Franz Kubat

Calculation and experimental verification of the acoustic stress at GHz frequencies in SAW resonators

High power applications of surface acoustic wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress are shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Damage analysis in Al thin films fatigued at ultrahigh frequencies

A quantitative damage analysis provides insight into the damage mechanisms and lifetimes of aluminum thin films fatigued at ultrahigh frequencies. Surface acoustic wave test devices were used to test continuous and patterned Al thin films up to more than 1014cycles. The analysis revealed increasing extrusion and void formation concentrated at grain boundaries. This finding and the observed grain growth indicate a high material flux at the grain boundaries induced by the cyclic load. A correlation between device degradation and defect density is established which is explained by a theoretical model. For stress amplitudes as low as 14MPa lifetime measurements showed no fatigue limit for 420nm Al thin films.

P-Matrix based calculations of the potential and kinetic power in resonating SAW-structures

Acoustomigration of metal structures is a phenomenon which may occur in Surface Acoustic Wave (SAW) devices operating at high power levels. This deteriorates the performance or can even destroy the SAW device. Acoustomigration is caused by the stored acoustic energy, which can be very high in resonating structures with good quality factors. We extended our P-Matrix based model to calculate. the stored energy in arbitrary SAW structures. We present a method to calculate the total power distribution, and the kinetic and potential power and their relative position within the structure. We discuss the corresponding time, behavior in resonating and non-resonating structures; Acoustomigration patterns have been compared to the calculated results.

A microscopic view on acoustomigration

Stress-induced 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/spl deg/ YX-lithium tantalate (YX-LiTaO/sub 3/) 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 acoustomigration.

Calculation of the SAW-induced stress distributions in an electrode of a SAW-device on LiTaO/sub 3/

Acoustomigration is a well-known phenomenon, which may occur in surface acoustic wave (SAW)-devices operating at high driving levels. Based on previously published reports (Roesler, U. et al., 1996; Kubat, F. et al., 2002; 2003) about the calculation of the acoustic power, dynamic stresses and displacements in a homogenous isotropic Al-layer for a given driving condition, we expand this method for the calculation of the SAW-induced stress in an electrode of a SAW-device. The quantitative calculation of this stress is based on the combination of the widely used P-matrix based model, which yields the distribution of the potential power (Kubat et al., 2002) for a given driving level and a FEM tool (Smole, P. et al., 2002), which provides the relative stresses inside an electrode.

In situ Observations and Quantitative Analysis of Short Circuit Probability Due to Ultrahigh Frequency Fatigue

Ultrahigh frequency fatigue (GHz) can lead to the formation of extrusions in aluminum thin film metallizations. Protruding from the surface, such extrusions can connect adjacent electrodes leading to the formation of short circuits. In situ observations presented in this paper verify this belief. Based on measurements from ex situ experiments, we present a defect-based probabilistic model to predict the failure probability, allowing to estimate the lifetime of test devices undergoing such load conditions.