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With buffered hydrogen fluoride (BHF) treatment, the etch rate of thermal oxide gradually increases over time. Thus, the old BHF must be frequently changed to new BHF in order to maintain the etch rate. As a result, considerable BHF and other chemicals to treat waste BHF are consumed and considerable waste such as sludge and wastewater is discharged. We have developed a new method to maintain the etch rate by supplying ammonia and water evaporated from BHF. The amount depends on the amount of NH4F/HF in BHF. Since the method can extend the usable lifetime of BHF, it will help to reduce BHF used and the chemicals required to treat waste BHF and decrease the discharge of wastewater and sludge.