Changzhi Shi

Piezoresistive Sensitivity, Linearity and Resistance Time Drift of Polysilicon Nanofilms with Different Deposition Temperatures

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.

Polycrystalline Silicon Piezoresistive Nano Thin Film Technology

The piezoresistive effect of semiconductor materials was discovered firstly in silicon and germanium (Smith, 1954). Dissimilar to the piezoresistive effect of metal materials induced from the change in geometric dimension, the piezoresistive phenomenon in silicon is due to that mechanical stress influences the energy band structure, thereby varying the carrier effective mass, the mobility and the conductivity (Herring, 1955). The gauge factor (GF) is used to characterize the piezoresistive sensitivity and defined as the ratio of the relative resistance change and the generated strain (nondimensional factor). Usually, the GF in silicon is around 100 and changes with stress direction, crystal orientation, doping concentration, etc. Recently, the giant piezoresistances were observed in silicon nanowires (He & Yang, 2006; Rowe, 2008) and metal-silicon hybrid structures (Rowe, et al., 2008), respectively. Although these homogeneous silicon based materials or structures possess high piezoresistive sensitivity, there are still several issues influencing their sensor applications, such as, p-n junction isolation, high temperature instability, high production cost and complex fabrication technologies. As another monatomic silicon material with unique microstructure, polycrystalline silicon has been investigated since the 1960s. The discovery of its piezoresistive effect (Onuma & Sekiya, 1974) built up a milestone that this material could be applied widely in field of sensors and MEMS devices. Moreover, polycrystalline silicon could be grown on various substrate materials by physical or chemical methods, which avoids p-n junction isolation and promotes further its applications for piezoresistive devices (Jaffe, 1983; Luder, 1986; Malhaire & Barbier, 2003). Among numerous preparation methods, the most popular technology is chemical vapour deposition (CVD), which includes APCVD, LPCVD, PECVD, etc. The PECVD method can deposit films on substrates at lower temperatures, but the stability and uniformity of as-deposited films are not good, and the samples could contain a large number of amorphous contents. Subsequently, the metal-induced lateral crystallization (MILC) technique was presented (Wang, et al., 2001). By enlarging grain size and improving crystallinity, the gauge factor of MILC polycrystalline silicon was increased to be about 60. But the MILC polycrystalline silicon-based devices could suffer the contamination from the metal catalyst layer (e.g. Ni, Al, etc.). Compared with the aforementioned technologies, the LPCVD process is a mature and stable CVD method with

Temperature characteristics of polysilicon piezoresistive nanofilm depending on film structure

The influence of film structure on temperature characteristics of polysilicon nanofilms (PSNFs) was reported in this paper. Samples were deposited by LPCVD with different film thickness and deposition temperature. The microstructure of films was characterized by SEM, TEM and XRD. By measuring the resistivity and the gauge factor of samples at different temperatures, temperature coefficients of the resistance and the gauge factor (TCR and TCGF) were investigated. Based on the analysis of tunneling piezoresistive effect, the results indicated that PSNFs of ultrahigh doping concentration (around 3times1020 cm-3) have better piezoresistive temperature characteristics than single crystal silicon. By controlling the process parameters like deposition temperature and film thickness, film structure was optimized to obtain a very low resistance temperature coefficient (about plusmn10-4/degC). Moreover TCGF was negative and almost not affected by deposition temperature and film thickness. These conclusions are useful for temperature compensation of polysilicon pressure sensors.

Piezoresistive linearity analysis of polysilicon nanofilms deposited at different temperatures based on interstitial-vacancy model

From our previous investigations, polysilicon nano-films (PSNFs) shows large gauge factor (>30) and lower temperature coefficients of resistance and gauge factor at high doping concentration, comparing with the common polysilicon films. The films are suitable for high temperature piezoresistive sensors. In this paper, the PSNFs doped highly (2×1020 cm-3) were prepared by LPCVD at different deposition temperatures, and the following measurements of resistivity, gauge factor and linearity of the films were performed. Based on as-established interstitial-vacancy model of grain boundaries, the structure, piezoresistive properties and linearity of PSNFs were analyzed. The influence of residual hydrogen atoms in films was also taken into consideration. The model is proved to have good agreement with experiment results. Finally, it can be obtained that using the optimized deposition temperature (620°C) and high temperature annealing, the PSNFs containing fewer amorphous phases and residual hydrogen atoms had better piezoresistive linearity.

Piezoresistive properties of heavily doped P-type polysilicon films

The different thickness polysilicon films were prepared by low pressure chemical vapor deposition. The microstructures of samples were observed by X-ray diffraction, scanning electron microscope and transmission electron microscope. The piezoresistive properties of samples were tested. The experimental results show that under high doping concentration, the gauge factor of polysilicon nanofilms is larger than that of common polysilicon films, which can not be explained reasonably by existing piezoresistive theories, but can be well explained by tunneling piezoresistive theory. The experimental results imply that the polysilicon nanofilms is a promising high temperature piezoresistive material.

DC electrical trimming characteristics of polysilicon nanofilms with different doping concentrations

Polysilicon nanofilms (less than 100nm in thickness) have been proved in our previous experiments to offer large gauge factor (≫30) and stable temperature characteristics. This promotes their applications in piezoresistive sensing devices. In order to improve the resistance matching of sensors after fabrication, it is necessary to perform resistor trimming. The electrical trimming is an effective method of correcting resistance error and mismatch. Therefore, in this paper, the electrical trimming characteristics of polysilicon nanofilm (PSNF) resistors with heavy doping concentrations were investigated. For the sample preparation, PSNFs were deposited on thermally oxidized Si substrates by LPCVD at 620°C and doped heavily at different doses by boron ion-implantation and post-annealing. The resistance changes of trimmed resistors were measured after a series of incremental DC current higher than the threshold current density is applied. Based on the as-established interstitial-vacancy (IV) model, it is considered that the phenomenon of electrical trimming is due to the recombination of IV pairs at grain boundaries under the energy excitation of Joule heat generated by high current conduction. Moreover, the occupation of implanted boron dopants to vacancies can restrain the recombination of IV pairs and influence the threshold current density. The experimental results indicate that elevating doping concentration can improve the trimming accuracy and decrease the trimming rate. It can be concluded that electrical trimming is suitable for the correction of resistance mismatch after device packaging.