Vapor phase deposition of uniform and ultrathin silanes

Uniform, conformal, and ultrathin coatings are needed on the surface of biomedical microdevices such as microfabricated silicon filters, in order to regulate hydrophilicity and minimize unspecific protein adsorption. Currently, the predominant coating methods involve the assembly of normally a silane "monolayer" onto silicon surfaces in organic solution. A typical prototype molecule is alkyltrichloro-silane. However, this type of molecule is very sensitive to moisture. Even trace amount of water in the organic solution or its environment lead to polymerization. This causes the formation of multilayers with variable thickness, and submicron aggregates or islands on the silicon surface. In this communication we present a vapor-phase coating method of forming a uniform and nanometer thick silanes on silicon surface at ambient pressure using nitrogen as a carrier gas. The modified surface is extremely smooth, with no detectable aggregates. The coatings and subsequent treatments are characterized with ellipsometry, scanning electron microscopy, contact angles, SFG spectroscopy (sum frequency generation), and zeta potential in water. The method of deposition is particularly advantageous whenever it is necessary to coat irregular shapes or channels in microdevices, where liquids may have difficult access due to capillary forces.