(1) Field of the Invention
The present invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming planarized isolation that combines local oxidation of silicon (LOCOS) and shallow trench isolation (STI) in the fabrication of integrated circuits.
(2) Description of the Prior Art
As device technology is scaled down to the quarter micron regime, the use of the conventional local oxidation of silicon (LOCOS) isolation will be confined by smaller channel-width encroachment (bird's beak). Shallow trench isolation (STI) can be used to eliminate these encroachments, especially in ultra large scale integrated (ULSI) circuit devices. To achieve good planarity after STI, chemical mechanical polishing (CMP) is often used. However, due to pad deformation, the trench open area is susceptible to dishing which causes oxide thinning in the wide trench.
FIG. 1 illustrates a partially completed integrated circuit device of the prior art. A pad oxide layer 12 has been grown or deposited over the surface of a semiconductor substrate 10. A silicon nitride layer 14 is deposited over the pad oxide layer. Trenches in the substrate have been filled with an oxide 17. Referring to FIG. 2, the oxide 17 is polished using CMP. Oxide dishing and the resulting lack of oxide uniformity can both be seen in wide area 19.
A combination of LOCOS in the wide areas and STI in the narrow areas may be used to resolve these problems. Various combinations of LOCOS and STI have been proposed in the art. U.S. Pat. No. 5,679,599 to Mehta teaches etching a deep trench and filling it with oxide, then using field oxidation both to form a LOCOS region and to smooth out the trench isolation region. U.S. Pat. No. 5,696,021 to Chan et al forms a FOX region which is then partially etched away. Trenches are etched at the edges of the FOX region which are then filled with further field oxidation. U.S. Pat. Nos. 5,683,932 and 5,411,913 both to Bashir et al disclose filling trenches with a oxide or polysilicon and etching back to planarize the surface while covering the surface in the wide areas with a photoresist mask. U.S. Pat. No. 5,350,941 to Madan teaches forming FOX regions, then etching through the FOX to form deep trenches of uniform width. The trenches can then be filled with oxide or polysilicon.