MEMS Pressure Sensor Research Articles

Optical MEMS Pressure Sensors Incorporating Dual Waveguide Bragg Gratings on Diaphragms

The wavelength shift of single Bragg grating integrated into a waveguide in optical MEMS pressure sensors gives inaccurate result due to the cross sensitivity caused by multiple external factors including temperature. Novel dual waveguide Bragg gratings (WBGs)-based microelectromechanical system pressure sensors with silicon micromachined diaphragm are presented here. The sensor consists of a curved waveguide with two identical WBGs located on the diaphragm. When pressure is applied on the diaphragm, the pitch of the gratings changes, and hence, there is a shift in the corresponding Bragg wavelength. The effect of temperature is assumed to be the same for both the gratings. As temperature shifts the Bragg wavelength of the two gratings equally, the error due to the temperature change on pressure sensitivity is eliminated. This method of pressure measurement effectively cancels out the temperature sensitivity. Three different configurations of the sensor with circular, square, and rectangular diaphragms are designed and simulated using COMSOL Multiphysics. Pressure sensitivity for the circular diaphragm configuration is found to be 2.1 and 0.744 pm/Pa for the two gratings, which are higher than the square and rectangular diaphragm configuration. Hence, the circular diaphragm configuration is better suited for multiplexing. The temperature independent properties of the sensors as well as the adjustability of sensitivity and measurement range by means of altering the dimensions of the sensors are studied. The temperature sensitivity is the same for both the gratings and is found to be 11.6 ppm/°C. Using modulation techniques and suitable designs, these sensors find applications in remote distributed sensing.

Effects of gas bubbles in protection gel on performance of MEMS pressure sensor

The process of dispensing gel above the silicon chip aimed at protecting chip surface and bonding wire is one of the most critical steps in the production of highly accurate low-pressure sensor modules. Numerous research efforts focused on the influences of dispensing process parameters, configuration and mechanical behavior of cured gel on the performance of sensors, while few concerns were taken on the air bubbles formed in protection gel that adversely influence the sensitivity and zero-drift in full temperature range. In this research, a series of hydrostatic fluid-element based 3D models in consideration of different positions and various diameters of gas bubble were conducted to explore this phenomenon. Results show that the existence of gas bubble brings in an extra load and changes the pressure distribution on the sensing membrane of the chip, which will result in zero-drift and sensitivity variation of packaged sensors. The influence degree is closely correlated with the position and size of gas bubble in protection gel. An experiment was conducted to verify the simulation, the results fit well. Due to the random distribution of bubble it is almost impossible to design signal modulation circuits to compensate for voltage drift in mass production. Therefore, in practical production, the gas bubble should be avoided especially in the region close to the sensing membrane of sensor chip.

Comparison of hermetic sealing using SAC and SnPb solder for a MEMS pressure sensor

In order to minimize the influence of packaging stress on the signal of MEMS pressure sensors, the pressure inlet can be reduced in footprint and mechanically decoupled from the mechanically moving parts. Moreover, the formation of a hermetic seal between the sensor inlet and an external inlet becomes more challenging as the footprint is reduced. Soldering is a preferred solution as a hermetic seal is achievable despite some surface roughness at the surfaces to be joined. However, the metallization on the MEMS, the solder and the metallization on the external inlet must be carefully matched to assure long term stability; the solder will react quickly with the metal layers deposited on the surfaces during the reflow process and later at a reduced rate during storage and application. The formation of intermetallic compounds (IMC) can catastrophically degrade the integrity of a joint if large amounts of voids are formed or the mechanical compliance significantly reduces as a result of the IMC formation. The metallization alternatives for the MEMS in this case were sputtered TiW/Au and NiCr/Au. The TiW and NiCr are adhesion layers whereas the Au is applied as a wetting layer which is normally fully consumed during the soldering process. A thick layer of plated Au, or a thick layer of plated Ni with a thin surface finish layer of Au, were possible metallization alternatives for the external inlet. Dewetting of solder from TiW is frequently mentioned in literature, but less conclusive work is published about soldering to NiCr/Au [1–3]. In particular, limited work has been published on long term effects of soldering to NiCr/Au surfaces using a SAC solder. In this work TiW and NiCr were compared as adhesion layers. In addition, SAC and SnPb were compared as solder, and Au and Ni/Au were compared as metallization on the external inlet. A total of 10–20 assemblies were prepared for each of 12 tested combinations. Half of the assemblies were exposed to high temperature storage (HTS) for ~300 hours at 130–150 °C. Shear testing and inspection of fracture surfaces and cross sections using light microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were performed for samples

New MEMS Pressure Sensor Element and Concept for Coronary Catheter

We report a capacitive pressure sensor to be integrated in a FFR catheter. The novelty presented in this work is based on two innovations: (1) VTT surface micromachined process used in fabrication of a ultrathin and narrow pressure sensor element and (2) mounting of the pressure sensor on the catheter sidewall instead of the guide wire enabling thus to use standard guide wires. Two design and fabrication runs were accomplished. The measurement results of the MEMS pressure sensor element correlate well with the calculated and simulated results.

Development of a CMOS MEMS pressure sensor with a mechanical force-displacement transduction structure

This study presents a capacitive pressure sensor with a mechanical force-displacement transduction structure based on the commercially available standard CMOS process (the TSMC 0.18 μm 1P6M CMOS process). The pressure sensor has a deformable diaphragm to support a movable plate with an embedded sensing electrode. As the diaphragm is deformed by the ambient pressure, the movable plate and its embedded sensing electrode are displaced. Thus, the pressure is detected from the capacitance change between the movable and fixed electrodes. The undeformed movable electrode will increase the effective sensing area between the sensing electrodes, thereby improving the sensitivity. Experimental results show that the proposed pressure sensor with a force-displacement transducer will increase the sensitivity by 126% within the 20 kPa–300 kPa absolute pressure range. Moreover, this study extends the design to add pillars inside the pressure sensor to further increase its sensing area as well as sensitivity. A sensitivity improvement of 117% is also demonstrated for a pressure sensor with an enlarged sensing electrode (the overlap area is increased two fold).

Automatic Drift Cancellation of Implanted Bladder Pressure Sensor.

Implanted pressure sensors suffer from long-term offset drift due to atmospheric changes, package moisture absorption, and patient factors such as posture, implant shift, and tissue overgrowth. Traditionally, wide dynamic range instrumentation is used to satisfy the full-scale and sensitivity requirements for a given application. Transmission of extra bits greatly increases the power draw of an implanted medical device, and simple AC-coupling cannot monitor static pressures. We present a mixed-signal offset cancellation loop to maximize the AC dynamic range of instrumentation circuitry. A digital implementation allows for designer control of the cancellation system time constant and was specifically designed for power-gated pressure sensors. Pressure offset is calculated by digital integration and a bipolar IDAC with coarse/fine tuning injects an offset-cancelling current into a standard piezoresistive MEMS pressure sensor. Test results showed a dynamic range increase of 2.9 bits using dynamic offset cancellation, for an effective sensing range of 11 bits using 8-bit instrumentation. The measured step response of the system showed an overall highpass response of 2.3 - 3.8 mHz. This approach is therefore relevant for bio-sensing of pressures in organs with a very slow physiologic response, e.g. the bladder.

Calibration of high frequency MEMS microphones and pressure sensors in the range 10 kHz–1 MHz

In the context of both nonlinear acoustics, and downscaled acoustic or aero-acoustic experiments, the characterization of the high frequency response of microphones and pressure sensors remains a critical challenge. In the case of the design of new MEMS microphones and shock pressure sensors with response in the frequency range of 10 kHz–1 MHz, this question was addressed by the definition of a new calibration method based on a spark source that generates spherical weak shock acoustic pulse. Waves are short duration non-symmetric N-waves with duration of about 40 microseconds and front shock rise time of the order of 0.1 microsecond. Taking advantage of recent works on the characterization of such pressure waves using an optical interferometer, and considering non linear propagation of weak shockwaves, we were able to estimate the incident pressure wave in the range of 10 kHz–1MHz. Hence, from the output voltage of the microphones, the frequency response was obtained in this range. The method applies whatever the transduction principle and the sensor mounting. [Work supported by the French National Agency for Research (SIMI 9, ANR 2010 BLANC 0905 03, and LabEx CeLyA ANR-10-LABX-60/ANR-11-IDEX-0007).]

Micro packaged MEMS pressure sensor for intracranial pressure measurement**Project supported by the National Natural Science Foundation of China (Nos. 61025021, 61434001), and the ‘Thousands Talents’ Program for Pioneer Researchers and Its Innovation Team, China.

This paper presents a micro packaged MEMS pressure sensor for intracranial pressure measurement which belongs to BioMEMS. It can be used in lumbar puncture surgery to measure intracranial pressure. Miniaturization is key for lumbar puncture surgery because the sensor must be small enough to allow it be placed in the reagent chamber of the lumbar puncture needle. The size of the sensor is decided by the size of the sensor chip and package. Our sensor chip is based on silicon piezoresistive effect and the size is 400 × 400 μm2. It is much smaller than the reported polymer intracranial pressure sensors such as liquid crystal polymer sensors. In terms of package, the traditional dual in-line package obviously could not match the size need, the minimal size of recently reported MEMS-based intracranial pressure sensors after packaging is 10 × 10 mm2. In this work, we are the first to introduce a quad flat no-lead package as the package form of piezoresistive intracranial pressure sensors, the whole size of the sensor is minimized to only 3 × 3 mm2. Considering the liquid measurement environment, the sensor is gummed and waterproof performance is tested; the sensitivity of the sensor is 0.9 × 10−2 mV/kPa.

LONG-TERM EVALUATION OF A NON-HERMETIC MICROPACKAGE TECHNOLOGY FOR MEMS-BASED, IMPLANTABLE PRESSURE SENSORS.

This paper reports long-term evaluation of a micropackage technology for an implantable MEMS pressure sensor. The all-polymer micropackage survived 160 days when subjected to accelerated lifetime testing at 85 °C in a 1% wt. saline solution. The package shows minimum effect on sensors' sensitivity and nonlinearity, which deviated by less than 5% and 0.3%, respectively. A 6-month in vivo evaluation of 16 MEMS-based pressure sensors demonstrated that the proposed micropackage has good biocompatibility and can protect the MEMS pressure sensor. To the best of our knowledge, these results establish new lifetime records for devices packaged using an all-polymer micropackaging approach.

Research on High-Precision, Low Cost Piezoresistive MEMS-Array Pressure Transmitters Based on Genetic Wavelet Neural Networks for Meteorological Measurements

This paper provides a novel and effective compensation method by improving the hardware design and software algorithm to achieve optimization of piezoresistive pressure sensors and corresponding measurement systems in order to measure pressure more accurately and stably, as well as to meet the application requirements of the meteorological industry. Specifically, GE NovaSensor MEMS piezoresistive pressure sensors within a thousandth of accuracy are selected to constitute an array. In the versatile compensation method, the hardware utilizes the array of MEMS pressure sensors to reduce random error caused by sensor creep, and the software adopts the data fusion technique based on the wavelet neural network (WNN) which is improved by genetic algorithm (GA) to analyze the data of sensors for the sake of obtaining accurate and complete information over the wide temperature and pressure ranges. The GA-WNN model is implemented in hardware by using the 32-bit STMicroelectronics (STM32) microcontroller combined with an embedded real-time operating system µC/OS-II to make the output of the array of MEMS sensors be a direct digital readout. The results of calibration and test experiments clearly show that the GA-WNN technique can be effectively applied to minimize the sensor errors due to the temperature drift, the hysteresis effect and the long-term drift because of aging and environmental changes. The maximum error of the low cost piezoresistive MEMS-array pressure transmitter proposed by us is within 0.04% of its full-scale value, and it can satisfy the meteorological pressure measurement.

Integration of MEMS Inertial and Pressure Sensors for Vertical Trajectory Determination

Integration of a low-cost global positioning system (GPS) with a microelectromechanical system-based inertial measurement unit (MEMS-IMU) is a widely used method that takes advantage of the individual superiority of each system to get a more accurate and robust navigation performance. However, because of poor observations as well as multipath effects, the GPS has low accuracy in the vertical direction. As a result, the navigation accuracy even in an integrated GPS/MEMS-IMU system is more challenged in the vertical direction than the horizontal direction. To overcome this problem, this paper investigates the integration of a MEMS barometric pressure sensor with the MEMS-IMU for vertical position/velocity tracking without the GPS that has applications in sports. A cascaded two-step Kalman filter consisting of separate orientation and position/velocity subsystems is proposed for this integration. Slow human movements in addition to more rapid sport activities such as vertical and step-down jumps can be tracked using the proposed algorithm. The height-tracking performance is benchmarked against a reference camera-based motion-tracking system and an error analysis is performed. The experimental results show that the vertical trajectory tracking error is less than 28.1 cm. For the determination of jump vertical height/drop, the proposed algorithm has an error of less than 5.8 cm.

Fabrication and characterization of sealed cavities realized by adhesive wafer bonding with dry etched Cyclotene™

Fabrication and characterization results of sealed cavities formed by adhesive wafer bonding with dry etched Cyclotene 3000™ are presented. The cavities were formed by patterning partially cured Cyclotene 3000™ on an SOI wafer. The cavities were sealed by bonding the patterned layer at low temperature (210 °C) to another partially cured thin Cyclotene 3000™ layer on a bare silicon wafer. The parameters for the vacuum bonding process were determined through an iterative process that involved optical and SEM inspection of the Cyclotene™ layers at the bonding surfaces to ensure a void and wrinkle free strong bond. The fabricated cavities were 28 µm wide and 800 nm high with a 10 µm wide supporting wall. Cavities of other dimensions can also be realized following the same procedure. The cavities can be used to realize MEMS microphones, MEMS pressure sensors, capacitive micromachined ultrasonic transducers (CMUT), resonant cavities, and also for protective encapsulation of MEMS and microelectronic dies.

Fabrication of electron beam physical vapor deposited polysilicon piezoresistive MEMS pressure sensor

MEMS pressure sensor fabrication introduce a number of challenges to handle the processed wafers in batch fabrication after bulk-micromachining. In this paper, fabrication process sequence was optimized where photolithography steps were reduced after the diaphragm formation. Reactive ion etching (RIE) and low pressure chemical vapor deposition (LPCVD) has been replaced during the modified process. Polysilicon thin film based on electron beam physical vapor deposition (EBPVD) was optimized for required sheet resistivity. MEMS pressure sensor rooted on EBPVD polysilicon piezoresistive sensing element was successfully fabricated and characterized in terms of sensitivity, linearity and repeatability.

All-Quartz High Accuracy MEMS Pressure Sensor Based on Double-Ended Tuning Fork Resonator

This paper reports an all-quartz high accuracy MEMS absolute pressure sensor based on high Q double-ended tuning fork (DETF) resonator, which is suitable for application in automotive, petroleum, meteorological, aerospace and spacecraft field. etc. The main parts of the MEMS pressure sensor, including the DETF resonator, diaphragm and back cavity structure, are all fabricated by quartz crystal using MEMS process. These pieces are bonded together as ‘sandwich’ structure to form the absolute pressure sensing unit using glass paste under low temperature and vacuum condition. This process could effectively eliminate the thermal stress effect and form the reference vacuum cavity. The experimental results show that the quality factor of the DETF resonator is up to 61000, the sensitivity is up to 7.35Hz/kPa in the operating range 0∼250kPa, the pressure error is only 0.021%FS over the temperature range -40̊C∼+60̊C.

Temperature measurement performance of silicon piezoresistive MEMS pressure sensors for industrial applications

Temperature and pressure are the most common parameters to be measured and monitored not only in industrial processes but in many other fields from vehicles and healthcare to household appliances. Silicon microelectromechanical (MEMS) piezoresistive pressure sensors are the first and the most successful MEMS sensors, offering high sensitivity, solid-state reliability and small dimensions at a low cost achieved by mass production. The inherent temperature dependence of the output signal of such sensors adversely affects their pressure measurement performance, necessitating the use of correction methods in a majority of cases. However, the same effect can be utilized for temperature measurement, thus enabling new sensor applications. In this paper we perform characterization of MEMS piezoresistive pressure sensors for temperature measurement, propose a sensor correction method, and demonstrate that the measurement error as low as ? 0.3?C can be achieved.

Encapsulation of implantable integrated MEMS pressure sensors using polyimide epoxy composite and atomic layer deposition

Abstract. Implantable MEMS sensors are an enabling technology for diagnostic analysis and therapy in medicine. The encapsulation of such miniaturized implants remains a largely unsolved problem. Medically approved encapsulation materials include titanium or ceramics; however, these result in bulky and thick-walled encapsulations which are not suitable for MEMS sensors. In particular, for MEMS pressure sensors the chip surface comprising the pressure membranes must be free of rigid encapsulation material and in direct contact with tissue or body fluids. This work describes a new kind of encapsulation approach for a capacitive pressure sensor module consisting of two integrated circuits. The micromechanical membrane of the pressure sensor may be covered only by very thin layers, to ensure high pressure sensitivity. A suitable passivation method for the high topography of the pressure sensor is atomic layer deposition (ALD) of aluminium oxide (Al2O3) and tantalum pentoxide (Ta2O5). It provides a hermetic passivation with a high conformity. Prior to ALD coating, a high-temperature resistant polyimide–epoxy composite was evaluated as a die attach material and sealing compound for bond wires and the chip surface. This can sustain the ALD deposition temperature of 275 °C for several hours without any measurable decomposition. Tests indicated that the ALD can be deposited on top of the polyimide–epoxy composite covering the entire sensor module. The encapsulated pressure sensor module was calibrated and tested in an environmental chamber at accelerated aging conditions. An accelerated life test at 60 °C indicated a maximum drift of 5% full scale after 1482 h. From accelerated life time testing at 120 °C a maximum stable life time of 3.3 years could be extrapolated.

Pressure sensing method based on the transient response of a thermally actuated micro-wire

This paper presents a novel kind of measurement method for MEMS pressure sensor based on the well known approach of the thermally actuated micro-wire. The originality of this work relies on the monitoring of the time-based transient response, particularly the step response of the heated wire – this is not a Pirani gauge. The sensor is investigated in the 1–11bar pressure range as well as in the 20–80°C ambient temperature range. The sensing part is a 1mm long, 4kΩ resistive micro-wire made of polysilicon and coated with silicon nitride. Its fabrication relies on a fully standardized industrial process, followed by a frontside anisotropic wet etching to release the micro-wire from the silicon substrate. The measurement relies on the time needed to reach a specified voltage and uses only a 1-bit ADC (i.e. a single voltage comparator), and a digital clock. Thus, in case of a readout circuitry monolithically integrated with the sensor, this measurement method will be very effective in terms of extra-power consumption, silicon area and design time. Performance were measured in nitrogen and gave: a sensitivity of approximately 100ns/mbar, at atmospheric pressure a precision of 0.2% and at 11bar a precision of 0.05%, a power consumption of approximately 1.5mW with 600μA, and showed highly linear characteristics with low drift with respect to ambient temperature. Those results are very encouraging given the proof feature of this work.

A Monolithically Integrated Pressure/Oxygen/Temperature Sensing SoC for Multimodality Intracranial Neuromonitoring

A fully integrated SoC for multimodality intracranial neuromonitoring is presented in this paper. Three sensors including a capacitive MEMS pressure sensor, an electrochemical oxygen sensor and a solid-state temperature sensor are integrated together in a single chip with their respective interface circuits. Chopper stabilization and dynamic element matching techniques are applied in sensor interface circuits to reduce circuit noise and offset. On-chip calibration is implemented for each sensor to compensate process variations. Measured sensitivity of the pressure, oxygen, and temperature sensors are 18.6 aF/mmHg, 194 pA/mmHg, and 2 mV/°C, respectively. Implemented in 0.18 m CMOS, the SoC occupies an area of 1.4 mm × 4 mm and consumes 166 μW DC power. A prototype catheter for intracranial pressure (ICP) monitoring has been implemented and the performance has been verified with ex vivo experiment.

Development of high temperature resistant of 500 °C employing silicon carbide (3C-SiC) based MEMS pressure sensor

This works reports a packaged MEMS capacitive pressure sensor (CPS) employing single crystal 3C-SiC thin film as a diaphragm. The details of the design and fabrication steps involved bulk micromachining process. The 3C-SiC-on-Si wafer is back-etched the bulk Si to leave 3C-SC thin film by applied ProTEK PSB coating as a newly photosensitive layer. The ProTEK PSB is exposed into desired pattern of MEMS capacitive pressure sensor and the exposed pattern is developed by developer (ethylene lactate). The photosensitive can be stripped off with strong combination acid such as 2-(1-methoxy)propyl acetate, ethyl acetoacetate and photoacid generator which is attack the exposed ProTEK PSB while unexposed ProTEK PSB areas remain contact the alignment on the wafer surfaces. The prototypes of a MEMS capacitive pressure is packaged for high temperature up to 500 °C and characterized under static pressure of 5.0 MPa in a stainless steel chamber with direct capacitance measurement using LCR meter. The diaphragm of 3C-SiC thin film has the thicknesses of 1.0 µm and the size of 2.0 × 2.0 mm. At room temperature (27 °C), the sensitivity of the sensor is 0.00962 pF/MPa in the range of (1.0---5.0 MPa), with nonlinearity of 0.49 %. At 300 °C, the sensitivity is 0.0127 pF/MPa, and nonlinearity of 0.46 %. The sensitivity increased by 0.0031 pF/MPa, corresponding temperature coefficient of sensitivity is 0.058 %/ °C. At 500 °C, the maximum temperature coefficient of output change is 0.073 %/ °C being red at 5.0 MPa. The main impact of this work is the ability of the sensor to operate up to 500 °C, compare to the previous work using similar 3C-SiC diaphragm that can operates only 400 °C. The main impact of this work is the ability our CPS to operate up to 500 °C and pressure of 5 MPa to surpass the performance of previous work at lower temperature and pressure. In addition, this CPS has reliable stainless steel (SS) o-ring packaging with a direct assembly approach to reduce manufacturing cost and easy installation and maintenance environment.

Process Control Challenges of Wet Etching Large MEMS Si Cavities

Anisotropic etching of silicon refers to the directional-dependent etching, usually by alkaline etchants like aqueous KOH, TMAH and other hydroxides like NaOH. With the strong dependence of the etch rate on crystal orientation and on etchant concentration and temperature, a large variety of silicon structures can be fabricated in a highly controllable and reproducible manner. Hence, anisotropic etching of <100> silicon has been a key process in common MEMS based technologies for realizing 3-D structures [1-4]. These structures include V-grooves for transistors, small holes for ink jets and diaphragms for MEMS pressure sensors as shown in Figure 1 [1]. The actual reaction mechanism has not been well understood and comprehensive physical and chemical models for the process have not yet been developed. With increasing numbers of MEMS applications, interest has grown in recent years for process modelling, simulation and software tools useful for the prediction of etched surface profiles [4-6].