An etching control system controls exposure of a silicon workpiece to a spontaneous etchant. The system determines an amount of material to be removed from the silicon workpiece, based on metrology information corresponding to the silicon workpiece. An estimated etch time duration is determined for removing the amount of the material upon exposing the silicon workpiece to the spontaneous etchant for the estimated etch time duration. In some embodiments, the system monitors a change in mass of the silicon workpiece caused by exposure of the silicon workpiece to the spontaneous etchant to determine when the amount of the material has been removed from the silicon workpiece. Exposure of the silicon workpiece to the spontaneous etchant is stopped when the change in the mass of the silicon workpiece indicates that the amount of the material has been removed.