Development of conductive SPM probes for applications in biology.

Abstract

We have developed insulated conductive SPM probes for scanning impedance microscopy characterization in liquid environment. The probes have been developed due to the need of characterizing olfactory biosensor devices within the European project BOND. We have made separate developments for DC probes (for DC electrical measurements in contact mode) and AC probes (to measure the AC response in dynamic mode)._x000D_ We have demonstrated the batch fabrication of conductive SPM probes (cantilevers integrated with sharp tetrahedral tips) insulated except at the very tip apex and the contact pad area with standard microfabrication processes. The tip radius of the fabricated probes obtained from topographic image resolution and electrical calibration techniques show very good agreement and gave tip radius values in the range of 30-50nm. These tip radius values satisfy the specification for the probes tips as required by the BOND project (which was tip radius value of less than 100nm). The force constant of the fabricated probes was calibrated using the thermal tuning technique. The force constant values of the characterized probes were found to satisfy the specification values for the probe cantilevers as required by the BOND project._x000D_ Capacitance gradient between the fabricated probes and a conductive substrate (HOPG) as a function of tip-substrate distance was measured by a setup with attofarad resolution. These measurements demonstrated that the stray capacitance between the different parts of the probes (cantilever, tip cone and probe substrate) and the conductive substrate was reduced to a non-detectable limit for both DC and AC probes. Static deflection of an AC probe cantilever due to the probe substrate capacitance coupling was measured as a function of DC applied voltage at constant tip-substrate distance (500 nm). Capacitance coupling between the fabricated AC probe cantilever and the conductive substrate was highly reduced compared to what will be between the conductive substrate and a probe cantilever with full metallic coating. These results are consistent with the design of the probes. We claim, to the best of our knowledge, that we are the 1st in fabricating insulated conductive SPM probes with much reduced stray capacitance contributions. Further EFM and tunneling AFM (TUNA) experiments were made with AC probes. Very good agreement between simultaneously taken EFM and topography images of silicon oxide nanostructures on highly doped silicon was observed. And linear relationship between TUNA current and DC applied voltage (between the probe and a conductive substrate) was observed. These results show the good electrical behavior of the probes. The dielectric coating is expected to minimize the parasitic conductance between the probe cantilever and samples in liquid media. Therefore, it is expected that the AC probes can be utilized for local electrical characterization of small signal samples in liquid environment._x000D_ In the DC probes, as the tip material is silicon, the native oxide on the tip blocked electrical contact between the probe tip and the conductive substrate in TUNA current measurement. To address the issue of native oxide, we performed titanium and tantalum silicide (as probe conducting layers) test runs. But we found that the silicon nitride (the dielectric layer used on the probes) etching conditions which are available at IMB-CNM clean room facility have poor selectivity to titanium and tantalum silicide layers. We believe that further test is needed to address the titanium or tantalum silicide selectivity problem to fabricate optimum DC probes for topographic and electrical characterization of samples in liquid environment.

Date of Award	19 Jun 2014
Original language	English
Supervisor	Joan Bausells Roige (Director)