Pixels and Speckles
WOLFGANG was sitting at the console of the Scanning Tunneling Microscope when George walked in. “Sorry I’m late,” he said, “the trigger group meeting ran way overtime.” George felt guilty about being late. Wolfgang must have been here for almost an hour and had the STM in full operation. The display showed what looked like the gate region of a failed field effect transistor.
George walked forward and peered through the thick glass window of the instrument’s sample holder. The silicon slab they had brought was clipped to a complicated positioning mechanism. The interior was close packed with unfamiliar shapes, but George imagined that he could make out the needle-point probe of the STM as it vibrated a few atomic diameters above the surface of the silicon, measuring its electrical conductivity on an atom-by-atom scale. The display screen showed a colorful contour map depicting a lumpy mountain-like terrain. “How’s it going, Wolfgang?” he asked.
“Schlecht,” said the other man. “It’s clear that your FET gate pinched off destructively for some reason, but its geometry is not significantly different from its neighbors, and also no different from the ones we used in the ATLAS detector.” He gestured at the colorful display.
George looked at the structures on the screen. The p-type and n-type regions of the silicon differed in elevation by a few atomic layers, enough to show up clearly on the STM. The gate region, which should have had an hour-glass shape, was instead two separated clumps with no connection between them. Around the region where the connection channel should have been, George noticed several clusters of white dots. “What are these,” he asked.
“Those speckles?” said Wolfgang. “I don’t know. There seem to be clusters of them near the gate region. Perhaps they are small particles made during the gate failure, debris from the catastrophe. There must have been some energy dissipated when the semiconductors decided to rearrange themselves.”
“Perhaps so,” said George. “Can we look at some gate that didn’t fail?”
“Of course,” said Wolfgang, typing into the keyboard of the STM control computer. The scene on the display began to shift to the right as the piezoelectric needle-position mechanism was given a new bias voltage. The stratified terrain passed by until an hour-glass shape appeared on the screen.
George studied it closely. “I see a few of the clusters near the gate region here also. There are fewer of them, though, and they seem to be lined up along the gate channel, like spectators along a parade route,” he said. “Was there anything similar on the ATLAS chips?”
Wolfgang shrugged. “I don’t think so,” he said. “I think we would have noticed.”
“What’s the scale here, a few microns?” George asked.
Wolfgang typed a command, and a graduated scale appeared along the lower part of the display. “Ja, the clusters are perhaps a micron across,” he said.
George thought for a moment. He had an idea. “The superconductor metallurgy group has a scanning electron microprobe. It scans an energetic electron beam over a surface, and records the x-rays produced at each spot. It can label any feature of a scan picture with its chemical composition of elements. Maybe we can schedule some time on it to find out what the clusters are.”
Wolfgang pursed his lips and nodded. “Ja, good idea.” he said.
George took out his cellphone and called Jerry Walton, the superconductor metallurgy group leader. George reserved two hours of microprobe time in the next blank spot in the schedule, Friday, June 18th. It was frustrating to have to wait more than a week, but there were no openings sooner. Jerry promised to call George or Wolfgang if anyone canceled.
They spent the rest of the afternoon moving the STM probe around on the micro-landscape of the detector, finding other blown FETs and clusters of speckles but learning nothing more.
At the end of the day, after a quick meal in the cafeteria, George returned to his room at the SSC Hostel feeling depressed. He spent the evening watching second-rate movies on the room’s TV wallscreen.