Want faster flash? Sprinkle in a little carbon. Researchers at Cornell University, in Ithaca, N.Y., suggest that a thin layer of buckminsterfullerene (C 60 ), a.k.a. buckyballs, embedded in an ordinary flash-memory cell, can increase how long the memory holds a bit, boost the speed at which a bit is written or erased, and decrease the memory's drain on a battery.
Like ordinary transistors, flash-memory cells are made up of a source, a drain, and a gate, whose voltage controls the flow of current between the two. The gate is separated from the rest of the transistor by a thin layer of insulation, the gate dielectric. The difference in flash is that the cell contains an additional, ”floating” gate embedded within the dielectric. Put enough voltage on the main gate and electrons will jump the dielectric barrier and get stuck inside the floating gate. Reverse the polarity of the voltage and the charges will jump back out. The stuck charge, or its absence, is the stored bit.
Tuo-Hung Hou, of the Cornell group, likens the floating gate to a tiny island inside the chip. To change the bit's value, the water level surrounding the island--the barrier that the gate dielectric presents to an electron trying to jump from the transistor channel into the floating gate--must be reduced so that the floating gate can be easily accessed.
Having followed Moore's Law for years, flash memory is bumping up against the physical limits of speed, size, and power consumption. To make flash chips that are faster and consume less power during the writing process, engineers must find a way to get electrons into and out of the floating gate using less voltage. One way is to make the insulation thinner so that it takes less voltage to induce an electron to jump across the insulation and less time for electrons to fill the floating gate.
Today the insulator is just 7 to 8 nanometers thick. But, says Sanjay Banerjee, director of the University of Texas at Austin's Microelectronics Research Center, if it gets any thinner, using the same insulation technology, ”that insulator becomes so thin that it inevitably has some defects, which causes the charge [holding the bit]...to leak away.”
Hou and his colleagues' solution was to build a layer of buckyballs into the dielectric between the floating gate and the transistor channel. ”The buckyballs are like putting stepping stones in the middle of the river,” Hou says. ”Then electrons can jump onto the stepping stones and then jump to the other side.”
Without any applied voltage, the dielectric barrier is still too big for electrons to cross. But introducing just a few volts reduces the barrier enough to make the buckyball stepping stones accessible, allowing electrons to move into and out of the floating gate more quickly and easily. Researchers had earlier made the stepping stones using silicon nanocrystals, but their effectiveness depends on the nanocrystals' size, which is difficult to keep consistent. Buckyballs, which act as a semiconductor in this case, are always the same size.
The Cornell group says that the presence of C 60 can cut the voltage needed to operate flash-memory chips from today's 10- to 13-volt range down to 5 V. Less voltage required for every bit being written or erased means substantially less drain on the battery, says Banerjee, who researches flash memory and consults for leading flash manufacturers. The improved writing speed could open up new kinds of applications that previously used faster memories, such as DRAM.
Banerjee thinks that adding buckyballs would add to the cost of manufacturing flash chips. ”But if it works the way we hope it does, then it should improve the performance so much that I think it'd be worth the cost.”