Focused ion beam machined nanostructures depth profiled by macrochannelling ion beam analysis

Abstract

High aspect ratio sub-μm periodic structures fabricated by focused ion beam (FIB) lithography have been characterised by Rutherford backscattering spectrometry (RBS) using the macrochannelling technique. The technique overcomes the limitations of complementary techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), which can provide images with sub-μm resolution of just the surface features and not of the deep sub-surface structures, without destructive cross sectioning of the sample. Here RBS macrochannelling with a 2MeV He+ ion beam is used to analyse a diffraction grating fabricated by FIB milling an array of 100nm wide trenches in a 300nm thick Ag film on a Si substrate. Using the surface structure imaged by SEM and AFM as a starting point, a numerical model for the RBS spectrum from the grating is fitted to the experimental spectrum as a function of the sub-surface structure. This process allows the width of the trenches to be determined as a function of depth even though the lateral structure is not resolved by the ion beam.