Abstract
Our previous research work indicated that highly boron doped polysilicon nanofilms (≤100 nm in thickness) have higher gauge factor (the maximum is ∼34 for 80 nm-thick films) and better temperature stability than common polysilicon films (≥ 200nm in thickness) at the same doping levels. Therefore, in order to further analyze the influence of deposition temperature on the film structure and piezoresistance performance, the piezoresistive sensitivity, piezoresistive linearity (PRL) and resistance time drift (RTD) of 80 nm-thick highly boron doped polysilicon nanofilms (PSNFs) with different deposition temperatures were studied here. The tunneling piezoresistive model was established to explain the relationship between the measured gauge factors (GFs) and deposition temperature. It was seen that the piezoresistance coefficient (PRC) of composite grain boundaries is higher than that of grains and the magnitude of GF is dependent on the resistivity of grain boundary (GB) barriers and the weight of the resistivity of composite GBs in the film resistivity. In the investigations on PRL and RTD, the interstitial-vacancy (IV) model was established to model GBs as the accumulation of IV pairs. And the recrystallization of metastable IV pairs caused by material deformation or current excitation is considered as the prime reason for piezoresistive nonlinearity (PRNL) and RTD. Finally, the optimal deposition temperature for the improvement of film performance and reliability is about 620 °C and the high temperature annealing is not very effective in improving the piezoresistive performance of PSNFs deposited at lower temperatures.
Highlights
For semiconductor materials, the piezoresistive effect was discovered firstly in Ge and Si by Smith in 1954 [1]
The piezoresistive sensitivity, piezoresistive linearity (PRL) and resistance time drift (RTD) of highly boron doped LPCVD polysilicon nanofilms (PSNFs) were studied, and the influences of deposition temperature and high temperature annealing on the above-mentioned properties were analyzed
A conclusion was drawn that the piezoresistance coefficient (PRC) of composite grain boundary (GB) is higher than that of grain neutral regions at high doping concentration
Summary
The piezoresistive effect was discovered firstly in Ge and Si by Smith in 1954 [1]. The experimental results reported by other researchers indicated that the GF of polysilicon common films (PSCFs, film thickness ≥ 200 nm) reaches the maximum as the doping concentration is at the level of 1019 cm-3, and decreases drastically as doping concentrations are increased further [9, 13,14,15] Based on this phenomenon, the existing piezoresistive theories of polysilicon were established during 1980s~1990s and used to predict the process steps for the optimization of device performance. In the early models proposed by Mikoshiba [16], Erskine [17] and Germer [18], the contribution of GBs to piezoresistive effect was neglected, thereby resulting in the discrepancy between experimental data and theoretical results at low doping levels. The influence of residual H atoms in polysilicon was taken into account for RTD
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