BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a method for fabricating semiconductor devices. More specifically, the present invention relates to a method for making recessed-gate Metal-Oxide-Semiconductor (MOS) transistor of Dynamic Random Access Memory (DRAM) devices.  
         [0003]     2. Description of the Prior Art  
         [0004]     Integrated circuit devices are continually being made smaller in order to increase speed, make the device more portable and to reduce the cost of manufacturing the device. However, certain designs have a minimum feature size, which cannot be reduced without compromising the integrity of electrical isolation between devices and consistent operation of the device. For example, dynamic random access memory devices (DRAMs), which use vertical metal oxide semiconductor field effect transistors (MOSFETs) with deep trench (DT) storage capacitors, have a minimum features size of approximately 90 nm˜0.15 μm. Below that size, the internal electric fields exceed the upper limit for storage node leakage, which decreases retention time below an acceptable level. Therefore, there is a need for different methods and/or different structures to further reduce the size of integrated circuit devices.  
         [0005]     With the continued reduction in device size, sub-micron scale MOS transistors have to overcome many technical challenges. As the MOS transistors become narrower, that is, their channel length decreases, problems such as junction leakage, source/drain breakdown voltage, and data retention time become more pronounced.  
         [0006]     One solution to decrease the physical dimension of ULSI circuits is to form recessed gate or “trench-type” transistors, which have a gate electrode buried in a groove formed in a semiconductor substrate. This type of transistor reduces short channel effects by effectively lengthening the effective channel length by having the gate extend into the semiconductor substrate.  
         [0007]     The recess-gate MOS transistor has a gate insulation layer formed on sidewalls and bottom surface of a recess etched into a substrate, a conductive filling the recess, contrary to a planar gate type transistor having a gate electrode formed on a planar surface of a substrate.  
         [0008]     However, the aforesaid recessed-gate technology has some shortcomings. For example, the recess for accommodating the recessed gate of the MOS transistor is etched into a semiconductor wafer by using conventional dry etching methods. It is difficult to control the dry etching and form recesses having the same depth across the wafer. A threshold voltage (Vt) control problem arises because of recess depth variation. Moreover, the recess lithography to DT (deep trench) overlay may also impact the Vt.  
       SUMMARY OF THE INVENTION  
       [0009]     It is one object of this invention to provide a method of fabricating a recess-gate MOS transistor of DRAM devices in order to solve the above-mentioned problems.  
         [0010]     According to the claimed invention, a method for fabricating a recessed gate MOS transistor device is provided. A semiconductor substrate having a main surface is provided. A pad layer is formed on the main surface. A plurality of trench capacitors is formed in the semiconductor substrate. Each trench capacitor is capped with a trench top oxide layer. The trench top oxide layer has a top surface higher than the main surface. A lithographic and etching process is performed to form a plurality of isolation trenches in the semiconductor substrate. An insulation layer is deposited on the semiconductor substrate and in the isolation trenches. The insulation layer fills the isolation trenches. The insulation layer is etched back such that a top surface of the insulation layer is lower than the top surface of the trench top oxide layer. The pad layer is stripped to expose the semiconductor substrate and the trench top oxide layer. A spacer is formed on sidewalls of the trench top oxide layer. Using the spacer as an etching hard mask, the semiconductor substrate is etched to form a gate trench. A gate dielectric layer is formed on interior surface of the gate trench. A gate material layer is formed on the gate dielectric layer, wherein the gate material layer fills the gate trench.  
         [0011]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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. In the drawings:  
         [0013]      FIG. 1  is a schematic top view showing the layout of the deep trench capacitors in the memory array area according to this invention;  
         [0014]      FIGS. 2-22  are schematic, cross-sectional diagrams illustrating an exemplary method of fabricating a recessed-gate MOS transistor in accordance with one preferred embodiment of this invention; and  
         [0015]      FIG. 23  is a schematic top view of the structure set forth in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0016]     Please refer to  FIGS. 1-22 .  FIG. 1  is a schematic top view showing the layout of the deep trench capacitors in the memory array area according to this invention.  FIGS. 2-22  are schematic, cross-sectional diagrams illustrating an exemplary method of fabricating a recessed-gate MOS transistor in accordance with one preferred embodiment of this invention. As shown in  FIGS. 1 and 2 , a semiconductor substrate  10  having thereon a pad oxide layer  14  and a pad nitride layer  16  is provided. The semiconductor substrate  10  may include but not limited to a silicon substrate, silicon epitaxital substrate or Silicon-On-Insulator (SOI) substrate. Deep trench capacitors  12  are formed within a memory array area  102  of the semiconductor substrate  10 . For the sake of clarity, a peripheral circuit area  104  and both of the I-I′ cross section and II-II′ cross section of the memory array area  102  in  FIG. 1  are shown in the subsequent drawings.  
         [0017]     As shown in  FIG. 2 , the deep trench capacitor  12  comprises a sidewall capacitor dielectric layer  24  and a doped polysilicon  26 . The deep trench capacitor  12  is fabricated using Single-Sided Buried Strap (SSBS) process. The doped polysilicon  26  functions as one electrode of the deep trench capacitor  12 . The method for fabricating the deep trench capacitor  12  is known in the art. For the sake of simplicity, only the upper portions of the deep trench capacitor  12  are shown in figures. It is understood that the deep trench capacitor  12  further comprises a buried plate acting as the other capacitor electrode, which is not shown.  
         [0018]     The aforesaid SSBS process generally comprises the steps of etching back the sidewall oxide dielectric layer and the doped polysilicon (or so-called Poly-2)  26  to a first depth; refilling the recess with another layer of polysilicon (or so-called Poly-3); etching back the Poly-3 to a second depth; forming an asymmetric spacer on interior sidewall of the recess; etching away the Poly-3 and Poly-2 that are not covered by the asymmetric spacer; filling the recess with TTO insulation layer; and chemical mechanical polishing the TTO insulation layer.  
         [0019]     As shown in  FIG. 3 , a silicon oxide layer is deposited over the semiconductor substrate  10  and fills the recesses on the deep trench capacitors  12 . Thereafter, using the pad nitride layer  16  as a polishing stop layer, a chemical mechanical polishing (CMP) process is carried out to planarize the silicon oxide layer, thereby forming a trench top oxide layer  18  on each deep trench capacitor  12 .  
         [0020]     As shown in  FIG. 4 , subsequently, a shallow trench isolation (STI) process is performed to form STI trenches  22  and  20  in the memory array area  102  and in the peripheral circuit area  104  respectively.  FIG. 23  shows a top view of the STI trench structure in  FIG. 4 .  
         [0021]     As shown in  FIG. 5 , a silicon nitride liner  32  is deposited on the semiconductor substrate  10 . The silicon nitride liner  32  has a thickness of about 5-150 angstroms. The silicon nitride liner  32  conformally covers the pad nitride layer  16 , the trench top oxide layer  18  and the interior surfaces of the STI trenches  22  and  20 .  
         [0022]     As shown in  FIG. 6 , after the deposition of the silicon nitride liner  32 , a silicon oxide layer  34  is deposited over the semiconductor substrate  10 . The silicon oxide layer  34  fills the STI trenches  22  and  20 . According to the preferred embodiments, the silicon oxide layer  34  is formed by Chemical Vapor Deposition (CVD) methods such as High-Density Plasma CVD (HDPCVD) process. The STI trenches may not be filled in one step. The STI fill process may include SOG etch back, SiN etch back and oxide fill. The STI fill material may be two or three layers.  
         [0023]     As shown in  FIG. 7 , using the silicon nitride liner  32  as a polishing stop layer, a CMP process is performed to planarize the silicon oxide layer  34 .  
         [0024]     As shown in  FIG. 8 , using the silicon nitride liner  32  as an etching hard mask, a dry etching process is carried out to recess etch the remaining silicon oxide layer  34  to a predetermined depth inside the STI trenches  22  and  20 , for example, 500-1100 angstroms. Preferably, after dry etching the top surface of the silicon oxide layer  34  inside the STI trenches  22  and  20  is lower than the top surface of the silicon nitride liner  32   32 .  
         [0025]     As shown in  FIG. 9 , the pad nitride layer  16  and the overlying silicon nitride liner  32  are stripped off from the surface of the semiconductor substrate  10  by using conventional etching methods such as wet etching involving the use of hot phosphoric acid solution, thereby exposing the pad oxide layer  14 . After the removal of the pad nitride layer  16 , the trench top oxide layer  18  protrudes from the main surface of the semiconductor substrate  10  with a height of about 150-1500 angstroms. An ion implantation process may be carried out to form doping regions of different conductivity types or ion wells (not shown) inside the semiconductor substrate  10 .  
         [0026]     As shown in  FIG. 10 , a conformal spacer layer  38  is blanket deposited over the semiconductor substrate  10  and on the upward protruding trench top oxide layer  18 . According to the preferred embodiments, the spacer layer  38  is a single layer of silicon nitride or a dual layer structure comprising silicon nitride and polysilicon.  
         [0027]     As shown in  FIG. 11 , a photolithographic process is performed to form a photoresist layer  40  that only masks the peripheral circuit area  104 . The photoresist layer  40  protects the spacer layer  38  in the peripheral circuit area  104  but exposes the spacer layer  38  in the memory array area  102 . Thereafter, using the photoresist layer  40  as an etching hard mask, a dry etching process is carried out to anisotropically etch the exposed spacer layer  38 , thereby forming spacer  42  at sidewall of the upward protruding trench top oxide layer  18 .  
         [0028]     As shown in  FIG. 12 , after the formation of the spacer  42 , another dry etching process is performed. Using the spacer  42 , the trench top oxide layer  18  and the silicon oxide layer  34  inside the STI trenches  22  and  20  as etching hard mask, gate trench  60  between the deep trench capacitors  12  is etched into the pad oxide layer  14  and the semiconductor substrate  10  in a self aligned fashion.  
         [0029]     As shown in  FIG. 13 , after etching the gate trench  60 , the photoresist layer  40  covering the peripheral circuit area  104  is removed. A wet etching process is performed to remove the spacer layer  38  in the peripheral circuit area  104  and the spacer  42  in the memory array area  102 . Simultaneously, the exposed silicon nitride liner  32  inside the gate trench  60  is also removed.  
         [0030]     As shown in  FIG. 14 , after removing the spacer layer, another wet etching process is carried out to remove the pad oxide layer  14 . A thermal oxidation process is performed to form a thick gate dielectric layer  62  on the exposed semiconductor substrate  10  and on the surface of the gate trench  60 . The aforesaid thermal oxidation process may be In-Situ Steam Growth (ISSG) process, but not limited thereto.  
         [0031]     As shown in  FIG. 15 , an anisotropic dry etching process is carried out to etch the thick gate dielectric layer  62 , thereby forming spacer  64  on the sidewall of the gate trench  60 . Subsequently, another thermal oxidation process such as ISSG process is performed to form a thin gate dielectric layer  66  on the exposed semiconductor substrate  10  and at the bottom of the gate trench  60 , as shown in  FIG. 16 . However, the gate dielectric layer  66  is not limited to oxide. For example, the gate dielectric layer  66  may be made of high-k dielectric materials.  
         [0032]     As shown in  FIG. 17 , a CVD process such as LPCVD or PECVD process is performed to deposit a polysilicon layer  70  over the semiconductor substrate  10  in the memory array area  102  and in the peripheral circuit area  104 . The polysilicon layer  70  fills the gate trench  60 . The polysilicon layer  70  may be made of metal gate materials such as W, TiN, HfN, Mo, or any combination thereof.  
         [0033]     As shown in  FIG. 18 , the polysilicon layer  70  is etched back by using a dry etching method or wet etching process. After etching, the top surface of the polysilicon layer  70  is lower than the top surface of the trench top oxide layer  18 . At this phase, except the upward protruding trench top oxide layer  18 , the other area of the semiconductor substrate  10  including the memory array area  102  and the peripheral circuit area  104  is covered with the polysilicon layer  70 .  
         [0034]     As shown in  FIG. 19 , after etching back the polysilicon layer  70 , another wet process such as wet etching is performed to etch the trench top oxide layer  18  protruding from the surface of the polysilcion layer  70 . For example, diluted hydrofluoric acid solution may be used to etch the trench top oxide layer  18 . The remaining trench top oxide layer  18  has a top surface that is approximately coplanar with the main surface of the semiconductor substrate  10  (slightly lower than the top surface of the remaining polysilcion layer  70 ).  
         [0035]     As shown in  FIG. 20 , after the etching of the trench top oxide layer  18 , a polysilicon layer  74  is blanket deposited over the semiconductor substrate  10 . A tungsten silicide layer  76  is then formed on the polysilicon layer  74 . A silicon nitride cap layer  78  is then deposited on the tungsten silicide layer  76 . The polysilicon layer  74  covers the polysilicon layer  70  and on the trench top oxide layer  18 . Preferably, the polysilicon layer  74  has a thickness of about 200-900 angstroms. The tungsten silicide layer  76  has a thickness of about 100-800 angstroms. The silicon nitride cap layer  78  has a thickness of about 800-1500 angstroms.  
         [0036]     As shown in  FIG. 21 , subsequently, a photolithographic process and an etching process are performed. A photoresist mask (not shown) is used to define the gate conductor pattern within the memory array area  102  and the logic gate pattern within the peripheral circuit area  104 . Using the photoresist mask as an etching hard mask, the silicon nitride cap layer  78 , tungsten silicide layer  76  and the polysilicon layers  70  and  74  that are not covered by the photoresist mask are etched away, thereby forming recessed gate  80  and gate conductor  82  in the memory array area  102  and forming gate structure  84  in the peripheral circuit area  104 .  
         [0037]     Finally, as shown in  FIG. 22 , after the patterning of the gates, a thermal oxidation process such as rapid thermal process (RTP) is performed to form insulation oxide  90  on respective sidewall of the gates including gate conductors  82  and the gate  84 . After the formation of the insulation oxide  90 , a spacer  96  is formed on sidewall of the gate conductors  82  and the gate  84 .  
         [0038]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
A method of fabricating self-aligned gate trench utilizing TTO spacer is disclosed. A semiconductor substrate having thereon a pad oxide layer and pad nitride layer is provided. Trench capacitors are formed in a memory array region of the semiconductor substrate. Each of the trench capacitors has a trench top oxide (TTO) that extrudes from a main surface of the semiconductor substrate. Spacers are formed on the extruding TTO and are used, after oxidized, as an etching hard mask for etching a recessed gate trench in close proximity to the trench capacitor.