Abstract
Surface contact quality of electrodes has been recognized as a key factor in Radio Frequency (RF) Microelectromechanical systems (MEMS) switch performance and reliability design.Topography of the surfaces and contact mechanics determine the quality of the contact.Surface topography contains complex and random signal features, which make measurement,surface characterization /modeling and contact simulations a big challenge in predicting the contact quality. The objective of our research is to develop a methodology to measure,characterize, and model topography for contact modeling and correlating contact properties to the surface characterization.To achieve this, an interdisciplinary approach, which integrates metrology, statistics,fractal mathematics, and contact mechanics, is proposed to achieve the research goal.• Multi-scale sampling plan design: A sampling plan is proposed for surface measurement using atomic force microscopy (AFM). Topography on RF MEMS switch contacting surfaces are scanned by atomic force microscopy (AFM) at different length scales (e.g. 1x1,10x10 and 60x60 μm2). A sample allocation plan is designed to maximize the spatial representative of the AFM scanning patches with different resolutions and uniformly distributed sample patches. The scanning data are used for characterizing, model estimation and contact analysis.• Topography characterization and statistical modeling: A flexible framework is created to accommodate various pattern structures. Regular patterns are found at coarser scales (e.g. 10x10 and 60x60 μm2). Random irregularity and the fractal structure are observed at finer scales (e.g. 1x1 μm2). A regular-fractal model is proposed to decompose and characterize the regular and fractal structures with two model components: one for the regular geometric pattern and the other for fractal irregularity. The former uses a 2D cosine functions to characterize dominant modes in the regular (larger scale) patterns. The later summarizes random irregularity in finer scales with a statistical fractal model estimated from the data on the scattered sample patches. The model validation is made through the comparisons of topography and conventional roughness parameters between the results of simulation from the proposed model and that derived from AFM scanned data.• Elasto-plastic contact analysis: A non-adhesive, frictionless contact analysis is carried out to investigate the contact aspects of RF MEMS capacitive switches by finite element commercial software COMSOL. Elastic-plastic contact model is employed based on the AFM data of different scales to represent the regular pattern dominate surface, regular-fractal,and fractal structure dominate surfaces. Loading and unloading cycles are applied during the contact process to simulate the working status of RF MEMS switches. Dimensionless contact area vs dimensionless load curve is determined and compared for different cycles. Dominant micro-scale regular patterns are found to significantly change the contact behavior. Contact areas mainly clusters around the regular pattern. The contribution from fractal structure is not significant. Under cyclic loading conditions, plastic deformation in 1st loading/unloading cycle smoothen the surface. The subsequent repetitive loading-unloading cycles undergo elastic contact without changing the morphology of the contacting surfaces.The work is expected to shed light on the quality of switch surface contact as well as optimum design of RF MEMS switch surfaces.