Surface Micromachined Pressure Sensor Research Articles

Toward a Self-Sensing Piezoresistive Pressure Sensor for All-SiC Monolithic Integration

This work focusses on the design and fabrication of surface micromachined pressure sensors, designed in a modular way for the integration with analog front-end read-out electronics. Polycrystalline 3C silicon carbide (SiC) was used to fabricate free-standing high topography cavities exploiting surface micromaching. The poly-SiC was in-situ doped and the membrane itself is used as piezoresistive element, thereby forming a so-called self-sensing membrane, easing fabrication. After sacrificial release, the cavity is sealed by conformal deposition of poly-SiC whereby the reference pressure of the absolute pressure sensor is determined. Aluminum and titanium metallizations were used and ohmic contacts were confirmed by wafer-scale measurements. Measurements were carried out on different devices ranging from 100 kPa down to 10 Pa at room temperature. The Wheatstone bridge yields a logarithmic response of 1.1 mVbar $^{-}1\text{V}^{-}1$ . A square 300 $\mu \text{m}$ device exhibits a logarithmic impedance behavior yielding a response of $\Delta {R} / {R}$ of $1.6\times 10^{-3}$ bar $^{-1}$ . The realized pressure devices are a first step toward a SiC ASIC + MEMS platform for intended operation in harsh environments, such as industrial process monitoring, combustion control or structural health monitoring. The future outlook of the integration concept implies extended functionality by front-end transducer read-out, signal amplification and communication.

Surface Micromachined Pressure Sensor with Substrate Internal Vacuum Cavity

A surface micromachined piezoresistive pressure sensor with a novel internal substrate vacuum cavity was developed. The proposed internal substrate...

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 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.

Capacitive pressure sensor with monolithically integrated CMOS readout circuit for high temperature applications

An integrated capacitive surface micromachined pressure sensor for high temperature applications was developed and characterized to work at temperatures up to 250 °C. The sensor and the CMOS readout circuit are processed on separation by implantation of oxygen (SIMOX) substrates. The readout circuit is programmable and allows calibration of linearity, offset and output range. The performance of a 20 and 50 bar sensor is discussed. Even without active temperature compensation the offset temperature coefficient (TCO) referred to full scale output (FSO) is less than 0.03%/°C between 25 and 150 °C and less than 0.09%/°C between 150 and 250 °C. The compensated linearity error is less than 0.4% FSO.

Determination of the effect of processing steps on the CMOS compatibilityof a surface micromachined pressure sensor

A surface micromachining process for the fabrication of apressure sensitive field effect transistor (FET), compatible withcomplementary metal oxide semiconductor (CMOS) processing, has beenestablished in which residual membrane stress can be tuned withoutchanging the underlying CMOS operation. The residual membrane stress mustbe controlled at a low tensile value for optimum operation of the pressureFET. Controlling this residual stress involves the development ofsuitable processing conditions, and the effect of alteration from standardCMOS processing on the underlying circuitry must be evaluated. Theeffects of different processing conditions for a surface micromachinedpolysilicon pressure sensing membrane on the CMOS characteristics arepresented. Variations in the polysilicon sensor layer deposition andimplant conditions in the surface micromachining process and back-end CMOSinterlayer dielectric reflow were examined as these strongly influence theresidual stress in the membrane. The electrical characteristics fordevices that had only CMOS processing did not vary significantly from theelectrical characteristics of devices that had the pressure sensor surfacemicromachining layer processing.

Design, optimization and fabrication of surface micromachined pressure sensors

Surface micromachined pressure sensors have been designed, optimized and fabricated. Polysilicon diaphragms with thickness of 2 μm have been manufactured by means of LPCVD (Low Pressure Chemical Vapor Deposition) and used as the core elements in micro pressure sensors. The diaphragms stand on top of micro vacuum cavities that are constructed by a specially designed, vacuum sealing process [1]. Pressure changes cause the diaphragms to deflect. The magnitudes of pressure are detected by using built-in piezoresistive resistors that are placed on the surface of the thin diaphragms. Both square- and circular-shape diaphragm pressure sensors with nominal widths (diameters) of 100 μm have been fabricated and tested. Optimized designs, including position, orientation and length of the piezoresistive resistors, have been analyzed. The fabrication process is fully compatible with IC (Integrated Circuit) processing. Therefore, micro electronics can be integrated directly with these micro sensors and microscale mechatronics systems can be realized. Prototype pressure sensors which are designed for 100 Psi in full operational scale have a measured sensitivity of 0.15 mV\\V\\Psi and a maximum linearity error of ±0.1% FSS (Full Scale Span).

Micromachined pressure sensors: review and recent developments

Since the discovery of piezoresistivity in silicon in the mid 1950s, silicon-based pressure sensors have been widely produced. Micromachining technology has greatly benefited from the success of the integrated circuit industry, borrowing materials, processes, and toolsets. Because of this, microelectromechanical systems (MEMS) are now poised to capture large segments of existing sensor markets and to catalyse the development of new markets. Given the emerging importance of MEMS, it is instructive to review the history of micromachined pressure sensors, and to examine new developments in the field. Pressure sensors will be the focus of this paper, starting from metal diaphragm sensors with bonded silicon strain gauges, and moving to present developments of surface-micromachined, optical, resonant, and smart pressure sensors. Considerations for diaphragm design will be discussed in detail, as well as additional considerations for capacitive and piezoresistive devices. Results from surface-micromachined pressure sensors developed by the authors will be presented. Finally, advantages of micromachined sensors will be discussed.

Microsensor techniques for cardiovascular systems

Summary. Progress in semiconductor technology has been very fast in the last few years. One important fabrication process is the CMOS (Complementary Metal Oxide Semiconductor) process. This technology has been developed for the fabrication of electronic circuits, but is now also used for the fabrication of microsensors and microsystems by adding some compatible process steps. This contribution reports upon some examples for the application of CMOS-compatible physical microsensors with on-chip integrated read-out electronics in a cardiovascular ambient. Especially monolithically integrated pressure sensors and flow sensors for blood pressure and blood flow measurement are presented, representing the state-of-the-art techniques of CMOS-compatible surface micromachining and bulk micromachining. Due to the flexible design possibilities, the surface micromachined pressure sensor with a chip size of 0.7 mm × 7 mm or even smaller can be placed in a heart catheter with a diameter of 1 mm for blood pressure measurement in arterial vessels. Additionally a temperature sensor is integrated on the chip. In the same catheter application a flow sensor chip based on a hot-film anemometry principle measures blood flow dynamically with a fast response and low power consumption combined with small dimensions. A system with integrated circuits used for data acquisition with wireless data transfer of cardiovascular sensor signals from inside the body is also discussed.

Surface micromachined piezoresistive pressure sensors with step-type bent and flat membrane structures

Surface micromachined pressure sensors with step-type bent and nearly flat membranes are compared. The influence of the membrane structure on the sensor characteristics is investigated. The sensors contain four polysilicon piezoresistors arranged at the underside of a polysilicon membrane. For the step-type bent version of the sensor polysilicon is used as sacrificial layer and aqueous TMAH solution for underetching. Devices with a nearly flat structure are fabricated by applying a combined TMAH and HF sacrificial layer etching technique of a polysilicon/oxide sandwich. Compared to step-type bent, nearly flat structures provide decisive advantages for fabrication and show improved performance. Pressure response, noise behavior as well as the thermal drift of both types of sensors is presented.

Surface micromachining technology applied to the fabrication of a FET pressure sensor

This paper presents a novel surface-micromachined pressure sensor based in a FET device. The diaphragm acts as the gate of the transistor and the gate - source voltage varies in a nearly linear form with pressure when keeping constant the current along the channel. Both theoretical characteristics and the technological process are analysed. Finally the advantages over existing microsensors are examined.

A surface micromachined capacitive absolute pressure sensor array on a glass substrate

Abstract The fabrication process and the characteristics of a new surface micromachined pressure sensor array on a glass substrate are presented. The array consists of electrically parallel individual sensors with composite SiO 2 -Cr-SiO 2 diaphragms and vacuum-sealed cavities underneath. The cavities are created by lateral etching of an Al sacrificial layer. Capacitive sensing of pressure is accomplished via chromic electrodes. The performance of the sensor array is described. In a pressure range of 800 kPa a terminal-based nonlinearity less than 4% was measured for the arrays. The pressure sensitivity of the array is about 0.28 fF hPa −1 .

Surface micromachined pressure sensors with integrated CMOS read-out electronics

Abstract In this paper a single chip pressure and tenperature sensor system with on chip electronics is presented. The capacitive pressure sensor is fabricated using a CMOS process with additional surface micromachining steps to form a membranes. The membrane dimensions have been optimized for a pressure range of 2, 3.5, 10 and 35 bars, respectively. The temperature sensor shows a straight linear output signal in a temperature range of 0 to 70 °C. For the signal processing switched capacitor circuits are used. The sensor signals are converted to a pulse width modulated output signal. The silicon chip has an active area of 3.5 mm 2 . Between 0 and 80 °C a temperature dependence of the pressure segnal of less than 200 ppm/°C referring to full scale was found.