The Spectral Colours of Nanometers

Contactless metrology becomes increasingly important for in-line or postprocess production quality control. Automotive industry, semiconductor industry, and manufacturers) of microsystems and sophisticated components built from glass and plastics need fast and accurate measurements of small structures, surface topography, and layer thickness. Typically, a lateral resolution of a few microns is demanded. The accuracy of the distance measured between the target and a reference plane usually needs to be better than 0,1 µm. Obviously, only optical systems are capable to cope with such demands. For most industrial applications mentioned above the chromatically coded distance detection method turned out to be very well suited. That method takes advantage from a lens error commonly known as chromatic aberration: the axial position of the focal point of an uncorrected lens depends on the color (wavelength) of the light to be focused. In the visible spectral region, the focal distance for blue light is minimal while it is maximal for red light. The focal points of other colors are located in between according to the row: red, orange, yellow, green, blue, violet. Depending on the distance of the target from the focusing lens, light of just a very small wavelength region ?1 is focused on the target's surface (Fig. 1). All other spectral components of the light source are illuminating a much wider area of the surface.