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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Study of tip wear in high-speed atomic force microscopes

20.12.2018

For realizing high throughput metrology of versatile micro- and nanostructures, a high-speed metrological large-range atomic force microscope (HS-Met. LR-AFM) has been investigated at PTB with measurement speeds up to 1 mm/s [1]. However, AFM tip abrasion is becoming a more crucial issue, particularly in high-speed measurements. The tip-sample interaction usually defines the resolution capability of AFM measurement systems and it critically depends on the tip geometry.

To investigate this challenging issue, PTB has performed a quantitative investigation on the tip abrasion of diamond-like-carbon (DLC) coated tips in the HS-Met. LR-AFM. Wear tests are conducted on four different surfaces made of silicon, niobium, aluminum and steel. During the tests, different scanning speeds up to 1 mm/s and different vertical load forces up to approximately 40 nN are applied. Different tip characterization techniques by using scanning electron microscopy (SEM), the blind-tip reconstruction (BTR) method [2] and a linewidth standard type IVPS100-PTB have been applied in a combined manner to quantitively and precisely characterize the change of the tip shape. The results of our experiment reveal that the tip height changes rather abruptly than progressively as shown in figure 1, indicating that tip breakage is the dominant reason for tip damage in HS-AFM measurements. In addition, the results indicate that the risk of tip breakage is much higher in measurements of structures with steep/sharp than smooth structures. In the latter case almost, no change of the tip height is detectable.

The investigations in this study reflect a fundamental limit of the AFM technique. The AFM technique typically applies its cantilever as the sensor unit, where the static deformation or dynamic vibration of the cantilever is measured to detect the tip-sample interaction force. However, as a cantilever type sensor, the AFM probe has dominant high sensitivity in its bending direction and consequently is a 1D sensor mainly. This is an inherent problem in measuring, for example, steep structures or 3D structures. To solve this problem, various AFM techniques for true 3D measurements of nanostructures are being intensively investigated at PTB.



Fig. 1. Experimental results of tip wear tests on an aluminum and a steel surface fabricated by lathing. The test points are marked with one of the following letters ‘A’,‘B’, ‘C’,‘D’, ‘E’,‘F’ and ‘G’, which stand for different scanning speeds of wear testing of 10 µm/s, 20 µm/s, 50 µm/s, 100 µm/s, 200 µm/s, 500 µm/s and 1000 µm/s, respectively. The whole test consists of four parts, separated by vertical red lines. The first part was performed on the aluminum sample with a normal load force of 10 nN. Afterwards the experiment was continued with wear tests on the steel sample with a normal load force of 10 nN. In the third and fourth steps, the probing forces were increased to 20 nN and 40 nN, respectively.

Reference:

[1] Dai G, Koenders L, Fluegge J and Hemmleb M, 2018, Fast and accurate: high-speed metrological large range AFM for surface and nanometrology, Meas. Sci. Technol. doi.org/10.1088/1361-6501/aaaf8a

[2] Flater E E, Zacharakis-Jutz G E, Dumba B G, White I A and Clifford C A, "Towards easy and reliable AFM tip shape determination using blind tip reconstruction," Ultramicroscopy, no. 146, pp. 130-143, 2014.

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