Desire to integrate electronic devices more closely into the everyday life of the consumer led to the emergence of flexible devices, which can conform to any shape surface. These flexible devices are lighter, thinner, and more resilient than their rigid counterparts. Since consumers these days are heavily concerned about the visual look of any product along with its technological features, the flexible devices also pave the way for innovation and creativity from the aesthetic point of view. As more and more electronics including wearable devices and displays are shifting towards the flexible regime, their basic components like sensors and integrated circuits are also required to be flexible and conformal. This paper reviews the development and evolution of several types of flexible MEMS sensors, accelerometers, and energy harvesters from the design, fabrication, and performance perspective.

We investigated the effect of channel hot carrier (CHC) stressing on the creation of gate oxide defect centers causing random telegraph signals (RTSs) in pMOSFETs. To identify these stress-induced hole traps in the oxide and fully characterize their trapping properties, variable temperature RTS measurements were performed from room temperature down to 215 K and repeated after each stressing interval, up to 1200 s. The trapping properties of the stress-induced defect sites and those present before stress are quantified, including the hole capture and emission activation energies, structural relaxation in the oxide network due to trapping/detrapping, and entropy change as well as the energy level and position of the trap in the oxide. Based on this information, disassociated III-Si and hydrogen bridge defects are identified as the hole trapping centers in SiO 2 . Difference was observed between the process- and stress-induced traps in the entropy change upon hole emission from the oxide trap to Si channel (electron capture from the Si valence band), implying the possibility of stress-induced structural defects being different than the native ones.

This chapter provides a brief background to the development of multifunctional sensors that can conform to any surface. The objective is to achieve what is now referred to, as “Smart Skin” (sensitive skin as defined by Vladimir Lumelsky, one of its pioneers) is a large area, flexible array of sensors with data processing capabilities. The microsensors are integrated to give the capability of sensing several stimuli, mimicking a real skin, including but not limited to temperature, touch, and flow. Other requirements are flexibility and ability to stretch and wrinkle without any degradation in its sensing figures of merit. Additional capabilities can be incorporated such as biochemical sensing and decision-making, and energy harvesting. When incorporated with thin film transistors fabricated on flexible plastic substrates, a complete “Smart Skin” is achieved.

A novel triboelectric energy harvesting (TEH) and sensing system scaled down to microelectromechanical systems (MEMS) size is presented. The design is structurally optimized for harvesting the highest average power and power density while ensuring the structural robustness. Unlike traditional triboelectric energy harvesters, this design results in a high operating frequency with a wide bandwidth. Adoption of MEMS fabrication techniques including the use of spin-coated Teflon AF rather than a Teflon sheet, adaptation of UV-LIGA (Ultra-Violet Lithographie, Galvanoformung, Abformung) with modifications and implementation of a thick polyimide sacrificial layer make the fabrication process unique. If excited by +/- 9.33 g external vibration with a frequency of 1.15 kHz, the TEH can generate 0.179 mu W average power and 0.597 mu W peak power at an optimum resistive load of 256 k Omega. The peak surface power density, volumetric power density and acceleration-normalized volumetric power density reach 3.98 mu Wcm(-2), 2.64 mWcm(-3) and 30.3 mu Wcm(-3)/g(2), respectively. While the surface power density of the presented TEH is moderate, the volumetric power density and acceleration-normalized volumetric power density are quite competitive among the state-of-the-art designs. The TEH also demonstrates a wide operating frequency bandwidth of 920 Hz. If operated as an accelerometer, the device shows a linear sensitivity of 43 mV/g. Although the simulation predicts the optimum operating frequency and load resistance of the system to be at 800 Hz and 10 M Omega, respectively, the experimental results demonstrate these values to be at 1150 Hz and 256 K Omega. A few fabrication anomalies, most notably the notching in the Teflon layer and bowing of the proof-mass, are responsible for this deviation. In addition, a distortion is observed in the simulated output voltage profile which is not present in the experimental output voltage profile due to the presence of the parasitic capacitance in the experimental circuit. The aforementioned triboelectric energy harvester can have specific applications in the sensor and actuator systems in the aircraft industry as well as in the automobile industry, micro-robotic systems, prosthetic systems, and sensor nodes in the internet of things (IoT) due to its operating frequency and bandwidth range. (C) 2020 Elsevier B.V. All rights reserved.

Although it is the worst degradation mechanism, the effect of channel hot carrier (CHC) stressing on random telegraph signals (RTS) has not been given enough attention in pMOSFETs. We report on the effect of CHC stressing on different RTS trap parameters namely screened scattering coefficient which controls the amount of charge carrier mobility fluctuations due to remote Coulomb scattering by the trap, RTS fluctuation amplitude, average capture time and capture cross-section. The generation of positive fixed oxide charge with stressing influences the screened Coulomb scattering of the channel carriers and therefore their mobility, in addition to the commonly accepted self-screening of the channel carriers. The two-dimensional mobility fluctuations model is adopted for analyzing the CHC effect on pMOSFETs. The comparison between theoretically and experimentally obtained screened scattering coefficients points toward the impact of the newly generated positive fixed oxide charge due to stressing. The decrease of the screened scattering coefficient results in increased RTS amplitude and decreased contribution of mobility fluctuations to RTS amplitude. Capture time and capture cross-section also change as the capture cross-section pre-factor has been impacted by the reduction of relaxation energy with stressing.

The effect of channel hot carrier stress has been investigated in pMOSFETs with respect to channel hole trapping by the gate oxide traps and the resultant Random Telegraph Signals (RTS) observed at the output. Process- and stress- induced oxide traps have been identified through RTS measurements. These traps are believed to be E' centers, acting as hole-attractive or repulsive centers, causing RTS through correlated carrier number and mobility fluctuations. Generation of E' centers as well as activation of the passivated oxide defects as a result of stress are thought to be the reasons behind observing the stress-induced RTS. Two of the observed defects: one process-induced and one stress-induced trap have disappeared and reappeared randomly with stress. We report the reaction of the defects with hydrogen to be responsible for the volatile nature of the traps. Hydrogen is either bound to the Si/SiO2 interface or trapped at the oxide defects. Applying the stress releases these hydrogen species, which then drift towards the gate through the oxide layer and react with the oxide defects to passivate them. Reaction with another hydrogen de-passivates the defect releasing H-2. This is the first article that reports hydrogen to deactivate and reactivate oxide traps responsible for RTS.

A MEMS wideband energy harvesting array design using linear and nonlinear generators is reported in this paper. An array comprising of four electrostatic energy harvesters using linear, softening, hardening springs and mechanical stoppers connected in parallel can work over a wide frequency range. Novel nonlinear spring design produces spring softening and hardening behaviors. Mechanical stoppers further enhance the operating frequency range of the hardening spring energy harvester. Mathematical model of the electromechanical system is derived and optimized using MATLAB. The system is built using Coventorware/MEMS+ ® . Simulation results indicated a maximum normalized power density of 1.97 μWs 4 /cm 3 -m 2 is achieved over 600 Hz-1350 Hz frequency range. Moreover, following the method developed in this work, the device size is reduced by 89% compared to traditional array design technique.

An array of four out-of-plane, gap-closing, electret-based electrostatic energy harvesters capable to harvest over a wide frequency range is designed. The novel design consists of energy harvesters with linear springs, softening springs, hardening springs and mechanical stoppers. This unique method of designing array with combination of linear and nonlinear generators allows the system to harvest significant amount of power even with shift in vibration frequency. A dynamic model of the system is derived and optimized using MATLAB. The optimized system is built using Coventorware / MEMS+ ® . A maximum power of 75 nW is obtained over a frequency range of 450-760 Hz. The design also takes 75% less volume compared to an array of linear energy harvesters to generate over the same frequency range.

A novel optimized design of a triboelectric vibrational energy harvester having small MEMS scale size and low mass is presented in this work. The triboelectric energy harvester is designed to harvest energy from high frequency mechanical vibrations of machineries such as the skin of an aircraft. The novelty of this work lies in the integrated design and modeling of the energy harvester which enables it to be fabricated and packaged as a MEMS device in a traditional cleanroom environment. Dynamic optimization has been implemented on its geometric model to maximize the output power and power density. Simulation shows that at an acceleration magnitude of 9.8 ms -2 and operating frequency of 800 Hz, the device can generate an average power of 196.91 nW with surface and volume power densities of 13.1 mWm -2 and 1544.4 Wm -3 , respectively. Due to its small size, low mass and comparatively high power density, this triboelectric energy harvester can have a significant impact in expanding the applications of the nano-sensors in wireless sensor nodes, in automobile industry, in space exploration programs, in micro- robotics and in prosthetics.

A complete investigation of the effect of Li doping on the physical, material, electromechanical and piezoelectric properties of ZnO nanowires (NWs) is presented. Low temperature hydrothermal growth technique is used to grow vertically aligned crystalline ZnO NWs doped with different concentrations of Li. Characterization techniques reveal considerable physical, material and electromechanical property modifications of the ZnO NWs due to the incorporation of Li dopants. Atomic Force Microscope is utilized to apply controlled amount of force on the fabricated NWs to assess their piezoelectric response. More than twenty two-fold improvement is observed in sensitivity due to the combined effect of modifications in NW geometry and piezoelectric properties with the addition of Li. Finite element method simulations were performed to decouple the individual effect of Li doping on the NW size and on the piezoelectric coefficient and to see how much each effect plays a role in the sensitivity improvement. It is estimated that the changes in the material and electromechanical properties alone are responsible for more than seven-fold improvement in the sensitivity. The impact of 'kick-out' diffusion mechanism of Li in ZnO is one of the major factors responsible behind this sensitivity improvement. It is also observed that there is an optimum level of Li doping concentration which can lead to the best piezoelectric performance by the ZnO NWs. The novelty of this work lies on the detailed analyses to illustrate the physics and impact of Li dopants in ZnO NW structures from a piezoelectric point of view towards improving their application as a nano-sensing and nano-energy harvesting element.

We report on the investigation of random telegraph signals (RTSs) in pMOSFETs at variable temperatures to identify and characterize the hole traps residing at the oxide-semiconductor interface. Attractive center defects as well as a repulsive center defect are identified with trap location, capture cross section, remote Coulomb scattering coefficient, capture barrier energy, trap binding enthalpy, relaxation energy, and change in entropy with hole emission. The attractive centers responsible for RTS were a pair of dissociated three-coordinated silicon (D-III-Si) defects while the repulsive center was identified as a puckered/back-projected oxygen vacancy defect, [a pair of under-coordinated silicon (III-Si) and over-coordinated oxygen (III-O) defects]. Both contain E gamma' centers. This is the first time when such detailed analysis is reported on the hole traps in SiO2 which are responsible for RTS.

This paper presents a wafer-level vacuum-packaged z-axis capacitive micro accelerometer fabricated on a flexible polyimide substrate and encapsulated by another polyimide superstrate enabling the flexibility, bendability, and portability of both the device and the package. The novel low-cost and low-temperature encapsulation procedure requires a fewer fabrication steps than the processes, where a thin epitaxial Si layer or a full Si wafer with cavity is used as a superstrate. It also provides a better pliability. The fabricated micro accelerometers demonstrated excellent integrity when bowed down to 2-cm radius of curvature with the sensitivity varying between 139 and 174 fF/g, depending on the design, under the application of ±4 g of acceleration. The resonant frequencies varied from 800 Hz for the largest device to 1050 Hz for the smallest device. The results reported here represent the best performing, waferlevel-packaged flexible micro accelerometers to date.

Surface micro-machined z-axis capacitive accelerometers were designed and fabricated on a flexible polyimide substrate, conformal down to 2 cm radius of curvature with the stresses sustained by all layers well below the yield strength for each material. A novel UV-LIGA fabrication technique was developed to realize a thicker proof-mass (8 mu m) compared to the spring (3 mu m), thus decoupling the two important parameters: the stiffness and the proof-mass to achieve higher sensitivity. Devices with three different sizes were fabricated with the capability of sustaining under distinct amount of acceleration and tuned to frequencies ranging from 600 Hz to 1100Hz. The largest device, having the area of 960 mu m x 960 mu m, showed a sensitivity of 187 fF/g with a SNR (Signal to Noise Ratio) of at least 100 when characterized at its resonance frequency of 800 Hz. The applied acceleration was +/- 4g in addition to gravitation. (C) 2017 Elsevier B.V. All rights reserved.

Flexible accelerometers, attached on robotic or prosthetic components are needed for sensing the motion or orientation to augment functionality and ensure safety of users. Flexibility, bendability and low profile enable ubiquitous presence without hindrance to the user. This paper presents design, simulation, fabrication and experimental characterization of MEMS accelerometers, wafer-level packaged between two polyimide layers and thus are bendable up to 1.0 cm radius of curvature with no performance degradation. The devices are compatible with robotic or prosthetic fingertips, which also have a similar curvature. Novelty lies in (1) wafer-level packaging with a polyimide substrate and a superstrate, (2) novel low-temperature fabrication process to maintain the integrity of the polyimide layers, (3) the use of double UV-LIGA process on a flexible substrate to achieve six-fold increase in sensitivity. The bendable packaged devices have increased sensitivity as high as 194 fF/g. The range of applied acceleration on the devices is ±4g.

As the demand for low- or no-power nano-sensors increases, new processing techniques gain importance to introduce novel sensing structures using well-known materials. ZnO presents substantial opportunities in this aspect due to its semiconducting and piezoelectric properties and ability to crystalize at relatively low temperatures. Many nanostructure configurations of ZnO can be easily synthesized, making it an ideal material for nano-sensors and nano-energy harvesters. This paper reviews design, fabrication and performance analysis of several nano-sensors and nano-energy harvesters based on ZnO. Several possible techniques are discussed that can further improve the performance of ZnO in these nano-sensors and nano-energy harvesters.

ZnO nanowires offer nanometer resolution with high piezoelectric coefficient as force sensors. There is, however, much room to improve the sensitivity with addition of substitutional impurities. Here, the effect of p-type Li doping on vertically-aligned crystalline piezoelectric ZnO nanowires (NWs) is investigated. The NWs are used as an elementary material to fabricate micro-Newton force sensors based on undoped and Li-doped ZnO nanostructures. The novelty of this work lies on the doping of ZnO NWs with Li that improves its performance by altering the physical and material properties. Scanning Electron Microscope (SEM) images illustrate that the force sensors can realize nanometer scale resolution. X-ray diffraction (XRD) and Raman spectroscopy depict that most of the Li atoms are incorporating into the ZnO structure as substitutional dopants, thus improving the piezoelectric properties. An Atomic Force Microscope (AFM) is operated in contact mode to apply pN range mechanical force to the nanowires. It is shown that the sensitivity of ZnO NW sensor increases from 0.033 mV/pN to 0.102 mV/pN when doped with Li.

Uncooled infrared detectors (IR) on a polyimide substrate have been demonstrated where amorphous silicon (a-Si) was used as the thermometer material. New concepts in uncooled microbolometers were implemented during the design and fabrication, such as the integration of a germanium long-pass optical filter with the device-level vacuum package and a double layer absorber structure. Polyimide was used for this preliminary work towards vacuum-packaged flexible microbolometers. The detectors were fabricated utilizing a carrier wafer and low adhesion strength release layer to hold the flexible polyimide substrate during fabrication in order to increase the release yield. The IR detectors showed a maximum detectivity of 4.54 × 106 cm Hz1/2/W at a 4 Hz chopper frequency and a minimum noise equivalent power (NEP) of 7.72 × 10−10 W/Hz1/2 at a biasing power of 5.71 pW measured over the infrared wavelength range of 8–14 μm for a 35 μm × 35 μm detector. These values are comparable to other flexible microbolometers with device-level vacuum packaging which are found in literature.

This paper presents a new statistical RTS model and a simulation tool to predict and replicate the RTS in time and frequency domains. This new developed RTS model and its simulation tool are constructed based on first-principles, taking into account Coulombic scattering effect, considering the non-uniformity in doping concentration in the channel and the non-uniformity in the trap distribution in the oxide, incorporating the effect of both linear and saturation regions of operation of MOSFETs, including the temperature effect on all temperature dependent parameters, and is fully scalable from single switching electron to ensembles 1/f noise. The RTS simulation (RTSSIM) generates the RTS in time domain based on the electrical properties that are associated with the identified trap. RTSSIM produces smooth transition from the RTS in time domain to 1/f noise and it takes into account the device scalability. The total power spectral density (PSD) for a number of active trap(s) less than 20 is computed directly from the generated RTS waveforms using Welch's PSD method. For 20+ active traps, RTSSIM evaluates the total PSD based on the summation of the RTS spectra.