Research On Key Technologies Of Polysilicon Nano Thin Films In Piezoresistive Sensor Applications

In piezoresistive sensor （PRS） aspect, compared with monocrystal silicon, polysili- con film device requires no p-n junction isolation and can work at high temperature; it also has the merits of simple process and low fabrication cost compared with SOI. But the common polysilicon PRS has the demerit of low sensitivity. The research results showed that the heavily boron-doped polysilicon nano thin films （PNTFs） thinner than 100nm present high gauge factor （GF≥34） and low temperature coefficients （TCs） of re- sistance and GF （TCR and TCGF）, while the GF of common polysilicon films with the same doping is 20²5 and TCR and TCGF are an order of magnitude higher. Therefore, PNTFs have good potential applications in the development of high-temperature PRSs.To achieve the application of PNTF in PRSs, the following issues must be solved: In the nanoscale, the uniformity of PNTF is poor and its resistance can have large devia- tions with the technical condition change, affecting sensor measurement precision great- ly. The conventional resistor trimming methods have the drawbacks of poor stability or chip-area waste, thus it is necessary to present a trimming method suited for PNTF PRSs. The conventional Al-based electrodes have large contact resistance to polysilicon, remarkable interdiffusion and electromigration, and their thermal stability is not good, affecting device performance seriously, so it is essential to study the multilayer ohmic contacts with low contact resistance, controlled alloying depth and good thermal stabili- ty. The PNTF ultra-thin structure makes its electrical properties sensitive to external charges, contaminations and surface oxidization, so it needs to passivate its surface. But the existing monolayer films do not have very good passivation quality and the tempera- ture of passivation technology can not be high considering sensor fabrication processes, so the study on low temperature high-quality passivation technology is very important. In this paper, the electrical trimming （ET）, ohmic contact and surface passivation techni- ques of heavily boron-doped LPCVD PNTFs are studied systemically.In resistor trimming, against the contradiction of no segregation in boron dopant and existing dopant segregation model, the current-induced recrystallization model and heat transfer model of grain boundary （GB） for ET were established based on interstitial- vacancy pair （IV pair） theory. Then, the ET was interpreted as the increase of mobility caused by cascade recrystallization of GB under applied large current. Based on the presented models, the ET properties of samples with different film parameters were analyzed, and the quantitative relationships between threshold current density and film parameters were given in a good agreement with experimental results. On this base, the DC current sequential trimming method was presented and the effects of ET on piezo- resistive and temperature properties were studied. The results show that this method can improve the trimming precision by 4 times and can not change markedly the piezoresis- tive sensitivity and TCs of PNTFs and can be applied for the resistor trimming of PNTF PRSs.In ohmic contact aspect, to solve the problems of low availability and measurement precision in existing model, the test structures of the existing linear and circular dot transmission line models （LTLM and CDTLM） were modified. Considering the effect of metal-layer resistance, the existing CDTLM was rectified based on double half-ring resistor network, and the orthogonal voltage measuring method was presented so that the structure availability and measurement precision were improved. Based on the modified testing structures and methods, the specific contact resistivity （SCR） and I-V charateristics of Al, Pd and Ni-based ohmic contacts under different alloying conditions were obtained. By XRD, EDX and SEM characterizations, the alloyed products at inter- face were analyzed. The results show that the silicide Pd2Si with low Si consumption was formed in the Ti/Pd/Au structure after alloying, and the SCR was two orders of magnitude lower than that of monolayer Al contact; The Pt/Pd/Au structure realized the nonalloyed ohmic contact with good thermal stability.In low temperature passivation technology aspect, to improve passivation quality and avoid the affects of surface oxidization, external charges and contaminations on film properties, SiO₂/Si₃N₄ compound films, polyimide （PI）/SiO₂ compound films and SiO₂ films with different binders were prepared by CVD, polymer coating and sol-gel methods, respectively. The surface morphologies and roughnesses of samples were cha- racterized and the minority carrier lifetime and surface recombination velocity were measured by micro-wave photoconductance delay. Finally, the optimal passivation tech- nologies were obtained: For the monolayer passivation, the PI/SiO₂ compound films with the SiO₂ content of 5g/L were adopted; For the double layer passivation, the com- pound film comprised of reserved LPCVD implantation buffer SiO₂ layer and PECVD Si₃N₄ layer was adopted. Compared with the reported passivation results, the passiva- tion effect of the samples based on the forenamed technologies were improved by 5⁶ times. The passivation mechanism analysis indicates that the wide energy gap of silicon nitride films makes the inversion layer formed at PNTF surface and the surface hole concentration is reduced, generating the field effect passivation at interface; defective SiO₂ can introduce positive fixed charges into interface and the minority carrier concen- tration increases at PNTF surface, thereby increasing the minority carrier lifetime.This work lays the theoretical and experimental foundation for the development of PNTF PRSs and also has some scientific reference value for the improvement of other semiconductor device performances.