Abstract

Thermal wave modulated optical reflectance is now the basis for a number of industrial optical inspection functions (1). We recently demonstrated that this method is capable of imaging crystallographic defects, such as dislocations and stacking faults, beneath the polished surface of silicon wafers (2). In this technique two lasers are employed in a pump-probe configuration. The pump beam, from an Ar⁺ laser, is acousto-optically modulated and focused to a 1-micron spot on the surface of the Si wafer. Absorption of these 2.5 eV photons perturbs the local electron-hole (or plasma) concentration and the temperature, both of which alter the optical reflectance (R) of the Si surface. A second, HeNe laser is employed to phase-synchronously probe the magnitude of the change in reflectance (ΔR), thus yielding the modulated optical reflectance (ΔR/R). Since subsurface damage or defects alter the plasma diffusion through recombination effects, the defects or damage can be detected with modulated optical reflectance measurements. We shall briefly review the metrology accomplishments of this new field of semiconductor optical inspection. We then describe modulated reflectance imaging of Si samples to illustrate detection of subsurface (a) stacking faults, (b) dislocations, (c) stress-induced slip, and (d) post-anneal implant damage.

© 1988 Optical Society of America