Besides the etchants, there’s also the dopants (arsenic, notably) and the compounds they have to be in to be useful. My favorites are arsine (AsH3) and phosphene (PH3), which are not only toxic as Hell but also hypergolic in air. Shortly behind, if you’re trying to deposit silica, is silane (SiH4) which is also hypergolic in air but also produces monomolecular silica, which makes your usual miners’ lung disease look benign.
The masking process necessarily requires some solvents that led to several Superfund sites back in the day and an entire generation of birth defects and later cancers such as leukemia. Plus the aforementioned fluorine compounds, which among other things can be unnoticed contact neurotoxins.
And in case anyone cares, I was in the industry in the early 70s and worked around this stuff. Sometimes I’m amazed I survived, but my oncologist suspects that my current leukemia might date back to those days.
