the preferred embodiment of the present invention will be described with reference to the case wherein it is applied to the manufacture of an n - channel mos lsi . it should be noted however that this invention is also applicable to various kinds of semiconductor devices such as bipolar transistor , p - channel mos , cmos and etc . ( i ) a masking material 12 such as a resist film ( for example 1 . 5 μm in thickness ) is deposited on a monocrystalline silicon semiconductor substrate ( p - type , 10 ω . cm in specific resistance ) 11 and patterned by photolithography or the like . using the resist film 12 as a mask , etching is performed to form grooves 13 for example , 3 μm in width , 1 μm in depth ( fig2 a ). in this case , it is possible to employ as the masking material al film , silicon nitride film , or a bi - layered film consisting for example of sio 2 and polycrystalline silicon . it is also possible as an etching method of the groove 13 to adopt a reactive ion etching or a conventional taper etching method . ( ii ) using the masking material 12 as a mask , an impurity ( e . g ., boron ) for preventing field inversion is implanted by , for example , ion implantation ( accelerated voltage of 80 kev , dosage of 5 × 10 12 cm - 2 ) to form a field channel stopper region 14 ( fig2 b ). the implantation of the impurity may be carried out after the following step ( iii ). depending on conditions such as the initial impurity concentration of the substrate or the like , the implantation of the impurity is not always required and may therefore be omitted . ( iii ) a first insulating film 15 , 1 μm in thickness for example ( e . g ., al 2 o 3 film , sio 2 film or the like ) is formed by a conventional method such as chemical vapor deposition method or evaporation method ( fig2 c ). ( iv ) the masking material 12 is etched away with , for example , a mixed solution of h 2 so 4 and h 2 o 2 to remove the first insulating film 15 , leaving it substantially only in the grooves 13 . as a result , as shown in fig2 d , a gap &# 34 ; a &# 34 ; ( for example , 1 μm in depth , 1 μm in width at the uppermost portion of the gap ) is formed between the insulating film 15 and each wall of the groove 13 . ( v ) a second insulating film comprising for example of a low melting point insulator 16 ( e . g ., boron phospho - silicate glass ( bpsg )) 1 to 2 μm in thickness is formed on the entire surface of the semiconductor substrate 11 including the groove 13 . the low melting point insulator 16 is then melted to fill in the gap &# 34 ; a &# 34 ; ( fig2 e ). it is possible to employ instead of the low melting point insulator , other insulating materials such as polyimide , a spin - coated sio 2 , cvd sio 2 or a resist material as a second insulating material . in the case of p - channel mos lsi , phospho - silicate glass ( psg ), arsenic silicate glass ( assg ) or the like may preferably be employed as a second insulating film . it is also possible to employ as a second insulating film the same material as that of a first insulating film 15 . ( vi ) the entire layer of the low melting point insulator 16 is eteched until the upper surface of the semiconductor substrate 11 is exposed ( fig2 f ). as a result , it is possible to obtain in a simple process a semiconductor device provided with a flat field region ( in general , insulative isolation region ) having the insulating film 15 and the low melting point insulator 16 buried therein and with an element region whose top surface is flush with that of the field region . ( vii ) a gate oxide film 17 and a gate electrode 18 are formed . then , an n + - type layer 19 is formed as a source or drain . an interlayer insulating film 20 is deposited in which is formed a contact hole 21 . an al wiring layer 22 is then formed to complete the main steps for completing an lsi ( fig2 g ). in step ( iii ) above , the first insulating film 15 of al 2 o 3 , sio 2 or the like is deposited . however , the present invention is not limited to this . for example , an oxidizable material layer may alternatively be formed and oxidized to provide a first insulating film . more specifically , as shown in fig3 a , a film 15 &# 39 ; may be formed of a material which has a faster oxidizing rate than the silicon substrate 11 ( e . g ., aluminum which can be oxidized faster than the silicon substrate by anodic oxidation , or porous silicon which can be oxidized faster than the silicon substrate by thermal oxidation ). then , as shown in fig3 b , the resist film 12 is etched away to remove the film 15 &# 39 ; formed thereon and to leave the film 15 &# 39 ; in the groove 13 . oxidation is performed such that the film 15 &# 39 ; buried in the groove 13 oxidizes faster than the substrate 11 ( e . g ., by anodic oxidation if the film 15 &# 39 ; is an al film , and by thermal oxidation if the film 15 &# 39 ; is a porous silicon film ) to convert the film 15 &# 39 ; to an oxide films 15 &# 34 ; ( an al 2 o 3 , alumina film if the film 15 &# 34 ; is an al film and an sio 2 film if the film 15 &# 34 ; is a porous silicon film ). during this step , a thin oxide film 23 is also formed on the substrate 11 ( fig3 c ). since the oxidizing rate of the substrate 11 is smaller than that of the film 15 &# 39 ;, the part of the substrate 11 contiguous with the film 15 &# 39 ; is not oxidized much , so that transverse undercutting of the field oxide film ( formation of the bird &# 39 ; s beak ) does not occur . the thin oxide film 23 is etched to expose the substrate 11 and to leave the oxide film 15 &# 34 ; only in the groove 13 ( fig3 d ). in step ( iii ) described above , the film 15 is an insulator film of al 2 o 3 or the like . however , the low melting point insulator as used in step ( v ) may also be used . the masking material may be other than the resist film 12 and may , for example as shown in fig4 comprise a bi - layered structure consisting of an sio 2 film 25 and a polycrystalline silicon film 26 . in this case , in order to remove the film 15 on the masking material , the polycrystalline silicon film 26 may be etched , leaving the sio 2 film 25 . before depositing the film 15 , a thin insulator film or a thin oxide film ( for example , 1 , 000 å in thickness ) may be formed in the groove 13 . the film 15 may comprise a bi - layered structure of two different materials . it is also possible to form , below the resist film 12 , an antioxidant film , such as an si 3 n 4 film . this si 3 n 4 film may be used as a mask during the field oxidation of the film 15 so as to prevent oxidation of the substrate 11 . for filling the low melting point insulator into the gap &# 34 ; a &# 34 ; in step ( v ) described above , an alternative step as shown in fig5 a may be performed . first , an insulating layer 16 &# 39 ; ( for example , 5 , 000 å in thickness ) containing an impurity such as boron is deposited . then , a low melting point insulating layer 16 &# 34 ; ( for example , 1 to 2 μm in thickness ) is deposited . these two layers are heated at a temperature of for example 1 , 000 ° c . for 40 minutes to form a p + - type layer 27 ( channel stopper region ) by diffusion on both walls of the groove 13 as shown in fig5 b . the entire surface of the structure is subjected to etching to leave the insulating layers 16 &# 39 ; and 16 &# 34 ; or to leave only the insulating layer 16 &# 39 ; ( in this case the insulating layer 16 &# 39 ; should be deposited relatively thick for instance , 1 μm ) in the groove 13 . the diffusion of an impurity to form the p + - type layer 27 may be carried out not necessarily at the melting step of the insulating layers 16 &# 39 ; and 16 &# 34 ;, but at any subsequent heating step . the formation of the p + - type layer 27 on each side of the gap &# 34 ; a &# 34 ; may also be conducted by a conventional ion implantation method . in step ( ii ), it is also possible to use the resist film 12 as a mask to ion - implant arsenic or phosphorus to form an n + - type wiring layer 28 ( a p + - type wiring layer if the ion - implanted impurity is of p - type conductivity ) below the field region ( fig6 ). before depositing the insulating layer in step ( iii ) described above , it is also possible to deposit a conductive layer 29 of a refractory metal such as molybdenum , silicide , polycrystalline silicon or the like , to deposit the insulating layer 15 thereover , and to treat the structure according to the steps after the etching of the resist film to provide a structure which includes the conductive layer below the field region ( fig7 ). the semiconductor substrate is not limited to that described above but may be a substrate of a p - type silicon substrate , an n - type silicon substrate , or a substrate of a compound such as gaas , with a monocrystalline semiconductor layer formed thereover by the epitaxial growing method . the semiconductor substrate may be a monocrystalline insulator substrate such as a sapphire substrate or a spinel substrate , with a monocrystalline semiconductor layer formed thereover . steps ( i ) to ( vii ) described above may be adopted without alteration for forming the element isolation region in this case . fig8 shows the case wherein a substrate of composite structure as described above is used . fig8 corresponds to fig2 except that a monocrystalline semiconductor layer 31 is deposited on a first substrate 30 of a compound such as gaas , p - type silicon , n - type silicon or an insulator such as sapphire . the rest of the configuration in fig8 is the same as shown in fig2 and the same reference numerals denote the same parts and the description thereof will be omitted . in the embodiment shown in fig2 d and 2e , the low melting point insulator 16 was directly buried in the gap &# 34 ; a &# 34 ;. however , the present invention is not limited to this . as shown in fig9 after step ( iii ), it is also possible to sufficiently fill an insulator 32 such as al 2 o 3 , sio 2 , or the like in the gap &# 34 ; a &# 34 ; by the cvd process and then to fill the gap &# 34 ; a &# 34 ; with the low melting point insulator 16 as in step ( iv ) described above . in the above embodiments , the insulating layers 16 , 16 &# 39 ; and 16 &# 34 ; are made flush with the surface of the substrate 11 by means of etching . however , such a flattening step can also be carried out physically by means of abrasion , e . g ., after the deposition of al 2 o 3 , sio 2 and the like . the embodiment of the present invention has been described with reference to an n - channel structure . however , the present invention is similarly applicable to a p - channel structure . however , it is also possible to use the phosphorus - or arsenic - doped polycrystalline silicon for the film 15 to be deposited over the n - type silicon substrate and to oxidize the film 15 by the wet oxidation ( especially when the temperature is 900 ° c . or lower , the difference in the oxidizing rates of the n - type polycrystalline silicon and the silicon substrate becomes greater .) in summary , the method of the present invention has various advantages to be described below : ( 1 ) since the substrate 11 is not oxidized much during the formation of the field oxide film , the bird &# 39 ; s beak does not form and the field region of finer pattern may be formed . ( 2 ) the heating period required for the field oxidation may be shortened . when the anodic oxidation is adopted , the heating step is not required in the field oxidation , so that the transverse diffusion of the field channel stopper region 14 may be suppressed to the minimum . if the groove 13 is formed deeper to form a deeper field channel stopper region 14 , the channel stopper region 14 may not extend into the element region even if the heat treatment in the field oxidation or in the subsequent steps is prolonged to a certain extent . ( 3 ) since a recess or gap is not formed between the insulating film 15 or 15 &# 34 ; constituting the field region and the side walls of the groove 13 , the element isolation layers of excellent characteristics may be formed with a good yield . furthermore , since the element region and the field region may be flattened , the subsequent photolithography step for forming a fine pattern may be facilitated .