ALL TUNGSTEN AFM CANTILEVER WITH < 40 NANOMETRE PYRAMIDAL TIP

We develop wear resistant, electrically conducting AFM probes. Here, we describe a new AFM probe made entirely out of tungsten (W). The fabrication involves microfabrication techniques such as surface moulding, thin-film structuring of tungsten and high aspectratio SU-8 patterning. The metallic cantilever and tip are attached to a SU-8 photopolymer body. The free-standing 500-nm-thick cantilevers are obtained by a wafer-through etching. Figure 2 shows a SEM image of the SU-8 body with W cantilevers and a close-up of a W tip having a radius < 40 nm. The 200-μm-thick SU-8 body is 1.8 x 3.2 mm. Cantilever resonance frequencies range between 35 kHz and 1 MHz as measured by optical interferometry, which corresponds well to theoretical values assuming Ew= 410 MPa, ρw= 19.3 g/cm3. The AFM probes have been designed for conducting mode imaging at the nanoscale whereas W has been chosen for its excellent properties in electrical conductivity and wear. First characterisation has demonstrated AFM imaging capability.

Microfabricated AFM probe with tungsten cantilever and < 40 nm pyramidal tip

Jürgen Brugger, Johannes Steen

In the past, conducting AFM probes have been made by coating standard Si-cantilevers and tips with a thin film of e.g. tungsten (W). Here, we describe a new AFM probe made entirely out of tungsten. The fabrication involves microfabrication techniques such as surface moulding, thin-film structuring of W and high aspect-ratio SU-8 patterning. The metallic cantilever and tip are attached to a SU-8 photopolymer body. The free-standing 500-nm-thick cantilevers are obtained by a wafer-through etching. Cantilever resonance frequencies range between 35 kHz and 1 MHz as measured by optical interferometry, which corresponds well to theoretical values assuming Ew= 410 MPa, ρw= 19.3 g/cm^3. The AFM probes have been designed for conducting mode imaging at the nanoscale whereas W has been chosen for its excellent properties in electrical conductivity and wear. First characterization has demonstrated AFM imaging capability. A critical parameter is stress-induced cantilever bending which can be minimized by optimization of process parameters such as W sputtering and annealing.

2004

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

Johannes Steen

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.

EPFL2007

Full tungsten and platinum nanoprobes for electrically conducting scanning probe methods

Jürgen Brugger, Johannes Steen

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).

2006

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

Johannes Steen

EPFL2007

Full tungsten and platinum nanoprobes for electrically conducting scanning probe methods

Jürgen Brugger, Johannes Steen

2006