Focused Ion Beam (FIB) technology allows to process various materials within a lateral range below 100 nm. The feasibility to mechanically sputter as well as to direct-write nanostructures and the fact that Ga-ions are utilized is unique for this method. The focused Ga-ions are used to locally induce a chemical vapor deposition of volatile precursor molecules adsorbed on a surface. Local deposition of metals and dielectrics has been achieved on a sub-µm scale utilizing a focused ion beam. This method is highly suitable for advanced microelectronic semiconductor fabrication. However, material specifications are narrow for these tailor-made applications. The effect of the Ga-ions implanted into the material both during sputtering and deposition has been realized as a key parameter for the function of FIB processed microelectronic devices. For Si-based semiconductors Ga can be used as dopant intentionally implanted into a Si substrate to locally modify the conductivity of Si. The results of locally confined ion irradiation on the surface roughness of a Si surface have been exploited by atomic force microscopy (AFM). Both local sputter depletion of the sample surface as well as sub-µm deposition of selected metals or dielectrics by ion-induced chemical vapor deposition (CVD) has been examined. The penetration depth and the distribution of Ga ions during the deposition process have been studied by simulation and experimentally by profiling with secondary ion mass spectroscopy (SIMS). Transmission Electron Microscopy (TEM) of cross-sections of the ion processed materials has revealed amorphisation of the crystalline substrate. For focused ion beam assisted deposition the effects of ion irradiation on the interface to the substrate and the local efficiency of the deposition are illustrated and discussed. The prospects of focused ion beam processing for modification of microelectronic devices in the sub-µm range and the limitations are demonstrated by the examples shown.