an embodiment of the present invention will be described as follows based on the drawings . like components are labeled with like reference numerals with respect the prior art , and the description of these components is not repeated . variations in sti level difference results from , of the element formation process shown in fig9 variations in the thickness of a silicon nitride film 3 deposited at the process step 1 , variations in the loss amount of the silicon nitride film 3 caused when a trench ( groove ) 13 is formed at the process step 2 , variations in the height of a silicon oxide 11 deposited in the trench 13 at the process step 4 , and variations in the loss amount of the silicon nitride film 3 and the silicon oxide 11 due to the leveling treatment at the process step 5 . therefore , in each process step , the thickness of the silicon nitride film 3 and the height of the silicon oxide 11 are measured , and the height of the silicon oxide 11 is amended based on the measured values so as to properly control the sti level difference when the element is completed . in the method of fabricating the trench type element isolation structure of the present embodiment , the height of the silicon oxide 11 is amended based on the measured values after the leveling treatment at the process step 5 so as to control the sti level difference at a proper size . [ 0077 ] fig1 and 2 shows respectively a cross sectional views and a flowchart of the process steps showing the method of fabricating the trench type element isolation structure of the present embodiment . first , at the process step 11 an underlying oxide film 2 and a silicon nitride film 3 are accumulated in this order onto a silicon substrate 1 , and the thickness t a of the silicon nitride film 3 is measured . at the process step 12 the trench 13 for element isolation is formed , and the thickness t b of the silicon nitride film 3 after the formation of the trench 13 is measured . at the process step 13 a thermal silicon oxide film 10 is formed on the inner wall of the trench 13 by thermal oxidation . at the process step 14 the silicon oxide 11 is deposited on the entire surface by the cvd method , and the height h a of the silicon oxide 11 is measured . at the process step 15 the silicon oxide 11 formed on the silicon nitride film 3 is removed by the cmp leveling method using the silicon nitride film 3 as a stopper , so as to leave the oxide 11 only inside the trench 13 . after this leveling process , the thickness t c of the silicon nitride film 3 and the height h b of the silicon oxide 11 after the leveling treatment are measured . at the process step 16 a predetermined amount of the silicon oxide 11 is etched , based on the values of the different states of thickness t a , t b and t c of the silicon nitride film 3 and the heights t a and t b of the silicon oxide 11 , so as to adjust the sti level difference at this point . at the process step 17 after the silicon nitride film 3 is removed by a hot phosphoric acid , the underlying oxide film 2 is removed by fluoric acid . consequently , a trench type element isolation structure with a controlled sti level difference is obtained . the thickness t a , t b and t c of the silicon nitride film 3 can be measured by using an optical method such as optical interference . the heights h a and h b of the silicon oxide 11 deposited in the trench 13 can also be measured by using an optical method such as optical interference . in that case , since the silicon oxide 11 and the thermal silicon oxide film 10 are made of the same material and the boundary between them can not be detected , the height h a and h b is detected as the height from the boundary between the thermal silicon oxide film 10 and the silicon substrate 1 . this value may be used as the heights h a and h b in case of optical measurement . the following is a description of a method of adjusting the sti level difference based on the different degrees of thickness t a , t b and t c of the silicon nitride film 3 and the heights h a and h b of the silicon oxide 11 . when variations in the thickness of the silicon nitride film 3 formed at the process step 11 are the main cause of variations in sti level difference , the value of the thickness t a of the silicon nitride film 3 is measured ; when variations in the loss amount of the silicon nitride film 3 caused in the formation of the trench 13 at the process step 12 are the main cause , the value of the thickness t b of the silicon nitride film 3 is measured ; and when variations in the height of the silicon oxide 11 deposited at the process step 14 are the main cause , the value of the height h a of the silicon oxide 11 is measured . the amount of the oxide 11 to be etched is adjusted in accordance with the sizes of these values t a , t b and h a . at the process step 15 , when the variations in the loss amount of the silicon nitride film 3 caused by the leveling treatment is the main cause , the value of the thickness t c of the silicon nitride film 3 is used ; and when the variations in the loss amount of the silicon oxide 11 caused in the same process is the main cause , the values of the heights h a and h b of the silicon oxide 11 are used to control the etching amount of the silicon oxide 11 in accordance with the loss amount due to the leveling treatment . table 1 shows an example of the amount of etching in the case where the height of the silicon oxide 11 is adjusted based on the thickness t c ( the thickness of the silicon nitride film 3 after the leveling treatment ) and the height h b ( the height of the silicon oxide 11 after the leveling treatment ). as shown in table 1 , the height of the silicon oxide 11 is adjusted by etching the oxide 11 in accordance with the thickness of the silicon nitride film 3 and the height of the silicon oxide 11 , thereby controlling the sti level difference at an appropriate size . when the variations in the thickness of the silicon nitride film 3 formed at the process step 11 ; the variations in the loss amount of the silicon nitride film 3 caused in the formation of the trench 13 at the process step 12 ; and the variations in the amount of leveling treatment at the process step 15 are all involved to a similar degree in the variations in the sti level difference , the amount of leveling treatment obtained by ( the thickness t b of the silicon nitride film 3 )−{( the height h a of the silicon oxide 11 )−( the height h b of the silicon oxide 11 )} is used to adjust the etching amount of the silicon oxide 11 in accordance with the amount of the leveling treatment , thereby making the sti level difference uniform in size . table 2 shows an example of the etching amount of the silicon oxide 11 with the amount of leveling treatment . as shown in table 2 , the sti level difference is adjusted by etching the silicon oxide 11 less when the amount of the leveling treatment is small and by etching the oxide 11 more when the amount of the leveling treatment is large . when the variations in the thickness of the silicon nitride film 3 formed at the process step 11 ; the variations in the loss amount of the silicon nitride film 3 caused in the formation of the trench 13 at the process step 12 ; the variations in the thickness of the silicon oxide film 11 buried at the process step 14 ; and the variations in the amount of leveling treatment at the process step 15 are all involved to a similar degree in the variations in the sti level difference , the values of the thickness t a of the silicon nitride film 3 and the height h b of the silicon oxide 11 are used to control the etching amount of the silicon oxide 11 at the process step 16 . [ 0083 ] fig3 and 4 shows respectively a cross sectional views and a flowchart of the process steps showing the method of fabricating the trench type element isolation structure of the present embodiment . the process steps 21 to 26 are equal to the process steps 11 to 16 shown in fig1 of the first embodiment . when the silicon nitride film 3 is removed by a hot phosphoric acid solution at the process step 27 , the degree of newness of the hot phosphoric acid solution to be used ( the number of wafers the hot phosphoric acid solution has processed so far ) causes variations in the loss amount of the height of the silicon oxide 11 . as a result , even if the etching amount of the silicon oxide 11 is adjusted based on the different degrees of thickness t a , t b and t c and the heights h a and h b at the process step 26 , the degree of newness of the heat phosphate solution causes variations in the height ( height h c ) of the silicon oxide 11 after the removal of the silicon nitride film 3 at the process step 27 . therefore , in the present embodiment , when the silicon nitride film 3 is removed by fluoric acid at the process step 28 based on the degree of newness of the hot phosphoric acid solution , the etching amount of the silicon oxide 11 is controlled at the same time . this realizes a trench type element isolation structure whose sti level difference is properly controlled in size , without dropping from the substrate surface . as an index to measure the degree of newness of the hot phosphoric acid solution , the amount of silicon dissolved in the solution can be used . when a small amount of silicon is dissolved in the heat phosphate solution , the underlying oxide film 2 is etched less than the standard , and when a large amount of silicon is dissolved , the film 2 is etched more than the standard at the process step 28 so as to control the amount of the oxide 11 to be etched as the same time . this realizes a trench type element isolation structure whose sti level difference is properly controlled in size , without dropping from the substrate surface . table 3 shows an example of etching time in the case where the number of processing the silicon substrate is used as the index of the degree of newness of the heat phosphate solution to control the etching time at the process step 28 , and also shows the amount of the silicon oxide 11 to be etched at the same time . according to the present embodiment , the level difference between the underlying oxide film 2 and the oxide 11 formed by removing the silicon nitride film 3 is measured , and the amount of the oxide 11 to be etched together with the underlying oxide film 2 is adjusted based on the measured value . [ 0086 ] fig5 and 6 shows respectively a cross sectional views and a flowchart of the process steps showing the method of fabricating the trench type element isolation structure of the present embodiment . at the process step 31 an underlying oxide film 2 and a silicon nitride film 3 are accumulated in this order onto a silicon substrate 1 . at the process step 32 a trench 13 for element isolation is formed in the silicon substrate 1 . at the process step 33 a thermal oxide film 10 is formed on the inner wall of the trench 13 by thermal oxidation . at the process step 34 the oxide 11 is deposited on the entire surface by the cvd method . at the process step 35 the oxide 11 formed on the silicon nitride film 3 is removed by the cmp method using the silicon nitride film 3 as a stopper , so as to leave the oxide 11 only inside the trench 13 . at the process step 36 the level difference sti formed when the silicon nitride film 3 is removed by the heat phosphate is measured . at the process step 37 the amount y of the silicon oxide 11 to be etched at the same time as the removal of the underlying oxide film 2 using the fluoric acid is controlled based on the measured value sti . consequently , a trench type element isolation structure whose level difference sti is properly controlled in size is realized . [ 0087 ] fig7 and 8 shows respectively a cross sectional views and a flowchart of the process steps showing the method of fabricating the trench type element isolation structure of the present embodiment . the present embodiment is an application of the first embodiment to the method of fabricating the conventional trench type element isolation structure shown in fig1 . in other words , in the process step 41 the formation of a polycrystalline silicon film 4 between an underlying oxide film 2 and a silicon nitride film 3 provides the method of fabricating the trench type element isolation structure of the first embodiment with the effect of expanding the formation region of a thermal oxide film 10 formed in the thermal oxidation treatment in the process step 43 , and with the effect of restricting the development of hollows caused in the removal of the underlying oxide film 2 in the process step 48 . the method of fabricating the trench type element isolation structure of the present embodiment will be described as follows with reference to fig7 . first , at the process step 41 the underlying oxide film 2 , the polycrystalline silicon film 4 and the silicon nitride film 3 are deposited in this order onto the silicon substrate 1 , and at the same time , the thickness t a of the silicon nitride film 3 is measured . at the process step 42 a trench 13 for element isolation is formed , and the thickness t b of the silicon nitride film 3 after the formation of the trench 13 is measured . at the process step 43 a thermal oxide film 10 is formed on the inner wall of the trench 13 by thermal oxidation . at the process step 44 a silicon oxide 11 is deposited on the entire surface by the cvd method , and then the height h a of the thermal silicon oxide 11 is measured . at the process step 45 the silicon oxide 11 formed on the silicon nitride film 3 is removed by the cmp method using the silicon nitride film 3 as a stopper , so as to leave the silicon oxide 11 only inside the trench 13 . at this moment , the thickness t c of the silicon nitride film 3 and the height h b of the silicon oxide 11 after the leveling treatment are measured . at the process step 46 a predetermined amount of the silicon oxide 11 is etched based on the values of the different degrees of thickness t a , t b and t c of the silicon nitride film 3 and the heights h a and h b of the silicon oxide 11 , so as to adjust the level difference sti at this moment . at the process step 47 the silicon nitride film 3 is removed by hot phosphoric acid , and the polycrystalline silicon film 4 is removed by ammonia water . later , the underlying oxide film 2 is removed with fluoric acid so as to obtain a trench type element isolation structure whose sti level difference is properly controlled . at this moment , the oxide film formed on the inner wall of the polycrystalline silicon can prevent the development of the hollows . the etching amount of the silicon oxide 11 at the process step 46 is controlled in the same manner as in the first embodiment . to be more specific , when variations in the thickness of the silicon nitride film 3 formed at the process step 41 are the main cause of variations in level difference sti , the value of the thickness t a of the silicon nitride film 3 is measured ; when variations in the loss amount of the silicon nitride film 3 caused in the formation of the trench 13 at the process step 42 are the main cause , the value of the thickness t b of the silicon nitride film 3 is measured ; and when variations in the height of the silicon oxide 11 formed at the process step 44 are the main cause , the value of the height h a of the oxide is measured . the etching amount of the silicon oxide 11 is adjusted in accordance with the sizes of these values . at the process step 45 when the variations in the loss amount of the silicon nitride film 3 caused by the leveling treatment is the main cause , the value of the thickness t c of the silicon nitride film 3 is used ; and when the variations in the loss amount of the silicon oxide 11 caused in the same process is the main cause , the values of the heights h a and h b of the silicon oxide 11 are used to control the etching amount of the silicon oxide 11 in accordance with the loss amount due to the leveling treatment . table 4 shows an example of the etching amount in the case where the height of the silicon oxide 11 is amended based on the thickness t c ( the thickness of the silicon nitride film 3 after the leveling treatment ) and the height h b ( the height of the silicon oxide 11 after the leveling treatment ). it must be noted that the polycrystalline silicon film 4 in this case has a thickness of 50 nm . when the variations in the thickness of the silicon nitride film 3 formed at the process step 41 ; the variations in the loss amount of the silicon nitride film 2 caused in the formation of the trench 13 at the process step 42 ; and the variations in the amount of leveling treatment at the process step 45 are all involved to a similar degree in the variations in the level difference sti , the amount of leveling treatment obtained by ( the thickness t b of the silicon nitride film 3 )−{( the height h a of the silicon oxide 11 )−( the height h b of the silicon oxide 11 )} is used to control the etching amount of the silicon oxide 11 in the process step 46 . table 5 shows an example of the etching amount in the case where the height of the silicon oxide 11 is amended using the amount of the leveling treatment . the present embodiment can be applied to the second embodiment . to be more specific , in etching the silicon nitride film 3 at the process step 47 the amount of silicon dissolved in the etching solution can be measured in order to adjust the etching amount ( etching time ) of the underlying oxide film 2 based on the measured value at the process step 48 . the present embodiment can be also applied to the third embodiment . in that case , the level difference between the polycrystalline silicon film 4 and the silicon oxide 11 formed by removing the silicon nitride film 3 is measured , and the etching amount at the process step 48 is controlled based on the measured value . moreover , the level difference between the silicon oxide 11 and the underlying oxide film 2 formed by removing the polycrystalline silicon film 4 is measured , so as to control the etching amount based on the measured value . according to the method of manufacturing a first aspect of the trench type semiconductor element isolation structure , height of the oxide is corrected on the basis of at least one measured value of the first , second and third thickness of the silicon nitride film and the first and second height of the silicon oxide . the height of the oxide measured from the surface of the silicon substrate can be controlled to be a suitable value . according to the method of manufacturing a second aspect of the trench type semiconductor element isolation structure , the silicon oxide as well as the oxide are etched by the predetermined amount according to the amount of silicon dissolved in etchant of the silicon nitride film . the height of the silicon oxide measured from the surface of the silicon substrate can be controlled to be a suitable value , and the height is not influenced by the degree of new or old of the etchant . according to the method of manufacturing a third aspect of the trench type semiconductor element isolation structure , the silicon oxide film and the oxide are etched by the predetermined amount according to the measured value of the level difference between the silicon oxide film and the oxide formed in the sixth step . accordingly , the height of the silicon oxide from the surface of the silicon substrate can be controlled to be a suitable value without being influenced by the degree of new or old of the etchant . according to the method of manufacturing a fourth aspect of the trench type semiconductor element isolation structure , the height of the silicon oxide is corrected on the basis of the measured value of at last one of each thickness of the first , second and third silicon nitride films and each of the first silicon oxide and second silicon oxide . accordingly , the height of the silicon oxide from the surface of the silicon substrate can be controlled to be a suitable value and the recess which is generated in the periphery of the silicon oxide can be prevented from being generated . according to the method of manufacturing a fifth aspect of the trench type semiconductor element isolation structure , the silicon oxide film and the silicon oxide are etched by the predetermined amount of the silicon oxide on the basis of the amount of silicon dissolved in the etchant of the silicon nitride film . accordingly , the height of the silicon oxide from the surface of the silicon substrate can be controlled to be a suitable value without being influenced by the degree of new or old of the etchant . according to the method of manufacturing a fifth aspect of the trench type semiconductor element isolation structure , the silicon oxide film and the silicon oxide are etched by the predetermined amount of the silicon oxide film and the oxide on the basis of the measured value of the level difference between the silicon oxide film and the oxide formed in the sixth step . accordingly , the height of the silicon oxide from the surface of the silicon substrate can be controlled to be a suitable value and the recess which is generated in the periphery of the oxide can be prevented from being generated without being influenced by the degree of new or old of the etchant . the forgoing is considered as illustrative only of the principles of the invention . further , because numerous modification and changes will readily occur to those skilled in the art , it is not desired to limit the invention to the exact construction and operation shown and described , and accordingly all suitable modifications and equivalents may be resorted to falling within the scope of the invention as definition by the claims which follow .