Abstract:
A semiconductor device includes a semiconductor pillar, an insulator, and an electrode. The semiconductor pillar has a semiconductor portion outwardly extending. The insulator extends along the semiconductor pillar. The insulator has an insulating portion outwardly extending along the semiconductor portion. The electrode extends along the insulator. The insulator is between the semiconductor pillar and the electrode. The electrode has an electrode portion overlapping the insulating portion in plain view. The electrode portion is under the insulating portion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device and a method of manufacturing the same. 
         [0003]    Priority is claimed on Japanese Patent Application No. 2009-010372, filed Jan. 20, 2009, the content of which is incorporated herein by reference. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently, semiconductor chips have been smaller and smaller in size to reduce costs. For example, a DRAM (Dynamic Random Access Memory) realizing the 4F2 memory cell size (F denotes a design rule) by using a vertical MOS transistor (3D pillar-type MOS transistor) has been proposed. 
         [0006]    Gate electrodes of such a vertical MOS transistor with a 4F2 memory cell size are formed by etching back. However, uneven etching causes a fluctuation in the gate length of the vertical MOS transistor. Consequently, the transistor characteristics of each vertical MOS transistor greatly fluctuate, thereby degrading their yield. 
         [0007]    As an example of a vertical MOS transistor, Japanese Patent, Laid-Open Publication No. 2007-201454 discloses a semiconductor device including a conductive structure, a first insulating film, and a first conductive film pattern. 
         [0008]    The conductive structure includes first to third portions. The second portion is disposed on the first portion and extends toward a first direction perpendicular to the plane of paper. The second portion is separated from another second portion in a second direction perpendicular to the first direction. The third portion is disposed on the second portion and is separated from another third portion in the first and second directions. The first insulating film covers sidewalls of the second portion. The first conductive film pattern covers the first insulating film. 
         [0009]    As another example of a vertical MOS transistor, Japanese Patent, Laid-Open Publication No. H10-326879 discloses a semiconductor device including at least two cells each including a vertical MOSFET and a capacitor on the vertical MOSFET. The vertical MOSFET includes a source, a drain, and a gate. The capacitor includes first and second electrodes. The source of one cell is connected to that of an adjacent cell. 
         [0010]    Regarding the above related arts, there have been demands for a semiconductor device and a method of manufacturing the same for achieving a smaller fluctuation in the gate length of a vertical MOS transistor to decrease a fluctuation in transistor characteristics. 
       SUMMARY 
       [0011]    In one embodiment, a semiconductor device includes a semiconductor pillar, an insulator, and an electrode. The semiconductor pillar has a semiconductor portion outwardly extending. The insulator extends along the semiconductor pillar. The insulator has an insulating portion outwardly extending along the semiconductor portion. The electrode extends along the insulator. The insulator is between the semiconductor pillar and the electrode. The electrode has an electrode portion overlapping the insulating portion in plain view. The electrode portion is under the insulating portion. 
         [0012]    In another embodiment, a semiconductor device includes a plurality of semiconductor pillars, a plurality of insulators, and an electrode. Each of the plurality of semiconductor pillars has a semiconductor portion outwardly extending. Each of the plurality of insulators extends along corresponding one of the plurality of semiconductor pillars. Each of the plurality of insulators has an insulating portion outwardly extending along the semiconductor portion of the corresponding one of the plurality of semiconductor pillars. The electrode extends along the plurality of insulators. Each of the plurality of insulators is between the corresponding one of the plurality of semiconductor pillars and the electrode. The electrode has a plurality of electrode portions each overlapping the insulating portion of corresponding one of the plurality of insulators in plain view. Each of the plurality of electrode portions is under the insulating portion of the corresponding one of the plurality of insulators. 
         [0013]    In still another embodiment, a method of manufacturing a semiconductor device includes the following processes. A semiconductor pillar having a semiconductor portion outwardly extending is formed. Then, an insulator extending along the semiconductor pillar is formed. The insulator has an insulating portion outwardly extending along the semiconductor portion. Then, an electrode extending along the insulator is formed. The insulator is between the semiconductor pillar and the electrode. The insulator insulates the electrode from the semiconductor pillar. The electrode has an electrode portion overlapping the insulating portion in plain view. The electrode portion is under the insulating portion. 
         [0014]    Accordingly, a fluctuation in the gate length can be prevented, thereby preventing a fluctuation in the transistor characteristics of the vertical MOS transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
           [0016]      FIG. 1A  is a plane view illustrating 4F2 memory cells of a semiconductor device according to a first embodiment of the present invention; 
           [0017]      FIG. 1B  is a cross-sectional view taken along a line A-A′ shown in  FIG. 1A ; 
           [0018]      FIGS. 2 to 11  cross-sectional views indicative of a process flow illustrating a method of manufacturing a vertical MOS transistor of the semiconductor device according to the first embodiment; 
           [0019]      FIGS. 12 and 13  are cross-sectional views illustrating examples of vertical MOS transistors when etching back is unevenly carried out; 
           [0020]      FIG. 14A  is a plane view illustrating a comparison example of 4F2 memory cells; 
           [0021]      FIG. 14B  is a cross-sectional view taken along a line B-B′ shown in  FIG. 14A ; and 
           [0022]      FIGS. 15 to 17  are cross-sectional views illustrating comparison examples of vertical MOS transistors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    The present invention will now be described herein with reference to illustrative embodiments. The accompanying drawings explain a semiconductor device and a method of manufacturing the semiconductor device in the embodiments. The size, the thickness, and the like of each illustrated portion might be different from those of each portion of an actual semiconductor device. 
         [0024]    Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the present invention is not limited to the embodiments illustrated herein for explanatory purposes. 
         [0025]      FIG. 1A  is a plane view illustrating 4F2 memory cells of a semiconductor device according to a first embodiment of the present invention.  FIG. 1B  is a cross-sectional view taken along a line A-A′ shown in  FIG. 1A . 
         [0026]    As shown in  FIG. 1A , the semiconductor device according to the first embodiment includes 4F2 memory cells including: pillars  1 ; first insulating films  2  each surrounding corresponding one of the pillars  1 ; and electrodes  3  each surrounding the first insulating films  2  while connecting the pillars  1  aligned in a straight line. 
         [0027]    The pillars  1  are arranged in a grid and are substantially ellipses having the different horizontal-to-vertical ratios. The centers of the pillars  1  are separated by 2F (F is a design rule) both in the horizontal and vertical directions. A vertical MOS transistor is formed at each of the pillars  1 . 
         [0028]    As shown in  FIG. 1B , the vertical MOS transistor includes: a substrate (semiconductor substrate)  10 ; the pillar  1  protruding from the substrate  10 ; impurity diffusion regions  15  and  16  on the bottom and top sides of the pillar  1 , respectively; the first insulating film  2  covering a side surface of the pillar  1 ; and the electrode  3  covering the first insulating film  2 . A contact plug  8  is connected to the top surface of the pillar  1 . An insulating film  5  covers a side surface of the contact plug  8 . 
         [0029]    For example, an n-type impurity is diffused in the impurity diffusion regions  15  and  16 . The impurity diffusion region  15  is connected to a power source. The impurity diffusion region  16  is connected to another power source through the contact plug  8 . 
         [0030]    For example, a p-type impurity is doped in the pillar  1 . The electrode  3  is connected to a gate power source. Thus, the pillar  1  functions as a channel of the vertical MOS transistor. 
         [0031]    The pillar  1  protrudes from the substrate  10 . The pillar  1  has a large-diameter portion  31  on the top side of the pillar  1  and a small-diameter portion  33  under the large-diameter portion  31 . Accordingly, a periphery  1   c  of the small-diameter portion  33  is inside a periphery  31   c  of the large-diameter portion  31  in plain view (i.e., when viewed in an extending direction of the pillar  1 ), thereby forming a recess  1   d.    
         [0032]    Although it is explained in the first embodiment that the pillar  1  is elliptical when viewed in the extending direction of the pillar  1 , the pillar  1  may be circular, polygonal, or the like. 
         [0033]    As will be explained in detail layer, the recess  1   d  is formed by etching a side surface of a pillar silicon substrate with a uniform thickness. The first insulating film  2  fills the recess  1   d.    
         [0034]    The electrode  3  includes first and second cylinders  41  and  42 . The first cylinder  41  covers the side surface  1   c  of the pillar  1 . The second cylinder  42  covers the first cylinder  42 . 
         [0035]    As will be explained in detail layer, the first and second cylinders  41  and  42  are formed by an electrode material film for forming the electrode  3  being etched. Therefore, the first and second cylinders  41  and  42  are made of the same material and integrated with each other. 
         [0036]    The first insulating film  2  fills the recess  1   d  and forms another recess  2   d  having a level smaller than that of the recess  1   d . The level of the recess  1   d  is measured from the upper surface of the first insulating layer  2  covering the substrate  10  to the lower surface of the large-diameter portion  31 . The level of the recess  2   d  is measured from the upper surface of the first insulating layer  2  covering the substrate  10  to a top surface  41   a  or  41   b  of the first cylinder  41  that will be explained later. 
         [0037]    The first cylinder  41  covers the side surface  1   c  of the pillar  1  through the first insulating film  2  so as to fill the recess  2   d . Accordingly, the gate length of the vertical MOS transistor is uniquely defined by the level of the recess  2   d , thereby achieving stable transistor characteristics. 
         [0038]    Top surfaces  42   a  and  42   b  of the second cylinder  42  are positioned lower than top surfaces  41   a  and  41   b  of the first cylinder  41 . Accordingly, a voltage is not applied from the second cylinder  42  to the pillar  1  through the first insulating film  2 , thereby keeping the transistor characteristics stable. 
         [0039]    The second cylinder  42  surrounds the first cylinder  41 , thereby making the electrode  3  thicker. Additionally, a voltage from the power source to the electrode  3  can be kept constant, and a voltage can be evenly applied to the side surface  1   c  of the pillar  1 . Moreover, the gate resistance can be reduced. 
         [0040]    The first insulating film  2  on the upper substrate  10  around the pillar  1  insulates the substrate  10  from the electrode  3 . The electrode  3  is made of, for example, polysilicon formed by in-situ doping phosphorus (P) at a concentration of 1×E 20 /cm 3 , a metal such as tungsten (W), titanium nitride (TiN), silicide such as cobalt silicide (CoSi 2 ), or the like. 
         [0041]      FIGS. 2 to 11  are cross-sectional views indicative of a process flow illustrating a method of manufacturing the vertical MOS transistor included in the semiconductor device according to the first embodiment. 
         [0042]    The method includes: a pillar-base formation process of forming a pillar base  32  for forming the pillar  1 ; a pillar formation process of forming the pillar  1 ; a first insulating film formation process of forming the first insulating film  2 ; and an electrode formation process of forming the electrode  3 . 
         [0043]    In the pillar base formation process, the second and third insulating films  4  and  5  are formed in this order over the substrate  10  as shown in  FIG. 2 . The substrate  10  is a semiconductor substrate, such as silicon substrate, into which a p-type or n-type impurity may be doped. 
         [0044]    The second insulating film  4  is, for example, a silicon oxide film that can be formed by, for example, thermally oxidizing a surface of the silicon substrate  10  or by CVD. The third insulating film  5  is, for example, a silicon nitride film that can be formed by, for example, CVD. 
         [0045]    Then, the large-diameter portion  31  is formed by photolithography and etching as shown in  FIG. 3 . Specifically, a resist material is applied over the third insulating film  5  and exposed using a mask. Then, the exposed portion is removed using a solution to form a resist mask. 
         [0046]    Then, the exposed second and third insulating films  4  and  5  are etched back using the resist mask to form a hard mask (nitride film) including the second and third insulating films  4  and  5 . 
         [0047]    Then, the upper surface of the substrate  10  is anisotropically dry etched using the hard mask as shown in  FIG. 3 . The resist mask may be removed before the etching process or simultaneously removed in the etching process. 
         [0048]    Then, a fourth insulating film  6  is formed so as to cover the entire surface as shown in  FIG. 4 . The fourth insulating film  6  is, for example, a silicon oxide film that can be formed by CVD. 
         [0049]    Then, a fifth insulating film  7  is formed so as to cover the fourth insulating film  6  as shown in  FIG. 5 . The fifth insulating film  7  is, for example, a silicon nitride film that can be formed by CVD. 
         [0050]    Then, the fifth insulating film  7  is etched back by anisotropic dry etching to form a sidewall  17  as shown in  FIG. 6 . Then, the fourth insulating film  6  on the substrate  10  is anisotropically etched. 
         [0051]    Then, the substrate  10  is anisotropically etched using a hard mask including the sidewall  17 , and the third and fourth insulating films  5  and  6  to form the pillar base  32  having a level C. 
         [0052]    In the pillar formation process, the side surface of the pillar base  32  and the substrate  10  which are not covered by the insulating film is isotropically wet-etched by a depth D to form the pillar  1 , as shown in  FIG. 8 . 
         [0053]    In this case, the substrate  10  is etched by the depth D. Consequently, a level of the exposed pillar base  32  is L′ (=C+D), and the diameter of the pillar base  32  decreases by 2D. However, the top side of the pillar  1  is covered by the hard mask including the sidewall  17 , and the third and fourth insulating films  5  and  6 , and therefore the shape of the top side is maintained. 
         [0054]    Thus, the large-diameter portion  31  remains on the top side of the pillar  1 , and the small-diameter portion  33  is formed under the large-diameter portion  31 . Accordingly, the recess  1   d  is formed on the side surface  1   c  of the pillar  1  due to the difference in the diameters of the portions  31  and  33 . 
         [0055]    In the first insulating film formation process, the sidewall  17  and the fourth insulating film  6  are removed as shown in  FIG. 9 . Then, the first insulating film  2  is formed so as to cover the exposed surfaces of the substrate  10  and the pillar  1  as shown in  FIG. 10 . 
         [0056]    The first insulating film  2  is, for example, a silicon oxide film that can be formed by thermally oxidizing the surface of the silicon substrate  10  or by CVD. By the first insulating film  2  filling the recess  1   d , the other recess  2   d  is formed. A level L of the recess  2   d  is bit smaller than the level L′ of the recess  1   d.    
         [0057]    In the electrode formation process, an electrode material film for forming the electrode  3  is formed so as to cover the side surface of the pillar  1  covered by the first insulating film  2  and the upper surface of the substrate  10  covered by the first insulating film  2 . 
         [0058]    Then, the electric material film is anisotropically dry etched until the first insulating film  2  covering the upper surface of the substrate  10  is exposed, thus forming the electrode  3  as shown in  FIG. 11 . 
         [0059]    The electrode  3  includes the first and second cylinders  41  and  42 . The first cylinder  41  fills the recess  2   d  so as to cover the first insulating film  2 . Accordingly, the level of the first cylinder  41  is uniquely defined by the level of the recess  2   d.    
         [0060]    The second cylinder  42  covers the first cylinder  41 . The first and second cylinders  41  and  42  are made of the same electrode material, and therefore are integrated with each other. 
         [0061]    According to the structure of the first and second cylinders  41  and  42 , the top ends  41   a  and  41   b  of the first cylinder  41  have the same level even if the etching rate varies, thereby uniquely defining the gate length of the MOS transistor as the level L of the recess  2   d , and therefore achieving stable transistor characteristics. 
         [0062]    The top ends  42   a  and  42   b  of the second cylinder  42  are positioned lower than the top ends  41   a  and  41   b  of the first cylinder  41 . Accordingly, a voltage is not applied from the second cylinder  42  to the pillar  1  through the first insulating film  2 , thereby achieving the stable transistor characteristics. 
         [0063]    The second cylinder  42  covers the first cylinder  41 , thereby making the electrode  3  thicker. For this reason, a voltage from the power source to the electrode  3  can be kept constant, thereby enabling a voltage to be evenly applied to the side surface  1   c  of the pillar  1 , and enabling the gate resistance to be reduced. The electrode  3  is insulated from the substrate  10  by the first insulating film  2  covering the substrate  10 . 
         [0064]    Then, the impurity diffusion regions  15  and  16  are formed on the top and bottom portions of the pillar  1 , respectively. For example, an impurity, such as arsenic, is ion implanted at a concentration of 1×E 15 /cm 2  at energy of 10 KeV by plasma doping from both sides of the third insulating film  5  into the top portion of the pillar  1 . 
         [0065]    Although it has been explained above that the impurity diffusion regions  15  and  16  are formed after the electrode  3  is formed, the impurity diffusion region  15  may be formed after the pillar  1  is formed by wet etching. Additionally, the impurity diffusion region  16  may be formed by ion implantation when a contact hole for the contact plug  8  is formed as will be explained layer. 
         [0066]    Finally, the contact plug  8  is formed so as to connect to the impurity diffusion region  16  after an inter-layer insulating film is formed so as to cover the upper portion of the pillar  1 . Thus, the vertical MOS transistor as shown in  FIG. 1B  is formed. 
         [0067]    Regarding the above method, the etching rate of the electrode material film varies depending on various conditions. For example, the etching rate of the electrode material film is smaller at a portion closer to the first insulating film  2 , thereby causing the aforementioned etching back to be unevenly carried out. 
         [0068]      FIG. 12  is a cross-sectional view illustrating an example of a vertical MOS transistor when the etching back is unevenly carried out. In this case, the top ends  42   a  and  42   b  of the second cylinder  42  have different heights. 
         [0069]    Even in this case, the first cylinder  41  is formed so as to fill the recess  2   d  and to cover the side surface  1   c  of the pillar  1  through the first insulating film  2 . Accordingly, the top ends  41   a  and  41   b  of the first cylinder  41  have the same level, thereby uniquely defining the gate length of the vertical MOS transistor as the level L of the recess  2   d , and therefore achieving stable transistor characteristics. 
         [0070]      FIG. 13  is a cross-sectional view illustrating another example of a vertical MOS transistor when the etching back is unevenly carried out. In this case, the top ends  42   a  and  42   b  of the second cylinder  42  are sloped. The top end  42   a  includes a flat portion  42   a   1  and a sloped portion  42   a   2 . The other top end  42   b  includes a flat portion  42   b   1  and a sloped portion  42   b   2 . The angles of the sloped portions  42   a   2  and  42   b   2  are substantially the same. The sloped surface of the sloped portion  42   a   2  is larger than that of the sloped portion  42   b   2 . 
         [0071]    Even in this case, the first cylinder  41  is formed so as to fill the recess  2   d  and to cover the side surface  1   c  of the pillar  1  through the first insulating film  2 . Accordingly, the top ends  41   a  and  41   b  of the first cylinder  41  have the same level, thereby uniquely defining the gate length of the vertical MOS transistor as the level L of the recess  2   d , and therefore achieving stable transistor characteristics. 
         [0072]    Although it has been explained above that the electrode  3  includes the first and second cylinders  41  and  42 , the second cylinder  42  may not be formed if only the first cylinder  41  can achieve stable transistor characteristics. 
         [0073]    As explained above, according to the semiconductor device of the first embodiment, a fluctuation in the gate length can be prevented, thereby preventing a fluctuation in the transistor characteristics of the vertical MOS transistor. 
         [0074]    Hereinafter, comparison examples are explained.  FIG. 14A  is a plane view illustrating a comparison example of 4F2 memory cells (semiconductor device).  FIG. 14B  is a cross-sectional view taken along a line B-B′ shown in  FIG. 14A . 
         [0075]    As shown in  FIG. 14A , the 4F2 memory cells include: pillars  101 ; first insulating films  102  each surrounding corresponding one of the pillars  101 ; and electrodes  103  each surrounding the first insulating films  102  while connecting the pillars  101  aligned in a straight line. 
         [0076]    The pillars  101  are arranged in a grid and are substantially ellipses having a different horizontal-to-vertical ratio. The centers of the pillars  101  are distanced by 2F (F is a design rule) both in horizontal and vertical directions. A vertical MOS transistor is formed at each of the pillars  101 . 
         [0077]    The vertical MOS transistor shown in  FIG. 14B  includes: a substrate  110 ; the pillar  101  protruding from the substrate  110 ; impurity diffusion regions  115  and  116  on the bottom and top sides of the pillar  101 , respectively; a gate oxide film  102  covering a side surface of the pillar  101 ; and a gate electrode  103  covering the gate oxide film  102 . A contact plug  108  is connected to the top surface of the pillar  101 . A nitride film  105  covers a side surface of the contact plug  108 . 
         [0078]    As shown in  FIG. 14B , the lower impurity diffusion region  115  is connected to an S/D power source. The upper impurity diffusion region  116  is connected to another S/D power source through the contact plug  108 . The gate electrode  103  is connected to a gate power source, thus forming the vertical MOS transistor. 
         [0079]      FIGS. 15 to 17  are cross-sectional views illustrating the gate electrode  103  of the vertical MOS transistor shown in  FIG. 14B  having been formed. When the gate electrode  103  is formed, a gate electrode material film having a predetermined thickness is formed by a known method so as to cover the pillar  101  and an upper surface  110   a  of the substrate  110 . 
         [0080]    Then, the gate electrode material film is evenly etched back by anisotropic dry etching from the top end  101   a  of the pillar  101  down to the level of the upper surface  110   a  of the substrate  110 , thus forming the cylindrical gate electrode  103  surrounding the gate oxide film  102 . 
         [0081]    In many cases, top ends  103   a  and  103   b  of the gate electrodes  103  have the same level. In these cases, a fluctuation in the transistor characteristics of the vertical MOS transistor is small. 
         [0082]    However, the etching rate of the electrode material film varies depending on various conditions. For example, the etching rate of the electrode material film is smaller at a portion closer to the gate oxide film  102 , thereby causing uneven etching back. 
         [0083]      FIG. 16  illustrates a case where the etching back has been unevenly carried out, and therefore the top ends  103   a  and  103   b  of the gate electrode  103  have different levels. 
         [0084]    The top end  103   b  is positioned lower by a value of t 1  than the top end  103   a . In this case, the gate length differs between the top ends  103   a  and  103   b , thereby causing a large fluctuation in the vertical MOS transistor. 
         [0085]      FIG. 17  illustrates another case where the etching back has been unevenly carried out, and therefore the top ends  103   a  and  103   b  of the gate electrode  103  have sloped portions. 
         [0086]    The top end  103   a  includes a flat portion  103   a   1  and a sloped portion  103   a   2 . The other top end  103   b  includes a flat portion  103   b   1  and a sloped portion  103   b   2 . The angles of the sloped portions  103   a   2  and  103   b   2  are substantially the same. The sloped surface of the sloped portion  103   a   2  is larger than that of the sloped portion  103   b   2 . 
         [0087]    For this reason, the top end  103   b  is positioned lower by a value of t 2  than the top end  103   a . Also in this case, the gate length differs between the top ends  103   a  and  103   b , thereby causing a large fluctuation in the vertical MOS transistor. 
         [0088]    As explained above, if the etching rate of the gate electrode material film of the vertical MOS transistor varies, the length of the gate electrode varies. Consequently, the gate length of the vertical MOS transistor varies, thereby causing a larger fluctuation in the transistor characteristics. 
         [0089]    The present invention is applicable to semiconductor device manufacturing industries. 
         [0090]    It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
         [0091]    As used herein, the following directional terms “forward, rearward, above, downward, vertical, horizontal, below, and transverse” as well as any other similar directional terms refer to those directions of an apparatus equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to an apparatus equipped with the present invention. 
         [0092]    The terms of degree such as “substantially,” “about,” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5 percents of the modified term if this deviation would not negate the meaning of the word it modifies.