[nwiswell]

I can't believe that worked.

Industrially (by which I mean how it was done circa 1970), silicon oxide and silicon nitride was etched using a buffered HF solution known as BOE (buffered oxide etchant). The buffer was typically ammonium fluoride; because of the presence of the buffer, the concentration of fluorine ions in solution stays constant even as some of the fluorine attacks the substrate to form e.g. hexafluorosilicic acid. Since the concentration of fluorine stays constant, so does the etch rate.

If you just pull some rust cleaner off the shelf at home depot, the etch rate will crash as the concentration of fluorine ions decreases. That's compounded by the fact that the HF concentration isn't very high in the first place.

As a result it would be very difficult to determine how long your wafer should remain in the etch bath. Underetching could easily cause "opens" in the circuits from unremoved insulator, and overetching and/or undercut can destroy the patterns you're trying to produce. Either way it can ruin the chip.

[ghghgfdfgh]

Yep, he used an ammonium fluoride buffer. > Instead of a standard HF etch, a buffered oxide etch of NH4F (Ammonium Fluoride) in HF can be used to control the etch rate and photoresist lifting. I use approximately 20-30g of 100% NH4F per 50mL of HF (stock whink rust remover)

Ammonium fluoride definitely isn’t as easily accessible as rust cleaner, but you could buy it for a somewhat cheap price on Amazon.

[xw38011]

It's the internet which means you're shown the one IC that worked but not the 20 that didn't... Just like a grad student paper! :)