Effect of STI shape and tunneling oxide thinning on cell Vth variation in the flash memory

We studied factors which affect cell Vth variation in the floating gate flash memory. By simulation and experiment, we showed that the shape of STI (shallow trench isolation) and the tunnel oxide thickness in the STI edge were the main control factors. For example, sharp and thin oxide in the STI edge caused an uncontrolled F-N gate current in the program or erase operation, which directly indicated the amount of threshold voltage in the flash memory. Furthermore, we found that tunnel oxide thinning was closely related to the activation energy in the oxidation process. Smaller activation energy resulted in better thinning and better cell Vth distribution.