Abstract:
A method for preparing an SRAM or DRAM structure on a substrate with an oppositely doped well therein, a field oxide region extending above and between the well and the substrate, first and second N-MOS transistors on the silicon substrate, and a P-MOS transistor on the silicon well. The source and drain regions of each of the P-MOS transistor and the first and second N-MOS transistors each have a polysilicon plug making contact therewith. Each polysilicon plug is isolated one from another by nitride spacers, has the same doping as the region with which it makes contact, and is self-aligned to the nitride spacers lining the passage of the polysilicon plugs to their respective contacts on either the silicon substrate or the silicon well. The self-aligned nature of the polysilicon plugs is due to the nitride spacers formed by etchant selectivities and photoresist masks.

Description:
This application is a continuation of U.S. patent application Ser. No. 08/931,822, filed on Sep. 16, 1997, now U.S. Pat. No. 5,864,156 which is a divisional of U.S. patent application Ser. No. 08/572,969, filed on Dec. 15, 1995, now U.S. Pat. No. 5,700,706, both 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 circuit devices. More particularly, the present invention is directed to a novel process which permits the formation of self-aligned polysilicon plugged contacts using nitride spacers and a polishing step to isolate the plugged contacts one from another. 
     2. The Relevant Technology 
     A challenge in current semiconductor design is the increasing pattern density and continuously smaller feature size during printing of DRAM arrays. This applies also for the printing and manufacturing of polysilicon plugs which are used in current DRAM technology to contact conductive layers e.g. the active area regions of a semiconductor to the conductive layers at a higher level. 
     In the current application this could be the connection between a source region of an access transistor in a memory cell to a tungsten plug of a bitline contact and the connection between the drain region of the access transistor to the storage container of the DRAM cell. Polysilicon plugs are required below every storage node and below every bitline contact. 
     The plugs have to meet two opposing requirements. Firstly, they have to be as large as possible to lower the electrical resistance and provide a good and easy alignment of the cell container or the bitline tungsten plug to the existing polysilicon plug. Secondly, they must be electrically isolated from each other, i.e. a space between the features during printing has to be maintained. Assuming all features are at minimum size, the space between the plugs has to be at the same size as the plug itself, which in turn limits the size of the plug. 
     One way to improve the situation is to print the polysilicon plug for the bitline contact with one mask and to print the polysilicon plugs for the cell containers using a different mask and alignment/exposure sequence. In this case, the pattern density is smaller and the polysilicon plugs can be enlarged without the risk of “photoresist scumming” during the exposure due to sub-minimum spaces between features. Highly accurate alignment becomes an important issue in this approach. 
     Consequently, it would be an advance in the art to overcome these problems by a creative processing sequence of masks, etchings, and order of fabrication process steps. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     In the novel process flow taught herein for fabrication of dynamic random access memory structures, at least two masks are required for the manufacturing of polysilicon plugs. A first mask opens up contacts for the formation of polysilicon plugs to the drain of an access transistor above which a cell capacitor will be formed. The second mask opens up contacts for the formation of polysilicon plugs to the source of an access transistor above which a tungsten plug will be formed to connect to the bitline of the memory device. The second mask can also be used to plug NMOS devices in the periphery. If PMOS devices should be connected with a p-doped polysilicon plug, a third mask is required. 
     The first and second masks must be self-aligned to the edge of the access transistor in the array on the foundational silicon substrate of structure and also to the adjacent polysilicon plug so as to avoid shorts between polysilicon plugs and the access transistor, as well as between adjacent doped plugs to be formed. Such shorts are the result of misalignment between the plug masks or large line size variations of the first and/or second plug(s) to be formed. 
     In the inventive process, shorts between misaligned polysilicon plugs are avoided by surrounding at least one of the two polysilicon plugs with a thin isolating nitride spacer. The spacers are formed by isotropically depositing a thin nitride film and etching it back using a very anisotropic etch leaving only the vertical parts of the nitride film. 
     The nitride spacer is formed at the vertical wall of the contact hole after the contact hole is etched into the oxide but before the contact hole is filled with doped polysilicon, thereby surrounding the polysilicon plug laterally. 
     The possibility to self-aligning the two polysilicon plugs against each other permits printing of the polysilicon plug below the cell capacitor as well as the printing of the plug below the bit line contact which will be larger than as if the plug where printed at the same time. Isolation between the plugs is not obtained by line size control and accurate alignment, but rather by the surrounding nitride spacer sealing the plugs laterally against each other. This permits effective spacings between plugs down to 200 Angstroms which is the spacer thickness, rather than 3000 Angstroms which would be the photolithographic limit. Vertical isolation between the plugs is obtained by CMP steps or etchback of the plug forming polysilicon deposited to fill the contact openings. 
     Another aspect of depositing the doped polysilicon plugs at the contacts in two masking steps is that the printing of the first mask (e.g. the cell plate node plug) with the smallest feature size can take advantage of contacts having only one size. Further, the first mask which opens up contacts for the cellnode plugs is nearly identical to the mask defining the cell container. Additionally, the lithography needs to be developed only once for two layers, and alignment of the two layers is more readily accomplished if the printing is similar. 
     In furtherance of the advantage of depositing the doped polysilicon plugs at the contacts in two masking steps, it can be stated that the process integration can take advantage of the contacts opened by the first and second masks to improve the transistors in the periphery by using source/drain implants as well as punch through suppression implants after the second mask opens its respective contacts, while maintaining low leakage junctions at the cellnode side by not implanting into these areas late in the processing steps. 
     In an alternative embodiment of the invention process flow taught herein for fabrication of dynamic random access memory structures (DRAM), a third mask is required. The third mask opens up contacts to P-MOS devices of the periphery. Like the second mask described above (e.g. the bitline contact plug mask), the printing of the third mask is similar to a conventional gate mask. 
     The self-aligning aspect of the invention enables very small spaces between the polysilicon plugs printed at different mask levels. Thereby the unusable space is minimized and the present inventive method is an advance in the exploitation of memory area. 
     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, 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 drawing 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 cross-sectional elevation view illustrating a semiconductor substrate having six transistors thereon, over which a passivation layer is situated, and a patterned photoresist layer is upon the passivation layer. 
     FIG. 2 is a cross-sectional elevation view of the structure seen in FIG. 1, to which an etching step has been performed, and the pattern photoresist layer removed, so as to create a series of cellnode contacts over which cell capacitors will be subsequently built. 
     FIG. 2A is an alternatively scaled top plan view of FIG.  2 . 
     FIG. 3 is a cross-sectional elevation view of the structure seen in FIG. 2, and shows the result of additional fabrication processing steps thereto, including the addition of a nitride spacer to remnants of the passivation layer, the addition of an N-doped polysilicon layer, the addition of a second passivation layer, and the addition of a second patterned photoresist layer. 
     FIG. 4 is a cross-sectional elevation view of the structure seen in FIG. 3, and shows the results of additional fabrication processing steps thereto, including etching through both the second passivation layer and the second N-doped polysilicon layer so as to contact two bitline contacts at the silicon substrate, and the cleaning off of the second patterned photoresist layer from the structure. 
     FIG. 4A is an alternatively scaled top plan view of FIG.  4 . 
     FIG. 5 shows the addition of a second polysilicon layer to the structure shown in FIG. 4 so as to make contact with the two exposed bitline contacts. 
     FIG. 6 shows the result of a chemical-mechanical polishing step to the structure seen in FIG. 5 that isolates each of the resultant polysilicon plugs one from another, which polysilicon plugs were polysilicon layers that made contact with either a cellnode contact or a bit contact. 
     FIGS. 6A and 6B are alternatively scaled top plan views of FIG.  6 . 
     FIG. 7 is a cross-sectional elevational partial view of the structure seen in FIG. 5, showing also P-MOS devices above an N-doped silicon well next to a junction with the P-doped polysilicon substrate, and shows the result of additional fabrication processing steps thereto, including the addition of a third patterned photoresist layer. 
     FIG. 8 is a cross-sectional elevation view of the structure seen in FIG. 7, and shows the results of additional fabrication processing steps thereto, including etching through both the third patterned photoresist layer, removing the photoresist, adding a nitride spacer to the transistor to avoid cross contamination between plugged contacts, and adding ion implanted regions in the N-doped silicon well to optimize the P-MOS devices. 
     FIG. 9 shows the deposition of a first P-doped polysilicon layer so as to contact the ion implanted active areas for the P-MOS devices. 
     FIG. 10 shows the result of a chemical-mechanical polishing step to the structure seen in FIG. 9 that isolates each of the resultant polysilicon plugs one from another, which polysilicon plugs were doped polysilicon layers that made contact with either a cellnode contact, a bit contact, or a P-MOS device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a semiconductor structure consisting of a P-doped polysilicon substrate  10  having MOS devices A, B, C, D, E, and F situated thereon. Each MOS device A-F, respectively, has an N-doped polysilicon gate electrode  12 ,  14 ,  16 ,  18 ,  20  and  22 . Each N-doped polysilicon gate electrode  12 - 22  has sealed thereover, respectively, a first nitride spacer  24 ,  26 ,  28 ,  30 ,  32 , and  34  each of which has vertically oriented sides in contact with P-doped silicon substrate  10 . 
     P-doped silicon substrate  10  is a surface over which a series of cellnode contacts  36 ,  37 ,  38 , and  39  are situated. Each cellnode contact  35 - 39  defines a locus at which a cellnode plug will be situated to serve as a foundation for a cell capacitor to be built thereon. P-doped silicon substrate  10  also features two bitline contacts  40 ,  42  thereon. 
     Situated above cellnode contacts  36 - 39 , bitline contacts  40 - 42 , and first nitride spacers  24 - 34  is a BPSG layer  46  that was deposited and flowed onto P-doped silicon substrate  10 . Situated over BPSG layer  46  is a first photoresist layer  48  having patterned therein a first photoresist pattern  50 . To make first photoresist pattern  50 , a mask  41 A is aligned with first photoresist layer  48 , followed by an exposure and a development of first photoresist layer  48  so as to form first photoresist patterns  50  seen in FIG.  1 . 
     FIG. 2 shows the result of additional processing steps to the structure seen in FIG.  1 . First, an etching step etches BPSG layer  46  with selectivity to nitride spacers  24 - 34 , N-doped polysilicon gate electrodes  12 - 22 , and P-doped polysilicon substrate  10 . The result of this etching step is a first etch pattern  52  seen in FIG.  2 . First etch pattern  52  exposes cellnode contacts  36 ,  37 ,  38 , and  39  over which cell capacitors will be subsequently built. 
     FIG. 3 shows the results of additional processing steps upon the structure seen in FIG.  2 . Initially, a substantially vertically oriented second nitride spacer  54  is formed over each remaining BPSG layer  46  by deposition and anisotropic etch of a thin nitride film (not shown) having a thickness from about 100 Å to about 300 Å. Next, an N-doped polysilicon layer  56  is deposited over exposed cellnode contacts  36 ,  37 ,  38 , and  39 , as well as over MOS devices A-F. An optional thin oxide layer  58  having a thickness from about 300 Å to about 1000 Å, is deposited over N-doped polysilicon layer  56 . A second photoresist layer  60  is then applied over thin oxide layer  58 . Second photoresist layer  60  is aligned with a mask, exposed to UV light, and is developed so as to form a second photoresist pattern  62  seen in FIG.  3 . 
     FIG. 4 shows the result of several processing steps applied to the structure shown in FIG.  3 . Particularly, a second etching step exposes bitline contacts  40  and  42  at P-doped silicon substrate  10  so as to form a second etch pattern  66  seen in FIG.  4 . 
     In the second etch step, a first etch component etches thin oxide layer  58 . A second etch component of the second etch step etches N-doped polysilicon layer  56  with selectivity to oxide layer  46  and second nitride spacer  54 . A third etch component then etches BPSG layer  46  with selectivity to second nitride spacer  54 , nitride spacers  24 - 34  (seen in FIG. 1) surrounding N-doped polysilicon electrodes  12 - 22 , and P-doped silicon substrate  10 . Next, a substantially vertically oriented third nitride spacer  64  is formed over N-doped polysilicon layer  56  remaining after the second etching step. Third nitride spacer  64  is formed by deposition and anisotropic back etch of a thin nitride film (not shown) having a thickness from about 100 Å to about 300 Å. 
     If the first and the second polysilicon plugs do not overlap as seen in the left half of second etch pattern  66  in FIG. 4, third nitride spacer  64  contains only a vertical component and is not essential for the isolation between the first and second polysilicon plugs. Isolation between these two plugs would have been obtained by remaining BPSG  46  but is enhanced by second nitride spacer  54  and third nitride spacer  64 . In case of overlapping of the first and second plugs, as seen in the right half of the second etch pattern  66 , spacers  54 ,  64  become desirable for the isolation between the two plugs. This, the spacers guarantee only a lateral isolation as opposed to a vertical isolation between the plugs. 
     In FIG. 4 there are two locations  65  where the anisotropic spacer etch removes the horizontal part of the deposited nitride and exposes the first polysilicon plug  56 . These undesired “shorts” will be removed by a CMP step as seen figures for further processing steps. Optionally, N-doped active areas  65  can be implanted by ion implants at bitline contacts  40 ,  42 . 
     Above electrode  16  of MOS device C at second nitride spacer  54 , a double spacer is seen in FIG.  4 . The double spacer, which is due to misalignment, will preferably have a thickness of about 200 Å. 
     The structure of FIG. 4 has been processed so as to result in that structure seen in FIG. 5, and is more particularly the addition of an N-doped polysilicon layer  68  so as to cover over the remainder of thin oxide layer  58 , third nitride spacer  64 , and bitline contacts  40 ,  42 . N-doped polysilicon layer  68  may be optionally dry back etched to stop on thin oxide layer  58 . This optional dry back etch step of N-doped polysilicon layer  58  helps to reduce stress therein. 
     In FIG. 6, a chemical-mechanical polishing step has been effected below oxide layer  58  so as to isolate N-doped polysilicon layers  56 ,  68  and avoid shorting especially at location  65  where the two plugs overlap. The remaining N-doped polysilicon layers  56 ,  68  are called plugs and make contact, respectively, with cellnode contacts  36 ,  37 ,  38 , and  39 , or bitline contacts  40 ,  42 . These plugs are separated either by a sequence of second nitride spacer  54 , BPSG layer  46 , third nitride spacer  64 , or are separated by second and third nitride spacers  54 ,  64  due to a misalignment of the aforedescribed two masking steps. Third nitride spacer  64  is seen above MOS devices C, F and immediately upon first and second nitride spacers  28 ,  34  as seen in FIG.  1 . 
     The depicted polysilicon plugs are separated either by a sequence of nitride spacers and BPSG layer, or are separated by a lone nitride spacer due to a misalignment of the aforedescribed first and second masking steps. Such a lone nitride spacer formed from misalignment is seen upon and vertically rising immediately above first nitride spacer  28  of MOS device C and second nitride spacer  34  above MOS device F as seen in FIG.  1 . 
     An alternative embodiment of the invention process flow taught herein for fabrication of dynamic random access memory structures (DRAM) is seen in FIGS. 7 through 10. In the alternative embodiment, a third mask is required. The third mask can be initiated following formation of polysilicon plugs  68 , or following the optional step of dry etchbacking N-doped polysilicon layer  68 . The purpose of the third mask is to open up contacts to P-MOS devices residing in an N-doped silicon well adjacent to a junction with P-doped silicon substrate  10  seen in FIGS. 1-6. 
     In FIG. 7, a field oxide region  74  is separate from MOS devices G and H which reside upon N-doped silicon well  72  and have, respectively, a doped polysilicon gate electrode  76 , 78  sealed under a fourth nitride spacer  80 ,  82  in contact with N-doped silicon well  72 . The structure seen in FIGS. 6 and 7 has a third photoresist layer  90  applied over BPSG layer  46 , N-doped polysilicon layer  68 , and N-doped polysilicon layer  56 . Third photoresist layer  90  is aligned with a mask, exposed to UV light, and is developed so as to form a third photoresist pattern  92 . 
     FIG. 8 shows the results of processing the structure seen in FIG. 7 with a third etching step that etches through oxide layer  46  to expose P-MOS device contacts  84 ,  86 ,  88  to MOS devices G, H in the periphery at N-doped silicon well  72  and to form a third etch pattern  94  seen in FIG.  8 . Optionally, a substantially vertically oriented fourth nitride spacer  96  is applied over BPSG layer  46  situated above MOS devices G, H remaining after the third etching step. Fourth nitride spacer  96 , which is formed by deposition and anisotropic backetch of a thin nitride film (not shown), having a thickness from about 100 Å to about 300 Å, serves to avoid cross contamination of the resultant P-doped polysilicon plugs from the surrounding BPSG. It is also optional to implant P-doped active areas  98 ,  100 ,  102  in the contacts  84 ,  86 ,  88  to MOS devices G, H in the periphery at N-doped silicon well  72  so as to optimize these P-MOS devices. 
     In FIG. 9, a P-doped polysilicon layer  104  is deposited over the contacts  84 ,  86 ,  88  to the P-MOS devices G, H in the periphery at N-doped silicon well  72  which will form the P-doped polysilicon plugs thereat. As a further optional step, P-doped polysilicon layer  104  can be dry etched back. 
     Following the foregoing steps of applying a third mask, FIG. 10 shows the results of a chemical-mechanical polishing step applied to all of the then exposed BPSG and polysilicon layers  46 ,  56 ,  68 , and  104  at the same rate so that the polysilicon plugs remaining from the first, second, and third masks are isolated one from another and will not short out. 
     The order of the three masks described herein can be varied. Preferably, the first masking step described above will be performed before the other two masking steps for reasons described below. Particularly, the first mask should precede the other masks because N-doped polysilicon layer  56  is the thinnest and introduces the least stress. Preferably, N-doped polysilicon layer  56  will having a thickness from about 1500 Å to about 4000 Å. Additionally, N-doped polysilicon layer  68  and P-doped polysilicon layer  104  are each thicker than N-doped polysilicon layer  56  and should stay on the exposed portions of P-doped silicon substrate  10  and N-doped silicon well  72  for an abbreviated period of time during fabrication processing steps. Preferably, N-doped polysilicon layer  68  will have a thickness from about 3000 Å to about 8000 Å, and P-doped polysilicon layer  104  will have a thickness from about 3000 Å to about 8000 Å. 
     The thickness of the polysilicon layers is determined by the size of the plugs which have to be filled. The first polysilicon layer  56  has to fill only the plugs connecting to the cell capacitors in the array, which are typically less than 0.4 micrometers wide and always have a simple shape of a square or rectangle. The second and third polysilicon layers  68  and  104  have to fill the plug for the bitline contact which is usually as small as the plug for the cell node, but in addition can and will be used to fill the plugs connecting the source/drain regions of NMOS and PMOS devices in the periphery of the DRAM to the tungsten plugs. These plugs can have twice the size of the plugs used in the DRAM array and can have more complicated shapes which can require thicker polysilicon films to obtain completely filled plugs. 
     The described process flow has also the following characteristics which makes the printing of the plugs connecting to the cell nodes first the preferable way of implementing the invention. As seen in FIGS. 1 to  6 , and more particularly in the top plan views of FIGS. 2A,  4 A,  6 A, and  6 B, it turns out that, in the case of overlapping polysilicon plugs, the pattern of the plug printed firstly is the one that remains unchanged in shape, while the ones that follow are reduced in size and shape by the first one in case of pattern overlap. Thus, the pattern etched in the first oxide etch  52  and protected by the spacer  54 , and the polysilicon plug  56  all remain unchanged throughout the process. Conversely, second etch patterns  66  are changed into patterns  68  seen in FIGS. 6A,  6 B by etching steps. Since the mask used for the cell capacitor formation is nearly identical to the mask used for the formation of the corresponding polysilicon plug, a better alignment between the cell capacitor and the underlying polysilicon plug is enabled by using the described ordering of the masks. 
     The ordering of the second and third masks described above is best determined based upon how MOS devices A through H and contacts thereto are formed upon P-doped silicon substrate  10  and N-doped silicon well  72 . 
     In case of non-overlapping polysilicon plugs, the nitride spacers are optional for the purpose of isolating polysilicon plugs from each other. In case of overlapping polysilicon plugs, at least one nitride spacer is required surrounding the polysilicon plug to provide for the self-aligned isolation. As it can be derived from FIGS. 1 to  6 B, either spacer  54 , spacer  64 , or both spacers  54  and  64  can be used to isolate the plugs laterally. The final CMP down to the level of the original BPSG accomplishes the vertical isolation at the locations  65 . Preferably, nitride spacers  54 ,  64 , and  96  will have a thickness from about 100 Angstroms to about 300 Angstroms. 
     Another advantage or reason for nitride spacers surrounding the BPSG layer laterally in the plug opening prior to polysilicon deposition is the excellent wet etch selectivity of the nitride spacers against oxide etches, especially HF-based chemicals. Typically, a  30  second long 100:1 HF etch is performed before plug polysilicon deposition to obtain good contact between the surface of the silicon substrate  10  or  72  and the plug. “Native” or remaining oxides have to be removed. This etch would also etch the BPSG laterally and open and widen the plug contacts in an undesired way. By using nitride spacers, this contact etch is avoided. 
     Another advantage or reason for the nitride spacers surrounding the doped polysilicon plugs is the excellent diffusion barrier of nitride films. Since the polysilicon plugs are surrounded by very highly doped BPSG which contains several percent of boron and phosphorus, the intentionally boron-doped polysilicon plugs could be contaminated/counterdoped with phosphorus outdiffusing from the BPSG and intentionally phosphorus or arsenic doped polysilicon plugs with boron from the BPSG film. This cross diffusion will not occur when nitride spacers are used. In order to decrease outdiffusing of dopants from BPSG layer  46  into the plugs  56 ,  68 ,  104 , seen in FIGS. 6 and 10, a thickness of not less than 100 Angstroms is preferable for nitride spacers  54 ,  64 ,  96 . 
     The foregoing novel process flow readily facilitates printing of the first, second, and third masks because the process separates the exposures of the first mask, which is similar to a contact mask requiring a high exposure dose, from the second mask, which is similar to a gate mask and requires a low exposure dose. Furthermore, the novel process flow permits a reduction of conventional pattern density in the MOS device array during the printing of the first and second masks. Stated otherwise, the inventive process splits up the polysilicon plug mask steps into two different masks. As such, the contact-like cell node polysilicon plug features are separated from the features of the bitline polysilicon plug and peripheral plugs. This separation of plug features betters the accuracy of the photolithography process in that the first mask can concentrate on printing the contacts using a high exposure dose, whereas the second exposure in the second mask can concentrate on printing larger trench-like features using lower exposure doses. 
     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.