A novel Characterization Method for electric Properties of single Silver-coated Polymer Spheres
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The electrical properties of isotropic conductive adhesives (ICAs) are dependent on the filler concentration and electrical properties of the conductive additives. ICAs with metal-coated polymer spheres (MPS) are an efficient way of reducing the amount of precious metals in the adhesive without compromising the condcuting properties. Today little is known about the electrical properties of a single MPS and how the thickness of the metal coating relates to the electrical properties, because no good method exists for measuring electrical properties of spherical thin films of small diameters. In this work, a method for probing silver-coated polymer spheres with micromanipulators, is developed and tested. The measurements were done in a dual beam focused ion beam (FIB)/scanning electron microscope (SEM) with a large vacuum chamber, allowing for manipulation and observation in the nanometer range. Polycrystalline tungsten probes are used for the measurements. They are shaped into flat punch probes with an ion beam and controlled by micromanipulators. Electrical measurements with both two and four probes/wires were performed, the latter found to give precise measurements of the silver coating investigated: The slope of the measured voltage drop divided by the total current was found, V/I, for simplicity called resistance throghout this study. The resistance decrease monotonically with increasing coating thickness. From 0.604 Ω for the 60 nm thick coating, to 0.118 Ω, 0.096 Ω, and 0.079 Ω for the 100 nm, 150 nm, and 270 nm respectively. Computer simulations of the same system conducted with COMSOL Multiphysics, supports the experimental findings. The simulations have resulted in a resistivity prediction model, which predicts the silver coating resistivity given the sphere's geometry and measured resistance. The proposed method proves it possible to conduct measurements on single micron sized silver-coated polymer spheres with a high precision. The promising method lays the foundation for further development of the four-wire setup in addition to even better understanding of the silver coatings.