A Focused Ion Beam Specimen Preparation Method to Minimize Gallium Ion Concentration in Copper Atom-Probe Tomography Specimen Tips

Atom-probe tomography (APT) is a quantitative technique that permits three-dimensional (3-D) spectroscopic characterization of interfaces and other nanometer-scale features within a material. Specimens for atom-probe tomography (APT) analysis of semiconductor devices and nanostructured materials are typically fabricated employing a focused ion beam (FIB) instrument [1 3] or a dual-beam FIB instrument [4 6]. One method used to fabricate APT specimen tips from bulk material is an in situ lift-out method that is adapted from a standard lift-out method for transmission electron microscopy (TEM) specimens [7]. This method employs a series of sequential steps to create a specimen tip with a radius < ~ 100 nm that is appropriate for APT analysis. The employment of a FIB instrument provides several advantages over conventional two-step electro-polishing methods to include applicability to all materials, reduction of preferential etching in multi-phase materials, reduction in use of hazardous chemicals, and increased control during specimen preparation. It also permits fabrication of APT specimen tips from alternative material geometries, such as thin sheets, flakes, ribbons, powders, and thin films that may be difficult to work with when using electro-polishing methods [5,6]. However, one well-known disadvantage of using the FIB to prepare specimen tips is the implantation of gallium (Ga) ions into the tip material. Ga ion implantation is observed as artifact peaks in the mass spectra at a massto-charge-state (m/n) ratio of 69 and 71. Excessive Ga ion implantation may hinder the compositional analysis of semiconductor devices and nanostructured materials. Additionally, it is known that copper (Cu), which is a commercially important material for semiconductor devices and other nanostructured materials, exhibits an irregular surface topography under a wide range of milling conditions and increased redeposition of sputtered material [7]. Thus, a FIB-based technique that minimizes Ga ion implantation and creates consistent specimen geometries for Cu is advantageous for APT analysis.