Method to solve the dishing issue in cmp planarization by using a nitride hard mask for local inverse etchback and cmp

Abstract

A method for forming planarized isolation using a nitride hard mask and two CMP steps is described. A first nitride layer is deposited over a pad oxide layer on the surface of a semiconductor substrate. The first nitride and pad oxide layers are etched through where they are not covered by a mask to provide at least one wide opening and at least one narrow opening where the surface of the substrate is exposed. Trenches are etched into the substrate where it is exposed. An oxide layer is deposited overlying the first nitride layer and within the trenches completely filling the narrow trench wherein a trough is formed over the wide trench. A second nitride layer is deposited over the oxide layer. The second nitride layer is polished away with a polish stop at the oxide layer whereby the second nitride layer is removed except: where it lies within the trough. The oxide layer is etched back where it is not covered by the second nitride layer wherein the oxide layer is planarized except where the oxide layer underlies the second nitride layer. The second nitride layer is removed within the trough whereby oxide horns forming the trough extend vertically upward from the planarized oxide surface. Thereafter, the oxide layer is polished away with a polish stop at the first nitride layer whereby the oxide horns are removed. The first nitride and the pad oxide layers are removed completing formation of the shallow trench isolation.