Experiments showed that increasing water concentration in the bath results in higher selectivity: more Si3N4 etched and less SiO2 etched. With the installations of a water concentration monitor and a water spiking apparatus, the bath is able to control the water concentration. The partial replacement of the chemical bath after each product lot reduces dissolved silicon concentration. The addition of H2SO4 saves initial conditioning time. With all of the efforts, the bath starts at a high selectivity and maintain this selectivity for an extended bath life.