Self-assembled monolayer (SAM) coatings of n-octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3] were deposited on Si micromolds for micro hot-embossing by dipping the Si molds into an anhydrous toluene solvent containing OTS. The coated samples were designated as OTS20, OTS40, OTS60 and OTS80 with respect to deposition time of 20, 40, 60 and 80 min to study the effect of deposition time on the coating quality. The composition, surface roughness, friction coefficient, thermal stability and surface energy were measured using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), nanotribological test and contact angle test, respectively. The thermal stability of the OTS coatings was determined by measuring water contact angle after heating at various temperatures. The XPS and AFM results showed that a prolonged deposition induced a denser and thicker coating structure and more aggregation of OTS, which also increased the surface roughness. A comparative study of the uncoated and OTS60 coated Si micromolds depicted that the OTS coatings had a good potential to improve the surface quality and efficiency of the molds. The OTS60 coating was evaluated after heat treatment at embossing temperature of 130 oC for a better understanding of the failure mechanism of the micromolds, which showed that the surface properties of the molds remained unchanged at the embossing temperature. Further characterization of the damaged Si micromolds showed that the peel-off of the coatings after a number of replications was the main reason for the failure of the molds. It was found that periodic re-cleaning of the micromolds and re-deposition of the OTS coatings on the coated Si micromolds could extend the lifetime of the molds up to about 112 embossing operations per mold.
Outcomes:
N-octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3] was able to form good quality self-assembled monolayers (SAMs) on Silicon (Si) micromolds, whose performance showed the dependence of the deposition time of the SAMs. XPS results revealed that longer deposition could produce denser and thicker OTS coatings. AFM results indicated that 20 min deposition was not sufficient to fully cover the Si substrate surface while 80 min deposition resulted in agglomerates spread over the OTS80 coating. A deposition for about 60 min could produce a uniform coverage of OTS coatings on the Si mold. All the OTS coatings were hydrophobic in nature and the OTS60 coated micromold showed a lowest surface energy of about 22.9 dyne/cm. The friction coefficient of the OTS20 sample increased continuously with the test duration as a thinner coating was easily worn out. A prolonged deposition (e.g. 40 min or above) could maintain a constant coefficient of friction up to about 1.57 m of sliding distance. A lowest friction coefficient of about 0.07 was observed from the OTS60 coated sample. The OTS coatings enhanced the replication performance of the Si micromolds and the OTS60 coated mold was successfully used for 27 times in a hot-embossing process for fabrication of PMMA microfluidic devices. The PMMA devices fabricated by using the OTS coated micromolds had a much better surface finishing than those produced by the bare Si micromold. Examination of both heat treated and damaged OTS60 coated micromolds showed that hot-embossing temperature used for molding the PMMA devices had no effect on the OTS coatings and no PMMA residues were left on the OTS coated Si micromolds. However, removal of the OTS coatings from the Si substrates during the demolding of the PMMA products was found to be responsible for the failure of the OTS coated molds after a certain number of replications. By periodic re-deposition of the OTS coatings on the coated Si micromolds, the lifetime of the OTS coated molds could be extended up to about 112 times per mold.
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