Apparatus and method for controlling silicon nitride etching tank

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,
• wherein said measuring silicon concentration includes circulating said phosphoric acid from and to said batch etching bath, heating said circulating phosphoric acid and measuring said circulating phosphoric acid by removing a sample of said circulating 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.

A method for controlling a silicon nitride etching bath comprising:

• providing a batch etching bath including an inner tank configured to process a batch of wafers therein and an outer tank, said batch etching bath containing phosphoric acid heated to an elevated bath temperature greater than about 120° C.;
• 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 to maintain a silicon nitride:silicon oxide etch selectivity of said batch etching bath within a desired range 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 nitride etch rate within said desired range, while maintaining said elevated bath temperature so that said elevated bath temperature does not deviate by greater than 5% from a steady state value of said elevated bath temperature,
• said fresh heated phosphoric acid heated by a dedicated phosphoric acid heater and delivered to said batch etching bath unmixed with other solutions,
• 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.