Fabrication of three-dimensional nanostructures by focused ion beam milling
Schematic drawing of an inverted woodpile photonic crystal structure. The , , and directions are indicated. The crystal consists of two 2D lattices of pores that run in the and crystallographic directions: the and pores, respectively. A unit cell of the 2D lattice is indicated, with lattice parameters and . The pore diameter is also indicated.
The holder, used to rotate the samples in the focused ion beam, was made from aluminum, with a copper clip to hold the sample in place. For the milling of the first pattern (the pores) the holder is placed in the position shown in (A). When the holder is rotated 180° (B) the other side of the sample is exposed to the beam and the second pattern (the pores) can be etched.
Specially milled marks [the two vertical lines on the upper left corner of the sample, indicated by (A)] were used to align the two orthogonal patterns of pores. A streamfile containing a cross-shaped pattern [indicated by (B)] was drawn on the screen of the focused ion beam apparatus, and the vertical bar of the cross was located exactly between the alignment marks. In this way we achieved an alignment accuracy of approximately 30 nm in the direction. The edge of the sample was aligned to the horizontal bar of the cross, as shown in the figure. The streamfile that contained the second pattern of holes [shown as the circles indicated by (C)] was then placed at known distances to the first streamfile.
SEM micrograph of a thin, three-dimensional photonic structure made in GaP using a focused ion beam. The structure is oriented in the same way as the structure in Fig. 1. The three squares underneath the slab are markers for the automatic drift correction. The two narrow lines on the right side of the slab are marks used to align the two orthogonal sets of pores with an accuracy of approximately 30 nm. The important parameters for the structure are , , and .
Average diameter of all pores in the slab as a function of the depth inside the pore (connected squares). The average diameter of five pores made in bulk material as a function of the depth is shown as connected circles. The tapering of the pores in the slab is much less than that of the pores in bulk material.
Diameters of the pores as a function of the distance to the front face of the porous slab, and the depth in the pore, measured from the top of the slab. The axis on top of the figure gives the distance of the pores to the nearest wall of the slab. Pores that are closer to one of the walls have a larger diameter than at the center of the slab.
Average diameter of the pores as a function of their distance to the top of the porous wall (black squares). The average thickness of the redeposited layers inside the pores is also indicated (red circles).
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