(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming field isolation.
(2) Description of Prior Art
The formation of integrated circuit devices on silicon substrates requires that a means be provided to electrically isolate the various circuit components from each other. In many cases p/n junctions themselves can be used to form at least part of the necessary isolation. Most isolation requirements, which cannot be resolved by circuit design, relate to the lateral isolation of devices. To some extent, junction isolation can be used here also. However, as device technology leads to smaller and smaller geometries as well as shallower structures, junction isolation technology becomes very limited.
The successful use of silicon for integrated circuits for the last thirty years can, in part, be attributed to the marvelous insulating properties of it's oxide. No other commonly known semiconductor material has this unique feature. Thus silicon oxide has become well established as the isolation material for integrated circuit chips. Earliest usage of this material involved growing it on the substrate in selected regions which are defined by an oxidation masking material. To this end, silicon nitride came into widespread use. Referring to FIG. 1, a pad silicon oxide film 22 several hundred Angstroms thick is grown on the surface of a silicon wafer 20. Next a film of silicon nitride 24 is deposited to a thickness of several thousand Angstroms using a chemical-vapor-deposition process. The pad oxide 22 is used as a buffer layer, preventing the highly stressed silicon nitride from causing dislocations in the silicon.
Using standard photolithographic techniques followed by etching, a pattern is defined in the oxide/nitride mask, leaving bare those regions 26 on the silicon which are to become isolation regions. Referring next to FIG. 2, the wafer 20 is subjected to an oxidizing ambient at elevated temperatures, during which the exposed areas are converted to silicon oxide 28. The oxidation proceeds at the oxide-silicon interface. Thereby pockets of silicon oxide 27 several thousand Angstroms deep can be formed in the patterned areas. These pockets 27 provide the necessary electrical isolation for the subsequently formed semiconductor devices.
The process just described has become known by several names, one of which is LOCOS an acronym for LOCal Oxidation of Silicon. LOCOS has been practiced for over twenty-five years and has been adapted to many specific applications. One of the problems with conventional LOCOS, illustrated in FIG. 2 is that the pad oxide 22 permits oxidation to take place under the silicon nitride mask, pushing up the edge of the mask and thereby leaving a raised portion of the oxide around the mask perimeter, known as "Birds Head" 28 and a protrusion of oxide under the edge of the oxidation mask 29, caused by diffusion of oxidant through the thin pad oxide 22, known as "Birds Beak". The birds head 28, being a protrusion above the surface, causes shadowing and thinning problems with subsequently deposited metallization stripes traversing across it. Thin areas in metal interconnection lines constitute "weak spots" and are subject to increased current density, heating, and subsequent electrical failure. The birds beak 29 can be reduced by using as thin as possible pad oxide 22 or by stiffening the mask by using a thicker silicon nitride layer 24.
In some instances, a sidewall oxidation barrier is formed along the mask edge to prevent penetration of the oxidant under the nitride mask. Ema et.al. U.S. Pat. No. 5,453,397 describes such a barrier wherein a nitride plug is formed in a recess produced by undercutting the pad oxide with a wet etch. Shimizu, et.al., U.S. Pat. No. 5,472,906 form a nitride barrier which extends over the edge of the pad oxide and over the edge of a subjacent depression in the silicon. Unfortunately, these practices lead to an abrupt feature 25 wherein high shearing stresses are produced in the silicon which lead to the formation of dislocations. These dislocations propagate diagonally into the device active regions causing junction failure.
Typically, the field oxide is grown at temperatures above 1,000.degree. C. At these temperatures, stresses which would be developed due to the volume expansion, are relieved by plastic flow of the oxide. However, upon completion of the oxidation, and when the wafer is cooled, sufficient plastic flow is no longer able to relieve stresses below about 960.degree. C. (Wolf, S.,"Silicon Processing for the VLSI Era", Vol.2, Lattice Press, Sunset Beach, Calif., (1990), p693). Thermal stresses now develop due to the difference in thermal expansion coefficients of the silicon oxide and the silicon. The profile of the field oxide is now the determining factor for the distribution of stresses within the silicon. Abrupt features in the silicon/silicon oxide profile, such as 25 in FIG. 2, are responsible for high values of x-y shearing stresses which produce dislocations. Such abrupt features 25 occur when a stiff mask edge is used to control the field oxidation.
The compressive stresses developed within the growing field oxide also have been observed to retard the oxidation rate of silicon. Known as the field oxide thinning effect, this phenomena is observed when field oxide is grown in mask openings below about 0.5 to 0.6 microns. See for example, P. Belluti and M. Zen in J. Electrochem. Soc. 143, (1996) p 2953.
Efforts to diffuse the abruptness of a stiff mask edge and to control the extent of birds beak encroachment into the silicon active area as well as the shape of the birds head, have been made using "L" shaped spacers which form a foot at the mask edge. Jang U.S. Pat. No. 5,397,733 describes several embodiments by which these spacers can be formed. An example of the use of such a spacer is shown in FIG. 3. The nitride foot 30 surrounding the periphery of the thicker nitride mask provides a region of more flexible mask at the edge of the opening, thereby allowing easier expansion of the growing field oxide under it. The thicker, and therefore stiffer, section 32 continues to constrict the birds beak.
By proper selection of the thickness and length of the mask foot 30 as well as the thickness of the more rigid portion 32, it becomes possible to tailor the mask edge to provide a field oxide profile 36 with reduced stress at the expense of minimal birds beak encroachment. Jang cites prior art difficulties in forming the L-shaped spacer, specifically, difficulties in controlling thickness and length of the spacer.
In one embodiment cited by Jang a nitride foot is formed by first depositing a silicon nitride layer over a pad oxide, thinning a region of the nitride layer to form a step, depositing a polysilicon layer over the step, and applying a spacer etch which forms a polysilicon spacer at the edge of the region. The nitride layer is then etched into the pad oxide using the polysilicon spacer to protect the nitride foot. The polysilicon spacer is subsequently removed from over the foot, permitting the foot to flex during the field oxidation.
The precise tailoring of the field oxidation mask edge becomes even more critical as device geometries shrink into the sub-half micron range. As field oxide widths approach these dimensions oxidation rates are significantly retarded and stress induced crystalline defects cause serious degradation of the semiconductor devices. The balance between reducing birds beak encroachment and preventing stress induced dislocations requires an even greater control of the field oxide oxidation mask edge. The invention herein described provides a foot with a continuous transition of flexibility from its tip to the main portion of the mask. The risk of cracking at the foot/mask interface is also reduced. In addition, the invention eliminates process steps required by Jang for spacer removal to provide a flexible foot for the field oxidation.
Chiu et.al., U.S. Pat. No. 5,470,783 shows the formation of a nitride foot at the base of the main nitride oxidation mask used for the formation of recessed oxide isolation. The foot is not tapered and functions to reduce the oxidation window of the opening, thereby reducing penetration of the recessed oxide under the main portion of the mask.