Key Engineering Materials Vol. 483

Abstract: Polysilicon is a very attractive material in microelectronics for thermal sensors. In this paper, the effect of humidity on thermal resistance of surface micromachined polysilicon is invested. It is found that the thermal conductivity variation of air due to humidity changes the suspended polysilicon resistor evidently. This property could be used as the working principle of relative humidity sensor.

Abstract: It is inevitable that measured signals are contaminated with noise when MEMS sensor is used for an outdoor measurement. So the obtained signals need noise elimination, which is one of key technologies for signal conditioning. In this paper, a novel noise elimination method based on algorithm of blind source separation (BSS) for MEMS sensor is proposed to separate the source signals from the mixed signals with noises. The BSS algorithm based on maximum signal noise ratio (SNR) is a method of global optimal property, using the characteristic that SNR is maximal when statistically independent source signals are completely separated.The algorithm has a low computational complexity for instantaneous linear mixture signals. It is effective to acquire the source signals from the mixed signals with noises obtained by MEMS sensor,and is successfully verified by simulation experiment of BSS algorithm.The result of noise elimination experiment has achieved signal conditioning and eliminates ambient noises for MEMS sensor signals.

Abstract: Micro-defects inside metal matrix composite structure seriously influence the mechanic properties of materials and can undermine machine operations. In order to detect the micro-defects, we can make use of one of the nonlinear acoustic features, i.e., harmonics, which usually appear together with ultrasound when propagating through a damaged material. Based on the relationship between the harmonics, a method of using nonlinear acoustic harmonic amplitude ratio to distinguish micro-defects in metal matrix composite structure is discussed. The test results show that acoustic features, such as second harmonic excitation efficiency were very obvious for the damaged group samples. The method presented is effective in detecting the micro-defects, which means that nonlinear acoustic waves may provide us a good solution for quantitative evaluation of the early damages in metal matrix composite structure.

Abstract: The piezoresistive and ohmic contact properties of polycrystalline silicon nano thin films were investigated in this paper. The polycrystalline silicon films with different thicknesses and doping concentrations were deposited by LPCVD and doped with boron highly, and then the cantilever beam samples were fabricated by photolithography and wet etching. By measuring the gauge factor and specific contact resistivity, the specific contact resistivity of Al contacts can reach 2.4×10^-3Ω·cm² after the alloying at 450 °C for 20 min; the enhanced piezoresistive effect of highly doped polycrystalline silicon nano thin films was discovered. The conclusions indicated that the enhanced piezoresistive sensitivity of PNTFs is due to the modification of depletion region barrier by ultra high doping and film thickness thinning and the enhancement of tunneling piezoresistive effect. The distinct piezoresistive phenomenon of PNTFs could be utilized for the development and fabrication of miniature piezoresistive sensors.