Improved scanning ion-conductance microscope using microfabricated probes

Abstract

The scanning ion-conductance microscope (SICM) is a member of the growing family of scanning probe microscopes, which includes the scanning tunneling microscope and the atomic force microscope. The SICM scans a small aperture over a sample immersed in electrolyte and measures the ionic current flowing through the aperture. This current serves as the feedback signal for standard scanning probe microscope electronics, which can then provide images of the topography of a sample surface, or the distribution of ionic current flow through the pores in a porous sample. The resolution of this microscope is essentially set by the size of the aperture. We have developed silicon microfabricated probes for the SICM that greatly improve its performance. Previous SICMs used fragile glass micropipettes as probes. The new microfabricated probes are end caps for a 1.5 mm glass capillary. A hollow tip is fabricated in the center of a silicon membrane. An aperture is formed at the tip apex using microfabrication techniques including photolithography and etching, giving typical minimum diameters of 250 nm. Unlike the glass micropipettes used previously, the new tips are compact and hence mechanically robust. The tips are mounted on a flexible membrane that allows the tip to deflect away from the surface in case of a collision with the sample surface. In addition, the microfabricated probes have been designed to have a high mechanical resonant frequency, allowing scan speeds up to 50 times faster than used with glass micropipette probes. With these new probes we have imaged the surface topography of a plastic diffraction grating, and the ion flow through porous polycarbonate membrane filters. Finally, the new microfabricated probe was designed as a general probe that may be adapted for use in other scanning probe microscopes.