Abstract:
A method of fabricating a dynamic random access memory is provided. A trench capacitor is formed in a substrate and an isolation structure is formed on the trench capacitor. A gate structure and a passing gate structure are formed on the substrate. The gate structure is on one side of the passing gate structure. A source region and a drain region are formed in the substrate on both sides of the gate structure. A dielectric layer is formed on the substrate. A contact is formed in the dielectric layer and the isolation structure, at the other side of the passing gate structure, and is coupled to the trench capacitor. Since the contact is formed at the other side of the passing gate structure, the contact would not coupled to the source and drain regions when misalignment occurs.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention relates to a structure and a fabrication method of a semiconductor device. More particularly, the present invention relates to a structure and a fabrication method of a dynamic random access memory (DRAM).  
         [0003]     2. Description of Related Art  
         [0004]     The DRAM is using a capacitor to store information. Data content of each memory cell is read by judging the charges stored in the capacitor. The size for the present memory cell is rather small. In order to increase the capacitance of the capacitor and reduce the possibility in error reading on data, and further reduce the refreshing frequency for increasing the operation efficiency, the usual way is increasing the surface area of lower electrode of the capacitor. By increasing the surface area of the capacitor, it can thus provide the sufficient storage capacitance. In order to satisfy the need in surface area of the capacitor and the integration of memory cells, the trench capacitor has been the popular option.  
         [0005]      FIG. 1A  is a top view, schematically illustrating the DRAM using a conventional trench capacitor.  FIG. 1B  is a cross-sectional view along the line I-I′ in  FIG. 1A . In  FIG. 1A  and  FIG. 1B , the DRAM includes a substrate  100 , a transistor  102 , a passing gate structure  104 , an isolation structure  106 , a trench capacitor  108 , an inter-layer dielectric layer  110 , and a contact  112 . The contact  112  is coupled to the trench capacitor  108 .  
         [0006]     However, when the contact window is under the photolithography process, due to rather small process, the misalignment easily occurs, causing the contact  112  to be electrically connected with the source region and drain region  114  of the transistor  102  at the circle  103 . As a result, the memory cell gets failure and the production yield of the memory is affected.  
       SUMMARY OF THE INVENTION  
       [0007]     With an objective, the invention provides a method for fabricating DRAM with improvement of production yield about the contact alignment.  
         [0008]     With another objective, the invention provides a method for fabricating DRAM, capable of increasing the yield of the memory device.  
         [0009]     The invention provides a DRAM, includes a substrate, a trench capacitor, a passing gate structure, a transistor, and a contact. The substrate has a trench. The trench capacitor is disposed in the trench. The passing gate structure is disposed on the trench capacitor. The transistor is disposed on the substrate at one side of the passing gate structure. The contact is disposed on the substrate at the other side of the passing gate structure, and the contact is coupled to the trench capacitor.  
         [0010]     In an embodiment, the foregoing trench capacitor includes a lower electrode, a capacitance dielectric layer, and an upper electrode. The lower electrode is disposed on the periphery of the trench in the substrate. The upper electrode fills the trench. The capacitance dielectric layer is disposed between the upper electrode and the lower electrode. A material for the capacitance dielectric material includes, for example, silicon oxide/silicon nitride/silicon oxide (ONO).  
         [0011]     In an embodiment, the foregoing DRAM further includes an isolation structure, disposed between the passing gate structure and the trench capacitor.  
         [0012]     The invention provides another DRAM, includes a substrate, a first transistor, a second transistor, a first trench capacitor, a second trench capacitor, a first passing gate structure, a second passing gate structure, a transistor, and a contact. The substrate has a device isolation structure. The first transistor is disposed on the substrate at one side of the device isolation structure. The second transistor is disposed on the substrate at the other side of the device isolation structure. The first trench capacitor is disposed between the first transistor and the device isolation structure. The second trench capacitor is disposed between the second transistor and the device isolation structure. The first passing gate structure is disposed on the first trench capacitor. The second passing gate structure is disposed on the second trench capacitor. The contact is disposed between the first passing gate structure and the second passing gate structure, and is coupled to the first trench capacitor and the second trench capacitor.  
         [0013]     In an embodiment, the foregoing first trench capacitor includes a first lower electrode, a first capacitance dielectric layer, and a first upper electrode. The first lower electrode is disposed in the substrate at the periphery of the upper electrode. The first capacitance dielectric layer is disposed between the first upper electrode and the first lower electrode. A material of the first capacitance dielectric layer includes silicon oxide/silicon nitride/silicon oxide (ONO).  
         [0014]     In an embodiment, the foregoing second trench capacitor includes a second lower electrode, a second capacitance dielectric layer, and a second upper electrode. The second lower electrode is disposed in the substrate at the periphery of the upper electrode. The second capacitance dielectric layer is disposed between the second upper electrode and the second lower electrode. A material of the second capacitance dielectric layer includes silicon oxide/silicon nitride/silicon oxide (ONO).  
         [0015]     In an embodiment, the foregoing DRAM further includes a first isolation structure, disposed between the first passing gate structure and the first trench capacitor. In addition, the foregoing DRAM further includes a second isolation structure, disposed between the second passing gate structure and the second trench capacitor.  
         [0016]     The invention further provides a method for fabricating a DRAM, including providing a substrate, and forming a trench capacitor in the substrate. Then, an isolation structure is formed on the trench capacitor. A gate structure and a passing gate structure are formed on the substrate. The passing gate structure is on the isolation structure, and the gate structure is at one side of the passing gate structure. Then, a source/drain region is formed in the substrate at each side of the gate structure. The gate structure with the source/drain region forms a transistor. A dielectric layer covers over the substrate. Then, a contact is formed in the dielectric layer and the isolation structure, at another side of the passing gate structure. Also and, the contact is coupled to the trench capacitor.  
         [0017]     In an embodiment, the method for forming the trench capacitor includes forming a trench in the substrate, and the substrate at the periphery of the trench serves as the lower electrode. Then, a conformal dielectric layer is formed over the substrate. This dielectric layer serves as a capacitance dielectric layer. After then, a conductive layer is formed over the substrate. This conductive layer fully fills the trench. A portion of the conductive layer and the dielectric layer other than the trench is removed, so that the upper electrode is formed. In addition, a material for the capacitance dielectric layer includes, for example, silicon oxide/silicon nitride/silicon oxide (ONO).  
         [0018]     The invention further provides a method for fabricating a DRAM, including providing a substrate and forming a device isolation structure in the substrate. Then, a first trench capacitor and a second trench capacitor are formed in the substrate at both sides of the device isolation structure. A first isolation structure and a second isolation structure are respectively formed on the first trench capacitor and the second trench capacitor. A first gate structure and a second gate structure are formed on the substrate. A first passing gate structure and a second passing gate structure are respectively formed on the first isolation structure and the second isolation structure. The first passing gate structure and the second passing gate structure are located between the first gate structure and the second gate structure. Several source/drain regions are formed in the substrate at sides of the first gate structure and the second gate structure. A dielectric layer is formed to cover over the substrate. A contact is formed in the dielectric layer between the first passing gate structure and the second passing gate structure, the first isolation structure, and the second isolation structure. The contact is coupled to the first trench capacitor and the second trench capacitor.  
         [0019]     In an embodiment, the method for forming the first trench capacitor and second trench capacitor includes forming a first trench and a second trench in the substrate. The substrate at the periphery of the first trench and the second trench serves as a first lower electrode and a second electrode. Then, a conformal dielectric layer and a conductive layer are sequentially formed over the substrate. The dielectric layer serves as a capacitance dielectric layer and the conductive layer fills the first trench and the second trench. After removing a portion of the dielectric layer and the conductive layer other than the first trench and the second trench, a first upper electrode and a second upper electrode are respectively formed in the first trench and the second trench. A material for the first capacitance dielectric layer and the second capacitance dielectric layer includes, for example, silicon oxide/silicon nitride/silicon oxide (ONO).  
         [0020]     In the invention, since the transistor is formed at one side of the passing gate structure and the contact is formed at the other side of the passing gate structure and coupling to the trench capacitor, the process window for the contact is therefore not limited by the source region and drain region. In addition, since the process for forming the trench capacitor is after the formation of the device isolation structure, the process window for the contact is not limited by the device isolation structure. In summary, the present invention can significantly improve the process window of contact, and further improve yield and reliability of the memory device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0022]      FIG. 1A  is a top view, schematically illustrating the DRAM using a conventional trench capacitor.  
         [0023]      FIG. 1B  is a cross-sectional view along the line I-I′ in  FIG. 1A .  
         [0024]      FIG. 2A  a top view, schematically illustrating a DRAM, according to an embodiment of the invention.  
         [0025]      FIG. 2B  is a cross-sectional view along the line II-II′in  FIG. 2A .  
         [0026]      FIG. 3A  is a top view, schematically illustrating a DRAM, according to another embodiment of the invention.  
         [0027]      FIG. 3B  is a cross-sectional view along the line II-II′ in  FIG. 3A .  
         [0028]      FIGS. 4A-4F  are cross-sectional views, schematically illustrating the fabrication processes to form a DRAM, according to an embodiment of the invention.  
         [0029]      FIG. 4G  is a cross-sectional view, schematically illustrating the fabrication processes to form a DRAM, according to another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]      FIG. 2A  a top view, schematically illustrating a DRAM, according to an embodiment of the invention.  FIG. 2B  is a cross-sectional view along the line II-II′ in  FIG. 2A . In  FIG. 2A  and  FIG. 2B , only a portion of the memory cell array in a DRAM is shown, however, the invention is not only limited to this portion. The arrangement between each of the device structures can be the repeat of structure in  FIG. 2  at the directions of up-to-down or left-to-right.  
         [0031]     In  FIG. 2A  and  FIG. 2B , the DRAM of the invention is formed from a substrate  200 , a device isolation structure  202 , a trench capacitor  216   a , a trench capacitor  216   b , a first isolation structure  218   a , a second isolation structure  216   b , a passing gate structure  222   a , a passing gate structure  222   b , a transistor  225   a , a transistor  225   b , and a contact  228 . The substrate is, for example, a silicon substrate, having trenches  204   a  and  204   b.    
         [0032]     In addition, the transistor  225   a  is disposed on the substrate  200  at one side of the device isolation structure  202 . The transistor  225   b  is disposed on the substrate  200  at the other side of the device isolation structure  202 . The transistor  225   a  includes a gate structure  220   a  and a source/drain region  224   a . The transistor  225   b  includes a gate structure  220   b  and a source/drain region  224   b . The gate structure  220   a  in the array of memory cells can be a part of the word line (W 1 ), and the gate structure  220   b  in the array of memory cells can be a part of the word line (W 2 ).  
         [0033]     In addition, the trench  204   a  is located between the transistor  225   a  and the device isolation structure  202 , and the trench  204   b  is located between the transistor  225   b  and the device isolation structure  202 . The trench capacitor  216   a  is disposed in the trench  204   a . The trench capacitor  216   a  includes the capacitance dielectric layer  210   a  and upper electrode  212   a . The capacitance dielectric layer  210   a  is disposed on the surface of the trench  204   a . A material for the capacitance dielectric layer  210   a  includes, for example, silicon oxide/silicon nitride/silicon oxide (ONO). The upper electrode  212   a  is disposed on the capacitance dielectric layer  210   a  and fills the trench  204   a . A material for the upper electrode  212   a  includes, for example, doped polysilicon. In addition, the substrate  200  at the periphery of the upper electrode  212   a  can be, for example, serving as a lower electrode  214   a . The trench capacitor  216   b  is disposed in the trench  204   b . The trench capacitor  216   b  includes the capacitance dielectric layer  210   b  and upper electrode  212   b . The capacitance dielectric layer  210   b  is disposed on the surface of the trench  204   b . A material for the capacitance dielectric layer  210   b  includes, for example, ONO. The upper electrode  212   b  is disposed on the capacitance dielectric layer  210   b  and fills the trench  204   b . A material for the upper electrode  212   b  includes, for example, doped polysilicon. In addition, the substrate  200  at the periphery of the upper electrode  212   b  can be, for example, serving as a lower electrode  214   b.    
         [0034]     Further, the first isolation structure  218   a  is disposed between the passing gate structure  222   a  and the trench capacitor  216   a , and the second isolation structure  218   b  is disposed between the passing gate structure  222   b  and the trench capacitor  216   b . A material for the first isolation structure  218   a  and the second isolation structure  218   b  is, for example, silicon oxide.  
         [0035]     On the other hand, the passing gate structure  222   a  is disposed on the trench capacitor  216   a . The passing gate structure  222   a  is disposed on the trench capacitor  216   b . The passing gate structure  222   a  in the array of memory cells can be a part of the word line (W 3 ), and the passing gate structure  222   b  in the array of memory cells can be a part of the word line (W 4 ).  
         [0036]     In addition, an inter-layer dielectric layer  226  is disposed over the substrate  200 . A material for the inter-layer dielectric layer  226  is, for example, silicon oxide. The contact  228  is formed in the first isolation structure  218   a  and the second isolation structure  218   b  between the passing gate structure  222   a  and the passing gate structure  222   b , and in the inter-layer dielectric layer  226 . the contact  228  is coupled to the trench capacitor  216   a  and the trench capacitor  216   b . A material for the contact  228  includes, for example, tungsten.  
         [0037]     Another embodiment of DRAM, similar to the previous DRAM, in the invention is described as follows.  
         [0038]      FIG. 3A  is a top view, schematically illustrating a DRAM, according to another embodiment of the invention.  FIG. 3B  is a cross-sectional view along the line II-II′ in  FIG. 3A . In  FIG. 3A  and  FIG. 3B , only a portion of the memory cell array in a DRAM is shown, however, the invention is not only limited to this portion. The arrangement between each of the device structures can be the repeat of structure in  FIG. 2  at the directions of up-to-down or left-to-right.  
         [0039]     In  FIG. 3A  and  FIG. 3B , the devices similar to those in  FIG. 2  and  FIG. 2B  are indicated with the same numerals, and the descriptions are omitted. Here, the differences are described. Since the upper electrodes  212   a ,  212  bunder operation of the memory have the same voltage level, a single contact  230  can be used in the DRAM of  FIG. 3A  and  FIG. 3B , to replace the contacts  228 . The contact  230  is disposed between the passing gate structure  222   a  and the passing gate structure  222   b , and is coupled to trench capacitors  216   a  and  216   b . A material for the contact  230  is, for example, tungsten.  
         [0040]     Since the transistor is disposed at one side of the passing gate structure and the contact is disposed at another side of the passing gate structure, it allows a larger tolerance for fabrication error, so as to prevent the contact from coupling to the source/drain region. Therefore, the DRAM of the invention has higher yield and reliability.  
         [0041]      FIGS. 4A-4F  are cross-sectional views, schematically illustrating the fabrication processes to form a DRAM, according to an embodiment of the invention.  
         [0042]     In  FIG. 4A , a substrate  400 , such as a silicon substrate, is provided. The substrate  400  has been formed with a device isolation structure  402 . The device isolation structure  402  is, for example, a shallow trench isolation structure and the material is, for example, silicon nitride.  
         [0043]     Then, in  FIG. 4B , the trenches  404   a  and  404   b  are formed in the substrate  400  at both side of the device isolation structure  402 . A portion of the device isolation structure  402  is removed during forming the trench  404   a  and the trench  404   b . The method for forming the trench  404   a  and the trench  404   b  includes, for example, sequentially forming a first dielectric layer (not shown) and a second dielectric layer (not shown) over the substrate  400 . The first dielectric layer is, for example, silicon oxide, and the second dielectric layer is, for example, silicon nitride. Then, the first dielectric layer and the second dielectric layer are patterned, so as to form a pad oxide layer  406   a  and a mask layer  408 . Then, it is the formed by performing an etching process by using the mask layer  408  as a mask.  
         [0044]     In  FIG. 4C , a conformal dielectric layer  410  is formed over the substrate  400 . A material layer for the dielectric layer  410  is, for example, ONO. The method for forming the dielectric layer  410  is, for example, chemical vapor deposition. Then, a conductive layer  412  is formed over the substrate  400 . The conductive layer  412  fills the trench  404   a  and the trench  404   b . A material for the conductive layer  412  is, for example, doped polysilicon. The method for forming the conductive layer  412  is, for example, chemical vapor deposition.  
         [0045]     In  FIG. 4D , a portion of the conductive layer  412  and the dielectric layer  410  other than the trench  404   a  and the trench  404   b  is removed. As a result, a capacitance dielectric layer  410   a  is formed on the surface of the trench  404   a , and an upper electrode  412   a  is formed in the trench  404   a . A capacitance dielectric layer  410   b  is formed on the surface of the trench  404   b , and an upper electrode  412   b  is formed in the trench  404   b . The method for removing the portion of the conductive layer  412  and the dielectric layer  410  other than the trench  404   a  and the trench  404   b  is, for example, performing a chemical mechanical polishing process by using the mask layer  408  as the polishing stop. The substrate  400  at the periphery of the trench  404   a  and  404   b  are respectively serving as a lower electrode  414   a  and a lower electrode  414   b . The capacitance dielectric layer  410   a , the upper electrode  412   a , and the lower electrode  414   a  form a trench capacitor  416   a ; the capacitance dielectric layer  410   b , the upper electrode  412   b , and the lower electrode  414   b  form a trench capacitor  416   b . An etching back process is performed on the upper electrode  412   a  and the upper electrode  412   b . The mask layer  408  and the pad oxide layer  406  are removed while a portion of the capacitance dielectric layers  410   a  and  410   b  is removed, too.  
         [0046]     In  FIG. 4E , an insulation layer (not shown) is formed over the substrate  400 . The insulation layer fully fills the trenches  404   a  and  404   b . A material for the insulation material is, for example, silicon oxide. After then, a portion of the insulation layer other than the trenches  404   a  and  404   b  is removed, so as to respectively form a first isolation structure  418   a  and a second isolation structure  418   b  on the trench capacitors  416   a  and  416   b . The method for removing the portion of the insulation layer other than the trenches  404   a  and  404   b  is, for example, chemical mechanical polishing process. The first isolation structure  418   a  and the second isolation structure  418   b  are serving as the passing gate isolation (PGI) structure. A gate structure  420   a  and a gate structure  420   b  are formed on the substrate  400 . A passing gate structure  422   a  and a passing gate structure  422   b  are also respectively formed on the first isolation structure  418   a  and the second isolation structure  418   b . The passing gate structure  422   a  and the passing gate structure  422   b  are located between the gate structure  420   a  and the gate structure  420   b . The source/drain regions  424   a  are formed in the substrate  400  at both sides of the gate structure  420   a , so as to form the transistor  425   a ; the source/drain regions  424   b  are formed in the substrate  400  at both sides of the gate structure  420   b , so as to form the transistor  425   b . The source/drain region  424   a  at one side of the gate structure  420   a  is coupled to trench capacitor  416   a , and the source/drain region  424   b  at one side of the gate structure  420   b  is coupled to trench capacitor  416   b.    
         [0047]     In  FIG. 4F , an inter-layer dielectric layer  426  is formed over the substrate  400  to cover the substrate  400 , the gate structure  420   a , the gate structure  420   b , the passing gate structure  422   a , the passing gate structure  422   b , the device isolation structure  402 , the first isolation structure  418   a , and the second isolation structure  418   b . A contact  428  is formed in the inter-layer dielectric layer  426  between the passing gate structure  422   a  and the passing gate structure  422   b , the first isolation structure  418   a , and the second isolation structure  418   b . The contact  228  is coupled to the upper electrodes  412   a  and  412   b . Remarkably, even under the situation of poor alignment, the contact  428  is still not contacting with the source/drain regions  424   a  and  424   b.    
         [0048]      FIG. 4G  is a cross-sectional view, schematically illustrating the fabrication processes to form a DRAM, according to another embodiment of the invention. Since the processes before forming the contact for the DRAM in  FIG. 4G  are the same as the processes for the DRAM in  FIG. 4F , the processes before forming the contact are not re-described.  
         [0049]     In  FIG. 4G , another contact  430  is formed over the substrate  400 , to replace the contact  428  in  FIG. 4F . The contact  430  is coupled to the trench capacitors  416   a  and  416   b . Even under the situation of poor alignment, the contact  430  is still not contacting with the source/drain regions  424   a  and  424   b.    
         [0050]     In summary, the DRAM of the invention at least has the advantages as follows.  
         [0051]     1. The transistor of the present invention is formed at one side of the passing gate structure, the contact is formed at another side of the passing gate structure with connecting to the trench capacitor. As a result, the process window is determined by the passing gate structure but not limited by the source/drain region. Thus, the present invention prevents the conventional issues about process window of contact, which is limited by both the passing gate structure and the source/drain region. In addition, since the contact of the adjacent two trench capacitors is incorporated in the same fabrication region, and there no other structure between the two contacts, the process window is significantly improved, and the yield of the memory device is improved.  
         [0052]     2. Since the process for forming the trench capacitor is performed after the device isolation structure having been formed, the conventional issues are significantly solved. As a result, the process window of the contact is not limited to the device isolation structure, and can be significantly improved.  
         [0053]     3. Since the invention changes the position of the contact, it can prevent the metal silicide at the interface of contact and the trench capacitor from extending to the source/drain region due to poor control of the fabrication process.  
         [0054]     4. Since the invention does not need the photolithography process to form the isolation structure above the trench capacitor, the fabrication process can be simplified.  
         [0055]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.