Selective Iterative Etching of Fused Silica with Gaseous Hydrofluoric Acid Francesco Venturini,* ,† Walter Navarrini, † Giuseppe Resnati, † Pierangelo Metrangolo, † Rebeca Martinez Vazquez, ‡ Roberto Osellame, ‡ and Giulio Cerullo ‡ Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Via Luigi Mancinelli, 7, 20133 Milan, Italy, and Istituto di Fotonica e NanotecnologiesCNR, Dipartimento di FisicasPolitecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milan, Italy ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: September 15, 2010 Femtosecond laser irradiation followed by chemical etching (FLICE) with hydrofluoric acid (HF) is an emerging technique for the fabrication of directly buried, three-dimensional microfluidic channels. With liquid HF, the etching process is diffusion-limited and is self-terminating, leading to maximum microchannel lengths of ≈1.5 mm. A strategy to overcome this limitation would be to perform iterative etching, periodically removing the exhausted products and replenishing the partially etched channel with fresh acid; this procedure is, however, quite cumbersome in the liquid phase. In this paper we present what is to our knowledge the first implementation of the FLICE technique with low-pressure gaseous HF etchant. The use of a gas-phase etchant naturally lends itself to the application of iterative etching techniques, since it is very easy to remove the etchant, by pumping vacuum in the reaction chamber after each etching step. We demonstrate that iterative etching in the gas phase overcomes the limitations of wet etching and allows to achieve nearly constant etching rate for a microchannel length up to ≈3 mm. Introduction Femtosecond laser micromachining has recently emerged as a powerful microfabrication technology for transparent materials. 1,2 Focused femtosecond laser pulses allow, by nonlinear absorp- tion, the deposition of energy inside the bulk of the material in a limited region around the focal volume, resulting in permanent alterations of the material properties that can be used to produce a variety of microdevices. Femtosecond microfabrication pre- sents some unique advantages over traditional approaches: 3–5 (i) it is a direct, maskless technique, which does not require a clean room facility and is thus very suited to rapid prototyping of devices; (ii) it has unique three-dimensional capabilities which are very valuable in manufacturing compact microsystems and in implementing innovative architectures. Nonlinear absorption of ultrashort pulses in fused silica can lead to two regimes of material processing: for a fluence just above the permanent modification threshold, a smooth refractive index change is achieved, which can be positive or negative depending on the irradiation parameters; 6 for higher fluence, subwavelength nanocracks 7,8 are formed, oriented perpendicu- larly to the writing laser polarization and with period ∼λ/2n, where λ is the laser wavelength and n the refractive index of the substrate. The former regime is typically exploited to produce optical waveguides and micro-optical elements (e.g., diffraction gratings, Fresnel lenses), while the latter regime, in combination with selective hydrofluoric acid (HF) etching, is used to manufacture directly buried microfluidic channels, 9–12 by a technique known as femtosecond laser irradiation followed by chemical etching (FLICE). The capability of femtosecond laser micromachining to produce both optical waveguides and microchannels is particularly interesting because it enables the fabrication of novel integrated devices for optofluidics, 13–16 which is a rapidly emerging field with far-reaching applica- tions. 17,18 The physicochemical mechanisms underlying FLICE, i.e., the reasons why femtosecond laser irradiated tracks in fused silica are preferentially etched by HF with respect to the pristine material, have been long discussed since the introduction of this technique. Initial papers attributed the selective etching of irradiated regions to the fact that femtosecond lasers can increase the local density of fused silica, thus reducing the bridging angle of the SiO 4 tetrahedrons; the decrease of the bridging angle increases the reactivity of oxygen and thus the etching rate of the irradiated regions. 9,10 Subsequent works demonstrated that femtosecond laser irradiation creates self-aligned nanocracks in fused silica, propagating in planes orthogonal to the laser polarization. 7,11 This discovery allowed a deeper understanding of the mechanism creating the microchannels during the etching step. It was indeed demonstrated that the aqueous HF solution penetrates the nanocracks and diffuses along the irradiated track, thus causing a faster removal of the fused silica in that region. The increased reactivity of the densified fused silica may still play a role, but the diffusion of HF in the nanocracks is the main driving mechanism, as demonstrated by the observation of very limited etching selectivity in tracks irradiated with a polarization parallel to the track (i.e., with the nanocracks aligned perpendicularly to the track). 11 A major limitation of FLICE with liquid HF etching in fused silica is the fact that the process is self-terminating, with a maximum achievable microchannel length of ≈1.5 mm, typi- cally obtained by etching for 3 h in 20% HF in water. 12,13 If longer etching is performed, the channel increases only in width but not in length, thus worsening the aspect ratio. It should be noted that recently KOH was proposed as an alternative etchant that does not present the self-termination problem. 19 For KOH the selectivity mechanism is completely different with respect to HF and is attributed to the structural changes induced in SiO 2 * Corresponding author. Tel. +39 02 23993035. E-mail address: francesco.venturini@chem.polimi.it. † Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano. ‡ Dipartimento di FisicasPolitecnico di Milano. J. Phys. Chem. C 2010, 114, 18712–18716 18712 10.1021/jp107055s  2010 American Chemical Society Published on Web 10/13/2010