Focused ion beam milling of three dimensional nanostructures with high precision

The fabrication of an extended three-dimensional nanostructure with dimensions much larger than the feature size using a focused ion beam is described. By milling two identical patterns of pores with a designed diameter of 460 nm in orthogonal directions, a photonic crystal with an inverse woodpile structure was made in a gallium phosphide single crystal. The patterns are aligned with an unprecedented accuracy of 30 nm with respect to each other. The influence of GaP redeposition on the depth, shape, and size of the pores is described. The work is published in J. Vac. Sci. Technol. B [1]. An inverse woodpiles tructure was milled in GaP. We chose GaP because of its high index of refraction and large electronic gap, with a view to eventually making photonic crystals for the visible range. There are few alternative methods to focused ion beam milling in GaP, in contrast to Si, for which many different processing techniques have been developed [2,3]. This makes the study of the milling of GaP with a focused ion beam relevant. The parameters of the inverted woodpile structure that was milled were chosen such that a photonic band gap would, in principle, be centered at wavelengths around 2000 nm. Therefore the diameter D of the pores was 460 nm, and the lattice parameters were a = 958 nm, and c = 677 nm. The thickness of the GaP walls between the pores is only 253 nm. When three-dimensional structures with dimensions that are large compared to their smallest features have to be fabricated, the mutual alignment of the various parts poses a challenge. The orthogonal components are aligned with an accuracy better than 30 nm by using simple 5 m μ x z y Figure 1: Three dimensional inverse woodpile nanostructure made by focused ion beam milling. The x, y, and z axes are indicated. 42 Workshop FIB for Photonics, Eindhoven 2008