Flexible a-Si:H-based Image Sensors Fabricated by Digital Lithography
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0989-A09-03
Flexible a-Si:H-based Image Sensors Fabricated by Digital Lithography William S. Wong1, TseNga Ng1, Michael L. Chabinyc1, Rene A. Lujan1, Raj B. Apte1, Sanjiv Sambandan1, Scott Limb2, and Robert A. Street1 1 Electronic Materials and Devices Lab, Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA, 94304 2 Hardware Systems Lab, Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA, 94304 ABSTRACT Amorphous silicon-based x-ray image sensor arrays were fabricated on poly-ethylene naphthalate substrates at process temperatures below 180°C. Patterning of the thin-film transistor backplane was accomplished using ink-jet printed etch masks. The sensor devices were found to be comparable to high-temperature processed devices. The integration of the sensor stack, TFT array and PEN substrate resulted in a flexible x-ray image sensor with 180×180 pixels with 75 dpi resolution.
INTRODUCTION The processing of electronic devices on flexible platforms possesses many intrinsic difficulties. Film stress, thermal cycling, environmental conditions, and handling are all issues that have a direct impact on the successful fabrication of flexible electronics. The incorporation of multiple thin-film layers over a wide range of thicknesses adds more complexity to managing the fabrication process. For example, in x-ray image sensor array applications, the thickness of the sensor layer itself may be more than twice the thickness of the thin-film transistor (TFT) device layer.1 The built-in stress for this layer may lead to strain-induced cracking of the sensor layer and the underlying TFT backplane.2 In order to reduce the built-in stress of these films, thinner layers and lower process temperatures may be used to minimize the possibility of cracking and distortion of the substrate and the thin film. To address potential misalignment problems, jet-printed etch masks may be more applicable for aligning to features on a distorted surface. The mask layers can be first aligned to a local reference mark and the non-contact jet-printed mask can then be placed directly onto the distorted feature to complete the masking process. We have combined the use of a jet-printed etchmask process, digital lithography, with 880 nm p-i-n sensor structures processed at < 200°C to integrate a-Si:H-based image sensor arrays onto flexible platforms.
EXPERIMENTAL DETAILS a-Si:H-based p-i-n sensors were first fabricated on glass substrates at temperatures ranging from 150°C to 250°C. The layers were deposited using plasma-enhanced chemical-vapor deposition. The heterostructure consisted of a bottom-metal electrode
followed by a 70 nm thick n-layer, i-layer, and a 10 nm thick p-layer. The i-layer thickness was varied between 500 nm to 1 micron. Current-voltage measurements were done using a Keithly 617 programmable electrometer. External quantum efficiency measurements were performed using irradiation from an Ar ion laser at a wavelength of 488 nm. The digital lithography process combines jet-printed lithography with digital imagi
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