Abstract:
A method for fabricating a triple well in a semiconductor device, includes the steps of forming a first well of a first conductive type with a first concentration lower than a first target concentration, wherein the first concentration is the minimum dose capable of isolating neighboring wells each other and forming a second well of a second conductive type with a second concentration higher than a second target concentration, wherein the second well includes a first region surrounded by the first well and a second region isolated from the first region by the first well.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for fabricating a semiconductor device; and, more particularly, to a method for fabricating a triple well in a semiconductor device.  
         DESCRIPTION OF RELATED ART  
         [0002]    Generally, semiconductor products are manufactured by a complementary metal-oxide-semiconductor field effect transistor (CMOSFET) technology, that is two kinds of field effect transistor, i.e., NMOSFET and PMOSFET, are configured on one wafer. In order to manufacture the NMOSFET and PMOSFET on one wafer at the same time, a well formation technology is required to divide the NMOSFET and PMOSFET.  
           [0003]    A typical well formation technology requires a thermal treatment process of a high temperature for a long time after an ion-implantation process is carried out with a low energy. Since the well formation technology requires a thermal treatment for a long time, there is a disadvantage for an aspect of a semiconductor device fabrication cost and, since a doping concentration of the ion is uniformly decreased in a vertically downward direction from a surface of a wafer. Thus, a control of the semiconductor device is limited.  
           [0004]    A profiled well formation technology is recently tried to solve the above problems. In accordance with the profiled well formation technology, a ions are implanted with high-energy to a desired depth, and then a simple thermal treatment process is carried out in order to prevent punchthrough, latch-up and the like.  
           [0005]    The well formation technology is classified with a twin-well and a triple well according to the number of kinds of wells formed on one wafer. The triple well is composed of two p-wells and one deep n-well surrounding one of the p-wells. The triple well has advantages that a property of an NMOSFET formed on the p-well can be differently controlled and the NMOSFET has a good property against an external noise. Accordingly, the well formation technology is recently changed from a diffusion twin-well process to the profiled triple well process.  
           [0006]    Generally, the NMOSFET formed on the p-well, which is surrounded by the n-well of the triple well, is used as a cell transistor. As a gate length of the cell transistor is decreased, threshold voltage thereof is also decreased, so that a threshold voltage roll-off is caused. Namely, a threshold voltage distribution is broadened with a lack of uniformity. The threshold voltage distribution is broadened, since gate length variation is increased as a size of the transistor becomes smaller and a threshold voltage roll-off of a small transistor is increased. Therefore, in order to secure a uniform characteristic of a cell transistor, it is preferable to have uniform threshold voltage distribution and to solve the threshold voltage roll-off.  
           [0007]    [0007]FIGS. 1A to  1 E are cross sectional views illustrating a fabrication process of a semiconductor device having a triple well according to the prior art.  
           [0008]    Referring to FIG. 1A, after a field oxide layer  12  is formed through a shallow trench isolation (STI) on a semiconductor substrate  11 , a photosensitive layer is coated and a first mask  13  with the photosensitive layer is formed through a patterning process including exposing and developing processes.  
           [0009]    Subsequently, the first mask  13  is used as an ion implantation mask, and n-type impurities are implanted into the semiconductor substrate  11  by using a high energy ion implanter. Thus, a first region  14  of a profiled n-well is formed in the semiconductor substrate  11 .  
           [0010]    Referring to FIG. 1B, a second mask  15  is formed by patterning a photosensitive layer coated on the semiconductor substrate  11  through exposing and developing processes, the second mask  15  is employed as an ion implantation mask and n-type impurities are implanted into the semiconductor substrate  11  by using a high energy ion implanter, so that second and third regions  16  and  17  of a profiled n-well are formed.  
           [0011]    The second region  16  of the profiled n-well is a middle n-well ion implantation region and the third region  17  is a p channel field stop ion implantation region. A numeral reference ‘ 18 ’ in FIG. 1B shows a doping profile of the profiled n-well.  
           [0012]    Referring to FIG. 1C, after stripping the first and second masks  13  and  15 , a third mask  19  is formed by patterning a photosensitive layer coated on the semiconductor substrate  11  through exposing and developing processes. The third mask  19  is employed as an ion implantation mask and p-type impurities are implanted into the semiconductor substrate  11  by using a high energy ion implanter, so that first and second regions  20 ,  21  are formed. The first and second regions  20 ,  21  represent a p-well ion implantation region and an n-channel field stop ion implantation region, respectively. A numeral reference  22  shows a doping profile of the profiled p-well.  
           [0013]    Referring to FIG. 1D, the profiled n-well and p-well are activated through a furnace annealing process, so that a triple well formation process is completed. The triple well is composed of a first p-well  23 , a deep n-well  24  adjacent to the first p-well  23 , a second p-well  25  surrounded by the deep n-well  24  and positioned at a predetermined distance from the first p-well  23 .  
           [0014]    A transistor to be formed on the second p-well  25  is independent of a transistor to be formed on the first p-well  23 . The second p-well  25  has an advantage that can be protected from an external voltage or noise suddenly applied, because the second p-well  25  is surrounded by the deep n-well  24 . For this reason, a cell transistor is formed on the second p-well  25 .  
           [0015]    A photosensitive layer is coated on the semiconductor substrate  11 , in which the triple well is completely formed, and a fourth mask  26 , exposing a portion of the semiconductor substrate  11  to form a cell transistor, is formed through a patterning process with exposing and developing the photosensitive layer. Thereafter, p-type impurities are implanted into the overall second p-well  25  to adjust a threshold voltage of the cell transistor by using the fourth mask  26  as an ion implantation mask, so that a threshold voltage ion implantation region  27  is formed.  
           [0016]    Referring to FIG. 1E, after removing the fourth mask  26 , a gate oxide layer  28  and a gate electrode  29 A are formed on a predetermined region of the semiconductor substrate  11 , and a spacer  29 B is formed on sidewalls of the gate electrode  29 A. Thereafter, impurities of n and p types are implanted to form an n +  source/drain region  30 A of an NMOSFET and a p +  pocket ion implantation region (source/drain region)  30 B for a PMOSFET in a peripheral circuit.  
           [0017]    [0017]FIG. 2 is a detailed cross sectional view along with line ‘X’ in FIG. 1E.  
           [0018]    Referring to FIG. 2, even though the p-type impurities are implanted to adjust the threshold voltage of the cell transistor according to the prior art as mentioned above, junction loss occurs due to a counter doping effect, since the threshold voltage ion implantation region  27  and the n+ source/drain region of cell junction are mostly overlapped. Thus, resistance and electric field are increased and a refresh time is reduced. Accordingly, reliability of a device is deteriorated.  
           [0019]    In order to adjust the threshold voltage necessary to an operation of the cell transistor, pre-determined p-type impurities are required to be implanted in a channel region. If the dose of the p-type impurities is increased, the threshold voltage is increased as much as required. However, the counter doping effect is considerably increased and a refresh characteristic is became worse. Therefore, there is a problem of selecting one appropriate ion implantation condition between to conditions, for forming the source/drain and adjusting the threshold voltage of the cell transistor which have a trade-off relation.  
           [0020]    [0020]FIG. 3 is a graph showing dopant profiles analyzed with secondary ion mass spectrometer (SIMS) after carrying out a thermal treatment process to the ion implantation region for the threshold voltage and the source/drain region.  
           [0021]    Referring to FIG. 3, if an ion-implantation dose of boron is increased in order to adjust the threshold voltage necessary to an operation of the cell transistor, the threshold voltage of the cell transistor may be increased to a desired voltage. However, a counter doping effect is also increased, so that a concentration of a source/drain region that is in a cell junction region is decreased and an electrical loss is largely caused.  
           [0022]    Accordingly, a refresh characteristic of a cell junction formed as above is more deteriorated.  
         SUMMARY OF THE INVENTION  
         [0023]    It is, therefore, an object of the present invention to provide a method for fabricating a triple well capable of obtaining a threshold voltage of a transistor and suppressing a counter doping effect in a semiconductor device.  
           [0024]    In accordance with an aspect of the present invention, there is provided to a method for fabricating a triple well in a semiconductor device, comprising the steps of: forming a first well of a first conductive type with a first concentration lower than a first target concentration, wherein the first concentration is the minimum dose capable of isolating neighboring wells each other; and forming a second well of a second conductive type with a second concentration higher than a second target concentration, wherein the second well includes a first region surrounded by the first well and a second region isolated from the first region by the first well.  
           [0025]    In accordance with another aspect of the present invention, there is provided a method forming a semiconductor device, comprising steps of: forming a first well of a first conductive type with a first concentration lower than a first target concentration in a semiconductor substrate, wherein the first concentration is the minimum dose capable of isolating neighboring wells each other; forming a second well of a second conductive type with a second concentration higher than a second target concentration, the second well surrounded by the first well; implanting impurities of the second conductive type to adjust a threshold voltage of a transistor to be formed on a surface of the semiconductor substrate; forming a gate oxide layer and a gate electrode on a predetermined region on the second well; and forming source/drain regions with a first conductive type at both sides of the gate electrode in the second well. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The above and other objects and features of the instant invention will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, in which:  
         [0027]    [0027]FIGS. 1A to  1 E are cross sectional views illustrating a fabrication process of a semiconductor device having a triple well according to the prior art;  
         [0028]    [0028]FIG. 2 is a detailed cross sectional view alone a line ‘X’ in FIG. 1E;  
         [0029]    [0029]FIG. 3 is a graph showing dopant profiles analyzed with SIMS after carrying out a thermal treatment process to the ion implantation region for the threshold voltage and the source/drain region;  
         [0030]    [0030]FIGS. 4A to  4 E are cross sectional views illustrating a fabricating process of a semiconductor device having a triple well structure in accordance with the preferred embodiment of the present invention;  
         [0031]    [0031]FIG. 5 is a graph showing a p+-junction/n-well leakage current characteristic according to a deep-well ion implantation process conditions; and  
         [0032]    [0032]FIG. 6 is a graph showing a threshold voltage roll-off characteristic of the second p-well according to p-well ion implantation process conditions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]    [0033]FIGS. 4A to  4 E are cross-sectional views illustrating a fabricating process of a semiconductor device having a triple well structure in accordance with the present invention.  
         [0034]    Referring to FIG. 4A, after a field oxide layer  32  for isolation between devices is formed on a semiconductor substrate  31  through a shallow trench isolation (STI) process, a photosensitive layer is coated on the semiconductor substrate  32  and then a first mask  33  is formed by a patterning process with exposing and developing processes. The first mask  33  is formed with a photosensitive layer suitable for a high energy ion implantation at a density of about 1 g/cm 3  to about 10 g/cm 3  and at a thickness of over 2.5 μm.  
         [0035]    Subsequently,  31 P ions of an n-type dopant are implanted in a predetermined portion of the semiconductor substrate  31 , which is exposed by the first mask  33 , with an ion implanter, so that a deep n-well ion implantation region  34  is formed. At this time, considering a junction leakage current, the ion implantation process is carried out with an ion implantation energy of about 0.6 MeV to about 1.6 MeV and with  31 p ion dose of about 5×10 12  ions/cm 2  to about 1.5×10 13  ions/cm 2  less than that a target dose. The reason using the dose less than that of the target dose is to reduce a junction leakage current, and the dose less than the target dose, is the minimum dose necessary for forming the n-well ion implantation region  34  isolating adjacent p-wells.  
         [0036]    Accordingly, dose reduction at the deep n-well ion implantation region  34  has an effect that dopant concentration of adjacent p-wells on which cell transistors will be formed, is increased.  
         [0037]    Referring to FIG. 4B, after a photosensitive layer is coated on the semiconductor substrate  31 , a second mask  35  is formed by a patterning process with exposing and developing processes. N-type dopant is implanted by using the first and second masks  33  and  35  as an ion implantation mask with a high energy ion implanter, so that a middle ion implantation region  36  and a p channel field stop ion implantation region  37  are formed.  
         [0038]    [0038] 31 P ions are used at the middle n-well ion implantation process. It is preferable that dose of the  31 P ions is of about 5×10 12  ions/cm 2  to about 2×10 13  ions/cm 2  and an ion implantation energy is of about 500 keV to about 600 keV. Also,  31 P ions are used at the p channel field stop ion implantation process. It is preferable that dose of the  31 P ions is of about 5×10 11  ions/cm 2  to about 2×10 13  ions/cm 2  and an ion implantation energy is of about 150 kev to about 300 keV. The n-well ion implantation region  34 , the middle n-well ion implantation region  36  and the p channel field stop ion implantation region  37  form a profiled n-well through the above implantation process. The numeral reference ‘ 38 ’ denoted in FIG. 4B represents a doping profile of the profiled n-well.  
         [0039]    Referring to FIG. 4C, after the second mask  35  is stripped, a third mask  39  is formed by a patterning process with exposing and developing processes after coating a photosensitive layer. A p-type dopant is implanted by using the third mask  39  as an ion implantation mask with a high energy ion implanter, so that a p-well ion implantation region  40  and an n-channel field stop ion implantation region  41  are formed. At this time, it preferable that the p-well implantation is carried out with boron (B) of a concentration of about 5×10 12  ions/cm 2  to about 3×10 13  ions/cm 2  and in an ion implantation energy of about 180 keV to about 300 keV. The n-channel field stop ion implantation process is carried out with boron (B) ions of a concentration of about 5×10 11  ions/cm 2  to about 1×10 13  ions/cm 2  and in an ion implantation energy of about 50 keV to about 80 keV. The p-well ion implantation region  40  and the n-channel field stop ion implantation region  41  form a profiled p-well. The numeral reference ‘ 42 ’ denoted in FIG. 4C represents a doping profile of the profiled p-well.  
         [0040]    Since the dose in the p-well ion implantation is reduced less than that in the n-well ion implantation, the dose of p-type dopant can be increased.  
         [0041]    Subsequently, referring to FIG. 4C, after the third mask  39  is stripped, the profiled n-well and the dopant implanted in the p-well are activated through a furnace annealing process so that a triple well formation process is completed. The triple well includes a first p-well  43 , a deep n-well  44  adjacent to the first p-well  43  and a second p-well  45  surrounded by the deep n-well  44  and positioned at a predetermined distance with the first p-well  43 .  
         [0042]    A transistor to be formed on the second p-well  45 , is independent of a transistor to be formed on the first p-well  43 . The second p-well  45  has an advantage that can be protected from an external voltage or noise suddenly applied, because the second p-well  45  is surrounded by the deep n-well  24 . For this reason, a cell transistor is to be formed on the second p-well  45 .  
         [0043]    Referring to FIG. 4D, a photosensitive layer is coated on the semiconductor substrate  31  and a fourth mask  46  is formed through a patterning process with exposing and developing processes. The fourth mask  46  exposes a portion of the semiconductor substrate  31 , i.e. the second p-well  45 , on which a cell transistor is to be formed. After forming the fourth mask  46 , p-type impurities are implanted into the overall second p-well  45  to adjust a threshold voltage of the cell transistor by using the fourth mask  46  as an ion implantation mask, so that a threshold voltage ion implantation region  47  is formed.  
         [0044]    Boron (B) or boron diflouride (BF 2 ) is implanted as the p-type impurities to adjust the threshold voltage of the cell transistor. At this time, Boron (B) or boron diflouride (BF 2 ) is implanted with dose of about 5×10 12  ions/cm 2  to about 1.5×10 13  ions/cm 2 . In case that the boron (B) is implanted, it is preferable that an ion implantation energy is about 15 keV to about 40 keV and, in case that the boron diflouride (BF 2 ) is implanted, it is preferable that an ion implantation energy is about 30 keV to about 40 keV. When the boron difluoride is implanted with an ion implantation energy of 30 keV, it has an identical doping profile to which the boron is implanted with an ion implantation energy of 6.7 keV.  
         [0045]    Referring to FIG. 4E, after removing the fourth mask  46 , a gate oxide layer  48  and a gate electrode  49 A are formed on a predetermined region of the semiconductor substrate  31 , and a spacer  49 B is formed on sidewalls of the gate electrode  49 A. Thereafter, impurities of n and p types are implanted with a blanket ion implantation without a mask process, so that an n +  source/drain region  50 A of an NMOSFET and a p +  pocket ion implantation region  50 B, on a region for a PMOSFET, are formed in the cell transistor and a peripheral circuit, respectively. At this time, a dose of about 1×10 13  ions/cm 2  to about 5×10 13  ions/cm 2  is used to form the n +  source/drain region  50   a.    
         [0046]    As mentioned the above, the threshold voltage of the cell transistor can be increased by controlling a concentration of the semiconductor substrate, that is, the dose of the p-well i in accordance with the present invention, in stead of controlling conditions of a threshold voltage ion implantation and a source/drain ion implantation for the cell transistor.  
         [0047]    The triple well has a shallow well profile, that is, the well profile is near to a channel region.  
         [0048]    [0048]FIG. 5 is a graph showing leakage current characteristic at a p+ junction/n-well according to a deepp-well ion implantation process conditions, such as the dose, an ion implantation energy and so on.  
         [0049]    Referring to FIG. 5, when the ion implantation energy is reduced from about 1.2 MeV to about 1.0 MeV, junction leakage characteristics are mostly similar each other. However, when the dose is reduced, a constant leakage current characteristic can be secured.  
         [0050]    As the dose of the deep n-well is reduced, an annealing process for removing defects caused from a deep n-well ion implantation can be omitted.  
         [0051]    [0051]FIG. 6 is a graph showing a threshold voltage roll-off characteristic of the second p-well according to p-well ion implantation process conditions, such as dose, ion implantation energy and so on. As the ion implantation energy is reduced, a boron concentration of a surface channel is increased so that the threshold voltage is increased.  
         [0052]    As shown in FIG. 6, even though the dose of the ion implantation for adjusting the threshold voltage is not increased, the desired threshold voltage can be obtained. Also, the counter doping effect with the cell junction is reduced and a resistance and an electric field are reduced, so that the refresh time is increased. Namely, a reliability of the device is improved.  
         [0053]    Accordingly, increase of the threshold voltage of the cell transistor and of the refresh time is expected. the process can be simplified in accordance with the present invention, since the well annealing process for removing defects can be omitted.  
         [0054]    While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.