Abstract:
It is intended to provide a method of producing a semiconductor device, comprising the steps of: providing a substrate on one side of which at least one semiconductor pillar stands; forming a first dielectric film to at least partially cover a surface of the at least one semiconductor pillar; forming a conductive film on the first dielectric film; removing by etching a portion of the conductive film located on a top surface and along an upper portion of a side surface of the semiconductor pillar; forming a protective film on at least a part of the top surface and the upper portion of the side surface of the semiconductor pillar; etching back the protective film to form a protective film-based sidewall on respective top surfaces of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar; forming a resist pattern for forming a gate line in such a manner that at least a portion of the resist pattern is formed on the top surface of the semiconductor pillar by applying a resist and using lithography; and partially removing by etching the conductive film using the resist pattern as a mask while protecting, by the protective film-based sidewall, the portions of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar, to form a gate electrode and a gate line extending from the gate electrode.

Description:
RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119(e), this application claims the benefit of the filing date of Provisional U.S. Patent Application Ser. No. 61/207,637 filed on Feb. 13, 2009. This application is a continuation application of PCT/JP2009/051465 filed on Jan. 29, 2009 which claims priority under 35 U.S.C. §365(a) to PCT/JP2008/051305 filed on Jan. 29, 2008. The entire contents of these applications are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a production method for a semiconductor, and more particularly to a structure and a production method for an SGT (Surrounding Gate Transistor) which is a vertical MOS transistor comprising a pillar-shaped semiconductor layer having a sidewall serving as a channel region, and a gate electrode formed to surround the channel region. 
     BACKGROUND ART 
     With a view to achieving higher integration and higher performance of a semiconductor device, a vertical transistor SGT has been proposed which comprises a pillar-shaped semiconductor layer formed on a surface of a semiconductor substrate, and a gate formed to surround a sidewall of the pillar-shaped semiconductor layer (see the following Patent Documents 1 and 2). In the SGT, a source, a gate and a drain are arranged in a vertical direction, so that an occupancy area can be significantly reduced as compared with a conventional planar transistor. In addition, the gate is formed to surround a channel region, so that, as a size of a pillar-shaped semiconductor layer is reduced, channel controllability of the gate can be effectively improved to obtain steep subthreshold characteristics. Furthermore, an improvement in carrier mobility based on electric field relaxation in the channel region can be expected by setting an impurity concentration and a size of the pillar-shaped semiconductor layer to allow the pillar-shaped semiconductor layer to become fully depleted. Therefore, the use of the SGT makes it possible to simultaneously achieve higher integration and higher performance as compared with the conventional planar transistor. 
       FIG. 177(   a ) shows a top plan view of a CMOS inverter designed using the SGT disclosed in the Patent Document 1, and  FIG. 177(   b ) is a sectional view taken along the cutting-plane line A-A′ in  FIG. 177(   b ). 
     Referring to  FIGS. 177(   a ) and  177 ( b ), an N-well  1302  and a P-well  1303  are formed in an upper region of a Si substrate  1301 . A pillar-shaped silicon layer  1305  forming a PMOS (PMOS-forming pillar-shaped silicon layer  1305 ) and a pillar-shaped silicon layer  1306  forming an NMOS (NMOS-forming pillar-shaped silicon layer  1306 ) are formed on a surface of the Si substrate, specifically on respective ones of the N-well region and the P-well region, and a gate  1308  is formed to surround the pillar-shaped silicon layers. Then, each of a P +  drain diffusion layer  1310  formed beneath the PMOS-forming pillar-shaped silicon layer, and a N +  drain diffusion layer  1312  formed beneath the NMOS-forming pillar-shaped silicon layer, is connected to an output terminal Vout  7 . A source diffusion layer  1309  formed in an upper portion of the PMOS-forming pillar-shaped silicon layer is connected to a power supply potential Vcc  7 , and a source diffusion layer  1311  formed in an upper portion of the NMOS-forming pillar-shaped silicon layer is connected to a ground potential Vss  7 . Further, the gate  1308  common to the PMOS and the NMOS is connected to an input terminal Vin  7 , and the diffusion layer ( 1310 ,  1312 ) beneath a respective one of the pillar-shaped silicon layers is connected to the output terminal Vout  7 . In this manner, the CMOS inverter is formed. 
       FIGS. 178(   a ) to  178 ( f ) show a schematic process flow for forming a pillar-shaped silicon layer and a gate electrode in the SGT disclosed in the Patent Document 1. In  FIG. 178(   a ), a pillar-shaped silicon layer  1401  is formed on a silicon substrate by etching. In  FIG. 178(   b ), a gate dielectric film  1402  is formed. In  FIG. 178(   c ), a gate conductive film  1403  is formed. In  FIG. 178(   d ), a resist  1404  for a gate line pattern is formed to be in contact with a portion of a gate conductive film surrounding the pillar-shaped silicon layer. In  FIG. 178(   e ), the gate conductive film  1403  is etched back to form a gate electrode  1403  and a gate line  1405  of an SGT. In  FIG. 178(   f ), the resist is released. In the above process flow, the gate electrode  1403  is formed around the pillar-shaped silicon layer  1401  by a desired film thickness, in a self-alignment manner, so that two pillar-shaped silicon layers each having a gate electrode to be applied with a different potential can be arranged side-by-side with a relatively small distance therebetween. 
     However, in the above process flow, the resist  1404  must be formed to be accurately in contact with the portion of the gate conductive film around a sidewall of the pillar-shaped silicon layer, in  FIG. 178(   d ). Therefore, a process margin in a lithography step of forming the gate line is small, which causes difficulty in stably fabricating the gate line. The following description will be made in regard to this point. 
       FIGS. 179(   a ) to  179 ( c ) illustrate a process flow in case where the resist  1404  is positionally deviated to the right side in  FIG. 178(   d ).  FIG. 179(   a ) shows a state after a resist  1414  for a gate line pattern is positionally deviated to the right side during alignment of a lithographic exposure. In this state, there arises a space between the resist  1414  and a sidewall of a pillar-shaped silicon layer  1411 . In  FIG. 179(   b ), a gate etch step is performed. In  FIG. 179(   c ), the resist is released. In this case, a gate electrode  1413  and a gate line  1415  of a resulting SGT are undesirably disconnected from each other. 
       FIGS. 180(   a ) to  180 ( c ) illustrate a process flow in case where the gate-line resist  1404  is positionally deviated to the left side in  FIG. 178(   d ).  FIG. 180(   a ) shows a state after a resist  1424  for a gate line pattern is positionally deviated to the left side during alignment of a lithographic exposure. In this state, there arises an overlapped area  1426  between the resist  1424  and a portion of a gate electrode on a top of a pillar-shaped silicon layer  1421 . In  FIG. 180(   b ), a gate etch step is performed. In  FIG. 180(   c ), the resist is released. In this case, a gate electrode  1423  of a resulting SGT undesirably has a shape abnormality  1427  on a side where the resist is formed. 
     A value of the above positional deviation of the resist arising from the alignment varies depending on a position on a wafer and a position in a chip, and thereby it is impossible to keep positional deviations in all patterns on a wafer, within a range free of the occurrence of the above problem. Thus, in the above SGT forming method, a process margin for forming the gate line becomes extremely small, and thereby it is impossible to produce an integrated circuit in high yield. 
     As one of the measures against the problem in the above SGT gateline forming method, the following Non-Patent Document 1 discloses an SGT gate-line forming method which is improved in process margin.  FIGS. 181(   a ) to  181 ( g ) illustrate a schematic process flow for forming a pillar-shaped silicon layer and a gate electrode of an SGT, which is disclosed in the Non-Patent Document 1. This process flow will be described below. In  FIG. 181(   a ), a silicon substrate is etched to form a pillar-shaped silicon layer  1501 . In  FIG. 181(   b ), a gate dielectric film  1502  is formed. In  FIG. 181(   c ), a gate conductive film is formed. In  FIG. 181(   d ), the gate conductive film, and a portion of the gate dielectric film on a top of the pillar-shaped silicon layer, are polished by chemical mechanical polishing (CMP). In  FIG. 181(   e ), the resulting gate conductive film is etched back in such a manner that a portion of the gate conductive film surrounding the pillar-shaped silicon layer is etched to have a desired gate length. In  FIG. 181(   f ), a resist for a gate line pattern is formed by lithography. In  FIG. 181(   g ), the gate conductive film is etched to form a gate electrode and a gate line. 
     In the above process flow, although a process margin in a lithography step of forming the gate line becomes larger, as compared with the process flow disclosed in the Patent Document 1, the gate electrode to be formed around the pillar-shaped silicon layer is not formed in a self-alignment manner, with respect to the pillar-shaped silicon layer. As a result, the gate electrode will be widely formed around the pillar-shaped silicon layer, and a film thickness of the gate electrode to be formed around the pillar-shaped silicon layer will vary depending on a deviation in alignment of a resist pattern and an error in size of the resist pattern. Thus, if a distance between two pillar-shaped silicon layers each having a gate electrode to be applied with a different potential is reduced, the respective gate electrodes will be short-circuited with each other. Therefore, an occupancy area of an SGT-based circuit is liable to become large.
         Patent Document 1: JP 2-188966A   Patent Document 2: JP 7-99311A   Non-Patent Document 1: Ruigang Li, et al., “50 nm Vertical Surrounding Gate MOSFET with S-factor of 75 mv/dec”, Device Research Conference, 2001, p. 63       

     As a prerequisite to achievement of an SGT applicable to a product comprising a highly-integrated and high-performance logic circuit, such as a CPU, it is necessary for a gate forming process to meet the following requirements. A first requirement is that it is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second requirement is that it is less vulnerable to a deviation in exposure alignment during gate line formation. A third requirement is that it is capable of accurately controlling a gate length to minimize a variation in gate length and increase a process margin. 
     In view of above problems, it is an object of the present invention to propose an SGT production method capable of solving the above problems. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of producing a semiconductor device. The method comprises: providing a substrate on one side of which at least one semiconductor pillar stands; forming a first dielectric film to at least partially cover a surface of the at least one semiconductor pillar; forming a conductive film on the first dielectric film; removing by etching a portion of the conductive film located on a top surface and along an upper portion of a side surface of the semiconductor pillar; forming a protective film on at least a part of the top surface and the upper portion of the side surface of the semiconductor pillar; etching back the protective film to form a protective film-based sidewall on respective top surfaces of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar; forming a resist pattern for forming a gate line in such a manner that at least a portion of the resist pattern is formed on the top surface of the semiconductor pillar by applying a resist and using lithography; and partially removing by etching the conductive film using the resist pattern as a mask while protecting, by the protective film-based sidewall, the portions of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar, to form a gate electrode and a gate line extending from the gate electrode. 
     For example, the step of removing by etching a portion of the conductive film located on a top surface and along a upper portion of a side surface of the semiconductor pillar includes the sub-steps of: forming a second dielectric film on the conductive film to allow the semiconductor pillar to be buried therein; flattening a top surface of the second dielectric film; and removing by etching a portion of the conductive film and the second dielectric film each located along the side surface of the semiconductor pillar to form the conductive film and the second dielectric film to have substantially the same height. 
     According to a second aspect of the present invention, there is provided a method of producing a semiconductor device. The method comprises: providing a substrate on one side of which at least one semiconductor pillar stands, the semiconductor pillar having a stopper film formed on a top surface thereof; forming a first dielectric film to at least partially cover a surface of the at least one semiconductor pillar; forming a conductive film on the first dielectric film; forming a second dielectric film on the conductive film to allow the semiconductor pillar to be buried therein; flattening a top surface of the resulting product by chemical mechanical polishing (CMP), using the stopper film as a CMP stopper; removing by etching a portion of the second dielectric film and the conductive film each located along an upper portion of a side surface of the semiconductor pillar to form the conductive film and the second dielectric film to have substantially the same height; forming a protective film on at least a part of the top surface and the upper portion of the side surface of the semiconductor pillar; etching back the protective film to form a protective film-based sidewall on respective top surfaces of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar; removing the second dielectric film; forming a resist pattern for forming a gate line in such a manner that at least a portion of the resist pattern is formed on the top surface of the semiconductor pillar by applying a resist and using lithography; and partially removing by etching the conductive film using the resist pattern as a mask while protecting, by the protective film-based sidewall, the portions of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar, to form a gate electrode and a gate line extending from the gate electrode. 
     According to a third aspect of the present invention, there is provided a method of producing a semiconductor device. The method comprises: providing a substrate on one side of which at least one semiconductor pillar stands; forming a first dielectric film to at least partially cover a surface of the at least one semiconductor pillar; forming a conductive film on the first dielectric film to allow the semiconductor pillar to be buried therein; etching an upper portion of the conductive film to remove a portion of the conductive film located on a top surface and along an upper portion of a side surface of the semiconductor pillar; forming a protective film on at least a part of the top surface and the upper portion of the side surface of the semiconductor pillar; etching back the protective film to form a protective film-based sidewall on respective top surfaces of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar; forming a resist pattern for forming a gate line in such a manner that at least a portion of the resist pattern is formed on the top surface of the semiconductor pillar by applying a resist and using lithography; and partially removing by etching, using the resist pattern as a mask, the conductive film to form at least a portion of the gate line, and partially removing by etching, using the protective film-based sidewall as a mask, the conductive film and the first dielectric film to form at least a portion of a gate electrode to have the desired film thickness. 
     For example, the method further comprises, as a preprocessing for the etching an upper portion of the conductive film to remove a portion of the conductive film located on a top surface and along an upper portion of a side surface of the semiconductor pillar, the step of flattening a top surface of the conductive film. 
     According to a fourth aspect of the present invention, there is provided a method of producing a semiconductor device. The method comprises: providing a substrate on one side of which at least one semiconductor pillar stands, the semiconductor pillar having a stopper film formed on a top surface thereof; forming a first dielectric film to at least partially cover a surface of the at least one semiconductor pillar; forming a conductive film on the first dielectric film to allow the semiconductor pillar to be buried therein; flattening a top surface of the resulting product by chemical mechanical polishing (CMP), using the stopper film as a CMP stopper; etching an upper portion of the conductive film to remove a portion of the conductive film located along an upper portion of a side surface of the semiconductor pillar; forming a protective film on at least a part of the top surface and the upper portion of the side surface of the semiconductor pillar; etching back the protective film to form a protective film-based sidewall on respective top surfaces of the conductive film and the first dielectric film each located along the side surface of the semiconductor pillar; forming a resist pattern for forming a gate line in such a manner that at least a portion of the resist pattern is formed on the top surface of the semiconductor pillar by applying a resist and using lithography; and partially removing by etching, using the resist pattern as a mask, the conductive film to form at least a portion of the gate line, and partially removing by etching, using the protective film-based sidewall as a mask, the conductive film and the first dielectric film to form at least a portion of a gate electrode to have the desired film thickness. 
     For example, the conductive film is a layered structure film comprising a thin metal film on the side of the first dielectric film, and a polysilicon film. 
     For example, the protective film is a silicon nitride film. 
     For example, each of the protective film and the stopper films is a silicon nitride film. 
     For example, the substrate further has a diffusion region formed in contact with a lower part of the semiconductor pillar. 
     For example, the method further comprises the step of forming, in an upper portion of the semiconductor pillar, a diffusion region having a same conductivity type as that of the diffusion region formed in contact with a lower part of the semiconductor pillar. 
     For example, the diffusion region formed beneath the pillar-shaped semiconductor layer is formed in a surface region of the substrate. 
     The term “on one side” appears in the present specification and claims in the form of “A is on one side of B”, which should be interpreted to mean either “A is situated in contact with B on one side of B,” or “A is situated away from B on one side of B,” whenever the context allows such an interpretation. 
     As described above, in the production method of the present invention, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed. Thus, the method is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and less vulnerable to a deviation in exposure alignment during gate line formation. This makes it possible to simultaneously solve both the following conventional problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner. 
     Further, the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. This makes it possible to simultaneously solve both the following conventional problems: a disconnection or open of a gate line and a variation in gate length e arising from a lithography step of forming a gate line; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a ) and  1 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to a first embodiment of the present invention. 
         FIGS. 2(   a ) and  2 ( b ) illustrate a part of a series of steps of the single-SGT production method according to the first embodiment. 
         FIGS. 3(   a ) and  3 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 4(   a ) and  4 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 5(   a ) and  5 ( b ) illustrate a part of the steps of the single-SGT production method according to the embodiment. 
         FIGS. 6(   a ) and  6 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 7(   a ) and  7 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 8(   a ) and  8 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 9(   a ) and  9 ( b ) illustrate a part of the steps of the single-SGT production method according to the embodiment. 
         FIGS. 10(   a ) and  10 ( b ) illustrate a part of the steps of the single-SGT production method according to first the embodiment. 
         FIGS. 11(   a ) and  11 ( b ) illustrate a part of the steps of the single-SGT production method according to the embodiment. 
         FIGS. 12(   a ) and  12 ( b ) illustrate a part of the steps of the SGT production method according to first the embodiment. 
         FIGS. 13(   a ) and  13 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 14(   a ) and  14 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 15(   a ) and  15 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 16(   a ) and  16 ( b ) illustrate a part of the steps of the single-SGT production method according to the first embodiment. 
         FIGS. 17(   a ) and  17 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to a second embodiment of the present invention. 
         FIGS. 18(   a ) and  18 ( b ) illustrate a part of a series of steps of the single-SGT production method according to the second embodiment. 
         FIGS. 19(   a ) and  19 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 20(   a ) and  20 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 21(   a ) and  21 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 22(   a ) and  22 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 23(   a ) and  23 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 24(   a ) and  24 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 25(   a ) and  25 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 26(   a ) and  26 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 27(   a ) and  27 ( b ) illustrate a part of the steps of the single-SGT production method according to the second embodiment. 
         FIGS. 28(   a ) to  28 ( d ) are explanatory diagrams showing a misalignment of gate line pattern occurring in the first embodiment. 
         FIGS. 29(   a ) to  29 ( d ) are explanatory diagrams showing a defect which is likely to occur in the first embodiment. 
         FIGS. 30(   a ) and  30 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to a third embodiment of the present invention. 
         FIGS. 31(   a ) and  31 ( b ) illustrate a part of a series of steps of the single-SGT production method according to the third embodiment. 
         FIGS. 32(   a ) and  32 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 33(   a ) and  33 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 34(   a ) and  34 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 35(   a ) and  35 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 36(   a ) and  36 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 37(   a ) and  37 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 38(   a ) and  38 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 39(   a ) and  39 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 40(   a ) and  40 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIGS. 41(   a ) and  41 ( b ) illustrate a part of the steps of the single-SGT production method according to the third embodiment. 
         FIG. 42  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to a fourth embodiment of the present invention. 
         FIG. 43  is a top plan view of the CMOS inverter formed by the production method according to the fourth embodiment. 
         FIGS. 44(   a ) and  44 ( b ) are sectional views of the CMOS inverter formed by the production method according to the fourth embodiment. 
         FIGS. 45(   a ) and  45 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 46(   a ) and  46 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 47(   a ) and  47 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 48(   a ) and  48 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 49(   a ) and  49 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 50(   a ) and  50 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 51(   a ) and  51 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 52(   a ) and  52 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 53(   a ) and  53 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 54(   a ) and  54 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 55(   a ) and  55 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 56(   a ) and  56 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 57(   a ) and  57 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 58(   a ) and  58 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 59(   a ) and  59 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 60(   a ) and  60 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 61(   a ) and  61 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 62(   a ) and  62 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIGS. 63(   a ) and  63 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fourth embodiment. 
         FIG. 64  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to a fifth embodiment of the present invention. 
         FIG. 65  is a top plan view of the CMOS inverter formed by the production method according to the fifth embodiment. 
         FIGS. 66(   a ) and  66 ( b ) are sectional views of the CMOS inverter formed by the production method according to the fifth embodiment. 
         FIGS. 67(   a ) and  67 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 68(   a ) and  68 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 69(   a ) and  69 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 70(   a ) and  70 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 71(   a ) and  71 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 72(   a ) and  72 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 73(   a ) and  73 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 74(   a ) and  74 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 75(   a ) and  75 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIGS. 76(   a ) and  76 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the fifth embodiment. 
         FIG. 77  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to a sixth embodiment of the present invention. 
         FIG. 78  is a top plan view of the CMOS inverter formed by the production method according to the sixth embodiment. 
         FIGS. 79(   a ) and  79 ( b ) are sectional views of the CMOS inverter formed by the production method according to the sixth embodiment. 
         FIGS. 80(   a ) and  80 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 81(   a ) and  81 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 82(   a ) and  82 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 83(   a ) and  83 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 84(   a ) and  84 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 85(   a ) and  85 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 86(   a ) and  86 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 87(   a ) and  87 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 88(   a ) and  88 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 89(   a ) and  89 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 90(   a ) and  90 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the sixth embodiment. 
         FIGS. 91(   a ) and  91 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to a seventh embodiment of the present invention. 
         FIGS. 92(   a ) and  92 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 93(   a ) and  93 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 94(   a ) and  94 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 95(   a ) and  95 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 96(   a ) and  96 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 97(   a ) and  97 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 98(   a ) and  98 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 99(   a ) and  99 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 100(   a ) and  100 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 101(   a ) and  101 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 102(   a ) and  102 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 103(   a ) and  103 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 104(   a ) and  104 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 105(   a ) and  105 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 106(   a ) and  106 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 107(   a ) and  107 ( b ) illustrate a part of the steps of the single-SGT production method according to the seventh embodiment. 
         FIGS. 108(   a ) and  108 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to an eighth embodiment of the present invention. 
         FIGS. 109(   a ) and  109 ( b ) illustrate a part of a series of steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 110(   a ) and  110 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 111(   a ) and  111 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 112(   a ) and  112 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 113(   a ) and  113 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 114(   a ) and  114 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 115(   a ) and  115 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 116(   a ) and  116 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 117(   a ) and  117 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 118(   a ) and  118 ( b ) illustrate a part of the steps of the single-SGT production method according to the eighth embodiment. 
         FIGS. 119(   a ) and  119 ( b ) are, respectively, a top plan view and a sectional view of a single SGT formed by a production method according to a ninth embodiment of the present invention. 
         FIGS. 120(   a ) and  120 ( b ) illustrate a part of a series of steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 121(   a ) and  121 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 122(   a ) and  122 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 123(   a ) and  123 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 124(   a ) and  124 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 125(   a ) and  125 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 126(   a ) and  126 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 127(   a ) and  127 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 128(   a ) and  128 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 129(   a ) and  129 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIGS. 130(   a ) and  130 ( b ) illustrate a part of the steps of the single-SGT production method according to the ninth embodiment. 
         FIG. 131  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to a tenth embodiment of the present invention. 
         FIG. 132  is a top plan view of the CMOS inverter formed by the production method according to the tenth embodiment. 
         FIGS. 133(   a ) and  133 ( b ) are sectional views of the CMOS inverter formed by the production method according to the tenth embodiment. 
         FIGS. 134(   a ) and  134 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 135(   a ) and  135 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 136(   a ) and  136 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 137(   a ) and  137 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 138(   a ) and  138 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 139(   a ) and  139 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 140(   a ) and  140 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 141(   a ) and  141 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 142(   a ) and  142 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 143(   a ) and  143 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 144(   a ) and  144 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 145(   a ) and  145 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 146(   a ) and  146 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 147(   a ) and  147 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 148(   a ) and  148 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIGS. 149(   a ) and  149 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the tenth embodiment. 
         FIG. 150  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to an eleventh embodiment of the present invention. 
         FIG. 151  is a top plan view of the CMOS inverter formed by the production method according to the eleventh embodiment. 
         FIGS. 152(   a ) and  152 ( b ) are sectional views of the CMOS inverter formed by the production method according to the eleventh embodiment. 
         FIGS. 153(   a ) and  153 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 154(   a ) and  154 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 155(   a ) and  155 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 156(   a ) and  156 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 157(   a ) and  157 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 158(   a ) and  158 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 159(   a ) and  159 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 160(   a ) and  160 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 161(   a ) and  161 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIGS. 162(   a ) and  162 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the eleventh embodiment. 
         FIG. 163  is an equivalent circuit diagram of a CMOS inverter formed by a production method according to a twelfth embodiment of the present invention. 
         FIG. 164  is a top plan view of the CMOS inverter formed by the production method according to the twelfth embodiment. 
         FIGS. 165(   a ) and  165 ( b ) are sectional views of the CMOS inverter formed by the production method according to the twelfth embodiment. 
         FIGS. 166(   a ) and  166 ( b ) illustrate a part of a series of steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 167(   a ) and  167 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 168(   a ) and  168 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 169(   a ) and  169 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 170(   a ) and  170 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 171(   a ) and  171 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 172(   a ) and  172 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 173(   a ) and  173 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 174(   a ) and  174 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 175(   a ) and  175 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 176(   a ) and  176 ( b ) illustrate a part of the steps of the CMOS inverter production method according to the twelfth embodiment. 
         FIGS. 177(   a ) and  177 ( b ) are, respectively, a top plan view and a sectional view of a conventional SGT. 
         FIGS. 178(   a ) to  178 ( f ) illustrate a series of steps of a conventional SGT production method. 
         FIGS. 179(   a ) to  179 ( c ) illustrate a part of the steps of the conventional SGT production method. 
         FIGS. 180(   a ) to  180 ( c ) illustrate a part of the steps of the conventional SGT production method. 
         FIGS. 181(   a ) to  181 ( g ) illustrate a series of steps of a conventional SGT production method. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIGS. 1(   a ) and  1 ( b ) are, respectively, a top plan view and a sectional view of an NMOS SGT formed by a SGT production method according to a first embodiment of the present invention. With reference to  FIGS. 1(   a ) and  1 ( b ), the NMOS SGT formed by the SGT production method according to the first embodiment will be described below. 
     A pillar-shaped silicon layer  102  is formed on a silicon substrate  101 , and a gate dielectric film  105  and a gate electrode  106   a  are formed around the pillar-shaped silicon layer  102 . An N +  drain diffusion layer  103  is formed beneath the pillar-shaped silicon layer  102 , and an N +  source diffusion layer  104  is formed in an upper portion of the pillar-shaped silicon layer  102 . A contact  107 , a contact  108 , and a contact  109 , are formed on the N +  drain diffusion layer  103 , the N +  source diffusion layer  104 , and a gate line  106   b  extending from the gate electrode  106   a , respectively. 
     Under conditions that the N +  source diffusion layer  104  is connected to a GND potential, and the N +  drain diffusion layer  103  is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode  106   a  to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 2(   a ) to  16 ( b ), one example of the SGT production method according to the first embodiment will be described below. In  FIGS. 2(   a ) to  16 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     Referring to  FIGS. 2(   a ) and  2 ( b ), a silicon nitride film  110  serving as a hard mask is formed on a silicon substrate  101  to have a film thickness of about 50 to 150 nm. 
     Referring to  FIGS. 3(   a ) and  3 ( b ), the hard mask  110  and the silicon substrate  101  are etched to form a pillar-shaped silicon layer  102 . The pillar-shaped silicon layer  102  is formed to have a height dimension of about 30 to 300 nm, and a diameter of about 5 to 100 nm. 
     Referring to  FIGS. 4(   a ) and  4 ( b ), an impurity, such as P or As, is introduced into a top surface of the silicon substrate, for example, by ion implantation, to form an N +  drain diffusion layer  103  therein. During this step, the silicon nitride film  110  on a top of the pillar-shaped silicon layer functions as a stopper for preventing the impurity from being injected into the top of the pillar-shaped silicon layer. 
     Referring to  FIGS. 5(   a ) and  5 ( b ), a gate dielectric film  105  and a gate conductive film  106  are formed by chemical vapor deposition (CVD) or atomic layer deposition (ALD). The gate conductive film  106  is formed to have a film thickness of about 10 to 100 nm. 
     Referring to  FIGS. 6(   a ) and  6 ( b ), a silicon oxide film  111  is formed to allow the pillar-shaped silicon layer to be buried therein. 
     Referring to  FIGS. 7(   a ) and  7 ( b ), the silicon oxide film  111 , and respective portions of the gate conductive film and the gate dielectric film above the pillar-shaped silicon layer, are polished by chemical mechanical polishing (CMP), to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, the silicon nitride film  110  on the top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  110  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. In place of the silicon nitride film, the film to be used as a CMP stopper may be any other suitable film capable of functioning as the CMP stopper film. This modification may also be made in after-mentioned embodiments. 
     Referring to  FIGS. 8(   a ) and  8 ( b ), the gate conductive film  106  and the silicon oxide film  111  are etched back, wherein the gate conductive film  106  is etched to fix a gate length. Preferably, etching conditions to be used in this step are set to allow the gate conductive film  106  and the silicon oxide film  111  to be etched at the same rate, and at a higher selectivity ratio relative to the silicon nitride film  110 . The etching of the gate conductive film  106  and the silicon oxide film  111  at the same rate makes it possible to suppress occurrence of a step between respective top surfaces of the two films, which improves a configuration of a silicon nitride film-based sidewall  112  to be formed in a next step. 
     Referring to  FIGS. 9(   a ) and  9 ( b ), a silicon nitride film  112   a  is formed by a film thickness required for the gate conductive film  106 . Subsequently, as shown in  FIGS. 10(   a ) and  10 ( b ), the silicon nitride film  112   a  is etched back to form a silicon nitride film-based sidewall  112 . In this step, a film thickness of the silicon nitride film-based sidewall  112  is controlled to become equal to that of the gate conductive film  106 , by adjusting a formed film thickness of the silicon nitride film  112   a , and then finely adjusting the formed film thickness based on an amount of the etch-back. A portion of the gate conductive film  106  covered by the silicon nitride film-based sidewall  112  will be protected during etching for forming a gate line in a subsequent step. This makes it possible to form the gate electrode in a self-alignment manner and with a desired film thickness, so as to reduce an occupancy area. In the first embodiment, the silicon nitride film is used as a sidewall protective film. Alternatively, any other suitable film capable of functioning as the sidewall protective film, such as a silicon oxide film, may also be used. This modification may also be made in the after-mentioned embodiments. 
     Referring to  FIGS. 11(   a ) and  11 ( b ), the silicon oxide film  111  remaining on the gate conductive film is removed by wet etching. 
     Referring to  FIGS. 12(   a ) and  12 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  113  by lithography. 
     Referring to  FIGS. 13(   a ) and  13 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  106   a  and a gate line  106   b.    
     Referring to  FIGS. 14(   a ) and  14 ( b ), the silicon nitride film  110  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  112 , are removed by wet etching. 
     Referring to  FIGS. 15(   a ) and  15 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  102 , for example, by ion implantation, to form an N +  source diffusion layer  104  therein. 
     Referring to  FIGS. 16(   a ) and  16 ( b ), an interlayer dielectric film is formed, and a contact ( 107 ,  108 ,  109 ) is formed on each of the drain diffusion layer in the upper region of the silicon substrate, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the first embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the first embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the first embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the first embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and adjust a film thickness of the gate electrode to be formed around the pillar-shaped silicon layer, based on a formed film thickness of a gate conductive film. Thus, two pillar-shaped silicon layers each having a gate electrode to be applied with a different potential can be arranged side-by-side with a relatively small distance therebetween, to reduce a circuit area. In cases where the gate conductive film is formed to have a relatively small film thickness, a resistance value thereof becomes higher. Thus, in the first embodiment, the gate conductive film is preferably comprised of a metal film. 
     Second Embodiments 
     A second embodiment of the present invention shows a gate forming process capable of reducing the number of steps and further increasing a process margin, as compared with the gate forming process in the first embodiment. 
       FIGS. 17(   a ) and  17 ( b ) are, respectively, a top plan view and a sectional view of an NMOS SGT formed by a SGT production method according to the second embodiment. With reference to  FIGS. 17(   a ) and  17 ( b ), the NMOS SGT formed by the SGT production method according to the second embodiment will be described below. 
     A pillar-shaped silicon layer  202  is formed on a silicon substrate  201 , and a gate dielectric film  205  and a gate electrode  206   a  are formed around the pillar-shaped silicon layer  202 . An N +  drain diffusion layer  203  is formed beneath the pillar-shaped silicon layer  202 , and an N +  source diffusion layer  204  is formed in an upper portion of the pillar-shaped silicon layer  202 . A contact  207 , a contact  208 , and a contact  209 , are formed on the N +  drain diffusion layer  203 , the N +  source diffusion layer  204 , and a gate line  206   b  extending from the gate electrode  206   a , respectively. 
     In the second embodiment, the gate electrode  206   a  and the gate line  206   b  are formed to be at the same height position. Specifically, the gate electrode is integrally formed with the gate line in such a manner that an entire area of a top surface of the integrated combination of the gate electrode and the gate line becomes parallel to the substrate. 
     Under conditions that the N +  source diffusion layer  204  is connected to a GND potential, and the N +  drain diffusion layer  203  is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode  206   a  to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 18(   a ) to  27 ( b ), one example of the SGT production method according to the second embodiment will be described below. In  FIGS. 18(   a ) to  27 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     In the second embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the first embodiment. Thus, the following description will be started from the step of forming a gate conductive film. 
     Referring to  FIGS. 18(   a ) and  18 ( b ), a gate conductive film  206  is formed by CVD or ALD, to allow a pillar-shaped silicon layer  202  to be buried therein. 
     Referring to  FIGS. 19(   a ) and  19 ( b ), the gate conductive film  206  is polished by CMP, to flatten a top of the gate conductive film. Through the flattening of the top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, a silicon nitride film  210  on a top of a pillar-shaped silicon layer  202  is used as a CMP stopper. The use of the silicon nitride film  210  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 20(   a ) and  20 ( b ), the gate conductive film  206  is etched back to fix a gate length. 
     Referring to  FIGS. 21(   a ) and  21 ( b ), a silicon nitride film  212   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 22(   a ) and  22 ( b ), the silicon nitride film  212   a  is etched back to form a silicon nitride film-based sidewall  212 . In the second embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  212 . Thus, a final film thickness of the silicon nitride film-based sidewall  212  is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  212   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 23(   a ) and  23 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  213  by lithography. 
     Referring to  FIGS. 24(   a ) and  24 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  206   a  and a gate line  206   b.    
     Referring to  FIGS. 25(   a ) and  25 ( b ), the silicon nitride film  210  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  212 , are removed by wet etching. 
     Referring to  FIGS. 26(   a ) and  26 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  202 , for example, by ion implantation, to form an N +  source diffusion layer  204  therein. 
     Referring to  FIGS. 27(   a ) and  27 ( b ), an interlayer dielectric film is formed, and a contact ( 207 ,  208 ,  209 ) is formed on each of the drain diffusion layer in the upper region of the silicon substrate, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the second embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the second embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the second embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the second embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. In the first embodiment, a film thickness of a gate electrode is controlled based on a formed film thickness of a gate conductive film. Differently, in the second embodiment, the film thickness of the gate electrode can be controlled based on a film thickness of the silicon nitride film-based sidewall  212 . Further, in the second embodiment, the gate line  206   b  has a relatively large film thickness as compared with that of the gate line in the first embodiment. Thus, the gate conductive film is not limited to a metal film, but may be made of a material having relatively high electrical resistance, such as polysilicon. 
     In the first embodiment, the silicon nitride film-based sidewall  112  must be formed to have a thickness approximately equal to that of the gate conductive film  106 . Thus, if the sidewall  112  is excessively thicker or thinner than the gate conductive film  106 , a problem is likely to occur. Specifically, as shown in  FIGS. 28(   a ) to  28 ( d ), in the case where the sidewall  112  is excessively thicker than the gate conductive film  106 , a silicon nitride film-based sidewall  112  having a film thickness greater than that of a gate conductive film  106  is formed ( FIG. 28(   a )), and a silicon oxide film  111  is removed by wet etching ( FIG. 28(   b )), whereafter a gate line pattern is formed by lithography ( FIG. 28(   c )), and a gate electrode  106   a  and a gate line  106   b  are formed by etching. In this case, the gate electrode has a protrusion  106   c  formed in a lower end thereof corresponding to a region which has not been covered by a resist  113 . If the protrusion becomes significantly large, such a defective structure is likely to become cause a problem, such as a short-circuiting between the protrusion  106   c  of the gate electrode and an adjacent contact. As shown in  FIGS. 29(   a ) to  29 ( d ), in the case where the sidewall  112  is excessively thinner than the gate conductive film  106 , a silicon nitride film-based sidewall  112  having a film thickness less than that of a gate conductive film  106  is formed ( FIG. 29(   a )), and a silicon oxide film  111  is removed by wet etching ( FIG. 29(   b )), whereafter a gate line pattern is formed by lithography ( FIG. 29(   c )), and a gate electrode  106   a  and a gate line  106   b  are formed by etching. In this case, a part of a top of the gate conductive film is not covered by a resist  113 , and thereby subjected to etching. Thus, a film thickness of the gate electrode is reduced. If the reduction in film thickness becomes significant, such a defective structure is likely to cause a problem, such as etching damage on a gate dielectric film, or a change in transistor characteristics. Differently, in the second embodiment, the gate electrode is formed to have a desired film thickness, in a self-alignment manner based on the silicon nitride film-based sidewall  112  having a film thickness equal to the desired film thickness of the gate electrode. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the first embodiment. 
     Third Embodiment 
     A NMOS SGT formed by a SGT production method according to a third embodiment of the present invention is different from the NMOS SGT in the second embodiment, in that a gate electrode and a gate line extending from the gate electrode are formed in a layered structure which comprises a thin metal film and a polysilicon layer. In a gate forming process in the third embodiment, the thin metal film is formed to be in contact with a gate dielectric film so as to suppress depletion of the gate electrode, and the polysilicon layer is formed to define respective top surfaces of the gate electrode and the gate line, so as to allow the SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
       FIGS. 30(   a ) and  30 ( b ) are, respectively, a top plan view and a sectional view of the NMOS SGT formed by the SGT production method according to the third embodiment. With reference to  FIGS. 30(   a ) and  30 ( b ), the NMOS SGT formed by the method according to the third embodiment will be described below. 
     A pillar-shaped silicon layer  302  is formed on a silicon substrate  301 , and a gate dielectric film  305  and a gate electrode  306   a  are formed around the pillar-shaped silicon layer  302 . The gate electrode has a layered structure which comprises a thin metal film  314  having a film thickness of about 1 to 10 nm, and a polysilicon layer  306   a  covering the metal film. An N +  drain diffusion layer  303  is formed beneath the pillar-shaped silicon layer  302 , and an N +  source diffusion layer  304  is formed in an upper portion of the pillar-shaped silicon layer  302 . A contact  307 , a contact  308 , and a contact  309 , are formed on the N +  drain diffusion layer  303 , the N +  source diffusion layer  304 , and a gate line  306   b  extending from the gate electrode  306   a , respectively. 
     In the third embodiment, the gate electrode  306   a  and the gate line  306   b  are formed to be at the same height position, in the same manner as that in the second embodiment. Specifically, the gate electrode is integrally formed with the gate line in such a manner that an entire area of a top surface of the integrated combination of the gate electrode and the gate line becomes parallel to the substrate. 
     Under conditions that the N +  source diffusion layer  304  is connected to a GND potential, and the N +  drain diffusion layer  303  is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode  306   a  to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 31(   a ) to  41 ( b ), one example of the SGT production method according to the third embodiment will be described below. In  FIGS. 31(   a ) to  41 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     In the third embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the second embodiment. Thus, the following description will be started from the step of forming a thin metal film and a polysilicon layer. 
     Referring to  FIGS. 31(   a ) and  31 ( b ), after forming a gate dielectric film  305 , a thin metal film  314  is formed to have a film thickness of about 1 to 10 nm, and then a polysilicon layer  306  is formed to allow a pillar-shaped silicon layer  302  to be buried therein. 
     Referring to  FIGS. 32(   a ) and  32 ( b ), the polysilicon layer  306 , and respective portions of the thin metal film  314  and the gate dielectric film  305  above the pillar-shaped silicon layer, are polished by CMP, to flatten respective top surfaces of the polysilicon layer  306  and the thin metal film  314 . Through the flattening of respective tops of the polysilicon layer  306  and the thin metal film  314  by CMP, respective configurations of the polysilicon layer  306  and the thin metal film  314  are improved to facilitate control of a gate length. During the CMP, a silicon nitride film  310  on a top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  310  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 33(   a ) and  33 ( b ), the polysilicon layer  306  and the thin metal film  314  are etched back to fix a gate length. 
     Referring to  FIGS. 34(   a ) and  34 ( b ), a silicon nitride film  312   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 35(   a ) and  35 ( b ), the silicon nitride film  312   a  is etched back to form a silicon nitride film-based sidewall  312 . In the third embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  312 . Thus, a final film thickness of the silicon nitride film-based sidewall  312  is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  312   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 36(   a ) and  36 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  313  by lithography. 
     Referring to  FIGS. 37(   a ) and  37 ( b ), the polysilicon layer, the thin metal film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  306   a  and a gate line  306   b.    
     Referring to  FIGS. 38(   a ) and  38 ( b ), the silicon nitride film  310  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  312 , are removed by wet etching. 
     Referring to  FIGS. 39(   a ) and  39 ( b ), a silicon nitride film is formed and then etched back to form a silicon nitride film  315 . The silicon nitride film  315  is formed to cover the thin metal film  314  of the gate electrode to keep a top surface of the thin metal film  314  from being exposed. This makes it possible to produce an intended SGT in the same production line as that for a transistor having a conventional polysilicon gate. 
     Referring to  FIGS. 40(   a ) and  40 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  302 , for example, by ion implantation, to form an N +  source diffusion layer  304  therein. 
     Referring to  FIGS. 41(   a ) and  41 ( b ), an interlayer dielectric film is formed, and a contact ( 307 ,  308 ,  309 ) is formed on each of the drain diffusion layer in the upper region of the silicon substrate, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the third embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the third embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening respective top surfaces of a polysilicon layer and a thin metal film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the third embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the third embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and control a film thickness of the gate electrode based on a film thickness of the silicon nitride film-based sidewall  312 , as with the second embodiment. 
     In the third embodiment, a gate is formed in a layered structure which comprises the thin metal film and the polysilicon layer, which is capable of suppressing depletion of the gate electrode, and allowing an intended SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
     In the first embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the third embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  312 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the first embodiment. 
     Fourth Embodiment 
     A fourth embodiment of the present invention shows a method of producing a CMOS inverter using the same gate forming process as that in the first embodiment. Thus, the method according to the fourth embodiment can be employed to obtain the same advantageous effects as those in the first embodiment. 
       FIG. 42  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the fourth embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  1  is applied to a gate of an NMOS Qn  1  and a gate of a PMOS Qp  1 . When the Vin  1  is “1”, the NMOS Qn  1  is placed in an ON state, and the PMOS Qp  1  is placed in an OFF state, so that an output signal Vout  1  becomes “0”. Reversely, when the Vin  1  is “0”, the NMOS Qn  1  is placed in an OFF state, and the PMOS Qp  1  is placed in an ON state, so that the Vout  1  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  1  to have a value opposite to that of the input signal Vin  1 . 
       FIG. 43  is a top plan view of the CMOS inverter formed by the method according to the fourth embodiment.  FIGS. 44(   a ) and  44 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 43 , respectively. With reference to  FIGS. 43 ,  44 ( a ) and  44 ( b ), a structure of the CMOS inverter will be described. 
     A P-well  402  and an N-well  403  are formed in an upper region of a silicon substrate  401 . A pillar-shaped silicon layer  407  forming an NMOS (NMOS-forming pillar-shaped silicon layer  407 ) and a pillar-shaped silicon layer  408  forming a PMOS (PMOS-forming pillar-shaped silicon layer  408 ) are formed on a surface of the silicon substrate, specifically on respective ones of the P-well region and the N-well region. A gate dielectric film  409  and a gate electrode ( 410   a ,  410   b ) are formed to surround the pillar-shaped silicon layers. Further, the gate electrodes  410   a ,  410   b  are connected to each other through a gate line  410   c  extending therefrom. 
     An N +  drain diffusion layer  404  is formed beneath the NMOS-forming pillar-shaped silicon layer  407 , and an N +  source diffusion layer  411  is formed in an upper portion of the NMOS-forming pillar-shaped silicon layer  407 . A P +  drain diffusion layer  405  is formed beneath the PMOS-forming pillar-shaped silicon layer  408 , and a P +  source diffusion layer  412  is formed in an upper portion of the PMOS-forming pillar-shaped silicon layer  408 . Each of the N +  drain diffusion layer  404  and the P +  drain diffusion layer  405  formed beneath respective ones of the pillar-shaped silicon layers  407 ,  408  is connected to the output terminal Vout  1  via a contact ( 416   a ,  416   b ). The N +  source diffusion layer  411  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  407  is connected to a ground potential Vss  1  via a contact  414 , and the P +  source diffusion layer  412  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  408  is connected to a power supply potential Vcc  1  via a contact  415 . Further, the gate line  410   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  1  via a contact  413 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 45(   a ) to  63 ( b ), one example of the SGT production method according to the fourth embodiment will be described below. In  FIGS. 45(   a ) to  63 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     Referring to  FIGS. 45(   a ) and  45 ( b ), a silicon nitride film  417  serving as a hard mask is formed on a silicon substrate  401  to have a film thickness of about 50 to 150 nm. 
     Referring to  FIGS. 46(   a ) and  46 ( b ), the hard mask  417  and the silicon substrate  401  are etched to form an element isolation region  406 . 
     Referring to  FIGS. 47(   a ) and  47 ( b ), a silicon oxide film  422  is filled in the element isolation region  406 . 
     Referring to  FIGS. 48(   a ) and  48 ( b ), a portion of the silicon oxide film  422  above the hard mask  417  is polished and flattened by CMP. 
     Referring to  FIGS. 49(   a ) and  49 ( b ), the silicon oxide film  422  filled in the element isolation region  406  is etched back in such a manner that a height position of the silicon oxide film  422  is adjusted to become equal to that of a drain diffusion layer which is to be formed in a subsequent step. 
     Referring to  FIGS. 50(   a ) and  50 ( b ), the hard mask  417  and the silicon substrate  401  are etched to form a pillar-shaped silicon layer ( 407 ,  408 ). 
     Referring to  FIGS. 51(   a ) and  51 ( b ), impurities are introduced into a surface of the silicon substrate, for example, by ion implantation, to form an N +  drain diffusion layer  404  and a P +  drain diffusion layer  405  therein. During this step, the silicon nitride film  417  on a top of each of the pillar-shaped silicon layers functions as a stopper for preventing the impurity from being injected into the top of the pillar-shaped silicon layer. 
     Referring to  FIGS. 52(   a ) and  52 ( b ), a gate dielectric film  409  and a gate conductive film  410  are formed by CVD or ALD. The gate conductive film  410  is formed to have a film thickness of about 10 to 100 nm. 
     Referring to  FIGS. 53(   a ) and  53 ( b ), a silicon oxide film  418  is formed to allow the pillar-shaped silicon layers to be buried therein. 
     Referring to  FIGS. 54(   a ) and  54 ( b ), the silicon oxide film  418 , and respective portions of the gate conductive film and the gate dielectric film above the pillar-shaped silicon layer, are polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, the silicon nitride film  417  on the top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  417  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 55(   a ) and  55 ( b ), the gate conductive film  410  and the silicon oxide film  418  are etched back, wherein the gate conductive film  410  is etched to fix a gate length. Preferably, etching conditions to be used in this step are set to allow the gate conductive film  410  and the silicon oxide film  418  to be etched at the same rate, and at a higher selectivity ratio relative to the silicon nitride film  417 . The etching of the gate conductive film  410  and the silicon oxide film  418  at the same rate makes it possible to suppress occurrence of a step between respective top surfaces of the two films, which improves a configuration of a silicon nitride film-based sidewall  112  to be formed in a next step. 
     Referring to  FIGS. 56(   a ) and  56 ( b ), a silicon nitride film  419   a  is formed by a film thickness required for the gate conductive film  410 . Subsequently, as shown in  FIGS. 57(   a ) and  57 ( b ), the silicon nitride film  419   a  is etched back to form a silicon nitride film-based sidewall  419 . In this step, a film thickness of the silicon nitride film-based sidewall  419  is controlled to become equal to that of the gate conductive film  410 , by adjusting a formed film thickness of the silicon nitride film  419   a , and then finely adjusting the formed film thickness based on an amount of the etch-back. A portion of the gate conductive film covered by the silicon nitride film-based sidewall  419  will be protected during etching for forming a gate line in a subsequent step. This makes it possible to form the gate electrode in a self-alignment manner and with a desired film thickness, so as to reduce an occupancy area. 
     Referring to  FIGS. 58(   a ) and  58 ( b ), the silicon oxide film  418  remaining on the gate conductive film is removed by wet etching. 
     Referring to  FIGS. 59(   a ) and  59 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  420  by lithography. 
     Referring to  FIGS. 60(   a ) and  60 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode ( 410   a ,  410   b ) and a gate line  410   c.    
     Referring to  FIGS. 61(   a ) and  61 ( b ), the silicon nitride film  417  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  419 , are removed by wet etching. 
     Referring to  FIGS. 62(   a ) and  62 ( b ), impurities are introduced into respective top portions of the pillar-shaped silicon layers  407 ,  408 , for example, by ion implantation, to form an N +  source diffusion layer  411  and a P +  source diffusion layer  412  therein. 
     Referring to  FIGS. 63(   a ) and  63 ( b ), an interlayer dielectric film is formed, and a contact ( 413 ,  414 ,  415 ,  416   a ,  416   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers, and the drain diffusion layers in the upper region of the silicon substrate. 
     In the method according to the fourth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the fourth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the fourth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the fourth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment mariner and with a desired film thickness, and adjust a film thickness of the gate electrode to be formed around the pillar-shaped silicon layer, based on a formed film thickness of a gate conductive film. Thus, a distance between a pillar-shaped silicon layer ( 410   a ,  410   b ) and a contact ( 416   a ,  416   b ) on a drain diffusion layer can be reduced so that an area of a circuit, such as an inverter circuit, can be reduced. In cases where the gate conductive film is formed to have a relatively small film thickness, a resistance value thereof becomes higher. Thus, in the fourth embodiment, the gate conductive film is preferably comprised of a metal film. 
     Although the SGT production method according to the fourth embodiment has been described based on one example where it is applied to a CMOS inverter, it is understood that the present invention may be applied to any suitable circuit other than the CMOS inverter, in just the same manner. 
     Fifth Embodiment 
     A fifth embodiment of the present invention shows a method of producing a CMOS inverter using the same gate forming process as that in the second embodiment. Thus, the method according to the fifth embodiment can be employed to obtain the same advantageous effects as those in the second embodiment. 
       FIG. 64  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the fifth embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  2  is applied to a gate of an NMOS Qn  2  and a gate of a PMOS Qp  2 . When the Vin  2  is “1”, the NMOS Qn  2  is placed in an ON state, and the PMOS Qp  2  is placed in an OFF state, so that an output signal Vout  2  becomes “0”. Reversely, when the Vin  2  is “0”, the NMOS Qn  2  is placed in an OFF state, and the PMOS Qp  2  is placed in an ON state, so that the Vout  2  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  2  to have a value opposite to that of the input signal Vin  2 . 
       FIG. 65  is a top plan view of the CMOS inverter formed by the method according to the fifth embodiment.  FIGS. 66(   a ) and  66 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 65 , respectively. With reference to  FIGS. 65 ,  66 ( a ) and  66 ( b ), a structure of the CMOS inverter will be described. 
     A P-well  502  and an N-well  503  are formed in an upper region of a silicon substrate  501 . A pillar-shaped silicon layer  507  forming an NMOS (NMOS-forming pillar-shaped silicon layer  507 ) and a pillar-shaped silicon layer  508  forming a PMOS (PMOS-forming pillar-shaped silicon layer  508 ) are formed on a surface of the silicon substrate, specifically on respective ones of the P-well region and the N-well region. A gate dielectric film  509  and a gate electrode ( 510   a ,  510   b ) are formed to surround the pillar-shaped silicon layers. Further, the gate electrodes  510   a ,  510   b  are connected to each other through a gate line  510   c  extending therefrom, and the gate electrode ( 510   a ,  510   b ) and the gate line  510   c  are formed to be at the same height position. An N +  drain diffusion layer  504  is formed beneath the NMOS-forming pillar-shaped silicon layer  507 , and an N +  source diffusion layer  511  is formed in an upper portion of the NMOS-forming pillar-shaped silicon layer  507 . A P +  drain diffusion layer  505  is formed beneath the PMOS-forming pillar-shaped silicon layer  508 , and a P +  source diffusion layer  512  is formed in an upper portion of the PMOS-forming pillar-shaped silicon layer  508 . 
     Each of the N +  drain diffusion layer  504  and the P +  drain diffusion layer  505  formed beneath respective ones of the pillar-shaped silicon layers  507 ,  508  is connected to the output terminal Vout  2  via a contact ( 516   a ,  516   b ). The N +  source diffusion layer  511  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  507  is connected to a ground potential Vss  2  via a contact  514 , and the P +  source diffusion layer  512  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  508  is connected to a power supply potential Vcc  2  via a contact  515 . Further, the gate line  510   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  2  via a contact  513 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 67(   a ) to  76 ( b ), one example of the SGT production method according to the fifth embodiment will be described below. In  FIGS. 67(   a ) to  76 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. In the fifth embodiment, any step before the step of forming a gate dielectric film is the same as those in the fourth embodiment. Thus, the following description will be started from the step of forming a gate conductive film. 
     Referring to  FIGS. 67(   a ) and  67 ( b ), a gate dielectric film  509  and a gate conductive film  510  are formed by CVD or ALD, wherein the gate conductive film  510  is formed to allow a pillar-shaped silicon layer ( 507 ,  508 ) to be buried therein. 
     Referring to  FIGS. 68(   a ) and  68 ( b ), the gate conductive film  510  is polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, a silicon nitride film  517  on a top of a pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  517  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 69(   a ) and  69 ( b ), the gate conductive film  510  is etched back to fix a gate length. 
     Referring to  FIGS. 70(   a ) and  70 ( b ), a silicon nitride film  519   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 71(   a ) and  71 ( b ), the silicon nitride film  519   a  is etched back to form a silicon nitride film-based sidewall  519 . In the fifth embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  519 . Thus, a final film thickness of the silicon nitride film-based sidewall is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  519   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 72(   a ) and  72 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  520  by lithography. 
     Referring to  FIGS. 73(   a ) and  73 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode ( 510   a ,  510   b ) and a gate line  510   c.    
     Referring to  FIGS. 74(   a ) and  74 ( b ), the silicon nitride film  517  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  519 , are removed by wet etching. 
     Referring to  FIGS. 75(   a ) and  75 ( b ), impurities are is introduced into respective top portions of the pillar-shaped silicon layers  507 ,  508 , for example, by ion implantation, to form an N +  source diffusion layer  511  and P +  source diffusion layer  512  therein. 
     Referring to  FIGS. 76(   a ) and  76 ( b ), an interlayer dielectric film is formed, and a contact ( 513 ,  514 ,  515 ,  516   a ,  516   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers and the drain diffusion layers in the upper region of the silicon substrate. 
     In the method according to the fifth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the fifth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the fifth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the fifth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. In the fourth embodiment, a film thickness of a gate electrode is controlled based on a formed film thickness of a gate conductive film. Differently, in the fifth embodiment, the film thickness of the gate electrode can be controlled based on a film thickness of the silicon nitride film-based sidewall  519 . Further, in the fifth embodiment, the gate line  510   c  has a relatively large film thickness as compared with that of the gate line in the fourth embodiment. Thus, the gate conductive film is not limited to a metal film, but may be made of a material having relatively high electrical resistance, such as polysilicon. 
     In the fourth embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the fifth embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  519 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the fourth embodiment. 
     Although the SGT production method according to the fifth embodiment has been described based on one example where it is applied to a CMOS inverter, it is understood that the present invention may be applied to any suitable circuit other than the CMOS inverter, in just the same manner. 
     Sixth Embodiment 
     A sixth embodiment of the present invention shows a method of producing a CMOS inverter using the same gate forming process as that in the third embodiment. Thus, the method according to the sixth embodiment can be employed to obtain the same advantageous effects as those in the third embodiment. 
       FIG. 77  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the sixth embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  3  is applied to a gate of an NMOS Qn  3  and a gate of a PMOS Qp  3 . When the Vin  3  is “1”, the NMOS Qn  3  is placed in an ON state, and the PMOS Qp  3  is placed in an OFF state, so that an output signal Vout  3  becomes “0”. Reversely, when the Vin  3  is “0”, the NMOS Qn  3  is placed in an OFF state, and the PMOS Qp  3  is placed in an ON state, so that the Vout  3  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  3  to have a value opposite to that of the input signal Vin  3 . 
       FIG. 78  is a top plan view of the CMOS inverter formed by the method according to the sixth embodiment.  FIGS. 79(   a ) and  79 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 78 , respectively. With reference to  FIGS. 78 ,  79 ( a ) and  79 ( b ), a structure of the CMOS inverter will be described. 
     A P-well  602  and an N-well  603  are formed in an upper region of a silicon substrate  601 . A pillar-shaped silicon layer  607  forming an NMOS (NMOS-forming pillar-shaped silicon layer  607 ) and a pillar-shaped silicon layer  608  forming a PMOS (PMOS-forming pillar-shaped silicon layer  608 ) are formed on a surface of the silicon substrate, specifically on respective ones of the P-well region and the N-well region. A gate dielectric film  609  and a gate electrode ( 610   a ,  610   b ) are formed to surround the pillar-shaped silicon layers. Each of the gate electrodes is formed in a layered structure which comprises a polysilicon layer defining a top surface thereof, and a thin metal film  623  in contact with the gate dielectric film. Further, the gate electrodes  610   a ,  610   b  are connected to each other through a gate line  610   c  extending therefrom, and the gate electrode ( 610   a ,  610   b ) and the gate line  610   c  are formed to be at the same height position. An N +  drain diffusion layer  604  is formed beneath the NMOS-forming pillar-shaped silicon layer  607 , and an N +  source diffusion layer  611  is formed in an upper portion of the NMOS-forming pillar-shaped silicon layer  607 . A P +  drain diffusion layer  605  is formed beneath the PMOS-forming pillar-shaped silicon layer  608 , and a P +  source diffusion layer  612  is formed in an upper portion of the PMOS-forming pillar-shaped silicon layer  608 . 
     Each of the N +  drain diffusion layer  604  and the P +  drain diffusion layer  605  formed beneath respective ones of the pillar-shaped silicon layers  607 ,  608  is connected to the output terminal Vout  3  via a contact ( 616   a ,  616   b ). The N +  source diffusion layer  611  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  607  is connected to a ground potential Vss  3  via a contact  614 , and the P +  source diffusion layer  612  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  608  is connected to a power supply potential Vcc  3  via a contact  615 . Further, the gate line  610   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  3  via a contact  613 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 80(   a ) to  90 ( b ), one example of the SGT production method according to the sixth embodiment will be described below. In  FIGS. 80(   a ) to  90 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. In the sixth embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the fourth embodiment. Thus, the following description will be started from the step of forming a thin metal film and a polysilicon layer. 
     Referring to  FIGS. 80(   a ) and  80 ( b ), after forming a gate dielectric film  609 , a thin metal film  623  is formed to have a film thickness of about 1 to 10 nm, and then a polysilicon layer  610  is formed to allow a pillar-shaped silicon layer ( 607 ,  608 ) to be buried therein. 
     Referring to  FIGS. 81(   a ) and  81 ( b ), the polysilicon layer  610 , and respective portions of the thin metal film  623  and the gate dielectric film  609  above the pillar-shaped silicon layer, are polished by CMP, to flatten respective top surfaces of the polysilicon layer  610  and the thin metal film  623 . Through the flattening of respective tops of the polysilicon layer  610  and the thin metal film  623  by CMP, respective configurations of the polysilicon layer  610  and the thin metal film  623  are improved to facilitate control of a gate length. During the CMP, a silicon nitride film  617  on a top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  617  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 82(   a ) and  82 ( b ), the polysilicon layer  610  and the thin metal film  623  are etched back to fix a gate length. 
     Referring to  FIGS. 83(   a ) and  83 ( b ), a silicon nitride film  619   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 84(   a ) and  84 ( b ), the silicon nitride film  619   a  is etched back to form a silicon nitride film-based sidewall  619 . In the sixth embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  619 . Thus, a final film thickness of the silicon nitride film-based sidewall is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  312   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 85(   a ) and  85 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  620  by lithography. 
     Referring to  FIGS. 86(   a ) and  86 ( b ), the polysilicon layer, the thin metal film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode ( 610   a ,  610   b ) and a gate line  610   c.    
     Referring to  FIGS. 87(   a ) and  87 ( b ), the silicon nitride film  617  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  619 , are removed by wet etching. 
     Referring to  FIGS. 88(   a ) and  88 ( b ), a silicon nitride film is formed and then etched back to form a silicon nitride film  624 . The silicon nitride film  624  is formed to cover the thin metal film  623  of the gate electrode to keep a top surface of the thin metal film  623  from being exposed. This makes it possible to produce an intended SGT in the same production line as that for a transistor having a conventional polysilicon gate. 
     Referring to  FIGS. 89(   a ) and  89 ( b ), impurities are introduced into respective top portions of the pillar-shaped silicon layers  607 ,  608 , for example, by ion implantation, to form an N +  source diffusion layer  611  and a P +  source diffusion layer  612  therein. 
     Referring to  FIGS. 90(   a ) and  90 ( b ), an interlayer dielectric film is formed, and a contact ( 613 ,  614 ,  615 ,  616   a ,  616   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers and the drain diffusion layers in the upper region of the silicon substrate. 
     In the method according to the sixth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the sixth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening respective top surfaces of a polysilicon layer and a thin metal film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the sixth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the sixth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and control a film thickness of the gate electrode based on a film thickness of the silicon nitride film-based sidewall  619 . 
     In the sixth embodiment, a gate is formed in a layered structure which comprises the thin metal film and the polysilicon layer, which is capable of suppressing depletion of the gate electrode, and allowing an intended SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
     In the fourth embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the sixth embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  619 . This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the fourth embodiment. 
     Although the SGT production method according to the sixth embodiment has been described based on one example where it is applied to a CMOS inverter, it is understood that the present invention may be applied to any suitable circuit other than the CMOS inverter, in just the same manner. 
     Seventh Embodiment 
     A seventh embodiment of the present invention shows a method of producing an NMOS SGT on an SOI substrate (SOI NMOS SGT), using the same gate forming process as that in the first embodiment. 
       FIGS. 91(   a ) and  91 ( b ) are, respectively, a top plan view and a sectional view of the SOI NMOS SGT formed by the SGT production method according to the seventh embodiment. With reference to  FIGS. 91(   a ) and  91 ( b ), the SOI NMOS SGT formed by the SGT production method according to the seventh embodiment will be described below. 
     A planar silicon layer  701  is formed on a buried oxide film layer  700 . A pillar-shaped silicon layer  702  is formed on the planar silicon layer  701 , and a gate dielectric film  705  and a gate electrode  706   a  are formed around the pillar-shaped silicon layer  702 . An N +  drain diffusion layer  703  is formed in the planar silicon layer  701  beneath the pillar-shaped silicon layer  702 , and an N +  source diffusion layer  704  is formed in an upper portion of the pillar-shaped silicon layer  702 . A contact  707 , a contact  708 , and a contact  709 , are formed on the N +  drain diffusion layer  703 , the N +  source diffusion layer  704 , and a gate line  706   b  extending from the gate electrode  706   a , respectively. 
     Under conditions that the N +  source diffusion layer is connected to a GND potential, and the N +  drain diffusion layer is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 92(   a ) to  107 ( b ), one example of the SGT production method according to the seventh embodiment will be described below. In  FIGS. 92(   a ) to  107 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     Referring to  FIGS. 92(   a ) and  92 ( b ), a silicon nitride film  710  serving as a hard mask is formed on a silicon layer  701   a  on a buried oxide film layer  700  in an SOI substrate, to have a film thickness of about 50 to 150 nm. 
     Referring to  FIGS. 93(   a ) and  93 ( b ), the hard mask  710  and the silicon layer  701   a  are etched to form a pillar-shaped silicon layer  702 . Through the etching, the pillar-shaped silicon layer  702  is formed to have a height dimension of about 30 to 300 nm, and a diameter of about 5 to 100 nm. Further, a planar silicon layer  701  is formed beneath the pillar-shaped silicon layer  702  to have a thickness of about 10 to 100 nm. 
     Referring to  FIGS. 94(   a ) and  94 ( b ), the planar silicon layer  701  is formed in an isolated structure by etching. 
     Referring to  FIGS. 95(   a ) and  95 ( b ), an impurity, such as P or As, is introduced into a top surface of planar silicon layer, for example, by ion implantation, to form an N +  drain diffusion layer  703  therein. During this step, the silicon nitride film  710  on a top of the pillar-shaped silicon layer functions as a stopper for preventing the impurity from being injected into the top of the pillar-shaped silicon layer. 
     Referring to  FIGS. 96(   a ) and  96 ( b ), a gate dielectric film  705  and a gate conductive film  706  are formed by CVD or ALD. The gate conductive film  706  is formed to have a film thickness of about 10 to 100 nm. 
     Referring to  FIGS. 97(   a ) and  97 ( b ), a silicon oxide film  711  is formed to allow the pillar-shaped silicon layer to be buried therein. 
     Referring to  FIGS. 98(   a ) and  98 ( b ), the silicon oxide film  711 , and respective portions of the gate conductive film and the gate dielectric film above the pillar-shaped silicon layer, are polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, the silicon nitride film  710  on the top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  710  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 99(   a ) and  99 ( b ), the gate conductive film  706  and the silicon oxide film  711  are etched back, wherein the gate conductive film  706  is etched to fix a gate length. Preferably, etching conditions to be used in this step are set to allow the gate conductive film  706  and the silicon oxide film  711  to be etched at the same rate, and at a higher selectivity ratio relative to the silicon nitride film  710 . The etching of the gate conductive film  706  and the silicon oxide film  711  at the same rate makes it possible to suppress occurrence of a step between respective top surfaces of the two films, which improves a configuration of a silicon nitride film-based sidewall  712  to be formed in a next step. 
     Referring to  FIGS. 100(   a ) and  100 ( b ), a silicon nitride film  712   a  is formed by a film thickness required for the gate conductive film  706 . Subsequently, as shown in  FIGS. 101(   a ) and  101 ( b ), the silicon nitride film  712   a  is etched back to form a silicon nitride film-based sidewall  712 . In this step, a film thickness of the silicon nitride film-based sidewall  712  is controlled to become equal to that of the gate conductive film  706 , by adjusting a formed film thickness of the silicon nitride film  712   a , and then finely adjusting the formed film thickness based on an amount of the etch-back. A portion of the gate conductive film  706  covered by the silicon nitride film-based sidewall  712  will be protected during etching for forming a gate line in a subsequent step. This makes it possible to form the gate electrode in a self-alignment manner and with a desired film thickness, so as to reduce an occupancy area. 
     Referring to  FIGS. 102(   a ) and  102 ( b ), the silicon oxide film  711  remaining on the gate conductive film is removed by wet etching. 
     Referring to  FIGS. 103(   a ) and  103 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  713  by lithography. 
     Referring to  FIGS. 104(   a ) and  104 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  706   a  and a gate line  706   b.    
     Referring to  FIGS. 105(   a ) and  105 ( b ), the silicon nitride film  710  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  712 , are removed by wet etching. 
     Referring to  FIGS. 106(   a ) and  106 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  702 , for example, by ion implantation, to form an N +  source diffusion layer  704  therein. 
     Referring to  FIGS. 107(   a ) and  107 ( b ), an interlayer dielectric film is formed, and a contact ( 707 ,  708 ,  709 ) is formed on each of the drain diffusion layer in the planar silicon layer, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the seventh embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the seventh embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the seventh embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the seventh embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and adjust a film thickness of the gate electrode to be formed around the pillar-shaped silicon layer, based on a formed film thickness of a gate conductive film. Thus, two pillar-shaped silicon layers each having a gate electrode to be applied with a different potential can be arranged side-by-side with a relatively small distance therebetween, to reduce a circuit area. In cases where the gate conductive film is formed to have a relatively small film thickness, a resistance value thereof becomes higher. Thus, in the seventh embodiment, the gate conductive film is preferably comprised of a metal film. 
     Eighth Embodiment 
     An eighth embodiment of the present invention shows a method of producing an NMOS SGT on an SOI substrate (SOI NMOS SGT), using the same gate forming process as that in the second embodiment. 
     The gate forming process in the eighth embodiment is capable of further reducing the number of steps and further increasing a process margin, as compared with the gate forming process in the seventh embodiment. 
       FIGS. 108(   a ) and  108 ( b ) are, respectively, a top plan view and a sectional view of the SOI NMOS SGT formed by the SGT production method according to the eighth embodiment. With reference to  FIGS. 108(   a ) and  108 ( b ), the SOI NMOS SGT formed by the SGT production method according to the eighth embodiment will be described below. 
     A planar silicon layer  801  is formed on a buried oxide film layer  800 . A pillar-shaped silicon layer  802  is formed on the planar silicon layer  801 , and a gate dielectric film  805  and a gate electrode  806   a  are formed around the pillar-shaped silicon layer  802 . An N +  drain diffusion layer  803  is formed in the planar silicon layer  801  beneath the pillar-shaped silicon layer  802 , and an N +  source diffusion layer  804  is formed in an upper portion of the pillar-shaped silicon layer. A contact  807 , a contact  808 , and a contact  809 , are formed on the N +  drain diffusion layer  803 , the N +  source diffusion layer  804 , and a gate line  806   b  extending from the gate electrode  806   a , respectively. In the eighth embodiment, the gate electrode  806   a  and the gate line  806   b  are formed to be at the same height position. 
     Under conditions that the N +  source diffusion layer is connected to a GND potential, and the N +  drain diffusion layer is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 109(   a ) to  118 ( b ), one example of the SGT production method according to the eighth embodiment will be described below. In  FIGS. 109(   a ) to  118 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     In the eighth embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the seventh embodiment. Thus, the following description will be started from the step of forming a gate conductive film. 
     Referring to  FIGS. 109(   a ) and  109 ( b ), a gate dielectric film  805  and a gate conductive film  806  is formed by CVD or ALD, wherein the gate conductive film  806  is formed to allow a pillar-shaped silicon layer  802  to be buried therein. 
     Referring to  FIGS. 110(   a ) and  110 ( b ), the gate conductive film  806  is polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, a silicon nitride film  810  on a top of a pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  810  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 111(   a ) and  111 ( b ), the gate conductive film  806  is etched back to fix a gate length. 
     Referring to  FIGS. 112(   a ) and  112 ( b ), a silicon nitride film  812   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 113(   a ) and  113 ( b ), the silicon nitride film  812   a  is etched back to form a silicon nitride film-based sidewall  812 . In the eighth embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  812 . Thus, a final film thickness of the silicon nitride film-based sidewall  812  is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  812   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 114(   a ) and  114 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  813  by lithography. 
     Referring to  FIGS. 115(   a ) and  115 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  806   a  and a gate line  806   b.    
     Referring to  FIGS. 116(   a ) and  116 ( b ), the silicon nitride film  810  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  812 , are removed by wet etching. 
     Referring to  FIGS. 117(   a ) and  117 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  802 , for example, by ion implantation, to form an N +  source diffusion layer  804  therein. 
     Referring to  FIGS. 118(   a ) and  118 ( b ), an interlayer dielectric film is formed, and a contact ( 807 ,  808 ,  809 ) is formed on each of the drain diffusion layer in the planar silicon layer, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the eighth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the eighth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the eighth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the eighth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. In the seventh embodiment, a film thickness of a gate electrode is controlled based on a formed film thickness of a gate conductive film. Differently, in the eighth embodiment, the film thickness of the gate electrode can be controlled based on a film thickness of the silicon nitride film-based sidewall  812 . Further, in the eighth embodiment, the gate line  806   b  has a relatively large film thickness as compared with that of the gate line in the seventh embodiment. Thus, the gate conductive film is not limited to a metal film, but may be made of a material having relatively high electrical resistance, such as polysilicon. 
     In the seventh embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the eighth embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  812 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the seventh embodiment. 
     Ninth Embodiment 
     A ninth embodiment of the present invention shows a method of producing an NMOS SGT on an SOI substrate (SOI NMOS SGT), using the same gate forming process as that in the third embodiment. 
     The gate forming process is different from that in the eighth embodiment, in that a gate electrode and a gate line extending from the gate electrode are formed in a layered structure which comprises a thin metal film and a polysilicon layer. In a gate forming process in the ninth embodiment, the thin metal film is formed to be in contact with a gate dielectric film so as to suppress depletion of the gate electrode, and the polysilicon layer is formed to define respective top surfaces of the gate electrode and the gate line, so as to allow the SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
       FIGS. 119(   a ) and  119 ( b ) are, respectively, a top plan view and a sectional view of the SOI NMOS SGT formed by the SGT production method according to the ninth embodiment. With reference to  FIGS. 119(   a ) and  119 ( b ), the SOI NMOS SGT formed by the method according to the ninth embodiment will be described below. 
     A planar silicon layer  901  is formed on a buried oxide film layer  900 . A pillar-shaped silicon layer  902  is formed on the planar silicon layer  901 , and a gate dielectric film  905  and a gate electrode  906   a  are formed around the pillar-shaped silicon layer  902 . The gate electrode has a layered structure which comprises a thin metal film  314  having a film thickness of about 1 to 10 nm, and a polysilicon layer  906   a  covering the metal film. An N +  drain diffusion layer  903  is formed in the planar silicon layer  901  beneath the pillar-shaped silicon layer  902 , and an N +  source diffusion layer  804  is formed in an upper portion of the pillar-shaped silicon layer  902 . A contact  907 , a contact  908 , and a contact  909 , are formed on the N +  drain diffusion layer  903 , the N +  source diffusion layer  904 , and a gate line  906   b  extending from the gate electrode  906   a , respectively. In the ninth embodiment, the gate electrode  906   a  and the gate line  906   b  are formed to be at the same height position. 
     Under conditions that the N +  source diffusion layer is connected to a GND potential, and the N +  drain diffusion layer is connected to a power supply potential Vcc, a potential ranging from zero to Vcc is applied to the gate electrode to allow the SGT to operate as a transistor. 
     With reference to  FIGS. 120(   a ) to  130 ( b ), one example of the SGT production method according to the ninth embodiment will be described below. In  FIGS. 120(   a ) to  130 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     In the ninth embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the seventh embodiment. Thus, the following description will be started from the step of forming a thin metal film and a polysilicon layer. 
     Referring to  FIGS. 120(   a ) and  120 ( b ), after forming a gate dielectric film  905 , a thin metal film  914  is formed to have a film thickness of about 1 to 10 nm, and then a polysilicon layer  906  is formed to allow a pillar-shaped silicon layer  902  to be buried therein. 
     Referring to  FIGS. 121(   a ) and  121 ( b ), the polysilicon layer  906 , and respective portions of the thin metal film  914  and the gate dielectric film  905  above the pillar-shaped silicon layer, are polished by CMP, to flatten respective top surfaces of the polysilicon layer  906  and the thin metal film  914 . Through the flattening of respective tops of the polysilicon layer  906  and the thin metal film  914  by CMP, respective configurations of the polysilicon layer  906  and the thin metal film  914  are improved to facilitate control of a gate length. During the CMP, a silicon nitride film  910  on a top of the pillar-shaped silicon layer is used as a CMP stopper. The use of the silicon nitride film  910  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 122(   a ) and  122 ( b ), the polysilicon layer  906  and the thin metal film  914  are etched back to fix a gate length. 
     Referring to  FIGS. 123(   a ) and  123 ( b ), a silicon nitride film  912   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 124(   a ) and  124 ( b ), the silicon nitride film  912   a  is etched back to form a silicon nitride film-based sidewall  912 . In the ninth embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  912 . Thus, a final film thickness of the silicon nitride film-based sidewall is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  912   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 125(   a ) and  125 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  913  by lithography. 
     Referring to  FIGS. 126(   a ) and  126 ( b ), the polysilicon layer, the thin metal film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode  906   a  and a gate line  906   b.    
     Referring to  FIGS. 127(   a ) and  127 ( b ), the silicon nitride film  910  on the top of the pillar-shaped silicon layer, and the silicon nitride film-based sidewall  912 , are removed by wet etching. 
     Referring to  FIGS. 128(   a ) and  128 ( b ), a silicon nitride film is formed and then etched back to form a silicon nitride film  915 . The silicon nitride film  915  is formed to cover the thin metal film  914  of the gate electrode to keep a top surface of the thin metal film  914  from being exposed. This makes it possible to produce an intended SGT in the same production line as that for a transistor having a conventional polysilicon gate. 
     Referring to  FIGS. 129(   a ) and  129 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  902 , for example, by ion implantation, to form an N +  source diffusion layer  904  therein. 
     Referring to  FIGS. 130(   a ) and  130 ( b ), an interlayer dielectric film is formed, and a contact ( 907 ,  908 ,  909 ) is formed on each of the drain diffusion layer in the planar silicon layer, the source diffusion layer in the upper portion of the pillar-shaped silicon layer, and the gate line. 
     In the method according to the ninth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the ninth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening respective top surfaces of a polysilicon layer and a thin metal film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the ninth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the ninth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and control a film thickness of the gate electrode based on a film thickness of the silicon nitride film-based sidewall  912 , as with the second embodiment. 
     In the ninth embodiment, a gate is formed in a layered structure which comprises the thin metal film and the polysilicon layer, which is capable of suppressing depletion of the gate electrode, and allowing an intended SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
     In the seventh embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the ninth embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  912 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the seventh embodiment. 
     Tenth Embodiment 
     A tenth embodiment of the present invention shows a method of producing a CMOS SGT on an SOI substrate (SOI CMOS SGT), using the same gate forming process as that in the seventh embodiment. Thus, the method according to the tenth embodiment can be employed to obtain the same advantageous effects as those in the seventh embodiment. 
       FIG. 131  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the tenth embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  4  is applied to a gate of an NMOS Qn  4  and a gate of a PMOS Qp  4 . When the Vin  4  is “1”, the NMOS Qn  4  is placed in an ON state, and the PMOS Qp  4  is placed in an OFF state, so that an output signal Vout  4  becomes “0”. Reversely, when the Vin  4  is “0”, the NMOS Qn  4  is placed in an OFF state, and the PMOS Qp  4  is placed in an ON state, so that the Vout  4  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  4  to have a value opposite to that of the input signal Vin  4 . 
       FIG. 132  is a top plan view of the CMOS inverter formed by the method according to the tenth embodiment.  FIGS. 133(   a ) and  133 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 132 , respectively. With reference to  FIGS. 132 ,  133 ( a ) and  133 ( b ), a structure of the CMOS inverter will be described. 
     A planar silicon layer ( 1002 ,  1003 ) is formed on a buried oxide film layer  1000 . A pillar-shaped silicon layer  1007  is formed on the planar silicon layer  1002 , and a pillar-shaped silicon layer  1008  is formed on the planar silicon layer  1003 . A gate dielectric film  1009  and a gate electrode ( 1010   a ,  1010   b ) are formed around the pillar-shaped silicon layers. The gate electrodes  1010   a ,  1010   b  are connected to each other through a gate line  1010   c  extending therefrom. An N +  drain diffusion layer  1004  is formed in the planar silicon layer  1002  beneath the pillar-shaped silicon layer  1007  forming an NMOS (NMOS-forming pillar-shaped silicon layer  1007 ), and an N +  source diffusion layer  1011  is formed in an upper portion of the pillar-shaped silicon layer  1007 . A P +  drain diffusion layer  1005  is formed in the planar silicon layer  1003  beneath the pillar-shaped silicon layer  1008  forming a PMOS (PMOS-forming pillar-shaped silicon layer  1008 ), and a P +  source diffusion layer  1012  is formed in an upper portion of the pillar-shaped silicon layer  1008 . 
     Each of the N +  drain diffusion layer  1004  and the P +  drain diffusion layer  1005  formed beneath respective ones of the pillar-shaped silicon layers  1007 ,  1008  is connected to the output terminal Vout  4  via a contact ( 1016   a ,  1016   b ). The N +  source diffusion layer  1011  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  1007  is connected to a ground potential Vss  4  via a contact  1014 , and the P +  source diffusion layer  1012  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  1008  is connected to a power supply potential Vcc  4  via a contact  1015 . Further, the gate line  1010   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  4  via a contact  1013 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 134(   a ) to  149 ( b ), one example of the SGT production method according to the tenth embodiment will be described below. In  FIGS. 134(   a ) to  149 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. 
     Referring to  FIGS. 134(   a ) and  134 ( b ), a silicon nitride film  1017  serving as a hard mask is formed on a silicon layer  1001   a  on a buried oxide film layer  1000  in an SOI substrate, to have a film thickness of about 50 to 150 nm. 
     Referring to  FIGS. 135(   a ) and  135 ( b ), the hard mask  1017  and the silicon layer  1001   a  are etched to form a pillar-shaped silicon layer ( 1007 ,  1008 ). Through the etching, the pillar-shaped silicon layer is formed to have a height dimension of about 30 to 300 nm, and a diameter of about 5 to 100 nm. A continuous planar silicon layer  1001  is also formed beneath the pillar-shaped silicon layer ( 1007 ,  1008 ) to have a thickness of about 10 to 100 nm. 
     Referring to  FIGS. 136(   a ) and  136 ( b ), the continuous planar silicon layer  1001  is formed with two isolated planar silicon layers  1002 ,  1003 , by etching. 
     Referring to  FIGS. 137(   a ) and  137 ( b ), impurities, such as P or As, are introduced into a top surface of respective top surfaces of the planar silicon sub-layers, for example, by ion implantation, to form an N +  drain diffusion layer  1004  and a P +  drain diffusion layer  1005  therein. During this step, the silicon nitride film  1017  on a top of the pillar-shaped silicon layer ( 1007 ,  1008 ) functions as a stopper for preventing the impurity from being injected into the top of the pillar-shaped silicon layer. 
     Referring to  FIGS. 138(   a ) and  138 ( b ), a gate dielectric film  1009  and a gate conductive film  1010  are formed by CVD or ALD. The gate conductive film  1010  is formed to have a film thickness of about 10 to 100 nm. 
     Referring to  FIGS. 139(   a ) and  139 ( b ), a silicon oxide film  1018  is formed to allow the pillar-shaped silicon layer ( 1007 ,  1008 ) to be buried therein. 
     Referring to  FIGS. 140(   a ) and  140 ( b ), the silicon oxide film  1018 , and respective portions of the gate conductive film and the gate dielectric film above of the pillar-shaped silicon layer ( 1007 ,  1008 ), are polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, the silicon nitride film  1017  on the top of the pillar-shaped silicon layer ( 1007 ,  1008 ) is used as a CMP stopper. The use of the silicon nitride film  1017  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 141(   a ) and  141 ( b ), the gate conductive film  1010  and the silicon oxide film  1018  are etched back, wherein the gate conductive film  1010  is etched to fix a gate length. Preferably, etching conditions to be used in this step are set to allow the gate conductive film  1010  and the silicon oxide film  1018  to be etched at the same rate, and at a higher selectivity ratio relative to the silicon nitride film  1017 . The etching of the gate conductive film  1010  and the silicon oxide film  1018  at the same rate makes it possible to suppress occurrence of a step between respective top surfaces of the two films, which improves a configuration of a silicon nitride film-based sidewall  1019  to be formed in a next step. 
     Referring to  FIGS. 142(   a ) and  142 ( b ), a silicon nitride film  1019   a  is formed by a film thickness required for the gate conductive film  1010 . Subsequently, as shown in  FIGS. 143(   a ) and  143 ( b ), the silicon nitride film  1019   a  is etched back to form a silicon nitride film-based sidewall  1019 . In this step, a film thickness of the silicon nitride film-based sidewall  1019  is controlled to become equal to that of the gate conductive film  1010 , by adjusting a formed film thickness of the silicon nitride film  1019   a , and then finely adjusting the formed film thickness based on an amount of the etch-back. A portion of the gate conductive film  1010  covered by the silicon nitride film-based sidewall  1019  will be protected during etching for forming a gate line in a subsequent step. This makes it possible to form the gate electrode in a self-alignment manner and with a desired film thickness, so as to reduce an occupancy area. 
     Referring to  FIGS. 144(   a ) and  144 ( b ), the silicon oxide film  1018  remaining on the gate conductive film is removed by wet etching. 
     Referring to  FIGS. 145(   a ) and  145 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  1020  by lithography. 
     Referring to  FIGS. 146(   a ) and  146 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode ( 1010   a ,  1010   b ) and a gate line  1010   c.    
     Referring to  FIGS. 147(   a ) and  147 ( b ), the silicon nitride film  1017  on the top of the pillar-shaped silicon layer ( 1007 ,  1008 ), and the silicon nitride film-based sidewall  1019 , are removed by wet etching. 
     Referring to  FIGS. 148(   a ) and  148 ( b ), an impurity, such as P or As, is introduced into a top portion of the pillar-shaped silicon layer  1007 , for example, by ion implantation, to form an N +  source diffusion layer  1011  therein. Further, an impurity, such as B or BF 2 , is introduced into a top portion of the pillar-shaped silicon layer  1008 , for example, by ion implantation, to form a P +  source diffusion layer  1012  therein. 
     Referring to  FIGS. 149(   a ) and  149 ( b ), an interlayer dielectric film is formed, and a contact ( 1013 ,  1014 ,  1015 ,  1016   a ,  1016   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers, and the drain diffusion layers in the planar silicon sub-layers. 
     In the method according to the tenth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the seventh embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the tenth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     Thus, the use of the method according to the tenth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the tenth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and adjust a film thickness of the gate electrode to be formed around the pillar-shaped silicon layer, based on a formed film thickness of a gate conductive film. Thus, two pillar-shaped silicon layers each having a gate electrode to be applied with a different potential can be arranged side-by-side with a relatively small distance therebetween, to reduce a circuit area. In cases where the gate conductive film is formed to have a relatively small film thickness, a resistance value thereof becomes higher. Thus, in the tenth embodiment, the gate conductive film is preferably comprised of a metal film. 
     Eleventh Embodiment 
     An eleventh embodiment of the present invention shows a method of producing a CMOS SGT on an SOI substrate (SOI CMOS SGT), using the same gate forming process as that in the eighth embodiment. Thus, the method according to the eleventh embodiment can be employed to obtain the same advantageous effects as those in the eighth embodiment. 
       FIG. 150  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the eleventh embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  5  is applied to a gate of an NMOS Qn  5  and a gate of a PMOS Qp  5 . When the Vin  5  is “1”, the NMOS Qn  5  is placed in an ON state, and the PMOS Qp  5  is placed in an OFF state, so that an output signal Vout  5  becomes “0”. Reversely, when the Vin  5  is “0”, the NMOS Qn  5  is placed in an OFF state, and the PMOS Qp  5  is placed in an ON state, so that the Vout  5  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  5  to have a value opposite to that of the input signal Vin  5 . 
       FIG. 151  is a top plan view of the CMOS inverter formed by the method according to the eleventh embodiment.  FIGS. 152(   a ) and  152 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 151 , respectively. With reference to  FIGS. 151 ,  152 ( a ) and  152 ( b ), a structure of the CMOS inverter will be described. 
     A planar silicon layer ( 1102 ,  1103 ) is formed on a buried oxide film layer  1100 . A pillar-shaped silicon layer  1107  is formed on the planar silicon layer  1102 , and a pillar-shaped silicon layer  1108  is formed on the planar silicon layer  1103 . A gate dielectric film  1109  and a gate electrode ( 1110   a ,  1110   b ) are formed around the pillar-shaped silicon layers. The gate electrodes  1110   a ,  1110   b  are connected to each other through a gate line  1110   c  extending therefrom. The gate electrode ( 1110   a ,  1110   b ) and the gate line  1106   c  are formed to be at the same height position. An N +  drain diffusion layer  1104  is formed in the planar silicon layer  1102  beneath the pillar-shaped silicon layer  1107  forming an NMOS (NMOS-forming pillar-shaped silicon layer  1107 ), and an N +  source diffusion layer  1111  is formed in an upper portion of the pillar-shaped silicon layer  1107 . A P +  drain diffusion layer  1105  is formed in the planar silicon layer  1103  beneath the pillar-shaped silicon layer  1108  forming a PMOS (PMOS-forming pillar-shaped silicon layer  1108 ), and a P +  source diffusion layer  1112  is formed in an upper portion of the pillar-shaped silicon layer  1108 . 
     Each of the N +  drain diffusion layer  1104  and the P +  drain diffusion layer  1105  formed beneath respective ones of the pillar-shaped silicon layers  1107 ,  1108  is connected to the output terminal Vout  5  via a contact ( 1116   a ,  1116   b ). The N +  source diffusion layer  1111  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  1107  is connected to a ground potential Vss  5  via a contact  1114 , and the P +  source diffusion layer  1112  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  1108  is connected to a power supply potential Vcc  5  via a contact  1115 . Further, the gate line  1110   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  5  via a contact  1113 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 153(   a ) to  162 ( b ), one example of the SGT production method according to the eleventh embodiment will be described below. In  FIGS. 153(   a ) to  162 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. In the eleventh embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the tenth embodiment. Thus, the following description will be started from the step of forming a gate conductive film. 
     Referring to  FIGS. 153(   a ) and  153 ( b ), a gate dielectric film  1109  and a gate conductive film  1110  is formed by CVD or ALD, wherein the gate conductive film  1110  is formed to allow a pillar-shaped silicon layer ( 1107 ,  1108 ) to be buried therein. 
     Referring to  FIGS. 154(   a ) and  154 ( b ), the gate conductive film  1110  is polished by CMP, to flatten a top surface of the gate conductive film. Through the flattening of a top of the gate conductive film by CMP, a configuration of the gate conductive film is improved to facilitate control of a gate length. During the CMP, a silicon nitride film  1117  on a top of a pillar-shaped silicon layer ( 1107 ,  1108 ) is used as a CMP stopper. The use of the silicon nitride film  1117  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 155(   a ) and  155 ( b ), the gate conductive film  1110  is etched back to fix a gate length. 
     Referring to  FIGS. 156(   a ) and  156 ( b ), a silicon nitride film  1119   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 157(   a ) and  157 ( b ), the silicon nitride film  1119   a  is etched back to form a silicon nitride film-based sidewall  1119 . In the eleventh embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  1119 . Thus, a final film thickness of the silicon nitride film-based sidewall is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  1119   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 158(   a ) and  158 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  1120  by lithography. 
     Referring to  FIGS. 159(   a ) and  159 ( b ), the gate conductive film and the gate dielectric film are etched using the resist as a mask, to form a gate electrode ( 1110   a ,  1110   b ) and a gate line  1110   c.    
     Referring to  FIGS. 160(   a ) and  160 ( b ), the silicon nitride film  1117  on the top of the pillar-shaped silicon layer ( 1107 ,  1108 ), and the silicon nitride film-based sidewall  1119 , are removed by wet etching. 
     Referring to  FIGS. 161(   a ) and  161 ( b ), impurities are introduced into respective top portions of the pillar-shaped silicon layers  1107 ,  1108 , for example, by ion implantation, to form an N +  source diffusion layer  1111  and a P +  source diffusion layer  1112  therein. 
     Referring to  FIGS. 162(   a ) and  162 ( b ), an interlayer dielectric film is formed, and a contact ( 1113 ,  1114 ,  1115 ,  1116   a ,  1116   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers, and the drain diffusion layers in the planar silicon layers. 
     In the method according to the eleventh embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the eleventh embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening a top surface of a gate conductive film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the eleventh embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the eleventh embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. In the tenth embodiment, a film thickness of a gate electrode is controlled based on a formed film thickness of a gate conductive film. Differently, in the eleventh embodiment, the film thickness of the gate electrode can be controlled based on a film thickness of the silicon nitride film-based sidewall  1119 . Further, in the eleventh embodiment, the gate line  1110   c  has a relatively large film thickness as compared with that of the gate line in the tenth embodiment. Thus, the gate conductive film is not limited to a metal film, but may be made of a material having relatively high electrical resistance, such as polysilicon. 
     In the tenth embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the eleventh embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  1119 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the tenth embodiment. 
     Although the SGT production method according to the eleventh embodiment has been described based on one example where it is applied to a CMOS inverter, it is understood that the present invention may be applied to any suitable circuit other than the CMOS inverter, in just the same manner. 
     Twelfth Embodiment 
     A twelfth embodiment of the present invention shows a method of producing a CMOS SGT on an SOI substrate (SOI CMOS SGT), using the same gate forming process as that in the ninth embodiment. Thus, the method according to the twelfth embodiment can be employed to obtain the same advantageous effects as those in the ninth embodiment. 
       FIG. 163  is an equivalent circuit diagram of a CMOS inverter formed by the method according to the twelfth embodiment. A circuit operation of the CMOS inverter will be described below. An input signal Vin  6  is applied to a gate of an NMOS Qn  6  and a gate of a PMOS Qp  6 . When the Vin  6  is “1”, the NMOS Qn  6  is placed in an ON state, and the PMOS Qp  6  is placed in an OFF state, so that an output signal Vout  6  becomes “0”. Reversely, when the Vin  6  is “0”, the NMOS Qn  6  is placed in an OFF state, and the PMOS Qp  6  is placed in an ON state, so that the Vout  6  becomes “1”. As above, the CMOS inverter is operable to allow the output signal Vout  6  to have a value opposite to that of the input signal Vin  6 . 
       FIG. 164  is a top plan view of the CMOS inverter formed by the method according to the twelfth embodiment.  FIGS. 165(   a ) and  165 ( b ) are sectional views taken along the cutting-plane line A-A′ and the cutting-plane line B-B′ in  FIG. 164 , respectively. With reference to  FIGS. 164 ,  165 ( a ) and  165 ( b ), the CMOS inverter formed by the method according to the twelfth embodiment will be described. 
     A planar silicon layer ( 1202 ,  1203 ) is formed on a buried oxide film layer  1200 . A pillar-shaped silicon layer  1207  is formed on the planar silicon layer  1202 , and a pillar-shaped silicon layer  1208  is formed on the planar silicon layer  1203 . A gate dielectric film  1209  and a gate electrode ( 1210   a ,  1210   b ) are formed around the pillar-shaped silicon layers. The gate electrode ( 1210   a ,  1210   b ) is formed in a layered structure which comprises a polysilicon layer defining a top surface thereof, and a thin metal film  1221  in contact with a gate dielectric film. The gate electrodes  1210   a ,  1210   b  are connected to each other through a gate line  1210   c  extending therefrom. The gate electrode ( 1210   a ,  1210   b ) and the gate line  1210   c  are formed to be at the same height position. An N +  drain diffusion layer  2104  is formed in the planar silicon layer  1202  beneath the pillar-shaped silicon layer  1207  forming an NMOS (NMOS-forming pillar-shaped silicon layer  1207 ), and an N +  source diffusion layer  1211  is formed in an upper portion of the pillar-shaped silicon layer  1207 . A P +  drain diffusion layer  1205  is formed in the planar silicon layer  1203  beneath the pillar-shaped silicon layer  1208  forming a PMOS (PMOS-forming pillar-shaped silicon layer  1208 ), and a P +  source diffusion layer  1212  is formed in an upper portion of the pillar-shaped silicon layer  1208 . 
     Each of the N +  drain diffusion layer  1204  and the P +  drain diffusion layer  1205  formed beneath respective ones of the pillar-shaped silicon layers  1207 ,  1208  is connected to the output terminal Vout  6  via a contact ( 1216   a ,  1216   b ). The N +  source diffusion layer  1211  formed in the upper portion of the NMOS-forming pillar-shaped silicon layer  1207  is connected to a ground potential Vss  6  via a contact  1214 , and the P +  source diffusion layer  1212  formed in the upper portion of the PMOS-forming pillar-shaped silicon layer  1208  is connected to a power supply potential Vcc  6  via a contact  1215 . Further, the gate line  1210   c  connecting between the gate electrodes for the PMOS and the NMOS is connected to the input terminal Vin  6  via a contact  1213 . In this manner, the CMOS inverter is formed. 
     With reference to  FIGS. 166(   a ) to  176 ( b ), one example of the SGT production method according to the twelfth embodiment will be described below. In  FIGS. 166(   a ) to  176 ( b ), the figure suffixed by (a) is a top plan view, and the figure suffixed by (b) is a sectional view taken along the line A-A′. In the twelfth embodiment, the step of forming a gate dielectric film and any step therebefore are the same as those in the tenth embodiment. Thus, the following description will be started from the step of forming a thin metal film and a polysilicon layer. 
     Referring to  FIGS. 166(   a ) and  166 ( b ), after forming a gate dielectric film  1209 , a thin metal film  1221  is formed to have a film thickness of about 1 to 10 nm, and then a polysilicon layer  1210  is formed to allow a pillar-shaped silicon layer ( 1207 ,  1208 ) to be buried therein. 
     Referring to  FIGS. 167(   a ) and  167 ( b ), the polysilicon layer  1210 , and respective portions of the thin metal film  1221  and the gate dielectric film  1209  above the pillar-shaped silicon layer, ( 1207 ,  1208 ) are polished by CMP, to flatten respective top surfaces of the polysilicon layer  1210  and the thin metal film  1221 . Through the flattening of respective tops of the polysilicon layer  1210  and the thin metal film  1221  by CMP, respective configurations of the polysilicon layer  1210  and the thin metal film  1221  are improved to facilitate control of a gate length. During the CMP, a silicon nitride film  1217  on a top of the pillar-shaped silicon layer ( 1207 ,  1208 ) is used as a CMP stopper. The use of the silicon nitride film  1217  as a CMP stopper makes it possible to control an amount of CMP with high repeatability. 
     Referring to  FIGS. 168(   a ) and  168 ( b ), the polysilicon layer  1210  and the thin metal film  1221  are etched back to fix a gate length. 
     Referring to  FIGS. 169(   a ) and  169 ( b ), a silicon nitride film  1219   a  is formed by a film thickness required for an after-mentioned gate electrode. Subsequently, as shown in  FIGS. 170(   a ) and  170 ( b ), the silicon nitride film  1219   a  is etched back to form a silicon nitride film-based sidewall  1219 . In the twelfth embodiment, a film thickness of the gate electrode is determined by a film thickness of the silicon nitride film-based sidewall  1219 . Thus, a final film thickness of the silicon nitride film-based sidewall is controlled to become equal to a desired film thickness of the gate electrode, by adjusting a formed film thickness of the silicon nitride film  1219   a  and then finely adjusting the formed film thickness based on an amount of the etch-back. 
     Referring to  FIGS. 171(   a ) and  171 ( b ), a resist or a multilayer resist is applied, and a gate line pattern is formed with a resist  1220  by lithography. 
     Referring to  FIGS. 172(   a ) and  172 ( b ), the polysilicon layer, the thin metal film and the gate dielectric film? are etched using the resist as a mask, to form a gate electrode ( 1210   a ,  1210   b ) and a gate line  1210   c.    
     Referring to  FIGS. 173(   a ) and  173 ( b ), the silicon nitride film  1217  on the top of the pillar-shaped silicon layer ( 1207 ,  1208 ), and the silicon nitride film-based sidewall  1219 , are removed by wet etching. 
     Referring to  FIGS. 174(   a ) and  174 ( b ), a silicon nitride film is formed and then etched back to form a silicon nitride film  1222 . The silicon nitride film  1222  is formed to cover the thin metal film  1221  of the gate electrode to keep a top surface of the thin metal film  1221  from being exposed. This makes it possible to produce an intended SGT in the same production line as that for a transistor having a conventional polysilicon gate. 
     Referring to  FIGS. 175(   a ) and  175 ( b ), impurities are introduced into respective top portions of the pillar-shaped silicon layers  1207 ,  1208 , for example, by ion implantation, to form an N +  source diffusion layer  1211  and a P +  source diffusion layer  1212  therein. 
     Referring to  FIGS. 176(   a ) and  176 ( b ), an interlayer dielectric film is formed, and a contact ( 1213 ,  1214 ,  1215 ,  1216   a ,  1216   b ) is formed on each of the gate line, the source diffusion layers in the upper portions of the pillar-shaped silicon layers, and the drain diffusion layers in the planar silicon layers?. 
     In the method according to the twelfth embodiment, the step of performing etching to fix a gate length, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed in the above manner. This makes it possible to achieve a gate forming process having the following features. 
     A first feature is that the process is capable of forming a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness. A second feature is that the process is less vulnerable to a deviation in exposure alignment during gate line formation. Thus, the use of the method according to the twelfth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line arising from a lithography step of forming a gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     A third feature is that the step of flattening respective top surfaces of a polysilicon layer and a thin metal film by CMP, using a structure which has a silicon nitride film formed on a top of a pillar-shaped silicon layer to serve as a hard mask, is provided before the step of performing etching to fix a gate length, and, after these steps, the step of forming a gate electrode-protecting silicon nitride film-based sidewall, the step of forming a gate line pattern, and the step of performing etching to form a gate line, are sequentially performed, whereby the gate length can be accurately controlled to achieve a process capable of minimizing a variation in gate length and increasing a process margin. Thus, the use of the method according to the twelfth embodiment makes it possible to simultaneously solve both the following problems: a disconnection or open of a gate line and a variation in gate length arising from a lithography step of forming the gate line, as the problem in the method disclosed in the Patent Document 1; and an incapability to form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner, as the problem in the method disclosed in the Non-Patent Document 1. 
     As described above, the method according to the twelfth embodiment can form a gate electrode around a pillar-shaped silicon layer in a self-alignment manner and with a desired film thickness, and control a film thickness of the gate electrode based on a film thickness of the silicon nitride film-based sidewall  1219 , as with the third embodiment. 
     In the twelfth embodiment, a gate is formed in a layered structure which comprises the thin metal film and the polysilicon layer, which is capable of suppressing depletion of the gate electrode, and allowing an intended SGT to be produced in the same production line as that for a transistor having a conventional polysilicon gate. 
     In the tenth embodiment, if the silicon nitride film-based sidewall has a film thickness largely different from that of the gate conductive film, the difference is likely to cause the problems as described in connection with the second embodiment. Differently, the gate forming process in the twelfth embodiment can form a gate electrode to have a desired film thickness, in a self-alignment manner according to a film thickness of the silicon nitride film-based sidewall  1219 , as with the second embodiment. This makes it possible to eliminate a risk of occurrence of the above problems, and further increase a process margin in the gate forming process, as compared with that in the tenth embodiment. 
     Although the SGT production method according to the twelfth embodiment has been described based on one example where it is applied to a CMOS inverter, it is understood that the present invention may be applied to any suitable circuit other than the CMOS inverter, in just the same manner.