Process for manufacturing an optical microelectromechanical device having a tiltable structure with an antireflective surface

Abstract

For manufacturing an optical microelectromechanical device, a first wafer of semiconductor material having a first surface and a second surface is machined to form a suspended mirror structure, a fixed structure surrounding the suspended mirror structure, elastic supporting elements which extend between the fixed structure and the suspended mirror structure, and an actuation structure coupled to the suspended mirror structure. A second wafer is machined separately to form a chamber delimited by a bottom wall having a through opening. The second wafer is bonded to the first surface of the first wafer in such a way that the chamber overlies the actuation structure and the through opening is aligned to the suspended mirror structure. Furthermore, 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 an optical microelectromechanical device.

Claims

1. A process for manufacturing an optical microelectromechanical device, comprising: forming, in a stacked arrangement, a work substrate and an insulating layer, the work substrate having a first work surface facing the insulating layer; forming a first opening in the insulating layer; forming a mask on top of the first work surface and the insulating layer to thereby form a second opening defined by the mask and being concentric with the first opening; removing part of the work substrate in a region corresponding to the second opening to create a first recess, and removing the mask thereafter; removing part of the work substrate to deepen the first recess and create a second recess surrounding the first recess and being wider than the first recess; removing remaining portions of the insulating layer to obtain an intermediate structure, with remaining portions of the first work surface of the work substrate forming a temporary contact surface having a smaller area than the first work surface and surrounding the second recess; thinning the work substrate while supporting the work substrate by temporarily bonding the work substrate to a supporting wafer at the temporary contact surface, the thinning serving to turn the second recess into a chamber delimited underneath by a bottom wall and serving to turn the first recess into a through recess; blackening an outer surface of the thinned work substrate; bonding the thinned work substrate to another temporary supporting wafer, with the bonding facing the outer surface of the thinned work substrate; flipping the thinned work substrate and bonding the thinned work substrate on a first surface of a silicon-on-insulator wafer; and removing the temporary supporting wafer.

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

3. The process of claim 1, wherein the mask comprises a resist layer, the resist layer being formed via deposition and lithographic definition.

4. The process of claim 1, wherein the part of the work substrate is removed in the region corresponding to the second opening via chemical etching to create the first recess.

5. The process of claim 4, wherein the part of the substrate removed has a first depth.

6. The process of claim 5, wherein the part of the work substrate is removed via chemical etching to deepen the first recess and create the second recess.

7. The process of claim 6, wherein the part of the work substrate removed to deepen the first recess and create the second recess has a second depth greater than the first depth.

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

9. The process of claim 1, wherein thinning the work substrate is performed via grinding.

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

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

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

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

14. The process of claim 1, wherein the silicon-on-insulator wafer has a mirror structure defined on the first surface thereof with actuation regions associated therewith; and wherein the bonding of the thinned work substrate on the first surface of a silicon-on-insulator wafer is done in such a way that the chamber surrounds and houses the actuation regions and faces the mirror structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For better understanding, an embodiment is now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:

(2) FIG. 1 is a perspective schematic representation of a known picoprojector;

(3) FIG. 2 is a top plan view of a microelectromechanical mirror device;

(4) FIG. 3 is a cross-sectional view of the device of FIG. 2, taken along the line of section III-III.

(5) FIGS. 4, 5A-5H, and 6-9 are cross-sectional views of wafers of semiconductor material in successive manufacturing steps of the microelectromechanical mirror device disclosed and described herein;

(6) FIG. 10 is a perspective view of the microelectromechanical mirror device disclosed and described herein;

(7) FIG. 11 is a block diagram of a picoprojector that uses the microelectromechanical mirror device disclosed and described herein;

(8) FIG. 12 is a perspective view regarding a possibility of coupling between the picoprojector of FIG. 10 and a portable electronic apparatus; and

(9) FIG. 13 is a perspective view regarding a possibility of coupling between the picoprojector of FIG. 10 and an augmented-reality viewing system.

DETAILED DESCRIPTION

(10) Described hereinafter are steps for manufacturing a microelectromechanical device, in particular a mirror device obtained with MEMS technology, which can be used in picoprojectors and can be inserted in mobile apparatuses, in particular mobile phones and augmented-reality and virtual-reality viewers.

(11) In particular, the manufacturing steps described below allow a microelectromechanical mirror device to be obtained having a general structure similar to the one illustrated in FIGS. 2 and 3 (but with substantial improvements made) and described briefly hereinafter. In particular, FIGS. 4, 5A-5H, and 6-9 refer to intermediate structures sectioned along lines of section corresponding to the line III-III of FIG. 2.

(12) FIG. 4 shows a SOI wafer 90 having a first surface 100A and a second surface 100A, 100B and comprising a first structural layer 90A and a second structural layer 90B, which are, for example, made of silicon, and an intermediate insulating layer 90C, which is, for example, made of silicon oxide. The SOI wafer 90 has already been subjected to first manufacturing steps that lead to formation, within the first structural layer 90A, of actuation regions 83 (which comprise bottom-electrode regions, top-electrode regions, a passivation layer, contact openings, and metal routing regions, similar to the homologous structures of FIG. 3 and here not illustrated). Moreover, by chemical etching of the first structural layer 90A of the SOI wafer 90, there have already been defined a tiltable structure 72; supporting elements 75A, 75B similar to the supporting elements 25A, 25B of FIG. 2, here visible only in part and delimited by a dashed line; elastic suspension elements similar to the elastic suspension elements 26A, 26B illustrated in FIG. 2 (here not illustrated); four driving arms (here just two driving arms 82A, 82C are visible); and four elastic decoupling elements, of which only two elastic decoupling elements 84A and 84C are visible. In addition, a reflective surface 72′ has already been formed on top of the tiltable structure 72, forming a mirror structure 86.

(13) Separately, a cap wafer 10, made of semiconductor material, for example silicon, comprising a work substrate 10′ and an insulating layer 10″, for example of silicon oxide is machined, as shown in FIG. 5A. The work substrate 10′ has a first work surface 10A, set between the work substrate 10′ and the insulating layer 10″, and a second work surface 10B, opposite to the first work surface 10A.

(14) In FIG. 5B, a first opening 11′ is obtained in the insulating layer 10″ via known lithographic steps and chemical-etching steps.

(15) A masking region 12 is then formed on top of the first work surface 10A and on top of the insulating layer 10″ of the cap wafer 10, via deposition and lithographic definition of a masking layer (for example, a resist layer); the masking region 12 forms an inner second opening 11″, for example concentric to the first opening 11′ (FIG. 5C).

(16) Via a first chemical etching and using the masking region 12, part of the work substrate 10′ is removed, for example for a depth of 50 μm, in a region corresponding to the second opening 11″, to create a first recess 13′. The masking region 12 is then removed (FIG. 5D).

(17) Using the remaining portions of the insulating layer 10″ as a mask, a second chemical etching is carried out that further removes, for example for a depth of 100 μm, part of the work substrate 10′, making the first recess 13′ deeper and creating a second recess 13″, wider than the first recess 13′. In practice, the first and second recesses 13′, 13″ are arranged underneath the first opening 11′, illustrated in FIG. 5B. The remaining portions of the insulating layer 10″ are then removed to obtain the intermediate structure of FIG. 5E. The remaining portions of the first surface 10A of the work substrate 10′ (FIG. 5A) form a temporary-contact surface 14 of the work substrate 10′, which has a smaller area than the first surface 10A and surrounds the second recess 13″. The angle α, defined between a bottom surface 13A of the second recess 13″ and a side wall 13B of the first recess 13′, which in FIG. 5E is 90°, can have values comprised between 10° and 90° according to the specific chemical etching used and the desired profile of the opening for passage of the light beam (onto the reflective surface 72′ illustrated in FIG. 4) in the final device, according to the application.

(18) The work substrate 10′ of FIG. 5E undergoes a thinning step, for example through a grinding process, as illustrated in FIG. 5F. To carry out this thinning process, the work substrate 10′ is conveniently supported according to techniques known to the person skilled in the art, for example by temporary bonding between the work substrate 10′, and a supporting wafer (here not illustrated), at the temporary-contact surface 14. In particular, the bonding process may envisage possible intermediate layers of material that facilitate the above process, in a way that is also known. In particular, the thinning process is carried out from the back, starting from the second work surface 10B of FIG. 5E and proceeds until the first recess 13′ is reached, to obtain a thinned substrate 15, having an outer surface 15A. Following upon the thinning step, the second recess 13″ forms a chamber 104 delimited underneath by a bottom wall 102, and the first recess 13′ becomes a through recess and forms an opening 103 for the passage of light beams in the finished microelectromechanical device (FIG. 9).

(19) Next, with reference to FIG. 5G, the outer surface 15A of the thinned substrate 15 undergoes a blackening process to make the outer surface 15A absorbant or diffusive in regard to light. For instance, the outer surface 15A may undergo an oxygen plasma etching (O.sub.2) or to laser etching, or some other work process such as to increase the roughness thereof, in a way known to the person skilled in the art. Alternatively, in a way equally known to the person skilled in the art, the outer surface 15A may be coated with thin layers of light absorbing materials or with dielectric multilayers designed for suppressing the reflected component of light.

(20) The thinned substrate 15 is then temporarily bonded to a temporary supporting wafer 16 via a layer of adhesive material 17, with the layer of adhesive material 17 facing the outer surface 15A of the thinned substrate 15 and the opening 103 (FIG. 5H).

(21) In FIG. 6, the thinned substrate 15 is turned over and bonded on the first surface 100A of the SOI wafer 90 of FIG. 4 in such a way that the chamber 104 surrounds and houses the actuation regions 83, and the opening 103 is aligned with, and faces, the mirror structure 86. A first composite wafer 109 is thus formed.

(22) In FIG. 7, the first composite wafer 109 undergoes chemical etches, starting from the second surface 100B of the SOI wafer 90 (which now delimits the first composite wafer 109 and hence will also be referred to as bottom surface 100B of the first composite wafer 109). In particular, the second structural layer 90B of the SOI wafer 90 and the intermediate insulating layer 90C are selectively removed to create a cavity 73 that delimits underneath, and releases, the tiltable structure 72, the driving arms 82A, 82C (as well as the other two arms corresponding to the driving arms 32B, 32D of FIG. 2, here not visible), the elastic decoupling elements 84A and 84C, the supporting elements 75A, 75B (visible only in part and delimited by a dashed line in FIG. 7), and elastic suspension elements similar to the elastic suspension elements 26A, 26B illustrated in FIG. 2 and here visible. Each driving arm (here, 82A, 82C) forms, together with the respective actuation region 83, an actuation structure 80. In this step, reinforcement elements 91 are also formed, which extend from the tiltable structure 72 within the cavity 73. Finally, in this step, a fixed structure 74 is defined, which surrounds the cavity 73 and is delimited in FIG. 7 by a dashed line.

(23) In FIG. 8, a supporting wafer 98 is bonded to the remaining portions of the bottom surface 100B of the first composite wafer 109 by a bonding layer 110 to form a second composite wafer 112. The supporting wafer 98 has been previously machined, in a way similar to what has been described with reference to FIG. 3, to form a rear recess 111 having a bottom surface 111′ which is blackened. In particular, the supporting wafer 98 is bonded in such a way that the rear recess 111 is located at the cavity 73, facing and underneath the tiltable structure 72.

(24) Finally, as shown in FIG. 9, the temporary supporting wafer 16 and the layer of adhesive material 17 are removed, and the second composite wafer 112 is diced to obtain a microelectromechanical mirror device 70. The microelectromechanical mirror device 70 thus comprises a sensor body 100, a protective cap 101, and a supporting body 99, obtained, respectively, from the SOI wafer 90, the cap wafer 10, and the supporting wafer 98.

(25) The resulting microelectromechanical mirror device 70 is illustrated also in FIG. 10. In detail, FIG. 10 shows the protective cap 101 bonded to the sensor body 100, with the opening 103 aligned to the mirror structure 86 and to the bottom surface 111′, which is blackened, of the supporting body 99. The protective cap 101 hence covers the actuation structures 80 (here not visible), leaving the control pads 88 free.

(26) In this way, the sensor body 100, the protective cap 101, and the supporting body 99 may be obtained using the technologies and machines for semiconductor work, in one and the same factory.

(27) Manufacturing the protective cap 101, now integrated in the process cycle as described above, moreover makes it possible to carry out the blackening of the outer surface 15A using efficient techniques, in particular in the case where the protective cap 101 is made of silicon. In this way, the outer surface 15A can absorb or diffuse the light beams 105 incident thereon, preventing undesired spurious reflections. In this way, the radiation reflected by the microelectromechanical device 70 is determined only by the light beams 105 reflected by the reflective surface 72′.

(28) Furthermore, the described manufacturing process allows for high freedom in the choice of the value of the angle α; in this way, it is possible to select in a controlled way the angle of incidence of the light beams 105 onto the tiltable structure 72, helping guarantee better control of operation of the microelectromechanical mirror device 70.

(29) The techniques described herein also help guarantee a high versatility in manufacturing the protective cap 101, preventing problems of alignment of the opening 103 with the tiltable structure 72 during post-manufacture back-end bonding, and hence possible malfunctioning of the microelectromechanical mirror device 70 during operation.

(30) The microelectromechanical device 70 can be used in a picoprojector 120 designed to be functionally coupled to portable electronic apparatuses, as illustrated schematically in FIGS. 11-13.

(31) In detail, the picoprojector 120 of FIG. 11 comprises a light source 122, for example a laser light source, designed to generate a light beam 123; the microelectromechanical mirror device 70, designed to receive the light beam 123 and to direct it toward a screen or display surface 125 (external to and set at a distance from the picoprojector 120 itself); a first driving circuit 126, designed to supply appropriate control signals to the light source 122, for generation of the light beam 123 as a function of an image to be projected; a second driving circuit 128, designed to supply driving signals to the microelectronic device 70; and a communication interface 129, designed to receive, from an external control unit 130, for example included in the portable apparatus 121, information on the image to be generated, for example in the form of a pixel array. This information is sent at input for driving the light source 122.

(32) The picoprojector 120 may be obtained as a separate and stand-alone accessory with respect to an associated portable apparatus or may be integrated therein.

(33) Consider, for example, FIG. 12, where the picoprojector 120 is arranged within a casing 133 of a portable electronic apparatus 121. In this case, the portable electronic apparatus 121 has a respective portion 132′ transparent to the light beam 123 coming from the microelectronic device 70. The picoprojector 120 is in this case for example coupled to a printed-circuit board present within the casing 133 of the portable electronic apparatus 121.

(34) In another configuration, thanks to the excellent optical properties of the present microelectromechanical device 70, this can be integrated safely for the user also in a viewer 142 configured for being worn at a close distance from the eyes and for providing augmented-reality or virtual-reality images, as illustrated in FIG. 13. In detail, the viewer 142 comprises, for example, sensors 143, 144 for recording both the reality external to the user and movements of the user himself, such as movements of his hands or gaze. The information gathered by the sensors 143, 144 can be processed by a processing unit 145 and supplied to the control unit 130 for projecting images specific to the desired application by means of the picoprojector 120 on a lens 146 operating as screen.

(35) Finally, it is clear that modifications and variations may be made to the microelectromechanical mirror device 70 and to the manufacturing process described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims.

(36) For instance, the mirror structure, the elastic suspension elements and the actuation system may have different shapes. Moreover, the protective cap may be manufactured starting from different materials, for example glass, using different blackening processes and may have a different shape. In addition, the opening of the protective cap may be obtained using different processes either of a mechanical type or of a chemical type, for example via deep chemical etching.

(37) The actuation system may be different, and may even not be a piezoelectric system.