Side-wall damage in a transmission electron microscopy specimen of crystalline Si prepared by focused ion beam etching

In focused ion beam (FIB) fabrication of cross-sectional transmission electron microscopy (X-TEM) specimens, highly accelerated ion beams sometimes cause serious damage. The damage can be induced in both the specimen surface and the side walls. We used the X-TEM technique to investigate the sidewall damage in crystalline Si. The damaged layer was found to be about 20 nm thick in the case of 30-keV-FIB etching. We evaluated several techniques for reducing the damage, such as gas-assisted etching (GAE) with iodine, low-energy FIB etching, and cleaning by broad argon ion milling or by wet etching. The damage depth was 19 nm for GAE and 10 nm for 10 keV FIB etching, and was reduced to 7 nm by 3 keV argon ion milling with a beam current of 20 μA and a tilt angle between the beam and the specimen of 4°. Wet etching using a mixture of nitric and hydrofluoric acid removed most of the damaged layer. The effect of the damaged layer on TEM observation was also investigated, and it was shown that removal of the damaged layer results in a high-resolution TEM image.