Relative humidity sensor using plasma polymerized methyl methacrylate (PPMMA)

Thin films of plasma polymerized methyl methacrylate (PPMMA) are deposited in an inductively coupled tubular type reactor using RF excitation and are used in humidity sensing devices. The films are deposited on a glass epoxy substrate having copper electrodes in interdigited form. They are deposited at different RF powers in the range from 10 to 45W keeping other plasma parameters fixed. Thickness is found to be varying between 150-250nm. Chemical structure of polymer is studied using FTIR technique. Basic response of the polymer to relative humidity in the range 0 to 92% is measured, in terms of capacitance variation for films deposited at different powers. The sensitivity, measured as change in capacitance with change in humidity, is found to be different in different humidity ranges. Sensitivity is low in lower region of humidity range and larger in higher range of humidity. For response and recovery time of the film, the change in resistance over time is recorded after every 30 s till the resistance showed saturation. They are found to be 118s and 63s respectively. The results are analyzed and compared with those available in the literature.