fig1 a to 1h are respectively cross sectional views showing processes for fabricating a semiconductor device according to a first embodiment of the present invention by utilizing a dual damascene method . as shown in fig1 a , an insulating film 2 is formed on a semiconductor substrate 1 on which a semiconductor element ( not shown ) is formed , and an interlayer insulating film 3 is formed on the insulating film 2 . a first copper wiring 5 is formed in the interlayer insulating film 3 in a state of being covered with a barrier metal layer 4 a . moreover , for example , an sin film is formed as a barrier insulating film 6 on an exposed portion of the interlayer insulating film 3 , and the first copper wiring 5 . here , the barrier insulating film 6 prevents copper diffusion and oxidation of the first copper wiring 5 , and also prevents a damage caused by etching made for an upper layer . next , as shown in fig1 b , an interlayer insulating film 7 made of a first material is formed on the barrier insulating film 6 . here , the first material is porous methyl siloxane having a si — ch 3 bond containing therein a methyl group . the interlayer insulating film 7 made of the first material has a relative dielectric constant that is not lower than 2 . 2 and not higher than 3 . 0 . incidentally , a material containing therein a siloxane bond as a main backbone and a hydrocarbon group bonded to si of the siloxane bond can be used as the first material in addition to porous methyl siloxane . next , as shown in fig1 c , a barrier metal film 4 b , a plug 13 and a second copper wiring 14 are formed in the interlayer insulating film 7 made of the first material . moreover , a cap layer 15 may be formed on an exposed surface of the second copper wiring 14 . here , the cap layer 15 is formed by utilizing a capmetal technique for selectively growing a metal such as cowb , or a photolithography technique and an etching technique , or the like for patterning an insulating film , after deposition of the insulating film made of sin , sicn , sic or the like . the cap layer 15 can prevent copper diffusion and oxidation of a copper wiring , and also can prevent a damage of the second copper wiring 14 caused by etching made for an upper layer . next , as shown in fig1 d , the interlayer insulating film 7 made of the first material is removed by using the second copper wiring 14 as a mask until the barrier insulating film 6 is exposed , thereby forming a trench 16 . at this time , a surface layer of the interlayer insulating film 7 made of the first material left under the second copper wiring 14 becomes a damaged layer 10 a since its methyl group is de - coupled by a damage caused during the etching . next , as shown in fig1 e , the damaged layer 10 a is subjected to repair processing using a sealing agent 11 of hmds gas or liquid to re - bond a hydrocarbon group such as a methyl group to a portion , having the de - coupled methyl group , of the interlayer insulating film 7 made of the first material , thereby forming a repaired layer 12 a . here , the repaired layer 12 a portion has a carbon concentration higher than that of the inside , of the interlayer insulating film 7 made of the first material , which has not been damaged . more specifically , the repaired layer 12 a portion has the carbon concentration of 5 atm % or more in a position which is 15 nm deep in a perpendicular direction from a surface layer of the repaired layer 12 a . after that , when hmds liquid is used as the sealing agent , a heat treatment is performed at temperatures of 100 to 200 ° c . in order to remove the remaining excessive hmds liquid . next , as shown in fig1 f , an interlayer insulating film 17 a made of a second material is deposited on the barrier insulating film 6 and the cap layer 15 . here , the second material is a porous insulating film having a relative dielectric constant lower than that of the first material . for example , a material such as porous methyl siloxane containing therein a siloxane bond as a main backbone and a hydrocarbon group bonded to si of the siloxane bond , a material such as porous silica or porous hydrogen silsesquioxane containing therein the siloxane bond , an organic polymeric material such as porous poly arylene ether or porous poly arylene , or a resin , becoming a precursor of a porous insulating film , which has a c — c bond or a c ═ c bond as a main backbone can be used as the second material . incidentally , in a next process , as shown in fig1 g , a heat treatment may be performed at temperatures of 300 to 450 ° c . to activate a dehydration polymerization reaction and volatilize porogen , thereby changing the interlayer insulating film 17 a made of the second material into a porous interlayer insulating film 17 b made of the second material . here , the porous interlayer insulating film 17 b made of the second material has a relative dielectric constant that is not lower than 2 . 0 and not higher than 2 . 5 . next , as shown in fig1 h , the porous interlayer insulating film 17 b made of the second material is polished , until the cap layer 15 is exposed , to be flattened by utilizing a cmp method or the like . now , when there is performed the heat treatment for changing the interlayer insulating film 17 a made of the second material into the porous interlayer insulating film 17 b made of the second material , the flattening processing may be performed before that heat treatment . that is to say , after the interlayer insulating film 17 b made of the second material is polished , until the cap layer 15 is exposed , to be flattened by utilizing the cmp method or the like , the heat treatment is performed at temperatures of 300 to 450 ° c . to activate the dehydration polymerization reaction and volatilize porogen , thereby changing the interlayer insulating film 17 a made of the second material into the porous interlayer insulating film 17 b made of the second material . although in this embodiment , the heat treatment is performed to activate the dehydration polymerization reaction and volatilize porogen , thereby changing the interlayer insulating film 17 a as the precursor into the porous interlayer insulating film 17 b made of the second material , a method of forming the porous interlayer insulating film 17 b is not limited to this method . according to the first embodiment , the damaged layer 10 a stimulating oxidation of the barrier metal 4 b in a high temperature state during the heat treatment in the process for forming the interconnection structure and the process for forming the interconnection structure of an upper layer , and during the operation of the semiconductor device is repaired through the repair processing . as a result , the film which has the low relative dielectric constant of 3 . 0 or less and thus which is easy to receive the etching damage can be used as the interlayer insulating film 7 made of the first material when the second copper wiring 14 is formed . also , since the interlayer insulating film 7 made of the first material is left to the last right under the wiring , use of the film having the low relative dielectric constant makes it possible to reduce the relative dielectric constant of the overall interlayer insulating films of the semiconductor device , thereby allowing signal delay to be suppressed . in addition , in the case where a wiring width is narrow when the trench is formed in the interlayer insulating film 7 made of the first material , it is feared that the interlayer insulating film right under the wiring becomes fragile due to the etching damage and becomes too weak to support the wiring and thus is destroyed . however , performing the repair processing results in that the interlayer insulating film right under the wiring recovers its mechanical strength and the like from the etching damage , and its resistance against the wet etching is strengthened . as a result , the interlayer insulating film right under the wiring becomes able to support the wiring . in addition , the interlayer insulating film 17 a made of the second material is deposited and formed in the trench without utilizing the etching technique , which results in that a film having a low relative dielectric constant and thus having low etching resistance can be used as the second material . in processes for fabricating a semiconductor device according to a second embodiment of the present invention , processes which will be described below are contained between the process shown in fig1 b and the process shown in fig1 c in the processes for fabricating the semiconductor device according to the first embodiment of the present invention . since processes other than the following processes are the same as those in the first embodiment of the present invention , its description is omitted here for the sake of simplicity . after as shown in fig1 b , the interlayer insulating film 7 made of the first material is formed on the barrier insulating film 6 , as shown in fig2 a , a via hole 8 and a wiring trench 9 are formed in the interlayer insulating film 7 made of the first material by utilizing a dry etching method using a fluorine system etching gas . at this time , a surface of the interlayer insulating film 7 made of the first material as a wall surface of the via hole 8 and the wiring trench 9 becomes a damaged layer 10 b since its methyl group is de - coupled due to a damage caused by the etching . next , as shown in fig2 b , a part of the barrier insulating film 6 corresponding to a bottom surface of the via hole 8 is etched away by utilizing the dry etching method to cause the via hole 8 to extend to the first copper wiring 5 . after that , an etching deposit and the like are removed by subjecting the via hole 8 and the wiring trench 9 to the wet etching processing . next , as shown in fig2 c , the damaged layer 10 b is subjected to repair processing using a sealing agent 11 of hmds gas or liquid to re - bond a hydrocarbon group such as a methyl group to a portion , having the de - coupled methyl group , of the interlayer insulating film 7 made of the first material , thereby forming a repaired layer 12 b . here , the repaired layer 12 b portion has a carbon concentration higher than that of the inside of the interlayer insulating film 7 , made of the first material , which has not been damaged . more specifically , the repaired layer 12 b portion has the carbon concentration of 5 atm % or more in a position which is 15 nm deep in a perpendicular direction from a surface layer of the repaired layer 12 b . after that , when hmds liquid is used as the sealing agent 11 , a heat treatment is then performed at temperatures of 100 to 200 ° c . in order to remove the remaining excessive hmds liquid . next , as shown in fig1 c , after tan is sputtered , cu is deposited thereon by utilizing an electrolytic plating method , and flattening is performed by utilizing the cmp method or the like , thereby forming the barrier metal film 4 b , the plug 13 and the second copper wiring 14 . the repair processing is also performed for the damaged layer 10 b of the via hole 8 and the wiring trench 9 which are formed in the interlayer insulating film 7 made of the first material , which results in that deterioration of the electrical characteristics of semiconductor device and degradation of the reliability thereof can be more effectively suppressed . fig3 a to 3j are respectively cross sectional views showing processes for fabricating a semiconductor device according to a third embodiment of the present invention by utilizing a single damascene method . as shown in fig3 a , an insulating film 2 is formed on a semiconductor substrate 1 on which a semiconductor element ( not shown ) is formed , and an interlayer insulating film 3 is formed on the insulating film 2 . a first copper wiring 5 is formed in the interlayer insulating film 3 in a state of being covered with a barrier metal layer 4 a . moreover , for example , an sin film is formed as a barrier insulating film 6 a on an exposed portion of the interlayer insulating film 3 , and the first copper wiring 5 . here , the barrier insulating film 6 a prevents copper diffusion and oxidation of a copper wiring , and also prevents a damage of the copper wiring caused by etching made for an upper layer . next , as shown in fig3 b , an interlayer insulating film 7 a made of a first material is formed on the barrier insulating film 6 a . here , the first material is porous methyl siloxane having a si — ch 3 bond containing therein a methyl group . the interlayer insulating film 7 a made of the first material has a relative dielectric constant that is not lower than 2 . 2 and not higher than 3 . 0 . next , as shown in fig3 c , a barrier metal film 4 c and a plug 13 are formed in the interlayer insulating film 7 a made of the first material . moreover , a barrier insulating film 6 b formed of , for example , an sin film is formed on the plug 13 and the interlayer insulating film 7 a made of the first material . next , as shown in fig3 d , an interlayer insulating film 7 b made of the first material is formed on the barrier insulating film 6 b . here , the first material is porous methyl siloxane having a si — ch 3 bond containing therein a methyl group . the interlayer insulating film 7 b made of the first material has a relative dielectric constant that is not lower than 2 . 2 and not higher than 3 . 0 . incidentally , in addition to porous methyl siloxane , a material containing therein a siloxane bond as a main backbone and a hydrocarbon group bonded to si of the siloxane bond can also be used as the first material of which each of the interlayer insulating films 7 a and 7 b is made . next , as shown in fig3 e , a barrier metal 4 d and a second copper wiring 14 are formed in the interlayer insulating film 7 b made of the first material . moreover , a cap layer 15 may be formed on the second copper wiring 14 . next , as shown in fig3 f , the interlayer insulating film 7 a made of the first material , the barrier insulating film 6 b and the interlayer insulating film 7 b made of the first material are removed by using the second copper wiring 14 as a mask until the barrier insulating film 6 a is exposed . at this time , a surface of the interlayer insulating film 7 a made of the first material left below the second copper wiring 14 becomes a damaged layer 10 a since its methyl group is de - coupled by a damage caused during the etching . next , as shown in fig3 g , the damaged layer 10 a is subjected to repair processing using a sealing agent 11 of hmds gas or liquid to re - bond a hydrocarbon group such as a methyl group to a portion , having the de - coupled methyl group , of the interlayer insulating film 7 a made of the first material , thereby forming a repaired layer 12 a . here , the repaired layer 12 a portion has a carbon concentration higher than that of the inside , of the interlayer insulating film 7 a made of the first material , which has not been damaged . more specifically , the repaired layer 12 a portion has the carbon concentration of 5 atom % or more in a position which is 15 nm deep in a perpendicular direction from a surface layer of the repaired layer 12 a . after that , when hmds liquid is used as the sealing agent 11 , a heat treatment is then performed at temperatures of 100 to 200 ° c . in order to remove the remaining excessive hmds liquid . next , as shown in fig3 h , an interlayer insulating film 17 a made of a second material is deposited on the barrier insulating film 6 a and the cap layer 15 . here , the second material is a porous insulating film having a relative dielectric constant lower than that of the first material . for example , a material such as porous methyl siloxane containing therein a siloxane bond as a main backbone and a hydrocarbon group bonded to si of the siloxane bond , a material such as porous silica or porous hydrogen silsesquioxane containing therein the siloxane bond , an organic polymeric material such as porous poly arylene ether or porous poly arylene , or a resin , becoming a precursor of a porous insulating film , which has a c — c bond or a c ═ c bond as a main backbone can be used as the second material . incidentally , in a next process , as shown in fig3 , a heat treatment may be performed at temperatures of 300 to 450 ° c . to activate a dehydration polymerization reaction and volatilize porogen , thereby changing the interlayer insulating film 17 a made of the second material into a porous interlayer insulating film 17 b made of the second material . here , the porous interlayer insulating film 17 b made of the second material has a relative dielectric constant that is not lower than 2 . 0 and not higher than 2 . 5 . next , as shown in fig3 j , the porous interlayer insulating film 17 b made of the second material is polished , until the cap layer 15 is exposed , to be flattened by utilizing the cmp method or the like . incidentally , when there is performed a heat treatment for changing the interlayer insulating film 17 a made of the second material into the porous interlayer insulating film 17 b made of the second material , the flattening processing may be performed before that heat treatment . that is to say , after the interlayer insulating film 17 a made of the second material is polished , until the cap layer 15 is exposed , to be flattened by utilizing the cmp method or the like , the heat treatment is performed at temperatures of 300 to 450 ° c . to activate a dehydration polymerization reaction and volatilize porogen , thereby changing the interlayer insulating film 17 a made of the second material into the porous interlayer insulating film 17 b made of the second material . although in this embodiment , the porous interlayer insulating film 17 b made of the second material is formed by activation of the dehydration polymerization reaction and volatilization of porogen through the heat treatment for the interlayer insulating film 17 a as the precursor , a method of forming the porous interlayer insulating film 17 b is not limited to this method . it should be noted that each of the above - mentioned embodiments is merely an embodiment , the present invention is not limited to those embodiments , and thus the various changes thereof can be implemented without departing from the gist of the invention . for example , although each of the above - mentioned embodiments has been described by giving the fabrication of the layers , of the semiconductor device having the multi - layer structure , from the second level on , the present invention is not limited to the number of layers described above . in addition , although each of the above - mentioned embodiments has been described by exemplifying cu as the metallic material for the wiring , the present invention is not limited thereto . that is to say , any other suitable material such as al , au , ag or w may also be used instead of cu . in addition , in the process for forming the plug 13 by utilizing the single damascene method as shown in fig3 c , the plug 13 may be formed after the damaged layer , of the surface of the interlayer insulating film 7 a made of the first material , caused in formation of the via hole is subjected to the repair processing using the sealing agent 11 of hmds gas or liquid . also , in the process for removing the interlayer insulating film 7 made of the first material to form the trench 16 as shown in fig1 d , when no barrier insulating film 6 is used , in order to protect the interlayer insulating film 3 , the interlayer insulating film 7 made of the first material may be left such that the interlayer insulating film 3 is not exposed without perfectly removing the interlayer insulating film 7 made of the first material . this is also applied to the process performed by utilizing the single damascene method as shown in fig3 f .