Recombination lifetime and minority-carrier lifetime are key performance parameters of semiconductor wafers used for microelectronics and solar cells. They are also particularly valuable for identifying low levels of impurities in wafers used for high-density integrated circuits. NREL has developed a contactless technique for measuring these properties that is superior to any previously available technique.
The apparatus uses a precision-tuned sensor system to detect eddy currents induced by light. For short-lifetime semiconductors such as metallurgical-grade silicon used in some solar cells, the system uses a pulsed laser. But for high-quality semiconductors such as epitaxial thin films or the silicon used for integrated circuits, the new induced-eddy-current-sensor system uses inexpensive conventional light sources. Unlike microwave reflection, eddy-current measurement is highly sensitive and highly linear over a wide range of injection levels and of sample size and conductivity. (Sensitivity at high as well as low injection levels makes the new NREL technology far better at detecting low-level impurities.) Photoluminescence, the only other contactless measurement, is not usable for silicon or other indirect-bandgap semiconductors.
NREL's improved technique for contactless measurement of recombination or minority-carrier lifetimes works for any semiconductor material. The apparatus measures the decay of photo-induced excess carriers by sensing induced eddy currents. The breakthrough from prior attempts to use eddy-current sensing is the addition of a precision tuning system that allows measurement of samples over a wide range of size and conductivity. The dynamic range of the apparatus has been demonstrated for measurement of samples ranging in size from 1 mm-×-1-mm-×-1-micron-thick films, to ingots of crystalline silicon approximately 6 in. in length by 4 in. in diameter, but will probably work well for much larger samples as well. The apparatus maintains very high sensitivity and high linearity for the wide range of samples described here, something that prior attempts to use either eddy-current sensing or microwave reflection the current commercial option were not capable of.
The new technique also allows the measurement of recombination lifetime over a wide range of injection levels. With this capacity for lifetime measurement at both high- and low-injection levels, the NREL eddy-current sensor can be used to identify low levels of impurities in high-purity silicon a capability that should prove highly valuable for the microelectronics industry. Such wide dynamic range is inherently impossible in the competitive contactless technology, which uses microwave reflection. The latter measurement is restricted to small signal/low-injection measurements because of intrinsic nonlinearity in the detection process.
NREL's induced eddy-current sensor has effectively measured lifetimes in semiconductors such as thin-film polycrystalline cadmium sulfide and single-crystal silicon carbide that could not be measured by other contactless techniques. And the high sensitivity of the apparatus makes it unnecessary to use a laser as an excitation source for high-quality materials, so an inexpensive conventional-light-source version of the apparatus is more than adequate for the most common commercially used semiconductors. For example, for high-quality silicon grown by the float zone technique, standard in the integrated circuit industry, a small xenon flash tube is highly effective. And for thin-film epitaxially grown compound semiconductors, a small light-emitting diode works quite well.