Patent ID: 8834671
Filing Date: 2014-09-16
Classification: H01L

Abstract:
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.