Redeposition Effects During the FIB Milling of Silicon

Abstract

The technique for the preparation of specimens for transmission electron microscopy (TEM) using the focused ion beam (FIB) miller typically consists of a series of milling steps performed over both sides of an area of interest until an electron transparent membrane is achieved [1]. This process is often accompanied by the formation of damage layers on the surfaces of the specimen. The origins of any damage layer are still not clear. On one hand the process of amorphisation of the target material by the highly energetic ion beam is well known. Alternatively, other workers have reported that this damage layer can be connected with redeposition of milled material. [2,3]. in this paper we have studied redeposition effects during FIB milling of silicon TEM specimens.

A FEI xP200 FIB system with a Ga⁺ ion source operating at 30 kV was used in this work. to study redeposition effects a row of trenches on a silicon specimen was milled under different beam currents ranging from 1000 to 6600 pA. The size of such trenches was 15x10 μm wide and 1 μm deep. The specimen was then removed from the FIB and sputter coated with a ∼50-100 nm thick Au film to preserve the trench surfaces from further damage during subsequent milling. The specimen was then placed back in the FIB system and a second set of trenches 5×8 μm wide and 0.6 μm deep was milled on the bottom of first set of trenches (Fig. 1a). The specimens were sputter coated with Au again and were placed back in the FIB system and the trenches were then covered with 1 μ thick Pt strips using the metal deposition facility of the FIB. The presence of these protection layers (Au and Pt) ensures that the final TEM specimen have unmodified original damage layers resulting from the initial milling steps. Cross-sectional TEM specimens of the trench walls were then prepared using normal FIB procedures (Fig. 1b) [2].