Four-Terminal Polycrystalline-Silicon Vertical Thin-Film Transistors on Glass Substrates for pH Sensor Applications
Authors: Suzuki Kosei , Nitta Masahide , Tabei Tetsuo , Hara Akito 
Published: December 2024

Thin-film transistors (TFTs) with small footprints, high performance, and low power consumption are essential for forming integrated circuits on glass substrates. Miniaturization of gate length is the most effective means of achieving high performance TFTs. However, owing to the thermal shrinkage of the glass substrate during the manufacturing process, miniaturizing the channel length to the nanometer scale is difficult, as is the case for MOSFETs on Si wafers. To achieve shorter channel lengths, TFTs with vertical, rather than planar, structures have been proposed. Vertical TFTs (VTFTs) are not limited by photolithography equipment because the channel length can be easily controlled by changing the thickness of the spacer layer, and their structure is tolerant to the thermal shrinkage of the glass substrate. Therefore, VTFTs are useful for realizing nanometer-scale channel lengths on glass substrates. In addition, a four-terminal (4T) structure has been reported to achieve higher TFT performance. This structure exhibits the best performance in a double-gate (DG) drive in which the top gate (TG) and bottom gate (BG) operate simultaneously. Moreover, threshold voltage (Vth) control is possible in a 4T drive in which the TG and BG are separated and driven independently. We recently developed a 4T poly-Si VTFT on a glass substrate. This structure can be applied to an extended-gate (EG) pH sensor. In this study, we applied the n-channel 4T poly-Si VTFT, that has both vertical and 4T structures, to pH sensors.
A fused quartz glass substrate was used in this study. First, 40 nm of Mo was sputtered, and a BG was formed using photolithography and wet etching. Then, using the BG as a hard mask, the fused quartz glass was etched 150 nm deep using reactive ion etching (RIE) to form sidewalls. Next, 50-nm thick SiO2 was deposited as the BG dielectric, and 50-nm thick non-doped amorphous silicon (a-Si) was deposited as the active region by plasma-enhanced chemical vapor deposition (PECVD). The non-doped a-Si was then crystallized by nickel metal-induced crystallization (Ni-MIC) at 580 ℃ for 8 h. Phosphorus (P) was implanted in the source–drain (S/D) region at a tilt angle of 30°. Next, a 50-nm thick SiO2 was deposited as the TG dielectric using PECVD, followed by activation heat treatment. A Mo film was then formed via sputtering, and the TG metal was formed via wet etching. Subsequently, a 200-nm thick SiO2 layer was deposited as an isolation layer, and the contact holes for the S/D, TG, and BG were opened by RIE. The electrodes were formed using aluminum sputtering and wet etching. Finally, hydrogenation treatment was performed at 380 ℃ for 2 h.
The fabricated 4T poly-Si VTFTs can be operated in four drive modes depending on the wiring connections: TG, BG, DG, and 4T. The pH sensor is operated by connecting the glass electrode to the TG. This measurement method is advantageous because the TG strongly interacts with the BG because of its structure. The variations in the drain current can be confirmed under BG driving when the TG voltage is varied in 20 mV steps. A variation of 20 mV in the TG voltage corresponds to a change of approximately 0.5 pH in glass electrode. The actual construction of the sensor circuit showed that the output voltage changed in accordance with the pH change, and the maximum sensitivity was confirmed to be greater than 59 mV/pH at Vdd = 1.0 V.

Figure 1