Abstract:
A SOI structure semiconductor device includes a silicon substrate ( 1 ), an insulating oxide layer ( 2 ) formed on the silicon substrate ( 1 ), a SOI layer ( 3 ) formed on the insulating oxide layer ( 2 ) a LOCOS oxide layer ( 4 ) formed on the insulating oxide layer ( 2 ) and contacting with the SOI layer ( 3 ) in order to insulate the SOI layer ( 3 ), a gate insulation layer ( 5 ) formed on the SOI layer ( 3 ) and a gate electrode ( 6 ) formed on the gate insulation layer ( 5 ). The SOI layer ( 3 ) has a sectional triangle portion ( 10 ) contacting with the LOCOS oxide layer ( 4 ). The sectional triangle has an oblique side ( 12 ) as a boundary between the SOI layer ( 4 ) and the LOCOS oxide layer ( 3 ), a height side ( 13 ) equal to the thickness of the SOI layer ( 3 ) and a base on the lower boundary of the SOI layer ( 3 ), in which the ratio of the height side ( 13 ) to the base is 4:1 or less.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a semiconductor device using a SOI (Silicon On Insulator) substrate, and more particularly to a device structure in which this feature is employed in the silicon layer portion of the semiconductor device. 
     In a SOI substrate, a silicon layer is formed on a layer with insulation properties, a so-called BOX oxide layer. This silicon layer is isolated by a trench structure or a LOCOS (Local Oxidation of Silicon) method in order to isolate the device. The trench method in which a groove is formed by the silicon layer being etched and an oxide layer being deposited in the groove is disclosed in “IEEE ELECTRON DEVICE LETTERS, VOL. 6, JUNE, 1995,” and others. The cost of isolation using the trench structure is high because the number of processes required for trench structure formation is greater than the LOCOS method. 
     The device isolation for SOI by the LOCOS method is disclosed in “Proceedings IEEE Intr. SOI conf., 116 (1995).” According to the LOCOS method, a thin silicon layer the sectional shape of which is a triangle is formed between a LOCOS oxide layer and a BOX oxide layer and this layer forms the parasite MOSFET. This parasite MOSFET influences significantly the current property of the original (on the assumption that there is no MOSFET) MOSFET. This influence is called a bump property because it looks like a bump is made on the current properties. The threshold voltage for which the parasite MOSFET exists is lower than the original MOSFET. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a semiconductor device of SOI structure which cannot easily form a parasite MOSFET. 
     To achieve the above described object, a SOI structure semiconductor device includes a silicon substrate, an insulating oxide layer formed on the silicon substrate, a SOI layer formed on the insulating oxide layer a LOCOS oxide layer formed on the insulating oxide layer and contacting with the SOI layer in order to insulate the SOI layer, a gate insulation layer formed on the SOI layer and a gate electrode formed on the gate insulation layer. The SOI layer has a sectional triangle portion contacting with the LOCOS oxide layer. The sectional triangle has an oblique side as a boundary between the SOI layer and the LOCOS oxide layer, a height side equal to the thickness of the SOI layer and a base on the lower boundary of the SOI layer, in which the ratio of the height side to the base is 4:1 or less. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a part of a sectional view of a SOI device structure of the first preferred embodiment of the present invention; 
     FIG. 2 is a view showing simulation results of current voltage characteristics of a SOI device structure of the first preferred embodiment; 
     FIGS. 3A-C is a sectional view showing a fabrication method of a SOI device structure of the first preferred embodiment; 
     FIGS. 4A-C is a sectional view showing fabrication processes contrasting with the first preferred embodiment; 
     FIGS. 5A-C is a sectional view showing another fabrication method of a SOI device structure of the first preferred embodiment; 
     FIGS. 6A-B is a part of a sectional view of a SOI device structure of the second preferred embodiment; 
     FIGS. 7A-D is a view showing simulation results of current voltage characteristics of a SOI device structure of the second preferred embodiment; 
     FIGS. 8A-D is a sectional view showing a fabrication method of a SOI device structure of the second preferred embodiment; 
     FIGS. 9A-B is a partial sectional view of a SOI device structure of the third preferred embodiment; 
     FIGS. 10A-B is a view showing simulation results of current voltage characteristics of a SOI device structure of the third preferred embodiment; 
     FIGS. 11A-C is a sectional view showing a fabrication method of a SOI substrate of the third preferred embodiment; 
     FIGS. 12A-D is a sectional view showing a fabrication method of a SOI device of the third preferred embodiment; 
     FIGS. 13A-B is a sectional view showing a fabrication method of a SOI device of the fourth preferred embodiment; 
     FIGS. 14A-B is a sectional view showing a modified example of a fabrication method of a SOI device of the fifth preferred embodiment; 
     FIG. 15 is a sectional view showing a fabrication method of a SOI device of the sixth preferred embodiment; and 
     FIG. 16 is a sectional view showing a modified example of a fabrication method of a SOI device of the sixth preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a part of a sectional view of a SOI device structure of the first preferred embodiment of the present invention. A SOI layer  3  is a silicon layer, the thickness of which is 400 to 500 angstroms is formed on a BOX oxide layer  2 , the thickness of which is approximately 400 to 500 angstroms, which is formed on a silicon substrate  1 . A part of the SOI layer  3  is oxidized by a LOCOS method and becomes a LOCOS oxide layer  4 , the layer thickness of which is approximately 400 angstroms. A gate oxide layer  5  the layer thickness of which is relatively thin (in the vicinity of approximately 70 angstroms) is formed on the SOI layer  3 . Polysilicon  6 , the layer thickness of which is 2500 to 3000 angstroms, functioning as the gate, is provided on the gate oxide layer  5 . 
     The boundary  10  between the SOI layer  3  and the LOCOS oxide layer  4  has a shape more likely to be perpendicular than the conventional boundary line  11 . To give an actual example, in the SOI device structure of the first embodiment, the ratio of the base (the boundary  12 ) to the height (the perpendicular line  13 ) of the triangle portion formed by the boundary  12  between the BOX oxide layer  2  and the SOI layer  3 , a perpendicular line  13  of the SOI layer  3  in the thickness direction, and the distance from a cross point of the perpendicular line and the bottom of the SOI layer  3  to the boundary  10  between the SOI layer  3  and the LOCOS oxide layer  4  is determined as 1:4 or the ratio of the base itself is less than this. 
     According to this type of structure, suppression of the influence caused by a parasite transistor may be expected. FIG. 2 is a view showing simulation results of a current voltage characteristic of a SOI device structure of the first preferred embodiment. Drain current Id is plotted on the vertical axis and gate bias current Vg is plotted on the horizontal axis. In FIG. 2, 1e-05 means 1×10 −5 . Compared with the ideal current properties “A” where the parasite transistor is not formed, the off-leak current of the current property “B” of the first embodiment is restrained within one digit. Compared with the current properties “C” where the ratio of the base to the height of the triangle portion of SOI structure is 1:1, a remarkable improvement is apparent and it is possible to verify according to FIG. 2 that this is the same as the ideal current property “A”. 
     FIGS.  3 (A) to  3 (C) are sectional views showing a fabrication method of a SOI device structure of the first preferred embodiment. Referring to these Figures, a fabrication method of a SOI device structure of the first preferred embodiment is explained hereinafter. 
     First of all, a SIMOX (Separation by Implementation of Oxygen) substrate is prepared in which the BOX oxide layer  2  the layer thickness of which is approximately 1000 to 1500 angstroms and the SOI layer  3  the layer thickness of which is approximately 500 angstroms are layered and formed. The gate oxide layer  5  the layer thickness of which is 70 angstroms and a nitride layer  7  the layer thickness of which is 500 angstroms are successively formed on the SOI layer  3  of the SIMOX substrate (FIG.  3 (A)). The layer thickness of the SOI layer  3  decreases to approximately 400 angstroms due to the gate oxide layer  5  being formed, successive processing, and so forth. 
     Next, a part of the SOI layer  3 , the gate oxide layer  5 , and the nitride layer  7  of the portion composing the LOCOS oxide layer is eliminated (FIG.  3 (B)). The volume of the SOI layer  3  eliminated corresponds to a thickness of approximately 300 angstroms which is ¾ of the original layer thickness of approximately 400 angstroms. Thus, the layer thickness of a SOI layer  3 A after elimination becomes 100 angstroms which is ¼ of the layer thickness of the original SOI layer  3 . 
     After this, the SOI layer  3 A is transformed to the LOCOS oxide layer  4  by implementation of the LOCOS oxide process (FIG.  3 (C)). The triangle portion of the SOI layer  2  formed in the boundary portion between the BOX oxide layer  2  and the transformed SOI layer  3  decreases and the ratio of the base to the height becomes 1:4 or the ratio of the base becomes smaller. 
     FIGS.  4 (A) and  4 (B) are sectional views showing fabrication processes contrasting with the first preferred embodiment. The process of FIG.  4 (A) corresponds to FIG.  3 (B) and the volume of the SOI layer  3  eliminated is defined as approximately 80 angstroms which is ⅕ of the original layer thickness of approximately 400 angstroms. Thus, the layer thickness of a SOI layer  3 B after elimination becomes approximately 320 angstroms which is ⅘ of the layer thickness of the original SOI layer  3 . 
     After this, the SOI layer  3 B is transformed to a LOCOS oxide layer  4 B by implementation of the LOCOS oxide process (FIG.  4 (B)). The triangle portion of the SOI layer  3  formed in the boundary portion between the SOI layer  3  and the transformed LOCOS oxide layer  4 B grows far into the LOCOS oxide layer  4 B, thus the ratio of the base to the height becomes approximately 1:1. 
     It is generally acknowledged that the volume ratio of silicon eroded into the oxide layer and extending upward is 0.44:0.56. Because of this, when the layer thickness of the SOI layer  3 A transformed into the LOCOS oxide layer  4  is thin, the length (the volume of the portion extending into the LOCOS oxide layer  4 ) of the base of the triangle portion of the boundary portion can be kept short. 
     FIGS.  5 (A) to  5 (C) are sectional views showing another fabrication method of a SOI device structure of the first preferred embodiment. Referring to these figures, another fabrication method of a SOI device structure of the first preferred embodiment is explained. 
     First of all, the SIMOX substrate is prepared, in which the SOI layer  3  the layer thickness of which is approximately 500 angstroms and the BOX oxide layer  2  the layer thickness of which is approximately 1000 to 1500 angstroms are layered and formed on the silicon substrate. The gate oxide layer  5  the layer thickness of which is approximately 70 angstroms and the nitride layer  7  the layer thickness of which is approximately 500 angstroms are formed successively on the SOI layer  3  of the SIMOX substrate (FIG.  5 (A)). The layer thickness of the SOI layer  3  decreases to approximately 400 angstroms because of the formation of the gate oxide layer  5  and the subsequent processing implemented. 
     Next, the part of the nitride layer  7  forming the LOCOS oxide layer, the gate oxide layer  5 , and the SOI layer  3  is removed. The nitride layer  7 , the gate oxide layer  5 , and the SOI layer  3  are eliminated by oblique etching (FIG.  5 (B)). The oblique etching may be implemented by a reactive ion etching method, etc. With regard to the volume of the SOI layer  3  eliminated by this oblique etching, it is not necessary that etching is implemented as far as approximately ¾ of the original layer thickness as already explained in the fabrication method shown in FIG.  3 . Although the volume of the SOI layer  3  being eliminated depends on the etching condition, it is appropriate that layer, the thickness of which is approximately 200 angstroms which is approximately ½ of the original layer thickness is eliminated. 
     After this, a part of the SOI layer  3 C is transformed to the LOCOS oxide layer  4 C by implementation of the LOCOS oxide process. Since the triangle portion of the SOI layer  3  formed in the boundary portion between the SOI layer  3  and the LOCOS oxide layer  4 C is over-etched by oblique etching, it decreases and the ratio of the base to the height becomes 1:4 or the ratio of the base becomes smaller than this ratio. 
     FIGS.  6 (A) and  6 (B) are partial sectional views showing a SOI device structure of the second preferred embodiment. As shown in FIG.  6 (A), for a SOI substrate  60  employed in the second embodiment, a nitride layer  63  the layer thickness of which is approximately 1150 angstroms is formed on a BOX oxide layer  62  the layer thickness of which is approximately 1000 to 1500 angstroms which is formed on a silicon substrate  61 . 
     A SOI layer  64  the layer thickness of which is approximately 500 angstroms is formed on the nitride layer  63 . A part of the SOI substrate  60  is oxidized by the LOCOS method and then a part of the SOI layer  64  becomes a LOCOS oxide layer  65  the layer thickness of which is approximately 400 angstroms. A gate oxide layer  66  the layer thickness of which is comparatively thin is formed on the SOI layer  64 , and polysilicon  67  functioning as a gate is formed on a gate oxide layer  66  (FIG.  6 (B)). 
     When the SOI layer  3  is oxidized and transformed to the LOCOS oxide layer  4 , according to the conventional method, the LOCOS oxide layer forms and is connected to the BOX oxide layer  2 . The SOI layer  3  as the device portion (channel portion) is then oxidized in the upward direction from the BOX oxide layer  2 . 
     As a result of this phenomenon, the SOI layer  3  becomes a so-called floating state, as the simulation results indicate. As shown in FIG.  7 (A) and FIG.  7 (B) which is a sectional view of the main parts of FIG.  7 (A), a thin SOI layer is formed in the boundary portion between the SOI layer  3  and the LOCOS oxide layer  4 . 
     For the SOI substrate employed in the second embodiment, however, the nitride layer  63  is formed on the BOX oxide layer  62 . Since the nitride layer does not accelerate oxidization of the silicon layer to the oxide layer, it cannot be verified that the phenomenon in which the SOI layer  64  of the device portion (channel portion) is oxidized in the upward direction from the nitride layer  63  does not occur. Thus, as the simulation results indicate as shown in FIG.  7 (C) and FIG.  7 (D) which is a sectional view of main parts of FIG.  7 (C), since oxidization from the lower portion is not implemented even in the boundary portion between the SOI layer  65  and the SOI layer  64 , the SOI layer  64  is formed comparatively thick. 
     As explained above, since the nitride layer  63  is formed between the BOX oxide layer  62  and the SOI layer  64 , oxidation the direction of which is from the lower portion of the SOI layer  64  is capable of being restrained. The layer thickness of the SOI layer  64  of the boundary portion between the SOI layer  64  and the LOCOS oxide layer  65  is therefore capable of being maintained. Further, since the BOX oxide layer  62  is under the nitride layer  63 , it is expected that leakage problems based on the rigid properties of the nitride layer can be decreased by stress relaxation due to the oxide layer. 
     FIGS.  8 (A) to  8 (D) are sectional views showing a fabrication method of a SOI device structure of the second embodiment. Referring to these figures, a fabrication method of a SOI device structure of the second preferred embodiment is explained. 
     First of all, an oxygen ion is injected into the silicon substrate  61  (FIG.  8 (A)) which has already been prepared. An oxygen containing layer  62 A is formed in a predetermined deep portion of the silicon substrate  61  by this ion injection, and a silicon layer  64 A remains on the surface (FIG.  8 (B)). The oxygen ion injection is controlled in order for the oxygen containing layer  62 A to be formed in a portion the depth of which is approximately 1650 to 3150 angstroms. Next, a nitride ion is injected into the silicon substrate  61  in which the oxygen containing layer  62 A has been formed. A nitride containing layer  63 A is formed on the oxygen containing layer  62 A by this ion injection (FIG.  8 (C)). Since the nitride layer  63  has a characteristic of being warped by heat treatment, the nitride ion injection is controlled to control the layer thickness of the nitride containing layer  63 A at approximately 1150 angstroms, which is 500 to 1650 angstroms from the surface. 
     Then, the oxygen containing layer  62 A is transformed into the BOX oxide layer  62 , the nitride containing layer  63 A is transformed into the nitride layer  63 , and the SOI substrate that is a prerequisite to FIG.  6 (A) is formed by the application of heat treatment (FIG.  8 (D)). 
     FIGS.  9 (A) and  9 (B) are partial sectional views of a SOI device structure of the third preferred embodiment. As shown in FIG.  9 (A), a nitride layer  93  the thickness of which is approximately 1150 angstroms is formed as a SOI substrate  90  employed in the preferred embodiment. A SOI layer  94  the layer thickness of which is approximately 500 angstroms is formed on the nitride layer  93 . A part of the SOI substrate  90  employed in the third preferred embodiment is oxidized by the LOCOS method, and a part of the SOI layer  94  becomes a LOCOS oxide layer  95  the layer thickness of which is 400 angstroms. A comparatively thin gate oxide layer  96  the layer thickness of which is approximately 70 angstroms is formed on the SOI layer  94 . A Polysilicon layer  97  functioning as the gate, the layer thickness of which is 2500 to 3000 angstroms is provided on the gate oxide layer  96 (FIG.  9 (B)). 
     As already explained in FIGS.  7 (A) and  7 (B), the thin SOI layer  3  is formed in the boundary portion between the SOI layer  3  and the LOCOS oxide layer  4 . The nitride layer  93  is formed on the SOI substrate employed in the third embodiment instead of the BOX oxide layer  2 . Since the nitride layer does not accelerate oxidization of the silicon layer to the oxide layer, a phenomenon in which the SOI layer  94  as the device portion (channel portion) is oxidized in the upward direction from the nitride layer  93  does not occur. Thus, as indicated in the simulation results shown in FIG.  10 (A) and FIG. 1 0 (B) which is the enlarged portion of FIG.  10 (A), since the boundary portion between the SOI layer  94  and the LOCOS oxide layer  95  is not oxidized in the direction from the lower portion, the SOI layer  94  is formed comparatively thick. 
     As shown in the above explanation, since the nitride layer  93  is formed in the third preferred embodiment instead of the BOX oxide layer, oxidation in the direction from the lower portion of the SOI layer  94  may be restrained. The result of this is the layer thickness of the SOI layer  94  in the boundary portion between the SOI layer  94  and the LOCOS oxide layer  95  may be maintained. Further, since the conventional BOX oxide layer is simply converted to the nitride layer, the fabricating processes do not increase and implementation may be easily done. 
     FIGS.  11 (A) to  11 (C) are sectional views showing a fabrication method of a SOI substrate of the third preferred embodiment. Referring to these Figures, the fabrication method of the SOI substrate employed in the third embodiment is explained. 
     First, the nitride ion is injected into a prepared silicon substrate  9  (FIG.  11 (A)). Due to this ion injection, a nitride containing layer  93 A is formed in a predetermined deep portion of the silicon substrate  61 , and a silicon layer  94 A remains on the surface (FIG.  11 (B)). The nitride ion injection is controlled so that the nitride containing layer  93 A is formed in a portion the depth of which is approximately 500 angstroms to 1650 angstroms from the surface. Since the nitride layer  93  has a characteristic of being warped by heat treatment, the nitride injection is controlled so that the layer thickness of the nitride containing layer  93 A is approximately 1150 angstroms. The nitride containing layer  93 A is transformed to the nitride layer  93  by heat treatment, and the SOI substrate  90  that is a prerequisite to FIG.  9 (A) is formed (FIG.  11 (C)). 
     The fabrication method of the SOI substrate employed in the third embodiment has an advantage where unexpected chemical reactions of oxygen, nitrogen and others can be avoided, compared with the fabrication method of the SOI substrate employed in the second preferred embodiment. Further, since the ion injection is applied only once, advantages exist as the processes are simplified and the processing cost is therefore reduced. 
     FIGS.  12 (A) to  12 (D) are sectional views showing a fabrication method of a SOI device of the fourth preferred embodiment. Referring to these figures, the fabrication method of the SOI device of the fourth preferred embodiment is explained. 
     A mask layer  123  is formed on a marginally larger area than a field oxide layer forming plan area  122  of the prepared silicon substrate  121  (FIG.  12 (A)). The mask layer  123  is not limited provided it does not allow passage of oxygen ions. The oxygen ion is injected into the silicon substrate  121  in which this mask layer  123  is formed. Due to this ion injection, the oxygen containing layer  124 A is formed in a predetermined deep portion of the silicon substrate  121  except the portion in which the mask layer  123  is formed, and a silicon layer  125 A remains on the surface (FIG.  12 (B)). The oxygen ion injection is controlled so that the oxygen containing layer  124 A is formed in a portion the depth of which is approximately 1650 to 3150 angstroms from the surface of the silicon substrate  121 . 
     The oxygen containing layer  124 A is then transformed to a BOX oxide layer  124  (FIG.  12 (C)) by application of heat treatment. Due to this heat treatment, when the oxygen containing layer  124 A is transformed to the BOX oxide layer  124 , the BOX oxide layer  124  grows in a lateral direction. Thus, the BOX oxide layer  124  is not formed in the lower portion of the field oxide layer forming plan area  122 . Following this, by implementing the LOCOS oxide process, a field oxide layer  126  grows in the downward direction of the silicon substrate  121 . The field oxide layer  126  will have a structure in which the BOX oxide layers  124  provided in the silicon substrate are connected (FIG.  12 (D)). Because of this, the final shape of the SOI device of the fourth embodiment will have almost the same shape as the case in which the SOI device is formed with the SIMOX substrate having the conventional BOX oxide layer. Observing a section in detail, as shown in FIG.  12 (D), the end portions of the field oxide layer  126 , namely, the boundary portions of the BOX oxide layer  124  formed in the silicon substrate  121  form a slit-like portion  129 . This shows the field oxide layer  126  growing in the downward direction. Since the field oxide layer grows in the downward direction, the growth in the lateral direction decreases compared with that when the conventional SIMOX substrate is employed. Because of this, oxidation in the direction from the lower portion is mitigated and a form with a relatively thick layer is obtained in the boundary condition between the field oxide layer  126  and a SOI layer  127 . 
     FIGS.  13 (A) and  13 (B) are sectional views showing a fabrication method of a SOI device of the fifth preferred embodiment. Referring to these figures, the fabrication method of the SOI device of the fifth preferred embodiment is explained. 
     A SIMOX substrate is prepared in which a SOI layer  133  the layer thickness of which is approximately 500 angstroms and a BOX oxide layer  132  the layer thickness of which is approximately 1000 to 1500 angstroms are layered and formed on a silicon substrate  131 . A gate oxide layer  135  the layer thickness of which is approximately 70 angstroms and a nitride layer  136  the layer thickness of which is approximately 500 angstroms are successively formed on the SOI layer  133  of this SIMOX substrate. The layer thickness of the SOI layer  133  decreases to approximately 400 angstroms because of the gate oxide layer  135  forming and subsequent processing, etc. Next, a part of the region of the SOI layer  133 , the gate oxide layer  135 , and the nitride layer  136  forming a LOCOS oxide layer  137  is eliminated. The volume of the SOI layer  133  eliminated is approximately 300 angstroms which is ¾ of the original layer thickness of approximately 400 angstroms, in the same way as the fabrication method of the first preferred embodiment. Thus, the layer thickness of the SOI layer after elimination becomes approximately 100 angstroms which is ¼ of the layer thickness of the original SOI layer  133 . 
     Following this, impurities are implanted into the whole substrate using the nitride layer  136  as the mask (FIG.  13 (A)). Due to this implant, impurities are introduced into the SOI layer  134  as the field oxide layer forming plan area which then becomes a high density region. The SOI layer  134  is transformed to the LOCOS oxide layer  137  by implementation of the LOCOS oxide process. Further, a gate polysilicon  138  is formed after the nitride layer  136  is eliminated, and the final SOI device structure is obtained (FIG.  13 (B)). A high density region  139  is formed in the lower portion of the triangle portion of the SOI layer  133  formed in the boundary portion between the SOI layer  133  and the transformed LOCOS oxide layer  137 . This is a portion of the high density region of the SOI layer  134  remaining. A part of the high density region does not work as a MOS because a channel portion becomes a high density region even if the parasite MOS structure is formed. 
     Since a high density region exists in a portion with a thin SOI layer  133  thickness, electric influence by the parasite MOS is eliminated, and the bump properties can be eliminated. Although the SOI layer as the field oxide layer forming plan area is reduced to ¼ of the original SOI layer in the same way as the fabrication method of the first preferred embodiment, in the fourth preferred embodiment this elimination volume is capable of being less than ¾ because the high density region is formed. 
     FIGS.  14 (A) and  14 (B) are sectional views showing a modified example of a fabrication method of a SOI device of the fifth preferred embodiment. Referring to these figures, the modified example of the fabrication method of the SOI device of the fifth preferred embodiment is explained. 
     After a gate oxide layer  145  and a nitride layer  147  are successively formed on the SIMOX substrate, elimination of a part of the nitride layer  147  as a LOCOS oxide layer forming plan area, a gate oxide layer  145 , and a SOI layer  143  is processed in the same way as in FIG.  13 (A). Impurities are obliquely implanted into the whole substrate using the nitride layer  147  as the mask (FIG.  14 (A)). By this oblique implantation, impurities are introduced in the end portions of the SOI layer  134  as the field oxide layer forming plan area and the SOI layer  133  as the channel portion, in order to form a high density region  146 . 
     Following this, the high density region  146  is transformed to a LOCOS oxide layer  144  by implementation of the LOCOS oxide process. Further, a gate polysilicon  148  is formed and a final SOI device structure is attained after the nitride layer  147  is eliminated (FIG.  14 (B)). A high density region  149  larger than the high density region  139  shown in FIG.  13 (B) is formed in the lower portion of the triangle portion of the SOI layer  143  formed in the boundary portion between the SOI layer  143  and the transformed LOCOS oxide layer  144 . This is because the high density region is formed in the end portion of the SOI layer  143  of the channel portion by the oblique implantation. The high density region does not function as MOS in the same way as shown in FIG.  13 (B) because the channel portion becomes the high density region even if the parasite MOS structure is formed. 
     Since the high density region the size of which is enlarged compared to that of FIG.  13 (B) exists in a portion with a thin layer SOI layer  143 , electric influence by the parasite MOS is reduced, and the bump properties can be improved. According to this modified example, it is no problem that the elimination volume of the SOI layer  143  may be decreased compared to that of the fifth preferred embodiment as shown in FIGS.  13 (A) and  13 (B). FIG. 15 is a sectional view showing a fabrication method of a SOI device of the sixth preferred embodiment. Referring to this Figure, the fabrication method of the SOI device of the sixth preferred embodiment is explained. A SOI device is formed on the SOI substrate employed in the second preferred embodiment in the same way as the second preferred embodiment. Thus, the SOI device structure of FIG. 15 is the same as FIG.  6 (B) except for forming the polysilicon  67 . The fabrication method of the sixth preferred embodiment applies a threshold value control implant to the SOI device, the structure of which is shown in FIG.  6 (B). Energy of this threshold value control implant is controlled so that the peak value of impurities is lower than the SOI layer  64  as shown in the graph on the right hand side of FIG.  15 . To give an actual example, a predetermined threshold voltage is capable of being obtained in the portion used as the channel of the SOI layer  64 . Further implantation is possible using a combination of implantation energy and the dosing volume of impurities in order for impurity density to become higher in the SOI layer  64  and especially in the lower portion of the boundary portion of a field oxide layer. 
     In the sixth embodiment, there is an advantage where threshold voltage control and hump property improvement are implemented at the same time due to the above described implantation processes being installed. It is acceptable that the whole lower portion of the SOI layer  64  becomes the high density impurity layer because the layer thickness of the SOI layer  64  necessary to control the threshold value is maintained. Although the SOI substrate of the sixth embodiment is the same as the SOI substrate of the second preferred embodiment, it is possible that the SOI substrate used in the third embodiment or a conventional SIMOX substrate can be used. 
     FIG. 16 is a sectional view showing a modified example of a fabrication method of a SOI device of the sixth preferred embodiment. Referring to the FIG. 16, the modified example of the fabrication method of the SOI device of the sixth preferred embodiment is explained. 
     In the modified example, the SOI device is formed in the same way as the sixth preferred embodiment in which the threshold value control implant is used. After this, the threshold value control implant is implemented as indicated by the reference number  160  of the graph of FIG. 16. A so-called counter dope, in which ions the polarity of which are opposite electrically to ions used in the threshold control implant are doped, is applied with an impurity density profile as shown in the reference number  161  of the graph of FIG.  16 . By implementing the impurity implants twice, the SOI layer  64  finally displays the impurity density profile as shown by the reference number  162  of the graph of FIG.  16 . 
     As shown in the above detailed explanation, according to the present invention, current influence, the so-called “hump property,” by the parasite MOS transistor formed in the boundary portion between the LOCOS oxide portion and the SOI layer is capable of being restrained.