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Half-bridge silicon strain gauges are widely used in the fabrication of diaphragm-type high-pressure sensors, but in some applications, they suffer from low output sensitivity because of mounting position constraints. Through a special design and fabrication approach, a new half-bridge silicon strain gauge comprising one arc gauge responding to tangential strain and another linear gauge measuring radial strain was developed using Silicon-on-Glass (SiOG) substrate technology. The tangential gauge consists of grid patterns, such as the reciprocating arc of silicon piezoresistors on a thin glass substrate. When two half-bridges are connected to form a full bridge with arc-shaped gauges that respond to tangential strain, they have the advantage of providing much higher output sensitivity than a conventional half-bridge. Pressure sensors tested under pressure ranging from 0 to 50 bar at five different temperatures indicate a linear output with a typical sensitivity of approximately 16 mV/V/bar, a maximum zero shift of 0.05% FS, and a span shift of 0.03% FS. The higher output level of pressure sensing gauges will provide greater signal strength, thus maintaining a better signal-to-noise ratio than conventional pressure sensors. The offset and span shift curves are quite linear across the operating temperature range, giving the end user the advantage of using very simple algorithms for temperature compensation of offset and span shift.

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This paper proposes a piezoresistive high-temperature absolute pressure sensor based on (100)/(111) hybrid SOI (silicon-on-insulator) silicon wafers, where the active layer is (100) silicon and the handle layer is (111) silicon. The 1.5 MPa ranged sensor chips are designed with the size as tiny as 0.5 × 0.5 mm, and the chips are fabricated only from the front side of the wafer for simple, high-yield and low-cost batch production. Herein, the (100) active layer is specifically used to form high-performance piezoresistors for high-temperature pressure sensing, while the (111) handle layer is used to single-side construct the pressure-sensing diaphragm and the pressure-reference cavity beneath the diaphragm. Benefitting from front-sided shallow dry etching and self-stop lateral wet etching inside the (111)-silicon substrate, the thickness of the pressure-sensing diaphragm is uniform and controllable, and the pressure-reference cavity is embedded into the handle layer of (111) silicon. Without the conventionally used double-sided etching, wafer bonding and cavity-SOI manufacturing, a very small sensor chip size of 0.5 × 0.5 mm is achieved. The measured performance of the 1.5 MPa ranged pressure sensor exhibits a full-scale output of approximately 59.55 mV/1500 kPa/3.3 VDC in room temperature and a high overall accuracy (combined with hysteresis, non-linearity and repeatability) of 0.17%FS within the temperature range of -55 °C to 350 °C. In addition, the thermal hysteresis is also evaluated as approximately 0.15%FS at 350 °C. The tiny-sized high temperature pressure sensors are promising in various industrial automatic control applications and wind tunnel testing systems.

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Currently, silicon-strain-gauge-based diaphragm pressure sensors use four single-gauge chips for high-output sensitivity. However, the four-single-gauge configuration increases the number of glass frit bonds and the number of aluminum wire bonds, reducing the long-term stability, reliability, and yield of the diaphragm pressure sensor. In this study, a new design of general-purpose silicon strain gauges was developed to improve the sensor output voltage while reducing the number of bonds. The new gauges consist grid patterns with a reciprocating arc of silicon piezoresistors on a thin glass backing. The gauges make handling easier in the bonding process due to the use of thin glass for the gauge backing. The pressure sensors were tested under pressure ranging from 0 to 50 bar at five different temperatures, with a linear output with a typical sensitivity of approximately 16 mV/V/bar and an offset shift of -6 mV to 2 mV. The new approach also opens the possibility to extend arc strain gauges to half-bridge and full-bridge configurations to further reduce the number of glass frit and Al wire bonds in the diaphragm pressure sensor.

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Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

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Silicon carbide (SiC) has promising potential for pressure sensing in a high temperature and harsh environment due to its outstanding material properties. In this work, a 4H-SiC piezoresistive pressure chip fabricated based on femtosecond laser technology was proposed. A 1030 nm, 200 fs Yb: KGW laser with laser average powers of 1.5, 3 and 5 W was used to drill blind micro holes for achieving circular sensor diaphragms. An accurate per lap feed of 16.2 µm was obtained under laser average power of 1.5 W. After serialized laser processing, the machining depth error of no more than 2% and the surface roughness as low as 153 nm of the blind hole were measured. The homoepitaxial piezoresistors with a doping concentration of 1019 cm-3 were connected by a closed-loop Wheatstone bridge after a rapid thermal annealing process, with a specific contact resistivity of 9.7 × 10-5 Ω cm2. Our research paved the way for the integration of femtosecond laser micromachining and SiC pressure sensor chips manufacturing.

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In this paper, we present a novel thermoresistive gas flow sensor with a high-yield and low-cost volume production by using front-side microfabricated technology. To best improve the thermal resistance, a micro-air-trench between the heater and the thermistors was opened to minimize the heat loss from the heater to the silicon substrate. Two types of gas flow sensors were designed with the optimal thermal-insulation configuration and fabricated by a single-wafer-based single-side process in (111) wafers, where the type A sensor has two thermistors while the type B sensor has four. Chip dimensions of both sensors are as small as 0.7 mm × 0.7 mm and the sensors achieve a short response time of 1.5 ms. Furthermore, without using any amplification, the normalized sensitivity of type A and type B sensors is 1.9 mV/(SLM)/mW and 3.9 mV/(SLM)/mW for nitrogen gas flow and the minimum detectable flow rate is estimated at about 0.53 and 0.26 standard cubic centimeter per minute (sccm), respectively.

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The stress state is a crucial parameter for the design of innovative microelectromechanical systems based on silicon carbide (SiC) material. Hence, mechanical properties of such structures highly depend on the fabrication process. Despite significant progresses in thin-film growth and fabrication process, monitoring the strain of the suspended SiC thin-films is still challenging. However, 3C-SiC membranes on silicon (Si) substrates have been demonstrated, but due to the low quality of the SiC/Si heteroepitaxy, high levels of residual strains were always observed. In order to achieve promising self-standing films with low residual stress, an alternative micromachining technique based on electrochemical etching of high quality homoepitaxy 4H-SiC layers was evaluated. This work is dedicated to the determination of their mechanical properties and more specifically, to the characterization of a 4H-SiC freestanding film with a circular shape. An inverse problem method was implemented, where experimental results obtained from bulge test are fitted with theoretical static load-deflection curves of the stressed membrane. To assess data validity, the dynamic behavior of the membrane was also investigated: Experimentally, by means of laser Doppler vibrometry (LDV) and theoretically, by means of finite element computations. The two methods provided very similar results since one obtained a Young's modulus of 410 GPa and a residual stress value of 41 MPa from bulge test against 400 GPa and 30 MPa for the LDV analysis. The determined Young's modulus is in good agreement with literature values. Moreover, residual stress values demonstrate that the fabrication of low-stressed SiC films is achievable thanks to the micromachining process developed.

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A gold-coated array of flow-through inverse pyramids applicable as substrate for entrapment and immobilization of micro-objects and for surface enhanced Raman spectroscopic measurements was fabricated using bulk micromachining techniques from silicon. Surface morphology, optical reflectance, immobilization properties, and surface enhanced Raman amplification of the array were modelled and characterized. It was found that the special perforated periodic 3D structure can be used for parallel particle and cell trapping and highly sensitive molecular analysis of the immobilized objects.

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A novel on-chip integration of pressure plus 2-axis (X/Z) acceleration composite sensors for upgraded production of automobile tire pressure monitoring system (TPMS) is proposed, developed, and characterized. Herein, the X-axis accelerometer is with the cantilever beam-mass structure and is used for automatically identifying and positioning each of the four wheels. The IC-Foundry-Compatible low-cost batch fabrication technique of MIS (i.e., Micro-openings Inter-etch and Sealing) is employed to only fabricate the device from the front side of (111) silicon wafer, without double-sided micromachining, wafer bonding, complex Cavity-SOI (Silicon on Insulator) processing, and expensive SOI-wafer needed. Benefited from the single-wafer front-side fabrication technique on ordinary single-polished wafers, the fabricated composite TPMS sensor has the advantages of a small chip-size of 1.9 mm × 1.9 mm, low cross-talk interference, low-cost, and compatible process with IC-foundries. The fabricated pressure sensors, X-axis accelerometer and Z-axis accelerometer, show linear sensing outputs, with the sensitivities as about 0.102 mV/kPa, 0.132 mV/kPa, and 0.136 mV/kPa, respectively. Fabricated with the low-cost front-side MIS process, the fabricated composite TPMS sensors are promising in automotive electronics and volume production.

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This article reports a process for batch fabrication of a fiber pigtailed optomechanical transducer platform with overhanging. The platform enables a new class of high bandwidth, high sensitivity, and highly integrated sensors that are, compact, robust, and small, with the potential potential for low cost batch fabrication inherent in Micro-Opto-Electro-Mechanical-Systems technology. This article provides a guide to the whole fabrication process and explains critical steps and process choices in detail. Possible alternative fabrication techniques and problems are discussed. The fabrication process consists of electron beam lithography, i-line stepper lithography, and back- and frontside mask aligner lithography. The goal of this article is to provide a comprehensive description of the fabrication process, presenting context and details which are highly relevant to the rational implementation and reliable repetition of the process. Moreover, this process makes use of equipment commonly found in nanofabrication facilities and research laboratories, facilitating the broad adaptation and application of the process. Therefore, while this article specifically informs users of the Center for Nanoscale Science and Technology (CNST) at the National Institute of Standards and Technology (NIST), we anticipate that this information will be generally useful for the nano- and microfabrication research communities at large.

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Bulk micromachining of Si is demonstrated by the well-known metal-assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 µm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 µm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post-partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low-cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (µTAS), and so forth.

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Using bulk micromachining, meander-shaped resistor elements consisting of 20 nm Cr and 200 nm Au were fabricated on 1 µm thick silicon nitride membranes, bridges, and cantilevers. The resistance change as a function of pressure depends strongly on the thermal resistance of the two metal lines connecting the heated resistor to the silicon bulk (cold junction) and on the thermal resistance of the silicon nitride. Relative resistance changes ranging from about 3% (small membrane) to 20% (bridge) per mW of input power were obtained when operating the devices in constant voltage mode. The pressure where maximum sensitivity of these gauges occurs depends on the distance 'd' between the periphery of the heated resistor element and the silicon cold junction. Devices with 'd' ranging from 50 µm to 1,200 µm were fabricated. Assuming that pressures can be reliably measured above the 10% and below the 90% points of the resistance versus pressure curve, the range of these devices is about two orders of magnitude. By integrating two devices, one with d = 65 µm and one with d = 1,200 µm on the same chip and connecting them in series, the range can be increased by about a factor of three. By fabricating the cantilever devices so that they curl upon release, it will be shown that these devices also exhibit larger range due to varying 'd'.

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A bulk micromachined inertial measurement unit (MIMU) is presented in this paper. Three single-axis accelerometers and three single-axis gyroscopes were simultaneously fabricated on a silicon wafer using a bulk micromachining process; the wafer is smaller than one square centimeter. In particular, a global area optimization method based on the relationship between the sensitivity and layout area was proposed to determine the layout configuration of the six sensors. The scale factors of the X/Y-axis accelerometer and Z-axis accelerometer are about 213.3 mV/g and 226.9 mV/g, respectively. The scale factors of the X/Y-axis gyroscope and Z-axis gyroscope are about 2.2 mV/°/s and 10.8 mV/°/s, respectively. The bias stability of the X/Y-axis gyroscope and the Z-axis gyroscope are about 2135 deg/h and 80 deg/h, respectively. Finally, the resolutions of X/Y-axis accelerometers, Z-axis accelerometers, X/Y-axis gyroscopes, and Z-axis gyroscopes are 0.0012 g/√Hz, 0.0011 g/√Hz, 0.314 °/s/√Hz, and 0.008 °/s/√Hz, respectively.

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In this paper, an optimal and robust design method to implement a two-chip out-of-plane microaccelerometer system is presented. The two-chip microsystem consists of a MEMS chip for sensing the external acceleration and a CMOS chip for signal processing. An optimized design method to determine the device thickness, the sacrificial gap, and the vertical gap length of the M EMS sensing element is applied to minimize the fundamental noise level and also to achieve the robustness to the fabrication variations. In order to cancel out the offset and gain variations due to parasitic capacitances and process variations, a digitally trimmable architecture consisting of an 11 bit capacitor array is adopted in the analog front-end of the CMOS capacitive readout circuit. The out-of-plane microaccelerometer has the scale factor of 372 mV/g∼389 mV/g, the output nonlinearity of 0.43% FSO∼0.60% FSO, the input range of ±2 g and a bias instability of 122 µg∼229 µg. The signal-to-noise ratio and the noise equivalent resolution are measured to be 74.00 dB∼75.23 dB and 180 µg/rtHz∼190 µg/rtHz, respectively. The in-plane cross-axis sensitivities are measured to be 1.1%∼1.9% and 0.3%∼0.7% of the out-of-plane sensitivity, respectively. The results show that the optimal and robust design method for the MEMS sensing element and the highly trimmable capacity of the CMOS capacitive readout circuit are suitable to enhance the die-to-die uniformity of the packaged microsystem, without compromising the performance characteristics.

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