Focused ion beam imaging of grain growth in copper thin films

Scanning ion microscopy (SIM) employing focused ion beam (FIB) imaging was used to study the grain structure of thin copper films as a function of annealing temperature from 20 to 500°C. Accurate measurement of grain size is obtained for grains as small as 60 nm, allowing the microstructure of copper to be analyzed on small-grained samples which show poor contrast in scanning electron microscopy. Moreover, the short sample preparation time provides an advantage over transmission electron microscopy (TEM). The growth and coalescence of small (<100 nm) grains in the initially bimodal grain size distribution occurs in the temperature range of 250-350°C in films of 1000 nm thickness. This grain growth takes place concurrently with the relaxation of compressive stress as observed by temperature-ramped stress measurement. Also, temperature-ramped in situ TEM examination confirms that coarsening of small grains is the dominant grain growth mechanism up to 500°C.