Three-Mask Polysilicon Thin-Film Transistor Biosensor

Biosensors are commonly produced using a siliconon-insulator (SOI) CMOS process and advanced lithography to define nanowires. In this paper, a simpler and cheaper junctionless three-mask process is investigated, which uses thin-film technology to avoid the use of SOI wafers, in situ doping to avoid the need for ion implantation and direct contact to a low-doped polysilicon film to eliminate the requirement for heavily doped source/drain contacts. Furthermore, TiN is used to contact the biosensor source/drain because it is a hard resilient material that allows the biosensor chip to be directly connected to a printed circuit board without wire bonding. pH sensing experiments, combined with device modeling, are used to investigate the effects of contact and series resistance on the biosensor performance, as this is a key issue when contacting directly to low-doped silicon. It is shown that in situ phosphorus doping concentrations in the range 4 × 10¹⁷-3 × 10¹⁹ cm^-3 can be achieved using 0.1% PH₃ flows between 4 and 20 sccm. Furthermore, TiN makes an ohmic contact to the polysilicon even at the bottom end of this doping range. Operation as a biosensor is demonstrated by the detection of C-reactive protein, an inflammatory biomarker for respiratory disease.