Ion thinning of TEM cross sections under beam switching control

Abstract

Ion milled, cross sectional TEM specimens are used extensively to study the microstructure of multilayered materials. Unfortunately, it is difficult to get all the layers in a cross section equally electron transparent because usually they mill at different rates and have different atomic numbers. Differential thinning can be minimized by milling at low angles so that the slow milling layers protect the fast milling layers from the ion beam. However, even at low angles uneven milling can still occur when the ion beam travels along the multilayer interface (glue line). This problem can be partially overcome by using a shield to cut off the ion beam in these directions. However, this solution is not completely satisfactory because material sputtered from the edge of the shield can contaminate the specimen. A better solution, is to rock the specimen over a range of angles while the ion beam is centered approximately about the perpendicular to the glue line. With this method there is a possibility of uneven milling at both ends of the rocking angle where the beam is stationary for a short period during the change in direction of specimen rotation. Another solution, which is reported here, is to rotate the specimen continuously in the normal way and to switch off the ion beam as it passes by along the specimen glue line.