Abstract:
A plurality of diffusion layers extending in a first direction is formed at a surface of a semiconductor substrate in a cell region to be provided with the memory cell transistors. A plurality of gate electrodes extending in a second direction perpendicular to the first direction is formed on the semiconductor substrate in the cell regions. An interlayer insulating film is formed on the semiconductor substrate. A first resist film is formed on the interlayer insulating film. The first resist film is provided with openings in positions in alignment with regions between adjacent diffusion layers among the plurality of diffusion layers. a second resist film provided with openings previously designed in an arbitrary manner is formed on the first resist film. Then ions are implanted in the cell region using the first and second resist films as a mask.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method of manufacturing a semiconductor device having memory cell transistors such as a mask ROM (Read Only Memory), and more particularly, to a method of manufacturing a semiconductor device having memory cell transistors by a reduced number of manufacturing steps.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1A is a plan view of a mask ROM in a flat cell structure, and FIG. 1B is an equivalent circuit diagram of the mask ROM.  
           [0005]    In the conventional flat cell type mask ROM, a plurality of N +  diffusion layers are formed in a line-and-space pattern at a surface of a semiconductor substrate (not shown). A plurality of gate electrodes  2  are formed perpendicularly to the N +  diffusion layers  1  also in a line-and-space pattern. The N +  diffusion layer  1  and the gate electrode  2  are insulated from each other by an insulating film (not shown). There is a gate insulating film (not shown) between the gate electrodes  2  and the semiconductor substrate. Thus, a memory cell transistor having the gate electrode  2 , the gate insulating film and two N +  diffusion layers is formed. The surface region of the semiconductor substrate under the gate insulating film corresponds to the channel of the memory cell transistor.  
           [0006]    A channel selected based on a request (requested design) by a customer is, for example, implanted with boron ions. The threshold value of the memory cell transistor having the channel increases. Thus, the mask ROM coding is performed. As a result, as shown in FIGS. 1A and 1B, a transistor  4   a  having a low threshold value and a transistor  4   b  having a high threshold value are formed. A mask used for implanting the boron ions is provided with an opening  3  designed based on the request by the customer as shown in FIG. 1A. The opening  3  is formed in a position in alignment with the channel of the transistor  4   b  having a high threshold value.  
           [0007]    A conventional method of manufacturing the mask ROM will be now described in conjunction with FIGS. 2A to  2 D. FIGS. 2A to  2 D are sectional views showing steps in the conventional method of manufacturing the mask ROM in the order of steps. Note that FIGS. 2A to  2 D are sectional views taken along line X-X in FIG. 1A.  
           [0008]    The semiconductor substrate  5  is defined to a region A having memory cell transistors, and a region D having a peripheral circuit for writing/reading data to/from the memory cell transistors. The region D has a region B having an N-channel MOS transistor, and a region C having a P-channel MOS transistor.  
           [0009]    As shown in FIG. 2A, in the region A, an N +  diffusion layer  1  is formed at the surface of the semiconductor substrate  5 . A gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In the region B, an N-type diffusion layer  16  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In the region C, a P-type diffusion layer  7  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . The gate oxide films or the gate oxide films are each formed at a time in some cases. Thereafter, an interlayer insulating film  6  is formed on the entire surface. The interlayer insulating film  6  is provided with a contact hole  6   a  extending to the N-type diffusion layer  16  and a contact hole  6   b  extending to the P-type diffusion layer  7 .  
           [0010]    As shown in FIG. 2B, phosphorus ions are implanted through the contact holes  6   a  and  6   b.  As a result, an N +  diffusion layer  17  is formed at the surface of the N-type diffusion layer  16  and the P-type diffusion layer  7 , and an N-channel transistor  11   a  is thus formed.  
           [0011]    As shown in FIG. 2C, a photoresist film  8  to expose only the region C is formed. Boron ions are then implanted. As a result, a P +  diffusion layer  9  is formed at the surface of the P-type diffusion layer  7  in place of the N +  diffusion layer  17 , and a P-channel transistor  11   b  is thus formed.  
           [0012]    Then, the photoresist film  8  is removed, and a photoresist film  18  covering the region D is formed as a ROM code mask instead. As shown in FIG. 2D, the photoresist film  18  is provided with openings  3   a  corresponding to the openings  3  in FIG. 1A. More specifically, the openings  3   a  are formed based on the design of the openings  3 . Then, boron ions are implanted through the openings  3   a.  As a result, code implantation layers  10  are selectively formed at the surface of the semiconductor substrate  5  in the region A. At the time, boron ions are not implanted into the transistors  11   a  and  11   b.    
           [0013]    Thereafter, the photoresist film  18  is removed, metal interconnections, bonding pads (not shown) and the like are formed to complete a semiconductor device.  
           [0014]    In the mask ROM, the flat cell structure is mainly used as a cell corresponding to high density integration.  
           [0015]    According to the above method (first prior art), cell transistors with a low threshold value are formed, and after the interlayer insulating film  6  is formed, a ROM code mask (photoresist film  18 ) having the openings  3   a  is formed according to the design. The ROM code mask is formed after the gate electrodes  2  are formed in some cases.  
           [0016]    However, the patterns of the ROM code masks are different depending upon the code content. The pattern density, i.e., the density of the openings  3   a  is different among chips in a single product. Therefore, if the opening  3   a  has a pattern size as designed in a location with a low mask pattern density, the pattern size of the opening  3   a  in a location with a high mask pattern density becomes larger than the designed value. In the mask ROM shown in FIG. 1A, for example, a transistor  4   a  with a low threshold value located in the second row from the top and the second column from the left is surrounded by eight transistors  4   b  with a high threshold value, and therefore the size of the opening  3   a  for the transistor  4   b  is larger than designed. As a result, code implantation layers (P-type diffusion layers)  10  are formed wider than the designed value, so that the threshold value of the transistor  4   a  surrounded by the transistors  4   b  is larger than designed. Consequently, the transistor  4   a  adjacent to the transistor  4   b  with a high threshold value and the transistor  4   a  adjacent to the transistor  4   a  with a low threshold have different threshold values.  
           [0017]    This is more noticeable as the distance between the memory cell transistors is reduced with the reduction of the element size. As the element size has been reduced, a fine pattern is necessary for the ROM code mask, so that a relatively expensive, high precision reticle requiring a long manufacturing period is necessary.  
           [0018]    In the field of the mask ROM, reduction in TAT (Turn Around Time) is a significant object and the use of such a high precision reticle requiring a long manufacturing period is not desirable. Therefore, there is a demand for a new type ROM code mask.  
           [0019]    A method directed to a solution to the difference in the size of the opening caused depending upon the pattern density of the ROM code mask is disclosed, for example, by Japanese Patent Laid-Open Publication No. Hei. 5-283653. The manufacturing method (second prior art) will be now described in conjunction with FIGS. 3A to  3 E. FIGS. 3A to  3 E are sectional views showing steps in the conventional method (second prior art) of manufacturing a mask ROM in the order of steps. FIGS. 3A to  3 E are sectional views taken along line X-X in FIG. 1A.  
           [0020]    As shown in FIG. 3A, in the region A, an N +  diffusion layer  1  is formed at the surface of the semiconductor substrate  5  and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In the region B, an N-type diffusion layer  16  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In the region C, a P-type diffusion layer  7  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . Note that the oxide films or the gate electrodes are each formed simultaneously in some cases. Then, an interlayer insulating film  6  is formed on the entire surface. The interlayer insulating film  6  is provided with a contact hole  6   a  extending to the N-type diffusion layer  16  and a contact hole  6   b  extending to the P-type diffusion layer  7 . The interlayer insulating film  6  is also provided with a contact hole  6   a  in alignment with the channel region in the region A.  
           [0021]    As shown in FIG. 3B, phosphorus ions are implanted through the contact holes  6   a,    6   b,  and  6   c.  As a result, an N +  diffusion layer  17  is formed at the surface of the N-type diffusion layer  16  and the P-type diffusion layer  7 , and an N-channel transistor  11   a  is thus formed.  
           [0022]    As shown in FIG. 3C, a photoresist film  8  to expose only the region C is formed. Boron ions are then implanted. As a result, a P +  diffusion layer  9  is formed in place of the N +  diffusion layer  17  at the surface of the P-type diffusion layer  7 , so that a P-channel transistor  11   b  is formed.  
           [0023]    The photoresist film  8  is then removed and a photoresist film  18  covering the region D is formed as a ROM code mask. As shown in FIG. 3D, the photoresist film  18  is provided with openings  3   a  corresponding to the openings  3  in FIG. 1A. Boron ions are then implanted through the openings  3   a.  As a result, code implantation layers  10  are selectively formed at the surface of the semiconductor substrate  5  in the region A. At the time, boron ions are not implanted into the transistors  11   a  and  11   b.    
           [0024]    The photoresist film  18  is then removed and a photoresist film  19  covering the region D is formed. As shown in FIG. 3E, the photoresist film  19  is patterned to expose contact holes  6   c.  An insulating film  12  is then deposited by liquid phase growth to fill the contact holes  6   c.  Then, using the photoresist film  19  as a mask, the insulating film  12  is etched back, so that the surface level of the insulating film  12  coincides with the surface level of the interlayer insulating film  6 .  
           [0025]    The photoresist film  19  is then removed, metal interconnections, bonding pads (not shown) and the like are formed and a semiconductor device is thus completed.  
           [0026]    According to the second conventional example, not only the photoresist film  18  but also the interlayer insulating film  6  serves as a ROM code mask. Therefore, ion implantation can be achieved through equal size openings.  
           [0027]    According to the second conventional example, however, there must be four masks in total for the ROM coding and the following steps. In other words, there must be a mask for the ROM coding (photoresist film  18 ), a mask for filling the contact hole  6   c  with an insulating film (photoresist film  19 ), a mask for forming metal interconnections (not shown), and a mask for forming pads (not shown). This increases the number of steps and the manufacturing cost as well.  
         SUMMARY OF THE INVENTION  
         [0028]    It is an object of the present invention to provide a method of manufacturing a semiconductor device having memory cell transistors with a reduced number of masks and reduced variation in the threshold values.  
           [0029]    According to the present invention, a method of manufacturing a semiconductor device having memory cell transistors comprises: forming a plurality of diffusion layers extending in a first direction at a surface of a semiconductor substrate in a cell region to be provided with the memory cell transistors; forming a plurality of gate electrodes extending in a second direction perpendicular to the first direction on the semiconductor substrate in the cell regions; forming an interlayer insulating film on the semiconductor substrate; forming a first resist film on the interlayer insulating film; forming a second resist film provided with openings previously designed in an arbitrary manner on the first resist film; and implanting ions in the cell region using the first and second resist films as a mask. The first resist film is provided with openings in positions in alignment with regions between adjacent diffusion layers among the plurality of diffusion layers.  
           [0030]    According to the present invention, variation in the threshold values can be suppressed regardless of the density of mask patterns. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1A is a plan view of a flat cell type mask ROM;  
         [0032]    [0032]FIG. 1B is an equivalent circuit diagram of the mask ROM;  
         [0033]    [0033]FIGS. 2A to  2 D are sectional views showing steps in a conventional method of manufacturing a mask ROM (first prior art) in the order of steps;  
         [0034]    [0034]FIGS. 3A to  3 E are sectional views showing steps in a conventional method of manufacturing a mask ROM (second prior art) in the order of steps;  
         [0035]    [0035]FIGS. 4A to  4 E are sectional views showing steps in a method of manufacturing a mask ROM according to a first embodiment of the present invention in the order of steps; and  
         [0036]    [0036]FIGS. 5A to  5 D are sectional views showing steps in a method of manufacturing a mask ROM according to a second embodiment of the present invention in the order of steps. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]    The preferred embodiments of the present invention will be now described in detail in conjunction with the accompanying drawings. FIGS. 4A to  4 E are sectional views showing steps in a method of manufacturing a mask ROM according to a first embodiment of the present invention in the order of steps. FIGS. 4A to  4 E are sections taken along line X-X in FIG. 1A.  
         [0038]    As shown in FIG. 4A, an N +  diffusion layer (impurity diffusion layer as a source/drain)  1  is formed at the surface of a semiconductor substrate  5  in a region A, and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In a region B, an N-type diffusion layer (impurity diffusion layer as a source/drain)  16  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In a region C, a P-type diffusion layer (impurity diffusion layer as a source/drain)  7  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . The gate oxide films or the gate electrodes may each be formed simultaneously. Then, an interlayer insulating film  6  is formed on the entire surface. The interlayer insulating film  6  is provided with a contact hole  6   a  extending to the N-type diffusion layer  16  and a contact hole  6   b  extending to the P-type diffusion layer  7 .  
         [0039]    As shown in FIG. 4B, phosphorus ions, for example, are implanted through the contact holes  6   a  and  6   b.  As a result, an N +  diffusion layer  17  is formed at the surface of the N-type diffusion layer  16  and the P-type diffusion layer  7 , and an N-channel transistor  11   a  is formed.  
         [0040]    As shown in FIG. 4C, a photoresist film  8  to expose only the region C is formed. Boron ions, for example, are implanted. As a result, a P +  diffusion layer  9  is formed in place of the N +  diffusion layer  17  at the surface of the p-type diffusion layer  7 , and a P-channel transistor  11   b  is formed.  
         [0041]    Then, the photoresist film  8  is removed, and a photoresist film (first resist film)  13  is deposited on the entire surface. The photoresist film  13  may be composed of, for example, photo-curing resin. As shown in FIG. 4D, openings  13   a  in alignment with channel regions in the region A are formed in the photoresist film  13  by patterning. The photoresist film  13  is cured by heating and ultraviolet-ray.  
         [0042]    Then, a photoresist film (second resist film)  20  covering a region D is formed as a ROM code mask on the photoresist film  13 . As shown in FIG. 4E, the photoresist film  20  is provided with openings  3   a  corresponding to the openings  3  in FIG. 1A. Boron ions, for example, are implanted through the openings  3   a.  As a result, code implantation layers  10  are selectively formed at the surface of the semiconductor substrate  5  in the region A. At the time, boron ions are not implanted into the transistors  11   a  and  11   b.    
         [0043]    Then, the photoresist films  20  and  13  are removed at a time, and metal interconnections, bonding pads (not shown) and the like are formed and a semiconductor device is completed.  
         [0044]    According to the first embodiment, not only the photoresist film  20  but also the photoresist film  13  serves as a ROM code mask. More specifically, the opening  3   a  allows the opening  13   a  to be selectively exposed, while variation in the size of the opening  3   a  does not affect the element characteristics. Since the opening  13   a  is formed on the channel regions of all the memory cell transistors, the density is uniform. Therefore, there is little variation in the size of the opening  13   a.  As a result, variation in the size of the code implantation layer  10  is extremely scarce. The transistors  4   a  with a low threshold value have a threshold value substantially uniform regardless of whether it is surrounded by the transistors  4   b  with a high threshold value or not.  
         [0045]    The steps required by the second prior art, i.e., the steps of forming an opening  6   c  in the interlayer insulating film  6 , filling the opening  6   c  with an interlayer insulating film  12 , and etching back the interlayer insulating film  12  are not necessary according to the present embodiment. Therefore, according to the present embodiment, the number of steps can be smaller than that of the second prior art. The number of masks is reduced by one as well. As a result, the TAT can be reduced.  
         [0046]    Furthermore, the photoresist film  13  as an underlying mask for the ROM code mask and the photoresist film  20  as the ROM code mask can be removed at a time, and therefore the number of steps can be prevented from increasing.  
         [0047]    A second embodiment of the present invention will be now described. FIGS. 5A to  5 D are sectional views showing steps in a method of manufacturing a mask ROM according to the second embodiment of the present invention in the order of steps. FIGS. 5A to  5 D are sectional views taken along line X-X in FIG. 1A.  
         [0048]    As shown in FIG. 5A, an N +  diffusion layer  1  is formed at the surface of a semiconductor substrate  5  in a region A, and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In a region B, an N-type diffusion layer  16  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . In a region C, a P-type diffusion layer  7  is formed at the surface of the semiconductor substrate  5 , and a gate oxide film (not shown) and a gate electrode  2  are formed on the semiconductor substrate  5 . Note that the gate oxide films or the gate electrodes may each be formed simultaneously. An interlayer insulating film  6  is then formed on the entire surface. The interlayer insulating film  6  is provided with a contact hole  6   a  extending to an N-type diffusion layer  16  and a contact hole  6   b  extending to the P-type diffusion layer  7 .  
         [0049]    As shown in FIG. 5B, phosphorus ions, for example, are implanted through the contact holes  6   a  and  6   b.  As a result, an N +  diffusion layer  17  is formed at the surface of the N type diffusion layer  16  and the P-type diffusion layer  7 , and an N-channel transistor  11   a  is formed.  
         [0050]    Then, as shown in FIG. 5C, a photoresist film (first resist film)  14  is formed on the interlayer insulating film  6 . The photoresist film  14  may be, for example, composed of photo-curing resin. Openings  14   a  are formed in the photoresist film  14  in alignment with the channel regions in the region and the photoresist film  14  in the region C is removed by patterning. As a result, the contact holes  6   b  is exposed. Boron ions, for example, are then implanted. A P +  diffusion layer  9  is formed at the surface of the P-type diffusion layer  7  in place of the N +  diffusion layer  17  as a result, and a P-channel transistor  11   b  is formed. At the time, the channel region in the region A is covered with the gate electrode  2  and the interlayer insulating film  6 , so that boron ions are not implanted into the channel region in the region A. Then, the photoresist film  14  is cured by heating and ultraviolet-ray.  
         [0051]    A photoresist film (second resist film)  15  covering a region D is then formed as a ROM code mask on the photoresist film  14 . As shown in FIG. 5D, the photoresist film  15  is provided with openings  3   a  corresponding to the openings  3  in FIG. 1A. Boron ions, for example, are implanted through the openings  3   a.  As a result, code implantation layers  10  are selectively formed at the surface of the semiconductor substrate  5  in the region A. At the time, boron ions are not implanted into the transistors  11   a  and  11   b.    
         [0052]    Thereafter, the photoresist films  15  and  14  are removed simultaneously, metal interconnections, bonding pads (not shown) and the like are formed to complete a semiconductor device.  
         [0053]    According to the second embodiment described above, not only the photoresist film  15  but also the photoresist film  14  serves as a ROM code mask, so that the same effects as those by the first embodiment can be provided. In addition, the photoresist film  14  may serve as a mask for ion implantation in the region C and therefore the number of masks can be reduced by one.