The Effect of Implanted Gallium on the Recrystallization of Amorphous Layers formed during FIB Milling of Silicon

Abstract

The focused ion beam (FIB) miller allows preparation of site-specific transmission electron microscopy (TEM) specimens from a wide range of materials in both cross-sectional and planar configurations [1,2]. However, radiation damage during exposure to the high-energy gallium beam may result in the formation of amorphous regions on thin film specimens. The thickness of such damage layers, on both sides of a TEM specimen, is comparable with the thickness required for lattice imaging. For example, the thickness of an amorphous layer in Si after 30 kV Ga⁺ FIB processing has been reported in the range from 15 [3] to 28 nm [4]. This problem limits the capabilities of FIB sample fabrication.

The aim of this study was to investigate, in detail, the structure, composition and the thickness of the damage layers in Si specimens after milling with a gallium ion beam. Using a FEI xP200 FIB system, with 30 kV Ga⁺ ions, a row of trenches on a silicon sample was milled under different beam currents ranging from 150 to 6600 pA. The average size of such trenches was 15×10 μm wide and 1 μm deep. The trenches were then removed from the FIB and sputter coated with a thick Au film to preserve the trench surfaces from further damage during subsequent TEM specimen preparation steps. Cross-sectional TEM specimens of the trench walls were then prepared using standard FIB procedures [5]. Observations were made using a Philips CM 200 Field Emission Gun TEM operating at an accelerating voltage of 200 kV.