Piezoresistive Silicon Nanowires Research Articles

Piezoresistive silicon nanowire resonators as embedded building blocks in thick SOI

The use of silicon nanowire resonators in nanoelectromechanical systems for new-generation sensing and communication devices faces integration challenges with higher-order structures. Monolithic and deterministic integration of such nanowires with the surrounding microscale architecture within the same thick crystal is a critical aspect for the improvement of throughput, reliability and device functionality. A monolithic and IC-compatible technology based on a tuned combination of etching and protection processes was recently introduced yielding silicon nanowires within a 10 m-thick device layer. Motivated by its success, the implications of the technology regarding the electromechanical resonance are studied within a particular setting, where the resonator is co-fabricated with all terminals and tuning electrodes. Frequency response is measured via piezoresistive readout with frequency down-mixing. Measurements indicate mechanical resonance with frequencies as high as 100 MHz exhibiting a Lorentzian behavior with proper transition to nonlinearity, while Allan deviation on the order of 3–8 ppm is achieved. Enabling the fabrication of silicon nanowires in thick silicon crystals using conventional semiconductor manufacturing, the present study thus demonstrates an alternative pathway to bottom-up and thin silicon-on-insulator approaches for silicon nanowire resonators.

A Monolithic Approach to Downscaling Silicon Piezoresistive Sensors

Multi-scale integration remains the primary challenge in the fabrication of miniature piezoresistive sensors, as the co-fabrication of a silicon nanowire along with a microscale shuttle is the main architecture facilitating high-sensitivity transduction. The efforts in this field are marred by the lack of batch techniques compatible with semiconductor manufacturing. A technology is introduced here that leads to the fabrication of a piezoresistive silicon nanowire sharing the same single-crystalline device layer of a thick silicon-on-insulator wafer as the microscale component. The approach is based on a combination of high-resolution lithography with a two-stage etching process. The demonstration is carried out by spanning an electrostatic comb-drive actuator and a micromechanical amplifier by a single nanowire. A gage factor range of 135-145 is obtained, corresponding to an almost 20% resistance change for a nanowire strain of 1.26 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The technique is shown to generate a two-order-of-magnitude scale difference within the same silicon crystal. It also provides ease of electrical access to the nanowire, as the nanowire does not remain buried underneath the thick micromechanical system. With the associated lack of high-temperature processes and its CMOS-compatibility, the technique is a promising enabler for future miniaturized piezoresistive sensors.

A feasible simulation method for vapor sensor based on polymer-coated NEMS diaphragm

To improve the performance of the vapor sensor based on polymer film-coated NEMS diaphragm embedded with piezoresistive silicon nanowires (SiNWs), a new method for calculating the inner strain distribution in a SiNWs and an investigation of the interactions between polymer film and NEMS diaphragm using finite element analysis is presented in this paper. In the model, the swelling stress of the polymer film due to absorption of the organic vapor is equivalent to a thermal expansion stress which leads to the deformation of the SiNWs. Meanwhile, an equivalent relation between temperature load and vapor concentration load is presented. Then, the simulation outputs of the NEMS vapor sensor are obtained. Compared with experimental results, the simulation results agree well with experimental results of the sensor. This method will be useful for optimizing the performance of this kind of vapor sensor.

MEMS Tri-Axial Tactile Sensor for Sensorized Guide Wire in Catheterization Application

This paper describes the integration of micro-electro-mechanical systems (MEMS) tri-axial force sensor using polyimide-based substrate for sensorised guide wire application. For tri-axial force sensor, piezoresistive silicon nanowires (SiNWs) are embedded into a cross cantilever design with a maneuverable stylus to allow the detection of force in all directions, and amplify the tactile forces at the tip for transverse directions. The electrical resistance changes in the four SiNWs are used to decode an arbitrary force applied onto the force sensor. Robustness of the force sensor is improved due to the novel design by incorporating a mechanical stopper at the tip of the stylus. Flip chip bonding using gold stud bumps is used to mount the force sensor on a substrate for characterization and to simplify the assembly process. The packaging process of the miniaturized sensorised guide wire was presented in this work.

MEMS tri-axial force sensor with an integrated mechanical stopper for guidewire applications

This paper describes the design and characterization of a micro-electro-mechanical systems tri-axial force sensor that can be mounted on the tip of an 1-French guidewire (0.014ź). Piezoresistive silicon nanowires (SiNWs) are embedded into four beams forming a cross-shape to allow the detection of forces in three axes. The electrical resistance changes in the four SiNWs are used to decode arbitrary force applied onto the force sensor. Finite element analysis was used in the structural design of the force sensor. Robustness of the force sensor is improved due to the novel design of incorporating a mechanical stopper on the tip of the stylus. Flip chip bonding, using gold stud bumps, is used to mount the force sensor on a substrate for characterization and to simplify the assembly process. The sensor is robust enough to withstand normal forces higher than 20 gf. The proposed sensor can be used for new medical applications in vascular interventions and robotic surgeries.

Characterization of Piezoresistive-Si-Nanowire-Based Pressure Sensors by Dynamic Cycling Test With Extralarge Compressive Strain

A novel pressure sensor using piezoresistive silicon nanowires (SiNWs) embedded in a suspended multilayered diaphragm is investigated by a probe-based dynamic cycling test combining the standard bulge testing setup. By utilizing the high fracture stress of the SiN <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> film, we explored the behavior of the SiNW under a level of extralarge compressive strain for the first time, including strain levels of more than 2.1% under the static testing and 1.5% under the dynamic testing. Drift of the initial resistances of the SiNW was observed at different time intervals during the dynamic testing under a compressive strain of higher than 1.3%, while the sensitivity of the pressure sensor basically keeps unchanged. However, there was almost no drift or degradation observed in the sensor characteristics when an equivalent point loading within the application working range is applied to the pressure sensor during the dynamic testing.

Piezoresistive silicon nanowire based nanoelectromechanical system cantilever air flow sensor

We present nanoelectromechanical system based cantilever air flow sensor using silicon nanowires (SiNWs). The cantilever is fabricated in the complementary metal-oxide-semiconductor compatible process with dimension of 90 μm × 20 μm × 3 μm. SiNWs with the size of 2 μm × 90 nm × 90 nm (length × width × height) are embedded at the edge of the cantilever fixed end to experience the maximum air flow induced strain. Compared with recently reported air flow sensors, our device shows a better sensitivity of 198 Ω/m/s and a flow sensing range up to 65 m/s. In addition, improvements in terms of linearity, hysteresis, and lower power consumption are reported as well.

Characterization of Silicon Nanowire Embedded in a MEMS Diaphragm Structure Within Large Compressive Strain Range

The characteristics of piezoresistive silicon nanowires (SiNWs) under compressive strain as large as 1.7% are reported. The SiNW is embedded in a multilayered diaphragm structure consisting of silicon nitride and silicon oxide. After leveraging the high fracture stress and intrinsic tensile stress of silicon nitride layer to produce a flat diaphragm, we can create large compressive strain to the SiNW without damaging the diaphragm. The relationship between SiNW resistance change and applied strain is measured and investigated with 2-μm and 5-μm SiNWs for both scientific and practical points of view. This approach demonstrates the validity to reveal the SiNW properties under large strain, and the exploration provides good reference for future SiNW-based MEMS sensor design.

Characteristics of NEMS Piezoresistive Silicon Nanowires Pressure Sensors With various Diaphragm Layers

Optimized diaphragm structures comprising silicon nitride (SiNx) film integrated with piezoresistive silicon nanowires (SiNW) are characterized for nanoelectromechanical system (NEMS) pressure sensors. Several improvements are done in comparison with our previously reported device [1]. Firstly, p-type SiNWs of 1 colonm with 1E14 implantation instead of 10 colonm 1E13 implantation are adopted to gain more uniform stress and show better linearity. Secondly, by using SiO2+SiNx bi-layer film instead of pure SiO2 film, the membrane is not only flat, but also toughened to be able to withstand pressure up to 40 atm. Finally, the real test shows that the sensitivity of pressure sensor is doubled by thinning down the SiNx film to half thickness.

Ultrasensitive nanowire pressure sensor makes its debut

A membrane pressure sensor with embedded piezoresistive silicon nanowires (NW) has been demonstrated to have an ultrasensitive piezoresistive response of (ΔR/R)/ΔP of 13 Pa−1. This was achieved through the effective tuning of the transverse electric field across the NW. The fabrication of the sensor is fully based on CMOS compatible technique. P-type 〈110〉 oriented NWs with a square cross-section of 100 nm were fabricated on silicon-on-insulator (SOI) wafers, acting as the sensing elements. The NWs’ exceptional properties and minute size will enable further shrinking of footprint of pressure sensors and other NEMS sensors with increased sensitivity, opening a way to new applications like implantable medical devices.