Atomic Force Microscopes scan and directly measure the topography of ruled gratings. From AFM images, the ruling angle, pitch, and surface texture of features are measurable. The images below are of of a ruled grating measured on a TT AFM.
The above image shows an analysis of a line profile taken from the same 4x4 micron vibrating mode image. Cursors 1 and 2 show that the grate angle is 19 degrees and cursors 2 and 4 show that the pitch is .376 nm.
3D AFM image of the same ruled frating, displayed in color scale with a light projection. This image readily facilitates visualization of the grating's irregularities.
Analysis of Polished Fused Silica Substrates
Once polished, fused silica substrates can have surface roughness values of 0.1 nm. With an AFM it is possible to visualize the surface of polished silica substrates, measure the surface texture, and measure the dimensions of structures created by the polishing process.
The image above is a Vibrating Mode AFM image of a fused silica substrate having an RMS roughness of 0.106 nm and an Ra of 0.083 nm. The scan size of the image is 1X2 microns. A residual polish mark is visible at the right of the image.
Above, a 1 X 2 micron AFM image of a substrate with an RMS roughness of .203 nm and an Ra of 0.159nm. Several polish marks in differing directions are visible in the image. The red line designates a line profile which allows measurement of the depth and the width of the polishing mark. The red line is detailed in the graph below.
Above, a line profile taken from the AFM image of the polished substrate having a .203 nm RMS roughness. The half width of the polish mark in the image is 19.4 nm and the depth of the feature is 0.0354 nm.
AFM is ideal for imaging coatings used for photonics devices. Beyond visualizing the overall surface topography, measurements of grain sizes and surface texture are possible. Below is an AFM image of Indium Tin Oxide. An advantage of imaging films such as ITO with an AFM is that a vacuum is not required.