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

Francesco Venturini,Giulio Cerullo,Maurizio Sansotera,Rebeca Martinez Vazquez,Walter Navarrini,Roberto Osellame

doi:10.3390/mi3040604

Abstract

Femtosecond laser irradiation followed by chemical etching (FLICE) with hydrogen fluoride (HF) is an emerging technique for the fabrication of directly buried, three-dimensional microfluidic channels in silica. The procedure, as described in literature, consists of irradiating a silica slab followed by chemical etching using hydrogen fluoride. With aqueous HF the etching process is diffusion-limited and is self-terminating, leading to maximum microchannel lengths of about 1.5 mm, while the use of low-pressure gaseous HF etchant can quickly produce 3 mm long channels with an aspect ratio (Length/Diameter) higher than 25. By utilizing this methodology the aspect ratio is not constant, but depends on the length of the channel. When the microchannel is short the aspect ratio increases quickly until it reaches a maximum length at around 1400 µm. Thereafter the aspect ratio starts to decrease slowly. In this paper we present a variation of the low-pressure gaseous HF etching method, which is based on the dynamic displacement of the etchant. This method results in a 13% increase in the aspect ratio (L/D = 29) at the expense of a low etching speed (4 µm/min).

Highlights

Microfluidic devices are interesting tools for synthetic chemistry and analysis
Irradiation followed by Chemical Etching (FLICE) [5,6,7]
We have recently studied a gaseous hydrogen fluoride (HF) implementation of the followed by chemical etching (FLICE) technique, obtaining an aspect ratio higher than 25 and a self-terminating length in excess of 3 mm [12]

Summary

Introduction

Microfluidic devices are interesting tools for synthetic chemistry and analysis. They are manufactured using principally three class of materials: metal, ceramics, glass/silicon or polymers. When compared to silicon, glass has a lower cost, a good transparency and a good corrosion resistance It remains a good candidate for being used as a main substrate in microfluidic devices, assuming a better control of the aspect ratio of the etched channels is possible. Irradiation followed by Chemical Etching (FLICE) [5,6,7] With this technique it is theoretically possible to produce three dimensional micro-channels, chambers and complex structures inside transparent solid materials [8]. 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]. Once the HF has reacted, the products (H2O, SiF4), the unreacted HF and the introduced nitrogen are removed by vacuum

Femtosecond Laser Irradiation

Gaseous HF Etching

HF Etching with Dynamic Displacement

Conclusions