Examination of etching agent and etching mechanism on femotosecond laser microfabrication of channels inside vitreous silica substrates

We examined the physical chemistry underlying a wet chemical etching-assisted femtosecond laser microfabrication technique. Close scrutiny of etching reagents and the etching process has led to further refinement of the method for practical use such as microchips for chemical total analysis systems (m-TAS). Microchannels as long as a centimeter scale with less than 60 diameter (aspect ratio of ∼200) were fabricated inside vitreous silica substrates. In this regard, we demonstrated that a concentrated aqueous solution of KOH is advantageous over commonly used aqueous HF because of least saturation behavior in elongating channel structures. Resultant nearly twice as large the etching selectivity of KOH as that of HF allowed substantial penetration depths within the laser-modified volume while leaving the unmodified regions practically intact. Furthermore, the mechanism of laser-modification that permits highly selective wet etching was investigated by photoluminescence and confocal Raman spectral measurements of the irradiated microvolumes. Based on the experimental results, we propose that the formation of a Si-rich structure is responsible for the remarkably enhanced etching rate of aqueous KOH, in addition to the densificaton of the silica structure previously advocated for the mechanism of waveguide formation.