Abstract:
A method for making a semiconductor integrated circuit device comprises the steps of: (a) depositing a first underlying film made of titanium nitride, on an insulating film having a plurality of through-holes; (b) depositing a tungsten film on the first underlying film, and etching the tungsten film back by means of a fluorine-containing plasma thereby leaving the tungsten film only in the connection holes; (c) sputter etching the surface of the first underlying film to remove the fluorine from the surface of the first underlying film; and (d) forming an aluminum film on the first underlying film. The semiconductor integrated circuit device obtained by the method is also described.

Description:
[0001]    This application is a Divisional application of ser. No. 09/245,743, filed Feb. 8, 1999, which is a Divisional application of Ser. No. 08/584,065, filed Jan. 11, 1996, now U.S. Pat. No. 5,904,556. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to a semiconductor integrated circuit device and also to a method for making the device. More particularly, the invention relates to a technique which is effective when applied to an interconnection structure and an interconnecting process of LSI having multiplayer interconnections.  
           [0003]    In recent years, integration of LSI has been in progress. This leads to increased aspect ratios (i.e., the depth of a connection hole formed on an inter-layer insulating film between a given Al interconnection and a low conductor layer, semiconductor region or a lower Al interconnection. In order to prevent the breakage of the Al interconnections in the inside of the connection holes, a so-called tungsten plug technique has been utilized wherein a W (tungsten) film is filled in the connection holes.  
           [0004]    To fill the W film up in the connection hole, a W film is deposited, according to the CVD method, on the entire surface of an insulating film in which connection holes have been formed. Subsequently, the W film on the insulating film is etched back, thereby leaving the W film only in the connection holes. For the etching back of the W film, a F (fluorine) plasma is used. In order to prevent the underlying insulating film (silicon oxide film) from being etched out with the F plasma, a underlying layer, which is constituted of stacked films including a Ti film and a TiN film, has been formed beneath the W film.  
           [0005]    The underlying film constituted of the Ti/TiN stacked films is very resistant to electromigration or stress migration, and has been employed for interconnection of LSI which is fabricated according to the design rule on the order of submicrons. Interconnections having such a stacked structure as of Ti/TiN/Al-Cu/TiN formed in this order and tungsten plug techniques for this are set out, for example,. in LVSI Multi-level Conference Jun. 7-8, 1994, pp. 36-43.  
         SUMMARY OF THE INVENTION  
         [0006]    The semiconductor integrated circuit device to which the invention is directed is of the type which comprises three-layered metallic interconnections including a first layer made of a tungsten film, and second and third layers made of an aluminium alloy film, respectively.  
           [0007]    A titanium (Ti) film and a titanium nitride (TiN) film, both serving as a underlying layer, are provided beneath the first-layered tungsten film. The interconnection for the first layer is constituted of a three-layered structure made of Ti/TiN/W formed In this order.  
           [0008]    Likewise, a titanium (Ti) underlying film, a titanium nitride (TiN) underlying film and a titanium (Ti) underlying film are provided beneath each of the second and third-layered aluminium alloy (Al-Si-Cu) layers. Ti/TiN cap films are provided on each of the second and third-layered aluminum alloy (Al-Si-Cu) layers. More particularly, a six-layered structure of Ti/TiN/Ti/Al-Si-Cu/Ti/TiN as viewed from the bottom is established. As a matter of course, a tungsten (W) film is filled in connection holes connecting the first and second layers and the second and third layers therewith. Each tungsten film exists in the connection hole between the titanium nitride (TiN) underlying film and the titanium (Ti) underlying film formed on the TiN film.  
           [0009]    We found that the semiconductor integrated circuit having such an interconnection structure as set out above has the following problems.  
           [0010]    (1) The process of filling the tungsten (W) film in the connection holes essentially requires removal of the W film from the insulating film through etching-back by use of a fluorine (F) plasma as set out hereinbefore. This permits part of the fluorine in the plasma to be left on the surface of the underlying film (Ti/TiN stacked film) formed on the insulating film and exposed by the etching-back step. The thus left fluorine reacts with titanium to provide a solid compound. Hence, the compound is left on the underlying film. When another underlying film (Ti film) is formed on the first-mentioned underlying film, or when an aluminium alloy film is deposited subsequently to the etching-back step, the bonding force at the interface between the underlying film on which the compound has remained and the film formed on this underlying film lowers by the influence of the fluorine (F) residue.  
           [0011]    Especially, the uppermost interconnection layer partly serves as a bonding pad. When a wire is bonded to the bonding pad, the pad may separate owing to the impact of the bonding. More particularly, it has been found that the underlying film on which the compound has been left separates from another underlying film formed thereon at the bonding pad portion.  
           [0012]    (2) The process of filling the W film in the connection holes includes the etching-back step wherein the W film is allowed to be left only in the connection holes. This requires over-etching in order to completely remove the W film from the surface of the insulating film. At the time, the W film in the connection holes is also etched out from the outer surface thereof. This leaves a step between the surface of the insulating film or the surface of the underlying film and the surface of the W film in each connection hole.  
           [0013]    In this condition, when an Al interconnection is formed on the insulating film, the Al interconnection is stepped at a surface portion just above the connection hole owing to the above-mentioned step. If a second connection hole is formed in the interlayer insulating film just above the first-mentioned connection hole in order to connect the Al interconnection and the upper Al interconnection therewith and the second connection hole is filled up with the W film, an insulating martial made of AlF 3  is formed in the second connection hole at the time of the formation of the W film. This presents the problem that the conduction failure takes place between the Al interconnection and the upper Al interconnection.  
           [0014]    Owing to the step appearing at the surface of the Al interconnection, the upper layer film formed on the Al interconnection suffers a coverage failure, thus Al being partially exposed from the upper layer film. The thus exposed Al reacts with F left at the time of the formation of the W film, thereby forming an insulating AlF 3  film. This is the reason why there arises the problem that the conduction failure or an increase in contact resistance between the Al interconnection and the upper Al interconnection takes place.  
           [0015]    (3) As having set out hereinabove, the Al interconnection is constituted of multi-layered interconnection (Ti/TiN/Ti/Al-Si-Cu/Ti/TiN). Usually, an uppermost interconnection is used as a bonding pad. However, if the uppermost interconnection is constituted of this type of multi-layered interconnection and part of a passivation film covering the uppermost interconnection therewith is removed by etching to form a bonding pad, a compound formed by reaction between Al and Ti is deposited at the interface between the Al film and the upper film (Ti/TiN stacked film) formed on the Al film. This compound is so hard that the bonding force between the bonding pad and a wire lowers. It should be noted that the etching of the passivation film does not make it possible to fully remove the compound of Al and Ti.  
           [0016]    (4) The Al interconnections are formed by depositing the Al composite film by sputtering and dry etching the deposited film. If the coverage of the Al film on deposition of the Al composite film lowers by the influence of the step formed in the underlying layer, the processing accuracy of the interconnection through dry etching unfavorably lowers. To avoid this, a so-called high temperature Al sputtering technique has been proposed. In the technique, a semiconductor substrate is maintained at high temperatures, and the Al film is deposited while re-flowing the Al film by application of heat from the substrate, thereby ensuring a good coverage of the Al.  
           [0017]    In this connection, however, when an Al film, particularly an Al-Si-Cu film or an Al-Cu film, is deposited according to the high temperature sputtering method, a reaction product is also precipitated in the film. The reaction product is left after dry etching, thus creating another cause of lowering the processing accuracy of the Al interconnection.  
           [0018]    It is therefore an object of the invention to provide a technique whereby a bonding pad constituted of multi-layered interconnection is prevented from separation.  
           [0019]    It is another object of the invention to provide a technique whereby an bonding force between a bonding pad constituted of multi-layered interconnection and a wire is improved.  
           [0020]    It is a further object of the invention to provide a technique which is able to realize a stack-on-plug structure wherein connection holes for an upper layer are located just above connection holes of an interlayer insulating film, respectively.  
           [0021]    It is a still further object of the invention to provide a technique wherein when an Al film is deposited according to a high temperature sputtering method, any reaction product is prevented from formation as precipitated in the Al film.  
           [0022]    These and other objects and novel features of the invention will become apparent from the following description and the accompanying drawings.  
           [0023]    Typical embodiments of the invention are summarized below.  
           [0024]    According to one embodiment of the invention, there is provided a method for making a semiconductor integrated circuit device which comprises the steps of:  
           [0025]    (a) forming a first insulating film formed on a semiconductor substrate and having a plurality of through-holes;  
           [0026]    (b) forming a first underlying film on the first insulating film and in the plurality of through-holes and forming a tungsten film on the underlying film in such a thickness that the through-holes are filled therewith;  
           [0027]    (c) etching the tungsten film to remove the tungsten film from said first insulating film thereby exposing the surface of the first underlying film and selectively leaving the tungsten film in the through-holes;  
           [0028]    (d) sputter etching the surface of the first underlying film;  
           [0029]    (e) forming a first metallic film on the sputter-etched first underlying film; and  
           [0030]    (f) electrically connecting a metallic wire to the first metallic film in regions other than regions where the through-holes are formed.  
           [0031]    According to another embodiment of the invention, there is also provided a semiconductor integrated circuit device which comprises:  
           [0032]    (a) a semiconductor substrate;  
           [0033]    (b) a first interconnection film formed on the semiconductor substrate;  
           [0034]    (c) an insulating film formed on the first interconnection film and having a plurality of through-holes;  
           [0035]    (d) a second interconnection film connected with the first interconnection film through the through-holes and formed on the insulating film; and  
           [0036]    (e) a bonding wire connected to the second interconnection film, wherein the first interconnection film is constituted of a first aluminium alloy film, a titanium film formed on the first aluminium film, and a first titanium nitride formed on the titanium film, and the second interconnection film is constituted of a second aluminium alloy film and a second titanium nitride film formed on the second aluminium alloy film.  
           [0037]    According to a further embodiment of the invention, there is provided a method for making a semiconductor integrated circuit device, which method comprising forming an aluminium film on a main surface of a semiconductor substrate by sputtering, characterized in that a first aluminium film is formed on the semiconductor substrate which is kept at a relatively low temperature, and a second aluminium film is formed at a substrate temperature which is higher than the first-mentioned temperature  
           [0038]    According to a still further embodiment of the invention, there is provided a method for making a semiconductor integrated circuit device, which comprises the steps of:  
           [0039]    (a) forming a first insulating film formed on a semiconductor substrate and having a plurality of first through-holes;  
           [0040]    (b) forming a tungsten film formed on the first insulating film and in the first through-holes in such a thickness that the first through-holes are filled with the tungsten film;  
           [0041]    (c) etching the tungsten film to remove it from the first insulating film until the surface of the first insulating film is exposed while selectively leaving the tungsten film in the individual first through-holes;  
           [0042]    (d) forming a first aluminium film on the exposed surface of the first insulating film and on the tungsten film in the first through-holes; and  
           [0043]    (e) re-flowing the first aluminium film at a given temperature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    FIGS.  1  to  11  and  15  to  17  are, respectively, a sectional view of an essential part of a semiconductor substrate which illustrates a method for making a semiconductor integrated circuit device according to one embodiment of the invention;  
         [0045]    [0045]FIG. 12 is an AES spectrum chart of the surface of a TiN film prior to sputter etching;  
         [0046]    [0046]FIG. 13 is a graph showing the relation between the content of F at the interface of Ti/TiN films and the thickness of a sputter etched titanium nitride film; and  
         [0047]    [0047]FIG. 14 is a graph showing the relation between the intensity of F ions at the interface of Ti/TiN films and the thickness of sputter-etched titanium nitride. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]    Embodiments of the invention are described in detail with reference to the accompanying drawings, in which like reference numerals indicate like parts or members throughout the specification.  
         [0049]    Reference is now made to FIGS.  1  to  17  which illustrate an embodiment of the invention applied to MOS-LSI having a three-layered interconnection structure.  
         [0050]    Initially, as shown in FIG. 1, a semiconductor substrate 1 made of p-type single crystal silicon is ion-implanted with a p-type impurity (boron) on the main surface thereof to form a p-type well  2 . Thereafter, a field oxide film  3  is formed on the main surface of the p-type well according to a selective oxidation (LOCOS) method. Subsequently, a gate oxide  5  is formed on the main surface of the p-type well  2  surrounded with the field oxide film  3  according to a thermal oxidation method, followed by ion implantation of a p-type impurity (boron) into the p-type well  2 , thereby creating a p-type channel stopper layer  4  in the p-type well  2  including the lower portion of the field oxide film  3 .  
         [0051]    Next, a polysilicon film and a silicon oxide film  9  are successively deposited on the semiconductor substrate  1  according to a CVD method, followed by patterning of this two-layered film by drying etching through a photoresist mask to form gate electrodes  6  of MISFET made of the polysilicon film. The polysilicon forming each gate electrode  6  is introduced with an n-type impurity (e.g. P) in order to reduce the resistance thereof. It will be noted that the gate electrodes  6  may be constituted of a polyside film which is made of a refractory metal silicide film, such as WSix, MoSix, TiSix or TaSix, built on the top of the polysilicon film.  
         [0052]    An n-type impurity (e.g. P) is ion-implanted into the p-type well  2  in self-aligned with the gate electrodes  6  so that a pair of n-type semiconductor regions  7 ,  7 , which constitute source and drain regions of the MISFET, are formed in the p-type well  2  at opposite sides of each gate electrode  6 .  
         [0053]    Thereafter, a silicon oxide film is deposited over the semiconductor substrate  1  by a CVD method, followed by anisotropic etching of the silicon oxide film by a reactive ion etching (RIE) method to form side wall spacers  8  at side walls of the gate electrode  6  respectively.  
         [0054]    Then, as shown in FIG. 2, an oxide film  10  and a BPSG film  11  are successively formed over the semiconductor substrate  1  by a CVD method, followed by dry etching the BPSG film  11  and the silicon oxide film  10  through a photoresist mask, thereby forming a connection hole  12  arriving at one of the paired semiconductor regions  7 ,  7  of the MISFET.  
         [0055]    As shown in FIG. 3, an underlying film comprising a first underlying Ti film  13  (30 nm in thickness) and a second underlying TiN film  14  (70 nm in thickness) is deposited on the BPSG film  11  including the inner surfaces of the connection hole  12  according to a sputtering method, followed by further deposition of a W film  15  (250 nm in thickness) on the TiN film  14  by a CVD method. Subsequently, as shown in FIG. 4, the W film  15  and the underlying film-(consisting of the TiN film  14  and the Ti film  13 ) are subjected to patterning through a photoresist mask, thereby forming a tungsten (W) interconnection  16  which is a first interconnection layer.  
         [0056]    The first underlying titanium (Ti) film  13  is provided for the following reason: the film  13  is in contact with the n-type and p-type semiconductor regions (not shown) formed on the main surface of the semiconductor substrate  1  to form titanium silicide (TiSi); and hence the contact resistance can be reduced.  
         [0057]    On the other hand, the second underlying titanium nitride (TiN) film  14  is provided in order to prevent the reaction between the gas (WF 6 ) used to form the tungsten film (W)  15  and the titanium film  13 .  
         [0058]    As shown in FIG. 5, a first interlayer insulating film  17  is deposited on the top of the W interconnection  16 . The interlayer insulating film  17  is constituted, for example, of a three-layer film made of a silicon oxide film deposited by a CVD method, a spin-on-glass film deposited by spin coating, and a silicon oxide film deposited by the CVD method.  
         [0059]    Next, a connection hole  18  is formed in the insulating film  17  on the W interconnection  16  by dry etching using a photoresist as a mask, followed by deposition, on the interlayer insulating film  17  including the inner surfaces of the connection hole  18 , of an underlying film consisting of a titanium (Ti) film  19  (30 nm in thickness), a titanium nitride (TiN) film  20  (100 nm in thickness) according to a sputtering method. Thereafter, a tungsten (W) film  21  (500 nm in thickness) is formed on the titanium nitride (TiN) film  20  by the CVD method.  
         [0060]    It will be noted that the underlying titanium (Ti) film  19  is provided so that it properly controls the crystal orientation of an aluminium alloy film to be subsequently formed, thereby imparting a high electromigration resistance thereto. Likewise, the underlying titanium nitride (TiN) film  20  is provided in order to prevent the reaction between the gas (WF 6 ) used to form the tungsten (W) film  21  and the titanium (Ti) film  19 , like the afore-stated titanium nitride (TiN) film  14 .  
         [0061]    As shown in FIG. 6, the tungsten (W) film  21  is etched back by use of a fluorine (F) plasma (e.g. SF 6  gas) to remove the tungsten (W) film  21  from the interlayer insulating film  17  but to leave the tungsten (W) film  21  only in the connection holes  18 . In order to completely remove the tungsten (W) film  21  from the interlayer insulating film  17 , the tungsten (W) film  21  has to be over-etched. This permits the tungsten (W) film  21  in each connection hole  18  to be removed to a degree, thereby establishing a step with the surface of the interlayer insulating film  17  or the underlying titanium nitride film  20 . The underlying film, particularly the TiN film  20 , formed on the interlayer insulating film  17  serves as an etching stopper at the time of the etching-back.  
         [0062]    Then, as shown in FIG. 7, a titanium (Ti) film  22  (10 nm in thickness) and an Al-Si-Cu film  23  (400 nm in thickness) are successively deposited, by a sputtering method, on the titanium nitride (TiN) film  20  exposed at the surface thereof on the interlayer insulating film  17 . At the time, the aluminium alloy (Al-Si-Cu) film  23  has a stepped surface at position just above the connection hole  18  formed in the insulating film  17 , correspondingly stepped between the surfaces of the interlayer insulating film  17  and the W film  21  in the connection hole  18 .  
         [0063]    To avoid this, according to this embodiment, the semiconductor substrate  1  is heated after deposition of the aluminium alloy (Al-Si-Cu) film  23  as is particularly shown in FIG. 8, so that the aluminium alloy (Al-Si-Cu) film  23  is re-flown thereby permitting the surface to be flattened. The re-flowing conditions include a substrate temperature of 450° C., a pressure of 1 mTorr, and a heating time of 180 seconds. The re-flown aluminium (Al-Si-Cu) film  23  has a surface reflectivity of 91% (wavelength: 365 nm) and is thus very flat.  
         [0064]    Next, as shown in FIG. 9, an upper film comprising a titanium (Ti) film  24  (10 nm in thickness) and a titanium nitride (TiN) film (60 nm in thickness) is deposited on the aluminium alloy (Al-Si-Cu) film  23  by a sputtering method, followed by patterning the titanium nitride (TiN) film  25 , titanium (Ti) film  24 , aluminium alloy (Al-Si-Cu) film  23 , titanium nitride (TiN) film  20 , titanium (Ti) film  19  by dry etching using a photoresist as a mask, thereby forming an aluminium (Al) interconnection  26  which is a second layer interconnection.  
         [0065]    The titanium nitride (TiN) upper film  25  serves as an antireflection film which prevents halation occurring during the course of the patterning of the aluminium second interconnection  26 . The titanium film  24  is provided in order to prevent formation of an aluminium nitride (Al 3 N) film when the titanium nitride (TiN) film  25  is formed on the aluminium alloy (Al-Si-Cu) film  23 .  
         [0066]    As shown in FIG. 10, a second interlayer insulating film  27  is deposited on the top of the aliminium (Al) interconnection  26 . The interlayer insulating film  27  is constituted, for example, of a three-layered film consisting of a silicon oxide film deposited by a CVD method, a spin-on-glass film deposited by a spin coating method, and a silicon oxide film deposited by a CVD method.  
         [0067]    Next, according to the dry etching using photoresist as a mask, a connection hole  28  is formed in the interlayer insulating film  27  at a position just above the connection hole  18  formed in the first interlayer insulating film  17 . The aluminium (Al) interconnection  26  is flattened on the surface thereof (i. e. the bottom of the connection hole  28  ) by the re-flowing. Accordingly, when the connection hole  28  is located at a position just above the connection hole  18  and then formed with a Ti film  29 , a TiN film  30  and a W film in this order, the conduction failure between the Al interconnection  26  and the upper Al interconnection layer can be appropriately prevented without formation of any insulating film in the connection hole  28 .  
         [0068]    Then, as shown in FIG. 11, an underlying film comprising a titanium (Ti) film  29  (30 nm in thickness) and a titanium nitride (TiN) film  30  (100 nm in thickness) is deposited on the interlayer insulating film  27  including the inner surfaces of the connection hole  28 . Thereafter, a tungsten (W) film  31  (500 nm in thickness) is deposited on the titanium nitride (TiN) film  30 . Subsequently, the tungsten (W) film  31  on the insulating film  27  is etched back by use of a fluorine (F) plasma to remove the film  31  from the film  27  while leaving the tungsten (W) film  31  within the connection hole  28 . Because part of F from the plasma undesirably remains on the surface of the titanium nitride (TiN) film  30  on the top of the interlayer insulating film  27  exposed by the etching-back, the titanium nitride (TiN) film  30  is subjected to sputter etching with argon (Ar) gas to an extent of approximately 15 nm, calculated as a thermally oxidized film (silicon oxide film), thereby removing the remaining fluorine (F).  
         [0069]    The reason why the titanium nitride (TiN) film  30  is sputter-etched on the surface thereof is that when the surface of the titanium nitride (TiN) film  30  is contaminated with the fluorine (F), bonding at the interface with a film to be further deposited lowers. More particularly, we have found that when a wire is bonded to a bonding pad in a subsequent step, separation takes place at the interface beneath the bonding pad. It will be noted that the underlying titanium nitride (TiN) film  30  may be replaced by a zirconium nitride (ZrN) film.  
         [0070]    [0070]FIG. 12 is a graph showing AES (Auger Electron Spectroscopy) spectra of the surface of the titanium nitride (TiN) film  30  prior to the sputter etching. From the spectral analysis, the content of fluorine (F) in or on the surfaces of the titanium nitride (TiN) film  30  is calculated as 12 atomic percent.  
         [0071]    [0071]FIG. 13 is a graph showing the relation between the content of fluorine (F) and the thickness of the sputter etched titanium nitride. The content of fluorine is determined by successively depositing, as will be described hereinafter, a titanium (Ti) film  32  and an aluminium alloy (Al-Si-Cu) film  33  on the titanium nitride (TiN) film  30  by a sputtering method and measuring the content of fluorine at the interface between the titanium nitride (TiN) film  30  and the titanium (Ti) film  32  by the SIMS analysis. For convenience&#39;s sake, the thickness of the sputter-etched titanium nitride (TiN) film  30  is indicated as a thickness of a sputter-etched silicon oxide film formed by thermal oxidation (wherein the sputter etching rate of the TiN film is 40% of the sputter etching rate of the silicon oxide film). From this, it has been calculated that the content of fluorine at the time when no sputter etching is conducted (A point in the figure) is 12 atomic percent, and the content of fluorine (F) at the time when the thickness of the sputter etched titanium nitride is 5 nm (B point in the figure) is 6 atomic percent.  
         [0072]    [0072]FIG. 14 is a graph showing the relation between the fluorine (F) ion intensity at the interface between the titanium nitride (TiN) film  30  and the titanium (Ti) film  32  and the thickness of sputter-etched titanium nitride film (calculated as a silicon oxide film). The relation is determined from the results of the AES spectra of FIG. 12 and the SIMS analysis of FIG. 13. The thickness of the sputter etched film and the bonding failure is shown in Table 1.  
                                                                                         TABLE 1                           Relation between the thickness of the sputter etched       film and the bonding failure                thickness of sputter etched film (nm)                    0   5   10   20   30   50                        bonding failure   x   ◯   ◯   ◯   ◯   ◯                          
 
         [0073]    As will be apparent from Table 1, when the titanium nitride (TiN) film  30  is not sputter etched on the surface thereof, separation in the bonding pad takes place. On the other hand, when the thickness of the sputter etched film is 5, 10, 20, 30 or 50 nm, no separation takes place. This reveals that the separation in the bonding pad can be prevented when the sputter etching is performed until the content of the fluorine (F) is 6 atomic percent or below (i.e. the thickness of the sputter etching is not smaller than 5 nm calculated as the silicon oxide film or not smaller than 2 nm for the titanium nitride film).  
         [0074]    Since any bonding pad is formed at the second-layered interconnection (Al interconnection  26 ), the above problem does not arise. However, when the titanium nitride (TiN) film  20  is contaminated with fluorine (F) on the surface thereof, the bonding force at the interface with the titanium (Ti) film  22  being deposited thereon lowers. Accordingly, it is preferred to subject the surface of the titanium nitride (TiN) film  20  to sputter etching prior to the deposition of the titanium (Ti) film  22 . The lowering of the bonding force by the action of the fluorine (F) does not take place only when the titanium (Ti) film is deposited on the titanium nitride (TiN) film. For instance, it will be highly possible that such a lowering occurs on direct deposition of the aluminium alloy (Al-Si-Cu) film on the titanium nitride (TiN) film  20 . In the case, the titanium nitride (TiN) film should preferably be sputter etched prior to the deposition of the aluminium alloy (Al-Si-Cu) film.  
         [0075]    Next, as shown in FIG. 15, a titanium (Ti) film  32  (20 nm in thickness) and an aluminium alloy (Al-Si-Cu) film  33  (600 nm in thickness) are successively deposited on the titanium nitride (TiN) film  30  by a sputtering method. In this embodiment, the aluminium alloy (Al-Si-Cu) film  33  is deposited at two stages. More particularly, the semiconductor substrate  1  is kept at a temperature not higher than 150° C. at which first-stage deposition is carried out at a sputtering rate of approximately 1300 to 1700 nm/minute (300 nm in thickness). Subsequently, the semiconductor substrate  1  is kept at a temperature of 250 to 350° C., at which second-stage deposition is performed at a sputtering rate of approximately 400 to 800 nm (300 nm in thickness).  
         [0076]    The sheet resistance and reflectivity of the aluminium alloy (Al-Si-Cu) film deposited under such conditions as set out hereinabove are shown in Table 2. Point A in Table 2 shows the case where the substrate temperature is maintained at 165° C., and the aluminium alloy (Al-Si-Cu) film  33  is deposited by one stage. With points B, C and D, the substrate temperature at the second stage is, respectively, maintained at 250° C., 300° C. and 350° C., and the respective aluminium alloy (Al-Si-Cu) films  33  are formed by two stages.  
                                                                                                     TABLE 2                           Resistance and reflectivity under different Al       sputtering conditions            Film   Sputtering   Temperature   Sheet   Reflect-       thickness   Rate (nm/minute)   (° C.)   Resist-   ivity (%)            of AlCuSi   first   second   first   second   ance   (Wavelength:       (nm)   stage   stage   stage   stage   (mΩ)   365 nm)                    A   600   1500   —   165   —   51.7   97.5       B   600   1500   600   165   250   50   97.9       C   600   1500   600   165   300   49.1   96.4       D   600   1500   600   165   350   49.6   83.3                  
 
         [0077]    The above results reveal that when the aluminium alloy (Al-Si-Cu) films  33  (B, C, D) are deposited by the two-stage sputtering process including a stage of a low temperature (165° C.) and a high sputtering rate (1500 nm/minute) and a stage of a high temperature (250 to 350° C.) and a low sputtering rate (600 nm/minute), they have sheet resistances and reflectivities almost the same as those of the film obtained by the one-stage sputtering process (A), but have reduced numbers of surface irregularities and precipitates of a reaction product in the film. Thus, the aluminium alloy (Al-Si-Cu) films  33  (B, C, D) exhibit a good coverage for all the cases.  
         [0078]    As shown in FIG. 16, an upper film is further deposited on the aluminium alloy (Al-Si-Cu) film  33 . The upper film is constituted of a single-layered titanium nitride (TiN) film  34  (60 nm in film thickness). In other words, any titanium (Ti) film is not formed on the aluminium alloy (Al-Si-Cu) film  33 . If a titanium (Ti) film is provided, the compound of titanium and the aluminium alloy is formed, thereby causing the bonding failure. As a mater of course, if a titanium (Ti) film is not provided, the compound of aluminium and the nitride is formed on the surface of the aluminium alloy film. However, this compound can be removed during the step of removing the titanium nitride at the time of making an opening for the bonding pad. After the deposition of the aluminium alloy (Al-Si-Cu) film  33 , the re-flowing as set out hereinbefore may be carried out to cause the surface to be more flattened. Alternatively, after the deposition of the aluminium alloy (Al-Si-Cu) film  33 , the semiconductor substrate may be removed to outside of the sputtering apparatus, and thus the aluminium alloy (Al-Si-Cu) film  33  may be exposed to the air to form an oxide film on the surface thereof. Thereafter, the upper film (TiN film  34 ) may be deposited thereon. In the case, the formation of the compound of aluminium and the nitride can be prevented.  
         [0079]    The procedure of forming the aluminium alloy film by the two-stage process may also be applied to the formation of the aluminium alloy film  23  of the second-layered aluminium interconnection  26 . In this case, the re-flowing step of the aluminium interconnection  26  may be omitted.  
         [0080]    Then, the titanium nitride (TiN) film  34 , aluminium alloy (Al-Si-Cu) film  33  titanium (Ti) film  32 , titanium nitride (TiN) film  30  and titanium (Ti) film  29  are, respectively, patterned by dry etching using photoresist as a mask to form an uppermost aluminium (Al) interconnection  35 , followed by further deposition of a passivation film  36  on the top of the aluminium (Al) interconnection  35 . The passivation film  36  is constituted, for example, of a two-layered film consisting of a silicon oxide film deposited by a CVD method and a silicon nitride film deposited by a CVD method.  
         [0081]    Next, as shown in FIG. 17, part of the passivation film  36  is made with a hole by dry etching using photoresist as a mask, thereby exposing part of the aluminium (Al) interconnection film  35  to form a bonding pad  37 . The upper film on the surface of the bonding pad  37  (Al interconnection  35 ) is constituted of the single-layered titanium nitride (TiN) film  34  (provided that where the Al-Si-Cu film  33  is oxidized on the surface thereof, it is made of TiN film and oxide film). Accordingly, the bonding pad  37  is not deposited with the compound of aluminium (Al) and titanium (Ti) unlike the case where the upper film is constituted of a builtup film of the titanium nitride (TiN) film and the titanium (Ti) film.  
         [0082]    Thus, according to this embodiment of the invention, when a gold (Au) wire  38  (i.e. a metallic wire) is bonded to the bonding pad  37 , good bonding force between the bonding pad  37  and the wire  38  is ensured.  
         [0083]    Moreover, according to the embodiment, the titanium nitride (TiN) film  30  which is a part of the uppermost aluminium (Al) interconnection is sputter etched on the surface thereof to remove the fluorine (F) therefrom, so that a satisfactory bonding force at the interface between the titanium nitride (TiN) film  30  and the titanium film (Ti) film  32  deposited thereon can be attained. Thus, the bonding pad  37  does not separate such as by impact of bonding of the wire  38  to the surface of the bonding pad  37 .  
         [0084]    In this embodiment, the upper film on the top of the uppermost interconnection is constituted of a titanium nitride (TiN) film. Accordingly, when part of the passivation film covering the uppermost interconnection is removed by etching to form the bonding pad, the reaction product of the aluminium alloy and titanium is prevented from precipitation at the interface between the Al film and the upper film.  
         [0085]    Furthermore, the underlying film is sputter etched on the surface thereof with use of Ar gas to remove fluorine from the surface. This contributes to improving the bonding force at the interface between the underlying film and the underlying film or aluminium alloy film deposited on the first-mentioned underlying film.  
         [0086]    The aluminium (Al) film is deposited by two stages including a first stage of deposing an aluminium (Al) film under low temperature and high sputtering rate conditions and a second stage of depositing another aluminium film under high temperature and low sputtering rate conditions. By this, the precipitation of a reaction product in the aluminium (Al) film can be prevented. Thus, the aluminium (Al) film obtained has a good coverage and a reduced degree of surface irregularities.  
         [0087]    In addition, after deposition of the aluminium (Al) film by a sputtering method, the film is re-flown at such high temperatures that the aluminium(Al) interconnection just above the connection hole filled up with the tungsten (W) film can be flattened.  
         [0088]    Having been described based on the embodiments, the invention should not be construed as limiting thereto. Many modifications and variations may be possible without departing from the scope of the invention.  
         [0089]    For instance, applications to MOS-LSI having a three-layered interconnection have been set out in the embodiments, and the invention is applicable to LSI having a four-layered or multi-layered interconnection.  
         [0090]    The effects attained by typical embodiments of the invention may be summarized below.  
         [0091]    (1) According to the invention, the bonding force between a bonding pad and a metallic wire increases, thereby improving the reliability of connection between the bonding pad and the wire  
         [0092]    (2) The bonding force at the interface between the underlying films at the uppermost interconnection increases, thereby preventing separation of the bonding pad.  
         [0093]    (3) An Al film is obtained as having a good coverage and a reduced degree of surface irregularities, thereby leading to improved processability of the Al interconnections.  
         [0094]    (4) A stack-on-plug structure wherein an upper connection hole is made in an interlayer insulating film at a position just above a lower connection hole is realized, thereby ensuring a reduced chip area.