Abstract:
Within an integrated circuit, a contact plug with a height not extending above the level of the gate/wordline nitride is nonetheless provided with a relatively large contact area or landing pad, significantly larger than the source/drain region to which the contact plug is electrically connected. Methods for producing the inventive contact plug include (1) use of a nitride facet etch, either (a) during a nitride spacer formation etch or (b) during a BPSG etch; (2) using at least one of (a) an isotropic photoresist etch or partial descum to narrow BPSG spacers above the gate/wordline nitride, and (b) a nitride step etch to etch the shoulder area of the gate/wordline nitride exposed by a BPSG etch; and (3) polishing a BPSG layer down to the top of a gate/wordline nitride before any doped polysilicon plug fill, masking for BPSG etch and performing a BPSG etch, etching the photoresist layer through a partial descum, and etching the shoulder area of the gate/wordline nitride exposed thereby.

Description:
RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Serial No. 09/225,593, filed on Jan. 6, 1999, titled “Self-Aligned Contact Plugs”, now U.S. Pat. No. 6,060,783, which is a continuation application of U.S. patent application Serial No. 08/948,935, filed on Oct. 10, 1997, titled “Formation of Large Self-Aligned Polyplugs for Relaxed Contact Alignment” now abn, which is a divisional application of U.S. Patent Application Serial No. 08/569,838, filed on Dec. 7, 1995, titled “Formation of Large Self-Aligned Polyplugs for Relaxed Contact Alignment”, now U.S. Pat. No. 5,858,865, all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to novel processes for forming self-aligned polyplugs having large contact areas. 
     2. The Relevant Technology 
     FIG. 1 is a flow diagram of selected steps of a conventional process flow for producing DRAM circuits on a silicon substrate, which steps are relevant to the present invention. 
     At the beginning of the process flow of FIG. 1, silicon wafers have already been processed to the point that gate structures of the DRAM circuits are formed upon the substrate. Nitride is then deposited in the first process step to enclose the gate structures. In the second step, the nitride layer is vertically anisotropically etched to leave only the vertical portions of the layer. In the third step, a thin layer of nitride is redeposited over the gate structures and the substrate. 
     According to the fourth step of the process flow of FIG. 1, a borophosphorous silica glass (“BPSG”) layer is then deposited on the surface of the wafer and flowed or densified to fill the surface features of the wafer. The BPSG forms a dielectric planarization layer. Upon the resulting relatively flat BPSG surface, a plug mask in the form of a patterned photoresist layer is produced in the fifth step using standard photolithography techniques. The resist layer is patterned to shield those areas in which no polysilicon plug is to be formed. In the sixth step, a vertical anisotropic BPSG etch removes the BPSG not vertically shielded by the patterned photoresist layer. In the seventh step, a vertical anisotropic nitride etch removes the nitride layer deposited in the third step, so that the polysilicon plugs can contact the source/drain regions. In the eighth step, the patterned photoresist layer is removed from the wafer. 
     In the ninth step of the process flow of FIG. 1, doped polysilicon is deposited upon the wafer surface to fill the areas left open by the BPSG and nitride etches. In the tenth step, the polysilicon on the wafer surface is gradually removed from the top downward by chemical mechanical polishing (“CMP”), or dry etch-back, to leave poly plugs only. 
     FIG. 2 shows a portion of a typical cross section of a partially formed DRAM device after processing through step  5  of the process of FIG.  1 . 
     With reference to FIG. 2, three gate stacks  20   a ,  20   b ,  20   c  have been formed upon a silicon substrate  10 . Gate stack  20   a  includes a doped polysilicon gate  24   a  topped with tungsten silicide  26   a  and a thick nitride layer  27   a . Polysilicon gate  24   a  is separated from silicon substrate  10  by gate oxide  22   a . The components of the gate stacks  20   b ,  20   c  are as described with respect to the like numbered components of gate stack  20   a.    
     Gate stacks  20   a ,  20   b ,  20   c  are enclosed laterally by nitride spacers  28 , which have been formed by nitride deposition followed by an anisotropic nitride etch. A second, thin layer nitride deposition has produced nitride film  29 . A BPSG layer  30  has been deposited on and flowed over nitride film  29 . A patterned photoresist layer  32  has been formed on BPSG layer  30  by photolithography techniques. 
     FIG. 3 shows the cross section of FIG. 2 after processing through step  6  of the process of FIG. 1, where step  6  is an anisotropic etch of the BPSG layer. The anisotropic BPSG etch has good selectivity of the BPSG layer  30  against the nitride of film  29 , leaving nitride film  29  intact with little or no etching thereof, while removing the portions of BPSG layer  30  not shielded by the photoresist. 
     FIG. 4 shows the cross section of FIG. 3 after processing through steps  7  and  8  of the process of FIG. 1, an anisotropic nitride etch and a photoresist strip. The nitride covering gates  24   a ,  24   b ,  24   c , is shown for convenience as an undifferentiated nitride layer  31 . The anisotropic nitride etch has uncovered source/drain contact regions  34 ,  36 ,  38 ,  40  in silicon substrate  10 , and etched small steps  42  into nitride layer  31  covering gates  24   a  and  24   c . Patterned photoresist layer  32  has been removed by the photoresist strip. 
     FIG. 5 shows the cross section of FIG. 4 after doped polysilicon plug fill and chemical mechanical polishing (CMP) or dry etch-back to a first depth in accordance with steps  9  and  10  or  10 A of the process of FIG.  1 . Polyplugs  44 ,  46 ,  48  have been formed. The CMP or dry etch-back has been applied long enough to isolate plugs that should be isolated according to the circuit layout, but short enough to permit, where desired, local interconnect across gates, such as with polyplug  46  which interconnects across gate  24   b . The top surfaces of polyplugs  44 ,  46 ,  48 , respectively, constitute landing pads  50 ,  52 ,  54  for metal contact by a subsequent metalization layer or for contact by other subsequent contact or interconnect layers. 
     FIG. 6 shows an alternative cross section of FIG. 5 processed according to steps  9  and  10  of the process of FIG. 1, but with the CMP or dry etch-back step performed to a second depth greater than the first depth of FIG.  5 . This second depth extends to the level of the top of nitride layer  31 . Polyplugs  56 ,  58 ,  60 ,  62  are formed having, respectively, landing pads  64 ,  66 ,  68 ,  70 . 
     CMP or dry etch-back to the second depth seen in FIG. 6 has certain advantages over CMP or dry etch-back stopping at the first depth seen in FIG.  5 . First, CMP or dry etch-back to the second depth results in polyplugs having landing pads which are self-aligned with the nitride-enclosed gate structures. Second, a major goal of CMP or dry etch-back is to provide a precisely planarized surface for subsequent processing steps. CMP to the second depth improves the flatness of the polished surface above that of typical CMP because nitride layer  28  polishes away at a much slower rate than both BPSG layer  30  and the doped polysilicon material which forms the polyplugs. Thus polishing is slowed in those areas in which the nitride is reached earliest, resulting in a more even polishing compared to polishing that extends only to the first depth seen in FIG.  5 . 
     The advantages of polishing to the second depth seen in FIG. 5 are tempered by the disadvantages of the small size of landing pads  64 ,  66 ,  68 ,  70 . Small landing pads increase the criticality of subsequent alignment steps. A contact etch generally etches both nitride and oxide, so that any misalignment can result in contacts which short between source/drain regions and gates/wordlines. Small landing pad areas also increase the resistance at the contact material/doped polysilicon interface. It would thus be a significant advance to provide the advantages of CMP to the second depth seen in FIG. 6 without the inherent disadvantages of small landing pads. 
     Even where CMP or dry etch-back is performed only to the first depth shown in FIG. 5, large landing pads do not result if the width of the gate stacks is already at the limit of the resolution of the photolithography system employed. At the resolution limit, the line width of patterned photoresist layer  32  is as shown by outline O in FIG.  2 . As seen in FIG. 2, the line width of patterned photoresist layer  32  at the resolution limit approaches the width of gate stacks  20   a ,  20   b ,  20   c . This results in narrower polyplugs, shown in FIG. 5 by outline N, having smaller landing pads, even with CMP or dry etch-back only to the first level shown in FIG.  5 . For gates or wordlines at the resolution limit, a method to increase the size of contact plug landing pads is thus needed, regardless of whether CMP to the second depth shown in FIG. 6 is employed. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     An object of the present invention is to provide source/drain contact plugs having large landing pad areas which are self-aligned to adjacent gates. 
     Another object of the present invention is to create self-aligned source/drain contact plugs having large landing pad areas in relatively few process steps. 
     Another object of the present invention is to improve the flatness of surfaces produced by CMP. 
     Another object of the present invention is to increase yield by decreasing the criticality of mask alignment subsequent to completion of source/drain contact plugs. 
     Another object of the present invention is to allow for smaller minimum feature size by increasing the size of landing pad areas of source/drain contact plugs. 
     In accordance with one presently preferred process of the present invention, transistors including gate/wordline stacks are formed upon a silicon substrate. Nitride is deposited over the gate/wordline stacks. The nitride is then etched to form nitride spacers enclosing the gates. The nitride etch process includes either a plasma sputter etch or a reactive ion etch with film-forming agents, such that facets are etched in the nitride at top corners of the gate/wordline stacks. Typical processing then follows, but with an optional isotropic etch of the plug mask layer to reduce the line width of the plug mask below the resolution limit of the photolithography system. The facets on the nitride spacers at the top corners of the gate/wordline stacks result in a polyplug having a wider landing pad area, even if CMP or dry etch-back is performed down to a depth of the top of the nitride spacers. 
     In accordance with another presently preferred process of the present invention, conventional processing is varied by an optional isotropic etch of the plug mask layer to reduce the line width of the plug mask below the resolution limit of the photolithography system, if needed, and by replacing a conventional anisotropic BPSG etch step with a step that both anisotropically etches BPSG and facet-etches the nitride enclosing the gate/wordline stacks. The resulting facets on the nitride spacers at the top corners of the gate/wordline stacks, after further processing, result in a polyplug having a wider landing pad area, even if CMP or dry etch-back is performed down to the nitride. 
     In accordance with yet another presently preferred process of the present invention, conventional processing is varied by isotropically etching the plug mask layer after it has been patterned, decreasing the effective line width of the plug mask layer. Subsequent standard processing results in wider polyplug landing areas if CMP or dry etch-back is performed down to a level above the top of the nitride. Alternatively, a step feature may be etched in the nitride at the corners of the gate/wordline stacks, which nitride at the corners of the gate/wordline stacks is left exposed after the BPSG etch. Isotropic etching of the plug mask may optionally be used to increase the area of nitride exposed at the corners of the gate/wordline stacks after the BPSG etch. Subsequent processing then results in wider polyplug landing areas even with CMP or dry etch-back performed down to the nitride enclosing the gate/wordline stacks. 
     In accordance with an additional presently preferred process of the present invention, conventional transistor structures including gate/wordline stacks and source/drain regions are formed upon a silicon substrate, and the gate/wordline stacks are enclosed in nitride spacers. A thin nitride layer is deposited over the entire surface. BPSG is then deposited over the entire surface and densified. A first CMP is then performed down to the level of the nitride enclosing the gate/wordline stacks, producing a flat surface. On the flat surface produced by the first CMP, a patterned plug mask layer is formed. The BPSG left exposed by the plug mask layer is then removed by an anisotropic etch. The plug mask layer is then optionally isotropically etched to increase the area left exposed thereby. The nitride layers left exposed are then anisotropically etched, removing a portion of the nitride at the top corners of the gate/wordline stacks and removing the nitride over the source/drain regions. Subsequent polyplug fill and CMP or dry etch-back down to the nitride result in wider polyplug landing areas and self-aligned polyplugs. 
     These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the above-recited and other advantages and objects of the invention are obtained may be more fully explained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a flow diagram illustrating a conventional process flow for producing doped polysilicon plugs for source/drain contacts in a DRAM device. 
     FIGS. 2-4 are cross-sectional elevation views of a portion of a partially formed DRAM device after performance of certain of the conventional processing steps of the process flow diagrammed in FIG.  1 . 
     FIG. 5 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIGS. 2-4 after completion of processing according to the conventional process flow of FIG.  1 . 
     FIG. 6 is an alternate cross-sectional elevation view of the portion of the partially formed DRAM device of FIGS. 2-4 after completion of processing according to the conventional process flow of FIG.  1 . 
     FIG. 7 is a flow diagram of a first process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. 
     FIG. 8 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through the first step of the process flow of FIG.  7 . 
     FIG. 8A is cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 8 after processing through a portion of one method of the second step of the process flow of FIG.  7 . 
     FIG. 9 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIG. 8 after processing through a complete second step of the process flow of FIG.  7 . 
     FIG. 9A is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIG. 8 after processing through the fifth step and optional step  5 . 5  of the process flow of FIG.  7 . 
     FIG. 10 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIG. 9 after completion of all processing according to the process flow of FIG.  7 . 
     FIG. 11 is a flow diagram of a second process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. 
     FIG. 12 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through the sixth step of the process flow of FIG.  11 . 
     FIG. 13 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIG. 12 after all processing according to the process flow of FIG.  11 . 
     FIG. 14 is a flow diagram of a third process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. 
     FIGS. 15 and 16 are cross-sectional elevation views of a portion of a partially formed DRAM device after processing through certain steps according to the process flow of FIG.  14 . 
     FIG. 17 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIGS. 15 and 16 after completion of processing according to the process flow of FIG.  14 . 
     FIG. 18 is an alternate cross-sectional elevation view of the portion of the partially formed DRAM device of FIGS. 15 and 16 after completion of processing according to the process flow of FIG.  14 . 
     FIG. 19 is a flow diagram of a fourth process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. 
     FIGS. 20 and 21 are cross-sectional elevation views of a portion of a partially formed DRAM device after processing through certain steps according to the process flow of FIG.  19 . 
     FIG. 22 is a cross-sectional elevation view of the portion of the partially formed DRAM device of FIGS. 20 and 21 after completion of processing according to the process flow of FIG.  19 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the present invention, polysilicon source/drain contact plugs are produced which, when polished by CMP or etched by dry etch-back down to the level of the gate/wordline nitride, have a contact area or landing pad substantially larger than the source/drain area with which said plugs are associated. The present invention also produces polysilicon source/drain contact plugs having a landing pad substantially larger than the source/drain area with which said plugs are associated even where the size of the associated gate stacks is at the minimum achievable by the photolithography system employed. The present invention includes several novel presently preferred process flows, with variations thereto, for producing large landing pad area contact plugs. While the present invention will be described below in terms of a DRAM device, the present invention is not limited to DRAM devices and will find also use in SRAM, bipolar, and other technologies as will be apparent to those of skill in the art. 
     A first preferred process flow for producing large landing pad area contact plugs is diagrammed in FIG.  7 . According to the process of FIG. 7, step  2  of the conventional process shown in FIG. 1, an anisotropic nitride etch, is replaced by a facet etch of the nitride layer. A facet etch etches preferentially at a 45° angle beginning at the portions of the nitride layer covering the corners of each gate structure. A facet etch also etches slowly in the vertical direction, and virtually not at all in the horizontal direction. 
     FIG. 8 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through step  1  of FIG. 7, i.e., after formation of transistors upon a silicon substrate, and after deposition of the spacer nitride, and before the facet etch of the spacer nitride which etch forms the desired nitride spacers. Substrate  110  has three gate structures formed thereon, each gate including a gate oxide  122   a ,  122   b ,  122   c  respectively, a polysilicon gate  124   a ,  124   b ,  124   c  respectively, with each polysilicon gate topped by a tungsten silicide layer  126   a ,  126   b ,  126   c , and a thick nitride layer  127   a ,  127   b ,  127   c , respectively. All of the gates are enclosed over substrate  110  by a nitride layer  128 . 
     FIG. 9 is the cross-sectional elevation view of FIG. 8 after processing through step  2  of FIG. 7, the nitride facet etch. The nitride spacers formed out of nitride layer  128  as a result of the nitride facet etch have much more material removed near the top corners of the gate structure than with conventional processing, as best seen by comparison of spacers  128  of FIG. 9 with spacers  28  of FIG.  2 . In FIG. 8A, even the top corners of thick nitride layers  127   a ,  127   b ,  127   c  have been removed. 
     FIG. 9A is the cross-sectional elevation view of FIG. 9 after processing through step  5  and, if needed, optional step  5 . 5  of FIG.  7 . If the photolithography process can directly form a patterned mask layer  132  of a desired line width as shown in FIG. 9A, no isotropic photoresist etch, optional step  5 . 5 , is required. But if the photolithography is already at the limits of its resolution when forming the gates, then the minimum sizes of the line widths of patterned mask layer  132  will be as illustrated by dashed outlines O. Then an isotropic photoresist etch step, step  5 . 5 , is required to reduce the size of the line widths of patterned mask layer  132  from the size of outlines O to the size shown in FIG. 9A in order to achieve a polyplug width beyond the printing capability of the photolithography. 
     FIG. 10 is the cross-sectional elevation view of FIG. 9A after all processing steps of FIG. 7 have been performed. A portion of BPSG layer  130  remains at the left of FIG.  10 . Doped polysilicon plugs  156 ,  158 ,  160 ,  162  have been formed in the spaces between the gate structures and have been polished down to the level of the gate/wordline nitride. The upward facing surfaces of plugs  156 ,  158 ,  160 ,  162  respectively constitute landing pads  164 ,  166 ,  168 ,  170 . The facet etch of the nitride spacers results in landing pads  164 ,  166 ,  168 ,  170  of substantially larger area than with conventional processing, as may be seen by comparison with FIG.  6 . 
     Step  2  of the process of FIG. 7, the facet etch of the spacer nitride, may be performed in at least two ways. 
     A first and presently preferred way to perform the facet etch of step  2  of the process of FIG. 7 is by sputter etching the nitride layer of step  1  in an argon plasma. During sputter etching in an argon plasma, energetic argon ions are made to impinge upon the wafer in a direction generally normal to the wafer surface. The argon ions thus impact upon and sputter only the horizontal surfaces of the nitride layer, and on those surfaces having a horizontal component. 
     With reference to FIG. 8 for example, those portions of nitride layer  128  located at the upper corners of gate/wordline stacks are more easily dislodged by the impinging argon ions than the nitride at other locations. The impinging argon ions perform a facet etch in which the portions of the nitride layer at the upper corners of the gate/wordline stacks are removed relatively rapidly at an angle of about 45°, while the more horizontal nitride surfaces are removed relatively slowly, and the vertical surfaces are left essentially intact. The selectivity of the etch for the 45° surfaces over the horizontal surfaces is sufficiently great that a separate conventional reactive ion etch (“RIE”) may be required to complete removal of nitride layer  128  from the trenches between the gates/wordlines, resulting in the structure of FIG.  9 . 
     The preferred sputter etch in an argon plasma may be performed, for example, in a Precision 5000 etcher, manufactured by Applied Materials, Inc., at moderate pressure and at high power. Pressure may be about 100 mTorr and power levels may be 600W or above. 
     A second and alternative way to perform the facet etch of step  2  of the process of FIG. 7 is by subjecting the wafer to an RIE in which film-forming agents are added to the processing chamber. The film-forming agents tend to form a film over all exposed surfaces of the wafer. The film is continuously removed from the horizontal surfaces by impinging ions so that etching may occur at the horizontal surfaces. The vertical surfaces are protected from chemical etch reactions by the film formed thereon. At areas where the nitride film to be etched makes a transition from a horizontal to a vertical surface, a facet etch occurs, producing a surface at approximately a 45° angle. The facet etch occurs both at the top and bottom corners of the gate/wordline stacks, resulting in the structure shown in FIG. 8A. A slight over-etch will remove the remaining nitride from the trenches between the gates/wordlines, resulting in the structure shown in FIG.  9 . 
     An appropriate RIE with film-forming agents may be performed for example in a Precision 5000 etcher at a power level of 600 W at a pressure of 200 mTorr with a 30 sccm flow of CF 4 , and a 30 sccm flow of CHF 3 . The CHF 3  causes a polymer film to form on the exposed nitride surfaces of the wafer. Increasing the polymer level in the RIE increases the relative amount of facet etch. Polymer levels may be increased by increasing the CHF 3  flow rate, increasing the pressure, decreasing the power, or any combination of these. 
     A second preferred process flow for producing the large contact area plugs of the present invention is diagrammed in FIG.  11 . In the process of FIG. 11, step  6  of the conventional process flow, an anisotropic BPSG etch, is replaced with a combined anisotropic BPSG etch and nitride facet etch. An optional step  5 . 5 , an isotropic photoresist etch, may precede step  6 . The isotropic photoresist etch, step  5 . 5 , is used if it is desired to increase the size of the polyplug beyond the printing capability of the photolithography, as discussed above with respect to FIG.  9 A and the process of FIG.  7 . 
     FIG. 12 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through step  6 , the combined anisotropic BPSG etch and facet nitride etch, of the process flow of FIG. 11. A BPSG layer  230  has been removed in those areas not vertically shielded by a patterned a photoresist layer  232 . A nitride layer  231  has been facet etched, and the portion of nitride layer  231  above source/drain regions  234   a ,  234   b ,  234   c ,  234   d  of substrate  210  has been removed. 
     FIG. 13 is the cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 12 after processing through all steps of the process flow of FIG. 11. A portion of BPSG layer  230  remains at the left of FIG.  13 . Doped polysilicon has been deposited in the regions above the source/drain regions of substrate  210 , and has been polished by CMP or etched by dry etch-back to the level of the nitride layer  231 , resulting in doped polysilicon contact plugs  256 ,  258 ,  260 ,  262 . The upward facing surfaces of doped polysilicon contact plugs  256 ,  258 ,  260 ,  262  respectively constitute landing pads  264 ,  266 ,  268 ,  270 . Because of the facet etch of the nitride layer  231 , landing pads  264 ,  266 ,  268 ,  270  are substantially larger than those produced by conventional processing as seen by comparison with FIG.  6 . 
     Step  6  of the process of FIG. 11 may be performed for example by an RIE in which the number and energy of ions vertically impinging on the wafer surface is sufficient to sputter the top corners of the nitride layer covering the gate/wordline stack, resulting in a facet etch of the nitride layer. An inert gas may be added to the standard RIE to relatively increase the sputtering effect. Standard etch chemistries that chemically etch nitride may also be employed. Such etchants will etch the BPSG at a faster rate than nitride, resulting in facet formation at the top corners of the nitride covering the gate/wordline stacks, where the nitride is first and longest exposed to the etchants. 
     FIG. 14 is a flow diagram of a third process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. In the process of FIG. 14, an optional step  5 . 5 , an Isotropic Photoresist Etch, is inserted after step  5 . Step  5 . 5  is used if it is necessary or merely desired to increase the size of the polyplug beyond the printing capabilities of the photolithography, as discussed above in reference to FIG.  9 A and the process of FIG.  7 . 
     FIG. 15 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through step  5 , the plug mask step of the process flow of FIG.  14 . Three gate structures have been formed upon a substrate  310 , each gate including a gate oxide  322   a ,  322   b ,  322   c  respectively, a polysilicon gate  324   a ,  324   b ,  324   c  respectively, with each polysilicon gate topped by a tungsten silicide layer  326   a ,  326   b ,  326   c , and a thick nitride layer  327   a ,  327   b ,  327   c  respectively. All of the gates have been enclosed laterally by spacers  328 , and laterally and vertically by a nitride film  329 , and a BPSG layer  330  has been deposited and densified over nitride film  329 . A patterned photoresist layer  332  has been formed on the surface of BPSG layer  330 . If the minimum line width of the patterned photoresist corresponds to that shown by outline O, the optional isotropic photoresist etch, step  5 . 5  of the process of FIG. 14, is used to reduce the patterned photoresist  332  to the size shown. 
     FIG. 16 is a cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 15 after processing through step  6 , the BPSG etch step, of the process flow of FIG.  14 . BPSG layer  330  has been etched vertically. 
     FIG. 17 is the cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 16 after processing through all steps of the process flow of FIG. 14, but with the CMP or dry etch-back not applied down to the level of nitride  328 . This results, with the use of optional step  5 . 5 , in contact plugs  356 ,  358 ,  360 ,  362  having relatively larger landing pads than possible with previously known processes, but does not take advantage of the more planar nature of the top surface possible with CMP down to the level of nitride  328 . 
     FIG. 18 shows an elevational cross section of the resulting structure after CMP or dry etch-back down to the nitride level in step  10  of the process flow of FIG.  14 . If the CMP or dry etch-back is to be performed down to the top of nitride  331 , then the optional nitride step etch must be performed by an anisotropic nitride etch in step  7  of sufficient depth to form steps  342  as shown in FIG. 18, and optional step  5 . 5  must also have been performed if necessary. This results in self-aligned polysilicon contact plugs  356 ,  358 ,  360 ,  362  have having landing pads  364 ,  366 ,  368 ,  370  larger than possible with conventional processing. FIG. 19 is a flow diagram of a fourth process flow according to the present invention for producing self-aligned polyplugs having large landing pad areas. In the process of FIG. 19, step  4 . 5 , a CMP down to the nitride level, has been added to the conventional process flow, as has optional step  5 . 5 , an isotropic photoresist etch or partial descum. The effects of these process modifications are illustrated in FIGS. 20-22. 
     FIG. 20 is a cross-sectional elevation view of a portion of a partially formed DRAM device after processing through step  5 , the plug mask step of the process flow of FIG.  19 . Three gate structures have been formed upon a substrate  410 , each gate including a gate oxide  422   a ,  422   b ,  422   c  respectively, a polysilicon gate  424   a ,  424   b ,  424   c  respectively, with each polysilicon gate topped by a tungsten silicide layer  426   a ,  426   b ,  426   c  respectively. All of the gate structures have been enclosed over substrate  410  by a nitride layer  431 , and a BPSG layer  430  has been deposited and densified over nitride layer  431 . BPSG layer  430  has been polished by CWP down to the top of nitride layer  431 . A patterned photoresist layer  432  has been formed on the CMP-produced surface. 
     FIG. 21 is the cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 20 after processing through optional step  5 . 5 , the isotropic photoresist etch step of the process flow of FIG.  19 . The BPSG layer  430  has been etched away in the locations not vertically shielded by patterned photoresist  432  shown in FIG.  20 . The photoresist itself has then been etched by an isotropic photoresist etch, such that material is removed from both the top and sides of each remaining portion of photoresist. The material removed corresponds to the space within outlines O. This results in a remaining patterned photoresist layer  432  with features potentially smaller than the smallest feature size achievable by photolithography alone. Nitride layer  431  remains virtually unaffected. If sufficiently small patterned photoresist such as patterned photoresist  432  of FIG. 21 can be formed directly by photolithography, optional step  5 . 5  is not required. 
     FIG. 22 is the cross-sectional elevation view of the portion of a partially formed DRAM device of FIG. 21 after processing through all steps of the process flow of FIG.  19 . Steps  442  have been etched into shoulder regions of nitride layer  431  in the locations unshielded by the portions of patterned photoresist layer remaining in FIG.  21 . The remaining photoresist has then been stripped, and the plug locations have been filled with doped polysilicon. CMP or dry etch-back has then been performed down to the level of nitride layer  431 . This results in contact plugs  456 ,  458 ,  460 ,  462  having landing pads  464 ,  466 ,  468 ,  470  substantially larger than possible with the conventional process flow of FIG.  1 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.