(Invited) Microcrystalline Silicon Based TFTs and Resistors for Reliable Flexible Electronics

Abstract

Silicon based flexible electronics is shown here as the right way when considering the reproducibility of the process and the long-term reliability, the only way leading to commercially available products. Microcrystalline silicon films are deposited directly on plastic substrates (PEN and Polyimide) at low temperature. Films are highly crystallized. Crystalline volume is higher than 73% for 50nm thick film as calculated from Raman spectra. Using such film, resistances are fabricated and checked as deformation gauges. The mechanical behaviour of microcrystalline silicon films under longitudinal as well as transversal tensile (Figure 1) and compressive stress is studied. The effect of the nature of the substrate and its thickness on this behaviour is studied also. These studies led to determine the maximum strain that can hold microcrystalline silicon film. This strain limit is reached by bending the film at different curvature radii depending on the thickness of the substrate. Lower curvature radius is reached with thinner substrate. Lower than 1 mm radius, leading to fold in half the film, is reached without breaking it (Figure 2). This high flexibility together with demonstrated reliability led the use of a matrix of microcrystalline silicon based gauges in the determination of any form and in the characterization of human pulse (Figure 3). Thin-film transistors are also fabricated on previous substrates at the same low temperature. Undoped microcrystalline silicon forms the active layer of these transistors. Phosphorus doped microcrystalline silicon is used as source and drain regions. The gate insulator is also deposited at low temperature. TFTs show very high flexibility. TFTs support lower than 1 mm curvature radius leading to compressive as well as tensile strain, without breaking (Figure 4). More than 200 times of such extreme bending are done without changing the TFT’s characteristics (Figure 5). Figure 1