Compliant substrates for thin-film transistor backplanes

The emergence of wearable electronics is leading away from glass substrates for the display backplane, to plastic and metal. At the same time the substrate thickness is reduced to make displays lighter. These two trends cooperate toward the development of compliant substrates, which are designed to offload mechanical stress from the active circuit onto the substrate (and encapsulation). Compliant substrates make the circuit particularly rugged against rolling and bending. Design principles for compliant substrates include: (a) Moving the circuit plane as close as possible to the neutral plane of the structure, and (b) Using substrate and encapsulation materials with low stiffness. Design principle (a) is demonstrated on thin-film transistors made on thin steel foil. Such transistors function well after the foils are rolled to small radii of curvature. Principle (b) of compliant substrates is demonstrated with bending experiments of a-Si:H TFTs made on thin substrates of polyimide foil. TFTs on 25-mu m thick polyimide foil may be bent to radii of curvature as low as 0.5 mm without failing. The reduction in bending radius, from R similar to 2 mm on same-thickness steel foil, agrees with the theoretical prediction that changing from a stiff to a compliant substrate reduces the bending strain in the device plane by a factor of up to 5