Object: This invention provides a method of refining an Rh-containing solution also containing at least one type of impurities of heavy metals, alkaline earth metals, and Ag. An etch stop dielectric layer may be formed on the gate dielectric layer, a first metal layer may be formed to fill the first and second trenches, and the first metal layer in the first region may be removed using the etch stop dielectric layer as an etch stopper. The coupling connects to another conduit which connects to a spigot. In this manner, sound quality with a good transient characteristic can be obtained. The value of the airflow is accurately controlled by adjustment of the power supply to the pump in order to obtain a match of the actual value of the airflow and the required value of the airflow.