Abstract:
A structure of dynamic random access memory with slanted active regions, comprising: a substrate; a plurality of slanted active regions formed on the substrate, wherein each of the plurality of slanted active regions has a bit line contact; a plurality of word line regions formed on the substrate to control transistors of the dynamic random access memory; a plurality of bit line regions formed on the substrate, wherein each of the bit line regions cross the bit line contact hole so that the bit line contact hole is completely covered by the bit line regions; a plurality of capacitors formed between the plurality of bit line regions.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a layout structure of dynamic random access memory, more specifically, to a layout structure of dynamic random access memory with slanted active regions. 
     BACKGROUND OF THE INVENTION 
     According to the fabrication of semiconductor devices or integrated circuits, dynamic random access memory (DRAM) is an important microelectronic device and it is a storage device for storing information. Typically, a DRAM cell consists of a capacitor and a comparator. The capacitor in a DRAM cell stores electrostatic charge and the comparator in the DRAM compares the voltage level in the capacitor with a standard voltage level to determine that the voltage level in the capacitor is high level or low level. In other words, the capacitor has a high-level charge storage, the data recorded in the capacitor is indicated as “1”. Similarly, the capacitor has a low-level charge storage, the data recorded in the capacitor is indicated as “0”. 
     The capacitor of a DRAM cell is fabricated on the drain region of the cell and the capacitor electrically connects to the active region of the cell by using a contact plug between the capacitor and the active region. The active region electrically connects to the bit lines of the DRAM cell by using a contact window. Consequently, the manufacture of a DRAM cell must use two etching masks, one etching mask is adapted for defining the electrical contact region of the capacitor region in order to align the capacitor region with the drain region of the cell, another etching mask is adapted for defining the capacitor node of a crown-type bottom electrode. 
     Referring to FIG. 6, a layout structure of a DRAM cell is shown. A DRAM has several active regions  400  that are formed on the semiconductor substrate and the active regions  400  have a square shape. Several word line regions  500  are formed on the substrate for controlling the transistors of the DRAM. Additionally, several bit line regions  100  are formed on the substrate and each of the bit line regions is protected by spacers  110  for insulating isolation. Besides, the spacers are formed from silicon nitride material. The bit line contact holes  200  are formed in interlayer dielectric layers of the DRAM and the contact holes  200  cross the bit line regions  100  and the active regions  400  for electrical coupling between the bit line regions  100  and the active regions  400 . 
     The bit line contact holes  200  are formed on the active regions  400  and the bit line regions then refill to cover on/in the bit line contact holes  200  for electrical coupling. Nevertheless, the bit line contact holes  200  are partially covers on the active regions  400  and the bit line regions  100  are partially on the bit line contact holes  200 . Thus, the bit line contact holes  200  between the bit line regions  100  and the active regions  400  is hard to be completely covered. Furthermore, the substrate between the bit line regions  100  is indicated as a capacitor regions  300 , in other words, the capacitors of the DRAM are formed on the active regions  400  between the bit line regions  100  for connecting the drain regions of the DRAM. During the etching process of the capacitor regions  300 , there is no good isolation reliability around the bit line contact holes  200  and it can not make sure the isolation between the capacitors and the bit lines. 
     The layout structure of the DRAM depicted in FIG. 6 includes two electrical contacts, the first electrical contact is located between the bit line regions and the active regions, the second electrical contact is located between the capacitors and the active regions of the DRAM. The first electrical contact is located at the contact hole regions of the bit lines and it is formed by using lithography process and the etching process to etch interlayer dielectric layers for forming contact holes exposing the partial portion of the active regions and the bit line regions. The conductive material is refilled into the contact holes to serve as the electrical connection between the bit lines and the active regions. The second electrical contact is formed by using lithography and etching process during the formation of the capacitor and the contact holes of the drain regions formed in the interlayer dielectric layer. Consequently, as the contact hole of the drain region is precisely defined in position, the electrical contacts of the drain regions have good reliability. The manufacture of the first electrical contact is to form a bit line contact hole for the connection between the bit lines and the active region and the electrical conductive material is adapted for the connection between the bit line and the active region. In prior art, the bit line region do not fully overlaps on the active region and a bad insulating isolation exists therein. 
     The trend of fabricating DRAM cells is to shrink the size of the cells, that is, to decrease the planar area of the cells for increasing the device integration on silicon wafers. Thus, the distance between the bit lines of DRAM is shrunken to increase the integrity of integrated circuits. As the distance between the bit lines is shrunken, the isolation reliability between the bit line regions and the contact plugs of the capacitors between the bit line regions is simultaneously degraded. At the same time, to simplify the fabrication of the DRAM is an important issue for fabricating the DRAM. Therefore, it is needed that the layout structure of the active regions and the bit line regions to improve the isolation reliability between the bit line regions and the capacitor regions, not to reduce the integrity density of the DRAM. 
     SUMMARY OF THE INVENTION 
     The present invention provides a layout structure of DRAM with slanted active regions and a method to manufacture the structure. Initially, slanted active regions are formed on a substrate and the active regions have a S-type shape. Afterwards, word line regions for controlling the transistors of DRAM are formed on the substrate. Bit line regions are formed on the substrate and cross the slanted active regions so that the first ends and the second ends of the active regions are respectively located on the first side and the second side of the bit line regions. Furthermore, there are bit line contact holes on the slanted active regions and the bit line regions fully cover on the bit line contact holes to form an electrical connection between the slanted active regions and the bit line regions. Finally, an etching process is performed on the interlayer dielectric layers between the bit line regions to form capacitor regions. 
     The present invention provides a structure of DRAM with slanted active regions. The bit line regions could fully cover on the bit line contact holes of the slanted active regions to prevent the bit line contact holes from the connection with the capacitor regions. 
     The present invention provides a layout structure of DRAM with slanted active regions. The bit line regions of DRAM cross the slanted active regions and the spacers of the bit line regions protect the bit line contact holes on the slanted active regions to prevent the bit line contact hole from being etched during the etching process of the capacitor regions of DRAM. 
     The present invention provides a method for fabricating DRAM with slanted active regions. Slanted active regions are defined in the semiconductor substrate and the bit line regions completely cover on the bit line contact holes on the active regions for protecting the bit line contact holes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a cross-section view of a substrate in accordance with the present invention, a polysilicon layer, a metal silicide layer and a silicon nitride layer are deposited on a semiconductor substrate to form a stacked structure, a photoresist layer is defined on the stacked structure to define a gate structure of integrated circuits, there are a memory cell region and a peripheral circuit region on the semiconductor substrate; 
     FIG. 2 shows a cross-section view of a substrate in accordance with the present invention, the stacked structure is etched by using a lithography process and an etching process to form a gate structure on the substrate, then silicon nitride insulating spacer structure are formed on the sidewalls of the gate structure; 
     FIG. 3 shows a cross-section view of a substrate in accordance with the present invention, a silicon nitride is conformally deposited on the gate structure and the semiconductor substrate, and the capacitor node of DRAM is defined by using a photolithography process and an etching process, the semiconductor substrate arounded the gate structure of the memory cell region is etched to form trenches; 
     FIG. 4 shows a cross-section view of a substrate in accordance with the present invention, a rugged polysilicon layer is deposited on the gate structure, the silicon oxide layer and the semiconductor substrate to finish the fabrication of a capacitor; 
     FIG. 5 shows a top view of the layout structure of DRAM in accordance with the present invention, the active regions in the layout structure of DRAM are slanted and a contact hole of the bit lines of DRAM is located between the two ends of a slanted active region so that the bitlines cross the slanted active regions and completely covers the contact holes of the bitlines for making sure that the isolation of the capacitors and the bitlines; and 
     FIG. 6 shows a top view of the layout structure of DRAM in accordance with the prior art, the active regions have a square shape and partially overlap with the contact holes of the bit lines of the memory cells so that the bitline contact holes between the bitlines and the active regions are not covered by the bitlines, the isolation effect between the capacitors and the bitline contact holes will be influenced, when the capacitor regions are formed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention discloses a layout structure of a DRAM with slanted active regions and a method for manufacturing the above mentioned layout structure. A substrate is served as a base of a DRAM. Slanted active regions are formed in the substrate and word line regions are formed on the substrate for controlling the transistors of DRAM. Bit line regions is formed on the substrate for crossing the slanted active regions and the bit line contact holes of the slanted active regions are fully covers by the bit line regions. Capacitor regions are formed on the substrate between the bit line regions and the fabrication of a DRAM is finished. Referring to FIGS. 1-5 and the following descriptions, the layout structure of the DRAM with slanted active regions and the method for fabricating the structure are explained in detail. 
     Referring to FIG. 1, a substrate  10  is provided for the base of DRAM and a polysilicon layer  20  is deposited on the substrate  10 . The polysilicon layer  20  is doped by using conducting dopants and has a thickness between from 500 to 1000 angstroms. A silicon tungsten layer  30 , having a thickness between about 1000 and 2000 angstroms, is then deposited on the polysilicon layer  20 . A silicon nitride layer  40  is deposited on the silicon tungsten layer  30  and the layer  40  has a thickness of between about 1500 and 3000 angstroms. After the above thin film layer is deposited on the substrate  10 , a photoresist pattern  1000  is defined on the silicon nitride layer  40  for defining a memory cell region M and the bit line region of a peripheral circuit region P. 
     Referring to FIG. 2, the silicon nitride layer  40 , the silicon tungsten layer  30  and the polysilicon layer  20  are etched by using anisotropical etching process to form the memory cell region M and the bit line region of the peripheral circuit region P. After the bit line structure is formed, a silicon nitride layer having a thickness of between about 500 to 1500 angstroms covers on the substrate  10  and, the surface and the sidewalls of the bit line structure. Subsequently, the silicon nitride layer is etched to remove the silicon nitride layer on the surface of the bit line structure and the substrate  10  for forming insulating spacers  45  on the sidewalls of the bit line structure. 
     The bit line structure as shown in FIG. 2, the polysilicon layer  20  is indicated as the bit line of the memory and the silicon nitride layer  40  and the insulating spacers  45  formed from silicon nitride material serve as an etching hard mask in following process steps for protecting the bit line structure from etching. 
     Referring to FIG. 3, an interlayer dielectric layer  50  is deposited on the surface of the silicon nitride layer  40 , the insulating spacers  45  and the substrate  10 . Furthermore, the interlayer dielectric layer  50  is formed from silicon oxide material. In a preferred embodiment of the present invention, the interlayer dielectric layer  50  is formed from boron-phosphorus-doped-silicon glass (BPSG) and has a thickness between from 6000 to 12000 angstroms. In virtue of the process design of a DRAM cell, the thickness of the interlayer dielectric layer  50  is determined by the capacitance of a capacitor. 
     Still referring to FIG. 3, an etching process is performed on the interlayer dielectric layer  50  by using a photolithography process and an etching process to define the opening of the capacitor of a memory cell. In the etching process, the silicon nitride layer  40  and the insulating spacers serve as an etching hard mask to protect the bit line region. The etching process is a reactive-ion-etching (RIE) process under a pressure of between from 30 to 50 mtorrs, by using a radio-frequency power of between about 1200 and 1800 watts and the reaction gas consisting of C 4 F 8 , Ar and CH 2 F 2 . The flow rate of C 4 F 8 , Ar and CH 2 F 2  is respectively between about 5 and 9 sccm, between about 400 and 600 sccm and between about 3 and 5 sccm. The interlayer dielectric layer  50  is etched by using the above recipe to maintain the high etching selectivity between silicon nitride material and silicon oxide material to prevent the silicon nitride layer  40  or the insulating spacers  45  from etching damage. After the opening of the capacitor is defined, the peripheral circuit region P is covered by the interlayer dielectric layer  50  to prevent the region P from any influence during the fabrication process of the capacitor in DRAM. In the present invention, an opening of a capacitor is formed in the interlayer dielectric layer and the capacitor is formed on and in the interlayer dielectric layer. The method for fabricating the capacitor above and in the bit line region is indicated as a fabricating method of a crown-type capacitor. The structure fabricated by the foregoing method is called as a capacitor-on-bitline (COB) structure. 
     Referring to FIG. 4, a rugged polysilicon layer  60  is deposited on the surface of the capacitor region and the interlayer dielectric layer. Subsequently, the rugged polysilicon layer  60  on the capacitor region is removed to define the electrode of the capacitor to increase the surface area of the electrode and the capacitance of the capacitor. At last, a cell plate  70  is defined on the opening of the capacitor to serve as another electrode of the capacitor. Additionally, a capacitor dielectric layer (not shown) is formed between the cell plate  70  and the rugged polysilicon layer  60  for maintain an electric field in the capacitor for charge storage. 
     In the above descriptions, a crown-type capacitor structure is formed on a substrate, more precisely, on a bit line region. The region of the capacitor and the bit line structure is shown in the layout structure according to FIG.  5 . Referring to FIG. 5, several slanted active regions  400  are formed in the substrate and word line regions  500  is formed on the substrate to control the transistors of the DRAM. Several bit line regions  100  is formed on the substrate and cross the slanted active regions  400 . Besides, the first end and the second end of the slanted active region  400  respectively locate on the first side and the second side of the bit line region  100 . One of the slanted active regions  400  has a bit line contact hole  200  that is covered by the bit line regions  100 . The overlap region of the bit line region  100  and the bit line contact hole  200  has a larger area than that in prior art, the bit line regions  100  can fully covers on the bit line contact holes  200 . 
     The capacitor regions  300  of the DRAM are defined in the interlayer dielectric layers between the two bit line regions  100 . The capacitor regions  300  is called as a crown-type capacitor. In the present invention, the bit line regions, the slanted active regions are adapted for solving the overlapping area between the slanted active regions and the capacitor regions. Each of the slanted active regions have S-type shape and has two ends, the first ends and the second ends. The first end and the second end are respectively located on the first side and the second side of the bit line regions  100 . Moreover, the bit line regions cross the bit line contact holes  200  of the slanted active regions  400  and the holes  200  is fully covered under the bit line regions  100 . By using the slanted active regions  400  in the present invention, the bit line contact holes  200  are completely covers by the bit line regions  100 . The bit line regions  100  are protected by insulating spacers  110  formed from silicon nitride material. According to the foregoing layout structure, during the etching process for defining the capacitors in DRAM, the bit line contact holes  200  are protected for the etching damage to make sure the isolation reliability between the bit line regions and the capacitor regions. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.