WHAT IS SURFACE ROUGHNESS? REALLY!
Surface roughness addresses the topography of a surface and quantifies the variations in its height on a micro-scale.
How is roughness defined? Roughness is defined by ISO standards. Depending on the type of roughness being studied, different ISO standards apply. Profile, or 2D, roughness is defined by ISO 21920. Ra and Rq are the profile values and are the arithmetic mean height and RMS surface height, respectively. For areal roughness, or 3D roughness, ISO 25178 is the standard. Sa is the arithmetic mean height over an area and Sq is the RMS surface height over an area.
Surface roughness is intrinsic to any surface; there are always slight surface height variations even on extremely flat surfaces. For flat optics, these variations are usually on the nanometer or Angstrom scale. They are different than cosmetic defects like scratches, digs, and pits, however they can be related. For example, a pit formed from the polishing process will show up in roughness measurements and affect the value. In addition, scratches ranging in size from nanometers to millimeters (depending on the polishing grit and material) can affect roughness measurements as well, depending where on the optic the measurement is taken.
Figure 1: A sample roughness measurement on plano SiC with x and y axes in microns. This particular measurement has an RMS surface height (Sq) value of 3.6 Angstroms. Standard low and high pass filters and form removal were applied.
Height Variation Types There are four different classes of optical surface height which are defined based on their spatial frequencies – form, waviness, roughness, and microroughness. Form is the general shape of the surface, like a plane, sphere, or cylinder. Generally, form is ≥ 10mm, but this can vary from optic to optic. The next finest is waviness, which is the general deviation from the form. Waviness can also be called mid-spatial frequency errors. The range can vary by company and size of the optic. At AOS, we refer to mid-spatial frequency errors on a scale between 1 and 10mm. Roughness, as described above, is the metric of interest. Roughness is the deviation from surface height, usually denoted as RMS surface height. These height variations are between 2.5um and 80um. Microroughness is the fine texture of a surface. This property can affect mild scattering and is generally smaller than 2.5um. Many times, roughness and microroughness are grouped together, but they are kept separate for the purposes of our work.
Figure 2: Measurement types with scale for lateral feature sizes. The figure is exaggerated for simplicity.
Why Measure Roughness? There are many parameters that are measured to determine the quality of an optical component such as cosmetic defects, laser-induced damage threshold, power spectral density, surface figure and reflected wavefront errors, optical aberrations, and so on. Surface roughness is simply one additional parameter. As previously stated, roughness affects how light scatters as it is reflected off the surface. The lower the surface roughness, the higher performance you can achieve. Less light will be scattered, and the optic will be more efficient as a result. At AOS, we guarantee 2nm RMS on off-axis parabolas and < 10 Angstrom RMS values for plano optics.