Testing the sensitivity of memory cells

Particle physics researchers go deep underground to escape the constant bombardment of cosmic radiation that creates background noise in their sensitive experiments. And what's good for particle physics, it turns out, is also good for programmable memory cells.

Xilinx is one of the world's leading providers of semi-conductor devices called field programmable gate arrays (FPGA). Based around a matrix of configurable logic blocks (CLBs) connected through programmable interconnects, FPGAs are designed and built using tens of millions of SRAM (static random access memory) cells, which can be sensitive to single event upsets (SEU). So, for the past year, Xilinx has been running tests on its FPGAs on the 4850 Level of Sanford Lab.

SEUs occur when the logic state of a SRAM memory cell is changed by ionized radiation. "When a neutron, proton, or alpha particle hits the silicon in the semiconductor, it leaves a trail of charged particles, which in some cases can cause a transistor of a SRAM cell to change its logic state," said John Latimer, senior director with Xilinx's Customer Quality Engineering division. "When that happens, it can potentially affect the design programmed into the FPGA, in rare cases causing it to function improperly."

FPGAs are used in such applications as 3-D video recording and movie projection, driver assistance, datacenters, cellular communication and networking, smart electricity grid management, avionics instrumentation, satellite instruments, and space vehicles like the Mars Rover. It's important that they function correctly.

What Xilinx is trying to understand is just what causes the single event upsets. "We need to separate out the cosmic radiation effects from the effects of the alpha particle-producing package material of the FPGA," Latimer said. "Placing the arrays deep underground allows us to block the cosmic radiation and only measure SEU events that are caused by the package material itself."

Latimer said that so far, "the results have been excellent with the alpha upset rates right in line with our predictions. We are very happy to be able use the Sanford Lab facilities and look forward to working with the ne sta at Sanford for many years to come."

The company plans to install and begin testing additional arrays over the next couple of months.