A microscopic view on acoustomigration

Stress-incluced material transport in surface acoustic wave devices, so-called acoustomigration, is a prominent failure mechanism, especially in high-power applications. We used scanning probe microscopy techniques to study acoustomigration of metal structures in-situ, i.e., during the high-power loading of the device. Scanning acoustic force microscopy (SAFM) allows for the simultaneous measurement of the acoustic wavefield and the topography with submicron lateral resolution. High-resolution microscopy is essential as acoustomigration is a phenomenon that not only results in the formation of more macroscopic voids and hillocks but also affects the microscopic grain structure of the film. We present acoustic wavefield and topographic image sequences giving a clear insight into the nature of the film damage on a submicron scale. The 900 MHz test structures were fabricated on 36 degrees YX-lithium tantalate (YX-LiTaO3) and incorporated 420-nm thick aluminium (Al) electrodes. By correlating the acoustic wavefield mapping and the local changes in topography, we confirmed model calculations that predict the correspondence of damage and stress (i.e., hillocks and voids) are preferentially formed in areas of high stress. The way the film is damaged does not significantly depend on the applied power (for typical power levels used in this study). Furthermore, acoustomigration leads to smoother surfaces via lateral grain growth. Another contribution to the grain dynamics comes from the apparent grain rotation in the highly anisotropic stress field of an acoustic wave. Thus, through in-situ scanning probe microscopy techniques, one can observe the initial changes of the grain structure in order to obtain a more detailed picture of the phenomenon of acoustornigration.