Piezoresistive Silicon Research Articles

Bio/chemical detection in liquid with self-sensing Pr-Oxi-Lever (piezo-resistive SiO 2 cantilever) sensors

The paper presents a self-sensing microcantilever for bio/chemical detection in liquid. The developed silicon piezoresistance encapsulated SiO 2 cantilever (Pr-Oxi-Lever) is with the electric interconnection wires insulated by a SU-8 coating layer for specifically detecting bio/chemical molecules in liquid environment. Functionalized with specific sensing terminals, siloxane-based molecule layer is directly modified on the cantilever SiO 2 surface via reliable Si–O covalent bond. Instead of the previously used thiol self-assembly molecule layers that show long-term instability, the modified siloxane sensing layer secures long-term sensitivity stability and long working life. The sensors have been successfully used for detection of 2.5% aqueous tetramethylammonium hydroxide (TMAH) and avidin at 10 −11 mol/mL trace level. An interesting phenomenon of opposite surface-stress sensing signals for different-sized target-molecules is experimentally observed. A simple model is proposed for preliminary explanation of this phenomenon.

Quad-cantilever microsensors with a low-cost single-sided micro-machining technique for trace chemical vapor detection

This paper presents a quad-cantilever microsensor for on-the-spot detection of ultra-low concentration chemical vapors. Compared with conventional dual-cantilever sensors, the quad-cantilever configuration can form a fully cantilever-formed Wheatstone-bridge that possesses a higher sensitivity of twofold and more balanced condition to compensate for environmental noise like temperature fluctuation or air flow. Besides, the four integrated micro-cantilevers are made of SiO 2, with each having a single crystal silicon piezoresistor fully encapsulated by SiO 2. Thus, the quad-lever sensor achieves a very low signal noise of 0.2 μV. For specific detection, sensitive molecule layer is self-assembled on two sensing cantilevers, with another two as reference. The sensors are micro-fabricated with a single-side process from the wafer front-side. With wet anisotropic etch used to release the piezoresistive cantilevers, the new fabrication technique features low-cost and high-yield. Self-assembled with a novel dual-branch specific mono-layer, about 100 ppt level trinitrotoluene (TNT) vapor has been rapidly and repeatedly detected.

Two clover-shaped piezoresistive silicon microphones for photo acoustic gas sensors

Low cost CO2 gas sensors for demand-controlled ventilation can lower the energy consumption and increase comfort and hence productivity in office buildings and schools. The photo aoustic principle offers very high sensitivity and selectivity when used for gas trace analysis. Current systems are too expensive and large for in-duct mounting. Here, the design, modeling, fabrication and characterization of two micromachined silicon microphones with piezoresistive readout designed for low cost photo acoustic gas sensors are presented. The microphones have been fabricated using a foundry MPW service. One of the microphones has been fabricated using an additional etching step that allows etching through membranes with large variations in thickness. To increase sensitivity and resolution, a design based on a released membrane suspended by four beams was chosen. The microphones have been characterized for frequencies up to 1 kHz and 100 Hz, respectively. Averaged sensitivities are measured to be 30 µV/(V × Pa) and 400 µV/(V × Pa). The presented microphones offer increased sensitivities compared to similar sensors.

A self-bended piezoresistive microcantilever flow sensor for low flow rate measurement

This paper presents the fabrication and characterization of a curved-up piezoresistive microcantilever flow sensor. The microcantilever sensor, fabricated with silicon-on-insulator (SOI) wafers, consists of two layers of silicon dioxide and a silicon piezoresistor in-between. The difference in the residual stresses between silicon and silicon dioxide layers curves the microcantilever upwards and the free-end bends out of plane. The curved-up microcantilever transfers fluidic momentum that acts on it to drag force, which bends the curved-up microcantilever and changes the resistance of the piezoresistor. This configuration has the advantage of high sensitivity for low flow rate measurement and allows the microcantilever to be integrated in microchannels to measure steady flow. The flow sensor is calibrated with respect to low flow rates at range of 0–20 cm/s. The sensitivity, repeatability, and zero drift are characterized in detail.

Integration of GaAs/In0.1Ga0.9As/AlAs resonance tunneling heterostructures into micro‐electro‐mechanical systems for sensor applications

AbstractDouble‐barrier quantum‐well resonant tunneling heterostructures of GaAs/In0.1Ga0.9As/AlAs have been used as pressure‐sensing elements of designed micro‐electro‐mechanical systems. The static experiments have been conducted on the heterostructure, in both positive and negative differential resistance (PDR and NDR) regions, showing a piezoresistive coefficient of 3.85 × 10−9 Pa−1, which is about seven times larger than that of piezoresistive silicon devices. At the same time, the devices also provided an improved dynamic response (119.6 µV · g−1 V−1) and signal‐to‐noise ratio (57 dB) in the NDR region.

Piezoresistive Cantilever Performance—Part II: Optimization

Piezoresistive silicon cantilevers fabricated by ion implantation are frequently used for force, displacement, and chemical sensors due to their low cost and electronic readout. However, the design of piezoresistive cantilevers is not a straightforward problem due to coupling between the design parameters, constraints, process conditions, and performance. We systematically analyzed the effect of design and process parameters on force resolution and then developed an optimization approach to improve force resolution while satisfying various design constraints using simulation results. The combined simulation and optimization approach is extensible to other doping methods beyond ion implantation in principle. The optimization results were validated by fabricating cantilevers with the optimized conditions and characterizing their performance. The measurement results demonstrate that the analytical model accurately predicts force and displacement resolution, and sensitivity and noise tradeoff in optimal cantilever performance. We also performed a comparison between our optimization technique and existing models and demonstrated eight times improvement in force resolution over simplified models.

A chemisorption-based microcantilever chemical sensor for the detection of trimethylamine

This paper presents a new method for detecting trimethylamine (TMA) in gas and liquid phases using a piezoresistive microcantilever sensor functionalized with a self-assembled monolayer (SAM). The microcantilever consists of two silicon dioxide layers and a single crystalline silicon piezoresistor in-between. This configuration allows microcantilevers to achieve high sensitivity because of the low stiffness of silicon dioxide and the high piezoresistive coefficients of single crystalline silicon. The surface of the microcantilever is chemically functionalized with a self-assembled monolayer of 11-mercaptoundecanoic acid (11-MUA) via Au–SH covalent bonding on a gold film deposited on the microcantilever. The 11-MUA SAM adsorbs TMA through hydrogen bonding between the SAM and TMA molecules, and the microcantilever is deflected by the changes in the surface stress as a result of the hydrogen bonding based chemisorption. Experimental results show that the deflection of the microcantilever depends on the concentration of TMA, and the minimum detection limits of 10 mg/L for liquid TMA and 1.65 μg/L for gas TMA are achieved. The selectivity of TMA with respect to ethanol, acetone, and butanone is also investigated.

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.

Fabrication and Characterization of Nanopillars for Silicon-Based Thermoelectrics

Si-based nanopillars of various sizes were fabricated by lateral structuring using anisotropic etching and thermal oxidation. We obtained pillars of diameter <500 nm, about 25 μm in height, with an aspect ratio of more than 50. The distance between pillars was varied from 500 nm to 10 μm. Besides the fabrication and structural characterization of silicon nanopillars, implementation of adequate metrology for measuring single pillars is described. Commercial tungsten probes, self-made gold probes, and piezoresistive silicon cantilever probes were used for measurements of nanopillars in a scanning electron microscope (SEM) equipped with nanomanipulators.

Design optimization of piezoresistive cantilevers for force sensing in air and water

Piezoresistive cantilevers fabricated from doped silicon or metal films are commonly used for force, topography, and chemical sensing at the micro- and macroscales. Proper design is required to optimize the achievable resolution by maximizing sensitivity while simultaneously minimizing the integrated noise over the bandwidth of interest. Existing analytical design methods are insufficient for modeling complex dopant profiles, design constraints, and nonlinear phenomena such as damping in fluid. Here we present an optimization method based on an analytical piezoresistive cantilever model. We use an existing iterative optimizer to minimimize a performance goal, such as minimum detectable force. The design tool is available as open source software. Optimal cantilever design and performance are found to strongly depend on the measurement bandwidth and the constraints applied. We discuss results for silicon piezoresistors fabricated by epitaxy and diffusion, but the method can be applied to any dopant profile or material which can be modeled in a similar fashion or extended to other microelectromechanical systems.

Design, fabrication and characterization of a two-step released silicon dioxide piezoresistive microcantilever immunosensor

This paper presents the design, fabrication and characterization of a silicon dioxide piezoresistive microcantilever immunosensor fabricated on silicon-on-insulator (SOI) wafers. The microcantilever consists of two strips of single crystalline silicon piezoresistors sandwiched in between two silicon dioxide layers. A theoretical model for the laminated microcantilever with a discontinuous layer is deduced using classic laminated beam theory. A two-step release method combining anisotropic and isotropic etching is developed to suspend the microcantilever, and the fabrication results show an excellent yield. The residual stress-induced free bending of the microcantilever and the stress caused by self-heating of the piezoresistors are discussed. The microcantilever sensor is characterized as an immunosensor using specific binding of antigen and antibody. These methods and some conclusions are also applicable to the development of other piezoresistive sensors that use laminated structures.

Ultra-sensitive shape sensor test structures based on piezoresistive doped nanocrystalline silicon

This paper describes the manufacture of a thin skin-like piezoresistor strain-sensing membrane and the miniaturization of dense piezoresistive sensor arrays based on n-type hydrogenated nanocrystalline silicon thin-films (nc-Si:H) deposited on flexible polyimide substrates (PI). The nc-Si:H thin-films, prepared by hot-wire chemical vapor deposition, have a piezoresistive gauge factor of −32.2. Six of the sensors batch-processed on the 15-μm thick membrane, were used in a test structure to track the simulated movement of the head of a bedridden patient. The sensors were glued to both sides of a 3-mm thick acrylic rectangular plate, to collect strain data from the tensile and compressive surfaces of the plate upon bending. The electrical output signal of the sensor was obtained by inserting the sensors into Wheatstone bridge circuits and recording the output voltage of the bridge as a function of the sensor/plate deformation. The results using quarter-, half-, and full-bridge configurations were compared. In order to give a further step toward a shape sensitive electronic textile, nine sensors were glued and interconnected using a machine-sewed conductive thread and preliminary tests on their output response under random loading conditions were performed.

A theoretical model for surface-stress piezoresistive microcantilever biosensors with discontinuous layers

A theoretical model for laminated piezoresistive microcantilevers with discontinuous layers is developed for surface stress sensitive biosensors. The microcantilever consists of two supporting layers of silicon dioxide and two silicon piezoresistor strips sandwiched in-between. To address the issue of discontinuity of the piezoresistive layer, the microcantilever is segmented and a closed-form equation is developed for each segment using classical laminated plate theory. A two-dimensional theoretical model for the discontinuous microcantilever is deduced by integrating all the segments and is validated using finite-element-method (FEM) simulation. The model allows the predication of the deflection and the sensitivity of piezoresistive microcantilever biosensors with discontinuous layers and surface stress. The effects of the dimension of the discontinuous piezoresistors on the performance of the microcantilevers are obtained using the segment model. Based on the model, guidelines for design and optimization of surface stress sensitive microcantilever biosensors are given.

Piezoresistive Cantilever Optimization and Applications

AbstractPiezoresistors are commonly used in microsystems for transducing force, displacement, pressure and acceleration. Silicon piezoresistors can be fabricated using ion implantation, diffusion or epitaxy and are widely used for their low cost and electronic readout. However, the design of piezoresistive cantilevers is complicated by coupling between design parameters as well as fabrication and application constraints. Here we discuss analytical models and design optimization for piezoresistive cantilevers, and describe several applications ranging from studying electron movement using scanning gate microscopy to measuring the biomechanics of whole organisms.

Piezoresistance in p-type silicon revisited

We calculate the shear piezocoefficient π44 in p-type Si with a 6×6 k⋅p Hamiltonian model using the Boltzmann transport equation in the relaxation-time approximation. Furthermore, we fabricate and characterize p-type silicon piezoresistors embedded in a (001) silicon substrate. We find that the relaxation-time model needs to include all scattering mechanisms in order to obtain correct temperature and acceptor density dependencies. The k⋅p results are compared to results obtained using a recent tight-binding (TB) model. The magnitude of the π44 piezocoefficient obtained from the TB model is a factor of 4 lower than experimental values; however, the temperature and acceptor density dependencies of the normalized values agree with experiments. The 6×6 Hamiltonian model shows good agreement between the absolute value of π44 and the temperature and acceptor density dependencies when compared to experiments. Finally, we present a fitting function of temperature and acceptor density to the 6×6 model that can be used to predict the piezoresistance effect in p-type silicon.

Modeling the stress enhancement of plasma enhanced chemical vapor deposited silicon nitride films by UV post treatment – impact of the film density

Plasma Enhanced Chemical Vapour Deposited (PECVD) tensile nitride liners have been introduced in the 90 nm CMOS technology node to generate mechanical strain within the transistor silicon channel. With strain, because of the silicon piezoresistance effect, the electron mobility is enhanced within the silicon channel, so the nMOS transistor performance. Since that time, significant work has been carried out to develop silicon nitride films with enhanced tensile stress values to increase the gain provided by these process induced stressors along the introduction of the new technologies. For the 45 nm node, an additional UV post treatment has been introduced to achieve highly tensile residual stress. This study determines the relevant as deposited PECVD nitride film properties that modulate the final stress achievable after UV cure. Thanks to a simple stress model, it is demonstrated that as deposited nitride films must present a nitrogen rich composition and an intrinsic low density to become highly tensile after post UV treatment. With these design rules, nitride films with final stress of 1.6 GPa have been synthesized.

Development of a prototype electronic tag for studying the migratory behaviour of marine species

Archival electronic tags can be used as standalone data loggers for sampling the ocean for gathering the environmental parameters and studying the migratory patterns of marine species, identifying their feeding and spawning grounds, etc. A prototype archival electronic tag for monitoring the ocean parameters like temperature, pressure and light intensity has been developed. A digital temperature sensor is used to sample the temperature from the tag's surroundings, while a micro machined piezoresistive silicon digital pressure sensor, which is capable of measuring absolute pressure levels upto 14 bars, provides the depth information. One of the important parameters to be measured is the geolocation of the species, which is computed from the ambient light intensities recorded by the digital light sensor in the tag. These parameters can be sampled and recorded in the memory at preset time intervals, as set at the time of deployment of the tag. This minuaturised tag provides the temperature data with 13 bit resolution, while the pressure and light intensity values have 15 bit resolutions. When used in fisheries studies, the size of the device has to be miniaturised, so that by way of attaching such devices, the normal behaviour of the species remain unaffected.

Investigation of conductivity and piezoresistance of n-type silicon on basis of quantum kinetic equation and model distribution function

Conductivity and first-order piezoresistance coefficient π 11 are investigated in detail for strained n-type silicon crystal. As scattering system were adopted impurities and longitudinal acoustic phonons. The consideration is based on quantum kinetic equation and corresponding precise balance equation. The main point of the used method is special model of non-equilibrium distribution function. Final results for mobility of unstrained crystal and piezoresistance coefficient π 11 are compared with experimental data and consequent theoretical results obtained by another method, grounded on the wide-spread isotropic τ-approximation. The latter consists in modelling of the collision integral by the simple form which contains scalar relaxation time depending on energy. Comparison shows the substantial numerical difference between values, calculated with the help of the two mentioned methods.

Fabrication of a strain sensor for bone implant failure detection based on piezoresistive doped nanocrystalline silicon

One common complication following total-hip-replacement arthroplasty – one of the most performed elective surgical procedures – is the loosening of the prosthetic stem and cup, responsible for more than 80% of non-successes. A combination of piezoelectric and piezoresistive materials in a sensing device for dynamical and quasi-static analysis of the implant internal environment is envisaged by the authors. In this paper, thin-film piezoresistive millimeter-long strain sensors fabricated on flexible plastic substrates using n- and p-type hydrogenated nanocrystalline silicon films with gauge factor GF∼−30 and +20, respectively, are reported. A maximal value of GF=−43 is theoretically predicted for isotropic n-type multicrystalline Si. A sensor consists of an array of piezoresistors, each connected to a Wheatstone bridge-type external circuit, and to the control electronics. A sensitivity of 30mV/μm was achieved in sensors bearing both longitudinal and transverse orientations of the resistors relative to the strain direction. Used as a shape sensor the device was able to map the contour of the hip implant. Cell growth tests show that osteoblasts grow faster on P-doped nc-Si:H thin films than on the control sample. Genotoxicity tests show that cell DNA is preserved if cultured in contact with n-type nanocrystalline silicon.

FEM assisted calibration of a MEMS-based dispensing system with integrated piezoresistive force sensors

The focus of this work is a liquid dispensing system relying on the use of piezoresistive silicon microcantilevers. The experimental calibration of newly developed devices, required for global characterization of their mechanical, electromechanical properties and other performance characteristics, is usually complicated due to the structures' small size as well as cost and time consumption. On the other hand, closed-form analytical solutions to the nonlinear behaviour of devices of non-canonical geometry incorporating residual stresses are not commonly available either. Therefore, virtual calibration by means of computational modelling and simulation is not only adequate but also highly desirable. The finite element method has repeatedly proven its worth as a useful and efficient technique for the investigation of various mechanical and multiphysical systems, and has been extensively used in the design and analysis of MEMS. In the present work, the method is employed for virtual calibration of mechanical and electromechanical properties of novel liquid dispensing cantilevers. Separately, mechanical and electromechanical simulation results are in good agreement with experiment. Combined, these simulation data provide the ultimate calibration tool, allowing control over the force applied by depositing devices onto the substrate, and thereby preventing stiction problems and damage of fragile depositing surfaces.