Very deep fused silica etching

Abstract

Fabrication processes for wet chemical and dry etching of hollow capillary leaky optical waveguides in high-purity fused silica for extended path cells for improved optical detection in analytical chemistry are described. We focus on microstructures with etch depths on the order of 80 µm. Special attention is paid to the preparation of the etch masks for the two different etch technologies. The fused silica wet chemical etching technique uses buffered hydrofluoric acid with ultrasonic agitation achieving etch rates > 100 nm/min. We succeeded in developing an etch process based on a singlelayer photoresist (AZ 5214E, Clariant Corp.) soft mask, which gives excellent results due to special adhesion promotion and a photoresist hardening cycle after the developing step. This procedure allows for the production of channels of nearly semi-cylindrical profiles with etch depths of up to 87 µm. For the dry etch process a ~10 µm thick Ni layer is used as a hard mask realized with electroplating and a thick photoresist. The etch process is performed in an ECR (Electron Cyclotron Resonance) chamber using CF 4 gas. The resulting etch rate for fused silica is about 138 nm/min. Etch depths of (accidentally also) 87 µm are achieved.

Ultrasonic‐assisted wet chemical etching of fused silica for high‐power laser systems

“…The dissolution rate of scratched samples plotted in Figure suggests that static etching (power density 0 W/cm 2 ) has an etching rate of ~467 μg/min, while the rate slightly goes up to ~501 μg/min (0.6 W/cm 2 ) (7% higher) and ~540 μg/min (2 W/cm 2 ) (16% higher) in acoustic power‐assisted etching, respectively. It can be concluded that acoustic power density ~0.6 and ~2 W/cm 2 has marginal effects on the etching rate, which is different from the results by Steingoettera but similar to Spierings . Steingoettera reported that ultrasonic agitation enhances the etch rate of fused silica markedly compared with the stirred solution, whereas Spierings mentioned in his paper that the dissolution rate of fused silica is limitedly affected by ultrasonic agitation of the etchant …”

“…It can be concluded that acoustic power density ~0.6 and ~2 W/cm 2 has marginal effects on the etching rate, which is different from the results by Steingoettera but similar to Spierings . Steingoettera reported that ultrasonic agitation enhances the etch rate of fused silica markedly compared with the stirred solution, whereas Spierings mentioned in his paper that the dissolution rate of fused silica is limitedly affected by ultrasonic agitation of the etchant …”

Deep multilevel wet etching of fused silica glass microstructures in BOE solution

“…The fabrication process complexity and required depths of fused silica microstructures determine the choice of mask materials for various devices. Thus, photoresist masks are easy to spin coat, but they have low adhesion and low resistance to HF solutions limiting etching depths at several tens of micrometers [19][20][21][22][23][24][25] . Si-based masks are highly resistant to hydrofluoric acid solution 4,[37][38][39][40][41] .…”

“…The etching rate increases with increasing HF content in the BOE solution. Adding an ammonium fluoride buffer NH 4 F to HF raises the etching resistance of photoresist masks and helps to maintain the etching rate 19 , but its dependence on the NH 4 F content is non-linear. The etching rate increases with a small addition of NH4F to a certain concentration, but with a further NH 4 F concentration increase, it starts decreasing 51 .…”

Etching a Micro-Trench with a Maximum Aspect Ratio of 60 on Silica Glass by Laser-Induced Backside Wet Etching (LIBWE)

“…Among industrial applications, HF etching [1][2][3] and plasma etching [3][4][5][6][7] are the most common methods used to structure silica glass. Both methods require patterning of a protection layer, typically a photoresist, by photolithography or electron beam lithography, which requires complicated procedures.…”