Many documents have pointed out that phosphoric acid is a hard evaporated acid, but in nanofabrication hot phosphoric acid is always used as an etchant. I want to know at high temperature (80-100 C ), if it is still a hard evaporated acid?
Yes, hot phosphoric acid is commonly used as a Silicon Nitride etch. First: be very careful with hot phosphoric acid. Second: typical 85% acid will be evaporating as well as the water and you should always use a reflux condenser during your etches. You should also do it in the properly vented hood. It i very important to use the condenser as during long etches concentration of the acid will change and as a result your etch rates and etch times will be changing as well. In my experience etch rates for 85% acid were on the order of 30 -40 A/min with temperature between 165 -175 C.
For my case, the 85% phosphoric acid had been used in a stainless steel container and in less than an hour, the stainless stell had a small hole and my phophoric acid appeared to be in a thick green paste (probably due to reaction with Iron in Stainless stell). Whereas, under the same condition, the temperature of phosphoric acid in a reflux setup appeared to be okay until 160 deg C
The boiling point of 85 wt% phosphoric acid has been stated as 158 ºC (atmospheric pressure) (*). It is displaced to higher temperature while the acid concentrates by nearly evaporating just water. When it reaches 180 ºC, the acid concentration should approach 90 wt% (**).
References: (*) ClearTech Industries Inc. (http://www.cleartech.ca/), ‘Phosphoric Acid 75-85%’ ─ Material Safety Data Sheet, 2012; Iowa State University, Chemistry Material Safety Data Sheets, http://avogadro.chem.iastate.edu/msds/h3po4.htm. (**) Transene Company (http://transene.com/), 'TRANSETCH N' (phosphoric acid; approx. 90%) ─ Material Safety Data Sheet, 2003.
On the vapour pressure of phosphoric acid(s):Article Vapor Pressure of Phosphoric Acids
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