Concentration Dependence of Thermal-Wave Contrast for Dopants in Silicon

Abstract

Thermal-wave images are displays of the amplitude or phase of ultrasonic waves created by a damped thermal wave produced by scanning a chopped electron or light beam over the surface of a solid sample. It has been shown that subsurface doped regions in silicon can be imaged by thermal-wave microscopy in the scanning-electron microscope (SEM). The dependence of feature contrast in the thermal-wave image upon dopant concentration is examined here experimentally with phosphorus as dopant, and theoretically for boron. It is found that for doping levels below about 10’8cm-3 characterization is difficult because the thermal conductivity of the doped region is a weak function of doping at room temperature. Calculations indicate that at 77K the lattice thermal conductivities for boron-doped silicon at concentrations of 1D’5and10’7cm-’ differ by only about 20%. Theoretical results and experimental findings obtained at room temperature and with a liquid-nitrogen- cooled SEM stage are presented.