Reflectance difference microscopy for nanometre thickness microstructure measurements.

The discontinuity of medium at the boundary produces optically anisotropic response which makes reflectance difference microscopy (RDM) a potential method for nanometre-thickness microstructure measurements. Here, we present the methodology of RDM for the edge measurement of ultrathin microstructure. The RD signal of microstructure's boundary is mathematically deduced according to boundary condition and polarization optics theory. A normal-incidence RDM setup was built simply with one linear polarizer, one liquid crystal variable retarder and one 5 × objective. Then, the performance of the developed setup was identified using homogenous reflection mirror and high quality linear polarizer. For demonstration, microstructures array with 100 nm step height was measured. The results show that the RD signal is sensitive to the edge and its sign reflects the change direction of the edge. Furthermore, a height sensitivity of better than 10 nm and a spatial resolution of ∼3 μm offer this technique a good candidate for characterizing ultrathin microstructures.