On the Limitations of the Confocal Scanning Optical Microscope as a Profilometer

We present a numerical study of the performance of a one-dimensional Confocal Scanning Optical Microscope (CSOM) when used as a profiler of highly reflecting surfaces. The study is conducted by means of a computer simulation that describes the interaction between the incident beam and the sample in a fairly rigorous manner. The degradation of the depth discrimination as a function of the local surface slope is illustrated with examples, and a criterion for the maximum allowable slope is proposed. We also show that the instrument has difficulties in profiling surfaces that have steps or sharp edges and it is argued that, in general, problems are encountered when the surface profile cannot be considered locally flat in the scale of the Point Spread Function (PSF) of the optical system. This implies that the lateral resolution of the instrument is far worse as a profilometer than when used as an imaging system, and that the unresolved features introduce spurious details on the estimated profile.