Abstract:
A method for manufacturing a semiconductor device according to the present invention includes the steps of providing a semiconductor substrate in which an element isolation region and active regions surrounded by the element isolation region are formed, forming a plurality of conductive lines disposed such that the conductive lines cross the active regions, forming an insulating film over the entire surface including the conductive lines, and etching away the insulating film situated over the active regions between the conductive lines so as to form contact holes. After an anti-etching film is formed to protect the surfaces in the contact holes, wet etching is conducted to remove the insulating film in the contact holes so as to form the contact holes.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for manufacturing a semiconductor device, and particularly to a method for manufacturing a semiconductor device suitable for forming a contact hole in a DRAM (Dynamic Random Access Memory) memory cell. 
   2. Description of the Related Art 
   Firstly, an example of a method for manufacturing a DRAM will be schematically described with reference to the cross-sectional view shown in  FIG. 1 . 
   An insulating film is buried in a predetermined region in a semiconductor substrate  1  made of silicon to form an element isolation region  2 . Then, transistors formed of gate insulating films  3 , gate electrodes  5 ,  6  and impurity diffusion layers  9 ,  10  are formed. Gate electrodes form word lines  4 ,  5 ,  6  and  7 . After each of the word lines is covered with an insulating film  8 , an interlayer insulating film  11  is formed. Then, contact plugs  12 ,  13  and  14  connected to the impurity diffusion layers  9 ,  10  are formed in the interlayer insulating film  11 . After an interlayer insulating film  15  is formed over the surface, a bit line contact plug  16  connected to the contact plug  13  is formed and a bit line  17  connected to the bit line contact plug  16  is formed. After an interlayer insulating film  18  is formed over the bit line, capacitance contact plugs  19  are formed through the interlayer insulating films  15  and  18  such that the capacitance contact plugs  19  are connected to the contact plugs  12  and  14 . Then, a silicon nitride film  20  and a silicon oxide film  21  are formed, and deep holes are formed such that the capacitance contact plugs  19  are exposed. A lower electrode  22 , a capacitance insulating film  23  and an upper electrode  24  that form a capacitor are formed in each of the deep holes. Thereafter, formation of another interlayer insulating film and formation of wiring is repeated to complete a DRAM element. 
   In recent years, as the degree of integration increases, the dimensions of lines and spaces of wiring and the diameters of contact holes decrease, resulting in considerable difficulty in processing wiring and contact holes. Among others, in forming the contact plugs  12 ,  13  and  14  shown in  FIG. 1 , a SAC (Self Aligned Contact) method has been used to achieve easier processing. The SAC method is described in Japanese Patent Laid-Open No. 2001-230383, No. 10-289951 and No. 10-65002. However, even when the SAC method is used to form the contact hole, the shoulder of the insulating film  8  is scraped, resulting in a serious problem of short circuit between the contact plug and the word line. 
   This short circuit problem of prior art will be described in more detail with reference to  FIGS. 2 to 6 .  FIG. 2  is a plan view of the memory cell corresponding to  FIG. 1 , and  FIGS. 3 to 6  are a series of cross-sectional views in the order of processing steps. 
   Firstly, the plan view of  FIG. 2  will be described. The word lines  4 ,  5 ,  6  and  7  have been formed in the vertical direction such that the word lines cross active regions surrounded by the element isolation region  2 . The portions between the word lines in the active regions correspond to the impurity diffusion layers  9 ,  10 . The circles  12   a ,  13   a  and  14   a  indicate the positions of the contact plugs  12 ,  13  and  14  that overlie the positions where the impurity diffusion layers are formed.  FIGS. 1 and 3  to  6  are cross-sectional views corresponding to the cross section taken along the line X-X shown in  FIG. 2 . 
   Next, a conventional method for forming contact plugs will be described with reference to a series of cross-sectional views of  FIGS. 3 to 6 . 
   Firstly, as shown in  FIG. 3 , after the word lines  4 ,  5 ,  6  and  7  are formed, the interlayer insulating film  11  is formed thereon. Specifically, the word line is formed of a polycrystalline silicon film  101 , a tungsten film  101   a , a silicon nitride film  102  and a silicon nitride film  103  that is formed in such a way that it surrounds the silicon nitride film  102  and the tungsten film  101   a . A silicon nitride film  104  is formed over the surface including the word lines. The interlayer insulating film  11  formed of a silicon oxide film is further formed, and CMP (Chemical Mechanical Polishing) method is used to planarize the surface of the interlayer insulating film  11 . In this description, the gate insulating film is omitted. 
   Then, as shown in  FIG. 4 , lithography is used to form a photoresist  105  as well as aperture patterns  12   a ,  13   a  and  14   a  corresponding to contact hole patterns in the photoresist  105 . These aperture patterns correspond to the positions of the contact plugs  12   a ,  13   a  and  14   a  shown in  FIG. 2 . 
   Thereafter, as shown in  FIG. 5 , dry etching using the photoresist  105  as a mask is conducted to form contact holes  109 . The etching of the interlayer insulating film  11  formed of a silicon oxide film is carried out by using fluorine-containing gas plasma. In this process, the etching rate of the silicon oxide film can be on the order of three times the etching rate of the silicon nitride film at maximum, so that the silicon nitride films  104 ,  103  and  102  are also disadvantageously etched during the etching of the interlayer insulating film  11 . The ratio of the etching rates shows a material-specific characteristic in dry etching, so that it is difficult to greatly change the characteristic even when the condition of the gas plasma is changed. 
   Then, as shown in  FIG. 6 , after a polycrystalline silicon film is formed such that the contact holes  109  are filled, the polycrystalline silicon film is etched back to form the contact plugs  12 ,  13  and  14  in the contact holes  109 . Consequently, as described above, the silicon nitride film at the shoulder of the word line has been etched during the formation of the contact hole, resulting in reduced thickness of the insulating film between the contact plug and the tungsten  101   a  that forms the word line and hence a possibility of short circuit. If short circuit occurs, the DRAM will not operate. 
   Patent Documents associated with the formation of the contact holes described above include Japanese Patent Laid-Open No. 2001-230383, No. 10-289951 and No. 10-65002. 
   As described above, the method for forming a contact hole by using dry etching reduces the thickness of the insulating film at the portion indicated by the circle A in  FIG. 6 , resulting in difficulty in preventing the short circuit between the contact plug and the word line. To ensure the normal operation of the DRAM, there is a desire to develop a method for forming a contact hole in which sufficient thickness of the insulating film between the contact plug and the word line is provided to prevent short circuit. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a method for manufacturing a semiconductor device in which a sufficiently thick film thickness of an insulating film is left between a contact plug and a conductive line such as a word line to prevent short circuit when a contact hole is formed. 
   According to the present invention, there is provided a method for manufacturing a semiconductor device including the steps of: 
   providing a semiconductor substrate in which an element isolation region and active regions surrounded by the element isolation region are formed; 
   forming a plurality of conductive lines disposed such that the conductive lines cross the active regions; 
   forming a first insulating film over the entire surface including the conductive lines; and 
   etching away the first insulating film situated over the active regions between the conductive lines so as to form contact holes, 
   wherein after an anti-etching film is formed to protect the surfaces in the contact holes, wet etching is conducted to remove the first insulating film in the contact holes so as to form the contact holes. 
   The manufacturing method described above can further include the step of: 
   after the conductive lines are formed and before the first insulating film is formed, forming a first silicon nitride film over the entire surface including the conductive lines, the active regions and the element isolation region; 
   after the first insulating film is formed, removing the first insulating film until the surfaces of the first silicon nitride films situated over the top surfaces of the conductive lines are exposed; 
   after the step of removing the first insulating film until the surfaces of the first silicon nitride films are exposed, forming a first resist mask including resist portions that are formed such that the resist portions are separately disposed corresponding to the active regions and cover contact hole formation regions in the active regions, 
   removing the first insulating film over the element isolation region by dry etching using the first resist mask; 
   after the first resist mask is removed, forming a second silicon nitride film over the entire surface; 
   forming a second insulating film over the second silicon nitride film and planarizing the surface of the second insulating film; 
   forming a third silicon nitride film on the planarized second insulating film; 
   forming a second resist mask having openings disposed at predetermined positions over the active regions; 
   dry etching the third silicon nitride film and the second insulating film using the second resist mask to form first contact holes and expose the surface of the second silicon nitride film in the holes; 
   after the second resist mask is removed, forming a fourth silicon nitride film over the entire surface; 
   dry etching the fourth silicon nitride film and the second silicon nitride film situated at the bottoms of the first contact holes to expose the surface of the first insulating film in the holes; 
   removing the first insulating film whose surface is exposed in the first contact holes by wet etching so as to form second contact holes that are situated under the first contact holes and communicate therewith; and 
   dry etching the first silicon nitride film exposed at the bottoms of the second contact holes so as to expose the surfaces of the active regions. 
   In the manufacturing method described above, the step of removing the first insulating film until the surface of the first silicon nitride film is exposed can be carried out by polishing to planarize the surface of the first insulating film. 
   In the manufacturing method described above, the first resist mask can have the same pattern as the pattern of the active regions and is disposed such that the first resist mask overlies the pattern of the active regions. 
   In the manufacturing method described above, the diameters of the openings in the second resist mask for forming the first contact holes may be smaller than the length of the first resist mask in the direction perpendicular to the longitudinal direction. 
   The manufacturing method described above can further include the step of filling the first contact holes and the second contact holes with conductive material to form contact plugs. 
   In the present invention, since wet etching is used to remove the insulating film necessary for contact hole formation, it is possible to significantly increase (for example, about 100 times) the etching rate of the insulating film, such as a silicon oxide film and a BPSG film, with respect to the etching rate of the anti-etching film, such as a silicon nitride film. Therefore, the insulating film formed of a silicon oxide film, a BPSG film or the like can be selectively removed substantially without etching the conductive line-protecting, anti-etching film, such as a silicon nitride film. As a result, it is possible to prevent short circuit between the contact plugs and the conductive lines such as word lines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view for explaining the configuration of a DRAM; 
       FIG. 2  is a plan view for explaining the positions of the contact holes shown in  FIG. 1 ; 
       FIG. 3  is a cross-sectional view taken along the line X-X in  FIG. 2  before the contact holes are formed and explains a problem of prior art; 
       FIG. 4  is a cross-sectional view of the process subsequent to  FIG. 3  and explains the problem of prior art; 
       FIG. 5  is a cross-sectional view of the process subsequent to  FIG. 4  and explains the problem of prior art; 
       FIG. 6  is a cross-sectional view of the process subsequent to  FIG. 5  and explains the problem of prior art; 
       FIG. 7A  is a cross-sectional view taken along the direction X-X in the plan view of  FIG. 9  and explains how to form contact plugs according to the present invention; 
       FIG. 7B  is a cross-sectional view taken along the direction Y-Y in the plan view of  FIG. 9  and explains how to form contact plugs according to the present invention; 
       FIG. 8A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 7A  and explains how to form contact plugs according to the present invention; 
       FIG. 8B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 7B  and explains how to form contact plugs according to the present invention; 
       FIG. 9  is a plan view for explaining the position of a photoresist pattern shown in  FIGS. 8A and 8B ; 
       FIG. 10A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 8A  and explains how to form contact plugs according to the present invention; 
       FIG. 10B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 8B  and explains how to form contact plugs according to the present invention; 
       FIG. 11A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 10A  and explains how to form contact plugs according to the present invention; 
       FIG. 11B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 10B  and explains how to form contact plugs according to the present invention; 
       FIG. 12A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 11A  and explains how to form contact plugs according to the present invention; 
       FIG. 12B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 11B  and explains how to form contact plugs according to the present invention; 
       FIG. 13A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 12A  and explains how to form contact plugs according to the present invention; 
       FIG. 13B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 12B  and explains how to form contact plugs according to the present invention; 
       FIG. 14A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 13A  and explains how to form contact plugs according to the present invention; 
       FIG. 14B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 13B  and explains how to form contact plugs according to the present invention; 
       FIG. 15A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 14A  and explains how to form contact plugs according to the present invention; 
       FIG. 15B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 14B  and explains how to form contact plugs according to the present invention; 
       FIG. 16A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 15A  and explains how to form contact plugs according to the present invention; 
       FIG. 16B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 15B  and explains how to form contact plugs according to the present invention; 
       FIG. 17A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 16A  and explains how to form contact plugs according to the present invention; 
       FIG. 17B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 16B  and explains how to form contact plugs according to the present invention; 
       FIG. 18A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 17A  and explains how to form contact plugs according to the present invention; 
       FIG. 18B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 17B  and explains how to form contact plugs according to the present invention; 
       FIG. 19A  is a cross-sectional view taken along the direction X-X subsequent to  FIG. 18A  and explains how to form contact plugs according to the present invention; and 
       FIG. 19B  is a cross-sectional view taken along the direction Y-Y subsequent to  FIG. 18B  and explains how to form contact plugs according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described below in detail with reference to the cross-sectional views of  FIGS. 7A to 8B , the plan view of  FIG. 9  and a series of cross-sectional views of  FIGS. 10A to 19B .  FIGS. 7A ,  8 A,  10 A,  11 A,  12 A,  13 A,  14 A,  15 A,  16 A,  17 A,  18 A and  19 A show the cross section taken along the line X-X shown in the plan view of  FIG. 9 .  FIGS. 7B ,  8 B,  10 B,  11 B,  12 B,  13 B,  14 B,  15 B,  16 B,  17 B,  18 B and  19 B show the cross section taken along the line Y-Y shown in the plan view of  FIG. 9 . 
   Firstly, as shown in  FIG. 7A , a known method is used to form an element isolation region  2  in a predetermined region in the surface of a semiconductor substrate  1  made of p-type monocrystalline silicon. After thermal oxidation is used to form a 4 nm-thick gate oxide film (not shown in the figure) on the surface of the semiconductor substrate, known CVD (Chemical Vapor Deposition) using monosilane (SiH 4 ) as raw material gas is used to deposit a polycrystalline silicon film  101 . The thickness of the polycrystalline silicon film  101  is 100 nm. Then, known sputtering is used to deposit a 70 nm-thick tungsten film  101  a, and known plasma CVD using monosilane and ammonium (NH 3 ) as raw material gas is used to deposit a 140 nm-thick silicon nitride film  102 . Thereafter, lithography and dry etching are used to etch and pattern the silicon nitride film  102  and the tungsten film  101   a  and expose the polycrystalline silicon film  101 . The etching of the silicon nitride film  102  and the tungsten film  101   a  is carried out by using fluorine-containing plasma and chlorine-containing plasma, respectively. 
   Then, known CVD using dichlorosilane (SiH 2 Cl 2 ) and ammonium as raw material gas is used to deposit a 15 nm-thick silicon nitride film  103 . Furthermore, dry etching is used to etch back the silicon nitride film  103  so as to form sidewall layers formed of the silicon nitride film  103  on the sidewalls of the silicon nitride film  102  and the tungsten film  101   a . This etch back process exposes the polycrystalline silicon film  101  in the area where there is no tungsten film  101   a.    
   Thereafter, chlorine-containing gas plasma is used to dry etch the exposed polycrystalline silicon film  101  so as to form word lines formed of the polycrystalline silicon film  101 , the tungsten film  101   a  and the silicon nitride films  102  and  103 . At this point, ion implantation may be used to form an impurity diffusion layer on the surface of the semiconductor substrate. It is noted that a tungsten nitride film is desirably formed as a barrier layer on the surface of the silicon film  101  before the tungsten film  101   a  is formed. 
   The word lines have been disposed along the vertical direction such that they cross active regions surrounded by the element isolation region  2 , as in the structure shown in  FIG. 2 . In each of the active regions, impurity diffusion layers  9  and  10  are provided between the word lines that cross the active region. In this embodiment, first contact holes are formed at the positions indicated by the circles  12   a ,  13   a  and  14   a  in  FIG. 2 . 
   After the word lines are formed, known CVD using tetraethoxysilane (Si(OC 2 H 5 ) 4 ) as raw material gas is used to deposit a 7 nm-thick silicon oxide film  104   a  over the surface, and plasma CVD is used to further deposit a 30 nm-thick first silicon nitride film  104  over the surface. 
   Then, a 600 nm-thick insulating film  11  made of BPSG (Boro-Phospho Silicate Glass) is formed such that the insulating film  11  covers all word lines. The BPSG is deposited by known plasma CVD using tetraethoxysilane, triethoxyboron (B(OC 2 H 5 ) 3 ) and triethoxyphosphine (P(OC 2 H 5 ) 3 ) as raw material gas as well as using ozone as an oxidant. Then, the resultant structure undergoes heat treatment in a steam atmosphere at 750° C. to fluidize and planarize the surface of the structure. Thereafter, CMP (Chemical Mechanical Polishing) method is used to polish the insulating film  11  until the surface of the first silicon nitride film  104  is exposed. At this point, the silicon oxide film  104   a , the first silicon nitride film  104  and the silicon oxide film  11  are present on the semiconductor substrate  1  as shown in the cross section taken along the direction Y-Y shown in  FIG. 7B . It is noted that after the silicon oxide film  104   a  is formed or after the first silicon nitride film  104  is formed, ion implantation is used to form an impurity diffusion layer on the surface of the semiconductor substrate. Instead of the silicon oxide film  104   a , a silicon oxynitride film may be formed. The silicon oxynitride film can be formed by CVD using dichlorosilane, ammonium and dinitrogen monoxide (N 2 O) as raw material gas. 
   Then, as shown in  FIGS. 8A and 8B , lithography is used to form a photoresist mask  105 .  FIG. 9  shows a plan layout diagram of the pattern of the photoresist mask  105  shown in  FIG. 8 . The pattern of the photoresist mask over the word lines  4 ,  5 ,  6  and  7  is the same as that of the active regions surrounded by the element isolation region  2  (see  FIG. 2 ) and disposed such that the pattern of the photoresist mask overlies the pattern of the active regions. 
   Thereafter, as shown in  FIGS. 10A and 10B , dry etching is used to etch the insulating film  11  in the area other than the area where the pattern of the photoresist mask has been formed. In this etching process, octafluorocyclopentane (C 5 F 8 ) is used as primary etching gas and the pressure is 40 mTorr. Since the ratio of the etching rate of the insulating film  11  to the etching rate of the silicon nitride film is about three at maximum, part of the first silicon nitride film  104 , silicon oxide film  104   a  and silicon nitride films  103  and  102  situated above the word lines at both ends that lie off the pattern of the photoresist mask  105  are also etched at the same time. However, the etched portions will not cause the problem of short circuit between the word lines and contact plugs, because no contact plug is formed in these portions. In this dry etching process, the photoresist mask  105  may not be sufficient as an etching mask in some cases. In this case, a silicon film, an amorphous carbon film or the like can be formed and used as a hard mask. For example, a silicon film can be formed before the photoresist mask  105  is formed, and the photoresist mask is used to etch the silicon film so as to process the silicon film into the same pattern as that of the photoresist mask. Then, the resultant silicon film can be effectively used as a mask. 
   Then, as shown in  FIGS. 11A and 11B , after the photoresist mask  105  is removed, known CVD using dichlorosilane and ammonium as raw material gas is used to deposit a 30 nm-thick second silicon nitride film  201  over the surface. 
   Thereafter, as shown in  FIGS. 12A and 12B , a 600 nm-thick interlayer insulating film  202  made of BPSG is deposited. Then, the resultant structure undergoes heat treatment in a steam atmosphere at 750° C. to fluidize and planarize the surface of the interlayer insulating film  202 . Thereafter, CMP method is used to planarize the surface of the interlayer insulating film  202 . Then, plasma CVD using monosilane and ammonium as raw material gas is used to deposit a 60 nm-thick third silicon nitride film  203 . 
   Then, as shown in  FIGS. 13A and 13B , lithography is used to form another photoresist mask  105  having contact hole patterns  12   a ,  13   a  and  14   a . These contact hole patterns correspond to the contact hole patterns  12   a ,  13   a  and  14   a  shown in the plan view of  FIG. 2 . 
   Thereafter, as shown in  FIGS. 14A and 14B , the photoresist mask  105  is used to dry etch the third silicon nitride film  203  and the interlayer insulating film  202  so as to expose the surface of the second silicon nitride film  201  and form first contact holes  205   a . In this process, when the contact hole patterns  12   a ,  13   a  and  14   a  lie off the pattern of the photoresist mask  105  shown in  FIG. 9  corresponding to the active regions in the vertical direction (direction of the line Y-Y), the contact hole patterns  12   a ,  13   a  and  14   a  will lie off the second silicon nitride film  201  situated on the top surface of the insulating film  11  in  FIG. 14B  in the lateral direction, disadvantageously resulting in deep grooves that reach the semiconductor substrate. Therefore, in the present invention, the diameters of the contact hole patterns  12   a ,  13   a  and  14   a  are preferably smaller than the length (width) of the pattern of the photoresist mask  105  shown in the plan view of  FIG. 9  in the direction perpendicular to the longitudinal direction and disposed in the area where the pattern of the photoresist mask  105  has been formed without lying off the mask pattern. 
   Then, as shown in  FIGS. 15A and 15B , after the photoresist mask  105  is removed, CVD using dichlorosilane and ammonium as raw material gas is used to deposit a 30 nm-thick fourth silicon nitride film  204 . 
   Thereafter, as shown in  FIGS. 16A and 16B , dry etching using fluorine-containing plasma is used to etch back the fourth silicon nitride film  204  and the second silicon nitride film  201  so as to expose the surface of the insulating film  11  formed of the BPSG film. In this process, although the third silicon nitride film  203  situated on the top is also etched back, the initial thickness of 60 nm ensures that an about 30 nm-thick silicon nitride film is left. 
   Then, as shown in  FIGS. 17A and 17B , a hydrofluoric acid-containing solution is used to etch the insulating film  11  formed of the BPSG film so as to form second contact holes  205  that are situated under the first contact holes  205   a  and communicate therewith. In this etching stage, since the side and bottom surfaces as well as other surfaces of the insulating film  11  are all covered with silicon nitride films that are etched by the hydrofluoric acid-containing solution at a slow etching rate, only the insulating film  11  formed of the BPSG film can be selectively removed. In this process, as the hydrofluoric acid-containing solution, BHF (Buffered HF) obtained by solving 113 g of NH 4 F (40% ammonium fluoride solution) in 28 ml of HF (50% hydrofluoric acid) and 170 ml of H 2 O is used. When such a solution is used, the silicon nitride film will be etched by only 1.5 nm during the etching of the insulating film  11  formed of the 300 nm-thick BPSG film. Therefore, the difference in etching rate is used to allow selective removal of the insulating film  11 . Although the BPSG film is used for the insulating film  11  in this embodiment, even a silicon oxide film that contains no B (boron) or P (phosphorus) can achieve similarly large difference in etching rate. It is known that the etching rate of a BPSG film differs from that of a silicon nitride film. 
   Thereafter, as shown in  FIGS. 18A and 18B , dry etching using fluorine-containing plasma is used to etch the first silicon nitride film  104  that is exposed at the bottom of the contact holes and the silicon oxide film  104   a  that is situated on the semiconductor substrate  1 , so as to expose the surface of the semiconductor substrate. Before or after the first silicon nitride film  104  and the silicon oxide film  104   a  are etched, ion implantation may be used to form desired impurity diffusion layers on the surface of the semiconductor substrate  1 . 
   Then, as shown in  FIGS. 19A and 19B , after a hydrofluoric acid-containing solution is used to clean the surface of the semiconductor substrate, CVD is used to form a phosphorus-containing silicon film such that the first contact holes  205   a  and the second contact holes  205  are filled. Thereafter, CMP method is used to remove the silicon film formed on the surface of the interlayer insulating film  202  so as to form contact plugs  12 ,  13  and  14 . Subsequently, as shown in  FIG. 1 , formation of a bit line, formation of capacitors, formation of wiring lines and the like can be performed to form a DRAM. 
   According to this embodiment, since the contact holes to be formed between the word lines are formed by wet etching, it is possible to prevent the shoulders of the top insulating films that form the word lines to be scraped, thereby eliminating the problem of short circuit between the contact plugs  12 ,  13 ,  14  and the tungsten layers  101   a  that form the word lines. 
   Alternatively, in the stage shown in  FIGS. 18A and 18B , after a silicon nitride film or a silicon oxide film is further formed over the surface and etched back so as to form sidewall layers of the silicon nitride film or the silicon oxide film on the sidewalls of the contact holes, ion implantation is used to form impurity diffusion layers on the surface of the semiconductor substrate, and then the contact plugs can be formed.