Abstract:
A structure and method for fabricating an integrated circuit crown structure for use in a DRAM cell on a substrate comprising a common source/drain region ( 18 ) disposed within a substrate ( 12 ), the common source/drain region ( 18 ) connected to a bitline ( 22 ), a gate oxide ( 28 ) disposed over the common source/drain region ( 18 ) and forming at least two wordline gates ( 30 ), at least two storage node source/drains ( 20 ) adjacent to said gates ( 30 ) and contacted by storage node contacts ( 38 ) and a storage node bowl ( 36 ), the bowl being formed within adjacent supporting layers formed over said wordline gates wherein the storage node bowl ( 36 ) is formed, and electrically isolated from, the bitline ( 22 ) without being exposed to etching agents during its formation and without forming a wine glass stem structure and a crown extending from the top of the storage node bowl ( 36 ), is disclosed.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to the field of integrated circuits, and more particularly, to the fabrication of semiconductor integrated circuit components such as a dynamic random access memory cell, and especially the cell&#39;s capacitor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Without limiting the scope of the invention, its background is described in connection with dynamic access random memory (DRAM) cells, as an example.  
           [0003]    As is well known in the art of integrated circuit design, layout and fabrication, the manufacturing cost of a given integrated circuit is largely dependent upon the chip area required to implement desired functions. The chip area is defined by the geometries and sizes of the active components disposed in the wafer substrate. Active components include gate electrodes in metal-oxide semiconductors (MOS) and diffused regions such as MOS source and drain regions and bipolar emitters, collectors and base regions. These geometries and sizes are often dependent upon the photolithographic resolution available for the particular equipment used for processing the integrated circuit.  
           [0004]    A significant problem of current techniques for the formation of integrated circuit structures as applied to very-large-scale integration (VLSI) as more and more layers are added, is that additional steps add additional complexities to the creation of circuits on the wafer surface. The resolution of small image sizes in photo-lithography becomes more difficult due to light reflection and the thinning of the photoresist during processing. In addition, the smaller patterns lead to increasing difficulties in the electrical isolation of the circuits. As the circuits shrink, the capacitor can become larger than the underlying circuitry, and thus the determining factor in the cell size, thereby requiring the capacitors to be stacked.  
           [0005]    As a two dimensional process used to achieve a three dimensional structure, the goal of photolithographic patterning is to establish the horizontal and vertical dimensions of the various devices and circuits used to create a pattern that meets design requirements, such as, the correct alignment of circuit patterns on the wafer surface. As line widths shrink, photolithography of patterns down to the nanometer level and smaller approach the limits of resolution of present equipment. These width lines, in the nanometer range, become increasingly more difficult to pattern because of the need to isolate the integrated circuit components.  
           [0006]    A DRAM cell generally consists of a transistor and a capacitor. A bitline is connected to one of the transistor source/drains and a wordline to its gate, with the other source/drain being connected to the capacitor. As the density of DRAM cells on a silicon chip increases, DRAM cells having three dimensional structures, such as stacked capacitors, have been developed to meet the increased need for miniaturization. The use of stacked three dimensional structures, for example, allows the DRAM designer to maximize the capacitance of storage nodes within the limited area of the DRAM cell.  
         SUMMARY OF THE INVENTION  
         [0007]    What is needed is a structure and method for using current integrated circuit processing techniques and manufacturing equipment that meet the demands of VLSI integrated circuits. One particular area in need of improvement is the fabrication of capacitors, and more particularly stack or crown capacitors, e.g., stack capacitors used in DRAM cells. As the circuits shrink, the capacitor can become larger than the underlying circuitry, and thus the determining factor in the cell size, thus the need for stack capacitors. The capacitor and cell designs must conform to current equipment and manufacturing techniques, and at the same time, provide the required increase in chip capacity and reliability.  
           [0008]    Unlike flatplate capacitors of the prior art, crown capacitors are three-dimensional and it is recognized herein that in the past, during some stage of fabrication, some of the partially constructed capacitors could be subjected to underetching due, e.g., to inherent variations in etching across the wafer which can remove part of the support of those capacitors and thus subject those partially constructed capacitors to damage during subsequent processing. Our recognition of this problem has led us to the process modification described herein, which significantly improves the support of the partially fabricated capacitor and significantly improves circuit yields.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0010]    [0010]FIG. 1 is a simplified cross-sectional view of a DRAM cell;  
         [0011]    [0011]FIGS. 2 through 15 show the layers and structures formed to create a crown capacitor.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0013]    The general features of a fully assembled pair of DRAM cells generally designated as  10  shown as a simplified cross-sectional view in FIG. 1. DRAM cells  10  have a substrate  12  that is formed having a conductivity type which is one of either a P-type conductivity or a N-type conductivity, if the substrate  12  is silicon based. Substrate  12  may be made of silicon, gallium arsenide, silicon on insulator (SOI) structures, epitaxial formations, germanium, germanium silicon, polysilicon, amorphous silicon, and/or like substrate, semi-conductive or conductive. The substrate  12  is typically made of single crystal silicon, and is usually lightly doped with boron, phosphorous or arsenic atoms.  
         [0014]    A moat  14  is shown disposed within a field oxide region  16  which has been grown from substrate  12 . The moat region  14 , generally formed by diffusion, has disposed therein storage source/drain regions  20 . The common source/drain  18  (common to both cells), located within moat  14 , is connected to bitline contact  38  that is etched through an insulating layer  26 .  
         [0015]    Disposed adjacent to the storage source/drain regions  20 , and the common source/drain  18 , are a gate oxides  28  and wordlines  30 . Portions of the wordlines  30  also function as gates, which define the field effect transistors (FET) of the DRAM cells  10 . The storage nodes  36  (which form one of the capacitor plates) of the DRAM cells  10  are electrically connected to the storage source/drains regions  20  by storage node contact  32 . A storage node  36 , a dielectric layer  34  disposed over the storage node  36  and grounded upper plate  37  form the capacitor. The various components of the DRAM cell  10  are electrically isolated by insulating layers  26 .  
         [0016]    The storage nodes  36  and the storage node contacts  32  together have a generally “wine glass” shape, with an upper “bowl” and a “stem” below (the storage node contact  32  generally makes up the steam). As will be seen below, a nitride layer used herein will provide a seal around the perimeter of the bowl and will generally prevent possible underetching which would weaken the bowl during fabrication.  
         [0017]    [0017]FIG. 2 depicts the first steps in the formation of the crown capacitor formation of the present invention. Three gate/wordline stacks  40  are shown composted of nitride caps  42  surrounding a silicide layers  44  and a polysilicon wordline/gates  30 , disposed over gate oxides  28 . The left “gate/wordline” stack  42  is not a gate, but leads to gates in adjacent cells. Note that FIGS. 2 through 15 show cells at the edge of a matrix and that “non-edge” cells would have an additional gate/wordline stack to the right on the three stacks  42  shown. As in FIG. 1, a moat  14  is disposed within a field oxide region  16  which has been grown from substrate  12 . The moat region  14  has disposed therein storage source/drain regions  20 . The common source/drain  18  (common to both cells) is located within moat  14 .  
         [0018]    Disposed over the gate stacks  40  is a glass layer  46 , which can be, e.g., a boro-, phospho- or borophospho-silicate glass that has been deposited over the gate stacks  40 . A photoresist has been deposited and patterned with openings  50  over the positions where the bitline and storage node contacts are to be etched.  
         [0019]    In FIG. 3, the sacrificial glass layer  46  is etched through to the storage source/drain regions  20  and the common source/drain  18  in a single etching step. This has the advantage of the use of a single patterning and etching step for the formation of the bitline and storage node contacts. A single patterning and a single etching step reduce the problems associated with misalignment of the mask patterns on the surface of the wafer  10  during the stepping operation. A single mask also allows for finer resolution between patterned components, for example, distance tolerances between the storage node contacts and the bitline. Also, the silicon nitride cap  42  helps direct etching into the conductive source/drain regions  20  and the common source/drain  18  while at the same time maintaining the isolation of the gates, thus making alignment less critical and improving yield.  
         [0020]    In FIG. 4, polysilicon  52  is deposited over the entire wafer  10  surface and fills the openings  50  created during the patterning and etching of the storage node and bitline contacts. In one embodiment, polysilicon  52  is made of two layers, a first doped polysilicon layer of, e.g., 500 angstroms is deposited before a subsequent deposition of undoped polysilicon. The undoped polysilicon layer  56  can have a thickness of, e.g., 5000 angstroms. One advantage of depositing two polysilicon layers, one being doped and one undoped, is that the doped polysilicon layer will help in the formation of the contact with the conductive source/drain regions  20  and the common source/drain  18 . Another advantage of depositing an undoped polysilicon layer is the more rapid deposition of an undoped polysilicon, thereby reducing processing time and cost.  
         [0021]    In FIG. 5 the top of the polysilicon layer  52  is etched back to a thickness of, e.g., 300 angstroms. Next, a silicide layer  58  is formed over the polysilicon layer  52 . The suicides can be, for example, titanium, tungsten, cobalt or nickel and may be used to dope the poly  52  with either a p or an n dopant. In one embodiment, the silicide is a tungsten silicide that can have, e.g., a thickness of about 1200 angstroms. Following the deposition of the silicide layer  58 , a first cap oxide layer  60 , which can be deposited using, e.g., chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD) of an oxysilane, such as, tetraethyl oxysilane. The thickness of the first cap oxide layer  60  can be, e.g., 2000 angstroms.  
         [0022]    [0022]FIG. 7 shows the structure that is formed following the deposition and patterning of first photoresist  48  pattern and etchback that will lead to the isolation of the bitline  22  and the bitline contact  38  from the storage node contacts  32 . The etchback can be accomplished in two steps, first an oxide etchback followed by a silicide-polysilicon etchback. The etchback can be, e.g., a 1500 angstrom etchback that exposes the top surface of the glass layer  46 . The etchback can also etch into the polysilicon layer  52  that was deposited to form the storage node contacts  32 .  
         [0023]    Following the etchback step depicted in FIG. 7, after removal of the first photoresist  48  a second cap oxide layer  62  is conformally deposited over the entire wafer  10  surface. The second cap oxide layer  62  completes the isolation of the bitline  22  from the storage node contacts  32 . The second cap oxide layer  62  can also be deposited using CVD or PECVD, and can have, e.g., a thickness of 500 angstroms. Next, a stopper silicon nitride layer  64  of, e.g., 500 angstroms, is deposited over second cap oxide  62 . As will become apparent in later figures, the stopper nitride layer  64  will both be used during a subsequent two-step etching and will seal to the perimeter of the bowl (of the wineglass shaped storage node) and generally prevent underetching of the lower portion of the bowl during removal of the dummy oxide. Following the depositions of the second cap oxide layer  62  and the nitride layer  64 , a dummy oxide  66  is deposited over the entire wafer  10 . The deposition of the dummy oxide  66  is depicted in FIG. 9. The dummy oxide  66  can have a thickness of, e.g., 5000 angstroms.  
         [0024]    [0024]FIG. 10 shows the pattern for a second photoresist  67  that will lead to the formation of the bowl on crown structures for the storage nodes or capacitors. Vias  68  are etched through to the stopper nitride layer  64 .  
         [0025]    [0025]FIG. 12 shows some important features of the present invention as relate to the build-up of layers in the previous figures and the etching step depicted in this figure. Both the stopper nitride layer  64  and the second cap oxide  62  are etched through via  68  to form the lower portion of the structure that will be part of the capacitor. This is a timed etch, which will etch the second cap oxide  62 , but will not etch through the combined thickness of second cap oxide  62  and first cap oxide  60  and thus will not open to (and cause a short to) bitline silicide  58 . The bowl lower portion, as will be formed in subsequent steps, will make contact with the storage node contact  32 , but importantly, will be formed below the surface of the stopper nitride layer  64  and the second cap oxide  62 . By forming the lower portion of the bowl structure below the stopper nitride layer  64  and the second cap oxide  62  the present invention generally eliminates the formation of vulnerable floating wine glass bowls. The structure disclosed herein prevents the formation of vulnerable floating wine glass type structures which float during a portion of the fabrication process by protecting the lower portion of the bowl from subsequent wet etching steps that, in the prior art, lead to floating bowl defects.  
         [0026]    The structure disclosed herein also eliminates an entire series of build-up and etching steps that were necessary to extend the length of the storage node contacts in order to isolate them from the bitline  22 . Isolation is accomplished by depositing a first cap oxide layer  60  and a second cap oxide layer  62 . Because the first cap oxide layer  60  is built-up over the bitline  22  but removed from over the storage node contact  32  during the etching step depicted in FIG. 12, the combined first and second cap oxide layers  60 ,  62  increases the oxide depth over the silicon  58  (which allows etching to expose the storage node contact  32  without exposing the bitline silicide  58 ). The extra distance provided by first cap oxide layer  60 , thus maintains the electrical isolation of silicide  58  (which is the top of the bitline  22 ) while allowing exposure of the top surface of the storage node contact  32 . This allows fabrication of a crown base which, unlike the prior art, is already isolated and thus does not require underetching during later steps of fabrication.  
         [0027]    In FIG. 13 a crown polysilicon  70  is conformally deposited over the wafer  10  and partially fills the opening  68  formed in the figures described hereinabove. To reduce the reflectivity of the crown polysilicon  70 , a non-reflective layer  72  is deposited over the crown polysilicon  70 . The crown polysilicon  70  can have a thickness of, e.g., 500 angstroms. The non-reflective layer  72 , also known as a bottom anti-reflective coating (BARC), can be any inorganic material that reduced the reflection of the stepper UV light source during subsequent patterning steps. Into the openings  68 , and over the non-reflective layer  72  and the crown polysilicon  70  is deposited and patterned a third photoresist  73 . The third photoresist  73  can be, for example, a positive tone photoresist. After a blanket exposure, the developer removes the photoresist from the entire wafer surface except for the resist at the bottom of the crown polysilicon  70 .  
         [0028]    Alternatively, the process can be modified by the application of thermally cured spin on glass, such as HSQ available from DuPont, after the crown polysilicon  70  layer is deposited. A blanket oxide may be applied resulting in the thermally cured spin on glass being disposed in the stem of the crown polysilicon  70 . The thermally cured spin on glass is then used to protect the stem of the crown polysilicon  70  during subsequent etches and is removed using either a wet or a dry etch after the completed crown  39  is formed.  
         [0029]    [0029]FIG. 14 depicts the structure following an etchback step in which the non-reflective layer  72  is removed along with the top surface of the crown polysilicon  70  as depicted in FIG. 14. Next, the photoresist  73  is ashed and cleaned, and, following the removal of the photoresist, the dummy oxide  66  is wet stripped using, e.g., a piranha etch. Piranha etching and photoresist ashing and clean-up are well known to those of skill in the art.  
         [0030]    The removal of the photoresist  73  and the dummy oxide  66 , leaves the top of the bowl extending above the stopper nitride layer  64 , as depicted in FIG. 15. The entire bottom of the polysilicon storage node bowl  70  is below the stopper nitride layer  64 . Next, a dielectric nitride layer  74  is blanket deposited over the wafer  10  by, for example, a low pressure CVD (LPCVD). The dielectric nitride layer  74  is then preferably converted into an oxynitride film by oxidizing the dielectric nitride layer  74 . The completion of formation of the crown capacitor follows conventional steps known to those in the art, such as isolation of the crown capacitor plate and formation of the grounded upper plate  37  depicted in FIG. 1. Thus, the bowl of capacitor storage node generally has a lower portion protected from the dummy oxide etch by its outer perimeter (the inside of the bowl being polysilicon is not significantly attacked by the dummy oxide etch).  
         [0031]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.