Surface contamination on both the tip and the sample complicates tip-sample interaction when operating an AFM in air. In the traditional non-contact mode using small vibration amplitudes, tip-sample spacing is maintained at a few nanometers, and the tip can be captured by the surface due to the capillary force. A popular method that prevents this tip- capture problem is to vibrate the tip at large amplitude, with the tip contacting the surface periodically. With a combined AFM/SEM system, the tips and samples were found to sustain damage by this periodic-contact. To minimize tip-sample damage and achieve maximal lateral resolution, we studied the surface contamination and the tip-sample interaction in air, and discovered a novel working mode producing one nanometer lateral resolution in air. This method uses a cantilever of large enough force constant to avoid the tip being captured, and small vibration amplitudes of only a few nanometers to avoid tip-sample damage. By combining stiff cantilever and small vibration amplitude, the tip can be maintained in the newly discovered 'near contact' region above the sample surface, so tip-sample spacing is minimized and tip sharpness preserved, achieving ultra-high lateral resolution in air. To explain the working mechanisms, we developed a microscopic model of the tip-sample interaction via the surface contamination layers.
The National Institute of Standards and Technology (NIST) is currently exploring the potential afforded by the incorporation of a commercial proximal probe microscope operating in the scanning tunneling or atomic force mode into a high resolution field emission scanning electron microscope (SEM). This instrument will be used in the development of NIST traceable standards for dimensional metrology at the nanometer level. The combination of the tow microscopic techniques provides: high precision probe placement, the capability of measuring and monitoring the probe geometry, monitoring the scanning of the probe across the feature of interest and an ability for comparative microscopy. The integration of the commercial instrument is the first step in the development of a custom NIST integrated SEM/SxM metrology instrument. This paper presents early results regarding the integration of the two instruments and the application of these instruments to the development of SRM 2090 and the SEM sharpness standard.