The ultimate in miniaturization is the atom — there are 10 million billion of them in a single grain of salt.
The scientist Richard Feynman suggested several decades ago that it would be possible to use single atoms to store bits of data. Researchers from the University of Wisconsin at Madison have taken a large step toward making the idea a reality with a prototype that uses single silicon atoms to represent the 1s and 0s of computing.
Practical atomic-scale memory would increase the amount of information that could be stored per square inch of recording material by several thousand times.
The researchers realized they had hit upon a mechanism for atomic memory when they discovered that scattering gold atoms on a silicon wafer caused the silicon atoms to assemble into tracks exactly five atoms wide. The pattern resembled the microstructure of a CD.
Making the tracks turned out to be relatively easy. “We can actually get atoms to assemble themselves… precisely, without any type of lithography,” said Himpsel. “It is actually quite simple, and my graduate students make the surfaces routinely now,” he said.
The breakthrough that made the prototype possible was working out a practical way to write data into the memory, Himpsel said. “In general, it is difficult to work with an individual atom in a controlled way, without affecting neighboring atoms,” he said.
The researchers initially tried to write information to the memory by moving atoms along the tracks. “That works well at very low temperatures with loosely-bound atoms, but not at room temperature where we wanted the memory to operate,” said Himpsel. Eventually, “instead of moving them, we [picked] up the atoms,” using a scanning tunneling microscope, he said.
The researchers found that the optimum spacing for each bit is a four-atom section of track. This makes the bit spacing 1.5 nm along the tracks and 1.7 nanometers between tracks, which amounts to a data storage density of 250 trillion bits per square inch. This is equivalent to storing the contents of 7,800 DVDs in one square inch of material. A nanometer is one millionth of a millimeter.
The researchers formatted the memory by depositing extra silicon atoms onto the tracks to make every bit a 1, then wrote data by removing atoms to represent 0s.
To read the data, they scanned along the tracks looking for the presence or absence of the extra atoms.
The approach bridges two broad camps of work on data storage, said Himpsel. “One group of researchers manipulates atoms with great precision at very low temperature,” he said. The other group improves existing memory devices, which use thousands of atoms per bit. “We attempt to bridge the gap between the two camps by sticking to the atomic density limit, but… obtaining realistic numbers for the ultimate performance limits of a memory, such as speed, error rate, and stability” at room temperature, he said.
The researcher’s prototype resembles the way nature stores data in DNA, said Himpsel. The memory structure self-assembles into the tracks. In addition, “the density and readout speed of DNA [is] quite similar to our silicon memory,” he said. While DNA uses 32 atoms to store one bit using one of four base molecules, the researcher’s silicon memory uses 20 atoms including the atoms between the individual atoms that store the bits, said Himpsel.
The prototype is clever, said Phillip First, an associate professor of physics at the Georgia Institute of technology.