Abstract:
A method for forming an interlayer insulating film is disclosed. This method comprises the steps of: forming an underlying insulating film on an object to be formed; and forming a porous SiO 2  film on said underlying insulating film by a Chemical Vapor Deposition that employs a source gas containing TEOS (tetraethoxy silane) and O 3  where the O 3  is contained in the source gas with first concentration that is lower than concentration necessary for oxidizing the TEOS. 
     Alternative method for forming an interlayer insulating film is also disclosed. This method comprises the step of: forming an underlying insulating film on an object to be formed; performing Cl (chlorine) plasma treatment for the underlying insulating film; and forming a porous SiO 2  film on the underlying insulating film by a Chemical Vapor Deposition that employs a source gas containing TEOS (tetraethoxy silane) and O 3 .

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for forming an interlayer insulating film and, more particularly, to a method for forming an interlayer insulating film having a low dielectric constant, which is necessary for a highly-integrated semiconductor device. A progress in high integration regarding the semiconductor device in recent years has resulted in a narrower interval between wiring lines. As the narrowed interval between the wiring lines causes an increase in capacitance between the wiring lines, a request has been made for formation of an interlayer insulating film, which has a low dielectric constant. 
     With recent progresses in high integration of an LSI device, the wiring line has been micronized and multilayered. There has also been an increase in capacitance between the wiring lines. Such an increase in capacitance has caused a great reduction, in an operating speed. Thus, improvement in this regard has been strongly demanded. As one of improvement measures, a method for reducing capacitance between the wiring lines has been studied. This method uses an interlayer insulating film, which has a dielectric constant lower than that of SiO 2  currently used for an interlayer insulating film. 
     Typical interlayer insulating films of low dielectric constants currently under study are {circumflex over (1)} an SiOF film, and {circumflex over (2)} an organic insulating film of a low dielectric constant. Description will now be made of these films. 
     {circle around (1)} SiOF Film 
     An SiOF film is formed by using source gas containing F and substituting Si—F bond for a portion of Si—O bond in SiO 2 . This SiOF film has a relative dielectric constant, which is monotonically reduced as concentration of F in the film increases. 
     For forming such SiOF films, several methods have been reported (see p.82 of monthly periodical “Semiconductor World”, February issue of 1996). Most promising among these methods is one for forming an SiOF film by using SiH 4 , O 2 , Ar and SiF 4  as source gases, and by a high-density plasma enhanced CVD method (HDPCVD method). A relative dielectric constant of an SiOF film formed by this method is in a range of 3.1 to 4.0 (varies depending on F concentration in the film). This value is lower than a relative dielectric constant 4.0 of SiO 2 , which has conventionally been used for the interlayer insulating film. 
     {circle around (2)} Organic Insulating Film of Low Dielectric Constant 
     As an insulating film which has a lower dielectric constant (3.0 or lower) compared with the SiOF film, an organic insulating film of a low dielectric constant is now a focus of attention. Table 1 shows a few organic insulating films of low dielectric constants, which have been reported, and respective relative dielectric constants and thermal decomposition temperatures thereof. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Relative 
                 Thermal 
                   
               
               
                 Organic 
                 Dielectric 
                 Decomposition 
               
               
                 Insulating Film 
                 Constant 
                 Temperature (° C.) 
                 Note 
               
               
                   
               
             
             
               
                 Fluorine- 
                 2.4 
                 420 
                 p. 82 of monthly 
               
               
                 containing resin 
                   
                   
                 periodical 
               
               
                   
                   
                   
                 “Semiconductor 
               
               
                   
                   
                   
                 World”, February 
               
               
                   
                   
                   
                 issue of 1997 
               
               
                 Cytop 
                 2.1 
                 400 
                 p. 90 of monthly 
               
               
                   
                   
                   
                 periodical 
               
               
                   
                   
                   
                 “Semiconductor 
               
               
                   
                   
                   
                 World”, February 
               
               
                   
                   
                   
                 issue of 1996 
               
               
                 Amorphous telon 
                 1.9 
                 400 
                 p. 91 of 
               
               
                   
                   
                   
                 monthly 
               
               
                   
                   
                   
                 periodical 
               
               
                   
                   
                   
                 “Semiconductor 
               
               
                   
                   
                   
                 World”, 
               
               
                   
                   
                   
                 February issue 
               
               
                   
                   
                   
                 of 1996 
               
               
                   
               
             
          
         
       
     
     However, the SiOF film is disadvantageous in that an increase in concentration of F in the film leads to a reduction in moisture absorption resistance. The reduced moisture absorption resistance poses a serious problem, because a transistor characteristic and adhesion of an upper barrier metal layer are affected. 
     Peeling-off easily occurs in the organic insulating film of a low dielectric constant, because of bad adhesion with a silicon wafer or the SiO 2  film. Furthermore, the organic insulating film is disadvantageous in that heat resistivity is low since a thermal decomposition temperature is around 400° C. The disadvantage of low heat resistivity poses a problem for annealing a wafer at a high temperature. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for forming an interlayer insulating film of a low dielectric constant, which has good moisture absorption resistance and heat resistivity. It is another object of the invention to provide a semiconductor device, which employs the above method. 
     In accordance with the method of the invention for forming an interlayer insulating film, first, porous SiO 2  film is formed on an object to be formed. This porous SiO 2  film is formed by using a Chemical Vapor Deposition method which employs source gases containing TEOS (tetraethoxy silane) and O 3 , where the concentration of the O 3  is lower than that necessary for oxidizing the TEOS. Accordingly, many voids are formed in the film. In other words, porosity is provided for the SiO 2  film formed in this manner. 
     Therefore, a dielectric constant of the porous SiO 2  film is smaller than that of a usual SiO 2  film having no porosity. 
     In addition, a SiO 2  film is formed on the porous SiO 2  film. This SiO 2  film is formed by a Chemical Vapor Deposition method which employs source gases containing TEOS and O 3  where the concentration of the O 3  is sufficient for oxidizing the TEOS. Accordingly, the SiO 2  film fore in this manner becomes a dense SiO 2  film that contains no CH and OH radicals. 
     Therefore, since the SiO 2  film formed on the porous SiO 2  film is dense, incursion of moisture into the porous SiO 2  film can be prevented, and an interlayer insulating film having good moisture resistance can be formed. 
     Furthermore, since these SiO 2  films consist mainly of Si and O, these films are expected to show better heat resistivity compared to the organic insulating films of the prior art. 
     Secondly, in accordance with the method of the present invention for forming an interlayer insulating film, Cl (chlorine) plasma treatment is performed for the object to be formed. Accordingly, Cl (chlorine) atoms are left on some portions of the surface of the object to be formed. Subsequently, an porous SiO 2  film is formed on the object to be formed by a Chemical Vapor Deposition method which contains TEOS and O 3  as source gases. At this time, the growth of the SiO 2  film is prevented on some portions of the surface on which the Cl (chlorine) atoms have been left. Accordingly, many voids are, formed in the SiO 2  film. In other words, porosity is provided for this SiO 2  film formed in this manner. 
     Therefore, a dielectric constant of the porous SiO 2  film is smaller than that of a usual SiO 2  film having no porosity. 
     Furthermore, since the porous SiO 2  film consists mainly of Si and O, heat resistivity of the film is expected to show better heat resistively compared to the organic insulating films of the prior art. 
     Thirdly, in accordance with the method of the present invention for forming an interlayer insulating film, a first insulating film is formed on the porous SiO 2  film, which has been formed on the object to be formed, the object having been subjected to the Cl (chlorine) plasma treatment. Then, after the first insulating film is etched to be planarized, a cover insulating film is formed thereon. 
     In other words, by the cover insulating film, incursion of moisture into the porous SiO 2  film can be prevented. Therefore, it is possible to form an interlayer insulating film, which has a planarized surface and good moisture absorption resistance and heat resistivity. 
     Furthermore, the method for forming the foregoing porous SiO 2  film can be applied to a damascene process. According to the damascene process, a Cu (copper) wiring layer having small electric resistance can be formed. By combining the Cu (copper) wiring layer with the foregoing porous SiO 2  film, it is possible to provide a semiconductor device where a parasitic capacitance of a wiring line is small, and a data processing speed is fast. 
     Fourthly, in accordance with the method of the present invention for forming an interlayer insulating film, after formation of the foregoing porous SiO 2  film, H (hydrogen) plasma treatment is performed. Accordingly, an Si—H bond is substituted for a dangling bond of Si in an Si—O bond in the surface of the void, and the surface of the void can be made stable. 
     Therefore, incursion of moisture from the surface of the void can be prevented, and it is possible to form an interlayer insulating film which has good moisture absorption resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 G are cross-sectional views, each of which shows a method for forming an interlayer insulating film according to a first embodiment of the present invention; 
     FIGS. 2A to  2 L are cross-sectional views, each of which shows a method for forming an interlayer insulating film according to a second embodiment of the invention; 
     FIGS. 3A to  3 I are cross-sectional views, each of which shows a method for forming an interlayer insulating film according to a third embodiment of the invention; and 
     FIGS. 4A to  4 N are cross-sectional views, each of which shows a method for forming an interlayer insulating film according to a fourth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, description will be made of the preferred embodiments of the present invention with reference to the accompanying drawings. 
     First Embodiment 
     FIGS. 1A to  1 G are cross-sectional views, each of which illustrates a first embodiment of the present invention. 
     First, as shown in FIG. 1A, a BPSG (borophosphosilicate glass) film  102  is formed on a silicon substrate  101 . Then, after an aluminum film is formed on the BPSG film  102 , an aluminum Wring layer  103  is formed by patterning the aluminum film. The silicon substrate  101 , the BPSG film  102  and the aluminum wiring layer  103  formed in this manner constitute an object  104  to be formed. 
     Then, as shown in FIG. 1B, an SiO 2  film  105  (underlying insulating film) is formed on the object  104  to be formed. This SiO 2  film  105  is formed by a plasma enhanced CVD method (plasma enhanced Chemical Vapor Deposition method), and SiH 4  and N 2 O are used as source gases. A film thickness of this SiO 2  film  105  is 100 nm. 
     Subsequently, as shown in FIG. 1C, a porous SiO 2  film  106  is formed on the SiO 2  film  105  (underlying insulating film). This porous SiO 2  film  106  is formed by an atmospheric CVD method (atmospheric Chemical Vapor Deposition method). TEOS (tetraethoxy silane), O 3  of low concentration, and O 2  are contained in the source gas for the CVD method. Here, the O 3  of low concentration is defined as the O 3  having concentration that is lower than that necessary for oxidizing the TEOS. Specifically, the flow rate of the TEOS is 25 sccm and that of O 2  is 7.5 slm. And O 3  of 1-2% by flow rate ratio is contained in the O 2 . 
     Furthermore, N 2  (nitrogen) with flow rate 1˜3 slm is also contained in the source gas. And the temperature of the silicon substrate  101  is maintained at 400° C. during the formation of SiO 2  film  106 . 
     Generally, in the case of the atmospheric CVD method which uses TEOS and O 3  as source gases, the following has been discovered for an SiO 2  film thereby formed. That is, as concentration of O 3  in the source gas is increased, oxidation of TEOS progresses faster on a wafer to form an SiO 2  film having flowability. Conversely, as concentration of O 3  is decreased, oxidation of TEOS is insufficient. Accordingly, if concentration of O 3  is low, many CH or OH radicals are left in an SiO 2  film formed on the wafer. Especially, if an underlying film is an SiO 2  film, an abnormal growth of an SiO 2  film having a rough surface occurs by employing O 3  of low concentration and TEOS. 
     The porous SiO 2  film  106  is formed by utilizing the aforementioned abnormal growth of the SiO 2  film, and many voids are formed in the film. 
     Then, as shown in FIG. 1D, H (hydrogen) plasma treatment is performed for the porous SiO 2  film  106 . 
     This H plasma treatment is performed by supplying H 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and power of 50 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. Further, during undergoing the H plasma treatment, the pressure in the chamber is 0.1˜10.2 Torr and the temperature of the silicon substrate  101  is maintained at 400° C. Still further, the time for the H plasma treatment is 60 sec. 
     The H plasma treatment substitutes Si—H bonds for dangling bonds of Si in an Si—O bond in the surface of the void. Therefore, OH radicals and water are made to be hard to bond to the dangling bonds of Si, which improves the moisture absorption resistance of the film. 
     Then, as shown in FIG. 1E, an SiO 2  film  107  is formed on the porous SiO 2  film  106 . This SiO 2  film  107  is formed by an atmospheric CVD method, for which the source gas containing O 2 , O 3 , and TEOS are used. At this time, a flow rate of TEOS is 25 sccm and that of O 2  is 7.5 slm. Further, O 2  contains O 3  of 5˜6% by flow rate ratio, which is sufficient for oxidizing the TEOS. Accordingly, as described above, the SiO 2  film  107  has flowability. Thus, even if the SiO 2  film  106  formed below has convexity and concavity in the surface, the SiO 2  film  107  is formed to have a nearly smooth surface shape, and self-planarizing is carried out. 
     Furthermore, N 2  (nitrogen) with flow rate 1˜3 is also contained in the source gas. And the temperature of the silicon substrate  101  is maintained at 400° C. during the formation of SiO 2  film  107 . 
     Subsequently, as shown in FIG. 1F, the SiO 2  film  107  and the porous SiO 2  film  106  formed above a convexity  103   a  of the aluminum wiring layer are polished to be planarized by a CMP method (Chemical Mechanical Polishing method). After completing the polishing, the SiO 2  film  105  (underlying insulating film) formed on the convexity  103   a  of the aluminum wiring layer and the porous SiO 2  film  106  formed in a concavity  103   b  of the same are exposed on the surfaces. 
     Then, as shown in FIG. 1G, an SiO 2  film  108  (cover insulating film) is formed on the SiO 2  film  105  (underlying insulating film) formed on the convexity  103   a  of the aluminum wiring layer and on the porous SiO 2  film  106  formed in the concavity  103   b  of the same. This SiO 2  film  108  is formed by the plasma enhanced CVD method. Sources gases used at this time are SiH 4  and N 2 O, and a film thickness of the SiO 2  film  108  is 100 nm. 
     The foregoing process of forming the SiO 2  films  105  (underlying insulating film),  106  and  108  (cover insulating film) results in formation, on the object  104  to be formed, of an interlayer insulating film of a low dielectric constant, which has good heat resistivity and moisture absorption resistance. That is, the SiO 2  film  106  has porosity, a dielectric constant thereof is 2.0 to 3.0. This value is smaller than a dielectric constant 4.0 of a usual SiO 2  film. Also, since the usual SiO 2  film  108  is formed on the porous SiO 2  film  106 , incursion of moisture into the SiO 2  film  106  can be prevented. 
     Further, the H plasma treatment for the SiO 2  film  106  can improve the moisture absorption resistance of the film  106 . 
     Still further, the SiO 2  films  105 ,  106  and  108  have better heat resistivity compared to the organic insulating film of the prior art, because these films consist mainly of Si and O. 
     Second Embodiment 
     FIGS. 2A to  2 L are cross-sectional views, each of which illustrates a second embodiment. 
     The second embodiment is a case of applying the first embodiment to a damascene process. 
     First, as shown in FIG. 2A, a BPSG (borophosphosilicate glass) film  202  is formed on a silicon substrate  201 . After an aluminum layer is formed on the BPSG film  202 , an aluminum wiring layer  203  is formed by patterning the aluminum layer. Then, the silicon substrate  201 , the BPSG film  202  and the aluminum wiring layer  203  constitute an object  204  to be formed. 
     Subsequently, as shown in FIG. 2B, an SiO 2  film  205  (underlying insulating film) having a film thickness of 100 nm is formed on the aluminum wiring layer  203 . This SiO 2  film  205  is formed by a plasma enhanced CVD method (plasma enhanced Chemical Vapor Deposition method), and SiH 4  and N 2 O are used as source gases. 
     Then, as shown in FIG. 2C, an SiO 2  film  206  having a film thickness of 500 nm is formed on the SiO 2  film  205  (underlying insulating film). This SiO 2  film  206  is formed by an atmospheric CVD method (atmospheric Chemical Vapor Deposition method) for which the source gas containing O 2 , O 3  of low concentration, and TEOS (tetraethoxy silane) are used. 
     Here, the O 3  of low concentration is defined as the O 3  having concentration that is lower than that necessary for oxidizing the TEOS. Specifically, the flow rate of the TEOS is 25 sccm and that of O 2  is 7.5 slm. And O 3  of 1-2% by flow rate ratio is contained in the O 2 . 
     As described above in the first embodiment, since O 3  of low concentration is used, the SiO 2  film  206  is provided with porosity. Therefore, many voids are formed in the SiO 2  film  206 . 
     It should be noted that N 2  (nitrogen) with flow rate 1˜3 slm is also contained in the source gas. And the temperature of the silicon substrate  201  is maintained at 400° C. during the formation of SiO 2  film  206 . 
     Subsequently, as shown in FIG. 2D, H (hydrogen) plasma treatment is performed for the SiO 2  film  206 . The process condition for the H plasma treatment is the same as explained in the first embodiment. Namely, it is performed by supplying H 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and power of 50 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. Further, during undergoing the H plasma treatment, the pressure in the chamber is 0.1˜0.2 Torr and the temperature of the silicon substrate  201  is maintained at 400° C. Still further, the time for the H plasma treatment is 60 sec. 
     The H plasma treatment substitutes Si—H bonds for dangling bonds of Si in an Si—O bond in the surface of the void. Therefore, OH radicals and water are made to be hard to bond to the dangling bonds of Si, which improves the moisture absorption resistance of the film. 
     Subsequently, as shown in FIG. 2E, patterning is performed for the SiO 2  film  205  and  206  to form a damascene trench  207 . This damascene trench  207  reaches the aluminum wiring layer  203  formed below the SiO 2  film  206 . 
     Then, as shown in FIG. 2F, an SiO 2  film  208  (second insulating film) is formed on the SiO 2  film  206  and on the side and bottom portions of the damascene trench  207 . This SiO 2  film  208  is formed by a plasma enhanced CVD method, and SiH 4  and N 2 O are used as source gases. By the SiO 2  film  208  formed on the side portion of the damascene trench  207 , Cu buried later in the damascene trench  207  can be prevented from being dispersed inside the porous SiO 2  film  206 . 
     Then, as is shown in FIG. 2G, anisotropic etching is performed for the SiO 2  film  208  (second insulating film). While this etching eliminates the SiO 2  film  208  formed on the bottom portion of the damascene trench  207 , the SiO 2  film  208  formed on the side portion of the damascene trench  207  is not eliminated in this etching. The remaining SiO 2  film  208  constitutes a sidewall insulating film on the side portion of the damascene trench  207 . 
     Subsequently, as shown in FIG. 2H, a Cu (copper)-plated film  209  is formed in the damascene trench  207  and on the SiO 2  film  206 . The Cu-plated film  209  formed in the damascene trench  207  is used as a Cu wiring line. 
     Then, as shown in FIG. 2I, the Cu-plated film  209  formed on the SiO 2  film  206  is polished and eliminated by a CMP method (Chemical Mechanical Polishing method). Accordingly, the Cu-plated film remains only in the damascene trench  207 . 
     Subsequently, as shown in FIG. 2J, a barrier metal TiN film  210  is formed above the damascene trench  207 . Accordingly, Cu in the damascene trench  207  can be prevented from being dispersed in an SiO 2  film formed later above the damascene trench  207 . 
     Then, as shown in FIG. 2K, patterning is performed to leave a TiN film  210   a  formed above the damascene trench  207 , and the TiN film  210  formed in the other portions is etched to be eliminated. 
     Subsequently, as shown in FIG. 2L, an SiO 2  film  211  (cover insulating film) is formed on the SiO 2  film  206  and the TiN film  210   a . This SiO 2  film  211  is formed by a plasma enhanced CVD method, and SiH 4  and N 2 O are used as source gases. 
     The foregoing process results in formation, on the object  204  to be formed, of an interlayer insulating film of a low dielectric constant, which has good heat resistivity and moisture absorption resistance. That is, the SiO 2  film  206  has porosity, and a dielectric constant thereof is 2.0 to 3.0. This value is smiler than a dielectric constant 4.0 of a usual SiO 2  film. Also, since the usual SiO 2  film  211  (cover insulating film) is formed on the porous SiO 2  film  206 , incursion of moisture into the SiO 2  film  206  can be prevented. 
     Further, the H plasma treatment for the SiO 2  film  206  can improve the moisture absorption resistance of the film  206 . 
     Still further, the, SiO 2  films  206  and  211  have better heat resistivity compared to the organic insulating film of the prior art, because these films consist mainly of Si and O. 
     Third Embodiment 
     FIGS. 3A to  3 I are cross-sectional views, each of which illustrates a third embodiment. 
     First, as shown in FIG. 3A, a BPSG (borophosphosilicate glass) film  302  is formed on a silicon substrate  301 . Then, after an aluminum film is formed on the BPSG film  302 , patterning is performed for the same to form an aluminum wiring layer  303 . The silicon substrate  301 , the BPSG film  302  and the aluminum wiring layer  303  formed in this manner constitute an object  304  to be formed. 
     Then, as shown in FIG. 3B, an SiO 2  film  305  (underlying insulating film) is formed on the object  304  to be formed. This SiO 2  film  305  is formed by a plasma enhanced CVD method (plasma enhanced Chemical Vapor Deposition method), and SiH 4  and N 2 O are used as source gases. A film thickness of the SiO 2  film  305  is 100 nm. 
     Subsequently, as shown in FIG. 3C, Cl (chlorine) plasma treatment is performed for the SiO 2  film  305  (underlying insulating film). 
     This Cl plasma treatment is performed by supplying Cl 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and power of 100 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. During undergoing the Cl plasma treatment the pressure in the chamber is about 0.2 Torr and the temperature of the silicon substrate  301  is maintained at 400° C. 
     This Cl plasma treatment leaves Cl (chlorine) atoms on some portions of the surface of the SiO 2  film  305 . 
     Then, as shown in FIG. 3D, an SiO 2  film  306  having a film thickness of 500 nm is formed on the SiO 2  film  305  (underlying insulating film) which has been subjected to the Cl (chlorine) plasma treatment. This SiO 2  film  306  is formed by an atmospheric CVD method (atmospheric Chemical Vapor Deposition method) for which the source gas containing O 2 , O 3 , and TEOS (tetraethoxy silane) are used. The flow rate of the TEOS is 25 sccm and that of O 2  is 7.5 slm. And O 3  of 4-6% by flow rate ratio is contained in the O 2 . Further, the source gas contains N 2  (nitrogen) of flow rate 1-3 slm. Still further, during the formation of the SiO 2  film  306  the temperature of the silicon substrate  301  is maintained at 400° C. 
     At this time, the SiO 2  film  306  is prevented from being grown on the portion of the surface of the SiO 2  film  305  where Cl (chlorine) has been left. Accordingly, many voids are formed in the SiO 2  film  306  to provide porosity for the same. 
     Subsequently, as shown in FIG. 3E, H (hydrogen) plasma treatment is performed for the porous SiO 2  film  306 . 
     The process condition for the H plasma treatment is the same as explained in the first and second embodiment. Namely, it is performed by supplying H 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and, power of 50 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. Further, during undergoing the H plasma treatment, the pressure in the chamber is 0.1˜0.2 Torr and the temperature of the silicon substrate  301  is maintained at 400° C. Still further, the time for the H plasma treatment is 60 sec. 
     The H plasma treatment substitutes Si—H bonds for dangling bonds of Si in an Si—O bond in the surface of the void. Therefore, OH radicals and water are made to be hard to bond to the dangling bonds of Si, which improves the moisture absorption resistance of the film. 
     Subsequently, as shown in FIG. 3F, an SiO 2  film  307  is formed on the porous SiO 2  film  306 . This SiO 2  film  307  is formed by a plasma enhanced CVD method. 
     Then, as shown in FIG. 3G, an SiO 2  film  308  (first insulating film) having a film thickness of 200 nm is formed on the SiO 2  film  307 . This SiO 2  film  308  is formed by an atmospheric CVD method, for which the source gas containing O 2 , O 3 , and TEOS are used. Since the concentration of O 3  in the source gas at this time is higher than usual, flowability is provided for the SiO 2  film  308 . Accordingly, even if the surface of the SiO 2  film  307  formed below has convexity and concavity, the SiO 2  film  308  is formed to have a nearly planarized surface, and self-planarizing is carried out. 
     In this case, by the previously formed SiO 2  film  307 , the SiO 2  film  308  having flowability can be prevented from entering the void of the porous SiO 2  film  306 . 
     Subsequently, as shown in FIG. 3H, in order for planarizing the surface, etching is performed for the SiO 2  films  307  and  308  (first insulating film). This etching should be carried out not to result in complete elimination of the SiO 2  film  308 . 
     Then, as shown in FIG. 3I, an SiO 2  film  309  (cover insulating film) is formed on the remaining SiO 2  films  307  and  308  (first insulating film), i.e., the portions of the films remaining without being eliminated by etching. This SiO 2  film  309  is formed by a plasma enhanced CVD method, and a film thickness thereof is 100 nm. 
     The foregoing process of forming the SiO 2  films  305  (underlying insulating film),  306 ,  307 ,  308  (first insulating film) and  309  (cover insulating film) results in formation, on the object  304  to be formed, an interlayer insulating film of a low dielectric constant, which has good heat resistivity and moisture absorption resistance. That is, the SiO 2  film  306  has porosity, and a dielectric constant thereof is 2.0 to 3.0. This value is smaller than a dielectric constant 4.0 of a usual SiO 2  film. 
     Further, the H plasma treatment for the SiO 2  film  306  can improve the moisture absorption resistance of the film  306 . 
     Also, since the usual SiO 2  films  307 ,  308  and  309  are formed on the porous SiO 2  film  306 , incursion of moisture into the SiO 2  film  306  can be prevented. 
     Moreover, the SiO 2  films  305 ,  306 ,  307 ,  308 , and  309  have better heat resistivity compared to the organic insulating film of the prior art, because these films consist mainly of Si and O. 
     Fourth Embodiment 
     A fourth embodiment is a case of applying the third embodiment to a damascene process. 
     FIGS. 4A to  4 N are cross-sectional views, each of which illustrates the fourth embodiment. 
     First, as shown in FIG. 4A, a BPSG (borophosphosilicate glass) film  402  is formed on a silicon substrate  401 . Then, after an aluminum layer is formed on the BPSG film  402 , patterning is performed for the aluminum layer to form an aluminum wiring layer  403 . The silicon substrate  401 , the BPSG film  402  and the aluminum wiring layer  403  constitute an, object  404  to be formed. 
     Subsequently, as shown in FIG. 4B, an SiO 2  film  405  (underlying insulating film) having a film thickness of 100 nm is formed on the aluminum wiring layer  403 . This SiO 2  film  405  is formed by a plasma enhanced CVD method (plasma enhanced Chemical Vapor Deposition method), and SiH 4  and N 2 O are used as source gases. 
     Then, as shown in FIG. 4C, Cl (chlorine) plasma treatment is performed for the SiO 2  film  405  (underlying insulating film). 
     This Cl plasma treatment is performed by supplying Cl 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and power of 100 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. During undergoing the cl plasma treatment the pressure in the chamber is about 0.2 Torr and the temperature of the silicon substrate  401  is maintained at 400° C. 
     This Cl plasma treatment leaves Cl (chlorine) atoms on some portions of the surface of the SiO 2  film  405 . 
     Then, as shown in FIG. 4D, an SiO 2  film  406  having a film thickness of 500 nm is formed on the SiO 2  film  405  (underlying insulating film) which has been subjected to the Cl (chlorine) plasma treatment. This SiO 2  film  406  is formed by an atmospheric CVD method (atmospheric Chemical Vapor Deposition method), for which the source gas containing O 2 , O 3 , and TEOS (tetraethoxy silane) are used. The flow rate of the TEOS is 25 sccm and that of O 2  is 7.5 slm. And O 3  of 4-6% by flow rate ratio is contained in the O 2 . Further, the source gas contains N 2  (nitrogen) of flow rate 1-3 slm. Still further, during the formation of the SiO 2  film  406  the temperature of the silicon substrate  401  is maintained at 400° C. 
     At this time, the SiO 2  film  406  is prevented from being grow on the portions of the surface of the SiO 2  film  405  where Cl (chlorine) has been left. Accordingly, many voids are formed in the SiO 2  film  406 , and porosity is provided for the SiO 2  film  406 . 
     Then, as shown in FIG. 4E, H (hydrogen) plasma treatment is performed for the porous SiO 2  film  406 . 
     The process condition for the H plasma treatment is the same as explained in the first to third embodiment. Namely, it is performed by supplying H 2  of 600 sccm to a chamber (not shown) and applying RF power to upper and lower electrodes (not shown) that is opposing each other in the chamber. And the RF power applied to the upper electrode has frequency of 13.56 MHz and power of 50 W. On the other hand, the RF power applied to the lower electrode has frequency of 400 kHz and power of 400 W. Further, during undergoing the H plasma treatment, the pressure in the chamber is 0.1˜0.2 Torr and the temperature of the silicon substrate  401  is maintained at 400° C. Still further, the time for the H plasma treatment is 60 sec. 
     The H plasma treatment substitutes Si—H bonds for dangling bonds of Si in an Si—O bond in the surface of the void. Therefore, OH radicals and water are made to be hard to bond to the dangling bonds of Si, which improves the moisture absorption resistance of the film. 
     Then, as shown in FIG. 4F, an SiO 2  film  407  is formed on the SiO 2  film  406 . This SiO 2  film  407  is formed by a plasma enhanced CVD method, and SiH 4  and N 2 O are used as source gases. By this SiO 2  film  407 , Cu of a Cu-plated film formed later on the SiO 2  film  407  can be prevented from being dispersed in the porous SiO 2  film  406 . 
     Subsequently, as shown in FIG. 4G, patterning is performed for the SiO 2  films  405  (underlying insulating film),  406  and  407  to form a damascene trench  408 . This damascene trench  408  reaches the aluminum wiring layer  403  formed below the SiO 2  film  405 . 
     Then, as shown in FIG. 4H, an SiO 2  film  409  (second insulating film) is formed on the SiO 2  film  407  and on the side and bottom portions of the damascene trench  408 . This SiO 2  film  409  is formed by a plasma enhanced CVD method. By the SiO 2  film  409  formed on the side portion of the damascene trench  408 , Cu buried later in the damascene trench  408  can be prevented from being dispersed in the porous SiO 2  film  406 . 
     Then, as shown in FIG. 4I, anisotropic etching is performed for the SiO 2  film  409  (second insulating film). Accordingly, the SiO 2  film  409  is eliminated except for the portion formed on the side portion of the damascene trench  408 , and a contact hole reaching the aluminum wiring layer  403  is formed in the lower portion of the damascene trench  408 . And the SiO 2  film  409  remaining on the side portion of the damascene trench  408  constitutes a sidewall insulating film. The SiO 2  film  407  is not eliminated by this etching and is left on the porous SiO 2  film  406 . 
     Subsequently, as shown in FIG. 4J, a Cu-plated film  410  is formed in the damascene trench  408  and on the SiO 2  film  407 . The Cu-plated film  410  formed in the damascene trench  408  is used as a Cu wiring line. 
     Then, as shown in FIG. 4K, the Cu-plated film  410  formed on the SiO 2  film  407  is polished and eliminated by a CMP method. Accordingly, the Cu-plated film  410  remains only in the damascene trench  408 . 
     Subsequently, as shown in FIG. 4L, a barrier metal TiN film  411  is formed above the damascene trench  408 . Accordingly, Cu in the damascene trench  408  can be prevented from being dispersed in an SiO 2  film later formed above the same. 
     Then, as shown in FIG. 4M, patterning is performed to leave a TiN film  411   a  formed above the damascene trench  408 , and the TiN film  411  formed in the other portions is etched to be eliminated. 
     Subsequently, as shown in FIG. 4N, an SiO 2  film  412  (cover insulating film) is formed on the SiO 2  film  407  and the TiN film  411   a . This SiO 2  film  412  is formed by a plasma enhanced CVD method, and SiH 4  and N 2 O are used as source gases. 
     The foregoing process results in formation, on the object  404  to be formed, an interlayer insulating film of a low dielectric constant, which has good heat resistivity and moisture absorption resistance. That is, the SiO 2  film  406  has porosity, and a dielectric constant thereof is 2.0 to 3.0. This value is smaller than a dielectric constant 4.0 of a usual SiO 2  film. 
     Further, the H plasma treatment for the SiO 2  film  406  can improve the moisture absorption resistance of the film  406 . 
     Also, since the usual SiO 2  films  407  and  412  (cover insulating film) are formed on the porous SiO 2  film  406 , incursion of moisture into the SiO 2  film  406  can be prevented. 
     Moreover, the SiO 2  films  406 ,  407 , and  412  have better heat resistivity compared to the organic insulating film of the prior art, because these films consist mainly of Si and O.