AFMs for the Photonics Industry

Atomic Force Microscopes (AFMs) offer substantially better horizontal and vertical resolution than optical and stylus profilers. For this reason, AFMs are ideal for the analysis of materials used in the photonics industry.

Some of the advantages from using an AFM for analysis of photonics materials include the following:

  • Three dimensional structures are measured: depth and angles are measurable.
  • Image is independent of optical properties.
  • No sample preparation required - no coatings, no cross sections.
  • Extreme contrast on flat samples.
  • Atomic Force Microscopy is non-destructive.

AFM applications in the photonics industry include:

  • Analysis of Polished Fused Silica Substrates
  • Ruled Gratings
  • Holographic Gratings

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, to measure the surface texture, and to measure the dimensions of structures created by the polishing process.

Vibrating AFM image of fused silica substrate 1 µm x 2 µmVibrating mode AFM image of a fused silica substrate having an RMS roughness of 0.106 nm. Scan size of the image is 1 X 2 microns. A residual polish mark is visible on the right.

Average value: 1.345 nm
Minimum: 0.705 nm
Maximum: 1.933 nm
Median: 1.346 nm
Ra (Sa): 0.083 nm
Rms (Sq): 0.106 nm

polish marks seen via AFM scan of fused silica substrate 1 X 2 micron AFM image of a substrate with an RMS roughness of .203 nm. Several polish marks in differing directions are visible in the image. A red line designates a line profile which allow measurement of the depth and the width of the polishing mark.

Average value: 0.743 nm
Minimum: 0.000 nm
Maximum: 1.723 nm
Median: 0.736 nm
Ra (Sa): 0.159 nm
Rms (Sq): 0.203 nm

Line profile from AFM scan of polished silica substrateLine 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.

Points  x[nm] y[nm] Length[um] Height[pm]
  307.0 789.6    
  287.7 434.8 -19.4 -354.8

Ruled Gratings

Atomic Force Microscopes scan and directly measure the topography of ruled grating. From AFM images the ruling angle, pitch, and surface texture of features are measurable. The image below is of a ruled grating measured on a TT-AFM.

AFMs for photonics, topography of a ruled grating

AFMs for photonics, 3-D topography of ruled grating3-D image of the ruled grating

Upper left: 4 X 4 micron color scale vibrating mode image of ruled grating. Notice the surface texture at the apex of each of the lines.

Upper Right: Analysis of line profile taken from the upper right image. Cursors 1 and 2 show that the graze angle is 19 degrees and cursors 2 and 4 show that the pitch is .376 nm.

Bottom Left: 3-D image of the ruled grading displayed in color scale with a light projection. This image readily facilitates visualization of the gratings irregularities.


Holographic Gratings

Specifications of holographic grating including heights must be maintained to very tight tolerances. Properly operated, an AFM can measure step heights as low as a few angstroms with very high precision and accuracy.

AFM scan of holographic grating
AFM scan of holographic grating

AFM height analysis of holographic grating


Upper left: 3D color scale image of a holographic grating.

Upper Right: Height analysis of the grating.

Bottom Left: Line profile of the holographic grating.