Piezoelectric microelectromechanical device and manufacturing method

Abstract

According to one exemplary aspect of the present invention, a method for manufacturing a microelectromechanical system (MEMS) device is provided, the method comprising the steps of: forming at least one cantilever from a highly doped silicon layer on a buried oxide layer, wherein the at least one cantilever has a first sidewall and a second sidewall, the first and second sidewalls being perpendicular to the interface between the highly doped silicon layer and the buried oxide layer; growing a lead-free piezoelectric layer on at least the first and second sidewalls; and depositing a conductive material on at least the lead-free piezoelectric layer on the first and second sidewalls in order to form a first conductive layer on the lead-free piezoelectric layer on the first sidewall and a second conductive layer on the lead-free piezoelectric layer on the second sidewall.

Description

[Technical Field]

[0001] This disclosure relates to the fields of microelectromechanical systems (MEMS) and piezoelectric actuation. [Background technology]

[0002] Among the most promising areas of microelectromechanical systems (MEMS) are piezoelectric actuation-based devices. Emerging technological trends such as the Internet of Things (IoT) and fifth-generation cellular network technology (5G) are driving increased interest in MEMS sensors and actuators. To meet this growing demand, MEMS devices need to improve in terms of size, cost, reliability, and sensing output. [Overview of the project] [Means for solving the problem]

[0003] In some aspects, the subject matter of the independent claims is provided. In the dependent claims, several embodiments are defined. The scope of protection required for various embodiments of the invention is presented by the independent claims. Any embodiments, examples, and features described herein that do not fall within the scope of the independent claims should be interpreted as examples that help to understand the various embodiments of the invention. A first aspect of the present disclosure provides a method for manufacturing a microelectromechanical system (MEMS) device, the method comprising the steps of: forming at least one cantilever from a highly doped silicon layer on a buried oxide layer, the at least one cantilever having a first sidewall and a second sidewall, the first and second sidewalls being perpendicular to the interface between the highly doped silicon layer and the buried oxide layer; growing a lead-free piezoelectric layer on at least the first and second sidewalls; and depositing a conductive material on at least the lead-free piezoelectric layer on the first and second sidewalls to form a first conductive layer on the lead-free piezoelectric layer on the first sidewall and a second conductive layer on the lead-free piezoelectric layer on the second sidewall.

[0004] A second aspect of the present disclosure provides a microelectromechanical system (MEMS) device, the device comprising: at least one cantilever, the at least one cantilever forming part of a silicon layer containing highly doped silicon, the at least one cantilever including a top surface, a bottom surface, a first sidewall, and a second sidewall, the first and second sidewalls connecting the top surface and the bottom surface in the thickness direction of the silicon layer; a first lead-free piezoelectric layer on the first sidewall of the at least one cantilever; a second lead-free piezoelectric layer on the second sidewall of the at least one cantilever; a first conductive layer on the first lead-free piezoelectric layer; and a second conductive layer on the second lead-free piezoelectric layer. [Brief explanation of the drawing]

[0005] [Figure 1] This invention presents a micro-electromechanical system comprising two cantilevers, according to at least some embodiments of the present invention. [Figure 2a] This invention presents a micro-electromechanical system comprising two cantilevers, according to at least some embodiments of the present invention. [Figure 2b] A side view of a microelectromechanical system including two cantilevers, according to at least some embodiments of the present invention, is shown. [Figure 3a] ~ [Figure 3i] The present invention describes a method for manufacturing a microelectromechanical system including at least one cantilever, according to at least some embodiments of the present invention. [Figure 3j] An isometric view of a microelectromechanical system including at least one cantilever, according to at least some embodiments of the present invention, is shown. [Figure 4] The following are scanning electron microscope (SEM) images of a microelectromechanical system including a cantilever according to at least some embodiments of the present invention. [Figure 5] A schematic diagram of two spring-mass-damper systems for a MEMS vibration gyroscope, according to at least some embodiments of the present invention, is shown. [Figure 6] Shows the piezoelectric thin film orientation of the cantilever according to at least some embodiments of the present invention. [Figure 7] Shows a two-dimensional view of the structure of a microelectromechanical system including two cantilevers according to at least some embodiments of the present invention.

Embodiments for Carrying Out the Invention

[0006] Microelectromechanical system (MEMS) devices can perform micro-scale sensing, control, and actuation, and can generate micro-scale effects. Examples of applications that require next-generation microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) include low-power wireless sensor networks for the Internet of Things (IoT), optical 3D systems for object recognition, RF components in 5G / 6G mobile communication networks, smart catheters for minimally invasive systems, etc. In these applications, it is necessary to improve the performance of current MEMS devices, for example, improve in terms of delay, accuracy, sensitivity, energy efficiency, reliability at the fail operational level, and miniaturization. In order to meet the high expectations and commercial potential currently faced by miniaturized sensor systems and actuator systems, new materials and evolved design concepts are required.

[0007] In the present disclosure, in-plane actuation microelectromechanical system (MEMS) devices and manufacturing methods are disclosed. The MEMS devices of the present disclosure include at least one cantilever. The cantilever is defined as a cantilever beam supported at one end.

[0008] Piezoelectric MEMS has less power consumption, strong electromechanical coupling, and potential for further miniaturization compared to electrostatic conversion-based devices. However, at present, there are design limitations in that vertical movement is mainly enabled by piezoelectric thin films on a plane. In the present disclosure, it is proposed to deposit piezoelectric aluminum nitride (AlN) on the vertical sidewalls of the MEMS structure, which enables more effective horizontal actuation and sensing. The currently used perspective line-of-sight physical vapor deposition (PVD) method does not have sufficient conformal coverage on the vertical sidewalls. In this specification, a high-quality piezoelectric AlN process (e.g., by metalorganic chemical vapor deposition (MOCVD)) for the vertical sidewalls of MEMS features is disclosed. Furthermore, atomic layer deposition (ALD), which also provides a highly conformal layer on the sidewalls even in the case of conductive materials, is also used.

[0009] If the deflection direction and the vertical direction are controlled, it is possible to combine their effects to generate high-precision three-dimensional (3D) displacement in a single integrated device structure. This enables more advanced integration and miniaturization of 3D MEMS actuators, sensing of rotation in all three axes, and tunable nanofotonics components.

[0010] Piezoelectric materials can be deformed when an electric field is applied. Conversely, when a piezoelectric material is deformed, an electric field is induced. In the present disclosure, horizontal electromechanical transducers utilize the piezoelectric properties of lead-free piezoelectric materials such as aluminum nitride (AlN). In the case of a conventional piezoelectrically driven cantilever, the piezoelectric layer grows on the plane of a wafer (e.g., made of silicon (Si)). When a voltage is applied between two electrodes sandwiching the piezoelectric material included in the cantilever, the cantilever deflects along the plane passing through the two electrodes and the piezoelectric material, that is, perpendicular to the surface of the wafer on which the piezoelectric layer grows. For horizontal movement, when a voltage is applied to electrodes of such a structure where the piezoelectric layer and the two electrodes grow on opposite sidewalls of a silicon cantilever, the cantilever moves horizontally, perpendicular to the direction of the sidewalls.

[0011] The horizontal electromechanical transducer of this disclosure is a cantilever structure having an aluminum nitride (AlN) layer on the sidewall of a highly doped silicon beam. The aluminum nitride (AlN) may be deposited on the sidewall by metal-organic chemical vapor deposition (MOCVD). The highly doped silicon beam acts as proof mass and, at the same time, acts as a reference electrode, sometimes called a bottom electrode (BE), in piezoelectric actuation. In this disclosure, the bottom electrode (BE) and the reference electrode are used interchangeably. Another electrode, sometimes called a top electrode (TE), is present so as to sandwich the piezoelectric material between the bottom electrode and the top electrode. The top electrode may include, for example, molybdenum (Mo), titanium nitride (TiN), or aluminum (Al). The electrode may be deposited, for example, by atomic layer deposition (ALD). In-plane motion of the cantilever is enabled when a voltage is applied between the top electrode (TE) and the bottom electrode (BE), which contributes to the deformation of the piezoelectric layer and the deflection of the cantilever. The deflection of the cantilever depends on the geometry of the cantilever and the thickness of the piezoelectric layer. Silicon-on-insulator (SOI) wafers may be used to manufacture a beam with one end clamped, i.e., a type of cantilever.

[0012] According to this disclosure, a method for manufacturing a microelectromechanical system (MEMS) device is provided, and this method is - A step of forming at least one cantilever from a highly doped silicon layer on a buried oxide layer, wherein the at least one cantilever has a first sidewall and a second sidewall, and the first and second sidewalls are perpendicular to the interface between the highly doped silicon layer and the buried oxide layer, - The step of growing a lead-free piezoelectric layer on at least the first and second side walls, - A step of depositing a conductive material on at least the lead-free piezoelectric layers on the first and second side walls in order to form a first conductive layer on the lead-free piezoelectric layer on the first side wall and a second conductive layer on the lead-free piezoelectric layer on the second side wall, Includes.

[0013] Figures 3a to 3i show a method for manufacturing a cantilever according to at least some embodiments of the present invention. In Figure 3a, a silicon-on-insulator is provided, which includes a device layer 360, an embedded oxide layer 320, and a substrate 380. The device layer includes highly doped silicon 310. In Figure 3b, an insulator mask 315 (preferably thermal silicon dioxide (SiO2)) is attached to achieve masking over the device layer. In Figure 3c, a portion of the device layer 360 is etched through to the embedded oxide layer 320. Furthermore, after a portion of the device layer has been etched, the insulator mask 315 may be removed. In Figure 3d, a lead-free piezoelectric layer 340 is deposited on the device layer 360, the embedded oxide layer 320, and optionally the insulator mask 315. Deposition may be carried out by metal-organic chemical vapor deposition (MOCVD). In Figure 3e, a conductive layer 370 is deposited. Deposition may be carried out by atomic layer deposition (ALD). In Figure 3f, an insulator 350 (e.g., aluminum oxide (Al2O3)) is deposited on the conductive layer 370. Deposition may be carried out by atomic layer deposition (ALD). In the final product, one end of the cantilever is connected to the device layer 360. The opposite end may be separated from the device layer 360, which is done by etching the corresponding portion of the device layer 360 through to the embedded oxide layer 320. In Figure 3g, the conductive layer 370 is patterned to obtain a pattern that extends over at least a portion of the sidewall of the cantilever. In Figure 3h, the lead-free piezoelectric layer 340 is patterned. In Figure 3i, the embedded oxide layer 320 is etched, thereby creating a structure for the cantilever. The structure is open, and the sidewalls include a piezoelectric layer 340 and conductive layers 371, 372. This etching may be performed with hydrofluoric acid (HF) or vapor-phase etching (VPE). Figure 3j shows the formed cantilever 300, which includes highly doped silicon 310, a lead-free piezoelectric layer 340, a first conductive layer 371, and a second conductive layer 372, with one end of the cantilever 300 attached to and supported by the device layer 360.

[0014] In preferred embodiments, the lead-free piezoelectric material includes aluminum nitride (AlN). The conductive layer may include titanium nitride (TiN). The conductive layer may include aluminum (Al). The conductive layer may include molybdenum (Mo).

[0015] In this disclosure, the highly doped silicon layer is characterized as doped silicon having a resistivity of less than 0.01 Ω·cm, or preferably doped silicon having a resistivity of less than 0.005 Ω·cm.

[0016] In one embodiment, highly doped silicon is included in the device layer of a silicon-on-insulator (SOI) substrate. Such a silicon-on-insulator (SOI) substrate consists of the device layer 360 and a buried oxide (BOX) layer 320. Such a buried oxide layer 320 may be made of, for example, silicon dioxide (SiO2). Furthermore, the silicon-on-insulator (SOI) may include a base layer 380, and the device layer 360 is separated from the base layer 380 by the buried oxide (BOX) layer 320. Such a base layer 380 may be, for example, silicon.

[0017] In one embodiment, the formation of at least one cantilever is achieved by etching at least one cavity on the device layer, which is done, for example, by performing a lithography process followed by at least one of wet etching and dry etching. The at least one cavity forms the first and second sidewalls of at least one cantilever. An example of wet etching is etching using potassium hydroxide (KOH). Another example of wet etching is etching using tetramethylammonium hydroxide (TMAH). An example of dry etching is inductively coupled plasma reactive ion etching using a sulfur-oxygen hexafluoride mixture (SF6 / O2). Furthermore, silicon dioxide (SiO2) may be used as a mask when etching at least one cavity. In such a case, silicon dioxide may also be used to separate aluminum nitride (AlN) from a reference electrode containing highly doped silicon. The silicon dioxide acting as a mask helps to prevent out-of-plane deflection of at least one cantilever.

[0018] The piezoelectric material is grown on the sidewall of at least one cantilever. Growth may include metal-organic chemical vapor deposition (MOCVD). An example of a lead-free piezoelectric material is aluminum nitride (AlN). A further conductive layer is deposited on at least a portion of the lead-free piezoelectric material. Examples of conductive layers include titanium nitride (TiN), aluminum (Al), and molybdenum (Mo). In some embodiments, an insulating layer is deposited. An example of an insulating layer is aluminum oxide (Al2O3). The insulating layer (including, for example, Al2O3) can act as a hard mask when etching the area connecting two or more antiparallel cantilevers (or so-called silicon bridges (Si bridges)). Alternatively, the hard mask may define the end portion of a single cantilever. So-called silicon bridges are useful in forming cantilevers because the unmasked ends of the cantilever are isolated from the surrounding device layer structure.

[0019] The formation of at least one cantilever may be partially separated from the surrounding device layer and partially separated from the embedded oxide (BOX) layer by etching. The embedded oxide (BOX) layer surrounding at least one cantilever may be etched with hydrofluoric acid (HF) or by vapor-phase etching (VPE), thereby forming at least one cantilever. Hydrofluoric acid (HF) has minimal or no effect on the layers containing AlN and TiN. This is because these materials are stable in HF and therefore do not require further protection.

[0020] This disclosure overcomes some of the limitations and shortcomings of currently available microelectromechanical systems (MEMS). Specifically, this disclosure addresses the lack of electrostatic and piezoelectric conversion in current MEMS. Examples of the advantages of this disclosure include electromechanical coupling of horizontal motion, integration of full 3D motion within a single MEMS element, high scalability, and vertical MEMS element arrays. Current inertial MEMS solutions require complex multimodal resonators to detect vertical forces. Embodiments of this disclosure eliminate the need for complex designs by having full 3D electromechanical coupling within a single MEMS element. Full 3D motion of the element can be achieved by integrating horizontal piezoelectric conversion with horizontal conversion. Integrating piezoelectric MEMS elements vertically enables high function density in the horizontal direction, which is advantageous for MEMS scalability. Finally, vertical MEMS arrays with high function density in the horizontal direction enable enhanced electromechanical coupling sensitivity, which is advantageous for high-sensitivity sensors where energy scavenging techniques are weak.

[0021] In one embodiment, at least two parallel cantilevers are formed, separated by a gap of less than 200 μm, where "parallel" refers to the orientation of the silicon beam and the open cantilever. In another embodiment, the gap may be 50 to 300 μm. Such a microelectromechanical system may be used, for example, as a gyroscope having drive electrodes and sensing electrodes on both cantilevers. In such a configuration, there may be three or more cantilevers. Furthermore, in some embodiments, cantilevers may be antiparallel or perpendicular to another cantilever. In some embodiments, three or more cantilevers may be formed.

[0022] The following describes MEMS devices and methods incorporating lead-free piezoelectric materials. Figure 1 shows a microelectromechanical system (MEMS) device according to at least some embodiments of the present disclosure. The cantilever includes highly doped silicon, a lead-free piezoelectric material on the surface of the highly doped silicon, and a conductive layer on the surface of the lead-free piezoelectric material. The highly doped silicon acts as a reference electrode. According to the present disclosure, a microelectromechanical system (MEMS) device is provided in which at least one cantilever includes highly doped silicon as a reference electrode, a first sidewall, a first lead-free piezoelectric layer on the first sidewall, a first electrode on the first lead-free piezoelectric layer, a second sidewall, a second lead-free piezoelectric layer on the second sidewall, and a second electrode on the second lead-free piezoelectric layer. The first and second lead-free piezoelectric layers may include, for example, aluminum nitride (AlN).

[0023] According to some embodiments, two cantilever mechanisms may be incorporated to obtain a tuning fork structure. The characteristics of such a two-cantilever MEMS system are described below.

[0024] Actuation mechanisms for MEMS devices can be implemented in various ways. Sensor designs include thermal, electromagnetic, and electrostatic conversion. Among these methods, piezoelectric actuation offers significant advantages, primarily due to its strong electromechanical coupling. As a result, piezoelectric devices consume less power. In contrast, electrostatically actuated sensors require an additional charge pump to increase the amplitude of the input voltage. Some of the most widely used piezoelectric materials for existing MEMS devices include aluminum nitride (AlN) and lead zirconate titanate (PZT). AlN is an environmentally friendly material and readily available, suitable for metal-oxide-semiconductor (CMOS) processes, while PZT is not. For these reasons, AlN is more promising in the design of vibration-based MEMS sensors.

[0025] Generally, commercially available inertial sensors are actuated electrostatically, and their detection output is measured by the change in capacitance in the detection direction. However, recent research has published a wide range of designs based on the properties of piezoelectric materials. For example, a MEMS piezoelectric solid-disk gyroscope is designed using elliptic bulk acoustic wave modes. Another piezoelectric MEMS gyroscope utilizes a beam-type structure in which the piezoelectric layer is sandwiched between two electrodes.

[0026] In AlN-based piezoelectric devices, the most common approach to utilizing out-of-plane deflection is by sputtering AlN onto a substrate. However, multi-axis sensors such as gyroscopes require the drive and detection directions to be perpendicular, thus increasing the demand for in-plane actuation. Furthermore, enhanced in-plane actuation and detection open up applications beyond inertial sensors. AlN-based energy harvesters can generate power by capturing minute amounts of energy from ambient vibrations. By implementing an AlN sidewall structure, it becomes possible to generate a voltage from in-plane deflection, thereby increasing the device's output. Moreover, in-plane motion allows energy harvesters to be stacked into combined arrays, thereby increasing the overall absorption efficiency. Simultaneously, piezoelectric in-plane actuation can also be attractive in high-quality switching of RF MEMS. The challenge in in-plane actuation and detection is depositing piezoelectric and electrode materials on vertical sidewalls with high crystalline quality. To overcome this challenge, deposition of piezoelectric AlN and metal electrodes was achieved by metal-organic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD). Figure 4 shows an SEM image of an in-plane cantilever with MOCVD-deposited piezoelectric AlN sidewalls fabricated on a silicon-on-insulator (SOI) wafer. A key feature of this structure is the ability to utilize a bimorph design with piezoelectric thin films on both sides of the cantilever. For actuation, a voltage is applied to the AlN thin film sidewall by a contact passing through the region of the metal electrode. Subsequently, the proposed sidewall structure utilizes the enlarged region of the metal electrode, thereby further improving the observable sensor output.

[0027] In this study, comprehensive FEM testing was conducted to fully understand the operating characteristics and overall picture of a MEMS design with a perpendicular piezoelectric AlN thin film structure. Multiphysics testing enables the implementation of the properties of anisotropic and piezoelectric materials, as well as the coupling of these properties with FEM software packages. To demonstrate the potential of the sidewall concept in inertial sensing applications, COMSOL FEM simulations were performed on a half-fork Coriolis oscillatory gyroscope (CVG) with a piezoelectric AlN sidewall structure.

[0028] After the simulation, the main characteristics of the proposed device (e.g., mechanical velocity sensitivity and scaling) were calculated to estimate the overall gyroscope performance. The output of the modeled gyroscope presented in this study can be compared to existing devices using numerous commercially available and research-based MEMS gyroscopes.

[0029] To investigate the potential performance of the sidewall piezoelectric structure of the MEMS, geometry optimization was performed. Multiparameter characterization was performed in COMSOL to obtain the mechanical sensitivity of the range of modified design options. Since the bimorph piezoelectric cantilever is the basic structure of the proposed MEMS gyroscope device, the initial geometric parameters were assumed by examining a fabricated sample of an open cantilever, shown in Figure 4.

[0030] The operating principle of a MEMS vibration gyroscope is based on the Coriolis acceleration effect. When a movable proof mass resonates at the applied angular velocity, the Coriolis force generates a displacement perpendicular to the linear velocity.

[0031] A basic single-axis Coriolis oscillator gyroscope (CVG) can be described as a resonator with two degrees of freedom. This consists of two mass-spring-damper systems, which are represented by the following equations of motion with respect to both the drive axis and the sensing axis.

[0032]

number

[0033] [Number]

[0034] However, with respect to the driving direction, m d is the mass, r d is the damping coefficient, k d is the stiffness, F d sin(ωt) is the applied force. On the other hand, with respect to detection, m s is the mass, r s is the damping coefficient, k s is the stiffness, F cor is the Coriolis force applied in the detection direction.

[0035] The amplitude of the Coriolis force is expressed as follows.

[0036] F cor = -2m(v d ×Ω ext )

[0037] However, m is the mass of the moving system, v d is the linear velocity, and Ω ext is the angular velocity. A concentrated gyroscope system is shown in FIG. 5.

[0038] Each motion component in the equations of motion for both the driving axis and the detection axis can contribute to COMSOL FEA modeling as follows. - m d , m s - The mass of the constructed geometry, material density - r d , r s - Presented as Rayleigh damping in the model. The value is based on the Q factor and the resonance frequency (detailed in Section 3.3). - kr d , kr s- The stiffness in drive mode and detection mode corresponds to the anisotropic properties of the materials used (e.g., elastic matrix and compliance matrix) and physical constraints (fixed surface constraints).

[0039] The proposed design fully utilizes a piezoelectric cantilever structure. The AlN vertical sidewalls are responsible for the driving direction, causing in-plane motion of the cantilever. When an external voltage is applied, the piezoelectric thin film deforms, causing cantilever deflection with a linear velocity in the driving direction. The sensing direction is opposite to the driving direction and can be achieved by having out-of-plane motion on the same cantilever. Therefore, angular velocity must be applied to observe the deflection in the sensing direction. Finally, the sensing piezoelectric thin film located on the upper surface of the cantilever converts the mechanical deformation caused by the Coriolis force into an output voltage.

[0040] Figure 1 shows a schematic diagram of a half-tuned fork CVG based on the fabricated AlN piezoelectric cantilever shown in Figure 4. A gyroscope with a width of 160 μm, a length of 420 μm, and an overall thickness of 50 μm is made of 0.04 Ω p-type silicon, which is also defined as the ground electrode. The mechanical structure of the resonator consists of two cantilevers connected to each other, with a deposited AlN layer. Furthermore, the following boundary conditions were applied to simulate the rotation of the gyroscope: a rotating coordinate system centered on the Y axis, with a specified angular velocity of va, and fixed constraints at the uniform ends of the sensor in the ZX plane.

[0041] To solve the FEA model, which includes anisotropic properties, it is necessary to determine the elastic constants. The elastic stiffness coefficient of the AlN wurtzite-type structure, obtained experimentally, is expressed by the stiffness tensor in the following equation.

[0042]

number

[0043] Furthermore, by utilizing the corresponding stress piezoelectric constant tensor, the modeled material was defined as piezoelectric.

[0044]

number

[0045] In the proposed model, Al electrodes are placed on each side of the cantilever to simulate piezoelectric actuation. The AC voltage applied to each drive electrode causes the two cantilevers to vibrate in opposite phases to each other.

[0046] When the gyroscope resonates along the X-axis, the rotating coordinate system around the Y-axis causes a displacement in the Z-axis direction due to the Coriolis force. Each cantilever has two Al electrodes to detect the electrical signal generated from the deformed piezoelectric layer. The out-of-plane deflections are in opposite directions. Therefore, the voltage difference between the two electrodes acts as an absolute value detection output.

[0047] The proposed design separates the drive direction and detection direction into the X-axis and X-axis, respectively. Two reference coordinate systems were created to obtain accurate behavior of the simulation model. Sample coordinate system (S xyz ) refers to the fixed coordinate system of the silicon base of the half-fork, while the crystal coordinate system (C abc The ) indicates the orientation of the AlN thin film. To observe separate driving and sensing directions, the driving crystal reference coordinate system is rotated relative to the sample reference plane. Further rotational systems were created to model the orientation of the above-mentioned layers in COMSOL. Meanwhile, axis switching was configured using Euler angles. Figure 6 shows the overall breakdown of the piezoelectric layer orientation.

[0048] The initial dimensions of the proposed design are shown in Figure 7. All geometric parameters are listed in the table below. [Table 1]

[0049] The COMSOL multiphysics eigenvalue solver uses the relationship between complex eigenvalues ​​and frequencies to determine the mode shape and value of the natural resonant frequencies of a modeled gyroscope. The frequencies of interest are the drive mode (in-plane, X-axis) and the sense mode (out-of-plane, Z-axis).

[0050] The proposed design operates in reverse phase to reduce orthogonal errors and provide a more accurate output. The reverse-phase mode was chosen because it allows for efficient utilization of the differential output, requiring only two sensing electrodes. In this case, the gyroscope's sensitivity is determined by the voltage difference between the two cantilevers, because the generated potentials have opposite signs.

[0051] While in in-phase mode, the signs of the sensing voltages on each cantilever are equal, resulting in the need for an additional neutral electrode and increased losses. In this example, the operating frequencies for the drive mode and sensing mode are 149.05 kHz and 256.74 kHz, respectively.

[0052] In this study, the drive mode of the proposed MEMS gyroscope is actuated by piezoelectric inverse effect. Both cantilevers of the resonator have AlN deposited thin film sidewalls with Al electrodes on their surfaces. To increase the drive displacement amplitude, the AC voltage has a 180-degree phase difference at the Al electrode sidewalls, while both cantilevers have mirrored orientations with respect to each other, thereby performing drive inverse phase deflection.

[0053] In practice, the drive input voltage amplitude is proportional to the displacement amplitude. As a result, the higher the source voltage, the higher the angular velocity sensitivity. However, the maximum voltage amplitude is limited by size and practical constraints. The vast majority of commercially available MEMS gyroscopes have an input voltage amplitude of 10-30V. Furthermore, some devices include a charge pump to increase the enumerated values. To adhere to possible constraints, 10V was chosen as the AC amplitude for the proposed CVG half-fork gyroscope. First, the damping boundary condition is presented as damping of the loss coefficient. This can be set up as a single ratio. However, this option does not take into account the differences in behavior between different resonant modes. Therefore, Rayleigh damping was chosen for further modeling to enhance the simulation results. This can be expressed as a linear combination of the mass matrix and the stiffness matrix and can be calculated analytically as follows:

[0054] C=α d MM+β d KK

[0055] In the above equation, parameter C is defined by two parameters α d M and β d K is defined as the fraction of mass and stiffness. The values ​​of these constants are determined by the drive frequency and the quality factor Q. d It can be derived from the value of .

[0056] MEMS gyroscopes, like other sensors such as accelerometers, require a high vacuum environment for improved performance. These conditions are strongly correlated with the structural behavior of the resonator, as the vacuum level affects the mechanical quality factor. A high quality factor in a tightly encapsulated system allows for more pronounced resonant displacement at lower input voltages. Experimental measurements and tests of highly sealed MEMS gyroscopes have shown their Q-factor values ​​to be in the range of 9,840 to 34,000. Furthermore, recent MEMS modeling studies suggest this parameter is between 10,000 and 30,000. However, analytical calculations for high-quality sensors suggest the quality factor can reach up to 50,000. Therefore, the quality factor for the proposed simulation was estimated at 20,000.

[0057] Mechanical velocity sensitivity or angular velocity sensitivity was selected as the output value for estimating the potential of the design being simulated. This parameter is provided for all devices with analog outputs and can be expressed as a relationship between output voltage (mV) and angular velocity (dps). The sensitivity value can be calculated by sweeping the angular velocity range and calculating the voltage generated by a velocity change of 1 dps.

[0058] To obtain sensitivity values, a COMSOL frequency domain test was performed. The gyroscope was swept at a fixed drive resonance frequency, focusing on various angular velocity values ​​in 8 dps steps from -64 dps to 64 dps.

[0059] The amplitude of the simulated Coriolis force was analyzed for three different cases (when a positive, negative, and zero rotating coordinate system was applied) to verify whether the behavior of the out-of-plane oscillations in opposite phases was correct.

[0060] The correlation between the applied rotating coordinate system and the detected voltage was tested using COMSOL's FEM through frequency domain testing. From the linear behavior of the resulting dependence, it is possible to evaluate the mechanical velocity sensitivity as a curve gradient. In this way, the sensitivity value of the proposed gyroscope design was determined to be 0.013 mV / dps.

[0061] Overall, these results indicate that the achieved sensitivity values ​​already fall within the lower limit of commercially available MEMS gyroscopes. Furthermore, this design offers significant advantages in the case of geometric scaling. To obtain quantitative scaling values, the surface area parameter (product of device length and width) was selected. This characteristic not only includes a crucial aspect of the sensor but can also serve as a scaling parameter for comparing various MEMS devices.

[0062] The optimization test aims to examine the proposed gyroscope design with AlN sidewalls. Geometry optimization reveals the interrelated parameters and their contributions to the gyroscope output. Simultaneously, the increased sensitivity makes this design more competitive among existing gyroscopes with analog outputs.

[0063] To explore possible improvements to the initial design, the model was parameterized. As mentioned earlier, the gyroscope's actuation is based on the piezoelectric effect, and the AlN layer is concentrated on the gyroscope's cantilever. Therefore, the AlN thickness, height, and width of the cantilever were selected as parameters for further optimization.

[0064] First, it is necessary to determine the resonant frequency for each gyroscope design according to the developed method. Therefore, COMSOL natural frequency testing was performed using parameter sweep to obtain natural modes for all geometry combinations. As mentioned above, this model includes fixed-plane boundary conditions, which inevitably lead to the appearance of torsional and rotational natural modes. To eliminate unwanted modes and simultaneously ensure that the necessary, out-of-phase in-plane drive modes appear after simulation, four natural frequencies were selected as the required number in the COMSOL test characteristics.

[0065] However, even within unimportant parameter ranges, several mode frequencies are generated (if each of the 64 possible designs has 4 modes, 256 frequencies will be shown). In this case, it is impossible to manually select the desired mode behavior.

[0066] First, the results of the natural frequency tests and the calculated contributing factors were exported from COMSOL for further analysis. The required drive modes have displacement in the X direction with the cantilevers in opposite phase, while unwanted mode shapes include torsion and rotation. Therefore, to eliminate unwanted mode shapes without manual selection, the built-in COMSOL contributing factors and effective mass coefficients were used as filtering variables. Furthermore, 64 frequencies for angular velocity sensitivity tests were selected from the initial dataset, each having the minimum effective mode mass in the Z direction and the maximum contributing factor in the X direction.

[0067] To determine angular velocity sensitivity, it is necessary to plot the detection curve and calculate the increment in detection voltage per 1 dps change in angular velocity.

[0068] The list of resonant frequencies obtained from previous tests makes it possible to calculate the mechanical velocity sensitivity for each design option. Therefore, the resonant frequencies were added to the swept list of parameters. In this test, all 64 geometry combinations were simulated at their respective intrinsic resonant frequencies.

[0069] This simulation demonstrated that the output results are susceptible to the cantilever geometry of the modeled gyroscope. The mechanical velocity sensitivity varies between 0.0125 mV / dps and 1.21 mV / dps.

[0070] According to the results of the simulations performed, the sensitivity value can be maximized when the height and width are minimum and the thickness of the AlN layer is maximum.

[0071] Further analysis was conducted to investigate the effect of gyroscope height on the gyroscope's sensing output. This analysis revealed that several geometric parameters are more strongly correlated with mechanical velocity sensitivity than others. To observe how closely these values ​​relate to each other, the Pearson correlation coefficient (r-Pearson) was calculated. The Pearson correlation coefficient characterizes the presence of a linear dependence between two values. Its range is from 1 to -1, where 1 indicates a perfectly positive linear relationship, -1 indicates a perfectly inverse linear relationship, and 0 indicates no linear correlation. The analysis revealed that the cantilever height has the greatest effect on the gyroscope output (Pearson value -0.4), the AlN thickness has a small effect on sensitivity (Pearson value 0.2), and some of the smallest parameters show no correlation at all.

[0072] The geometry dependencies obtained from optimized simulations allow for the analysis of potential improvements. The model in this study shows a significant increase in output sensitivity from the initial range of 0.013–1.23 mV / dps, which was already comparable to commercially available devices.

[0073] The primary objective of this study was to investigate the potential of AlN sidewall structures in state-of-the-art MEMS design. A half-tuned fork gyroscope with vertical AlN sidewalls was modeled to study how it couples with inertial sensors. FEM analysis was performed using COMSOL multiphysics to analyze the input, resonant, and output characteristics of the proposed device. This included natural frequency testing, sensitivity analysis, and optimization.

[0074] Furthermore, the proposed FEM model provides a direct approach to comparing the simulated gyroscope with existing devices. As a result, there is a promising outlook for implementing AlN sidewalls in this piezoelectric MEMS design. The first model of the proposed MEMS gyroscope achieved 0.013 mV / dps, which is within the lower limit of commercially available devices. This design has a significant advantage in scaling factor despite its lower sensitivity. The square dimensions of the gyroscope design are several orders of magnitude smaller than those of existing research and commercially available gyroscopes. To make the proposed design more competitive in terms of angular velocity sensitivity, the following parameterizations were performed through optimization. According to FEM multiparameter simulations, a design with a lower height and a thicker AlN layer could potentially achieve sensitivity values ​​exceeding 1.2 mV / dps.

[0075] The achieved results of the simulated gyroscope, and its remarkable scalability advantages, provide a solid foundation for enabling the implementation of piezoelectric sidewall structures beyond inertial sensor MEMS.

[0076] According to this disclosure, a microelectromechanical system (MEMS) device is provided, and this device is -At least one cantilever 100, the at least one cantilever 100 forming part of a silicon layer containing highly doped silicon, the at least one cantilever 100 including an upper surface 130, a lower surface, a first side wall 110, and a second side wall 120, the first and second side walls connecting the upper surface and the lower surface in the thickness direction of the silicon layer, - A first lead-free piezoelectric layer on the first side wall 110 of at least one cantilever 100, - A second lead-free piezoelectric layer on the second side wall 120 of at least one cantilever 100, - The first conductive layer 111 on the first lead-free piezoelectric layer, - The second conductive layer 121 on the second lead-free piezoelectric layer, Includes.

[0077] The first and second conductive layers 111 and 121 may be considered electrodes, and they can be connected to an external voltage source to cause deflection of the first lead-free piezoelectric layer between the first conductive layer 111 and the highly doped silicon, and the second lead-free piezoelectric layer between the second conductive layer 112 and the highly doped silicon. Conversely, it is possible to measure the voltage generated between the first conductive layer and the highly doped silicon, and the voltage generated between the second conductive layer and the highly doped silicon, caused by the deflection of the first and second piezoelectric layers.

[0078] In one embodiment, the resistivity of the highly doped silicon is less than 0.01 Ω·cm, in another embodiment, it is less than 0.005 Ω·cm, and in yet another embodiment, it is less than 0.004 Ω·cm. This resistivity is used to enable the highly doped silicon contained in at least one cantilever to be used as a reference electrode.

[0079] In a preferred embodiment, the lead-free piezoelectric layer includes aluminum nitride (AlN).

[0080] In one embodiment, the first conductive layer 111 and the second conductive layer 121 contain molybdenum (Mo), aluminum (Al), and / or titanium nitride (TiN). In one embodiment, the resistivity of the conductive layer is 65 to 670 μΩ·cm. In another embodiment, the resistivity of the conductive layer is less than 670 μΩ·cm, or less than 500 μΩ·cm, or less than 100 μΩ·cm, or less than 70 μΩ·cm.

[0081] In one embodiment, a third lead-free piezoelectric layer and a third conductive layer 131 are provided on the upper surface 130 of at least one cantilever 100. Such a third conductive layer 131 can function as an electrode to acquire measurement information regarding physical deflection perpendicular to the first and second sidewalls. Conversely, the third electrode can assist in the generation of deflection actuated by the third lead-free piezoelectric material when a voltage is applied between the third conductive layer and the highly doped silicon. The voltage that may be generated by such out-of-plane deflection can be measured using a contact and the highly doped silicon material acting as a reference electrode of the cantilever. Out-of-plane should be understood as a plane perpendicular to a virtual plane passing through the first and second sidewalls (i.e., perpendicular to a plane considered in-plane). Such a third electrode can be used as a so-called sensing electrode to quantify deflection occurring perpendicular to the driving direction (i.e., the in-plane direction).

[0082] Figures 2a and 2b show a microelectromechanical system including two cantilevers according to at least some embodiments. The embodiments of Figures 2a and 2b further include third conductive layers 231a, 231b, which may be useful for utilizing a Coriolis oscillatory gyroscope (CVG) in a microelectromechanical system (MEMS) including a first cantilever 200a and a second cantilever 200b parallel to the first cantilever 200a and separated from the first cantilever 200a by a gap. In the first cantilever 200a, a first voltage may be applied between the first conductive layer 211a and the highly doped silicon 250a, and a second voltage of opposite polarity to the first voltage may be applied between the second conductive layer 221a and the highly doped silicon 250a. In the second cantilever 200b, a third voltage may be applied between the first conductive layer 210b and the highly doped silicon 250b, and a fourth voltage with the opposite polarity to the third voltage may be applied between the second conductive layer 220b and the highly doped silicon 250a. In one embodiment, the first, second, third, and fourth voltages have the same amplitude, and the signs of the first and third voltages are opposite to the signs of the second and fourth voltages. When the first, second, third, and fourth voltages alternate periodically, the piezoelectric layer periodically physically deflects the first and second cantilevers. This deflection occurs in the so-called driving direction (i.e., in-plane direction). When a microelectromechanical system (MEMS) containing a cantilever that periodically vibrates in-plane rotates along an axis parallel to the first and second cantilevers 200b, the Coriolis force generates a deflection perpendicular to the so-called driving direction (i.e., the out-of-plane direction) or the so-called sensing direction. The out-of-plane deflection can be measured using a third lead-free piezoelectric layer, a third conductive layer 231a, 231b, and highly doped silicon 250a, 250b, because the third lead-free piezoelectric layer generates a measurable voltage between the third conductive layer and the highly doped silicon.

[0083] In one embodiment, the cantilever has a side wall width of 20 to 100 μm (e.g., 20 to 40 μm) and a height of 20 to 100 μm (e.g., 20 to 40 μm). The length of the cantilever may be 200 μm to 1 mm (e.g., 350 to 500 μm). Furthermore, the piezoelectric layer may have a thickness of 500 nm to 2 μm (e.g., 500 to 700 nm). Furthermore, the conductive layer may have a thickness of 100 to 500 nm (e.g., 200 to 400 nm).

[0084] Naturally, the embodiments of the invention disclosed herein are not limited to the specific structures, processing procedures, or materials disclosed herein, but extend to their equivalents, as will be understood by those skilled in the art. Furthermore, naturally, the terminology used herein is used solely for the purpose of describing specific embodiments and is not intended to be limiting.

[0085] Any reference to "one embodiment" or "an embodiment" throughout this specification means that certain features, structures, or characteristics described in relation to that embodiment are included in at least one embodiment of the present invention. Therefore, not all occurrences of the phrase "in one embodiment" or "in an embodiment" throughout this specification necessarily refer to the same embodiment. For example, where numerical values ​​are referenced using phrases such as "about" or "substantially," the exact numerical values ​​are also disclosed.

[0086] Multiple items, structural elements, compositional elements, and / or materials used herein may be included in general lists for convenience. However, these lists should be interpreted as if each element in the list were individually identified as a distinct and unique element. Accordingly, each element in such lists should be interpreted as a de facto equivalent of any other element in the same list, based solely on their presence in a general group, unless otherwise indicated. Furthermore, various embodiments and examples of the invention may be referred to herein in conjunction with alternative forms with respect to their various components. Naturally, such embodiments, examples, and alternative forms should not be interpreted as de facto equivalents of each other, but should be considered distinct and independent expressions of the invention.

[0087] Furthermore, the described features, structures, or properties may be combined in any suitable manner in one or more embodiments. The description so far has provided various specific details, such as examples of length, width, and shape, to ensure a full understanding of embodiments of the present invention. However, as those skilled in the art will understand, the present invention can be implemented without one or more of these specific details, or with other methods, components, materials, etc. In other examples, well-known structures, materials, or operations are not illustrated or described in detail, so as not to obscure aspects of the present invention.

[0088] The embodiments described above illustrate the principles of the present invention in one or more specific applications. However, as will be apparent to those skilled in the art, various modifications can be made to the form, usage, and details of the implementation without exercising inventive ability and without departing from the principles and concepts of the present invention. Accordingly, the present invention is not limited except as defined by the claims described below.

[0089] In this document, the verbs “to comprise” and “to include” are used as open limitations, neither excluding nor requiring the existence of features not described. Features described in dependent claims may be freely combined with each other unless otherwise specified. Furthermore, naturally, the use of “a” or “an,” i.e., the singular form, does not exclude plurality throughout this document. [Industrial applicability]

[0090] At least some embodiments of this disclosure have industrial applications in the manufacture and use of microelectromechanical systems (MEMS), and in particular in electrostatic and piezoelectric conversion of MEMS. List of acronyms ALD atomic layer deposition BOX embedded oxide film layer C abc Crystal coordinate system CVG Coriolis Oscillating Gyroscope CVD (Chemical Vapor Deposition) CMOS complementary metal-oxide-semiconductor dps degree / second (deg / s) FEA finite element analysis FEM finite element method HF Hydrofluoric Acid IoT (Internet of Things) MEMS (Micro Electromechanical Systems) MOCVD (Metal-Organometallic Vapor Deposition) NEMS Nanoelectromechanical Systems PVD (Physical Vapor Deposition) PZT Lead Zirconate Titanate RF radio frequency Scanning electron microscopy (SEM) S xyz Sample coordinate system Q Quality Factor m d Coordinate density of built geometry in the driving direction m sCoordinate density of built geometry in the driving direction r d Rayleigh damping coefficient in the driving direction r s Rayleigh attenuation coefficient in the detection direction k d Rigidity in drive mode k s Rigidity in detection mode F cor Coriolis force F d Force applied in the driving direction VPE vapor phase etching [Explanation of symbols]

[0091] 100, 200, 300 Cantilever 110, 210 First lateral wall of the cantilever 120, 220 Second side wall of the cantilever 130, 230 Cantilever top surface 111, 371 First conductive layer 121, 372 Second conductive layer 131 Third conductive layer 140, 340 Lead-free piezoelectric layer 141 First lead-free piezoelectric layer 142 Second lead-free piezoelectric layer 143 Third lead-free piezoelectric layer 150, 310 High-doped silicon 160, 260 fixed restraint 200a First Cantilever 200b Second Cantilever 210a First side wall in the first cantilever 210b First lateral wall in the second cantilever 220a Second side wall in the first cantilever 220b Second lateral wall in the second cantilever 211a First electric layer in the first cantilever 211b First electric layer in the second cantilever 221a Second electric layer in the first cantilever 221b Second electric layer in the second cantilever 230a Upper surface of the first cantilever 230b Upper surface of the second cantilever 231a Third electric layer in the first cantilever 231b Third electric layer in the second cantilever 250a Highly doped silicon in the first cantilever 250b High-doped silicon in the second cantilever 315 Insulator Mask 320 Embedded oxide layer 350 Insulators 360 device layer 370 Conductive layer 380 Base material 510 Drive direction 520 Detection direction 530 proof mass

Claims

1. A method for manufacturing a microelectromechanical system (MEMS) device, A step of forming at least one cantilever from a highly doped silicon layer on a buried oxide layer, wherein the at least one cantilever has a first sidewall and a second sidewall, and the first and second sidewalls are perpendicular to the interface between the highly doped silicon layer and the buried oxide layer, The steps include growing a lead-free piezoelectric layer on at least the first and second side walls, The steps of depositing a conductive material on at least the lead-free piezoelectric layer on the first side wall and the second side wall in order to form a first conductive layer on the lead-free piezoelectric layer on the first side wall and a second conductive layer on the lead-free piezoelectric layer on the second side wall, A method that includes this.

2. The method according to claim 1, wherein the forming step includes etching at least one cavity down to the highly doped silicon layer to remove the embedded oxide layer.

3. The method according to claim 1 or 2, wherein the highly doped silicon layer has a resistivity of less than 0.01 Ω·cm or less than 0.005 Ω·cm.

4. The method according to any one of claims 1 to 3, wherein the highly doped silicon layer is a device layer of a silicon-on-insulator (SOI) substrate.

5. The method according to any one of claims 1 to 4, wherein the first conductive layer and / or the second conductive layer comprises at least one of molybdenum (Mo), titanium nitride (TiN), and aluminum (Al).

6. The aforementioned embedded oxide film layer is silicon dioxide (SiO ２ The method according to any one of claims 1 to 5, including )

7. The method according to any one of claims 1 to 6, wherein the lead-free piezoelectric layer comprises aluminum nitride (AlN).

8. The step of growing the lead-free piezoelectric layer includes the step of growing a portion of the lead-free piezoelectric layer on the upper surface of at least one silicon beam, The method further includes the step of depositing a third conductive layer on the portion of the lead-free piezoelectric layer. The method according to any one of claims 1 to 7.

9. The method according to any one of claims 1 to 8, wherein at least two parallel silicon beams are open and separated by a gap, the gap being 50 to 300 μm.

10. The method according to any one of claims 1 to 9, wherein the at least one silicon cantilever has a width of 20 to 100 μm, a height of 20 to 100 μm, and a length of 200 μm to 1 mm, and the lead-free piezoelectric layer has a thickness of 500 nm to 2 μm.

11. At least one cantilever, the at least one cantilever forms part of a silicon layer containing highly doped silicon, the at least one cantilever includes a top surface, a bottom surface, a first side wall, and a second side wall, the first and second side walls connecting the top surface and the bottom surface in the thickness direction of the silicon layer, The first lead-free piezoelectric layer on the first side wall of the at least one cantilever and A second lead-free piezoelectric layer on the second side wall of at least one cantilever, The first conductive layer on the first lead-free piezoelectric layer, The second conductive layer on the second lead-free piezoelectric layer, Microelectromechanical systems (MEMS) devices, including those mentioned above.

12. The microelectromechanical system (MEMS) device according to claim 11, wherein the at least one cantilever comprises highly doped silicon with resistivity less than 0.01 Ω·cm or less than 0.005 Ω·cm.

13. The microelectromechanical system (MEMS) device according to claim 11 or 12, wherein the upper surface is perpendicular to the first side wall and the second side wall.

14. The aforementioned device further, A third lead-free piezoelectric layer on the upper surface of at least one cantilever, A third conductive layer on the third lead-free piezoelectric layer, A microelectromechanical system (MEMS) device according to any one of claims 11 to 13, including the following:

15. A microelectromechanical system (MEMS) device according to any one of claims 11 to 14, wherein at least two parallel cantilevers are separated by a gap, the gap being 50 to 300 μm.

16. The microelectromechanical system (MEMS) device according to any one of claims 11 to 15, wherein the first and second lead-free piezoelectric layers comprise aluminum nitride (AlN).

17. The microelectromechanical system (MEMS) device according to any one of claims 11 to 16, wherein the at least one silicon cantilever has a width of 20 to 100 μm, a height of 20 to 100 μm, and a length of 200 μm to 1 mm, and the lead-free piezoelectric layer has a thickness of 500 nm to 2 nm.

18. The aforementioned device further, A first contact electrically connected to the first conductive layer, A second contact electrically connected to the second conductive layer, A reference contact electrically connected to the highly doped silicon of the at least one cantilever, A microelectromechanical system (MEMS) device according to any one of claims 11 to 17, comprising:

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