Abstract:
A method for forming a buried split word line structure is provided. The method comprises the following steps. At first, a substrate having a trench therein is provided. Two liners are formed to a first thickness on sidewalls of the trench. Then, the trench is filled with a first insulating layer to a first height. The two liners are removed. Finally, a conductive material is deposited to a second height between and adjacent to the first insulating layer and the trench. Here, the first height is greater than the second height.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the fabrication of semiconductor integrated circuit structures, and more particularly to the formation of buried split word line structures in memory cells. 
     2. Description of the Related Art 
     Semiconductor memories store bits of information in arrays of memory cells. For example, a dynamic random access memory (DRAM) cell typically includes an access field effect transistor (FET) and a storage capacitor. Some types of memory cells have buried word and bit lines. Memory cell word and bit lines may be buried by forming trenches in a semiconductor substrate and filling the trench with metal. Storage capacitors can be formed on the substrate surface or in the metal layers disposed above the substrate. For example, some types of DRAM cells have buried split word lines formed above buried bit lines. The buried split word lines extend in trenches orthogonal to the buried bit lines. 
     In fabricating semiconductor devices such as DRAMs, buried split word line structure is used to provide the memory cells in the adjacent rows for separately gating the access FETs therein.  FIGS. 1A-E  describe various processing techniques of a method of fabricating a buried split word line structure according to prior art. Referring to  FIG. 1A , a crystalline silicon substrate  102  is covered with a layer of pad nitride  104 , such as silicon nitride (Si 3 N 4 ). Here, the pad nitride  104  serves as a hard mask. A photoresist (not shown) is deposited over the hard mask. The photoresist is exposed, patterned and etched to remove exposed portions. Then, the semiconductor wafer  100  is exposed to an etch process to transfer the photoresist pattern to the hard mask. Portions of the wafer  100  not covered by the hard mask are etched to form word line trenches within the wafer  100  using the hard mask to pattern the word line trenches. The substrate  102  is etched off to a preset depth, which forms the word line trenches  120 . The photoresist is then removed prior to any further processing steps. 
     Referring to  FIG. 1B , gate oxide (silicon dioxide, SiO 2 )  106  is formed on the exposed sidewalls  122  and bottom portions  124  in respective trenches  120 , such as by In-situ steam generation (ISSG) oxidation. A glue layer  108 , such as TiN, is formed on gate oxide  106 . A conductive layer  110  is then formed over the working surface of the wafer  100 , including filling the word line trenches  120  by chemical vapor deposition (CVD) of a refractory metal, such as tungsten or polysilicon. The working surface is then planarized, such as by chemical mechanical polishing/planarization (CMP). The glue layer  108  and the conductive layer  110  are dry etched (RIE) to form recesses in the word line trenches  120 . 
     Referring to  FIG. 1C , an oxide layer  112  is then deposited to fill the trenches  120 . Portions of the oxide layer  112  are removed, such as by isotropic etching, leaving oxide spacers  112   a  and  112   b  along sidewalls  122  of the trenches  120 . Afterward, the trenches  120  are etched through the conductive layer  110  and the glue layer  108  and particularly into the substrate  102  to form recesses in the substrate  102 . Thus, the conductive layer  110  is split into two halves  110   a  and  110   b  and the glue layer  108  is split into two halves  108   a  and  108   b  along sidewalls  122  of the trenches  120  as shown in  FIG. 1D . In  FIG. 1E , isolation material  114 , such as oxide, is formed over the working surface of the wafer  100 , filling the trenches  120 . CMP or other suitable planarization process is used to remove portions of isolation material  114  above the nitride layer  104 . 
     In general, the height y of the split word lines ( 110   a  and  110   b ) is related to the channel length of the access FET while the width x of the split word lines ( 110   a  and  110   b ) is related to the sheet resistance of the gate region as shown in  FIG. 1E . The etched depth of the glue layer  108  and the conductive layer  110  determine the height y of the split word lines. Consistency of the etched depth of the glue layer  108  and the conductive layer  110  depends on the capability of manufacturing equipments. In other words, the height y may vary greatly from equipment to equipment. On the other hand, both the thickness of the oxide spacers  112   a  and  112   b  and the lateral etching rate during the above tungsten/silicon etching process determine the width x of the split word lines. However, it is difficult to precisely control the above factors during etching processes. Thus, there is a need in the art to provide a more stable and consistent dimension of the buried split word line structure. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problems, an object of the invention is to provide a method for forming a buried split word line structure to ensure a more stable and consistent dimension of the buried split word line structure. 
     According to an embodiment of the invention, a method for forming a buried split word line structure is provided. The method comprises the following steps. At first, a substrate having a trench therein is provided. Then, a conductive material conformal to the trench is formed. A first insulating layer within the trench adjacent the conductive material is deposited. Next, the first insulating layer is etched back to a first etched depth. The conductive material is etched back to a second etched depth in accordance with the first etched depth to thereby form a U-shaped gate conductor. Then, a second insulating layer is deposited above the U-shaped gate conductor and conformal to the trench. Finally, portions of a third insulating layer, the U-shaped gate conductor and the substrate are etched away to form a recess in substrate so that the U-shaped gate conductor is split into two halves. Here, the third layer is formed above the U-shaped gate conductor and the second etched depth is greater than the first etched depth. 
     According to another embodiment of the invention, a method for forming a buried split word line structure is provided. The method comprises the following steps. At first, a substrate having a trench therein is provided. Two liners are formed to a first thickness on sidewalls of the trench. Then, the trench is filled with a first insulating layer to a first height. The two liners are removed. Finally, a conductive material is deposited to a second height between and adjacent to the first insulating layer and the trench. Here, the first height is greater than the second height. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIGS. 1A-E  describe generally various processing techniques of a method of fabricating a buried split word line structure according to prior art. 
         FIG. 2  is a flow chart illustrating a method for forming a buried split word line structure in accordance with an embodiment of the invention. 
         FIGS. 3A-G  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 2 . 
         FIGS. 4A-4B  illustrate additional steps used to form the buried split word line structure according to another embodiment. 
         FIG. 5  is a flow chart illustrating a method for forming a buried split word line structure in accordance with another embodiment of the invention. 
         FIGS. 6A-G  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 5 . 
         FIG. 7  is a flow chart illustrating a method for forming a buried split word line structure in accordance with another embodiment of the invention. 
         FIGS. 8A-F  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the present invention will be discussed, followed by a discussion of some advantage of the invention. Only one trench is shown in each figure, although many trenches and other components of a memory cell are present in the semiconductor memories. 
       FIG. 2  is a flow chart illustrating a method for forming a buried split word line structure in accordance with an embodiment of the invention.  FIGS. 3A-G  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 2 . The steps of  FIG. 2  are illustrated in  FIGS. 3A-G . 
     In  FIG. 3A , the processing steps described above with respect to  FIG. 1A  have been carried out. Gate oxide  302  is grown on the exposed sidewalls  122  and bottom portions  124  in respective trenches  120  in step  212 . A conformal glue layer  304 , such as TiN, is deposited to a first thickness on gate oxide  302 , such as by atomic layer deposition (ALD). A conformal conductive layer  306 , such as tungsten or polysilicon, is then deposited to a second thickness on the glue layer  304 , such as by ALD, to form a word line structure in step  214 . Other suitable conductive materials could also be used. In step  214 , the first thickness of the conformal glue layer  304  and the second thickness of the conductive layer  306  are equivalent to the width x of the split word lines, so they are well defined before ALD is performed. It should be noted that ALD is only utilized as embodiments and not limitation of the invention. In the actual implementations, any other depositions that can be used to form the above conformal glue layer  304  and conformal conductive layer  306  also fall in the scope of the invention. 
     Next, in step  216 , center oxide  308  is formed over the working surface of the wafer  300 , including filling the trenches  120 . CMP or other suitable planarization process is used to remove portions of center oxide  308  above conductive layer  306 . Center oxide  308  is then recessed back to a depth of y 1  in the trenches  120  by wet chemical etching, such as in a solution of hydrofluoric acid (HF), as shown in  FIG. 3B . The etched depth y 1  of center oxide  308  is measured prior to any further processing steps. 
     Then, in step  218 , according to the etched depth y 1  of the oxide recesses as shown in  FIG. 3C , the glue layer  304  and the conductive layer  306  are recessed back to a etched depth y 2  by wet chemical etching, such as in a solution of HF. Here, the etched depth y 2  is greater than the etched depth y 1 . According to the invention, the etched depth y 1  of the oxide recesses is used as a reference level to dynamically adjust the etched depth y 2  of the glue layer  304  and the conductive layer  306 . For example, if the etched depth y 1  is too deep, the manufacturing equipment is dynamically adjusted to etch the glue layer  304  and the conductive layer  306  to a relatively shallow depth. By contrast, if the etched depth y 1  is too shallow, the manufacturing equipment is dynamically adjusted to etch the glue layer  304  and the conductive layer  306  to a relatively deep depth. Here, the depth y 2  increases as the etched depth y 1  decreases. In this way, independent of the capability of the manufacturing equipments, the height y of word lines is determined by the two processing steps  216  and  218 . 
     In step  220 , center oxide  308  is removed by wet chemical etching, such as in a solution of HF, as shown in  FIG. 3D . Then, in step  222 , a conformal oxide layer  310  is formed over the trenches  120 , such as by ALD, as shown in  FIG. 3E . Next, in step  224 , an oxide spacer etch is performed on the oxide layer  310 , leaving oxide spacers  310   a  and  310   b . Referring to  FIG. 3F , the trenches  102  are etched through the oxide layer  310 , the conductive layer  306 , the glue layer  304  and particularly into the substrate  102  to form a pair of split word lines ( 306   a  and  306   b ) and a pair of split glue layers ( 304   a  and  304   b ). Next, in step  226 , isolation material  312 , such as SiO 2 , is deposited to fill the trenches  120 . Finally, in step  228 , CMP or other suitable planarization technique is used to remove portions of isolation material  312  above the surface of the nitride layer  104  as shown in  FIG. 3G . 
     It should be noted that the step  220  is optional. In an alternative embodiment, the step  220  is omitted from the flow chart of  FIG. 2 , thus representing in dotted lines in  FIG. 2 .  FIGS. 4A-4B  illustrate additional steps used to form the buried split word line structure according to another embodiment. As mentioned above, in an alternative embodiment, the step  220  is omitted from the flow chart of  FIG. 2 . Accordingly, the step  218  is directly followed by the process steps  222 - 228  described below with respect to  FIGS. 4A-4B  and  3 G. 
     In  FIG. 4A , a conformal oxide layer  402  is formed over the trenches  120  (step  222 ), such as by ALD. Since center oxide  308  is not removed in this embodiment, the depth of the opening of the trench  120  in  FIG. 4A  is shallower than that in  FIG. 3E . Next, in step  224 , an oxide spacer etch is performed on the oxide layer  402  and center oxide  308 , leaving oxide spacers  402   a  and  402   b . Referring to  FIG. 4B , the trenches  120  are etched through the oxide layer  402 , center oxide  308 , the conductive layer  306 , the glue layer  304  and particularly into the substrate  102  to form a pair of split word lines ( 306   a  and  306   b ) and a pair of split glue layers ( 304   a  and  304   b ). Next, in step  226 , isolation material  312 , such as SiO 2 , is deposited to fill the trenches  120 . Finally, in step  228 , the working surface is planarized, such as by CMP, as shown in  FIG. 3G . 
     In the above two embodiments, grouping control (steps  216  and  218 ) determines the height y of the split word lines ( 110   a  and  110   b ) and the step  214  (i.e., depositing conformal glue layer/conductive layer) determines the width x of the word line ( 110   a  or  110   b ). The lateral etching is significantly reduced during the whole processing steps. Compared with convention fabrication process, the invention provides a more stable and consistent dimension of the buried split word line structure. 
       FIG. 5  is a flow chart illustrating a method for forming a buried split word line structure in accordance with another embodiment of the invention.  FIGS. 6A-G  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 5 . The steps of  FIG. 5  are illustrated in  FIGS. 6A-G . 
     In  FIG. 6A , the processing steps described above with respect to  FIG. 1A  have been carried out. A conformal insulating layer  602 , such as oxide layer (hereinafter referred to as “conformal oxide layer  602 ”), is formed over the trenches  120 , such as by ALD, in step  512 . In this step, the thickness of the conformal oxide layer  602  determines the width x of the split word lines, so the thickness of the conformal oxide layer  602  needs to be well defined before ALD is performed. It should be noted that ALD is only utilized as embodiments and not limitation of the invention. In the actual implementations, any other depositions that can be used to form the above conformal insulating layer  602  also fall in the scope of the invention. 
     Next, in step  514 , an oxide spacer etch is performed on the oxide layer  602  to form oxide spacers  602   a  and  602   b  as shown in  FIG. 6B . The trenches  102  are etched through the oxide layer  602  and particularly into the substrate  102  to form recesses in the substrate  102 . The next step  516  is to form bottom oxide  604  at the bottom of the recesses and top oxide  605  over the nitride layer  104 , such as by In-situ steam generation (ISSG) oxidation as shown in  FIG. 6C . Here, top oxide  605  serves to protect the nitride layer  104 . Then, in step  518 , center nitride  606  is deposited to fill the trenches  120 . portions of center nitride  606  above the nitride layer  104  is removed by wet chemical etching, such as in a solution of Phosphoric acid (H 3 PO 4 ). Center nitride  606  is then recessed back to a depth by wet chemical etching, such as in a solution of H 3 PO 4 , as shown in  FIG. 6D . The shallower the etched depth, the better the isolation of the split word lines  610   a  and  610   b . By contrast, the deeper the etched depth, the easier the deposition of the conductive layer  610 . 
     Then, in step  520 , the oxide layer  602  and top oxide  605  are removed by wet chemical etch, such as in a solution of HF, as shown in  FIG. 6E . Referring to  FIG. 6F , the next step  522  is to form gate oxide  608  on the exposed sidewalls  122  and bottom portions  124  in respective trenches  120  and then form a glue layer  609 , such as TiN, on gate oxide  608 . Next, a conductive layer  610 , such as tungsten or polysilicon, is deposited to fill the trenches  120 . The conductive layer  610  is then etched back to form buried split word lines  610   a  and  610   b  in the trenches  120 . Next, in step  526 , isolation material  612 , such as SiO 2 , is deposited to fill the trenches  120 . Finally, in step  528 , the working surface is planarized, such as by CMP, as shown in  FIG. 6G . 
       FIG. 7  is a flow chart illustrating a method for forming a buried split word line structure in accordance with another embodiment of the invention.  FIGS. 8A-F  describe generally various processing techniques of a method of fabricating a buried split word line structure with reference to  FIG. 7 . The steps of  FIG. 7  are illustrated in  FIGS. 8A-F . 
     In  FIG. 8A , the processing steps described above with respect to  FIG. 1A  have been carried out. In step  718 , an insulating layer  802  (e.g., oxide, hereinafter referred to as “oxide layer  802 ”) is deposited to fill the trenches  120 . CMP or other suitable planarization process is used to remove portions of the oxide layer  802  above the nitride layer  104 . The oxide layer  802  is then etched back to form recesses in the trenches  102 . Afterward, a conformal hard mask  804 , such as titanium nitride (TiN), is formed over the trenches  102 , such as by ALD, in step  714 . In this step, the thickness of the hard mask  804  determines the width x of the split word lines, so the thickness of the conformal hard mask  804  needs to be well defined before ALD is performed. It should be noted that TiN and ALD are utilized as embodiments and not limitations of the invention. In the actual implementations, other suitable materials could also be used as hard mask and any other depositions that can be used to form the above conformal hard mask  804  also fall in the scope of the invention. 
     In step  716 , referring to  FIG. 8B , a spacer etch is performed on the hard mask  804  to form spacers  804   a  and  804   b . The trenches  120  are etched through the hard mask  804  and the oxide layer  802  and particularly into the substrate  102  to form recesses in substrate  102 . The next step  718  is to form bottom oxide  806  at the bottom of the recesses and top oxide  807  over the nitride layer  104 , such as by ISSG oxidation as shown in  FIG. 8C . Here, top oxide  807  serves to protect the nitride layer  104 . Then, in step  720 , center nitride  808  is deposited to fill the trenches  120 . Portions of center nitride  808  above the nitride layer  104  is removed by wet chemical etching, such as in a solution of Phosphoric acid (H 3 PO 4 ). Center nitride  808  is then recessed back to a depth by wet chemical etching, such as in a solution of H 3 PO 4 , as shown in  FIG. 8D . The shallower the etched depth, the better the isolation of the split word lines  810   a  and  810   b . By contrast, the deeper the etched depth, the easier the deposition of the conductive layer  810 . 
     Then, in step  722 , the oxide layer  802 , the hard mask  804 , and top oxide  807  are removed, such as by wet etch, such as in a solution of HF, as shown in  FIG. 8E . Referring to  FIG. 8F , the next step  724  is to form gate oxide  811  on the exposed sidewalls  122  and bottom portions  124  in respective trenches  120  and then form a glue layer  809 , such as TiN, on gate oxide  811 . Next, a conductive layer  810 , such as tungsten or polysilicon, is deposited to fill the trenches  120 , in step  726 . The conductive layer  810  is then etched back to form buried split word lines  810   a  and  810   b  in the trenches  120 . Next, in step  728 , isolation material  812 , such as SiO 2 , is deposited to fill the trenches  120 . Finally, in step  730 , the working surface is planarized, such as by CMP, as shown in  FIG. 8F . 
     As can be observed from  FIG. 6C , a liner ( 602   a  or  602   b ) along each sidewall  122  of the trench  120  comprises an oxide layer  602  only. By comparison, in  FIG. 8C , a liner ( 803   a  or  803   b ) along each sidewall  122  of the trench  120  comprises two layers, namely an oxide layer ( 802   a  or  802   b ) and a hard mask ( 804   a  or  804   b ). In operation, since the etching process of the oxide layer  802  does not affect the hard mask layer  804 , the profile of the hard mask spacers  804   a  and  804   b  is better than that of the oxide spacers  602   a  and  602   b . With respect to the embodiments of  FIGS. 5 and 7 , it should be noted that the invention is not limited to the number of layers forming the liner. In the actual implementations, other suitable number of layers, such as three layers, could also be used to form the liner. In the embodiment of  FIGS. 5 and 7 , the width x of the split word lines is defined in steps  512  and  714 , respectively, such as by ALD. After buried split word lines are formed or deposited in the trenches  120  (in steps  524  and  726 ), no further etching process will affect the width x of the split word lines. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention should not be limited to the specific construction and arrangement shown and described, since various other modifications may occur to those ordinarily skilled in the art.