Univ of Tokyo Develops 'Organic Flash Memory'

Dec 15, 2009
Tetsuo Nozawa, Nikkei Electronics
The polyethylene naphthalate (PEN) resin sheet with a memory array
The polyethylene naphthalate (PEN) resin sheet with a memory array
[Click to enlarge image]
The memory array. The white parts are control gates, and the watery blue parts under the three electrodes are pentacene.
The memory array. The white parts are control gates, and the watery blue parts under the three electrodes are pentacene.
[Click to enlarge image]
The structure of the intelligent pressure sensor. From the bottom, they are an organic memory sheet, a pressure-sensitive rubber sheet and an electrode sheet.
The structure of the intelligent pressure sensor. From the bottom, they are an organic memory sheet, a pressure-sensitive rubber sheet and an electrode sheet.
[Click to enlarge image]

The University of Tokyo developed the "organic flash memory," a non-volatile memory that has the same basic structure as a flash memory and is made with organic materials.

The erasing and reading voltages of the new flash memory are as low as 6V and 1V, respectively. Data can be written in and erased from the memory more than 1,000 times.

With its flexibility, the flash memory can be used for large-area sensors, electronic paper and other large-area electronic devices if its memory retention time can be extended, the university said.

The organic flash memory was developed by a research group led by Takeo Someya and Tsuyoshi Sekitani, professor and research associate at the Department of Electrical Engineering and Information Systems, Graduate School of Engineering, the University of Tokyo. The research results were published on the 11th December 2009 issue of Science magazine.

The research group used a polyethylene naphthalate (PEN) resin sheet as a substrate and arranged 26 x 26 2T memory cells in an array structure on it. The sheet is flexible, and the university confirmed that it can be bent until its curvature radius reaches 6mm without causing mechanical or electrical degradation. The university also confirmed that the sheet can be used as a non-volatile memory.

Furthermore, the research group made an "intelligent pressure sensor," which can retain a pressure image in the sheet, by integrating the memory array and a pressure sensor.

The new memory is called "organic flash memory" because it is equipped with floating-gate transistors, which are also used for silicon-based flash memories. Specifically, the PEN substrate is mounted with aluminum (Al) control gate electrodes, insulating films, aluminum floating gates, organic semiconductor pentacene, and source and gate electrodes made of gold (Au).

The insulating film was made by using a self-assembled monolayer (SAM), which is made of a kind of phosphoric acid having an alkyl chain (CH2-CH2-CH2-...), in addition to AlOx. The SAM is as thin as 2nm.

There are non-volatile memories developed in the past. One is made by using ferroelectric materials, and the other has a floating-gate structure like the latest non-volatile memory. However, it is difficult to lower the writing and erasing voltages of the former memory than 20V. The latter memory also has a high erasing voltage. And it becomes unstable when exposed to air because its memory properties fluctuate due to the nonconstant thickness of the insulating film.

This time, the university researchers employed an SAM whose insulating film does not require thickness control to reduce the variation in memory properties. Moreover, this SAM is stable in the atmosphere.

As a disadvantage, the new organic flash memory has a memory retention time of only one day. But this can be drastically improved by reducing the size of the element and employing an SAM with a long molecular length, the university said.