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 method, comprising: forming a piezoelectric actuator on a first surface of a body of semiconductor material further having a second surface, a first portion, and a second portion; and removing selective portions of the body from the second surface of the body to form a cavity extending in the body, and to form a deformable portion at the first portion of the body, the deformable portion including a first region having a first thickness in a thickness direction and a second region having 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 first portion of the body.

2. The method according to claim 1, comprising, prior to forming the piezoelectric actuator, forming the body by forming a stiffening element and a structural layer on a substrate of semiconductor material, and wherein removing selective portions of the body comprises removing portions of the substrate to form the cavity and a through opening in the stiffening element, the through opening and the cavity being mutually aligned and contiguous and having respective areas in in respective planes transverse to the thickness direction, the area of the through opening being smaller than the area of the cavity.

3. The method according to claim 2, wherein forming the stiffening element comprises: removing selective portions of the substrate to form a recess and a projecting portion defined by the recess; forming an etch-stop layer on side surfaces of the projecting portion and in the recess; and forming a stiffening layer in the recess and at sides of the projecting portion.

4. The method according to claim 3, wherein forming the stiffening layer comprises growing an epitaxial layer and thinning the epitaxial layer as far as the projecting portion.

5. The method according to claim 3, further comprising: forming an insulating layer on the projecting portion and on the stiffening element; and forming the structural layer on the insulating layer. comprises: opening; and

6. The method according to claim 2, wherein forming the stiffening element forming a etch-stop layer on a surface of the substrate; removing a portion of the etch-stop layer to form a coating layer that delimits an forming a stiffening layer on the coating layer.

7. The method according to claim 6, wherein forming the stiffening layer comprises growing an epitaxial layer on the coating layer and on the substrate, the epitaxial layer filling the opening of the coating layer.

8. The method according to claim 1, wherein forming the piezoelectric actuator comprises patterning a stack of layers into an annular shape, the piezoelectric actuator exposing the second portion of the body.

9. A method, comprising: forming a protective layer on a first surface of a substrate; forming a side recess and forming a projecting portion at the surface of the substrate surrounded by the side recess by removing selective portions of the substrate and the protective layer; forming an etch stop layer including a portion of the protective layer by forming an etch stop material along a sidewall of the projecting portion and in the side recess; forming an epitaxial layer on the etch stop layer, in the side recess, and overlapping the projecting portion; removing a portion of the epitaxial layer exposing a portion of the etch stop layer on the projecting portion; forming a plurality of layers on a portion of the epitaxial layer in the side recess and on the surface of the projecting portion; forming a piezoelectric actuator on the plurality of layers; and forming a cavity extending into a second surface of the substrate opposite to the first surface of the substrate and extending to the plurality of layers.

10. The method of claim 9, wherein forming the plurality of layers includes: forming an insulating layer on the epitaxial layer in the side recess and on the surface of the projecting region; and forming a structural layer on the insulating layer.

11. The method of claim 10, wherein forming the piezoelectric actuator on the plurality of layers includes forming the piezoelectric actuator on the structural layer.

12. The method of claim 9, wherein forming the cavity includes: removing the projecting region; and removing a portion of the substrate overlapped by the projecting region and overlapped by the etch stop layer.

13. The method of claim 12, wherein removing the portion of the substrate overlapped by the projecting region and overlapped by the etch stop layer further includes forming a sidewall of the substrate that delimits that cavity.

14. The method of claim 12, wherein removing the projecting region and removing the portion of the substrate overlapped by the projecting region and overlapped by the etch stop layer exposes the etch stop layer.

15. The method of claim 12, further comprising forming a through hole extending through the plurality of layers to the cavity.

16. The method of claim 9, wherein forming the insulating layer further includes forming the insulating layer to include the portion of the etch stop layer.

17. A method, comprising: forming a protective layer on a first surface of a substrate; exposing a region of the first surface of the substrate by forming an opening through the protective layer; forming an epitaxial layer on the region of the first surface of the substrate, in the opening, and on the protective layer; forming a plurality of layers on a surface of the epitaxial layer; forming a piezoelectric actuator on the plurality of layers; and forming a cavity extending into a second surface of the substrate opposite to the first surface of the substrate and extending to the plurality of layers.

18. The method of claim 17, wherein forming the plurality of layers includes: forming an insulating layer on the epitaxial layer; and forming a structural layer on the insulating layer.

19. The method of claim 18, wherein forming the piezoelectric actuator on the plurality of layers further includes forming the piezoelectric actuator on the structural layer.

20. The method of claim 17, wherein forming the cavity further includes: forming a sidewall of the substrate that delimits the cavity; and exposing a surface of the protective layer that delimits the cavity.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0039] 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:

[0040] FIGS. 1A and 1B show simplified side views of a known MEMS piezoelectric actuator, in a rest condition and in a deformed condition, respectively;

[0041] 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;

[0042] FIG. 3 shows a simplified cross-section of a known MEMS piezoelectric actuator;

[0043] FIG. 4 shows a cross-section of a radiofrequency switch including a known MEMS actuator;

[0044] FIG. 5 shows a simplified cross-section of the present MEMS device according to one embodiment;

[0045] FIG. 6 shows the MEMS device of FIG. 5 in top plan view;

[0046] FIGS. 7-14 show successive steps of the manufacturing process of the MEMS device of FIGS. 5-6;

[0047] FIG. 15 shows a simplified cross-section of the present MEMS device according to another embodiment;

[0048] FIG. 16 shows the MEMS device of FIG. 15 in top plan view;

[0049] FIG. 17 shows a simplified cross-section of the present MEMS device according to a further embodiment; and

[0050] FIG. 18 shows a block diagram of an electronic apparatus including the MEMS device of FIGS. 5-6 and 15-16.

[0051] 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.

[0052] 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.

[0053] 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).

[0054] 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.

[0055] 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.

[0056] The stiffening element 170 has a thickness (stiffening thickness T.sub.1), for example between 1 μm and 20 μm.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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).

[0062] 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.

[0063] 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.

[0064] 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.

[0065] The manufacturing steps of the MEMS device 130 are illustrated schematically in FIGS. 7-14.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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).

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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).

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] The MEMS device 530 communicates with the microprocessor 701 via the ASIC 504.

[0094] 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.

[0095] The present MEMS device and the manufacturing process thereof have various advantages.

[0096] 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.

[0097] Moreover, the present MEMS device 130, 330 is manufactured according to a simple and far from costly manufacturing process.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.