Selective etching of fused silica
The channels in many commercial devices are currently fabricated in glass or engraved into silicon using technologies borrowed from semiconductor processing. These are intrinsically two-dimensional techniques that creates surface trenches that needs to be covered with a glass slab in order to obtain the microfluidic device. Due to its low cost, transparency and good corrosion resistance, glass is the preferred material for hosting microfluidic devices but in the standard micro-fabrication the trenches are mainly etched in silicon while glass is relegated mainly as a top seal for silicon chips. The use of glass as the only substrate for microfluidic devices is relegated to small applications because, on the opposite of silicon, glass is isotropically etched and the absence of a preferential etching direction doesn’t allow to fabricate complex micro-devices inside it. It has been discovered recently that by coupling a femtosecond laser-irradiation with a etching technique it is possible to produce three dimensional micro-channels, chambers and complex structures inside transparent solid materials. This is possible because the laser irradiated volume acquire an increased reactivity to acids such as hydrofluoridric acid. In the most implemented method, a preparation of 3D structures consists of two steps: first, a photomodifcation is executed by irradiating a train of pulses of a focused femtosecond laser beam into the transparent materials placed on a piezoelectric or motorized stage. Second, the irradiated material is immersed in aqueous hydrofluoridric acid for a specified period of time.
Photo of microchannel obtained applying pulses of gaseous hydrogen fluoride and nitrogen. Length = 1.4 mm; aspect ratio = 29. The dark bubble inside the channel is air while the white part is filled with water.
We have identified several flaws in the etching procedures reported in literature that prevents to quickly obtain high aspect ratio micro-channels. To overcome this limits we have studied the first implementation of the femtosecond laser assisted micro-machining (FLICE) technique using gaseous hydrogen fluoride etching. This approach allowed us to quickly obtain double-side etched channels with an unprecedented length and aspect ratio for HF etching of fused silica. The advantage of this technique is that it does not require a “clean room” while having the potential to replace, in some applications, the photo-lithographic approach and the use of photo-sensitive glass.
