Abstract:
A masking and etching technique during the formation of a memory cell capacitor which simultaneously separates storage poly into individual storage poly nodes and etches recesses into the storage poly nodes which increase the surface area of the storage poly nodes and thereby increase the capacitance of a completed memory cell without additional processing steps.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/233,820, filed Jan. 20, 1999, pending, which is a divisional of application Ser. No. 08/759,464, filed Dec. 5, 1996, now U.S. Pat. No. 6,010,932, issued Jan. 4, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a memory cell or storage capacitor and method for forming the memory cell capacitor. More particularly, the present invention relates to a one-step masking and etching technique which simultaneously separates storage poly into individual storage poly nodes and etches recesses into the storage poly node to increase the surface area of the storage poly node. 
     2. State of the Art 
     A widely-utilized DRAM (Dynamic Random Access Memory) manufacturing process utilizes CMOS (Complimentary Metal Oxide Semiconductor) technology to produce DRAM circuits which comprise an array of unit memory cells, each including one capacitor and one transistor, such as a field effect transistor (“FET”). In the most common circuit designs, one side of the transistor is connected to one side of the capacitor, the other side of the transistor and the transistor gate are connected to external circuit lines called the bitline and the wordline, and the other side of the capacitor is connected to a reference voltage that is typically ½ the internal circuit voltage. In such memory cells, an electrical signal charge is stored in a storage node of the capacitor connected to the transistor, which opens and closes to charge and discharge the circuit lines of the capacitor. 
     Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. The advantages of increased miniaturization of components include: reduced-bulk electronic equipment, improved reliability by reducing the number of solder or plug connections, lower assembly and packaging costs, and improved circuit performance. In pursuit of increased miniaturization, DRAM chips have been continually redesigned to achieve ever higher degrees of integration. However, as the dimensions of the DRAM chips are reduced, the occupation area of each unit memory cell of a DRAM chip must be reduced. This reduction in occupied area necessarily results in a reduction of the dimensions of the capacitor, which, in turn, makes it difficult to ensure required storage capacitance for transmitting a desired signal without malfunction. However, the ability to densely pack the unit memory cells while maintaining required capacitance levels is a crucial requirement of semiconductor manufacturing if future generations of DRAM chips are to be successfully manufactured. 
     In order to minimize such a decrease in storage capacitance caused by the reduced occupied area of the capacitor, the capacitor should have a relatively large surface area within the limited region defined on a semiconductor substrate. The drive to produce smaller DRAM circuits has given rise to a great deal of capacitor development. However, for reasons of available capacitance, reliability, and ease of fabrication, most capacitors are stacked capacitors in which the capacitor covers nearly the entire area of a cell and in which vertical portions of the capacitor contribute significantly to the total charge storage capacity. In such designs, the side of the capacitor connected to the transistor is generally called the “storage node” or “storage poly” since the material out of which it is formed is doped polysilicon, while the polysilicon layer defining the side of the capacitor connected to the reference voltage mentioned above is called the “cell poly”. 
     Furthermore, a variety of methods are used for increasing the surface area of the capacitor. These methods include forming the capacitor with various three-dimensional shapes extending from the capacitor. Such shapes include fins, cylinders, and cubes, as well as forming rough or irregular surfaces on these shapes. 
     FIGS. 7-10 illustrate a prior art technique for forming a capacitor for a memory cell. FIG. 7 illustrates an intermediate structure  200  in the production of a memory cell. This intermediate structure  200  comprises a substrate  202 , such as a lightly doped P-type crystal silicon substrate, which has been oxidized to form thick field oxide areas  204  and exposed to implantation processes to form drain regions  208  and a source region  210  of N+ doping. Transistor gate members  212  are formed on the surface of the substrate  202  and span between the drain regions  208  and source region  210 . The transistor gate members  212  each comprise a thin gate oxide layer  206  separating a gate conducting layer or wordline  216  of the transistor gate member  212  from the substrate  202 . Transistor insulating spacer members  218  are formed on either side of each transistor gate member  212 . A lower insulating layer  220  is applied over the transistor gate members  212  and the substrate  202 . After application, the lower insulating layer  220  is planarized. 
     The planarized lower insulating layer  220  is then masked and etched to form a channel therethrough to the source region  210 . A bitline  222  is then formed to contact the source region  210  and extend to other source regions (not shown) on the planarized surface  224  of the planarized lower insulating layer  220 . An upper insulating layer  226  is then applied over the lower insulating layer  220  and the bitlines  222 . After application, the upper insulating layer  226  is planarized. 
     The planarized upper insulating layer  226  is then masked and etched to form channels through the upper insulating layer  226  and the lower insulating layer  220  to respective drain regions  208 . A storage poly  228  is then deposited over the planarized upper insulating layer  226  such that the storage poly  228  extends through the channels to contact the drain regions  208 . 
     As shown in FIG. 8, the storage poly  228  is then masked with a resist layer  230  and etched to separate the storage poly  228  into storage poly nodes  232  (shown in FIG.  9 ). A capacitor or cell dielectric  234  (shown in FIG. 10) is deposited over the storage poly nodes  232  and the upper insulating layer  226 . A cell poly or plate electrode  236  is then disposed over the capacitor or cell dielectric  234  to form the capacitor  238  of each memory cell, as shown in FIG.  10 . 
     If the surface area of the capacitor  238  needs to be increased, further processing steps would be required to form an irregular or rough surface on the storage poly nodes  232  prior to the addition of the capacitor or cell dielectric  234  and the cell poly or plate electrode  236 . 
     U.S. Pat. No. 5,457,063 issued Oct. 10, 1995 to Park (“the Park patent”) teaches a method of increasing the surface area of a capacitor. The Park patent illustrates a prior art method of first etching recesses in the polysilicon layer to form the storage poly nodes, then again etching the polysilicon layer to separate individual storage poly nodes. The Park patent teaches using the same prior art two-step method to form the storage poly nodes, but also forms polysilicon sidewalls to exploit the empty space around the periphery of the storage poly node. Although both the prior art method and the method of the Park patent increase the surface area of a capacitor, each require numerous processing steps to achieve this goal. The additional process steps result in increased production costs which, in turn, result in increased semiconductor chip costs. 
     Therefore, it would be advantageous to develop a technique for forming a high surface area capacitor, while using inexpensive, commercially available, widely practiced semiconductor device fabrication techniques and apparatus without requiring such additional processing steps. 
     SUMMARY OF THE INVENTION 
     The present invention is a one-step masking and etching technique utilized during each formation of a memory cell capacitor which simultaneously separates storage poly into individual storage poly nodes and which etches recesses into the storage poly node in order to increase the surface area of the storage poly node. The increase in the storage poly node surface area increases memory cell capacitance to an adequate degree without additional processing steps. Thus, the overall size of the storage poly node can be reduced while maintaining a predetermined capacitance, which feature ultimately results in higher density DRAMs. 
     The method of the present invention occurs after formation of an intermediate structure comprising transistor gates on a silicon substrate which has been oxidized to form thick field oxide areas and exposed to implantation processes to form drain and source regions. The intermediate structure further comprises a lower insulating layer which substantially covers the transistor gates and the silicon substrate. Bitlines are formed on the lower insulating layer and extend through the lower insulating layer to contact the silicon substrate source regions. The intermediate structure also includes an upper insulating layer covering the lower insulating layer and the bitlines. A storage poly layer covers the upper insulating layer and extends through the upper insulating layer and lower insulating layer to contact the silicon substrate drain regions, completing the intermediate structure. 
     The method of the present invention comprises simultaneously etching the storage poly to separate the storage poly into storage poly nodes and etching recesses into the storage poly nodes to increase the surface area of the storage poly nodes. The simultaneous etching is achieved with a mask or resist pattern which has apertures for separating the storage poly into storage poly nodes and at least one recess forming aperture positioned over each storage poly node. The size (i.e., width, diameter, or surface area) of the recess apertures should be less than the size (i.e., width, diameter, or surface area) of the separation apertures. With the recess apertures smaller than the separation apertures, the etching solution has a lower diffusion rate in the recess apertures than the separation apertures. Thus etching occurs more slowly in the recess apertures than the separation apertures. Therefore, when the etching step to separate the storage poly into storage poly nodes is complete, the etching in the recess apertures will merely create recesses in the storage poly rather than etching completely through. The recess apertures can be of any shape, size, and/or density to achieve a desired surface area of the storage poly nodes. 
     Once the storage poly node is etched, a capacitor or cell dielectric is deposited over the storage poly node and the upper insulating layer. A cell poly or plate electrode is then disposed over the capacitor dielectric to form the capacitor of each memory cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
     FIGS. 1-4 illustrate a method of fabricating a forked capacitor for a memory cell according to the present invention; 
     FIG. 5 illustrates an oblique view of an etching mask of the present invention; 
     FIG. 6 illustrates an oblique view of another etching mask of the present invention; and 
     FIGS. 7-10 illustrate a prior art method of fabricating a capacitor for a memory cell. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4 illustrate a technique for forming a forked capacitor for a memory cell. FIG. 1 illustrates an intermediate structure  100  in the production of a memory cell. This intermediate structure  100  comprises a semiconductor substrate  102 , such as a lightly doped P-type crystal silicon substrate, which has been oxidized to form thick field oxide areas  104  and exposed to implantation processes to form drain regions  108  and a source region  110  of N+ doping. Transistor gate members  112  are formed on the surface of the semiconductor substrate  102  between the drain regions  108  and the source region  110 . The transistor gate members  112  each comprise a patterned thin gate oxide layer  106  (preferably silicon dioxide) separating a gate conducting layer or wordline  116  of the transistor gate member  112  and the semiconductor substrate  102 . Transistor insulating spacer members  118 , preferably silicon dioxide, are formed on either side of each transistor gate member  112 . A lower insulating layer  120  (such as boro-phospho-silicate glass—BPSG, silicon nitride, silicon dioxide, etc.) is applied over the transistor gate members  112  and the semiconductor substrate  102 . After application, the lower insulating layer  120  is planarized, preferably by abrasion, such as with a CMP (chemical/mechanical polishing) process, as known in the art. 
     The planarized lower insulating layer  120  is then masked and etched to form a channel therethrough to the source region  110 . A bitline  122  is then formed to contact the source region  110  and extend to other source regions (not shown) on the planarized surface  124  of the planarized lower insulating layer  120 . An upper insulating layer  126  is then applied over the lower insulating layer  120  and the bitlines  122 . After application, the upper insulating layer  126  is also planarized. 
     The planarized upper insulating layer  126  is then masked and etched, preferably utilizing a storage poly node photo-mask process as known in the art, to form channels through the upper insulating layer  126  and the lower insulating layer  120  to respective drain regions  108 . A storage poly  128 , such as doped silicon, is then deposited over the planarized upper insulating layer  126  such that the storage poly  128  extends through the channels to contact the drain regions  108 . 
     The storage poly  128  is masked and etched to separate the storage poly  128  into storage poly nodes  132  (FIG.  3 ). However, as shown in FIGS. 2 and 3, the present invention differs from previously known manufacturing techniques in that the mask  130  or resist used to separate the storage poly  128  into storage poly nodes  132  includes at least one aperture  140  positioned over the proposed or intended location of each storage poly node  132 . The area of the apertures  140  should be less than the area of boundary openings  142  in the mask  130  for separating the storage poly  128  into storage poly nodes  132 . With the apertures  140  being smaller than the boundary openings  142 , the etching solution has a lower diffusion rate in the apertures  140  than in the boundary openings  142 , thus the etching occurs more slowly in the apertures  140  than the boundary openings  142 . Therefore, when the etching step conducted through boundary openings  142  to separate the storage poly  128  into storage poly nodes  132  is complete, the etching conducted through the apertures  140  will merely create recesses  144  in the storage poly  128  rather than etch completely therethrough. It is, of course, understood that the apertures  140  can be of any shape (circular, triangular, rectangular, etc.), size, and/or dimension to achieve a desired surface area on the storage poly nodes  132 . 
     As shown in FIG. 4, a capacitor or cell dielectric  134 , preferably silicon dioxide, silicon nitride or the like, is subsequently deposited over the storage poly nodes  132  and the upper insulating layer  126 . A cell poly or plate electrode  136  is disposed over the capacitor or cell dielectric  134  to form the capacitor  138  of each memory cell. 
     The formation of the recesses  144  in the storage poly nodes  132  results in an increase in the surface area of the capacitor  138 . This results in lower refresh rates, easier sensing of the contents of the cell due to a greater potential difference between bitlines, and a reduction in the pitch constraints on the layout of the memory cells due to smaller capacitors fabricated by the method of the present invention being able to achieve a predetermined capacitance. 
     FIGS. 5 and 6 illustrate oblique views of two embodiments of the mask  130  as shown in FIG.  2 . FIG. 5 illustrates an embodiment where the apertures  140  are holes through the mask  130  and the boundary openings  142  are avenues, wherein the diameter of the holes of the apertures  140  is smaller than the width of the avenues of the boundary openings  142 . FIG. 6 illustrates an embodiment where both the apertures  140  and the boundary openings  142  in the mask  130  are avenues, wherein the avenues of the boundary openings  142  are wider than the avenues of the apertures  140 . 
     This invention could also be used to fabricate decoupling and pump capacitors during the same processing steps as are used to form the cell capacitors of the array. This increase in capacitance would be advantageous for the decoupling and pump capacitors to save space on the die and potentially decrease the overall die size. 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope thereof.