Piezoelectric MEMS devices and methods of forming thereof

In a non-limiting embodiment, a device may include a substrate, and a hybrid active structure disposed over the substrate. The hybrid active structure may include an anchor region and a free region. The hybrid active structure may be connected to the substrate at least at the anchor region. The anchor region may include at least a segment of a piezoelectric stack portion. The piezoelectric stack portion may include a first electrode layer, a piezoelectric layer over the first electrode layer, and a second electrode layer over the piezoelectric layer. The free region may include at least a segment of a mechanical portion. The piezoelectric stack portion may overlap the mechanical portion at edges of the piezoelectric stack portion.

TECHNICAL FIELD

The present disclosure relates generally to microelectromechanical system (MEMS) devices and methods of forming MEMS devices.

BACKGROUND

Piezoelectric MEMS devices such as a MEMS microphone, may employ a piezoelectric thin film to convert acoustic waves to electric signals. However, existing process integration and structure cannot meet high yield due to limitations in the capability of the deposition tool for forming the MEMS device. For example, due to limitations in the deposition process and film residue, the piezoelectric thin film has a high vertical stress gradient and poor stress uniformity as well as high deflection of the cantilever or membrane structure, which impacts device performance (e.g., sensitivity) significantly. In addition, cantilever/membrane in existing MEMS microphone which is formed of piezoelectric thin film and electrode film exhibits high deflection and mismatch due to non-uniform distribution of stress in the piezoelectric film and/or large membrane size. The membrane size may be reduced in an attempt to reduce deflection and mismatch which may facilitate on low frequency response, however the sensitivity of the MEMS microphone would be undesirably lowered, and when the sensitivity of the MEMS device is too low, it is not suitable for most applications. In some cases, a bi-layer piezoelectric film with different stress may be employed to mitigate the vertical stress gradient, however such design is not good enough, particularly in the case when cantilever/membrane length is longer.

From the foregoing discussion, it is desirable to provide MEMS devices having an active structure for sensing with high sensitivity and with reduced deflection.

SUMMARY

Embodiments generally relate to MEMS devices and methods for forming the MEMS devices. According to various non-limiting embodiments, a device may include a substrate, and a hybrid active structure disposed over the substrate. The hybrid active structure may include an anchor region and a free region. The hybrid active structure may be connected to the substrate at least at the anchor region. The anchor region may include at least a segment of a piezoelectric stack portion. The piezoelectric stack portion may include a first electrode layer, a piezoelectric layer over the first electrode layer, and a second electrode layer over the piezoelectric layer. The free region may include at least a segment of a mechanical portion. The piezoelectric stack portion may overlap the mechanical portion at edges of the piezoelectric stack portion.

According to various non-limiting embodiments, a method of forming the MEMS device is provided. The method may include providing a substrate, and arranging a hybrid active structure over the substrate. The hybrid active structure may include an anchor region and a free region. The hybrid active structure may be connected to the substrate at least at the anchor region. The anchor region may include at least a segment of a piezoelectric stack portion. The piezoelectric stack portion may include a first electrode layer, a piezoelectric layer over the first electrode layer, and a second electrode layer over the piezoelectric layer. The free region may include at least a segment of a mechanical portion. The piezoelectric stack portion may overlap the mechanical portion at edges of the piezoelectric stack portion.

DETAILED DESCRIPTION

As used herein, the term “connected,” when used to refer to two physical elements, means a direct connection between the two physical elements. The term “coupled,” however, can mean a direct connection or a connection through one or more intermediary elements.

Embodiments of the present disclosure generally relate to microelectromechanical (MEMS) devices. In various non-limiting embodiments, the MEMS device may include a hybrid active structure. The hybrid active structure may include a piezoelectric stack portion and a mechanical portion. The piezoelectric stack portion may include one or more piezoelectric layers, each of which may be disposed in between two electrode layers. The piezoelectric stack portion may overlap the mechanical portion at edges of the piezoelectric stack portion. For example, the hybrid active structure may have a cantilever (or beam) or membrane (or diaphragm) configuration. In various non-limiting embodiments, the MEMS device may include an anchor region which includes at least a segment of the piezoelectric stack portion, and a free region which includes at least a segment of the mechanical portion. For example, the piezoelectric stack portion may be arranged nearer to (and/or at) the anchor region, while the mechanical portion may be arranged nearer to (and/or at) the free region of the MEMS device. The MEMS device having the hybrid active structure, according to various embodiments of the present invention, allows effective sensing and generation of electrical signal by the piezoelectric stack portion (the piezoelectric stack portion is arranged nearer to the anchor region and stress is concentrated nearer to the anchor region), while reducing or eliminating deflection of the cantilever or membrane by providing at least a segment of the mechanical portion (e.g., non-piezoelectric material with low stress gradient) in the free region of the hybrid active structure without trade-off on device performance.

The hybrid active structure may be incorporated into or used with various types of MEMS devices, such as a microphone, PMUT, resonator, energy harvester, pressure sensor, accelerometer, in various non-limiting examples. The MEMS device may advantageously reduce deflection in the active structure having one or more of the piezoelectric layers (i.e., hybrid active structure), without trade-off on device performance. Further, the hybrid active structure may be able to maintain the same area of the cantilever/membrane in the MEMS device to achieve high sensitivity, and reduce deflection effectively which advantageously enables both high yield and high sensitivity of the device.

FIG.1shows a simplified cross-sectional view of an embodiment of a device100. In various non-limiting embodiments, the device100may be, or include, a MEMS device. The MEMS device may include a substrate105. The substrate105may be a semiconductor substrate, such as a silicon substrate. Other types of semiconductor substrates, such as a silicon germanium substrate, may also be used.

A hybrid active structure110may be disposed over the substrate105. In various non-limiting embodiments, the hybrid active structure110may be used to generate electrical signals by piezoelectric effect. For example, the hybrid active structure may convert acoustic waves into electrical signals. For example, the hybrid active structure110may have a cantilever or membrane configuration. In various non-limiting embodiments, the hybrid active structure110may include a piezoelectric stack portion120and a mechanical portion130. For example, the piezoelectric stack portion120may serve as a sensing part of the hybrid active structure110to generate electrical signal by piezoelectric effect while the mechanical portion130may provide an area for applying pressure or for tuning stiffness of the hybrid active structure110.

The piezoelectric stack portion120may include one or more piezoelectric layers. Each piezoelectric layer of the one or more piezoelectric layers may be disposed between two electrode layers. In various non-limiting embodiments, the piezoelectric stack portion120may include a first electrode layer122, a piezoelectric layer124over the first electrode layer122, and a second electrode layer126over the piezoelectric layer124. The first electrode layer122, for example, may be a bottom electrode while the second electrode layer126may be a top electrode of the piezoelectric stack portion120. In other embodiments, the piezoelectric stack portion120may include a further piezoelectric layer127over the second electrode layer126, and a further electrode layer128over the further piezoelectric layer127. For example, the first electrode layer122may be a bottom electrode, the second electrode layer126may be a middle electrode, and the further electrode layer128may be a top electrode of the piezoelectric stack portion120. In yet other embodiments, the piezoelectric stack portion120may include any number of piezoelectric layers and electrode layers. In various non-limiting embodiments, the piezoelectric stack portion120may further include a seed piezoelectric layer121. The seed piezoelectric layer121may be disposed under the first electrode layer122. The seed piezoelectric layer121, for example, may facilitate the electrode layer and device layer (e.g., piezoelectric layer) arranged over the seed piezoelectric layer121having a good crystal orientation.

In a non-limiting example, the one or more piezoelectric layers may be formed of a piezoelectric material, including but not limited to, aluminum nitride (AlN), scandium-doped AlN (ScAlN), germanium-doped AlN (GeAlN), titanium-doped AlN (TiAlN), piezoelectric ceramic lead zirconate and titanate (PZT), zinc oxide (ZnO), or combinations thereof. In a non-limiting example, the electrode layers may be formed of an electrically conductive material, including but not limited to, molybdenum (Mo), platinum (Pt), titanium (Ti), or combinations thereof. As for the mechanical portion130, it may be formed of a non-piezoelectric material such as silicon, polysilicon, silicon nitride, silicon carbide, polymers, in a non-limiting example. The non-piezoelectric material of the mechanical portion130may be chosen such that it is able to withstand a release etch process (e.g., hydrofluoric acid (HF) etching). For example, in the case an oxide layer is used as a sacrificial layer in the fabrication of the hybrid active structure110of the MEMS device, a non-oxide and non-piezoelectric material may be used for the mechanical portion130. Additionally, the mechanical portion130may be formed of a non-piezoelectric material that exhibits a low stress gradient. This advantageously ensures reduced or no deflection of the mechanical portion130of the hybrid active structure110during operation.

As illustrated inFIG.1, the hybrid active structure110may include an anchor region115and a free region117. The hybrid active structure110may be connected to the substrate105at least at the anchor region115. The anchor region115may include at least a segment of the piezoelectric stack portion120. The free region117may include at least a segment of the mechanical portion130. For example, the mechanical portion130may form a free end or center of a cantilever/membrane, in various non-limiting embodiments of the MEMS device.

As illustrated inFIG.1, the piezoelectric stack portion120overlaps the mechanical portion130. In various non-limiting embodiments, the piezoelectric stack portion120overlaps the mechanical portion130at edges of the piezoelectric stack portion120. In various non-limiting embodiments, the piezoelectric stack portion120may abut (or contacts) the mechanical portion130in the areas of overlap or overlap region150. For example, the piezoelectric stack portion120overlaps the mechanical portion130without an additional adhesive layer therebetween (e.g., without additional layer to the one or more piezoelectric layers and electrode layers). In various non-limiting embodiments, a piezoelectric layer (e.g., seed piezoelectric layer121) of the piezoelectric stack portion120contacts the mechanical portion130in the areas of overlap between the piezoelectric stack portion120and the mechanical portion130(overlap region150of the piezoelectric stack portion120and the mechanical portion130). In the case where a seed piezoelectric layer121is not arranged in the piezoelectric stack portion120, an electrode layer (e.g., first electrode layer122) of the piezoelectric stack portion120contacts the mechanical portion130in the areas of overlap between the piezoelectric stack portion120and the mechanical portion130(overlap region150of the piezoelectric stack portion120and the mechanical portion130).

In various non-limiting embodiments, an edge of the mechanical portion130in the overlap region150of the hybrid active structure110may have a sloped portion with an angled/sloped profile and a substantially flat portion with a substantially flat profile. For example, the sloped portion with the angled profile may range from about 5° to about 45° with respect to the substantially flat portion. The piezoelectric stack portion120may have a profile which is conformal to the profile of the mechanical portion130in the overlap region150. In various non-limiting embodiments, the piezoelectric stack portion120may overlap the mechanical portion130at the free region117by a predetermined length l. The predetermined length l may range from about 1 um to about 50 um, in a non-limiting example. Providing the mechanical portion130with the sloped portion having the angled profile allows good coverage of the piezoelectric stack portion120in the overlap region150. This advantageously obviates mechanical reliability issue in the fabricated hybrid active structure110of the MEMS device.

In various non-limiting embodiments, the anchor region115may further include interconnects and bond pads. For example, one or more via contacts160may be disposed through the piezoelectric stack portion120. The one or more via contacts160may provide electrical connection to the electrode layers in the piezoelectric stack portion120. For example, the one or more via contacts160may electrically connect the first electrode layer and the second electrode layer122and126. The one or more via contacts160may be electrically connected to one or more bond pads162.

In various non-limiting embodiments, the MEMS device may further include a cavity175arranged under the hybrid active structure110. In other embodiments, a cavity may not be provided under the hybrid active structure110. In various non-limiting embodiments, the MEMS device may further include a sacrificial layer180. For example, the sacrificial layer may be remaining sacrificial material after a release process of the hybrid active structure in the fabrication of the MEMS device. The sacrificial layer may be disposed between the hybrid active structure110and the substrate105. The sacrificial layer may be an oxide layer, in a non-limiting example.

It is noted that stress of a cantilever/membrane of a MEMS device, for example, due to pressure/vibration may be concentrated nearer to the anchor region115of the cantilever/membrane, while the free region117does not contribute to generation of electrical signal by piezoelectric effect. In addition, deflections are higher at the free region of conventional cantilever/membrane structures, and particularly highest at the tip (e.g., centermost area) of the cantilever/membrane. Accordingly, the MEMS device having the hybrid active structure110according to various embodiments of the present invention allows effective sensing and generation of electrical signal by the piezoelectric stack portion120(since stress is concentrated nearer to the anchor region115and the piezoelectric stack portion120is arranged nearer to the anchor region115), while reducing or eliminating deflection of the cantilever/membrane by providing at least a segment of the mechanical portion130(e.g., non-piezoelectric material with low stress gradient) in the free region117of the hybrid active structure110. Further, deflection of the cantilever/membrane may be advantageously reduced while maintaining sensitivity and without reducing the size or length of the cantilever/membrane of the MEMS device. The MEMS device having the hybrid active structure110may be able to gain the same signal compared to MEMS device using conventional piezoelectric layer since stress is concentrated nearer to the anchor region and the hybrid active structure110has the piezoelectric stack portion120arranged nearer to and/or at the anchor region115. Further, the overall deflection of the hybrid active structure110may be tunable by configuring the length and thickness of the mechanical portion130in the free region117. For example, the length and thickness of the mechanical portion130in the free region117may be configured depending on the desired stiffness of the hybrid active structure110to meet different application requirements (e.g., various Acoustic Overload Point).

FIGS.2A-2Fshow simplified top views of various exemplary embodiments of the device100. The device100may include the hybrid active structure110as described with respect toFIG.1.

FIGS.2A-2Dshow an exemplary top views of the hybrid active structure110with a cantilever configuration. According to various non-limiting embodiments, the hybrid active structure110may be configured to be a plurality of cantilevered beams (only one beam is illustrated). For example, each beam may be connected to the substrate105at least at one end or edge of the beam. Referring toFIGS.2A-2B, the hybrid active structure110may be connected to the substrate105at one of its end. As illustrated, the mechanical portion130forms a free end of the cantilevered beam. In other words, a free region117of the beam may be formed by the mechanical portion130. Referring toFIGS.2C-2D, the hybrid active structure110may be connected to the substrate105at both of ends. For example, the piezoelectric stack portion includes a first piezoelectric stack portion1201and a second piezoelectric stack portion1202respectively anchored or connected to the substrate105. At least a segment of the mechanical portion130may be disposed in between the first piezoelectric stack portion1201and the second piezoelectric stack portion1202. In various non-limiting embodiments, the sidewalls of the piezoelectric stack portion120and the mechanical portion130may be aligned, as illustrated inFIGS.2A and2C. In other embodiments, the mechanical portion130may further surround sidewalls of the piezoelectric stack portion120or extend over the sidewalls of the piezoelectric stack portion120(e.g., the mechanical portion130having a higher width relative to the piezoelectric stack portion120), as illustrated inFIGS.2B and2D.

FIGS.2E-2Fshow an exemplary top views of the hybrid active structure110with a membrane configuration. As illustrated, the mechanical portion130forms a center of the membrane. The piezoelectric stack portion120surrounds the mechanical portion130. For example, the piezoelectric stack portion120may be arranged at the edge of the hybrid active structure110(e.g., anchor region) while the mechanical portion130may be arranged at the center of the hybrid active structure110(e.g., free region117). Referring toFIG.2F, the piezoelectric stack portion120may not completely surround the mechanical portion130. In a non-limiting embodiment, the hybrid active structure110forms a diaphragm of an acoustic sensor.

FIGS.3A-3Mshow simplified cross-sectional views of an embodiment of a process300for forming a device. According to various non-limiting embodiments, the process forms a hybrid active structure of a MEMS device. The device formed, for example, is similar or the same as that shown and described inFIG.1andFIGS.2A-2F. As such, common elements may not be described or described in detail.

In various non-limiting embodiments, a wafer or substrate105may be provided. The substrate may be a semiconductor substrate, such as a silicon substrate. Other types of semiconductor substrates, such as a silicon germanium substrate, may also be used. In various non-limiting embodiments, a sacrificial layer307may be formed on the substrate105. The sacrificial layer307may be formed by performing thermal oxidation on the substrate105, in a non-limiting embodiment. In various non-limiting embodiments, the sacrificial layer307may be an oxide layer, such as silicon oxide, in a non-limiting example. The sacrificial layer307may have a thickness ranging from about 0.1 um to about 5 um, in a non-limiting example.

A hybrid active structure may be arranged (or formed) over the substrate105. Referring toFIG.3A, a non-piezoelectric layer330may be deposited over the substrate105for forming a mechanical portion of the hybrid active structure. In various non-limiting embodiments, the non-piezoelectric layer330may be a non-piezoelectric material as described above. The non-piezoelectric layer330may be formed over the substrate105by deposition such as physical vapor deposition, in a non-limiting example. The non-piezoelectric layer330may have a thickness ranging from about 0.1 um to about 5 um in a non-limiting example. An anneal may be performed to reduce the stress gradient in the non-piezoelectric layer330. In other embodiments, the substrate105may be a crystalline-on-insulator (COI) substrate, such as a silicon-on-insulator (SOI) substrate, in a non-limiting example. A COI substrate includes a surface crystalline layer separated from a bulk crystalline by an insulator layer. The insulator layer, for example, may be formed of a dielectric insulating material, such as silicon oxide. In the case a COI substrate is used, the insulator layer of the COI substrate may be used as the sacrificial layer307, while the surface crystalline layer may be used as the non-piezoelectric layer330. An anneal need not be performed in the case the COI substrate is used.

The non-piezoelectric layer may be patterned to form the mechanical portion130of the hybrid active structure. As illustrated inFIG.3B, the non-piezoelectric layer330may be patterned to form sloped portions and substantially flat portions. For example, a sloped portion may have an angled profile and a substantially flat portion may have a substantially flat profile. The non-piezoelectric layer may be patterned by mask and etch techniques, in a non-limiting embodiment. For example, the non-piezoelectric layer may be patterned such that the sloped portion may have an angled profile (or sloped profile) ranging from about 5° to about 45° with respect to a substantially planar (or flat) top surface of the non-piezoelectric layer. The sacrificial layer307may serve as an etch stop layer for patterning the non-piezoelectric layer.

A piezoelectric stack portion may be formed over the patterned mechanical portion130of the hybrid active structure.FIGS.3C-3Fillustrate various exemplary piezoelectric and electrode layers deposited over the substrate105for forming the piezoelectric stack portion. Referring toFIG.3C, a first electrode layer122may be deposited over the substrate105. The first electrode layer122may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. The first electrode layer122may be patterned, for example, using mask and etch techniques to form a bottom electrode of the piezoelectric stack portion. In some embodiments, a seed piezoelectric layer121may be deposited over the substrate105prior to deposition of the first electrode layer122. The seed piezoelectric layer121may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. In other embodiments, the seed piezoelectric layer121may not be required for forming the piezoelectric stack portion.

A piezoelectric layer124may be deposited over the substrate105, as illustrated inFIG.3D. The piezoelectric layer124may have a thickness ranging from about 100 nm to about 1000 nm, in a non-limiting example. In various non-limiting embodiments, a second electrode layer126may be deposited over the piezoelectric layer124. The second electrode layer126may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. As illustrated inFIG.3E, the second electrode layer126may be patterned, for example, using mask and etch techniques to form a top electrode or middle electrode of the piezoelectric stack portion. In various non-limiting embodiments, a further piezoelectric layer127may be deposited over the second electrode layer126, and a further electrode layer128may be deposited over the further piezoelectric layer127. The further piezoelectric layer127may have a thickness ranging from about 100 nm to about 1000 nm, in a non-limiting example. The further electrode layer128may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. The further electrode layer128may be patterned, for example, using mask and etch techniques to form a top electrode of the piezoelectric stack portion in the case the second electrode layer126serves as the middle electrode. The further piezoelectric layer127and the further electrode layer128may be optional depending on the design of the piezoelectric stack portion and may not be required in some embodiments. As illustrated, the layers of the piezoelectric stack portion may have a profile which is conformal to the profile of the mechanical portion130. A passivation layer340may be deposited over the layers of the piezoelectric stack portion. The passivation layer340may be used to protect the electrode in the piezoelectric stack portion (e.g., top electrode) during and after the device release process. The passivation layer340may be, or include, AlN, silicon nitride, polysilicon, aluminum, titanium nitride, etc., in a non-limiting example. The passivation layer340may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. In various non-limiting embodiments, a sacrificial layer such as a TEOS layer, in a non-limiting example, may be formed over the passivation layer340to protect the passivation layer340before the device release process (not shown). The sacrificial layer over the passivation layer340may be removed during the device release. The sacrificial layer may have a thickness ranging from about 20 nm to about 200 nm, in a non-limiting example. In other embodiments, the sacrificial layer may not be required.

A first via opening350may formed in the piezoelectric stack portion, as illustrated inFIG.3G. For example, the first via opening350may be patterned by a mask and etch technique, using the second electrode layer126as an etch stop layer. A second via opening355may be formed in the piezoelectric stack portion, as illustrated inFIG.3H. For example, the second via opening355may be patterned by a mask and etch technique, using the first electrode layer122as an etch stop layer. In various non-limiting embodiments, an opening360may be formed in the center of the piezoelectric stack portion. For example, the opening360may be formed simultaneously during patterning of the second via opening355(e.g., higher etch rate at a bigger pattern for forming opening360). The patterning for the opening360may stop upon reaching the mechanical portion. The process continues with patterning of the piezoelectric stack portion to form an opening for a bond pad (not shown).

A conductive layer may be deposited over the substrate105to form a bond pad and interconnect. The conductive layer may have a thickness ranging from about 100 nm to about 1000 nm, in a non-limiting example. The conductive layer may be Al, in a non-limiting example. The conductive layer may be patterned to form a bond pad370and interconnect372.

The process then continues to pattern the top electrode (e.g., denoted by a gap380) of the piezoelectric stack portion depending on the design and electrical connection of the top electrode, as illustrated inFIG.3I. In various non-limiting embodiments, the mechanical portion130of the hybrid active structure may be further patterned. For example, in the case the hybrid active structure serves as a plurality of cantilevers, the mechanical portion130may be patterned to form a gap382.

In various non-limiting embodiments, a cavity may be formed in the substrate105below the hybrid active structure. As illustrated inFIG.3J, a protective layer385may be deposited over the substrate105to protect the piezoelectric stack portion and for wafer bow control prior to forming the cavity. In a non-limiting example, the protective layer385may be a thick oxide layer. For example, the protective layer385may have a thickness ranging from about 500 nm to about 5000 nm. The substrate may be then flipped over for backgrinding a bottom surface of the substrate in the backgrinding is required. Backgrinding may be optional depending on any packaging requirements. For example, the substrate105may be backgrinded to a final thickness of about 200 um to about 600 um.

The substrate105may be patterned to form a cavity390, as illustrated inFIG.3L. The cavity390may be formed by patterning the bottom surface of the substrate using mask and etch techniques. For example, a patterned hard mask may be used to expose an area of the substrate corresponding to the cavity to be formed and the substrate may be etched by a deep reactive-ion etching (DRIE) to form the cavity390. The sacrificial layer307may be used as etch stop layer for the etching. In other embodiments, the MEMS device may be fabricated without forming the cavity below the hybrid active structure. For example, the process steps as described with respect toFIGS.3J-3Lmay not be required.

A release etch process may be performed to release the hybrid active structure110of the MEMS device. The release etch process may be a vapor release etch (e.g., using HF etching) to remove a portion of the sacrificial layer307exposed by the cavity390, releasing the hybrid active structure110over the substrate as illustrated inFIG.3M.