Temperature sensitivity in silicon piezoresistive pressure transducers

The various mechanisms responsible for temperature sensitivity in silicon piezoresistive pressure sensors are described. As a representative transducer, a full-bridge device having a 1-mm-square 23-µm-thick diaphragm is used. The 200 Ω/square, 2K-Ω bridge resistors produce a pressure sensitivity of 13.3 µV/V.mmHg with a temperature coefficient of -1300 ppm/°C. Variability in this sensitivity is most strongly influenced by the diaphragm thickness and the absolute resistor tolerance. A new technique-the electrochemical EDP etch-stop-is found to offer significant advantages over alternative schemes for diaphragm formation. Temperature sensitivity in electrostatically-bonded, vacuum-sealed devices is dominated by resistor match, with oxide stress and junction leakage current playing relatively minor roles over the -40 to + 180°C temperature range. While individual pressure trims for offset and sensitivity will continue to be required, individual temperature trims may be eliminated in these devices for many applications as increasingly precise resistor processes are used.     

Low-pressure sensor with bossed diaphragm

A novel design of piezoresistive, low-pressure sensors is presented. This design exhibits a number of advantages compared to conventional ones. The main objective of this development was realizing a sensor with high sensitivity, high pressure overload, and low non-linearity. This paper describes the theory of designing a piezoresistive low-pressure sensor. Through the application of a square diaphragm bossed in the center, a piezoresistive low-pressure sensor for the pressure range of ±10 kPa could be realized, exhibiting excellent sensitivity and low non-linearity of 35 mV/VF.S.O. and <|±0.05|%, respectively.     

Piezoresistive effect in GaAs/InxGa1−xAs/AlAs resonant tunneling diodes for application in micromechanical sensors

The current–voltage characteristics of GaAs/In_xGa_1−xAs/AlAs resonant tunneling diodes (RTDs) are a function of stress, and the current–voltage changes of RTDs with stress are attributed to the piezoresistive effect in RTDs. In order to study the piezoresistive effect in RTDs for application in micromachined mechanical sensors, the beam-mass structure based on RTDs is designed, fabricated and tested by the Wheatstone bridge test circuit. The test results show that the piezoresistive sensitivity of RTDs can be adjusted through the bias voltage, and the maximal piezoresistive sensitivity of RTDs with bias voltage at 0.618 V is 7.61×10⁻¹¹ Pa⁻¹, which is two orders higher than the minimal piezoresistive sensitivity (2.03×10⁻¹³ Pa⁻¹) of RTDs with bias voltage at 0.656 V, and is also higher than the piezoresistive sensitivity of silicon material (5.52×10⁻¹¹ Pa⁻¹).     

1D/2D Nanomaterials Synergistic,Compressible, and Response Rapidly 3D Graphene Aerogel for Piezoresistive Sensor

Graphene‐based aerogels have been widely studied for their high porosity, good compressibility, and electrical conductivity as piezoresistive sensors. However, the fabrication of graphene aerogel sensors with good mechanical properties and excellent sensing properties simultaneously remains a challenge. Therefore, in this study, a novel nanofiber reinforced graphene aerogel (aPANF/GA) which has a 3D interconnected hierarchical microstructure with surface‐treated PAN nanofiber as a support scaffold throughout the entire graphene network is designed. This 3D interconnected microporous aPANF/GA aerogel combines an excellent compressive stress of 43.50 kPa and a high piezoresistive sensitivity of 28.62 kPa^?1 as well as a wide range (0–14 kPa) linear sensitivity. When aPANF/GA is used as a piezoresistive sensor, the compression resilience is excellent, the response time is fast at about 37 ms at 3 Pa, and the structural stability and sensing durability are good after 2600 cycles. Indeed, the current signal value is 91.57% of the initial signal value at 20% compressive strain. Furthermore, the assembled sensors can monitor the real time movement of throat, wrist pulse, fingers, wrist, and knee joints of the human body at good sensitivity. These excellent features enable potential applications in health detection.     

Scaling limits in batch-fabricated silicon pressure sensors

The scaling properties of silicon capacitive and piezoresistive pressure sensors are described. An evaluation of the various noise mechanisms and pressure offsets in the scaled devices is presented, including Brownian noise, electrical noise, electrostatic pressure variations and pressure offset due to resistor mismatch. The analysis of diaphragm deflection includes the effects of intrinsic stress and the transition from plate theory to membrane theory. Both ultraminiature and ultrasensitive sensors are considered. Ultraminiature piezoresistive sensors with diaphragms measuring 100 µm in length and resolving 1 mmHg should be possible using present technology as well as ultrasensitive capacitive sensors that resolve 1 µmHg.     

SENSIM: A simulation program for solid-state pressure sensors

A simulation program is described which is capable of calculating the output response of silicon piezoresistive or capacitive pressure sensors as a function of both pressure and temperature. A thermoelastic plane-stress formulation is used in calculating the stress and deflection of the transducer diaphragm. Both analytical and finite-difference solution methods are available, depending on the sensor structure. Diaphragm thickness taper, oxide and package stress, and rim effects are simulated. For capacitive structures, the program accurately predicts the diaphragm deflection and pressure sensitivity as a function of pressure and temperature. Stepped diaphragm structures are shown to be capable of improving pressure sensitivity by as much as 50 percent. The package-induced thermal drift for electrostatically sealed glass-silicon devices is typically less than 0.05 mmHg/°C.     

Rational Design of Soft Yet Elastic Lamellar Graphene Aerogels via Bidirectional Freezing for Ultrasensitive Pressure and Bending Sensors

To enhance the sensitivity of graphene aerogel-based piezoresistive sensors by weakening their compressive strength while keeping their elasticity, lightweight and lamellar graphene aerogels (LGAs) with high elasticity and satisfactory electrical conductance networks are fabricated by bidirectional-freezing of aqueous suspensions of graphene oxide in the presence of small amounts of organic solvents, followed by lyophilizing and thermal annealing. Because of the lamellar structure of the LGA, its compressive strength along the direction perpendicular to the lamellar surface is much lower than those of both isotropic and unidirectionally aligned graphene aerogels with similar apparent densities, leading to an ultrasensitive LGA-based piezoresistive sensor with a high sensitivity of −3.69 kPa⁻¹ and a low detection limit of 0.15 Pa. The ultrahigh sensitivity and low detection limit of LGA-based piezoresistive sensor contribute to detecting subtle pressure at room temperature and in liquid nitrogen with ability to detect dynamic force frequency and sound vibration. Besides, thanks to the fewer junction points between the graphene lamellae, LGAs slices can be integrated as a wide-range and sensitive bending sensor, which can detect arbitrary bending angles from 0° to 180° with a low detection limit of 0.29°, and is efficient in detecting biosignals of wrist pulse and finger bending.     

Multiscale Wrinkled Microstructures for Piezoresistive Fibers

Even though flexible piezoresistive materials are widely researched in recent years, it is full of challenges to simultaneously satisfy high conductivity, stretchability, and sensitivity. In this study, core–shell conductive fibers with high conductivity (10^?4–10^?5 Ω cm), stretchability (400%), and durability (more than 1200 s ultrasonic treatment) are presented and multiscale wrinkled microstructures (about 1.7 μm in height and 2.6 μm in length) are built on the surfaces of fibers via simply writing silver nanowires ink on prestrained commercial polyurethane fibers. The as prepared core–shell elastic fibers are twisted to construct flexible piezoresistive fibers, which show desirable sensitivity to pressure and bending deformations (0.12 kPa^?1 and 0.012 Rad^?1), fast response and relaxation time (35 and 15 ms), very low detection limit (10 mg), and excellent working stability (>4000 loading/unloading cycles). The wrinkled microstructures to overcome the viscoelastic delay of polymer composites are observed, making substantial contributions to improve the responsiveness. The investigations to the sensing mechanism indicate that increasing the contact points inner the flexible piezoresistive fibers will significantly improve the sensitivity. Finally, the potential applications of the flexible piezoresistive fibers as wearable devices and smart fabrics are demonstrated.     

Light‐Boosting Highly Sensitive Pressure Sensors Based on Bioinspired Multiscale Surface Structures

Pressure sensors have attracted tremendous attention because of their potential applications in the fields of health monitoring, human–machine interfaces, artificial intelligence, and so on. Improving pressure‐sensing performances, especially the sensitivity and the detection limit, is of great importance to expand the related applications, however it is still an enormous challenge so far. Herein, highly sensitive piezoresistive pressure sensors are reported with novel light‐boosting sensing performances. Rose petal–templated positive multiscale millimeter/micro/nanostructures combined with surface wrinkling nanopatterns endow the assembled pressure sensors with outstanding pressure sensing performance, e.g. an ultrahigh sensitivity (70 KPa^?1, <0.5 KPa), an ultralow detection limit (0.88 Pa), a wide pressure detect ion range (from 0.88 Pa to 32 KPa), and a fast response time (30 ms). Remarkably, simple light illumination further enhances the sensitivity to 120 KPa^?1 (<0.5 KPa) and lowers the detection limit to 0.41 Pa. Furthermore, the flexible light illumination offers unprecedented capabilities to spatiotemporally control any target in multiplexed pressure sensors for optically enhanced/tailorable sensing performances. This light‐control strategy coupled with the introduction of bioinspired multiscale structures is expected to help design next generation advanced wearable electronic devices for unprecedented smart applications.     

Low-temperature conductance measurements of surface states in HfO2–Si structures with different gate materials

Metal-oxide-semiconductor capacitors based on HfO₂ gate stack with different metal and metal compound gates (Al, TiN, NiSi and NiAlN) are compared to study the effect of the gate electrode material on the trap density at the insulator–semiconductor interface.C–V and G–ω measurements were made in the frequency range from 1 kHz to 1 MHz in the temperature range 180–300 K. From the maximum of the plot G/ω vs. ln(ω) the density of interface states was calculated, and from its position on the frequency axis the trap cross-section was found. Reducing temperature makes it possible to decrease leakage current through the dielectric and to investigate the states located closer to the band edge.The structures under study were shown to contain significant interface trap densities located near the valence band edge (around 2×10¹¹ cm⁻²eV⁻¹ for Al and up to (3.5–5.5)×10¹² cm⁻² eV⁻¹ for other gate materials). The peak in the surface state distribution is situated at 0.18 eV above the valence band edge for Al electrode. The capture cross-section is 5.8×10⁻¹⁷ cm² at 200 K for Al–HfO₂–Si structure.     

Electrically active defects induced by hydrogen and helium implantations in Ge

Results of a study of electrically active defects induced in Sb-doped Ge crystals by implantations of hydrogen and helium ions (protons and alpha particles) with energies in the range from 500 keV to 1 MeV and doses in the range 1×10¹⁰–1×10¹⁴ cm⁻² are presented in this work. Transformations of the defects upon post-implantation isochronal anneals in the temperature range 50–350 °C have also been studied. The results have been obtained by means of capacitance–voltage (C–V) measurements and deep-level transient spectroscopy (DLTS).It was found from an analysis of DLTS spectra that low doses (<5×10¹⁰ cm⁻²) of H and He ion implantations resulted in the introduction of damage similar to that observed after MeV electron irradiation. The Sb–vacancy complex was the dominant deep-level defect in the lightly implanted samples. After implantations with doses higher than 5×10¹⁰ cm⁻² peaks due to more complex defects were observed in the DLTS spectra. Implantations with heavy (5×10¹³ cm⁻²) doses of both H and He ions caused the formation of a sub-surface layer with a high (up to 1×10¹⁷ cm⁻³) concentration of donors. These donors were eliminated by anneals at temperatures in the range 100–200 °C. Heat treatments of the heavy proton-implanted Ge samples in the temperature range 250–300 °C resulted in the formation of shallow hydrogen-related donors, the concentration of which was the highest in a region close to the projected depth of implanted protons. The maximum peak concentration of the H-related donors was higher than 1×10¹⁵ cm⁻³ for a proton implantation dose of 1×10¹⁴ cm⁻².     

Hydrogen incorporation, diffusivity and evolution in bulk ZnO

Hydrogen is readily incorporated into bulk, single-crystal ZnO during exposure to plasmas at moderate (100–300°C) temperatures. Incorporation depths of >25 μm were obtained in 0.5 h at 300°C, producing a diffusivity of 8 × 10⁻¹⁰ cm²/V s at this temperature. The activation energy for diffusion is 0.17 ± 0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C is sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of Secondary Ion Mass Spectrometry (<5 × 10¹⁵ cm⁻³). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage.     

Evaluation of MOS structures processed on 4H–SiC layers grown by PVT epitaxy

MOS capacitors have been fabricated on 4H–SiC epilayers grown by physical vapor transport (PVT) epitaxy. The properties were compared with those on similar structures based on chemical vapor deposition (CVD) layers. Capacitance–voltage (C–V) and conductance measurements (G–V) were performed in the frequency range of 1 kHz to 1 MHz and also at temperatures up to 475 K. Detailed investigations of the PVT structures indicate a stable behaviour of the interface traps from room temperature up to 475 K. The amount of positive oxide charge Q_O is 6.83 × 10⁹ cm⁻² at room temperature and decreases with temperature increase. This suggests that the processed devices are temperature stable. The density of interface states D_it obtained by Nicollian–Brews conductance method is lower in the structure based on the PVT grown sample.     

A piezoresistive microcantilever magnetic-field sensor with on-chip self-calibration function integrated

A micromachined piezoresistive cantilever magnetometer, with a self-calibration function on-chip integrated is presented . When the cantilever is subjected to a magnetic field to be measured, the magnetic force will exert upon the magnetized nickel thin-film pattern that is located at the cantilever end. The magnetic force bends the micromechanical cantilever, which is further read out by an integrated piezoresistor. For realizing the self-calibration function, an aluminum spiral is integrated around the cantilever to provide an artificial magnetic field, when an electric current flows through the spiral coil. The artificial magnetic field can be used to drive the cantilever bending and causes a self-calibration output signal. With this on-chip self-calibration scheme, the detection of magnetic field can be immune to the long-term drift in remanence of the magnetized nickel pattern, thereby, improving the sensing stability. Bulk micromachining technologies are used to fabricate the sensors. The formed sensor is used for magnetic-field measurement, resulting in the piezoresistive sensitivity as 1.06×10⁻⁴/mT and the sensing resolution 4.58 μT.     

Optimisation of a thick-film 10–400 N force sensor

The ring-on-ring bending principle allows the fabrication of simple, low-cost thick-film piezoresistive sensors for compressive forces in the 10–400 N range. However, some imperfections are encountered in its basic embodiment, such as relatively high force-signal hysteresis and nonrepeatability (up to ca. 5%). These shortcomings were studied in this work, and major improvements have been achieved. Hysteresis was found to be mainly due to friction at the outer support ring, and was considerably reduced by inserting a compliant silicone glue ring. The same glue ring was used to permanently bond the sensor to a rigid base, thereby giving well-defined and constant boundary conditions and also considerably improving the repeatability of the sensitivity. Overall, hysteresis and repeatability error were reduced down to a level of ca. 1%.     

Investigation of high-K gate stacks with epitaxial Gd2O3 and FUSI NiSi metal gates down to CET=0.86 nm

Novel gate stacks with epitaxial gadolinium oxide (Gd₂O₃) high-k dielectrics and fully silicided (FUSI) nickel silicide (NiSi) gate electrodes are investigated. Ultra-low leakage current densities down to 10^–7 A cm^–2 are observed at a capacitance equivalent oxide thickness of CET=1.8 nm. The influence of a titanium nitride (TiN) capping layer during silicidation is studied. Furthermore, films with an ultra-thin CET of 0.86 nm at a Gd₂O₃ thickness of 3.1 nm yield current densities down to 0.5 A cm⁻² at V_g=+1 V. The extracted dielectric constant for these gate stacks ranges from k=13 to 14. These results emphasize the potential of NiSi/Gd₂O₃ gate stacks for future material-based scaling of CMOS technology.     

High temperature characterization of high-κ dielectrics on SiC

We have fabricated thin catalytic metal–insulator–silicon carbide based structure with palladium (Pd) gates using TiO₂ as the dielectric. The temperature stability of the capacitor is of critical importance for use in the fabrication of electronics for deployment in extreme environments. We have evaluated the response to temperatures in excess of 450 °C in air and observed that the characteristics are stable. Results of high temperature characterization are presented here with extraction of interface state density up to 650 °C. The results show that at temperatures below 400 °C the capacitors are stable, with a density of interface traps of approximately 6×10¹¹ cm² eV⁻¹. Above this temperature the C–V and G–V characteristics show the influence of a second set of traps, with a density around 1×10¹³ cm² eV⁻¹, which is close to that observed for slow states near the conduction band edge. The study of breakdown field as a function of temperature shows two distinct regions, below 300 °C where the breakdown voltage has a strong temperature dependence and above 300, where it is weaker. We hypothesize that the oxide layer dominates the breakdown voltage at low temperature and the TiO₂ layer above 300 °C. These results at high temperatures confirms the suitability of the Pd/TiO₂/SiO₂/SiC capacitor structure for stable operation in high temperature environments.     

Micro cantilever probe array integrated with Piezoresistive sensor

A micro cantilever-tip silicon probe-array with integrated electro-thermal nano-tip and piezoresistive sensor has been presented for NEMS high-density data storage. After its fundamental working principle has been illustrated, such a 1×10 probe-array has been designed. Both analysis and FEM simulation are used for modeling and designing with their results agreeing well with tolerance of only 5%. The device has been fabricated with silicon bulk micromachining technologies. The relationship between the heating resistance and tip temperature was experimentally obtained and fitted with second order polynomial function. Based on those, the microsecond-instantaneous electro-thermal performance of the device has been gained and the tested results were in agreement with the simulated ones. Under the 4 V pulse power and 3 μs heating time, the tested results were indicative of the 463.15 K temperature on the tip, the 6.2 μs decreasing-temperature constant of the heating resistor and the nearly 100 KHz reading-writing velocity. The sensitivity of piezoresistivity was up to 5.4×10⁴ under the force of 2×10⁻⁷ N, which was sufficient to read out the data from the polymer indent.     

Noise due to Brownian motion in ultrasensitive solid-state pressure sensors

Noise due to Brownian motion of diaphragm in ultrasensitive solid-state capacitive and piezoresistive pressure sensors operating at sub-millimeters of mercury pressures in a gaseous ambient is considered. The statistical properties and spectral characteristics of the noise are obtained as functions of the diaphragm dimensions, temperature, and applied pressure. The results show that the Brownian equivalent pressure noise is substantially less than has been previously reported and is well below 1µmHg for most practical cases of interest. Thus it is not a limiting factor in setting device performance when compared to circuit noise sources.     

Structural and electrical properties of brush plated ZnTe films

Zinc telluride thin films were deposited by the brush plating technique at a potential of −0.90 V (SCE) on conducting glass and titanium substrates at different temperatures in the range 30–90 °C. The films were polycrystalline in nature with peaks corresponding to the cubic phase. Direct band gap of 2.30 eV was observed. XPS studiers indicated the formation of ZnTe. Depth profiling studies indicated a uniform distribution of Zn and Te throughout the entire thickness. EDAX measurements were made on the films and it was found that there was a slight excess of Te. The carrier concentration was found to vary from 10¹⁴–10¹⁵ cm⁻³ with increase of substrate temperature. The mobility was found to vary from 5 to 60 cm² V⁻¹ s⁻¹.