OXYGEN-IMPERVIOUS PAD STRUCTURE ON A SEMICONDUCTOR SUBSTRATE

Abstract: An oxygen-impervious pad structure (100) which reduces the bird's beak profiles in semi-recessed oxide isolation regions. The sidewalls of a conventional silicon oxide (12) - silicon nitride (14) pad are coated with a thick layer (20) of oxynitride. A thin layer (18) of oxynitride is grown on the substrate surface prior to deposition of the thick oxynitride layer. The thick oxynitride layer prevents lateral oxidizing specie diffusion through the oxide layer of the conventional pad, and the thin oxynitride layer prevents oxidizing specie diffusion through the pad-substrate interface into the substrate region beneath the pad.

Description: The invention relates to an oxygen-impervious pad structure adapted for application in semi-recessed oxidation isolation schemes in which bird's beak formations are substantially eliminated. For many years, oxide isolation regions have been used to electrically isolate adjacent active devices formed on a semiconductor substrate. These structures can be fully recessed ("ROX"), wherein the upper surface of the oxide region is substantially planar with the upper surface of the substrate; or semi-recessed, wherein the upper surface of the oxide region extends above the upper surface of the substrate. One common method of forming these regions includes the steps of forming a silicon oxide layer on the substrate, forming a silicon nitride layer on the oxide, and patterning these two layers in order to form an oxygen-impervious "pad". The pad is formed in an area of the substrate where the active devices will be formed. When the substrate is subsequently oxidized, the portions of the substrate left unprotected by the pad oxidize much faster than does the pad itself, thus forming oxide isolation regions between areas of the substrate where the active devices will be formed. A problem with the above method of forming oxide isolation regions is the formation of the so-called "bird's beak structures". While the unprotected regions of the substrate are oxidized, oxygen impurities (generally referred to as "oxidizing specie") laterally diffuse through the silicon oxide layer of the pad into the protected portions of the substrate. This diffusion causes the sidewalls of the oxide isolation regions to slope inwardly into the protected substrate portions, such that the sidewall profiles resemble a bird's beak. Several methods have been proposed for eliminating or reducing the formation of these bird's beak profiles. In an article by Ho et al ("Elimination of Bird's Beak in ROI Process", IBM Technical Disclosure Bulletin , Vol. 24, No. 2, July 1981, pp. 1290-1291) a method of reducing bird's beak is disclosed in which the pad is made up of silicon oxide, silicon nitride and silicon oxide layers. The sides of the pad are protected by a layer of silicon nitride. Thus, when the surrounding substrate is oxidized, the silicon nitride on the sidewalls of the pad greatly reduces the lateral diffusion of oxidizing specie through the SiO₂ layer of the pad. The idea of using silicon nitride as a protective coating on the sidewalls of a pad is also shown in Chui et al, "A Bird's Beak Free Local Oxidation Technology Feasible for VLSI Circuits Fabrication", IEEE Transactions on Electron Devices , Vol. ED-29, No. 4, April, 1982, pp. 536-540. While these methods do result in a reduction of bird's beak profiles, they present several difficulties. In general, silicon nitride produces film stresses when it is deposited on a silicon substrate and subjected to heating. These stresses (in the order of 1 x 10⁹ Pa (1 x 10¹⁰ dynes/cm²)) are large enough to disturb the crystal lattice of the underlying silicon. In both the Ho and Chui articles, the silicon nitride which is formed on the sidewalls of the pad abuts the silicon substrate, subjecting the substrate to these high film stresses. However, from EP-A-0 071 203 a process using recessed oxidation isolation is known wherein a silicon oxynitride layer confronting the substrate with a silicon nitride layer is provided. The silicon oxynitride layer is disposed between the substrate and the silicon nitride layer in order to prevent tensile stresses from generating defects in adjacent silicon wafer structures. EP-A-0 075 875 also discloses a method of reducing lateral field oxidation in the vicinity of the active regions of a silicon substrate. The method utilizes a first thin layer of silicon dioxide in contact with the active region of the substrate and a second thick layer of silicon nitride overlaying the thin layer of silicon dioxide. In this method silicon oxynitride at the periphery of the silicon dioxide layer serves as a barrier to the diffusion of oxygen therethrough. The invention as claimed solves the problem of how to construct an oxygen-impervious pad in which the pad sidewalls are coated with a material which prohibits lateral diffusion of oxygen while not subjecting the underlying substrate to high film stresses and which pad structures allows to produce oxide isolation regions which are substantially free of bird's beak profiles. In summary, the invention starts from a conventional silicon oxide - silicon nitride pad being formed in regions of the substrate where active devices are to be provided. An anneal is then performed in order to grow a thin layer of oxynitride on the substrate. A thick layer of oxynitride is then deposited over the substrate as well as the pad. The thick oxynitride is etched to form sidewall pacers which protect the sidewalls of the pad. The thin oxynitride forms a boundary layer between the thick oxynitride sidewall spacers and the underlying substrate. The thick oxynitride prohibits lateral oxidizing specie diffusion into the pad, and the thin oxynitride prohibits oxidizing specie diffusion through the silicon-pad interface into the portion of the substrate lying beneath the pad. Moreover, since oxynitride does not produce the high film stresses associated with silicon nitride, disruption of the crystal lattice of the underlying silicon is substantially eliminated. The invention will become more apparent upon a detailed description of the best mode for carrying out the invention. In the description to follow, reference will be made to the accompanying drawing, in which: Figs. 1-6 are cross-sectional views of a silicon substrate undergoing the various process steps of the invention. As shown in Fig. 1, an oxide layer 12 is formed on a silicon substrate 10. The oxide is grown in a steam ambient, using conventional temperatures and pressures. Oxide layer 12 must be thick enough (e.g. 25-45nm (250-450Å)) to protect the underlying substrate 10 from high film stresses produced by the silicon nitride subsequently formed on the oxide layer 12, as described below. A layer of silicon nitride 14 is then deposited on oxide layer 12 using conventional LPCVD (Low Pressure Chemical Vapor Deposition) techniques. Silicon nitride layer 14 must be thick enough (typically 30-100nm (300-1000Å)) to prevent diffusion of oxidizing specie into the underlying layers. Silicon nitride layer 14 and oxide layer 12 are then etched to form a conventional oxygen-impervious pad structure 100 on the surface of substrate 10. A photoresist layer is applied, exposed and developed to form a mask 16, and the underlying layers are reactive ion etched (RIE) through the mask. By way of example, the nitride is etched in a CF₄ atmosphere and the oxide is etched in a CHF₃ atmosphere. Other gases known to etch these layers could be used. As shown in Fig. 2, the resulting pad structure has substantially vertical sidewalls. The photoresist mask 16 is removed after completion of the RIE etch step. After this etch step is completed, the substrate is briefly annealed in a furnace having an NH₃ atmosphere for a time sufficient to form a thin (5nm (50Å)) layer 18 of silicon oxynitride (Si x O y N z ) on the surface of the substrate 10. In a conventional furnace, the anneal is carried out at 1000°C-1200°C for 15-60 minutes. Other techniques of annealing may be used, so long as the parameters are adjusted to produce a relatively thin oxynitride layer (a thicker layer of oxynitride could be formed, but it is unnecessary). For example, in using the so-called "rapid thermal annealing" method, the anneal is carried out for approximately 1 minute. There is a possibility that under the proper conditions (e.g. absolutely pure NH₃ used as the anneal gas) silicon nitride may be formed instead of oxynitride; however, the probability of this substitution is extremely small. Moreover, even if this substitution takes place, the 5nm (50Å) thickness and high deposition temperature of this layer would appreciably reduce the resulting film stresses. As shown in Fig. 3, the substrate is then coated with a second layer 20 of silicon oxynitride. The oxynitride can be deposited by using either LPCVD or PECVD (Plasma Enhanced CVD) techniques. Using LPCVD the deposition pressure should be approximately 40 Pa (0.3 Torr) and the gasses SiH₂Cl₂, NH₃ and N₂O should be introduced into the chamber during deposition. The resulting oxynitride layer should be thick enough to retard diffusion of oxidizing specie. Accordingly, the resulting oxynitride layer should be approximately 50-100nm (500-1000Å) in thickness, and should have a refractive index n in the order of 1.75-1.80. The resulting structure is then anisotropically etched such that oxynitride layer 20 remains on the sidewalls of pad structure 100 (to form sidewall spacers 20A) and oxynitride layer 18 remains between oxynitride layer 20 and substrate 10, resulting in the pad of the invention. The region of the sidewall spacers 20A near substrate 10 is approximately 50-100nm (500-1000Å) thick. See Fig. 4. These layers must be reactive ion etched (RIE) in an atmosphere which results in a highly selective etch (i.e. etching must cease as soon as the underlying substrate 10 is exposed). An example of such an atmosphere is CF₄ + O₂ + H₂. Freon 23 (CHF₃) or Freon 12 (CCl₂F₂) may be substituted for the carbon tetrafloride (CF₄). "Freon 12" and "Freon 23" are both trade names of the Du Pont Corporation of Wilmington, Delaware. Prior to oxidation, the substrate 10 is implanted with boron in order to raise the threshold voltage of the resulting oxide isolation regions. Alternatively, boron implanatation could be carried out immediately after formation of pad structure 100. During boron implantation, the nitride layer 14 atop oxide layer 12 serves as an implantation mask. As shown in Fig. 5, the substrate is then subjected to an oxidizing ambient in order to form oxide isolation regions 24. During the growth of the oxide regions 24, the sidewall spacers 20A and oxynitride 18 undergo a chemical reaction by which they are at least partially converted into silicon oxide. The rate of this conversion is much slower than the oxidation rate of the implanted silicon. Thus, although the sidewall spacers 20A are slowly converted into oxides, they prevent the lateral diffusion of oxidizing specie through silicon oxide layer 12 during most of the long oxidation cycle used to grow oxide regions 24. Note that even if oxynitride layer 18 was etched so that it remains only beneath spacers 20A, it will prevent lateral diffusion of oxidizing specie through the pad-substrate interface into the portion of substrate 10 lying beneath the pad, because it is at least 50-100nm (500-1000Å) thick in the lateral direction. A three-step process is then used to remove the pad. First, the pad is dip etched in a buffered hydrofluoric (BHF) solvent in order to remove a portion of the sidewall spacers 20A. That is, since the sidewall spacers are now made up of oxide, they can be removed in oxide etchants such as BHF. Then the pad 100 is RIE or plasma etched in a CF₄ atmosphere in order to remove the nitride layer 14. Finally, the remaining structure is again dip etched in BHF in order to remove the remaining sidewall spacer 20A, oxide (formerly oxynitride) layer 18 and oxide layer 12. The resulting oxide regions are shown in Fig. 6. Thus, as disclosed above, the invention provides an improved oxygen-impervious pad having a first deposited oxynitride layer coated on the sidewalls of an oxide-nitride pad structure, and a second thermally grown oxynitride layer disposed between the first oxynitride layer and the substrate. The first oxynitride layer protects the pad from lateral oxidizing specie diffusion through the pad oxide layer, and the second oxynitride layer protects the portion of the substrate lying beneath the pad from oxidizing specie penetration. Moreover, since oxynitride produces a negligible amount of film stress (as compared to silicon nitride) on the silicon substrate, the silicon crystal lattice is not degraded as in the case of the silicon nitride spacers disclosed in the prior art.

Claims: An oxygen-impervious pad on a silicon substrate for integrated circuit devices, the pad preventing the formation of bird's beak profiles in recessed oxide isolation (ROX) structures, comprising: a layer (12) of silicon oxide overlaying the silicon substrate (10); a layer (14) of silicon nitride overlaying said layer of silicon oxide, said layers of silicon nitride and silicon oxide forming a pad structure (100) having substantially vertical sidewalls; a first thick oxygen-impervious layer (20) coated on said sidewalls of said structure to form spacers (20A) thereon, said first oxygen-impervious layer producing a negligible amount of film stress as compared to a silicon nitride layer; and a second thin oxygen-impervious layer (18) disposed between said first oxygen-impervious layer (20) and the silicon substrate (10), said second oxygen-impervious layer producing a negligible amount of film stress as compared to a silicon nitride layer; said first oxygen-impervious layer preventing diffusion of oxygen through said sidewalls of said structure and said second oxygen-impervious layer preventing diffusion of oxygen into the regions of the substrate lying beneath the pad. The pad as recited in claim 1, wherein said first (20) and second (18) oxygen-impervious layers are comprised of oxynitride. The pad as recited in claims 1 or 2, wherein the thickness of said first oxygen-impervious layer (20) is greater than that of said second oxygen-impervious layer (18) with a preferable relation being about 50 to 100nm (500 to 1000Å) relative to about 5nm (50Å). The pad as recited in one of the preceding claims, wherein said oxygen-impervious layer has a refractive index in the order of 1.75 to 1.80. A method of forming an oxygen-impervious pad on a semiconductor substrate, comprising the steps of: forming an oxide layer (12) on a surface of the semiconductor substrate (10); forming a layer (14) of silicon nitride on said oxide layer; etching said layer of silicon nitride and said oxide layer through a mask (16) to define a structure (100) having substantially vertical sidewalls; forming a thin oxygen-impervious layer (18) on the semiconductor substrate (10) said thin oxygen-impervious layer producing a negligible amount of film stress as compared to a silicon nitride layer; forming a thick oxygen-impervious layer (20) on said structure and on said thin oxygen-impervious layer (18) said thick oxygen-impervious layer producing a negligible amount of film stress as compared to a silicon nitride layer; and anisotropically etching said thick oxygen-impervious layer (20) to form spacers (20A) on said vertical sidewalls of said structure (100), said spacers overlying portions of said thin oxygen-impervious layer (18) to protect said portions from removal during said step of etching said thick oxygen-impervious layer (20). The method as recited in claim 5, wherein said thin oxygen-impervious layer (18) is formed in an anneal step of said semiconductor substrate. The method as recited in claims 5 or 6, wherein said thin and thick oxygen-impervious layers (18, 20) are comprised of oxynitride. The method as recited in claims 5 to 7, with the thickness ratio of said thin and thick oxygen-impervious layers (18, 20) being in the order of about 5nm/50-100nm (50Å/500-1000Å). The method as recited in one of claims 5 to 8, wherein said oxygen-impervious layer has a refractive index in the order of 1.75 to 1.80. The method as recited in one of claims 5 to 9, further comprising the steps of: growing oxide isolation regions (24) on the substrate (10), said spacers (20A) preventing diffusion of oxygen between the layers of said pad structure (100) and said thin oxygen-impervious layer preventing diffusion of oxygen beneath said pad structure. The method as recited in claim 10, wherein prior to growing said oxide isolation regions (24) a doping step is performed using the nitride layer (14) atop oxide layer (12) as a mask to raise the threshold voltage of the resulting oxide isolation regions.

Inventors: GEIPEL, HENRY JOHN, JR., PAN, PAI-HUNG
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21