An international research group has given new life to charge pumping, a former mainstay of semiconductor testing that had been nearly abandoned when its sensitivity failed to keep pace with ever-shrinking chips.
A handful of single-atom defects in a semiconductor chip—a few holes in the lattice, dangling bonds, an single foreign atom impurity, or even an extra interstitial silicon atom shoe-horned in to disrupt the structure—can degrade the chip’s performance. When they occur at the junction of a transistor gate and the dielectric layer, these “interface traps” badly distort the device’s response as applied electrons fill and drain from these backwater pools.
Finding such defects in metal-oxide field effect transistors quickly and inexpensively is essential to maintaining quality.
In traditional charge pumping, testers apply a square-wave input signal to the transistor gate, cycling the voltage to change the gap bias and generate a signal that can be measured on the far side of the substrate (which is also connected to the source and drain).
In a normal transistor, the output signal echoes the input in a consistent way. In a defective chip, though, the electrons eddying in the interface traps degrade the response. A single input signal won’t find all of the defects, though: Different traps fill and empty at different rates, so the engineers repeat the test with a number of different input signal frequencies.
The test is further complicated by current leaking through the dielectric layer, which adds a leakage current to the charge pumping signal. This was a negligible problem at first; leakage through the relatively thick dielectric layers of earlier transistors was relatively small. But chips got smaller, and so did the charge pumping signal; but the leakage current grew. And about a decade ago, the usable signal vanished almost completely amid the leakage noise. Charge pumping ceased to be a reliable technique for evaluating the most advanced devices.
In IEEE Transactions on Electron Devices, researchers at NIST in Gaithersburg, Md.—with colleagues from Peking University in Beijing, IBM Research in Albany, N.Y., and TSMC in Hsinchu, Taiwan—report on a revision of the technique that both revives charge pumping’s utility and simplifies the measurements.
Traditional charge-pumping applied a square wave input at a single frequency and measured a direct current output. Individual measurements at multiple frequencies were needed, and output data had to be manipulated off-line to subtract out the leakage signal.
NIST researchers Jason T. Ryan and Kin P. Cheung (the corresponding author) and their collaborators have developed an elegant solution that both cancels the leakage current error and generates results directly, without manipulation. They have replaced the series of single-frequency square waves with a frequency modulated input signal that combines two alternating test frequencies in a single input. If the higher first input signal is f1 and the second is f2, the output frequency, f3, is the difference between them (f3=f1-f2, the familiar “beat” of musical notes that are not quite in tune). The combination cancels the leakage current, so that the combined AC output current reflects the charge pumping alone.
In the process, the researchers also found a systematic error in the traditional test. The classical method assumed that leakage current did not vary with the input frequency, while the charge pumping output did. The earlier chip-testers plotted total output signal against the multiple input frequencies, extrapolated the curve to zero frequency, took that value to be the leakage current, and subtracted it from their other results to find the true charge pumping output.
Ryan, Huang, and their collaborators found that while it is technically true that leakage current is independent of the input frequency of an ideal square wave, in the real world, the leakage current does, in fact, change with frequency. Because square waves are, of course, not absolutely square, there is a finite time during which the voltage is rising, and another during which it is falling. These intervals do slightly alter the amount of current flowing through the dielectric.
This article was edited 21 June 2015 to include NIST’s Kin P. Cheung as corresponding author, rather than the last author, Peking University’s Ru Huang.