Investigation of aperture SNOM levers fabricated by FIB patterning and wet chemical etching

Aperture probes for near-field optical microscopy are currently limited in tip geometry due to the etching process involved in the production of fiber tips [1] or the principal crystallographic planes when dealing with silicon-based SNOM tips integrated into a microfabricated cantilever [2]. Also, the reproducibility in tip production is still small. Therefore, a novel concept for the micofabrication of aperture SNOM probes with user-defined shapes and aperture sizes has been proposed by Schmidt, Bischoff and Eng [3]. These tips are directly incorporated into a cantilever which properties may be varied in a broad range. We believe that the SNOM-levers will offer novel applications in biology and material science.
To produce such cantilevers with an integrated optical tip, focused ion beam (FIB) 3D-patterning was used to define both the tip and cantilever as a monolithic structure in the silicon substrate. By varying the ion dose of implanted gallium ions we are able to construct levers with various force constants at small cantilever lengths of < 20 µm. A point-like FIB irradiation of Si, leading to hole erosion by sputtering, allows us to produce tips having a truncated Gaussian shape of high aspect ratio. Various forms are possible, including open and closed tips. This method allows to achieve hollow tips of less than 100 nm in diameter. The cantilever and tip structure predefined by Ga+-FIB implantation and sputtering is subsequently etched in KOH:H2O solutions to remove the surrounding silicon, not irradiated by the FIB. Ga-doped areas are more resistant against the etchant during the wet anisotropic etching process so that the SNOM lever is finally formed as a free-standing structure.
We present investigations of the mechanical and optical properties of the levers and the tips, respectively. The micromechanical cantilever structures with lateral dimensions of a few microns and a thickness of only some tens of nanometers were tested interferometrically to deduce their lowest mechanical resonance frequency which was found to be in the range of 0.5 to 5 MHz, depending on their lateral dimensions and the cross section shape. The corresponding spring constants range from 0.01 to ~1 N/m offering a lot of new applications. For studying the optical properties the light transmission through as-constructed apertures within a flat extended support was investigated for different apertures and cone sizes.

References
[1] R. Stöckle, C. Fokas, V. Deckert, B Sick, B. Hecht and U.P. Wild, Appl. Phys. Lett. 75, 160 (1999)
[2] Witec GmbH, Ulm (Germany), http://www.witec.de/snom.html
[3] Patentanmeldung 100 57 656.7 (21.11.2000)
[4] B. Schmidt, L. Bischoff, and J. Teichert, Sensors and Actuators A 61, 369 (1997)