Surface scanning with photonic force microscopy in tapping mode

The present work describes the implementation of a new measurement mode of surface imag- ing photonic force microscopy. The aim of the new measurement mode is to make the method more robust in order to extend its field of application and to establish it as a reliable mea- surement method. Photonic force microscopy utilises the effect of optical tweezers to capture nanoparticles in the range from 50 nm to 10 µm in a focussed beam of light and move them in a controlled manner. The position of the nanoparticle is measured using an interferometric metrology method with nanometer precision. The trapped nanoparticle is used as a probe to analyse the surface of a sample. Via the displacement of the probe, the interaction force between the probe and the sample is measured and the surface profile of the sample can be determined. The force exerted by the probe on the sample is in the range of pN. This measuring method is therefore a very soft probing technique and particularly suitable for researching bio- logical samples. However, these small forces also induce an easily disturbed system. Strongly attracting forces can cause the probe to adhere permanently to the sample and thus render a measurement invalid. To avoid this, the probe-sample contact is controlled in the newly intro- duced tapping mode. For this purpose, the probe is kept permanently at distance to the sample and is only brought into contact with the sample for a few milliseconds for each measurement point. A mathematical model to describe the dynamic tapping mode is presented and the mea- surement parameters tapping frequency, trap stiffness and sample distance are discussed. The model is in accordance with the experimentally determined values and suitable for optimisation or adaptation of the measurement parameters to the sample and environment. Deviations in the measurement results of the new moder are corrected by improved data evaluation and lead to an enhancement for both measurement modes. When used on test structures, the tapping mode shows no disadvantages against the currently used contact mode in the determination of height profiles and reliably resolves nanoparticle structures up to 235 nm. At steep edges, the contact mode shows a measurement artefact which does not occur with tapping mode due to the probing from above. The reference measurement required for evaluating the height profiles can be replaced in tapping mode. This reduces the measurement time by 20 %. Finally, both measurement modes are applied to macrophage cells. Sample structures with a maximum thickness of 200 nm are measured. Also, the tapping mode shows a significantly higher success rate on experiments with cells. The tapping mode is therefore a useful addition to photonic force microscopy that solves some of the difficulties of the contact mode and provides a promising tool for surface height profiling of vital biological and non-biological samples.