are made of this by George Marsh

That certain polymer materiaLs could be made to

conduct and hence exhibit electrical activity has been

known since the 1970s. The award Last year of the

Nobel Prize for Chemistry to A[an Heeger, Alan

MacDiarmid and Hideki Shirakawa for their 1977

discovery that polymers such as po[yacetylene couLd

be chemica[Ly doped to allow the flow of current,

emphasized the importance of this knowledge. With

ever more eLectricalLy active polymers now being

demonstrated in Laboratories around the wor[d,

'plastic electronics' Look set to cha[[enge the present

monopoLy of microeLectronics by silicon.

Image above shows an array o[~lexible polymer memory devices. (Courtesy of Thin Film Electronics.)

Traditional semiconductor memory falLs into two

categories - volatile and non-volatile. Volatile

memories, such as SRAM (static random access

memory) and DRAM (dynamic random access

memory), lose their contents when power is removed.

RAM memories are easy to use and perform well, but

require a continuous power source - not ideal for

battery-powered portable devices. Non-volatile

memories retain their contents when power is

removed and those in current use are derived from

ROM (read-only memory). However, non-volatile

memories like EEPROM (electrically erasable

programmable ROM) and Flash are difficult to write,

wear out after a few over-writes and guzzle power.

All these semiconductor memories rely on full

transistor switching - but memory can be based on

simpler bipolar action, which is where electrically

active polymers come in. A number of companies

hoping to provide the next generation of memory, by

combining the best of volatile and non-volatile

memory attributes, are developing polymeric

semiconductors that show promise in this role.

Since polymer memory can, in principle, be fabricated as continuous film, it holds out the prospect of low cost. Manufacturing imperfections need not be critical given the possibilities for bit redundancy and incorporation of electronic code checking, as used on traditional compact disc storage media. Using stacked multiple layers of film can yield volumetric memory of extremely high capacity for a given surface area.

34 ~ September/October 2001

flPPUCRTIOnS FEF:ITLIRC

Horeover, the appeal of a medium that is rugged, flexible and can accept bending without damage is clear. According to Stefan

Lai, vice president of technology and manufacturing at Inte[,

"The industry has been searching for a new kind of memory that

would be the Holy Graft of memories."

Polymer possibi|ities A particular form of polymer storage that Lai and his

colleagues at Intel have been exploring is polymeric ferroelectric RAM (PFRAM), which uses two electrode layers

of metal strands running perpendicularly to each other

separated by a thin polymer layer (Fig. 1). The complete

polymer film can be laid down by printing or by a simple,

inexpensive spin-on process. Lai explains that a memory cell

is formed at each intersection of the cross-point electrode

matrix and a data bit is stored by changing the polarization of the polymer between two addressed lines. The matrix

architecture is not restricted to read-only memory function

as it would be with a metal-programmed cross-point silicon

memory. Instead, data can be written, read or erased

according to the voltage level applied.

Bit line--

Fig. 7: DetaiLs of polymeric memory being developed by Inte! Corporation in collaboration with Scandinavian technology originator Thin Film Electronics/Opticom. (Courtesy of lnteL )

The simplicity of this storage mechanism avoids the need

for the NAND and NOR transistor-based logic gates that are

the basis of current Flash memory. (NAND Flash is used

mainly for mass data and NOR Flash for program code).

Space normally taken u|) by transistors therefore becomes

available for more compact memory elements so that bit

storage density is increased. Density is further boosted by

the use of multiple film layers to create memory in

3-dimensions. Write speeds exceed those of both NAND and

NOR Flash, while read times compare well - 0.1-10 I~S,

depending on the voltage applied, in destructive mode or

<50 ns in non-destructive mode. Power consumption is low,

with no cell standby power or refresh required.

Currently, active electronic functions necessary for memory control and coordination would be provided by a

conventional CMOS (complementary metal-oxide silicon)

base wafer over which multiple layers of polymer memory

would be laid, separated by polymer insulation films. Polymer

memory, says Intel, is readily integrated with CMOS.

However, current progress towards achieving organic field

effect transistors (for example, Lucent Technologies' work on

pentacene) suggests that integrated memory chips that are

entirely polymer will one day be possible.

For the last three years Inlet has been using a polymer technology developed by Thin Film Electronics, a subsidiary

of Norwegian company Opticom. Achievement of a viable

system, even on a lab-scale, has required that substantial

technology challenges be met. At the core of the Thin Film

Electronics/Opticom technology is the use of an organic film,

which incorporates transistor properties like switchability and

addressabiiity with charge storage (providing bistability and

non-volatility). Because the memory array is 100% passively

addressed, it can store data at the highest possible density -

and volumetrically (i.e. in 3-dimensions). Although Intel and Opticom are reluctant to reveal the precise nature of the

polymers they are usin~ poiythiophenes are known to have

been among those investigated by Thin Film Electronics.

Test memory arrays and complete operational CMOS/polymer chips have now been fabricated, but Intel is

reluctant to give estimates of when commercial-scale

production might commence. Currently Inte[ predicts manufacturing costs about an eighth of those for

conventional memory. Based on work to date, the company

thinks that polymer memory will be capable of billions of

read/write cycles with minimal fatigue.

September/October ZOO1 ~ 35

RPPUCRTIOnS FCRTURE

Table 1: Comparison of conductivities of various materials, including conductive pot vmer~ The best stable conducting plastics have conductivities of 100 to 1000 Scm- 1. Conductivities of up to 700,000 Scm- 1 have now been reatisec;, but the p/astics that demonstrate these values are unstab/e. (Courtes~ o/ PhG'ps Research.)

Material Conductivity/Scrn -1

2

Other companies are also investigating the use of

conjugated polymers as a basis for integrated circuits -

including memory devices. As Philips point out conductivities

ranging from fully insulating to levels associated with

metallic conductors can be achieved with polymers (Table 1).

Because semiconductor polymers were, in their original

form, extremely difficult to process, Phi[ips substituted large

flexible side chains for the previous unreactive rigid flat

chains to make the polymers soluble. Solution processing of

organic semiconductors, combined with large-area stamping

or printing to eliminate lithography, is seen as the most

economical manufacturing route. Varying the precise chemical composition of the side chains enables the mix of

solubility, conductivity and durability properties to be

optimized for particular applications. Philips sees modified

conjugated polymers as key to non-volatile memory in

applications where performance is subsidiary to price. It has

produced prototypes of programmable plastic memory chips

Data storage region

Fig. 2: Ovonyx uni#ed memory (OUhf) based on thin -#tin chalcogenide materials. (Courtesy o[ InteL )

that continue to function even when folded double. But Philips believes that conventional chips based on silicon wil l

have the edge for switching speed and performance for many

years to come. IBM has also made progress in organic thin-f i lm

electronics based on conjugated organic molecules, long-

chain polymers, shorter-chain o[igomers and organic- inorganic hybrids such as perovskites I. Research efforts on

materials such as thiophene polymers and oligomers, plus the

small pentacene molecule, have led to improvements in

charge mobil ity of five orders of magnitude over the last IS

years. Further conductivity enhancements are likely to follow

from advances in synthesis and ordering or self-assembly of

these materials. Improvements in performance of active materials that can

be processed at low temperatures over large areas on plastic

film or paper will, it is suggested, help address a growing

need for pervasive computing and enhanced connectivity.

Techniques such as vacuum evaporation, solution casting,

ink-jet printing and stamping wil l lend themselves to roll-to-

roll manufacturing of low-cost products such as memory,

logic for smart cards and information displays.

Competing technologies In combining the electronic properties of metals and

semiconductors with the processability and mechanical

properties of plastics, conducting polymers offer great hope

for economical bulk memory and integrated electronics.

However, if polymer memories are to realize their potential,

they must compete with inorganic material alternatives such

as magnetic RAM (MRAM), ferroelectric RAM (FRAM) and

Ovonyx's technology Ovonic Unified Memory (OUM).

In MRAM, the polarity of each memory cell on a chip is switched electrically, changes in polarity are then sensed as

resistance changes. The system is non-volatile; offering high

density, low voltage and power. It is fast (read/write speeds

of <50 ns) and has unlimited read/write endurance, but

material compatibil ity with CMOS is a key challenge.

FRAM is based on ferroe[ectric materials with a bistable

center atom, typically PZT ([ead-zirconate-titanate). Data is stored by applying a voltage via external electrodes to

polarize the internal dipoles - caused by the movement of

the central atom in the electric field - either 'up' or 'down'.

This non-volatile system offers fast random read access, and

fast write with very low power consumption. However, a

36 ~ September/October'2001

flPPLICflTiOnS FEATURE

Fig. 3: Memory technologies comparison, showing how polymer memory could bridge the gap in both cost and performance, between rotating and conventional semiconductor media. (Courtesy Intel).

destructive read causes material property degeneration,

giving a Limited number of read/write cycles.

OUM relies on the thin-film chaLcogenide material alloys

that are used in re-writable CDs and DVDs (Fig. 2).

Transistors control the electronic conversion of the material

from crystalline (conductive) to amorphous (resistive) phases

as the data storage method. Intel, which has been developing

the system with Ovonyx, says that it offers non-volatility,

high density, non-destructive read, more than 10 billion

write/erase cycles, low voltage and Low power. It is less

expensive than either MRAM or FRAM, but is slower. It is,

says the company, easy to integrate with conventional

semiconductor logic. Versatile OUM can serve most

applications (hence 'unified'), with only a small amount of

supplementary DRAM/SRAM likely to be needed for cache

and other frequent-write functions.

Despite their individual advantages, none of these

memory technologies can be manufactured from solution, as

polymeric memory can, and are restricted to single-layer

storage.

Intel says it is keeping its options open, but can foresee a

future for both polymer and inorganic chaicogenide

memories in particular, the former as Low-cost data storage

and OUM for code and data (Fig. 3). When Intei's Gordon

Moore, one of the pioneers of Flash non-volatile memory,

propounded his now famous law that the number of

transistors that can be incorporated on a chip would be

doubted every 18 months, he knew that this would one day

reach limits imposed by the fundamental physics of existing

semiconductor materials. New high performance engineered

(smart) polymers offer a technology that could overcome

these limits, lIT

FURTHER READING 1. IBM Journal of Research and Development, Volume 45, No1 ZOO1

2. The Chemist. June 2001

September/October 2001 ~ 37