Abstract:
A self-aligned inner gate recess channel in a semiconductor substrate includes a recess trench formed in an active region of the substrate, a gate dielectric layer formed on a bottom portion of the recess trench, recess inner sidewall spacers formed on sidewalls of the recess trench, a gate formed in the recess trench so that an upper portion of the gate protrudes above an upper surface of the substrate, wherein a thickness of the recess inner sidewall spacers causes a center portion of the gate to have a smaller width than the protruding upper portion and a lower portion of the gate, a gate mask formed on the gate layer, gate sidewall spacers formed on the protruding upper portion of gate and the gate mask, and a source/drain region formed in the active region of the substrate adjacent the gate sidewall spacers.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This is a divisional application based on application Ser. No. 10/730,996, filed Dec. 10, 2003 now U.S. Pat. No. 7,154,144, the entire contents of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a recess channel transistor and a method of forming the same. More particularly, the present invention relates to a self-aligned recess channel transistor having an inner gate with a center portion that has a smaller width than a protruding upper portion and a lower portion of the gate and a method of forming the same. 
   2. Description of the Related Art 
   Conventionally, a recess transistor is used to achieve a higher density as a cell size is scaled down to the sub-micron range. As a planar gate size narrows in a recess transistor, however, short channel effects, junction leakage, and source/drain breakdown voltage become significant issues. Conventional recess gates have several disadvantages. A disadvantage of a conventional recess transistor, for example, is a large gate oxide area, which increases a gate loading capacitance. Additionally, another disadvantage of a conventional recess transistor structure is a deep junction depth, which increases a gate to source/drain overlap capacitance and a bit line loading capacitance. A conventional recess gate structure has a further disadvantage of having a weak photo misalign margin. 
   SUMMARY OF THE INVENTION 
   In an effort to overcome at least some of the problems described above, the present invention provides a self-aligned recess channel transistor having a gate with a center portion that has a smaller width than a protruding upper portion and a lower portion of the gate and a method of forming the same. 
   It is a feature of an embodiment of the present invention to provide a self-aligned inner gate recess channel in a semiconductor substrate including a recess trench formed in an active region of the substrate, a gate dielectric layer formed on a bottom portion of the recess trench, recess inner sidewall spacers formed on sidewalls of the recess trench, a gate formed in the recess trench so that an upper portion of the gate protrudes above an upper surface of the substrate, wherein a thickness of the recess inner sidewall spacers causes a center portion of the gate to have a smaller width than the protruding upper portion and a lower portion of the gate, a gate mask formed on the gate layer, gate sidewall spacers formed on the protruding upper portion of gate and the gate mask, and a source/drain region formed in the active region of the substrate adjacent the gate sidewall spacers. 
   Preferably, the recess trench has a width at an opening thereof of about 900 Å and a depth of between about 1300-1800 Å. Preferably, the shallow trench isolation region has a depth of approximately 3000 Å. 
   Preferably, the substrate includes a shallow trench isolation region and the active region includes a well region, a threshold voltage control region, and a source/drain region. 
   The gate dielectric layer may be an oxide layer, an oxynitride layer, an alumina (Al2O3) layer, or a ruthenium oxide (RuO) layer and have a thickness of about 50 Å. 
   Preferably, the recess inner sidewall spacers have a thickness of about 200 Å and are formed of either silicon oxide or silicon nitride. 
   The gate formed in the recess trench may include a first gate layer formed in a bottom portion of the recess trench and a second gate layer formed on the first gate layer in an upper portion of the recess trench, the second gate layer having a lower portion within the recess trench and an upper portion that protrudes above the upper surface of the substrate, wherein a thickness of the recess inner sidewall spacers causes the lower portion of the second gate layer to have a smaller width than the protruding upper portion of the second gate layer and the first gate layer. The first gate layer may be a poly gate layer and have a thickness of about 800 Å. The second gate layer may be a poly gate layer. 
   Preferably, the source/drain region in the active region of the substrate is an n+ source/drain region. 
   It is another feature of an embodiment of the present invention to provide a method of forming a self-aligned inner gate recess channel in a semiconductor substrate including sequentially depositing an oxide mask layer, a poly mask layer, and a photoresist layer on the substrate having an active region, etching the poly mask layer, the oxide mask layer and the active region of the substrate to form a recess trench, forming recess inner sidewall spacers on sidewalls of the recess trench, etching a bottom portion of the recess trench to increase a depth of the recess trench, enlarging a width of the bottom portion of the recess trench below the recess inner sidewall spacers, forming a gate dielectric on the bottom portion of the recess trench, forming a gate in the recess trench so that an upper portion of the gate protrudes above an upper surface of the substrate, wherein a thickness of the recess inner sidewall spacers causes a center portion of the gate to have a smaller width than the protruding upper portion and a lower portion of the gate, forming a gate mask on the gate, forming gate sidewall spacers on the protruding upper portion of the gate and the gate mask, and performing an ion implantation process to form a source/drain region in the active region of the substrate adjacent the gate sidewall spacers. 
   The method may further include enlarging a lower portion of the recess trench, prior to forming the recess inner sidewall spacers on sidewalls of the recess trench. 
   Preferably, etching the poly mask layer, the oxide mask layer and the active region of the substrate to form the recess trench includes etching the poly mask layer using the photoresist layer as a mask and removing the photoresist layer and etching the active region of the substrate using the etched poly mask layer as a mask to form the recess trench and removing the poly mask layer. 
   Preferably, forming the recess inner sidewall spacers includes depositing a spacer layer on the substrate and the recess trench using a LPCVD process and anisotropically etching the spacer layer to form the recess inner sidewall spacers on the sidewalls of the recess trench. 
   It is still another feature of an embodiment of the present invention to provide a method of forming a self-aligned inner gate recess channel in a semiconductor substrate including sequentially depositing an oxide mask layer, a poly mask layer, and a photoresist layer on the substrate having an active region, etching the poly mask layer, the oxide mask layer and the active region of the substrate to form a recess trench, forming a gate dielectric layer within the recess trench, forming and etching a first gate layer to partially fill a lower portion of the recess trench, forming recess inner sidewall spacers on sidewalls of an upper portion of the recess trench above the first gate layer, forming and etching a second gate layer on the first gate layer to form a gate in the recess trench so that an upper portion of the second gate layer protrudes above an upper surface of the substrate, wherein a thickness of the recess inner sidewall spacers causes a lower portion of the second gate layer to have a smaller width than the protruding upper portion of the second gate layer and the first gate layer, forming a gate mask on the second gate layer, forming gate sidewall spacers on the protruding upper portion of the second gate layer and the gate mask, and performing an ion implantation process to form a source/drain region in the active region of the substrate adjacent the gate sidewall spacers. 
   The method may further include enlarging a lower portion of the recess trench, prior to forming the recess inner sidewall spacers on sidewalls of the recess trench. 
   Preferably, forming the first gate layer includes depositing a first gate layer on the substrate and recess trench to fill the recess trench and performing an etchback process on the first gate layer until the first gate layer remains only in the lower portion of the recess trench. 
   Preferably, forming the recess inner sidewall spacers includes depositing a spacer layer on the substrate and the recess trench using a LPCVD process and anisotropically etching the spacer layer to form the recess inner sidewall spacers on the sidewalls of the recess trench. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  illustrates a top view of a layout of a self-aligned recess channel transistor according to a first embodiment of the present invention; 
       FIGS. 2 through 11  illustrate cross-sectional views of stages in a method of forming self-aligned inner gate recess channel transistor according to the first embodiment of the present invention, wherein  FIGS. 2 through 9  and  11  illustrate cross-sectional views taken along line I-I′ of  FIG. 1  and  FIG. 10  illustrates a cross-sectional view taken along line II-II′ of  FIG. 1 ; 
       FIG. 12  illustrates a top view of a layout of a self-aligned recess channel transistor according to a second embodiment of the present invention; 
       FIGS. 13 through 22  illustrate cross-sectional views of stages in a method of forming a self-aligned inner gate recess channel transistor according to the second embodiment of the present invention, wherein  FIGS. 13 through 20  and  22  illustrate cross-sectional views taken along line I-I′ of  FIG. 12  and  FIG. 21  illustrates a cross-sectional view taken along line II-II′ of  FIG. 12 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred and alternate embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Like reference numerals and characters refer to like elements throughout. 
   First Embodiment 
     FIG. 1  illustrates a top view of a layout of a self-aligned recess channel transistor according to a first embodiment of the present invention.  FIGS. 2 through 11  illustrate cross-sectional views of stages in a method of forming a self-aligned inner gate recess channel transistor according to the first embodiment of the present invention. More specifically,  FIGS. 2 through 9  and  11  illustrate cross-sectional views taken along line I-I′ of  FIG. 1  and  FIG. 10  illustrates a cross-sectional view taken along line II-II′ of  FIG. 1 . 
     FIG. 1  illustrates a gate layer  122  formed over an active area  108  and a field area  102  of a semiconductor substrate.  FIG. 1  includes cross-sectional lines I-I′ and II-II′ taken along an x-direction and a y-direction, respectively, of the semiconductor substrate. 
   Referring now to  FIG. 2 ,  FIG. 2  illustrates a cross-sectional view, taken along line I-I′ of  FIG. 1 , of an initial stage in the method of forming the self-aligned inner gate recess channel transistor according to the first embodiment of the present invention. A substrate  100 , including a well region  104 , is provided. Preferably, the well region  104  is formed using an ion implantation process. A shallow trench isolation (STI) region  102  is conventionally formed by anisotropically etching a trench and filling the trench with an insulation layer. The STI region  102  forms a field area of the semiconductor substrate. Preferably, the STI trench has a depth of about 3000 Å. Threshold voltage (Vt) control ions are implanted by an ion implantation process on the well region  104  to form a Vt control region  106 . Subsequently, source/drain (S/D) ions are implanted by an ion implantation process to form a source/drain (S/D) region  108 . The source/drain (S/D) region  108  forms an active area of the semiconductor substrate. Preferably, the source/drain (S/D) region  108  is an N− source/drain (S/D) region. 
   Referring to  FIG. 3 , an oxide mask layer  110  is formed on the active area  108  and the field area  102 . Preferably, the oxide mask layer  110  is formed to a thickness of about 200 Å. Next, a poly mask layer  112  is formed on the oxide mask layer  110  using a low-pressure chemical vapor deposition (LPCVD). Preferably, the poly mask layer  112  is a ploy hard mask layer and is formed to a thickness of about 1000 Å. A recess pattern is then formed on the poly mask layer  112  by depositing a photoresist  114  and performing a photolithographic process. 
   Referring to  FIG. 4 , a first recess hole  116  is formed in the active area  108  using a two-step isotropical etching process. In a first step, the poly hard mask  112  is etched using the photoresist ( 114  of  FIG. 3 ) as a mask and then the photoresist is removed using a photoresist stripping process. In a second step, the active area  108  of the substrate is etched using the etched poly hard mask ( 114  of  FIG. 3 ) as a pattern to form the first recess hole  116 . Subsequently, the poly hard mask  112  is removed. Preferably, the first recess hole  116  has a depth of about 1000 Å. 
   In  FIG. 5 , an optional etching process is performed to enlarge a width of a lower portion of the first recess hole  116 . The etching process may use a chemical dry etch (CDE) process. The first recess hole  116  is preferably enlarged by a thickness of about 200 Å. Preferably, a width of an opening of the first recess hole  116  is about 900 Å. 
   Referring to  FIG. 6 , a spacer layer is deposited on the sides of the first recess hole  116 , the active area  108 , and the field area  102  using a LPCVD process. The spacer layer is then anisotropically etched to form a recess inner oxide spacer  118 . Preferably, the recess inner oxide spacer  118  has a thickness of about 200 Å. Although the inner spacer  118  is described as an inner oxide spacer, the spacer layer may be formed of either silicon oxide (SiO) or silicon nitride (SiN). 
   As shown in  FIG. 7 , a second recess hole  117  is formed by anisotropically etching a bottom of the first recess hole  116  below the recess inner oxide spacer  118 . Preferably, the second recess hole  117  has a depth of about 300 Å. A width L 1  of the first recess hole  116  and the second hole recess  117  is approximately 500 Å. 
   Referring to  FIG. 8 , the initial width L 1  of the second recess hole  117  is enlarged using a chemical dry etching (CDE) process. Preferably, the enlarged width L 2  of the second recess hole  117  is increased to a width of about 900 Å. A final total recess depth of the first recess hole  116  and the second recess hole  117  is preferably about 1300-1800 Å. 
   As shown in  FIG. 9 , a gate dielectric layer  120  is formed on sidewalls of the second recess hole  117 . The gate dielectric layer  120  may be an oxide layer, an oxynitride layer, an alumina (Al2O3) layer, or a ruthenium oxide (RuO) layer. The gate dielectric layer  120  may be formed using a thermal oxidation process. Preferably, the gate dielectric layer  120  has a thickness of about 50 Å. Subsequently, a gate layer and a gate mask layer are formed on the active area  108 , the field area  102 , and the first recess hole  116  using a LPCVD process. A gate  122  and a gate mask  124  are then formed using photolithography and an etching process. An upper portion  125  of the gate  122 , which additionally includes the gate mask  124 , protrudes above an upper surface of the substrate. 
     FIG. 10  illustrates a cross-sectional view, taken along line II-II′ of  FIG. 1 , of the stage in the method of forming the recess gate shown in  FIG. 9 .  FIG. 10  further illustrates a thickness L 3  of the recess inner oxide spacer  118 , which is preferably 200 Å. 
     FIG. 11  illustrates a completed structure of a recess channel according to the first embodiment the present invention. As shown in  FIG. 11 , a gate spacer layer is deposited on the gate mask  124 , the active area  108  and the field area  102 . The gate spacer layer is then anisotropically etched to form a sidewall spacer  128  on the gate  122  and the gate mask  124 . Subsequently, source/drain (S/D) ions  130  are implanted using an ion implantation process on the active area  108  to form an S/D region  130 . Preferably, the S/D ions are N+ ions implanted to form an N+ S/D region. 
   In the first embodiment of the present invention, a recess inner oxide spacer is a relatively thick oxide layer, which decreases a gate loading capacitance and a bit line loading capacitance, thereby increasing a refresh time. In addition, the recess inner oxide spacer improves the photo misalign margin and reduces a S/D junction capacitance, thereby increasing a device speed. 
   Second Embodiment 
     FIG. 12  illustrates a top view of a layout of a self-aligned recess channel transistor according to a second embodiment of the present invention.  FIGS. 13 through 22  illustrate cross-sectional views of stages in a method of forming a self-aligned inner gate recess channel transistor according to the second embodiment of the present invention. More specifically,  FIGS. 13 through 20  and  22  illustrate cross-sectional views taken along line I-I′ of  FIG. 12  and  FIG. 21  illustrates a cross-sectional view taken along line II-II′ of  FIG. 12 . 
     FIG. 12  illustrates a gate layer  222  formed over an active area  208  and a field area  202  of a semiconductor substrate.  FIG. 12  includes cross-sectional lines I-I′ and II-II′ taken along an x-direction and a y-direction, respectively, of the semiconductor substrate. 
   Referring to  FIG. 13 ,  FIG. 13  illustrates a cross-sectional view, taken along line I-I′ of  FIG. 12 , of an initial stage in the method of forming the self-aligned inner gate recess channel transistor according to the second embodiment of the present invention. A substrate  200 , including a well region  204 , is provided. Preferably, the well region  204  is formed using an ion implantation process. A shallow trench isolation (STI) region  202  is conventionally formed by anisotropically etching a trench and filling the trench with an insulation layer. The STI region  202  forms a field area of the semiconductor substrate. Preferably, the STI trench has a depth of about 3000 Å. Threshold voltage (Vt) control ions are implanted by an ion implantation process on the well region  204  to form a Vt control region  206 . Subsequently, source/drain (S/D) ions are implanted by an ion implantation process to form a source/drain (S/D) region  208 . The source/drain (S/D) region  208  forms an active area of the semiconductor substrate. Preferably, the source/drain (S/D) region  208  is an N− source/drain (S/D) region. 
   Referring to  FIG. 14 , an oxide mask layer  210  is formed on the active area  208  and the field area  202 . Preferably, the oxide mask layer  210  is formed to a thickness of about 200 Å. Next, a poly mask layer  212  is formed on the oxide mask layer  210  using a low-pressure chemical vapor deposition (LPCVD). Preferably, the poly hard mask layer  212  is a poly hard mask and is formed to a thickness of about 1000 Å. A recess pattern is then formed on the poly mask layer  212  by depositing a photoresist  214  and performing a photolithographic process. 
   Referring to  FIG. 15 , a recess hole  216  is formed in the active area  208  using an isotropical etching process. Preferably, the recess hole  216  has a depth of about 1500 Å and a width at an opening of about 900 Å. Subsequently, the photoresist ( 214  of  FIG. 14 ) is removed using a photoresist stripping process. 
   In  FIG. 16 , an optional etching process is performed to enlarge a width of a lower portion of the recess hole  216 . The etching process may be a chemical dry etch (CDE) process. The recess hole  216  is preferably enlarged to a width of about 900 Å. 
   As shown in  FIG. 17 , a gate dielectric layer  217  is formed on sidewalls of the recess hole  216 . The gate dielectric layer  217  may be an oxide layer, an oxynitride layer, an alumina (Al2O3) layer, or a ruthenium oxide (RuO) layer. The gate dielectric layer  217  may be formed using a thermal oxidation process. Preferably, the gate dielectric layer  217  has a thickness of about 50 Å. 
   Referring to  FIG. 18 , a first poly gate layer  219  is deposited in the recess hole  216  using an LPCVD process and an etchback process. After the etchback process, the first poly gate layer  219  preferably has a remaining thickness of about 800 Å. 
   As shown in  FIG. 19 , a spacer layer is deposited on the sides of the recess hole  216 , the active area  208 , and the field area  202  using a LPCVD process. The spacer layer is then anisotropically etched to form a recess inner oxide spacer  218 . Preferably, the recess inner oxide spacer  218  has a thickness of about 200 Å. Although the inner spacer  218  is described as an inner oxide spacer, the spacer layer may be formed of either silicon oxide (SiO) or silicon nitride (SiN). 
   Subsequently, as shown in  FIG. 20 , a gate layer and a gate mask layer are formed on the active area  208 , the field area  202 , and the recess hole  216  using a LPCVD process. A gate  222  and a gate mask  224  are then formed using photolithography and an etching process. An upper portion  225  of the gate  222 , which additionally includes the gate mask  224 , protrudes above an upper surface of the substrate. 
     FIG. 21  illustrates a cross-sectional view, taken along line II-II′ of  FIG. 12 , of the stage in the method of forming the recess gate shown in  FIG. 20 . 
     FIG. 22  illustrates a completed structure of a recess channel according to the second embodiment the present invention. As shown in  FIG. 22 , a gate spacer layer is deposited on the gate mask  224 , the active area  208  and the field area  202 . The gate spacer layer is then anisotropically etched to form a sidewall spacer  228  on the gate  222  and the gate mask  224 . Subsequently, source/drain (S/D) ions are implanted using an ion implantation process on the active area  208  to form an S/D region  230 . Preferably, the S/D ions are N+ ions implanted to form an N+ S/D region. 
   By way of comparison, whereas the first embodiment of the present invention discloses a two-step etching process to form the recess hole and a single step deposition process to form the gate, the second embodiment of the present invention discloses a single step etching process to form the recess hole and a two-step deposition process to form the gate. 
   Similar to the first embodiment of the present invention, in the second embodiment, a recess inner oxide spacer is a relatively thick oxide layer, which decreases a gate loading capacitance and a bit line loading capacitance, thereby increasing a refresh time. In addition, the recess inner oxide spacer improves the photo misalign margin and reduces a S/D junction capacitance, thereby increasing a device speed. 
   Preferred embodiments of the present invention have been disclosed herein and, although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.