Adsorption and growth of dip-coating prepolymer films on silicon wafers. An atomic force microscope study

Abstract

Ultrathin films of a prepolymer of the dicyanate of bisphenole A were made by dip-coating on commercial silicon wafers. The initial stages of film formation could be characterized in detail by AFM (atomic force microscopy) in the tapping mode and in a newly developed deformation mapping mode. The interpretation is assisted by simple molecular modelling as well as surface analytical investigations. The thinnest films are discontinuous and have a mean thickness <1 nm (ellipsometry). They consist primarily of an amorphous deformable phase which probably represents monomer. Even for the tiny roughness of the wafers (0.13 nm RMS), the initial adsorption is preferred in substrate depressions. High resolution imaging could be reproducibly achieved even for this amorphous film surface. An extended contact time of the wafer with the prepolymer solution induces the adsorption of adsorbates, whose heights correspond to the next larger species of the prepolymer, the trimer. Further increase in contact time results in a close packing of these adsorbates and such with double the height. After a contact time of about one minute, a steady state of the film morphology is reached. Then, the mean film thickness can only be raised by increasing the concentration of the solution. In addition, all films comprise large prepolymer agglomerates which represent conglomerates already present in the solution above a small, critical concentration.