Ion milling involves a sputtering process that is capable of removing very fine quantities of sample material. This method is often used as a final polish for TEM specimens that have been previously polished by mechanical means. Ion milling utilizes an inert gas such as Argon which passes through an electric field to generate a stream of plasma that is directed at the region of interest on the sample. Acceleration energies are generally from a few hundred volts to a few kilovolts. The TEM sample may be rotated to promote even polishing as well as tilted to control the angle of milling. The sputtering rate of ion milling is limited to a few tens of microns per hour, limiting the method to already pre-thinned samples requiring only final milling.
More recently, methods involving the use of focused ion beam tools (FIB) have been used to prepare samples for TEM examination from larger bulk materials. A principal advantage of the FIB technique is that the nanometer-scale resolution allows for a precise region of interest to be chosen for thinning. The drawback of FIB milling is that it causes surface damage in the form of amorphization as well as Ga implantation. The damaged layer can be minimized by FIB milling with lower acceleration voltages, or by subsequent milling with a low-voltage Argon ion beam.
Mechanical polishing is one methodology often used for preparing TEM samples. Polishing needs to be performed to a high degree of accuracy and quality to ensure uniformity of thickness, smooth, and defect free region of interest. Dimpling and tripod polishing are generally utilized in conjunction with hard abrasives (films and slurries) composed of such materials as diamond, aluminum oxide, and cubic boron nitride. Even after polishing, additional thinning methods such as ion milling may be required in some cases.