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