Patent ID: 7976718

Claim:
A method of etching silicon nitride from at least one substrate comprising: providing a closed-loop circulation system having a process chamber and a recirculation line fluidly coupled to the process chamber; supplying sulfuric acid, phosphoric acid, and water to the closed-loop circulation system in predetermined amounts so as to form a mixture having a predetermined concentration ratio and a predetermined volume, the mixture filling the process chamber and overflowing into the recirculation line; submerging at least one substrate in the mixture within the process chamber; circulating the mixture through the closed-loop circulation system; continuously measuring concentration ratio of the mixture with a concentration sensor; comparing the measured concentration ratio to the predetermined concentration value to determine whether the measured concentration value is within a predetermined range of the predetermined concentration ratio; and upon determining that the measured concentration ratio is not within the predetermined range of the predetermined concentration ratio, automatically feeding a volume of sulfuric acid, phosphoric acid, and/or water into the closed-loop circulation system while bleeding a substantially equal volume of the mixture from the closed-loop circulation system to return the concentration ratio of the mixture back within the predetermined range, the feeding and the bleeding occurring during processing of the at least one substrate.