Dense Memory Circuit Sized Way Down
Don’t throw away your laptop yet, but there’s a promising new high-tech invention being announced this week. Researchers have created a memory circuit the size of a white blood cell that has enough capacity to store the Declaration of Independence and have space left over. With 160 kilobits of capacity, it’s the densest memory circuit ever fabricated. Announcing the achievement in the January 25 issue of the journal Nature, the team led by chemistry professor James Heath of the California Institute of Technology says that the memory circuit is a milestone in manufacturing, even if it’s not anywhere near readiness for the market. “It’s the sort of device that Intel would contemplate making in the year 2020,” says Heath, who is the Gilloon Professor at Caltech. “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 micrometers, 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. However, the Caltech-UCLA team points out in their Nature article that manufacturers can see no clear way at present of extending this miniaturization beyond the year 2013. The new approach of the Heath team, therefore, will show the potential for making integrated circuits at smaller and smaller dimensions. “Whether it’s actually possible to get this new memory circuit into a laptop, I don’t know,” says Heath. “But we have time.” The 160,000 memory bits are arranged like a large tic-tac-toe board: 400 silicon wires crossed by 400 titanium wires, with a layer of molecular switches sandwiched between the crossing wires. Each wire crossing defines a bit, and a single bit is only 15 nanometers wide, or about one ten-thousandth the diameter of a human hair. By contrast, the most dense memory devices currently available are approximately 140 nanometers in width. The molecular switches, called rotaxanes, comprise two interlocking components — a molecular ring encircling a dumbbell-shaped molecule — that together are similar to a wedding band on a finger. When the molecular switch is electronically triggered, the ring slides between two locations on the dumbbell. Switching, then, arises from the different conductivities of the molecular switch with respect to the ring position. Heath’s group manufactured the memory circuit in a clean-room facility in their labs at Caltech, and the molecular switches were prepared by J. Fraser Stoddart, who holds UCLA’s Fred Kavli Chair in Nanosystems Sciences, and his group. The circuit has a bit density of 100 gigabit per square centimeter, which Heath’s fellow lead author Jonathan Green says sets the record for integration density in a man-made object. “We showed we can increase the density to nearly 1,000 gigabits per square centimeter, but, beyond that, there is almost no point, because you begin to run out of molecules,” says Green, a Caltech graduate student in chemistry and applied physics. 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. 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.