WE CAN now watch electricity as it flows through even the tiniest circuits. By scanning the magnetic field generated as electric currents flow through objects, physicists have managed to picture the progress of the currents. The technology will allow manufacturers to scan microchips for faults, as well as revealing microscopic defects in anything from aircraft to banknotes.
Gang Xiao and Ben Schrag at Brown University in Providence, Rhode Island, visualise the current by measuring subtle changes in the magnetic field of an object and converting the information into a colour picture showing the density of current at each point (see Diagram).
Their sensor is adapted from an existing piece of technology that is used to measure large magnetic fields in computer hard drives. 鈥淲e redesigned the magnetic sensor to make it capable of measuring very weak changes in magnetic fields,鈥� says Xiao.
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The resulting device is capable of detecting a current as weak as 10 microamperes, even when the wire is buried deep within a chip, and it shows up features as small as 40 nanometres across (Applied Physics Letters, vol 82, p 3272).
At present, engineers looking for defects in a chip have to peel off the layers and examine the circuits visually; this is one of the obstacles to making chips any smaller. But the new magnetic microscope is sensitive enough to look inside chips and reveal faults such as short circuits, nicks in the wires or electromigration 鈥� where a dense area of current picks up surrounding atoms and moves them along. 鈥淚t is like watching a river flow,鈥� explains Xiao.
As well as scanning tiny circuits, the microscope can be used to reveal the internal structure of any object capable of conducting electricity. For example, it could look directly at microscopic cracks in an aeroplane鈥檚 fuselage, faults in the metal strip of a forged banknote or bacteria in a water sample. The technique cannot yet pick up electrical activity in the human brain because the current there is too small, but Xiao doesn鈥檛 rule it out in the future. 鈥淚 can never say never,鈥� he says.
Although the researchers have only just made the technical details of the microscope public, it is already on sale, from electronics company Micro Magnetics in Fall River, Massachusetts. It is currently the size of a refrigerator and takes several minutes to scan a circuit, but Xiao and Schrag are working to shrink it to the size of a desktop computer and cut the scanning time to 30 seconds.