P. I. Hsu

Thin-film transistor circuits on large-area spherical surfaces

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.

Spherical deformation of compliant substrates with semiconductor device islands

conventional methods on flat foil substrates, into a spherically shaped cap after the device fabrication process. In contrast to rolling, with spherical deformation, the surface is in tension on both the concave and convex sides of the substrate and thinning the substrate cannot be used to reduce the strain. Because inorganic semiconductor materials are brittle, the uniform layers of device materials crack during the substrate deformation. Thus, spherical deformation is fundamentally more difficult than cylindrical deformation because the deformation inherently involves stretching the substrate and devices on it, independent of the substrate thickness. In this article, Sec. II explains our approach to plastically deform thin foil substrates into spherical dome shapes. Section III demonstrates that by patterning device materials into isolated islands, ‘‘hard’’ device islands can remain crack free after deformation. Finally, Sec. IV discusses that the strain distribution in the device islands for two different substrate structures, and why patterning brittle materials into islands suppresses fracture in the devices.

Amorphous Si TFTs on plastically deformed spherical domes

There is a growing interest in the design and fabrication of flexible and rugged electronics particularly for large-area displays and sensor arrays. In this work, we describe the fabrication of amorphous silicon (a-Si:H) thin film transistors (TFTs) on a Kapton substrate which can be permanently deformed into a spherical cap shape. This level of strain would crack uniform a-Si:H device films. To prevent fractures in our TFT structure, the silicon and silicon nitride layers of the TFTs are patterned to create isolated device islands. After deformation, these brittle islands can remain crack-free, and the TFTs achieve comparable device behavior despite average strain in the substrate in excess of 5%.

Three-Dimensional Electronic Surfaces

There is an increasing interest in electronics functionality on surfaces which are not planar. This paper examines the critical technologies for fabricating electronic surfaces which have a three-dimensional shape. Two different approaches for achieving such a goal are examined. One can fabricate electronics using conventional technologies on a flat surface, and then after fabrication deform that surface into the desired shape (e.g. a spherical cap). In an alternative approach, one can directly fabricate onto substrates with an arbitrary shape. In this case one must address the issue of pattern formation and transfer on the curved surfaces. The scaling of letterpress printing to micron-scale features on flat and spherically curved surfaces is demonstrated.

Plastic Deformation of Thin Foil Substrates with Amorphous Silicon Islands into Spherical Shapes

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 µm 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.

Amorphous Si TFTs on plastically-deformed substrates with 3-D shapes

In this paper we report the first transistors fabricated on a substrate that is then plastically deformed. Using amorphous silicon (a-Si) device islands on a polyimide substrate, TFTs can withstand an average substrate strain of 6%, as the substrate is deformed into a spherical cap shape subtending angles as large as 66/spl deg/ (1 steradian solid angle).