Apparatus and method for controlling silicon nitride etching tank

摘要

A method and apparatus for controlling a silicon nitride etching bath provides the etching bath including phosphoric acid heated to an elevated temperature. The concentration of silicon in the phosphoric acid is controlled to maintain a desired level associated with a desired silicon nitride/silicon oxide etch selectivity. Silicon concentration is measured while the silicon remains in soluble form and prior to silica precipitation. Responsive to the measuring, fresh heated phosphoric acid is added to the etching bath when necessary to maintain the desired concentration and silicon nitride:silicon oxide etch selectivity and prevent silica precipitation. The addition of fresh heated phosphoric acid enables the etching bath to remain at a steady state temperature. Atomic absorption spectroscopy may be used to monitor the silicon concentration which may be obtained by diluting a sample of phosphoric acid with cold deionized water and measuring before silica precipitation occurs.

主权项

1. An apparatus comprising:
an etching bath containing phosphoric acid at an elevated bath temperature; a measuring system configured to measure silicon concentration of silicon in soluble form in said phosphoric acid; a heater capable of heating fresh phosphoric acid; a system capable of adding said heated fresh phosphoric acid to said etching bath; and a controller configured to control said silicon concentration by causing said heated fresh phosphoric acid to be added to said etching bath if necessary to maintain a silicon nitride:silicon oxide etch selectivity within a desired range and prevent precipitates of silica, said controller in communication with said measuring system, wherein said measuring system is configured to cause a sample of said phosphoric acid removed from recirculating phosphoric acid, to be diluted with unheated deionized water thereby producing a diluted sample and wherein said measuring system is configured to measure said diluted sample before silica precipitation occurs in said diluted sample.