Maintaining a Stable Etch Selectivity between Silicon Nitride and Silicon Dioxide in a Hot Phosphoric Acid Bath

Phosphoric acid (H3PO4) - water (H2O) mixtures at high temperature have been used for many years to etch silicon nitride (Si3N4) selective to silicon dioxide (SiO2) layers. The production requires removing the Si3N4 completely while keeping SiO2 loss to a minimum. The challenge for batch wafer cleaning is how to maintain a high etching selectivity of Si3N4 to SiO2 for a longer bath life. 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.