Air-gap polysilicon thin film transistors on glass substrates

Surface micro-machined polycrystalline silicon micro-bridge is used as a suspended gate in an air-gap polysilicon thin film transistor (TFT) made on glass substrate. Here, polycrystalline low pressure chemical vapour deposition (LPCVD) germanium film as sacrificial layer is compared to the conventional silicon oxide. Besides its good step coverage, germanium shows a great etch selectivity against silicon oxide, silicon nitride, polysilicon and even glass substrates, which represent the most used materials in the IC technology. Firstly, phosphorous and boron in situ doped LPCVD silicon micro-bridges are investigated in view of their use as a suspended gate of thin film transistor. They are compared in terms of the maximum free-standing micro-bridge length before deformation. The mechanical behavior of micro-bridges is studied using micro-Raman technique and scanning electron microscope (SEM). Based on these investigations, a transistor fabrication process, that is fully compatible with both IC technology and low-cost glass substrates, is proposed. In order to stabilize and improve the process, germanium is used as a sacrificial layer, and the channel is passivated by a very thin layer of silicon nitride (Si3N4). Notice that an air-Gap TFT can be used as a high sensitive ambiance sensor by involving a thin layer of a dielectric gas-sensitive material that can be deposited on the channel. (C) 2004 Elsevier B.V. All rights reserved.