Abstract

Subwavelength surface profiling demands interferometers with a high degree of stability, usually not available in interferometers with separated beam paths. It takes a ΔT~ 1.44°C between the two beam paths to generate random phase noise of ~2π when the beam’s separation is ~8 in. Elimination of such random noise is essential to achieving a subwavelength profiling capability. We have an interferometer capable of achieving this. The part of the beam that gets through the hole acts as the reference beam whereas the portion that is focused by the lens is the object or probe beam. The latter carries local surface (phase) information on return, whereas the reference beam wavefront returns with an average phase value. The detector output can be represented as S = S_o(1 +αcosϕ). In the quadrature condition, small changes in ϕ, dϕ, caused by the surface features, are directly proportional to variation in detector output dS. With this interferometer, surface features smaller than 50 Å have been measured with minimal precautions against environmental perturbations.

© 1987 Optical Society of America