Patent Publication Number: US-7223668-B2

Title: Method of etching metallic thin film on thin film resistor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 10/131,683 filed Apr. 24, 2002, now U.S. Pat. No. 6,809,034 which is a divisional of U.S. patent application Ser. No. 09/361,980 filed Jul. 28, 1999, which is now U.S. Pat. No. 6,770,564 which is based upon and claims the benefit of Japanese Patent Applications No. 10-214495 filed on Jul. 29, 1998, No. 10-217725 filed on Jul. 31, 1998, and No. 10-276083 filed on Sep. 29, 1998, the contents of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a method of manufacturing a semiconductor device having a thin film resistor, and particularly to a method of etching a metallic thin film on the thin film resistor. 
   2. Description of the Related Art 
   A conventional method of manufacturing a CrSi resistor device including a CrSi resistor and a TiW barrier metal will be explained referring to  FIGS. 1A to 1D . First, as shown in  FIG. 1A , a CrSi film is deposited on a silicon substrate  101  through a silicon oxide film  102  interposed therebetween, and is patterned in to a shape of a CrSi resistor  103 . Then, as shown in  FIG. 1B , a TiW film  104  as barrier metal is deposited to cover the CrSi resistor  103 , and an Al film  105  is deposited as electrodes. After that, as shown in  FIG. 1C , the Al film  105  is patterned by etching using a photo-resist  106  as a mask. As shown in  FIG. 1D , an unnecessary part of the TiW film  104  is further removed by wet etching using the photo-resist  106  as a mask again. 
     FIG. 2  is an enlarged view showing a portion around the CrSi resistor  103  formed by the conventional method described above. As shown in the figure, the conventional method is accompanied by Al over-hanging such that the Al film  105  is inversely tapered by the wet etching for the TiW film  104 , and TiW under-cut such that the etching of the TiW film  104  progresses under the Al film  105 . As a result, the Al film  105  and the TiW film  104  form an inversely tapered cross-sectional shape as a whole. The inversely tapered cross-sectional shape adversely affect a shape of an intermediate insulation film which covers the CrSi resistor device, and further affects not only a shape of a wiring pattern formed on the insulation film but also a shape of a protective film for covering the wiring pattern. This may result in breakage of the wiring pattern and cracks in the protective film. 
   Further, as shown in  FIG. 3 , when the under-cut amount of the TiW film  104  is large, it becomes difficult for an intermediate insulation film  108  to fill the under-cut portion. This results in deterioration of step-coverage, and allows invasion of water or the like. As a result, the reliability of the device is lowered. Incidentally,  FIG. 3  omits the over-hanging portion of the Al film  105 . 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above problems. An object of the present invention is to prevent under-cut of a barrier metal from being produced by etching of the barrier metal. Another object of the present invention is to prevent over-hanging of a conductive film from being produced by etching of the barrier metal. Still another object of the present invention is to provide a semiconductor device including a thin film resistor with high reliability. 
   According to a first aspect of the present invention, a first opening is formed in a conductive film to expose a metallic film (barrier metal) that is formed on a thin film resistor, and then a mask is formed on the conductive film with a second opening having an opening area smaller than that of the first opening and open in the first opening to expose the metallic film therefrom. Then, the metallic film is etched through the second opening. Accordingly, because the metallic film is etched from an inner portion more than the opening end of the first opening, the metallic film underlying the conductive film is hardly etched, thereby preventing under-cut of the metallic film. 
   According to a second aspect of the present invention, after a first part of a metallic film is dry-etched through an opening of a conductive film, a second part of the metallic film directly contacting a thin film resistor is wet-etched. Because a side etching amount produced by dry-etching is smaller than that produced by wet-etching, a variation in the side etching amount produced during the etching of the metallic film is decreased. As a result, a variation in a contact width between the metallic film and the thin film resistor is decreased, thereby achieving high reliability of a semiconductor device. 
   According to a third aspect of the present invention, a conductive film is formed on a metallic film to have a thickness equal to or less than 300 nm, and the conductive film is patterned to have an upper surface area, a ratio of which relative to an upper surface area of a thin film resistor is equal to or more than 0.02. Then, the metallic film is etched through an opening of the conductive film. Accordingly, a variation in an etching amount produced due to a battery effect between the conductive film and the metallic film is decreased to prevent over-hanging of the conductive film and under-cut of the metallic film. 
   According to a fourth aspect of the present invention, after a surface portion of a metallic film is oxidized to form a surface oxide layer, a conductive film is formed on the surface oxide layer. Then, the surface oxide layer and the metallic film are wet-etched through an opening of the conductive film. In this case, a potential difference produced between the metallic film and the conductive film at the wet-etching step is decreased by the surface oxide layer. As a result, the conductive film is prevented from being etched during the wet-etching step, thereby preventing the over-hanging of the conductive film. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
       FIGS. 1A to 1D  are cross-sectional views for explaining a conventional method of manufacturing a semiconductor device in a stepwise manner; 
       FIG. 2  is a schematic view for explaining problems caused by the conventional method; 
       FIG. 3  is a schematic view for explaining another problem caused by the conventional method; 
       FIG. 4  is a cross-sectional view showing a semiconductor device manufactured by a method according to a first preferred embodiment of the present invention; 
       FIGS. 5A to 5F  are cross-sectional views for explaining the method in the first embodiment in a stepwise manner; 
       FIG. 6  is a partially enlarged cross-sectional view showing a part around a barrier metal when only wet etching is performed; 
       FIG. 7  is a partially enlarged cross-sectional view showing a part around the barrier metal when dry etching and wet etching are successively performed in the first embodiment; 
       FIG. 8  is a graph showing a relationship between a dry etching amount and a side etching amount in the first embodiment; 
       FIG. 9  is a cross-sectional view showing a CrSi resistor device manufacture by a method according to a second preferred embodiment; 
       FIGS. 10A to 10D  are cross-sectional views for explaining the method of manufacturing the CrSi resistor device in the second embodiment in a stepwise manner; 
       FIGS. 11A and 11B  are plan and cross-sectional views, respectively, for explaining a relationship in area between a CrSi resistor and an Al film in the second embodiment; 
       FIG. 12  is a graph showing a change in a TiW under-cut amount relative to a ratio of an upper surface area the Al film relative to an upper surface area of the CrSi resistor in the second embodiment; 
       FIG. 13  is a schematic plan view for explaining a case where segments of the Al film disposed on both ends of the CrSi resistor have areas different from each other; 
       FIG. 14  is a graph showing a relationship between a thickness of the Al film and an etching amount of the Al film; 
       FIG. 15  is a graph showing a change in an over-hanging amount of the Al film relative to a ratio between the upper surface area of the CrSi resistor and the upper surface area of the Al film; 
       FIGS. 16A to 16E  are cross-sectional views showing a process of manufacturing the CrSi resistor device in a stepwise manner according to a modified embodiment of the second embodiment; 
       FIG. 17  is a schematic view for explaining a mechanism of over-hanging of the Al film; 
       FIG. 18  is a cross-sectional view partially showing a circuit part including a MOSFET part and a CrSi resistor part in a third preferred embodiment; 
       FIGS. 19 to 29  are cross-sectional views showing a method of manufacturing the integrated circuit shown in  FIG. 18  in a stepwise manner in the third embodiment; and 
       FIG. 30 to 33  are cross-sectional view showing a method of manufacturing the integrated circuit in a stepwise manner in a fourth preferred embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   First, a structure of a semiconductor device including a metallic thin film resistor which is manufactured by a method in a first preferred embodiment will be explained referring to  FIG. 4 . The semiconductor device has a circuit part including a PN junction element such as diode or transistor, and the metallic thin film resistor  3  made of CrSi or CrSiN is formed on a semiconductor substrate  1  including the circuit part. 
   Specifically, an n +  type diffusion layer  10   a  and a p +  type diffusion layer  11   a  are provided in a surface region of the silicon semiconductor substrate  1  at a circuit part side, thereby forming a PN junction between the n +  type diffusion layer  10   a  and the p +  type diffusion layer  11   a,  which constitutes the PN junction element. An insulation film  2  such as a BPSG film including boron (B) and phosphorus (P) is deposited on the semiconductor substrate  1 , and the metallic thin film resistor  3  is formed on the insulation film  2 . Al electrodes  5   a  are provided on both end portions of the metallic thin film resistor  3  through a barrier metal  4  made of TiW or the like. The insulation film  2  has contact holes  3   a  therein on the upper portion of the PN junction. Al wiring segments  5   b  are electrically connected to the PN junction element through the contact holes  3   a.  The metallic thin film resistor  3 , the barrier metal  4 , the Al electrodes  5   a,  and the Al wiring segments  5   b  are entirely covered with a protective film (intermediate insulation film)  6  such as a TEOS oxide film. Thus, the semiconductor device is constituted. 
   Next, a method of manufacturing the semiconductor device shown in  FIG. 4  will be explained referring to  FIGS. 5A to 5F . 
   At a step shown in  FIG. 5A , first, the insulation film  2  is formed on the silicon semiconductor substrate  1  by plasma enhanced CVD, atmospheric pressure CVD, thermal oxidation, or the like. The PN junction element composed of the N +  type diffusion layer  10   a  and the p +  type diffusion layer  11   a  are formed in the semiconductor substrate  1  before forming the insulation film  2 . Next, the metallic thin film resistor  3  made of CrSi or CrSiN is formed on the insulation film  2  by a sputtering method to have a thickness of approximately 200 Å. The barrier metal  4  is then formed from TiW with a thickness of approximately 2000 Å. 
   At a step shown in  FIG. 5B , the metallic thin film resistor  3  and the barrier metal  4  are patterned by dry etching using gas such as CF 4 , and a photo-resist  210  serving as a mask. At a step shown in  FIG. 5C , the contact holes  3   a  are formed in the insulation film  2  through a photo-lithography step for the electrical connection with the PN junction at the circuit part. Successively, an Al film  5  made of Al or AlSi are entirely formed with a thickness of approximately 1.0 μm. 
   At a step shown in  FIG. 5D , dry etching is carried out using CCl 4  or the like as etching gas and a photo-resist  220  as an etching mask to pattern the Al film  5 , thereby forming the Al electrodes  5   a  for the metallic thin film resistor  3  and the Al wiring segments  5   b  for the circuit part. Simultaneously, the Al film  5  disposed above the metallic thin film resistor  3  is removed to form an opening portion  5   c  in the Al film  5 . 
   At a step shown in  FIG. 5E , first, a photo-resist  230  is formed with an opening portion  230   a  therein exposing the barrier metal  4 . At that time, a length between an opening end of the opening portion  230   a  defined in the photo-resist  230  and an opening end of the opening portion  5   c  defined in the Al film  5 , i.e., the thickness of the photo-resist  230  formed on the inner wall of in the opening portion  5   c  is controlled to be approximately 2 μm. Then, the barrier metal  4  is removed through the photo-resist  230  serving as a mask. In this case, because the photo-resist  230  is formed inside the opening portion  5   c  of the Al film  5 , the barrier metal  4  is removed from an inner side more than the opening end of the Al film  5 . Therefore, the barrier metal  4  underlying the Al film  5  is hardly removed even by wet etching. 
   Thus, because the photo-lithography step is carried out twice, the barrier metal  4  underlying the Al film  5 , i.e., the barrier metal  4  inside the opening end of the opening portion  5   c  is not under-cut. Therefore, it is not necessary to form the protective film  6  under the Al film  5  at the successive step. As a result, the step-coverage of the protective film  6  is improved, resulting in high reliability of the semiconductor device. 
   In view of this point, it is conceivable that the barrier metal  4  is removed only by wet etching. When the barrier metal  4  is removed only by the wet etching, however, referring to  FIG. 6 , it is difficult to control a side etching amount S. Variation in the side etching amount S causes variation in a contact length between the barrier metal  4  and the metallic thin film resistor  3 , i.e., causes variation in a substantial resistor length of the metallic thin film resistor  3 . 
   Therefore, at the step shown in  FIG. 5E , first, the barrier metal  4  is dry-etched through the opening portion  23 c of the photo-resist  230  with CF 4  or the like so that it is thinned by a thickness of approximately 1000 Å. Accordingly, the thickness of the barrier metal  4  becomes approximately 1000 Å. Because the barrier metal  4  is dry-etched, the side etching amount produced at this step is small. 
   Next, at a step shown in  FIG. 5F , wet etching is carried out using the photo-resist  230  as a mask again, and using etching solution mainly including H 2 O 2 . As a result, the remaining barrier metal  4  is removed so that the metallic thin film resistor  3  is exposed.  FIG. 7  schematically shows a portion around the barrier metal  4  at this stage. 
   As described above, when the barrier metal  4  is removed only by wet etching, the barrier metal  4  is removed in a lateral direction by side etching. On the other hand, according to the present embodiment, because the barrier metal  4  is removed by successively carried out the dry etching and the wet etching, the barrier metal  4  has a step shape as a result of performing the two etching steps. At that time, because the dry etching is carried out first, the wet etching capable of easily increasing the side etching amount is required to remove only the remaining portion. Therefore, the total side etching amount is decreased as compared to the case where the barrier metal is removed only by the wet etching. The side etching amount of the barrier metal  4  can be accurately controlled, and accordingly, the variation in the contact length between the barrier metal  4  and the metallic thin film resistor  3  can be decreased. As a result, the substantial resistor length of the metallic thin film resistor  3  is obtained with high controllability. 
   Incidentally, only for decreasing the variation in the contact length between the barrier metal  4  and the metallic thin film resistor  3 , it is conceivable that the removal of the barrier metal  4  is done only by the dry etching without performing the wet etching. However, it is not preferable because the etching gas such as CF 4  used in the dry etching damages the thin film resistor  3  to extremely increase the sheet resistance of the thin film resistor  3 . Because of this, the part of the barrier metal  4  directly contacting the upper surface of the metallic thin film resistor  3  should be removed by the wet etching. Finally, after the protective film  6  is formed, a heat treatment is carried out under nitrogen atmosphere at 450° C. for 20 min., thereby forming the semiconductor device including the thin film resistor  3 . 
   In addition to the effects described above, according to the first embodiment, as shown in  FIG. 5E , the photo-resist  230  is disposed with a specific thickness on the inner wall of the opening portion  5   a  of the Al film  5  to form the opening portion  230   a  having a diameter smaller than that of the opening portion  5   a.  Then, the barrier metal  4  is etched from the inner side more than the opening portion  5   c  of the Al film  5 . Therefore, the barrier metal  4  underlying the Al film  5  is not largely under-etched. 
   Further, because the Al film  5  is covered with the photo-resist  230  during the wet etching, the Al film  5  and the barrier metal  4  are not exposed to the etching solution at the same time. Accordingly, undesirable etching of the Al film  5  (elution of Al), which is caused by a battery effect, does not occur. Incidentally, the battery effect occurs when the Al film  5  and the barrier metal  4 , which have ionization tendencies different from each other, are exposed to the etching solution at the same time as in the conventional manner in which the Al film is not covered during the wet etching. 
   In the first embodiment, the thickness of the barrier metal  4  is controlled to be approximately 2000 Å; however, it is not limited to that and may be changed if necessary. A preferable minimum thickness of the barrier metal  4  for preventing mutual diffusion between the Al film  5  and the metallic thin film resistor  3  is approximately 500 Å. 
   The etching amount of the barrier metal  4  by the dry etching is controlled to be approximately 1000 Å when the initial thickness of the barrier metal  4  is approximately 2000 Å. The etching amount is determined to prevent the effect of the dry etching from eliminating by the wet etching. When the etching amount by the dry etching is too small, the adverse effect by the wet etching prominently appears, and the wet etching cannot follow the shape formed by the dry etching. As a result, the effects described above do not effectively appear. For instance, the side etching amount S cannot be accurately controlled. 
     FIG. 8  is an experimental result indicating the side etching amounts with respect to the dry etching amount. Specifically, the dry etching amount represents the dry etching amount relative to the sum of the dry etching amount and the wet etching amount, i.e., relative to the total etching amount. According to the figure, it is found that, for instance, when the variation in the side etching amount is aimed to be equal to or less than 2 μm, the dry etching amount should be approximately 20% or more of the barrier metal  4  in thickness. The etching amount described above was fixed in this way. 
   The larger the dry etching amount becomes, the smaller the adverse effect by the wet etching becomes. However, as described above, the dry etching can increase the sheet resistance of the metallic thin film resistor  3 . Therefore, it is preferable that the barrier metal  4  is left with a thickness equal to or more than 100 Å by the dry etching in consideration of the variation in the etching amount and the like. 
   Further, according to the first embodiment described above, both the dry etching and the wet etching are carried out through the opening portion  230   a  of the photo-resist  230  having an inner diameter smaller than that of the opening portion  5   a  of the Al film  5 . However, it is sufficient that only the wet etching is carried out through the small opening portion  230   a  of the photo-resist  230 , and the dry etching can be carried out through the opening portion  5   c  of the Al film  5 . In this case, after the dry etching is carried out, the photo-resist  230  having the opening portion  230   a  is formed to serve as the mask for the wet etching as a finish etching. Also, although the Al electrodes  5   a  and the Al wiring segments  5   b  are formed from the Al film  5  in the first embodiment, they may be formed from separate Al films. 
   Second Embodiment 
   Al over-hanging is considered to be produced by a battery effect between an Al film and a TiW film, and TiW under-cut is considered to be produced by a battery effect between the TiW film and a CrSi resistor and by wet-etching in a lateral direction. Further, when the amount of the TiW under-cut becomes large to expose an interface between the Al film and the TiW film, the generation of the Al over-hanging is accelerated. In a second preferred embodiment, the Al over-hanging and the TiW under-cut are prevented based on the considerations described above without performing two etching steps as in the first embodiment. 
   First, a constitution of a semiconductor device in the second embodiment will be explained referring to  FIG. 9 . As shown in  FIG. 9 , a CrSi resistor  13  as a thin film resistive member is formed on a silicon substrate  11 , in which a semiconductor element not shown in provided, through a silicon oxide film  12 . An Al film  15  is disposed as electrodes on both ends of the CrSi resistor  13  through a TiW film  14  as a barrier metal. 
   The edge portions of the TiW film  14  and the Al film  15  are not inversely tapered as a whole, and are approximately perpendicular to the surface of the CrSi resistor  13 . An intermediate insulation film  16  is disposed on the entire surface of the silicon substrate  11  to cover the CrSi resistor  13 . Via holes  16   a  are formed in the intermediate insulation film  16 , and an Al wiring layer  17  is electrically connected to the Al film  15  through the via holes  16   a.  A protective film  18  is further disposed on the entire surface of the silicon substrate  11  to cover the Al wiring layer  17  and the like. The shapes of the Al wiring layer  17  and the protective film  18  follow the shapes of the TiW film  14  and the Al film  15 . Therefore, these films are not inversely tapered as well. Thus, a CrSi resistor device is constituted. 
   Next, a method of manufacturing the semiconductor device shown in  FIG. 9  will be described referring to  FIGS. 10A to 10D . 
   First, at a step shown in  FIG. 10A , after the semiconductor element is formed in the silicon substrate  11 , the silicon oxide film  12  is formed on the silicon substrate  11 . A CrSi film as thin film resistor material is formed on the silicon oxide film  12  with a thickness in a range of approximately 10 nm to 20 nm, and is patterned to form the CrSi resistor  13 . At a step shown in  FIG. 10B , the TiW film  14  as barrier metal material is deposited to cover the CrSi resistor  13 . Further, the Al film  15  as electrode material is deposited on the TiW film  14 . The thickness of the Al film  15  is equal to or less than 300 nm. The reason of determining this thickness will be explained later. 
   Next, at a step shown in  FIG. 10C , a photo-resist  10  is deposited on the Al film  15 , and is partially removed to remain on a specific portion. Then, the Al film  15  is patterned using the photo-resist  10  as a mask. At that time, a ratio of the upper surface area of the patterned Al film  15  relative to the upper surface area of the CrSi resistor  13  is controlled to be equal to or larger than 0.02 and not to exceed 2.0. 
   Then, at a step shown in  FIG. 10D , the TiW film  14  is patterned by wet etching using the photo-resist  10  as a mask. The wet etching uses etching solution of NH 4 OH:H 2 O 2 :H 2 O=5: 100:400 as a composition ratio. Ionization tendency of metal changes according to kinds of solution. In the solution described above, ionization tendencies satisfy a relationship of Al&gt;TiW&gt;CrSi. Therefore, the etching of the TiW film  14  using the above etching solution progresses in a state where the ionization tendency of Al is larger than that of TiW. 
   Next, the reason why the ratio in area of the Al film  5  relative to the CrSi resistor  13  is set equal to or larger than 0.02 as described above will be explained in connection with the etching property. 
   Generally, an etching (corrosion) amount by a battery effect occurring when two metals are exposed to etching solution is decreased when a metal having ionization tendency larger than that of the other metal has an exposed area (solution contact area) larger than that of the other metal. Because of this, the under-cut of the TiW film  14  produced by the battery effect between the TiW film  14  and the CrSi resistor  13  can be decreased by setting the ratio of the solution contact area of the TiW film  14  relative to that of the CrSi resistor  13  large. 
   Referring to  FIGS. 11A and 11B , the solution contact areas of the CrSi resistor  13  and the TiW film  14  in the present embodiment are considered in more detail. The solution contact area of the CrSi resistor  13  approximately corresponds to the upper surface area of the CrSi resistor  13  because the thickness of the CrSi resistor  13  is in a range of 10 nm to 20 nm, which is very thin. 
   On the other hand, the solution contact area of the TiW film  14  corresponds to a circumferential area thereof because the upper surface of the TiW film  14  is covered with the Al film  15 . Therefore, the solution contact area of the TiW film  14  can be represented by the product of the circumferential length and the thickness of the TiW film  14 , which correlates with the upper surface area of the TiW film  14 . Further, the upper surface area of the TiW film  14  is approximately equal to the upper surface area of the Al film  15  that serves as a mask when the TiW film  14  is patterned. Therefore, there is a relationship that the circumferential area of the TiW film  14  ∝ the upper surface area of the TiW film  14  ≈ the upper surface area of the Al film  15 . Accordingly, the under-cut amount by the battery effect can be estimated based on the ratio of the upper surface area of the Al film  15  relative to the upper surface area of the CrSi resistor  13 . 
     FIG. 12  shows a change in under-cut amount of the TiW film  14  when the ratio of the upper surface area of the Al film  15  relative to the upper surface area of the CrSi resistor  13  is changed. In the figure, the case in which the under-cut amount is larger than zero means that the TiW film  14  is under-cut inwardly from the Al film  15 . The case in which the under-cut amount is smaller than zero means that the TiW film  14  protrudes outwardly from the Al film  15  without being under-cut. 
   As shown in  FIG. 12 , when the ratio of the upper surface area of the Al film  15  relative to that of the CrSi resistor  13  (Al/CrSi area ratio) is equal to or larger than 0.02, the under-cut amount of the TiW film  14  becomes equal to or less than zero. That is, in this case, the under-cut of the TiW film  14  is not produced by the battery effect occurring between the CrSi resistor  13  and the TiW film  14 . 
   The increase in the under-cut amount of the TiW film  14  means an increase in an exposed area of the interface between the TiW film  14  and the Al film  15 , resulting in an increase in over-etching of the Al film  15  that is produced by the battery effect between the TiW film  14  and the Al film  15 . To the contrary, the prevention of the under-cut of the TiW film  14  prevents the exposure of the interface between the TiW film  14  and the Al film  15 , resulting in prevention of over-hanging of the Al film  15 . As shown in  FIG. 13 , when the Al film  15  includes two Al segments disposed on both ends of the CrSi resistor  13  and having upper surface areas different from each other, each of the upper surface areas of the Al segments is set to fall within the range described above relative to the upper surface area of the CrSi resistor  13 . 
   Next, the reason why the thickness of the Al film  15  is set equal to or less than 300 nm will be explained. As described above, the over-hanging of the Al film  15  occurs due to elution of Al caused by the battery effect between the Al film  15  and the TiW film  14 . In the conventional CrSi resistor device shown in  FIG. 2 , the etching amount of the Al film  105  is large around the interface between the Al film  105  and the TiW film  104 , and is small around the surface of the Al film  105 . It is assumed that this difference in the etching amount of the Al film  105  produces the over-hanging of the Al film  105 . The etching amount of the Al film  105  changes according to the distance from the interface between the Al film  105  and the TiW film  104 . That is, it is considered that the larger the thickness of the Al film  105  becomes, the larger the over-hanging becomes. 
   In view of this point, the over-hanging amount was examined while changing the thickness of the Al film  15  in the present embodiment. The result is shown in  FIG. 14 . Accordingly, it is found that the etching amount of the Al film  15  relates to the thickness of the Al film  15 , and has variation with an inclination which changes at a thickness of approximately 300 nm as an inflection point. Therefore, the thickness of the Al film  15  is set equal to or less than 300 nm in the present embodiment so that the etching amount difference in the Al film  15  is decreased, thereby reducing the over-hanging amount of the Al film  15 . 
   Further, a change in the over-hanging amount of the Al film  15  was examined with respect to the ratio of the upper surface area of the Al film  15  relative to the upper surface area of the CrSi resistor  13 . The result is shown in  FIG. 15 . It is found that the over-hanging amount become large when the ratio of the upper surface area of the Al film  15  relative to the upper surface area of the CrSi resistor  13  exceeds 2.0. Based on this result, the ratio of the upper surface area of the Al film  15  relative to the upper surface area of the CrSi resistor  13  is suppressed not to exceed 2.0 in the present embodiment. 
   Accordingly, the present embodiment can reduce not only the under-cut of the TiW film  14  produced by the battery effect between the CrSi resistor  13  and the TiW film  5  but also the over-hanging of the Al film  15  produced by the battery effect between the Al film  15  and the TiW film  14  when the TiW film  14  is patterned. As a result, the TiW film  14  and the Al film  15  are not inversely tapered in cross-section, thereby preventing the breakage of the wiring layer  17  formed above the CrSi resistor  13  and cracks in the protective film  18 . 
   Incidentally, because the over-hanging amount of the Al film  15  is suppressed, the upper surface area of the TiW film  14  can be roughly calculated with the upper surface area of the Al film  15  to prevent the under-cut of the TiW film  14 . Further, the photo-resist  10  remaining when the TiW film  14  is patterned reduces the solution contact area of the Al film  15 . This makes the etching of the Al film  15  by the battery effect between the Al film  15  and the TiW film  14  easier than that of the TiW film  14 , and therefore, contributes to the prevention of the under-etching of the TiW film  14 . 
   Then, after the step shown in  FIG. 10D , the intermediate insulation film  16  is disposed on the entire upper surface of the silicon substrate  11  to cover the Al film  15  and the CrSi resistor  13 . Then, the via holes  16   a  are formed in the intermediate insulation film  16 , and the Al wiring layer  17  and the protective film  18  are successively formed. As a result, the CrSi resistor device shown in  FIG. 9  is completed. According to the second embodiment described above, the over-hanging of the Al film  15  and the under-cut of the TiW film  14  can be prevented without increasing a number of manufacturing process as compared to the conventional method. 
   In the second embodiment, after the CrSi resistor  13  is patterned, the TiW film  14  as barrier metal and the Al film  15  as electrode material are deposited to cover the CrSi resistor  13 ; however, the manufacturing process shown in  FIGS. 16A to 16E  are adoptable as well. 
   Specifically, after the CrSi film  3  for the CrSi resistor and the TiW film  14  are deposited on the silicon substrate  11  through the silicon oxide film  12  as shown in  FIG. 16A , the CrSi film  3  and the TiW film  14  are simultaneously patterned using an identical mask as shown in  FIG. 16B . Next, as shown in  FIG. 16C , after the Al film  15  is deposited to cover the CrSi resistor  13  and the TiW film  14 , as shown in  FIG. 16D , the Al film  15  is patterned using a photo-resist  19  as a mask. After that, as shown in  FIG. 16E , the TiW film  14  is etched. Thus, the Al film  15  can be deposited after the CrSi resistor  13  and the TiW film  14  are patterned. The other features are the same as those in the second embodiment. Accordingly, the same effects as those in the second embodiment can be provided. 
   In addition, according to the procedure shown in  FIGS. 16A to 16E , because the Al film  15  is disposed directly on the silicon oxide film  12 , the Al film  15  can be utilized as an element for another device, or a wiring member for connecting the CrSi resistor  13  with another device. When the CrSi resistor device is manufactured in the procedure described above, however, it is confirmed that the over-hanging of the Al film  15  is liable to become large as compared to the case where it is manufactured in the procedure shown in  FIGS. 10A to 10D . 
   The reason is considered as follows. That is, the Al film  15  directly formed on the silicon oxide film  12  can contact the silicon substrate  11  through contact holes or the like provided in the silicon oxide film  12 . Accordingly, when the etching occurs due to the battery effect between the Al film  15  and the TiW film  14 , electrons remaining in the Al film  15  flows into the silicon substrate  11  to further accelerate the etching of the Al film  15 . 
   This phenomenon will be specifically explained in connection with the over-hanging mechanism of the Al film  15  referring to  FIG. 17  in which it is assumed that the semiconductor device is immersed into alkaline aqueous solution  100  having high conductivity for wet-etching the TiW film  14 . In such a case, an electromotive force is produced between the TiW film  14  and the Al film  15  to provide a battery effect therebetween. Al is a base metal as compared to TiW, and accordingly is dissolved by the battery effect. This results in the over-hanging of the Al film  15 . 
   When the Al film  15  is dissolved, the following chemical reactions (1) to (3) occur;
 
Al→Al 3+ +3e −   (1)
 
3/2H 2 O+3/4O 2 +3e − →3OH −   (2)
 
Al 3+ +3OH − →Al(OH) 3    (3)
 
   in which reaction (1) occurs at the base metal (Al) side, reaction (2) occurs at the noble metal (TiW) side, and reaction (3) occurs in the solution. 
   When the Al film  15  contacts the silicon substrate  11  or other conductive members, electrons produced by reaction (1) flows into the silicon substrate  11  or the like to promote reaction (1). As a result, the etching of the Al film  15  is accelerated. To prevent this problem, the Al film  15  should be prevented from contacting the silicon substrate  11  and the other wiring members when the TiW film  14  is patterned. 
   Third Embodiment 
   In a third preferred embodiment, referring again to  FIG. 17 , the potential difference produced between the Al film and the TiW film is lowered to prevent Al from being dissolved. First, a constitution of a MOSFET in the third embodiment will be explained referring to  FIG. 18 . 
     FIG. 18  shows a part of an integrated circuit of the MOSFET. A silicon substrate  21  is formed with a SOI (Silicon on Insulator) structure, in which a high impurity concentration n type layer  21   c  and a low impurity concentration n type layer  21   d  are provided on a high impurity concentration p type substrate  21   a  through a silicon oxide film  21   b.  A trench is formed in the silicon substrate  21 , and is filled with a silicon oxide film  22   a  and a polysilicon layer  22   b . Accordingly, an element (MOSFET) formation region  23  and a thin film resistor formation region  24  are isolated from each other. 
   In the element formation region  23 , a p type well layer  23   a  is formed by implanting p type impurities into the n −  type layer  21   d,  and n type source region  25   a  and an n type drain region  25   b  are provided in a surface region of the p type well layer  23   a.  A gate oxide film  26  is formed on the surface of the p type well layer  23   a  between the source region  25   a  and the drain region  25   b.  A LOCOS film  27  is formed on the surface of the silicon substrate  21  to isolate the thin film resistor formation region  24  from the element formation region  23 . A gate electrode  28  is formed on the gate oxide film  26 , and the gate electrode  28  is covered with a BPSG insulation film  29 . The source region  25   a  and the drain region  25   b  are electrically connected to a TiN film  30  and an AlSiCu film  31  as a 1st Al film (source electrode and drain electrode) through contact holes. Incidentally, wiring patterns  32   a,    32   b  provided on the BPSG film  29  within the thin film resistor formation region  29  are formed simultaneously when the AlSiCu film  31  is formed. 
   The gate electrode  28  and the source (drain) electrode  31  are covered with a P—SiN film  33 , a TEOS film  34 , a SOG (Spin on Glass)  35 , and a silicon oxide film  36 . Then, a thin film resistor  37  is disposed at a specific position on the silicon oxide film  36 . The thin film resistor  37  is composed of a CrSi film with a thickness of approximately 15 nm. The LOCOS film  27  has an irregularly shaped part  27   a  which underlies the thin film resistor  37  for scattering laser beam when laser trimming is performed to adjust a value of resistance of the thin film resistor  37 . The irregularly shaped part  27   a  prevents interference of the leaser beam and the like so that the thin film resistor  37  can be desirably fused and cut. 
   A barrier metal  38  is disposed on both ends of the thin film resistor  37 , and an Al thin film  39  as electrodes is disposed on the barrier metal  38  through an alloy layer  38   b,  which is transformed from a surface oxide layer  38   a  as described below. The surface oxide layer  38   a  is formed by, for instance, oxidizing the barrier metal  38 . This surface oxide layer  38  prevents the Al thin film  39  and the barrier metal  38  from being inversely tapered by wet-etching for patterning the Al thin film  39  and the barrier metal  38 . The surface oxide layer  38   a  is alloyed with the Al thin film  39  during a heat treatment (for instance, Al sintering) for patterning the barrier metal  38 , thereby transforming into the alloy layer  38   b  having high conductivity. 
   Further, a silicon oxide film  40  is formed as an intermediate insulation film to cover the entire surface of the silicon substrate  21 . An Al thin film  41  is formed as a 2nd Al film to form a wiring pattern filling via holes  40   a  defined in the silicon oxide film  40 . The upper surface of the silicon substrate  21  including the Al thin film  41  is entirely covered with a protective film  42  composed of a P—SiN film. 
   Next, a method of manufacturing the integrated circuit of the MOSFET will be explained referring to  FIGS. 19–29 . First, at a step shown in  FIG. 19 , the silicon substrate  21  in which the high impurity concentration n type layer  21   c  and the low impurity concentration n type layer  21   d  are disposed on the high impurity concentration p type substrate  21   a  through the silicon oxide film  21   b  is prepared. Then, the trench is formed to reach the silicon oxide film  21   b  at the interface between the elements. The silicon oxide film  22   a  is disposed on the side wall of the trench, and the gap defined by the silicon oxide film  22   a  is filled with the polysilicon layer  22   b,  thereby forming the element isolation structure. 
   Next, selective ion implantation is carried out so that the p type well layer  23   a  is formed in the surface region of the n −  type layer  21   d  in the MOSFET formation region  23 . Then, the LOCOS oxide film  27  is formed on the trench by LOCOS oxidation to have the irregularly shaped part  27   a  in the thin film resistor formation region for improving the workability of the laser trimming for the thin film resistor  37  (see  FIG. 18 ). 
   After the gate oxide film  26  is formed on the p type well layer  23   a,  polysilicon is deposited thereon. The gate electrode  28  is formed by patterning the polysilicon. Then, ion implantation is carried out using the gate electrode  28  as a mask, and then a heat treatment is performed. Consequently, the source region  25   a  and the drain region  25   b  are formed. After that, the BPSG film  29  is formed on the entire surface of the silicon substrate  21  as an intermediate insulation film by a CVD method or the like. At a step shown in  FIG. 20 , after a contact hole  29   a  is formed in the BPSG film  29 , a reflow treatment is carried out at a temperature in a range of approximately 900° C. to 950° C. to make an edge portion of the contact hole  29   a  smooth. 
   Then, at a step shown in  FIG. 21 , the TiN film  30  as barrier metal is formed with a thickness of approximately 100 nm. Then, after the AlSiCu film is deposited with a thickness of approximately 0.45 μm, the 1st Al film  31  is patterned by ECR (Electron cyclotron resonance) dry etching. 
   At a step shown in  FIG. 22 , after the P—SiN film  33  is deposited, the first TEOS film  34  is formed with a thickness of approximately 0.2 μm. Further, after coating the SOG, the irregular portion of the surface of the silicon substrate  21  is filled with the SOG  35  by baking and etch-back treatments so that the surface of the silicon substrate  21  is flattened. Further, the second TEOS film  36  is deposited with a thickness of approximately 0.3 μm by the CVD method. 
   Then, at a step shown in  FIG. 23 , the CrSi film is deposited by sputtering with a thickness of approximately 15 nm, and is patterned to form the thin film resistor  37 . Further, the barrier metal  38  composed of a TiW film is deposited on the enter surface of the silicon substrate  21  including the thin film resistor  27  with a thickness of approximately 1000 Å. 
   After that, at a step shown in  FIG. 24 , the surface of the barrier metal  38  undergoes an oxidation treatment including water cleaning, heat treatment, O 2  ashing, and the like, thereby forming the surface oxide layer  38   a  on the surface of the barrier metal  38 . The surface oxide layer  38  is composed of a TiO 2  layer. Next, at a step shown in  FIG. 25 , an Al film for forming the electrodes of the thin film resistor  37  is deposited with a thickness of approximately 2000 Å. Then, a patterned photo-resist  45  is disposed on both ends of the thin film resistor  37  for performing a photo-lithography step. 
   At a step shown in  FIG. 26 , the Al film is patterned to form the Al thin film  39  by wet etching using the photo-resist  45  as a mask. Successively, at a step shown in  FIG. 27 , the surface oxide layer  38   a  and the barrier metal  38  are patterned by wet etching using the photo-resist  45  as a mask again. In this wet etching, H 2 O 2 /NH 4 OH based solution is used. Therefore, a high etching rate can be realized to prevent the photo-resist  45  from floating. 
   At that time, because the surface oxide film  38   a  is formed on the surface of the barrier metal  38  at the step shown in  FIG. 24 , a potential difference produced between the Al thin film  39  and the barrier metal  38  during the wet etching is decreased. Specifically, the surface oxide layer  38   a  intervening between the Al thin film  39  and the barrier metal  38  serves as a barrier for disturbing a flow of electrons between the Al thin film  39  and the barrier metal  38 , thereby reducing the potential difference therebetween. Consequently, the battery effect caused by the potential difference between the Al thin film  39  and the barrier metal  38  is reduced to prevent the Al etching rate around the interface of the barrier metal  38  from becoming large. Therefore, the Al thin film  39  is not inversely tapered by the wet etching. After that, the photo-resist  45  is removed. 
   Next, at a step shown in  FIG. 28 , the surface of the silicon substrate  21  is entirely covered with the silicon oxide (SiO 2 ) film  40 . Because the Al thin film  39  is not inversely tapered, the silicon oxide film  40  is not inversely tapered as well. Then, Al sintering is carried out at 450° C. for 30 min. so that the TiAl 3  alloy layer  38   b  is formed at the interface between the Al thin film  39  and the barrier metal  38  as a result of reaction (Al+TiO 2 →TiAl 3 ) between the Al thin film  39  and the surface oxide layer  38   a.  TiAl 3  constituting the TiAl 3  alloy layer  38   b  is formed by a heat treatment in a range of approximately 400° C. to 500° C. The TiAl 3  alloy layer  38   b  improves the contact property between the barrier metal  38  and the Al thin film  39 . 
   Then, at a step shown in  FIG. 29 , after the via holes  40   a  are formed in the silicon oxide film  40 , the patterned AlSi film  41  as the 2nd Al film is disposed in the via holes  40   a.  At that time, because the silicon oxide film  40  is not inversely tapered, the AlSi film  41  can be formed with a desirable pattern without having breakage. 
   Further, the upper surface of the silicon substrate  21  including the AlSi film  40  is entirely covered with the protective film  42  composed of a P—SiN film, and an annealing treatment is carried out. This annealing treatment can improve the contact property between the surface oxide layer  38   a  and the Al thin film  39  as well as the Al sintering performed at the step shown in  FIG. 28 . Therefore, the thickness of the surface oxide film  38   a  is preferably controlled to be almost reacted by these treatments. 
   Thus, the surface oxide layer  38   a  intervening between the Al thin film  39  and the barrier metal  38  prevents the Al thin film  39  from being inversely tapered, and accordingly, the breakage of the wiring pattern (AlSi film  41 ) formed on the Al thin film  39  does not occur. Further, the contact property between the surface oxide layer  38   a  and the Al thin film  39  is improved by the heat treatment. 
   Fourth Embodiment 
   A fourth preferred embodiment differs from the third embodiment in a method for patterning the barrier metal  38 . Only points different from the method of the third embodiment will be explained referring to  FIGS. 30–33 . 
   First, the steps shown in  FIGS. 19–22  are successively performed as in the third embodiment. Then, at a step shown in  FIG. 30 , a CrSi film  51  is deposited by sputtering with a thickness of approximately 15 nm, and a TiW film  52  is deposited on the CrSi film  51  with a thickness of approximately 1000 Å. Then, a photo-resist  53  is deposited, and is patterned to remain only at a portion where a thin film resistor  37  (see  FIG. 18 ) is to be formed. 
   At a step shown in  FIG. 31 , the CrSi film  51  is patterned together with the TiW film  52 . Accordingly, the thin film resistor  37  and the barrier metal  38  are formed. After that, water cleaning, heat treatment, O 2  ashing, and the like are performed to oxidize the surface of the barrier metal  38 , thereby forming the surface oxide layer  38   a.  The surface oxide layer is made of TiO 2 . 
   Next, at a step shown in  FIG. 32 , the Al thin film  39  are deposited with a thickness of approximately 2000 Å as electrodes for the thin film resistor. A photo-resist  55  is formed and is patterned to remain on both end portions of the thin film resistor  37  for performing a photo-lithography step. After that, dry etching is performed using the photo-resist  55  as a mask so that the Al thin film  39  is patterned. At a step shown in  FIG. 33 , wet etching is further performed using the photo-resist  55  as a mask again, so that the surface oxide layer  38   a  and the barrier metal  38  are patterned. The wet etching uses H 2 O 2 /NH 4 OH based etching solution. At that time, because the surface oxide layer  38   a  is formed on the surface of the barrier metal  38  at the step shown in  FIG. 31 , the Al thin film  39  is not inversely tapered as in the third embodiment. Then, the integrated circuit of the MOSFET is finished through the steps shown in  FIGS. 18–19 . Thus, even when the thin film resistor  37  and the barrier metal  38  are patterned using the identical mask, the same effects as those in the third embodiment can be provided. The third and fourth embodiments can be combined with the first and second embodiments to enhance the effects described above. 
   Although the present invention is applied to prevent the under-cut of the barrier metal and the over-hanging of the Al thin film in the method of manufacturing the semiconductor device with the thin film resistor, it can be widely applied to prevent dissolution (elution) of a metallic layer by a battery effect. Specifically, in a case where two laminated metallic films are exposed to a solution to produce an electrode potential difference therebetween which can cause a battery effect in the solution to dissolve one of the metallic layers, the above-described present invention can be applied to prevent the dissolution of the one of the metallic layers. Therefore, the present invention is not necessarily applied to an wet-etching step, and is sufficient to be applied to a wet-processing step using a specific solution to provide the effects described above. 
   While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.