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Cheap, Plastic Memory for Flexible Devices

A new type of flash could be used in e-readers.

Cheap and plastic aren’t words often associated with cutting-edge technology. But researchers in Tokyo have created a new kind of plastic low-cost flash memory that could find its way into novel flexible electronics.

Flexible flash: This plastic sheet is arrayed with 676 flash memory cells.

Flash memory stores data electrically, in specially designed silicon transistors. Information can be recorded and read quickly and is retained even when the power is off. This makes flash ideal for MP3 players, cameras, memory cards, and USB drives. But the technology is still more expensive than conventional hard disks.

The prototype plastic flash memory cannot match silicon’s storage density, long-term stability, or number of rewrite cycles. But its low cost could make it possible to integrate flash memory into more unconventional electronics. For example, cheap plastic memory devices might be incorporated into e-paper or disposable sensor tags.

“Organic materials offer the capability to significantly lower the price of memory,” because they can be processed much more cheaply than silicon, says Yang Yang, professor of materials science and engineering at the University of California, Los Angeles, who was not involved with the work. The demonstration of plastic flash “is a very important milestone in organic memory,” says Yang.

The plastic memory was made by a team of researchers at the University of Tokyo led by electrical engineering professor Takao Someya. The key to making the plastic memory device work, says Someya, is a hybrid insulating layer made of a polymer and a metal oxide. This layer electrically isolates the metal gate in which charges are stored. An applied voltage causes the metal gates to accumulate charge–charged and uncharged gates represents binary 1s and 0s, as in silicon flash. The better the insulator works, the longer the data can be stored before the electrons leak away and the data degrades.

Someya’s group starts by placing metal transistor gates on top of a plastic substrate. Then a thin layer of aluminum oxide is deposited on top and the plastic film is submerged in a solution containing an insulating polymer. The polymer finally self-assembles on the surface of the aluminum oxide. The plastic devices can endure 1,000 writing and reading cycles. In contrast, silicon flash can be written to about 100,000 times.

To demonstrate the memory, Someya’s group integrated a 676-memory-cell device with a rubber pressure sensor. The flexible sensor-memory device, which is less than 700 micrometers thick, can record pressure patterns and retain them for up to a day. The plastic device is described today in the journal Science.

Sensory memory: Plastic flash memory cells were combined with a rubber pressure sensor. The impressions made by a roll of tape and by fingers (left) are retained entirely after 20 minutes (middle). After 12 hours (right), the data begins to degrade.

“The attractive feature of organics would be the cost,” says Victor Zhirnov, a program manager at the Semiconductor Research Corporation, a consortium of United States chip manufacturers. “But organics don’t operate as well as silicon,” he points out.

Zhirnov believes other new memory technologies, such as phase-change memory, may have more potential. Phase-change memory, which is being developed by companies including Samsung and Intel, uses heat to flip glassy units between an electrically insulating crystalline state and a conductive amorphous state. The technology offers about 100 million read-write cycles and greater overall stability.

Ethan Miller, professor of computer science at the University of California, Santa Cruz, says that plastic memory might be incorporated into e-paper. “Suppose you have a sheet with memory and a pressure sensor underneath it–you could write something and store the data, without a scanner,” he says.

Someya believes the performance of plastic flash can be improved further. In the meantime, says Yang, “there are things silicon won’t touch–lower-end applications where you want disposable memory.” Cheap organic memory devices could fill this niche. They could be used to record temperature or environmental pollution and be incorporated into pharmaceutical and food packaging for tracking purposes, he says.

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