1. Floating Gate Devices
Kyle Craig

2. Flash Memory Cells – An overview
Paolo Pavan, Roberto Bez, Piero Olivo and Enrico Zanoni

3. Motivation! Predicted Worldwide Memory Market
Flash prediction, 6% of total memory market…

4. According to Gartner Research in 2006 flash consisted of 33% of the market

5. FGMOSFET
If a charge can be forced onto the floating gate, it will remain there.
Charge on the floating gate shifts the VT of the device
Two VT device

6. By having two VTs depending on the charge of the floating gate, device can be used as a memory.
No charge on floating gate = logic 1
Charge on floating gate = logic 0

7. Hot Electron Injection
Electrons travel laterally from source to drain with applied voltage. Voltage on gate gives enough energy to inject through the thin oxide onto the floating gate
Three principles
“lucky” enough to gain
enough energy
No collisions in substrate
No collisions in oxide

8. Fowler Nordheim Tunneling
With an applied electric field, electrons are able to tunnel through the oxide.
The thicker the oxide, the greater the applied voltage needs to be
10nm oxide is considered standard
Variation in this oxide will lead to
wide distribution of VT values

9. Side effects
HEI and FN Tunneling can lead to charge being trapped in the oxide
Change in the VTs of the device
Inability to add or remove charge from floating gate

10. Flash Memory: Programming
Assumed device starts with no charge on the floating gate i.e., storing a “1”
Use HEI to put charge onto floating gate
Shifts VT of device
Depends on:
Channel Length, Time, Temp, drain voltage

11. Flash Memory: Erasing
Use Fowler-Nordheim Tunneling to pull electrons off of floating gate to the source
Depends on:
Oxide thickness, applied voltage
Need to worry about breakdown of the source/substrate junction
Limits Scaling!

12. Program, Erase, Read Source Control Gate (WL) Drain (BL) Read GND Vcc
Vread
Program
Vpp
Vdd
Erase
Floating
Typical Values: Vcc = 5V Vpp = 12V Vdd = 5V Vread = 1V

13. Programming Disturbs Gate Disturbs – Cells not selected with active WL
DC Erasing
If cell has charge store on it, electrons can tunnel from the FG to the Control Gate
DC Programming
If the cell has no charge on it, electrons can tunnel from the substrate to the FG

14. Programming Disturbs Drain Disturbs Lowers the high VT value
Electrons can tunnel from the FG to the drain
Holes generated by impact-ionization in substrate then injected into FG
Lowers the high VT value

15. Retention/Endurance Retention: Change in charge on FG
Intrinsic: field-assisted electron emission, thermionic emission
Extrinsic: Oxide defects, Ionic contamination
Endurance: Change in threshold values based on number of cycles

16. Scaling Issues with scaling
Decreasing L will increase performance but also increase number of disturbs
Physical voltage constraints
3.2 eV energy barrier and 8-9MV/cm for FN
Oxide thickness limit

17. NOR vs NAND NOR similar in structure to SRAM
NAND more comparable to Harddrives

18. **From Micron NAND vs NOR Comparison

19. A 6V Embedded 90nm Silicon Nanocrystal Nonvolatile Memory
R. Muralidhar, R.F. Steimle, M. Sadd, R.Rao, C.T. Swift, E.J. Prinz, J. Yater, L. Grieve, K. Harber, B. Hradsky, S. Straub, B. Acred, W. Paulson, W. Chen, L. Parker, S.G.H. Anderson, M. Rossow, T. Merchant, M. Paransky, T. Huynh, D. Hadad, KO-Min Chang, and B.E. White Jr.

20. Motivation
Scaling of conventional floating gate memories is limited due to high voltages that are needed
Use of Silicon Nanocrystals has some benefits over floating gate
Immune to oxide defects during program/erase
Reduction of oxide thickness
Reduction of operating voltage

21. Replaces floating gate of traditional cell with discrete Nanocrystal particles
Produced using a conventional CMOS process flow with only 4 additional masks compared to logic
Control Gate

22. Current Characteristics
Two Bits/cell operation is possible with proper nanocyrstal isolation
Without needed isolation functions like a normal FG

23. Mask Adder

24. Endurance and Retention

25. Can be erased from the top oxide (between FG and control gate) or the bottom oxide (between nanocrystals and substrate)
Erasing through the top oxide produces lower VT

26. Cycling on VT
After cycling 1000 cycles Erase and Program VTs maintain tight distribution.

27. Summary Produced in 90nm .25um technology Operation voltages 6V
90% yield on 4 MB array