Abstract:
A method for forming an interlayer insulating film is disclosed. This method includes the steps of: forming a first insulating film on a substrate, the film containing at least one of H 2 O, C and a hydrocarbon; forming pores in the first insulating film by heat treatment of the first insulating film to discharge the H 2 O, C or hydrocarbon therefrom; and forming a second insulating film on the porous first insulating film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field 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. 
     2. Description of the Related Art 
     Progress in high integration of semiconductor devices 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 need has been created for an interlayer insulating film which has a low dielectric constant. 
     More specifically, recent progress in high integration of an LSI device, has led to the wiring lines being micronized and multilayered which, in turn, has led to an increase in capacitance between the wiring lines. Such an increase in capacitance has caused a great reduction in operating speed. Thus, improvement in this regard has been strongly demanded. As one improvement measure, 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 {circle around (1)} an SiOF film, and {circle around (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 bonds for a portion of the Si—O bonds in SiO 2 . This SiOF film has a relative dielectric constant which is 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 in a high-density plasma enhanced CVD method (HDPCVD method). The 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 the 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 
               
               
                   
               
               
                 Organic 
                 Relative 
                 Thermal 
                   
               
               
                 Insulating 
                 Dielectric 
                 Decomposition 
               
               
                 Film 
                 Constant 
                 Temperature (° C.) 
                 Note 
               
               
                   
               
             
             
               
                 Fluorine-contain- 
                 2.4 
                 420 
                 P. 82 of monthly 
               
               
                 ing resin 
                   
                   
                 periodical “Semi- 
               
               
                   
                   
                   
                 conductor World”, 
               
               
                   
                   
                   
                 February issue of 
               
               
                   
                   
                   
                 1997 
               
               
                 Cytop 
                 2.1 
                 400 
                 P. 90 of monthly 
               
               
                   
                   
                   
                 periodical “Semi- 
               
               
                   
                   
                   
                 conductor World”, 
               
               
                   
                   
                   
                 February issue of 
               
               
                   
                   
                   
                 1996 
               
               
                 Amorphous telon 
                 1.0 
                 400 
                 P. 91 of monthly 
               
               
                   
                   
                   
                 periodical “Semi- 
               
               
                   
                   
                   
                 conductor 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 a superimposed barrier metal layer are affected. 
     The organic insulating film of a low dielectric constant is easily peeled off, because of poor adhesion to a silicon wafer or the SiO 2  film. Furthermore, the organic insulating film is disadvantageous in that heat resistivity is low since its 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 made by the above method. 
     According to the method of the present invention for forming an interlayer insulating film, first, an SiO 2  film containing H 2 O, C or hydrocarbon is formed on a substrate. Then, this SiO 2  film is subjected to plasma or vacuum annealing. The vacuum annealing is performed by heating in a vacuum, i.e., at a pressure of 0.1 Torr or lower. If the pressure is 0.1 Torr or lower, a small quantity of N 2  or Ar may be contained in the atmosphere. 
     Then, by the annealing, gas contained in the SiO 2  film is discharged from the film, and the SiO 2  film becomes a porous SiO 2  film. 
     By experiment, the present inventor confirmed that the dielectric constant of the porous SiO 2  film lies in a range of 2.0 to 3.0. This value is smaller than the dielectric constant 4.0 of a conventional SiO 2  film having no porosity. 
     Since the porous SiO 2  film is formed by a conventional chemical vapor deposition method, better heat resistivity is provided. 
     After formation of the porous SiO 2  film, its surface can be made more stable by H (hydrogen) plasma treatment. In other words, by substituting Si—H bonds for dangling Si—O bonds in the surface, adsorption of water can be prevented. 
     Then, by forming a conventional SiO 2  film on the porous SiO 2  film, adsorption of water can be further prevented. 
     In a second embodiment of the method of the present invention for forming an interlayer insulating film, a first film is formed in concavities of a surface which has concavities and convexities. A first insulating film is then formed on the first film. The first insulating film has an etching rate lower than that of the first film. Then, holes are bored in the first insulating film, and selective etching of the first film is performed through the holes to eliminate corresponding areas of the first film. Then, a second insulating film is formed on the first insulating film to close the holes formed in the first insulating film. 
     Then, a cavity is formed within each concavity of the substrate and the first and second insulating films thereby become porous. Accordingly, an interlayer insulating film having cavities is formed on the substrate. The dielectric constant of this interlayer insulating film having cavities is apparently lower than that of a similar film without cavities. By experiment, the present inventor has confirmed that the dielectric constant of the interlayer insulating film having cavities was about 2.0. This value is lower than the dielectric constant 4.0 of a conventional SiO 2  film having no cavities. In addition, since the cavities are surrounded by the substrate and the conventional insulating film, no water adsorption occurs in the cavity. In other words, the above process results in formation, on the substrate, of an interlayer insulating film of a low dielectric constant, which has good moisture absorption resistance. 
     In a third embodiment of the method of the present invention for forming an interlayer insulating film, a first film is formed on a substrate. Then, a pattern is formed in the first film, i.e., a damascene trench which reaches the substrate. Then, a first insulating film is formed on the first film, on a side portion of the damascene trench and on a bottom portion of the same. Anisotropic etching is then performed for the first insulating film to eliminate the first insulating film formed on the bottom portion of the damascene trench while leaving the same formed on the side portion of the damascene trench. Subsequently, a Cu-plated film is buried in the damascene trench. In this case, by the first insulating film formed previously in the side portion of the damascene trench, a component in the first film can be prevented from dispersing into the Cu-plated film. Then, a barrier metal film is formed on the Cu-plated film. Due to this barrier metal film, components of a film formed on the Cu-plated film are prevented from dispersing into the Cu-plated film. Then, a second insulating film is formed on the first film and the barrier metal film, and holes are bored. Thereafter, selective etching of the first film is performed through the holes to eliminate corresponding areas of same. Accordingly, cavities are formed in the first film. Then, a second insulating film is formed on the first insulating film to close the holes. This process results in formation of an interlayer insulating film having cavities. 
     The dielectric constant of the interlayer insulating film having cavities formed in the above manner is apparently lower than that of an otherwise similar interlayer insulating film having no cavities. By experiment, the inventor confirmed that the dielectric constant of the interlayer insulating film having cavities was about 2.0. This value is lower than the dielectric constant 4.0 of a conventional SiO 2  film having no cavities. Moreover, since the cavities are surrounded by the conventional insulating film, no water adsorption occurs in the cavities. In other words, the process results in formation of an interlayer insulating film of a low dielectric constant, which has good moisture absorption resistance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to  1 F are cross-sectional views showing a method for forming an interlayer insulating film according to a first embodiment of the present invention; 
     FIGS. 2A to  2 F are cross-sectional views showing a method for forming an interlayer insulating film according to a second embodiment of the invention; 
     FIGS. 3A to  3 F are cross-sectional views showing a method for forming an interlayer insulating film according to a third embodiment of the invention; 
     FIGS. 4A to  4 F are cross-sectional views showing a method for forming an interlayer insulating film according to a fourth embodiment of the invention; 
     FIGS. 5A to  5 H are cross-sectional views showing a method for forming an interlayer insulating film according to a fourth embodiment of the invention; and 
     FIGS. 6A to  6 N are cross-sectional views showing a method for forming an interlayer insulating film according to a sixth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (1) First Embodiment (FIGS.  1 A to  1 F) 
     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 , a pattern is formed in the aluminum to produce a wiring layer  103 . The silicon substrate  101 , the BPSG film  102  and the aluminum wiring layer  103  constitute a substrate  104  used in the method of the present invention. 
     Then, as shown in FIG. 1B, a SiO 2  film  105  is formed on the substrate  104 . This SiO 2  film  105  is formed by using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  101  at 400° C., in a CVD method (chemical vapor deposition method). The SiO 2  film  105  can prevent H 2 O from being absorbed in the aluminum wiring layer  103 . 
     Subsequently, as shown in FIG. 1C, a SiO 2  film  106  is formed on the SiO 2  film  105 . This SiO 2  film  106  is formed by using TEOS (tetraethoxysilane), O 2  and a small quantity of H 2 O as source gases, while keeping the silicon substrate  101  at 100° C., with application of RF power having a frequency of 13.56 MHz, and using a plasma enhanced CVD method at a pressure of 1 Torr. Flow rates of source gases at this time are 30˜50 sccm, 100˜600 sccm and 50˜60 sccm, respectively, for TEOS, O 2  and H 2 O. In this case, the use of the small quantity of H 2 O as a source gas results in inclusion of OH radicals and small quantity of H 2 O in the SiO 2  film  106 . It should be noted that TMS (trimethylsilane) may also be used instead of the TEOS, at a flow rate of 30˜50 sccm. 
     Then, as shown in FIG. 1D, vacuum annealing is performed for the SiO 2  film  106  at a pressure of 0.1 Torr while keeping the silicon substrate  101  at 400° C. The vacuum annealing is defined as an annealing that is performed in an atmosphere of reduced pressure. The annealing serves to discharge OH radicals and H 2 O from the SiO 2  film  106  to form a number of voids therein. 
     Instead of the aforementioned vacuum annealing, a plasma annealing may be performed for the SiO 2  film  106 . The plasma annealing is defined an annealing that is performed in a plasmanized atmosphere. In the present embodiment, RF power is applied to the atmosphere via an upper electrode (not shown) and a lower electrode (not shown) so as to convert the atmosphere to a plasma. The process parameters of the plasma annealing are as follows: RF power having a frequency of 13.56 MHz and a power of 100 W is applied to the upper electrode, RF power having a frequency of 400 kHz and a power of 400 W is applied to the lower electrode, pressure is 0.2 Torr, temperature is 400° C., time of annealing is 60˜120 sec, and O 2  is introduced into the atmosphere at a flow rate of 600 sccm. 
     Then, as shown in FIG. 1E, a H hydrogen) plasma treatment is applied to the SiO 2  film  106 . The H (hydrogen) plasma is generated by applying RF power to a H (hydrogen) containing atmosphere via the upper electrode (not shown) and the lower electrode (not shown). In the present embodiment the RF power applied to the upper electrode has a frequency of 13.56 MHz and a power of 50 W, and the RF power applied to the lower electrode has a frequency of 400 kHz and a power of 400 W. Further, the pressure of the H containing atmosphere is 0.1˜0.2 Torr, the flow rate of H is 600 sccm, and the time of the H plasma treatment is 60 sec. During the H plasma treatment the temperature of the silicon substrate  101  is maintained at 400° C. 
     At this time, plasma H atoms enter the voids formed in the SiO 2  film  106 , and SiH bonds are formed on the surface of the voids by the H atoms and Si atoms at the surface. Accordingly, the surfaces of the voids become stable, and absorption of water into the SiO 2  film  106  can be slowed pending further processing. Also, since the interiors of the voids are filled with H 2  molecules, which have no dipole moment, the dielectric constant of the SiO 2  film  106  is 3.0 or lower. This value is smaller than the dielectric constant 4.0 of a conventional SiO 2  film. 
     In the H (hydrogen) plasma treatment, the plasma H atoms do not enter the SiO 2  film  105 , formed below the SiO 2  film  106 , very deeply. Accordingly, the H atoms can be prevented from affecting the substrate  104  formed below the SiO 2  film  105 . 
     Then, as shown in FIG. 1F, a SiO 2  film  107  is formed on the SiO 2  film  106 . This SiO 2  film  107  is formed by a CVD method, using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  101  at 400° C. The SiO 2  film  107  serves to prevent incursion of water into the previously formed porous SiO 2  film  106 , and the H 2  atoms filling the interiors of the voids of the SiO 2  film  106  can be prevented from being discharged from the film. 
     The foregoing process of forming the SiO 2  films  105 ,  106  and  107  results in formation of an interlayer insulating film of a low dielectric constant on the substrate  104 . In other words, since the SiO 2  film  106  has porosity and the insides of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant thereof is smaller than that of a conventional SiO 2  film. Also, the SiO 2  films  107  and  105  are formed on and below SiO 2  film  106 , respectively. These films can prevent incursion of water into the porous SiO 2  film  106 , and the H 2  molecules inside the voids from being discharged from the film. 
     (2) Second Embodiment (FIGS.  2 A to  2 F) 
     The second embodiment is different from the first embodiment in that SiH 4  is used as source gas instead of TEOS for forming a porous SiO 2  film. 
     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 , a pattern is formed in the aluminum to form an aluminum wiring layer  203 . Then, the silicon substrate  201 , the BPSG film  202  and the aluminum wiring layer  203  constitute the substrate  204  used in the method of the present invention. 
     Then, as shown in FIG. 2B, an SiO 2  film  205  is formed on the substrate  204 . This SiO 2  film  205  is formed by a CVD method (chemical vapor deposition method), which employs SiH 4  and N 2 O as source gases, while keeping the silicon substrate  201  at 400° C. The SiO 2  film  205  can prevent H 2 O from dispersing into the aluminum wiring layer  203 . 
     Subsequently, as shown in FIG. 2C, a SiO 2  film  206  is formed on the SiO 2  film  205 . This SiO 2  film  206  is formed by a CVD method, which employs SiH 4 , O 2  and a small quantity of H 2 O as source gases, while keeping the silicon substrate  201  at 100° C., and at a pressure of 3 Torr. Flow rates of source gases are 30˜50 sccm. 90˜100 sccm, 30˜50 sccm and 200˜600 sccm respectively for SiH 4 , O 2 , H 2 O and Ar. In this case, the use of the small quantity of H 2 O as a source gas results in inclusion of small quantity of H 2 O in the SiO 2  film  206 . 
     Then, as shown in FIG. 2D, vacuum annealing is performed for SiO 2  film  206  by keeping the silicon substrate  201  at 400° C. Ad at a pressure of 0.1 Torr. Accordingly, H 2 O contained in the SiO 2  film  206  is discharged therefrom, and a number of voids are formed therein. 
     Instead of performing the vacuum annealing, a plasma annealing may be performed for the SiO 2  film  206 . In the present embodiment, RF power is applied to the atmosphere via an upper electrode (not shown) and a lower electrode (not shown) so as to convert the atmosphere to a plasma. And the process conditions of the plasma annealing are as follows: RF power having a frequency of 13.56 MHz and a power of 100 W is applied to the upper electrode, RF power having a frequency of 400 kHz and a power of 400 W is applied to the lower electrode, pressure is 0.2 Torr, temperature is 400° C., time of annealing is 60˜120 sec, and O 2  is introduced into the atmosphere at a flow rate of 600 sccm. 
     Then, as shown in FIG. 2E, H (hydrogen) plasma treatment is performed for the SiO 2  film  206  in the same way as in the first embodiment. That is, the H (hydrogen) plasma is generated by applying RF power to a H (hydrogen) containing atmosphere via the upper electrode (not shown) and the lower electrode (not shown). The RF power applied to the upper electrode has a frequency of 13.56 MHz and a power of 50 W, and the RF power applied to the lower electrode has a frequency of 400 kHz and a power of 400 W. Further, the pressure of the H containing atmosphere is 0.1˜0.2 Torr, the flow rate of H is 600 sccm, and the time of the H plasma treatment is 60 sec. During the H plasma treatment the temperature of the silicon substrate  201  is maintained at 400° C. 
     At this time, plasma H atoms enter the voids formed in the SiO 2  film  206 . By reaction of the H atoms with the Si atoms on the surfaces of the voids, Si—H bonds are formed on the surfaces of the same. Accordingly, the surfaces of the voids are stabilized, and incursion of water into the SiO 2  film  206  can be slowed down pending further processing. Also, since the insides of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  film  206  lies in a range of 2.0 to 3.0, which is smaller than the dielectric constant 4.0 of a conventional SiO 2  film. 
     In the H (hydrogen) plasma treatment, the plasma H atoms do not enter the SiO 2  film  205  formed below the SiO 2  film  206  very deeply. Accordingly, the H atoms can be prevented from affecting the substrate  204  formed below the SiO 2  film  205 . 
     Then, as shown in FIG. 2F, a SiO 2  film  207  is formed on the SiO 2  film  206 . This SiO 2  film  207  is formed by a CVD method, using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  201  at 400° C. The SiO 2  film  207  prevents incursion of water into the previously formed porous SiO 2  film  206  and H 2 O molecules filling the inside of the voids of the SiO 2  film  206  can be prevented from being discharged from the film. 
     As in the case of the first embodiment, the foregoing process of forming the SiO 2  films  205 ,  206  and  207  results in formation of an interlayer insulating film of a low dielectric constant on the substrate  204 . In other words, since the SiO 2  film  206  has porosity and the insides of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  flim  206  is smaller than that of a conventional SiO 2  film. Also, the conventional SiO 2  films  205  and  207  are formed below and on the SiO 2  film  206 . Accordingly, incursion of water into the porous SiO 2  film  206  can be prevented, and H 2  molecules inside the voids can be prevented from being discharged from the film. 
     (3) Third Embodiment (FIGS.  3 A to  3 F) 
     The third embodiment is different from the first and second embodiments in that the porous insulating film is formed using B 2 H 6  is used as the source gas. 
     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 thereon, a pattern is formed in the aluminum film to produce an aluminum wiring layer  303 . The silicon substrate  301 , the BPSG film  302  and the aluminwn wiring layer  303  constitute the substrate  304  used in the method of the present invention. 
     Then, as shown in FIG. 3B, a SiO 2  film  305  is formed on the substrate  304 . This SiO 2  film  305  is formed by a CVD method (chemical vapor deposition method), using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  301  at 400° C. The SiO 2  film  305  can prevent H 2 O from dispersing into the aluminum wiring layer  203 . 
     Then, as shown in FIG. 3C, a B (boron)-containing SiO 2  film  306  is formed on the SiO 2  film  305 . This SiO 2  film  306  is formed by using SiH 4 , O 2  and B 2 H 6  as source gases while keeping the silicon substrate  301  at 100° C., applying an RF power having a frequency of 13.56 MHz, using a plasma enhanced CVD method at a source gas pressure of 1 Torr. Flow rates of source gases are 30˜50 sccm, 120 sccm and 24˜30 sccm respectively for SiH 4 , O 2  and B 2 H 6 . N 2 O at a flow rate of 1000 sccm can also be contained in the source gases. During the formation of the film  305 , O 2  of the source gas and H contained in SiH 4  or B 2 H 6  generate H 2 O. Accordingly, the B (boron)-containing SiO 2  film  306  contains a small quantity of H 2 O. 
     Then, as shown in FIG. 3D, vacuum annealing may also be performed for the SiO 2  film  306  by keeping the silicon substrate  301  at 400° C. Ad at a pressure of 0.1 Torr. Accordingly, H 2 O and B (boron) contained in the SiO 2  film  306  are discharged from the film, and a number of voids are formed therein. 
     Instead of performing the vacuum annealing, a plasma annealing may be performed for the SiO 2  film  306 . In the present embodiment, RF power is applied to the atmosphere via an upper electrode (not shown) and a lower electrode (not shown) so as to convert the atmosphere into a plasma And the process parameters of the plasma annealing are as follows: RF power having a frequency of 13.56 MHz and a power of 100 W is applied to the upper electrode, RF power having a frequency of 400 kHz and a power of 400 W is applied to the lower electrode, pressure is 0.2 Torr, temperature is 400° C., time of annealing is 60˜120 sec, and O 2  is contained in the atmosphere with a flow rate of 600 sccm. 
     Then, as shown in FIG. 3E, a H (hydrogen) plasma treatment is performed for the SiO 2  film  306  in the same way as in the first and second embodiments. That is, the H (hydrogen) plasma is generated by applying RF power to a H (hydrogen) containing atmosphere via the upper electrode (not shown) and the lower electrode (not shown). The RF power applied to the upper electrode has a frequency of 13.56 MHz and a power of 50 W, and the RF power applied to the lower electrode has a frequency of 400 kHz and a power of 400 W. Further, the pressure of the H containing atmosphere is 0.1 0.2 Torr, the flow rate of H is 600 sccm, and the time of the H plasma treatment is 60 sec. During the H plasma treatment the temperature of the silicon substrate  301  is maintained at 400° C. 
     At this time, plasma H atoms enter a large number of voids formed in the SiO 2  film  306 . By reaction of the H atoms with the Si atoms on the surfaces of the voids, Si—H bonds are formed on the surfaces of the voids. Accordingly, the surfaces of the voids are stabilized, and incursion of water into the SiO 2  film  306  can be slowed down pending further processing. Also, since the inside of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  film  306  is in a range of 2.0 to 3.0, which is smaller than the dielectric constant 4.0 of a conventional SiO 2  film. 
     Then, as shown in FIG. 3F, a SiO 2  film  307  is formed on the SiO 2  film  306 . This SiO 2  film  307  is formed by a CVD method, which uses SiH 4  and N 2 O as source gases, while keeping the silicon substrate  301  at 400° C. The SiO 2  film  307  prevents incursion of water into the previously formed SiO 2  film  306  and H 2  filling the inside of the voids of the SiO 2  film  305  can be prevented from being discharged from the film. 
     The foregoing process of forming the SiO 2  film  305 ,  306  and  307  results in formation of an interlayer insulating film of a low dielectric constant on the substrate  304 . In other words, since the SiO 2  film  306  has porosity and the interior of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  film  306  is smaller than that of a conventional SiO 2  film. Also, the SiO 2  film  307  is formed on the SiO 2  film  306 . Since this film  307  is a conventional SiO 2  film, incursion of water into the porous SiO 2  film  306  can be prevented, and H 2  molecules in the voids can be prevented from being discharged from the film. 
     (4) Fourth Embodiment (FIGS.  4 A to  4 F) 
     The fourth embodiment is different from the first to third embodiments in that C 2 F 6  is used as a source gas for forming a porous SiO 2  film. 
     First, as shown in FIG. 4A, a BPSG (borophosphosilicate glass) film  402  is formed on a silicon substrate  401 . Then, after an aluminum film is formed thereon, a pattern is formed in the aluminum film to produce an aluminum wiring layer  403 . The silicon substrate  401 , the BPSG film  402  and the aluminum wiring layer  403  constitute a substrate  404  for use in the method of the present invention. 
     Then, as shown in FIG. 4B, a SiO 2  film  405  is formed on the substrate  404 . This SiO 2  film  405  is formed by a CVD method (chemical vapor deposition method), using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  401  at 400° C. The SiO 2  film  405  prevents H 2 O from dispersing into the aluminum wiring layer  403 . 
     Subsequently, as shown in FIG. 4C, a F (fluorine)-containing SiO 2  film  406 , which will later be converted to a porous insulating film, is formed on the SiO 2  film  405 . This SiO 2  film  406  is formed by using TEOS (tetraethoxysilane), O 2  and C 2 F 6  as source gases while keeping the silicon substrate  401  at 100° C., applying RF power having a frequency of 13.56 MHz, and by a plasma enhanced CVD method at a source gas pressure of 1 Torr. Flow rates of source gases at this time are 30˜50 sccm, 600 sccm and 40˜60 sccm respectively for TEOS, O 2  and C 2 F 6 . N 2 O with a flow rate of 1000 sccm can also be contained in the source gases. During the formation of the film  406 , O 2  of the source gas and C contained in TEOS or C 2 F 6  generate hydrocarbon, and O 2  of the source gas and H contained in TEOS generate H 2 O. Accordingly, the SiO 2  film  406  contains hydrocarbon and H 2 O. It should be noted that TMS (trimethylsilane) may be used instead of the TEOS. The flow rate of the TMS is 30˜50 sccm. 
     Then, as shown in FIG. 4D, vacuum annealing is performed for the SiO 2  film  406  by heating the silicon substrate  401  at 400° C. Ad at pressure of 0.1 Torr. Accordingly, hydrocarbon, H 2 O and F (fluorine) contained in the SiO 2  film  406  are discharged from the film, and a number of voids are formed therein. 
     Instead of performing the vacuum annealing, a plasma annealing may be applied to the SiO 2  film  406 . In the present embodiment, RF power is applied to the atmosphere via an upper electrode (not shown) and a lower electrode (not shown) so as to convert the atmosphere to a plasma. And the process conditions of the plasma annealing are as follows: RF power having a frequency of 13.56 MHz and a power of 100 W is applied to the upper electrode, RF power having a frequency of 400 kHz and a power of 400 W is applied to the lower electrode, pressure is 0.2 Torr, temperature is 400° C., time of annealing is 60˜120 sec, and O 2  is introduced into the atmosphere at a flow rate of 600 sccm. 
     Then, as shown in FIG. 4E, H (hydrogen) plasma treatment is performed for the SiO 2  film  406  in the same way as in the first to third embodiments. That is, a H (hydrogen) plasma is generated by applying RF power to a H (hydrogen) containing atmosphere via the upper electrode (not shown) and the lower electrode (not shown). The RF power applied to the upper electrode has a frequency of 13.56 MHz and a power of 50 W, and the RF power applied to the lower electrode has a frequency of 400 kHz and a power of 400 W. Further, the pressure of the H containing atmosphere is 0.1˜0.2 Torr, the flow rate of H is 600 sccm, and the time for the H plasma treatment is 60 sec. During the H plasma treatment the temperature of the silicon substrate  301  is maintained at 400° C. 
     At this time, plasma H atoms enter a large number of voids formed in the SiO 2  film  406 . By reaction of the H atoms with the Si atoms on the surface of the voids, Si—H bonds are formed on the surfaces of the voids. Accordingly, the surfaces of the voids are stabilized, and incursion of water into the SiO 2  film  406  can be slowed down pending further processing. Also, since the interiors of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  film  406  lies in a range of 2.0 to 3.0, which is smaller than the dielectric constant 4.0 of a conventional SiO 2  film. 
     In the H (hydrogen) plasma treatment, the plasma H atoms do not enter the SiO 2  film  405  formed below the SiO 2  film  406  very deeply. Thus, the H atoms can be prevented from affecting the substrate  404  below the SiO 2  film  405 . 
     Then, as shown in FIG. 4F, a SiO 2  film  407  is formed on the SiO 2  film  406 . This SiO 2  film  407  is formed by a CVD method, using SiH 4  and N 2 O as source gases, while keeping the silicon substrate  401  at 400° C. The SiO 2  film  407  prevents incursion of water into the previously formed porous SiO 2  film  406  and H 2  molecules filling the interiors of the voids of the SiO 2  film  406  can be prevented from being discharged from the film. 
     The foregoing process of forming the SiO 2  films  405 ,  406  and  407  results in formation of an interlayer insulating film of a low dielectric constant on the substrate  404 . In other words, since the SiO 2  film  406  has porosity and the interiors of the voids are filled with H 2  molecules having no dipole moment, the dielectric constant of the SiO 2  film  406  is smaller than that of a conventional SiO 2  film. Also, because the conventional SiO 2  films  405  and  407  are formed below and on the porous SiO 2  film  406 , incursion of water into the porous SiO 2  film  406  can be prevented, and H 2  molecules inside the voids can be prevented from being discharged from the film. 
     (5) Fifth Embodiment (FIGS.  5 A to  5 M) 
     According to the fifth embodiment, a photoresist is buried in a substrate and, by etching, cavities are formed in an interlayer insulating film, as described below. 
     First, as shown in FIG. 5A, a BPSG (borophosphosilicate glass) film  502  is formed on a silicon substrate  501 . Then, after an aluminum film is formed thereon, a pattern is formed in the aluminum film to produce an aluminum wiring layer  503 . The silicon substrate  501 , the BPSG film  502  and the aluminum wiring layer  503  constitute substrate  504  to be used in the method of the present invention. 
     Then, as shown in FIG. 5B, a photoresist  505  is coated on the substrate  504  so as to cover the convexities  503   a  of the wiring layer. The photoresist covering the convexities  503   a  of the wiring layer will be eliminated later. Thus, the photoresist  505  should have a thickness which facilitates the elimination. 
     Then, as shown in FIG. 5C, the photoresist  505  is irradiated uniformly with ultraviolet radiation. Ultraviolet radiation of low intensity should be used, which eliminates only the photoresist covering the convexities  503   a  of the wiring layer, but not the photoresist in concavities  503   b  of the wiring layer in the following step. 
     Subsequently, as shown in FIG.  5 D. the photoresist covering the convexities  503   a  of the wiring layer is developed and eliminated. 
     Then, as shown in FIG. 5E, a SiO 2  film  506  is formed on the convexities  503   a  of the wiring layer and the remaining photoresist  505  by a plasma enhanced CVD method (chemical vapor deposition method). Accordingly, the remaining photoresist  505  is confined between the concavities  503   b  of the wiring layer and the SiO 2  film  506 . 
     Then, as shown in FIG. 5F. a hole  506   a  is formed in the SiO 2  film  506  formed on the remaining photoresist  505 . This hole  506   a  will be used later for etching the confined photoresist  505 . After etching, the hole will be closed by another SiO 2  film. Thus, the diameter of the hole should be set small enough for closing later. 
     Subsequently, as shown in FIG. 5G, the photoresist  505  confined between the concavity  503   b  of the wiring layer and the SiO 2  film  506  is etched by O plasma through the hole  506   a.    
     Then, as shown in FIG. 5H, an SiO 2  film  507  having a film thickness of 300 nm is formed on the SiO 2  film  506  by a plasma enhanced CVD method. Accordingly, the hole  506   a  is closed, and a cavity  508  is formed in an area surrounded by the concavity  503   b  of the wiring layer and the SiO 2  films  506  and  507 . 
     The foregoing process results in formation of an interlayer insulating film of SiO 2  having cavities on the substrate  504 . This interlayer insulating film has a dielectric constant smaller than that of a conventional SiO 2  interlayer insulating film. In other words, since the dielectric constant of the cavity portion is smaller than that of SiO 2 , the overall dielectric constant of the entire film is about 2.0, which is smaller than the dielectric constant 4.0 of a similar film without cavities. 
     (6) Sixth Embodiment (FIGS.  6 A to  6 N) 
     The sixth embodiment applies the fifth embodiment to a damascene process. 
     First, as shown in FIG. 6A, a BPSG (borophosphosilicate glass) film  602  is formed on a silicon substrate  601 . Then, after an aluminum layer is formed thereon, a pattern is formed in the aluminum layer to produce an aluminum wiring layer  603 . It should be noted that the aluminum wiring layer  603  in the figures is not shown patterned for convenience. The silicon substrate  601 , the BPSG film  602  and the aluminum wiring layer  603  serve as a substrate  604  in the method of the present invention. 
     Then, as shown in FIG. 6B, a SiO 2  film  605  having a film thickness of 50 nm is formed on the aluminum wiring layer  603  by a CVD method (chemical vapor deposition method). 
     Subsequently, as shown in FIG. 6C, a polyimide film  606  having a film thickness of 500 nm is formed on the SiO 2  film  605 . This polyimide film  606  will be subjected to etching later as in the case of the fifth embodiment, and used for forming cavities in an insulating film. 
     Then, as shown in FIG. 6D, a pattern is formed in the SiO 2  film  605  and the polyimide film  606  to form a damascene trench  607  reaching the aluminum wiring layer  603 . 
     Subsequently, as shown in FIG. 6E, a SiO 2  film  608  having a film thickness of 100 nm is formed on the polyimide film  606  by a plasma enhanced CVD method. In this case, the SiO 2  film  608  is also formed on the side and bottom portions of the damascene trench  607 . 
     Then, as shown in FIG. 6F, anisotropic etching is performed for the SiO 2  film  608 . Accordingly, the SiO 2  film  608  formed on the bottom portion of the damascene trench  607  is eliminated, and a contact hole  609  reaching the aluminum wiring layer  603  is formed. In this case, the SiO 2  film  608  formed on the side portion of the damascene trench  607  is left intact. 
     Then, as shown in FIG. 6G, a Cu film  610  is plated on the SiO 2  film  608  and in the damascene trench  607 . The Cu-plated film in the damascene trench  607  is used as a Cu wiring line. 
     Then, as shown in FIG. 6H, the Cu-plated film  610  is polished by a CMP method (chemical mechanical polishing method), and superfluous Cu formed on the SiO 2  film  608  is eliminated. Accordingly, Cu is left only in the damascene trench  607 . 
     Then, as shown in FIG. 61, a barrier metal TiN film  611  is formed above the damascene trench  607 . Accordingly, Cu in the damascene trench  607  can be prevented from being dispersed into an SiO 2  film formed later above the damascene trench  607 . 
     Subsequently, as shown in FIG. 6J, patterning leaves a TiN film  611   a  formed above the damascene trench  607 , and the TiN film  611  formed in the other portions is etched and thereby eliminated. 
     Then, as shown in FIG. 6K, a SiO 2  film  612  having a film thickness of 100 nm is formed on the SiO 2  film  608  and the TiN film  611   a  by a plasma enhanced CVD method. 
     Then, as shown in FIG. 6L, a pattern is formed in the SiO 2  film  608  and the SiO 2  film  612  formed thereon to bore holes  613  which are used for etching the polyimide film  606 . Thus, the holes  613  should be formed in an area other than above the damascene trench  607 , i.e., in an area where the polyimide film  606  remains, and the hole diameter should be small enough for closing later. 
     Subsequently, as shown in FIG. 6M, the polyimide film  606  is etched by O plasma through the hole  613 . 
     Then, as shown in FIG. 6N, a SiO 2  film  614  having a film thickness of 400 nm is formed on the SiO 2  film  612  by a plasma enhanced CVD method. This SiO 2  film  614  is used for closing the holes  613 . Accordingly, a cavity  615  is formed in an area surrounded by the SiO 2  films  605 ,  608  and  614 . 
     The foregoing process results in formation of an interlayer insulating film of SiO 2 , having cavities, on the substrate  604 . This interlayer insulating film has a dielectric constant which is smaller than that of the conventional SiO 2  interlayer insulating film. In other words, since the dielectric constant of the cavity portion is smaller than that of SiO 2 , the dielectric constant of the overall film is about 2.0, which is smaller than the dielectric constant 4.0 without cavities.