An optical probe of thin film and surface contamination based on surface plasmon resonance.

Abstract

The microcontamination of optical surfaces or optical thin films affects many of their properties. In this work, we investigated several measurement systems to detect many types of surface contamination of coatings based on the surface plasmon resonance (SPR) phenomenon. The attenuated total reflection (ATR) coupling, also known as the Kretschmann configuration, excited the nonradiative surface plasmon wave for SPR measurement. Several microcontamination layers thinner than 10 nm were studied. The results showed that in all the cases SPR curves shifted to larger incident angles. From the amount of angle shift, the thickness of contamination was determined with a sensitivity of as little as one angstrom. The optical constants of those contamination layers were also derived. The shifts of the SPR curves served as an index for the efficiency of cleaning processes. It was found that the contamination by moisture can be removed with Iso-propyl alcohol by the ultrasonic cleaning process, while acetone was the more effective solvent in removing the contamination left by strippable coating residue show that the contamination layer was roughened by ultrasonic cleaning. In studies of island-like discontinuous thin layers of Ag, Al, and MgF₂, we found that the refractive index of MgF₂, a dielectric film material, slightly decreased as the thickness decreased, but for discontinuous metal films, the optical constants changed rapidly and became more dielectric in nature. Direct detection of contamination by coating processes in a small vacuum chamber was also carried out. In a chamber with high backstreaming from a diffusion pump, a broad SPR curve for an Ag film revealed obvious optical constant changes. Measuring and comparing the shift indicates that a significant amount of contamination was occurring right after the coating was completed. This suggests that for good evaporated optical thin films, it is important to have a more tightly controlled evaporation process. Finally, particulate, as well as layered, contamination can also be measured. A radiative SPR wave was generated by illuminating a contaminated surface. Similar radiative SPR waves also can be observed by adding a rough contaminant layer on an Ag film.