Characterization of postfabricated parylene C coatings inside PDMS microdevices

Abstract

This work investigates the extent to which parylene can coat the interior of PDMS structures after assembly. Parylene coatings have historically been used as conformal, pinhole-free, gas-impermeable coatings on a wide range of devices. However, this work is the first to systematically characterize how the porous PDMS substrate can affect parylene coating quality. Our work shows that thin coatings of less than 1 μm thickness may contain pinholes, which permit the permeation of gases and solvents into the surrounding bulk PDMS structure. The penetration of parylene into PDMS matrix that occurs during deposition was measured using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The thickness uniformity and penetration depth of vapor-deposited parylene C coatings into narrow PDMS channels was measured using profilometry and correlated with an analytical model. The coating along the length of a microchannel is not homogeneous. The parylene film thickness decreases with increasing distance from the channel access holes. Parylene coatings greater than 1 μm form an airtight seal, and air bubbles trapped in dead end channels do not escape through this wall. Fluorescence of rhodamine diffusing beyond the coating barrier in a channel was used to confirm the parylene penetration distance, as well as determine the position at which pinholes appear in the coating. We identify a characteristic length scale which allows us to calculate a dimensionless penetration length for a given set of deposition parameters. This result is applicable to other geometries, to enable optimal design of structures which require this coating.