Trapping-free string select transistors and ground select transistors for Vg-type 3D NAND Flash memory

In SONOS 3D NAND, if the string select transistor (SST) and the ground select transistor (GST) use memory cell´s trapping dielectric as their gate dielectrics, they could suffer abnormal Vt shift due to unwanted charge injection during programming/erasing (P/E). To solve this, we remove the blocking oxide and the trapping nitride, and use the BE-SONOS´ ONO tunneling dielectric and an additional LPCVD oxide as the gate dielectric of SST and GST. The nitride in the ONO tunnel dielectric is thin (<;30A) so it is trapping-free. After repeatedly programming/erasing the NAND strings for more than 3,000 times, the Vt shifts of the select transistors are less than 200mV. The trapping-free SST and GST can prolong the cycling endurance of SONOS 3D NAND Flash memory.