Cross section investigations of compositions and sub-structures of tips obtained by focused electron beam induced deposition

The spatial evolution of compositions and sub-structures inside focused-electron-beam-deposited tips from dicobalt-octacarbonyl Co2(CO)8 precursor at 25 keV and varying beam current (20 pA - 3 A) is extensively studied for the first time by means of energy dispersive X-ray spectroscopy, transmission electron microscopy, back-scattered electron imaging, and ion-induced secondary electron imaging. Transverse and longitudinal tip cross sections and lamellae were prepared by focused ion beam milling. Two sub-structure types can be distinguished: a nano-composite sub-structure is grown during the initial deposition stage (small-aspect-ratio tips). It consists of cobalt nano-crystals embedded in a carbonaceous matrix. A second distinct cobalt-grain-rich sub-structure develops in high-aspect ratio tips. Both sub-structures vary in appearance and composition with increasing beam current: the initial nano-composite sub-structure increases in cobalt content and nano-crystal size, and the cobalt-grain sub-structure develops polycrystal-, texture-, whisker-, or platelet-like habits. The directed precursor flux from a micro-tube prevents a radial symmetry of the sub-structures with respect to the impinging focused electron beam, at medium to high beam current. Homogeneous nano-composite high-resolution tips with small diameter and length were obtained at low beam current. Observations suggest an additional contribution to pure electron induced precursor molecule decomposition. The influence of electron beam heating and related chemical reactions is discussed.