Recently, the control of particles measured in the tens of nanometers has become indispensable for manufacturing semiconductor devices. In sulfuric acid - hydrogen peroxide mixture (SPM) processes, it is increasingly important to reduce particles on the wafer. A batch type re-circulation bath has been generally used with a fluoroplastic filter built into the equipment. The pore size of the filter used has gradually become smaller. The particle levels on the wafer have decreased after the filter was changed from a 100nm rating to a 30nm rating.