Silicon nitride is often used in MEMS as a masking layer along with silicon dioxide for deep cavity etch using KOH, as oxide may not be enough to mask through the entire process. Silicon nitride is also used a passivation layer for protecting the circuit side of the device. Hot dilute phosphoric acid (H3PO4) is generally used for etching silicon nitride, especially due to it good selectivity over silicon dioxide.
The boiling point of phosphoric acid depends directly on the the percentage concentration by weight of the acid in the etchant. To ensure constant etch rate it is necessary to maintain the temperature and thereby the concentration of the etchant. Evaporating water content will increase the concentration of the acid and hence it is necessary to reflux this evaporating liquid by using a closed etch chamber or by adding DI water to replace water at the same rate as it is evaporated. The temperature of the etchant should be regulated by regulating the concentration and not by adjusting the heat source.
This design interface can be used to determine the etch rate of silicon nitride and thermal oxide at a given temperature and acid concentration. The actual etch rate will depend also on the stoichiometry of the nitride, the deposition temperature and density of film. The etch rate increases with temperature and for a fixed acid concentration it increases till it reaches the boiling point of the mixture. It is not possible to increase the temperature of the etchant with constant concentration beyond its boiling point just by adding further heat. When the temperature is fixed, the etch rate is higher for a more dilute mixture. But for a given temperature, the amount of water that can be added is limited by the boiling point of that mixture. The etch rate for thermally grown silicon dioxide at the same concentration and temperature is found to be much lesser.
The plot shows the variation of etch rate with etchant temperature for the given concentration. The etch rate is found to increase with temperature. Using the crosshair, the etch rate corresponding to the given temperature can be read out. The maximum etch rate corresponds to the boiling point of that etchant of the given concentration.