  Fused silica glass is a material of choice for micromechanical, microfluidic,
and optical devices due to its ultimate chemical resistance, optical,
electrical, and mechanical performance. Wet etching in hydrofluoric solutions
especially a buffered oxide etching (BOE) solution is still the key method for
fabricating fused silica glass-based microdevices. It is well known that
protective mask integrity during deep fused silica wet etching is a big
challenge due to chemical stability of fused glass and extremely aggressive BOE
properties. Here, we propose a multilevel fused silica glass microstructures
fabrication route based on deep wet etching through a stepped mask with just a
one grayscale photolithography step. First, we provide a deep comprehensive
analysis of a fused quartz dissolution mechanism in BOE solution and calculate
the main fluoride fractions like $HF^-_2$, $F^-$, $(HF)_2$ components in a BOE
solution as a function of pH and $NH_4F:HF$ ratio at room temperature. Then, we
experimentally investigate the influence of BOE concentration ($NH_4F:HF$ from
1:1 to 14:1) on the mask resistance, etch rate and profile isotropy during
fused silica 60 minutes etching through a metal/photoresist mask. Finally, we
demonstrate a high-quality multilevel over-200 um isotropic wet etching process
with the rate up to 3 um/min, which could be of a great interest for advanced
fused silica microdevices with flexure suspensions, inertial masses,
microchannels, and through-wafer holes.
