MICROELECTROMECHANICAL MIRROR DEVICE WITH PIEZOELECTRIC ACTUATION AND OPTIMIZED SIZE

A microelectromechanical device has a first tiltable mirror structure extending in a horizontal plane defined by first and second horizontal axes and includes a fixed structure defining a frame delimiting a cavity, a tiltable element carrying a reflecting region, elastically suspended above the cavity having first and second median axes of symmetry, elastically coupled to the frame by first and second coupling structures on opposite sides of the second horizontal axis. The first tiltable mirror structure has a driving structure coupled to the tiltable element to cause rotation around the first horizontal axis. The first tiltable mirror structure is asymmetrical with respect to the second horizontal axis and has, along the first horizontal axis, a first extension on a first side of the second horizontal axis, and a second extension greater than the first extension, on a second side of the second horizontal axis opposite to the first side.

PRIORITY CLAIM

This application claims the priority benefit of Italian application for Patent No. 102022000012884 filed on Jun. 17, 2022, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

TECHNICAL FIELD

This disclosure relates to a microelectromechanical mirror device (made using MEMS—Micro-Electro-Mechanical System—technology) having an optimized size.

BACKGROUND

Microelectromechanical mirror devices are used in portable apparatuses, such as for example smartphones, tablets, notebooks, PDAs, for optical applications, in particular to direct light radiation beams generated by a light source (for example a laser) with desired patterns. Owing to their small size, these devices allow stringent requirements regarding space occupation, in terms of area and thickness, to be compiled with.

For example, microelectromechanical mirror devices are used in optoelectronic apparatuses, such as miniaturized projectors (so-called picoprojectors), capable of projecting images from a distance and generating desired light patterns, for example for augmented or virtual reality applications.

Microelectromechanical mirror devices generally include a tiltable structure carrying a suitable reflecting (or mirror) surface, elastically supported above a cavity and made from a body of semiconductor material so as to be movable, for example with tilt or rotation movement out of a corresponding main extension plane, to direct an impinging light beam in a desired manner.

In particular, in the case of microelectromechanical mirror devices with biaxial projection, a deflection of the light beam along two axes is required, which may be provided by two uniaxial tiltable structures.

In this regard,FIG.1schematically shows a picoprojector1comprising a light source2, for example a laser source, generating a light beam which is deflected by a microelectromechanical mirror device3towards a screen4.

In the example schematically shown in the aforementionedFIG.1, the microelectromechanical mirror device3comprises: a first tiltable mirror structure3a, of a uniaxial type, controlled so that it rotates around a rotation axis A with resonance movement, to generate a fast horizontal scan; and a second tiltable mirror structure3b, also of a uniaxial type, controlled so that it rotates around a respective rotation axis A′ with linear or quasi-static movement (i.e., at a frequency much lower than the frequency of the resonance movement), to generate a slow vertical scan, for example of a sawtooth type. The aforementioned respective rotation axis A′ is transverse, for example orthogonal or inclined by a certain non-zero angle with respect to the rotation axis A.

The first and the second tiltable mirror structures3a,3bcooperate to generate, on the screen4, a scanning pattern, which is schematically shown and indicated by5in the sameFIG.1. In particular, the first tiltable mirror structure3a, rotating around the rotation axis A, “draws” a horizontal line on the second tiltable mirror structure3b; and the same second tiltable mirror structure3b, rotating around the respective rotation axis A′, directs the projection onto a desired rectangular surface on the screen4.

The rotation of the tiltable mirror structures3a,3bis controlled by a respective actuation system which may be for example of electrostatic, electromagnetic or piezoelectric type.

Electrostatic actuation systems generally have the disadvantage of requiring high operating voltages, while electromagnetic actuation systems generally entail a high power consumption; it has therefore been proposed to control the movement of the tiltable mirror structure with piezoelectric actuation.

Microelectromechanical mirror devices with piezoelectric actuation have the advantage of requiring reduced actuation voltages and power consumption with respect to devices with electrostatic or electromagnetic actuation. Furthermore, piezoresistive (PZR) sensor elements for sensing the driving condition of the mirror (in terms of the applied stress or the displacement or position assumed) and for providing a feedback signal to allow a feedback control of the same driving, may be easily provided.

A general request is to reduce the size of the tiltable mirror structures for the aforementioned microelectromechanical mirror devices, in order to obtain a reduced overall area occupation. For example, this need is particularly felt when the mirror devices are used in glasses or headsets for virtual reality or augmented reality applications.

In the case of microelectromechanical mirror devices with biaxial projection, size reduction entails not only a reduction in the size of the single tiltable mirror structures, but also a suitable mutual arrangement of the same structures in order to generally optimize the occupied volume.

In this regard,FIG.2schematically shows a microelectromechanical mirror device with biaxial projection, denoted again by3, with a possible arrangement of the corresponding first and second tiltable mirror structures3a,3b, placed in a close position in order to reduce the occupied volume.

A problem that may occur with a similar configuration of the microelectromechanical mirror device3is represented by the possibility that the output light beam (OUT) reflected by the second tiltable mirror structure3bis intercepted, even if partially, by the bulk of the first tiltable mirror structure3a, creating a clipping of the light projection.

There is accordingly a need in the art to provide a microelectromechanical mirror device which allows the previously highlighted problems to be overcome and which has a reduced area occupation, having optimized size.

SUMMARY

Disclosed herein is an electronic device including a microelectromechanical mirror device with a first tiltable mirror structure in a first die of semiconductor material, extending in a horizontal plane defined by first and second horizontal axes. The first tiltable mirror structure features a fixed structure that defines a frame delimiting a cavity, a tiltable element carrying a reflecting region elastically suspended above the cavity with first and second median axes of symmetry parallel to the first and second horizontal axes, and a driving structure coupled to the tiltable element for rotation around the first horizontal axis with a resonance movement. The first tiltable mirror structure is asymmetrical with respect to the second horizontal axis and has different extension dimensions along the first horizontal axis on opposing sides of the second horizontal axis.

The electronic device includes a first coupling structure on the first side of the second horizontal axis with a single torsional spring connected to the tiltable element and the frame, extending linearly along the first horizontal axis. The second coupling structure features first and second torsional springs, a constraint element between the springs, with the springs connected to the tiltable element, constraint element, and frame along the first horizontal axis.

The single torsional spring of the first coupling structure has a first width along the second horizontal axis, smaller than a corresponding second width of the first torsional spring of the second coupling structure.

The single torsional spring of the first coupling structure has a first torsional stiffness, and the first torsional spring of the second coupling structure has a second torsional stiffness, with a ratio between the first and second torsional stiffnesses lying between 0.55 and 0.65.

The driving structure is entirely positioned on the second side of the second horizontal axis, on the same side as the second coupling structure.

The driving structure includes a single pair of driving arms coupled to the tiltable element, formed by first and second driving arms arranged symmetrically with respect to the first horizontal axis and the second coupling structure. The first and second driving arms are integrally coupled to the frame of the fixed structure, suspended above the cavity, and carry respective piezoelectric structures on their top surfaces, opposite to the cavity.

The driving structure also features first and second displacement transfer structures arranged symmetrically with respect to the first horizontal axis and interposed between the second end of the first and second driving arms and respective end portions of the constraint element of the second coupling structure. Each displacement transfer structure is configured to convey driving of the first or second driving arm to the respective end portion of the constraint element.

Each of the first and second displacement transfer structures includes a first arm extending linearly along the first horizontal axis and coupled between the second end of a corresponding driving arm and a rigid connecting element near the tiltable element, and a second arm extending linearly along the first horizontal axis, parallel to the first arm, and coupled between the rigid connecting element, near the tiltable element, and the respective end portion of the constraint element of the second coupling structure.

The second end of the single torsional spring of the first coupling structure is connected to the frame by first and second coupling elastic elements, extending parallel to the second horizontal axis, transversely to the single torsional spring, from the second end towards a respective long side of the frame.

The second end of the single torsional spring of the first coupling structure is connected to the frame by first and second coupling elastic elements, which are of a folded type and have a general extension along the first horizontal axis, connecting the second end of the single torsional spring to a first short side of the frame.

The electronic device has first and second coupling elastic elements and part of the single torsional spring extending inside a recess provided in the frame at the first short side.

A reinforcement structure is coupled below the tiltable element of the first tiltable mirror structure as a mechanical reinforcement for the tiltable element.

The device also includes a second tiltable mirror structure with a tiltable element that rotates around a rotation axis with linear or quasi-static movement. The second tiltable mirror structure cooperates with the first tiltable mirror structure's tiltable element to direct an impinging light beam. The second tiltable mirror structure is provided in a second die of semiconductor material with a fixed structure defining a frame that delimits a cavity housing the tiltable element. The frame defines an outer side surface of the second die with a concavely patterned shape, forming a recess that accommodates at least part of the first die of the first tiltable mirror structure.

The second tiltable mirror structure is arranged with its horizontal plane at a certain angle, less than 90°, relative to the horizontal plane of the first tiltable mirror structure.

The recess has a basin shape and is delimited by a base portion extending parallel to the first horizontal axis and by wall portions, inclined or orthogonal, with respect to the base portion.

The second tiltable mirror structure further includes an actuation structure, coupled to the tiltable element and configured to cause it to rotate around the rotation axis. The actuation structure features first and second pairs of driving arms, each formed by first and second driving arms, arranged symmetrically with respect to the rotation axis. Each driving arm has a first end integrally coupled to the frame and a second end elastically coupled to the tiltable element by a respective decoupling elastic element. The outer side surface is arranged in a close position, with a reduced separation gap, with respect to the driving arms and the tiltable element throughout the entire extension along the first horizontal axis.

The electronic device is an optoelectronic device that includes a light source for generating a light beam. The microelectromechanical mirror device acts as a mirror module with biaxial projection for receiving the light beam and directing it towards an external screen or display surface placed at a distance from the optoelectronic device.

DETAILED DESCRIPTION

FIG.3illustrates a tiltable mirror structure, based on MEMS technology, generally denoted by10; this tiltable mirror structure10is designed to provide a resonance fast scanning movement (thus corresponding, for example, to the first tiltable mirror structure3aof the aforementioned microelectromechanical mirror device3of the picoprojector1ofFIG.1).

As will be described in detail below, according to an aspect of this disclosure, the tiltable mirror structure10has an asymmetrical configuration along a main extension axis thereof, in order to obtain a size reduction along the same main extension axis.

The tiltable mirror structure10is formed in a die11of semiconductor material, in particular silicon, and has a tiltable element12, having (at rest) a main extension in a horizontal plane xy and being arranged so that it rotates with fast movement, in resonance, around a first axis A, parallel to a first horizontal axis x of the aforementioned horizontal plane xy (this first horizontal axis x represents in this case the aforementioned main extension axis of the tiltable mirror structure10).

The first axis A represents a median axis of symmetry for the tiltable element12and in general for the tiltable mirror structure10.

A second axis B, orthogonal to the aforementioned first axis A and intersecting the same first axis A at a geometric center O of the tiltable element12in the horizontal plane xy, represents a further median axis of symmetry for the same tiltable element12. This second axis B is parallel to a second horizontal axis y, orthogonal to the first horizontal axis x and defining, with the same first horizontal axis x, the horizontal plane xy.

As previously indicated, and as will be described in detail below, according to an aspect of this disclosure, the tiltable mirror structure10is generally asymmetrical with respect to this second axis B (and with respect to the second horizontal axis y) and has, along the first horizontal axis x: a first extension dimension d1, on a first side of the aforementioned second axis B; and a second extension dimension d2, greater with respect to the first extension dimension d1, on a second side of the aforementioned axis B, opposite to the first side (a total extension of the tiltable mirror structure10along the same first horizontal axis x being indicated by d, where d=d1+d2).

Purely by way of example, the aforementioned first extension dimension d1may be equal to 3.5 mm and the aforementioned second extension dimension d2may be equal to 4.5 mm (with the total extension d equal to 8 mm); in general, the first extension dimension d1may be in the example comprised between 3 mm and 4.5 mm (the lower limit being related to a mechanical stress limit bearable by elastic elements).

The aforementioned tiltable element12is suspended above a cavity13, provided in the die11and defines a supporting element, which carries a reflecting region12′ (for example of aluminum, or gold, depending on whether the projection is in the visible or in the infrared region), so as to define a mirror element.

The tiltable element12is elastically coupled to a fixed structure14, formed in the same die11and defining, in the horizontal plane xy, a frame14′; this frame14′ has a generally rectangular shape in the aforementioned horizontal plane xy and delimits and surrounds the aforementioned cavity13.

In particular, the tiltable element12is elastically coupled to the frame14′ by first and second coupling structures15a,15b, which extend longitudinally along the first horizontal axis x, suspended above the cavity13, between the frame14′ and the tiltable element12, on opposite sides of the same tiltable element12with respect to the second axis B.

In detail, the first coupling structure15a, arranged on the aforementioned first side of the second axis B, is in this case formed by a single torsional spring16a, having a first end coupled to the tiltable element12and a second end coupled to the frame14′ (in particular to a corresponding first short side). In the illustrated embodiment, this torsional spring16ahas a linear-beam shape with extension along the first horizontal axis x.

The second coupling structure15b, conversely, comprises a first torsional spring16b′ and a second torsional spring16b″, having a linear-beam shape with extension along the first horizontal axis x, and a constraint element18, interposed between the aforementioned first and second torsional springs16b′,16b″.

In detail, the first torsional spring16b′ has a first end coupled to the tiltable element12and a second end coupled to the constraint element18. The second torsional spring16b″ has a first end coupled to the constraint element18(on an opposite side with respect to the first torsional spring16b′ along the first horizontal axis x) and a second end coupled to the frame14′ (in particular to a corresponding second short side, opposite to the aforementioned first short side); the second torsional spring16b″ has a length along the first horizontal axis x that is smaller, in particular much smaller, than a corresponding length of the first torsional spring16b′.

In general, the aforementioned torsional springs16a,16b′,16b″ have high stiffness to bending along the first and the second horizontal axes x, y of the horizontal plane xy and are yielding to torsion around the axis A, so as to allow the rotation of the tiltable element12.

The aforementioned constraint element18is stiff and in the example has a generally rectangular shape in the horizontal plane xy with a width (in a direction parallel to the second horizontal axis y) much greater with respect to the torsional springs16b′,16b″ and a length (in a direction parallel to the first horizontal axis x) comparable, in the example, to that of the second torsional spring16b″. The same constraint element18traverses the first axis A, having first and second end portions along the second horizontal axis y arranged on opposite sides of the same first axis A.

The tiltable mirror structure10further comprises a driving structure20, coupled to the tiltable element12and configured so as to cause it to rotate around the first axis A; this driving structure20is entirely arranged on the aforementioned second side of the second axis B, that is on the same side as the second coupling structure15b.

According to an aspect of this disclosure, the tiltable mirror structure10does not include driving structure arranged on the aforementioned first side of the second axis B, which is on the same side as the first coupling structure15a.

In detail, the aforementioned driving structure20comprises a single pair of driving arms formed by first and second driving arms22a,22b, arranged on opposites side of, and symmetrically with respect to, the first axis A and the second coupling structure15b, and having a longitudinal extension parallel to the first horizontal axis x.

In the embodiment illustrated inFIG.1, the driving arms22a,22bhave a generically rectangular shape with a first end integrally coupled to the frame14′ of the fixed structure14, are suspended above the cavity13and carry, at a respective top surface (opposite to the same cavity13) a respective piezoelectric structure23(in particular including PZT—Lead Zirconate Titanate), having for example substantially the same extension in the horizontal plane xy with respect to the corresponding driving arm22a,22b.

This piezoelectric structure23(in a manner not illustrated in detail) is formed by superimposing a bottom electrode region, of a suitable conductive material, arranged above the corresponding driving arm22a,22b; a region of piezoelectric material (for example formed by a PZT thin film) arranged on the aforementioned bottom electrode region; and a top electrode region arranged on the piezoelectric material region.

The aforementioned driving structure20further comprises first and second displacement transfer structures25a,25b, arranged symmetrically to each other with respect to the first axis A and interposed between a second end of the first, respectively, the second driving arm22a,22band a respective end portion of the constraint element18of the second coupling structure15b.

Each of the first and second displacement transfer structures25a,25bcomprises a first arm26, having linear extension along the first horizontal axis x and coupled between the second end of the corresponding driving arm22a,22band a rigid connecting element27, arranged in proximity to the tiltable element12; and a second arm28, also having linear extension along the first horizontal axis x, parallel to the first arm26and coupled between the same rigid connecting element27, in proximity to the tiltable element12, and the first, or second, end portion of the constraint element18of the coupling structure15b. This second arm28is therefore interposed between the first torsional spring16b′ of the second coupling structure15band the aforementioned first arm26.

The tiltable mirror structure10further comprises a piezoresistive (PZR) sensor30, suitably arranged so that it provides a sensing signal associated with the rotation of the tiltable element12around the first axis A; this sensing signal may be provided outside the microelectromechanical mirror device1to implement a feedback control for driving of the same tiltable element12.

In the embodiment illustrated inFIG.3, this piezoresistive sensor30is provided (for example by surface diffusion of doping atoms) in the frame14′, at the region of coupling of the same frame14′ to the second torsional spring16b″ of the second coupling structure15b. This piezoresistive sensor30is arranged so that it senses the stress associated with the torsion of the aforementioned second torsional spring16b″ and therefore provides an indication relating to the rotation movement of the tiltable element12.

The tiltable mirror structure10further comprises a plurality of electrical contact pads32, carried by the fixed structure14at the frame14′, electrically connected (in a manner not illustrated in detail in the sameFIG.3) to the piezoelectric structures23of the driving arms22a,22bby respective electrical connection tracks, to allow the electrical biasing thereof by electrical signals coming from outside of the microelectromechanical mirror device1(for example provided by a biasing device of an electronic apparatus wherein the tiltable mirror structure10is integrated). The aforementioned electrical contact pads32are also connected to the piezoresistive sensor30, to output the aforementioned sensing signal.

During operation of the tiltable mirror structure10, as schematically illustrated inFIG.4, the application of a biasing voltage to the piezoelectric structure23of the first driving arm22a(having a positive value with respect to the biasing of the piezoelectric structure23of the second driving arm22b, which may for example be connected to a ground reference potential), may cause a rotation of a positive angle around the first axis A.

Correspondingly, the application of a biasing voltage to the piezoelectric structure23of the second driving arm22b(having a positive value with respect to the bias of the piezoelectric structure23of the first driving arm22a, which may for example in this case be connected to a ground reference potential), may cause a corresponding rotation of a negative angle around the same first axis A.

In particular, driving of the first driving arm22ain a first direction along the orthogonal axis z (for example downwards, as illustrated in the aforementionedFIG.4) is conveyed to the first end portion of the constraint element18by the first displacement transfer structure25a; similarly, the driving of the second driving arm22ain a second direction (for example downwards) of the same orthogonal axis z is conveyed to the second end portion of the constraint element18by the second displacement transfer structure25b, thereby causing torsion of the first torsional spring16b′ and the consequent rotation of the tiltable element12.

In greater detail, according to an aspect of this disclosure, the torsional spring16aof the first coupling structure15ahas a first width t1along the second horizontal axis y, which is smaller with respect to a corresponding second width t2of the first torsional spring16b′ of the second coupling structure15b.

In particular, the torsional spring16ahas a torsional stiffness k1even 40% lower with respect to a corresponding torsional stiffness k2of the first torsional spring16b′; in the embodiment illustrated inFIG.3, this different torsional stiffness is mainly due to the different width, since the length of the aforementioned torsional springs is substantially the same (in order to maintain a similar stress distribution).

In general, the ratio between the aforementioned torsional stiffnesses (k1/k2) is preferably comprised between 0.55 and 0.65; in other words, the torsional stiffness k1of the single torsional spring16ais comprised between 55% and 65% of the torsional stiffness k2of the first torsional spring16b′.

FIGS.5A and5Bschematically show a different embodiment of the tiltable mirror structure10, which envisages, coupled below the tiltable element12, the presence of a reinforcement structure33, acting as a mechanical reinforcement for the same tiltable element12(and also for ensuring the flatness thereof, in the horizontal plane xy, in a rest condition); this reinforcement structure33may have, for example, a ring shape and be arranged at the periphery of the tiltable element12(the reinforcement structure33is essentially formed on the back of the die11).

In particular, in the embodiment shown inFIG.5B, the die11is of a SOI (Silicon on Insulator) type, with the tiltable element12and the first and the second coupling structures15a,15bprovided in an active layer34aof the die11and the aforementioned reinforcement structure33provided in a support layer34bof the same die11. In this case, the aforementioned frame14′ is provided in both the active and support layers34a,34bof the die11and also in a corresponding insulating layer34c, interposed between the same active and support layers34a,34b.

A support wafer37, coupled for example by bonding, is also present below the die11.

In this embodiment, owing to the presence of the aforementioned reinforcement structure33, it is possible to further reduce the size of the tiltable mirror structure10, in particular to reduce the length of the first coupling structure15aand of the corresponding torsional spring16a(i.e., the first extension dimension d1), while maintaining a same operating frequency.

In the illustrated example, the aforementioned first extension dimension d1of the tiltable mirror structure10is for example equal to 2.5 mm and the aforementioned second extension dimension d2is equal to 4 mm (with the total extension d equal to 6.5 mm), with a same optical performance (in particular, a same opening angle) of the structure described with reference toFIG.3.

In this embodiment, the frame14′ may also be provided in a further asymmetrical manner, having the aforementioned first and second short sides (i.e., the portions on the opposite sides of the aforementioned second axis B directed along the second horizontal axis y) having a different width along the first horizontal axis x. In particular, the side portion coupled to the first coupling structure15a(the aforementioned first short side) has in this case a width w1smaller with respect to a width w2of the side portion coupled to the second coupling structure15b(the aforementioned second short side).

With reference toFIGS.6A-6Cfurther variant embodiments of the tiltable mirror structure10are now illustrated, aimed at reducing the size of the same tiltable mirror structure10and the corresponding area occupation.

In particular, in the embodiment illustrated inFIG.6A, the second end of the single torsional spring16aof the first coupling structure15ais not directly coupled to the frame14′. Conversely, this second end is coupled to the aforementioned frame14′ by first and second coupling elastic elements40a,40b, of a linear type, having extension parallel to the second horizontal axis y, transversely to the same torsional spring16a, from the aforementioned second end towards a respective long side of the frame14′ (it should be noted that in this case, the aforementioned torsional spring16ais therefore not coupled to the first short side of the same frame14′).

Advantageously, this variant embodiment allows the length of the torsional spring16aof the first coupling structure15ato be reduced.

In the variant embodiment illustrated inFIG.6B, the aforementioned first and second coupling elastic elements40a,40bare of a folded type and have general extension along the first horizontal axis x, coupling the aforementioned second end of the torsional spring16aof the first coupling structure15a, again to the first short side of the frame14′.

Owing to the folded configuration of the same first and second coupling elastic elements40a,40bit is possible to obtain an even greater reduction of the first extension dimension d1of the tiltable mirror structure10, on the first side of the second axis B; in general, the space occupation of the die is optimized.

In the variant embodiment ofFIG.6C, the aforementioned first and second coupling elastic elements40a,40b, again having a folded-type configuration, and furthermore most of the torsional spring16aof the first coupling structure15aextend inside a recess45provided in the frame14′ at the corresponding first short side. In this manner, it is possible to obtain an even greater reduction of the first extension dimension d1of the tiltable mirror structure10, on the first side of the second axis B.

As previously discussed, the tiltable mirror structure10may be used as a first tiltable mirror structure3a(see the aforementionedFIGS.1and2), driven in resonance to generate a fast scan, in a microelectromechanical mirror device3(defining for example a mirror module with biaxial projection of a picoprojector), further comprising a second tiltable mirror structure3b, controlled so as to rotate around a respective rotation axis with linear or quasi-static movement, to generate a slow scan.

The size reduction obtained, as discussed in detail, for the first tiltable mirror structure3ahelps reducing the general volume occupation of the microelectromechanical mirror device3.

However, it has been realized that, for the purposes of this volume reduction, optimizing the size of the second tiltable mirror structure3bas well and also providing for an optimized joint arrangement of the same first and second tiltable mirror structures3a,3b, is recommended.

A further aspect of this disclosure therefore provides, in a microelectromechanical mirror device3with biaxial projection, comprising the tiltable mirror structure10previously described (acting as a first tiltable mirror structure3a), for optimization of the size and arrangement of the associated second tiltable mirror structure3b.

As will be discussed in detail below, this aspect of this disclosure envisages a suitable patterning of a die wherein the second tiltable mirror structure3bis formed, aimed not only at reducing the size, but also at facilitating a mutual positioning with the first tiltable mirror structure3a.

Referring now toFIG.7A, an embodiment of a tiltable mirror structure50is described, which may be employed as the aforementioned second tiltable mirror structure3bin the microelectromechanical mirror device3.

In general, this tiltable mirror structure50is for example provided as described in detail in United States Patent Publication No. 2020/0192199 (corresponding to European patent application EP366672A1) which is incorporated herein by reference.

The tiltable mirror structure50has a completely symmetrical shape with respect to a first x′ and a second y′ horizontal axis of a respective horizontal plane x′y′ and is made in a respective die51of semiconductor material, in particular silicon.

The tiltable mirror structure50is provided with a respective tiltable element52, which is arranged so that it rotates (with a quasi-static movement) around a respective rotation axis A′ (parallel to the first horizontal axis x′) and carries a reflecting surface52′. In the example illustrated, the tiltable element52has a rectangular shape in the horizontal plane x′y′, elongated along the respective rotation axis A′.

In particular, the die51comprises a fixed structure54defining a frame54′ which delimits and surrounds a cavity53wherein the tiltable element52is accommodated; first and second support (or anchoring) elements55a,55b, integral with the frame54′, extend from the same frame54′, inside the cavity53along the aforementioned respective rotation axis A′, on opposite sides with respect to the tiltable element52.

The tiltable element52is elastically coupled to the first and the second support elements55a,55bby first and second suspension elastic elements56a,56b, having a high stiffness with respect to movements outside the horizontal plane and yielding with respect to torsion around the respective rotation axis A′. These first and second suspension elastic elements56a,56balso extend along the respective rotation axis A′, as an extension of the first, respectively the second, support elements55a,55b.

The tiltable mirror structure50further comprises an actuation structure60, coupled to the tiltable element52and configured to causes its rotation around the respective rotation axis A; the actuation structure60is interposed between the tiltable element52and the fixed structure54and contributes to supporting the tiltable element52above the cavity53.

This actuation structure60comprises first and second pairs of driving arms, each formed by first and second driving arms62a,62b, arranged on opposite sides of, and symmetrically with respect to, the respective rotation axis A′ and a respective one of the first and the second support elements55a,55b.

Each driving arm62a,62bis suspended above the cavity53and carries a respective piezoelectric structure63(in particular including PZT); each driving arm62a,62bhas a first end integrally coupled to the frame54′ and a second end elastically coupled to the tiltable element52by a respective decoupling elastic element64a,64b, in the example of linear type (having a high stiffness with respect to movements outside the horizontal plane and yielding with respect to torsion).

According to a particular aspect of this disclosure, the frame54′ of the die51, which defines an outer side surface51′ of the same die51has, at its longitudinal extension, parallel to the respective rotation axis A′, a suitably patterned shape. In other words, the aforementioned frame54′ does not have, in the horizontal plane x′y′, a rectangular or square profile.

In particular, this outer side surface51′ is patterned in a concave manner, so that it defines, on both sides of the tiltable element52with respect to the first horizontal axis x′, a respective recess66.

In the embodiment illustrated in the aforementionedFIG.7A, each recess66has a basin shape and is delimited by: a base portion66aextending along the first horizontal axis x′, externally to the tiltable element52; and by wall portions66b, inclined (in a ‘V’ pattern) with respect to the base portion66a.

In the different embodiment illustrated inFIG.7B, the same recess66has instead, in the horizontal plane x′y′, a ‘U’ shape, with the base portion66aextending along the first horizontal axis x′ and the wall portions66bin this case extending orthogonally along the second horizontal axis y′.

In both embodiments, the outer side surface51′ is arranged in a very close position, with a small or minimum separation gap, with respect to the driving arms62a,62band to the tiltable element52, through an entire extension along the first horizontal axis x′, so as to optimize the area occupation in the horizontal plane x′y′.

In other words, the tiltable mirror structure50thereby minimizes the extension of non-active areas (i.e., the empty spaces not having a specific function in the same structure).

It should be noted that in the embodiment illustrated inFIG.7B, the aforementioned driving arms62a,62bextend along the second horizontal axis y′ (instead of along the first horizontal axis x′), giving the tiltable mirror structure50an overall ‘H’ shape in the horizontal plane x′y′.

As schematically illustrated inFIG.8A(relative to the embodiment ofFIG.7A) and inFIG.8B(relative to the embodiment ofFIG.7B), according to a particular aspect of this disclosure, assembling of the microelectromechanical mirror device3provides that the first tiltable mirror structure3a(i.e., the aforementioned tiltable mirror structure10) is accommodated at least in part in the recess66of the tiltable mirror structure50(which provides, as previously indicated, the second tiltable mirror structure3b), so as to optimize the total volume occupation and also obtain a very close arrangement between the same first and second tiltable mirror structures3a,3b.

In particular, the second tiltable mirror structure3bis arranged with the respective horizontal plane x′y′ at a certain angle (lower than 90°) with respect to the horizontal plane xy of the first tiltable mirror structure3a.

In this manner it is possible to obtain a very compact final assembly and also to reduce the size of the first and the second tiltable mirror structures3a,3bwith the same optical requirements (for example in terms of an opening angle) for the same first and second tiltable mirror structures3a,3b.

From the manufacturing point of view, manufacturing of the die51of the tiltable mirror structure50, given the patterning of the corresponding outer side surface51′, entails defining, on the wafers of semiconductor material, scribing lines that are not parallel. To this end, stealth dicing technique may be used and/or “dummy” structures (non-functional) may be provided between the dies51before dicing.

By way of example,FIG.9shows a portion of a wafer70of semiconductor material, in particular silicon, wherein a plurality of dies51have been provided, each integrating a corresponding tiltable mirror structure50(according to the embodiment previously discussed with reference toFIG.7A). In addition to the scribing lines, indicated by LT, being non-parallel, dummy dies are highlighted, indicated by72, interposed between the dies51, in this case along the second horizontal axis y′.

In order to optimize the manufacturing costs, alternative arrangements of the dies51in the wafer70may be provided, which do not require the presence of the aforementioned dummy dies72.

By way of example,FIG.10shows a respective wafer70of semiconductor material, in particular silicon, wherein a plurality of dies51have been provided, each integrating a corresponding tiltable mirror structure50(this time according to the embodiment previously discussed with reference toFIG.7B). In addition to the scribing lines, indicated by LT, non-parallel, in this case the absence of the aforementioned dummy dies72is highlighted, given the contiguous and adjacent arrangement of dies51.

As schematically illustrated inFIG.11, the microelectromechanical mirror device3may be advantageously used in an optoelectronic device, such as a picoprojector,80, for example to be functionally coupled to a portable electronic apparatus81(such as a smartphone or augmented or virtual reality glasses or headset).

In detail, the optoelectronic device80comprises a light source82, for example of a laser type, for generating a light beam83; the microelectromechanical mirror device3, acting as a mirror module with biaxial projection and for receiving the light beam83and directing it towards a screen or display surface85(external and placed at a distance from the same optoelectronic device80); a first driving circuit86, for providing suitable control signals to the light source82, for generation of the light beam83, as a function of an image to be projected; a second driving circuit88, for providing suitable control signals to the actuation structure of the microelectronic mirror device3; and an interface89, for receiving, from a control unit90, in this case being external, for example included in the portable electronic apparatus81, first control signals to control the first driving circuit86, and second control signals to control the second driving circuit88.

The control unit90also receives, through the interface89, a feedback signal, provided by the microelectromechanical mirror device3, for a feedback control of the driving of the same tiltable structure2.

The advantages are clear from the preceding description.

In any case, it is again highlighted that the described asymmetrical embodiment of the tiltable mirror structure10(3a) allows obtaining a reduced area occupation of the same tiltable mirror structure10, in particular a reduction of the aforementioned first extension dimension d1on the first side of the second axis B.

This embodiment therefore allows avoiding clipping of the light projection, due to the close arrangement between the tiltable mirror structures3a,3bof a microelectromechanical mirror device3with biaxial projection, as described with reference to the prior art.

Furthermore, the shaping of the die51of the tiltable mirror structure50(3b) and the resulting assembly with optimized volume occupation of the resulting microelectromechanical mirror device3(owing to the close arrangement of the tiltable mirror structures3a,3b) is also advantageous.

In general, this disclosure allows exploiting the advantages of the piezoelectric actuation (i.e., the use of reduced biasing voltages with a reduced energy consumption to obtain high displacements) and of the mirror actuation piezoresistive sensing, while having improved mechanical and electrical performance with respect to known solutions.

Finally, it is clear that modifications and variations may be made to what has been described and illustrated without thereby departing from the scope of the present invention, as defined in the attached claims.

For example, it is highlighted that the described asymmetrical embodiment of the tiltable mirror structure10may also find advantageous application for different configurations of the same tiltable mirror structure10, for example in case of structures having quasi-static movement, therefore intended to provide a slow scan, for example a sawtooth scan, onto a corresponding projection screen. This may, for example, be advantageous, in case the required opening angle is not high and reducing the overall space occupation of the microelectromechanical mirror device is important.