A Novel Random Telegraph Signal Method to Study Program/Erase Charge Lateral Spread and Retention Loss in a SONOS Flash Memory

A novel random telegraph signal (RTS) method is proposed to characterize the lateral distribution of injected charge in program and erase states in a nor-type silicon-oxide-nitride-oxide-silicon Flash memory. The concept of this method is to use RTS to extract an oxide trap position in the channel and then to use the trap and RTS as internal probe to detect a local channel potential change resulting from injected charge during program/erase. By using this method, the lateral width of the injected charge-induced channel potential barrier is shown to be around 20 nm in channel hot electron (CHE) program. Our method also confirms that Channel Initiated Secondary ELectron (CHISEL) program has a broader injected charge distribution than CHE program. A mismatch of CHE program electrons and band-to-band tunneling erase holes is observed. Program-state retention loss models, charge vertical loss versus lateral migration, are reexamined by using this method. The polarity of a program-state charge distribution along the channel is explored within 10-20 program/erase cycles. Nitride charge vertical loss is verified by this method.