Referenzmetrologie für flächenhaft-messende Oberflächenmessgeräte ausgezeichnet mit „Best Paper Award“

Since a decade, a metrological large range atomic force microscope (Met. LR-AFM) has been built up at PTB based on an ultra-high precision positioning stage referred to as the nano measuring and positioning machine (NMM). Recently, the Met. LR-AFM has been further upgraded concerning two aspects: the reduction of measurement noise and the improvement of measurement speed. Some key components of the NMM such as its interferometers and the angle sensors have been upgraded for easier adjustment, better thermal behaviour and better stability. The geometry of the corner mirror has been improved to fix samples so that the stress introduced into the optical component due to the sample fixing is greatly minimised. A new motorised spring mechanism has been installed for the weight compensation of the motion stage, thus reducing the heat generation for better temperature stabilisation. In addition, an improved instrument chamber for better thermal and acoustic insulation and an improved vibration damping stage are applied. With these improvements, the noise level of the metrology tool has been reduced significantly, for instance, the positioning noise along the z-axis has been improved from 1σ = 0.52 nm to 1σ = 0.13 nm, both measured at the sampling frequency of 6.25 kHz.

Till now the low measurement speed remains as a major shortcoming of the LR-AFMs. It leads not only to a low measurement throughput, but also to a significant measurement drift over the long measurement time (up to hours or even days), particularly for measurements over a large area. To overcome this challenging issue, a new high-speed measurement mode is realised in the Met. LR-AFM. In its design, the contact AFM mode is applied instead of the intermittent and non-contact modes, which offers shorter AFM response time and larger AFM sensing range. During measurements, the sample is scanned in the xy-plane solely by the NMM (such a motion usually has a constant velocity, therefore, high dynamics of the xy-scanner is not needed), meanwhile a high dynamic z-motion of the sample is realised by a combined piezo stage and the z-sage of the NMM controlled in parallel. The AFM output signal is combined with the position readouts of the piezo stage and the NMM to derive the surface topography. The combination of these readouts offers a large bandwidth of measurement signals, thus provides high speed measurement capability.

Owing to its outstanding metrology features, the developed Met. LR-AFM becomes a powerful tool for offering reference metrology for quantitative areal surface measuring 3D-microscopy. As an example, Fig. 1 illustrates some AFM data measured on several standards defined in ISO 25178 using the Met. LR-AFM. These data sets can be applied as the reference data either to calibrate surface topography measuring instruments in terms of amplification, linearity, squareness, flatness as well as noise, or to investigate the topography fidelity of its measurements.



Fig.1. AFM images measured by the Met. LR-AFM on standards defined in ISO 25178 type (a) ARS, (b) AIR, (c) ACG and (d) CIN.

Recently, a new material measure has been developed at PTB for the characterisation of the instrument transfer function (ITF) of areal surface topography measuring instruments. Two main innovative ideas are implemented in the sample design. Firstly, the sample is based on a kind of circular structure pattern. Such a rotational symmetric pattern is preferred for characterising the ITF features in different angular directions to detect angular-dependent asymmetry. Secondly, sample features are arranged in a linear chirp function with respect to its radial distance r from the pattern centre (x_"c" ,y_"c" ). Thus, it well represents a surface topography with a quasi-flat amplitude over a given bandwidth, which is suitable for characterising the ITF. Some application examples are performed which indicate the promising capability of the new sample for characterising the ITF of a laser scanning confocal microscope (LSCM) with a 100x objective as well as its angular-dependent asymmetry.

The research progress has received the “best paper award” in the International Symposium on Measurement Technology and Intelligent Instruments (ISMTII) held on September 1-4, 2019 in Niigata, Japan.

Reference:

Gaoliang Dai, Frank Pohlenz, Xiukun Hu, Thomas Weimann, Andre Felgner, Dorothee Hüser, Proceeding of the 14th International Symposium on Measurement Technology and Intelligent Instruments, September 1 ~ 4, 2019, Niigata, Japan