Microscale Laser Shock Peening of Thin Films, Part 1: Experiment, Modeling and Simulation

Abstract

More recently, reliability and failure problems in MEMS have attracted increasing attention @1,2#. While the dominant material in MEMS is silicon, metal and metallic thin film structures are often indispensable. Non-metal plates suffer from electrostatic stiction, which has become an important cause of MEMS failure. For metal structures, electrostatic stiction can be avoided and metal connection line can be integrated naturally with the metal structure. Aluminum thin film microwave switch was demon- strated to have very low current loss due to its small dimension and its metal structure @3#. These metallic thin film structures are typically made by patterning the thin film first and then sacrificing part of the supporting substrate. Wearing of rubbing surfaces is the major cause of failure for silicon-based micro-engines @4#, while tungsten-coated polysilicon micro-engines show much higher wear resistance than pure poly- silicon structures @5#. The strength of silicon is determined by the integrity of the lattice. When defects or micro-cracks are present, the strength of silicon decreases sharply @6#. Thus, when the struc- ture is irregular or when the aspect ratio of the structure is high, high quality lithography of silicon is difficult to achieve and de- fects in the structure may lead to serious reliability problems. Thicker microstructures ( .10 microns) and higher aspect-ratio microstructure can be more economically and conveniently fabri- cated using electroforming ~electroplating! techniques @7,8# .W ei et al. @9# demonstrated that high aspect ratio and complex geom- etry nickel microstructures could be obtained using electroform- ing techniques. Electroforming ~electroplating! techniques are pri- marily for metals. Some of these metal microstructures, such as micro- electromechanical actuators, metal gears, and metal switches, ex- perience cyclic loads in service. Wear resistance and fatigue per- formance of these metal structures should be improved to increase the reliability of the system. Microscale Laser Shock Peening ~LSP!, also known as Laser Shock processing, has been shown to efficiently induce favorable residual stress distributions in bulk metal targets with micron-level spatial resolution @10,11# .I t may potentially be used to improve the wear resistance and fatigue performance of metal film structures. The most popular metallic MEMS materials are copper, nickel and aluminum. Microscale LSP will not bring adverse effects to the micro-devices, since thermal effects are typically shielded by an ablative coating, and the water curtain typically used in LSP will carry away possible contamination of vaporized materials. Water does not have com- patibility problems with MEMS materials and structures. To apply microscale LSP to metallic components in microsys- tems, metallic films on silicon substrates need to be considered because LSP needs to be applied to the film before these compo- nents are patterned on the film and the silicon substrate is then partially sacrificed. It is necessary to understand how the thin film material on silicon substrate responds to LSP. The application of microscale LSP to metal films with a thickness less than 10 mi- crons has not been studied in the literature. The microscale also poses challenges in terms of material characterization. Conven- tional X-ray diffractometry does not provide the spatial resolution necessary to characterize residual stress distribution with micros- cale resolution. Recent advances in X-ray microdiffraction offer promise and need to be investigated. In Part 1 of this study, experiments of microscale LSP of copper thin films on silicon substrates are carried out. Preliminary curva- ture measurements prove that favorable compressive residual stress can be induced by microscale LSP. The improved modeling work of microscale LSP is described and simulation results of stress/strain analysis of layered thin film structures are presented. High-spatial resolution characterization of the stress/strain field in thin films will be presented in Part II of this study. X-ray micro- diffraction @12# and instrumented nanoindentation @13# are used for the first time to measure the stress/strain variations in the shock treated thin films with micron-level spatial resolution.