Blood-Cell-Sized Memory Circuit Created

Researchers have created an ultradense memory device the size of a white blood cell that has enough capacity to store the Declaration of Independence and still have space left over. The accomplishment represents an important step toward the creation of molecular computers that are much smaller and could be more powerful than today's silicon-based computers.

The bluish-grey area in the center of the photo contains the 160,000-bit memory array developed by researchers at the California Institute of Technology and the University of California, Los Angeles. The greenish circles nearby are white blood cells. (Photo: Jonathan Green, John Nagarah and Habib Ahmad, California Institute of Technology)

"It's the sort of device that Intel would contemplate making in the year 2020," said James R. Heath, the Elizabeth W. Gilloon Professor of Chemistry at the California Institute of Technology in Pasadena and co-author of a paper on the research. "But at the moment it furthers our goal of learning how to manufacture functional electronic circuitry at molecular dimensions."

The 2020 date assumes the validity of Moore's law, which states that the complexity of an integrated circuit will typically double every year. Current memory cell size is .0408 square µm, so Moore's law assumes that the electronics industry will achieve a device density comparable to the Heath team's memory circuit in about 13 years. Manufacturers currently see no way to extend the miniaturization beyond the year 2013, according to reports.

Heath's group manufactured the memory circuit in a cleanroom facility in their labs at Caltech, and the molecular switches were prepared by J. Fraser Stoddart, the University of California, Los Angeles' Fred Kavli Chair in Nanosystems Sciences, and his group. Stoddart and Heath are pioneers in molecular electronics -- using nanoscale molecules as key components in computers and other electronic devices.

"Using molecular components for memory or computation or to replace other electronic components holds tremendous promise," said Stoddart, who also directs the California NanoSystems Institute.

The circuit has a bit density of 100 Gb per square centimeter, which Heath's fellow lead author Jonathan Green said sets the record for integration density in a man-made object.

"We showed we can increase the density to nearly 1000 gigabits per square centimeter, but, beyond that, there is almost no point, because you begin to run out of molecules," said Green, a Caltech graduate student in chemistry and applied physics.

A bit, or binary digit, is the basic unit of information storage and communication in digital computing. A kilobit is equal to 1000 bits and is commonly used for measuring the amount of data that is transferred in one second between two telecommunication points.

The 160,000 memory bits on the memory device are arranged like a large tic-tac-toe board: 400 silicon wires crossed by 400 titanium wires, each 16 nm wide, with a layer of dumbbell-shaped molecular switches -- called [2]rotaxanes-- sandwiched between the crossing wires. Each wire crossing defines a bit, and a single bit is only 15 nm wide, or about one ten-thousandth the diameter of a human hair. By contrast, the most dense memory devices currently available are approximately 140 nm in width.

"This research is the best example -- indeed one of the only examples -- of building large molecular memory in a chip at an extremely high density, testing it and working in an architecture that is practical, where it is obvious how information can be written and read," Stoddart said

"Our goal was not to demonstrate a robust technology; the memory circuit we have reported on is hardly that," said Heath. "Instead, our goal was to demonstrate that large-scale, working electronic circuits could be constructed at a density that is well-beyond (10-15 years) where many of the most optimistic projections say is possible."

Molecular switches (right) called [2]rotaxanes are made of two interlocking components -- a molecular ring encircling a dumbbell-shaped molecule. When the switch is triggered, the ring slides between two locations on the dumbbell to control conductivity. Designed in the UCLA laboratory of J. Fraser Stoddart, the switches store information in an ultra-dense 160-kilobit memory made up of a 400 x 400 grid of nanowires (left). (Image: J. Fraser Stoddart Supramolecular Chemistry Group, UCLA)

The capability to manufacture electronic circuitry at such extreme dimensions opens up a host of new applications, ranging from extremely sensitive chemical and biological sensors, energy-efficient logic circuits, and a class of high-performance energy-conversion materials known as thermoelectrics.

"We have shown that if a wire is broken or misaligned, the unaffected bits still function effectively; thus, this architecture is a great example of 'defect tolerance,' which is a fundamental issue in both nanoscience and in solving problems of the semiconductor industry. This research is the culmination of a long-standing dream that these bistable rotaxane molecules could be used for information storage," said Stoddart.

A variety of molecular electronic components have been demonstrated, the researchers said. For example, logic gates, memory circuits, sensors and many other fundamental components have been reported.

"However, few of these components have been demonstrated to work in practical, highly dense device arrays before," Stoddart said.

"One of the most exciting features of this research is that it moves beyond the testing of molecular electronic components in individual, non-scalable device formats and demonstrates a large, integrated array of working molecular devices," said William R. Dichtel, a researcher who is a member of both Stoddart's and Heath's research teams. "In targeting a large memory array, many fundamental issues of how information is stored and retrieved had to be addressed."

"Whether it's actually possible to get this new memory circuit into a laptop, I don't know," said Heath. "But we have time."

"In 1959, physicist Richard Feynman said it should be possible some day to store all of the Encyclopedia Britannica on the tip of a needle," Stoddart said. "We're not there yet, but we're not far off."

The other lead author of the paper is Jang Wook Choi, a graduate student in chemical engineering at Caltech. The other authors are Akram Boukai, Yuri Bunimovich, Ezekiel Johnston-Halperin, Erica DeIonno, Yi Luo, Bonnie Sheriff, Ke Xu, and Young Shik Shin, all graduate students in Caltech's Division of Chemistry and Chemical Engineering; and Hsian-Rong Tseng and Stoddart, both of UCLA.

The research was funded primarily by the National Science Foundation and DARPA and is reported in the Jan. 25 issue of the journal Nature.

For more information, visit: www.caltech.edu or www.ucla.edu