1. Non-MOSFET Based Memory
Alex Rodriguez-Triana
Terence Frederick
April 21, 2008

2. Outline MOSFET Based RAM Memory Problems with MOSFET Memory
DRAM, SRAM, FLASH
Problems with MOSFET Memory
Scaling
Alternative Memory
MRAM
FeRAM
PCRAM
Summary

3. History of MOSFET Memory
Concept goes back to the 1960s
People were speculative
BJT was more advanced and faster
Leakage current
They were attractive
Simple Processing
Layout Advantages
Leads to high-density integrated circuits

4. History of MOSFET Memory
SRAM were proposed
six MOSFET’s per cell
SRAM began to be used in the mid-70s
DRAM patented in 1968
1 MOSFET, 1 Capacitor
First commercial DRAM
1971 by Intel

5. Dynamic RAM Most common type of RAM memory Arranged in a square array
one capacitor and transistor per cell
Stores one bit per cell
Recharging/Refreshing : capacitors lose their charge
Rows: Word Lines
Columns: Bit Lines

6. Advantages/Disadvantages of DRAM
Cost
Small
1T & 1C vs. 6T for SRAM
Number of Read/Write Cycles
> 10^15
Disadvantages
Slow
Need to refresh
Volatile
Data is lost when memory is not powered

7. Static RAM Memory cell uses flip-flop to store bit
Requires 6 transistors
Each bit is stored on 4 transistors that form two inverters
Two other transistors control the access to a cell during read and write operations
This storage cell has two stable states
0 and 1

8. Advantages/Disadvantages of SRAM
Performance better than DRAM
Faster
Less Power Hungry
Number of Read/Write Cycles
> 10^15
Disadvantages
Cost
More than DRAM
Volatile
Data is lost when memory is not powered

9. FLASH Memory Invented by Dr. Fujio Masuoka at Toshiba in 1984
Stores information in an array of memory cells made from floating-gate transistors
Single-Level Cell Devices - each cell stores only one bit

10. Advantages/Disadvantages of FLASH
Cost
Non-Volatile
Does not lose information when the power is off
Low Power
Fast Erase
Large blocks rather than one word at a time
Disadvantages
Number of Read/Write Cycles
~ 10^6
Slow Write
Entire block must be read, word updated, then entire block written back

11. Future of MOSFET Memory
Current memory technologies are nearing the end
Main issue with MOSFET RAMs
Scalability
Designers put more components onto each chip
Width of the smallest features is shrinking
130 nm in 2000 to 45 nm today
Existing memory technologies will be good for several more generations
Unlikely to make the transition to 22 nm (scheduled for 2011) or 16 nm (2018)
New types of technologies
MRAM, FeRAM, PCRAM

12. MOSFET Scaling Late 1990s Reasons for smaller MOSFETs
Scaling resulted in great improvement in MOSFET circuit operation
Reasons for smaller MOSFETs
Same functionality in a smaller area
Reduces cost per chip
Smaller ICs allow for more chips on a wafer
Fab costs for wafer are relatively fixed

13. MOSFET Scaling Problems when scaling too small Slower chip speed
Greater delay due to interconnects
Operational problems
Higher sub-threshold, increased gate-oxide and junction leakage, lower transconductance, heat production, and process variation
Simulation
Difficult to predict what the final device will look like
Modeling of physical processes
Microscopic variations in structure due to the probabilistic nature of atomic processes require statistical predictions

14. Alternative Technologies
Magnetic RAM (MRAM)
Ferroelectric RAM (FeRAM)
Phase Change RAM (PCRAM)

15. Magnetoresistive RAM Under development since the 1990s
Data is stored by magnetic storage elements
Formed from two ferromagnetic plates
Plates can hold a magnetic field
Polarization doesn’t leak away with time like charge
Less wear since switching states doesn’t involve movement of electron or atoms
One plates is a permanent magnet
Set to a certain polarity
Second plate’s field will change to match that of an external field
A memory device is built from a grid of "cells"

16. 4MB MRAM 1st commercial available MRAM
Based on 1T and 1 magnetic tunnel junction
Isolates read and write path
Separates programming components from the sense circuit
Improved performance

17. Read and Write of MRAM Read Write Current is passed through the bit
resistance of the bit is sensed
Write
Current is passed through the programming lines
Induced magnetic field is created at the junction, which the writable plate picks up

18. MRAM Cell works in a toggling mode Same direction Opposite direction
Low resistance state (0)
Opposite direction
High resistance state (1)

19. MRAM in Embedded Systems
Inserted late in the SC fabrication process
Low temperature
Compatible with CMOS processing
Consolidate multiple MRAM into one
highly reliable NVRAM
Less complexity
High performance RD/WR

20. Advantages/Disadvantages of MRAM
Non-volatile
Does not require programming sequences or block erasing
Very fast RD/WR and unlimited endurance
Simple device Architecture and easy software development
Due to easy write and overwrite
Disadvantages
Scalability of magnetic domain?
Might have the same problems as a transistor
Disturbance of neighboring cells when put close together
Leads to false writes
High power needed to write

21. Ferroelectric RAM Borrows concepts from DRAM
most popular design follows the 1T1C design concept
similar/same write process
write accomplished by applying charge that is stored in capacitor
Similarity to Floating Gate Design
1T design
Also reminiscent of MRAM
focuses on ferroelectric properties, whereas MRAM techniques often focus on ferromagnetic properties
both characteristics take form of hysteresis loop

22. Structure 1T type 1T1C type Similar to normal transistor
Identical to floating gate design where floating gate is ferroelectric material
1T1C type
ferroelectric material serves ONLY as capacitor

23. “Recent Progress in Ferroelectric Memory Technology”
by Hiroshi Ishiwara

24. Introduction Two major focuses in the paper
developing a better material to deal with leakage currents in 1T1C FeRAM
replace some Fe in lattice with Mn
Improve upon 1T FeRAM design
create MFIS-FET
Introduce a new 1T2C FeRAM design

25. Results I

26. 1T2C Design
2 Ferroelectric capacitors of the same size connected to the gate of the transistor
capacitors polarized opposite the gate
Good performance
non-destructive data reads
good data retention time
high on/off current ratio

27. Advantages/Disadvantages of FeRAM
lower power usage
faster write speed
greater number of rewrites
already being mass-produced
Disadvantages
still more research to be done on reliability (i.e. high NRE cost)
only applicable to a small niche

28. “Study of Phase Change Random Access Memory (PCRAM) at the Nano-Scale”
by R. Zhao, L.P. Shi, W.J. Wang, H.X. Yang, H.K. Lee, K.G. Lim, E.G. Yeo, E.K. Chua and T.C. Chong

29. Introduction
RAM based on floating-gate design (i.e. Flash memory) will soon meet physical limitations
interpoly tunneling
intercell crosstalk
Flash memory is the most prevalent non-volatile memory on the market
a viable option must be found soon
PCRAM may be that option

30. Fabrication/Design “Bybrid” process used to etch the layers
Electronic Beam Lithography (EBL)
Optical Lithography
Electrodes made of TiW
Dielectric is common SiO2
Phase Change material is Ge2Sb5Te2
Feature size refers to contact between PC and bottom electrode

31. How it Works Unique Phase Change material has two states
Crystalline state has low resistance and represents a stored ‘1’
Amorphous state has high resistance and represents a stored ‘0’
To change bit from 1 to 0 (i.e. RESET), a relatively high voltage is applied for a short time such that the compound melts but is not able to recrystallize
To change bit from 0 to 1 (i.e. SET), a lower voltage is applied for a longer time so that compound can crystallize

32. Simulation Pulse generator created to produce short (<10ns) signal
Known resistance placed in circuit
Voltages measured to determine drop across resistor
Current into PCRAM approximately (V1-V2)/Rload
Cells with feature sizes ranging from 40 to 200 nm created
same wafer used

33. Results

34. Advantages/Disadvantages of PCRAM
great scalability
fast for both reads and writes
low current required to program
Disadvantages
as of yet, only in the research phase
still limited read/write accesses (108)

35. Summary SRAM DRAM FLASH MRAM FeRAM PCRAM Read Speed Fast Medium
SRAM
DRAM
FLASH
MRAM
FeRAM
PCRAM
Read Speed
Fast
Medium
Write Speed
Slow
Non-Volatile No
Yes
Endurance
Infinite
Limited
Low Voltage 35