Ion beam mixing for enhanced electron tunneling in metal-oxide-silicon structures

Abstract

Metal-oxide-silicon (MOS) structures were fabricated to investigate enhanced Fowler–Nordheim tunneling in thin oxides due to ion beam mixing. Ions of germanium were implanted into a 100 nm polycrystalline silicon layer deposited on 10 nm thermal silicon dioxide such that the tail of the implant profile contains the thin oxide. Besides simple MOS capacitors and transistors, flash electrically erasable programmable read only memory (flash EEPROM) cells were fabricated for the first time using this technique. Using 1×1015 cm−2 at 80 keV the Fowler–Nordheim tunneling barrier reduced by about 0.9 eV at the polycrystalline gate/oxide interface and by 1.3 eV at the oxide/substrate interface. The consequently lower program/erase voltages in the flash EEPROM were measured. Flash EEPROM charge retention measurements show that the discharge process is logarithmic in time. This leads to the possibility of flash EEPROMs being programmed and erased at low voltages and having sufficient charge retention for several years operation.