some embodiments of the present invention will be described below referring to the drawings . fig1 a through 1d and 2 a through 2 d are schematic sectional views illustrating a method for manufacturing a semiconductor device according to first embodiment in the order of process steps . the structure and the manufacturing method of a semiconductor device according to first embodiment will be described below referring to fig1 a through 1d and 2 a through 2 d . first , an interlayer insulating film 2 is formed on a conductive layer 1 so as to cover the conductive layer 1 with the interlayer insulating film 2 . here , the conductive layer 1 is a semiconductor substrate or a wiring layer formed on a semiconductor substrate . next , the interlayer insulating film 2 is selectively removed to form a contact hole 7 ( opening ) reaching the conductive layer 1 . thereafter , a barrier metal film ( first adhering layer ) composing of a laminated film consisting of a titanium film 3 and a titanium nitride film 4 is formed on the internal wall of the contact hole 7 and on the interlayer insulating film 2 , and a tungsten film 5 ( first conductive film ) is formed on the barrier metal film to fill the contact hole 7 . at this time a seam portion 6 ( gap ) is formed in the tungsten film 5 in the contact hole 7 . this state is shown in fig1 a . next , as fig1 b shows , etch back is performed using the titanium nitride film 4 as a stopper . thereby , the tungsten film 5 is removed from the interlayer insulating film 2 , and a predetermined quantity of the tungsten film 5 in the contact hole 7 is removed . and as fig1 b shows , a recess 8 having a depth d 1 from the upper surface of the titanium nitride film 4 and a diameter d is formed . thereby , the seam portion 6 formed in the contact hole 7 is exposed outward . next , as fig1 c shows , a tungsten film 9 ( second conductive film ) is formed to fill the recess 8 . thereby , the tungsten film 9 is laminated on the tungsten film 5 in the contact hole 7 , and the exposed seam portion 6 is sealed by the tungsten film 9 . next , as fig1 d shows , the tungsten film 9 is polished using cmp . here , since the recess 8 is a shallow hole , the coverage of the tungsten film 9 on the bottom of the recess 8 is substantially the same as the coverage on the side of the recess 8 . therefore , when a tungsten film 9 is formed in the recess 8 in the process step shown in fig1 c , the tungsten film 9 is deposited upward from the bottom of the recess 8 , as well as in the lateral direction from the side of the recess 8 , and a seam portion 9 a is also formed on the tungsten film 9 . as fig1 c shows , in the state where the seam portion 9 a is adhered , since a tungsten film 9 of a thickness of d / 2 is deposited from the side wall of the recess 8 , and the coverage is uniform , the lower end of the seam portion 9 a is positioned above the bottom of the recess 8 by d / 2 . therefore , if the recess 8 is formed in the state of fig1 b so as to be d / 2 & gt ; d 1 , the lower end of the seam portion 9 a is always above the upper surface of the titanium nitride film 4 , and the seam portion 9 a is never exposed upward by polishing using cmp shown in fig1 d . thus , if the shape of the recess 8 is established so as to be d / 2 & gt ; d 1 , the presence of the remaining seam portion 9 a on the tungsten film 9 in the state of fig1 d can be inhibited . thus , a tungsten plug ( conductor ) of a two - stage structure consisting of tungsten films 5 and 9 can be formed in the contact hole 7 without leaving the seam portion 9 a on the upper surface . even if the condition of d / 2 & gt ; d 1 is not satisfied , since the depth d 1 of the recess 8 is shallow , the recess 8 can be filled by the film thickness of the tungsten film 9 . therefore , even if the diameter and depth of the recess 8 is not specified , the occurrence of the seam portion on the tungsten film 9 can be prevented because the plug has a two - stage structure . since the tungsten plug can be made to have a two - stage structure , and the upward exposure of the seam portion 6 can be prevented , the contact area of the tungsten plug with the metal wiring can be secured sufficiently when the overlying metal wiring to be connected to the tungsten plug is formed . therefore , the electrical resistance of the contact between the tungsten plug and the metal wiring can be lowered , and the reliability of the contact , such as em resistance , can be improved . also , by preventing the upward exposure of the seam portion 6 , the corrosion of the tungsten plug during polishing using cmp , or following cleaning or the like can be prevented . next , the step for forming the alignment mark and the superposition test mark formed together with the tungsten plug of fig1 a through 1d will be described below referring to fig2 a through 2d . fig2 a through 2d are schematic sectional views showing the alignment mark portion and the superposition test mark portion for photoengraving ( hereafter referred to “ mark portion ”) formed in another region on the semiconductor substrate . first , as fig2 a shows , an interlayer insulating film 2 is formed on a conductive layer 1 , and by selectively removing the interlayer insulating film 2 , an opening 11 extending to the conductive layer 1 is formed . thereafter , a barrier metal film 12 is formed on the internal wall of the opening 11 and on the interlayer insulating film 2 , and a tungsten film 5 is formed on the barrier metal film 12 to fill the opening 11 . here the barrier metal film 12 is composed of a laminated film consisting of a titanium film 3 and a titanium nitride film 4 as fig1 a through 1d show . this process step corresponds to the process step of fig1 a . next , as fig2 b shows , etch back is performed using the barrier metal film 12 as a stopper . thereby , the tungsten film 5 is removed from the interlayer insulating film 2 and the opening 11 , and the barrier metal film 12 on the bottom of the opening 11 is exposed . in the opening 11 , the tungsten film 5 remains on a part of the sidewall . this process step corresponds to the process step of fig1 b . thus , in the mark portion , since the opening 11 of a width larger than the depth of the interlayer insulating film 2 is usually formed , after the etch back of the tungsten film 5 , the tungsten film 5 on the bottom of the opening 11 is completely etched back , and the underlying barrier metal film 12 is exposed as fig2 b shows . next , as fig2 c shows , a tungsten film 9 is formed to cover the tungsten film 5 and the barrier metal film 12 in the opening 11 . this process step corresponds to the process step of fig1 c . next , as fig2 d shows , the tungsten film 9 and the barrier metal film 12 on the interlayer insulating film 2 are removed by polishing using cmp . this process step corresponds to the process step of fig1 d . thereby , the tungsten film 9 is left only in the opening 11 . in the process step shown in fig2 b , in order to etch back until the barrier metal film 12 in the opening 11 is exposed , a step 9 a of a sufficient depth (= d 2 ) is formed on the surface of the tungsten film 9 , as fig2 d shows . in particular , since the tungsten film 9 is formed only for filling the recess 8 of the contact hole 7 shown in fig1 a through 1d , it is sufficient to determine the film thickness of the tungsten film 9 to be the film thickness of the recess 8 or below . thereby , as fig2 d shows , the step 9 a on the surface of the tungsten film 9 can be deepened even after the tungsten film 9 has been polished . therefore , it is ensured that the step 9 a is formed in the mark portion , and the alignment and super position test for photoengraving can be performed at a high accuracy . according to first embodiment , as described above , since the tungsten plug is made to be a two - stage structure consisting of a tungsten film 5 and a tungsten film 9 , the seam portion 6 in the contact hole 7 can be sealed tightly . therefore , decrease in the contact area with the overlying wiring connected to the tungsten plug by the seam portion 6 can be prevented . thereby , it can be ensured that the contact area of the tungsten plug with the overlying wiring is sufficiently widened , and decrease in electrical resistance in the contacting portion with the overlying wiring can be achieved . also , since the contact area of the tungsten plug with the overlying wiring can be widened , reliability such as em resistance can be improved . in addition , since the seam portion 6 is tightly sealed , the permeation of the polishing liquid used in cmp , the etching solution in the following process steps , and the cleaning solution into the seam portion 6 can be inhibited , and the corrosion of the tungsten plug can be prevented . furthermore , in the alignment mark portion and the superposition test mark portion for photoengraving , since the tungsten film 5 is removed by etch back until the bottom of the opening 11 is exposed , and the tungsten film 9 of the thickness substantially the same as the depth of the recess 8 in the region to form the contact hole 7 is formed , the tungsten film 9 can be formed along the internal wall of the opening 11 . therefore , the step 9 a on the surface of the tungsten film 9 can be made sufficiently deep , it can be ensured that the step 9 a is detected . thereby , the accuracy of the alignment adjustment and the superposition test in photoengraving can be improved significantly . fig3 a through 3c are schematic sectional views illustrating a method for manufacturing a semiconductor device according to second embodiment in the order of process steps . the structure and the manufacturing method of a semiconductor device according to second embodiment will be described below referring to fig3 a through 3c . in fig3 a through 3c , the same reference numerals are used for the same constituting components as in first embodiment . in the manufacturing process of second embodiment , the process step shown in fig1 a of first embodiment is carried out in the same manner as in first embodiment . fig3 a shows the state where the tungsten film 5 has been etched back after the process step shown in fig1 a of first embodiment . here , in second embodiment , the interlayer insulating film 2 is used as the stopper for etch back . therefore , as shown in fig3 a , in the contact hole 7 , the titanium film 3 and the titanium nitride film 4 above the upper surface of the tungsten film 5 have been removed . also , in the region other than the contact hole 7 , the titanium film 3 and the titanium nitride film 4 on the interlayer insulating film 2 have been removed , and the interlayer insulating film 2 has been exposed . in the state shown in fig3 a , a seam portion 6 is formed in the tungsten film 5 as in first embodiment . after the process step shown in fig3 a , as fig3 b shows , a titanium film 13 and a titanium nitride film 14 are sequentially formed on the tungsten film 5 and the interlayer insulating film 2 in the contact hole 7 , and a barrier metal film ( second adhering layer ) consisting of the titanium film 13 and the titanium nitride film 14 is formed . then , a tungsten film 15 ( second conductive film ) is formed again on the titanium nitride film 14 . thereby , the seam portion 6 that has been exposed upward is tightly sealed . next , as fig3 c shows , the tungsten film 15 , the titanium nitride film 14 , and the titanium film 13 are removed from the interlayer insulating film 2 are removed by polishing using cmp , and the interlayer insulating film 2 is exposed . thereby , the tungsten plug of second embodiment is completed . in second embodiment , since the titanium film 13 and the titanium nitride film 14 are formed between the tungsten film 5 and the tungsten film 15 , the adhesion of the tungsten film 5 and the tungsten film 15 can be enhanced . also , when the tungsten film 5 and the tungsten film 15 are substituted by two kinds of different conductive materials as the materials for the plug , ohmic properties between these different materials can be improved , and the diffusion of conductive materials to each other can be prevented . according to second embodiment , since the tungsten plug is made to be a two - stage structure consisting of a tungsten film 5 and a tungsten film 15 , the seam portion 6 formed in the tungsten film 5 can be tightly sealed . therefore , as in first embodiment , the contact area with the overlying wiring can be widened , decrease in electrical resistance and the improvement of reliability such as em resistance can be achieved , and the corrosion of the tungsten plug can be prevented . furthermore , by the etch back of the tungsten film 5 , the step of the mark portion can be deepened as in first embodiment , and the accuracy of alignment and superposition test can be improved . fig4 is a schematic sectional view illustrating a semiconductor device according to third embodiment . a semiconductor device of third embodiment will be described below referring to fig4 . in fig4 the same reference numerals are used for the same constituting components as in first and second embodiments . fig4 shows the state where only a titanium nitride film 11 has been formed as a barrier metal film after a tungsten film 5 has been etched back in the process step shown in fig3 b of second embodiment . other structures are identical to those of second embodiment . thus , by forming the barrier metal film only from a titanium nitride film 11 , the dissolution of titanium in the barrier metal film can be prevented during cleaning with a hydrogen fluoride solution after polishing the tungsten film 15 using cmp . since the tungsten plug is made to be a two - stage structure , as in first embodiment , the contact area with the overlying wiring can be widened , decrease in electrical resistance and the improvement of reliability such as em resistance can be achieved , and the corrosion of the tungsten plug can be prevented . furthermore , the step of the mark portion can be deepened as in first embodiment , and the accuracy of alignment and superposition test can be improved . fig5 a and 5b are schematic sectional views illustrating a method for manufacturing a semiconductor device according to fourth embodiment in the order of process steps . the structure and the manufacturing method of a semiconductor device according to fourth embodiment will be described below referring to fig5 a and 5b . in fig5 a and 5b , the same reference numerals are used for the same constituting components as in first and second embodiments . in the manufacturing process of fourth embodiment , the process step shown in fig1 a of first embodiment is carried out in the same manner as in first embodiment . fig5 a shows the state where the tungsten film 5 has been etched back after the process step shown in fig1 a of first embodiment . here , in fourth embodiment , the interlayer insulating film 2 is used as the stopper for etch back . therefore , as shown in fig5 a , in the contact hole 7 , the titanium film 3 and the titanium nitride film 4 above the upper surface of the tungsten film 5 have been removed . also , in the region other than the contact hole 7 , the titanium film 3 and the titanium nitride film 4 on the interlayer insulating film 2 have been removed , and the interlayer insulating film 2 has been exposed . as in first embodiment , a seam portion 6 is formed in the tungsten film 5 . in fourth embodiment , after the tungsten film 5 has been etched back , the titanium film 10 on the sidewall of the contact hole 7 is oxidized by oxygen ( o 2 ) plasma treatment , or by annealing in an oxygen atmosphere . thereby , a titanium oxide ( ti x o y ) film 16 is formed on the titanium film 10 , and the upward exposure of the titanium film 10 is prevented . thereafter , as fig5 b shows , a barrier metal film consisting of a titanium film 13 and a titanium nitride film 14 is formed as in second embodiment , and a tungsten film 15 is formed to seal the seam portion 6 . then , the tungsten film 15 is polished using cmp , and cleaning with a hydrogen fluoride solution is performed . in cleaning with the hydrogen fluoride solution , the titanium film 13 exposed to the top is dissolved , and a gap 17 is formed . however , underneath the titanium film 13 , since a titanium oxide film 16 is formed along the internal wall of the contact hole 7 , the dissolution of titanium stops when the gap 17 reaches the titanium oxide film 16 . therefore , the dissolution of the titanium film 3 under the titanium oxide film 16 in the hydrogen fluoride solution can be inhibited , and the reaching of the gap 17 to the underlying conductive layer 1 can be prevented . since the tungsten plug is made to be a two - stage structure , as in first embodiment , the contact area with the overlying wiring can be widened , decrease in electrical resistance and the improvement of reliability such as em resistance can be achieved , and the corrosion of the tungsten plug can be prevented . furthermore , the step of the mark portion can be deepened as in first embodiment , and the accuracy of alignment and superposition test can be improved . fig6 a and 6b are schematic sectional views illustrating a method for manufacturing a semiconductor device according to fifth embodiment in the order of process steps . the structure and the manufacturing method of a semiconductor device according to fifth embodiment will be described below referring to fig6 a and 6b . in fig6 a and 6b , the same reference numerals are used for the same constituting components as in first embodiment . in the manufacturing process of fifth embodiment , the process step shown in fig1 a of first embodiment is carried out in the same manner as in first embodiment . fig6 a shows the state where the tungsten film 5 has been etched back after the process step shown in fig1 a of first embodiment . here , in fifth embodiment , the interlayer insulating film 2 is used as the stopper for etch back . therefore , as shown in fig6 a , in the contact hole 7 , the titanium film 3 and the titanium nitride film 4 above the upper surface of the tungsten film 5 have been removed . also , in the region other than the contact hole 7 , the titanium film 3 and the titanium nitride film 4 on the interlayer insulating film 2 have been removed , and the interlayer insulating film 2 has been exposed . as in first embodiment , a seam portion 6 is formed in the tungsten film 5 . in fifth embodiment , after the tungsten film 5 has been etched back , the titanium film 10 on the sidewall of the contact hole 7 is nitrogenized by nitrogen ( n 2 ) plasma treatment , or by annealing in an nitrogen atmosphere at a temperature of 600 ° c . or above . thereby , a titanium nitride ( ti x n y ) film 18 is formed on the titanium film 10 , and the upward exposure of the titanium film 10 is prevented . thereafter , as fig6 b shows , a barrier metal film consisting of a titanium film 13 and a titanium nitride film 14 is formed as in second embodiment , and a tungsten film 15 is formed to seal the seam portion 6 . then , the tungsten film 15 is polished using cmp , and cleaning with a hydrogen fluoride solution is performed . in cleaning with the hydrogen fluoride solution , the titanium film 13 exposed to the top is dissolved , and a gap 19 is formed . however , underneath the titanium film 13 , since a titanium nitride film 18 is formed along the internal wall of the contact hole 7 , the dissolution of titanium stops when the gap 19 reaches the titanium nitride film 18 . therefore , the dissolution of the titanium film 3 under the titanium nitride film 18 in the hydrogen fluoride solution can be inhibited , and the reaching of the gap 19 to the underlying conductive layer 1 can be prevented . since the tungsten plug is made to be a two - stage structure , as in first embodiment , the contact area with the overlying wiring can be widened , decrease in electrical resistance and the improvement of reliability such as em resistance can be achieved , and the corrosion of the tungsten plug can be prevented . furthermore , the step of the mark portion can be deepened as in first embodiment , and the accuracy of alignment and superposition test can be improved . in the above - described embodiments , although tungsten , which has favorable filling properties , is used as an example of a material for a plug , copper , which has lower resistance , can also be used in place of tungsten . also , although a laminated film consisting of a titanium film and a titanium nitride film is used as an example of a barriar metal film , a laminated film consisting of a tantalum film and a tantalum nitride film , or a three - layer laminated film consisting of a tantalum film , a tantalum nitride film , and a tantalum film can also be used . furthermore , a single layer tantalum film , or a single layer tantalum nitride film can also be used . since the present invention is constituted as described above , the following effects can be obtained . since the conductor for filling the opening is made to be a two - stage structure consisting of a first conductive film and a second conductive film , the gap ( seam portion ) within the opening can be tightly sealed . therefore , decrease in the contact area with the overlying wiring connected to the conductor by the gap can be inhibited . thereby , the contact resistance with the overlying wiring can be reduced , and reliability such as em resistance can be improved . since the diameter of the opening is made larger than the depth from the upper surface of the insulating film to the upper end of the first conductive film , the formation of a gap ( seam portion ) on the surface of the second conductive film can be inhibited . since a first adhering layer is formed on the internal wall and the bottom of the opening , the adhesion of the conductor to the inside of the opening can be enhanced , and the diffusion of the conductive materials constituting the conductor to other layers can be inhibited . since a second adhering layer is formed so as to cover the side and the lower surface of the second conductive film , the adhesion and ohmic properties of the first conductive film and the second conductive film can be improved , and the mutual diffusion of the conductive materials constituting the first conductive film and the second conductive film can be inhibited . since the upper end portion of the titanium film of the first adhering layer in the upper portion of the internal wall of the opening is oxidized or nitrogenized , the dissolution of the underlying titanium film due to the following cleaning step , the etching step , and the like , can be inhibited , and the formation of voids in the further underlying conductive layer and semiconductor substrate can be prevented . since the first conductive film is formed only on the circumferential portion along the internal wall of the opening , and the first conductive film is removed from a part of the bottom of the opening , a step can be formed on the upper surface in the vicinity of the center of the opening in formation of the second insulating film . thereby on aligning , the step can be surely detected , and the adjustment of alignment and the accuracy of the superposition test in photoengraving can be significantly improved . since tungsten films are used as the first and second conductive films , it is ensured that even an opening having a large aspect ratio is completely filled . also , by using copper films as the first and second conductive films , decrease in the resistance of the conductor can be achieved . obviously many modifications and variations of the present invention are possible in the light of the above teachings . it is therefore to be understood that within the scope of the appended claims the invention may by practiced otherwise than as specifically described . the entire disclosure of a japanese patent application no . 2001 - 361243 , filed on nov . 27 , 2001 including specification , claims , drawings and summary , on which the convention priority of the present application is based , are incorporated herein by reference in its entirety .