Phonon excitation and its significance for electrochemically grown copper layers

Abstract

Electrochemically grown fine and coarsely grained copper films have been exposed to low and strongly damped 5 MHz shear oscillations that travel through them with normal incidence. The real parts of the electrical components such as inductivity L, capacitance C and Ohmic resistance R were evaluated from impedance spectroscopy performed on copper films affixed to 5 MHz quartz oscillators. L, C, R-data were used to calculate the inverse quality factor 1/ Q which is related to the energy dissipated during a single oscillation cycle. Recently, phonon activation has been identified as a major route to explain energy dissipation that occur between sliding surfaces. With the reasonable assumption that shear movement across a grained metal film causes sliding across the grain boundary the sliding distance a s and sliding velocity v s can be evaluated. Their presentation as 1/ Q vs. a s and v s plots prove that energy dissipation is limited to very small ranges for a s and v s. On the basis of this observation atomic scale sliding is subdivided into non-sliding, effective sliding and non-effective sliding. The observed energy dissipation for effective sliding is attributed to the formation of localized phonons which cause the resistivity to increase. As a consequence two conducting states are formed defined as conductivity ground and excited state. One is characterized by a small 1/ Q-value and is attributed to a conducting state which is purely determined by the Ohmic film resistance as prepared. For this situation the current flow through the sample occurs normal to the surface. On the other hand, the conducting state associated with a large 1/ Q-value results from phonon excitation that arise due to the motion across the grain boundary. The resulting increase of the Ohmic resistance causes a change of the current direction from normal to parallel to the surface. These two conducting states coexist during shear oscillation. Their individual contributions to the overall conductivity depends on the number of grains which increase with the layer thickness. The formation of sliding atomic contact points their significance for conductivity and reinforcement between copper grains and between monoatomic and bulk copper layers are discussed as well.