Single-crystal Silicon Diaphragm Research Articles

Design, development and characterization of MEMS silicon diaphragm force sensor

The aim of the paper is to describe the design, development and experimental characterization of a MEMS silicon diaphragm based force sensor. In the presented design, a unique combination of a thin single crystal silicon diaphragm has been used as a mechanical sensing element. The force sensor is based on the principle of conversion of applied force to an electrical quantity (e.g. voltage) consists of four piezoresistors in a Wheatstone-bridge configuration. The sensor has been modeled with the help of modern CAD tools and, after several iterations, the diaphragm size and thickness have been optimized to obtain a high sensitivity against an applied load in the designed range. The finite element analysis (FEA) has been carried out for computational investigations to have an approximate evaluation in regard of mechanical design and features. The important parameters like stress, strain and deflection are found to be within permissible limits. The fabricated sensor has been characterized by its metrological capabilities. The relative error due to repeatability is found to be <1% in the working range of the sensor. The sensitivity has been found to be in order of 0.35–0.40 mV/V/N for 10 N–50 N force range.

Optical fiber sensor for temperature measurement based on Silicon thermo-optics effect

A temperature sensor based on the thermo-optics effect of single crystal silicon diaphragm was proposed and studied. The temperature measurement mechanism by using single crystal silicon diaphragm was theoretically analyzed and found the relation between temperature measurement range and the thickness of single crystal diaphragm. It was found that when the temperature changed from 20°C to 48°C, the reflected resonant peak wavelength was shifted which was detected by using the dual-wavelength signal demodulation method. Experimental results for temperature measurement demonstrated reasonable linearity and repeatability, related linear regression coefficient of the experimental result is 99.94%. The study would be of utility value for the design and fabrication of silicon-based sensors which were used to measure pressure or the other physical quantities.

Modeling and Demonstration of Thermally Stable High-Sensitivity Reproducible Acoustic Sensors

A thermally stable high-sensitivity compact fiber acoustic sensor with a large bandwidth and a high dynamic range is introduced. The device is based on a photonic crystal fabricated on a compliant single-crystal silicon diaphragm, which is placed near the metalized end of a single-mode fiber to form a Fabry-Perot (FP) cavity. High reproducibility in operating wavelength (±1 nm) is enabled by assembling the sensor in a SiO2 chip using room-temperature sodium silicate oxide bonding. We demonstrate ten FP sensors with measured displacement sensitivities within ±0.6 dB. The response is shown to be polarization independent and thermally stable, with a thermal coefficient of the operating wavelength of 2.9pm/°C over more than 50 °C. An experimental sensor is shown to measure acoustic pressures down to a record low of 5.6 μPa/√Hz at 12.5 kHz with a flatband response greater than 8 kHz and a sensitivity extending down to at least 100 Hz. The dynamic range in pressure is greater than 100 dB. An electromechanical model of the device response is presented and shown to be in good agreement with experimental results.

A multifunctional integrated silicon tactile imager with arrays of strain and temperature sensors on single crystal silicon diaphragm

In this study, a silicon multifunctional tactile imager with the simultaneous detection abilities of ‘3D surface shape’ and ‘temperature distribution’ of touching object has been realized with silicon LSI/MEMS technology. The pixel circuit array including a piezoresistor and a temperature sensor is integrated on elastic surface of pneumatically swollen silicon diaphragm. Also, elasticity detection of touching object has been simultaneously realized with signal processing in frequency domain. Prototype tactile imagers with a 3.1 mm × 3.1 mm sensor diaphragm integrating 420-μm pitch multifunctional sensor pixel array have been fabricated. In the evaluation experiments, the shape of a 1.5 mmϕ soft tube was successfully detected. Also, simultaneous measurement of surface shape and temperature distribution was performed using a heated metal pipe. The detectable lower-limit of vibration in this tactile imager was below 1.0 μm with signal bandwidth more than 2 kHz. Using the ability of vibration detection, the three kinds of materials with different elasticity (0.59–7.04 mN/μm) were successfully distinguished with the tactile imager. The highly sensitive and multifunctional tactile imager with versatile detection ability has been realized by sophisticated integration ability of silicon-MEMS technology.

High temperature smart-cut SOI pressure sensor

Piezoresistive pressure sensors based on SMART CUT ® SOI wafer have been developed, which can be used in extreme high temperature environments. It has been demonstrated that the resistance value of a heavily doped thin film (∼0.34 μm) resistor increases monotonically with temperature up to 600 °C. This is much higher than the maximum temperature of 330 °C normally shown in bulk silicon resistors. An analytical model is developed to explain how to extend the maximum operating temperature range based on doping effects and minority-carrier exclusion effects. Two types of packaging have been developed for different applications; one is for low pressure, high accuracy application, the other is for high pressure, high temperature application. The former is fully characterized across the range of 0.5–25 psi and −55 to 300 °C and the latter is calibrated across the range of 16–600 psi and −55 to 500 °C. A digitized curve fitting technique is used to calibrate the sensors by use of on-chip temperature signals. After curve fitting, the accuracy is <0.05% F.S. for the first type of the pressure sensor and <0.25% F.S. for the second type of the pressure sensor. A very low pressure hysteresis (<0.1% FS) at 500 °C indicates that the single crystal silicon diaphragm is capable of operating at very high temperature without creep or plastic deformation.

A silicon directly bonded capacitive absolute pressure sensor

A silicon directly bonded capacitive absolute pressure sensor is presented in this paper. The sensor consists of a vacuum-sealed cavity and a parallel plate capacitance microstructure fabricated by silicon direct bonding. The flexible single crystal silicon diaphragm whose thickness can be well controlled was completed by two steps of silicon direct bonding, and the sealed cavity was formed at the same time. Lead-transfer was realized by the thermo-compression bonding. The performance of the sensor was simulated by ANSYS software. The experimental results were compared with the theoretical values. It shows that the sensor provides a dynamic range of 1040–520mbar, has a sensitivity of 1.58fF/mbar. The nonlinearity of the full range (FSO) is less than 6.47%, while the nonlinearity in 520–680mbar, 680–840mbar and 840–1040mbar is less than 0.68%, 2.03% and 1.28%, respectively. The maximum hysteresis error occurs in the range of 680–840mbar, the error over the capacitance change of the full range is about 2.43%, and the sensor has a TCO of 181.5ppm/°C.

Pumping capacity and reliability of cryogenic micro-pump for micro-satellite applications

In micro-satellites, delicate instruments are compacted into a limited space. This raises concerns of active cooling and remote cooling. Silicon based micro-pump arrays are employed thanks to manufacturing simplicity, a small cryogen charge, etc, and keep the instruments within a narrow cryogenic temperature range. The pumping capacity and reliability of the micro-pump are critical in terms of heat balance calculation and lifetime evaluation. The pumping capacity is associated with the diaphragm deflection while the reliability is associated with stress and fatigue. Both of them heavily depend on the silicon diaphragm, one of the key components. This paper examines the pumping capacity and reliability of the micro-pump under cryogenic temperature for micro-satellite applications. In this work, differential pressure was used for the actuation of a single-crystal silicon diaphragm. Diaphragm deflection and stress distribution were achieved using interferometry and micro-Raman spectroscopy, respectively. As a result, smaller pumping capacity was derived under cryogenic temperature, compared to that under room temperature, indicating a stiffer material. From stress mapping, the edge centers were believed to be the most vulnerable to fracture, which was further validated by analyzing the fracture diaphragm. Moreover, a fatigue testing was conducted for 1.8 million cycles with no damage found, verifying silicon as a viable material for long time operation in a cryogenic environment.

A new method of forming a thin single-crystal silicon diaphragm using merged epitaxial lateral overgrowth for sensor applications

Merged epitaxial lateral overgrowth (MELO) of silicon was combined with an SiO/sub 2/ etch stop to form a 9- mu m-thick and 250- mu m*1000- mu m single-crystal Si membrane for micromechanical sensors. When epitaxial lateral overgrowth (ELO) silicon merges on SiO/sub 2/ islands, it forms a local silicon-on-insulator (SOI) film of moderate doping concentration. The SiO/sub 2/ island then acts as a near-perfect etch top in a KOH- or ethylenediamine-based solution. The silicon diaphragm thickness over a 3-in wafer has a standard deviation of 0.5 mu m and is precisely controlled by the epitaxial silicon growth rate ( approximately=0.1 mu m/min) rather than by conventional etching techniques. Diodes fabricated in the substrate and over MELO regions have nearly identical reverse-bias currents, indicating good quality silicon in the membrane. >

A novel pressure sensor structure for integrated sensors

A novel pressure sensor ‘silicon box’ structure is proposed, fabricated by one-sided processing technology. Tests and analyses show that, while the excellent performance of single crystal silicon diaphragm sensors is maintained, the new technique has advantages for both discrete and integrated pressure sensors of high yield, good uniformity, low cost and feasibility of mass production. This structure also provides some reduction of thermal stress caused by the mismatch between the chip and the substrate occurring in a silicon cup structure.

Silicon Diffused-Element Piezoresistive Diaphragms

The properties of a pressure transducer consisting of a single-crystal silicon diaphragm having stress-sensitive piezoresistive regions formed by the localized diffusion of impurities have been theoretically and experimentally investigated. The longitudinal and transverse piezoresistance effects are discussed and the results are applied to the stress pattern of a deformed diaphragm. The conditions under which the stress in the diaphragm varies linearly with applied pressure are discussed and good agreement between the predicted and measured sensitivity is found in both the linear and nonlinear cases.