Patent ID: 7091104

Claim:
A method for forming an isolation structure on a semiconductor substrate, the method comprising: forming a pad oxide layer over the substrate; forming a nitride layer over the pad oxide; opening a portion of the nitride layer, the nitride layer opening defining a first edge; opening a portion of a pad oxide layer exposed by the nitride opening step using a process gas including an etchant gas and a polymer-forming gas to form a polymer residue at a second edge of the opened portion of the pad oxide, the second edge of the pad oxide layer being substantially aligned to the first edge of the nitride layer; etching a portion of the substrate exposed by the opening step to form a trench having a first slope and a second slope, the first slope being greater than 45 degrees and the second slope being less than 45 degrees; forming a first liner oxide layer over the trench prior to the isotropic etching of the nitride layer to protect the substrate from being damaged during a subsequent etch step; etching the nitride layer using an isotropic etch to cause the first edge of the nitride layer to recede outwardly from the trench, the first liner oxide protecting the substrate during the isotropic nitride etch; and filling the trench to form the isolation structure.