Polysilicon Diaphragm Research Articles

Ultra-small high‐temperature pressure sensor chips fabricated in single‐layer (111) SOI wafers

This paper presents a 0.5 mm × 0.5 mm tiny-sized high-temperature piezoresistive pressure sensor fabricated with a thin-film under bulk single-sided micromachining process from the front-side of single-layer (111) silicon on insulator (SOI) wafer. The (111)-device layer of the SOI wafer is specifically selected to optimize the piezoresistor performance where the SiO2 buried layer isolates the piezoresistors from the handle substrate. And the (111)-handle layer is employed to subtly construct a single-crystalline-silicon beam-island combining with a very thin but uniform poly-silicon diaphragm for realization of both low nonlinearity and high sensitivity. Without double-sided micromachining process and wafer bonding used, a tiny sensor-chip size of 0.5 mm × 0.5 mm is achieved, which is required in wind-tunnel systems for measuring pressure distribution at high temperature. The testing results of the fabricated sensor show high sensitivity of 0.15 mV V−1 kPa−1 for 150 kPa measure range and satisfactory linearity of ±0.11% FS (full scale) at ambient temperature. At 350 °C, the overall accuracy is 0.17% FS and the thermal hysteresis is 0.22% FS. The temperature coefficient of zero-point offset of the sensor is tested as low as 0.01% °C−1 FS and the temperature coefficient of sensitivity is −0.07% °C−1 FS within the whole temperature range from −55 °C to 350 °C. Featuring the advantages of high accuracy and high-yield low-cost fabrication, the tiny-sized high-temperature pressure sensors exhibit promising perspectives in the field of aerospace industry including wind-tunnel applications.

A Novel MEMS Capacitive Microphone with Semiconstrained Diaphragm Supported with Center and Peripheral Backplate Protrusions.

Audio applications such as mobile phones, hearing aids, true wireless stereo earphones, and Internet of Things devices demand small size, high performance, and reduced cost. Microelectromechanical system (MEMS) capacitive microphones fulfill these requirements with improved reliability and specifications related to sensitivity, signal-to-noise ratio (SNR), distortion, and dynamic range when compared to their electret condenser microphone counterparts. We present the design and modeling of a semiconstrained polysilicon diaphragm with flexible springs that are simply supported under bias voltage with a center and eight peripheral protrusions extending from the backplate. The flexible springs attached to the diaphragm reduce the residual film stress effect more effectively compared to constrained diaphragms. The center and peripheral protrusions from the backplate further increase the effective area, linearity, and sensitivity of the diaphragm when the diaphragm engages with these protrusions under an applied bias voltage. Finite element modeling approaches have been implemented to estimate deflection, compliance, and resonance. We report an 85% increase in the effective area of the diaphragm in this configuration with respect to a constrained diaphragm and a 48% increase with respect to a simply supported diaphragm without the center protrusion. Under the applied bias, the effective area further increases by an additional 15% as compared to the unbiased diaphragm effective area. A lumped element model has been also developed to predict the mechanical and electrical behavior of the microphone. With an applied bias, the microphone has a sensitivity of −38 dB (ref. 1 V/Pa at 1 kHz) and an SNR of 67 dBA measured in a 3.25 mm × 1.9 mm × 0.9 mm package including an analog ASIC.

Ultra-small pressure sensors fabricated using a scar-free microhole inter-etch and sealing (MIS) process

This paper presents a tiny 0.4 mm × 0.4 mm piezoresistive absolute pressure sensor chip with a fabrication cost as low as 0.01 US$/die. With the thin-film under bulk-silicon technique, a very thin but uniform poly-silicon pressure-sensing diaphragm is formed beneath a bulk-silicon beam-island structure to accommodate piezoresistors. The thin diaphragm exhibits a high sensitivity, and the beam-island-reinforced structure helps reduce deflection and improve linearity. The sensor is fabricated using a novel scar-free micro-hole inter-etch and sealing (MIS) process. In this newly developed process, seals are removed from the diaphragm area and placed on both the single-crystalline silicon island and single-crystalline silicon frame surrounding the poly-silicon diaphragm. Thus, the thin diaphragm is kept flat and smooth, thereby enhancing the sensing performance and fabrication yield. More importantly, the diaphragm-beam-island structure is small enough to fabricate sensor chips as small as 0.4 mm × 0.4 mm. With the high-yield process, a very low fabrication cost of 0.01 US$/die is realised owing to a high throughput of 90 000 die per 6 inch wafer. The ultra-small and low-cost pressure sensor exhibits sensitivity of 0.88 mV kPa−1/3.3 V, hysteresis of 0.15% full scale (FS), repeatability error of 0.04% FS, and non-linearity of 0.10% FS. Without any additional thermal compensation method, the sensor exhibits a low temperature coefficient of zero-point offset of −0.064%/°C FS and a temperature coefficient of sensitivity (TCS) of −0.22%/°C within the temperature range from −25 °C to +85 °C, at a full measurement range (i.e. full scale) of 100 kPa. With the new scar-free MIS process, the presented pressure sensor looks promising for smartphones, drones, and other consumer-electronic applications.

High Sensitive MEMS Intraocular Capacitive Pressure Sensor (Glaucoma)

Micro Electro Mechanical Systems (MEMS) are a small-scale technology that was largely adopted by the IC industry and applied to miniaturize of all systems (electrical systems, mechanical, optical, fluidic, magnetic, etc.). Minimization has been accomplished with small manufacturing processes. A Capacitive pressure sensor is simply a diaphragm-type device in which the diaphragm displacement is determined by measuring the capacitance change between the diaphragm and a metal plate that is close to it.
 For this purpose, intraocular pressure sensors are important in detection and treatment of an incurable disease called glaucoma. To improve the sensitivity of the capacitive pressure sensor, low stress doped polysilicon material is used as a biocompatible material. Glaucoma is a group of eye diseases that occurs by high intraocular pressure (IOP). IOP is the pressure exerted by the ocular fluid called aqueous humor (the clear fluid inside the eye) that fills the anterior chamber of the eye
 The results Shows the simulated relation between capacitance and pressure for clamped ++silicon and polysilicon clamped. It can be seen from figure that the initial capacitance for clamped p++ silicon is about 1.81 pF the capacitance varies from 1.81 to 2.162 pF for clamped p++silicon and clamped polysilicon diaphragm, respectively, so the total variation of the capacitance. This result shows the use of poly silicon material in diaphragm is high sensitivity than the p++ silicon.

High-Performance Low-Range Differential Pressure Sensors Formed With a Thin-Film Under Bulk Micromachining Technology

In the microelectromechanical systems (MEMS) field, low-range (e.g., kPa or sub-kPa) differential pressure sensors have remained difficult for decades. Generally, there are two unsolved problems that hinder volume production and wide applications in consumer-electronics. First, high sensitivity required very thin and large sensing diaphragm normally fabricated with low yield, and thus, high selling price. Second, high sensitivity required large deflection of the diaphragm easily causeing severe output of non-linearity that degrades the detecting precision. This paper reports a so-called thin-film under bulk (TUB) MEMS technique to high-yield fabricate high-performance low-range pressure sensors for smart central air-conditioning systems. With the single-wafer-based TUB structure, pressure-induced stress is highly concentrated in a bulk-silicon beam-island structure for piezoresistive readout, where a very thin but uniform poly-silicon diaphragm can be formed beneath the bulk-silicon structure to sustain the pressure. The beam-island reinforced structure brings about decreased deflection and highly linear output. Micro-columns are also formed under the bulk-silicon islands as over-range protection stoppers that are important for low-range pressure sensor. After the description about the 1.2 mm $\times 1.2$ mm tiny-sized sensor-chip design and its single-wafer fabrication process, a test is conducted that results in 22-mV output for 1.2-kPa measure range and very good linearity of ±0.05% (FS). About $100\times $ over-range protection capability of the sensor is also achieved. The novel high-yield TUB process can be used for low-cost production of high-performance pressure sensors for the applications of new-generation smart central air-conditional systems and spirometers, etc, and the bulk/surface combined TUB micromachining technique promises to be widely used for the formation of various complex MEMS structures. [2016-0309]

Singularly perturbed BVP to estimation of diaphragm deflection in MEMS capacitive microphone: An application of ADM

In this paper, we implement the Adomian decomposition method (ADM) to approximate the deflection of polysilicon diaphragm with small flexural rigidity of Micro Electro Mechanical System (MEMS) capacitive microphone. We prove the existence and the uniqueness of the solution of considered problem with the help of the theory of semi-group. Special attention is given to prove the convergence of the implemented method. Numerical simulation and comparison between the approximate solutions by using the ADM with the variational iteration method are given.

Design and performance analysis of MEMS capacitive pressure sensor array for measurement of heart rate

In this paper, a diaphragm based MEMS capacitive pressure sensor array in conjunction with signal conditioning circuitry is designed for measuring Heart Rate (HR) of human being. In order to measure the heart rate, an array of capacitive sensors are mounted on wrist in such a way that the radial artery blood flow during contraction and expansion of the heart, impinges on the sensor diaphragms. Due to blood flow in the wrist artery, the designed circular diaphragm of the sensor is being deflected and thus induces a corresponding change in the original capacitance value. A proposed schematic of the system to measure the change in capacitance consists of signal conditioning circuitry along with the sensors. The structure of the sensor consists of parallel plates having 1.5 micron air gap in between. This change in capacitance is detected by a CMOS based capacitance-to-voltage converter which is designed as a precision interface with the sensor array. The designed MEMS sensor has a Poly Silicon diaphragm and is capable of identifying HR (60–100 bpm) which corresponds to a specific change in absolute pressure from 10 to 100 kPa for 5 micron thick diaphragm with 3600 µm area. The design of the sensor along with its fabrication steps and characteristics analysis are performed in FEM based simulation software.

A surface micromachined pressure sensor based on polysilicon nanofilm piezoresistors

In order to effectively apply the piezoresistive properties of the polysilicon nanofilm and the surface micromachined technology, a surface micromachined pressure sensor based on polysilicon nanofilm piezoresistors and vacuum cavity is presented. The design of the sensor structure is carried out by finite element analysis for improving its performance. Residual stress relaxation in the polysilicon diaphragm and prevention of adhesion in the sensor cavity are carried out in the process of the sensor fabrication. The sensor sample is fabricated according to the structure designed. The sample's measurement results show that a full scale pressure of 2.5MPa, a sensitivity of 2.896×10−2mV/V/kPa at 25°C, an overpressure of 7 times higher than the full scale pressure at 25°C, a thermal zero drift of −0.01%FS/°C and a thermal sensitivity drift of −0.1%FS/°C in a temperature range of −40 to 200°C are achieved. The measurement performance is excellent and agrees with the predictions of the design method.

A broadband, capacitive, surface-micromachined, omnidirectional microphone with more than 200 kHz bandwidth.

A surface micromachined microphone is presented with 230 kHz bandwidth. The structure uses a 2.25 μm thick, 315 μm radius polysilicon diaphragm suspended above an 11 μm gap to form a variable parallel-plate capacitance. The back cavity of the microphone consists of the 11 μm thick air volume immediately behind the moving diaphragm and also an extended lateral cavity with a radius of 504 μm. The dynamic frequency response of the sensor in response to electrostatic signals is presented using laser Doppler vibrometry and indicates a system compliance of 0.4 nm/Pa in the flat-band of the response. The sensor is configured for acoustic signal detection using a charge amplifier, and signal-to-noise ratio measurements and simulations are presented. A resolution of 0.80 mPa/√Hz (32 dB sound pressure level in a 1 Hz bin) is achieved in the flat-band portion of the response extending from 10 kHz to 230 kHz. The proposed sensor design is motivated by defense and intelligence gathering applications that require broadband, airborne signal detection.

Rotational Capacitive Micromachined Ultrasonic Transducers (cMUTs)

A rotational capacitive micromachined ultrasonic transducer is introduced. Two pressure-sensitive diaphragms atop vacuum-sealed cavities are mechanically coupled via a rocking beam structure. The rocking structure is designed to reduce deflection of the vacuum-sealed diaphragms under atmospheric pressure while introducing ideally zero rotational stiffness to differential diaphragm pressure. The rocking structure responds to in-plane pressure gradients and is therefore anticipated to have dipole-type directivity to ultrasound. Prototypes employing 200 μm diameter polysilicon diaphragms are presented with a fundamental resonance frequency of 250 kHz.

Finite Element Analysis and Modeling of Mems Pressure Sensor for Intraocular Pressure

This paper presents the Finite Element Analysis (FEA) of MEMS based capacitive pressure sensor to monitor intraocular pressure. The sensor composed of a square Polysilicon diaphragm that deflects in response to applied pressure. Applied pressure deflects the 2 µm diaphragm, changing the capacitance between the Polysilicon diaphragm and the bottom electrode Si substrate. The square diaphragm produces highest induced stress resulting in high sensitivity. Intellisuite FEA modeling software was used to predict sensor actuation, capacitance output and the avoidance of material failure. The FEA modeling and simulated data results is generated. The simulation of the MEMS capacitive pressure sensor achieves good linearity in the intraocular pressure range. The geometry and dimensions are selected to produce optimal performance of the sensor.

Modeling and Analysis of Thin Film PolySi Diaphragm Pressure Sensor

Micromachining technology has greatly benefited from the success of developments in implantable biomedical micro devices. In this paper, a simulation solution of micro-electro mechanical systems (MEMS) capacitive pressure sensor operating for biomedical applications in the range of 20–400 mmHg was designed. Employing the micro electro mechanical systems (MEMS) technology, high sensor sensitivities and resolutions have been achieved. This paper provides initial data on the design and simulation of such a sensor. Capacitive sensing uses the diaphragm deformation-induced capacitance change. The sensor composed of a rectangular Polysilicon diaphragm that deflects due to pressure applied over it. Applied pressure deflects the 2 μm diaphragm changing the capacitance between the Polysilicon diaphragm and gold flat electrode deposited on a glass pyrex substrate. The simulation of the MEMS capacitive pressure sensor achieves good linearity and large operating pressure range. The static and thermo electro mechanical analysis was performed. The FEA modeling and simulated data results were generated. The capacitive response of the sensor performed as expected according to the relationship of the spacing of the plates. Intellisuite software is used for modeling and simulation of MEMS capacitive pressure sensor to optimize the design, improve the performance and to reduce the time of fabricating process of the device.

Analysis and Optimization of Sensitivity of a MEMS Peizoresistive Pressure Sensor

This paper presents a MEMS Piezoresistive pressure sensor which utilizes a circular shaped polysilicon diaphragm with a nanowire to enhance the sensitivity of the pressure sensor. The polysilicon nanowire is fabricated in such a way that it forms a bridge between the circular polysilicon diaphragm and the substrate. The high Piezoresistive effect of Silicon nanowires is used to enhance the sensitivity. A circular polysilicon nanowire piezoresistor was fabricated by means of reactive ion etching. This paper describes the performance analysis, structural design and fabrication of piezoresistive pressure sensor using simulation technique. The polysilicon nanowire pressure sensor has a circular diaphragm of 500nm radius and has a thickness about 10nm. Finite element method (FEM) is adopted to optimize the sensor output and to improve the sensitivity of the circular shaped diaphragm of a polysilicon nanowire Piezoresistive pressure sensor. The best position to place the Polysilicon nanowires to receive maximum stress was also considered during the design process..The fabricated polysilicon nanowire has high sensitivity of about 133 mV/VKPa.

Simulation of Low Pressure MEMS Sensor for Biomedical Application

Micromachining technology has greatly benefited from the success of developments in implantable biomedical microdevices. In this paper, microelectromechanical systems (MEMS) capacitive pressure sensor operating for biomedical applications in the range of 20–400 mm Hg was designed. Employing the microelectromechanical systems technology, high sensor sensitivities and resolutions have been achieved. Capacitive sensing uses the diaphragm deformation-induced capacitance change. The sensor composed of a rectangular polysilicon diaphragm that deflects due to pressure applied over it. Applied pressure deflects the 2 µm diaphragm changing the capacitance between the polysilicon diaphragm and gold flat electrode deposited on a glass Pyrex substrate. The MEMS capacitive pressure sensor achieves good linearity and large operating pressure range. The static and thermo electromechanical analysis were performed. The finite element analysis data results were generated. The capacitive response of the sensor performed as expected according to the relationship of the spacing of the plates.

Parametric study of flip-chip packaging for an MEMS device with diaphragm

In present study, a backside-etched silicon chip with a polysilicon diaphragm flip-chip attached on a printed wiring board (PWB) and globally bumped on a FR4 substrate was investigated based on finite element analysis (FEA) for determining three key parameters of flip chip chip size packaging (FC-CSP), namely, the size of solder bump, the thickness of PWB substrate, with/without U8437-3 underfill. In order to improve the reliability and the yield of MEMS packaging, the thermal cycle of environmental test ranging from −40° to 125° was utilized for determination of the influence of key parameters on the thermal-induced stresses and the deflection in the diaphragm as well as the warp of PWB substrate. In addition, the thermal stress occurred at the interface between solder bump and silicon chip was also analyzed to understand the failure mechanism of solder bump. According to the simulation results, we concluded three phenomena as follow: (1) As the thickness of substrate increased, the thermally-induced stress in the diaphragm also increased, but the deflections of diaphragm and substrate decreased instead. (2) As the size of solder bump increased, both of stress and deflection in the diaphragm decreased, however, the warp of substrate was insensitive to the size of solder bump. (3) If the underfill material encapsulated the solder bump between chip and substrate, the induced stress occurred at the interface between solder joint and chip was decreased, however, the stress in the diaphragm was increased instead. On the other hand, the deflection was smaller in the diaphragm but larger in the substrate as the underfill existed in the model. In general, the parametric study can provide the basis for the flip chip package of MEMS device with diaphragm, such as MEMS microphone, MEMS pressure sensor, etc.

Micromachined optical microphone structures with low thermal-mechanical noise levels

Micromachined microphones with diffraction-based optical displacement detection have been introduced previously [Hall et al., J. Acoust. Soc. Am. 118, 3000-3009 (2005)]. The approach has the advantage of providing high displacement detection resolution of the microphone diaphragm independent of device size and capacitance-creating an unconstrained design space for the mechanical structure itself. Micromachined microphone structures with 1.5-mm-diam polysilicon diaphragms and monolithically integrated diffraction grating electrodes are presented in this work with backplate architectures that deviate substantially from traditional perforated plate designs. These structures have been designed for broadband frequency response and low thermal mechanical noise levels. Rigorous experimental characterization indicates a diaphragm displacement detection resolution of 20 fm radicalHz and a thermal mechanical induced diaphragm displacement noise density of 60 fm radicalHz, corresponding to an A-weighted sound pressure level detection limit of 24 dB(A) for these structures. Measured thermal mechanical displacement noise spectra are in excellent agreement with simulations based on system parameters derived from dynamic frequency response characterization measurements, which show a diaphragm resonance limited bandwidth of approximately 20 kHz. These designs are substantial improvements over initial prototypes presented previously. The high performance-to-size ratio achievable with this technology is expected to have an impact on a variety of instrumentation and hearing applications.

The surface and bulk microfabrication of optical seismometers and vibrometers using Sandia National Laboratories’ silicon micromachining technology

The microfabrication of optical seismometer and vibrometer structures using Sandia’s silicon-based process technology is presented. The structure employs a 1.5-mm-diameter polysilicon diaphragm anchored to a 200- to 400-μm-thick bulk silicon proof mass, and suspended by surface micromachined polysilicon springs. Motion of the diaphragm is detected using an integrated polysilicon surface micromachined grating-based optical interferometer. Initial dynamic characterization of these structures in vacuum shows a fundamental resonant frequency of 250 Hz. Based on the high force sensitivity of these structures combined with interferometric motion detection, 1-μg/√Hz detection resolution in the 1-Hz frequency range is estimated. Microfabricated structures employing electrostatic actuation electrodes for the diaphragm are also presented, which enable force feedback operation for high dynamic range and enhanced dc stability. These characteristics may make these sensors well-suited for seismic and inertial navigation applications.

A diode-based two-wire solution for temperature-compensated piezoresistive pressure sensors

A novel two-wire solution for ultraminiaturized temperature-compensated piezoresistive pressure sensors is presented. The technique makes it possible to measure both pressure and temperature separately by using bias-controlled diodes as switches between the pressure and temperature sensing elements. The p-n junction diodes are forward-biased or reverse-biased depending on the polarization of the voltage supply. The new diode-based two-wire technique was evaluated using a surface micromachined pressure sensor where the pressure-sensing element consists of a piezoresistor on an (80/spl times/40/spl times/1 /spl mu/m) double end supported force- transducing beam. The beam is located beneath a (100/spl times/100/spl times/2 /spl mu/m) square polysilicon diaphragm having its ends attached to the diaphragm and to the cavity edge. The thermal compensation piezoresistor is also located in the cavity on a (100/spl times/40/spl times/1 /spl mu/m) beam. Both ends of this beam are attached to the cavity edge and are therefore pressure-insensitive. The new detection solution enables a reduction of conducting leads from three to two in combination with high pressure sensitivity (0.7 /spl mu/V/V/mmHg) and environmental isolation compared to a commercialized traditional piezoresistive pressure sensor. Its simplicity and the potential for size reduction due to fewer bonding pads and conducting wires make it ideal for applications such as disposable blood pressure sensors where cost and size are critical parameters.

A temperature compensated dual beam pressure sensor

The first temperature compensated leverage beam pressure sensor fabricated by surface micromachining is presented. The pressure-sensing element consists of a piezoresistor on an 80μm×40μm×1 μm double end supported force transducing beam. The beam is located beneath a (100μm×100μm×2 μm) square polysilicon diaphragm having its ends attached to the diaphragm and to the cavity edge, thus, entirely enclosed inside the vacuum cavity. The thermal compensation piezoresistor is also located in the cavity on a (100μm×40μm×1 μm beam to achieve nearly identical thermal conditions. Both ends of this beam are attached to the cavity edge and is therefore, pressure-insensitive. The new design enables a combination of high pressure sensitivity (0.8 μV/(V mmHg)), environmental isolation and a decrease of the relative temperature dependency mismatch from 6 to 3% compared to a commercialized traditional piezoresistive pressure sensor.

A free-hanging strain-gauge for ultraminiaturized pressure sensors

Abstract The first entirely surface micromachined free-hanging strain-gauge pressure sensor is presented. The sensing element consists of a 80 μm long H-shaped force transducing beam supported at both ends. The H-shape enables the strain-gauge to be a part of the beam without the need for additional layers. The beam is located beneath and attached at one end to a square polysilicon diaphragm and at the other end to the cavity edge. The diaphragm of the fabricated sensor is 2 μm thick and has a side length of only 100 μm. The new design enables a combination of high pressure sensitivity and miniature chip size as well as good environmental isolation. The pressure sensitivity of the sensor with a H-shaped 0.4 μm thick force transducing beam was measured to be 5 μV/V/mmHg.