Abstract:
The present invention relates to a manufacturing method for a trench capacitor having an isolation collar which is electrically connected with a substrate on a single side via a buried contact, particularly for use in a semiconductor memory cell. More specifically, the present invention relates to a manufacturing method for a trench capacitor having an isolation collar with a metal conductive fill in the collar region connected to a metal fill in the capacitor region.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates to a manufacturing method for a trench capacitor, and in particular having an isolation collar which is electrically connected with a substrate on a single side via a buried contact for use in a semiconductor memory cell.  
       BACKGROUND OF THE INVENTION  
       [0002]     Although in principle applicable to arbitrary integrated circuits, the following invention and the underlying problems will be explained with respect to integrated memory circuits in silicon technology.  
         [0003]      FIG. 1  shows a schematic cross-section of a semiconductor memory cell having a trench capacitor and a planar selection transistor connected thereto.  
         [0004]     In  FIG. 1 , reference sign  1  denotes a silicon substrate. In the semiconductor substrate  1 , trench capacitors GK 1 , GK 2  are provided which include trenches G 1 , G 2 , the electrically conductive fillings  20   a ,  20   b  thereof forming capacitor electrodes. The conductive fillings  20   a ,  20   b  are isolated by a dielectric  30   a ,  30   b  with respect to the semiconductor substrate  1  in the lower and medium trench region, said substrate  1  forming the second capacitor electrodes (e.g. in form of a buried plate not shown).  
         [0005]     In the medium and upper region of the trenches G 1 , G 2 , surrounding isolation collars  10   a ,  10   b  are provided, above which buried contacts  15   a ,  15   b  are provided which are in electrical contact with said conductive fillings  20   a ,  20   b  and the adjoining semiconductor substrate  1 . The buried contacts  15   a ,  15   b  are connected to the semiconductor substrate on a single side only (cmp.  FIG. 2   a,b ). Isolation regions  16   a ,  16   b  isolate the other substrate side with respect to the buried contacts  15   a ,  15   b  and isolate the buried contacts  15   a ,  15   b  to the top of the trenches G 1 , G 2 .  
         [0006]     This arrangement allows a very high packing density of the trench capacitors GK 1 , GK 2  and the associated selection transistors which will be explained in the following. In this respect, the selection transistor associated to the trench capacitor GK 2  will be mainly referred to, because only the drain region D 1  and the source region S 3 , respectively, of the neighbor selection transistors are depicted. The selection transistor associated to trench capacitor GK 2  comprises a source region S 2 , a channel region K 2  and a drain region D 2 . The source region S 2  is connected to a (not shown) bit line arranged above an isolation layer I via a bit line contact BLK. The drain region D 2  is connected to said buried contact  15   b  on a single side. Above the channel region K 2 , a word line WL 2  is located which comprises a gate stack GS 2  and a surrounding gate isolator GI 2 . The word line WL 2  is an active word line for the selection transistor of the trench capacitor GK 2 .  
         [0007]     Word lines WL 1  comprising gate stack GS 1  and gate isolator GI 1  and word line WL 3  comprising gate stack GS 3  and gate isolator GI 3  are running in parallel to the word line WL 2 , word lines WL 1  and WL 3  being passive word lines for the selection transistor of the trench capacitor GK 2 . The word lines WL 1 , WL 3  are adapted for controlling selection transistors which are shifted in the third dimension with respect to said cross-section.  
         [0008]     It may be obtained from  FIG. 1  that this kind of single-sided connection of the buried contact allows a direct arrangement of the trenches and the neighboring source regions or drain regions of the associated selection transistors next to each other. Thus, the length of a memory cell can amount to only 4 F, and the width can amount to only 2 F, F being the minimum length unit which may be realized in a technological respect (cmp.  FIG. 2   a ,  2   b ).  
         [0009]      FIG. 2A  shows a view from above onto a memory cell field having memory cells according  FIG. 1  in a first arrangement possibility.  
         [0010]     Reference sign DT in  FIG. 2A  denotes trenches which are arranged in rows having a distance of 3 F and in columns having a distance of 2 F to each other. Neighboring rows are shifted by 2 F with respect to each other. UC in  FIG. 2A  denotes the area of the unit cell which amounts to 4 F×2 F=8 F 2 . STI denotes isolation trenches which are arranged in row direction in a distance of 1 F to each other and which isolate neighboring active regions from each other. Also, with a distance of 1 F to each other, bit lines BL are arranged in row direction, whereas said word lines in column direction have a distance of 1 F to each other. In this arrangement example, all trenches DT have a contact region KS of the buried contact to the substrate on the left side and an isolation region IS on the right side (regions  15   a,b  and  16   a,b , respectively, in  FIG. 1 ).  
         [0011]      FIG. 2B  shows a view from above onto a memory cell field having memory cells according to  FIG. 1  in a second arrangement possibility.  
         [0012]     In this second arrangement possibility, the rows of the trenches have alternating connection regions or isolation regions of the buried contacts. Thus, in the lowest row of  FIG. 2B , the buried contacts have on the left side a contact region KS 1  and on the right side a contact region IS 1 . On the other hand, in the row laying above, all trenches DT have an isolation region IS 2  on the left side and a contact region KS 2  on the right side. This arrangement alternates in column direction.  
         [0013]     For DRAM memory devices having trench capacitors in sub-100 nm technologies, the resistance of the trench and the buried contact make up a main contribution to the total RC delay and determine the speed of the DRAM. Caused by the relatively low conductivity and the pinch-off which is produced by an overlay displacement of the STI etch, the series resistance in the trench is enhanced dramatically.  
         [0014]     This problem was diminished by the introduction of polysilicon highly doped with arsenic, an improvement of the overlay between the active regions and the trench, the introduction of a self-aligned manufacture of a buried contact with single-sided connection, and a thinning of the nitride contact region of the buried contact. Particularly, the upper region of the polysilicon filling highly doped with arsenic in the trench constitutes a big problem for the sub-100 nm technologies, because the doping level cannot be further enhanced and the diameter is influenced by the STI trench formation (STI=Shallow Trench Isolation).  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention relates to a manufacturing method for a trench capacitor having an isolation collar which is electrically connected with a substrate on a single side via a buried contact, particularly for use in a semiconductor memory cell. More specifically, the present invention relates to a manufacturing method for a trench capacitor having an isolation collar with a metal conductive fill in the collar region connected to a metal fill in the capacitor region.  
         [0016]     The present invention provides an improved manufacturing method for a single-sided connected trench capacitor having lower RC delay.  
         [0017]     The present invention provides a process where metal regions having low resistance are provided in the lower and upper trench region.  
         [0018]     According to a preferred embodiment, the metallic region is a second metallic liner and a third conductive filling is provided within the second metallic liner which is etched-back to the upper side of the etched-back second metallic liner.  
         [0019]     According to another preferred embodiment, the metallic region is a metallic filling.  
         [0020]     According to another preferred embodiment, after partly removing the second conductive filling, an isolation filling is provided in the upper trench region at least up to the upper side of the substrate.  
         [0021]     According to another preferred embodiment, the isolation filling is provided in an STI trench forming process.  
         [0022]     According to another preferred embodiment, the first metallic liner and the second metallic region are made of one of the following materials: TiN, TaN, HfN, W, HfAlN, C.  
         [0023]     According to another preferred embodiment, the first conductive filling and the second conductive filling are made of polysilicon.  
         [0024]     According to another preferred embodiment, the third conductive filling is made of polysilicon. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Preferred embodiments of the invention are depicted in the drawings and explained in the following description.  
         [0026]     In the Figures:  
         [0027]      FIG. 1  shows a schematic cross-section of a semiconductor memory cell having a trench capacitor and a planar selection transistor connected thereto.  
         [0028]      FIG. 2A ,B show a respective view from above onto a memory cell field having memory cells according to  FIG. 1  in a first and second arrangement possibility.  
         [0029]     FIGS.  3 A-G are schematic drawings of subsequent method steps of a manufacturing method as a first embodiment of the present invention.  
         [0030]     FIGS.  4 A-C are schematic drawings of subsequent method steps of a manufacturing method as second embodiment of the present invention. 
     
    
       [0031]     In the Figures, identical reference signs denote equivalent or functionally equivalent components.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     In the embodiments described below, an explanation of the manufacture of the planar selection transistors is omitted for the sake of clarity, and only the formation of the single-sided connected buried contact of the trench capacitor is explained in detail. The steps of manufacturing the planar selection transistors are the same as known from the state of the art, except it is explicitly described differently.  
         [0033]     FIGS.  3 A-G are schematic drawings of subsequent method steps of a manufacturing method as a first embodiment of the present invention.  
         [0034]     In  FIG. 3A , reference sign  5  denotes a trench which is provided in a silicon semiconductor substrate  1 . On the upper side OS of the semiconductor substrate  1 , there is a hard mask consisting of a padoxide layer  2  and a padnitride layer  3 . A dielectric layer  30  is provided in the trench which isolates an electrically conductive filling  20  with respect to the surrounding semiconductor substrate  1 . After providing the dielectric layer  30 , a first metal liner layer  40  is deposited over the structure. In this example, the first metal liner layer  40  is made of TiN. Thereafter, a doped polysilicon filling  20  is provided in the trench  5  by depositing and back-etching polysilicon. The filling  20  fills about the lower half of the trench  5 , as schematically shown in  FIG. 3A .  
         [0035]     According to  FIG. 3B , the dielectric layer  30  and the first metal liner layer  40  are etched back below the upper surface of the conductive filling  20 . This etching is performed by two different etching steps which are selective with respect to the conductive filling  20 . In particular, the metal liner layer  40  is etched back in NH 4 OH/H 2 O 2  solution which is selective with respect to polysilicon and silicon oxide.  
         [0036]     In the next step, as shown in  FIG. 3C , the first metal liner layer  40  is further etched back below the dielectric layer  30  in order to prevent later electrical short circuits.  
         [0037]     In the next step which is shown in  FIG. 3D , an isolation collar  10  may be of silicon dioxide and is provided in the upper half of the trench  5  above the dielectric layer  30 , the first metal liner layer  40  and the conductive filling  20 . The width extension of the isolation collar  10  is such that it fully covers the dielectric layer  30 , the first metal liner layer  40  and partly covers the conductive filling  20  of polysilicon.  
         [0038]     In a subsequent process step which is illustrated in  FIG. 3E , the conductive filling  20  of polysilicon is etched back below the upper side of the first metal liner layer  40 . Then, a second metal liner layer  50  made of TiN is deposited over the resulting structure. It should be noted that the second metal liner layer  50  of TiN is in electrical contact with the first metal liner layer  40 .  
         [0039]     Subsequently, as depicted in  FIG. 3F , a second conductive filling  60  of silicon is provided which completely fills the trench within the second metal liner layer  50 . Thereafter, the second metal liner layer  50  and the second conductive filling  60  are etched back below the upper side or else of the substrate, and then also the isolation collar  10  is etched back below the upper side or else of the substrate  1 .  
         [0040]     In particular, the second metal liner layer  60  is etched back in NH 4 OH/H 2 O 2  solution which is selective with respect to polysilicon and silicon oxide.  
         [0041]     As a result, the upper side of the second metal liner layer  50  and the second conductive filling  60  is below the upper side of the etched-back isolation collar  10 .  
         [0042]     In  FIG. 3G , it is shown that starting from the process state of  FIG. 3 , the trench is completely filled with a third conductive filling  70  of polysilicon which is polished back down to the surface of the pad nitride layer  3 . In order to provide a single-sided buried contact having a contact region KS only on a single side, a part of the third conductive filling  70  is removed and replaced by an isolation filling  80 . In this example, this isolation filling  80  is provided in an STI trench process. In the region where the third conductive filling  70  has been removed, the isolation filling  80  which in this example is made of silicon oxide provides an isolation region of the capacitor contact with respect to the substrate  1 .  
         [0043]     As may be obtained from  FIG. 3G , the major part of the inner capacitor electrode is formed of the first and second metal liner layer  30 ,  40 , and therefore it is possible to reduce the RC delay of this trench capacitor dramatically in comparison to prior art solutions where the inner capacity electrode is made of polysilicon only.  
         [0044]     FIGS.  4 A-C are schematic drawings of subsequent process steps of the manufacturing method as second embodiment of the present invention.  
         [0045]     The starting point of  FIG. 4A  follows the process status of  FIG. 3B  of the above described first embodiment.  
         [0046]     Instead of depositing a second metal liner layer  50 , the second embodiment is characterized by a complete metal fill  50 ′ of the trench, said metal fill  50 ′ being made of TiN as well.  
         [0047]     Further, as shown in  FIG. 4B , the metal filling  50 ′ is etched below the upper side OS of the substrate  1 , and thereafter the isolation collar  10  is also etched below the upper side OS of the substrate  1  such that the upper side of the etched-back metal filling  50 ′ is below the upper side of the etched-back isolation collar  10 . In particular, the metal filling  50 ′ is etched back in NH 4 OH/H 2 O 2  solution which is selective with respect to polysilicon and silicon oxide.  
         [0048]     Finally, as shown in  FIG. 4C , the third conductive filling  70  made of polysilicon is provided in the trench in order to establish the contact region KS to the surrounding substrate  1 . Then, a part of the third conductive filling  70  is removed and replaced by the isolation filling  80  made of silicon dioxide in order to provide the isolation region IS.  
         [0049]     Also in this embodiment, the major part of the inner capacitor electrode is made of the first metal liner layer  40  and the metal filling  50 ′ which are made of TiN. Thus, the RC delay of the trench capacitor according to this embodiment is also remarkably lowered in comparison to the prior art.  
         [0050]     Although the present invention has been described with respect to two preferred embodiments, it is not limited thereto, but can be modified in various manners which are obvious for the person skilled in the art.  
         [0051]     Particularly, the selection of the materials is only an example and can be varied variously.