Abstract:
According to the present invention, there is provided a semiconductor device comprising: a plurality of transistors each having a semiconductor substrate, a first-conductivity-type semiconductor layer formed on said semiconductor substrate via a first insulating film, and having a single-crystal structure, a second-conductivity-type source region and second-conductivity-type drain region formed in said semiconductor layer, a first-conductivity-type body region formed between said source region and said drain region in said semiconductor layer, and electrically floating, and a gate electrode formed on a central portion of a surface of said body region via a second insulating film; an element isolation insulating film which isolates said body regions in adjacent transistors of said plurality of transistors; a word line which connects said gate electrodes of said plurality of transistors together; a bit line electrically connected to said drain region; and a source line electrically connected to said source region, wherein in a section along said word line, an area in which said body region contacts said second insulating film is smaller than an area in which said body region contacts said first insulating film.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims benefit of priority under 35 USC §119 from the Japanese Patent Application No. 2004-262320, filed on Sep. 9, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device and a method of fabricating the same.  
         [0003]     Semiconductor memories such as DRAMs are more and more down-scaled in recent years. 1T-1C (1 Transistor-1 Capacitor) type DRAM, however, needs a certain area for the capacitor to ensure a storage capacitance of the capacitor. This limits the degree of down-scaling of the 1T-1C DRAM.  
         [0004]     Also, since a capacitor must be formed in the 1T-1C DRAM, the fabrication process complicates, and the cost increases.  
         [0005]     To solve these problems, a technique which forms a DRAM on an SOI (Silicon On Insulator) substrate is developed. For example, patent reference 1 or 2 (to be described later) discloses a DRAM formed by using an FBC (Floating Body Cell). The FBC is a memory cell formed by one transistor by using an SOI substrate.  
         [0006]     This FBC is formed as a MOS transistor on an SOI substrate. A source region, drain region, and body region are formed in an SOI layer. The body region sandwiched between the source and drain regions is electrically floating, and data can be stored by charging or discharging this region.  
         [0007]     In the FBC as described above, the data holding time prolongs and the yield increases as the capacitance between the body region and a fixed-potential element such as a support substrate increases.  
         [0008]     In an FBC shown in  FIG. 32  of patent reference 1, the capacitance between the body region and the support substrate is increased by using an SOI substrate having a thin buried oxide film (to be referred to as a BOX layer hereinafter).  
         [0009]     On the other hand, in an FBC shown in  FIG. 25  of patent reference 1, a back gate electrode is formed in a thick BOX layer to increase the capacitance between the body region and this back gate electrode.  
         [0010]     Unfortunately, even when these structures are used, it is difficult to form an FBC having a sufficiently long data holding time and high yield.  
         [0011]     References disclosing the conventional semiconductor memories are as follows. 
        1: Japanese Patent Laid-Open No. 2002-246571     2: Japanese Patent Laid-Open No. 2002-343886        
 
       SUMMARY OF THE INVENTION  
       [0014]     According to one aspect of the invention, there is provided a semiconductor device comprising: 
        a plurality of transistors each having     a semiconductor substrate,     a first-conductivity-type semiconductor layer formed on said semiconductor substrate via a first insulating film, and having a single-crystal structure,     a second-conductivity-type source region and second-conductivity-type drain region formed in said semiconductor layer,     a first-conductivity-type body region formed between said source region and said drain region in said semiconductor layer, and floating, and     a gate electrode formed on a central portion of a surface of said body region via a second insulating film;     an element isolation insulating film which isolates said body regions in adjacent transistors of said plurality of transistors;     a word line which connects said gate electrodes of said plurality of transistors together;     a bit line electrically connected to said drain region; and     a source line electrically connected to said source region,     wherein in a section along said word line, an area in which said body region contacts said second insulating film is smaller than an area in which said body region contacts said first insulating film.        
 
         [0026]     According to one aspect of the invention, there is provided a semiconductor device comprising: 
        a plurality of transistors each having     a semiconductor substrate,     a first-conductivity-type semiconductor layer formed on said semiconductor substrate via a first insulating film, and having a single-crystal structure,     a second-conductivity-type source region and second-conductivity-type drain region formed in said semiconductor layer,     a first-conductivity-type body region formed between said source region and said drain region in said semiconductor layer, and floating, and     a first gate electrode formed on a central portion of a surface of said body region via a second insulating film;     an element isolation insulating film which isolates said body regions in adjacent transistors of said plurality of transistors;     a word line which connects said first gate electrodes of said plurality of transistors together;     a bit line electrically connected to said drain region;     a source line electrically connected to said source region; and     a second gate electrode formed on said semiconductor substrate, below said body region in said plurality of transistors, so as to oppose said body region via a third insulating film,     wherein in a section along said word line, an area in which said body region contacts said second insulating film is smaller than an area in which said body region contacts said third insulating film.        
 
         [0039]     According to one aspect of the invention, there is provided a method of fabricating a semiconductor device including a transistor having a first-conductivity-type semiconductor layer separated from a semiconductor substrate by a first insulating film and having a single-crystal structure, a gate electrode formed on the semiconductor substrate via a second insulating film, a second-conductivity-type source region and second-conductivity-type drain region formed in the semiconductor layer, and a body region positioned between the source region and the drain region, comprising the steps of: 
        forming a mask on the semiconductor layer formed on the semiconductor substrate via the first insulating film, and dividing the semiconductor layer by patterning using the mask, thereby partially exposing a surface of the first insulating film;     depositing an insulating film for element isolation so as to cover surfaces of the mask, semiconductor layer, and first insulating film;     planarizing the insulating film by using the mask as a stopper, and removing the mask, thereby covering corners at two ends of the surface of each of the divided semiconductor layers with a shoulder at one side of a T-shape of an element isolation insulating film made of the insulating film;     forming the second insulating film on a central portion of the surface of each semiconductor layer, which is not covered with the element isolation insulating film;     depositing and patterning an electrode material to form the gate electrodes on the semiconductor layers via the second insulating film, and forming a word line which connects the gate electrodes together; and     ion-implanting an impurity in each semiconductor layer by using the gate electrode as a mask, thereby forming the second-conductivity-type source region and second-conductivity-type drain region at two end portions of each semiconductor layer,     wherein an area in which the body region between the source region and the drain region in the semiconductor layer contacts the second insulating film is smaller than an area in which the body region contacts the first insulating film.        
 
         [0047]     According to one aspect of the invention, there is provided a method of fabricating a semiconductor device including a transistor having a first-conductivity-type semiconductor layer separated from a semiconductor substrate by a first insulating film and having a single-crystal structure, a first gate electrode formed on the semiconductor substrate via a second insulating film, a second-conductivity-type source region and second-conductivity-type drain region formed in the semiconductor layer, a body region positioned between the source region and the drain region, and a second gate electrode formed on the semiconductor substrate so as to oppose the body region via a third insulating film, comprising the steps of: 
        forming, on the semiconductor layer formed on the semiconductor substrate via the first insulating film, a mask having an opening in a position where a word line formation region in which a word line which connects the first gate electrodes together is to be formed intersects an element isolation insulating film formation region in which an element isolation insulating film for isolating a plurality of transistors is to be formed;     patterning the first insulating film by using the mask as an etching stopper such that the first insulating film remains along the word line formation region below the source region and drain region in the semiconductor layer, thereby forming a hollow portion below the body region of the semiconductor device, and dividing the semiconductor layer into the plurality of transistors by patterning;     forming the third insulating film on a surface of the semiconductor layer in the hollow portion;     forming the second gate electrode, along the word line formation region, on the third insulating film in the hollow portion;     depositing an insulating film for element isolation so as to cover surfaces of the mask, semiconductor layer, and first insulating film;     planarizing the insulating film by using the mask as a stopper, and removing the mask, thereby covering corners at two ends of the surface of each of the divided semiconductor layers with a shoulder at one side of a T-shape of an element isolation insulating film made of the insulating film;     forming the second insulating film on a central portion of the surface of each semiconductor layer, which is not covered with the element isolation insulating film;     depositing and patterning an electrode material to form the gate electrodes on the semiconductor layers via the second insulating film, and forming a word line which connects the gate electrodes together; and     ion-implanting an impurity in each semiconductor layer by using the gate electrode as a mask, thereby forming the second-conductivity-type source region and second-conductivity-type drain region at two end portions of each semiconductor layer,     wherein an area in which the body region between the source region and the drain region in the semiconductor layer contacts the second insulating film is smaller than an area in which the body region contacts the third insulating film.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0058]      FIG. 1  is a plan view showing the planar arrangement of a semiconductor device according to the first embodiment of the present invention;  
         [0059]      FIG. 2  is a longitudinal sectional view showing a sectional structure taken along a line B 1 -B 1  in  FIG. 1 ;  
         [0060]      FIG. 3  is a longitudinal sectional view showing a sectional structure taken along a line C 1 -C 1  in  FIG. 1 ;  
         [0061]      FIG. 4  is a plan view showing the planar arrangement of a peripheral logic circuit of the same semiconductor device;  
         [0062]      FIG. 5  is a longitudinal sectional view showing a sectional structure taken along a line F 1 -F 1  in  FIG. 4 ;  
         [0063]      FIG. 6  is a graph showing changes in potential when data is written and held in the same semiconductor device;  
         [0064]      FIG. 7  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0065]      FIG. 8  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0066]      FIG. 9  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0067]      FIG. 10  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0068]      FIG. 11  is a longitudinal sectional view showing a section taken along the line E 1 -E 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0069]      FIG. 12  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0070]      FIG. 13  is a longitudinal sectional view showing a section taken along the line E 1 -E 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0071]      FIG. 14  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0072]      FIG. 15  is a longitudinal sectional view showing a section taken along the line E 1 -E 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0073]      FIG. 16  is a longitudinal sectional view showing a section taken along the line B 1 -B 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0074]      FIG. 17  is a longitudinal sectional view showing a section taken along the line F 1 -F 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0075]      FIG. 18  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0076]      FIG. 19  is a longitudinal sectional view showing a section taken along the line E 1 -E 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0077]      FIG. 20  is a longitudinal sectional view showing a section taken along the line B 1 -B 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0078]      FIG. 21  is a longitudinal sectional view showing a section taken along the line F 1 -F 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0079]      FIG. 22  is a longitudinal sectional view showing a section taken along the line C 1 -C 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0080]      FIG. 23  is a longitudinal sectional view showing a section taken along the line E 1 -E 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0081]      FIG. 24  is a longitudinal sectional view showing a section taken along the line B 1 -B 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0082]      FIG. 25  is a longitudinal sectional view showing a section taken along the line F 1 -F 1  in  FIG. 1 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0083]      FIG. 26  is a plan view showing the planar arrangement of a semiconductor device according to the second embodiment of the present invention;  
         [0084]      FIG. 27  is a longitudinal sectional view showing a sectional structure taken along a line A 2 -A 2  in  FIG. 26 ;  
         [0085]      FIG. 28  is a longitudinal sectional view showing a sectional structure taken along a line C 2 -C 2  in  FIG. 26 ;  
         [0086]      FIG. 29  is a longitudinal sectional view showing a sectional structure taken along a line B 2 -B 2  in  FIG. 26 ;  
         [0087]      FIG. 30  is a longitudinal sectional view showing a sectional structure taken along a line D 2 -D 2  in  FIG. 26 ;  
         [0088]      FIG. 31  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0089]      FIG. 32  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0090]      FIG. 33  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0091]      FIG. 34  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0092]      FIG. 35  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0093]      FIG. 36  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0094]      FIG. 37  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0095]      FIG. 38  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0096]      FIG. 39  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0097]      FIG. 40  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0098]      FIG. 41  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0099]      FIG. 42  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0100]      FIG. 43  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0101]      FIG. 44  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0102]      FIG. 45  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0103]      FIG. 46  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0104]      FIG. 47  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0105]      FIG. 48  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0106]      FIG. 49  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0107]      FIG. 50  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0108]      FIG. 51  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0109]      FIG. 52  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0110]      FIG. 53  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0111]      FIG. 54  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0112]      FIG. 55  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0113]      FIG. 56  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0114]      FIG. 57  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0115]      FIG. 58  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0116]      FIG. 59  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0117]      FIG. 60  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0118]      FIG. 61  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0119]      FIG. 62  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0120]      FIG. 63  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0121]      FIG. 64  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0122]      FIG. 65  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0123]      FIG. 66  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0124]      FIG. 67  is a longitudinal sectional view showing a section taken along the line A 2 -A 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0125]      FIG. 68  is a longitudinal sectional view showing a section taken along the line C 2 -C 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0126]      FIG. 69  is a longitudinal sectional view showing a section taken along the line B 2 -B 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0127]      FIG. 70  is a longitudinal sectional view showing a section taken along the line D 2 -D 2  in  FIG. 26 , as the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0128]      FIG. 71  is a longitudinal sectional view showing the sectional structure, along a word line, of a semiconductor device according to the third embodiment of the present invention;  
         [0129]      FIG. 72  is a graph showing the impurity concentration profile in the same semiconductor device;  
         [0130]      FIG. 73  is a graph showing the acceptor impurity concentration profile in a body region of the same semiconductor device;  
         [0131]      FIG. 74  is a graph showing the relationship between the dose of arsenic as a counter impurity and the threshold voltage;  
         [0132]      FIG. 75  is a longitudinal sectional view showing the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0133]      FIG. 76  is a longitudinal sectional view showing the sectional structure of an element in a semiconductor device according to the fourth embodiment of the present invention;  
         [0134]      FIG. 77  is a longitudinal sectional view showing the sectional structure, along a bit line, of the same semiconductor device;  
         [0135]      FIG. 78  is a longitudinal sectional view showing the sectional structure, along a word line, of the same semiconductor device;  
         [0136]      FIG. 79  is a graph showing input waveforms to the same semiconductor device and to a semiconductor device according to a comparative example;  
         [0137]      FIG. 80  is a graph showing the difference between drain currents when data “1” and “0” are read out from the same semiconductor device and the semiconductor device according to the comparative example;  
         [0138]      FIG. 81  is a longitudinal sectional view showing the sectional structure of an element in one step of a method of fabricating the semiconductor device according to the fourth embodiment of the present invention;  
         [0139]      FIG. 82  is a longitudinal sectional view showing the sectional structure of an element in one step of another method of fabricating the semiconductor device according to the fourth embodiment of the present invention;  
         [0140]      FIG. 83  is a longitudinal sectional view showing the sectional structure of an element in one step of a method of fabricating the same semiconductor device;  
         [0141]      FIG. 84  is a view for explaining the principle when data “1” is written in an FBC;  
         [0142]      FIG. 85  is a view for explaining the principle when data “0” is written in the FBC;  
         [0143]      FIG. 86  is a view for explaining the principle when data written in the FBC is held; and  
         [0144]      FIG. 87  is a graph showing the difference between drain currents when data “1” and “0” are written in the FBC. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0145]     First, the principle of an operation of writing data in an FBC will be described below.  
         [0146]     The FBC is formed as a MOS transistor on an SOI substrate. As shown in  FIG. 84 , a source region SR, a drain region DR, and a body region BR between them are formed in an SOI layer. The body region BR sandwiched between the source region SR and the drain region DR is electrically floating. Data can be stored by charging or discharging the body region BR.  
         [0147]     To write data “1” in the FBC, as shown in  FIG. 84 , 1.5 V and 1.5 or 2 V, for example, are applied to a word line WL and the drain region DR, respectively, thereby biasing the FBC to a saturated state. This induces impact ionization in the body region BR, and holes are stored in the body region BR. Data “1” is written when a large number of holes are stored in the body region BR and the body potential rises.  
         [0148]     To write data “0” in the FBC, as shown in  FIG. 85 , 1.5 V and −1 or −1.5 V, for example, are applied to the word line WL and drain region DR, respectively, thereby forwardly biasing a p-n junction between a p-type body region contained in the body region BR and an n-type drain region. This removes holes stored in the body region BR to the drain region DR. Consequently, the number of holes in the body region BR decreases, the potential of the body region BR lowers, and data “0” is written in the FBC.  
         [0149]     To hold the body potentials corresponding to data “0” and data “1”, as shown in  FIG. 86 , −1.5 V, for example, are applied as a negative potential to the word line WL, and the drain region DR is set at 0 V.  
         [0150]     To read out data from the FBC, 0.2 V are applied to the drain region DR, and 1.5 V, for example, are applied to the word line WL to detect a drain current I.  
         [0151]     As shown in  FIG. 87 , data “0” and data “1” can be distinguished from each other by a difference ΔI between a drain current I 0  when the body potential corresponding to data “0” is held and a drain current I 1  when the body potential corresponding to data “1” is held.  
         [0152]     Generally, in the FBC as described above, the data holding time prolongs and the yield increases as the capacitance between the body region BR and a fixed-potential element such as a support substrate increases.  
         [0153]     Semiconductor devices according to embodiments of the present invention will be described below with reference to the accompanying drawings.  
         [0000]     (1) Semiconductor Device According to First Embodiment  
         [0154]      FIG. 1  shows the planar arrangement of a DRAM  100  as a semiconductor device according to the first embodiment of the present invention.  
         [0155]     A peripheral circuit for controlling the DRAM  100  can also be formed in the periphery of the DRAM  100 . The DRAM  100  includes word lines WL, source lines SL, and bit lines BL (not shown).  
         [0156]     The word lines WL and source lines SL run substantially parallel to each other, and the bit lines BL run in a direction substantially perpendicular to the word lines WL and source lines SL.  
         [0157]     Bit line contacts BC which electrically connect the bit lines BL (not shown) and drain regions (not shown) are formed below the bit lines BL.  
         [0158]      FIG. 2  shows a longitudinal sectional structure when an element region ER shown in  FIG. 1  is cut along a line B 1 -B 1 .  FIG. 3  shows a longitudinal sectional structure when the word line WL shown in  FIG. 1  is cut along a line C 1 -C 1 .  
         [0159]     Also,  FIG. 4  shows the planar arrangement of an N-channel MOS transistor in the region of a peripheral circuit or logic circuit (to be referred to as a peripheral logic circuit hereinafter).  FIG. 5  shows a longitudinal sectional structure when a gate electrode shown in  FIG. 4  is cut along a line E 1 -E 1 .  
         [0160]     A source region and drain region are formed in the two end portions of the element region ER, and a gate electrode GE is formed on a channel region between these source and drain regions via a gate insulating film. In an interlayer dielectric film  170 , a source contact SC, drain contact DC, and gate contact GC (none of them is shown) are formed on the source region, drain region, and gate electrode GE, respectively.  
         [0161]     The longitudinal sectional structure, along the word line WL, of the FBC included in the DRAM according to the first embodiment will be explained below with reference to  FIG. 3 .  
         [0162]     A body region  136  is formed on a support substrate  110  via a buried oxide film (to be referred to as a BOX layer hereinafter)  120  (an example of a first insulating film). A first gate insulating film  140  (an example of a second insulating film) is formed on the upper surface of the body region  136 . The gate insulating film on the upper surface of the body region in this FBC is the first gate insulating film  140 , and the gate insulating film in the peripheral circuit is a second gate insulating film  141 .  
         [0163]     The body region  136  except for the first gate insulating film  140  is covered with an STI oxide film  211  (an example of an element isolation insulating film) having a T-shape, a height H and a shoulder width S.  
         [0164]     The word line WL is formed on the upper surfaces of the first gate insulating film  140  and STI oxide film  211 .  
         [0165]     A silicide film  162  for reducing the resistance is formed on the upper surface of the word line WL, and the interlayer dielectric film  170  is formed on the silicide film  162 . The bit lines BL are formed in the surface portion of the interlayer dielectric film  170  in the direction perpendicular to the paper.  
         [0166]     In this embodiment, the STI oxide film  211  having a large thickness (height H) is formed, and this reduces the capacitance (to be referred to as Cg hereinafter) between the body region  136  of the FBC and the word line WL. Accordingly, it is possible to increase the ratio of the capacitance (to be referred to as Csub hereinafter) between the body region  136  and the support substrate  110  to the capacitance Cg.  
         [0167]     This facilitates distinguishing between data “1” and data “0”, and realizes a DRAM having a high yield and a long data holding time.  
         [0168]     When this FBC region is compared with the peripheral logic circuit region shown in  FIGS. 4 and 5 , the height of an STI oxide film  212  with respect to a body region  137  in the peripheral logic circuit is smaller than that of the STI oxide film  211  in the FBC region, in the section along the gate electrode GE.  
         [0169]     This makes it possible to eliminate residual polysilicon when the gate electrode material is etched in the peripheral logic circuit region, and provide a DRAM having a high yield.  
         [0170]     Note that the body region  137  in a transistor of the peripheral logic circuit is not necessarily electrically floating.  
         [0171]     The longitudinal sectional structure, along the bit line BL, of the FBC shown in  FIG. 2  will be described below. An n-type source region  132  and n-type drain region  134  are formed in an SOI layer  130 , and the p-type body region  136  is formed between the source region  132  and the drain region  134 .  
         [0172]     The first gate insulating film  140  (an example of a second insulating film) is formed on the p-type body region  136 , and the word line WL is formed on the first gate insulating film  140 . The word line WL is insulated from the body region  136  by the first gate insulating film  140 .  
         [0173]     The first gate insulating film  140  is made of, e.g., SiO 2 .  
         [0174]     The film thickness of the BOX layer  120  (an example of a first insulating film) is, e.g., 5 to 25 nm. The film thickness of the body region  136  is, e.g., 75 nm.  
         [0175]     A suicide  160  is formed on the surface of the source region  132  in contact with the source contact SC, thereby reducing the contact resistance.  
         [0176]     The upper surface of the word line WL is covered with the silicide  162 , and this reduces the resistance of the word line WL.  
         [0177]     A silicide  164  is formed on the surface of the drain region  134  in contact with the bit line contact BC, thereby reducing the contact resistance.  
         [0178]     A gap between the word line WL and the bit line BL is filled with the interlayer dielectric film  170  made of an insulating material such as SiO 2 .  
         [0179]     Furthermore, as shown in  FIG. 3 , in the longitudinal sectional structure along the word line WL, the STI oxide film  211  is formed in the front and rear of the body region  136 . The STI oxide film  211  is made of an insulating material such as SiO 2 .  
         [0180]     In this structure, the body region  136  is electrically floating because it is surrounded by the insulating material and the semiconductor materials different in conductivity type.  
         [0181]     The potential of the body region  136 , therefore, changes in accordance with the potentials of the support substrate  110 , word line WL, source region  132 , and drain region  134 .  
         [0182]      FIG. 6  shows the potential (to be referred to as a body potential hereinafter) of the body region  136  when data “1” or data “0” is written and held.  
         [0183]     V 1  denotes the body potential for data “1”; and V 0 , the body potential for data “0”. The abscissa indicates the time.  
         [0184]     From time  0  to time t 1 , data “1” is written in a certain body region  136  by setting the potentials of the word line WL and bit line BL at, e.g., 1.5 V, and data “0” is written in another body region  136  by setting the potential of the word line WL at, e.g., 1.5 V, and the potential of the bit line BL at, e.g., −1.5 V.  
         [0185]     At time t 1 , the bit line BL is returned to the holding state (e.g., 0 V).  
         [0186]     At time t 2 , the word line WL is returned to the holding state (e.g., −1.5 V).  
         [0187]     Of curves indicated by the body potentials V 1  and V 0 , curves L 2  indicated by the broken lines represent the body potentials of a conventional DRAM (to be referred to as a DRAM  10  for the sake of convenience) not including the STI oxide film  211  of this embodiment, and curves L 1  indicated by the solid lines represent the body potentials of the DRAM  100  according to the first embodiment.  
         [0188]     During data write from time  0  to time t 1 , the body potentials of the DRAM  10  and DRAM  100  are substantially the same.  
         [0189]     In this state, the body potential difference between data “1” and data “0” is large, so these data can be readily distinguished from each other.  
         [0190]     When the bit line BL is returned to the holding state from time t 1  to time t 2 , however, the body potential V 1  lowers, and the body potential V 0  rises. Accordingly, the body potential difference between data “1” and data “0” decreases.  
         [0191]     Furthermore, when the word line WL is returned to the holding state from time t 2  to time t 3 , the body potential V 0  lowers, and the body potential V 1  lowers more than the body potential V 0 . This further decreases the body potential difference between data “1” and data “0”.  
         [0192]     As is apparent from this graph shown in  FIG. 6 , in the conventional DRAM  10 , a body potential difference d 0  between data “1” and data “0” at time t 1  lowers to a potential difference d 10 . In the DRAM  100  according to the first embodiment, the body potential difference d 0  lowers to a potential difference d 100 .  
         [0193]     The potential difference d 100  of the DRAM  100  of this embodiment is larger than the potential difference d 10  of the conventional DRAM  10 .  
         [0194]     Generally, the larger the difference between the body potentials V 1  and V 0 , the more easily data “1” and data “0” are distinguished from each other, and the higher the yield.  
         [0195]     Accordingly, the DRAM  100  of this embodiment facilitates distinguishing between data “1” and data “0” and increases the yield, compared to the conventional DRAM  10 .  
         [0196]     The reason is as follows. In the DRAM  100  of this embodiment, as shown in  FIG. 3 , the STI oxide film  211  makes that area of the bottom surface of the body region  136 , which opposes the support substrate  110  larger than that area of the upper surface of the body region  136 , which opposes the word line WL. That is, in the section along the word line, a width Wb of the bottom portion of the body region is larger by width S×2 than a channel width Wg. Therefore, a ratio Csub/Cg of the capacitance Csub between the support substrate  110  and the body region  136  to the capacitance Cg between the body region  136  and the word line WL is higher than the capacitance ratio Csub/Cg of the conventional DRAM  10 .  
         [0197]     This reason will be explained in more detail below. Letting Csub denote the capacitance between the support substrate  110  and the body region  136 , Cd denote the capacitance between the drain region  134  and the body region  136 , Cs denote the capacitance between the source region  132  and the body region  136 , and Cg denote the capacitance between the word line WL and the body region  136 .  
         [0198]     A ratio R at which the capacitance Csub between the support substrate  110  and the body region  136  contributes to the body region  136  is represented by Csub/(Csub+Cd+Cs+Cg).  
         [0199]     A negative fixed potential is given to the support substrate  110 . Therefore, if Csub greatly contributes to the body region  136 , the body potentials V 1  and V 0  become more stable.  
         [0200]     Accordingly, the higher the ratio R, i.e., the larger the capacitance Csub, the more stable the body potentials V 1  and V 0 .  
         [0201]     As a consequence, even after the bit line BL and word line WL are returned to the holding state after time t 2  shown in  FIG. 6 , the potential difference d 100  in the DRAM  100  of this embodiment is maintained close to the potential difference d 0  during data write from time  0  to time t 1 .  
         [0202]     For example, the potential difference between the body potentials V 1  and V 0  is about 1.5 V during data write. However, after the bit line BL is returned to the holding state after time t 1 , the body potential difference (V 1 -V 0 ) is about 1.5 V×(Csub/(Csub+Cd+Cs+Cg)).  
         [0203]     If the ratio at which Cg contributes to the body region  136  is high, the potential difference between data “1” and data “0” largely decreases when the word line WL is returned to the holding state (from time t 2  to time t 3 ).  
         [0204]     For example, the decrease in body potential V 1  is larger by 1.5 V×(Cg/(Csub+Cd+Cs+Cg)) than that in body potential V 0 .  
         [0205]     This is so because the transistor threshold voltage for data “1” differs by 1.5 V from that for data “0”, and so the degrees of capacitance coupling between the word line WL and the body region  136  have a difference of 1.5 V.  
         [0206]     This expression (1.5 V×(Cg/(Csub+Cd+Cs+Cg))) also indicates that the body potential difference (V 1 -V 0 ) is increased by decreasing the capacitance Cg between the word line WL and the body region  136 .  
         [0207]     Each of the DRAM  100  of this embodiment and the conventional DRAM  10  has a BOX layer about 25 nm thick. In the DRAM  100  of this embodiment, however, the contact area between the body region  136  and the support substrate  110  increases, and this makes Csub of the DRAM  100  larger than that of DRAM  10 . Accordingly, the potential difference d 100  in the DRAM  100  of this embodiment is larger than the potential difference d 10  in the conventional DRAM  10 .  
         [0208]     The individual capacitances described above will be estimated in detail below.  
         [0209]     Referring to  FIG. 3 , assume that the channel width Wg is 100 nm, the width of the STI oxide film  211  is 100 nm, and a width (gate length) L of the word line WL is 100 nm.  
         [0210]     Assume also that the impurity concentration in the body region  136  is 1×10 18  cm −3 , the film thickness of the BOX layer  120  is 25 nm, the film thickness of the gate insulating film  140  is 5 nm, and the film thickness of the body region  136  is 75 nm.  
         [0211]     In the conventional DRAM  10  in which the corners of the upper surface of the body region  136  are not covered with the STI oxide film  211  having a T-shape according to this embodiment, both the capacitances Cd and Cs are 0.021 fF.  
         [0212]     The capacitance Csub is 0.014 fF. The depletion layer capacitance below the channel is 0.03 fF, the capacitance of the gate insulating film is 0.069 fF, and the capacitance Cg obtained by connecting these two capacitances in series is 0.021 fF.  
         [0213]     In this structure, the value (0.014 fF) of Csub is smaller than Cd, Cs, and Cg (each of which is 0.021 fF).  
         [0214]     The capacitance Csub is proportional to the width Wb and first gate length L of the body region  136 , and inversely proportional to a film thickness Tbox of the BOX layer  120 .  
         [0215]     That is, a relation Csub ∞ body region width Wb×first gate length L/BOX layer film thickness Tbox holds.  
         [0216]     On the other hand, the capacitances Cs and Cd are proportional to the width Wb of the body region and a film thickness Tsoi of the SOI layer, respectively. That is, 
 
 Cs∞Wb×Tsoi  
 
 Cd∞Wb×Tsoi  
 
         [0217]     Also, the capacitance Cg is proportional to the channel width Wg, the first gate length L, and a first gate capacitance Cgo (0.021×10 −4  fF/nm 2 ) per unit area.  
         [0218]     That is, 
 
 Cg∞Wg×L×Cgo  
 
         [0219]     By using the above numerical values and expressions, it is possible to increase the ratio of contribution of the capacitance Csub, and thereby increase the body potential difference d 100 .  
         [0220]     For example, letting Tbox denote the film thickness of the BOX layer  120 , Csub can be made larger than Cd and Cs by satisfying 
 
 Tsoi×Tbox/L&lt; 12.5 nm (1) 
 
         [0221]     Also, Csub can be made larger than Cg by satisfying 
 
 Tbox×Cgo×Wg/Wb&lt; 3.5×10 −5 /nm (2) 
 
         [0222]     In the conventional DRAM  10 , the channel width Wg and the width Wb of the body region are equal. To make the contribution ratio of Csub higher than that of Cg, therefore, it is necessary to decrease the film thickness of the BOX layer, or increase the thickness of the first gate insulating film.  
         [0223]     If the film thickness of the BOX layer is decreased, however, the parasitic capacitance of the peripheral logic circuit increases, and this increases the power consumption of the circuit or decrease the speed of the circuit.  
         [0224]     When the first gate insulating film is made thick, the FBC drain current greatly varies to decrease the yield.  
         [0225]     The DRAM  100  of this embodiment, therefore, has a structure in which the STI oxide film  211  covering the corners of the upper surface of the body region  136  makes the width Wg of the channel region smaller than the width Wb of the body region.  
         [0226]     For example, if the DRAM  100  has a shoulder width S of 25 nm, Wg is 100 nm, i.e., the same value as in the conventional device, whereas the width Wb of the body region in the interface of the BOX layer is 150 nm.  
         [0227]     In this structure, the capacitance Csub is 1.5 times that of the conventional DRAM  10 , i.e., 0.021 fF.  
         [0228]     That is, expression (2) presented earlier is satisfied by making the shoulder width S larger than 25 nm.  
         [0229]     More specifically, the shoulder width S on one side is increased to about 25% or more of the channel width Wg. This facilitates distinguishing between data “0” and data “1”, and extends the data holding time.  
         [0230]     The height H of the STI oxide film  211  is 10 times or more the film thickness of the gate insulating film  140 , i.e., at least 50 nm.  
         [0231]     This makes it possible to decrease the capacitance coupling formed between the first gate and the body region  136  via the STI oxide film  211  at the corners of the upper surface of the body region  136 .  
         [0232]     Also, if the DRAM  100  has an SOI film thickness of 50 nm, the capacitances Cd and Cs are 0.021 fF.  
         [0233]     That is, expression (1) presented earlier is satisfied by making the SOI film thickness smaller than 50 nm.  
         [0234]     Note that the impurity concentration in the body region  136  is 1×10 18  cm −3 .  
         [0235]     When a gate voltage equal to or higher than the threshold voltage is applied to this transistor, a depletion layer width formed below the channel, i.e., a so-called maximum depletion layer width is about 35 nm.  
         [0236]     Accordingly, when the SOI film thickness is equal to or larger than 35 nm, the FBC is a so-called “partially-depleted device”, i.e., a device in which the body is not entirely depleted and a neutral region is formed on the bottom.  
         [0237]     For example, if the impurity concentration in the body region is 1×10 17  cm −3 , the thickness of the depletion layer formed below the channel is about 100 nm.  
         [0238]     In this case, an FBC having an SOI film thickness of 100 nm or less is a so-called “fully-depleted device”, i.e., a device in which the entire body is depleted.  
         [0239]     The fully-depleted device can perform the FBC operation described above by sufficiently pulling the potential of the body bottom portion toward the negative side by applying a negative voltage to the support substrate, i.e., by setting the body bottom portion in a surface accumulation state.  
         [0240]     Accordingly, the capacitance Csub plays an important role even in this fully-depleted device, so the data holding time can be extended by the FBC structure according to this embodiment.  
         [0000]     (2) Method of Fabricating Semiconductor Device According to First Embodiment  
         [0241]     A method of fabricating the semiconductor device (DRAM  100 ) according to the first embodiment of the present invention will be described below with reference to FIGS.  7  to  25  showing the sections of elements in different steps.  
         [0242]     FIGS.  7  to  9  are sectional views each showing the element structure of a longitudinal section taken along the word line WL along the line C 1 -C 1  in  FIG. 1 .  
         [0243]     Of FIGS.  10  to  25 , each of  FIGS. 10, 12 ,  14 ,  18 , and  22  shows the longitudinal sectional structure of the FBC taken along the word line WL along the line C 1 -C 1  in  FIG. 1 .  
         [0244]     Each of  FIGS. 16, 20 , and  24  shows the longitudinal section of the FBC taken along the element region ER along the line B 1 -B 1  in  FIG. 1 .  
         [0245]     Each of  FIGS. 11, 13 ,  15 ,  19 , and  23  shows a longitudinal section along the gate electrode of an N-channel transistor included in the peripheral logic circuit taken along the line E 1 -E 1  in  FIG. 4 .  
         [0246]     Each of  FIGS. 17, 21 , and  25  shows a longitudinal section including the source contact SC and drain contact DC taken along a line F 1 -F 1  in  FIG. 4 .  
         [0247]     First, as shown in  FIG. 7 , an SOI substrate having a support substrate  110 , BOX layer  120 , and SOI layer  130  is prepared.  
         [0248]     The thickness of the BOX layer  120  is 25 nm, and that of the SOI layer  130  is 60 nm.  
         [0249]     A silicon oxide film  201  is formed by oxidizing the upper surface of the SOI layer  130 .  
         [0250]     CVD or the like is used to deposit a silicon nitride film  203  on the silicon oxide film  201 , and a silicon oxide film  205  on the silicon nitride film  203 .  
         [0251]     The silicon oxide film  205  is coated with a resist, and this resist is patterned by using photolithography to form a resist film  207 .  
         [0252]     As shown in  FIG. 8 , the resist film  207  is used as a mask to etch the silicon oxide film  205 , silicon nitride film  203 , and silicon oxide film  201  by RIE or the like.  
         [0253]     A silicon oxide film  206  is then deposited. The film thickness of the silicon oxide film  206  is adjusted not to fill the spacing between the nitride films  203  with the silicon oxide film  206 .  
         [0254]     For example, when the spacing between the silicon nitride films  203  is 200 nm, the film thickness of the silicon oxide film  206  is 80 nm.  
         [0255]     The silicon oxide film  206  is etched by RIE or the like.  
         [0256]     As shown in  FIG. 9 , the SOI layer  130  is etched by using the silicon oxide films  205  and  206  as masks by RIE or the like.  
         [0257]     Then, the silicon oxide films  205  and  206  are selectively removed. After that, a silicon oxide film  211  is deposited so as to fill the spacings between the adjacent SOI layers  130  by using CVD or the like.  
         [0258]     CMP or the like is used to polish the silicon oxide film  211  and planarize its upper surface by using the silicon nitride film  203  as a stopper.  
         [0259]     As shown in  FIGS. 10 and 11 , the silicon nitride film  203  as an etching stopper is removed.  
         [0260]     A resist film (not shown) having an opening in an FBC formation region is formed. This resist film is used as a mask to ion-implant boron at an acceleration energy of 130 keV and a dose of 1×10 14  cm −2 . In addition, boron is ion-implanted at an acceleration energy of 10 keV and a dose of 1×10 13  cm −2 , and the resist film is removed.  
         [0261]     Annealing is then performed for about 10 min in a nitrogen ambient at 1,200° C. Consequently, boron having an impurity concentration of about 1×10 18  cm −3  evenly distributes in the FBC formation region in the SOI layer  130 .  
         [0262]     Simultaneously, boron having a concentration of about 1×10 18  cm −3  evenly distributes in the FBC formation region in the support substrate  110 .  
         [0263]     The boron diffusion layer formed in the support substrate  110  extends outside the cell array. This boron diffusion layer is connected to an interconnection by a contact plug (not shown) formed on the diffusion layer, and given a fixed voltage.  
         [0264]     After that, an appropriate impurity is doped in the SOI layer and support substrate in an n-channel MOS transistor region and p-channel MOS transistor region in a peripheral logic circuit. RTA (Rapid Thermal Annealing) is then performed to activate the doped impurity.  
         [0265]     As shown in  FIGS. 12 and 13 , gate insulating films  140  and  141  are formed on the SOI  130 . In this case, a resist pattern (not shown) having an opening in the peripheral logic circuit is formed, and the STI oxide film  211  is etched by ammonium fluoride.  
         [0266]     In this stage, in the FBC formation region shown in  FIG. 12 , a height H and shoulder width S of the STI oxide film  211  are about 50 nm and 50 nm, respectively. These values depend upon the film thickness of the silicon oxide film  206 .  
         [0267]     On the other hand, in the peripheral logic circuit shown in  FIG. 13 , the height H and shoulder width S of the STI oxide film  211  are substantially 0.  
         [0268]     In the FBC region shown in  FIG. 12 , the thickness of the first gate insulating film  140  is, e.g., 6 nm in order to increase the drain current difference.  
         [0269]     On the other hand, in the peripheral logic circuit shown in  FIG. 13 , the thickness of the second gate insulating film  141  is very small, e.g., 1 nm, in order to increase the processing speed and reduce the power consumption.  
         [0270]     As shown in FIGS.  14  to  17 , polysilicon as a gate electrode material is deposited on the gate insulating films in the FBC region and peripheral logic circuit region.  
         [0271]     As shown in  FIGS. 14 and 16 , photolithography is used to form a resist film  175  (not shown) having the pattern of word lines WL in the FBC region.  
         [0272]     In this state, the peripheral logic circuit region shown in  FIGS. 15 and 17  is covered with the resist film  175 , so no gate electrode patterning is performed in this region.  
         [0273]     The resist film  175  is used as a mask to form word lines WL in the FBC region by RIE.  
         [0274]     Since the height H of the STI oxide film  211  is as large as 50 nm, sufficient overetching is necessary. In the FBC region, an overetching amount can be increased because the thickness of the first gate insulating film  140  is as large as 6 nm.  
         [0275]     As shown in  FIGS. 18 and 20 , in the FBC region, the word lines WL are used as masks to ion-implant an impurity in self-alignment in the SOI layer  130 . In this manner, a lightly doped drain of about 10 18  cm −3  can be formed.  
         [0276]     The side surfaces of the word lines WL are covered with spacers  213 . The spacers  213  can be formed by using a silicon oxide film or silicon nitride film.  
         [0277]     As shown in  FIGS. 19 and 21 , to form a gate electrode GE in the peripheral logic circuit region, a resist film  176  which covers the FBC region and has the pattern of the gate electrode GE in the peripheral logic circuit is formed. The resist film  176  is used as a mask to perform RIE on polysilicon.  
         [0278]     In the peripheral logic circuit region, an overetching amount cannot be increased because the second gate insulating film  141  having a small thickness of 1 nm is formed. Therefore, if in the peripheral logic circuit the height H of the STI oxide film  211  is 50 nm as in the FBC region, a polysilicon residue is produced to decrease the yield of the DRAM.  
         [0279]     In this embodiment, however, the height of the STI oxide film  211  in the peripheral logic circuit is smaller than the height H of the STI oxide film in the FBC region. Therefore, no polysilicon residue is produced, and a high-yield DRAM can be implemented.  
         [0280]     As shown in FIGS.  22  to  25 , the word lines WL and spacers  213  are used as masks to ion-implant an impurity in self-alignment in the SOI layer  130 .  
         [0281]     In this embodiment, phosphorus or arsenic is ion-implanted in the SOI layer  130 . Consequently, as shown in  FIG. 24 , an n-type source region  132  and n-type drain region  134  are formed.  
         [0282]     A p-type body region  136  is formed between the source region  132  and the drain region  134 . A metal is deposited on the upper surfaces of the word lines WL, thereby forming silicide layers  160 ,  162 , and  164 . The silicide layers  160 ,  162 , and  164  may also be formed by using, e.g., cobalt silicide.  
         [0283]     Subsequently, an interlayer dielectric film  170  is deposited on the silicide layers  160 ,  162 , and  164 . A plug (not shown) to be electrically connected to the silicide layer  164  is formed in the interlayer dielectric film  170 .  
         [0284]     In addition, source lines SL to be connected to source plugs (not shown) are formed parallel to the word lines WL, and bit lines BL are formed substantially perpendicularly to the word lines WL. The source lines SL and bit lines BL may also be formed by using a metal material such as copper, aluminum, or tungsten.  
         [0285]     The DRAM  100  of this embodiment can be fabricated by the above method.  
         [0000]     (3) Semiconductor Device According to Second Embodiment  
         [0286]      FIG. 26  shows the planar arrangement of a semiconductor device (DRAM  200 ) according to the second embodiment of the present invention.  
         [0287]     The DRAM  200  has back gate lines BGL in addition to the constituent elements of the DRAM  100 . The back gate lines BGL run substantially parallel to word lines WL.  
         [0288]     A front gate electrode FGE (not shown) of a unit transistor is connected to the word line WL, and a back gate electrode BGE (not shown) is connected to the back gate line BGL.  
         [0289]      FIG. 27  shows a longitudinal section taken along a line A 2 -A 2  in  FIG. 26 .  
         [0290]     Likewise,  FIGS. 28, 29 , and  30  illustrate longitudinal sections taken along lines C 2 -C 2 , B 2 -B 2 , and D 2 -D 2 , respectively, in  FIG. 26 .  
         [0291]     As shown in FIGS.  27  to  33 , particularly, in  FIG. 29 , in the semiconductor device according to the second embodiment, the back gate electrode BGE is formed below a body region  336  in the longitudinal section along the word lines WL.  
         [0292]     A back gate insulating film BGI (an example of a first insulating film) is formed between the body region and the back gate electrode. Also, a front gate insulating film FGI (an example of a second insulating film) is formed between the body region and the front gate electrode.  
         [0293]     In addition, a T-shaped STI oxide film  311  is formed. This makes the contact area between the body region  336  and the front gate insulating film FGI smaller than that between the body region  336  and the back gate insulating film BGI. That is, in the section along the word lines, the channel width is set smaller than the bottom width of the body region. The capacitance between the body region  336  and the front gate electrode FGE corresponds to the capacitance Cg explained in the first embodiment. Also, the capacitance between the body region  336  and the back gate electrode BGE corresponds to the capacitance Csub explained in the first embodiment.  
         [0294]     More specifically, as described in detail in the first embodiment, the ratio of the capacitance Csub to the capacitance Cg is higher than the capacitance ratio in the conventional device. This facilitates distinguishing between data “1” and data “0”, and implements a DRAM having a high yield and a long data holding time.  
         [0000]     (4) Method of Fabricating Semiconductor Device According to Second Embodiment  
         [0295]     A method of fabricating the semiconductor device according to the second embodiment described above will be explained below.  
         [0296]      FIGS. 31, 35 ,  39 ,  43 ,  47 ,  51 ,  55 ,  59 ,  63 , and  67  illustrate longitudinal sections, in different steps, taken along the line A 2 -A 2  in  FIG. 26 .  
         [0297]     Similarly,  FIGS. 32, 36 ,  40 ,  44 ,  48 ,  52 ,  56 ,  60 ,  64 , and  68  illustrate longitudinal sections, in different steps, taken along the line C 2 -C 2  in  FIG. 26 .  
         [0298]      FIGS. 33, 37 ,  41 ,  45 ,  49 ,  53 ,  57 ,  61 ,  65 , and  69  illustrate longitudinal sections, in different steps, taken along the line B 2 -B 2  in  FIG. 26 .  
         [0299]      FIGS. 34, 38 ,  42 ,  46 ,  50 ,  54 ,  58 ,  62 ,  66 , and  70  illustrate longitudinal sections, in different steps, taken along the line D 2 -D 2  in  FIG. 26 .  
         [0300]     First, as shown in FIGS.  31  to  34 , an SOI substrate having a BOX layer  320  and SOI layer  330  on the upper surface of a support substrate  310  is prepared. The thickness of the BOX layer  320  is, e.g., 150 nm.  
         [0301]     On the SOI layer  330 , a silicon nitride film  340  and a silicon oxide film (not shown) are deposited in order.  
         [0302]     A resist film (not shown) is used as a mask to perform reactive ion etching, thereby selectively removing the silicon oxide film, silicon nitride film  340 , and SOI layer  330 , and selectively removing the BOX layer  320  to the middle of its depth. After that, the resist film is peeled, and the silicon oxide film is removed.  
         [0303]     On the side surfaces of the SOI layer  330 , a thermal oxide film (not shown) about 2 nm thick is formed.  
         [0304]     As shown in FIGS.  35  to  38 , a silicon nitride film  350  as an etching stopper about 10 nm thick is formed by CVD so as to cover the surfaces of the silicon nitride film  340 , SOI layer  330 , and BOX layer  320 .  
         [0305]     A resist film  360  for reducing steps is formed on the silicon nitride film  350 , and an SOG (Spin On Glass) film  370  is formed on the resist film  360 .  
         [0306]     Then, a resist film  380  is formed on the SOG film  370 . The resist film  380  has openings in positions where portions corresponding to word line formation regions intersect portions corresponding to regions in which an STI oxide film for isolating element regions of a plurality of transistors is to be formed.  
         [0307]     As shown in FIGS.  39  to  42 , the resist film  380  is used as a mask to perform reactive ion etching, thereby selectively removing the SOG film  370 , resist film, silicon nitride film  350 , and BOX layer  320 .  
         [0308]     As shown in FIGS.  43  to  46 , the resist film  380 , SOG film  370 , and resist film  360  are removed.  
         [0309]     Etching stoppers are formed by the silicon nitride films  340  and  350 . These etching stoppers thus formed are used to remove the BOX layer  320  by wet etching using ammonium fluoride. In this manner, hollow portions  321  are formed below the SOI layer  330  where channel bodies are to be formed.  
         [0310]     As shown in FIGS.  47  to  50 , the silicon nitride film  340  is anisotropically etched. In addition, thermal oxidation is performed to form a silicon oxide film  360  on the exposed surfaces of the silicon substrate  310  and SOI layer  330 .  
         [0311]     The silicon oxide film in contact with the SOI layer  330  exposed to the hollow portions  321  is the back gate insulating film BGI. The thickness of the back gate insulating film BGI is about 10 nm.  
         [0312]     Then, CVD is used to deposit an arsenic-doped amorphous silicon film  370 .  
         [0313]     As shown in FIGS.  51  to  54 , the amorphous silicon film  370  is etched to form an amorphous silicon film  371 .  
         [0314]     As shown in FIGS.  55  to  58 , the exposed silicon oxide film is removed by using ammonium fluoride, and an amorphous silicon film  372  as an example of an arsenic-doped conductive film is deposited by CVD.  
         [0315]     As shown in FIGS.  59  to  62 , the amorphous silicon film  372  is etched back by anisotropic etching with the etching stoppers made of the silicon nitride films  340  and  350  left behind between adjacent element isolating portion formation regions, i.e., between adjacent element formation regions.  
         [0316]     In this manner, back gate lines are patterned while the film thickness of the amorphous silicon film  372  is decreased in the element isolating portion formation regions.  
         [0317]     That is, in order to form back gate electrodes via the back gate insulating film BGI, back gate lines for connecting the back gate electrodes together are patterned along the direction of word line formation regions.  
         [0318]     After the back gate lines are thus patterned, as shown in FIGS.  63  to  66 , hot phosphoric acid is used to etch the silicon nitride film  340  until it recedes by about 60 nm.  
         [0319]     This etching amount determines the cover amount of the silicon oxide film which covers the corners of the upper surface of a body region, i.e., determines the shoulder width S. Therefore, the etching amount must be appropriately set in accordance with the characteristics of the FBC.  
         [0320]     As shown in FIGS.  67  to  70 , a silicon oxide film  311  as an element isolating portion is formed on the entire surface of the SOI substrate by CVD.  
         [0321]     Then, the silicon oxide film  311  is planarized by CMP (Chemical Mechanical Polishing) by using the silicon nitride film  340  as a stopper.  
         [0322]     The subsequent steps are the same as in the first embodiment, so a detailed explanation thereof will be omitted.  
         [0000]     (5) Semiconductor Device According to Third Embodiment  
         [0323]     A DRAM  300  according to the third embodiment of the present invention will be described below with reference to the accompanying drawings.  
         [0324]      FIG. 71  shows the longitudinal sectional structure, cut along the longitudinal direction of word lines WL, of an FBC included in the DRAM  300 .  
         [0325]     In the DRAM  300 , as in the first embodiment, the corners on the two sides of the upper surface of a body region are covered with shoulders having a width S and height H on the two sides of a T-shaped STI oxide film  411 .  
         [0326]     This increases the difference between drain currents when data “0” and “1” are read out, and extends the data holding time.  
         [0327]     Also, that central upper surface portion of a body region  436 , which is not covered with the shoulders of the STI oxide film  411  contains an impurity (counter impurity)  480  having a conductivity type opposite to that of the bottom portion. This lowers the threshold voltage of a transistor, and implements a low-power-consumption DRAM.  
         [0328]     The principle of reducing the power consumption will be explained below. To increase the difference between drain currents when data “0” and data “1” are read out, it is possible to increase the difference between body potentials during holding, or increase the body effect. The body effect is a phenomenon in which the body potential modulates the drain current.  
         [0329]     When the impurity concentration in the body region is increased, the thickness of a depletion layer below the gate decreases, and this strengthens the capacitance coupling between the body region and a channel region, and increases the body effect. The difference between drain currents can be increased by increasing the body effect.  
         [0330]     If, however, the acceptor impurity concentration in the body region rises, the threshold voltage of the transistor rises. Accordingly, it is necessary to rise the gate voltage during data read and write. This poses the problem that the power consumption of the DRAM increases.  
         [0331]     To solve this problem, in this embodiment, the acceptor concentration in the bottom region of the body region  436  is 1×10 18  cm −3  or more.  
         [0332]     In the central upper surface portion  480  of the body region  436 , an impurity (counter impurity) having a conductivity type opposite to that in the bottom region is doped to set the net acceptor impurity concentration at 1×10 17  cm −3 .  
         [0333]     The central upper surface portion  480  is a region from the surface to a depth of about 20 nm.  
         [0334]     When a gate voltage equal to or higher than the threshold voltage is applied to a transistor having an acceptor concentration of 1×10 17  cm −3 , a depletion layer width formed below the channel region, i.e., a so-called maximum depletion layer width is about 100 nm. Therefore, the central upper surface portion  480  having a depth of 20 nm is depleted. The threshold voltage of a transistor is determined by the acceptor concentration in the surface portion. In this embodiment, therefore, the threshold voltage of a transistor can be lowered.  
         [0335]     Also, the addition of the central surface portion  480  corresponding to a lightly doped surface layer 20 nm thick increases the maximum depletion layer width below the first gate insulating film.  
         [0336]     Since, however, the bottom region of the body region  436  has a concentration of 1×10 18  cm −3  or more, the increase is about 1/10 the thickness of the lightly doped surface layer. This slightly lowers the body effect.  
         [0337]      FIG. 72  shows the distribution of each impurity concentration in the direction of depth. That is, curves L 1 , L 2 , and L 3  indicates the impurity concentrations of arsenic, boron, and phosphorus, respectively, in a gate electrode GE, a gate insulating film GEI, the body region  436 , a BOX layer  420 , and a support substrate  410 .  
         [0338]     As shown in  FIG. 72 , the counter impurity region  480  is formed in self-alignment in the center of the body region  436 , i.e., in a first gate insulating film formation region which is not covered with the shoulders of the STI buried oxide film  411 .  
         [0339]     To realize this structure, it is desirable to dope boron and arsenic as a counter impurity in the bottom region of the body region  436 .  
         [0340]     Since boron is a relatively light element, the distribution curve after ion implantation is moderate. By contrast, arsenic is a heavy element, so the distribution after ion implantation forms a steep curve.  
         [0341]     Also, the diffusion coefficient of arsenic is smaller than that of boron. After a heating step, therefore, boron evenly distributes in the body region by diffusion, but arsenic hardly diffuses and does not change the distribution shape.  
         [0342]      FIG. 73  shows the result of simulation of the net acceptor concentration distribution of the sum of arsenic, boron, and phosphorus in the direction of depth in particularly the central portion of the body region  436 .  
         [0343]     Assume that the acceleration voltage and dose of boron as an impurity are 15 keV and 1×10 13  cm −2 , respectively, the thickness of the body region  436  is 55 nm, the thickness of the BOX layer  420  is 25 nm, the thickness of the gate oxide film is 6 nm, the gate length is 0.15 μm, and the drain voltage is 1.5 V.  
         [0344]     The acceptor concentration is 1.2×10 18  cm −3  in the bottom region and 1×10 17  cm −3  or less in the central surface region of the body region  436 .  
         [0345]      FIG. 74  shows the result of simulation performed on the relationship between the dose of arsenic doped as a counter impurity and the threshold voltage. As shown in  FIG. 74 , as the dose of arsenic increases, the threshold voltage of the transistor lowers.  
         [0346]     In this embodiment, an impurity having a conductivity type opposite to that of an impurity doped in the bottom portion is doped as a counter impurity in the central surface portion of the body region  436 . This makes it possible to lower the threshold voltage of the transistor and reduce the power consumption.  
         [0000]     (6) Method of Fabricating Semiconductor Device According to Third Embodiment  
         [0347]     A method of fabricating the semiconductor device according to the third embodiment of the present invention will be explained below.  
         [0348]     As shown in  FIG. 75 , an SOI substrate having, on a support substrate  401 , a 25-nm thick BOX layer  420  and a 60-nm thick SOI layer, in which a body region  436  is to be formed, is prepared. A structure having a gate insulating film GEI and an STI insulating film  411  is obtained through the same fabrication steps as in the first embodiment described earlier.  
         [0349]     As indicated by the arrows, arsenic is ion-implanted at an acceleration energy of 20 keV and a dose of 3×10 12  cm −2 .  
         [0350]     In this state, the corners of the upper surface of the body region  436  are covered with the 60-nm high STI oxide film  411 . Therefore, arsenic can be doped in self-alignment in a region where the first gate insulating film GEI is formed. After that, the semiconductor device can be fabricated through the same steps as in the fabrication method according to the first embodiment.  
         [0000]     (7) Semiconductor Device According to Fourth Embodiment  
         [0351]      FIG. 76  is a longitudinal sectional view, cut along word lines WL, of an FBC in a DRAM  400  according to the fourth embodiment of the present invention.  
         [0352]     In the DRAM  400 , as in the first embodiment, the corners of the upper surface of a body region  536  are covered with shoulders having a height H and a width S of an STI oxide film  511 .  
         [0353]     The body region  536  contains an impurity at a concentration of about 1×10 18  cm −3  as an acceptor impurity concentration by which the whole body region  536  is not depleted even when a voltage equal to or higher than the threshold voltage is applied to the gate.  
         [0354]     Also, a central region  580  of the body region  536 , which is not covered with the shoulders of the STI oxide film  511  has an impurity concentration of, e.g., 1×10 17  cm −3  as an impurity concentration by which the entire body region  536  is depleted when a voltage equal to or higher than the threshold voltage of the transistor is applied to the gate.  
         [0355]     The film thickness of the body region  536  is 55 nm. The central region  580  contains boron at an impurity concentration of, e.g., 1×10 17  cm −3 . When a gate voltage equal to or higher than the threshold voltage is applied to a transistor having an acceptor concentration of 1×10 17  cm −3 , a depletion layer formed below the channel is about 100 nm. Therefore, the 55-nm thick body region  536  is entirely depleted.  
         [0356]     Also, end regions  581  of the channel contain boron at an impurity concentration of, e.g., 2×10 18  cm −3 .  
         [0357]     Since a depletion layer at this impurity concentration is about 25 nm, the 55-nm thick end regions  581  are not entirely depleted, but have neutral regions.  
         [0358]     The following effects are obtained by the DRAM  400  of this embodiment.  
         [0359]     First, the threshold voltage of the transistor can be lowered.  
         [0360]     This is so because the central region  580  of the body region  536 , which is not covered with the shoulders of the STI oxide film  511  has a low acceptor impurity concentration.  
         [0361]     Consequently, it is possible to lower the gate voltage during data read and write, and reduce the power consumption of the DRAM.  
         [0362]     Second, the reverse bias p-n junction current can be reduced.  
         [0363]     In the conventional fully-depleted FBC, the body bottom portion must be set in a surface accumulation state by well pulling the potential of this body bottom portion toward the negative side by applying a negative voltage to the support substrate.  
         [0364]     Since this increases the electric field of the p-n junction formed by the body and the source/drain, a large p-n junction current flows, and the data holding time shortens.  
         [0365]     By contrast, in the DRAM  400  according to this embodiment, while the body central region  580  having a low impurity concentration is fully-depleted, the body peripheral regions  581  having a high impurity concentration have neutral regions.  
         [0366]     In the FBC having this structure, data “0” and data “1” can be distinguished from each other by the number of holes in the neutral regions of the body peripheral regions  581 , without applying any large negative potential to the support substrate  510 .  
         [0367]     As a consequence, the DRAM  400  can raise the voltage of the support substrate  510 , weaken the p-n junction electric field, and reduce the p-n junction current, when compared to the conventional fully-depleted FBC.  
         [0368]     The third effect is that the difference between drain currents when data “0” and data “1” are read out can be increased.  
         [0369]     The neutral regions which are not fully-depleted are formed in the body peripheral regions  581  covered with the thick STI oxide film  511 . As described in the first embodiment, therefore, the capacitance coupling Cg between the gate electrode and the neutral regions can be decreased. Consequently, the difference between drain currents when data “0” and data “1” are read out can be made larger than that in a structure in which no T-shaped STI oxide film having shoulders with a height H is formed.  
         [0370]     Also, since the neutral regions are formed in self-alignment in the body peripheral regions  581 , variations in characteristics can be suppressed.  
         [0371]     The fourth effect is the same as the third effect, i.e., the difference between drain currents when data “0” and data “1” are read out can be increased.  
         [0372]     The body central region  580  not covered with the shoulders of the STI oxide film  511  is fully-depleted. In the fully-depleted device, the strength of the body effect is inversely proportional to the SOI film thickness. Accordingly, the difference between drain currents when data “0” and data “1” are read out can be increased by decreasing the SOI film thickness, i.e., the film thickness of the body region  536 .  
         [0373]     The results of simulation of write and read operations performed in the DRAM  400  according to the fourth embodiment will be explained below.  
         [0374]      FIGS. 77 and 78  illustrate the longitudinal sectional structures, along bit lines and word lines, respectively, of the FBC in the DRAM  400 .  FIG. 78  shows a longitudinal sectional structure along word lines. The film thickness of the SOI layer, i.e., the body region  536  is 35 nm, the film thickness of the gate insulating film GEI is 6 nm, and the film thickness of the BOX oxide film  520  is 20 nm.  
         [0375]     Also, as shown in  FIG. 78 , a channel width Wg is 110 nm, and a region below the channel contains an impurity at a concentration of 1×10 17  cm −3 . The width S of the shoulder of the STI oxide film  511  is 60 nm, and an impurity is contained in the shoulder region at a concentration of 2×10 18  cm −3 .  
         [0376]     The results of comparison between the DRAM  400  according to the fourth embodiment and a conventional DRAM, in which the channel width Wg and the width Wb of the body region are equally 230 nm, and which contains an impurity at an impurity concentration of 1×10 17  cm −3 , will be explained below.  
         [0377]      FIG. 79  shows input waveforms. A curve L 11  indicates the drain voltage, and a curve L 12  indicates the gate voltage. The support substrate is fixed to −1 V, while the source region is fixed to 0 V.  
         [0378]     Data “1” is written from time 12 nsec to time 30 nsec. Data “0” is written from time 42 nsec to time 60 nsec.  
         [0379]     Data is read out from time 38 nsec to time 40 nsec, and from time 68 nsec to time 70 nsec.  
         [0380]      FIG. 80  shows the results of simulation of a drain current Id as a function of a gate voltage Vg when data is read out from time 38 nsec to time 40 nsec, and from time 68 nsec to time 70 nsec. The difference between a curve L 21  when data “1” is read out and a curve L 22  when data “0” is read out from the DRAM of this embodiment, i.e., the difference between drain currents Id of data “0” and data “1” is larger than the difference between a curve L 31  when data “1” is read out and a curve L 32  when data “0” is read out from the DRAM as a comparative example.  
         [0000]     (8) Method of Fabricating Semiconductor Device According to Fourth Embodiment  
         [0381]     A method of fabricating the semiconductor device according to the fourth embodiment of the present invention will be explained below.  
         [0382]     An SOI substrate having a 25-nm thick BOX layer 520 and 60-nm thick SOI layer on a support substrate  510  is prepared, and a structure shown in  FIG. 81  is obtained through the same fabrication steps as in the first embodiment.  
         [0383]     Then, a resist film (not shown) which has an opening in an FBC formation region and covers a peripheral circuit formation region is formed.  
         [0384]     This resist film is used as a mask to ion-implant boron at an acceleration energy of 130 keV and a dose of 1×10 14  cm −2 . After that, the resist film is removed. Annealing is then performed for about 10 min in a nitrogen ambient at 1,200° C.  
         [0385]     Consequently, boron evenly distributes at an impurity concentration of 1×10 18  cm −3  in the FBC formation region of the support substrate  510 .  
         [0386]     The boron diffusion layer formed in the support substrate  510  extends outside the FBC cell array. This boron diffusion layer is connected to an interconnection by a contact plug (not shown) formed on the diffusion layer, and given a fixed voltage.  
         [0387]     A resist film having an opening in the FBC formation region is used as a mask to ion-implant boron at an acceleration energy of 30 keV and a dose of 2×10 13  cm −2 .  
         [0388]     In this state, the corners of the upper surface of the body region are covered with shoulders, having a height H of 60 nm, of an STI oxide film  511 .  
         [0389]     As indicated by regions  510 A in  FIG. 81 , therefore, boron is mostly implanted into the support substrate  510  in regions where a gate insulating film is to be formed. In regions covered with the STI oxide film  511 , boron is mostly implanted into the SOI layer as indicated by regions  536 A.  
         [0390]     Accordingly, the boron concentration in the SOI layer in the regions covered with the STI oxide film  511  can be made higher, by self-alignment, than that in the SOI layer in the regions where a gate insulating film is to be formed.  
         [0391]     After that, the semiconductor device of this embodiment can be completed through the same steps as in the fabrication method according to the first embodiment.  
         [0392]     Another method of fabricating the semiconductor device according to the fourth embodiment will be described below.  
         [0393]     An SOI substrate having a 25-nm thick BOX layer 520 and 60-nm thick SOI layer on a support substrate  510  is prepared. A structure in which an SOI layer  536  is patterned as shown in  FIG. 82  is obtained through the same fabrication steps as in the first embodiment.  
         [0394]     Then, a resist film (not shown) having an opening in an FBC formation region is formed.  
         [0395]     This resist film is used as a mask to ion-implant boron at an acceleration energy of 260 keV and a dose of 1.5×10 14  cm −2 . After that, a resist film  590  is removed.  
         [0396]     Annealing is then performed for about 10 min in a nitrogen ambient at 1,200° C.  
         [0397]     Consequently, boron evenly distributes at an impurity concentration of 1×10 18  cm −3  in the FBC formation region of the support substrate  510 .  
         [0398]     The boron diffusion layer formed in the support substrate  510  extends outside the FBC cell array. This boron diffusion layer is connected to an interconnection by a contact plug (not shown) formed on the diffusion layer, and given a fixed voltage.  
         [0399]     As shown in  FIG. 83 , a resist film having an opening in the FBC formation region and a silicon nitride film  590  corresponding to the silicon nitride film  211  in the first embodiment are used as masks to ion-implant boron at an acceleration energy of 3 keV and a dose of 1×10 13  cm −2 . After that, the resist film  590  is removed.  
         [0400]     RTA (Rapid Thermal Annealing) is then performed to activate the doped impurity.  
         [0401]     As a consequence, boron is doped at a concentration of 1×10 18  cm −3  in the two end portions of the body region  536 .  
         [0402]     After that, the semiconductor device of this embodiment can be completed through the same steps as in the fabrication method according to the first embodiment.  
         [0403]     Of the steps of forming the structure shown in  FIGS. 10 and 11 , however, the step of doping boron at a concentration of 1×10 18  cm −3  in the support substrate  510  is unnecessary.  
         [0404]     Also, boron is doped at a concentration of 1×10 17  cm −3  in the SOI layer in order to form a fully-depleted device in the region below the gate insulating film.  
         [0405]     In the fabrication method of this embodiment, the silicon nitride film  590  formed in the region where the gate insulating film is to be formed is used as a mask to ion-implant boron in the two end portions of the body region  536 . Therefore, the boron concentration in regions except for the region where the gate insulating film is to be formed can be increased with high controllability.  
         [0406]     Accordingly, it is possible to suppress variations in difference between drain currents corresponding to data “0” and data “1”, and provide a high-yield DRAM Also, in this fabrication method, the silicon nitride film  590  formed in the region where the first gate insulating film is to be formed is used as a mask to ion-implant boron in the two end portions of the SOI layer. This obviates the need to limit the ion implantation region of boron by forming a very small pattern by using the advanced lithography technique. As a consequence, the fabrication cost can be reduced.  
         [0407]     As described above, the semiconductor devices and the methods of fabricating the same according to the embodiments of the present invention can provide an FBC having a long data holding time, and this increases the yield.  
         [0408]     The embodiments described above are merely examples, and can be variously modified within the technical scope of the present invention.