CN115268059A - Method for manufacturing a microelectromechanical mirror device and microelectromechanical mirror device - Google Patents

Abstract

Embodiments of the present disclosure relate to a method of manufacturing a microelectromechanical mirror device and a microelectromechanical mirror device. A process for manufacturing a microelectromechanical mirror device, comprising: in a semiconductor wafer, a support frame, a plate connected to the support frame so as to be orientable about at least one axis of rotation, and a cantilever structure extending from the support frame and coupled to the plate, such that bending of the cantilever structure causes rotation of the plate about the at least one axis of rotation. The process also includes forming a piezoelectric actuator on the cantilever structure, forming a pad on the support frame, and forming a spacer structure that protrudes from the support frame more than both the layer stack and the pad forming the piezoelectric actuator.

Description

Method for manufacturing a microelectromechanical mirror device and microelectromechanical mirror device

Cross Reference to Related Applications

The present application claims priority to italian patent application No.102021000011039 filed on 30/4/2021, the contents of which are incorporated by reference herein in their entirety to the maximum extent allowed by law.

Technical Field

The present disclosure relates to a method for manufacturing a microelectromechanical mirror device and a microelectromechanical mirror device so manufactured.

Background

As is known, the so-called shadow mask technique is widely used in the manufacture of micro-electromechanical micromirror devices and is particularly appreciated due to its simplicity of implementation and the high reflectivity values that can be achieved. This technique is applied in a final manufacturing step, after the support and actuation structures, usually of the piezoelectric type, have been defined in the semiconductor wafer and the metallization and passivation layers have been deposited and shaped.

Basically, a mask, in particular called a "shadow" mask, is formed separately from the semiconductor wafer and has openings corresponding in shape and arrangement to the micromirrors to be formed. The shadow mask is aligned and applied to the wafer being processed and then a sputter deposition process is used to deposit, for example, gold/aluminum, forming the micro mirrors through openings in the shadow mask itself.

Problems often occur when shadow masks are applied to semiconductor wafers to deposit micro-mirrors, or when the masks themselves are subsequently removed. The wafer being processed has a protruding structure on which a shadow mask is placed. The most protruding part is usually the stack of layers corresponding to the contacts of the piezoelectric actuation structure and thus performs a critical function for the operation of the micromirror device. Some of the structures in the contact may be relatively vulnerable to damage when the shadow mask is applied and removed, thereby compromising the functionality of the entire device. Thus, the yield of the manufacturing process may not be satisfactory.

Therefore, there is a need to further develop manufacturing techniques in this field.

There is a need in the art to provide a method for manufacturing a microelectromechanical mirror device and a microelectromechanical mirror device so manufactured, allowing to overcome or at least alleviate the described limitations.

Disclosure of Invention

In one embodiment, a method for fabricating a microelectromechanical mirror device includes: defining in a semiconductor wafer: a support frame, a plate connected to the support frame so as to be orientable about at least one axis of rotation, and a cantilever structure extending from the support frame and coupled to the plate such that bending of the cantilever structure causes rotation of the plate about the at least one axis of rotation; forming a piezoelectric actuator on the cantilever structure; forming a pad on the support frame; and forming spacer structures protruding from the support frame, the spacer structures protruding more than the stack of layers and pads forming the piezoelectric actuator.

The method may further comprise: applying a shadow mask to the spacer structure; forming micro-mirrors on the plate using the shadow mask; and removing the shadow mask.

The spacer structure may protrude further from the support frame than the stack and pads forming the piezoelectric actuator.

Forming the piezoelectric actuator may include: successively depositing a bottom electrode layer, a piezoelectric layer and a top electrode layer on a semiconductor wafer; and for each piezoelectric actuator, forming an actuator bottom electrode from the bottom electrode layer, an actuator piezoelectric region from the piezoelectric layer, and an actuator top electrode from the top electrode layer, respectively.

For each spacer structure, forming the spacer structure may include forming a dummy actuator.

Forming the dummy actuator may include forming a dummy bottom electrode from the bottom electrode layer, forming a dummy piezoelectric region from the piezoelectric layer, and forming a dummy top electrode from the top electrode layer, respectively.

The method may further comprise: depositing a first passivation layer on the top electrode layer; opening a contact window in the first passivation layer; depositing a wiring metallization layer; forming an actuator contact in the contact window from the wiring metallization layer; and depositing a second passivation layer covering the first passivation layer and the actuator contact.

For each spacer structure, forming the spacer structure may include: forming a dummy line from the wiring metallization layer on a portion of the first passivation layer covering the corresponding dummy actuator; and covering the dummy line with a corresponding portion of the second passivation layer.

The method may further comprise: depositing an adhesion layer on the second passivation layer, and depositing a pad metallization layer on the adhesion layer; and wherein forming the spacer structure includes forming an adhesion region from the adhesion layer and a dummy contact from the pad metallization layer, respectively, on the dummy actuator.

Forming the pad may include forming the pad from a pad metallization layer.

In one embodiment, a microelectromechanical mirror device includes: a support frame formed of a semiconductor material; a plate connected to the support frame so as to be orientable about at least one axis of rotation; a micro mirror on the plate; a cantilever structure extending from the support frame and coupled to the plate such that bending of the cantilever structure causes rotation of the plate about the at least one axis of rotation; a piezoelectric actuator on the cantilever structure; a pad on the support frame; and spacer structures protruding more from the support frame than both the stack of layers and the pads forming the piezoelectric actuator.

The spacer structure may protrude further from the support frame than the pads and stacks forming the piezoelectric actuator.

Each spacer structure may include a dummy actuator. Each piezoelectric actuator may include an actuator bottom electrode, an actuator piezoelectric region, and an actuator top electrode. Each dummy actuator may include a dummy bottom electrode, a dummy piezoelectric region, and a dummy top electrode.

The apparatus may include: a first passivation layer at least partially over the piezoelectric actuators and the dummy actuators; a second passivation layer on the first passivation layer; and a connection line between the first passivation layer and the second passivation layer. The piezoelectric actuator may comprise actuator contacts connected to respective connecting lines through the first passivation layer, and the spacer structure may comprise dummy lines between portions of the first passivation layer covering respective dummy actuators and the second passivation layer.

In another embodiment, a pico projector apparatus includes: a control unit; a micro-electromechanical mirror device as described above controlled by a control unit; and a light source directed towards the micro-electromechanical mirror and controlled by the control unit to generate a light beam based on an image to be generated.

Embodiments herein also include a portable electronic device comprising a system processor and a pico projector device as described above coupled to the system processor.

Drawings

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

FIG. 1 is a top plan view, with portions removed, of a micro-electromechanical mirror device according to an embodiment of the present disclosure;

FIG. 2 is a cross section through the micro-electromechanical mirror device of FIG. 1 taken along line II-II of FIG. 1;

FIG. 3 is a cross section through the micro-electromechanical mirror device of FIG. 1 taken along line III-III of FIG. 1;

fig. 4, 6, 8, 10, 12 show cross-sections through a semiconductor wafer corresponding to line II-II of fig. 1 in successive steps of a manufacturing process according to an embodiment of the present disclosure;

fig. 5, 7, 9, 11, 13 show cross-sections through the semiconductor wafer of fig. 4 corresponding to line III-III of fig. 1 in successive steps of a manufacturing process according to the present disclosure;

FIG. 14 is a first cross section through a micro-electromechanical mirror device in various embodiments of the present disclosure;

FIG. 15 is a second cross section through the micro-electromechanical mirror device of FIG. 12;

FIG. 16 is a simplified block diagram of a micro-electromechanical projector apparatus incorporating a micro-electromechanical mirror apparatus according to an embodiment of the present disclosure; and

fig. 17 is a schematic diagram of a portable electronic device using the pico projector device of fig. 16.

Detailed Description

Referring to fig. 1-3, a microelectromechanical mirror device based on microelectromechanical systems (MEMS) technology formed in accordance with an embodiment is indicated generally by reference numeral 1. In the non-limiting example shown, the microelectromechanical mirror device 1 is of the uniaxial type and is formed in a die of semiconductor material, in particular silicon.

The microelectromechanical mirror device 1 comprises a support frame 2 and a plate 5 defining a cavity 3. A cover 4 is joined to the support frame 2 and is arranged to close the cavity 3 on the side opposite to the plate 5.

The plate 5 partially encloses the cavity 3 and is connected to the anchor 2a of the support frame 2 by means of an elastic element 6 so as to be orientable about a rotation axis X, which is also the median axis of the plate 5.

In the embodiment of fig. 1, the plate 5 has a substantially elliptical shape and is symmetrical with respect to the rotation axis X and with respect to a Y-axis perpendicular to the rotation axis X, and the Y-axis and the rotation axis X define an XY-plane parallel to the plate 5. The shape of the plate 5 is not to be understood as limiting. For example, the plate 5 may be a quadrangle or a polygon with a different number of sides, or a circle.

A micromirror 7 defined by a layer of reflective material, for example gold or aluminum, occupies the central portion of the face of the plate 5 opposite the cavity 3. The plate 5 is provided with a reinforcing structure 5a, for example in the form of one or more ribs, extending into the cavity 3.

The microelectromechanical mirror device 1 also comprises a motion actuator assembly 8 configured to orient the plate 5 about an axis of rotation X, Y. In the embodiment of fig. 1, there are four actuator assemblies, arranged in respective quadrants in a mirror-symmetrical manner with respect to the centre 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, which are arranged on the respective cantilever structure 9. The end of the cantilever structure 9 is connected to the plate 5 by a command spring element 11. The contact pads 12 are coupled to the respective piezoelectric actuators 10 by connection lines 13, partially shown in fig. 1, for applying electrical command signals. The actuator assemblies 8 are independent of each other and can be operated to orient the plate 5 in a controlled manner with respect to the axis of rotation X, as explained, for example, in U.S. patent publication No.2020/0192199 (corresponding to published european patent application EP3666727 A1), the contents of which are incorporated by reference in their entirety to the maximum extent allowed by law.

In detail, each piezoelectric actuator 10 comprises a stack of layers including a bottom electrode 15, for example a piezoelectric region 16 of PZT (lead zirconate titanate) and a top electrode 17, extending on the respective cantilever structure 9. Here and below, the "bottom electrode" indicates an electrode formed between the surface of the corresponding cantilever and the corresponding piezoelectric region, and the "top electrode" indicates an electrode formed on the corresponding piezoelectric region and opposite to the corresponding bottom electrode. The bottom electrode 15 and the top electrode 17 are coupled to respective contact pads 12 on the support frame 2 by means of connection lines 13. The stack of layers forming the piezoelectric actuator 10 further comprises in the following order: a portion 20 of the first passivation layer; portions of the wiring metallization layer forming the actuator contacts 21 through the openings in the first passivation layer 20 and the connection lines 13; and a portion of the second passivation layer 22 covering the first passivation layer 20, the actuator contacts 21 and the connection lines 13.

The microelectromechanical mirror device 1 comprises a spacer structure 25, which spacer structure 25 is arranged on the support frame 2 around the cavity 3 and protrudes more from the surface of the support frame 2 itself than the other structures of the microelectromechanical mirror device 1, in particular the stack of layers of the piezoelectric actuator 10. In one embodiment, in particular, as a stack of layers of the piezoelectric actuator 10, the spacer structure 25 comprises a dummy actuator 27 with a dummy bottom electrode 28, a dummy piezoelectric region 29 and a dummy top electrode 30, a portion of the first passivation layer 20 covering the dummy actuator 27, a dummy line 31 formed by a respective portion of the wiring metallization layer and a portion of the second passivation layer 22 covering the dummy line 31. Furthermore, the spacer structure 25 comprises, on the portion of the second passivation layer 22 covering the dummy line 31, a respective adhesion area 33 and dummy contacts 35 formed by the metallization layer from which the pads 12 are also obtained. The adhesion layer 28 and the dummy contact 35 are protruding with respect to other structures of the microelectromechanical mirror device 1, in particular with respect to the layer stack of the piezoelectric actuator 10. For this reason, the spacer structure 25 provides certain advantages in the manufacturing process of the microelectromechanical mirror device 1, as will be shown in the following description.

Referring to fig. 4 and 5, a semiconductor wafer 50 of silicon-on-insulator (SOI) type includes a first semiconductor layer 51 (e.g., a single crystal epitaxial or polycrystalline dummy epitaxial layer) and a second semiconductor layer 52 (e.g., a single crystal substrate). The first semiconductor layer 51 and the second semiconductor layer 52 are separated by a dielectric layer 53, the dielectric layer 53 also extending on the face of the second semiconductor layer 52 opposite the first semiconductor layer 51. A thermal oxide layer 54 is initially grown on the first semiconductor layer 54 and then successively deposited and defines a bottom electrode layer 55, a piezoelectric layer 56 and a top electrode layer 57. The remaining part of the stack formed by the bottom electrode layer 55, the piezoelectric layer 56 and the top electrode layer 57 defines the piezoelectric actuator 10 (bottom electrode 15, piezoelectric area 16 and top electrode 17, respectively) and the dummy actuator 27 (bottom electrode 28, piezoelectric area 29 and top electrode 30, respectively).

Subsequently (referring now to fig. 6 and 7), a first passivation layer 20 is deposited and a contact window 58 is opened on the piezoelectric actuator 10. A wiring metallization layer, here indicated with numeral 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.

Referring to fig. 8 and 9, a second passivation layer 22 is deposited and covers the connection line 13, the actuator contact 21 and the dummy line 31. Then, an adhesion layer 60 and a pad metallization layer 62 are deposited and shaped to form the adhesion areas 33, the pads 12 (not visible in fig. 8 and 9; see fig. 1), the connection lines 13 and the dummy contacts 35. Specifically, the dummy contact is formed on top of the dummy actuator 27. The spacer structure 25 is thus completed. The spacer structures 25 protrude more from the surface of the support frame 2 than both the pads 12 formed on the wiring metallization layer without interposing the piezoelectric material and the stack of piezoelectric actuators 10 (where there is no dummy contact).

As shown in fig. 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 define a portion of the support frame 2, the board 5, and the anchors 2 a. In this step, the passivation layers 20, 22 and the thermal oxide layer 54 are also removed on the portion intended to form the first semiconductor layer 51 of the plate 5.

At this point (fig. 12 and 13), a shadow mask 70 is defined and applied to semiconductor wafer 1 on one side of plate 5. Shadow mask 70 actually rests on spacer structures 25, spacer structures 25 protruding more than other structures present on semiconductor wafer 1, and first it is not in contact with electrical connection structures and piezoelectric actuators 10. Shadow mask 70 has an opening 71 at a position corresponding to plate 5. Micromirror 7 is then formed through opening 71 of shadow mask 70, for example by depositing aluminum or gold by sputtering. Shadow mask 70 is then removed. Finally, a cover 4 is attached to the support frame 2 to close the cavity 3 on the back side of the semiconductor wafer 1, which is then cut to obtain a copy of the microelectromechanical device 1 of fig. 1 and 3.

The use of spacer structures 25 is particularly advantageous because the microelectromechanical mirror device 1 is protected when the shadow mask 70 is applied and removed. During this step, spacer structures 25 in contact with shadow mask 70 may be damaged without exposing other structures to risk. On the other hand, the spacer structure 25 does not play a role in the use of the microelectromechanical mirror device 1, and therefore its damage or even complete destruction is completely irrelevant. Portions for handling are instead protected and therefore the application and removal of shadow mask 70, which may typically result in a high percentage of rejects, does not pose a significant risk. The overall yield of the process is high.

Another advantage derives from the fact that the spacer structure 25 comprises the same layer sequence as the piezoelectric actuator and furthermore comprises a portion of the metallization layer for the pads 12. On the one hand, in practice, the sequence of the layers helps to ensure that the spacer structure 25 protrudes more from the support frame 2 than other structures of the microelectromechanical mirror device 1, in particular more from the support frame 2 than the piezoelectric actuators 10. On the other hand, the spacer structure 25 is manufactured without using additional process steps and therefore does not affect the production cost. The layers forming the spacer structure 25 are used for other structures of the microelectromechanical mirror device 1 and may be shaped in the same process step and with the same appropriately designed mask as defined for the other structures.

Although this is particularly advantageous, the spacer structure does not, however, have to be formed from the same layer as the other structures from which the microelectromechanical mirror device 1 is obtained. For example, the spacer structure may be made in whole or in part of other materials, such as a part comprising a polymeric material, as shown in fig. 14, where parts that are identical to parts already shown are indicated by the same reference numerals. In this case, the microelectromechanical mirror device 100 comprises a spacer structure 125 of a polymeric material, which defines a maximum protrusion with respect to the support frame 2.

Fig. 16 shows a pico projector device 200 comprising a micro-electromechanical mirror device 1, a control unit 202, a light source 203 and an interface 204 for connection to an electronic device such as a desktop or portable computer, a tablet computer or a mobile phone.

The control unit 201 controls the light beam emitted by the light source 203 and the orientation of the plate 5 based on the image to be projected in order to coordinate the projection of the sequence of image points and the two-dimensional scanning process of the image area.

Fig. 17 shows an electronic device 250, in particular a mobile phone, coupled to the pico projector device 200 through an interface 204. The electronic device 250 is provided with a system processor 251, the system processor 251 providing an image signal, such as a file in a standard image format, to the pico projector apparatus 200.

In one embodiment, the pico projector apparatus may be integrated into a portable device.

Finally, it is apparent that modifications and variations may be made to the processes and apparatus described without departing from the scope of the disclosure.

In particular, the micro-electromechanical mirror device may be of the biaxial type. Further, the shapes of the plate and the micromirror may be freely defined according to design preference. For example, the plates and micro-mirrors may be circular, quadrilateral or more generally polygonal. Further, the micromirrors may not have the same shape as the plate.

For this process, the micromirrors may be formed on the plate through a shadow mask after etching the second semiconductor layer.

Claims (16)

1. A method for manufacturing a microelectromechanical mirror device, comprising:

in a semiconductor wafer, defining a support frame, a plate connected to the support frame so as to be orientable about at least one axis of rotation, and a cantilever structure extending from the support frame and coupled to the plate such that bending of the cantilever structure causes rotation of the plate about the at least one axis of rotation;

forming a piezoelectric actuator on the cantilever structure;

forming a pad on the support frame; and

forming a spacer structure that protrudes more from the support frame than both the layer stack forming the piezoelectric actuator and the pad.

2. The method of claim 1, comprising: applying a shadow mask to the spacer structure; forming micro-mirrors on the plate using the shadow mask; and removing the shadow mask.

3. The method of claim 1, wherein the spacer structures protrude further from the support frame than the layer stack and the pads forming the piezoelectric actuator.

4. The method of claim 1, wherein forming the piezoelectric actuator comprises:

successively depositing a bottom electrode layer, a piezoelectric layer, and a top electrode layer on the semiconductor wafer; and

for each piezoelectric actuator, an actuator bottom electrode, an actuator piezoelectric region, and an actuator top electrode are formed from the bottom electrode layer, from the piezoelectric layer, and from the top electrode layer, respectively.

5. The method of claim 4, wherein forming the spacer structure comprises: a dummy actuator is formed for each spacer structure.

6. The method of claim 5, wherein forming the dummy actuator comprises: a dummy bottom electrode, a dummy piezoelectric region, and a dummy top electrode are formed from the bottom electrode layer, from the piezoelectric layer, and from the top electrode layer, respectively.

7. The method of claim 5, further comprising:

depositing a first passivation layer on the top electrode layer;

opening a contact window in the first passivation layer;

depositing a wiring metallization layer;

forming an actuator contact in the contact window from the wiring metallization layer; and

depositing a second passivation layer covering the first passivation layer and the actuator contact.

8. The method of claim 7, wherein forming the spacer structure comprises: for each spacer structure:

forming a dummy line from the wire metallization layer on a portion of the first passivation layer covering the respective dummy actuator; and

covering the dummy line with a corresponding portion of the second passivation layer.

9. The method of claim 8, further comprising: depositing an adhesion layer on the second passivation layer and a pad metallization layer on the adhesion layer; and wherein forming the spacer structure comprises: forming an adhesion region and a dummy contact on the dummy actuator from the adhesion layer and from the pad metallization layer, respectively.

10. The method of claim 9, wherein forming the pad comprises: the pad is formed from the pad metallization layer.

11. A microelectromechanical mirror device comprising:

a support frame formed of a semiconductor material;

a plate connected to the support frame so as to be orientable about at least one axis of rotation;

a micro-mirror on the board;

a cantilever structure extending from the support frame and coupled to the plate such that bending of the cantilever structure causes rotation of the plate about the at least one axis of rotation;

a piezoelectric actuator located on the cantilever structure;

a pad on the support frame; and

spacer structures protruding more from the support frame than both the layer stack forming the piezoelectric actuator and the pads.

12. The apparatus of claim 11, wherein the spacer structures protrude further from the support frame than the layer stack and the pads forming the piezoelectric actuator.

13. The apparatus of claim 11, wherein:

each spacer structure includes a dummy actuator;

each piezoelectric actuator comprises an actuator bottom electrode, an actuator piezoelectric area and an actuator top electrode; and

each dummy actuator includes a dummy bottom electrode, a dummy piezoelectric region, and a dummy top electrode.

14. The apparatus of claim 13, further comprising:

a first passivation layer at least partially on the piezoelectric actuator and the dummy actuator;

a second passivation layer on the first passivation layer; and

a connection line between the first passivation layer and the second passivation layer;

wherein the piezoelectric actuator comprises an actuator contact connected to a respective connection line through the first passivation layer; and

wherein the spacer structure comprises a dummy line between the portion of the first passivation layer covering the respective dummy actuator and the second passivation layer.

15. A pico projector apparatus, comprising:

a control unit;

a microelectromechanical mirror device according to claim 11, controlled by said control unit; and

a light source oriented towards the micro-electromechanical mirror and controlled by the control unit to generate a light beam based on an image to be generated.

16. A portable electronic device, comprising: a system processor and the pico projector apparatus of claim 15 coupled to the system processor.

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