Abstract:
The present invention relates to a semiconductor device and a method for isolating the same. The semiconductor device includes: a silicon substrate provided with a trench including at least one silicon pillar at a bottom portion of the trench, wherein the silicon pillar become sidewalls of micro trenches; and a device isolation layer selectively and partially filled into the plurality of micro trenches.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor device; and, more particularly, to a method for isolating device elements of a semiconductor device by applying a shallow trench isolation process and a semiconductor device provided with a plurality of devices isolated by the method.  
       DESCRIPTION OF RELATED ARTS  
       [0002]     As known, a device isolation technology has been employed to electrically isolate individual device elements such as transistors and capacitors during fabrication of a semiconductor integration circuit. Among various processes of the device isolation technology, a local oxidation of silicon (LOCOS) process and a shallow trench isolation (STI) process are commonly adopted.  
         [0003]     The LOCOS method is a method of forming a nitride layer-based mask pattern on an active region of a silicon substrate and thermally oxidating the silicon substrate with use of the mask pattern as a mask. However, the LOCOS method has a disadvantage that a bird&#39;s beak phenomenon occurs at an interfacial surface between the oxide layer and the silicon substrate. Thus, it is limited to apply the LOCOS method to highly integrated device elements. As a result of this limitation, the STI method is more widely employed in highly integrated device elements since the STI method forms a device isolation region by forming a shallow trench in a substrate and then burying an oxide layer into the trench.  
         [0004]      FIGS. 1A  to  1 C are cross-sectional views for describing a device isolation method by applying a conventional STI process.  
         [0005]     Referring to  FIG. 1A , an oxide layer  11  and a nitride layer  12  are sequentially deposited on a substrate  10 . The oxide layer  11  and the nitride layer  12  are then patterned by employing an etching process with use of a device isolation mask to thereby make a device isolation region of the substrate  10  exposed. Herein, the oxide layer  11  serves as a buffer layer for preventing generation of stress caused by a direct contact between the substrate  10  and the nitride layer  12 . The nitride layer  12  serves as a mask for an etching process for forming a trench and an etch stop layer during a subsequent chemical mechanical polishing (CMP) process.  
         [0006]     Next, the exposed substrate  10  is etched to a predetermined depth to thereby form a trench. Then, optionally, a liner nitride layer  14  is formed along the above resulting structure under interposition of a lateral oxide layer  13  on sidewalls of the trench. Herein, the liner nitride layer  14  is formed to improve device characteristics by suppressing a boron segregation phenomenon and prevent induction of stress caused by an additional oxidation of the sidewalls during a subsequent oxidation process. Thereafter, a device isolation layer  15  is deposited on an entire surface of the above substrate structure in a manner to completely fill the trench on which the liner nitride layer  14  is formed. That is, the device isolation layer  15  has a gap-fill property.  
         [0007]     Referring to  FIG. 1B , the above resulting structure including the device isolation layer  15  is subjected to a CMP process which makes a surface of the nitride layer  12  exposed.  
         [0008]     Referring to  FIG. 1C , the nitride layer  12  is removed by a wet etching process.  
         [0009]     The above described STI process is capable of overcoming limitations of the LOCOS method, and thus, being applicable to highly integrated device elements. Hence, the STI process is the most widely applied device isolation method. However, the STI process still has limitations in its applicability. Particularly, although inside of a device isolation layer, e.g., a trench, should be filled with an insulation layer such as an oxide layer by employing a chemical vapor deposition (CVD) method, it is not easy to fill the trench with the insulation layer through the use of a conventional method due to the decreased widths of device elements to be isolated.  
         [0010]     As the width of a trench type device isolation layer decreases, it is highly probable that electric fields from adjacently disposed cells affect other neighboring cells. Thus, scaling down in device integration conversely results in the higher probability that the electric fields from the neighboring cells causes carriers to be inversed along sidewalls of a device isolation layer, and thus further resulting in degradation of refresh characteristic.  
         [0011]     Meanwhile, in a sub-0.1 μm dynamic random access memory device (DRAM) technology, a metal shield embedded (MSE)-STI structure is contrived to obtain an effect of shielding electric fields. The MSE-STI structure is formed by filling the trench with polysilicon. Particularly,  FIG. 2  shows the MSE-STI structure disclosed in an article by Yongjik Park et. al., entitled “COB Stack DRAM Cell Technology beyond 100 nm Technology Node,”  Technology Digest of IEDM,  391-394 (2001), the entire contents of which being incorporated herein by reference.  
         [0012]     With respect to a fabrication process of a device isolation layer, an etch-back process is accompanied to fill a trench with polysilicon. However, after the etch-back process, polysilicon residues are created on sidewalls of a pad nitride layer, degrading device characteristics. Also, it is necessary to form an oxide layer for device isolation on top of the polysilicon after the etch-back process by performing a thermal oxidation process. However, during the thermal oxidation process, the polysilicon expands extensively, and this volume expansion might severely exert stress to the substrate.  
       SUMMARY OF THE INVENTION  
       [0013]     It is, therefore, an object of the present invention to provide a device isolation structure in a semiconductor device capable of preventing leakage current generation caused by an inversion occurring along sidewalls of a device isolation layer and of reducing resistance capacitance (RC) delays by forming a cavity having a small capacitance within a trench.  
         [0014]     It is another object of the present invention to provide a method for isolating devices in a semiconductor device capable of easily filling a trench having a high aspect ratio with a device isolation layer, of preventing silicon residues from being generated on sidewalls of a pad nitride layer by filling silicon for shielding an electric field within a trench and of preventing stress exertion caused by a thermal oxidation process.  
         [0015]     In accordance with an aspect of the present invention, there is provided a semiconductor device, including: a silicon substrate provided with a trench including at least one silicon pillar at a bottom portion of the trench, wherein the silicon pillar become sidewalls of micro trenches; and a device isolation layer selectively and partially filled into the trench and the plurality of micro trenches.  
         [0016]     In accordance with another aspect of the present invention, there is also provided a method for isolating devices of a semiconductor device, including the steps of: forming a primary trench by etching a silicon substrate in a device isolation region; forming at least one secondary trench by etching plural places of the silicon substrate disposed at a bottom surface of the primary trench; and burying a device isolation layer within the primary trench and the secondary trench.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above and other objects and features of the present invention will become better understood with regard to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0018]      FIGS. 1A  to  1 C are cross-sectional views for describing a conventional method for forming a device isolation layer in a semiconductor device;  
         [0019]      FIG. 2  shows a conventional metal shield embedded-shallow trench isolation (MSE-STI) structure;  
         [0020]      FIG. 3  is a cross-sectional view showing a device isolation structure in a semiconductor device in accordance with a preferred embodiment of the present invention; and  
         [0021]      FIGS. 4A  to  4 F are cross-sectional views for describing a method for isolating device elements of a semiconductor device in accordance with the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     Hereinafter, a preferred embodiment of the present invention will be described in detail along with the accompanying drawings.  
         [0023]      FIG. 3  is a cross-sectional view showing a device isolation structure in accordance with a preferred embodiment of the present invention.  
         [0024]     As shown, a trench T is formed in a device isolation region of a silicon substrate  301 . The trench T includes a primary trench T 1  having the same width as that of a designated device isolation region and a plurality of secondary trenches T 2  formed at a bottom of the primary trench T 1 . The primary trench T 1  is defined by a device isolation mask and an etching, while the secondary trench T 2  is defined by finely etching predetermined regions of the silicon substrate  301  disposed at a bottom surface of the primary trench T 1 . As simultaneous to the formation of the plurality of secondary trenches T 2 , a plurality of silicon pillars  320 A extended upwardly from a dotted line D become existed between the secondary trenches T 2 .  
         [0025]     A first oxide layer  303  is covered with the inner side surface of the trench T, and a second oxide layer  304  is formed on top of the first oxide layer  303  to fill the trench T. Herein, the first oxide layer  303  is a thermal oxide layer, while the second oxide layer  304  is an oxide layer deposited by a chemical vapor deposition (CVD) method.  
         [0026]     As shown, if the silicon pillars  320 A exist between the secondary trenches T 2 , it is possible to have an effect of shielding electric fields as simultaneously as to prevent leakage currents from being generated due to an inversion occurring along sidewalls of a device isolation layer because of an elongated distance between isolated devices. That is, it is possible to attain effects of shielding electric fields and preventing the leakage current generation even within a limited area of an ultra-highly integrated device requiring a fine device isolation structure. Also, since the silicon pillars  320 A are not formed through a deposition process and an etch-back process inside of trench T, it is possible to solve a problem in silicon residue generation as described in  FIG. 2 .  
         [0027]     Since the width of each secondary trench T 2  is very small, the second oxide layer  304  is not completely filled thereinto. As a result, there is formed a plurality of cavities C. This cavity C has a very small capacitance, and thus, resulting in an effect of reducing a capacitance that causes resistance capacitance (RC) delays. The cavities C can be formed only in a partial number of the secondary trenches T 2 . In addition, it is easier to fill the trench T with the second oxide layer  304  since it is not necessary to completely fill the secondary trenches T 2 .  
         [0028]     Because a conducting layer such as a polysilicon layer is not used for filling the trench in this preferred embodiment, there is no necessity for performing a thermal oxidation process after the trench-filling process. Therefore, it is possible to prevent stress induction caused by the thermal oxidation performed after the trench-filling process.  
         [0029]      FIGS. 4A  to  4 F are cross-sectional views for describing a method for isolating device elements of a semiconductor device in accordance with the preferred embodiment of the present invention.  
         [0030]     Referring to  FIG. 4A , a buffer oxide layer  402  and a pad nitride layer  403  are sequentially formed on a silicon substrate  401  to be used as an etch mask for use in an etching process for forming a trench and as an etch stop layer during a chemical mechanical polishing (CMP) process.  
         [0031]     The buffer oxide layer  402  plays a role in reducing a stress created by a direct contact made between the silicon substrate  401  and the pad nitride layer  403 . Although the preferred embodiment exemplifies the buffer oxide layer  402  formed in only one layer, it is still possible to form a stack of layers of polysilicon and oxide and an oxynitride layer for the same purpose. Also, it is possible to omit the formation of the buffer oxide layer  402 . Also, in addition to the use of a silicon wafer, the substrate  401  can be formed of a silicon epitaxial layer without departing from the scope and sprit of the present invention.  
         [0032]     The pad nitride layer  403  and the buffer oxide layer  402  are patterned by performing a lithography process with use of a device isolation mask, thereby obtaining a patterned pad nitride layer  403 A and a patterned buffer oxide layer  402 A as shown in  FIG. 4B . The silicon substrate  401  is then etched to a predetermined depth by using the patterned pad nitride layer  403 A as an etch mask. From this etching, a primary trench  404  is formed. In accordance with the preferred embodiment of the present invention, the depth and the width of the primary trench  404  is formed in consideration of the total intended depth of the finally formed trench.  
         [0033]     In more detail of the formation of the primary trench  404 , a photoresist is coated on the pad nitride layer  403  and is patterned by performing a photo-exposure and developing process with use of the device isolation mask. Then, the pad nitride layer  403  and the buffer oxide layer  402  are etched by using the photoresist pattern as a mask. This etching continues until a portion of the silicon substrate  401  is etched by using the patterned pad nitride layer  403 A as an etch mask. Thereafter, the remaining photoresist pattern is removed.  
         [0034]     Referring to  FIG. 4C , a first oxide layer  405  is formed on inner walls of the primary trench  404  by performing a thermal oxidation process. Hemispherical silicon grains (HSG)  406  are formed on the above resulting structure. The first oxide layer  405  serves as an etch mask for forming a secondary trench. In addition to the use of oxide for being used as the etch mask, it is still possible to use a thin layer having a different etch selectivity from silicon. Examples of such thin layer are an oxide layer and a stack layer of nitride and oxide.  
         [0035]     Referring to  FIG. 4D , an anisotropic etching process is performed to etch the first oxide layer  405  by using the HSG  406  as a mask. More specifically, the first oxide layer  405  disposed on a bottom surface of the primary trench  404  is locally etched at several regions to thereby form a first oxide layer pattern  405 A. Geometrically, the HSG  406  has groove parts corresponding to grain boundaries and mounted parts corresponding to silicon grains, and the first oxide layer  405  is etched at the groove parts.  
         [0036]     Referring to  FIG. 4E , the silicon substrate  401  exposed at the bottom surface of the trench  404  is then etched by using the first oxide layer pattern  405 A as a mask. From this etching, a plurality of secondary trenches  407  and a plurality of silicon pillars  401 A disposed between a first secondary trench  407 A and a second secondary trench  407 B are formed.  
         [0037]     Referring to  FIG. 4F , a second oxide layer  408  is thinly grown along a trench profile by carrying out a thermal oxidation process. Afterwards, a third oxide layer  409  is filled into the primary trench  404  and the secondary trenches  407  by employing a CVD method. Thereafter, conventionally performed processes such as a CMP process and a removal of the patterned pad nitride layer  403 A proceed to complete the formation of the device isolation layer.  
         [0038]     Since the secondary trenches  407  have a fine structure, in accordance with the preferred embodiment of the present invention, the third oxide layer  409  partially fills each of the secondary trenches  407 , thereby forming a plurality of cavities  410 . Especially, the CMP process continues until a surface of the patterned pad nitride layer  403 A is exposed. Subsequent to the CMP process, the patterned pad nitride layer  403 A is removed by being dipped into a wet etching solution for use in a removal of a nitride layer. For instance, phosphoric acid (H 3 PO 4 ) is an example of such wet etching solution.  
         [0039]     Although  FIG. 3  and  FIGS. 4A  to  4 F show three of the silicon pillars  401 A, the number of the silicon pillars is not important but it should be noted that there exists at least one silicon pillar disposed between the secondary trenches. As a result, it is possible to provide an effect of shielding electric fields. Also, a distance between isolated devices is elongated and thus, it is possible to prevent the leakage current generation caused by an inversion occurring along sidewalls of the trench even in a very highly integrated device.  
         [0040]     In addition, the cavity formation caused by an incomplete filling of the secondary trenches provides an additional effect of decreasing a capacitance. Furthermore, since it is necessary to completely fill only the primary trench with the oxide layer for use in the device isolation, i.e., the third oxide layer in this preferred embodiment, a depth of a hollow to be filled is also decreased, thereby easing the burial of the oxide layer for use in a device isolation layer.  
         [0041]     The present application contains subject matter related to Korean patent application No. KR 2003-0098450, filed in the Korean Patent Office on Dec. 29, 2003, the entire contents of which being incorporated herein by reference.  
         [0042]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope and spirit of the invention as defined in the following claims.