Vapor phase deposition of fluoroalkyl trichlorosilanes on silicon and glass: Influence of deposition conditions and chain length on wettability and adhesion forces

Vapor phase deposition (VPD) of a homologous series of fluoroalkyl silanes (FAS), containing 3 to 10 carbon atoms in a molecule, on silicon and glass substrates was investigated. Two deposition procedures were compared: 1) VPD carried out at atmospheric pressure, where silanization reactions occur in an unrestricted presence of FAS vapors and atmospheric water and 2) VPD carried out under evacuated conditions. Significant differences between topographies of the coatings obtained in the two VPD processes were found, suggesting that silanization conditions highly influenced molecular packing densities of the coatings. This, in turn, was considered to influence the trends in the acid-base (AB) component of surface free energy (SFE), macroscopic work of water-substrate adhesion and locally measured adhesion forces on these surfaces. The AB interactions were found to significantly influence wetting and adhesive properties of coatings obtained from short chain precursors by atmospheric pressure VPD (AP-VPD). Wettability and adhesive force on coatings from longer chain precursors deposited by AP-VPD and on coatings from the whole range of tested FAS precursors deposited in low-pressure VPD (LP-VPD) were found to be predominantly governed by dispersive interactions. Correlations between locally measured adhesive force and macroscopic work of adhesion were found. Considering the net effect of FAS deposition procedure on the adhesive force strength, LP-VPD was found to more effectively reduce adhesive force on Si substrates, and both deposition methods were found to be equally effective on glass substrates. Advantages and disadvantages of both deposition methods were highlighted, with references to their most suitable applications.