Microfabricated electrically conducting and wear-resistant nanoprobes for scanning probe applications

Abstract

The era of scanning probe microscopy (SPM) started twenty years ago. A cut gold cantilever with a glued diamond stylus served as the scanning probe. Within a few years the crystalline silicon (Si) became the – up to today – predominant SPM probe material. The rise of micromachining provided straightforward silicon probe fabrication to yield probes with nanometre-sized tip radii. For electrically conducting SPM applications Si probes are metal coated. Wear, and melting of this coating at the tip extremity at high current densities in the order of 108 A/cm2 disable the probe and thus are serious limitations. This thesis addresses and tackles these problems. The approach consisted of a surface micromachining technique using an oxidation sharpened SiO2 mould to fabricate full metal cantilevers with integrated metal nanometrically sharp tip. The tip mould sharpness can be can be tuned by an isotropic wet etch. The used probe material should be hard, have a high melting temperature and a good electrical conduction. In addition a good resistance to oxidation is necessary. The thesis describes the design, microfabrication, characterisation and demonstrates applications of the fabricated electrically conducting probes. The body of the probe consists of the photo-polymer SU-8. The probe metals used in this work are tungsten (W), platinum (Pt) and a developed platinum-tungsten alloy, yielding tip diameters below 40 nm, 40 nm and 23 nm, respectively. The tungsten probes with initial electrical resistance of 35 Ω were 14 GPa hard though showed oxidation upon IV spectroscopy. The Pt probes with 8,1 Ω resistance did not oxidise and with limited tip melting did retain a sharpness of 40 nm tip diameter at current densities up to 1,2x108 A/cm2. The Pt probe wear resistance however, with a 7,2 GPa thin film hardness, was limited. The PtW probes were fabricated in two versions: Pt89W11 and Pt74W26 with a hardness of 11,6 GPa and 13,7 GPa, respectively. The Pt89W11 probes with 42 Ω resistance did not show any oxidation at applied current densities up to 3,1x108 A/cm2 (5 mA), while equally retaining a 40 nm tip diameter. The Pt74W26 probes with initial 76 Ω resistance oxidised at current densities between 0,6x108 A/cm2 (1 mA) and 1,4x108 A/cm2 (2,5 mA). Pt89W11 and Pt74W26 probes showed wear characteristics comparable to Si probe wear. The Pt74W26 probes outperformed the Pt89W11 probes. The feasibility of SPM measurements in (electrically conducting) contact and dynamic mode was demonstrated with the Pt and the PtW probes. As such, with a high hardness, good electrical conductivity and resistance to oxidation at elevated current densities, the Pt89W11 probes meet all requirements for a wear resistant electrically conducting SPM probe and offer an alternative to coated Si probes.