Morphology and Wettability of Polystyrene film on QCM Sensor Caused by Oxygen Plasma with DC Bias

Surface wettability and morphology are important aspects of developing QCM sensors for chemical sensors and biosensors. The functionality and morphology of the sensitive coating on the sensor surface affect the sensor sensitivity and selectivity. Various materials and methods to improve the functionality and morphology of the sensor surface have been developed, one of which is plasma. Species and parameters of the generated plasma play an important role in the treatment. One parameter is the DC-bias voltage of RF plasma, which plays an important role in plasma generation and bombardment on the surface being modified. This work shows the effect of the DC bias of the 2 MHz RF plasma with oxygen species. Surface functionality and morphology modification of the polystyrene film on the QCM sensor was done by surface modification using oxygen plasma. Partial etching of the polystyrene film occurred when treated with oxygen plasma generated using 2 MHz with negative DC-bias. C^II, CO+, and CO were detected during the exposure of oxygen plasma on polystyrene film, indicating a chemical reaction between the plasma species, oxygen plasma, and the polystyrene. Observation by optical microscope and SEM showed the occurrence of partial etching on the polystyrene film. As there is no masking, the partial etching occurred randomly. The average size of the etched film increased with the DC bias voltage, but the percentage of the etched area was not. The oxygen plasma converts the polystyrene film from hydrophobic to hydrophilic, but hydrophobicity increases with the DC bias voltage.