Research
Materials for Printed MEMS
Project Overview
The structural layers we are investigating for printing MEMS are sintered metal nanoparticles inks (e.g. silver, copper, gold, alloys). These inks have been developed and optimized for their electrical properties, but have not yet been characterized and optimized for their mechanical properties. Nanoparticle inks are stable dispersions of nanoparticles in a liquid carrier. The surface of the nanoparticles have been modified with a polymer capping layer so that the particles form a stable colloidal dispersion, and the surface tension and viscosity of the dispersion have been optimized for inkjet printing.
Due to the small size of the nanoparticles they have a reduced melting point, as shown in the figure below (left), allowing the particles to be sintered at temperatures that are much lower than the melting temperature of the related bulk materials . This enables printing of conducting features on low-cost substrates such as paper, plastic and glass, and the reduced thermal budget may enable integration of printed MEMS with microelectronics. The thermal processing of nanoparticle inks is shown on the right in the figure below.
(Left) Reduced melting temperature of nanoparticles. (Right) Resistivity as a function of curing temperature (from Cabot-PEDS).
When the particles are printed and dried at room temperature they are electrically insulated from each other by their polymeric shells, as shown in the figure below (left, top). Low temperature (125C) curing allows the shells to flow and the nanoparticles to come into electrical contact with each other (left, bottom). Once the polymer shells have reflowed an electrically continuous percolation network is formed that is conducting. Increasing the curing temperature from 125C to 250C decreases the resistivity and further volatizes the polymer shell, which is eventually eliminated. A high resolution micrograph of sintered silver nanoparticles is shown in the figure below (right).
(Left) Schematic cross-section of the low-temperature sintering process. (Top) After room temperature deposition the nanoparticles (silver) are insulated from each other by the polymeric shells (red). (Bottom) After a low-temperature anneal at 125C they are in electrical contact. (Right) High-resolution SEM image of AG-IJ-G-100-S1 silver nanoparticle ink oven-cured at 150C for 30 minutes (from Cabot-PEDS).