Nonlinear femtosecond laser processing of alkylsiloxane monolayers on surface-oxidized silicon substrates

Femtosecond laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates via single-pulse processing at λ=800 nm, τ<30 fs, and ambient conditions has been investigated. Depending on the laser pulse fluence, local irradiation results in circular spots of distinct size and morphology. At high fluences, a particular rich complexity of distinct surface morphologies is observed including hole, rim, and ripple formation, and a faint boundary area where monolayer decomposition sets in. At low fluences, subwavelength patterning of the organic monolayer is feasible. In particular, at a 1/e laser spot diameter of 1.3 μm, surface spots with a width down to 300 nm are fabricated. Selective processing of the organic monolayer, though, is restricted to a very narrow range of fluences between 1.1 and 1.2 J/cm2. A significantly larger parameter range for selective processing is anticipated in the case of functional monolayers that incorporate aromatic groups. Promising perspectives in femtosecond laser processing of organic monolayers are discussed.