Swelling-Etching Characterization of PDMS-Copper Particle for the Development of the Nano/Micro-Particle Polymer Composite MEMS Corrosion Sensor

The concept of using Micro-Electro-Mechanical Systems (MEMS) for in-situ corrosion sensing and for long-term applications has been proposed and is currently under development by our research lab. This is a new type of sensing using MEMS technology and, to the knowledge of our team, has not been explored previously. The MEMS corrosion sensor is based on the oxidation of metal nano/micro-particle embedded in elastomeric polymer to form a composite sensing element. The polymer controls the diffusion into and out of the sensor while the corrosion of the metal particles inhibits electrical conduction which is used as the detection signal. The work presented here is based on the experimental wet-chemistry method developed for the removal of native and process-induced metal oxides. A major aspect is the study of the swelling dynamics of the polymer matrix (polydimethylsiloxane, PDMS) and its influence on the material transport of etchants into and reaction products out of the composite during oxide removal. The understanding of this process is important to the ultimate development of the MEMS corrosion sensor, where the swelling must be controlled sufficiently to prevent stress-induced delamination of the sensing element from the substrate. For the first time, the simultaneous swelling (toluene) and oxide removal (acetic acid) characterization of the copper particles embedded in PDMS will be published. The experimental setup consists of small disk samples of the composite (6.35mm diameter × 1mm thick) dropped into a 50mL beaker filled with the swelling agent (the amount of which is part of the parametric study). This is then followed by the addition of the etching agent after reaching the swelling time constants characterized in prior work. The full process is captured through a HD webcam set inverted underneath the beaker. The swelling and contraction process is analyzed in real-time, while post processing of the recorded video is available to verify unexpected responses. Data gather so far indicate interesting swelling recovery phenomenon that is mostly consistent with the two part kinetics of swelling and etching. However, some data also seem to suggest a third mechanism that may be explained by material stiffness changes as etching and swelling proceed simultaneously. A full set of data currently being gathered will provide more clues and aid in the development of a consistent theoretical explanation. The final goal of this series of experiment is to provide mathematical models of the observed phenomenon so they can be used to aid the development of the fabrication processes of the MEMS corrosion sensor.