Phase Change Memory(PCM)

“No one will need more than 637KB of memory for a personal computer. 640KB ought to be enough for anybody,” :Bill Gates (1981)

Jibin George Mathews,06142,S7 EA,

Department of Electronics and Communication

History

• Dr. Ovshinsky -1960s

• He formed his company ECD(Energy conversion devices)

• Article in 1970 September 28 edition of Electronics magazine

by him & Gordon Moore titled “non volatile & reprogrammable

”

• 2000 – STMicroelectronics & ovonyx

What is Phase Change Memory ?

”PCM/PRAM uses the unique behavior of chalcogenide glass, which can

be "switched" between two states, crystalline and amorphous, with the

application of heat.”

• Technology to produce high purity thin films

• Cost

• Numerous breakthroughs in chalcogenide materials.

• Scaling - Less material to heat –less energy reqd.

• Flash memory will soon reach its scaling limit.

Why PCM is becoming attractive now ??

A review of memory basics !

What is a computer memory ?

Hard disk ?? No,Its simply a type of storage(permanent)“Out of memory” message on computer indicates RAM.

Memory is commonly known as RAM.

• Two types: volatile & nonvolatile(NVRAM)• Volatile : Static & Dynamic

• Historically RAM and Hard disk were called primary and secondary storages respectively

• NVRAM also called CMOS RAMon-board semiconductor chip

powered by a CMOS battery inside computers that stores information such as the system time and

system settings for

your computer.

Memory basics(contd)Hierarchy of computer storage

• Primary,

• Secondary

• Tertiary

Why we use volatile RAM as primary ?

Memory basics(contd)

• You turn computer ON

• CPU derives data from ROM &performs POST

• Loads BIOS from ROM

• Loads OS from HDD into RAM

• Opening applications will load them into RAM.Saving them causes them to be written to the storage device and file deleted from RAM

Why does a computer need so many memory systems ?

4 groups

Flash memory

Non volatile

Problems related to flash

• Nand & Nor flash technologies reaching scaling limit

• If size of code/data increased by a byte that space has to be doubled

• It can be written to in bytes only ie can be overwritten only if an

entire block is erased. So cant be used for small random writes of

processor.

• So a complement of NVRAM & RAM had to be used.

• Only good for 100k-1M writes

•Answer is PCM !!– No longer should the code and data be separately stored in NVM

and RAM

Current NVM

• density is improving!

• for how long?

performance is

stagnating!

forever!

Answer is PCM !!

PCM to the rescue !

Contributes attributes of NOR,NAND & RAM

• Byte alterable

• Faster writes

• Faster execution

• Non volatile

• Can store both code and data

PCM performance• Fast (~50 ns)

• Low voltage (0.4-2 V)• Scaling: good

• Medium endurance (109-1013)

NVM/Flash performance• Slow (s-ms)

• High voltage (10-15 V)• Scaling: bad

• Short endurance (105-106)

1.PCM - Introduction

Physical characteristics• Chemical formula: GexSbyTez

• Uses chalcogenide glass

• Varies between two states:– Crystalline – low resistance, represents binary 0– Amorphous – high resistance, represents binary 1

• Can switch on the order of nanoseconds

1.PCM uses a reversible structural phase-change (between amorphous phase & crystalline phase)

2.The small volume of active media in each memory cell acts as a fast programmable resistor.

2.PCM-Technology concept

Technology concept(contd)

3.PCM-Basic structure

Amorphous orC rystalline Chalcogenide

C rystalline Chalcogenide

Res

istiv

e H

eate

r

4.PCM-Cell element characteristics

Basic Device Operation

Figure 2 – Entire cell diagram

5.PCM-operating principle

The PCM cell is programmed by application of a current pulse at a

voltage above the switching threshold.

PCM devices are programmed by electrically altering the structure

(amorphous or crystalline)of a small volume of chalcogenide alloy

The programming pulse drives the memory cell into a high or low

resistance state(phase transition process), depending on current

magnitude.

Phase transition process can be completed in as quickly as 5

nanoseconds. Information stored in the cell is read out by measurement

of the cell’s resistance.

PCM-operating principle(contd)

• A simple scalable device:• An access transistor and a programmable element (PE)• High switching speed (~ns)• Read/write endurance: >1012 (Flash: 106) PE

word

-lin

es

bit-lines

Memory array with NMOS transistors:

• PE based on a switching resistance

• Phase-change materials amorphous phase: ‘high’-Ohmic

crystalline phase: ‘low’-Ohmic

• Fast switching between amorphous and crystalline phase

•.

Switching

17

Temperature

Time

MeltingTemperature

CrystalTemperature

amorphization pulse

Electric pulses induce Joule heating

RESET pulse:

- T > Tmelt

- Rapid cooling down amorphization

Temperature

Time

MeltingTemperature

CrystalTemperature

crystallization pulse

SET pulse:

- T > Tcryst

- Longer pulse crystallization

V-I characteristics

•. At low voltages, the device exhibits

either a low resistance (~1k) or

high resistance (>100k), depending

on its programmed state : READ

region

For a reset device : V >Vth to program

The reciprocal slope of I-V curve in the dynamic on state is the series device resistance

R-I Characteristics

shows the device read resistance

resulting from application of the

programming current pulse amplitude.

low amplitude pulses at voltages less

than Vth do not set the device. Once Vth

is surpassed, the device switches to the

dynamic on state and programmed

resistance is dramatically reduced as

crystallization of the material is

achieved.

The slope of the right side of the curve is the device design parameter and can be

adjusted to enable a multi‐ state memory cell.

About Chalcogenide alloy

Two types : Nucleation dominant material &

fast growth material

Chalcogenide or phase change alloys is a

ternary system of Gallium, Antimony and

Tellurium. Chemically it is Ge2Sb2Te5.

Production Process: Powders for the phase change targets are produced by

state‐of –the art alloying through melting of the raw material and

subsequent milling. This achieves the defined particle size distribution.

Then powders are processed to discs through Hot Isotactic Pressing

PCM - Advantages

PCM uses a reversible structural phase-change scaled device has been demonstrated

Cost/Bit Reduction• Small active storage medium• Small cell size – small die size• Simple manufacturing process – low step count• Simple planar device structure• Low voltage – single supply• Reduced assembly and test costs

• Highly Scalable• Performance improves with scaling• Only lithography limited• Low voltage operation• Multi-state demonstrated

PCM Today

• 2004 :Samsung Prototyped a 512 MB module

• 2006 :Intel created a mass producible 128 module

• 2008: Intel discovered 2 additional states effectively

doubling the Capacity

• 2008 End: Intel begins shipping beta version called

Alverstone

• Multilevel recording on optical CDs

Challenges

• Challenges include :Management of proximity

heating with declining cell space.

• Increased set/reset resistance and decreased read

current/set current margin with scaling.

Emerging technologies & PCM

Conclusion

Near ideal memory qualities

Broadens system applications

– Embedded, System-On-a-Chip (SOC), other

products

Highly scalable

Risk factors have been identified

Time to productize

More companies looking forward to OUM

That might completely replace FLASH.

References

www.ovonyx.com www.ieee.org http://www.xbitlabs.com/news/memory/display/2005091221

2649.html

http://www.theinquirer.net/default.aspx?article=36841 http://www.tgdaily.com/2008/09/13/bitmicro_rolls_out_155_g

ig_solid/

http://www.storagesearch.com/semico-art1.html http://www.samsung.com/he/presscenter/pressrelease/pres

srelease_20060524_0000257996.asp http://www.intel.com/pressroom/archive/releases/20070530

corp.htm Objective analysis pcm white paper August 2009\\

PCM – a 180 nm non volatile memory cell element technology forstand alone and embedded applications – Stefan Lai and Tyler Lowrey

• Current status of Phase change memory – Stefan Lai

THANK YOU !!

Any Questions ?!