Optical microelectromechanical devices and fabrication processes thereof

Abstract

The present disclosure relates to a process for fabricating an optical microelectromechanical device having a tiltable structure using an antireflective surface. To fabricate the optical microelectromechanical device, a first wafer of semiconductor material having a first surface and a second surface is processed to form a suspended mirror structure, a fixed structure surrounding the suspended mirror structure, an elastic support element extending between the fixed structure and the suspended mirror structure, and an actuation structure coupled to the suspended mirror structure. A second wafer is separately processed to form a chamber defined by a bottom wall having a through opening. The second wafer is bonded to the first surface of the first wafer in a manner such that the chamber overlies the actuation structure and the through opening is aligned with the suspended mirror structure. Further, a third wafer is bonded to the second surface of the first wafer to form a composite wafer device. The composite wafer device is then diced to form the optical microelectromechanical device.

Description

[0001] CLAIM OF PRIORITY

[0002] This application claims the priority benefit of Italian Patent Application No. 102019000025042, filed December 20, 2019, the contents of which are incorporated herein in their entirety to the maximum extent legally permissible. TECHNICAL FIELD

[0003] The present disclosure relates to a process for manufacturing optical microelectromechanical devices with tiltable structures, exploiting antireflection surfaces. In particular, hereinafter reference will be made to the manufacturing of micro-mirrors using MEMS (Microelectromechanical System) technology. BACKGROUND

[0004] As is known, micro-mirrors can be produced starting from a semiconductor material. In fact, the small size of MEMS semiconductor devices enables them to be integrated in portable devices for optical applications, such as augmented reality and virtual reality viewers, portable computers, laptops, notebooks, PDAs, tablets, mobile phones and smartphones.

[0005] Typically, such microelectromechanical devices are inserted in miniaturized projector modules, so-called micro projectors, which are able to project an image at a certain distance, or generate a desired light pattern, and have, for example, the structure shown in Figure 1

[0006] Figure 1 is a schematic view of a micro-projector 1 comprising a light source 2 (typically a laser source), a light beam 3, a micro-mirror 4 and a screen 5. Here, the micro-mirror 4 directs the light beam 3 coming from the light source 2 onto the screen 5.

[0007] In the example shown in Figure 1 , the micro-mirror 4 is a mirror of the biaxial type, i.e. able to rotate about two axes R’ and R” transversal to each other, for example perpendicular to each other, to direct the light beam 3 onto the surface of the screen 5. Alternatively, two uniaxial mirrors can be used, each of which is able to rotate about only one axis.

[0008] Micro-mirrors obtained with MEMS technology typically comprise a reflecting structure suspended above a cavity and elastically supported by arms that enable the reflecting structure to be tilted or rotated with respect to a resting plane. The movement of the micro-mirror is guaranteed by an actuation system, for example of the electrostatic, piezoelectric or electromagnetic type.

[0009] Electrostatic actuation systems typically require high operating voltages, while electromagnetic actuation systems typically involve high power consumption. For this reason, piezoelectric actuation systems are increasingly used. ​

[0010] As an example of a microelectromechanical mirror device with piezoelectric type actuation, reference can be made to US patent application publication no. 20200192199 (corresponding to Italian patent application no. 102018000011112 filed on 14 December 2018 and European patent application no. 19165958.0 filed on 28 March 2019), the entire content of which is incorporated by reference, which US patent describes a microelectromechanical mirror device comprising a micro-mirror, which can be of the biaxial type or of the uniaxial type. Figure 2 A top view is schematically represented of one of the possible embodiments of the above microelectromechanical device of the uniaxial type. In detail, the microelectromechanical device, indicated with 20, is formed in a die of a semiconductor material, in particular silicon, and is provided with a tiltable structure 22 having a main extension in a horizontal plane XY of a Cartesian reference system XYZ, the origin of which is at the center of the microelectromechanical device 20, and arranged to rotate about a rotation axis A, coinciding with a first horizontal axis X of the above horizontal plane XY (for example, said rotation axis A corresponds to the rotation axis R’ represented in the middle). Figure 1 The tiltable structure 22 is suspended above a cavity 23, obtained in the die, and in the embodiment shown has a substantially elliptical shape in the horizontal plane XY. The tiltable structure 22 carries, at the top, a reflective surface 22’ to define a mirror structure 36.

[0011] The tiltable structure 22 is elastically coupled to a fixed structure 24, defined by the chip itself. In particular, the fixed structure 24 forms a frame 24’ in the horizontal plane XY, for example having a substantially rectangular shape in the plane XY, which delimits and surrounds the above cavity 23. The frame 24’ carries a first support element 25A and a second support element 25B, which extend from the frame 24’ itself along the axis X and are suspended above the cavity 23 on opposite sides of the tiltable structure 22.

[0012] The tiltable structure 22 is supported by the first and second support elements 25A, 25B, to which it is elastically coupled by a first and second elastic suspension element 26A, 26B, respectively.

[0013] The tiltable structure 22 is supported by the first and second support elements 25A, 25B, to which it is elastically coupled by a first and second elastic suspension element 26A, 26B, respectively.

[0014] The microelectromechanical device 20 further comprises an actuation structure 30 coupled to the tiltable structure 22 and configured in such a way as to cause the tiltable structure 22 to rotate about the rotation axis A. The actuation structure 30 is arranged between the tiltable structure 22 and the frame 24’ and, in addition, contributes to supporting the tiltable structure 22 above the cavity 23.

[0015] The actuation structure 30 comprises four drive arms 32A-32D, which are grouped together into two pairs. The first pair comprises a first drive arm 32A and a second drive arm 32B. The second pair comprises a third drive arm 32A and a fourth drive arm 32D. The two pairs are identical to each other and arranged symmetrically with respect to the second axis Y of the reference system XYZ.

[0016] Each drive arm 32A-32D is suspended above the cavity 23 and has a first end fixedly coupled to the frame 24’ and a second end elastically coupled to the tiltable structure 22 through a respective elastic decoupling element 34A-34D. In addition, each drive arm 32A-32D carries, on its top surface opposite the cavity 23, a respective actuation region 33 of the piezoelectric type.

[0017] As Figure 2 schematically shown in Figure 2 , the microelectromechanical device 20 further comprises a plurality of electrical contact pads 38 carried by the fixed structure 24 at the frame 24’ and electrically connected (in a manner not shown in detail in

[0018] During operation of the microelectromechanical device 20, the application of a bias voltage V to the actuation regions 33 of the first drive arm 32A and the third drive arm 32C (with positive sign with respect to the bias of the actuation regions 33 of the second drive arm 32B and the fourth drive arm 32D, for example, the actuation regions 33 of the second drive arm 32B and the fourth drive arm 32D can be connected to a ground reference potential) causes the mirror structure 36 to rotate in a first direction about the rotation axis A.

[0019] Correspondingly, the application of a bias voltage V to the actuation regions 33 of the second drive arm 32B and the fourth drive arm 32D (with positive sign with respect to the bias of the actuation regions 33 of the first drive arm 32A and the third drive arm 32C, for example, the actuation regions 33 of the first drive arm 32A and the third drive arm 32C can be connected to a ground reference potential) causes the mirror structure 36 itself to rotate in the opposite direction about the rotation axis A.

[0020] With reference to Figure 3The microelectromechanical device 20 is composed of two main bodies: a sensor body 50 and a support body 49. In a possible implementation, a protective cover 51 is fixed to the microelectromechanical device 20.

[0021] The sensor body 50 generally has a parallelepiped shape, with a first surface 50A and a second surface 50B, and houses the mirror structure 36 and the corresponding support elements 25A, 25B, the actuation structure 30, the elastic decoupling elements 34A-34D, and the elastic suspension elements 26A, 26B, as described above with reference to the microelectromechanical device 20. Figure 2

[0022] The support body 49 is made, for example, of a semiconductor material and is coupled to the second surface 50B of the sensor body 50 by means of a (for example, silicon oxide) bonding layer 60 at the frame 24'. The support body 49 has a recess 61 facing the tiltable structure 22, so as to enable the tiltable structure 22 to rotate during use. The recess 61 has a bottom surface 61' which is generally blackened for reasons discussed below.

[0023] In the possible implementation considered, the protective cover 51 is made of molded plastic, for example with liquid crystal polymer (LCP), and is bonded to the first surface 50A of the sensor body 50. The protective cover 51 is substantially shaped like an inverted cup and has a chamber 54 closed at the top by a bottom wall 52 arranged at a distance from the first surface 50A of the sensor body 50. Furthermore, the protective cover 51 has an opening 53 in the bottom wall 52 facing the reflective surface 22'. The bottom wall 52 and the opening 53 limit the light beam 62 that can reach the sensor body 50.

[0024] The blackened bottom surface 61' and the protective cover 51 that limits the light beam 62 allow the reflective properties of the microelectromechanical device 20 to be determined by the reflective surface 22' and the movement of the tiltable structure 22 in use.

[0025] This is particularly useful in cases where the microelectromechanical device 20 is used within an augmented reality or virtual reality viewer, or more generally in all those head-mounted systems (HMS) or head-mounted displays (HMD) in which it is desirable to obtain excellent optical properties, in particular reflective properties, in order to help guarantee the optimal performance of the device itself and protect the safety of the user.

[0026] In fact, in particular with regard to the safety of the user, there is a problem with the metal surfaces exposed by the sensor structure 50. In particular, stray reflections of the light rays incident on the microelectromechanical device 20 can accidentally focus on the user's retina, causing irritation to the retina.

[0027] ​However, it is not simple to meet the above safety expectations using the aforementioned microelectromechanical device 20, in which the two main bodies 49 to 50 are manufactured separately and joined to the protective cover 51 in the final stage (the so-called back-end stage).

[0028] In detail, according to the Italian patent application cited above, the sensor body 50 is obtained starting from an SOI (silicon-on-insulator) wafer 40, which is formed of two layers of semiconductor material (e.g., silicon) (hereinafter referred to as the first structural layer 40A and the second structural layer 40B) and an intermediate insulating layer 40C (e.g., silicon oxide).

[0029] Defined by chemical etching in the first structural layer 40A (e.g., having a thickness of 20 μm), are a tiltable structure 22, a fixed structure 24, elastic decoupling elements 34A to 34D, and elastic suspension elements 26A and 26B. Figure 3 (not shown in the image), support elements 25A and 25B ( Figure 3 (Not shown in the image) and drive arms 32A to 32D. Cavity 23 is formed by chemical etching of selective portions of the second structural layer 40B (e.g., having a thickness of 140 μm) and the intermediate insulating layer 40C.

[0030] Below the tiltable structure 22, after etching to form the cavity 23, there is still a reinforcing element 41, which has an extension along the orthogonal axis Z of the reference system XYZ and has a mechanical reinforcing function.

[0031] Formed on the top surface 40' of the first structural layer 40A of the SOI wafer 40 are: a reflective surface 22' made of a suitable material (e.g., aluminum or gold, depending on whether the projection is visible light or infrared light) at the tiltable structure 22; and also, a bottom electrode region 42 made of a suitable conductive material at the drive arms 32A to 32D.

[0032] Then, a region of piezoelectric material 43 (composed of a thin film of PZT-lead zirconate titanate) is obtained on top of the bottom electrode region 42, and a top electrode region 44 is obtained on top of the region of piezoelectric material 43, thereby forming an actuation region 33.

[0033] A passivation layer 45 made of a suitable dielectric material is formed on top of the actuation region 33 as a cover, and a contact opening 46 extends through the passivation layer 45 to approach the bottom electrode region 42 and the top electrode region 44.

[0034] Then, a metal wiring region 47 is formed on the passivation layer 45 to contact the bottom electrode region 42 and the top electrode region 44 through the contact opening 46, and further extends to the corresponding electrical contact pad 38 (not shown here).

[0035] Other wafers of semiconductor material, for example silicon, are configured to form a support body 49, which is selectively etched to obtain recesses 61 and to obtain blackening of the bottom surfaces 61'of the recesses 61. For example, the bottom surfaces 61'can be processed to increase their roughness in a manner known to the person skilled in the art.

[0036] The other wafers of semiconductor material are then bonded to the SOI wafer 40 via the bonding layer 60 and the composite wafer is then cut to form the microelectromechanical device 20.

[0037] In the possible implementation considered, the protective cover 51 is molded by the manufacturer of the apparatus in which the microelectromechanical device 20 is to be installed, or by a representative thereof, who will therefore also fix the protective body 51 to the microelectromechanical device 20.

[0038] As is evident from the foregoing, with the implementation described, the process of fixing the protective cover 51 (capping step) performed in the back-end phase is complex and not standardized, since it is performed in a factory different from the one in which the microelectromechanical device 20 is produced, and depends on the specific application and the technology used.

[0039] Furthermore, the use of a protective cover 51 made of plastic in the back-end can easily cause alignment problems with the microelectromechanical device 20, in particular between the opening 53 of the protective cover 51 and the tiltable structure 22 of the sensor body 50, causing operational defects of the microelectromechanical device.

[0040] Therefore, further developments are needed to provide a microelectromechanical device that allows overcoming the drawbacks of the prior art. SUMMARY

[0041] The embodiments herein relate to a process for manufacturing an optical microelectromechanical device and an optical microelectromechanical device.

[0042] In fact, described herein is a process for manufacturing an optical microelectromechanical device, comprising: forming, in a first wafer of semiconductor material having a first surface and a second surface, a suspended mirror structure, a fixed structure surrounding the suspended mirror structure, an elastic support element extending between the fixed structure and the suspended mirror structure, and an actuation structure coupled to the suspended mirror structure; forming, in a second wafer, a chamber delimited by a bottom wall having a through opening; bonding the second wafer to the first surface of the first wafer and bonding a third wafer to the second surface of the first wafer, so that the chamber covers the actuation structure and so that the through opening is aligned with the suspended mirror structure, thereby forming a device composite wafer; and cutting the device composite wafer to form the optical microelectromechanical device.

[0043] Before bonding the third wafer to the second surface of the first wafer, the process may include forming an internal anti-reflective surface on the third wafer facing the suspended mirror structure.

[0044] The bonding of the second wafer to the first surface of the first wafer can be performed before the step of bonding the third wafer to the second surface of the first wafer.

[0045] The forming chamber may include: selectively removing a portion of a working wafer having a first surface and a second surface, starting from the first surface, to form a first recess and a second recess, wherein the first recess is surrounded by the second recess and extends within the working wafer from the second recess; and thinning the working wafer from the second surface until reaching the first recess to form a second wafer having a through opening and an outer surface opposite to the first surface.

[0046] The outer surface of the second wafer can undergo blackening so that the outer surface absorbs or diffuses light.

[0047] Darkening can include increasing the roughness of the outer surface.

[0048] Blackening can include depositing an absorption layer or dielectric layer onto the outer surface.

[0049] The second chip can be made of semiconductor materials.

[0050] Forming the suspended mirror structure may include selective chemical etching to release the suspended mirror structure, which is performed after the second wafer is bonded to the first surface of the first wafer.

[0051] Forming the suspended mirror structure may include selective chemical etching to release the suspended mirror structure, which is performed before bonding the third wafer to the second surface of the first wafer.

[0052] The actuation structure can be piezoelectric.

[0053] The bottom surface of the second recess of the second wafer and the sidewall of the first recess of the second wafer can form an angle α, which is included between 10° and 90°.

[0054] Also disclosed herein is an optical microelectromechanical device, which can comprise: a sensor body of semiconductor material, having a first surface and a second surface, and having a suspended mirror structure, a fixed structure surrounding the suspended mirror structure, an elastic support element extending between the fixed structure and the suspended mirror structure, and an actuation structure coupled to the suspended mirror structure; a protective cover of semiconductor material or glass, bonded to the first surface of the sensor body, and having a chamber overlying the actuation structure, the chamber being delimited by a wall having a through opening and having an outer surface that is absorbing or diffusing to light, and the through opening being aligned with the suspended mirror structure; and a support body bonded to the second surface of the sensor body.

[0055] The support body can have an internal anti-reflection surface facing the suspended mirror structure.

[0056] Also disclosed herein is a pico-projector device for use in a portable electronic device, comprising: a light source operable for generating a light beam according to an image to be generated; the above optical microelectromechanical device; and a drive circuit configured to supply an electrical drive signal for rotating the suspended mirror structure. The portable electronic device can be a viewer for augmented reality or virtual reality. BRIEF DESCRIPTION OF DRAWINGS

[0057] For a better understanding, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0058] Figure 1 is a perspective schematic representation of a known pico-projector;

[0059] Figure 2 is a top view of a microelectromechanical mirror device;

[0060] Figure 3 is Figure 2 is a cross-sectional view of the device of

[0061] Figure 4 , Figures 5A to 5H and Figures 6 to 9 are cross-sectional views of a wafer of semiconductor material in successive manufacturing steps of the microelectromechanical mirror device disclosed and described herein;

[0062] Figure 10 is a perspective view of the microelectromechanical mirror device disclosed and described herein;

[0063] Figure 11 is a block diagram of a pico-projector using the microelectromechanical mirror device disclosed and described herein;

[0064] Figure 12 is a block diagram of a portable electronic device using the microelectromechanical mirror device disclosed and described herein; Figure 10a perspective view of the possibility of coupling between a micro-projector and a portable electronic device; and

[0065] Figure 13 is about the possibility of coupling between a micro-projector and an augmented reality viewing system. Figure 10 is a perspective view of the possibility of coupling between a micro-projector and an augmented reality viewing system. DETAILED DESCRIPTION

[0066] The following describes steps for manufacturing a microelectromechanical device, in particular a mirror device obtained with MEMS technology, which can be used in a micro-projector and can be inserted in mobile devices, in particular mobile phones and augmented reality and virtual reality viewers.

[0067] In particular, the manufacturing steps described below allow to obtain a microelectromechanical mirror device having a structure similar to that shown in Figure 2 and Figure 3 , and of which the general structure, briefly described below, is made (but with substantial improvements). In particular, Figure 4 , Figures 5A to 5H and Figures 6 to 9 refer to an intermediate structure taken along a cross-sectional line corresponding to line III-III of Figure 2 .

[0068] Figure 4 An SOI wafer 90 is shown, having a first surface 100A and a second surface 100B, and comprising a first structural layer 90A and a second structural layer 90B, for example made of silicon, and an intermediate insulating layer 90C, for example made of silicon oxide. The SOI wafer 90 has already undergone a first manufacturing step which has caused the formation of an actuation region 83 (comprising a bottom electrode region, a top electrode region, a passivation layer, a contact opening and a metal wiring region, similar to the analogous structure of Figure 3 and not shown here) within the first structural layer 90A. Furthermore, by chemical etching of the first structural layer 90A of the SOI wafer 90, a tiltable structure 72 has been defined; support elements 75A, 75B, similar to the support elements 25A, 25B of Figure 2 , here only partially visible and delimited by dashed lines; elastic suspension elements (not shown here), similar to the elastic suspension elements 26A, 26B shown in Figure 2 ; four drive arms (here only two drive arms 82A, 82C are visible); and four elastic decoupling elements, of which only two, 84A and 84C, are visible. Furthermore, a reflective surface 72' has been formed on top of the tiltable structure 72, thus forming a mirror structure 86.

[0069] Individually, as Figure 5AA lid wafer 10 made of a semiconductor material (e.g. silicon) is shown, the lid wafer 10 comprising a working substrate 10' and an insulating layer 10" (e.g. of silicon oxide). The working substrate 10' has a first working surface 10A disposed between the working substrate 10' and the insulating layer 10", and a second working surface 10B opposite the first working surface 10A.

[0070] In Figure 5B , via known photolithography and chemical etching steps, a first opening 11' is obtained in the insulating layer 10".

[0071] Then, via deposition and photolithography of a mask layer (e.g. a resist layer), a mask region 12 is formed on top of the first working surface 10A of the lid wafer 10 and on top of the insulating layer 10"; the mask region 12 forms an inner second opening 11", e.g. concentric with the first opening 11' (see Figure 5C ).

[0072] Via a first chemical etching and using the mask region 12, portions of the working substrate 10' are removed (e.g. to a depth of 50 pm) in the region corresponding to the second opening 11", to create a first recess 13'. The mask region 12 is then removed Figure 5D .

[0073] Using the remaining portion of the insulating layer 10" as a mask, a second chemical etching is performed, which further removes portions of the working substrate 10' (e.g. to a depth of 100 pm), thus making the first recess 13' deeper and creating a second recess 13" wider than the first recess 13". In practice, the first recess 13' and the second recess 13" are arranged below the first opening 11' as shown in Figure 5B . The remaining portion of the insulating layer 10" is then removed, to obtain the intermediate structure of Figure 5E . The remaining portion of the first working surface 10A of the working substrate 10' (see Figure 5A ) forms a temporary contact surface 14 of the working substrate 10', having a smaller area than the first working surface 10A and surrounding the second recess 13". An angle a is defined between the bottom surface 13A of the second recess 13" and the sidewall 13B of the first recess 13', the angle a being 90° in Figure 5E , according to the application, according to the specific chemical etching used and according to the desired profile of the opening for the passage of the light beam (onto the reflective surface 72' shown in Figure 4 ) in the final device, the angle a can have a value comprised between 10° and 90°.

[0074] As shown in Figure 5F , Figure 5EThe working substrate 10' undergoes a thinning step, for example, by a grinding process. To perform this thinning process, according to techniques known to those skilled in the art, the working substrate 10' is conveniently supported at a temporary contact surface 14, for example, by a temporary bonding between the working substrate 10' and a support wafer (not shown). In particular, the bonding process can be conceived in a manner also known to facilitate the aforementioned process using possible intermediate material layers. Specifically, from... Figure 5E The thinning process begins on the second working surface 10B from the back side and continues until the first recess 13' is reached to obtain a thinned substrate 15 with an outer surface 15A. After the thinning process, the second recess 13' forms a chamber 104, which is defined below by a bottom wall 102, and the first recess 13' becomes a through recess and forms an opening 103 for use in the finished microelectromechanical device (MEMS). Figure 9 The light beam passes through it.

[0075] Next, refer to Figure 5G The outer surface 15A of the thinned substrate 15 undergoes a blackening process to cause it to absorb or diffuse light. For example, in a manner known to those skilled in the art, the outer surface 15A may undergo oxygen plasma etching (O2) or laser etching, or some other working process (such as increasing its roughness). Alternatively, in a manner also known to those skilled in the art, the outer surface 15A may be coated with a thin layer of light-absorbing material, or with a dielectric multilayer designed to suppress the reflective component of light.

[0076] Then, the thinned substrate 15 is temporarily bonded to the temporary support wafer 16 via an adhesive layer 17, wherein the adhesive layer 17 faces the outer surface 15A and the opening 103 of the thinned substrate 15. Figure 5H ).

[0077] exist Figure 6 In the middle, the thinned substrate 15 is flipped over and bonded to the substrate in the following manner. Figure 4 On the first surface 100A of the SOI wafer 90, a chamber 104 surrounds and accommodates the actuation region 83, and an opening 103 is aligned with and faces the mirror structure 86. The first composite wafer 109 is thus formed.

[0078] exist Figure 7 In this process, the first composite wafer 109 undergoes chemical etching starting from the second surface 100B of the SOI wafer 90 (which now defines the first composite wafer 109 and is therefore also referred to as the bottom surface of the first composite wafer 109). Specifically, the second structural layer 90B and the intermediate insulating layer 90C of the SOI wafer 90 are selectively removed to create a cavity 73, which defines and releases below the tiltable structure 72, drive arms 82A, 82C (and related components). Figure 2corresponding to the other two arms of the drive arms 32B, 32D, not visible here), the elastic decoupling elements 84A and 84C, the support elements 75A, 75B (only partially visible in Figure 7 and delimited by dashed lines), and the elastic suspension elements similar to the elastic suspension elements 26A, 26B shown in Figure 2 and visible here. Each drive arm, here 82A, 82C, forms a drive structure 80 with the corresponding actuation area 83. In this step, a stiffening element 91 is also formed, which extends from the tiltable structure 72 within the cavity 73. Finally, in this step, a fixation structure 74 is defined, which surrounds the cavity 73 and is delimited by dashed lines in Figure 7 .

[0079] In Figure 8 , the support wafer 98 is bonded to the remaining part of the bottom surface of the first composite wafer 109 by means of the bonding layer 110, to form a second composite wafer 112. The support wafer 98 has previously been processed in a similar manner to that described with reference to Figure 3 , to form a back recess 111 with a darkened bottom surface 111’. In particular, the support wafer 98 is bonded in such a way that the back recess 111 is located at the cavity 73, facing and below the tiltable structure 72.

[0080] Finally, as shown in Figure 9 , the temporary support wafer 16 and the adhesive material layer 17 are removed, and the second composite wafer 112 is cut to obtain the microelectromechanical mirror device 70. The microelectromechanical mirror device 70 thus comprises the sensor body 100, the protective cover 101 and the support body 99, obtained from the SOI wafer 90, the cover wafer 10 and the support wafer 98, respectively.

[0081] The microelectromechanical mirror device 70 thus produced is also shown in Figure 10 . In detail, Figure 10 the protective cover 101 is shown bonded to the sensor body 100, with the opening 103 aligned with the mirror structure 86 and the darkened bottom surface 111’ of the support body 99. The protective cover 101 thus covers the drive structure 80 (not visible here), leaving the control pads 88 free.

[0082] In this way, the sensor body 100, the protective cover 101 and the support body 99 can be obtained in the same factory using technologies and machines for semiconductor work.

[0083] Moreover, the manufacturing of the protective cover 101, now integrated in the above-mentioned process cycle, makes it possible to use highly efficient techniques to perform the blackening of the outer surface 15A, in particular in the case in which the protective cover 101 is made of silicon. In this way, the outer surface 15A can absorb or diffuse the light beams 105 incident thereon, thus preventing unwanted stray reflections. In this way, the radiation reflected by the microelectromechanical mirror device 70 is determined only by the light beams 105 reflected by the reflecting surface 72’.

[0084] Moreover, the manufacturing process allows a high degree of freedom in the choice of the value of the angle of incidence a; in this way, it is possible to select in a controlled manner the angle at which the light beams 105 are incident on the tiltable structure 72, thus helping to guarantee better control of the operation of the microelectromechanical mirror device 70.

[0085] The techniques described herein also help to guarantee a high versatility in the manufacturing of the protective cover 101, thus preventing problems of alignment of the opening 103 with the tiltable structure 72 during the manufacturing back-end bonding, and therefore possible malfunctions of the microelectromechanical mirror device 70 during operation.

[0086] As Figures 11 to 13 As schematically shown in Fig. 6, the microelectromechanical mirror device 70 can be used in a micro-projector 120 designed to be functionally coupled to a portable electronic device.

[0087] In detail, Figure 11 The micro-projector 120 of Fig. 6 comprises a light source 122, for example a laser light source, designed to generate light beams 123; a microelectromechanical mirror device 70 designed to receive the light beams 123 and direct them towards a screen or display surface 125 (external to the micro-projector 120 itself and arranged at a distance from the micro-projector 120 itself); a first driving circuit 126 designed to supply the light source 122 with appropriate control signals for generating the light beams 123 according to the image to be projected; a second driving circuit 128 designed to supply the microelectromechanical mirror device 70 with driving signals; and a communication interface 129 designed to receive information about the image to be generated (for example in the form of an array of pixels) from an external control unit 130 (for example included in the portable electronic device 121). This information is sent at the input for driving the light source 122.

[0088] The micro-projector 120 can be obtained as a separate and independent accessory with respect to the associated portable device, or can be integrated in the associated portable device.

[0089] For example, considering Figure 12wherein the micro projector 120 is arranged within the housing 133 of the portable electronic device 121. In this case, the portable electronic device 121 has a respective portion 132' which is transparent to the light beam 123 coming from the micro electro-mechanical mirror device 70. In this case, the micro projector 120 is coupled, for example, to a printed circuit board present within the housing 133 of the portable electronic device 121.

[0090] In another configuration, thanks to the excellent optical properties of the present micro electro-mechanical mirror device 70, this can be safely integrated in a viewer 142 also for the user, as Figure 13 As shown in Fig. 1 1, the viewer 142 is configured for being worn at a distance close to the eye, and is configured for providing augmented reality or virtual reality images. In detail, for example, the viewer 142 comprises sensors 143, 144 for recording both the reality external to the user and the movements of the user himself, such as the movements of his hands or the gaze. The information collected by the sensors 143, 144 can be processed by a processing unit 145 and supplied to the control unit 130 for projecting, by means of the micro projector 120, images specific to the desired application on the lens 146 operating as a screen.

[0091] Finally, it is clear that modifications and changes can be made to the micro electro-mechanical mirror device 70 and to the manufacturing process described and illustrated herein without thereby departing from the scope of the present application as defined in the appended claims.

[0092] For example, the mirror structure, the elastic suspension elements and the actuation system can have different shapes. Moreover, the protective cover can be manufactured starting from different materials (for example glass) using different blackening processes, and can have different shapes. Moreover, different processes of mechanical type or of chemical type (for example via deep chemical etching) can be used to obtain the openings of the protective cover.

[0093] The actuation system can be different, and can even not be a piezoelectric system.

Claims

1. A method for manufacturing optical microelectromechanical devices, comprising: A working substrate and an insulating layer are formed by stacking, wherein the working substrate has a first working surface facing the insulating layer; A first opening is formed in the insulating layer; A mask is formed on the top of the first working surface and the insulating layer, thereby forming a second opening defined by the mask, and the second opening is concentric with the first opening; Remove a portion of the working substrate in the region corresponding to the second opening to create the first recess, and then remove the mask thereafter. A portion of the working substrate is removed to deepen the first recess, and a second recess is created around the first recess, the second recess being wider than the first recess; The remaining portion of the insulating layer is removed to obtain an intermediate structure, wherein the remaining portion of the first working surface of the working substrate forms a temporary contact surface having an area smaller than the first working surface and surrounding the second recess; While supporting the working substrate by temporarily bonding the working substrate to the support wafer at the temporary contact surface, the working substrate is thinned, the thinning being used to transform the second recess into a chamber defined below by the bottom wall, and to transform the first recess into a through recess. The outer surface of the thinned working substrate is blackened; The thinned working substrate is bonded to another temporary support wafer, wherein the bond faces the outer surface of the thinned working substrate; The thinned working substrate is flipped over and bonded to the first surface of the silicon-on-insulator wafer; as well as Remove the temporary support chip.

2. The method of claim 1, wherein the first opening is formed in the insulating layer by photolithography and chemical etching.

3. The method of claim 1, wherein the mask comprises a resist layer formed by deposition and photolithography.

4. The method of claim 1, wherein the portion of the working substrate is removed by chemical etching in the region corresponding to the second opening to create the first recess.

5. The method of claim 4, wherein the portion of the removed substrate has a first depth.

6. The method of claim 5, wherein the portion of the working substrate is removed by chemical etching to deepen the first recess and create the second recess.

7. The method of claim 6, wherein the portion of the working substrate that is removed to deepen the first recess and create the second recess has a second depth greater than the first depth.

8. The method of claim 1, wherein the angle defined between the bottom surface of the second recess and the sidewall of the first recess is between 10° and 90°.

9. The method of claim 1, wherein the thinning of the working substrate is performed by polishing.

10. The method of claim 9, wherein the grinding is performed from a second working surface of the working substrate opposite to the first working surface toward the first working surface until the first recess is reached.

11. The method of claim 1, wherein temporarily bonding the working substrate to the support wafer at the temporary contact surface is performed by temporarily bonding the working substrate to the support wafer at the temporary contact surface using an adhesive.

12. The method of claim 1, wherein the blackening of the outer surface is performed via plasma etching.

13. The method of claim 1, wherein the blackening of the outer surface is performed via laser etching.

14. The method of claim 1, wherein the silicon-on-insulator wafer has a mirror structure defined on its first surface, the mirror structure having an associated actuation region; and wherein the bonding of the thinned working substrate on the first surface of the silicon-on-insulator wafer is accomplished such that the chamber surrounds and accommodates the actuation region and faces the mirror structure.

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