the voltage difference ΔV to the driving electrodes of the sets Set1 and Set4, with zero biasing of the driving electrodes of the sets Set2 and Set3 leads to a positive rotation about the same rotation axis A1; application of the voltage difference ΔV, for example once again positive, to the driving electrodes of the sets Set1 and Set3, with zero biasing of the driving electrodes of the sets Set2 and Set4 leads to a negative rotation about the rotation axis A2; and application of the voltage difference ΔV to the driving electrodes of the sets Set2 and Set4, with zero biasing of the driving electrodes of the sets Set1 and Set3 leads to a positive rotation about the same rotation axis A2.

By way of example,FIG.3shows the latter position of the tiltable structure12, with positive rotation about the rotation axis A2, and moreover shows the deformation of the various elastic elements involved in the same rotation, as described in detail previously.

As mentioned, in different implementations, biasing of the four sets of driving electrodes may be such as to cause the tiltable structure12to assume desired positions within its range of movement.

FIG.4shows schematically, by way of example, the displacement of the microelectromechanical device10, along a section IV-IV ofFIG.3, with a positive rotation about the rotation axis A2. InFIG.4, F designates the driving force due to the piezoelectric effect acting at the points of coupling of the lever elements16with the respective elastic driving elements24,25(which are, in fact, configured so as to transmit the force in the direction of the vertical axis Z to the aforesaid lever elements16) causing the lever movement of the lever elements16about the lever rotation axis L; and0designates the consequent angle of rotation of the tiltable structure12with respect to the vertical axis Z.

As shown schematically in the aforesaidFIG.2, the microelectromechanical device10further comprises one or more piezoresistive (PZR) sensors29, appropriately arranged so as to provide one or more detection signals associated to the rotation of the tiltable structure12about the first and second rotation axes A1, A2; these detection signal can be supplied as a feedback at the output from the microelectromechanical device10allowing implementing of an appropriate closed control loop.

The above piezoresistive sensors29are obtained (for example, by surface diffusion of dopant atoms) on one or more of the connecting arms15bof the connecting structure15(different arrangements for the piezoresistive sensors29may, however, be envisaged). Advantageously, the elastic connecting elements20are able to transmit the stresses to the connecting arms15band therefore toward the piezoresistive sensors29, thus enabling detection of the rotation of the tiltable structure12.

In a manner not described in detail, but that will be evident, two arrangements of piezoresistive sensors29may be used, in a Wheatstone-bridge configuration, for detecting rotations of the tiltable structure12about the first and second rotation axes A1, A2(and therefore the overall rotation of the tiltable structure12).

Alternatively, as it is described in detail in the aforementioned document EP 3,666,727 A1, a mechanical amplification structure may moreover be introduced, designed to attempt to maximize, by an appropriate lever mechanism, the stress detected by the piezoresistive sensors29and, therefore, the corresponding sensitivity.

In a manner not illustrated, other types of detection of the rotations of the tiltable structure12may moreover be provided, for example by piezoelectric sensors.

FIG.5shows a simplified sectional view of the microelectromechanical device10, where some elements are omitted for simplicity and others are represented schematically.

In particular, the above sectional view shows that the frame14′ is coupled at the bottom to a supporting wafer (the so-called “handling wafer”)31, which has, underneath the cavity13and the tiltable structure12, a recess32, for enabling rotation of the same tiltable structure12.

In this embodiment, the tiltable structure12, as likewise the actuation structure22, is made in the active layer34of a SOI wafer, a supporting layer35of which, separated from the active layer34by a dielectric layer36, is coupled at the bottom to the aforesaid handling wafer31. The tiltable structure12moreover has at the bottom, in contact with the surface opposite to the reflecting surface12′, reinforcement elements37, having extension along the vertical axis Z.

As shown inFIG.6, a possible variant embodiment envisages, instead, that the tiltable structure12is formed at a higher level with respect to the actuation structure22, in a stacked manner. In particular, in this case the tiltable structure12is coupled in a stacked manner to a base portion40, formed at the same level as the actuation structure22(separated from the latter by the cavity13), by a connection pillar41having extension along the vertical axis Z. In this case, advantageously, an amplification of the lever effect implemented by the lever elements16(here not illustrated) is obtained, thus achieving a wider angle of inclination of the tiltable structure12, with a more compact and sturdier solution.

The tiltable structure12is formed in this case in a different wafer of semiconductor material, which can be coupled to the base surface40for example by wafer bonding in front-end stages of the manufacturing process.

The advantages of this disclosure emerge clearly from the foregoing description.

In any case, it is highlighted that the described embodiments provide robust and efficient microelectromechanical mirror devices, in particular of the bi-dimensional vector-scanner type (the so-called MEMS vector scanners).

The embodiments described have a high linearity, wide angles of aperture for rotation of the mirror, and a compact structure.

For example, the microelectromechanical device10can be advantageously used in a projection apparatus40designed to be operatively coupled to a portable electronic apparatus41, as illustrated schematically with reference toFIG.7.

The projection apparatus40comprises a source42, designed to generate an image43; the microelectromechanical device10, acting as mirror and designed to receive the image43and to direct it toward a screen45(external to, and set at a distance from, the same projection apparatus40); a first driving circuit46, designed to provide driving signals to the source42, for generation of the image43to be projected; a second driving circuit48, designed to supply driving signals to the actuation structure22of the microelectromechanical device10; and a communication interface49, designed to receive, from an external control unit50, for example included in the portable electronic apparatus41, information on the image to be projected, for example in the form of an array of pixels, which are sent at an input of the source42for driving it.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of this disclosure.

In particular, the described embodiments and approaches can also be used to provide actuators for microelectromechanical speaker devices (the so-called μ-speakers), by varying the combination of biasing of the piezoelectric material regions23carried by the driving arms22a,22bof the first and second pairs.

In this case, the piezoelectric material regions23of the driving arms22aof the first pairs are electrically connected together, and moreover the piezoelectric material regions23of the driving arms22bof the second pairs are electrically connected together.

With reference toFIG.8, by applying a same voltage difference ΔV, for example positive, to the driving electrodes22bof the second pairs, with zero biasing of the driving electrodes22aof the first pairs, an upward displacement along the vertical axis Z of the tiltable structure12is obtained. Likewise, by applying a same voltage difference ΔV to the driving electrodes22aof the first pairs, with zero biasing of the driving electrodes22bof the second pairs, a downward displacement along the vertical axis Z of the same tiltable structure12is obtained.

Basically, a piston-like displacement is achieved for the tiltable structure12, which can thus be used as actuator, for example coupled to an appropriate deformable membrane (in a manner not illustrated), for generation of sound waves in a microelectromechanical speaker device.

Advantageously, this allows obtainment of vertical movements of the tiltable structure12, with high linearity and excellent resistance to stress and shocks.

Furthermore, it is evident that variants may be envisaged regarding the shape and arrangement of the elements that form the microelectromechanical device10, for example different shapes of the tiltable structure12(and of the corresponding reflecting surface12′).

In this regard,FIG.9shows a further embodiment of the microelectromechanical device10, which differs from the one illustrated previously in that the rotation axes A1and A2are inclined by 45° with respect to the first and second horizontal axes x and y, i.e., to the sides of the frame14′ of the fixed structure14and of the corresponding die.

Consequently, the lever elements16are in this case aligned with the aforesaid horizontal axes x and y, whereas the central arms15aof the supporting structure15are oriented along the diagonal directions (i.e., in this case, along the aforesaid rotation axes A1and A2).

The tiltable structure12moreover has here a substantially square shape in the horizontal plane XY and the first portion18a, once again of a folded type, of each of the elastic suspension elements18is doubled and connected to two end portions of a respective side of the tiltable structure12.

In addition, in the embodiment illustrated, the first and second elastic decoupling elements24,25have a linear and non-folded arrangement, being in each case rigid to movements out of the horizontal plane XY and compliant to rotation.

Otherwise, the configuration and operation of the microelectromechanical device10does not differ substantially from what has been discussed previously for the first embodiment.

By way of example,FIG.10shows a rotation of the tiltable structure12of the microelectromechanical device10ofFIG.9, which is a positive rotation about the first rotation axis A1, and moreover represents the deformation of the various elastic elements involved in the same rotation.