title: Design of experiment for optimization of plasma-polymerized octafluorocyclobutane coating on very high aspect ratio silicon molds.
abstract: As-fabricated deep reactive ion etched (DRIE) silicon mold with very high aspect ratio (>10) feature patterns is unsuitable for poly(dimethylsiloxane) (PDMS) replication because of the strong interaction between the Si surface and the replica and the corrugated mold sidewalls. The silicon mold can be conveniently passivated via plasma polymerization of octafluorocyclobutane (C4F8), which is also employed in the DRIE process itself, to enable the mold to be used repeatedly. To optimize the passivation conditions, we have undertaken a Box-Behnken experimental design on the basis of three passivation process parameters (plasma power, C4F8 flow rate, and deposition time). The measured responses were fluorinated film thickness, demolding status/success, demolding force, and fluorine/carbon ratio on the fifth replica surface. The optimal passivation process conditions were predicted to be an input power of 195 W, a C4F8 flow rate of 57 sccm, and a deposition time of 364 s; these were verified experimentally to have high accuracy. Demolding success requires medium-deposited film thickness (66-91 nm), and the thickness of the deposited films correlated strongly with deposition time. At moderate to high ranges, increased plasma power or gas flow rate promoted polymerization over reactive etching of the film. It was also found that small quantities of the fluorinated surface were transferred from the Si mold to the PDMS at each replication, entailing progressive wear of the fluorinated layer.
authors.affiliation: The Singapore-MIT Alliance, Innovation in Manufacturing Systems and Technology Program, School of Chemical and Biomedical Engineering, and School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore.
authors.forename: L P
authors.initials: LP
authors.lastname: Yeo
authors.forename: Y H
authors.initials: YH
authors.lastname: Yan
authors.forename: Y C
authors.initials: YC
authors.lastname: Lam
authors.forename: Mary B
authors.initials: MB
authors.lastname: Chan-Park
mesh_terms: D001672:Biocompatible Materials / Q000737:chemistry*; D017402:Chlorofluorocarbons; D005617:Chlorofluorocarbons, Methane / Q000737:chemistry*; D020099:Coated Materials, Biocompatible / Q000737:chemistry; D004129:Dimethylpolysiloxanes / Q000737:chemistry; D004867:Equipment Design; D005461:Fluorine / Q000737:chemistry; D008855:Microscopy, Electron, Scanning; D015233:Models, Statistical; D008962:Models, Theoretical; D009757:Nylons / Q000737:chemistry; D012825:Silicon / Q000737:chemistry*; D013052:Spectrometry, X-Ray Emission; D013499:Surface Properties; D005466:Fluorocarbons
chemical_list: D001672:Biocompatible Materials; D017402:Chlorofluorocarbons; D005617:Chlorofluorocarbons, Methane; D020099:Coated Materials, Biocompatible; D004129:Dimethylpolysiloxanes; D005461:Fluorine; D009757:Nylons; D012825:Silicon; C085707:poly(dimethylsiloxane)-polyamide copolymer; C007785:octafluorocyclobutane; D005466:Fluorocarbons