(Invited) Self-Aligned Four-Terminal Low-Temperature Polycrystalline-Silicon Thin-Film Transistors on Glass Substrate Using Continuous-Wave Laser Lateral Crystallization
Authors: Hara Akito , Ohsawa Hiroki 
Published: February 2020

Low-temperature (LT) polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) are indispensable for applications that involve integrating high-speed and low-power dissipation complementary metal-oxide-semiconductor (CMOS) circuits onto a glass substrate. The on-current of the LT poly-Si TFTs can be enhanced by fabricating a high-quality poly-Si film on the glass substrate. In the current study, the fabrication of these transistors was achieved using continuous-wave laser lateral crystallization (CLC). This method enabled us to form a lateral poly-Si film with high quality and large grain size (the grain area was approximately 100 times larger than that of the conventional excimer-laser crystallization (ELC) poly-Si film) without damaging the substrate.

A possible approach to achieve low-power dissipation is by controlling the threshold voltage (Vth). In order to control the Vth of the LT poly-Si TFTs, we fabricated self-aligned four-terminal (4T) LT poly-Si TFTs using high-quality CLC poly-Si film. These TFTs comprise metal gates on the top and bottom. The bottom metal gate was  embedded in the glass substrate using a chemical mechanical polish. The top metal gate was fabricated by a self-aligned process of back surface exposure using the bottom metal gate as a photomask. This TFT was fabricated on a fused quartz glass at 550°C. The top and bottom metal gates acted as drive and control gates, respectively, or vice versa.  The performance of the fabricated n-channel (n-ch) and p-channel (p-ch) self-aligned 4T CLC LT poly-Si TFTs was evaluated. To evaluate their feasibility, an E/D inverter was utilized by connecting two identical n-ch TFTs on different glass substrates, which were fabricated by identical processes.

The self-aligned 4T CLC LT poly-Si TFTs showed excellent Vth controllability. The variation of the Vth of the drive gate TFT, with respect to small variation in the control gate voltage, was found to closely match the theoretically predicted values of the top and bottom gate drives for both n- and p-ch TFTs. The output characteristics of the drive TFTs were found to be sensitive to the varying magnitude of the control gate voltage. The mobility of the n-ch TFTs in the top and bottom drive TFTs was above 300 cm2/Vs. This remarkable performance may be attributed to the high-quality CLC poly-Si film. The mobility varied sensitively against the strength of the vertical electric field, which was changed by varying the control gate voltage. The s.s. values were also found to vary depending on the control gate voltage, and this variation is explained by the change in the position of the channel layer towards the side of either the drive or the control gate. By exploiting the high controllability of the 4T TFTs, an E/D inverter was fabricated and successfully operated at 2.0 V.

The Vth of the self-aligned 4T CLC LT poly-Si TFTs was found to be sensitive to a 20-meV variation of the control gate voltage. Utilizing this sensitivity, we further fabricated an extended-gate pH sensor, where the control gate was connected to the glass electrodes, and the pH of the solution varied from 4.1 to 8.8. Consequently, we were able to successfully observe the variation of the drain current in the drive gate TFT depending on the strength of pH.

In summary, the results suggest that the high controllability of the self-aligned 4T CLC LT poly-Si TFTs could enable us to fabricate high-speed and low-power-dissipation circuits on a glass substrate. Further, our study suggests the promising potential of such TFTs for applications involving sensor fabrication.