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3D Roughness Metrology

Working Group 5.14

Scattered light methods

Everyone who tries to clean a mirror illuminated by a halogen lamp can observe how sensitive the scattering of light reacts to irregularities on the surfaces. The light scattering effect can also be quantitatively evaluated. This, however, always requires assumptions to be made. The evaluation of the angle-resolved scattered light measurement and the conversion into the surface parameter Rq are, for example, based on the model that the surface amplitudes are smaller than λ/4 but larger than λ/100. As light is available in a large wavelength region, the scattered light measuring procedures have developed into a sensitive and effective procedure of surface measuring technique which, depending on the wavelength used, cover a large range of surface roughness.

 

Figure: Set up for scattered light measurement

The expanded beam of the HeNe laser (λ=633 nm) is focussed onto the detector. On the sample, the beam diameter amounts to 3 mm or 5 mm, depending on the expansion objective used. Turning of the specimen on the goniometer allows the angle of incidenceλ to be varied between 0° and +85°. The light scattered by the specimen is recorded by the receiver under the scan angle β. The scan angle may be varied by 360° around the specimen so that the scattered light can be measured both in reflection and in transmission. So the bidirectional scattered light function (BRDF, BTDF) can be evaluated, i.e. it is possible to determine both the reflectivity and the transmissibility of a specimen.
The distribution function measured allows the power density curve (PSD) to be determined, and subsequently an RMS value for the surface roughness to be calculated..

Some parameters of the measuring set-up for scattered light:

Wavelength: 632,8 nm
Minimum roughness evaluable in Rq: 0.1 nm
Polarization plane: preferred s-polarization, laser-typical
The set-up is not provided with a separate polarisation filter
Angle resolution of the goniometer (Detector): 0.001°
Stop diameter on the detector: 376 µm, 1524 µm, 3969 µm und 12903 µm
Dynamic range of the receiver: lock-in technique with stop control
15 decades

The specimen is mounted on a xy-table with a displacement range of ± 75 mm in x- and y- direction.