Patent Description:
In a known manner, 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 laser) in desired manners. Thanks to their small size, these devices allow stringent requirements as to space occupation, in terms of area and thickness, to be complied 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.

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 relative main extension plane, to direct an impinging light beam in a desired manner.

The rotation of the mirror device is controlled through an 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 high power consumption; it has therefore been proposed to control the movement of the tiltable mirror structure with piezoelectric mode.

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, it is possible to exploit the inverse piezoelectric effect to form piezoresistive (PZR) sensor elements for sensing the driving condition of the mirror (in terms of the stress applied or the displacement or position assumed) and providing a feedback signal to allow a feedback control of the driving operation.

However, these piezoresistive sensor elements require a dedicated calibration, in order to know precisely the corresponding detection sensitivity, and therefore the sensitivity of the mirror, and thus be able to have accurate information on the displacement of the same mirror to achieve the desired control.

The present Applicant has realized that known solutions for the calibration of the aforementioned piezoresistive sensor elements have some limitations.

Known solutions provide for a complex calibration setup comprising, for example: a driving unit to control the operation of the mirror; a laser source to direct a light beam onto the mirror; a projection screen whereon the mirror projects a light pattern due to the reflection of the laser beam and the driving movement of the same mirror; a camera to detect the angle of aperture, or opening angle (that is, the extent of the rotation out of the horizontal plane) of the mirror from the analysis of the projected light pattern; and an oscilloscope (or similar measuring instrument) for acquiring the detection signals provided by the piezoresistive sensor elements.

A processing unit is then configured to jointly process the information obtained from the analysis of the images acquired by the camera and the information obtained from the signals sensed by the piezoresistive elements, in order to obtain information for the calibration of the same piezoresistive elements (in particular to determine their detection sensitivity).

The calibration operations are therefore particularly onerous, in terms of time and economic resources required and are difficult to apply to large-scale manufacturing processes.

<CIT>, <CIT> and <CIT> disclose known solutions for microelectromechanical mirror devices provided with piezoresistive sensors.

The aim of the present solution is therefore to provide a microelectromechanical mirror device which allows the calibration problems previously highlighted to be overcome, in particular being provided with self-calibration properties.

According to the present solution, a microelectromechanical mirror device is provided, as defined in the attached claims.

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

<FIG> schematically illustrates a microelectromechanical mirror device, based on MEMS technology, generally denoted with <NUM>; this device generally has the structure disclosed in <CIT> in the name of the same Applicant.

The microelectromechanical device <NUM> is formed in a die <NUM>' of semiconductor material, in particular silicon, and is provided with a tiltable structure <NUM>, having a main extension in a horizontal plane xy and arranged to rotate around a rotation axis, parallel to a first horizontal axis x of the aforementioned horizontal plane xy.

The rotation axis represents a first median axis of symmetry X for the microelectromechanical device <NUM>; a second median axis of symmetry Y for the same microelectromechanical device <NUM> 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.

The tiltable structure <NUM> is suspended above a cavity <NUM>, formed in the die <NUM>' and defines a support structure, which carries a reflecting region <NUM>' (for example of aluminum, or gold, depending on whether the projection is in the visible or in the infrared) at the top, to define a mirror structure.

The tiltable structure <NUM> is elastically coupled to a fixed structure <NUM>, defined in the same die <NUM>'. In particular, the fixed structure <NUM> forms, in the horizontal plane xy, a frame <NUM>', which delimits and surrounds the aforementioned cavity <NUM>, and also has a first and a second supporting (or anchoring) element 5a, 5b, extending longitudinally along the first median axis of symmetry X inside the cavity <NUM> from the same frame <NUM>', on opposite sides of the tiltable structure <NUM> (along the first horizontal axis x).

The tiltable structure <NUM> is supported by the first and the second supporting elements 5a, 5b, to which it is elastically coupled through a first and, respectively, a second elastic suspension element 6a, 6b, having high stiffness with respect to movements out of the horizontal plane xy (along an orthogonal axis z, transverse to this horizontal plane xy) and yielding with respect to torsion around the first horizontal axis x. The first and the second elastic suspension elements 6a, 6b extend overall along the first median axis of symmetry X, between the first, respectively, the second supporting elements 5a, 5b and a facing side of the tiltable structure <NUM>, being coupled at a central portion thereof. In the illustrated embodiment, the first and the second elastic suspension elements 6a, 6b are of a linear type.

The first and the second elastic suspension elements 6a, 6b couple the tiltable structure <NUM> to the fixed structure <NUM>, allowing it to rotate around the first rotation axis and providing high stiffness with respect to movements out of the plane, thus ensuring a high ratio between the frequencies of spurious movements out of the horizontal plane xy and the rotation frequency around the first rotation axis.

The microelectromechanical device <NUM> further comprises an actuation structure <NUM>, coupled to the tiltable structure <NUM> and configured to cause the rotation thereof around the first rotation axis; the actuation structure <NUM> being interposed between the tiltable structure <NUM> and the fixed structure <NUM> and also contributing to supporting the tiltable structure <NUM> above the cavity <NUM>.

This actuation structure <NUM> comprises a first pair of driving arms formed by a first and a second driving arm 12a, 12b, arranged on opposite sides of, and symmetrically with respect to, the first median axis of symmetry X and the first supporting element 5a, and having a longitudinal extension parallel to the first horizontal axis x and to the aforementioned first supporting element 5a.

In the embodiment illustrated in <FIG>, the driving arms 12a, 12b have a generically trapezoidal (or "fin-like") shape, with major side directed parallel to the second horizontal axis y and integrally coupled to the frame <NUM>' of the fixed structure <NUM>; and minor side directed parallel to the same second horizontal axis y and elastically coupled to the tiltable structure <NUM>. Each driving arm 12a, 12b therefore has a respective first end integrally coupled to the frame <NUM>' of the fixed structure <NUM> and a respective second end elastically coupled to the tiltable structure <NUM>, through a first, respectively a second, elastic decoupling element 14a, 14b.

Each driving arm 12a, 12b is suspended above the cavity <NUM> and carries, at a top surface thereof (opposite to the same cavity <NUM>) a respective piezoelectric structure <NUM> (in particular including Lead Zirconate Titanate - PZT), having for example substantially the same extension in the horizontal plane xy with respect to the driving arm 12a, 12b.

This piezoelectric structure <NUM> (in a manner not illustrated in detail) is formed by superimposing: a bottom electrode region, of a suitable conductive material, arranged above the relative driving arm 12a, 12b; a piezoelectric material region (for example made by a thin PZT film) arranged on the aforementioned bottom electrode region; and a top electrode region arranged on the piezoelectric material region.

The aforementioned first and second elastic decoupling elements 14a, 14b have high stiffness with respect to movements out of the horizontal plane xy (along the orthogonal axis z) and are yielding to torsion (around a rotation axis parallel to the first horizontal axis x). The first and the second elastic decoupling elements 14a, 14b extend parallel to the first horizontal axis x, between the first, respectively, the second driving arm 12a, 12b and a same facing side of the tiltable structure <NUM>.

The first and the second elastic decoupling elements 14a, 14b are coupled to the tiltable structure <NUM> at a respective coupling point Pa, Pb, which is located in proximity to the first median axis of symmetry X, at a short distance from the same first median axis of symmetry X. For example, this distance may be comprised between <NUM> and <NUM> in a typical embodiment and may also be generally comprised between <NUM>/<NUM> and <NUM>/<NUM> of a main dimension (in the example along the second median axis of symmetry Y) of the tiltable structure <NUM>.

In any case, the distance between the respective coupling point Pa, Pb and the first median axis of symmetry X is preferably smaller, in particular much smaller, than the distance between the same coupling point Pa, Pb and end or edge portions (considered along the second median axis of symmetry Y) of the tiltable structure <NUM>. In fact, the closer these coupling points Pa, Pb are to the first rotation axis, the greater the ratio between the vertical displacement of the end of the tiltable structure <NUM> and the vertical displacement of the driving arms 12a, 12b, due to the piezoelectric effect.

In the embodiment illustrated in <FIG>, the first and the second elastic decoupling elements 14a, 14b are of a folded type, being formed by a plurality of arms having a longitudinal extension parallel to the first horizontal axis x, connected two by two through connecting elements having extension parallel to the second horizontal axis y (in a different embodiment, the elastic decoupling elements 14a, 14b may alternatively be of a linear type).

The aforementioned actuation structure <NUM> further comprises a second pair of driving arms formed by a third and a fourth driving arm 12c, 12d, arranged on opposite side of the first median axis of symmetry X and, this time, of the second supporting element 5b and having a longitudinal extension parallel to the first horizontal axis x and to the aforementioned second supporting element 5b (the second pair of driving arms 12c, 12d is therefore arranged symmetrically to the first pair of driving arms 12a, 12b with respect to the second median axis of symmetry Y).

Similarly to what has been discussed for the first pair of driving arms 12a, 12b, each driving arm 12c, 12d of the second pair carries, at a top surface thereof, a respective piezoelectric structure <NUM> (in particular including PZT - Lead Zirconate Titanate) and has a respective first end integrally coupled to the frame <NUM>' of the fixed structure <NUM> and a respective second end elastically coupled to the tiltable structure <NUM>, through a respective third and fourth elastic decoupling element 14c, 14d (arranged on opposite side of the first and second elastic decoupling elements 12a, 12b with respect to the second median axis of symmetry Y).

The aforementioned third and fourth elastic decoupling elements 14c, 14d also have high stiffness with respect to movements out of the horizontal plane xy (along the orthogonal axis z) and are yielding to torsion (around a rotation axis parallel to the first horizontal axis x).

As illustrated in the aforementioned <FIG>, also the third and the fourth elastic decoupling elements 14c, 14d are coupled to the tiltable structure <NUM> at a respective coupling point Pc, Pd, which is located in proximity to the first rotation axis, at the short distance d from the same first rotation axis. Furthermore, the third and the fourth elastic decoupling elements 14c, 14d are also of a folded type.

The microelectromechanical device <NUM> further comprises a plurality of electrical contact pads <NUM>, carried by the fixed structure <NUM> at the frame <NUM>', electrically connected (in a manner not illustrated in detail in the same <FIG>) to the piezoelectric structures <NUM> of the driving arms 12a-12d through electrical connection tracks, to allow electrical biasing thereof through electrical signals coming from the outside of the same electromechanical device <NUM> (for example provided by a biasing device of an electronic apparatus having the electromechanical device <NUM> integrated therein).

During operation of the microelectromechanical device <NUM>, the application of a biasing voltage to the piezoelectric structure <NUM> of the first driving arm 12a (having a positive value with respect to the biasing of the piezoelectric structure <NUM> of the second driving arm 12b, which may for example be connected to a ground reference potential), causes a rotation of a positive angle around the first rotation axis (parallel to the first horizontal axis x).

In a corresponding manner, the application of a biasing voltage to the piezoelectric structure <NUM> of the second driving arm 12b (having a positive value with respect to the biasing of the piezoelectric structure <NUM> of the first driving arm 12a, which may for example in this case be connected to a ground reference potential), causes a corresponding rotation of a negative angle around the same first rotation axis.

It should be noted that the same biasing voltage may be applied to the piezoelectric structure <NUM> of both the first driving arm 12a and the third driving arm 12c, and, likewise, in order to cause rotation in the opposite direction, to the piezoelectric structure <NUM> of both the second driving arm 12b and the fourth driving arm 12d, to contribute in a corresponding manner to the rotation of the tiltable structure <NUM> around the first rotation axis (as on the other hand will be apparent from the foregoing description).

The elastic decoupling elements 14a-14d elastically decouple the displacement of the driving arms 12a-12d along the orthogonal axis z due to the piezoelectric effect, from the consequent rotation of the tiltable structure <NUM> along the first rotation axis.

In particular, by virtue of the proximity to the rotation axis of the coupling points Pa-Pd between the same elastic decoupling elements 14a-14d and the tiltable structure <NUM>, a wide angle of aperture (i.e. angle of rotation of the tiltable structure <NUM> around the first rotation axis), or, likewise, a large displacement out of the horizontal plane xy of the end portions (considered along the second horizontal axis y) of the same tiltable structure <NUM> corresponds to a small displacement out of the horizontal plane xy of the aforementioned driving arms 12a -12d; for example, the ratio between the extent of these displacements may be equal to five in a possible embodiment.

The tiltable structure <NUM> may thereby reach wide angles of aperture (for example > <NUM>°) with a low value of the biasing voltage (for example < <NUM> V).

The maximum amount of stress occurs in the elastic suspension elements 6a, 6b that couple the tiltable structure <NUM> to the fixed structure <NUM>.

<FIG> shows a schematic cross-section of the microelectromechanical device <NUM>. In particular, this section (parallel to the first horizontal axis x) shows that the thickness (along the orthogonal axis z) of the elastic decoupling elements 14a-14d (and, in a manner not illustrated, also of the elastic suspension elements 6a, 6b) is equal to the thickness of the driving arms 12a-12d and also corresponds to the thickness of the tiltable structure <NUM>, for example being equal to <NUM>, this thickness being hereinafter referred to as the first thickness (the aforementioned elements are essentially formed at the front of the die <NUM>').

A reinforcement structure <NUM> is coupled underneath the same tiltable structure <NUM>, having the function of mechanical reinforcement for the same tiltable structure <NUM> (and also for ensuring a flatness thereof, in the horizontal plane xy, in a rest condition). This reinforcement structure <NUM> has a second thickness along the orthogonal axis z, which is greater than the first thickness, for example being equal to <NUM>, and may have a ring shape and be arranged at the periphery of the tiltable structure <NUM> (the same reinforcement structure <NUM> is substantially formed on the back of the die <NUM>').

The fixed structure <NUM> of the microelectromechanical device <NUM> (in particular, the corresponding frame <NUM>') has a thickness, along the orthogonal axis z, substantially equal to the sum of the aforementioned first and second thicknesses.

As shown in the same <FIG>, a base body <NUM> is also coupled underneath the fixed structure <NUM> and has, underneath the cavity <NUM> and corresponding to the movable structure <NUM>, a recess <NUM>', to allow the rotation of the same movable structure <NUM>. In particular, the frame <NUM>' is coupled to this supporting body <NUM> through suitable bonding material regions.

As illustrated again in <FIG>, the microelectromechanical device <NUM> further comprises a piezoresistive (PZR) sensor <NUM>, suitably arranged to provide a detection signal associated with the rotation of the tiltable structure <NUM> around the first rotation axis; this detection signal may be provided as a feedback to the outside of the microelectromechanical device <NUM>, through at least one of the electrical contact pads <NUM>.

In the embodiment illustrated in <FIG>, this piezoresistive sensor <NUM> is provided (for example by surface diffusion of dopant atoms) at the second supporting element 5b (different arrangements may, however, be provided for the same piezoresistive sensor <NUM>, which may for example be similarly provided at the first supporting element 5a).

In general, the piezoresistive sensor <NUM> may be arranged in proximity to the elastic suspension elements 6a, 6b, to detect the stress associated with their torsion and therefore provide an indication relating to the displacement of the tiltable structure <NUM>.

Advantageously, the elastic suspension elements 6a, 6b are capable of transmitting the stress to the supporting elements 5a, 5b and hence towards the piezoresistive sensor <NUM>, enabling arrangement of the latter at the same supporting elements 5a, 5b and a consequent simplification of routing of the electrical connections to the electrical contact pads <NUM>.

As will also be discussed below, the aforementioned piezoresistive sensor <NUM> may for example be made by four piezoresistor elements which are arranged and connected in a Wheatstone bridge configuration, at the end of the corresponding supporting element (in the example of the second supporting element 5b) that is coupled to the corresponding elastic suspension element (in the example to the second elastic suspension element 6b). Electrical connection tracks (not illustrated in detail) extend from the aforementioned piezoresistor elements along the corresponding supporting element, to reach (in a manner not illustrated here) the electrical contact pads <NUM>.

The present Applicant has realized that the sensitivity of the piezoresistive sensor <NUM> has a variability factor dependent on the stress felt, which is a function of possible geometry variations in the manufacturing of the structure, in particular in the manufacturing of the elastic elements for transmitting the stress to the same piezoresistive sensor <NUM> (i.e., in the illustrated embodiment, the elastic suspension elements 6a, 6b).

Variations in the dimension, in particular in the transverse width, of the elastic suspension elements 6a, 6b may occur, due to the so-called "Critical Dimension - CD - loss" error; as a result of this error, for example, the dimensions of the elements formed by etching do not correspond to the dimensions of the photolithographic etching masks used for their manufacturing.

The present Applicant has realized that such geometric variations entail a variation of the operating (torsional) frequency of the tiltable structure <NUM> of the microelectromechanical device <NUM>; however, this variation is due to the geometric variations not only of the elastic suspension elements 6a, 6b, but also, for example, of the reflecting region <NUM>' carried by the tiltable structure <NUM> and of the reinforcement structure <NUM> coupled underneath the tiltable structure <NUM>.

As a result, it is not possible to obtain information about the sensitivity variation of the piezoresistive sensor <NUM> from the variation of the torsional frequency of the tiltable structure <NUM>.

One aspect of the present solution provides for integrating, in the die <NUM>' of the microelectromechanical device <NUM>, at least one test structure <NUM> (schematically indicated in <FIG>) configured to provide information about the sensitivity variation of the piezoresistive sensor <NUM>, in particular comprising (as will be discussed in detail below) at least one movable or tiltable mass, operable at a resonance frequency. In particular, this test structure <NUM> is configured so that a variation of this resonance frequency is associated with the geometric variations in a substantially exclusive manner, due to the "CD loss" error, so as to be correlated to the sensitivity variation of the piezoresistive sensor <NUM>. In other words, from the variation of resonance frequency of the test structure <NUM>, the sensitivity variation of the piezoresistive sensor <NUM> may be determined.

The information obtained from the test structure <NUM> may therefore be used in a suitable manner for the calibration of the sensitivity of the piezoresistive sensor <NUM>.

In a possible embodiment, the test structure <NUM> comprises a torsional elastic element which elastically supports the movable mass; the characteristics of the aforementioned torsional elastic element are such that they substantially match the characteristics of the elastic suspension elements 6a, 6b of the tiltable structure <NUM> (in terms of dimensions, i.e. length and width, of the configuration and arrangement), such that the variation of resonance frequency of this torsional elastic element provides a reliable indication on the dimensional variations of the same elastic suspension elements 6a, 6b (due to CD loss) and, consequently, an indication of the correlated sensitivity variation of the piezoresistive sensor <NUM>.

In detail, in a possible embodiment, illustrated in <FIG>, the test structure <NUM> comprises:.

This torsional elastic element <NUM>, as previously indicated, has characteristics matching those of the first and second elastic suspension elements 6a, 6b, having a high stiffness with respect to movements out of the horizontal plane xy and being yielding with respect to torsion around its own longitudinal axis (in the example along the second horizontal axis y), thus allowing the rotation of the movable mass <NUM>.

The torsional elastic element <NUM> thus extends, in the example with linear extension, along the second horizontal axis y, between a central portion of the supporting beam <NUM> and a facing central portion of the movable mass <NUM>.

The test structure <NUM> also comprises a first and a second piezoelectric actuator <NUM>, <NUM> (in particular including PZT - Lead Zirconate Titanate), coupled on top of the supporting beam <NUM>, on opposite sides with respect to the central part coupled to the torsional elastic element <NUM>.

In a manner not illustrated in detail, these piezoelectric actuators <NUM>, <NUM> are formed by superposing a bottom electrode region, of a suitable conductive material, arranged above the corresponding portion of the supporting beam <NUM>; a piezoelectric material region (for example made by a thin PZT film) arranged on the aforementioned bottom electrode region; and a top electrode region arranged on the piezoelectric material region.

As schematically indicated in <FIG> by the dashed boxes, an empty area <NUM>', comprised in the horizontal plane xy between the supporting beam <NUM> and the movable mass <NUM>, at the sides of the torsional elastic element <NUM>, may have a dimension substantially equivalent to the corresponding empty area <NUM>" (see <FIG> and the corresponding dashed boxes) interposed between each elastic suspension element 6a, 6b and the associated elastic decoupling elements 14a, 14b (respectively 14c, 14d), laterally to the same elastic suspension elements 6a, 6b.

This matching of empty areas <NUM>', <NUM>" contributes to increasing the similarity of behavior between the test structure <NUM> and the mirror structure, thereby further improving the resulting accuracy of the calibration of sensitivity of the piezoresistive sensor <NUM>.

As shown in <FIG>, in a possible embodiment, the microelectromechanical device <NUM> may comprise a plurality of test structures <NUM> (each made as previously discussed in detail), in the example four in number, arranged in respective cavities <NUM> formed in the frame <NUM>', at the side, at the bottom and at the top, with respect to the tiltable structure <NUM>.

In general, the presence of a plurality of test structures <NUM> allows to increase the reliability of the calibration of sensitivity of the piezoresistive sensor <NUM>, for example implementing an averaging of the calibration indications provided by each of the same test structures <NUM>.

In a possible embodiment, the resonance frequency associated with the test structure <NUM> (from which the information for sensitivity calibration of the piezoresistive sensor <NUM> are obtained) may be determined by an impedance spectroscopy technique.

In particular, and with reference to <FIG>, a suitable actuation voltage (for example with a value of <NUM> V) is applied to the first and/or the second piezoelectric actuators <NUM>, <NUM> at a variable frequency in a certain range (around a design value, for example equal to <NUM>) and the impedance associated with the same piezoelectric actuators <NUM>, <NUM> (for example, the capacitance between the corresponding top and bottom electrode regions) is measured.

As shown in the aforementioned <FIG>, a characteristic pattern (with two consecutive peaks, negative and positive) occurs in the trend of the capacitance value, at the resonance frequency of the test structure <NUM>, indicated with fr; this pattern may be easily detected to obtain the value of the same resonance frequency fr.

The variation between this resonance frequency fr and the design frequency is, as previously discussed, attributable only to the geometric variations that occur in the formation of the test structure <NUM>, and therefore provides the desired indication relating to the CD loss and, consequently, to the sensitivity variation of the piezoresistive sensor <NUM> (the same piezoresistive sensor <NUM> being substantially affected by the same geometric variations).

It is highlighted that this measurement technique is simple and inexpensive to implement, providing for a reduced number of further electrical contact pads <NUM>, carried by the fixed structure <NUM> at the frame <NUM>', electrically connected (in a manner not illustrated in detail) to the piezoelectric actuators <NUM>, <NUM> and in particular to the corresponding top and/or bottom electrode regions.

As previously indicated, the piezoresistive sensor <NUM> may be made of four piezoresistor elements arranged and connected in a Wheatstone bridge configuration.

A further aspect of the present solution provides for a suitable arrangement of the aforementioned piezoresistor elements, aimed at minimizing the effects that possible misalignments of the same piezoresistor elements with respect to the longitudinal axis of the elastic suspension elements 6a, 6b may have on the detection sensitivity of the same piezoresistive sensor <NUM>. In this regard, the present Applicant has found, in known solutions, a possible sensitivity variation > <NUM>%/µm, with misalignment values that may even reach <NUM> or <NUM>.

As illustrated in <FIG>, one aspect of the present solution therefore provides for "splitting" the Wheatstone bridge, with a first half of the piezoresistor elements (a first and a second piezoresistor element 20a, 20b) arranged at the end of the first supporting element 5a coupled to the first elastic suspension element 6a; and a second half of the piezoresistor elements (a third and a fourth piezoresistor element 20c, 20d) arranged at the end of the second supporting element 5b coupled to the second elastic suspension element 6b.

This arrangement of the piezoresistor elements 20a-20d substantially eliminates (or in any case substantially reduces) the sensitivity variation due to possible misalignments along the first horizontal axis x, given that a moving away/approaching of the piezoresistor elements 20c, 20d of the second pair from/to the corresponding second elastic suspension element 6b corresponds to an identical approaching/moving away of the piezoresistor elements 20a, 20b of the first pair to/from the corresponding first elastic suspension element 6a.

A further aspect of the present solution provides for separating the piezoresistor elements (20a-20b and 20c-20d) of each pair along the direction of the second horizontal axis y, such that the same piezoresistor elements are arranged at zones with reduced variation of stress distribution.

As shown schematically in the same <FIG>, the stress distribution lines have, at the supporting elements 5a, 5b, due to the torsion of the elastic suspension elements 6a, 6b, a semicircle configuration, with an increasing radius as the distance from the same elastic suspension elements 6a, 6b increases. In particular, it is apparent that a slope of the stress distribution lines (with respect to the first horizontal axis x) is maximum at the first horizontal axis x and decreases as it moves away from the same first horizontal axis x along the second horizontal axis y.

The piezoresistor elements of each pair (20a-20b and 20c-20d) are thus placed at a suitable separation distance along the second horizontal axis y, each being arranged at an edge portion of the corresponding supporting element 5a, 5b.

In the illustrated example, the piezoresistor elements 20a-20d are also placed so that they have a substantially orthogonal arrangement with respect to the aforementioned stress distribution lines.

The present Applicant has demonstrated the possibility of obtaining a value of sensitivity variation of the piezoresistive sensor <NUM> lower than <NUM> %/µm with respect to misalignments along the second horizontal axis y, for example due to misalignments of the photolithographic etching masks.

With all of the aforementioned solutions and the suitable self-calibration of the sensitivity of the piezoresistive sensor <NUM>, the present Applicant has found that errors in detecting the position of the tiltable structure <NUM> due to the sensitivity variation of the same piezoresistive sensor <NUM> may be less than <NUM> mdeg with respect to angles of aperture of the mirror which are greater than <NUM> deg.

A further aspect of the present solution provides that the test structure provides, in alternative or in addition to the indication relating to the geometry variations on the front of the die <NUM>' (as previously discussed), an indication relating to the geometry variations that may occur on the back of the same die <NUM>'.

In particular, for example again due to the CD loss error, dimensional variations of the structures formed on the back of the die <NUM>' may occur, which may cause, in certain embodiments, a variation in detection sensitivity of the piezoresistive sensor <NUM>.

In this regard, <FIG> and <FIG> show a possible further embodiment of the microelectromechanical mirror device, here indicated with <NUM>, in this case of a resonant type for high frequency applications.

This microelectromechanical device is described in detail for example in <CIT> in the name of the same Applicant.

In general, the configuration of the microelectromechanical device <NUM> is similar to what has been previously discussed for the microelectromechanical device <NUM>, the device indeed comprising:.

The elastic suspension elements 6a, 6b are in this case interposed between the frame <NUM>' of the fixed structure <NUM> and the supporting elements 5a, 5b, the latter being coupled to the tiltable structure <NUM> and having themselves torsional elastic characteristics.

In this case, the piezoresistive sensor <NUM> is placed at the portion of the frame <NUM>' coupled to the second elastic suspension element 6b, so as to provide, again, a detection signal associated with the rotation of the tiltable structure <NUM> around the rotation axis (usable for feedback control of the mirror).

As also highlighted by <FIG>, in this case the sensitivity of the piezoresistive sensor <NUM>, due to its placing in proximity to the frame <NUM>', is also affected by the effects of CD loss that may occur on the back of the die <NUM>' (the frame <NUM>' is in fact also defined on the back of the same die <NUM>').

A further aspect of the present solution therefore provides for introducing, advantageously in addition to the test structure(s) <NUM>, at least one further test structure, indicated schematically with <NUM>', suitably configured to provide, again on the basis of the variation of a relative resonance frequency, indications about the geometric variations on the back of the die <NUM>' due to the CD loss (and therefore to the associated sensitivity variations of the piezoresistive sensor <NUM>).

In a possible embodiment, shown in <FIG>, this further test structure <NUM>' comprises a beam element <NUM>, suspended above a respective cavity <NUM> formed through the frame <NUM>', in a suitable position, for example at one of the corners of the same frame <NUM>' (see also <FIG>).

The beam element <NUM> is anchored to the fixed structure <NUM> of the die <NUM>' of the microelectromechanical device <NUM>, in particular to the corresponding frame <NUM>', through an anchoring portion 40a and carries at the top, above a cantilever portion 40b, a respective piezoelectric actuator <NUM>, for example formed by superimposing a bottom electrode region, of a suitable conductive material; a piezoelectric material region (for example made by a thin PZT film) arranged on the aforementioned bottom electrode region; and a top electrode region arranged on the piezoelectric material region.

The beam element <NUM> also has at the bottom, formed on the back of the die <NUM>', a reinforcement portion <NUM>, which is coupled, at a first end thereof, underneath the anchoring portion 40a, and is arranged, at a second end thereof longitudinally opposite to the first end, in proximity to the cantilever portion 40b of the same beam element <NUM>.

In particular, the longitudinal extension of the aforementioned reinforcement portion <NUM> determines the corresponding extension of the cantilever portion 40b of the beam element <NUM>.

During operation, biasing of the piezoelectric actuator <NUM> causes oscillation of the cantilever portion 40b of the beam element <NUM>, in the vertical direction, at a given resonance frequency (the cantilever portion 40b being in this case the movable mass of the test structure <NUM>').

In particular, the CD loss on the back of the die <NUM>' may cause a variation of the longitudinal dimension of the reinforcement portion <NUM> and, consequently, of the cantilever portion 40b of the beam element <NUM>, causing a variation of the resonance frequency.

Since there are no other variation factors that determine the resonance frequency in the test structure <NUM>', the value of the same resonance frequency may be directly correlated to the dimensional variations on the back (due to the CD loss), thus providing an indication usable for the calibration of the sensitivity variations of the piezoresistive sensor <NUM> due to the same dimensional variations on the back.

Also in this case, the microelectromechanical device <NUM> may advantageously comprise a plurality of further test structures <NUM>', to increase the calibration precision, for example by implementing suitable averaging operations based on the indications provided by the same test structures <NUM>'. For example, in a manner not illustrated, four test structures <NUM>' might be provided, arranged at the corners of the frame <NUM>' of the microelectromechanical device <NUM>.

As illustrated schematically in <FIG>, the microelectromechanical device <NUM>, <NUM> may be advantageously used in an optoelectronic device, such as a picoprojector, <NUM>, for example to be functionally coupled to a portable electronic apparatus <NUM> (such as a smartphone or augmented reality glasses).

In detail, the optoelectronic device <NUM> comprises a light source <NUM>, for example of a laser type, for generating a light beam <NUM>; the microelectronic device <NUM>, <NUM>, acting as a mirror, for receiving the light beam <NUM> and for directing it towards a screen or display surface <NUM> (external and placed at a distance from the same pico-projector <NUM>); a first driving circuit <NUM>, for providing suitable command signals to the light source <NUM>, for the generation of the light beam <NUM>, according to an image to be projected; a second driving circuit <NUM>, for providing suitable command signals to the actuation structure of the microelectronic device <NUM>, <NUM> (and also, according to an aspect of the present solution, to the test structure(s) <NUM>, <NUM>' which is/are integrated in the same microelectronic device <NUM>,<NUM>); and an interface <NUM>, for receiving, from a control unit <NUM>, in this case external, for example included in the portable apparatus <NUM>, first control signals Sd1, for controlling the first driving circuit <NUM>, and second control signals Sd2, for controlling the second driving circuit <NUM>.

The control unit <NUM> also receives, through the interface <NUM>, a feedback signal Sr, provided by the piezoresistive sensor <NUM> of the microelectromechanical device <NUM>, <NUM>, indicative of the position of the tiltable structure <NUM>, so as to implement a feedback control of the operation of the same tiltable structure <NUM>.

In particular, according to a possible implementation, the control unit <NUM> may also receive a calibration signal Sc, provided by the test structure <NUM>, <NUM>' integrated in the microelectromechanical device <NUM>, <NUM>, indicative of the sensitivity variation of the piezoresistive sensor <NUM>, so as to actuate, based on this calibration signal Sc, a suitable self-calibration operation, for the correction of the position of the tiltable structure <NUM> determined according to the feedback signal Sr.

Alternatively, the calibration operation may be performed by electronics external to the portable apparatus <NUM> (again using the calibration signal Sc provided by the test structure <NUM>, <NUM>' for the correction of the feedback signal Sr), in a calibration procedure which, advantageously, does not require in any case the complex optical measurements required by known solutions.

The advantages of the present solution are clear from the foregoing description.

In any case, it is again underlined that the solution described allows to avoid the use of complex and expensive (in terms of money and time) calibration set-ups that require feedbacks of an optical nature, via cameras or the like.

In fact, the present solution provides a test structure integrated in the same microelectromechanical mirror device, which provides an indication about the possible sensitivity variation of the piezoresistive sensor, for the self-calibration of the detection signals provided as a feedback for controlling the mirror actuation.

It is highlighted that, thanks to the present solution, this self-calibration may also be performed in real time, that is during operation of the microelectromechanical mirror device, or in a dedicated calibration step, in any case requiring simple and effective calibration operations.

Furthermore, as discussed above, the particular arrangement of the piezoresistor elements of the piezoresistive sensor <NUM> is advantageous, allowing to reduce the variations of detection sensitivity (and therefore facilitating the aforementioned self-calibration operations).

In general, the present solution allows to exploit the advantages of the piezoelectric actuation (i.e., the use of reduced biasing voltages with a reduced energy consumption to obtain large displacements) and of the piezoresistive sensing of the mirror actuation, 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, the solution described may also be applied in the case of a biaxial embodiment of the microelectromechanical mirror device (in a manner similar to what has been described in detail in the aforementioned <CIT>), that is in the case wherein the tiltable structure <NUM> is capable of performing rotation movements both around the first rotation axis (coinciding with the first median axis of symmetry X parallel to the first horizontal axis x), and around a second rotation axis (coinciding with the second median axis of symmetry Y parallel to the second horizontal axis y).

Furthermore, variations may generally be provided as regards the shape of the elements constituting the microelectromechanical mirror device <NUM>, <NUM> for example different shapes of the tiltable structure <NUM> (and of the corresponding reflecting surface <NUM>'), or different shapes and/or arrangements of the driving arms 12a-12d.

Claim 1:
A microelectromechanical mirror device (<NUM>; <NUM>), comprising a die (<NUM>') of semiconductor material having:
a fixed structure (<NUM>) defining a cavity (<NUM>);
a tiltable structure (<NUM>) carrying a reflecting region (<NUM>'), elastically suspended above the cavity (<NUM>) and having a main extension in a horizontal plane (xy);
at least one first pair of driving arms (12a, 12b), coupled to the tiltable structure (<NUM>) and carrying respective piezoelectric material regions (<NUM>), configured to be biased to cause a rotation of the tiltable structure (<NUM>) around a rotation axis (X) parallel to a first horizontal axis (x) of said horizontal plane;
elastic suspension elements (6a, 6b), configured to elastically couple said tiltable structure (<NUM>) to said fixed structure (<NUM>) at said rotation axis (X), being stiff with respect to movements out of the horizontal plane (xy) and yielding with respect to torsion around said rotation axis (X); and
a piezoresistive sensor (<NUM>), configured to provide a detection signal (Sr) indicative of the rotation of the tiltable structure (<NUM>) around the rotation axis (X),
characterized by comprising at least one test structure (<NUM>, <NUM>'), integrated in said die (<NUM>') and configured to provide a calibration signal (Sc) indicative of a sensitivity variation of the piezoresistive sensor (<NUM>), configured to be used for calibration of said detection signal (Sr),
wherein the sensitivity variation of the piezoresistive sensor (<NUM>) is due to geometry variations; and wherein said test structure (<NUM>, <NUM>') comprises piezoelectric actuators (<NUM>,<NUM>; <NUM>) and a movable mass (<NUM>; <NUM>) configured to be operated in resonance by actuation of the piezoelectric actuators (<NUM>,<NUM>; <NUM>), the resonance frequency associated with said test structure (<NUM>; <NUM>') having a variation which is a function of said geometry variations and correlated to a variation of said sensitivity of the piezoresistive sensor (<NUM>); wherein said calibration signal (Sc) is indicative of the variation of said resonance frequency.