Poly-Oxide/Nitride/Oxide Structures for Highly Reliable EPROM Cells

In advanced high density submicron EPROMs such as 4Mb and beyond, it is particularly important to realize thin 1st and 2nd gate insulators to scale down device dimensions and to achieve faster write/read speed. Usually, the 2nd gate insulator of EPROM is formed by thermal oxidation of doped poly-Si. However, highly reliable thin poly-oxide formation by usual thermal oxidation is very difficult because of high oxidation rate of doped poly-Si and larger conductivity through the oxide due to thickness fluctuation and asperity at poly-Si/poly-oxide interface. In this paper, new technology to form thin and less conductive 2nd gate insulator film, poly-oxide/nitride/oxide is presented. Using these structures, 1 μm EPROM cells are fabricated, and the device characteristics and reliability are examined. Good charge retention and writing characteristics are obtained.