Abstract:
There is disclosed an object to supply a power MOSFET semiconductor device high in pressure resistance and low in resistance at a low cost and in a short manufacture turnaround time. In planar-type power MOSFET, a manufacture method comprises forming a trench in a drift region, and forming a body diffusion layer on a trench side wall and bottom portion (forming the trench and subsequently performing diffusion) to obtain a structure. Deep body diffusion formation is effective for obtaining the high pressure resistance and low resistance, but to attain the structure, usually epitaxial growth and selective formation of a deep body region have to be performed a plurality of times, and with an increase of manufacture steps, soaring of manufacture cost and lengthening of manufacture period are caused. However, the present structure can further simply bring about the similar effect. It is possible to supply the power MOSFET semiconductor device at the low cost and in the short manufacture turnaround time.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    i) Field of the Invention  
           [0002]    The present invention relates to a structure of a power MOSFET semiconductor device high in pressure resistance and low in resistance and a method of manufacturing the structure.  
           [0003]    ii) Description of Related Art  
           [0004]    [0004]FIG. 6 is a sectional view of a conventional power MOSFET. In order to attain high pressure resistance and low on-resistance, so-called body diffusion is partially incorporated into a drift region of a drain in a structure. During MOSFET off, void layers extend from both sides of the deep body diffusion to contact each other in middle. Specifically, in this case the drift region under a gate electrode completely constitutes the void layer to a depth substantially equal to that of the deep body diffusion. Since a void layer width is very large, electric field relaxing action is large, and pressure resistance can be enhanced without decreasing an impurity density of the drift region. On the other hand, since the density of the drift region does not need to be lowered, it is unnecessary to lower a drift parasitic resistance during an on state, and it is also possible to keep MOSFET on-resistance to be low.  
           [0005]    To achieve a conventional structure, however, epitaxial growth and selective formation of a deep body region have to be performed a plurality of times, and with an increase of manufacture steps, soaring of a manufacture cost and lengthening of a manufacture period are caused.  
           [0006]    For example, when a drain pressure resistance of several hundreds of volts or more is realized, the deep body region needs a depth of five to a dozen micrometers, but in this case the epitaxial growth and selective formation of the deep body region need to be repeated around six times.  
         SUMMARY OF THE INVENTION  
         [0007]    In order to solve the aforementioned problems, the present invention uses the following means.  
           [0008]    (1) There is provided a semiconductor device comprising: a high-density one-conductive type semiconductor substrate; a low-density one-conductive type semiconductor layer formed on a surface layer of the semiconductor substrate; a trench selectively formed in the low-density semiconductor layer from a surface; a low-density reverse-conductive type semiconductor diffusion layer formed on a side wall and a bottom portion of the trench; a relatively shallow low-density reverse-conductive type semiconductor diffusion layer partially overlapped with the reverse-conductive type semiconductor diffusion layer and selectively formed on the surface layer of the low-density one-conductive type semiconductor; a high-density one-conductive type semiconductor diffusion layer selectively formed in the relatively shallow low-density reverse-conductive type semiconductor diffusion layer; a gate insulation film formed on the low-density one-conductive type semiconductor layer and the relatively shallow low-density reverse-conductive type semiconductor diffusion layer; and a gate electrode selectively formed on the gate insulation film.  
           [0009]    (2) The inside of the trench formed in the low-density one-conductive type semiconductor layer is filled with an insulation film in the semiconductor device.  
           [0010]    (3) The inside of the trench formed in the low-density one-conductive type semiconductor layer is filled with one-conductive type polycrystalline silicon in the semiconductor device.  
           [0011]    (4) A manufacture method of a semiconductor device, comprising steps of: forming a low-density one-conductive type semiconductor layer on a high-density one-conductive type semiconductor substrate by epitaxial growth; selectively forming a trench in the low-density semiconductor layer from a surface; forming a low-density reverse-conductive type semiconductor diffusion layer on a side wall and a bottom portion of the trench; partially overlapping a relatively shallow low-density reverse-conductive type semiconductor diffusion layer with the reverse-conductive type semiconductor diffusion layer disposed on the side wall and the bottom portion of the trench and selectively forming the relatively shallow low-density reverse-conductive type semiconductor diffusion layer in the low-density one-conductive type semiconductor layer; selectively forming a high-density one-conductive type semiconductor diffusion layer in the relatively shallow low-density reverse-conductive type semiconductor diffusion layer; forming a gate insulation film on the low-density one-conductive type semiconductor layer and the relatively shallow low-density reverse-conductive type semiconductor diffusion layer; and selectively forming a gate electrode on the gate insulation film.  
           [0012]    (5) The manufacture method of the semiconductor device further comprises a step of filling the inside of the trench formed in the low-density one-conductive type semiconductor layer with an insulation film.  
           [0013]    (6) The manufacture method of the semiconductor device further comprises a step of filling the inside of the trench formed in the low-density one-conductive type semiconductor layer with polycrystalline silicon.  
           [0014]    (7) The step of forming the low-density reverse-conductive type semiconductor diffusion layer on the side wall and the bottom portion of the trench comprises solid phase diffusion from an oxide film including an impurity in the manufacture method of the semiconductor device.  
           [0015]    (8) The step of forming the low-density reverse-conductive type semiconductor diffusion layer on the side wall and the bottom portion of the trench comprises solid phase diffusion from polycrystalline silicon including an impurity in the manufacture method of the semiconductor device.  
           [0016]    (9) The step of forming the low-density reverse-conductive type semiconductor diffusion layer on the side wall and the bottom portion of the trench comprises a molecular layer doping process in the manufacture method of the semiconductor device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic sectional view showing a first embodiment of a semiconductor device of the present invention.  
         [0018]    [0018]FIG. 2 is a schematic sectional view showing a second embodiment of the semiconductor device of the present invention.  
         [0019]    [0019]FIGS. 3A to  3 G are sectional views in order of steps, showing a first manufacture method of the first embodiment of the semiconductor device of the present invention.  
         [0020]    [0020]FIGS. 4A to  4 C are sectional views in order of steps, showing a second manufacture method of the first embodiment of the semiconductor device of the present invention.  
         [0021]    [0021]FIGS. 5A to  5 E are sectional views in order of steps, showing the first manufacture method of the second embodiment of the semiconductor device of the present invention.  
         [0022]    [0022]FIG. 6 is a schematic sectional view showing one example of a conventional semiconductor device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Embodiments of the present invention will be described hereinafter with reference to the drawings.  
         [0024]    [0024]FIG. 1 is a schematic sectional view showing a first embodiment of a semiconductor device of the present invention. After a low-density drift layer  102  is disposed on a semiconductor substrate  101  as high-density single-crystal silicon, a trench  103  selectively formed on the drift layer, a diffusion layer  104  formed on a side wall and bottom portion of the trench, and an insulation film  109  for filling the inside of the trench are formed, and further to constitute power MOSFET, a source  106 , a body diffusion layer  105 , a gate insulation film  107 , and a gate electrode  108  are formed. The body diffusion layer  105  is partially overlapped with the diffusion layer  104 .  
         [0025]    When the power MOSFET is NMOS, for example, a single crystal silicon substrate including antimony or arsenic with a density of 1×10 19 /cm 3  to 1×10 20 /cm 3  is used, and for example, an epitaxial layer with a phosphorus density of 1×10 14 /cm 3  to 5×10 16 /cm 3  is used as the drift layer. A thickness of the epitaxial layer differs with a required pressure resistance, but is usually in a range of five to a dozen micrometers with an operation voltage up to about several hundreds of volts. The thickness of the trench depends on the required pressure resistance similarly as the epitaxial layer thickness, but is in a range of about 3 to 10 μm and is slightly shallower than the epitaxial layer. The density of the diffusion layer formed on the side wall and bottom portion of the trench is usually in a range of 1×10 16 /cm 3  to 1×10 18 /cm 3 , and the diffusion of the depth and transverse directions is of the order of 0.5 to 2 μm. Parameters such as the density, depth, and thickness of the body diffusion layer, source and gate insulation film indicate numeric values similar to those of the usual power MOSFET.  
         [0026]    In FIG. 1 the structure produces element performance effect. Specifically, when MOSFET is off, void layers extend from both sides of the body diffusion layer formed on the trench side wall to contact each other in a middle so that a drift region under the gate electrode is completely formed into the void layer to a depth substantially equal to that of deep body diffusion. Since a void layer width is very large, electric field relaxation action is large and pressure resistance can be enhanced without lowering an impurity density of the drift region. Since the density of the drift region does not need to be lowered, a drift parasitic resistance during MOSFET on does not have to be lowered, MOSFET on-resistance can be kept to be low. These effects are obtained similarly as the conventional example. Additionally, as compared with the conventional method, the epitaxial growth and selective formation of the deep body region do not need to be performed a plurality of times, and the formation of the trench and diffusion layer may be performed once, so that with considerable simplification of manufacture steps, effects such as cost reduction and manufacture period reduction are brought about.  
         [0027]    Furthermore, in the embodiment of FIG. 1, the diffusion layer  104  can be formed simultaneously with the body diffusion layer  105 , and the effects are enlarged in this case. Details will be described later.  
         [0028]    [0028]FIG. 2 is a schematic sectional view showing a second embodiment of the semiconductor device of the present invention. A basic concept is similar to that of the embodiment of FIG. 1, but the second embodiment is characterized in that the inside of the trench  103  is filled with polycrystalline silicon  110  including an impurity. By employing such structure, the diffusion layer disposed on the side wall and bottom portion of the trench can be formed by diffusing the impurity from the polycrystalline silicon  110 , and further step reduction is possible. In this case, it is necessary to use a method of performing a doped poly-process or the like to introduce the impurity beforehand into the polycrystalline silicon  110  simultaneously during filling with polycrystalline silicon. A manufacture method of the present embodiment will be described later in detail.  
         [0029]    [0029]FIG. 3 shows sectional views in order of steps, showing a first manufacture method of the first embodiment of the semiconductor device of the present invention. As an example, N-type power MOSFET is used.  
         [0030]    [0030]FIG. 3A shows a method comprising: forming the low-density drift layer  102  with a density of phosphorus as an N-type impurity in a range of 1×10 14 /cm 3  to 5×10 16 /cm 3  and with a thickness of about 5 μm to a dozen micrometers on the high-density semiconductor substrate  101  including antimony or arsenic as the N-type impurity with a density of 1×10 19 /cm 3  to 1×10 20 /cm 3  by an epitaxial growth process; subsequently growing an oxide film  111  by about 500 angstroms by oxidation with an electric furnace or the like, subsequently depositing a nitride film  112  by about 1000 angstroms to 2000 angstroms by a chemical vapor development process (CVD); further depositing a mask oxide film  113  by about 2000 angstroms to 1 μm by the CVD process; subsequently patterning the mask oxide film  113  by a photolithography process and etching process; and stripping a resist and using the patterned mask oxide film  113  as a mask to form the trench  103  in the nitride film  112 , oxide film  111  and low-density drift layer  102  by a dry etching process. A narrower width of the trench  103  is more advantageous in respect of an area, but the width is appropriately in a range of about 0.5 μm to 2 μm in consideration of the subsequent filling inside the trench and the doping of the impurity into the trench bottom portion and side wall. Moreover, for a trench depth, since the bottom portion needs to stay in the low-density drift layer, a depth of the order of 3 μm to 10 μm is appropriate.  
         [0031]    The dry etching of the nitride film  112 , oxide film  111  and trench  103  using the mask oxide film  113  as the mask can be performed by changing gas for each material to be processed. Moreover, the mask oxide film  113  may be an oxide film of NSG, PSG or TEOS.  
         [0032]    Subsequently as shown in FIG. 3B, for example, by using the ion injection process to introduce boron as a P-type impurity into the trench side wall and bottom portion by angle injection or rotation injection and subsequently performing heat treatment, the diffusion layer  104  is formed. Moreover, the diffusion layer  104  can be formed as shown in the drawing even by a molecular layer doping process. The boron density of the diffusion layer  104  is usually in a range of 1×10 16 /cm 3  to 1×10 18 /cm 3 , and diffusion in depth and transverse directions is of the order of 0.5 to 2 μm.  
         [0033]    Subsequently, as shown in FIG. 3C, after selectively stripping the mask oxide film  113  by wet etching, the insulation film  109  is deposited inside the trench  103  and on the nitride film  112  by CVD process. From a viewpoint of coverage, when TEOS oxide film is used, the insulation film  109  can easily fill the inside of the trench. In this case, since the thickness needs to be equal to or more than a trench width, deposition is performed in a range of about 0.5 μm to 2 μm. When this thickness is impossible in one step, the deposition may be performed a plurality of times in a divided manner.  
         [0034]    Subsequently, as shown in FIG. 3D, the insulation film  109  is etched back by the dry etching process. The etching is ended by end point detection upon exposing of the nitride film  112 . Moreover, this step may be performed by a chemical machine polishing process (CMP).  
         [0035]    Subsequently, by removing the nitride film  112  by the wet etching process or the dry etching process by phosphoric acid, further removing the oxide film  111  by wet etching, and subsequently forming the gate insulation film  107  by oxidation in the electric furnace, a structure shown in FIG. 3E is obtained. The thickness of the gate oxide film depends on the required pressure resistance, but is usually in a range of 200 angstroms to 800 angstroms.  
         [0036]    Subsequently, as shown in FIG. 3F, by patterning polycrystalline silicon with the impurity doped therein in a high density by the photolithography process and dry etching process to form the gate electrode  108 , and using the gate electrode  108  as the mask, the body diffusion layer  105  as a power MOSFET body is selectively formed in the low-density drift layer  102  by ion injection and heat treatment. For the density and diffusion amounts of vertical and transverse directions of the body diffusion layer  105 , when boron is used as the P-type impurity similarly as the diffusion layer  104 , the density is of the order of 1×10 16 /cm 3  to 1×10 18 /cm 3 , and the diffusion amount is of the order of 0.5 to 2 μm. In the formation, for example, BF 2  ion is injected with a dose amount of about 1×10 13  to 5×10 14 /cm 2 , the heat treatment of 1000° C. to 1100° C. is performed for several tens of minutes, and other conditions are used. Moreover, the body diffusion layer  105  is securely brought into contact with the previously formed diffusion layer  104 .  
         [0037]    Subsequently, as shown in FIG. 3G, the power MOSFET source  106  is formed by using the gate electrode  108  as the mask and performing the ion injection and heat treatment. Arsenic is used as the N-type impurity, and the density is of the order of 1×10 19 /cm 3  to 1×10 20 /cm 3 .  
         [0038]    The structure shown in the first embodiment of the present invention is obtained by the aforementioned manufacture method.  
         [0039]    [0039]FIG. 4 shows sectional views in order of steps, showing a second manufacture method of the semiconductor device of the first embodiment according to the present invention.  
         [0040]    By performing steps similar to those of FIG. 3A until the trench formation, and subsequently selectively stripping the mask oxide film  113  by wet etching as shown in FIG. 4A, an insulation film  114  including the impurity is formed inside the trench  103  and on the nitride film  112  by the CVD process or a spin on glass (SOG) process. With the N-type power MOSFET, as the insulation film  114  including the impurity, for example, BSG, that is, the oxide film including boron is used.  
         [0041]    Subsequently, as shown in FIG. 4B, the insulation film  114  including the impurity is removed by etching back or CMP until the nitride film  112  is exposed.  
         [0042]    Subsequently, by performing the heat treatment to diffuse boron from the insulation film including the impurity, the diffusion layer  104  is formed as shown in FIG. 4C. Thereafter, similarly as the manufacture method described with reference to FIG. 3, while the insulation film  114  including the impurity is left in the trench, the nitride film and oxide film are removed and the gate oxide film, gate electrode, body diffusion layer, and source may successively be formed. Alternatively, after the insulation film  114  including the impurity is removed once by wet etching, the filling step of the insulation film  109  may be performed similarly as FIG. 3. Unless a particular trouble arises, and since the number of steps is small, advancement to the subsequent step with the insulation film  114  including the impurity left in the trench is advantageous in respect of cost and work period.  
         [0043]    [0043]FIG. 5 shows sectional views in order of steps, showing the first manufacture method of a second embodiment of the semiconductor device according to the present invention.  
         [0044]    [0044]FIG. 5A shows a method comprising: forming the N-type low-density drift layer  102  on the N-type high-density semiconductor substrate  101  by the epitaxial growth process; subsequently depositing the mask oxide film  113  by about 2000 angstroms to 1 μm by the CVD process; subsequently patterning the mask oxide film  113  by the photolithography process and etching process; and subsequently stripping a resist and using the patterned mask oxide film  113  as the mask to form the trench  103  in the low-density drift layer  102  by the dry etching process.  
         [0045]    The density of the high-density semiconductor substrate and the density and thickness of the low-density drift layer, and further the width and depth of the trench are similar to those in the embodiment shown in FIG. 3.  
         [0046]    Moreover, similarly as FIG. 3, the mask oxide film  113  may be an oxide film of NSG, PSG or TEOS.  
         [0047]    Subsequently, as shown in FIG. 5B, by introducing the impurity by the ion injection process or the molecular layer doping process and subsequently performing the heat treatment, the P-type diffusion layer  104  is formed on the side wall and bottom portion of the trench. For the density and diffusion amount of the diffusion layer  104 , similarly as the embodiment of FIG. 3, the density is of the order of 1×10 16 /cm 3  to 1×10 18 /cm 3  and the diffusion amount is of the order of 0.5 to 2 μm.  
         [0048]    Subsequently, as shown in FIG. 5C, the polycrystalline silicon  110  is deposited inside the trench  103  and on the mask oxide film  113  by the CVD process. In this case, since the thickness of polycrystalline silicon needs to be equal to or more than the trench width, the deposition of the order of 0.5 μm to 2 μm is performed. For polycrystalline silicon, film stress is large, warp of the semiconductor substrate sometimes becomes large with one deposition, and to avoid this the deposition may be performed a plurality of times in a divided manner.  
         [0049]    Subsequently, as shown in FIG. 5D, the polycrystalline silicon  110  is etched back by the dry etching process. The etching is ended by end point detection upon exposing of the mask oxide film  113 . Moreover, this step may be performed by the chemical machine polishing (CMP) process.  
         [0050]    Subsequently, by removing the mask oxide film  113  and successively forming the gate oxide film, gate electrode body diffusion layer, and source similarly as the embodiment of FIG. 3, the structure of the second embodiment of the semiconductor device of the present invention shown in FIG. 5E can be formed.  
         [0051]    In the manufacture method shown in FIG. 5 the mask step relating to the trench formation and filling may be performed only once, and as compared with the embodiment of FIG. 3 there is an advantage that the number of steps is reduced.  
         [0052]    Moreover, in the embodiment shown in FIG. 5 the diffusion layer  104  is formed after the trench formation, but similarly as the embodiment of FIG. 4, even by using polycrystalline silicon including the impurity, that is, using the doped poly-process to embed polycrystalline silicon into the trench, subsequently performing the heat treatment to diffuse the impurity from polycrystalline silicon, forming the diffusion layer on the trench side wall and bottom portion, and performing the subsequent steps while embedded polycrystalline silicon is left as it is, it is possible to obtain the structure shown in the second embodiment of the semiconductor device of the present invention shown in FIG. 2.  
         [0053]    The aforementioned embodiments have been described by illustrating the N-type power MOSFET, but additionally the manufacture of the P-type power MOSFET is also possible by reversing the conductive type.  
         [0054]    As described above, according to the structure and manufacture method of the power MOSFET of the present invention, it is possible to supply the power MOSFET semiconductor device high in pressure resistance and low in resistance at a low cost and in a short manufacture turnaround time.