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Novel material measure for characterising the two-dimensional instrument transfer functions of areal surface topography measuring instruments

22.12.2020

 Measurement fidelity is a challenging issue for various optical areal surface topography measuring instruments such as phase shifting interference, confocal, white light interference (WLI) and focus variation microscopes. To address this challenge, there are three major issues to be considered in measurements: scale issue, light-surface interaction and bandwidth characteristics. Among them, the investigation of the bandwidth characteristics is a very complicated task. As the surface topography can be understood as a composition of components with different spatial frequencies, it can only be measured correctly if the measurement instrument can exactly respond to its spatial frequencies. The Instrument Transfer Function (ITF) aims to describe the bandwidth characteristics of optical areal surface topography measuring instruments.The PTB has recently developed a new kind of material measure for characterising 2D ITF of areal surface topography measuring instruments. Several innovative ideas have been realised in the design of the material measure. For instance, the sample is designed with circular structure patterns. Such rotational symmetric patterns are advantageous for characterising ITFs along different angular directions, thus for characterising the angular anisotropy of measuring instruments. Three different types of patterns are implemented in the design: circular step (CS) pattern, circular chirp (CC) pattern and circular discreate grating (CDG) pattern. They are designed to complementary represent three kinds of spatial signals for characterising ITFs. In addition, the material measure consists of 25 circular patterns with radii from 30 µm to 300 µm, and wavelengths from 0.1 µm to 150 µm. These patterns can be applied combinedly to meet various measurement demands for miscellaneous instruments which may have very different bandwidth characteristics, field of view (FOV) etc. This design thus offers a high level of flexibility for application. The material measure is manufactured in collaboration with working group 2.44 using the e-beam writing lithography facilities at PTB.As an example, the ITF characterisation of a phase shift interference microscope type PLu SneoX (with a 50x objective and an optical light source with a wavelength λ = 530 nm) is demonstrated in Fig.1. A measured image on a CC pattern is shown in Fig. 1(a) as the raw data after 1st levelling; two radial profiles at the marked positions are depicted in Fig. 1(b). As can be seen in the figure, the amplitude of the measured feature decreases as the spatial frequency of the feature increases. As the real feature heights are almost uniform over all spatial frequencies as calibrated by the metrological large-range AFM of PTB, this phenomenon indicates the impact of bandwidth characteristics of the optical tool. Figure 1(c) depicts the evaluated ITF of the SneoX from three CC patterns “C31”, “C32” and “C33” and five CDG patterns “Z1” … “Z5”. It can be seen that the fitted ITF curves of both CC and CDG patterns have excellent agreement. Short-term repeatability and reproducibility tests show that the method has excellent repeatability and robustness. The standard deviation of the fitted parameters reaches approx. 1 % ~ 5 %.


     

Fig.1. (a) Image of a circular chirp pattern on the developed material measure measured by a phase shifting interference microscope type “PLu SneoX” using a 50x optical objective (NA = 0.55) with a wavelength λ = 530 nm; (b) two radial profiles of the measured pattern at the marked positions in (a); (c) ITF curves of the instrument evaluated from 8 circular patterns with three circular chirp patterns (C31 to C33) and five circular discrete grating (CDG) patterns (Z1 to Z5).

In summary, we conclude that the developed material measure has the following advantages:

(i) Capable of determining 2D ITF: It is capable of characterising angular-dependent bandwidth characteristics of areal surface topography measuring instruments;
(ii) Moderate costs: The design of the material measures is suitable for batch production, e.g. using the state-of-the-art lithography techniques;
(iii) Excellent flexibility: The designed material measure is applicable for characterising different instruments with different bandwidth characteristics and different sizes of field of view (FOV)
(iv) Ease of use: The material measure, calibration method and data evaluation software offer a package solution for convenient characterisation of 2D ITF of measurement tools. Calibration of the 2D ITF of an optical instrument can be performed within an hour as shown in our application tests.
(v) High measurement repeatability and robustness.

Reference:
1) Gaoliang Dai, Ziyang Jiao, Lanting Xiang et al. 2020 A novel material measure for characterising two-dimensional instrument transfer functions of areal surface topography measuring instruments, Surf. Topogr.: Metrol. Prop. 8 045025 

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