Full tungsten and platinum nanoprobes for electrically conducting scanning probe methods

Recently electrically conducting SPM probes were used as read/write sensor of magneto-resistive nanopillars and ferroelectric domains in the development of >1 Tb/inch2 data storage. Since metal coated (platinum (Pt) or Pt/iridium) silicon (Si) probes are not suitable at high current densities, probes with an entire metallic tip form a solution. We present the fabrication and characterisation of hybrid SPM probes with fully metallic cantilever and tip. One method is based on a backside opening etch in KOH solution. The other method uses a dry under-etch probe release. Fully metallic cantilevers with integrated tips of tungsten (W) - and Pt have been fabricated. The body of the probes consists of the photopolymer SU-8. A tip sharpness controlling method is presented. Both processes are based on a surface moulding technique where a pyramidal mould made in KOH solution is used to form the tip. Oxidation sharpening of the mould yields very sharp tips with measured tip radius below 20 nm. Sputter deposition guarantees with its good step coverage an excellent mould filling with the desired metal (W or Pt). Stress control in the metal layer is important for straight cantilevers and is done by adjustment of deposition parameters. After a dry structuring of the cantilever, a 200-m-thick SU-8 body for the probe is formed on top of the metal, which can be spun in a single step. Proper surface treatment prior to SU-8 spinning ensures a good adhesion between the SU-8 and the metal. The use of SU-8 forms a substantial advantage over bonding or Si-etching techniques. A backside release etch in KOH solution is done for the W probes using a vacuum chuck. For the Pt probes, with an open honey-comb body structure, a top-side dry etch release is done. Figure 1 shows released Pt probes after a final BHF SiO2 removal. AFM imaging has been shown and IV analysis of the probes on a gold sample was performed. The results are shown for the W-probe and Pt-probe in figure 2 and 3, respectively. The measured contact resistance between tip and substrate values around 30 ohm for the W-probe and 10 ohm for the Pt probe. This is one order of magnitude lower than for metal coated probes. The W probe shows oxidation in air, but the Pt probe shows reproducible curves at currents higher than 10 mA, as needed for switching of magneto-resistive nanopillars. Besides use in future data storage, the probes are promising in contact and non contact mode (e.g. Kelvin Probe Microscopy).