A new scalable floating-gate EEPROM cell

In a Floating-gate Thin Oxide (FLOTOX) MOS transistor, the dimension of the tunnel window and surrounding active area is a limiting factor in cell scaling. However, this limiting factor can be eliminated by opening a tunnel window which overlaps the field oxide. Experimental results show that the presence of the field oxide edges does not pose a reliability concern for the tunnel oxide quality in the new cell. Since the tunnel region is defined by the intersection of the tunnel window mask and the active area, the new cell eliminates the spacing between the tunnel window and the field oxide. Therefore, cell characteristics are insensitive to the tunnel window misalignment with respect to the field edges. Because the tunnel region area of the new cell is scalable, the floating gate transistor drain area and the overall cell size become scalable as the peripheral circuit device dimension is scaled down. Due to a smaller achievable drain area, the new cell shows a better programmability. For a target threshold voltage window, the required programming voltage as well as the isolation spacing between cells can be reduced. By controlling the doping profile around the tunnel region, a scalable EEPROM cell with reliability and uniformity comparable to the conventional FLOTOX cell has been achieved.