| 摘要 |
Apparatus and method are described for etching in liquid acids at high temperatures, notably silicon nitride in phosphoric acid. This has been achieved by providing an apparatus in which the liquid acid evaporant is condensed and returned to the main volume of acid by way of a pH meter whose output is used to control the flow of additional pure water into the main system. Alternatively, a conductivity meter, located in the etch bath, may be used for the same purpose. Undiluted acid is automatically added to the main system, as needed, under the control of a level sensor that senses the quantity of liquid in the main volume. Thus the pH or conductivity of the etchant is constantly being monitored and maintained at the desired level. Since the aqueous concentration remains steady throughout, sudden unexpected, and potentially dangerous, increases in the rate of boiling do not occur.
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