Self-Aligned Four-Terminal Planar Metal Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors for System-on-Glass

Self-aligned four-terminal (4T) planar metal double-gate (DG) polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) were fabricated on a glass substrate at a low temperature (LT), which is below 550◦C, to realize high performance and low power dissipation system-onglass (SoG). The top gate (TG) and bottom gate (BG) were formed from tungsten (W); the BG was embedded in the glass substrate and the TG was fabricated by a self-alignment process using the BG as a photomask. This structure is called embedded metal double-gate (E-MeDG) in this paper. The poly-Si channel with lateral large grains was fabricated using a continuous-wave laser lateral crystallization (CLC). The self-aligned 4T E-MeDG LT poly-Si TFT, with a gate length of 5 μm and TG and BG SiO2 thicknesses of 50 and 100 nm, respectively, exhibited a subthreshold swing of 120mV/dec and a threshold voltage (Vth) of −0.5V in the connecting DG mode; i.e. when TG is connected to BG. In the TG operation at various BG control voltage, a threshold voltage modulation factor (γ = ∆Vth/∆VBG) of 0.47 at negative BG control voltage and 0.60 at positive BG control voltage are demonstrated, which values are nearly equal to theoretical prediction of 0.40 and 0.75. Trend of subthreshold swing (s.s.) of TG operation under different BG control voltage are also consistent with theoretical prediction. In addition to TG operation, successful BG operation under various TG control voltages was confirmed. Field-effect mobility derived from gm also varied depending on control gate voltage. The high controllability of device parameter of individual LT poly-Si TFTs is caused by excellent crystalline quality of CLC poly-Si film and will enable us to the fabrication of high-speed and low power-dissipation SoG. key words: poly-Si, TFT, double-gate, four-terminal, system-on-glass