Roll-to-roll process prints thousands of cheap, flexible memory elements

Jul 31, 2012 by Lisa Zyga feature
Photograph of a printed WORM memory bank with 26 bits, with an optical microscope image of the bit layout at right. The bit size is approximately 200 x 300 micrometers. Image credit: Leppäniemi, et al. ©2012 IOP Publishing Ltd

(Phys.org) -- In an attempt to lower the cost of making flexible write-once-read-many (WORM) memory devices, a team of researchers from Finland has developed a fabrication process that can mass-print thousands of these memories on a flexible substrate. Since they cannot be rewritten, WORM memories are particularly useful for tamper-proof applications, such as electronic voting and storing medical records.

The researchers, Jaakko Leppäniemi, Tomi Mattila, Terho Kololuoma, Mika Suhonen, and Ari Alastalo at the VTT Technical Research Centre of Finland, have published their paper on the WORM printing process in a recent issue of Nanotechnology.

The WORM memory is a resistive memory in which data is written using a high-resistive ‘0’ state and a low-resistive ‘1’ state. Readout can be performed with a device that measures the different resistances by physical contact or capacitively by sweeping over the memory without making contact.

To realize a resistive WORM memory, the researchers prepared bits out of a silver nanopaste mixture that combines the advantages of two different commercial inks by Advanced Nano Products Ltd. One of the inks, called DGP, has the advantage of being writable with moderate , but has the disadvantage of instability due to the high-resistive ‘0’ state losing . The second ink, called DGH, has the opposite properties: it requires a high electrical power for writing but has improved long-term stability. Although neither ink by itself is optimal for making memory bits, the researchers found that combining them offers the best of both worlds: moderate electrical power for writing and good long-term stability.

(a) Illustration of the roll-to-roll printing process. After printing, the roll containing the WORM memory banks is fed into a die cutter, and some of the cut WORM memory banks are laminated. (b) VTT’s roll-to-roll printing line “ROKO.” Image credit: Leppäniemi, et al. ©2012 IOP Publishing Ltd

When combined, the two inks form a self-organized network, which allows the researchers to tune the initial conductivity and ‘0’ state resistance of the bits. The researchers could control the resistance by controlling how sintered and close together the nanoparticles are: resistance is high when the particles are unsintered and well separated, but decreases when the particles sinter and coalesce. To control the inter-particle distance, the researchers employed nanoparticles encapsulated with ligands to prevent agglomeration and create the high-resistive ‘0’ state. To decrease the resistance, and thus switch from the ‘0’ state to the ‘1’ state, the researchers removed the encapsulation by heating with electrical current. The heat melts the ligand, which allows the unencapsulated particles to coalesce and sinter, where they achieve higher conductivity and lower resistance.

In this way, the technique allows for selectively and irreversibly modifying the resistance of the bits, enabling a WORM memory function. As the scientists explained, such a memory has important advantages for real-world use.

“The advantage of the memory lies in the processability,” Leppäniemi told Phys.org. “The memories can be printed with high-throughput methods and the bit properties can be tailored by changing the composition of the bit ink. Also, the resistive memory provides simple, non-destructive readout when compared, for example, to printable ferroelectric random access memories.”

(a) A questionnaire card with a printed 12-bit WORM memory bank, flexible battery, and LED. (b) Roll-to-roll printing of the WORM memory banks for the card with VTT’s “ROKO” printing line. (c) WORM memory bank pre-sintering and the readout device. Image credit: Leppäniemi, et al. ©2012 IOP Publishing Ltd

In terms of stability, the researchers observed the onset of a slow decline in the resistance after being stored for four months in the dark with a desiccant at ambient conditions. The decline resulted from the less-stable ‘0’ state due to self-sintering, which decreased its resistance. However, even after the 19 months of monitoring time, the researchers described the bits as maintaining a good ‘0’ state stability. In contrast, when exposed to a high temperature of 85 °C (185 °F) and a high relative humidity of 85%, the resistance underwent a rapid drop in less than three hours.

To demonstrate a simple application of the WORM memory, the researchers, together with Stora Enso Oyj, manufactured 1,000 electrical questionnaire cards for the Printed Electronics Europe 2011 conference that attendees used to vote for the best booth at the conference. A flexible printed battery supplied the small write voltage (less than 10 volts), and an LED inside the card indicated the successful push of a button. The questionnaire cards represent just one possible use of the WORM memory, which the researchers hope to further improve in the future.

“The goal is to provide printed memory addressing logic and reach higher bit amounts,” Leppäniemi said. “Also, improving the long-term stability via proper encapsulation requires further attention.”

Explore further: World's smallest propeller could be used for microscopic medicine

More information: Jaakko Leppäniemi, et al. “Roll-to-roll printed resistive WORM memory on a flexible substrate.” Nanotechnology 23 (2012) 305204 (12pp). DOI: 10.1088/0957-4484/23/30/305204

Journal reference: Nanotechnology search and more info website

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User comments : 12

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Sonhouse
not rated yet Jul 31, 2012
26 bits? A bit low for applications. Even a thousand times more would be a bit light, great for 1970 though.
cdt
not rated yet Aug 01, 2012
26 bits is all you'd need in an election for 26 positions with 2 candidates running for each position.
antialias_physorg
not rated yet Aug 01, 2012
In this way, the technique allows for selectively and irreversibly modifying the resistance of the bits...

Maybe I'm missing something here, but aren't they just saying that once a bit is heated (turned into the "1" state) it is no longer chnageable. But the "0" bits still remain changeable after the fact by reheating them.

So you could well turn a 1000 into a 1111 at any time later on (so if I were a candidate who was out to do some large scale vote mannipulation of this "write once" technology I'd just make sure that my code was all 1's)
Deathclock
1 / 5 (1) Aug 01, 2012
So... Write-once Read-many (WORM) is just a new hip way to say ROM?
PPihkala
not rated yet Aug 01, 2012
So... Write-once Read-many (WORM) is just a new hip way to say ROM?

No it is not. In WORM you can alter the memory after manufacture, but in ROM you manufacture the memory to contain the information. Variations of ROM exist of course, like UV-EPROM, which is Ultra Violet light Erasable Programmable Read Only Memory. Or EEPROM, which is Electrically Erasable PROM.
Deathclock
1 / 5 (1) Aug 01, 2012
The only difference is ROM is program once at manufacture, and WORM is program once after manufacture... Maybe I'm not being creative but I don't see a practical difference, in either case you cannot use this as general purpose memory, only as permanent storage.

I guess the analogy would be CD-ROM versus CD-R... the distinction here is meaningful but only because the capacity is significant... the article is talking about tens of bits, which consumers have no use for.
antialias_physorg
not rated yet Aug 01, 2012
Maybe I'm not being creative but I don't see a practical difference,

Think about using one to sign a cast ballot. It would be nice to sign it AFTER the ballot has cast - not at the time of manufacture (unless you're into major vote manipulation).
Eikka
not rated yet Aug 02, 2012
the article is talking about tens of bits, which consumers have no use for.


You do realize that things like barcodes, hotel room keycards, the magnetic stripe on a credit card, a bus fare card etc. are all applications that make use of just a handful of bits for the "payload"?

Deathclock
1 / 5 (1) Aug 02, 2012
the article is talking about tens of bits, which consumers have no use for.


You do realize that things like barcodes, hotel room keycards, the magnetic stripe on a credit card, a bus fare card etc. are all applications that make use of just a handful of bits for the "payload"?



You do realize that consumers do not program those devices, they are programmed once when they are created...
Eikka
not rated yet Aug 03, 2012
You do realize that consumers do not program those devices, they are programmed once when they are created...


Not necessarily.

For example, a bus ticket, instead of having a microchip and all that jazz, would simply have a strip of memory that works like a punch card. Once all the bits are flipped, no more rides. Or the hotel room keycard, which is programmed at the reception. It would be far cheaper to have those things printed on cheap cardstock instead.

The definition of "consumer" can vary as well. For example, credit cards are made from blank stock, and the consumer is the bank who presses the names and numbers on the card and magnetizes the strip on the back.
antialias_physorg
not rated yet Aug 03, 2012
For example, a bus ticket,

That's a pretty good idea right there. Some cities still use punchcards or mechanisms that print something onto your card. Both methods will sooner or later require maintenance or refiling of the ink used. A WORM type memory strip on your card would only require a brief, no-contact exposure to heat to 'punch' it - giving you potentially indefinite lifetime of the machine.
Sonhouse
not rated yet Aug 03, 2012
In this way, the technique allows for selectively and irreversibly modifying the resistance of the bits...

Maybe I'm missing something here, but aren't they just saying that once a bit is heated (turned into the "1" state) it is no longer chnageable. But the "0" bits still remain changeable after the fact by reheating them.

So you could well turn a 1000 into a 1111 at any time later on (so if I were a candidate who was out to do some large scale vote mannipulation of this "write once" technology I'd just make sure that my code was all 1's)

Or just provide as many bits as needed for a specific purpose and no more....