Characteristic Research On Acceleration Sensor Based On Nc-Si:H TFTs

Acceleration sensor is one of the predominant components in the micro-inertia measurement system. In the domain of the navigation system, automation, automobile,seismic survey, military affairs as well as space system, acceleration sensor manufactured by micro-electromechanical systems(MEMS) has a widespread application. Among various sorts of acceleration sensor, piezoresistive acceleration sensor appeals to great attention due to its small volume, a wide range of tested frequency and acceleration. Additionally, it has capacity for the output of the voltage signal, simple interface, batch production, and compatibility with IC processing. This work studies on the acceleration sensor based on nc-Si:H TFTs that underpins the high sensitivity, miniazation and multifunctional research of sensors. The main content is displayed by the following.1. Essential configuration and operational theory of the acceleration sensor based on nc-Si:H TFTsBy means of CMOS processing, the Wheatstone bridge comprising four nc-Si:H TFTs was fabricated at the end of the cantilever beam with the proof mass to fulfill the acceleration sensor based on nc-Si:H TFTs, where two nc-Si:H TFTs are along the orientation < 011 > while the others along the orientation <011>. The cantilever, when suffering from the applied acceleration, will bend now that the bending moment gives rise to stress. Thus, the variation of the equivalent channel resistances of four nc-Si:H TFTs originates from the stress at the terminal of the beam. The channel resistance of nc-Si:H TFTs along < 011 > contributes to the positive change while the channel resistance of nc-Si:H TFTs along the orientation <011> facilitates the negative one.Owing to both arms of the Wheatstone bridge in nonequilibrium, the output voltage arises, where the nonelectrical signal shifts into the electrical output. That nc-Si:H TFTs at the end of the cantilever of the acceleration sensor based on nc-Si:H TFTs are subject to stress is analyzed, dependent on the piezoresistive theory in two aspects P=0 and P≠0for I-V characteristics.2. Simulation, chip design, fabrication and package of the acceleration sensor based on nc-Si:H TFTsFor the sake of research on characteristics of nc-Si:H TFTs and implementation of the acceleration sensor based on nc-Si:H TFTs with high performance, simulation models of the nc-Si:H TFT and acceleration sensor were set up by SILVACO and ANSYS. Allowing for the hot-carrier energy balance transport theory, IDS-VDS characteristics of the nc-Si:H TFT have negative resistance phenomena at VGS=3V, 6V and 9V as the channel length of the nc-Si:H TFT is shrinked up to 10μm. Extracted from the two-dimensional model established by ATLAS, the parameters including the carrier recombination rate at the interface between the gate oxide and nc-Si:H film,transverse electrical field strength parallel to the surface of the channel and longitudinal electrical field strength at the interface of the nc-Si:H film and oxide layer are employed to figure out the effect of the hot carriers in the pinch-off region on the negative resistance characteristics. ANSYS acting as the finite element simulation tool was to establish the finite element model of the acceleration sensor with the cantilever length6000μm, width 1500μm and thickness 62μm. Apart from that, the whole chip dimension is built to be 10mm×10mm×500μm. The static analysis of the acceleration sensor indicates that the maximum of δXX and δYY can be obtained away from the end of the cantilever beam 150μm. It can be seen as the optimum region for the piezoresistive effect, where the Wheatstone bridge composed of four nc-Si:H TFTs can be ensured so as to optimize the design and performance of the acceleration sensor based on nc-Si:H TFTs. The static, modal and harmonic analyses were simulated for fabrication of the high-sensitivity acceleration sensor.Based on the simulation, the chip design, fabrication and package of the acceleration sensor based on nc-Si:H TFTs are presented in details. On the n-type high-resistive(ρ>100Ω·cm) silicon substrate with <100> orientation, CMOS processing was utilized to fabricate four nc-Si:H TFTs with the aspect ratio 80μm/40μm at the terminal of the beam. Two of them were made in < 011 > orientation while the others were along the oriention <011>. ICP etching was employed to form the cantilever with6000μm long and 1500μm wide, together with the mass at the free end of the beam so that the acceleration sensor based on nc-Si:H TFTs with the chip size 10mm×10mm can be performed.3. Storge circuit for output of the acceleration sensorTo perform the digital storage of the output signal of the acceleration sensor based on nc-Si:H TFTs, this work makes further efforts to investigate into analog-to-digital conversion and storage of the acceleration sensor. A 1T-1R one-bit storage cell circuit based on a bistable resistive switching ITO/PVK/Al was designed and manufactured,which provides a novel method for digital storage of sensors. In addition to the A/D conversion of the acceleration sensor, the unconjugated polymer poly(N-vinylcarbazole)(PVK), as memory layer, is used to investigate in not only a bistable resistive switching ITO/PVK/Al but a 1T-1R one-bit storage cell circuit composed of an n-MOSFET and a bistable resistive switching ITO/PVK/Al. ITO/PVK/Al demonstrates a bistably nonvolatile resistive switching, whose threshold voltage is-1V and ON/OFF current ratio can approach 104. At ambient temperature, the 1T-1R one-bit storage cell circuit consisting of n-MOSFET and organic resistive switching has high stability during the programming time 104 s, which provides a reasonable scenario for the digital memory based on organic materials of output data of the acceleration sensor. Therefore, it can implement the data storage from analog to digital signal for the piezoresistive acceleration sensor.4. Research on characteristics of the acceleration sensor based on nc-Si:H TFTsX-ray Diffraction(XRD), Raman spectra, Atomic Force Microscope(AFM) and Scanning Electron Microscope(SEM) were employed to characterize nano silicon films made by LPCVD. IDS-VDS characteristics of nc-Si:H TFTs with aspect ratio 80μm/40μm,160μm/40μm and 240μm/40μm were tested. As for the frequency characteristics of the acceleration sensor with 90 nm thick nano-silicon film, the resonant frequency grows from 458.72 Hz to 1054.06 Hz when the thickness of the cantilever changes from 62μm to 160μm. The resonant frequency declines to 458.71 Hz for the acceleration sensor with the thickness of the beam 160μm when the mass of the proof rises. However, the delay time 1.08 ms between the input and output signals can be generated for the accelerationsensor whose sensitivity can touch 0.198V/g/5V through the instrumentation amplifier.The resonant frequency of the acceleration sensor is associated with the frequency of the applied vibration signal, independent of the waveform of the input stimulus signal.