Device Degradation on a Full-Frame CCD Image Sensor with a Transparent Gate Electrode
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DEVICE DEGRADATION ON A FULL-FRAME CCD IMAGE SENSOR WITH A TRANSPARENT GATE ELECTRODE BIAY-CHENG HSEIH, S. KOSMAN, Y.C. LO, K. JAYAKAR, M. MEHRA, P. ROSELLE, AND W.C. CHANG, Microelectronics Technology Division, Eastman Kodak Company, Rochester, NY 146502019 ABSTRACT The electrical and optical properties of Indium-Tin-Oxide (ITO) films, deposited by radio frequency (r.f.) magnetron sputtering, were studied. ITO films, when deposited using optimum sputtering conditions, were reproducibly prepared with resistivity as low as 1.5 x 10-4 fl-cm and optical transmissivity higher than 80% over the wavelength range of interest. Device stability when ITO is used as a replacement for polysilicon as a gate electrode in silicon charge-coupled device (CCD) image sensors was also studied. After an anneal process at 950 'C in N2 the device degraded. The degradation can be attributed to the generation of oxide charge and interface states in the ITO/SiO 2/Si system. INTRODUCTION Optically transparent and electrically conducting films, especially Indium-Tin-Oxide (ITO), have been applied to photosensitive devices for transparent gates on charge-coupled devices (CCD's) [I], flat panel displays [2], and for transparent barrier layers for solar cells [3]. For solid-state image sensors, transparent gates are desirable because they offer higher quantum efficiency. Imagers with ITO gates have higher quantum efficiency, compared to devices with polysilicon gates, because ITO transmissivity is typically above 80% over the wavelength range of interest. This is particularly true in the blue region (- 400 - 500 nm) of the visible spectrum where ITO can offer substantially improved response over polysilicon, which has lower transmissivity in this region. One of the major concerns of ITO gate devices is the generation of defects during the r.f. sputtering deposition of the ITO films and diffusion of impurities from ITO films during the subsequent heat treatments. The interface state density at the SiO 2/Si interface in an ITO gate MOS structure is especially important, since it influences the short wavelength quantum efficiency of the devices. In this paper, properties of ITO films prepared by magnetron sputtering are reported. The optical and electrical properties of the layers were characterized and the deposition conditions optimized. More importantly, the device parameters of ITO gate Si devices in a megapixel frame transfer CCD imager were investigated in detail. EXPERIMENTAL
The experiments were carried out in an MRC 603 II in-line sputtering system using magnetron cathodes. The ITO films were deposited using reactive r.f. magnetron sputtering in either an inert Ar gas or a 0 - 10% O2/Ar gas mixture. The oxygen concentration in the chamber was varied between zero and ten volume percent using an MKS 254A flow ratio control system.. The composition of the hot pressed ITO oxide target was 90 mol% In203 and 10 mol% Sn02. The substrate temperature during a deposition cycle was varied between 300 'C and 80 *C. For most depositions the substrates
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