New buried channel flash memory cell with symmetrical source/drain structure

In flash memory, the writing operation is achieved by channel hot electron injection and the erasing operation is achieved by Fowler-Nordheim tunneling. Therefore, high voltage must be applied in both operations. In memory cells, the source/drain structures are asymmetrical in order to efficiently carry out writing/erasing operations. However, the asymmetrical structures and high voltage operation are serious obstacles in designing devices with microscopic structures. In order to overcome these obstacles, a memory cell with buried channel nMOSFETs was fabricated. This device had a symmetrical source/drain structure in order to achieve efficient hot carrier injection and low voltage operation. In addition, the reliability of the tunnel oxide layer was improved by implanting nitrogen into the tunnel oxide layer.