Segmented Amorphous Silicon n-i-p Photodiodes on Stainless-Steel Foils for Flexible Imaging Arrays
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0989-A12-02
Segmented Amorphous Silicon n-i-p Photodiodes on Stainless-Steel Foils for Flexible Imaging Arrays Y. Vygranenko1, R. Kerr2, K. H. Kim1, J. H. Chang1, D. Striakhilev1, A. Nathan1,3, G. Heiler2, and T. Tredwell2 1 Dept. of Electrical and Computer Engineering, University of Waterloo, 200 University Ave West, Waterloo, N2L 3G1, Canada 2 Eastman Kodak Company, Rochester, NY, 14650-23487 3 London Centre for Nanotechnology, University College, London, WC1H OAH, United Kingdom
ABSTRACT This paper reports the first successful attempt to fabricate amorphous silicon (a-Si:H) n-i-p photodiodes on a thin stainless-steel foil substrate for medical X-ray imaging applications. Two architectures of the n-i-p-photosensor, where the top electrode is based on amorphous or polycrystalline ITO, have been developed and characterized. The impact of critical fabrication steps including the deposition of semiconductor layers, dry etch of the NIP stack, diode passivation and encapsulation, as well as a contact formation on the device performance is presented and discussed. The test structures comprising segmented photodiodes with a junction area ranged from 0.126 × 0.126 to 1 × 1 mm2 have been fabricated on stainless-steel foils and on glass substrates for the purposes of process characterization. The fabricated samples are evaluated in terms of current-voltage, capacitance-voltage, and spectral response characteristics. INTRODUCTION Flexible stainless-steel foils present an excellent alternative to fragile glass substrates conventionally used in large area electronics. Compared to polymer films, another alternative substrate material, advantages of metal foil are in high-temperature process capability, chemical and moisture barrier properties [1]. A rapid progress in development of thin-film transistors (TFTs) on flexible substrates enables a fabrication of light, flexible and rugged displays [2-4]. It is also very attractive to apply this novel substrate technology to fabricate mechanically flexible and robust imagers for medical applications. This paper discusses the integration issues specific for flexible substrates and reports the basic characteristics of amorphous silicon (a-Si:H) n-i-p photodiodes on a thin stainless-steel foil substrate. EXPERIMENT An integration issue for stainless-steel foils is a surface roughness, which should be below 20 nm. The foils available on the market are too rough to use them as substrates for the fabrication of electronic devices, and it is a technical challenge to polish metal foils with a required roughness at low cost. A simple solution of this problem is the use of stainless-steel foils of moderate surface roughness covered by a planarization layer. A process has been developed by Eastman Kodak Company. The first step is an electro-polishing of the raw stainless-steel foil to reduce the RMS roughness from 0.4 µm down to 0.15 µm. Fig. 1 includes
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(b) Figure 1. Surface of the stainless-steel foil before (a) and after (b) electro-polishing.
Surface Profile (nm)
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