Abstract:
A method for manufacturing a semiconductor device is provided including the steps of covering the first nitride film formed on the first oxide film within the first region where the first gate insulation film for the first transistor that operates at one voltage is to be formed; performing plasma nitridation for the second nitride film, having a thickness virtually the same as that of the first nitride film, formed on the second oxide film within the second region where the second gate insulation film for the second transistor that operates at another voltage lower than the one voltage is to be formed; and growing the second nitride film.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims priority to Japanese Patent Application No. 2003-424702 filed Dec. 22, 2003 which is hereby expressly incorporated by reference herein in its entirety.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for manufacturing a semiconductor device comprising a plurality of transistors that operate at different voltages.  
         [0004]     2. Related Art  
         [0005]     Conventionally, there is a semiconductor device, as one of the semiconductor devices described above, comprising a transistor that operates at a low voltage (hereinafter referred to as “low-voltage transistor”) and a transistor that operates at a high voltage (hereinafter referred to as “high-voltage transistor”). Both transistors are required to have a gate insulation film having insulating characteristics in accordance with the above operating voltages.  
         [0006]      FIG. 6  is a cross-sectional view of a method for manufacturing the above conventional semiconductor device. The method for manufacturing the conventional semiconductor device will now be described in detail referring to  FIG. 6 .  
         [0007]     Step S 30 : After forming an element isolation film  101  on a silicon substrate  100 , deposit an oxide film (not illustrated) on the entire surface of the silicon substrate  100  by means of thermal oxidation. Then by performing photolithography and etching, form an oxide film  130  within a region  110  where the gate insulation film of the low-voltage transistor is to be formed, as illustrated, and further form a gate oxide film  140  of the high-voltage transistor within a region  120  where the gate insulation film of the high-voltage transistor is to be formed.  
         [0008]     Step S 31 : After applying a photoresist  102  on the entire surface of the silicon substrate  100 , make an opening at a portion  102   a  that approximately corresponds to the oxide film  130  by means of photolithography and, at the same time, leave a portion  102   b  that approximately corresponds to the gate oxide film  140 .  
         [0009]     Step S 32 : After removing the oxide film  130  by performing etching, remove the photoresist  102   b.    
         [0010]     Step S 33 : By performing thermal oxidation, form a gate oxide film  150  of the low-voltage transistor, which is thinner than the gate oxide film  120 , and, at the same time, grow the gate oxide film  140  of the high-voltage transistor slightly.  
         [0011]     Step S 34 : By performing plasma nitridation, form a gate nitride film  160  on the gate oxide film  150  and, at the same time, form a gate nitride film  170 , having a thickness approximately the same as that of the gate nitride film  160 , on the gate oxide film  140 . That is, in the conventional semiconductor device, the gate insulation film of the high-voltage transistor comprises the gate oxide film  140  and the gate nitride film  170 , and the gate insulation film of the low-voltage transistor comprises the gate oxide film  150  and the gate nitride film  160  having a thickness approximately the same as that of the gate nitride film  170 .  
         [0012]     Generally, from the viewpoint that a low-voltage transistor must operate at a low voltage, “thinness” is required for a gate insulation film of the low-voltage transistor. That is, thinness is required for the gate oxide film  150 . On the other hand, from the viewpoints of gate leak and reliability (including aging due to negative bias temperature instability (NBTI)) as well as prevention of impurity penetration, “thickness” is required.  
         [0013]     Here, “gate leak” means that an electric current runs through the gate insulation film and between a source electrode and a gate electrode because the gate insulation film is thin. Also, “impurity penetration” means that impurities penetrate through the gate insulation film and reach a semiconductor substrate, because the gate insulation film is thin, when the impurities are driven into the gate electrode from above in order to reduce the resistance of the gate electrode or when the transistor starts to operate.  
         [0014]     However, in the above method for manufacturing the conventional semiconductor device, the thickness of the gate nitride film  160  of the low-voltage transistor cannot be made thicker than that of the nitride film  170  of the high-voltage transistor. Therefore, in the low-voltage transistor, there has been a problem that the reduction of the above-described gate leak becomes insufficient because the gate oxide film  150  and gate nitride film  160  of the low-voltage transistor are thin.  
       SUMMARY  
       [0015]     In order to solve the above problem, a method for manufacturing the first semiconductor device according to the present invention comprises the steps of covering the first nitride film formed on the first oxide film within the first region where the first gate insulation film for the first transistor that operates at one voltage is to be formed; performing plasma nitridation for the second nitride film, having a thickness virtually the same as that of the first nitride film, formed on the second oxide film within the second region where the second gate insulation film for the second transistor that operates at another voltage lower than the one voltage is to be formed; and growing the second nitride film.  
         [0016]     Based on the method for manufacturing the first semiconductor device according to the present invention, by performing plasma nitridation for the second nitride film for the second transistor that operates at a relatively low voltage, with the first nitride film covered, the thickness of the second nitride film is made thicker than that of the first nitride film, and therefore the occurrence of the above-described gate leak and impurity penetration can be reduced.  
         [0017]     A method for manufacturing the second semiconductor device according to the present invention comprises the steps of covering the first oxide film within the first region, where the first gate insulation film for the first transistor that operates at one voltage is to be formed, with a mask having a plurality of holes piercing from the top surface through to the bottom surface, by placing the bottom surface of the mask facing the first oxide film; forming the first nitride film on the first oxide film by performing plasma nitridation for the first oxide film and the second oxide film within the second region where the second gate insulation film for the second transistor that operates at another voltage lower than the one voltage is to be formed; and forming, on the second oxide film, the second nitride film thicker than the first nitride film.  
         [0018]     Based on the method for manufacturing the second semiconductor device according to the present invention, by performing plasma nitridation for the first oxide film and the second oxide film for the second transistor that operates at a relatively low voltage, with the first oxide film covered with the mask having a plurality of holes, the thickness of the second nitride film formed on the second oxide film is made thicker than that of the first nitride film formed on the first oxide film, and therefore the occurrence of gate leak and impurity penetration can be reduced in the same manner as described in the method for manufacturing the first semiconductor device according to the present invention.  
         [0019]     In the above method for manufacturing the second semiconductor device according to the present invention, it is desirable that the plurality of holes of the mask are formed in a shape of a slit or a lattice. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a drawing of a method for manufacturing a semiconductor device according to the first embodiment (the first half).  
         [0021]      FIG. 2  is a drawing of a method for manufacturing a semiconductor device according to the first embodiment (the second half).  
         [0022]      FIG. 3  is a drawing of a method for manufacturing a semiconductor device according to the second embodiment (the first half).  
         [0023]      FIG. 4  is a drawing of a method for manufacturing a semiconductor device according to the second embodiment (the second half).  
         [0024]      FIGS. 5A  and B are drawings of a mask according to the second embodiment.  
         [0025]      FIG. 6  is a drawing of a method for manufacturing the conventional semiconductor device. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Embodiments of the method for manufacturing a semiconductor device according to the present invention will now be described in detail referring to the accompanying drawings. Semiconductor devices according to the first embodiment and the second embodiment have, the same as the semiconductor device described in Background, a low-voltage transistor and a high-voltage transistor, while the thickness of a gate nitride film for the low-voltage transistor is thicker than that of the high-voltage transistor.  
       FIRST EMBODIMENT  
       [0027]      FIG. 1  and  FIG. 2  are cross-sectional views of a method for manufacturing a semiconductor device according to the first embodiment. The method for manufacturing a semiconductor device according to the first embodiment will now be described in detail referring to  FIG. 1 .  
         [0028]     Step S 10 : On a surface-polished silicon substrate  10 , on which an element isolation film of a shallow trench isolation (STI) film  11  is formed, form an oxide film (not illustrated) that is the original of a gate oxide film  30  of the high-voltage transistor by means of thermal oxidation. Then, by perform photolithography and etching for the relevant oxide film, form an oxide film  20  and a gate oxide film  30  of the high-voltage transistor, each of which takes an approximate rectangular shape as illustrated, within a region where a gate insulation film of the low-voltage transistor is to be formed and a region where a gate insulation film of the high-voltage transistor is to be formed.  
         [0029]     Step S 11 : After applying photoresist (not illustrated) on the entire surfaces of the silicon substrate  10 , oxide film  20  and gate oxide film  30 , followed by exposure using a mask (not illustrated) that is patterned so that only a part of the photoresist  31  approximately corresponding to the gate oxide film  30  may remain, perform etching. Thus, make the oxide film  20  exposed and, at the same time, coat the gate oxide film  30  with the photoresist  31 .  
         [0030]     Step S 12 : By performing wet etching using an etching solution including, for example, hydrofluoric acid on the entire surface of the silicon substrate  10 , remove the oxide film  20 . Further, dissolve and remove the photoresist  31 .  
         [0031]     Step S 13 : By performing thermal oxidation, deposit a gate oxide film  21  of the low-voltage transistor and, at the same time, grow the gate oxide film  30  of the high-voltage transistor slightly.  
         [0032]     Step S 14 : By performing plasma nitridation, grow a gate nitride film  22  and a gate nitride film  32 , which have the same thickness, on the gate oxide film  21  of the low-voltage transistor and the gate oxide film  30  of the high-voltage transistor.  
         [0033]     Step S 15 : After applying photoresist (not illustrated) on the entire surfaces of the silicon substrate  10 , gate nitride film  22  and gate nitride film  32 , followed by exposure using a mask (not illustrated) that is patterned so that only a part of the photoresist  33  approximately corresponding to the gate nitride film  32  may remain, perform etching. Thus, make the gate nitride film  22  of the low-voltage transistor exposed and, at the same time, coat the nitride film  32  of the high-voltage transistor with the photoresist  33 .  
         [0034]     Step S 16 : By performing the same plasma nitridation as that in Step S 14  and thus growing the gate nitride film  22  of the low-voltage transistor, make the gate nitride film  22  of the low-voltage transistor thicker than the gate nitride film  32  of the high-voltage transistor.  
         [0035]     Step S 17 : Remove the photoresist  33 .  
         [0036]     As described above, in the method for manufacturing a semiconductor device according to the first embodiment, since the gate nitride film  32  of the high-voltage transistor is coated with the photoresist  33  and, with the nitride film  22  of the low-voltage transistor exposed, plasma nitridation is performed on the entire surface of the silicon substrate  10 , the gate nitride film  22  of the low-voltage transistor can be grown without growing the gate nitride film  32  of the high-voltage transistor. That is, the gate nitride film  22  of the low-voltage transistor can be made thicker than the gate nitride film  32  of the high-voltage transistor, which makes it possible to control the occurrence of gate leak and impurity penetration on the low-voltage transistor.  
       SECOND EMBODIMENT  
       [0037]      FIG. 3  and  FIG. 4  are cross-sectional views of a method for manufacturing a semiconductor device according to the second embodiment. The method for manufacturing a semiconductor device according to the second embodiment will now be described in detail referring to  FIG. 3  and  FIG. 4 .  
         [0038]     Step S 20 : On a surface-polished silicon substrate  40 , on which an STI film  41  is formed, form an oxide film (not illustrated) that is the original of a gate oxide film  40  of the high-voltage transistor by means of thermal oxidation. Then, by perform photolithography and etching for the relevant oxide film, form an oxide film  50  and a gate oxide film  60  of the high-voltage transistor, each of which takes an approximate rectangular shape as illustrated, within a region where a gate insulation film of the low-voltage transistor is to be formed and a region where a gate insulation film of the high-voltage transistor is to be formed.  
         [0039]     Step S 21 : After applying photoresist (not illustrated) on the entire surfaces of the silicon substrate  40 , oxide film  50  and gate oxide film  60 , followed by exposure using a mask (not illustrated) that is patterned so that only a portion approximately corresponding to the gate oxide film  60  may remain, perform etching. Thus, make the oxide film  50  exposed and, meanwhile, coat the gate oxide film  60  of the high-voltage transistor with the photoresist  61 .  
         [0040]     Step S 22 : By performing wet etching on the entire surface of the silicon substrate  40 , remove the oxide film  50 . Further, remove the photoresist  61 .  
         [0041]     Step S 23 : By performing thermal oxidation, deposit a gate oxide film  51  of the low-voltage transistor.  
         [0042]     Step S 24 : Cover the gate oxide film  60  with a mask having a plurality of holes piercing from the top surface to the bottom surface, more specifically, a mask  62 , which is a mask  62   a  having a plurality of parallel slits as shown in  FIG. 5A , or a mask  62   b  having a plurality of rectangular holes that are placed in a lattice shape as shown in  FIG. 5B .  
         [0043]     Step S 25 : Perform plasma nitridation with the gate oxide film  50  entirely exposed and the gate oxide film  60  partially exposed by the mask  62 . Thus, form a gate nitride film  52  on the entire surface of the gate oxide film  50  and, at the same time, dispersively form a gate nitride film  63  on the surface of the gate oxide film  60 .  
         [0044]     Step S 26 : Remove the mask  62 .  
         [0045]     Step S 27 : In order to diffuse and stabilize the gate nitride film  63  on the gate oxide film  60 , perform annealing.  
         [0046]     As described above, in the method for manufacturing a semiconductor device according to the second embodiment, since plasma nitridation is performed with the gate oxide film  60  of the high-voltage transistor covered with the mask  62  having a plurality of holes piercing between the top surface and the bottom surface and also with the gate oxide film  50  of the low-voltage transistor exposed, the gate nitride film  63  having a thickness in accordance with the size and layout pattern of slits or lattice-shaped holes can be formed on the gate oxide film  60  of the high-voltage transistor, and further the gate nitride film  52  thicker than the gate nitride film  63  of the high-voltage transistor can be formed on the gate oxide film  50  of the low-voltage transistor. Thus, the same as the method for manufacturing a semiconductor device according to the first embodiment, it becomes possible to control the occurrence of gate leak and impurity penetration on the low-voltage transistor.