the invention will be now described herein with reference to illustrative embodiments . those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes . referring to the attached drawings , a manufacturing method of semiconductor device according to embodiments of the present invention will be described bellow . fig1 is a plan view of a semiconductor wafer 1 according to an embodiment of the present invention . the semiconductor wafer 1 includes a plurality of pellet corresponding areas 2 . each of the pellet corresponding areas 2 includes a memory cell region 4 in which memory cells are formed and an outside of the memory cell region , 3 as a region other than the memory cell region 4 . each of the pellet corresponding areas 2 corresponds to one pellet . each pellet is packaged to provide a semiconductor device . sram ( static random access memory ) is an example of the semiconductor device . fig2 shows a plan view of a transistor formed in the outside 3 . fig2 shows an x1 axis and an y1 axis which are orthogonal to each other . the direction of channel length of the transistor is parallel to the x1 axis . the direction of channel width of the transistor is parallel to the y1 axis . in the outside 3 , a gate electrode 71 of the transistor is provided above a diffusion layer 81 . the gate electrode 71 extends along a straight line parallel to the y1 axis . fig3 shows a plan view of a transistor formed in the memory cell region 4 . fig3 shows an x2 axis and an y2 axis which are orthogonal to each other . the direction of channel length of the transistor is parallel to the x2 axis . the direction of channel width of the transistor is parallel to the y2 axis . in the memory cell region 4 , a gate electrode 72 of the transistor is provided above a diffusion layer 82 . the gate electrode 72 extends along a straight line parallel to the y2 axis . the diffusion layer 82 extends along a straight line parallel to the x2 axis . hereinafter , referring to fig4 a to 4e and fig5 a to 5e , a process for manufacturing the semiconductor wafer 1 shown in fig1 to 3 will be described . fig4 a to 4e show sectional views of the semiconductor wafer 1 in the outside 3 in order of the process . fig5 a to 5e show sectional views of the semiconductor wafer 1 in the memory cell region 4 in order of the process . here , fig4 a to 4d are sectional views along a cut line a - a ′ shown in fig2 . the cut line a - a ′ is parallel to the y1 axis . fig4 e is a sectional view along a cut line c - c ′ shown in fig2 . the cut line c - c ′ is parallel to the x1 axis . fig5 a to 5d are sectional views along a cut line b - b ′ shown in fig3 . the cut line b - b ′ is parallel to the y2 axis . fig5 e is the sectional view along a cut line d - d ′ shown in fig3 . the cut line d - d ′ is parallel to the x2 axis . as shown in fig4 a and 5a , a silicon oxide film 20 is formed on a silicon substrate 10 , and a silicon nitride film 30 is formed on the silicon oxide film 20 . the silicon oxide film 20 is formed by thermally oxidizing the silicon substrate 10 for example . the silicon substrate 10 includes a first region corresponding to the outside 3 and a second region corresponding to the memory cell region 4 . the first region is a region to be a portion of the outside 3 . the second region is a region to be a portion of the memory cell region 4 . then , trenches 41 and 42 are formed which reach from the silicon nitride film 30 to the silicon substrate 10 . as shown in fig4 a , the trenches 41 are formed in the first region of the silicon substrate 10 . two of the trenches 41 are provided to be adjacent to each other in the direction of the y1 axis . the first region of the silicon substrate 10 includes a first portion 11 as a portion between the two trenches 41 adjacent to each other . a width w 11 along the y1 axis of a portion of the silicon nitride film 30 above the first portion 11 is substantially equal to a width along the y1 axis of the first portion 11 . the width along the y1 axis of the first portion 11 is substantially equal to the channel width of the transistor to be formed in the outside 3 . as shown in fig5 a , the trenches 42 are formed in the second region of the silicon substrate 10 . two of the trenches 42 are provided to be adjacent to each other in the direction of the y2 axis . the second region of the silicon substrate 10 includes a second portion 12 as a portion between the two trenches 42 adjacent to each other . a width w 12 along the y2 axis of a portion of the silicon nitride film 30 above the second portion 12 is substantially equal to a width along the y2 axis of the second portion 12 . the width along the y2 axis of the second portion 12 is substantially equal to the channel width of the transistor to be formed in the memory cell region 4 . the channel width of the transistor to be formed in the outside 3 is wider than the channel width of the transistor to be formed in the memory cell region 4 . after that , as shown in fig4 b and 5b , the silicon nitride film 30 is retreated , an buried oxide film 50 is formed to be buried in the trenches 41 and 42 , the buried oxide film 50 is polished by cmp with the silicon nitride film 30 being used as a stopper , and then wet etching is applied to the buried oxide film 50 . the buried oxide film 50 is a dielectric film such as a silicon oxide film . portions of the buried oxide film 50 buried in the trenches 41 and portions of the buried oxide film 50 buried in the trenches 42 function for element isolation . the retreat of the silicon nitride film 30 is performed such that width w 41 in the y1 direction of the trench 41 at the silicon nitride film 30 is widened and width w 42 in the y2 direction of the trench 42 at the silicon nitride film 30 is widened . for example , the silicon nitride film 30 is retreated by isotropic dry etching . at this time , the portion of the silicon nitride film 30 above the first portion 11 is left and the portion of the silicon nitride film 30 above the second portion 12 is removed . that is , the silicon nitride film 30 is retreated such that double of retreat length δ along the y1 axis of the portion of the silicon nitride film 30 above the first portion 11 is larger than the width w 12 ( fig5 a ) along the y2 axis of the portion of the silicon nitride film 30 above the second portion 12 before the retreat . here , the width w 11 along the y1 axis of the portion of the silicon nitride film 30 above the first portion 11 after the retreat shown in fig4 b is smaller than the width w 11 before the retreat shown in fig4 b by 2δ . since the channel width of the transistor to be formed in the outside 3 is larger than the channel width of the transistor to be formed in the memory cell region 4 , it is easy to leave the portion of the silicon nitride film 30 above the first portion 11 and remove the portion of the silicon nitride film 30 above the second portion 12 . by wet - etching the buried oxide film 50 , a thickness h of a portion of the buried oxide film 50 on the silicon oxide film 20 is adjusted to be a desired value . as shown in fig4 b , the silicon oxide film 20 on the first portion includes an adjacent portion adjacent to the trench 41 and a central portion far from the trench 41 . the adjacent portion is covered by the buried oxide film 50 . the central portion is covered by the retreated silicon nitride film 30 . meanwhile , as shown in fig5 b , whole of the silicon oxide film 20 on the second portion 12 , which includes an adjacent portion adjacent to the trench 42 and a central portion far from the trench 42 , is covered by the buried oxide film 50 . after that , as shown in fig4 c and 5c , the portion of the silicon nitride film 30 above the first portion 11 and the portion of the silicon oxide film 20 under that portion are removed , and impurity for adjusting a threshold voltage of the transistor to be formed in the outside 3 is implanted into the first portion 11 to form the diffusion layer 81 in the first portion 11 . the portion of the silicon nitride film 30 above the first portion 11 and the portion of the silicon oxide film 20 under that portion are removed by wet etching , for example . at this time , the portion of the buried oxide film 50 above the first portion 11 and the portion of the silicon oxide film 20 under that portion are not removed ; and the portion of the buried oxide film 50 above the second portion 12 and the portion of the silicon oxide film 20 under that portion are not removed . as a result , an opening is formed on the first portion 11 by removing the portion of the silicon nitride film 30 and the portion of the silicon oxide film 20 under that portion . meanwhile , no opening is formed on the second portion 12 . the impurity is implanted from the opening into the first portion 11 . at this time , the implantation of impurity into the second portion 12 is prevented by the portion of the buried oxide film 50 above the second portion 12 . after that , as shown in fig4 d and 5d , after forming a new oxide film and wet - etching , the gate electrode 71 is formed above the first portion 11 and the gate electrode 72 is formed above the second portion 12 . the new oxide film , the silicon oxide film 20 and the buried oxide film 50 are shown as a silicon oxide film 60 . a portion of the silicon oxide film 60 between the first portion 11 and the gate electrode 71 and a portion of the silicon oxide film 60 between the second portion 12 and the gate electrode 72 function as gate dielectric films . after that , as shown in fig5 e , impurity for adjusting a threshold voltage of the transistor to be formed in the memory cell region 4 is implanted into the second portion 12 to form the diffusion layer 82 in the second portion 12 . the impurity is implanted into the second portion 12 by ( rotation ) oblique ion implantation . in the oblique ion implantation , the impurity is implanted in the oblique direction with the gate electrode 72 being used as a mask . as a result , concentration distribution of the impurity in the diffusion layer 82 has peaks at deep locations beneath a source end and a drain end of the channel . therefore , a short channel effect is suppressed . a width w 71 in the direction of the x1 axis of the gate electrode 71 is substantially equal to the channel length of the transistor to be formed in the outside 3 . a width w 72 in the direction of the x2 axis of the gate electrode 72 is substantially equal to the channel length of the transistor to be formed in the memory cell region 4 . when the channel length of the transistor to be formed in the memory cell region 4 is shorter than the channel length of the transistor to be formed in the outside 3 , it is relatively easy to implant the impurity into the second portion 12 by the oblique ion implantation . when the impurity is implanted into the second portion 12 as shown in fig5 e , no impurity is implanted into the first portion 11 as shown in fig4 e . in the embodiment of the present invention , since the silicon nitride film 30 is retreated , generation of a divot in the oxide film buried in the trenches 41 and 42 is prevented . further , since the implantation of the impurity into the first portion 11 is performed separately from the implantation of the impurity into the second portion 12 , it is easy to adjust both the threshold voltage of the transistor to be formed in the outside 3 and the threshold voltage of the transistor to be formed in the memory cell region 4 to be desired values . it is apparent that the present invention is not limited to the above embodiments , but may be modified and changed without departing from the scope and spirit of the invention .