Piezoelectric MEMS device with a suspended membrane having high mechanical shock resistance and manufacturing process thereof

Abstract

A MEMS device having a body with a first and a second surface, a first portion and a second portion. The MEMS device further has a cavity extending in the body from the second surface; a deformable portion between the first surface and the cavity; and a piezoelectric actuator arranged on the first surface, on the deformable portion. The deformable portion has a first region with a first thickness and a second region with a second thickness greater than the first thickness. The second region is adjacent to the first region and to the first portion of the body.

Claims

1. A microelectromechanical system (MEMS) device, comprising: a body having a first surface and a second surface, the body having a support portion and a deformable portion supported by the support portion extending between the first surface and a cavity that extends in the body from the second surface, the deformable portion includes: a stiffening element including: an end of in the cavity; a stiffening layer including a sidewall at the end, the sidewall is transverse to the first and second surfaces; and a coating layer between the stiffening layer and the support portion, the coating layer covering the sidewall of the stiffening layer; a piezoelectric actuator extending over the first surface of the body, the piezoelectric actuator is on the deformable portion; wherein the deformable portion has a first region with a first thickness in a thickness direction and a second region with a second thickness in the thickness direction, the second thickness is greater than the first thickness, the second region being adjacent to the first region of the deformable portion and being adjacent to the support portion of the body.

2. The device according to claim 1, wherein the deformable portion further comprises a structural layer on the stiffening element, the stiffening element having a through opening aligned with the cavity, the through opening and the cavity having respective areas in respective planes transverse to the thickness direction, the area of the through opening being smaller than the area of the cavity.

3. The device according to claim 2, wherein the stiffening element extends between the structural layer and the second surface of the body.

4. The device according to claim 3, wherein the body comprises: a substrate having a top delimited by a third surface and a bottom delimited by the second surface, the substrate being crossed by the cavity and forming the support portion of the body; and an insulating layer extending between the stiffening element and the structural layer, wherein the stiffening element extends on the third surface of the substrate, between the substrate and the insulating layer, and towards an inside of the cavity.

5. The device according to claim 4, wherein the coating layer is an insulating material, and the coating layer is interposed between the stiffening layer and the third surface of the substrate and delimiting at least in part an end of the cavity.

6. The device according to claim 5, wherein the stiffening layer is of polysilicon.

7. The device according to claim 1, wherein the piezoelectric actuator has an annular shape and exposes the deformable portion of the body.

8. The device according to claim 7, comprising a through hole extending through the first region of the deformable portion, the through hole being in communication with the cavity.

9. The device according to claim 1, wherein the device is a device chosen among: an autofocus device, a switch, a cantilever, and a manipulator device.

10. An electronic apparatus, comprising: an application-specific integrated circuit (ASIC); a memory block; an input/output interface; a microprocessor electrically coupled to the ASIC, the memory block, and the input/output interface; and a microelectromechanical system (MEMS) device coupled to the ASIC and including: a body having a first surface and a second surface, the body having a support portion and a deformable portion supported by the support portions extending between the first surface and a cavity that extends in the body from the second surface, the deformable portion including: a stiffening element including: an end in the cavity; a stiffening layer including a sidewall at the end, the sidewall is transverse to the first and second surfaces; and a coating layer between the stiffening layer and the support portion, the coating layer covering the sidewall of the stiffening layer; a piezoelectric actuator extending over the first surface of the body, on the deformable portion; wherein the deformable portion has a first region with a first thickness in a thickness direction and a second region with a second thickness greater than the first thickness, in the thickness direction, the second region being adjacent to the first region of the deformable portion and being adjacent to the support portion of the body.

11. The electronic apparatus according to claim 10, wherein the deformable portion further comprises a structural layer, the stiffening element having a through opening aligned with the cavity, the through opening and the cavity having respective areas in respective planes transverse to the thickness direction, the area of the through opening being smaller than the area of the cavity.

12. The electronic apparatus according to claim 10, wherein: the piezoelectric actuator has an annular shape and exposes the deformable portion of the body; and the body includes a through hole extending through the first region of the deformable portion, the through hole being in communication with the cavity.

13. A microelectromechanical system (MEMS) device, comprising: a central axis; a substrate including a first surface, a second surface opposite to the first surface, and a wall that is transverse to the first and second surfaces and that extends from the first surface to the second surface; a deformable portion including: a stiffening element on the second surface of the substrate, the stiffening element including: a stiffening layer including a sidewall spaced apart from the wall of the substrate, the sidewall of the stiffening layer is closer to the central axis than the wall; a coating layer on the sidewall of the stiffening layer and between the stiffening layer and the second surface of the substrate; a deformable element that extends inward from the stiffening element towards the central axis; a cavity delimited by the deformable portion and the substrate, the cavity extends into the first surface of the substrate to the deformable portion; and a piezoelectric actuator on the deformable portion, the piezoelectric actuator at least partially overlaps the stiffening element.

14. The device of claim 13, further comprising a through opening that extends through the stiffening element to the deformable element of the deformable portion, and the through opening is in fluid communication with the cavity.

15. The device of claim 14, wherein the cavity has a first dimension that extends across the cavity in a direction transverse to the wall of the substrate and the through opening has a second dimension that extends across the through opening in the direction, the first dimension being greater than the second dimension.

16. The device of claim 15, wherein the through opening has a first circular profile and cavity has a second circular profile, and the first dimension of the cavity is a first diameter and the second dimension of the through opening is a second diameter.

17. The device of claim 14, wherein a portion of the coating layer on the sidewall of the stiffening layer delimits the through opening.

18. The device of claim 13, wherein the stiffening element is in the cavity and the coating layer is exposed to the cavity.

19. The device of claim 13, wherein the deformable element includes an insulating layer on the stiffening element and extending across the cavity, the insulating layer being exposed to the cavity.

20. The device of claim 19, wherein the deformable element further includes a structural layer on the insulating layer and extending across the cavity, the structural layer is separated from the cavity by the insulating layer.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

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

(2) FIGS. 1A and 1B show simplified side views of a known MEMS piezoelectric actuator, in a rest condition and in a deformed condition, respectively;

(3) FIGS. 2A and 2B show simplified side views of another known MEMS piezoelectric actuator, used in an optical device, in a rest condition and in a deformed condition, respectively;

(4) FIG. 3 shows a simplified cross-section of a known MEMS piezoelectric actuator;

(5) FIG. 4 shows a cross-section of a radiofrequency switch including a known MEMS actuator;

(6) FIG. 5 shows a simplified cross-section of the present MEMS device according to one embodiment;

(7) FIG. 6 shows the MEMS device of FIG. 5 in top plan view;

(8) FIGS. 7-14 show successive steps of the manufacturing process of the MEMS device of FIGS. 5-6;

(9) FIG. 15 shows a simplified cross-section of the present MEMS device according to another embodiment;

(10) FIG. 16 shows the MEMS device of FIG. 15 in top plan view;

(11) FIG. 17 shows a simplified cross-section of the present MEMS device according to a further embodiment; and

(12) FIG. 18 shows a block diagram of an electronic apparatus including the MEMS device of FIGS. 5-6 and 15-16.

(13) FIGS. 5 and 6 show a MEMS device 130 according to an embodiment. In particular, the MEMS device 130 has a quadrangular shape in top plan view (FIG. 6), with center O lying on a central axis S parallel to an axis Z of the Cartesian reference system XYZ.

(14) With reference to FIG. 5, the MEMS device 130 comprises a body 131, extending in thickness in a thickness parallel to the axis Z and has a first and a second surface 131A, 131B. In particular, the body 131 comprises a substrate 132, of semiconductor material (e.g., silicon), delimited at the top by a third surface 132A and at the bottom by the second surface 131B. The substrate 132 has a cavity 134 extending throughout the entire thickness of the substrate 132; in particular, the cavity 134 is delimited at the side by a wall 134A. Moreover, the cavity 134 has, in top plan view (FIG. 6), a circular shape with center O and diameter D.sub.1.

(15) The body 131 further comprises a stiffening element 170 extending on the third surface 132A. In particular, the stiffening element 170 comprises a stiffening layer 171 of semiconductor (e.g., polysilicon), and a coating layer 172, of insulating material (e.g., silicon oxide).

(16) The stiffening element 170 has at the center an opening 170A, which is concentric with respect to the cavity 134 and has a diameter D.sub.2, smaller than the diameter D.sub.1 of the cavity 134. Consequently (FIG. 5), the stiffening element 170 projects, with respect to the substrate 132, towards the inside of the cavity 134, to form a first part of end surface 170B of the cavity 134.

(17) The coating layer 172 coats the stiffening layer 171 on the side facing the substrate 170 and fixes the stiffening element 170 to the substrate 132. Moreover, the coating layer 172 coats the inner wall of the opening 170A.

(18) The stiffening element 170 has a thickness (stiffening thickness T.sub.1), for example between 1 μm and 20 μm.

(19) The body 131 further comprises an insulating layer 139, for example of silicon oxide, extending on the third surface 132A; and a structural layer 141, for example of BPSG, extending on the insulating layer 139. In particular, the structural layer 141 is delimited at the top by the first surface 131A of the body 131, and the ensemble formed by the insulating layer 139 and the structural layer 141 has a structural thickness T.sub.2, for example comprised between 5 μm and 50 μm. Moreover, the portion of insulating layer 139 exposed by the opening 170A delimits the cavity 134 at the top, to form a second part of end surface 134A of the cavity 134.

(20) The coating layer 172, the stiffening layer 171, the insulating layer 139 and the structural layer 141 form a membrane 137 of variable thickness, suspended over the cavity 134. In particular, the membrane 137 has a first and a second portion 180, 181. The first portion 180 is formed only by the insulating layer 139 and by the structural layer 141, is surrounded by the second portion 181 of the membrane 137, and has a thickness equal to the structural thickness T.sub.2. The second portion 181 is formed by the insulating layer 139, the structural layer 141, the stiffening layer 171 and the coating layer 172, is arranged adjacent to a peripheral portion 145 of the body 131, and has a thickness T.sub.3 equal to the sum of the stiffening thickness T.sub.1 and the structural thickness T.sub.2.

(21) Moreover, as may be noted from the top plan view of FIG. 6, the membrane 137 has a circular shape with a diameter equal to the diameter D.sub.2 of the opening 170A.

(22) A piezoelectric actuator 150 having, in top plan view (FIG. 6), an annular shape with center O (and thus concentric with the cavity 134 and to the opening 170A) extends on the structural layer 141 and specifically on the first surface 131A of the body 131. In practice, in top plan view, the piezoelectric actuator 150 extends on the peripheral portion 145 of the body 131 and surrounds a central portion 143 of the body 131.

(23) In particular, the piezoelectric actuator 150 is formed by a stack of layers, comprising a first electrode 160, of conductive material; a piezoelectric material layer 161, for example PZT (Pb, Zr, TiO.sub.2), aluminum nitride (AlN), potassium-sodium niobate (KNN) or barium titanate (BaTiO.sub.3), extending on the first electrode 160; and a second electrode 162, of conductive material, extending on the piezoelectric material layer 161. In particular, the first and the second electrodes 160, 162 are electrically coupled to respective voltage sources (not illustrated) by conductive paths (not illustrated).

(24) Moreover, a dielectric layer (not illustrated) extends between the structural layer 141 and the first electrode 161 so as to physically and electrically isolate them from one another.

(25) In use, the MEMS device 130 operates according to the modalities described with reference to the MEMS device 30 of FIG. 3; in particular, the MEMS device 130 of FIGS. 5-6 is advantageously used, for example, in optical applications.

(26) The stiffening element 170 allows an increase in the resistance to mechanical shock of the membrane 137 of the MEMS device 130 in its peripheral portion (first portion 180) without increasing the thickness in the operative central region (second portion 181) of the membrane 137, and thus without reducing the performance of the MEMS device 130. In fact, the stiffening element 170 is of a material (here, polysilicon) capable of withstanding a high tensile stress (e.g., ranging between 1 GPa and 2 GPa). In this way, when a high force due to a mechanical shock (e.g., when the device is dropped) acts on the MEMS device 130, causing a high stress in the second portion 181 of the membrane 137, the stiffening element 170 is capable of absorbing said force (and, thus, the stress localized in the second portion 181) at least in part. In this way, it is possible to limit sharp deflection of the membrane 137 and prevent possible failures. In other words, the stiffening element 170 locally thickens the membrane 137 in the points of greater concentration of the mechanical stress.

(27) The manufacturing steps of the MEMS device 130 are illustrated schematically in FIGS. 7-14.

(28) Initially (FIG. 7), a protective layer 210, for example of silicon oxide, is thermally grown on a semiconductor wafer designed to form the substrate 132 of FIGS. 5-6, and thus designated by the same reference number, using known growth techniques. As a whole, the wafer thus formed is designated by the reference number 200. The wafer 200 has a bottom surface forming the second surface 131B of the body 131 of FIGS. 5-6.

(29) With reference to FIG. 8, selective portions of the wafer 200 are removed using known etching techniques (e.g., deep silicon etching), thus forming a side recess 215, which surrounds a projecting portion 220. In particular, the side recess 215 has, in top plan view, a circular shape of a diameter equal to the diameter D.sub.2 of the opening 170A illustrated in FIGS. 5-6, is delimited at the bottom by the third surface 132A of the substrate 132, and in practice defines the dimensions of the first portion 180. The side recess 215 in the first wafer 200 moreover has a first thickness equal to T.sub.1, i.e. equal to the stiffening element 170 to be formed.

(30) With reference to FIG. 9, a first etch-stop layer 225, for example of silicon oxide, is deposited on the surface of the wafer 200 for a thickness, for example, comprised between 0.6 μm and 1 μm; then, the first etch-stop layer 225 is subjected to thermal densification according to known techniques, thus forming (on the walls and on the bottom of the side recess 215) the coating layer 172 of the stiffening element 170. The first etch-stop layer 225 incorporates the protective layer 210 on the projecting portion 220 of the side recess 215.

(31) Next, FIG. 10, using known techniques, an epitaxial layer 230, filling the side recess 215, is grown on the surface of the wafer 200. The epitaxial layer 230 is grown for a thickness, in a direction parallel to the axis Z, for example, of approximately 30 μm.

(32) In FIG. 11, the epitaxial layer 230 is thinned out and planarized according to known techniques. In particular, the epitaxial layer 230 is reduced to a thickness greater by a few micrometers than the first thickness T.sub.1 using a grinding step. Then, the surface of the epitaxial layer 230 is polished using known techniques, such as CMP (Chemical-Mechanical Polishing). In this way, the thickness of the epitaxial layer 230 is further reduced by approximately 5 μm, up to approximately 10 μm, and planarized. In the polishing step, the first etch-stop layer 225 allows the polishing operation to stop, thus functioning as a hard stop. In this way, the remaining portions of the epitaxial layer 230 form the stiffening layer 171 of the stiffening element 170.

(33) With reference to FIG. 12, a second etch-stop layer (not illustrated) is deposited on the epitaxial layer 230 thus thinned out. Next, the second etch-stop layer is subjected to densification, thus forming the insulating layer 139 that, on the projecting portion 220, incorporates the first etch-stop layer 225.

(34) Next, FIG. 13, the structural layer 141 is grown on the surface of the insulating layer 139 for a thickness, for example, comprised between 2 μm and 30 μm in a per se known manner. Then, a dielectric layer (not illustrated) may be deposited on the structural layer 141 and subjected to densification in a per se known manner to form the insulator interposed between the structural layer 141 and the first electrode 162.

(35) Next, FIG. 14, the first electrode 160 is formed on the structural layer 141 using known deposition and masking techniques. Then, a piezoelectric layer and an electrode layer, which are defined by known masking and definition techniques s to form the piezoelectric material layer 161 and the second electrode 162, are deposited in succession. In this way, the piezoelectric actuator 150 is formed.

(36) Next, a mask layer (not illustrated) is deposited and patterned on the second surface 132B of the substrate 132, which is etched from the back using known photolithographic and etching techniques (e.g., through anisotropic etching, such as DRIE—Deep Reactive Ion Etching) so as to form the cavity 134 and the opening 170A, thus releasing the membrane 137.

(37) At the end of the process, the mask layer is removed, and the wafer 200 is diced so as to obtain the MEMS device 130 of FIGS. 5-6.

(38) In a variant of the manufacturing process of the stiffening element 170 illustrated in FIGS. 8-11, the stiffening element 170 of the acoustic transducer 130 is formed according to the manufacturing steps illustrated in FIGS. 8A and 9A. In particular, FIGS. 8A and 9A show manufacturing steps similar to the ones illustrated in FIGS. 8-11 so that parts similar to those illustrated and described with reference to FIGS. 8-11 are designated in FIGS. 8A and 9A by the same reference numbers increased by 1000 and will not be described any further.

(39) With reference to FIG. 8A, a portion of the protective layer 1210 is removed using known etching techniques (e.g., dry etching, wet etching, or sputter etching) so as to form part of the coating 1172, and thus an opening 1173 having a shape and area (in top plan view) corresponding to those of the opening 170A of FIG. 5.

(40) Next, FIG. 9A, using known techniques, a further epitaxial layer 1250 intended to form the stiffening layer 1171 is grown on the surface of the wafer 1200. In particular, the further epitaxial layer 1250 is grown on the coating 1172 and fills the opening 1173. The further epitaxial layer 1250 is grown for a thickness, in a direction parallel to the Z axis, greater by a few micrometers than the first thickness T.sub.1.

(41) Next, the further epitaxial layer 1250 is planarized and polished according to known techniques, in a way similar to what described with reference to FIG. 11.

(42) The further manufacturing steps are similar to the ones described in FIGS. 12-14. In particular, the coating 1172, formed in the step of FIG. 8A, functions as hard stop for the process of back etching described with reference to FIG. 14.

(43) FIGS. 15-16 show another embodiment of the present MEMS device. In detail, FIGS. 15-16 show a MEMS device 330 having a general structure similar to the MEMS device 130 of FIGS. 5-6, so that parts similar to the ones illustrated and described with reference to FIGS. 5-6 are designated in FIGS. 15-16 by the same reference numbers increased by 200 and will not be described any further.

(44) In particular, the MEMS device 330 has a through hole 385, extending through the structural layer 341 and the insulating layer 339 at the first portion 380 of the membrane 337. In particular, the through hole 385 has, in top plan view (FIG. 16), a circular shape, is concentric with the cavity 334 and the opening 370A, and has a diameter D.sub.3. In particular, the diameter D.sub.3 is smaller than the diameter D.sub.1 of the cavity 334 and the diameter D.sub.2 of the opening 370A.

(45) Moreover, in the present embodiment, the structural layer 341 may, for example, be of polysilicon, silicon, BPSG or metal (such as copper, Cu, aluminum, Al, platinum, Pt, gold, Au).

(46) An electrical contact (not illustrated) similar to the electrical contact 774 of FIG. 4) may be provided on the first portion 380 of the membrane 337.

(47) The MEMS device 330 may advantageously be used, for example, for acoustic applications (e.g., such as microphone), as a valve, or as an RF switch in a way similar to what discussed for the switch 770 of FIG. 4.

(48) The MEMS device 330 of FIGS. 15-16 is manufactured with a manufacturing process similar to the one described with reference to FIGS. 7-14. In particular, the through hole 385 is obtained by known photolithographic and etching techniques (e.g., Si dry etching, laser ablation, or cutting techniques such as sand-and-water cutting) at the end of the step described with reference to FIG. 14.

(49) FIG. 17 shows another embodiment of the present MEMS device. In detail, FIG. 17 shows a MEMS device 1330 having a general structure similar to the MEMS device 330 of FIGS. 15-16, so that parts that are similar to the ones illustrated and described with reference to FIGS. 15-16 are designated in FIG. 17 by the same reference numbers increased by 1000, and will not be described any further.

(50) In particular, the membrane 1337 forms a cantilever, suspended over the cavity 1334. Moreover, in the present embodiment, the membrane 1337 has, for example, a quadrangular (e.g., rectangular) shape in top plan view (not illustrated); in addition, the piezoelectric actuator 1350 has, for example, a quadrangular (e.g., rectangular) shape in top plan view (not illustrated).

(51) In use, the MEMS device 1330 operates according to the operating modalities described with reference to the MEMS devices 130, 330 of FIGS. 5-6 and 15-16.

(52) Moreover, the MEMS device 1330 is obtained in a way similar to what described with reference to the manufacturing steps illustrated in FIGS. 7-14.

(53) FIG. 18 shows an electronic device 500 that uses the MEMS device 130, 330 according to one of the embodiments illustrated in FIGS. 5-6 and 15-16, respectively. In particular, hereinafter the MEMS device 130, 330 is designated as a whole in FIG. 18 by the reference number 530.

(54) The electronic device 500 comprises, in addition to the MEMS device 530, a microprocessor (CPU) 501, a memory block 502, connected to the microprocessor 501, and an input/output interface 503, for example a keyboard and/or a display, also connected to the microprocessor 501. An application-specific integrated circuit (ASIC) 504 may be integrated in the MEMS device 530 or, as illustrated in FIG. 17, may be arranged outside the MEMS device 530 and operatively coupled thereto.

(55) The MEMS device 530 communicates with the microprocessor 701 via the ASIC 504.

(56) The electronic device 500 is, for example, a mobile communication device, such as a mobile phone or smartphone, a PDA, or a computer, but may also be a voice recorder, a player of audio files with voice-recording capacity, a console for video games, and the like.

(57) The present MEMS device and the manufacturing process thereof have various advantages.

(58) In particular, the presence of the stiffening element 170, 370 allows to reduce the impact of mechanical shock, for instance when the MEMS device 30, 330 is dropped. In particular, the stiffening element 170, 370 is arranged in the second portion 181, 381 of the membrane 137, 337, where there is a high stress in the case of a high force. In addition, the stiffening element 170, 370 is of a material capable of withstanding high tensile stresses (e.g., polysilicon). In this way, in the presence of mechanical shocks, the deflection of the membrane 137, 337 of the MEMS device 30, 330 is limited in so far as the stiffening element 170, 370 (and thus the second portion 181, 381 of the membrane 137, 337) is able to absorb at least in part the force (and, thus, the stress) and consequently reduce the risk of failure or weakening of the membrane due to mechanical shock.

(59) Moreover, the present MEMS device 130, 330 is manufactured according to a simple and far from costly manufacturing process.

(60) Finally, it is clear that modifications and variations may be made to the MEMS device and to the manufacturing process described and illustrated herein, without thereby departing from the scope of the present disclosure, as defined in the attached claims.

(61) For instance, the stiffening element 170, 370, 1370 may be provided in MEMS devices of the type described in U.S. Patent Publication No. 2018/0190895, which describes a piezoelectric micro-actuator formed by a beam element of semiconductor material and by a piezoelectric region, extending over the beam. In particular, one end of the beam element is fixed and may be provided with the stiffening element 180, 380, 1380; the other end is connected to a hinge element of a constraint structure that is not deformable in the thickness direction of the beam.

(62) Moreover, the present stiffening element 170, 370, 1370 may be provided in MEMS devices of the type described in U.S. Patent Publication No. US2019/0240844, which describes a MEMS device of a piezoelectric type having a first and a second manipulation arm formed by a control arm and an articulated arm. The control arm of both of the manipulation arms may be provided with the stiffening element described herein.

(63) The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.