Nomarski microscopy, also known as Nomarski interference contrast (NIC) or differential interference contrast (DIC) microscopy, was invented by Georges Nomarski in the mid-1950s. It is used to analyse planar semiconductor processing or even image live and unstained biological samples. For the analysis of semiconductor defects or contamination existing on top of the samples, it is more visible compared with conventional optical microscopy. The concept of polarization is introduced into DIC microscopy to image the specimen’s surface with a shadowed-like three dimensional appearance. A Wollaston prism is used to separate one polarized light beam, which is generated by a lamp and passed by a polarizer, into two orthogonally polarized beams.
These two coherent parts travel into the sample surface and are recombined by another Wollaston prism. As a result of the optical path difference, through changes such as sample thickness and reflective index, two beams are incapable of interfering with each other.
In order to bring the beams into the same plane, an adjustable offset phase is designed by adding a polarizer to modulate the position at zero optical path difference, and the contrast is recorded as the shear distance, which is marked in corresponding intensity or colour. The surface information is observed with a pseudo 3-dimensional appearance. It is one of the most common and convenient ex-situ tools for the characterization of epilayers.
We are proud to house in the QNC-MBE lab one of the Nomarski microscopes from Bell Labs used by Al Cho, the father of MBE. The vintage Nikon Optiphot-66 has been refurbished and updated with a SPOT digital camera, and serves as our starting point for wafer inspection.