Abstract:
An existing stacked capacitor fabrication process is modified to construct a three-dimensional stacked capacitor, referred to as a Double Ring Stacked Cell or DRSC. The DRSC design defines a capacitor storage cell that in the present invention is used in a DRAM process. The DRSC is made up of a polysilicon storage node structure having circular polysilicon ringed upper portion centered about a lower portion that makes contact to an active area via a buried contact. The polysilicon storage node structure is overlaid by polysilicon with a dielectric sandwiched in between to form a completed DRSC capacitor. The novel 3-dimensional shaped polysilicon storage node plate having double polysilicon rings, allows substantial capacitor plate surface area of 200% or more to be gained at the storage node over that of a conventional STC.

Description:
FIELD OF THE INVENTION 
     This invention relates to semiconductor circuit memory storage devices and more particularly to a process for fabricating three-dimensional stacked cell capacitors used in high-density dynamic random access memory (DRAM) arrays. 
     BACKGROUND OF THE INVENTION 
     In dynamic semiconductor memory storage devices it is essential that storage node capacitor cell plates be large enough to retain an adequate charge or capacitance in spite of parasitic capacitances and noise that may be present during circuit operation. As is the case for most semiconductor integrated circuitry, circuit density is continuing to increase at a fairly constant rate. The issue of maintaining storage node capacitance is particularly important as the density of DRAM arrays continues to increase for future generations of memory devices. 
     The ability to densely pack storage cells while maintaining required capacitance levels is a crucial requirement of semiconductor manufacturing technologies if future generations of expanded memory array devices are to be successfully manufactured. 
     One method of maintaining, as well as increasing, storage node size in densely packed memory devices is through the use of &#34;stacked storage cell&#34; design. With this technology, two or more layers of a conductive material such as polycrystalline silicon (polysilicon or poly) are deposited over an access device on a silicon wafer, with dielectric layers sandwiched between each poly layer. A cell constructed in this manner is known as a stacked capacitor cell (STC). Such a cell utilizes the space over the access device for capacitor plates, has a low soft error rate (SER) and may be used in conjunction with inter-plate insulative layers having a high dielectric constant. 
     However, it is difficult to obtain sufficient storage capacitance with a conventional STC capacitor as the storage electrode area is confined within the limits of its own cell area. Also, maintaining good dielectric breakdown characteristics between poly layers in the STC capacitor becomes a major concern once insulator thickness is appropriately scaled. 
     A paper submitted by N. Shinmura, et al., entitled &#34;A Stacked Capacitor Cell with Ring Structure,&#34; Extended Abstracts of the 22nd International Conference on Solid State Devices and Materials, 1990, pp. 833-836, herein incorporated by reference, discusses a 3-dimensional stacked capacitor incorporating a ring structure around the main electrode to effectively double the capacitance of a conventional stacked capacitor. 
     The ring structure and its development is shown in FIGS. 1(c) through 1(g), pp. 834 of the article mentioned above. FIG. 1(a), on the same page shows a bird&#39;s eyeview of storage electrodes. The storage node is formed by two polysilicon layers that form a core electrode encircled by a ring structure. Capacitor dielectric film surrounds the whole surface of the storage node electrode and then is covered with a third polysilicon layer to form the top capacitor electrode and completes the storage cell. This design can be fabricated using current methods and increases storage capacitance by as much as 200%. However, the process flow needed to realize this ring capacitor structure requires precise alignment between two adjacent word lines and digits lines. 
     The present invention develops an existing stacked capacitor fabrication process without adding any photolithography step to construct a three-dimensional double ring stacked capacitor cell. The capacitor&#39;s bottom plate (or storage node plate) is centered over the buried contact (or node contact) connected to an access transistor&#39;s active area. The buried contact may be self-aligned depending on the process approach selected in the present invention. Additionally, the storage node does not need to be critically aligned on the node contact which allows the manufacture of a smaller cell area for a given lithographic resolution. 
     SUMMARY OF THE INVENTION 
     The invention is directed to maximizing storage cell surface area in a high density/high volume DRAM (dynamic random access memory) fabrication process. An existing stacked capacitor fabrication process is modified to construct a three-dimensional double ring stacked capacitor, referred to hereinafter as a Double Ring Stacked Cell or DRSC. The DRSC design defines a capacitor storage cell that in the present invention is used in a DRAM process, however it will be evident to one skilled in the art to incorporate these steps into other processes requiring memory cells such as VRAMs or the like. 
     After a silicon wafer is prepared using conventional process steps, the present invention develops the DRSC by depositing and patterning two interconnecting layers of polysilicon separating at specific areas to form the storage node structure having a double ringed, circular shape centered about the storage node&#39;s buried contact. The double ring polysilicon structure results in increased capacitor plate surface area for each storage cell. Such a structure is a vast improvement over the conventional STC cell by maximizing the area available for a storage node. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top planar view of a portion of an in-process wafer showing digit lines, word lines and storage capacitors; 
     FIG. 2 is a cross-sectional view through broken line A--A of FIG. 1; 
     FIG. 3 is a cross-sectional view through broken line B--B of FIG. 1; 
     FIG. 4a is a cross-sectional view of the in-process wafer portion of FIG. 3 after a conformal deposition of a thick oxide and planarization followed by a conformal deposition of thin nitride; 
     FIG. 4b is a cross-sectional view of the in-process wafer portion of FIG. 3 after conformal depositions of a thin oxide layer and a thin nitride layer, respectively; 
     FIG. 5a is a cross-sectional view of the in-process wafer portion of FIG. 4a after patterning and etch of storage node buried contacts; 
     FIG. 5b is a cross-sectional view of the in-process wafer portion of FIG. 4b after patterning and etch of self aligned storage node buried contacts; 
     FIG. 6 is a cross-sectional view of the in-process wafer portion of FIG. 5 after a deposition and patterning of a thick oxide followed by a deposition of thin nitride and a nitride space etch; 
     FIG. 7 is a cross-sectional view of an in-process wafer portion of FIG. 6 following a polysilicon dry etch and a wet oxide etch, respectively; 
     FIG. 8 is a cross-sectional view of the in-process wafer portion of FIG. 7 after a conformal deposition of polysilicon; 
     FIG. 9 is a cross-sectional view of the in-process wafer portion of FIG. 8 an anisotropic polysilicon etch, a wet nitride etch and polysilicon doping; 
     FIG. 10 is a cross-sectional view of the in-process wafer portion FIG. 9 following blanket depositions of conformal cell dielectric and polysilicon, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is directed to maximizing storage cell surface area in a high density/high volume DRAM fabrication process, in a sequence shown in FIGS. 1-10. 
     A silicon wafer is prepared using conventional process steps up to the point of defining a cell array. Fabrication of word lines, digit lines and a capacitor cell of the present invention will now follow. 
     The capacitor of each cell will make contact with a buried contact within the cell and aligned directly to its associated buried contact. Each active area within the array is isolated from one another by a thick field oxide. The active areas can be arranged in interdigitated columns and non-interdigitated rows or simply parallel and in line to one another in both the vertical and horizontal directions. The active areas are used to form active MOS transistors that can be doped as NMOS or PMOS type FETs depending on the desired use. 
     FIG. 1 shows a top planar view portion of a completed multilayered memory array with the main building blocks comprising digit lines 11, word lines 12 and a storage node plate 13 of an DRSC capacitor. 
     As shown in FIG. 2, poly 22, covered with silicide 23 and dielectric 24 (either oxide or nitride) are patterned to serve as word lines 12. Silicide 23 may be selected from such silicides as tungsten silicide (WSi 2 ), titanium silicide (TiSi 2 ) or the like. Poly 22 is conductively doped in order to combine with silicide 23 to form the conductor for word lines 12. Word lines 12 are further isolated from one another as well as subsequent conductive layers by dielectric spacers 26 (also either oxide or nitride). Word lines 12 have been previously deposited over a thin layer of gate oxide 25 or a thick layer of field oxide 27. Active areas 21 have been appropriately doped to a desired conductivity type which penetrates into the bulk silicon wafer 20, by conventional process steps. Now the wafer is ready for digit line formation that will run perpendicular to and over word lines 12. 
     FIG. 3 shows the formation of digit lines 11. A conformal layer of dielectric 31 is deposited over the existing wafer surface to isolate previously formed active areas 21 from the subsequent formation of digit lines 11. First, buried digit line contacts are patterned and etched through dielectric 31 allowing access to active areas 21 at the desired locations (the buried digit line contacts cannot be seen in the cross-section of FIG. 3). Second, digit lines 11 are made up of patterned layers consisting of polysilicon 32, silicide 33 and dielectric 34 following the location of the buried digit line contacts. Dielectric 34 can be either nitride or oxide and is deposited by chemical vapor deposition (CVD) which is preferred for its excellent conformity. Polysilicon 32 has previously been conductively doped to electrically couple with silicide 33 to serve as the conductor for digit lines 11. Silicide 33 may be selected from such silicides as tungsten silicide (WSi 2 ), titanium silicide (TiSi 2 ) or the like. Digit lines 11 run perpendicular to and over the top of word lines 12 (shown in FIG. 2) and conform to the wafer surface resulting in a waveform-like topology running in both the digit line and word line directions. A second dielectric, such as nitride or oxide is now deposited, followed by an anisotropic etch to form vertical dielectric spacers 35. 
     As shown in FIG. 4a, digit lines 11 and their subsequent isolation layers are then covered with a thick layer of oxide 41, preferably by CVD. Following dielectric 41 deposition, a conformal layer of thin nitride 42 is then deposited. 
     Alternately, as shown in FIG. 4b, digit lines 11 and their subsequent isolation layers are then covered with a thin layer of oxide 41 at a thickness of 200 to 500 angstroms, preferably by CVD. Following dielectric 41 deposition, a conformal layer of thin nitride 42 is then deposited. The thin oxide is deposited if the process is to incorporate self aligned storage node buried contacts. Otherwise, using thick oxide will require aligning the storage node buried contacts by applying a photoresist and the appropriate mask (as shown in FIG. 5a) to allow a subsequent anisotropic etch to open storage node buried contacts 51. 
     In conjunction with FIG. 4b, FIG. 5b shows the formation of self aligned buried contacts 51. Buried contacts 51 are aligned to digit lines 11 by covering all of the wafer surface area with photoresist 52. After applying an appropriate photomask, a buried contact anisotropic etch provides the openings to locate buried contacts 51. 
     Up to this point, process flow has followed that of an array comprising conventional stacked capacitor cells. From this point forward, the process is unique to an array having DRSC-type storage capacitors. The invention will be described from hereon from FIGS. 6-10 in conjunction with FIGS. 4a and 5a as it is the preferred method. However, if one skilled in the art chooses to use the invention with self aligned storage node buried contacts (developed in FIGS. 4b and 5b) the resultant storage node capacitor will be the same as the one shown in FIG. 10, except that it will possess self aligned storage node buried contacts. 
     As shown in FIG. 6, photoresist 52 (of FIG. 5a) has been stripped and a conformal layer of thick polysilicon 61 is deposited thereby connecting to active area 21 via buried contact 51. Thick poly 61 is then etched back to a planarized surface. Following poly 61 planarization, a conformal layer of thick oxide 62 is deposited, preferably by CVD. The height of oxide 62 deposited is controlled as it plays an important role in determining the overall surface area of the DRSC to be completed later in the process. Oxide 62 is patterned into circular shapes thereby leaving oxide 62 aligned, preferably directly over buried contacts 51. Now a thin layer of conformal nitride is deposited followed by an anisotropic etch to form nitride spacers 63. 
     As shown in FIG. 7, a polysilicon dry etch is performed to separate poly 61 into individual poly structures that will eventually become individual storage node plates developed later in the process. Following poly 61 etch, oxide 62 (of FIG. 6) is etched away leaving circular shaped nitride spacers 63 residing vertically on the outer edges of patterned poly 61. 
     As shown in FIG. 8, polysilicon layer 81 is deposited thereby connecting to all exposed areas of polysilicon 61 and enclosing nitride spacers 63. 
     As shown in FIG. 9, a poly anisotropic etch is performed to separate poly 81 to form double ring, poly spacers 91. A wet nitride etch is the performed to remove nitride spacers 63 and nitride 42 (seen in FIG. 8). Poly spacers 91 and patterned poly 61 are conductively doped to the desired conductivity type (p-type or n-type) to become double ring, poly storage node plates 92. As can be seen in this view the height that polysilicon spacers 91 extend upward from polysilicon 61 is determined by the thickness of previously deposited and patterned oxide 62. 
     As shown in FIG. 10, a dielectric layer of nitride 101 is deposited that conforms to poly storage node plate 92. Nitride 101 serves as a capacitor dielectric for the DRSC. Although nitride 101 is selected as the capacitor dielectric in the preferred embodiment any material having a high dielectric constant such as Ta 2  O 5  or SrTiO 3  could be used. Also, the nitride or Ta 2  O 5  layers can be oxidized. Following nitride 101 deposition, a blanket deposition of conformal poly 102 is deposited. Poly 102 is conductively doped either n-type or p-type depending on the conductivity type desired for active area 21. Poly 102 now serves as a top poly capacitor cell plate of the DRSC storage capacitor and also becomes a common cell plate to all DRSC storage capacitors in the array. All conformal polysilicon layers used to create the DRSC capacitor are preferably deposited by CVD although other conformal deposition methods could be used as well. 
     With the unique shape of poly storage node plate 92 along with top poly capacitor cell plate 102 that envelops plate 92, substantial capacitor plate surface area is gained at the storage node. Since capacitance is mainly effected by the surface area of a capacitor&#39;s cell plates the additional area gained by a 3-dimensional double ring cell structure can provide an additional 200% or more increase in capacitance over that of a conventional STC capacitor, without more space than that required for defining a stacked capacitor storage cell. Higher density for a given capacitance is possible as a smaller cell area is realized. 
     It is to be understood that although the present invention has been described with reference to a preferred embodiment, various modifications, known to those skilled in the art, may be made to the structures and process steps presented herein without departing from the invention as recited in the several claims appended hereto.