Abstract:
Provided is a metal oxide semiconductor (MOS) capacitor, in which trenches ( 3 ) are formed in a charge accumulation region ( 6 ) of a p-type silicon substrate ( 1 ) to reduce a contact area between the p-type silicon substrate ( 1 ) and a lightly doped n-type well region ( 2 ), thereby reducing a leak current from the lightly doped n-type well region ( 2 ) to the p-type silicon substrate ( 1 ).

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. JP2008-215125 filed on Aug. 25, 2008, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device in which a leak current from a metal-oxide-semiconductor (MOS) capacitor to a silicon substrate is suppressed. 
     2. Description of the Related Art 
     When a MOS capacitor is formed on a silicon substrate and used at the same voltage as that applied to the silicon substrate, an electrode for the silicon substrate side of the MOS capacitor must be formed on a well region of an opposite conductivity to the silicon substrate. When the MOS capacitor has a large area and is used under a high temperature, in particular, the leak current becomes considerably large between the silicon substrate and the well region in which the electrode for the silicon substrate side is formed, causing a problem in circuit construction. 
     Conventional ways of avoiding the above-mentioned leak current include a method involving forming a capacitor with a first polysilicon layer and a second polysilicon layer being opposing electrodes, and a method involving separating, as in silicon on insulator (SOI), the silicon substrate and the well region in which the electrode on the silicon substrate side is formed, by an oxide film. 
     Apart from the problem of the leak current, as a way of realizing high integration of capacitors in a dynamic random access memory (DRAM) cell, conventionally, a trench capacitor which is formed by utilizing a recess surface of a trench formed in a silicon substrate has been used as described in JP 02-165663 A. 
     As has been described above, as a way of suppressing the leak current parasitically flowing from the capacitor to another circuit, when the capacitor is formed of two polysilicon layers, there is a need to add a step of forming the second polysilicon layer, and because the leak current between the electrodes is large compared with a capacitor formed of a silicon substrate and polysilicon interposing a gate oxide film therebetween, quality of an insulating film between the first polysilicon layer and the second polysilicon layer needs to be optimized. Further, when the well and the silicon substrate are separated by the oxide film using a SOI substrate, a cost for the substrate increases, becoming a problem. 
     SUMMARY OF THE INVENTION 
     Instead of using the two polysilicon layers or the SOI substrate as described above, the present invention utilizes a trench capacitor to reduce a contact area between a silicon substrate and a well which serves as an electrode on a silicon substrate side of a capacitor, thereby suppressing a leak current between the silicon substrate and the well which serves as the electrode on the silicon substrate side. 
     Specifically, the present invention provides a semiconductor device including a metal oxide semiconductor (MOS) capacitor, comprising: a silicon substrate of a first conductivity type; a lightly doped well region of a second conductivity type, which is formed by diffusing impurities into the silicon substrate; a charge accumulation region formed in the lightly doped well region of the second conductivity type; trenches formed in the charge accumulation region; a heavily doped region of the second conductivity type, which is formed outside the charge accumulation region and has a higher impurity concentration than an impurity concentration of the lightly doped well region of the second conductivity type; an oxide film formed in the trenches formed in the charge accumulation region and on a surface of the silicon substrate of the first conductivity type; a polysilicon electrode formed on the oxide film; and a substrate-side electrode formed to make contact with the heavily doped region of the second conductivity type. 
     The present invention also provides a semiconductor device including a MOS capacitor, comprising: a silicon substrate of a first conductivity type; a lightly doped well region of a second conductivity type, which is formed by diffusing impurities into the silicon substrate; a charge accumulation region formed in the lightly doped well region of the second conductivity type; trenches formed in the charge accumulation region; a heavily doped region of the second conductivity type, which is formed outside the charge accumulation region and has a higher impurity concentration than an impurity concentration of the lightly doped well region of the second conductivity type; a heavily doped charge accumulation region of the second conductivity type formed in the trenches formed in the charge accumulation region and on a surface of the silicon substrate of the first conductivity type; an oxide film formed on the heavily doped charge accumulation region of the second conductivity type; a polysilicon electrode formed on the oxide film; and a substrate-side electrode formed to make contact with the heavily doped region of the second conductivity type. 
     With the above-mentioned means, the contact area between the silicon substrate of the first conductivity type and the well region of the second conductivity type may be reduced, and hence the leak current between the silicon substrate of the first conductivity type and the well region of the second conductivity type may be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a cross-sectional view of a semiconductor device according to a first embodiment of the present invention; and 
         FIG. 2  is a cross-sectional view of a semiconductor device according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are described below with reference to the accompanying drawings. 
       FIG. 1  is a cross-sectional view of a semiconductor device  100  according to a first embodiment of the present invention. The semiconductor device  100  has the following structure. For example, in a p-type silicon substrate  1  having a resistance of 20 to 30 Ωcm, a lightly doped n-type well region  2  is formed to a depth of 20 μm with an impurity such as phosphorus at a concentration of about 1×10 16  cm −3 . Further, a heavily doped n-type region  7  to make contact with a substrate-side electrode  8  is formed on a part of a surface of the lightly doped n-type well region  2 . The heavily doped n-type region  7  has a concentration of 1×10 24  cm −3  and uses phosphorus or arsenic as an impurity species. 
     Subsequently, a plurality of trenches  3  each having a depth of 5 to 10 μm and an opening width of 2 to 3 μm are formed on a surface of the silicon substrate  1 . After forming the trenches  3 , the silicon substrate  1  is thermally oxidized to form an oxide film  4  with a thickness of 500 Å on the surface of the silicon substrate  1  and inner walls of the trenches  3 . On the oxide film  4 , a polysilicon film is deposited to a thickness of 4,000 Å, impurities are introduced to the polysilicon film to impart electrical conductivity, and then the polysilicon film is patterned to form a polysilicon electrode  5  on the lightly doped n-type well region  2  including the plurality of trenches  3 . A region below the electrode  5  is called a charge accumulation region  6  and serves as a capacitor. Then, an aluminum alloy is formed to a thickness of approximately 5,000 Å as the substrate-side electrode  8  on the heavily doped n-type region  7 . 
     By forming the trenches  3  in the charge accumulation region  6  as described above, a contact area between the p-type silicon substrate  1  and the lightly doped n-type well region  2  may be reduced, and hence a leak current between the p-type silicon substrate  1  and the lightly doped n-type well region  2  may be reduced. Note that the substrate and the well region have been described to be p-type and n-type, respectively, but the conductivity type may be opposite so that the substrate is n-type and the well region is p-type. 
       FIG. 2  is a cross-sectional view of a semiconductor device  101  according to a second embodiment of the present invention. The semiconductor device  101  has the following structure. For example, in a p-type silicon substrate  1  having a resistance of 20 to 30 Ωcm, a lightly doped n-type well region  2  is formed to a depth of 20 μm with an impurity such as phosphorus at a concentration of about 1×10 16  cm −3 . Further, a heavily doped n-type region  7  to make contact with a substrate-side electrode  8  is formed on a part of a surface of the lightly doped n-type well region  2 . The heavily doped n-type region  7  has a concentration of 1×10 20  cm −3  and uses phosphorus or arsenic as an impurity species. 
     Subsequently, a plurality of trenches  3  each having a depth of 5 to 10 μm and an opening width of 2 to 3 μm are formed in a surface of the silicon substrate  1 . On inner walls of the trenches  3  and the surface of the silicon substrate  1 , a heavily doped n-type charge accumulation region  9  is formed. Note that the heavily doped n-type charge accumulation region  9  has a concentration of 1×10 18  to 1×10 20  cm −3 . Then, the silicon substrate  1  is thermally oxidized to form an oxide film  4  with a thickness of 500 Å on the surface of the silicon substrate  1  and the inner walls of the trenches  3 . On the oxide film  4 , a polysilicon film is deposited to a thickness of 4,000 Å, impurities are introduced to the polysilicon film to impart electrical conductivity, and then the polysilicon film is patterned to form a polysilicon electrode  5  on the lightly doped n-type well region  2  including the plurality of trenches  3 . The electrode  5  is formed to have the same size as a size of the heavily doped n-type charge accumulation region  9 . Then, an aluminum alloy is formed to a thickness of approximately 5,000 Å as the substrate-side electrode  8  on the heavily doped n-type region  7 . 
     By forming the trenches  3  in the charge accumulation region  6  as described above, a contact area between the p-type silicon substrate  1  and the lightly doped n-type well region  2  may be reduced, and hence a leak current between the p-type silicon substrate  1  and the lightly doped n-type well region  2  may be reduced. Further, depletion of the polysilicon electrode  5  during application of a voltage may be prevented by forming the heavily doped n-type charge accumulation region  9 .