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Transcript
NON-MOSFET BASED MEMORY
Alex Rodriguez-Triana
Terence Frederick
April 21, 2008
OUTLINE

MOSFET Based RAM Memory


Problems with MOSFET Memory


Scaling
Alternative Memory




DRAM, SRAM, FLASH
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



Number of Read/Write Cycles


1T & 1C vs. 6T for SRAM
> 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


Existing memory technologies will be good for
several more generations


130 nm in 2000 to 45 nm today
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


Operational problems


Greater delay due to interconnects
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?


Disturbance of neighboring cells when put close
together


Might have the same problems as a transistor
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



Similarity to Floating Gate Design


write accomplished by applying charge that is stored in
capacitor
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


Improve upon 1T FeRAM design


replace some Fe in lattice with Mn
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