Nanobits - exchangable and customisable scanning probe tips

Invention of atomic force microscopy (AFM) pioneered a novel aspect for the surface metrology concept. A range of scanning probe methods have been developed over the years based on different sorts of tip-surface interaction: electrical, optical, thermal, force. Reproducible and fast fabrication of microcantilevers and probes together with alternative probing modes ease AFM’s adaptation to altering technological needs. The need to constantly adapt to the ever-altering device architecture and perpetual size shrinkage calls for enhancements to address specific needs, like specialised probes. Device miniaturisation requires the scanning probes to adapt into finer geometries to provide higher lateral resolution. To meet these needs critical dimension AFM (CD-AFM) and deep trench AFM (DT-AFM) were invented, which use different types of AFM tips: high-aspect-ratio tips for DT-AFM and CD tips for CD-AFM. Unfortunately, these advanced tip types are fabricated by vertical means of fabrication which results in low throughput, and thus, high-cost and batch-incompatible processes. Another drawback is the wear problem of the silicon tips which reduces the efficiency since after a certain number of scans, tip diameter becomes wider losing its sharpness. Both changing of the sample (topography) and the blunting of the tips require replacement of the AFM tip with a newer, sharper or application-specific one. AFM can benefit from availability of adequate, surface-specific tips and AFM probes available for in situ replacement could greatly increase the efficiency and adaptability of a CD system. In this PhD study, NanoBits – nano-sized customisable and exchangeable scanning probe tips – were developed to meet the demands of current AFM applications. Two different methods were followed for the fabrication of NanoBits; lateral nanolithography which utilises the focused ion beam milling of freestanding NEMbranes and microfabrication, where the NanoBits and hybrid structures were defined by electron beam lithography. The design of the NanoBits considered challenging application topographies like deep trench and critical dimensions: tips suitable for imaging high-aspect ratio structures and sidewall profiles were designed. Tip diameters in the order of 30 nm were reproducibly obtained with the FIB milling and the smallest tip diameter achieved was <15 nm, with aspect ratios of 45 being possible. Scanning electron microscope investigation showed that the polycrystalline silicon NanoBits obtained by microfabrication had tip diameters of 15–30 nm at surface but increased to 275 nm along the thickness of the NanoBit, indicating wedge sidewalls. Therefore, post-processing sharpening of the microfabricated NanoBits was tested by physical etching methods: reactive ion etching (RIE), ion beam etching (IBE), and FIB milling. RIE and FIB milling were observed to be promising, resulting in increased overall tip sharpness while IBE experiments were inconclusive. The out-of-plane bending of NanoBits which would accelerate the assembly process by providing direct picking up of the NanoBits by the AFM probe was investigated. Two different bending mechanisms were studied for out-of-plane bending studies: FIB irradiation- and the residual stress-driven bending in bimorph structures. With FIB irradiation studies, NanoBits were demonstrated to bend close to 90° and to a certain degree window (30°–60°). In addition, it was shown that the bending angle can be adjusted by controlling the irradiation dose. Tip angles varying from ∼25° to ∼130° were achieved by bending due to residual stress in bimorph structures. Further experiments with silicon nitride substrate and their comparison to theoretical modelling were conducted to understand mechanism and factors in the bending process. The results showed that the main driving force for the bilayer bending is the intrinsic stress built up in the system. It was additionally demonstrated that the tip angle can be fine-tuned by thermal actuation such that up to 150°C the bending is reversible and the NanoBits’ angle can be modified within ±10°. At higher temperatures reversal of stress from tensile to compressive, buckling, and changing of surface colour were observed, pointing to silicidation of the structure which may be starting at 170°C. The fabricated NanoBits were assembled and their performance as AFM probes were tested at OFFIS. The NanoBits were successfully picked up by a microgripper, collected in a cartridge and mounted to an AFM probe. Performances of the assembled high-aspect-ratio NanoBits were investigated by imaging optical gratings. In most cases, the NanoBits showed better performance than the standard pyramid AFM probes, and long scan experiments proved that (i) the NanoBits did not get blunt even after 100 scans (continuous 30 h imaging), and (ii) stiction into the slit on the plateau tip was sufficiently strong. As a result, NanoBits can offer an unprecedented freedom in adapting to ever-altering surface topographies providing fast exchangeability and reliable outcome owing to their low-cost production (per NanoBit) and versatile capabilities.