Process for manufacturing a microelectromechanical mirror device and microelectromechanical mirror device

Abstract

A process for manufacturing a microelectromechanical mirror device includes, in a semiconductor wafer, defining a support frame, a plate connected to the support frame so as to be orientable around at least one rotation axis, and cantilever structures extending from the support frame and coupled to the plate so that bending of the cantilever structures causes rotations of the plate around the at least one rotation axis. The process further includes forming piezoelectric actuators on the cantilever structures, forming pads on the support frame, and forming spacer structures protruding from the support frame more than both the pads and the stacks of layers forming the piezoelectric actuators.

Claims

1. A microelectromechanical mirror device, comprising: a support frame formed of semiconductor material; a plate connected to the support frame so as to be orientable around at least one rotation axis; a micro-mirror on the plate; cantilever structures extending from the support frame and coupled to the plate so that bending of the cantilever structures causes rotation of the plate around the at least one rotation axis; piezoelectric actuators on the cantilever structures; pads on the support frame; and spacer structures protruding from the support frame more than both the pads and stacks of layers forming the piezoelectric actuator; wherein the spacer structures comprise a dummy actuator having a dummy bottom electrode, a dummy piezoelectric region and a dummy top electrode, portions of a first passivation layer overlaying the dummy actuator, dummy lines formed by respective portions of a routing metallization layer, and portions of a second passivation layer covering the dummy lines, and wherein the spacer structures further comprise, on the portions of the second passivation layer covering the dummy lines, adhesion regions and dummy contacts formed from a metallization layer.

2. The device according to claim 1, wherein the spacer structures protrude from the support frame farther than the pads and the stacks of layers forming the piezoelectric actuators.

3. The device according to claim 1, wherein: each piezoelectric actuator comprises an actuator bottom electrode, an actuator piezoelectric region and an actuator top electrode.

4. The device according to claim 3: wherein the first passivation layer is at least partially overlaying the piezoelectric actuators and the dummy actuators; and further comprising connection lines between the first passivation layer and the second passivation layer; and wherein the piezoelectric actuators comprise actuator contacts connected to respective connection lines through the first passivation layer.

5. A picoprojector apparatus, comprising: a control unit; the microelectromechanical mirror device according to claim 1, controlled by the control unit; and a light source, oriented towards the microelectromechanical mirror and controlled by the control unit to generate a light beam based on an image to be generated.

6. A portable electronic apparatus comprising a system processor and the picoprojector apparatus according to claim 5 coupled to the system processor.

7. The device according to claim 1, wherein the plate includes a reinforcement structure comprising at least one rib extending into a cavity delimited by the support frame.

8. The device according to claim 1, further comprising a cap bonded to the support frame, wherein the cap and the support frame define a cavity enclosing at least a portion of the plate.

9. The device according to claim 1, wherein the cantilever structures are arranged symmetrically with respect to a center of the plate in respective quadrants.

10. A microelectromechanical scanning device, comprising: a semiconductor support structure defining a cavity; a movable plate partially closing the cavity and connected to the semiconductor support structure through elastic elements; a reflective element disposed on the movable plate; a plurality of actuator assemblies extending from the semiconductor support structure toward the movable plate, each actuator assembly comprising: a cantilever arm having a fixed end coupled to the semiconductor support structure and a free end coupled to the movable plate; and a piezoelectric stack on the cantilever arm; electrical contact pads disposed on the semiconductor support structure and electrically coupled to the piezoelectric stacks; and protective spacer elements disposed on the semiconductor support structure and protruding farther from a surface of the semiconductor support structure than both the electrical contact pads and the piezoelectric stacks: wherein the protective spacer elements comprise a dummy actuator having a dummy bottom electrode, a dummy piezoelectric region and a dummy top electrode, portions of a first passivation layer overlaying the dummy actuator, dummy lines formed by respective portions of a routing metallization layer, and portions of a second passivation layer covering the dummy lines, and wherein the protective spacer elements further comprise, on the portions of the second passivation layer covering the dummy lines, adhesion regions and dummy contacts formed from a metallization layer.

11. The device of claim 10, wherein the protective spacer elements further comprise polymeric material.

12. The device of claim 10, wherein: the movable plate is orientable around at least one rotation axis; and the plurality of actuator assemblies are arranged symmetrically with respect to the at least one rotation axis.

13. The device of claim 10, further comprising a cap bonded to the semiconductor support structure and arranged to close the cavity on a side opposite to the movable plate.

14. The device of claim 10, wherein the movable plate includes a reinforcement structure extending into the cavity.

15. The device of claim 10, wherein the plurality of actuator assemblies are independently controllable to orient the movable plate in multiple directions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding, some embodiments will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:

(2) FIG. 1 is a top plan view, with parts removed, of a microelectromechanical mirror device according to an embodiment of this disclosure;

(3) FIG. 2 is a cross-section through the microelectromechanical mirror device of FIG. 1, taken along line II-II of FIG. 1;

(4) FIG. 3 is a cross-section through the microelectromechanical mirror device of FIG. 1, taken along line of FIG. 1;

(5) FIGS. 4, 6, 8, 10, 12 show a cross-section, corresponding to line II-II of FIG. 1, through a semiconductor wafer in successive steps of a manufacturing process according to an embodiment of this disclosure;

(6) FIGS. 5, 7, 9, 11, 13 show a cross-section, corresponding to line of FIG. 1, through the semiconductor wafer of FIG. 4 in successive steps of the manufacturing process according to this disclosure;

(7) FIG. 14 is a first cross-section through a microelectromechanical mirror device in a different embodiment of this disclosure;

(8) FIG. 15 is a second cross-section through the microelectromechanical mirror device of FIG. 12;

(9) FIG. 16 is a simplified block diagram of a picoprojector device incorporating a microelectromechanical mirror device according to an embodiment of this disclosure; and

(10) FIG. 17 is a schematic view of a portable electronic device using the picoprojector device of FIG. 16.

DETAILED DESCRIPTION

(11) With reference to FIGS. 1-3, a microelectromechanical mirror device based on microelectromechanical system (MEMS) technology and formed according to an embodiment is indicated as a whole with the reference 1. In the illustrated non-limiting example, the microelectromechanical mirror device 1 is of monoaxial type and is formed in a die of semiconductor material, in particular silicon.

(12) The microelectromechanical mirror device 1 comprises a support frame 2, which delimits a cavity 3, and a plate 5. A cap 4 is bonded to the support frame 2 and is arranged to close the cavity 3 on a side opposite to the plate 5.

(13) The plate 5 partially closes the cavity 3 and is connected to anchors 2a of the support frame 2 through elastic elements 6, so as to be orientable around a rotation axis X, which is also a median axis of the plate 5.

(14) In the embodiment of FIG. 1, the plate 5 has a substantially elliptical shape and is symmetrical with respect to the rotation axis X, as well as with respect to a Y-axis perpendicular to the rotation axis X and defining, with the rotation axis X, an XY-plane parallel to the plate 5. The shape of the plate 5 is not to be construed as limiting. For example, the plate 5 might be quadrangular or polygonal with a different number of sides, or circular.

(15) A micro-mirror 7 defined by a layer of reflecting material, for example gold or aluminum, occupies a central portion of a face of the plate 5 opposite to the cavity 3. The plate 5 is provided with a reinforcement structure 5a, for example in the form of one or more ribs, extending into the cavity 3.

(16) The microelectromechanical mirror device 1 further comprises motion actuator assemblies 8 configured to orient the plate 5 around the rotation axes X, Y. In the embodiment of FIG. 1, there are four actuator assemblies arranged in a specular symmetrical manner in respective quadrants with respect to the center of the plate 5. In detail, the actuator assemblies extend from the support frame 2 towards the plate 5 and each motion actuator assembly 8 comprises a cantilever structure 9 and a piezoelectric actuator 10, arranged on the respective cantilever structure 9. The ends of the cantilever structures 9 are connected to the plate 5 through command elastic elements 11. Contact pads 12 are coupled to respective piezoelectric actuators 10 through connection lines 13, partially shown in FIG. 1, to apply electrical command signals. The actuator assemblies 8 are independent from each other and may be operated to orient the plate 5 with respect to the rotation axis X in a controlled manner, as explained for example in United States Patent Publication No. 2020/0192199 (corresponding to the published European Patent Application EP 3,666,727A1), the contents of which are incorporated by reference in their entirety to the maximum extent allowable under law.

(17) In detail, each piezoelectric actuator 10 comprises a stack of layers including a bottom electrode 15, which extends on the respective cantilever structure 9, a piezoelectric region 16, for example of PZT (Lead Zirconate Titanate), and a top electrode 17. Here and below, bottom electrode indicates an electrode formed between the surface of the respective cantilever and the respective piezoelectric region, while top electrode indicates an electrode formed on the respective piezoelectric region and opposite to a corresponding bottom electrode. The bottom electrode 15 and the top electrode 17 are coupled to respective contact pads 12 on the support frame 2 through the connection lines 13. The stack of layers forming the piezoelectric actuators 10 also includes, in the following order: portions of a first passivation layer 20; portions of a routing metallization layer forming actuator contacts 21 through openings in the first passivation layer 20 and the connection lines 13; and portions of a second passivation layer 22 covering the first passivation layer 20, the actuator contacts 21 and the connection lines 13.

(18) The microelectromechanical mirror device 1 comprises spacer structures 25 which are arranged on the support frame 2 around the cavity 3 and protrude from the surface of the support frame 2 itself more than the other structures of the microelectromechanical mirror device 1, in particular the stacks of layers of the piezoelectric actuators 10. In one embodiment, in particular, the spacer structures 25 comprise, as the stacks of layers of the piezoelectric actuators 10, a dummy actuator 27 with a dummy bottom electrode 28, a dummy piezoelectric region 29 and a dummy top electrode 30, portions of the first passivation layer 20 overlaying the dummy actuator 27, dummy lines 31 formed by respective portions of the routing metallization layer and portions of the second passivation layer 22 covering the dummy lines 31. Furthermore, the spacer structures 25 comprise, on the portions of the second passivation layer 22 covering the dummy lines 31, respective adhesion regions 33 and dummy contacts 35, formed from a metallization layer wherefrom the pads 12 are also obtained. The adhesion layers 28 and the dummy contacts 35 are prominent with respect to the other structures of the microelectromechanical mirror device 1, in particular, to the stacks of layers of the piezoelectric actuators 10. For this reason, the spacer structures 25 offer certain advantages during the manufacture of the microelectromechanical mirror device 1, as shown by the description below.

(19) With reference to FIGS. 4 and 5, a semiconductor wafer 50 of silicon on insulator (SOI) type comprises a first semiconductor layer 51, for example a monocrystalline epitaxial or polycrystalline pseudo-epitaxial layer, and a second semiconductor layer 52, for example a monocrystalline substrate. The first semiconductor layer 51 and the second semiconductor layer 52 are separated by a dielectric layer 53, which also extends on a face of the second semiconductor layer 52 opposite to the first semiconductor layer 51. A thermal-oxide layer 54 is initially grown on the first semiconductor layer 54, then a bottom electrode layer 55, a piezoelectric layer 56 and a top electrode layer 57 are deposited in succession and defined. Residual portions of the stack formed by the bottom electrode layer 55, the piezoelectric layer 56 and the top electrode layer 57 define the piezoelectric actuators 10 (the bottom electrode 15, the piezoelectric region 16 and the top electrode 17, respectively) and the dummy actuators 27 (the bottom electrode 28, the piezoelectric region 29 and the top electrode 30, respectively).

(20) Subsequently (referring now to FIGS. 6 and 7), the first passivation layer 20 is deposited and contact windows 58 are opened on the piezoelectric actuators 10. The routing metallization layer, here indicated with the number 59, is deposited and shaped to form the connection lines 13, the actuator contacts 21 in the contact windows 58 and the dummy lines 31 on the dummy actuators 27.

(21) With reference to FIGS. 8 and 9, the second passivation layer 22 is deposited and covers the connection lines 13, the actuator contacts 21 and the dummy lines 31. Then, an adhesion layer 60 and a pad metallization layer 62 are deposited and shaped to form the adhesion regions 33, the pads 12 (not visible in FIGS. 8 and 9; see FIG. 1), the connection lines 13 and the dummy contacts 35. In particular, the dummy contacts are formed on top of the dummy actuators 27. The spacer structures 25 are thus completed. The spacer structures 25 protrude from the surface of the support frame 2 more than both the pads 12, which are formed on the routing metallization layer, but without the interposition of piezoelectric material, and the stacks of the piezoelectric actuators 10, where the dummy contacts are absent.

(22) As shown in FIGS. 10 and 11, the passivation layers 20, 22, the thermal-oxide layer 54, the first semiconductor layer 51 and the dielectric layer 53 (between the first semiconductor layer 51 and the second semiconductor layer 52) are anisotropically etched and a portion of the support frame 2, the plate 5 and the anchors 2a are defined. In this step, the passivation layers 20, 22 and the thermal-oxide layer 54 are also removed on the portion of the first semiconductor layer 51 intended to form the plate 5.

(23) At this point (FIGS. 12 and 13), a shadow mask 70 is defined and applied to the semiconductor wafer 1 on the side of the plate 5. The shadow mask 70 in practice rests on the spacer structures 25, which protrude more than the other structures present on the semiconductor wafer 1, and above all it does not come into contact with electrical connection structures and with the piezoelectric actuators 10. The shadow mask 70 has an opening 71 in a position corresponding to the plate 5. The micro-mirror 7 is then formed through the opening 71 of the shadow mask 70 for example by deposition of aluminum or gold by sputtering. The shadow mask 70 is then removed. Finally, the cap 4 is joined to the support frame 2 to close the cavity 3 on the back of the semiconductor wafer 1, which is then diced to obtain copies of the microelectromechanical device 1 of FIGS. 1 and 3.

(24) The use of the spacer structures 25 is particularly advantageous since the microelectromechanical mirror device 1 is protected when the shadow mask 70 is applied and removed. During this step, the spacer structures 25, which are in contact with the shadow mask 70, may be damaged, whereas the other structures are not exposed to risks. On the other hand, the spacer structures 25 are not functional to the use of the microelectromechanical mirror device 1 and therefore damage or even the complete destruction thereof are totally irrelevant. The parts used for operation are instead safeguarded and therefore the application and the removal of the shadow mask 70, which might normally cause a high percentage of reject, do not entail significant risks. The overall yield of the process is therefore high.

(25) A further advantage results from the fact that the spacer structures 25 include the same sequence of layers as the piezoelectric actuators and, in addition, a portion of the metallization layer used for the pads 12. On the one hand, in fact, the sequence of the layers helps ensure that the spacer structures 25 protrude from the support frame 2 more than the other structures of the microelectromechanical mirror device 1, in particular more than the piezoelectric actuators 10. On the other hand, the manufacture of the spacer structures 25 does not utilize additional process step and therefore does not affect the production costs. The layers that form the spacer structures 25 are used for other structures of the microelectromechanical mirror device 1 and may be shaped in the same process steps and with the same suitably designed masks with which the other structures are defined.

(26) Although this is particularly advantageous, however, it is not essential that the spacer structures are formed from the same layers wherefrom the other structures of the microelectromechanical mirror device 1 are obtained. For example, the spacer structures might be made in whole or in part of other materials, for example including parts of polymeric materials, as shown in FIG. 14, where parts equal to those already shown are indicated with the same reference numbers. In this case, a microelectromechanical mirror device 100 comprises spacer structures 125 of polymeric material which define the maximum protrusions with respect to the support frame 2.

(27) FIG. 16 illustrates a picoprojector device 200 which comprises the microelectromechanical mirror device 1, a control unit 202, a light source 203 and an interface 204 for the connection to an electronic apparatus, such as a desktop or portable computer, a tablet or a mobile phone.

(28) The control unit 201 controls, on the basis of an image to be projected, a light beam emitted by the light source 203 and the orientation of the plate 5 so as to coordinate the projection of a sequence of image points and a two-dimensional scanning procedure of an image area.

(29) FIG. 17 illustrates an electronic apparatus 250, in particular a mobile phone, coupled to the picoprojector device 200 through the interface 204. The electronic apparatus 250 is provided with a system processor 251 which provides the picoprojector device 200 with an image signal, for example a file in a standard image format.

(30) In one embodiment, the picoprojector device may be integrated into the portable apparatus.

(31) Finally, it is apparent that modifications and variations may be made to the described process and device, without departing from the scope of this disclosure.

(32) In particular, the microelectromechanical mirror device may be of biaxial type. Furthermore, the shape of the plate and the micro-mirror may be freely defined according to the design preferences. For example, the plate and the micro-mirror may be circular, quadrangular or more generally polygonal. Furthermore, the micro-mirror may not have the same shape as the plate.

(33) As for the process, the micro-mirror may be formed on the plate through the shadow mask after the etching of the second semiconductor layer.