Fabrication and Analysis of Bottom Gate Nanocrystalline Silicon Thin Film Transistors

Abstract

Thin film transistors (TFTs) have brought prominent growth in both variety and utility of large area electronics market over the past few decades. Nanocrystalline silicon (nc-Si:H) TFTs have attracted attention recently, due to high-performance and low-cost, as an alternative of amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) TFTs. The nc-Si:H TFTs has higher carrier mobility and better device stability than a-Si:H TFTs while lower manufacturing cost than poly-Si TFTs. However, current nc-Si:TFTs have several challenging issues on materials and devices, on which this thesis focuses. In the material study, the gate quality silicon nitride (a-SiNx) films and doped nc-Si:H contacts based on conventional plasma enhanced chemical vapor deposition (PECVD) are investigated. The feasibility of a-SiNx on TFT application is discussed with current-voltage (I-V)/capacitance-voltage(C-V) measurement and Fourier Transform Infrared Spectroscopy (FTIR) results which demonstrate 4.3 MV/cm, relative permittivity of 6.15 and nitrogen rich composition. The doped nc-Si:H for contact layer of TFTs is characterized with Raman Spectroscopy and I-V measurements to reveal 56 % of crystalinity and 0.42 S/cm of dark conductivity. Inverted staggered TFT structure is fabricated for nc-Si:H TFT device research using fully wet etch fabrication process which requires five lithography steps. The process steps are described in detail as well as adaptation of the fabrication process to a backplane fabrication for direct conversion X-ray imagers. The modification of TFT process for backplane fabrication involves two more lithography steps for mushroom electrode formation while other pixel components is incorporated into the five lithography step TFT process. The TFTs are electrically characterized demonstrating 7.22 V of threshold voltage, 0.63 S/decade of subthreshold slope, 0.07 cm2/V•s of field effect mobility, and 106 of on/off ratio. The transfer characteristics of TFTs reveal a severe effect of parasitic resistance which is induced from channel layer itself, a contact between channel layer and doped nc-Si:H contact layer, the resistance of doped nc-Si:H contact layer, and a contact between the doped nc-Si:H layer and source/drain metal electrodes. The parasitic resistance effect is investigated using numerical simulation method by various parasitic resistances, channel length of the TFT, and intrinsic properties of nc-Si:H channel layer. It reveals the parasitic resistance effect become severe when the channel is short and has better quality, therefore, several further research topics on improving contact nc-Si:H quality and process adjustment are required.