32 IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 4, NO. 1, MARCH 2004 Effect of P/E Cycling on Drain Disturb in Flash EEPROMs Under CHE and CHISEL Operation Deleep R. Nair, Student Member, IEEE, Nihar R. Mohapatra, Member, IEEE, Souvik Mahapatra, Member, IEEE, Shoji Shukuri, and Jeff D. Bude, Associate Member, IEEE Abstract—Drain disturb is studied in NOR flash EEPROM cells under CHE and CHISEL programming operation, before and after repeated program/erase (P/E) cycling. Drain disturb is shown to originate from band-to-band tunneling under CHISEL operation, unlike under CHE operation where it originates from source-drain leakage. Under identical initial programming time, CHISEL operation always shows slightly lower program/ disturb (P/D) margin before cycling but similar P/D margin after repetitive P/E cycling when compared to CHE operation. The degradation of gate coupling coefficient that affects source/drain leakage and the increase in trap-assisted band-to-band tunneling seems to explain well the behavior of CHE and CHISEL drain disturb after cycling. Index Terms—Band-to-band tunneling, CHE, CHISEL, cycling endurance, drain disturb, flash EEPROM. I. INTRODUCTION N OR FLASH EEPROM cells are programmed either by channel hot electron (CHE) [1] or by CHannel Initiated Secondary ELectron (CHISEL) [2]–[14] injection mechanisms. CHE injection involves injection of lateral field heated channel electrons into the floating gate (FG). Note that the CHE process does not rely on impact ionization. On the other hand, CHISEL injection relies on impact ionization feedback and is activated by the application of a negative substrate bias that results in a high transverse field near the drain junction. This causes high-energy electron injection into the FG over a spatially broad area in the channel. The schematic of CHE and CHISEL injection is shown in Fig. 1. Compared to the CHE process, CHISEL injection provides faster programming time under equivalent programming power and lower voltage and lower power operation for equiva- lent . CHISEL injection also offers self-convergent program- ming leading to excellent threshold voltage control and a unique recovery procedure for over erased cells not available under conventional CHE programming [2]–[13]. CHISEL pro- gramming also shows better program/erase (P/E) cycling en- durance of window and lower degradation compared to CHE operation [14], [15]. Most of the studies done so far were Manuscript received September 11, 2003; revised November 13, 2003. The work of D. R. Nair was supported in part by Siemens AG. D. R. Nair, N. R. Mohapatra, and S. Mahapatra are with the Department of Electrical Engineering, Indian Institute of Technology Bombay, India (e-mail: drnair@ee.iitb.ac.in; nihar@ ee.iitb.ac.in; souvik@ee.iitb.ac.in). S. Shukuri is with the Semiconductors and Integrated Circuits Group, Hitachi Ltd., Japan (e-mail: shoji-shukuri@sic. hitachi.co.jp). J. D. Bude is with Agere Systems, Allentown, PA 18109 USA (e-mail: bude@agere.com). Digital Object Identifier 10.1109/TDMR.2004.824371 Fig. 1. Schematic of CHE (top) and CHISEL (bottom) injection mechanisms. Filled and open circles represent electrons and holes, respectively. Impact ionization is present during CHE operation (not shown), but does not contribute to the injection process. Fig. 2. Schematic of a NOR flash memory array showing cells under program (Cell A) and drain disturb (Cell B) condition. concentrated on the effect of P/E cycling on the CHE/CHISEL programming performance of memory cells. Drain disturb (during programming) is an important relia- bility issue for NOR flash EEPROMs [16], [17]. It causes shift in cells that have unselected wordline (WL) but share the same bitline (BL) of a cell being programmed, as shown in Fig. 2. The application of during CHISEL operation results in a larger across the drain junction, which can aggra- vate drain disturb. Furthermore, drain disturb can worsen after P/E cycling and, therefore, requires careful attention. This paper 1530-4388/04$20.00 © 2004 IEEE Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on October 24, 2008 at 00:36 from IEEE Xplore. Restrictions apply.