<title>New technologies of photosensitivity improvement and VOD shutter voltage reduction for CCD image sensors</title>

New technologies to increase the photo-sensitivity and reduce the shutter voltage of the vertical over-flow-drain (VOD) have been developed for CCD image sensors. A 40 percent photo-sensitivity increase was obtained by forming an anti-reflection film over the photodiode, in addition to reducing the thickness of the P⁺ layer formed at the photodiode surface. A VOD shutter voltage reduction from 31 V to 18 V was successfully obtained by using an epitaxially grown substrate with double impurity concentration layers. We found that a stacked film structure of Si₃N₄ on SiO₂ film was suitable for the anti-reflection to obtain the maximum increase in sensitivity. A suitable film thickness was estimated by using an optical multiple- reflection analysis simulator, resulting in a 10 nm SiO₂ and a 50 nm Si₃N₄ films. As a result, a 30 percent higher photo-sensitivity than that of the conventional structure was obtained. Additionally, by reducing the depth of the P⁺ junction formed at the photodiode surface, a 10 percent photo-sensitivity increase was obtained for a 15 percent depth reduction. The VOD shutter voltage reduction was performed by preventing the photodiode depletion layer depth from spreading deeply into the substrate. An epitaxially grown substrate with double impurity concentration layers, whose impurity concentration of the bottom layer is 10 times higher than that of the top layer, was adopted.