Determination of the topographic spatial resolution of optical 3D microscopes depending on the aspect ratio of the structures to be measured

Optical microscopes are more and more widely used in industry for surface quality control due to their advantages of fast, areal and non-destructive measurements. However optical roughness measurements are often not traceable because of the complexity of the strong interaction of the surface characteristics to be measured and the measuring system. For surfaces with higher aspect ratios large measurement deviations, batwings, and multi-scattering effects are observed. The aspect ratio of structures together with their slopes and curvatures need to be incorporated to the topographic spatial resolution of optical microscopes for areal roughness measurements. Recently in ISO standards ISO 25178-600 a new term “topography fidelity” TFi is introduced which describes the method or instrument influenced measurement deviation of a topography related to the correct measurement of surface features, i.e. the width and the height of the feature. The small-scale fidelity limit, the smallest lateral surface feature (e.g. surface wavelength) for which the reported topography parameters (e.g. height) deviate by less than a specified fraction from the reference values will be used to determine the topographic spatial resolution.

To investigate the topography fidelity of optical surface measurement instruments, a chirp-calibration standard developed at PTB is used as a reference artefact. The chirp standard is made of low-oxygen copper with a 100  μm thick nickel-phosphorus layer and manufactured by a single diamond turning process. The standard contains four kinds of chirp structures:

• profile1: 1 µm amplitude with 10 wavelengths (12 µm – 5 µm)
• profile2: 0.7 µm amplitude with 12 wavelengths (12 µm – 4.2 µm)
• profile3: 0.4 µm amplitude with 15 wavelengths (12 µm – 3.2 µm)
• profile4: 8 µm wavelength, but with 12 different amplitudes (2 µm – 0.1 µm)

The chirp sample has been measured by a 3D optical microscope with confocal and white light interferometer principles, respectively. Figure1 and Figure2 show typical measurement results of the chirp structures with a fixed amplitude of 400 nm measured by an optical microscope with white light interference mode and confocal mode. Experimental investigations on the chirp reference standards indicate that larger deviation appears in the measurements using white light interference microscopes. However, for topography measurement using confocal microscopy, this phenomenon has smaller contribution to the measurement results.

Further investigations especially for the interpretation of surface topography measurements in terms of aspect ratio, slope and curvature and how to incorporate areal topographic resolution and topography fidelity into uncertainty budgets need to be performed.

(a) Surface topography	(b) x-profile

Figure1 Surface topography and the corresponding x-profile of a chirp structure with 400 nm amplitude measured by an optical microscope with white light interference mode (50x objective, numerical aperture 0.55 )

(a) Measured by white light interference mode (50x objective, numerical aperture 0.55)	(b) Measured by confocal mode (50x objective, numerical aperture 0.8)

Figure2 Topography fidelity curves of a chirp structure with 400 nm amplitude measured by an optical microscope with white light interference mode and confocal mode, respectively.