The importance of minimizing stress-induced voiding for reliable VLSI metalization is now well appreciated. For detection of subsurface voids in a nondestructive manner, thermal wave modulated reflectance (TW) imaging is increasingly utilized.
Although the TW response is vectorial (with components that are in-phaSfi. and 90 degrees out-of-phase with the laser modulation), heretofore only the magnitude of this signal was utilized in this application. For optimum imaging of deep voids, it is desirable to selectively display a signal that responds to both phase-delay and magnitude information simultaneously. The phase delay originates from the round-trip propagation time of the thermal wave from the surface to the void.
In this paper, we demonstrate the use of this technique to (a) improve the detection (i.e. signal-to-noise ratio) of subsurface voids (b) distinguish between near-surface artifacts and deep voids, and (c) demonstrate the general utility of this technique by exhibiting improved S/N in crystalline silicon defects.