This article explores, through experiments and finite element analysis, the ability to plastically deform thin-film semiconductor structures on deformable substrates to spherical cap shapes without cracking the semiconductor layers. The major challenge involves contending with the large strain due to extreme deformation that will crack uniform stiff layers, such as silicon or silicon nitride. By patterning amorphous silicon and silicon nitride layers into islands, such problems can be avoided despite average strains in the substrate in excess of 5%. The strain in the device islands after deformation is a function of the island structure, size, and substrate material properties. Although the substrate is plastically expanded to a spherical dome, device islands can experience either tension or compression depending on the structure.

We report amorphous silicon (a-Si:H) thin-film transistors (TFTs) fabricated on a planar foil substrate, which is then permanently deformed to a spherical dome, where they are interconnected to inverter circuits. This dome subtends as much as 66° ( ∼ 1 sr) with the tensile strain reaching a maximum value of ∼ 6% on its top. Functional TFTs are obtained if design rules are followed to make stiff TFT islands of limited size on compliant substrates. Photoresist patterns for island interconnects are made on the flat structure, are plastically deformed during the shaping of the dome, and then serve to delineate interconnects deposited after deformation by lift-off. We describe the effect of deformation on the TFTs before and after deformation and the performance of TFT inverter circuits. Our results demonstrate that the concept of stiff circuit islands fabricated on deformable foil substrates is a promising approach to electronics on surfaces with arbitrary shapes

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There is a growing interest in the application of large area electronics on curved surfaces. One approach towards realizing this goal is to fabricate circuits on planar substrates of thin plastic or metal foil, which are subsequently deformed into arbitrary shapes. The problem that we consider here is the deformation of substrates into a spherical shape, where the strain is determined by geometry and cannot be reduced by simply using a thinner substrate. The goal is to achieve permanent, plastic deformation in the substrates, without exceeding fracture or buckling limits in the device materials.Our experiments consist of the planar fabrication of amorphous silicon device structures onto stainless steel or Kapton® polyimide substrates, followed by permanent deformation into a spherical shape. We will present empirical experiments showing the dependence of the results on the island/line size of the device materials and the deformation temperature. We have successfully deformed Kapton® polyimide substrates with 100 [.proportional]m wide amorphous silicon islands into a one steradian spherical cap, which subtends 66 degrees, without degradation of the silicon. This work demonstrates the feasibility of building semiconductor devices on plastically deformed substrates despite a 5% average biaxial strain in the substrate after deformation.

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