Density of Thin Evaporated Aluminum Films

The mass per unit area of evaporated aluminum films was measured with a piezoelectric quartz crystal microbalance. The thickness of the films was measured by multiple beam interferometry. The density of the films was determined from the sensitivity of the microbalance for a set of aluminum films ranging in thickness from a few hundred to a few thousand angstroms. The sensitivity of the microbalance was determined for a set of AT-cut quartz crystals ranging in fundamental frequency from 2 to 10 Mc/sec. The density of the thinner films is less than the thicker films but rapidly approaches a value of 2.61±0.03 g/cm3 for films in excess of 500 Å. This value is about 3% less than the value of 2.70 g/cm3 for bulk aluminum. The aluminum films were evaporated in a pressure of 2×10−6 Torr with normal incidence on unheated glass and quartz substrates. The low density of the films cannot be explained on the basis of oxide formation in the residual atmosphere. The most likely explanation is that the films are porous which is partially confirmed by surface replication transmission electron micrographs.