Selective Iterative Etching of Fused Silica with Gaseous Hydrofluoric Acid

Abstract

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.

Femtosecond laser microstructuring: an enabling tool for optofluidic lab‐on‐chips

“…As the channel is progressively produced starting from the end face, it is necessary to remove the reaction products and provide fresh acid, which has to diffuse along the channel. With increasing channel length, the amount of fresh acid able to reach its end reduces and the etch rate gradually decreases [76,77]. This limits the maximum achievable channel length and also results in microchannels with conical shapes, with larger radius at the channel entrance as compared to the buried end (see Fig.…”

Micromanufacturing in Fused Silica via Femtosecond Laser Irradiation Followed by Gas-Phase Chemical Etching

“…The experimental setup used for the laser irradiation process is shown in Figure 1 and extensively documented [11,12]; it is based on a femtosecond laser microfabrication apparatus initially described in [13,14]. It starts with a regeneratively amplified Ti:sapphire laser (model CPA-1 from Clark Instrumentation), generating 150 fs, 500 μJ pulses at 1 kHz repetition rate and 790 nm wavelength.…”

“…The use of low-pressure gaseous HF etchants and the related comparison between liquid and gas phase etching has been extensively discussed by Venturini et al [12]. The aspect ratio of the microchannel obtained using the gaseous etching procedure (described in Section 2.2) with τR = 2min and τD = 3 min is shown in Figure 3 and a picture of the starting channel used for the dynamic displacement tests is shown in Figure 4.…”

“…With this technique it is theoretically possible to produce three dimensional micro-channels, chambers and complex structures inside transparent solid materials [8]. The procedure described in literature consists in a focused laser irradiation of the silica slab followed by a chemical etching using hydrofluoridric acid, either in aqueous [9][10][11] or gas phase [12]. The photomodification is executed by irradiating the transparent material placed on a motorized translation stage with a train of pulses of a focused femtosecond laser beam.…”

“…With aqueous HF the etching process is diffusion-limited and is self-terminating, leading to maximum micro-channel lengths of about 1.5 mm with a poor aspect ratio (Length/Diameter); with aqueous potassium hydroxide the etching process is very slow but do not show self-termination [11]. To overcome these limitations, we have recently studied a gaseous HF implementation of the FLICE technique, obtaining an aspect ratio higher than 25 and a self-terminating length in excess of 3 mm [12]. In this work we further improved the aspect ratio by removing the unreacted HF from the channel inlet and by contemporaneously increasing the HF concentration at the bottom of the channel; this approach has been followed in order to overcome the convective transport ineffectiveness previously observed above a certain channel length.…”