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Metrological large range AFM with high measurement speed up to 1 mm/s

01.12.2015

Highly accurate dimensional metrology of nanostructures are increasingly demanded for quality assurance in various nanomanufacturing industries. To satisfy these demands, a metrological large range AFM [1] has been successfully developed based on an ultra-precision nanopositioning and nanomeasuring machine (NMM), and is being applied as main instrument at PTB for versatile traceable calibrations of nanostructures with sub-nm accuracy [2]. However, operated in the conventional raster scan mode, i.e. in a kind of serial measurement technique, the conventional AFM measurement is usually rather slow. This fact leads to not only a low measurement throughput, but also to significant measurement drift due to the long measurement time needed, which results in a challenging issue and a bottleneck for expanding its applications.
To address this challenging issue, we recently have developed a method for significantly improving the scanning speed of a Met. LR-AFM [3]. In its design, the z scanner is realised by combining a piezo stage and a large range mechanical stage (referred to as a nanopositioning and nanomeasuring machine, NMM) which move the sample in parallel, thus providing both a high dynamic positioning capability and a large motion range. A contact mode AFM which offers both, a short response time and a large sensing range is applied for measuring the surfaces of interest. Sensor signals from the AFM, the piezo stage and the NMM are combined to derive the surface topography, thus offering high bandwidth for high speed measurements.
However, measurement at high speed may result in distorted profiles and thus degrade the image quality and measurement accuracy. As shown in figure 1(a) and 1(b), some indications of distorted artefacts were observed in the derived surface profiles if the high speed AFM is not properly optimised. We conclude that the distortion is mainly due to two issues. The first concerns the calibration of the sensors. Only after the sensors are correctly calibrated, they can be meaningfully combined for deriving surface profiles. The second is related to the time delay (or phase lag) between different signals. To reduce the distortion, the high speed AFM is further improved so that the time delay of sensors is carefully determined and corrected for as well as the displacements of the AFM and the piezo stage are traceably calibrated referring to the z-interferometer of the NMM in situ. Figure 1(c) and 1(d) shows the AFM image measured after the high speed AFM had been optimised, indicating a significantly improved image quality.
The new high speed large area measurement capability of the instrument is demonstrated in figure 2. In this investigation, a 2D grating (2D10000) with a nominal pitch of 10 μm is measured over an area of 500 x 100 μm2 with a scan speed of 500 μm/s. The large area scanning is performed directly, i.e. no imaging stiching is needed. The obtained image containing 5000 pixels x 500 lines is shown in the figure as the raw data after 1st order leveling. The result indicates that the instrument is capable of delivering high quality images at high scan speeds. The measurement repeatability at different scan speeds ranging from 10 μm/s to 1000 μm/s on step height and lateral grating standards has been investigated. The results indicate a high repeatability of below ± 0.7 nm and ± 5x10-6 for step height and pitch measurements, respectively. The amendments have improved the measurement speed of the Met. LR-AFM by a factor of more than 20, thus significantly enhanced the measurement throughput and reduced the measurement drift.


Fig.1 (a) AFM images measured on a 2D10000 grating with a scan speed of 500 μm/s, shown as an image obtained without correction and (b) its cross-sectional profile at the marked position (white line in (a));(c) image obtained with correction and (d) its cross-sectional profile at the marked position (white line in (c)).



Fig.2 AFM image with a size of 500 μm x 100 μm measured on a 2D10000 grating with a scan speed of 500 μm/s.

Reference:

  1. Dai G et al 2009 A metrological large range atomic force microscope with improved performance, Rev. Sci. Instrum. 80, 043702
  2. Dai G et al 2007 Accurate and traceable calibration of two-dimensional gratings, Meas. Sci. Technol. 18, 415–421
  3. Dai G et al 2015 High-speed metrological large range AFM, Meas. Sci. Technol. 26, 095402

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