| Zusammenfassung |
A novel instrument named hybrid microscope (HM) for in-situ reference areal surface metrology is presented in this doctoral work. The hybrid in its name indicates that the developed instrument synergically combines different surface measuring modes such as atomic force microscopic mode (AFM) and interference microscopic mode (IM). With these modes in combination, topography can be measured firstly with the IM-mode to have a quick overlook of the sample surface in a large measurement area. Then the instrument is switched to the AFM-mode for measuring with higher resolution whenever and wherever needed. Two of the most essential advantages of this instrument are (I) in-situ reference data can be obtained at the very same position without changing and transporting samples from one instrument to another, and (II) the local topography fidelity as well as lateral resolution of the IM measurement results can be enhanced by the AFM measurement results.
The HM is developed on a framework of an optical interference microscope, facilitating the IM measurements through the integration of both phase-shifting interferometric- (PSI) and white-light interferometric- (WLI) techniques. The HM system is employed with Mirau-type objectives manufactured by the company Nikon, with magnifications of 20x and 100x, respectively. These objectives enable high-precision measurements, demonstrated by a low noise level, e.g., the measurement noise of the IM-mode using 20x objective is characterized as low as approximately 0.27 nm (RMS). Furthermore,the IM-mode of the HM exhibits good isotropic bandwidth characteristics, whose angular-dependent relative difference is characterized lower than 8%, and excellent measurement repeatability, where a standard deviation less than 0.5 nm is obtained across five repeated IM measurements.
The AFM-mode is integrated into the HM system without relying on a conventional optical-lever readout system. Instead, by positioning the AFM cantilever within the camera’s field of view (FOV) at the objective’s focal plane, interference fringe is generated between a measurement light beam reflected from the back side of the AFM cantilever and a reference beam reflected from the reference mirror of the Mirau-objective. The generated interference fringe represents the topography of the back side of the AFM cantilever. During AFM measurements, as the AFM tip interacts with sample surfaces and impacted by tip-sample interaction forces, deflection of the AFM cantilever will be detected from the changes of the interference fringe. More specifically, the phase shift of the fringe represents the z-motion of the AFM tip, and the distortion of the fringe indicates the torsion of the AFM cantilever. In such a way, AFM-mode can be realized in a compact but high synergic design manner. The AFM sensor system developed in this study exhibits a lateral measurement resolution comparable to that of commercial AFM instruments (down to 10 nm, depending on the AFM tip used), and a noise level of 0.04 nm, which is both theoretically predicted and experimentally confirmed in this work. The measurement traceability of the HM is ensured by calibrations using nanoscale standards, which were calibrated by a metrological AFM developed at the Physikalisch-Technische Bundesanstalt (PTB).
Application examples are demonstrated in this dissertation, showing a high application potential of the developed HM for reference surface metrology. One of the most potential applications is to offer reference topography data for optical measurement instruments. To demonstrate this application, a set of sample surfaces made of different materials and manufactured by different surface processing techniques are investigated using AFM- and IM-modes of the HM. AFM and IM datasets are then correlated and compared. The results show that the sample structures can not only be resolved with more details by AFM measurements than that of IM, but also the optical artifacts in IM measurements can be easily detected by using the AFM data as a reference. Thus, by using the AFM data as a reference, the topography fidelity of optical measurements can be characterized, which also offers hints for potential improvements of optical measurement techniques. |
