Abstract:
According to the present invention, there is provided a semiconductor device fabrication method comprising: forming an interlayer dielectric film on a semiconductor substrate; removing a predetermined region of the interlayer dielectric film, and forming a film by depositing a conductive material on the semiconductor substrate and interlayer dielectric film so as to fill the removed region; planarizing the film such that the film has substantially the same height as the interlayer dielectric film, thereby burying the conductive material to form a first conductive layer; and performing processing using a dilute aqueous choline solution on an upper surface of the buried first conductive layer.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims benefit of priority under 35 USC §119 from the Japanese Patent Application No. 2004-233405, filed on Aug. 10, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device fabrication method.  
         [0003]     In the semiconductor fabrication step, an interlayer dielectric film is formed on a semiconductor substrate in which a semiconductor element such as a MISFET is formed, and a contact plug for making contact with the semiconductor substrate surface is formed in this interlayer dielectric film. Another interlayer dielectric film is formed on these interlayer dielectric film and contact plug.  
         [0004]     The other interlayer dielectric film is coated with a photoresist, and the photoresist is exposed and developed to form a resist mask having a pattern which opens above the upper surface of the contact plug.  
         [0005]     This resist mask is used as a mask to etch away the surface portion of the interlayer dielectric film by a predetermined depth, thereby forming an interconnection trench in this interlayer dielectric film, and exposing the upper surface of the contact plug.  
         [0006]     After the resist mask is removed by oxidation, deposits such as the resist residue are removed by using an organic fluoric chemical prepared by adding NH 4 F to an organic solvent.  
         [0007]     When performed using this organic fluoric chemical, etching progresses in the lateral direction of the interconnection trench, and this increases the width of the trench. As a consequence, if a copper interconnection is formed to connect to the contact plug by burying copper in this trench, this copper interconnection becomes wider than the mask pattern. This makes the wiring resistance different from the design value, and produces variations in characteristics.  
         [0008]     A reference pertaining to the removal of the resist residue is as follows.  
         [0009]     PCT(WO) 2002-520812  
       SUMMARY OF THE INVENTION  
       [0010]     According to one aspect of the invention, there is provided a semiconductor device fabrication method comprising:  
         [0011]     forming an interlayer dielectric film above a semiconductor substrate;  
         [0012]     removing a predetermined region of the interlayer dielectric film, and forming a film by depositing a conductive material so as to fill the removed region;  
         [0013]     planarizing the film such that the film has substantially the same height as the interlayer dielectric film, thereby burying the conductive material to form a first conductive layer; and  
         [0014]     performing processing using a dilute aqueous choline solution on an upper surface of the buried first conductive layer.  
         [0015]     The bottom of the removed region of the interlayer dielectric film is a surface of a semiconductor substrate, a conductive layer or a dielectric film, etc.  
         [0016]     According to one aspect of the invention, there is provided a semiconductor device fabrication method comprising:  
         [0017]     forming a first interlayer dielectric film above a semiconductor substrate;  
         [0018]     removing a predetermined region of the first interlayer dielectric film, and forming a film by depositing a conductive material so as to fill the removed region;  
         [0019]     planarizing the film such that the film has substantially the same height as the first interlayer dielectric film, thereby burying the conductive material to form a conductive layer;  
         [0020]     forming a second interlayer dielectric film on the first interlayer dielectric film and buried conductive layer;  
         [0021]     forming, on the second interlayer dielectric film, a mask having a pattern which opens above part or a whole of an upper surface of the conductive layer;  
         [0022]     exposing the upper surface of the conductive layer by etching the second interlayer dielectric film by using the mask; and  
         [0023]     performing processing using a dilute aqueous choline solution on the upper surface of the exposed conductive layer.  
         [0024]     The bottom of the removed region of the first interlayer dielectric film is a surface of a semiconductor substrate, a conductive layer or a dielectric film, etc.  
         [0025]     According to one aspect of the invention, there is provided a semiconductor device fabrication method comprising:  
         [0026]     forming an interlayer dielectric film above a semiconductor substrate;  
         [0027]     removing a plug formation region for forming a plug of the interlayer dielectric film;  
         [0028]     removing an interconnection formation region for forming an interconnection of the interlayer dielectric film, thereby removing the interlayer dielectric film to a predetermined depth;  
         [0029]     forming a film by depositing a conductive material so as to fill the plug formation region and interconnection formation region;  
         [0030]     planarizing the film such that the film has substantially the same height as the interlayer dielectric film, thereby forming the plug and interconnection; and  
         [0031]     processing the interconnection by using a dilute aqueous choline solution.  
         [0032]     The bottom of the removed plug formation region of the interlayer dielectric film is a surface of a semiconductor substrate, a conductive layer, or a dielectric film, etc.  
         [0033]     According to one aspect of the invention, there is provided a semiconductor device fabrication method comprising:  
         [0034]     performing processing using a dilute aqueous choline solution on an upper surface of a buried conductive layer in an interlayer dielectric film formed above a semiconductor substrate.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a longitudinal sectional view showing an element sectional structure in a predetermined step of a semiconductor device fabrication method according to the first embodiment of the present invention;  
         [0036]      FIG. 2  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0037]      FIG. 3  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0038]      FIG. 4  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0039]      FIG. 5  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0040]      FIG. 6  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0041]      FIG. 7  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0042]      FIG. 8  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0043]      FIG. 9  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0044]      FIG. 10  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0045]      FIG. 11  is a graph showing the film thickness of a tungsten oxide film before and after processing is performed using a dilute aqueous choline solution;  
         [0046]      FIG. 12  is a view showing the relationship between an interlayer dielectric film and its etching amount;  
         [0047]      FIG. 13  is a longitudinal sectional view showing an element sectional structure in a predetermined step of a semiconductor device fabrication method according to the second embodiment of the present invention;  
         [0048]      FIG. 14  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0049]      FIG. 15  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0050]      FIG. 16  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0051]      FIG. 17  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0052]      FIG. 18  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0053]      FIG. 19  is a longitudinal sectional view showing an element sectional structure in a predetermined step of a semiconductor device fabrication method according to the third embodiment of the present invention;  
         [0054]      FIG. 20  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0055]      FIG. 21  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0056]      FIG. 22  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0057]      FIG. 23  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0058]      FIG. 24  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0059]      FIG. 25  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0060]      FIG. 26  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method;  
         [0061]      FIG. 27  is a longitudinal sectional view showing an element sectional structure in a predetermined step of the semiconductor device fabrication method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0062]     Embodiments of the present invention will be described below with reference to the accompanying drawings.  
       (1) First Embodiment  
       [0063]     FIGS.  1  to  10  illustrate a semiconductor device fabrication method according to the first embodiment of the present invention. As shown in  FIG. 1 , an interlayer dielectric film  20  made of, e.g., a silicon oxide (SiO 2 ) film is formed on a semiconductor substrate  10  in which a semiconductor element such as a MISFET (not shown) is formed, and the surface of the interlayer dielectric film  20  is planarized by CMP (Chemical Mechanical Polishing) or the like.  
         [0064]     To avoid the problem of a wiring delay, the interlayer dielectric film  20  may also be a low-dielectric-constant film (low-k film) having a dielectric constant lower than that of a silicon oxide (SiO 2 ) film. As this low-dielectric-constant film, it is possible to use an organic low-dielectric-constant film made of an organic material, an SiOF film formed by doping fluorine in a silicon oxide (SiO 2 ) film, an SiOC film formed by doping a few % of carbon in a silicon oxide (SiO 2 ) film, a porous SiOC film, a porous organic film, or an SiCN film. It is also possible to combine two or more types of these films by stacking them.  
         [0065]     Contact holes are formed by removing predetermined regions of the interlayer dielectric film  20 . The bottom of the removed region of the interlayer dielectric film  20  can be a surface of the semiconductor substrate  10 , a conductive layer or a dielectric film formed on the substrate  10 , etc. After that, a tungsten film is formed by depositing tungsten (W) as a conductive material on the semiconductor substrate  10  and interlayer dielectric film  20  so as to fill the contact holes.  
         [0066]     This tungsten film is then planarized to form tungsten plugs  30  in the interlayer dielectric film  20 , as plugs which connect the surface of the semiconductor substrate  10  to an interconnecting layer. Note those plugs are not limited to the tungsten plugs  30 , and it is also possible to form polysilicon plugs, or other metal plugs such as titanium plugs. Alternatively, plugs containing at least one of tungsten and titanium may also be formed. When metal plugs made of tungsten or the like are to be formed, a barrier metal is desirably stacked as an underlying layer.  
         [0067]     For example, as a barrier metal of tungsten, titanium (Ti) and titanium nitride (TiN) can be used singly or in combination.  
         [0068]     By natural oxidation when or after the tungsten film is planarized, the upper surfaces of the tungsten plugs  30  oxidize to form tungsten oxide films  35  on them. The tungsten oxide films  35  are desirably removed because they increase the contact resistance.  
         [0069]     As shown in  FIG. 2 , the upper surfaces of the tungsten plugs  30  are processed by using a dilute aqueous choline solution prepared by diluting choline (2-hydroxyethyltrimethyl ammonium hydroxide) with pure water, thereby etching away the tungsten oxide films  35 . When this processing is performed using the dilute aqueous choline solution, deposits such as the resist residue can also be removed.  
         [0070]     This makes it possible to avoid the rise of the contact resistance, and thereby prevent variations in characteristics and increase the yield. Note that processing conditions for effectively removing the tungsten oxide films  35  will be explained later.  
         [0071]     As shown in  FIG. 3 , an interlayer dielectric film  40  made of, e.g., a silicon oxide (SiO 2 ) film is deposited on the interlayer dielectric film  20  and tungsten plugs  30 . Similar to the interlayer dielectric film  20 , the interlayer dielectric film  40  may also be a low-dielectric-constant film (low-k film) having a dielectric constant lower than that of a silicon oxide (SiO 2 ) film. As this low-dielectric-constant film, it is possible to use, e.g., an organic low-dielectric-constant film, SiOF film, SiOC film, porous SiOC film, porous organic film, or SiCN film. Two or more types of these films may also be combined by stacking them.  
         [0072]     As shown in  FIG. 4 , the interlayer dielectric film  40  is coated with a photoresist, and the photoresist is exposed and developed to form a resist mask  50  having a pattern which opens above the upper surfaces of the tungsten plugs  30 .  
         [0073]     As shown in  FIG. 5 , the resist mask  50  is used as a mask to etch away the interlayer dielectric film  40  to a depth on substantially the same level as the upper ends of the tungsten plugs  30 , thereby forming interconnection trenches  60  in the interlayer dielectric film  40 , and exposing the upper surfaces of the tungsten plugs  30 .  
         [0074]     As shown in  FIG. 6 , ashing is performed to remove the resist mask  50  by oxidation. During this ashing, the exposed upper surfaces of the tungsten plugs  30  oxidize to form tungsten oxide films  70  on them. The tungsten oxide films  70  are desirably removed because they increase the contact resistance.  
         [0075]     It is also possible to deposit a different film serving as a hard mask on the interlayer dielectric film  40  shown in  FIG. 4 , process this hard mask by using the resist mask  50  to once transfer the pattern of the resist mask  50  to the hard mask, and then remove the resist mask  50  by ashing or the like. In this case, the upper surfaces of the tungsten plugs  30  can be exposed by removing, by using the hard mask as a mask, the interlayer dielectric film  40  to the depth on substantially the same level as the upper ends of the tungsten plugs  30 . During this processing, tungsten oxide films  70  are formed by natural oxidation on the upper surfaces of the tungsten plugs  30 .  
         [0076]     As shown in  FIG. 7 , the tungsten oxide films  70  are etched away by processing the upper surfaces of the tungsten plugs  30  by using a dilute aqueous choline solution prepared by diluting choline with deionozed water.  
         [0077]     Methods of removing the tungsten oxide films  70  by using the dilute aqueous choline solution are as follows. In single wafer processing, the tungsten oxide films  70  are removed by spraying the dilute aqueous choline solution against the upper surfaces of the tungsten plugs  30 . In batch processing, the tungsten oxide films  70  are removed by dipping the semiconductor substrate  10  into the dilute aqueous choline solution.  
         [0078]     As processing conditions for effectively removing the tungsten oxide films  70 , the concentration of the dilute aqueous choline solution is desirably 0.01 to 10 wt %. Especially in single wafer processing, the concentration of the dilute aqueous choline solution is desirably 0.1 to 0.5 wt %, and the temperature is desirably 40° C. to 80° C. However, the temperature of the dilute aqueous choline solution need only be 20° C. to 100° C.  
         [0079]     For example, when processing is performed at a temperature of 80° C. for 90 sec by using a dilute aqueous choline solution having a concentration of 0.1 to 0.5 wt %, as shown in  FIG. 11 , the tungsten oxide films  70  can be removed by about 9 nm. Referring to  FIG. 11 , the abscissa indicates a position in the radial direction on a circular substrate surface 200 mm in diameter. On a line passing through the center of the substrate, one end point is position  1 , the center is position  11 , and the other end point is position  21 .  
         [0080]     That is, as shown in  FIG. 11 , the film thickness of the tungsten oxide films  70  was about 9 nm before processing was performed using the dilute aqueous choline solution, and about 0 nm after that.  
         [0081]     When the dilute aqueous choline solution is used as an etching solution, the tungsten oxide films  70  are etched more easily than the silicon oxide (SiO 2 ) film forming the interlayer dielectric film  40 , because the etching selectivity is high, i.e., the etching rate of the former are higher than those of the latter.  
         [0082]     Accordingly, when, for example, processing is performed at a temperature of 80° C. for 120 sec by using a dilute aqueous choline solution having a concentration of 0.1 to 0.5 wt %, the etching amount of the tungsten oxide films  70  is about 9 nm, whereas the etching amount of the interlayer dielectric film  40  can be decreased to 1 nm or less, as shown in  FIG. 12 , regardless of the type of the interlayer dielectric film  40 .  
         [0083]     More specifically, the etching amount of the interlayer dielectric film  40  is 0.198 nm when the interlayer dielectric film  40  is a silicon oxide (SiO 2 ) film, 0.031 nm when it is an organic low-dielectric-constant film, 0.027 nm when it is an SiOC film, 0.332 nm when it is a porous SiOC film, and 0.046 nm when it is an SiCN film.  
         [0084]     By contrast, when an organic fluoric chemical is used as an etching solution, the etching rate of the interlayer dielectric film increases, so the etching amount of the interlayer dielectric film also increases. Therefore, when, for example, processing is performed for 120 sec by using an organic fluoric chemical, the etching amount of the interlayer dielectric film is about 2 to 3 nm if it is a silicon oxide (SiO 2 ) film.  
         [0085]     When a dilute aqueous choline solution is used as an etching solution as described above, the tungsten oxide films  70  can be removed, and the etching amount of the interlayer dielectric film  40  can be reduced. Accordingly, the tungsten films  70  can be removed without increasing the width of the interconnection trenches  60  formed in the interlayer dielectric film  40 , i.e., without increasing the width of copper interconnections to be formed later.  
         [0086]     Note that in single wafer processing, hot water may also be sprayed together with the dilute aqueous choline solution when it is sprayed. In this case, the temperature of the hot water can be selected from room temperature to 100° C.  
         [0087]     Also, by adding a slight amount of hydrogen fluoride (HF) or a fluorine compound (e.g., ammonium fluoride (NH 4 F) or an organic fluorine compound) to the dilute aqueous choline solution, it is possible to remove those portions of the surfaces of the interlayer dielectric films  20  and  40 , which are modified by various processes such as the etching step and ashing step. It is also possible to perform the process using dilute HF simultaneously with the process using the dilute aqueous choline solution, or to sequentially perform these processes. In this case, the HF concentration is preferably 10 wt % or less, and particularly preferably, 0.01 to 0.1 wt %, in order to suppress etching of the interlayer dielectric films. When processes were actually sequentially performed for 30 sec by using HF having a concentration of about 0.05 wt % and for 30 sec by using an aqueous choline solution having a concentration of about 0.1 wt %, the resist residue was removed better.  
         [0088]     As shown in  FIG. 8 , a barrier metal film  80  and a seed copper (Cu) film  90  serving as a seed layer for plating are sequentially formed on all the surfaces of the interlayer dielectric films  20  and  40  by sputtering. After that, as shown in  FIG. 9 , a film mainly containing copper is formed on the entire surface by plating, thereby forming the barrier metal film  80  and a copper film  100 .  
         [0089]     As this barrier metal, tantalum (Ta), tantalum nitride (TaN), titanium (Ti), titanium nitride (TiN), and the like can be used singly or in combination.  
         [0090]     As shown in  FIG. 10 , the barrier metal film  80  and copper film  100  are polished by CMP to form copper interconnections  110 . In this manner, the copper interconnections  110  having the same width as the photomask can be formed, so the wiring resistance can be made equal to the design value. Also, the spacing between the adjacent copper interconnections  110  can be ensured, so a shortcircuit between them can be avoided. Note that metal interconnections are not limited to the copper interconnections  110 , and it is also possible to form metal interconnections by using a material containing at least one of aluminum (Al), tungsten, and copper, or by using another metal.  
         [0091]     Interconnections may also be formed instead of forming the plugs  30  on the semiconductor substrate  10 . Alternatively, both plugs and interconnections may also be formed on the semiconductor substrate  10 .  
         [0092]     In addition, instead of the copper interconnections  110 , plugs or both interconnections and plugs may also be formed.  
         [0093]     Furthermore, the material of the copper interconnections  110  or the plugs or both the plugs and interconnections formed instead of the copper interconnections is not limited to copper. That is, these interconnections and plugs can be formed by using a material containing at least one of metal materials such as tungsten, titanium, tantalum, and aluminum, or by using another film.  
       (2) Second Embodiment  
       [0094]     FIGS.  13  to  18  illustrate a semiconductor device fabrication method according to the second embodiment of the present invention. First, as shown in  FIG. 13 , an interlayer dielectric film  210  made of, e.g., a silicon oxide (SiO 2 ) film is formed on a semiconductor substrate  200 , and the surface of the interlayer dielectric film  210  is planarized by CMP or the like.  
         [0095]     Contact holes are formed by removing predetermined regions of the interlayer dielectric film  210 .  
         [0096]     The bottom of the removed region of the interlayer dielectric film  210  can be a surface of the semiconductor substrate  200 , a conductive layer or a dielectric film formed on the substrate  200 , etc. After that, a tungsten (W) film is deposited on the semiconductor substrate  200  and interlayer dielectric film  210  so as to fill the contact holes. This tungsten film is then planarized to form tungsten plugs  220  as contact plugs in the interlayer dielectric film  210 .  
         [0097]     As a barrier metal of tungsten, titanium (Ti), titanium nitride (TiN), and the like can be used singly or in combination.  
         [0098]     By natural oxidation when or after the tungsten film is planarized, the upper surfaces of the tungsten plugs  220  oxidize to form tungsten oxide films  230  on them. The tungsten oxide films  230  are desirably removed because they increase the contact resistance.  
         [0099]     As shown in  FIG. 14 , as in the first embodiment, the upper surfaces of the tungsten plugs  220  are processed by using a dilute aqueous choline solution prepared by diluting choline with deionozed water, thereby etching away the tungsten oxide films  230 . This makes it possible to avoid the rise of the contact resistance, and thereby prevent variations in characteristics and increase the yield. Note that processing conditions and the like for effectively removing the tungsten oxide films  230  are the same as in the first embodiment.  
         [0100]     As shown in  FIG. 15 , a barrier metal film  240  is formed on the interlayer dielectric film  210  and tungsten plugs  220  by sputtering. After that, an aluminum (Al) film  250  as an interconnecting material is formed on the barrier metal film  240 , and a barrier metal film  260  is formed on the aluminum (Al) film  250 .  
         [0101]     In the formation of the barrier metal films  240  and  260 , titanium (Ti), titanium nitride (TiN), and the like can be used singly or in combination.  
         [0102]     Note that the film of the interconnecting material formed on the interlayer dielectric film  210  and tungsten plugs  220  via the barrier metal film  240  is not limited to the aluminum (Al) film  250 , and it is also possible to form a film of various interconnecting materials such as tungsten. Note also that, of the barrier metal films  240  and  260  as the lower and upper layers, it is not particularly necessary to form the barrier metal film  260  as the upper layer.  
         [0103]     As shown in  FIG. 16 , the barrier metal film  260  is coated with a photoresist, and the photoresist is exposed and developed to form a resist mask  270  having a pattern corresponding to the tungsten plugs  220 .  
         [0104]     As shown in  FIG. 17 , the resist mask  270  is used as a mask to etch away desired regions of the barrier metal film  240 , aluminum (Al) film  250 , and barrier metal film  260 , thereby forming aluminum interconnections  290  on the tungsten plugs  220 .  
         [0105]     As shown in  FIG. 18 , ashing is performed to remove the resist mask  270  by oxidation. Furthermore, tungsten plugs and aluminum interconnections are sequentially formed on the aluminum interconnections  290  to stack aluminum interconnections, thereby forming multilayered interconnections.  
         [0106]     In this embodiment as described above, it is possible to avoid the rise of the contact resistance, and thereby prevent variations in characteristics and increase the yield.  
         [0107]     Although the interconnections  290  are formed on the upper surfaces of the plugs  220  in this embodiment, plugs may also be formed on the upper surfaces of the plugs, instead of the interconnections.  
       (3) Third Embodiment  
       [0108]     FIGS.  19  to  27  illustrate a semiconductor device fabrication method according to the third embodiment of the present invention. First, as shown in  FIG. 19 , an interlayer dielectric film  310  made of, e.g., a silicon oxide (SiO 2 ) film is formed on a semiconductor substrate  300 , and the surface of the interlayer dielectric film  310  is planarized by CMP or the like.  
         [0109]     A resist mask for forming contact holes is formed on the interlayer dielectric film  310 . This resist mask is used as a mask to etch away plug formation regions for forming plugs of the interlayer dielectric film  310 , thereby forming contact holes  315 . After that, the resist mask for contact hole formation is removed.  
         [0110]     Then, a resist mask for forming interconnection trenches is formed. After an etching time is designated, this resist mask is used to further etch away interconnection formation regions for forming interconnections of the interlayer dielectric film  310 , thereby removing the interlayer dielectric film  310  to a predetermined depth to form interconnection trenches  316 . After that, the resist mask for forming interconnection trenches is removed.  
         [0111]     The bottom of the removed interconnection trenches or plug formation region of the interlayer dielectric film  310  can be a surface of the semiconductor substrate  300 , a conductive layer or a dielectric film formed on the substrate  300 , etc.  
         [0112]     A barrier metal film  320  is formed on the inner surfaces of the contact holes  315  and interconnection trenches  316 , and a tungsten (W) film is deposited to bury the barrier metal film  320 . The barrier metal film  320  and tungsten film are then planarized to form tungsten plugs  330  as contact plugs and tungsten interconnections  340  in the interlayer dielectric film  310 .  
         [0113]     In the formation of the barrier metal film  320 , titanium (Ti), titanium nitride (TiN), and the like can be used singly or in combination.  
         [0114]     By natural oxidation when or after the tungsten film is planarized, the upper surfaces of the tungsten interconnections  340  oxidize to form tungsten oxide films  350  on them. The tungsten oxide films  350  are desirably removed because they increase the contact resistance.  
         [0115]     As shown in  FIG. 20 , the upper surfaces of the tungsten interconnections  340  are processed by using a dilute aqueous choline solution prepared by diluting choline with deionozed water, thereby etching away the tungsten oxide films  350 . This makes it possible to avoid the rise of the contact resistance, and thereby prevent variations in characteristics and increase the yield. Note that processing conditions and the like for effectively removing the tungsten oxide films  350  are the same as in the first embodiment.  
         [0116]     As shown in  FIG. 21 , an interlayer dielectric film  360  made of, e.g., a silicon oxide (SiO 2 ) film is deposited on the interlayer dielectric film  310 , barrier metal film  320 , and tungsten interconnections  340 . As shown in  FIG. 22 , the interlayer dielectric film  360  is coated with a photoresist, and the photoresist is exposed and developed to form a resist mask  370  having a pattern which opens above the upper surfaces of the tungsten interconnections  340 .  
         [0117]     In the formation of the interlayer dielectric film, it is also possible to use a low-dielectric-constant film such as an organic low-dielectric-constant film, SiOF film, SiOC film, porous SiOC film, SiCN film, or porous organic film. It is also possible to combine two or more types of these films by stacking them.  
         [0118]     As shown in  FIG. 23 , the resist mask  370  is used as a mask to etch away the interlayer dielectric film  360  to a depth on substantially the same level as the upper ends of the tungsten interconnections  340 , thereby forming contact holes  380  in the interlayer dielectric film  360 , and partially exposing the upper surfaces of the tungsten interconnections  340 .  
         [0119]     As shown in  FIG. 24 , ashing for removing the resist mask  370  by oxidation is performed. During this ashing, the exposed upper surfaces of the tungsten interconnections  340  oxidize to form tungsten oxide films  390  on portions of the upper surfaces of the tungsten interconnections  340 . The tungsten oxide films  390  are desirably removed because they increase the contact resistance.  
         [0120]     As shown in  FIG. 25 , the upper surfaces of the tungsten interconnections  340  are processed by using a dilute aqueous choline solution prepared by diluting choline with deionozed water, thereby etching away the tungsten oxide films  390 . Note that practical processing conditions and the like for effectively removing the tungsten oxide films  390  are the same as in the first embodiment.  
         [0121]     When the dilute aqueous choline solution is used as an etching solution, it is possible to remove the tungsten oxide films  390 , and, as in the first embodiment, reduce the etching amount of the interlayer dielectric film  360 . Accordingly, the tungsten oxide films  390  can be removed without increasing the width of the contact holes  380  formed in the interlayer dielectric film  360 , i.e., the width of tungsten plugs to be formed later.  
         [0122]     As shown in  FIG. 26 , a barrier metal film  400  is formed on the interlayer dielectric film  360  and tungsten interconnections  340  by sputtering, and a tungsten film  410  is formed on the entire surface by CVD.  
         [0123]     As shown in  FIG. 27 , the barrier metal film  400  and tungsten film  410  are polished by CMP to form tungsten plugs  420 . This makes it possible to form the tungsten plugs  420  having a width corresponding to the photomask, and prevent variations in characteristics.  
       (4) Other Embodiments  
       [0124]     Note that the above embodiments are merely examples, and do not limit the present invention. For example, when the tungsten plugs  30  or  220  or the tungsten interconnections  340  are processed by using a dilute aqueous choline solution having a concentration of 0.1 to 0.5 wt %, the temperature is desirably 20° C. to 100° C., and can be freely selected where necessary.  
         [0125]     It is also possible to add, to the dilute aqueous choline solution, a slight amount of HF or a fluorine compound, a surfactant for improving the wettability, an organic solvent for improving the resist removability, and the like.  
         [0126]     The semiconductor device fabrication methods of the above embodiments can prevent variations in characteristics and increase the yield.