Semiconductor designers today are attempting to design atomically accurate materials using the scanning tunneling microscope (STM) to image individual atoms. Unfortunately, only still images could be made. Now IBM has reinvented STM to work like pulsed lasers, permitting measurements to be made on a sub-nanosecond time scale, resulting in videolike movies of atoms made at rates of billions of frames per second .
IBM invented the STM in the 1980s to create topographical maps of the locations of individual atoms on semiconductor surfaces. Resembling a raised-relief map of Earth, the STM is now used in every semiconductor lab worldwide. Upping the ante, IBM has now reinvented the pulsed-STM to make measurements a million times faster.
“Ordinarily, an STM measurement is a very slow process because it is measuring a very small current,” said Andreas Heinrich, a physicist in IBM’s Almaden Lab. “Our main breakthrough was to turn this situation around, continuing to measure current in the same way, but getting our time dependence from the pump-probe technique.”
STMs work by applying a voltage to a tiny tip held a few nanometers above the material being characterized. The voltage induces “quantum tunneling,” whereby electrons teleport across the gap—disappearing from the tip and reappearing on the material without traversing the space in between. The new pulsed-STM improves on this technique by preceding measurements with a pump pulse followed by a probe pulse to read out the results.
Scanning tunneling microscope topograph of an iron atom (large yellow) on a nitride-covered substrate (blue), which someday may enable single-atom bit cells for memory chips.
“The pump pulse is basically the hammer that hits the bell and drives it into an excited ‘ringing’ state, and then the probe pulse is like a finger touching the bell to see if it is still ringing or not,” said Heinrich.
In its demonstration, the pump signal set an iron atom’s electrons into the “down-spin,” or “1,” state, and then, by virtue of quantum tunneling of the magnetization, relaxes back into the “up-spin,” or “0,” state without passing through the intermediate angles between up and down. By performing the test over and over, each time with a slightly longer interval between pump and probe pulse, IBM was able to determine the “refresh” time of future single-atom memory chips.
Compared with traditional dynamic random access memory (DRAM) memory chips, whose data values must be refreshed every 50 milliseconds or so, IBM’s characterization of single atoms of iron used as the bit cells in future DRAMs would require refreshing much more quickly, about every 50 to 250 nanoseconds.
Scanning tunneling microscopes create raised-relief maps of the surface on a semiconductor chip.
Besides characterizing possible atomic memory chip materials, IBM hopes that solar cell researchers will use its pulsed-STM method to improve their formulations—using a flash of light as the pump pulse to “ring the bell,” followed by a normal electronic probe pulse to measure the photovoltaic material’s efficiency.
Scanning tunneling microscope topograph of a test chip’s surface (green) with individual atoms (bumps) placed within cut-outs (blue) for characterization.
“What we need to do is control the amount of quantum mechanics,” said Heinrich. “I see myself as being at a crossroads. One way is to have less quantum mechanics, which will make our single-atom bit cells more stable. But the other way is to have more quantum mechanics. Then we can start to perform quantum computations on the atomic scale too.”(smartertechnology)