Mechanical Theory of the Film-on-Substrate-Foil Structure: Curvature and Overlay Alignment in Amorphous Silicon Thin-Film Devices Fabricated on Free-Standing Foil Substrates

Abstract

Flexible electronics will have inorganic devices grown at elevated temperatures on free-standing foil substrates. The thermal contraction mismatch between the substrate and the deposited device films, and the built-in stresses in these films, cause curving and a change in the in-plane dimensions of the workpiece. This change causes misalignment between the device layers. The thinner and more compliant the substrate, the larger the curvature and the misalignment. We model this situation with the theory of a bimetallic strip, which suggests that the misalignment can be minimized by tailoring the built-in stress introduced during film growth. Amorphous silicon thin-film transistors (a-Si:H TFTs) fabricated on stainless steel or polyimide (PI) (Kapton E^®) foils need tensile built-in stress to compensate for the differential thermal contraction between the silicon films and the substrate. Experiments show that by varying the built-in stress in just one device layer, the gate silicon nitride (SiN_x), one can reduce the misalignment between the source/drain and the gate levels from ∼400 parts-per-million to ∼100 parts-per-million.