The invention relates to the fabrication of semiconductor devices with recessed oxide regions and, more particularly, to a method of forming coplanar integrated circuit structures having recessed oxide regions.
It is well-known that in the fabrication of integrated circuits, dielectric isolation between devices is achieved by forming recessed oxide regions extending into the silicon substrate from the surface. These thick oxide regions are delineated by using a silicon dioxide-silicon nitride composite mask. In such recessed oxide formation, the silicon dioxide-silicon nitride mask is first used as an etch barrier while recesses are etched through the mask openings in the silicon substrate and is then used as an oxidation mask during thermal oxidation of recessed isolation oxide regions.
One of the problems associated with the above technique of achieving dielectric isolation is that on growing the thick oxide region a conspicuous thin protuberance also grows underneath the silicon nitride oxidation mask. The protuberance is usually called a bird's beak. Associated with the bird's beak is the bird's head, an undesirable bump in the order of 4,000-5,000 Angstroms in height formed in the edge of the thick oxide layer adjoining the dual mask.
Typically, the bird's beak is formed in the recessed oxide during oxidation and results from lateral diffusion underneath the silicon dioxide (also known as pad oxide) which is formed between the silicon substrate and the silicon nitride. The bird's head results from the higher specific volume of the silicon dioxide than silicon and the lateral oxidation which occurs at the sidewalls of the recess. The silicon dioxide pad is used to prevent damage to the underlying silicon substrate by minimizing the stresses created on the substrate by the silicon nitride-silicon interface. Such stresses induce dislocations in the silicon substrate which result in undesirable leakage current channels and otherwise have a deleterious effect on the electrical characteristics of the interface. In other words, the use of the silicon dioxide pad to prevent damage to the underlying substrate results in the undesirable bird's beak and bird's head.
The existence of the bird's beak or bird's head incurs many disadvantages. The device active area is reduced and the field isolation area is increased by the bird's beak. This is disadvantageous since chip real estate, particularly in very large scale integrated circuits, is at a premium. Because of the bird's beak, it is difficult to achieve well-defined lateral isolation boundaries. Because of the abrupt inclinations of the recessed oxide due to the bird's head, electrode interconnections formed on the recessed oxide regions are prone to be fractured and disconnected. Another disadvantage of having the bird's head is that the close contact with a mask pattern necessary for photolithographic processing is difficult to achieve.
The foregoing are representative of the numerous disadvantages associated with the bird's beak and bird's head. It is, thus, readily apparent that it is desirable to reduce or eliminate the bird's beak and bird's head.
The publications, "Local Oxidation of Silicon: New Technology Aspects" by J. A. Appels et al., Philips Research Report 26, pages 157-165, June, 1971 and "Selective Oxidation of Silicon and its Device Application", E. Kooi et al., Semiconductor Silicon, 1973, published by Electrochemical Society, edited by H. R. Huff and R. R. Burgess, pages 860-879, are representative of the recognition in the prior art of the bird's beak and bird's head problems associated with silicon dioxide-silicon nitride composite masks when used in the formation of recessed silicon dioxide by thermal oxidation. The Appels et al. publication also concerns the bird's beak and bird's head problems associated with a nitride mask alone.
Attempts have been made in the prior art to reduce the bird's beak. For example, U.S. Pat. No. 3,900,350, issued Aug. 18, 1975, to Appels et al. teaches an approach to reducing the bird's beak by using a polysilicon layer under the silicon nitride mask instead of the silicon dioxode.
U.S. Pat. No. 3,961,999 issued June 8, 1976, to Antipov also describes a method for minimizing the bird's beak problem. In this patent, the conventional silicon dioxide pad is formed between the silicon substrate and the silicon nitride layer. The method disclosed in this patent involves etching openings through the silicon dioxide corresponding to the openings in the nitride to enable the undercutting and exposure of the underside of the silicon nitride at the periphery of the openings. A layer of silicon is then deposited in the recesses covering the undercut portions of the nitride followed by thermal oxidation of the deposited silicon to form the recessed oxide regions.
U.S. Pat. No. 4,098,618 issued July 4, 1978, to Crowder et al. attempts to solve the bird's beak problem by eliminating the intermediate pad oxide between the nitride mask and the substrate. The technique taught in this patent involves initially damaging the surface of the silicon substrate by ion implantation to a controlled depth and then annealing to generate a dense dislocation network array layer which prevents the stress induced defect propagation from the nitride mask. The nitride oxidation mask is then directly applied on the resultant substrate surface and patterned to define the recessed oxide regions. Thereafter, the recessed oxide is formed.
Despite the improvements in suppressing the bird's beak and/or bird's head exemplified by the above patents, there is need for further improvement in achieving well-defined dielectric isolation, particularly in view of the stringent requirements of the ever-demanding design rules in microelectronics industry.