Abstract:
A semiconductor memory cell structure and method for forming the same. The memory cell is formed on a surface of a substrate and includes an active region formed in the substrate, an epitaxial post formed on the surface of the substrate over the active region. The epitaxial post has at least one surface extending outwardly from the surface of the substrate and another surface opposite of the surface of the substrate. A gate structure is formed adjacent to at least a portion of all the outwardly extending surfaces of the epitaxial post, and a capacitor formed on an exposed surface of the epitaxial post.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a divisional of U.S. patent application Ser. No. 10/855,705, filed May 26, 2004 now U.S. Pat. No. 6,921,935, which is a continuation of U.S. patent application Ser. No. 10/643,269, filed Aug. 18, 2003, now U.S. Pat. No. 6,797,573, which is divisional of U.S. patent application Ser. No. 10/177,228, filed Jun. 21, 2002, now U.S. Pat. No. 6,756,625. 

   TECHNICAL FIELD 
   The present invention relates in general to memory circuits, and more particularly, to dynamic random access memory cells and a method for forming the same. 
   BACKGROUND OF THE INVENTION 
   Random access memory (“RAM”) cell densities have increased dramatically with each generation of new designs and have served as one of the principal technology drivers for ultra large scale integration (“ULSI”) in integrated circuit (“IC”) manufacturing. However, in order to accommodate continuing consumer demand for integrated circuits that perform the same or additional functions and yet have a reduced size as compared with available circuits, circuit designers continually search for ways to reduce the size of the memory arrays within these circuits without sacrificing array performance. 
   With respect to memory ICs, the area required for each memory cell in a memory array partially determines the capacity of a memory IC. This area is a function of the number of elements in each memory cell and the size of each of the elements. For example,  FIG. 1  illustrates an array  100  of memory cells  110  for a conventional dynamic random access memory (DRAM) device. Memory cells  110  such as these are typically formed in adjacent pairs, where each pair is formed in a common active region  120  and share a common source/drain region that is connected to a respective digit line via a digit line contact  124 . The area of the memory cells  110  are said to be 8F 2 , where F represents a minimum feature size for photolithographically-defined features. For conventional 8F 2  memory cells, the dimension of the cell area is 2F×4F. The dimensions of a conventional 8F 2  memory cell are measured along a first axis from the center of a shared digit line contact  124  (½F), across a word line  128  that represents an access transistor (1F), a storage capacitor  132  (1F), an adjacent word line  136  (1F), and half of an isolation region  140  (½F) separating the active region  120  of an adjacent pair of memory cells (i.e., resulting in a total of 4F). The dimensions along a second perpendicular axis are half of an isolation region  150  on one side of the active region  120  (½F), the digit line contact  124  (1F), and half of another isolation region  154  on the other side of the active region  120  (½F) (i.e., resulting in a total of 2F). 
   In some state-of-the-art memory devices, the memory cells for megabit DRAM have cell areas approaching 6F 2 . Although this is approximately a 25% improvement in memory cell area relative to conventional 8F 2  memory cells, as previously described, a further reduction in memory cell size is still desirable. Therefore, there is a need for a compact memory cell structure and method for forming the same. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a semiconductor memory cell structure. The memory cell is formed on a surface of a substrate and includes an active region formed in the substrate, an epitaxial post formed on the surface of the substrate over the active region. The epitaxial post has at least one surface extending outwardly from the surface of the substrate and another surface opposite of the surface of the substrate. A vertical transistor is formed in the epitaxial post having a gate structure that is formed adjacent to at least a portion of all the outwardly extending surfaces of the epitaxial post. The memory cell further includes a memory cell capacitor formed on an exposed surface of the epitaxial post. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified top plan view of conventional memory cells. 
       FIG. 2A  is a simplified top plan view of memory cells according to an embodiment of the present invention, and  FIG. 2B  is a simplified cross-sectional view of a pair of memory cells according to the embodiment shown in  FIG. 2A . 
       FIG. 3  is a simplified cross-sectional view of a semiconductor substrate that can be processed to form the memory cell of  FIG. 2 , in accordance with an embodiment of the present invention. 
       FIG. 4  is a simplified cross-sectional view of the substrate of  FIG. 3  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 5  is a simplified cross-sectional view of the substrate of  FIG. 4  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 6  is a simplified cross-sectional view of the substrate of  FIG. 5  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 7  is a simplified cross-sectional view of the substrate of  FIG. 6  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 8  is a simplified cross-sectional view of the substrate of  FIG. 7  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 9  is a simplified cross-sectional view of the structure of  FIG. 2B  at a later point in processing, in accordance with an embodiment of the present invention. 
       FIG. 10  is a simplified cross-sectional view of a pair of memory cell according to an alternative embodiment. 
       FIG. 11  is a functional block diagram of a memory circuit that includes memory cells according to an embodiment of the present invention. 
       FIG. 12  is a functional block diagram of a computer system including a memory device according to the embodiment shown in  FIG. 11 . 
   

   As is conventional in the field of integrated circuit representation, the lateral sizes and thicknesses of the various layers are not drawn to scale, and portions of the various layers may have been arbitrarily enlarged or reduced to improve drawing legibility. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2A  is a top plan view of an array of memory cells  200  according to an embodiment of the present invention. As shown in  FIG. 2A , capacitors have not been illustrated in order to avoid unnecessarily obscuring the other structures of the memory cell  200 . The dimensions of the cell  200  are 4F 2 . That is, the cell  200  measures 2F along a first axis, starting with half of a digit line contact (½F), and extending over an epitaxial post on which a capacitor is formed (1F) and half of an isolation region (½F). Along a second perpendicular axis, the cell  200  measures 2F, starting with half of an isolation region (½F), and extending over the digit line contact (1F), and half of another isolation region (½F).  FIG. 2B  is a simplified cross-sectional view of the memory cell  200  ( FIG. 2A ) along A—A at a stage of processing. A more detailed description of the memory cell  200  will be provided with respect to  FIGS. 3 through 10 , which illustrate the memory cell  200  at various stages of processing. 
     FIG. 3  is a simplified cross-sectional view of the memory cell  200  ( FIG. 2 ) at a stage of processing. Formed in a p-type substrate  204  is an n-type active region  206  in which a pair of memory cells  200  are formed. The active region  206  is isolated from adjacent active regions by isolation regions  202 . The active region  206  and the isolation regions  202  can be formed using conventional methods, for example, conventional masking, deposition, implant and drive-in processes. Following the formation of the isolation regions  202  and the active region  206 , a layer of insulating material is deposited onto the substrate  204 , masked and etched to form sacrificial structures  208   a–c  on the substrate  204 . The insulating material from which the sacrificial structures  208   a–c  are formed is silicon nitride, or alternatively, as will be explained in more detail below, other insulating material to which subsequent etch processes are selective. 
     FIG. 4  is a simplified cross-sectional view of the structure shown in  FIG. 3  at a later point in processing, in accordance with an embodiment of the present invention. An insulating material is deposited over the substrate  204  and the sacrificial structures  208   a–c  and subsequently etched back using an anisotropic etch process. Suitable etch processes are known in the art. Sidewalls  210   a–c ,  212   a–c  are formed as a result of the deposition and etch back processes. The insulating layer can be formed from a silicon-oxide material, and the etch back process should be selective to the silicon nitride of the sacrificial structures  208   a–c . A p-type epitaxial layer is formed on the exposed regions of the substrate  204 , and etched to selectively form epitaxial “posts”  220 ,  222  within the trench region between the sacrificial nitride structures  208   a ,  208   b , and  208   b ,  208   c , respectively. As will be described in more detail below, the epitaxial posts  220 ,  222  represent the material in which vertical access transistors (i.e., word lines) will be formed and to which memory cell capacitors are electrically coupled. 
     FIG. 5  is a simplified cross-sectional view of the structure shown in  FIG. 4  at a later point in processing, in accordance with an embodiment of the present invention. An etch process selective to the nitride sacrificial structures  208   a–c  and the epitaxial posts  220 ,  222  is performed to remove the oxide sidewalls  210   a–c ,  212   a–c . Gate oxide  230  is then formed over the epitaxial posts  220 ,  222  and the exposed regions of the substrate  204 . The material of the sacrificial structures  208   a–c  is such that oxide does not form thereon during the formation of the gate oxide  230 . 
     FIG. 6  is a simplified cross-sectional view of the structure shown in  FIG. 5  at a later point in processing, in accordance with an embodiment of the present invention. A polysilicon layer is formed over the structure of  FIG. 5  followed by a masking and etch process to selectively remove portions of the polysilicon layer. An anisotropic etch back process is then performed to remove additional portions of polysilicon layer in order to form gates  240 ,  242  of vertical transistors  250 ,  252 , respectively. The etch back process recesses the gates  240 ,  242  to below the height of the epitaxial posts  220 ,  222 , respectively. Although shown in cross-section in  FIG. 6 , the gates  240 ,  242  surround the respective posts  220 ,  222 . This is apparent from  FIG. 2A , which illustrates that the gate  242  is part of a continuous polysilicon wordline that is formed around each of the epitaxial posts associated with the memory cells of that row. 
     FIG. 7  is a simplified cross-sectional view of the structure shown in  FIG. 6  at a later point in processing, in accordance with an embodiment of the present invention. An insulating layer is formed over the structure shown in  FIG. 6  and subsequently etched back to form a relatively planar surface. Although a conventional chemical-mechanical polishing process can be used for the etch back step, it will be appreciated that other suitable etch back processes may be used as well. The etch back process results in the formation of insulating spacers  256  to isolate the gates  240 ,  242  of the vertical transistors  250 ,  252 . The insulating layer  258 , and consequently, the insulating spacers  256 , can be formed from a silicon oxide material, or other material, that is selective to a silicon nitride etch process. 
     FIG. 8  is a simplified cross-sectional view of the structure shown in  FIG. 7  at a later point in processing, in accordance with an embodiment of the present invention. An etch process is used to remove the silicon nitride sacrificial structures  208   a–c  to leave the epitaxial posts  220 ,  222 , the vertical transistors  250 ,  252 , and the insulating spacers  256 . An insulating material is then deposited over the remaining structure and anisotropically etched back to form sidewalls  260  that isolate the gates  240 ,  242  of the vertical transistors  250 ,  252 , respectively. As shown in  FIG. 2B , a dielectric interlayer  264  is subsequently deposited over the existing structure and etched back to form a planar surface on which digit lines and storage capacitors can be formed. Still with reference to  FIG. 2B , a via  270  is formed through the dielectric interlayer  246  to expose a portion the active region  206 . A conductive material  272  is subsequently deposited over the structure and in the via  270  to electrically contact the active region  206 . The conductive material  272  is masked and etched to form a digit line contact. 
     FIG. 9 . is a simplified cross-sectional view of the structure shown in  FIG. 2B  at a later point in processing, in accordance with an embodiment of the present invention. A second dielectric interlayer  274  is deposited over the structure, and using conventional methods, container shaped memory cell capacitors  280  are formed in the second dielectric interlayer  274  and have a first capacitor plate  282  electrically coupled to a respective epitaxial post  220 ,  222 . The first capacitor plate  282  can be formed from a highly doped polysilicon material, however, it will be appreciated that other suitable materials may be used as well. Following the formation of the first capacitor plates  282  of the memory cell capacitors  280 , dopants from the highly doped polysilicon layer are diffused into the respective epitaxial post  220 ,  222  by heating the substrate  204 . As a result, lightly doped conductive regions  284  are created in the epitaxial posts  220 ,  222  in a region adjacent the insulating spacers  256 . The lightly doped conductive regions  284  provide a conductive path between a memory cell capacitor  280  and the respective gate  240 ,  242  of the vertical transistors  250 ,  252 . Thus, when a vertical transistor is activated, the memory cell capacitor  280  can be electrically coupled to the active region  206 . 
   Although embodiments of the present invention have been described as including container shaped memory cell capacitors  280 , it will be appreciated that alternative capacitor structures can also be used as well without departing from the scope of the present invention. For example, conventional stacked capacitor structures electrically coupled to the epitaxial posts  220 ,  222  could be used in an alternative embodiment of the present invention. Alternatively, capacitors having a first capacitor plate with multiple polysilicon layers, that is, a “finned” capacitor, could also be used. Moreover, other modifications can be made to the memory cell capacitors  280  as well and still remain within the scope of the present invention. An example of such a modification includes forming memory cell capacitors  280  having a rough surface such as a hemispherical silicon grain (HSG) layer (not shown). Consequently, the present invention is not limited to the specific embodiments described herein. 
     FIG. 10  illustrates a pair of memory cells  1000  according to an alternative embodiment of the present invention. Whereas memory cells  200  ( FIG. 9 ) includes a digit line contact formed from a conductive material  272 , the memory cell  1000  includes a buried digit line  1006 . Formation of the buried digit line  1006  is well known in the art and can be formed using conventional processing methods. 
   It will be appreciated that the description provided herein is sufficient to enable those of ordinary skill in the art to practice the invention. Selecting specific process parameters, including temperature, doping levels, thicknesses, and the like, are well within the understanding of those ordinarily skilled in the art. Particular details such as these have been omitted from herein in order to avoid unnecessarily obscuring the present invention. It will be further appreciated that additional processing steps can be performed in fabricating the memory cells  200  without departing from the scope of the present invention. For example, in forming the isolation regions  202 , an implant process can be performed to create a junction region below the isolation region  202  to minimize leakage currents between adjacent active regions. Another example of such a modification is performing an implant step prior to deposition of the conductive material  272  to create a highly doped region in the active region  206  to promote conductivity to the digit line contact. 
     FIG. 11  is a functional block diagram of one embodiment of a memory circuit  60 , Which includes memory banks  62   a  and  62   b . These memory banks each incorporate a memory array according to an embodiment of the present invention. In one embodiment, the memory circuit  60  is a synchronous DRAM (SDRAM), although it may be another type of memory in other embodiments. 
   The memory circuit  60  includes an address register  64 , which receives an address from an ADDRESS bus. A control logic circuit  66  receives a clock (CLK) signal receives clock enable (CKE), chip select (CS), row address strobe (RAS), column address strobe (CAS), and write enable (WE) signals from the COMMAND bus, and communicates with the other circuits of the memory device  60 . A row-address multiplexer  68  receives the address signal from the address register  64  and provides the row address to the row-address latch-and-decode circuits  70   a  and  70   b  for the memory bank  62   a  or the memory bank  62   b , respectively. During read and write cycles, the row-address latch-and-decode circuits  70   a  and  70   b  activate the word lines of the addressed rows of memory cells in the memory banks  62   a  and  62   b , respectively. Read/write circuits  72   a  and  72   b  read data from the addressed memory cells in the memory banks  62   a  and  62   b , respectively, during a read cycle, and write data to the addressed memory cells during a write cycle. A column-address latch-and-decode circuit  74  receives the address from the address register  64  and provides the column address of the selected memory cells to the read/write circuits  72   a  and  72   b . For clarity, the address register  64 , the row-address multiplexer  68 , the row-address latch-and-decode circuits  70   a  and  70   b , and the column-address latch-and-decode circuit  74  can be collectively referred to as an address decoder. 
   A data input/output (I/O) circuit  76  includes a plurality of input buffers  78 . During a write cycle, the buffers  78  receive and store data from the DATA bus, and the read/write circuits  72   a  and  72   b  provide the stored data to the memory banks  62   a  and  62   b , respectively. The data I/O circuit  76  also includes a plurality of output drivers  80 . During a read cycle, the read/write circuits  72   a  and  72   b  provide data from the memory banks  62   a  and  62   b , respectively, to the drivers  80 , which in turn provide this data to the DATA bus. 
   A refresh counter  82  stores the address of the row of memory cells to be refreshed either during a conventional auto-refresh mode or self-refresh mode. After the row is refreshed, a refresh controller  84  updates the address in the refresh counter  82 , typically by either incrementing or decrementing, the contents of the refresh counter  82  by one. Although shown separately, the refresh controller  84  may be part of the control logic  66  in other embodiments of the memory device  60 . The memory device  60  may also include an optional charge pump  86 , which steps up the power-supply voltage V DD  to a voltage V DDP . In one embodiment, the pump  86  generates V DDP  approximately 1–1.5 V higher than V DD . The memory circuit  60  may also use V DDP  to conventionally overdrive selected internal transistors. 
     FIG. 12  is a block diagram of an electronic system  1212 , such as a computer system, that incorporates the memory circuit  60  of  FIG. 11 . The system  1212  also includes computer circuitry  1214  for performing computer functions, such as executing software to perform desired calculations and tasks. The circuitry  1214  typically includes a processor  1216  and the memory circuit  60 , which is coupled to the processor  1216 . One or more input devices  1218 , such as a keyboard or a mouse, are coupled to the computer circuitry  1214  and allow an operator (not shown) to manually input data thereto. One or more output devices  1220  are coupled to the computer circuitry  1214  to provide to the operator data generated by the computer circuitry  1214 . Examples of such output devices  1220  include a printer and a video display unit. One or more data-storage devices  1222  are coupled to the computer circuitry  1214  to store data on or retrieve data from external storage media (not shown). Examples of the storage devices  1222  and the corresponding storage media include drives that accept hard and floppy disks, tape cassettes, and compact disk read-only memories (CD-ROMs). Typically, the computer circuitry  1214  includes address data and command buses and a clock line that are respectively coupled to the ADDRESS, DATA, and COMMAND buses, and the CLK line of the memory device  60 . 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, the memory cell  200  has been illustrated as having epitaxial posts with a rectangular or quadrilateral cross-sectional area. However, the epitaxial posts can be formed having a generally circular cross-sectional area or a generally polygonal cross-sectional area as well. Accordingly, the invention is not limited except as by the appended claims.