Study of Semi-Insulating Polysilicon Films and Application to Low Temperature Polycrystalline Silicon Thin Film Field Effect Transistors.

Large flat panel displays for portable computers and other applications require active matrix addressing in order to obtain adequate contrast. The choice of a process for fabrication of thin film switching transistors in these displays involves a number of difficult compromises. Low temperature processes are preferred so that low cost glass substrates can be used with minimal warpage during process. However, the gate insulator has so far prevented further reductions in the process temperatures and transistor drift also becomes a serious problem. In this thesis, we will discuss the progress toward a process which will allow fabrication of field effect transistors in polycrystalline silicon with the use of Semi-insulating polysilicon (SIPOS) deposited at 600 ^circC as the gate insulator. As a result of prolonged exposure to wet oxygen at 600 ^circC, the dielectric constant of SIPOS film approaches that of silicon dioxide and electron diffraction indicates a similar structure. Electrical measurements show moderate interface state density and low leakage current and small drift. By combining the oxidized SIPOS gate insulator with a technique for producing large grain crystallized silicon films previously reported ^{19}, we are able to fabricate TFTs with excellent characteristics. Even though no process temperature exceeds 600^circC. In addition, reasonably high mobilities (~ 40 cm^2/V-s), low threshold voltages (~3 V) and an ON/OFF current ratio >5 times 10 ^5 has been achieved after hydrogen passivation. Finally, we examine the stability of our polysilicon TFTs under different bias conditions. The devices do not show threshold voltage shift associated with oxide states. In addition, the long term reliability of polysilicon TFTs due to hot electron stress was also studied. The observations are consistent with avalanche injection studies on MIS capacitors with an oxidized SIPOS dielectric. Consequently, the results so far indicate that TFTs made using these process steps will be suitable for use in the active matrix and possibly also in peripheral circuits on low melting point glass substrates.