We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer
(SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers,
which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the
characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides
the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear
geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of
480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear
conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision
and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence
and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic
coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated
glass substrates with a film thickness of 1-2 nm and 40 nm respectively.
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.