Enhanced imaging of biomolecules with electron beam deposited tips for scanning force microscopy

Tip/sample interaction on the scanning force microscope (SFM) is a particularly difficult problem with biological materials. One major factor affecting image quality is the tip shape. Improved electron beam induced deposition technique with a scanning electron microscope (SEM) was used for the reproducible fabrication of carbon sharp tips on the end of commercially available silicon nitride cantilevers for scanning force microscopy. By aligning a fine focused beam of 20 nm diameter directly down the axis of the pyramidal tip at electron energy of 20 kV, carbon deposits grow with full cone angle of about 25°, cone length of 2 μm, and radii of curvature down to 10 nm, making these e-beam tips suitable for biomolecules imaging. The tip dimensions also were controlled by adjusting the beam parameters. Three different types of SFM tips were used to image tobacco mosaic virus (TMV). Conventional pyramidal tips appeared generally worse for imaging helical particles of TMV than SEM-deposited tips which were found more robust than commercially available conical tips. The use of sharper tips for SFM imaging of protein DNA revealed a 25% improvement in lateral resolution.