Piezoelectric micro-electro-mechanical device and manufacturing method thereof

Abstract

According to an example aspect of the present invention, a method for manufacturing a piezoelectric MEMS transducer (1) is presented, the method comprising forming at least one cantilever (10) from a silicon-on-insulator (SOI) wafer, where the at least one cantilever (10) comprises a first sidewall, a second sidewall substantially parallel to the first sidewall, an aluminum nitride (AlN) layer (40) and a top electrode (50), and where the at least one cantilever (10) comprises a second sidewall substantially parallel to the first sidewall, the second sidewall, the AlN layer (40) and the top electrode (50) are electrically connected to each other. The top electrode (50) is connected to at least the first sidewall and / or the second sidewall via an AlN layer (40) therebetween.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the field of micro- and nano-electromechanical systems (MEMS, NEMS) and piezoelectric actuation. Generally, the invention relates to micro-electromechanical systems (MEMS). In particular, but not exclusively, the invention relates to inertial MEMS and electromechanical transducers in the field of energy harvesting. BACKGROUND

[0002] Three-dimensional motion (3D motion) represents a finite motion without obstruction in the x, y and z axis directions, where the x, y (lateral) axes lie in the plane of the device and the z axis lies in the out-of-plane (vertical) direction. Compact actuator designs and fabrication enabling full 3D motion at the micro- and nanometer scale are complex. Micro-electromechanical systems (MEMS) enabling 3D sensing and actuation typically incorporate several devices. Each of these devices only moves in one direction, and therefore such MEMS typically occupy a large space on the chip. Attempts to create small size coupled 3D moving devices result in an ever increasing cross-axis sensitivity or so-called "mechanical cross-talk" between the different channels. The present disclosure proposes a method of fabricating a MEMS / NEMS device that allows to enable pure in-plane motion by fabricating a piezoelectric transducer on the vertical surface of the MEMS / NEMS element. SUMMARY

[0003] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0004] According to a first aspect of the invention, a method for fabricating a MEMS transducer is provided, the method comprising forming at least one cantilever from a silicon-on-insulator (SOI) wafer, wherein the at least one cantilever comprises a first sidewall, a second sidewall substantially parallel to the first sidewall, an aluminum nitride (AlN) layer, and a top electrode, and wherein the top electrode is connected to the at least first sidewall and / or second sidewall via the AlN layer therebetween.

[0005] According to a second aspect of the invention, a piezoelectric MEMS transducer is provided, the piezoelectric MEMS transducer comprising: a longitudinally highly doped silicon cantilever with a first end fixed and a second end free, the longitudinal cantilever comprising a first sidewall, a second sidewall substantially parallel to the first sidewall, and a top portion substantially perpendicular to the first sidewall and the second sidewall; and a first electrode and a second electrode connected to the first sidewall and the second sidewall, respectively, via an aluminum nitride (AlN) layer therebetween. BRIEF DESCRIPTION OF DRAWINGS

[0006] Fig. 1 is an illustration of device performance. Figure 1A The concept of a device is shown. Figure 1BThe movement of the cantilever when a voltage is applied between the top and bottom electrodes is shown, as well as the deformation of the AlN under applied voltage.

[0007] Figure 2A Illustration of the performance of a multi-axis MEMS device: 3D piezoelectric actuator.

[0008] Figure 2B Illustration of the performance of a multi-axis MEMS device: multi-axis energy harvester shows harvesting in in-plane (x-y plane) and out-of-plane (y-z plane) directions.

[0009] Figure 3 is the name of the main components of the device on the schematic illustration of the structure. Figure 3A : The cantilever comprises a Si (111) n++ device layer, a TiN top electrode, a buried oxide layer BOX SiO2, an operational wafer, and AlN grown by MOVPE on the sidewalls of the silicon cantilever. Figure 3B : The two cantilevers are shown separated by a silicon bridge region and cavity.

[0010] Figure 4 is a SEM micrograph of the fabricated device. Figure 4A The released cantilever is depicted, while Figure 4B The two released cantilevers are depicted.

[0011] Figure 5A and Figure 5B Illustration of the device production line and picture of the device area. DETAILED DESCRIPTION

[0012] This novel design of lateral motion and sensing using aluminum nitride, AlN, piezoelectric thin films deposited on vertical surfaces is a step towards emerging multi-axis MEMS. This work demonstrates the fabrication process and potential applications of an in-plane moving piezoelectric actuator. The actuator utilizes the converse piezoelectric effect of the AlN thin film grown on the vertical surfaces of a Si cantilever. When a voltage is applied between the top and bottom electrodes (highly doped Si and titanium nitride, TiN, thin films) of the device, it enables the lateral motion of the actuator.

[0013] The development of novel architectures for 3D motion in MEMS and nano-electromechanical systems (NEMS) opens up a breakthrough prospect for the fabrication of state-of-the-art devices and technologies. Enhanced designs not only bring opportunities for more accurate inertial sensing in autonomous vehicles or augmented and virtual reality AR / VR systems, but also for novel MEMS / NEMS. For example, nanoscale multi-axis motion introduces new approaches for complex designed nanoswitches, beam steering devices, and gyroscopes. In addition, nanoscale 3D motion can play a key role in solving the problem of multifunctional nanomanipulators for moving and positioning nano-sized particles, cells, and other biological objects. In addition, compact 3D MEMS will introduce novel inertial MEMS with superior accuracy and high functional density, meaning that a large number of devices can be installed on a small surface area. For example, the piezoelectric half-hinge MEMS gyroscope demonstrated in the study by Gabrelian et al. requires only 0.00846 mm 2 , while other demonstrated gyroscopes with similar sensitivity occupy 2 mm 2 to 72 mm 2 .

[0014] MEMS devices actuated with the piezoelectric effect (piezoelectric actuators) have several strong points due to direct electromechanical coupling, small size, and the ability to apply small voltages for driving. Excitation of piezoelectric actuators does not require high-voltage driving or DC biasing, as the output signal is proportional to the change in stress. Moreover, piezoelectric-based MEMS (piezoelectric MEMS) are less sensitive to the viscous drag that is common in comb-driven devices. The manufacturing technology for out-of-plane moving piezoelectric actuators has been well demonstrated previously, but the structural design for achieving 3D motion remains difficult to implement. One of the main challenges in manufacturing coupled 3D piezoelectric MEMS devices is generating lateral motion. In-plane piezoelectric actuators must have a piezoelectric layer with a certain crystal orientation deposited on the vertical surface of the moving cantilever and have a suitable electrical connection to the layer. The crystal orientation of the piezoelectric material must be chosen to maximize the piezoelectric effect capability. Torsional or tilting cannot be considered purely lateral motion, as in this case there is still a vertical motion component. Therefore, the design development of piezoelectric-based actuators for lateral driving is a key link in the manufacture of novel compact 3D piezoelectric MEMS.

[0015] Aluminum nitride, AlN, is a well-known lead-free piezoelectric material. Using AlN thin films grown on the vertical surface of a silicon cantilever, a natural bimorph and highly symmetrical structure can be obtained, which can provide higher robustness and energy efficiency for the system. Lateral moving AlN-based actuators can have a small static amplitude, which can be used for nanoscale measurements, switching, and manipulation.

[0016] In this study, piezoelectrically driven in-plane actuators based on AlN are introduced for the first time. The fabrication process of the actuator is easily compatible with mass production and the structure can be used as a platform for a wide range of devices from energy harvesters and compact gyroscopes to nanoswitches and manipulators.

[0017] Deposition of AlN thin films on planar surfaces has been a well-optimized and studied process for out-of-plane motion. But 3D piezoelectric MEMS will require the use of piezoelectric thin films on high aspect ratio structures of patterned substrates for in-plane motion. Metals and oxides grown on vertical surfaces have been integrated into some MEMS devices and can also be used in, for example, through silicon vias. At the same time, deposition of piezoelectric films on vertical surfaces is a more challenging task and has not been sufficiently studied for AlN. AlN thin films must meet certain criteria for piezoelectric MEMS fabrication to achieve the most efficient motion in the in-plane direction.

[0018] First, the film should be conformal, i.e. have a uniform thickness, over the entire surface determined for deposition, without gaps or pinholes. Non-proportional thin films increase the likelihood of dielectric breakdown and limit the maximum driving voltage. Second, the AlN film must have good crystal quality and maintain the c-axis orientation perpendicular to the vertical surface of the device. AlN with large grain size and narrow X-ray diffraction (XRD), rocking curve (XRC), c-axis orientation around the (0002) reflection exhibits the best piezoelectric properties. The correct orientation of the AlN film on the vertical surface is important for the efficient operation of the device, determined by the non-centrosymmetric wurtzite structure of AlN along the c-axis or parallel to the

[0001] direction. Therefore, to achieve maximum efficiency of the electromechanical coupling, the electric field should be aligned parallel to the c-axis of the AlN. A large fraction of the grains of the film have the c-axis oriented perpendicular to the vertical surface of the Si. Therefore, when an electric field is applied, the total deformation of the film is proportional to the applied voltage. The piezoelectric effect in AlN is averaged over all grains, so a decrease in crystal quality and orientation respectively leads to a decrease in electromechanical coupling and device performance.

[0019] It has been previously demonstrated that the tilt of the AlN grains significantly affects the piezoelectric response in sputtered AlN films. It is also worth adding that AlN cannot be polarized in the same way as ferroelectric materials, so the orientation of the AlN film can only be controlled by growth parameters.

[0020] Previous studies have demonstrated that metal organic chemical vapor deposition, MOCVD, and AlN deposition on patterned Si(110) substrates exhibit strong c-axis orientation perpendicular to the Si(111), but lack thickness uniformity. However, MOCVD AlN film thickness on vertical surfaces is greatly reduced within the growth area, and the film is typically thicker at the top portion of the structure closer to the reactor showerhead. There are several reasons for the poor conformality of MOCVD AlN films grown on vertical surfaces: low temperature, high pressure, and insufficient speed of precursor to reach the bottom of the trench (when deposition is for a wafer with cavities). Additionally, MOCVD AlN films often have pinholes even when grown on planar surfaces. Also, for AlN, the temperature required for growth exceeds 1000 °C, which limits the materials that can be used for fabrication, for example, limiting the materials that can be used as bottom electrodes. Additionally, high temperature deposition results in large residual stresses in the film after deposition. Furthermore, when using MOCVD, the crystal growth of the film is largely dependent on the substrate orientation. This is critical for patterned substrates whose structures are formed by different etching methods, and whose surfaces can not be of ideal smoothness. Therefore, a rough surface in the case of a crystal lattice mismatch will result in misorientation of the AlN grains. Despite some limitations of this method, MOCVD AlN films maintain c-axis orientation and have higher crystal quality overall compared to other deposition methods when the substrate surface is well prepared for growth.

[0021] High conformality is a major advantage of the atomic layer deposition (ALD) process compared to other CVD and PVD methods. Despite the fact that ALD is a time-consuming and relatively costly process, it has no equivalent due to its repeatability, large-area uniformity, low-temperature deposition possibility, and stress-free films. However, previously obtained ALD AlN films have a rather weak piezoelectric response due to the presence of randomly oriented grains in addition to the preferential

[0001] oriented grains. Also, a close relationship between the underlying layer and the AlN orientation was reported.

[0022] In this study, MOCVD was used as a method for AlN deposition because of its exceptional ability to grow high-quality textured c-axis oriented AlN on vertical surfaces.

[0023] Figure 1A and Figure 1B A schematic illustration of the working principle and the in-plane motion concept of the actuator is shown. The device 1 can be a cantilever 10 structure, where the AlN thin film 40 is sandwiched between a top electrode TE 50 and a bottom electrode BE 30 on the vertical walls of the Si cantilever 5, as shown in Figure 1A When an electric field E is applied, the crystal lattice of the AlN 40 is deformed due to the charge separation along the c-axis, which enhances the in-plane motion of the cantilever, asFigure 1B The voltage source 60 is connected to the top electrode 50 and the bottom electrode 30. Figure 1B

[0024] Figure 2A and Figure 2B The performance of the multi-axis MEMS device 1 is shown. Figure 2B The performance of the multi-axis MEMS device 1 as a multi-axis energy harvester is shown, showing harvesting in the in-plane (x-y plane) and out-of-plane (y-z plane) directions.

[0025] Deposition of piezoelectric AlN 40 on the lateral surfaces is required to obtain 3D motion. Additional TE / AlN films 55, 40 can be placed on top of the lateral surfaces of the cantilever 5 in the same way as shown in Figure 2A When a voltage is applied on the top electrode (53 in Figure 2A ), the AlN 40 on top of the cantilever 5 enables out-of-plane motion, while when a voltage is applied on the sidewall drive electrodes (51 and 52 in Figure 2A ), in-plane motion can be achieved. The piezoelectric effect is reversed, so the piezoelectric actuator 1 can be turned into an energy harvester by adding a proof mass 11 at the free end of the cantilever 10, as shown in Figure 2B If the harvesters are placed in an array, the harvested energy output can be maximized.

[0026] Figure 3A and Figure 3B The main parts of the device 1 are shown. In Figure 3A the cantilever (10) comprises a Si (111) n++ device layer 20, a TiN top electrode 50, a buried oxide layer BOX SiO2 31, an operational wafer 20, and AlN 40 grown by MOVPE on the sidewalls of the silicon cantilever 5. In Figure 3B two cantilevers 10A, 10B are shown separated by a silicon bridge region 70 and a cavity 80.

[0027] ​For the fabrication of piezoelectric actuators and collectors, a silicon-on-insulator (SOI) wafer can be used as a starting platform. Cantilevers 10 can be formed in the device layer of SOI, and the wafer can be etched at the end of the fabrication process to release the device 1. Special terminology is used herein to describe different parts during the device fabrication. Figure 3 shows all parts of a piezoelectric actuator 1. As mentioned in the section on AlN 40 growth on vertical surfaces, in this study, MOCVD was used to deposit the AlN film 40. MOCVD requires high temperature to achieve high quality AlN growth. Therefore, to avoid using high melting point metals, an SOI wafer with a highly doped device layer was used. The highly doped Si simultaneously functions as the substrate for cantilever formation and the BE.

[0028] The cavity-first approach is a modern concept that has been incorporated into the fabrication flow of piezoelectric actuators. This approach requires the formation of cavities in the device layer of SOI prior to thin film deposition, whereas the traditional fabrication path operates in the manner of depositing thin films on planar surfaces first and then performing cavity etching and element release. The areas between the cavities form future cantilevers, as shown in Figure 3B Figure 3. The silicon bridge region 70 separates the two cantilevers 10A, 10B and provides isolation for the future TEs 50 on the left and right sides of the cantilevers 10A, 10B. Figure 4 shows an SEM micrograph of a released piezoelectric actuator.

[0029] Table 1 presents the results obtained from high-resolution XRD scans on MOCVD AlN / Si(111) structures. As can be seen from the table, the AlN film is a high-quality crystal with c-axis orientation. In the study by et al., the c-axis oriented growth of MOCVD AlN on a blank Si(111) wafer indirectly demonstrates that the AlN film remains c-axis oriented when grown on a vertical surface with a surface roughness of 2.0 ± 1.4 nm using the same growth parameters. Therefore, the same deposition parameters were adopted when growing AlN on the vertical surface of a piezoelectric cantilever.

[0030]

[0031] The profile of the AlN film on the vertical sidewalls was measured and indicated that after 7000 s of growth, the film thickness decreased from 750 nm at the top of the cantilever wall to 166 nm at the bottom. The AlN film thickness profile remained the same for both types of samples when deposited on patterns etched in ICP-RIE or KOH. EBSD results indicated that only for the sample with KOH-etched cavities, the AlN film retained its crystal quality and c-axis texture on the vertical sidewalls, whereas the AlN grown on the sample with dry-etched cavities had poor crystal quality.

[0032] Table 2 summarizes the characteristic frequency f of the piezoelectric actuator obtained for different device length and width r The simulated values of the characteristic frequency f of the piezoelectric actuator for different device length and width are summarized in Table 2. Thus, the required operating frequency is in the range of 0.024 (0.028) to 1.694 (1.800) MHz.

[0033] FEA simulation results for in-plane directional displacement of the cantilever with different length and width were obtained when the driving voltage was equal to 10 V. The results show that the maximum amplitude of the deflection of the piezoelectric actuator along the length varies significantly with the change of the thickness of the AlN and TE. The maximum displacement of the piezoelectric actuator with 300 nm TiN electrode and 1000 nm thick AlN piezoelectric layer is 1.5 times the maximum displacement of the actuator with the same geometry but with thinner TiN / AlN films. The decrease in the deflection value of the actuator occurs because of the increase in the overall thickness of the structure when thicker AlN and TE are deposited, thus the cantilever becomes stiffer and more resistant to deformation. The same behavior is exhibited by the actuator when only the width of the cantilever is increased. The in-plane deflection increases exponentially when the length of the cantilever increases. In addition, the out-of-plane displacement is negligible with respect to the in-plane displacement, especially for long cantilevers, thus it is not represented on the graph. Therefore, the presented actuator should perform lateral motion only.

[0034]

[0035] This study has demonstrated the fabrication flow of the first AlN driven in-plane actuator. The study has examined the effect of the cantilever geometry on the maximum displacement amplitude and resonance frequency of the actuator using finite element method simulation. The main goal of the study was to demonstrate the complete design of the device that utilizes the piezoelectric properties of the AlN thin film deposited on the vertical surface as the driving force to perform lateral motion of the released cantilever. The design of the presented device can be adapted to fabricate more complex 3D piezoelectric MEMS.

[0036] Figure 5A and Figure 5B are a schematic of the device production line and a picture of the device area.

[0037] The fabrication of the piezoelectric actuator is described. A 1 pm thick wet thermal SiO2layer is formed on the device layer 22 of the SOI wafer 18 and the handle wafer 20. Two different methods are used to achieve the formation of the cavity in the device layer 22: dry etching and wet etching. The dry etching uses cryogenic etching with inductively coupled plasma RIE, ICP-RIE, Plasmalab 100. The etching is performed in a plasma of sulfur hexafluoride (40 seem, standard cubic meters per minute) and oxygen O2(6 seem) at -110 °C. The wet etching is performed in a 40 wt% aqueous potassium hydroxide (KOH) solution at 70 °C for 66 minutes.

[0038] Afterwards, a textured AlN film 40 is deposited on the patterned substrate using Aixtron 1x4” close coupled showerhead MOVPE. First, the wafer 20 is annealed in hydrogen (H2) for 5 minutes and then in disilane Si2H6 for 10 minutes or more at 1025 °C and 300 mbar reactor pressure. Nitridation is performed in an ammonia (NH3) atmosphere at 100 mbar and a substrate temperature of 980 °C for 15 s. The NH3gas flow is 15 seem. Trimethylaluminium TMAl and NH3are used as precursors for aluminium and nitrogen, respectively. The AlN 40 is grown with H2carrier gas at a substrate temperature of 1085 °C and a pressure of 100 mbar. The V / III ratio is kept at 336.05 for both the low temperature (LT) and the high temperature (HT) growth. The substrate temperature is set to 980 °C and 1085 °C for the LT and HT AlN, respectively.

[0039] On top of the MOCVD AlN 40, TiN 55 is deposited in an ALD reactor Picosun SUNALER-200 Advanced. The deposition is realized in the thermal mode of the reactor. As precursors, titanium tetrachloride TiCl4and TMAl are used. At a flow of 80 seem, 0.1 s pulses of TiCl4and 6 s purging are used. At a flow of 60 seem, 0.1 s pulses of TMAl and 3.6 s purging are used. The deposition is realized at 450 °C.

[0040] Furthermore, as Figure 5A and Figure 5B As shown in steps g-h, the Si bridge region 70 is etched using a thick layer of SU-8 75, thereby patterning the structure. First, the AlN / TiN layers 40, 55 are etched using chlorine (18 seem) and argon (6 seem) in an ICP-RIE. Then, the bottom Si layer is etched using a BOSCH process in a STS Deep RIE Etcher. The etching step is performed in SF6(130 seem) and O2(13 seem) with a coil power P coil = 600 W and a bias power P bias = 12 W. For the passivation step octafluorocyclobutane (C4F8) is used. At P coil = 600 W and P bias = 0 W, the C4F8gas flow is 85 seem. After that, the wafer is patterned for TE formation as Figure 5BSteps i-j are shown. For TE formation, AlN 40 and TiN 55 are etched using the same procedure as described for Si bridge etching. Cantilever 10 is released via DRIE of handle wafer 20 in SF6 / O2 using the same procedure as for Si bridge etching.

[0041] The crystal quality of the AlN thin film on a blank Si (111) wafer was measured using XRD Rigaku SmartLab. Symmetrical 2theta-omega and XRC were measured around the 0002 reflection. The thickness of the film was measured using a spectroscopic ellipsometer. For the orientation evaluation of the film on the sidewall, the electron backscatter diffraction, EBSD, method was used.

[0042] To study the effect of the actuator geometry on the resonant frequency and deflection values of the device, finite element modeling, FEM, was used in the “solid mechanics” and “electrostatic” interfaces of COMSOL Multiphysics. The design parameters were taken from Table 3.

[0043]

[0044] The motion of the manufactured actuators was tested using an Imina Technologies nano-probing station for in-situ electrical measurements. Displacement measurements were performed in a Zeiss Supra 40 SEM chamber at vacuum level.

[0045] According to the present disclosure, there is provided a method of manufacturing a MEMS transducer, the method comprising forming at least one cantilever from a silicon-on-insulator, SOI, wafer, wherein the at least one cantilever comprises a first sidewall, a second sidewall substantially parallel to the first sidewall, an aluminum nitride, AlN, layer, and a top electrode, and wherein the top electrode is connected to at least the first sidewall and / or the second sidewall via the AlN layer therebetween.

[0046] According to the present disclosure, there is provided a piezoelectric MEMS transducer, the piezoelectric MEMS transducer comprising: a longitudinally highly doped silicon cantilever with a first end fixed and a second end free, the longitudinal cantilever comprising a first sidewall, a second sidewall substantially parallel to the first sidewall, and a top portion substantially perpendicular to the first sidewall and the second sidewall; and a first electrode and a second electrode connected to the first sidewall and the second sidewall, respectively, via an aluminum nitride, AlN, layer therebetween.

[0047] In an embodiment, there is provided a method for manufacturing a microelectromechanical system (MEMS) device, the method comprising steps a-k, such that:

[0048] - in step a, an SOI wafer 18 is obtained, comprising a highly doped silicon (Sin++) device layer 22, a BOX SiO2 31 and a silicon handling wafer 20;

[0049] - in step b, a thermal oxide layer SiO2 25 is grown on the highly doped silicon device layer 22, having a thickness of 1 pm;

[0050] - in step c, a cavity 80 is formed in the device layer 22 using the thermal SiO2 25 as a hard mask;

[0051] - in step d, the SiO2 is etched in a buffered oxide etchant BHF;

[0052] - in step e, aluminum nitride AlN 40 is grown using metal organic vapor phase epitaxy MOVPE;

[0053] - in step f, titanium nitride TiN 55 is deposited using atomic layer deposition;

[0054] - in step g, SU-8 photoresist 75 is deposited and patterned onto the grown and / or deposited layers, such that the silicon bridge area is patterned;

[0055] - in step h, the silicon bridge area 70 is etched such that parts of the BOX SiO2 layer 31 are exposed;

[0056] - in step i, the TiN layer 55 and the AlN layer 40 are patterned using the SU-8 photoresist 75;

[0057] - in step j, the TiN layer 55 and the AlN layer 40 are etched using inductively coupled plasma reactive ion etching (ICP-RIE);

[0058] - in step k, the handling wafer 20 is etched using deep reactive ion etching DRIE and parts of the buried oxide layer 31 and the handling wafer 20 are removed, such that the cantilever 10 is released, forming the MEMS transducer 1.

[0059] Industrial applicability

[0060] At least some embodiments of the present disclosure have industrial applicability at least in the manufacture and use of micro- and nano-electromechanical systems (MEMS, NEMS), and in particular of electrostatic and piezoelectric NEMS / MEMS transducing and NEMS / MEMS actuating.

[0061] List of abbreviations

[0062] ALD atomic layer deposition

[0063] AR augmented reality

[0064] BE bottom electrode

[0065] BHF buffered oxide etch, buffered hydrofluoric acid etch

[0066] CVD chemical vapor deposition

[0067] DC direct current

[0068] DRIE deep reactive ion etching

[0069] E electric field

[0070] EBSD electron backscatter diffraction

[0071] FEA finite element analysis

[0072] FEM finite element modeling

[0073] f r feature frequency

[0074] HT high temperature

[0075] ICP-RIE inductively coupled plasma reactive ion etching

[0076] ICP inductively coupled plasma

[0077] LT low temperature

[0078] MEMS microelectromechanical system

[0079] MOCVD metal organic chemical vapor deposition

[0080] MOVPE metal organic vapor phase epitaxy

[0081] NEMS nanoelectromechanical system piezoelectric MEMS piezoelectric-based MEMS

[0082] PVD physical vapor deposition

[0083] SF6 sulfur hexafluoride

[0084] SEM scanning electron microscope

[0085] SOI silicon on insulator

[0086] SU-8 epoxy-based negative photoresist

[0087] TE top electrode

[0088] TMAl trimethylaluminum

[0089] VR virtual reality

[0090] XRD X-ray diffraction

[0091] XRC X-ray diffraction rocking curve

[0092] List of reference signs

[0093] 1 MEMS transducer

[0094] 5, 5A, 5B silicon cantilever

[0095] 10, 10A, 10B cantilever

[0096] 11 proof mass

[0097] 18 silicon-on-insulator SOI wafer

[0098] 20 handle wafer

[0099] 22 device layer

[0100] 25 SiO2 thermal oxide

[0101] 30 bottom electrode, BE

[0102] 31 buried oxide layer SiO2 layer

[0103] 40 aluminum nitride layer AlN

[0104] 50 top electrode

[0105] 51 first top electrode

[0106] 52 second top electrode

[0107] 53 third top electrode

[0108] 55 titanium nitride layer TiN

[0109] 60 voltage source

[0110] 70 silicon bridge region

[0111] 75 SU-8 photoresist

[0112] 80 cavity

[0113] List of references

[0114] Non-patent literature

[0115] A. Gabrelian, G. Ross, K. Bespalova, M. Paulasto-Krockel, Materials Today Communications 2022, 33, p. 104522.

[0116] E. S. Suihkonen, G. Ross, A. Torkkeli, H. Kuisma, M. Paulasto-Krockel, Journal of Crystal Growth 2020, 531, p. 125345.

Claims

1. A method for manufacturing a piezoelectric MEMS transducer (1), the method comprising forming at least one cantilever (10) from a silicon-on-insulator (SOI) wafer (18), wherein, The at least one cantilever (10) includes a first sidewall, a second sidewall substantially parallel to the first sidewall, an aluminum nitride AlN layer (40), and a top electrode (50), wherein the top electrode (50) is connected to at least the first sidewall and / or the second sidewall via the AlN layer (40) therebetween.

2. The method according to claim 1, wherein, The formation of the at least one cantilever (10) includes: - Obtain a silicon-on-insulator (SOI) wafer (18), the silicon-on-insulator (SOI) wafer (18) comprising a highly doped silicon device layer (22), a buried oxide layer (31) and an operating wafer (20); - Partially etch the highly doped silicon device layer (22) to form at least one cavity (80) in the highly doped device layer (22), the at least one cavity (80) exposing a portion of the buried oxide layer (31), and wherein the at least one cavity (80) is at least partially defined by the exposed portion of the buried oxide layer (31) and the first sidewall and the second sidewall of the highly doped silicon device layer (22); - At least an aluminum nitride AlN layer (40) is deposited on the exposed portion of the highly doped silicon device layer (22), the first sidewall, the second sidewall, and the buried oxide layer (31); -At least a titanium nitride TiN layer (55) is deposited on the AlN layer (40); - The top electrode (50) is formed from the TiN layer (55); and -Release at least one cantilever (10).

3. The method according to claim 2, wherein, The at least one cavity (80) is also defined by a silicon bridge region (70), and the method further includes etching the silicon bridge region (70) prior to forming the top electrode (50) to expose a portion of the buried oxide layer (31).

4. The method according to claim 2 or 3, wherein, Etching of the at least one cavity (80) includes dry etching of the highly doped silicon device layer (22) at low temperature by using inductively coupled plasma reactive ion etching (ICP-RIE) with sulfur hexafluoride (SF6) and oxygen (O2).

5. The method according to claim 2 or 3, wherein, Etching the at least one cavity (80) includes wet etching the highly doped silicon device layer (22) with an aqueous solution of potassium hydroxide (KOH).

6. The method according to any one of claims 2 to 5, wherein, After etching the at least one cavity (80), the surface roughness of the sidewall is 0.6 nm to 3.4 nm.

7. The method according to any one of claims 2 to 6, wherein, Depositing the aluminum nitride AlN layer (40) involves using metal-organic vapor phase epitaxy (MOVPE), and the deposition of the AlN layer (40) includes: - The SOI wafer (18) is annealed in hydrogen (H2); - The SOI wafer (18) was annealed in silane SiH6; - The SOI wafer (18) was nitrided under an ammonia (NH3) atmosphere; - An AlN layer (40) is grown at low temperature on the SOI wafer (18) in H2 carrier gas; and - An AlN layer (40) is grown at high temperature on the SOI wafer (18) in H2 carrier gas.

8. The method according to any one of claims 2 to 7, wherein, The AlN layer (40) comprises a substantially c-axis crystal orientation.

9. The method according to any one of claims 2 to 8, wherein, Depositing the titanium nitride TiN layer (55) involves thermally atomically depositing titanium nitride TiN on top of the AlN layer (40).

10. The method according to any one of claims 2 to 9, wherein, Etching the silicon bridge region (70) includes: - The AlN layer (40) and the TiN layer (55) on the silicon bridge region (70) are etched using inductively coupled plasma reactive ion etching (ICP-RIE) with chlorine and argon gas, exposing the bottom portion of the highly doped silicon device layer (22); and - The exposed bottom portion of the highly doped silicon device layer (22) is etched, wherein the etching is a reactive ion etching in SF6 and O2.

11. The method according to any one of claims 2 to 10, wherein, The top electrode (50) is formed by etching the aluminum nitride AlN layer (40) and the titanium nitride TiN layer (55) including: - The AlN layer (40) and the TiN layer (55) are etched using inductively coupled plasma reactive ion etching (ICP-RIE) with chlorine and argon gas, exposing the bottom portion of the highly doped silicon device layer (22); and - The exposed bottom portion of the highly doped silicon device layer (22) is subjected to reactive ion etching in SF6 and O2.

12. The method according to any one of claims 2 to 11, wherein, Releasing the at least one cantilever (10) includes: - Deep reactive ion etching (DRIE) is performed on the buried oxide layer (31) and the operational wafer (20) in SF6 and O2.

13. The method according to any one of the preceding claims, wherein, The highly doped silicon device layer (22) includes a surface having a substantially Si(111) crystal orientation.

14. The method according to any one of claims 2 to 13, wherein, Forming the top electrode (50) includes: - A first electrode (51) is formed on the first sidewall; - A second electrode (52) is formed on the second sidewall, the second sidewall being substantially parallel to the first sidewall; and - A third electrode (53) is formed on the top portion of the cantilever, wherein the top portion is connected to the first sidewall and the second sidewall and the top portion is perpendicular to the first sidewall and the second sidewall.

15. The method according to any one of claims 2 to 14, wherein, Before etching the at least one cavity (80), the method includes forming a wet-heat oxidized SiO2 layer (25) on the highly doped silicon device layer (22).

16. The method according to claim 15, wherein, Before depositing the aluminum nitride AlN layer (40), the wet-heat oxidized SiO2 layer (25) is removed using buried oxide etching BHF.

17. A piezoelectric MEMS transducer (1), the piezoelectric MEMS transducer (1) comprising: - A longitudinally highly doped silicon cantilever (5) with a fixed first end and a free second end, the longitudinal cantilever (5) comprising a first sidewall, a second sidewall substantially parallel to the first sidewall, and a top portion substantially perpendicular to the first and second sidewalls, and - A first electrode (51) and a second electrode (52), the first electrode (51) and the second electrode (52) being connected to the first sidewall and the second sidewall respectively via an aluminum nitride AlN layer (40) therebetween.

18. The method according to claim 17, wherein, The third electrode (53) is connected to the top portion via an aluminum nitride AlN layer (40) therebetween.

19. The piezoelectric MEMS transducer according to claim 17 or 18, wherein, The longitudinal cantilever (5) includes an inspection quality component (11) attached to the second end.

20. The piezoelectric MEMS transducer according to any one of claims 17 to 18, wherein, The aluminum nitride AlN layer (40) has a thickness of 300 nm to 1000 nm.

21. The piezoelectric MEMS transducer according to any one of claims 17 to 20, wherein, The thickness of the aluminum nitride AlN layer (40) on the first sidewall and the second sidewall is 166 nm to 750 nm.

22. The piezoelectric MEMS transducer according to any one of claims 17 to 21, wherein, The length of the vertically highly doped silicon cantilever is 200 μm to 1000 μm.

23. The piezoelectric MEMS transducer according to any one of claims 17 to 22, wherein, The width of the vertically highly doped silicon cantilever is 20 μm to 50 μm.

24. The piezoelectric MEMS transducer according to any one of claims 17 to 23, wherein, The thickness of the vertically highly doped silicon cantilever (5) is 50 μm.

25. The piezoelectric MEMS transducer according to any one of claims 17 to 24, wherein, The first, second, and third electrodes have a thickness of 100 nm to 300 nm.

26. The piezoelectric MEMS transducer according to any one of claims 17 to 25, wherein, The first electrode and / or the second electrode comprise TiN, Mo, or Al.

Images