Flash is designed to last a decade or more of use. A lot of the gadgets that rely on it are not. Shady recyclers have spotted opportunity in that mismatch, stripping out used chips and selling them as new. But engineers at the University of Alabama have come up with a straightforward electronic examination that can tell if a flash chip is new or recycled, even if that chip has only seen 5 percent or less of its life. And the technique is so straightforward that a smartphone app could run it on its own memory.
“Most of the time, recycled memory is thrown into the market when it is functional,” explains Biswajit Ray, assistant professor of electrical and computer engineering at the University of Alabama in Huntsville. “And it will pass most tests as a good chip even though it is recycled.” Until now, there was no method that could tell a working recycled chip with most of its life left ahead of it from a brand new one.
A flash memory cell is like an ordinary transistor; it has a source and a drain and a channel through which current flows under the control of voltage on the gate electrode. The difference is that the gate is split into several layers—the control gate, the blocking oxide, the floating gate, and the tunneling oxide. Voltage on the control gate causes electrons to tunnel through that bottom oxide and get stuck inside the floating gate. This charge or its absence is the stored bit. It alters how much voltage you need to turn the transistor on in a way that you can easily measure. Erasing the bit is done by reversing the voltage and driving the charge out of the floating gate.
Ray and his team took advantage of the rather high voltages—about plus or minus 20 volts—needed to program and erase flash. The more you program and erase a cell, the more defects will accumulate in the oxide, he explains. These defects lead to an increase in the amount of current that leaks through the transistor when it’s supposed to be off, and it also reduces the rate at which charge moves through the device. These effects show up as a slowdown in the memory’s erase time. Ray and his colleagues Preeti Kumari, Bahar Talukder, Sadman Sakib, and Tauhidur Rahman examined other metrics, but “we found that slower erase is the best metric to get [a chip's] age.”
In research reported this week at the IEEE International Symposium on Hardware Oriented Security and Trust, in Washington, D.C., the Alabama engineers’ erase-time technique was able to identify recycled flash with as little as 3 percent usage with 100 percent confidence.
But Ray thinks they can do better. In research not yet published, they use a different metric called program disturb. It turns out that if you “hammer” a page (about 16 kilobytes of memory) by programming it over and over again, a certain number of bits will flip. Data that should be zeros will be ones, and vice versa. Older flash, even if it’s only gone through 1 percent of its lifetime, will reliably have more of these flipped bits than new flash will, his team discovered.
The erase-time method could fairly easily allow an app on a smartphone to examine its own storage for recycled parts, says Ray. The page-disturb method, which relies on less-standard methods, will “take some work,” he says.