Preparation of organic monolayers on uniform and patterned silicon substrates

The growth of self-assembled alkylsiloxane monolayers on uniform and patterned silicon substrates has been investigated at room temperature using atomic force microscopy (AFM), contact angle measurements and quartz crystal microbalance (QCM) gravimetry. Immersion of oxidized silicon substrates in a millimolar solution of octadecyltrichlorosilane (OTS) results in the formation of ordered octadecylsiloxane islands with a height close to 2.5 nm. In the area between these islands an additional - presumably disordered - adsorbate layer with a height of about 0.6 nm can be identified. The overall uptake-curves show subtle but significant deviations from the generally assumed first-order Langmuir adsorption kinetics. A nearly perfect fit, however, can be achieved on the basis of a simple model considering the adsorption of initially disordered species which subsequently transform into ordered islands. In this model, the disordered species are believed to occupy a larger surface area per entity and hence prevent adsorption of further molecules before rearrangement takes place. In contrast to oxidized silicon substrates, H-terminated areas on silicon substrates appear to remain uncoated after immersion into an OTS solution. Considering these results, a laser direct writing technique has been used in order to create arbitrarily patterned silicon substrates which expose H-terminated as well as oxidized areas. Starting with a uniformly H-terminated silicon surface this technique allows for writing oxide lines with a lateral resolution arround 500 nm suitable for the selective coating with an alkylsiloxane monolayer.