Patent Publication Number: US-5294563-A

Title: Sidewall-sealed and sandwiched poly-buffered locos isolation methods

Description:
This application is a continuation of application Ser. No. 07/693,821, filed Apr. 30, 1991 now abandoned. 
    
    
     NOTICE 
     (C) Copyright, Texas Instruments Incorporated 1991. A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The coassigned patent application entitled &#34;Sidewall-Sealed Poly-Buffered LOCOS Isolation&#34;, Ser. No. 244,968, filed Sept. 15, 1988 is her incorporated herein by reference as background and supporting information to the subject matter discussed. 
     FIELD OF THE INVENTION 
     The present invention relates generally to semiconductor isolation methods and regions and, more particularly, to LOCOS isolation methods and regions. 
     BACKGROUND OF THE INVENTION 
     Advanced VLSI-class semiconductor devices are being designed with sub-micron moat-to-moat spacing. Working (or active) semiconductor devices are fabricated on the moat region on the semiconductor wafer. These devices must be adequately isolated from one another to prevent adverse interaction (e.g., Leakage current or shorting) between these devices. A well known technique for forming isolation regions between the moat regions is the LOCOS field oxidation technique, which has become an industry mainstay. Conventional LOCOS techniques, however, have been inadequate or marginal at best when used with sub-micron VLSI products. In particular, &#34;moat encroachment&#34; (also known as &#34;bird&#39;s beaking&#34;) is a major problem. It is caused be lateral oxidation into the moat regions during field oxide growth. This encroachment at moat edges and corners results in drastic decrease of moat dimensions and is severely detrimental for maintaining sub-micron moat-to-moat geometries. 
     Several isolation techniques have been developed in recent years for use with VLSI CMOS and BiCMOS products. For instance, the Poly-Buffered LOCOS (PBL) Isolation technique is currently used with VLSI designs having 1.2 micron minimum moat-to-moat spacing. Poly-buffered LOCOS is a modification to the industry standard LOCOS isolation process. The modification includes the step of adding a thin buffer layer of polysilicon (poly) between the underlying pad oxide and the overlying moat nitride film. The polysilicon layer allows the use of a thick moat nitride film during the field oxidation process, by relieving stresses in the silicon lattice normally present during LOCOS oxidation. 
     SUMMARY OF THE INVENTION 
     In accordance with the teaching of the present invention, the disclosed sidewall-sealed and sandwiched poly buffered LOCOS process reduces or eliminates the disadvantages and shortcomings associated with relevant prior art methods for forming localized isolation regions for VLSI technology. In one embodiment, the present invention comprises the steps of forming a pad layer of silicon dioxide on a semiconductor body or substrate; a first silicon nitride layer is thereafter formed on the silicon dioxide layer; a polysilicon layer is thereafter formed on the first silicon nitride layer; a second silicon nitride layer is thereafter formed on the polysilicon layer. Next, the isolation regions are defined, using LOCOS field oxidation techniques to grow the isolation oxide. In an alternative embodiment, a nitride sidewall seal may be formed around the perimeter of the semiconductor moat regions. If nitride sidewall seals are used, a preferred embodiment places a pad oxide buffer layer between the nitride sidewall and the semiconductor body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages will become apparent from the following and more particular description of the various embodiments of the invention, as illustrated in the accompanying drawing, wherein: 
     FIG. 1, is a cross-sectional view of a wafer illustrating an intermediate step in the formation of the sidewall-sealed and sandwiched LOCOS isolation regions; 
     FIG. 1a is a flow diagram of the process of key manufacturing steps involved in the manufacture of the sidewall-sealed and sandwiched LOCOS isolation regions; 
     FIG. 2, is a cross-sectional view of the FIG. 1 structure showing semiconductor material (e.g. nitride) disposed on the wafer for formation of a sidewall seal; 
     FIG. 3, is a cross-sectional view of the formed FIG. 2 structure showing the sidewall seal; 
     FIG. 4, is a cross-sectional view of FIG. 1 illustrating the bird&#39;s beaking locations after the growth of the field oxide in the isolation region; 
     FIG. 5, is a cross-sectional view of an alternative embodiment of the FIG. 3 wafer illustrating the elimination of pad oxide layer from the inverse moat region; and 
     FIG. 6, is a cross-sectional view of FIG. 3 illustrating the bird&#39;s beaking locations after the growth of the field oxide in the isolation region. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although an improvement over the conventional LOCOS technique, PBL has been found to have its own set of problems and limitations. Specifically, in addition to the usual bird&#39;s beak oxide (termed hereafter as the first bird&#39;s beak), the PBL technique results in the formation of a &#34;second bird&#39;s beak&#34; oxide, associated with lateral encroachment into moat regions during field oxidation. The first bird&#39;s beak oxide is due to the combined effects of oxidation of moat silicon as well as oxidation of the bottom surface of poly buffer film, at the moat edges and corners. The second bird&#39;s beak oxide is the result of oxidation of the top surface of the poly buffer film at moat edges and corners. Additionally, during moat etch sequence, which usually follows field oxidation, this second bird&#39;s beak oxide acts as a mask during high-selectivity plasma-etching of the poly film, thus hindering successful removal of the poly film at moat edges and corners. This phenomenon of second bird&#39;s beak oxide thus severely limits the applicability of PBL to about 1.2 to 1.0 micron moat-to-moat widths. 
     The present inventor addressed this second bird&#39;s beak phenomenon in his co-pending patent application entitled &#34;Sidewall-Sealed Poly-Buffered Locos Isolation&#34;, Ser. No. 244,968, filed Sept. 15, 1988 and assigned to applicant&#39;s assignee. The contents of said application are hereby incorporated herein by reference. The Sidewall-Sealed Poly Buffered LOCOS isolation (SSPBL) technique is a modification of the original PBL process. The modifications include forming nitride sidewalls all along the moat perimeter prior to field oxidation. In the SSPBL technique, stress related defects are generally avoided in the underlying silicon. In addition, SSPBL also includes other modifications to the original PBL process, such as the use of a thicker pad oxide, optional high pressure oxidation during field oxidation, and also an improved moat etch sequence. 
     The sidewall nitride in the SSPBL technique essentially eliminates formation of the second bird&#39;s beak by acting as an oxidation mask during field oxidation. The absence of this second bird&#39;s beak permits successful plasma etching of poly-buffer film during the post-field oxidation moat etch sequence. Although the first bird&#39;s beak oxide is still present, it can be sufficiently removed by a controlled deglaze in a dilute (10%) HF solution. Excessive deglaze, on the other hand, will result in undesirable retrograde profiles of the field oxide/moat interface regions. 
     The sidewall-sealed poly buffered LOCOS technique is applicable for VLSI designs having 1.2 to 0.5-micron moat-to-moat widths. Improved semiconductor isolation process techniques are needed for moat dimensions below 0.5 um without degrading performance or the useful area of the active region. For such applications, improvements to SSPBL have been made as described in this invention. 
     Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, FIG. 1 depicts a resultant intermediate structure in the process for forming sidewall-sealed and sandwiched polybuffered LOCOS isolation regions. Similarly, FIG. 1a is a flow diagram of the process of manufacturing steps involved in the manufacture of the sandwiched LOCOS isolation regions. As shown in FIG. 1, preferably a 12-15 nanometer pad silicon dioxide layer 12 is grown on a conventional silicon semiconductor substrate or body 10. Subsequently, a first nitride layer 40 is deposited over said pad silicon dioxide layer 12 to approximately 5 to 45 nanometers, using low-pressure chemical vapor deposition (LPCVD) techniques. Then, a polysilicon layer 14 is deposited over said first nitride layer by low-pressure chemical vapor deposition (LPCVD) to approximately 50 nanometers. Thereafter, a second silicon nitride layer 16 is deposited over polysilicon layer 14 by low-pressure chemical vapor deposition to a thickness of approximately 240 to 300 nanometers. Next, an inverse-moat region 18 is patterned in accordance with standard photo-lithographic techniques; the area 26 beneath the non-exposed photoresist 22 defines the active device moat region. The nitride and polysilicon layers over the inverse-moat region 24 are thereafter plasma etched to expose the pad oxide over the inverse moat regions 24, as shown. Next, a boron channel-stop implant 20 is performed in the inverse-moat regions 18. The photoresist 22 is thereafter removed by a plasma-ash, piranha-strip and pre-furnace clean-up. 
     FIG. 2 illustrates an intermediate step in the process to form sidewall seals around the sacrificial films of first nitride, polysilicon and second nitride layers. In particular, FIG. 2 shows a 30-90 nanometer layer of silicon nitride 28 deposited over the wafer by low-pressure chemical-vapor deposition. The wafer is thereafter subjected to an anisotropic blanket etch. This step etches away the silicon nitride and pad oxide in the inverse moat regions, leaving a thin 30-90 nanometer silicon nitride sidewall seal 30 along the perimeter of the moat region 26. The nitride sidewall generally seals the moat edges from oxygen diffusion during formation of the isolation field oxide in the inverse moat region. The sealing of the moat region substantially reduces the amount of oxide encroachment associated with conventional LOCOS processing. 
     FIG. 3 illustrates the preferred embodiment of the sidewall-sealed and sandwiched Poly-buffered LOCOS isolation regions. As shown in FIG. 3, the anisotropic etch results in a pad oxide edge 32 remaining beneath the nitride sidewall which aids in relieving stresses in the underlying silicon 10 during the formation of the isolation field oxide. 
     As shown in FIGS. 4 and 6, the isolation field oxide 34 is grown to approximately 800 to 850 nanometers in accordance with the present method, by subjecting the wafer to a high-pressure, approximately 10 Atm., oxidation at an elevated temperature of approximately 975 degrees C. This combination of pressure and temperature aids to minimize both the time required as well as the encroachment, and deters the spreading of the channel-stop implant into the active moat regions. Conventional steam oxidation at 900 degrees C. may also be used in accordance with the present invention, if boron dopant spreading is not a major concern. Channel-stop implants may also be formed in said inverse-moat regions after anisotropically etching said silicon nitride layer or after growing the field oxide in said inverse moat region, as required to optimize the overall process to achieve desired set of device electrical parameters. 
     FIG. 4 shows a special case of the use of sandwiched PBL (SPBL) isolation, using the 1st nitride layer only, without the sidewall-seal nitride. As shown in FIG. 4, the two dimensional oxidation of the two edges comprising a corner, also generally causes encroachment (bird&#39;s beaking) at the narrow moat corners. FIG. 4 is a cross-sectional view of FIG. 1 illustrating the bird&#39;s beaking locations on the sandwiched poly-buffered LOCOS isolation regions without sidewalls, after the growth of the field oxide 34 in the isolation region. The first bird&#39;s beak location BB1 41 grows between the pad silicon dioxide layer 12 and the substrate 10. The second bird&#39;s beak location BB2 42 occurs between the upper surface of the first nitride region 40 and the lower surface of the polysilicon layer 14. The third bird&#39;s beak location BB3 43 occurs between the upper surface of the polysilicon 14 and the second nitride region 16. The first nitride layer 40 suppresses the formation of bird&#39;s beaks at locations BB2 and BB3. 
     On the other hand, FIG. 6 shows the present invention (SSSPBL) with use of both the sandwich (nitride-1) as well as the sidewall-seal nitride films. FIG. 6 is a cross-sectional view of FIG. 3 illustrating the bird&#39;s beak locations after the growth of the field oxide in the isolation region. Thus, FIG. 6 shows the bird&#39;s beak locations, if both sidewall seals 30 and sandwiched nitride layers 40 and 16 are used. In addition to the sandwiched nitride layer&#39;s effects on suppressing bird&#39;s beaks, the nitride sidewall 30 significantly reduces oxidation at the BB3 43 and the BB2 42 locations. The use of the sandwiched nitride layers 40 and 16 in conjunction with the sidewall seals 30 thus significantly reduces the formation of various bird&#39;s beak at the locations BB4, 44, BB3 43 and BB2 42, and BB1 41, associated with the various versions of poly buffered LOCOS processes described previously, namely PBL, SPBL, and SSPBL. The reduction of these encroachments or bird&#39;s beaks greatly simplifies the successful removal of polysilicon layer 14 at the active moat edges during a subsequent high-selectivity plasma etch to expose the active moat region 26. 
     In prior art methods (PBL isolation technology), the oxide on top of the polysilicon layer, comprising the second bird&#39;s beak oxide at the edges, shields the edges of the polysilicon and inhibits its successful removal. The polysilicon layer over the middle of the active region is completely removed before all the edge polysilicon can be eliminated. These edge particles of polysilicon leave small foot-prints, referred to as &#34;scalloping&#34;, around the moat edges, especially at narrow moat corners where the severity of bird&#39;s beak formation is the worst. 
     An alternative embodiment of the present invention is shown in FIG. 5. As shown, the pad oxide in the inverse-moat regions may be completely removed by plasma etching after removal of the silicon nitride and polysilicon layers. Thus, the subsequent sidewall formation will not include edge portion 32 of the pad oxide depicted in FIG. 3. In another alternative embodiment, the sidewalls may be formed from an LPCVD deposition of polysilicon or oxide films, however, a nitride sidewall is preferred. 
     In accordance with the teachings of the present invention, the features of the active moat regions are further improved by a novel processing sequence to expose the active moat region. The field oxidation step is very likely to form a thin oxynitride skin over the moat nitride layer (about 17 nanometers thick, following the 975 degrees C., 10 Atm. high-pressure oxidation). When processing sequences have attempted to remove this skin concurrently with the removal of the deposited top nitride layer, crust formations occur along the edges of the active region due to unsuccessful and incomplete stripping of the top nitride film from moat regions. This crusting further degrades the amount and quality of the active moat region. This crusting can be eliminated by a sufficient deglazing of the wafer to strip off all of the oxynitride skin, before attempting the removal of the moat nitride layer. This can be performed by subjecting the wafer to 10% HF at room temperature for approximately 2 minutes. This deglaze will ensure that all of the oxynitride skin following the oxidation is removed. 
     After the deglaze, the second moat nitride is removed by acid stripping in a hot phosphoric bath of approximately 175 to 182 degrees C. for approximately 90 minutes. The longer etch time at this temperature, in comparison to the conventional 60 minute etch of conventional PBL, ensures that all of the moat nitride layer and the sidewall nitride are removed. 
     After the removal of the sidewall nitride, the wafer is subjected to a high-selectivity isotropic blanket plasma etch with SF 6  (Sulfur-Hexa-Fluoride) gas to remove the polysilicon layer. This plasma-etch step in combination with the use of the thicker pad oxide layer helps to prevent silicon pitting (detrimental etching) of the active moat region. The use of the sandwiched nitride layers 40 and 16 enhances the present poly-buffered LOCOS process by minimizing the second and third bird&#39;s beak oxide 43 and 44 (i.e., the oxide that forms on both the upper and lower surfaces of the polysilicon film at the edges, due to the combined oxidation of the moat silicon and the underside of the poly buffer film at the moat-field oxide interfaces) and to some extent the upper subcomponent of the first bird&#39;s beak at location 41 oxide (i.e., the oxide that forms on the upper surface of the pad oxide). The use of the nitride sidewall deposition after the inverse moat oxide plasma etch further enhances the present poly-buffered LOCOS process by minimizing the second and the third bird&#39;s beak oxide at locations 42 and 43, respectively. 
     After the high-selectivity isotropic blanket plasma etch, the first moat nitride 40 is removed by acid stripping in a hot phosphoric bath of approximately 175 to 182 degrees C. for approximately 15 minutes. Since there is only a thin moat nitride layer 40 to be removed at this step, the shorter etch time of 15 minutes at this temperature is sufficient. A longer etch time was required in the previous step during removal of the thicker moat nitride 16 and the sidewalls 30. 
     After the removal of the first moat nitride 40, the moat pad oxide 12 is stripped off and the wafer is subjected to a megasonic clean up to ensure that any un-oxidized and un-etched polysilicon granulites are effectively removed. The active region, 26 in FIG. 1, is now ready for the formation of the sacrificial gate oxide, Vt-adjust implant, and the subsequent gate oxidation and LPCVD polysilicon gate deposition. 
     Another embodiment of the present invention is to avoid the formation of any thin oxide layers between the moat polysilicon and nitride films. This can be achieved in accordance with the teachings of the present invention by avoiding any piranha or megasonic (RCA-type) NH 4  OH-H 2  O 2  -H 2  O (Ammonium-Hydroxide-Hydrogen-Peroxide-Water) like cleanups before the inverse-moat nitride deposition or after the moat nitride strip. This prevents any excessive native oxide build-up on the top surface of the polysilicon buffer film. If there are any oxides on top of the poly film, they may not etch off during the SF 6  (Sulfur-Hexa-Fluoride) high-selectivity polysilicon plasma-etch process, thus making it difficult to remove the underlying polysilicon film 14 during the moat-etch sequence. 
     Additionally, the use of the sidewall seal and the sandwiched nitride layers prior to field oxidation aids the polysilicon etch process (during the moat etch sequence), by avoiding the second and third bird&#39;s beak oxide that has occurred in the prior-art on the poly surfaces around the edges. This additionally avoids any scalloping problems in the narrow moat corners which has been experienced in the prior-art conventional poly buffered LOCOS. 
     The values and ranges disclosed herein are given for a preferred embodiment of the present invention, of course these values may vary as the parameters are changed. Therefore it is to be understood that many changes in detail may be made as a matter of engineering choices without departing from the spirit and scope of the invention, as defined by the appended claims. Other dopants may be used for channel stop implant, such as a Phosphorous, Germanium, and Germanium/Boron. The present sidewall-sealed and sandwiched poly-buffered LOCOS process can be used in conjunction with Germanium./Boron channel stop implant to fabricate high-voltage VLSI devices such as EPROMS, EEPROMS. In addition, the channel-stop implant can be placed into the silicon at one or more of the various steps involved in device fabrication. For example, the channel-stop implant can be done right after plasma-etching of the inverse-moat stack (of nitride, poly, nitride layers), with the photo-resist still in place. Optionally, the channel-stop implant may be placed alternatively after the field oxidation process, using high-energy implants. The choice of placement of channel stop implant in the overall process sequence depends on the need to optimize the device electrical parameters. 
     TECHNICAL ADVANTAGES OF THE INVENTION 
     From the foregoing the fabrication steps of an improved poly-buffered LOCOS (PBL) isolation region has been disclosed. A technical advantage of the disclosed isolation process is that a substantial amount of the lateral encroachment generally known as Bird&#39;s Beak has been eliminated without degrading the operational characteristics of the active region. Therefore, an attendant technical advantage of the present encroachment reduction process is that it facilitates the successful fabrication of VLSI technologies with smaller design rules. 
     Another technical advantage of the invention presented is that detrimental processing steps, which result in excessive pitting, scalloping and crusting of the active region and which reduce the amount of usable active moat area, have been eliminated or reduced. Furthermore, the excessive corner loss of active device regions experienced in prior art processes due to two-dimensional oxidation has been substantially suppressed with the present invention. Another attendant technical advantage is that little or no unusable moat area need be compensated for in the processing sequence.