Wiring forming method
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates to a wiring forming method used in semiconductor integrated circuit or the like. 
     b) Description of the Related Art 
     In a process of manufacturing a semiconductor integrated circuit, wirings for connecting respective elements are formed after the respective elements are formed. Process of forming wirings, generally, is achieved by such procedure as follows. First, contact holes are formed to expose electrode portions of respective elements (such as diffusion layers, polycrystalline silicon regions or the like) through an insulating layer which covers the whole surface of the substrate. Then, after depositing a metal layer as of aluminum (Al) over the whole surface of the insulating layer, selective etching of the metal layer using a mask of photo-resist is carried out to form wirings connecting the electrodes of the respective elements. Further, an inter-layer insulating film is formed, and an upper level wiring for connecting the wirings is formed by a similar procedure. Here, to obtain a normally operable semiconductor integrated circuit, it is necessary to obtain a perfect electrical contact between the substrate or a lower level wiring which has exposed portion to be contacted via contact hole and the wiring. Following methods are employed heretofore to satisfy this requirement. 
     (1) Blanket Tungsten Chemical Vapor Deposition 
     First, one or a plurality of adhesion layers selected from a group of Ti, TiN, TiW, and WSi is formed on a conductive portion to be contacted and on an insulating layer. Then, a tungsten film is laminated on a whole surface of the adhesion layer by chemical vapor deposition (CVD). Next, the whole surface of tungsten film is etched until the tungsten film becomes flash with the one or plurality of adhesion layers so that the tungsten filmremains only at inside of the contact holes. Thereafter, a wiring layer as of Al is deposited by sputtering on the whole surface. Etching is performed to produce wirings. 
     (2) Selective Tungsten Chemical Vapor Deposition 
     Tungsten layer is selectively grown only on the conductive portions exposed inside of the contact holes by CVD. Here, deposition of Lungs ten layers of the same thickness can only be obtained at respective contact holes even when there exist a plurality of contact holes with different depths. Thereafter, a wiring layer is deposited thereon and selectively etched to form wirings using the same method as described with reference to the method (1). 
     (3) Reflow Type Sputtering 
     A wiring layer is deposited over the whole surface of a substrate by sputtering under a condition that the substrate is heated to a high temperature. According to this method, the sputtered wiring layer is molten to flow into contact holes. 
     However, in prior art wiring forming method as described above, there were such problems as follows. 
     First, according to method (1), as shown in FIG. 3A, there has been a case wherein an unnecessary tungsten 13a could remain at a step-like portion of adhesion layer 12 even though etching was performed to the extent that a tungsten layer 13 within a contact hole 11 reached the same level as the adhesion layer 12. An excessive etching or an over-etching exceeding the optimum level has to be performed to remove the unnecessary tungsten 13a. Thus, as shown in FIG. 3B, the surface of the tungsten layer 13 has recessed to a depth of about D 1  =0.3 to 0.5 μm from an upper end portion of the contact hole 11. When a wiring layer as of Al alloy was formed by usual sputtering, the wiring layer is also recessed approximately the same amount as the depth D 1  of the surface of the tungsten layer 13 within the contact hole 11, as shown in FIG. 3C. 
     Next, according to method (2), as shown in FIG. 4, when there were formed contact holes 11a and 11b each having different depth, a tungsten layer 13 having a thickness corresponding to the shallower contact hole 11b fills the holes. Accordingly, a gap with a depth from an upper end portion of the hole 11a to the surface of the tungsten layer 13 of about D 2  =0.3 to 0.5 μm is yielded to the contact hole 11a which is deeper than the contact hole 11b. Thus, when a wiring layer 14 is deposited, a surface of the wiring layer 14 is recessed at a part corresponding to the contact hole 11a. 
     As described above, according to method (1) and method (2), there has been a problem that a sufficient coverage or degree of coverage cannot be obtained when the wiring layer 14 is deposited. 
     Next, according to method (3), as shown in FIG. 5, when an aspect ratio V/H, defined by a horizontal dimension (diameter) II of a contact hole 11 in a direction parallel to a bottom surface thereof and by a vertical dimension (height) V thereof in a direction perpendicular to the bottom surface, is large (which means that it is equal to or larger than 1), there is a case wherein a void VD is formed and remains in a wiring layer 14 within the contact hole 11. Thus, there was a problem that a breakage occurs at the time of flowing electricity. The method 3 also had a problem that, because sputtering is performed in a high temperature, quality of layers is deteriorated and durability against, electro-migration is lowered, and hence breakage and short circuiting can occur. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a wiring forming method for forming a wiring layer having a good coverage, a good quality, and a flat surface. 
     According to one aspect of the present invention, a wiring forming method comprising steps of forming an insulating layer covering a semiconductor wafer having a surface portion to be contacted, forming a contact hole through the insulating layer at a portion corresponding to the surface portion, forming an adhesion layer formed of at least one material selected from a group consisting of Ti, TiN, TiW, and WSi on the surface portion to be contacted and on a whole surface of the insulating layer, forming a tungsten layer on the adhesion layer, etching a whole surface of the tungsten layer until a small gap is formed over the contact hole to leave the tungsten layer only at an inside of the contact hole, forming a wiring layer on the whole surface of the insulating layer and at the inside of the contact hole provided with the remaining tungsten layer by reflow-sputtering conductive wiring material at a low temperature compared with a temperature at which the contact hole without tungsten can be refilled by reflow. 
     The reflow sputtering mentioned above is preferably performed at a temperature range from 400° C. to 500° C. 
     The portion to be contacted, in this specification, is an impurity diffusion region or a wiring, and is also a portion to be connected electrically with an upper level wiring. 
     Because a sufficient etching is to be performed to have a tungsten layer only in the inside of the contact hole, the inside of the contact hole is over-etched producing an unfilled portion in the contact hole. A wiring layer, however, fills inside of the contact hole, and a surface of the wiring layer formed thereon becomes flat because the wiring layer is formed by a reflow-type sputtering. Further, since the tungsten layer is etched up to a level that only a small gap is left at an upper end portion of the contact hole and the contact hole becomes shallow, the aspect ratio becomes small, and, consequently, a reflow-type sputtering can be performed under a comparatively low temperature. Thus, a deterioration of quality of the layer can be prevented. Here, an adhesion layer selected from the group of Ti, TiN, TiW, and WSi prevents peeling off of the tungsten layer. 
     By etching to leave the tungsten layer only at the inside of the contact hole, the inside of the contact hole is over-etched. However, formation of a wiring layer by reflow-sputtering a conductive wiring material at a comparatively low temperature on the whole surface of the insulating layer and on the inside of the contact hole provided with the tungsten layer, the wiring layer embeded fully to the inside of the contact hole and the surface of the wiring layer becomes flat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to 1K are longitudinal sectional views of a semiconductor device for illustrating steps of a wiring forming method according to a first embodiment of the present invention. 
     FIGS. 2A to 2J are longitudinal sectional views of a semiconductor device for illustrating steps of a wiring forming method according to a second embodiment of the present invention. 
     FIGS. 3A to 3C arc sectional views of a semiconductor device for illustrating a prior art wiring forming method employing a blanket tungsten CVD method. 
     FIG. 4 is a sectional view of a semiconductor device for illustrating a prior art wiring forming method employing a selective tungsten CVD method. 
     FIG. 5 is a sectional view of a semiconductor device for illustrating a prior art wiring forming method employing a reflow-type sputtering method. 
     FIGS. 6A to 6C are sectional views of a semiconductor device illustrating results of analyses of wiring forming experiments on a blanket tungsten CVD method performed by the inventor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present inventor has achieved various experiments and discussions and has Found that there is a novel problem in the aforementioned method (1). A tungsten layer 13 formed by CVD using the hydrogen reduction of WF 6  on an adhesion layer 12, as shown in FIG. 6A, includes largely grown crystal grains and form unevenness on the surface of the tungsten layer 13 which results in surface asperity. For example, a difference of height D3 between the top of a convex part 13b and the bottom of a concave part 13c of the tungsten layer 13 is approximately 30 nm. 
     For leaving tungsten only at contact hole, the tungsten layer 13 is etched back isotropically using an etching gas with a large selectivity ratio between the base adhesion layer 12 and the tungsten layer 13 e.g. of 1:10. As shown in FIG. 6B, tungsten at the concave part 13c is etched off firstly to expose the adhesion layer 12. At this stage, tungsten at the convex part 13b still remains. It is thus necessary to further continue etching. By further continuing etching, exposed part of the adhesion layer 12 is also etched even if the etching rate is low. When overetching is finished, as shown in FIG. 6C, surface of the adhesion layer 12 is provided with an unevenness resulting from depressions generated through the overetching. Such unevenness is smaller than that of the surface of the tungsten layer 13 before etching. For example, when the difference of height D 3  between the top of a convex part 13b and the bottom of a concave part 13c of the tungsten layer is approximately 300 nm, a difference of height between the top of a convex part and the bottom of a concave part of the adhesion layer 13 is approximately 30 nm. When a wiring layer, as of Al alloy, is formed on the surface of the uneven adhesion layer 12, by usual sputtering, the wiring layer will have the same unevenness as is provided on the surface of the adhesion layer 12. At the time of exposure of a photo-resist in the step of patterning a wiring, such unevenness formed on the surface of the wiring layer will result in an irregular reflection of exposing light on the surface of the wiring layer, and will disable formation of desired mask, meaning that it is difficult to form a desired wiring pattern. 
     The present invention will be described specifically hereinunder. The spirit and scope of the present invention is not limited to embodiments which arc to be disclosed below. 
     FIGS. 1A to 1J show a series of steps of wiring forming method according to a first embodiment of the present invention. 
     In the first embodiment, a first level insulating layer 2 formed of an insulating film as of SiO 2  is formed on a semiconductor substrate or wafer 1, as shown in FIG. 1A. A polycrystalline Si wiring 3 is formed on the first level insulating layer 2. Further, on the polycrystalline Si wiring 3 and the first level insulating layer 2, a second level insulating layer 4 of an insulating film as of SiO 2  is formed. The second level insulating layer 4 has an undulated surface. 
     Next, as shown in FIG. 1B, an Al alloy layer as of AlSiCu is deposited on the second level insulating layer 4, and a lower level wiring 5 is formed by etching using photolithography. In the present specification, the term &#34;Al alloy&#34; is used to include pure aluminum, an alloy of alumninum and silicon, and an alloy of aluminum, silicon and copper. 
     Thereafter, as shown in FIG. 1C, a third level insulating layer 6 formed of an insulating film as of SiO 2  is formed on the second level insulating layer 4 and on the lower level wiring 5. In the third level insulating layer 6, contact holes 11 is formed by etching. The Al alloy layer 5 also has an undulated surface. When plural contact holes above a convex portion of the Al alloy layer 5 and a concave portion of the Al alloy layer 5 are to be formed through the third level insulating layer 6, depths of the contact holes differ because of the undulation. 
     Further, as shown in FIG. 1D, on a surface of the contact holes 11 and a whole surface of the third level insulating layer 6, an adhesion layer 12 formed of one or more materials selected from a group of Ti, TiN, TiW, and WSi is formed by using sputtering or CVD method. The adhesion layer 12 firmly adheres to a tungsten layer to be successively formed thereon. To accomplish this purpose, the adhesion layer 12 may be formed by laminating plural layers respectively comprising a material selected from a group of Ti, TiN, TiW and WSi (for example, laminating a Ti layer and a TiN layer). When the adhesion layer 12 of Ti is formed by sputtering, the sputtering conditions, for example, are as follows. The substrate heated to approximately 200° C. is located in an argon gas atmosphere at a pressure of 4 mTorr, and a flow rate of 20 sccm, approximately. Source material of an adhesion layer 12 is introduced to deposit a layer on the substrate 1 with a growth rate of approximately 100 nm/min until the layer has a thickness of approximately 20  nm. When the adhesion layer 12 of TiN is formed by sputtering, the sputtering conditions, for example, arc as follows. The substrate heated to approximately 200° C. is placed in an argon gas atmosphere at a pressure of 4 mTorr, and a flow rate of 20 sccm, approximately. Source material of an adhesion layer 12 is introduced to deposit a layer on the substrate 1 with a growth rate of approximately 100 nm/min until the layer has a thickness of approximately 100 nm. When the adhesion layer 12 of WSi is formed by sputtering, the sputtering conditions, For example, arc as follows. The substrate heated to approximately 200° C. is located in an argon gas atmosphere at a pressure of 8 mTorr, and a flow rate of 20 sccm, approximately. Source material of an adhesion layer 12 is introduced to deposit a layer on the substrate 1 with a growth rate of approximately 200 nm/min until the layer has a thickness of approximately 50 nm. 
     Next, as shown in FIG. 1E, a blanket tungsten layer 13 is formed on the adhesion layer 12 by such method as CVD. The contact holes 11 are filled with the blanket tungsten layer 13. The CVD conditions for forming the tungsten layer by CVD method, for example, are as follows. The substrate heated to approximately 450° C. is located in a WF 6  gas atmosphere at a pressure of 8 mTorr, and a flow rate of 80 sccm, approximately. Source material of an adhesion layer 12 is introduced to deposit a layer on the substrate 1 with a growth rate of approximately 300 to 500 nm/min until the tungsten layer 13 has a thickness of approximately 600 nm. 
     Next, the tungsten layer 13 is etched by an etching gas, e.g. SF 6 , or CBrF 3  which has an etching selectivity ratio of the adhesion layer 12 and the tungsten layer 13 at 1:10 to 30, to leave the tungsten layer 13 only in the inside of the contact holes 11. In this step, when etching is insufficient, unnecessary tungsten is left at cavities other than contact holes on the surface of the adhesion layer 12. For example, when there is a step on the third level insulating layer 6, the adhesion layer 12 formed thereon will have a step. Thus an unnecessary tungsten will be heft at a lower part of the adhesion layer 12. Therefore, an etching to be performed here, as shown in FIG. 1F, should be an over-etching on the whole surface of the tungsten layer until a gap is formed at the upper end portion of the contact hole 11 so that the surface of the tungsten layer 13 is positioned a little depth lower from the top end portion of the contact hole 11. At the maximum, the gap will be half depth of the contact hole. As a result, the contact holes 11 are partially filled with tungsten. 
     Thereafter, as shown in FIG. 1G, an upper level wiring layer 15, formed of an Al alloy film, etc. is formed on a whole surface of the insulating layer 6 and the inside of the contact hole 11 filled with the tungsten layer 13, by reflow-sputtering such conductive wiring material as AlSiCu (with 1 wt % of Si, 0.5 wt % of Cu, and remainder of Al) or the like at 500° C. or lower temperature. 
     The reflow-type sputtering mentioned above is such a method wherein, while heating a substrate on which an upper level wiring layer 15 is to be formed, a conductive wiring material is deposited on the substrate by sputtering, to flow the deposited conductive wiring material, thereby obtaining a flat surface of the deposited layer, or alternatively wherein after depositing a layer formed of a conductive wiring material on a substrate 1 by sputtering, the substrate 1 is heated to flatter the surface of the deposit. 
     When a reflow-type sputtering is performed over 500° C., durability against migration is reduced, and grain size of conductive wiring material becomes too large. This results in a deterioration of surface shape of the upper wiring layer 15. And, moreover, when AlSiCu is employed as a conductive wiring material, silicon and copper may be crystallized because of the high temperature and will be left as residua when removing unnecessary portions in the step of patterning the upper level wiring 15a. The residua may short-circuit the wirings. 
     Further, reflow sputtering is preferably performed at a temperature range of 400° to 500° C., and more preferably at a temperature range of 400° to 450° C. In these temperature ranges, the unevenness of a surface of the adhesion layer 12 can be compensated while keeping a flatness of the upper level wiring layer 15. This is because reflowed conductive material of the upper level wiring layer 15 fills depressions generated through the overetching process and further the conductive material is fluidized through the reflow-sputtering process so that the surface of the conductive material is smoothed. 
     When the temperature of the reflow-type sputtering is lower than 400° C., fluidization of the conductive material of the upper level wiring layer 15 is insufficient, thus, flatness of the upper level of wiring layer 15 is deteriorated. 
     The conditions of forming an upper level wiring layer 15 by reflow-type sputtering method, for example, are as follows. The substrate 1 heated to approximately 150° C. is placed in an argon gas atmosphere at a pressure of 2 mTorr, and a flow rate of 20 sccm, approximately. AlSiCu is deposited on the substrate 1 with a growth rate of approximately 1 μm/min until the deposited layer has a thickness of approximately 500 nm, and the substrate 1 provided with AlSiCu wiring is heated at 450° C. for 120 sec. 
     As shown in FIG. 1H, a photo-resist film 20 is spin-coated on the upper level wiring layer 15, which photo-resist film 20 is irradiated by an ultraviolet ray 21 through a mask to expose patterns selectively. Since the upper level wiring layer 15 is provided with a flat surface, a desired wiring pattern can be transferred with good fidelity. 
     As shown in FIG. 1I, by developing the photo-resist film 20, unnecessary part of the photo-resist film is removed to form resist patterns 20a and 20b. 
     As shown in FIG. 1J, etching is performed utilizing the resist patterns 20a and 20b as masks to remove unnecessary portions of the upper level wiring layer 15. Thus upper level wiring patterns 15a and 15b are formed. The resist patterns 20a  and 20b are thereafter removed by such method as ashing. 
     And, as shown in FIG. 1K, on the upper level wiring patterns 15a and 15b and on the whole surface of the third level insulating layer 6, a protecting layer 16 formed in SiN or the like is formed. 
     In the wiring forming method according to the first embodiment, a third level insulating layer 6 is formed covering a lower level wiring layer 5, contact hole 11 corresponding to the lower level wiring 5 is formed in the third level insulating layer 6. Further, after forming a tungsten layer 13 on the adhesion layer 12, the whole surface of the tungsten layer 13 is etched back until a small gap is formed at the upper end portion of the contact hole 11 to leave the tungsten layer 13 only at the inside of the contact hole 11. The inside of the contact hole 11 are over-etched at this stage. Thereafter, an Al alloy layer is reflow-sputtered on the whole surface of third level insulating layer 6 and the inside of the contact hole 11 at 500° C. or lower temperature of form an upper level wiring layer 15. Thereby, the inside of the contact holes are filled with the upper level wiring layer 15 and the surface of the upper level wiring layer becomes flat. The aspect ratio before reflow-sputtering is also reduced, because the tungsten layer 13 is etched to the degree wherein a small step is formed at the upper end of the contact hole 11 and hence the contact hole 11 becomes shallow. The reflow-type sputtering can be performed under a comparatively low temperature. Thus, a deterioration of the quality of layer can be prevented. 
     When formation of the upper level wiring layer by reflow-type sputtering of conductive material at 500° C. or lower temperature enables to fill uneven portions even when there have been produced unevennesses with different depth such as steps formed at the upper end portion of the contact hole 11, step-like portions on the adhesion layers 12, unevenness on the surface of the adhesion layer 12 formed by the over-etching of the tungsten layer 13. It also enables to achieve a flat surface of the upper level wiring layer. 
     Further, the unevenness formed by the over-etch of the tungsten layer 13 can be prevented from being transferred to the surface morphology of the upper level wiring layer 15, and thus providing a flat surface of the upper level wiring layer. This flat surface can reduce irregular reflection of exposing light beam at the surface of the upper level wiring layer 15 at the time of exposure of photo-resist to provide a desired pattern of an etching mask. Thus, a desired shape of an upper wiring 15 can be obtained. 
     Consequently, the wiring forming method according to the first embodiment can form an upper level wiring layer 15 having a good coverage, a good quality of layer, and a flat surface, and, further, can form an upper level wiring layer 15 having a predetermined shape. 
     FIGS. 2A to 2J show steps of wiring forming method according to a second embodiment of the present invention. The wiring forming method according to the second embodiment differs from the wiring forming method according to the first embodiment as shown in FIGS. 1A to 1K in that, while the first embodiment connects a wiring of a level to a wiring of another level, this embodiment connects a substrate to a wiring. 
     In the second embodiment, as shown in FIG. 2A, a first level insulating layer 2 formed of an insulating film of SiO 2  or the like is formed on a semiconductor substrate 1 formed with an impurity diffusion region 17. Contact hole 11 is formed in the first level insulating layer 2 by etching using a photolithography method. 
     Next, as shown in FIG. 2B, an adhesion layer 12 is formed on a surface of the contact hole 11 and on the whole surface of the first level insulating layer 2 in the same manner as was performed in the first embodiment. 
     Then, as shown in FIG. 2C, a tungsten layer 13 is formed on the adhesion layer 12 in the same manner as was performed in the first embodiment. 
     Then, the tungsten layer 13 is etched using an etching gas similar to the gas used for the first embodiment to leave the tungsten layer 13 only in the inside of the contact hole 11. Further, as shown in FIG. 2E, the whole surface of the tungsten layer is over-etched until a small step or hollow is formed at the upper end portion of the contact hole 11. In other words the upper surface of the tungsten layer 13 is positioned at a level slightly below the top end of the contact hole 11. Thereafter, as shown in FIG. 2F, an Al alloy layer is reflow-sputtered on the whole surface of the first insulating layer 2 and the inside of the contact hole 11 filed with the tungsten layer 13, to form a wiring layer 18 formed of Al alloy layer at a temperature range of 400° to 500° C . The Al alloy may be similar conductive wiring material as used in the first embodiment. 
     As shown in FIG. 2G, a photo-resist film 22 is spin-coated on the wiring layer 18. An ultraviolet ray is irradiated through a mask to exposure the photo-resist selectively. 
     As shown in FIG. 2H, the resist film 22 is developed to form a resist pattern 22a. Using the resist pattern 22a as a mask, the wiring layer 18 is etched selectively. After the etching, the resist pattern 22a is removed by such method as ashing or the like to leave a wiring 19 as shown in FIG. 2I. A second insulating layer 4 formed of an insulating film of SiO 2  or the like is formed on the wiring 19 and on the whole surface of the first insulating layer 2 as shown in FIG. 2J. 
     In the method according to the second embodiment, a first level insulating layer 2 is formed to cover a semiconductor substrate 1, a contact hole 5 registered to a contact area on the semiconductor substrate 1 is formed in the first insulating layer 2. An adhesion layer 12 is formed on an impurity diffusion region 17 and a whole surface of the first level insulating layer 2. Further, after forming a tungsten layer 13 on the adhesion layer 12, the whole surface of the tungsten layer 13 is etched back until a small step or hollow is formed at the upper end of the contact hole 11, to leave the tungsten layer 13 only in the inside of the contact hole 11. The inside of the contact hole 11 is over-etched at this stage. Thereafter, a wiring layer of an Al alloy is reflow-sputtered on the whole surface of first level insulating layer 2 and the inside of the contact hole 11 at a comparatively low temperature to form a wiring layer 18. Thereby, the inside of the contact hole 11 is filled with the wiring layer 18 and a flat surface of the wiring layer 18 is obtained. The aspect ratio before reflow sputtering is also reduced, because the tungsten layer 13 is etched to a degree wherein a small step is provided at the upper end portion of the contact hole 11, and the contact hole 11 becomes shallow. The reflow-type sputtering can, therefore, be performed under a comparatively Low temperature. Thus, deterioration of the quality of the wiring layer can be prevented. 
     The formation of the wiring layer 18 by reflow-type sputtering of conductive material at a comparatively low temperature enables to smoothen uneven portions even when there have been formed unevennesses with different depths such as a step or hollow at the upper end portion of the contact hole 11, a step-like portion on the adhesion layers 12, unevennesses on the surface of the adhesion layer 12 formed by the over-etching of the tungsten layer 13. It also enables to provide a flat surface of the upper level wiring layer to be formed thereby. 
     Further, the unevenness formed by the over-etch of the tungsten layer 13 can be prevented from transferring to the wiring layer 18. Thus, a flat surface of the layer can be provided. This flat surface can reduce irregular reflection of exposing light on the surface of the upper level wiring layer 15 at the time of exposure of photo-resist, thereby enhancing the precision of photo lithography and providing a desired pattern of the etching mask. Thus, a desired pattern of wiring 19 can be obtained. 
     Therefore, the wiring forming method according to the second embodiment can provide similar effects as those of the wiring forming method according to the first embodiment. 
     The present invention has been described above in relation to preferred embodiments. The present invention is not limited to what have been described in connection with the embodiments. It will be apparent to those skilled in arts that various changes, substitution, combinations and improvements can be made within the scope and spirit of the appended claims.