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NON-MOSFET BASED MEMORYAlex Rodriguez-TrianaTerence Frederick
April 21, 2008
OUTLINE
MOSFET Based RAM Memory DRAM, SRAM, FLASH
Problems with MOSFET Memory Scaling
Alternative Memory MRAM FeRAM PCRAM
Summary
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
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
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
ADVANTAGES/DISADVANTAGES OF DRAM
Advantages 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
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
ADVANTAGES/DISADVANTAGES OF SRAM
Advantages 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
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
ADVANTAGES/DISADVANTAGES OF FLASH
Advantages 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
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
MOSFET SCALING
Late 1990s 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
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
ALTERNATIVE TECHNOLOGIES
Magnetic RAM (MRAM)
Ferroelectric RAM (FeRAM)
Phase Change RAM (PCRAM)
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"
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
READ AND WRITE OF MRAM
Read 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
MRAM
Cell works in a toggling mode Same direction
Low resistance state (0) Opposite direction
High resistance state (1)
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
ADVANTAGES/DISADVANTAGES OF MRAM Advantages
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
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
Structure 1T type
Similar to normal transistor
Identical to floating gate design where floating gate is ferroelectric material
1T1C type ferroelectric material
serves ONLY as capacitor
“Recent Progress in Ferroelectric Memory Technology”
by Hiroshi Ishiwara
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
Results I
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
Advantages/Disadvantages of FeRAM
Advantages 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
“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
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
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
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
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
Results
Advantages/Disadvantages of PCRAM
Advantages 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)
SUMMARY
SRAM DRAM FLASH MRAM FeRAM PCRAM
Read Speed Fast Medium Fast Fast Fast Fast
Write Speed Fast Medium Slow Fast Medium Fast
Non-Volatile No No Yes Yes Yes Yes
Endurance Infinite Infinite Limited Infinite Limited Limited
Low Voltage Yes Limited Limited Yes Limited No