Abstract:
The semiconductor device comprises a MOSFET including a pair of impurity diffused regions formed on both sides of a gate formed on a semiconductor substrate; an insulation film covering a top of the MOSFET and having a through-hole opened on one of the impurity diffused regions formed in; and a capacitor formed at at least a part of an inside of the through-hole, the through-hole having a larger diameter inside than at a surface thereof or having a larger diameter at an intermediate part between the surface thereof and a bottom thereof than the surface and the bottom thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of Ser. No. 08/592,481, filed on Jan. 26, 1996. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a semiconductor device and a method for fabricating the same, more specifically to a high-integration DRAM and a method for fabricating the same.  
           [0003]    The present invention can realize a large capacitor and a micronized memory while securing an alignment allowance in a lithography step and electric isolation.  
           [0004]    With reference to FIGS. 3A and 3B, a conventional semiconductor device and a method for fabricating the same will be explained.  
           [0005]    A field oxide film  2  for defining a device region is formed on a semiconductor substrate  1 . A gate  4  of memory cell transistor is formed through a gate oxide film  3  in the device region defined by the field oxide film  2 . A part of the gate  4  is extended on the field oxide film  2 . An insulation film  5  is formed on the side walls and the upper surface of the gate  4 . A silicon nitride (Si 3 N 4 ) film  6  which function as an etching stopper and an inter-layer insulation film  12  are formed on the semiconductor substrate  1  having memory cell transistor. A through-hole is formed through these films down to the semiconductor substrate  1 . A storage electrode  9  of a capacitor connected to the semiconductor substrate  1  is formed on the inside wall and the bottom of the through-hole. An opposed electrode  11  of the capacitor is formed through a dielectric film  10  on the surface of the storage electrode  9 .  
           [0006]    In the exemplified conventional semiconductor device, the side wall of the through-hole for the capacitor contact is vertical or tapered. In such capacitor, in a case that the through-hole has a large diameter for a larger capacitance, the opening is subjected to restrictions for an alignment allowance for a bit line contact and for electric isolation.  
           [0007]    In a case that a hole diameter and a disalignment are large, as shown in FIG. 3A, the field oxide film  2  is adversely dug even though the contact is formed in self-alignment with the gate  4  to resultantly form a projection in the capacitor, with a resultant problem that an electric field tends to be concentrated to cause dielectric breakdown of the dielectric film  10 . Increase of a hole diameter is restricted.  
           [0008]    In a case that, as shown in FIG. 3B, the contact is formed only by an optical alignment, a hole diameter must be smaller because the storage electrode of the capacitor tends to short-circuit with the gate electrode.  
           [0009]    Accordingly, in a case that the capacitor has such configuration, it is restricted to simply increase a hole diameter for a larger capacitance. As means for effectively omitting such restriction to the opening, the bit line contact is formed before, and then the capacitor is formed. A problem with this means is fabrication step increase.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is to provide a larger capacitance while the requirements of alignment allowance for the alignment of an opening for a capacitor contact and a bit line contact, and of electric isolation from an adjacent conducting films.  
           [0011]    The above-described object is achieved by a semiconductor device comprising: a MOSFET including a pair of impurity diffused regions formed on both sides of a gate formed on a semiconductor substrate: an insulation film covering a top of the MOSFET, a through-hole opened on one of the impurity diffused regions being formed in the insulation film; and a capacitor formed at at least a part of an inside of the through-hole, the through-hole having a larger diameter inside than at a surface thereof.  
           [0012]    The above-described object is also achieved by a semiconductor device comprising: a MOSFET including a pair of impurity diffused regions formed on both sides of a gate formed on a semiconductor substrate: an insulation film covering a top of the MOSFET, a through-hole opened on one of the impurity diffused regions being formed in the insulation film; and a capacitor formed at at least a part of an inside of the through-hole, the through-hole having a larger diameter at an intermediate part thereof between a surface thereof and a bottom thereof than at the surface and the bottom thereof.  
           [0013]    The above-described object is also achieved by a method for fabricating a semiconductor device comprising: a MOSFET forming step of forming on a semiconductor substrate a MOSFET including a gate and a pair of impurity diffused regions on both sides of the gate; an insulation film forming step of forming an insulation film for covering the MOSFET; a through-hole forming step of forming in the insulation film a through-hole having a larger diameter inside than at a surface thereof, the through-hole being opened on one of the impurity diffused regions; and a capacitor forming step of forming a capacitor at at least a part of an inside of the through-hole.  
           [0014]    The above-described object is also achieved by a method for fabricating a semiconductor device comprising: a MOSFET forming step of forming on a semiconductor substrate a MOSFET including a gate and a pair of impurity diffused regions on both sides of the gate; an insulation film forming step of forming an insulation film for covering the MOSFET; a through-hole forming step of forming in the insulation film a through-hole having a larger diameter at an intermediate part between a surface thereof and a bottom thereof than the surface and the bottom thereof, the through-hole being opened on one of the impurity diffused regions; and a capacitor forming step of forming a capacitor at at least a part of an inside of the through-hole.  
           [0015]    The above-described object is also achieved by a method for fabricating a semiconductor device comprising: a MOSFET forming step of forming on a semiconductor substrate a MOSFET including a gate and a pair of impurity diffused regions on both sides of the gate; an insulation film forming step of forming a plurality of insulation films being laminated for covering the MOSFET; a through-hole forming step of forming in the insulation films a through-hole having a larger diameter inside than at a surface thereof, the through-hole being opened on one of the impurity diffused regions; and a capacitor forming step of forming a capacitor at at least a part of an inside of the through-hole.  
           [0016]    In the above-described method for fabricating the semiconductor device, it is preferable that the insulation film forming step includes a step of forming a first insulation film, and a step of forming on the first insulation film a second insulation film having etching characteristics different from those of the first insulation film; and the through-hole forming step includes a step of forming in the first insulation film and the second insulation film the through-hole having substantially the same opening diameter, and a step of retreating the first insulation film inside the through-hole by the use of an etchant having a higher etching rate with respect to the first insulation film than with respect to the second insulation film.  
           [0017]    The above-described object is also achieved by a method for fabricating a semiconductor device comprising: a MOSFET forming step of forming on a semiconductor substrate a MOSFET including a gate and a pair of impurity diffused regions on both sides of the gate; an insulation film forming step of forming a plurality of insulation films being laminated for covering the MOSFET; a through-hole forming step of forming in the insulation films a through-hole having a larger diameter at an intermediate part between a surface thereof and a bottom thereof than the surface and the bottom thereof, the through-hole being opened on one of the impurity diffused regions; and a capacitor forming step of forming a capacitor at at least a part of an inside of the through-hole.  
           [0018]    In the above-described method for fabricating the semiconductor device, it is preferable that the insulation film forming step includes a step of forming a first insulation film, a step of forming on the first insulation film a second insulation film having etching characteristics different from those of the first insulation film, and a step of forming on the second insulation film a third insulation film having etching characteristics different from those of the second insulation film; and the through-hole forming step includes a step of forming in the first insulation film, the second insulation film and the third insulation film the through-hole having substantially the same opening diameter, and a step of retreating the second insulation film inside the through-holes by the use of an etchant having a higher etching rate with respect to the second insulation film than with respect to the first and the third insulation film.  
           [0019]    In the above-described method for fabricating the semiconductor device, it is preferable that the insulation film forming step includes a step of forming a first insulation film, a step of forming on the first insulation film a second insulation film having etching characteristics different from those of the first insulation film, and a step of forming on the second insulation film a third insulation film having etching characteristics different from those of the second insulation film; and the through-hole forming step includes a step of etching the second and the third insulation films by the use of an etchant which has a higher etching rate with respect to the second insulation film than with respect to the third insulation film and substantially anisotropically contains an isotropic component, and a step of isotropically etching the first insulation film by the use of an etchant having a higher etching rate with respect to the first insulation film than with respect to the second insulation film.  
           [0020]    In the above-described method for fabricating the semiconductor device, it is preferable that the etchant for etching the first insulation film has a higher etching rate with respect to the second insulation film than with respect to the third insulation film.  
           [0021]    In the above-described method for fabricating the semiconductor device, it is preferable that the first insulation film is a silicon nitride film; the second insulation film is a silicon oxide film containing boron and/or phosphorus: the third insulation film is a non-doped silicon oxide film. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIGS. 1A and 1B are diagrammatic sectional views of the semiconductor device, which explain the principle of the present invention.  
         [0023]    FIGS.  2 A- 2 E are sectional views of the semiconductor device according to a first embodiment of the present invention in the steps of the method for fabricating the same, which explain the same and the method for fabricating the same.  
         [0024]    [0024]FIGS. 3A and 3B are sectional views of the conventional semiconductor device, which explain the problems of the conventional semiconductor device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    The principle of the present invention will be explained with reference to FIGS. 1A and 1B.  
         [0026]    A field oxide film  2  for defining a device region is formed on a semiconductor substrate  1 . A gate  4  of a memory cell transistor is formed on a gate oxide film  3  in the device region defined by the field oxide film  2 . A part of the gate  4  is extended on the field oxide film  2 . An insulation film  5  is formed on the side wall and the upper surface of the gate  4 . On the semiconductor substrate  1  with the memory cell transistor including the gate covered with the insulation film  5  are formed a silicon nitride (Si 3 N 4 ) film  6  which is to function as an etching stopper, an inter-layer insulation film  7  of boro-phospho-silicate glass (BPSG) film, and inter-layer insulation film  8  of silicon dioxide (SiO 2 ) film are formed. A through-hole is formed through the silicon nitride film  6  and the inter-layer insulation films  7 ,  8 . The through-hole has a larger diameter in the inter-layer insulation film  7  than in the silicon nitride film  6  and the inter-layer insulation film  8 . A storage electrode  9  of a capacitor connected to the semiconductor substrate  1  is formed on the inside wall and the bottom of the through-hole. An opposed electrode  11  of the capacitor is formed on the surface of the storage electrode  9  through a dielectric film  10 .  
         [0027]    As described above, the present invention is characterized in that, as shown in FIG. 1A, the through-hole for the capacitor contact has a larger diameter at an intermediate part thereof between the opening (the surface) and the contact (the bottom) than at the opening and the contact thereof.  
         [0028]    An inside diameter of the through-hole can be increased up to a size which permits electric isolation from the adjacent conducting film without considering an alignment allowance.  
         [0029]    The through-hole is formed by forming the inter-layer insulation film of two or more insulation layers having etching rates different from each other, anisotropically etching the inter-layer insulation film, and isotropically etching the inter-layer insulation film by the use of an etching rate difference between insulation layers.  
         [0030]    A configuration of the through-hole can be decided arbitrarily depending on a structure of the inter-layer insulation film, and can be as shown in FIG. 1B.  
         [0031]    As described above, the capacitor can have an increased capacitance while the requirements of an alignment allowance for alignment of the opening and the contact of the capacitor contact, and electric isolation from the adjacent conducting film are met.  
       A First Embodiment  
       [0032]    FIGS.  2 A- 2 E are explanatory views of a first embodiment of the present invention.  
         [0033]    As shown in FIG. 2A, a 200 nm-thick field oxide film  2  is formed on a p-type silicon (p-Si) substrate  1  by local oxidation method, and an about 7 nm-thick gate oxide film  3  is formed by thermal oxidation in an active region surrounded by the field oxide film  2 .  
         [0034]    Then, a 150 nm-thick polycrystalline silicon film  4  containing phosphorus is grown by chemical vapor deposition (CVD) method, and a 100 nm-thick SiO 2  film  5 A is grown on the polycrystalline silicon film  4 . Then, the polycrystalline silicon film  4  and the SiO 2  film  5 A are patterned by lithography step and anisotropic etching to form a gate  4  of a MOSFET.  
         [0035]    Then, with the gate  4  and the field oxide film  2  as a mask, a 1E14 cm −2  dose of phosphorus ions (P + ) is implanted at a 20 keV energy to form a source/drain diffused layer  1 A of the MOSFET.  
         [0036]    As shown in FIG. 2B, a 100 nm-thick SiO 2  film is grown and is subjected to anisotropic etching to form a sidewall spacer  5 .  
         [0037]    As shown in FIG. 2C, a 100 nm-thick Si 3 N 4  film  6  and a 2 μm-thick BPSG film  7  are grown by CVD method.  
         [0038]    Then, the BPSG film  7  is reflowed by a thermal treatment for about 15 minutes in a 850° C. nitrogen atmosphere to planarize the surface of the substrate.  
         [0039]    It is possible that a SiO 2  film may be provided below the Si 3 N 4  film  6 .  
         [0040]    Then, a 200 nm-thick SiO 2  film  8  is grown by CVD method. The SiO 2  film  8  may function as a hard mask for forming a through-hole. A conducting film, e.g. a polycrystalline silicon film, may be also applicable for the hard mask, but in this case it should be noted that a capacitor and a bit line do not short-circuit with each other.  
         [0041]    Then, a resist pattern for forming the through-hole is formed by photolithography step.  
         [0042]    Then, the SiO 2  film  8  and the BPSG film  7  are etched with an etching gas (e.g., C 4 F 8 +Ar+CO+O 2 ) having an etching selectivity with respect to the Si 3 N 4  film  6 . Then, the Si 3 N 4  film  6 , which has functioned as the etching stopper is anisotropically etched to form the through-hole  13 .  
         [0043]    Then, the BPSG film  7  is selectively etched to be retreated by the use of a etching rate difference by isotopic etching, such as hydrofluoric acid treatment or others to form a larger-diameter portion in the through-hole. The larger-diameter portion may have a size which permits electric isolation from the adjacent conducting film.  
         [0044]    The through-hole for exposing the source/drain diffused region  1 A is extended on the gate  4 . Unless the Si 3 N 4  film  6  is etched under good control, a storage electrode to be formed in the through-hole and the gate  4  short-circuit with each other. This must be kept in mind.  
         [0045]    In a case that a contact diameter is large, or a disalignment in the photolithography is large, the through-hole is adversely extended even on the field oxide film. In etching the Si 3 N 4  film  6 , a projection of the capacitor in the field oxide film  2  as shown in FIG. 3A is formed, and dielectric breakdown of the dielectric film of the capacitor tends to occur due to electric field concentration. The projection must be avoided. A size of the contact is thus limited, and a contact diameter is generally determined by both an alignment allowance and the electric isolation.  
         [0046]    As shown in FIG. 2D, a 100 nm-thick polycrystalline silicon film containing phosphorus is grown by CVD method. The polycrystalline silicon film except that inside the through-holes  13  is removed by mechanical chemical polishing to form the storage electrodes  9  of the capacitors in respective through-holes  13 . Reference numeral  14  is a lead portion for a bit line.  
         [0047]    A 5 nm-thick Si 3 N 4  film  10  is grown on the surface of the storage electrode  9  by CVD method. Then, the Si 3 N 4  film  10  is oxidized to form the dielectric film  10  of a silicon oxynitride film. Then, a 100 nm-thick polycrystalline silicon film containing phosphorus is grown to form the opposed electrode  11  of the capacitor.  
         [0048]    Then, the polycrystalline silicon film is etched by lithography step to form an opening  15  of the Si 3 N 4  film of the lead portion of a bit line.  
         [0049]    As shown in FIG. 2E, a 350 nm-thick BPSG film  16  is grown on the entire surface of the substrate by CVD method. Then, the BPSG film  16  is reflowed under the above-described conditions to planarize the surface of the substrate.  
         [0050]    It is possible that an SiO 2  film is grown in place of the BPSG film and is planarized by mechanical chemical polishing.  
         [0051]    Then, the BPSG film  16  is etched by lithography step to form an opening  17  in the BPSG film  16  in the lead portion for a bit line.  
         [0052]    Although a portion of a larger-hole diameter is present inside the capacitor, the opening of the through-hole has the same diameter as the conventional opening, and an alignment allowance of the bit line contact is as conventional.  
         [0053]    Then, Ti film, TiN film and W film are grown by CVD method in the stated order and patterned by lithography step to form a bit line  18 .  
         [0054]    Thus, hole diameters of the opening and the contact are defined in accordance with the requirements of an alignment allowance and electric isolation, but a through-hole whose inside diameter is determined by the requirement of electric isolation alone is formed, whereby the capacitor can easily have an increased capacitance with the conventional alignment allowance.  
       A Second Embodiment  
       [0055]    The second embodiment is different from the first embodiment in the formation of the through-hole of the first embodiment shown in FIG. 2C.  
         [0056]    As shown in FIG. 2C, a resist pattern for forming a through-hole is formed. Then, an SiO 2  film  8  and a BPSG film  7  are etched with an etching gas having a etching selectivity with respect to an Si 3 N 4  film  6 . Here the etching gas which has an etching rate having a relationship of 
         BPSG&gt;SiO 2 &gt;Si 3 N 4 , 
         [0057]    and is anisotropic and has a trace of an isotopic component, e.g., C 4 F 8 +Ar+CO+O 2 , is used, whereby the BPSG film  7  is a little transversely extended. An extension depends on a film thickness of the BPSG film  7 .  
         [0058]    Then, the Si 3  N 4  film  6  is isotopically etched to form a through-hole  13  for exposing a source/drain diffused region  1 A of a MOSFET.  
         [0059]    In a case that the Si 3 N 4  film  6  is isotopically etched, an etching gas having an etching rate having a relationship of 
         Si 3 N 4 &gt;BPSG&gt;SiO 2 , 
         [0060]    e.g., SF 6 +HBr, is used, and in a case that in etching the SiO 2  film  8  and the BPSG film  7 , the BPSG film  7  is transversely much extended, the through-hole  13  has a configuration having a larger diameter inside as shown in FIG. 1A.  
         [0061]    In a case that a transverse extension is small, the through-hole  13  has a reversely tapered configuration as shown in FIG. 1B. In this case, because the etching step of increasing an inside diameter also etches the Si 3 N 4  film  6 , no addition is made to a step number.  
       A Third Embodiment  
       [0062]    In the first and the second embodiments, the capacitor and the lead portion for a bit line are concurrently formed, but the lead portion for a bit line may be opened after the capacitor is formed.  
         [0063]    The contact of the capacitor is formed by self-alignment with the gate but may be formed by the usual alignment.  
       A Fourth Embodiment  
       [0064]    In the first and the second embodiments, the capacitor and the lead portion for a bit line are concurrently opened, but the capacitor may be formed after the lead portion for a bit line is opened and the bit line is formed.  
         [0065]    The contact of the capacitor is formed by self-alignment with the gate but may be formed by the usual alignment.  
         [0066]    In forming the through-hole in the inter-layer insulation film by etching, the BPSG film on the Si 3 N 4  film is anisotropically etched up to midway, then isotopically etched and again anisotropically etched to thereby form a larger-diameter in the through-hole. It is possible to further repeat the anisotropical etching and the isotopic etching to thereby form a plurality of a larger diameter. In this case, to define a diameter of the opening in the surface, the initial etching is anisotropic.  
         [0067]    According to the present invention, the capacitor can have an increase capacitance while the requirements of an alignment allowance for alignment of the opening of the capacitor contact and the bit line contact and electric isolation from the adjacent conducting film.