Patent ID: 8409997

Claim:
A method for controlling an etching bath comprising: providing a batch etching bath configured to process a batch of wafers therein, said batch etching bath containing phosphoric acid heated to an elevated bath temperature, said batch etching bath including an inner tank for processing said batch of wafers therein, and an outer tank; measuring silicon concentration of silicon in said phosphoric acid contained in said batch etching bath while said silicon is in soluble form; and responsive to said measuring, controlling said silicon concentration by adding fresh heated phosphoric acid to said phosphoric acid in said outer tank of said batch etching bath when necessary to maintain said silicon concentration within a desired range, said fresh heated phosphoric acid heated by a dedicated phosphoric acid heater and wherein said measuring includes removing a sample of said phosphoric acid and diluting said sample with deionized water to produce a diluted sample and measuring said diluted sample before silica precipitation in said diluted sample, said deionized water having a temperature no greater than room temperature.