Three-Dimensional Measurements of Nanometer-Thick Lubricant Films Using Low-Coherence Phase-Shifting Interferometry

We have developed a method utilizing low-coherence phase-shifting interferometry (LCPSI) to directly visualize molecularly thin lubricant films coated on diamond-like carbon (DLC) surfaces. In contrast to conventional point-by-point scanning methods, this method straightforwardly measures lubricant distributions in three dimensions using phase-shifted two-dimensional interferograms which are generated by a Michelson interferometer and a robust feedback controlled phase-shifting system. To effectively suppress the image noise caused by unwanted interference, we employed a low-coherence light source rather than a laser. Moreover, an image subtraction method, in which the pseudo lubricant thickness attributed to surface roughness and system noise is first measured on a non-lubricated disk and is then subtracted from the result obtained on the lubricated disk, was introduced to eliminate the influence of surface roughness and system noise. By measuring nanometer-thick perfluoropolyether films coated on DLC surfaces, we demonstrated that the LCPSI is capable of providing real-time three-dimensional measurements with a thickness resolution of sub-nanometer and a spatial resolution of several micrometers.