Measurement of Diffused Semiconductor Surface Concentrations by Infrared Plasma Reflection

When infrared radiation is reflected from the surface of a degenerate semiconductor, a minimum is obtained in the reflection spectrum. This plasma minimum is related to the free carrier concentration, effective mass, and dielectric constant of the semiconductor. In a diffused semiconductor with high surface concentration, the absorption constant is high and consequently the reflected plasma minimum is directly related to the surface concentration of free carriers. By calibrating the plasma minima vs known impurity concentrations, it is possible to determine surface concentration in n- and p-type silicon ranging from 5 × 1018 to 5 × 1020 cm−3 using an infrared spectrometer with a wavelength span of 2.5 to 25 μ.