Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-204122, filed Sep. 20, 2011; the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    Embodiments described herein relate to a semiconductor device. 
       BACKGROUND 
       [0003]    In recent years, in addition to the switching power supply market which requires devices with high current and high voltage capability, demand for a power MOSFET in an energy-saving switching power supply market, first for laptops and now in mobile communication equipment and the like, has arisen. A power MOSFET is used for synchronous rectification in AC-DC converters in a power supply. In this case, in addition to a breakdown voltage of about 80-250 V, low on-resistance structure and switching loss reduction are required. 
         [0004]    Here, a MOSFET having a trench MOS structure is used in order to reduce on-resistance of the power MOSFET. This MOSFET of trench MOS structure has a plurality of trenches at predetermined interval on a semiconductor layer which becomes a channel region. On an inner wall of this trench, an insulating film which is the gate insulating film is formed, and through this insulating film, a conductive film providing the gate electrode is formed inside the trench. By miniaturizing the width of the trench or the width of the semiconductor layer between the trenches, channel density in internal elements can be improved. 
         [0005]    In the case where a MOSFET having reduced on-resistance using the structure described above, the breakdown voltage of an end region adjacent thereto has to be ensured, which has been problematic in current designs. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]      FIGS. 1A and 1B  are cross-sectional views of the semiconductor device according to a first, prior art, comparative example. 
           [0007]      FIGS. 2A and 2B  are graphs showing the impurity concentration of the semiconductor device according to the first, prior art, comparative example. 
           [0008]      FIGS. 3A and 3B  are cross-sectional views of the semiconductor device according to a second, prior art, comparative example. 
           [0009]      FIGS. 4A and 4B  are graphs showing the impurity concentration of the semiconductor device according to the second, prior art, comparative example. 
           [0010]      FIGS. 5A and 5B  are cross-sectional views of a semiconductor device according to a first embodiment. 
           [0011]      FIGS. 6A and 6B  are graphs showing the impurity concentration of the semiconductor device according to the first embodiment. 
           [0012]      FIGS. 7A and 7B  are cross-sectional views of a semiconductor device according to a second embodiment. 
           [0013]      FIGS. 8A and 8B  are graphs showing the impurity concentration of the semiconductor device according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In general, according to one embodiment, the semiconductor device according to the first embodiment will be explained by referring to the attached and referenced drawing figures. After having explained the schematic configuration of a semiconductor device according to the first and the second prior art comparative examples, a semiconductor device according to the embodiments, will be described. 
         [0015]    According to the embodiment, there is provided a semiconductor device which enables an improvement of breakdown voltage and a reduction in on-resistance. 
         [0016]    A semiconductor device according to a first embodiment: includes: a first region which functions as a MOSFET; and a second region which is adjacent to the first region; the first region comprising, a drain electrode of the MOSFET; a semiconductor substrate of a first conductivity type which has a first impurity concentration while being electrically connected to the drain electrode; a first semiconductor layer (formed on top of the semiconductor substrate) of the first conductivity type which has a second impurity concentration which is lower than the first impurity concentration; a second semiconductor layer (formed on the surface of the first semiconductor layer) of the first conductivity type which has a third impurity concentration which is lower than the first impurity concentration but higher than the second impurity concentration; a plurality of first trenches formed on the upper side of the second semiconductor layer; a third semiconductor layer (formed on the surface of the second semiconductor layer) of the second conductivity type, which is adjacent to the first trenches; a fourth semiconductor layer (formed on the surface of the third semiconductor layer) of the first conductivity type which is adjacent to the first trenches; a first insulating layer which is formed along inner walls of the first trenches; a gate electrode layer (provided in the middle of the insulating layer) which functions as a MOSFET gate electrode and is opposed to the third semiconductor layer through the first insulating layer; a trench source electrode layer which is formed in order to embed the first trenches through the first insulating layer; and a MOSFET source electrode which contacts the fourth semiconductor layer and which is electrically connected to the trench source electrode layer, and the second region comprising: the semiconductor substrate; the first semiconductor layer; the first insulating layer formed in order to extend to the upper face of the first semiconductor layer; and the source electrode formed in order to extend to the upper face of the first insulating layer, wherein the first semiconductor layer of the second region has the second impurity concentration. 
       Comparative Example 1 
       [0017]      FIGS. 1A and 1B , explain the semiconductor device according to the first comparative example. As shown in  FIG. 1A  and  FIG. 1B , the semiconductor device according to the first comparative example, includes a cell unit which functions as a MOSFET and a termination unit provided in the periphery of the cell unit. 
         [0018]    First, the cell unit will be described. As shown in  FIG. 1B , the cell unit includes a drain electrode  11 , an n+ type semiconductor substrate  12 , an n− type epitaxial layer  13  and multiple trenches  14  extending inwardly of the n− type epitaxial layer and provided therein in predetermined intervals in direction X. 
         [0019]    The n+ type semiconductor substrate  12  is provided on drain electrode  11  and is electrically connected to drain electrode  11 . The n+ type semiconductor substrate  12  can have an impurity concentration of 1×10 20  [atoms/cm 3 ]. The n− type epitaxial layer  13  is formed on n+ type semiconductor substrate  12 . The n− type epitaxial layer  13  is smaller than n+ type semiconductor substrate  12 , it can have an impurity concentration of 1×10 15  [atoms/cm 3 ] for example. Each trench  14  extends from the upper side of n− type epitaxial layer  13  toward the lower, substrate  12  side of the n− type epitaxial layer, but terminates within the n− type epitaxial layer  13 . As shown in  FIG. 1B , the cell unit includes p type base layer  15 , n+ type source layer  16  and p+ type contact layer  17 . P type base layer  15  is adjacent to trenches  14  and is formed on n− type epitaxial layer  13  on the side thereof opposite to substrate  12 . p type base layer  15  can have a degree of impurity concentration of, 1×10 16  to 1×10 17  [atoms/cm 3 ]. p type base layer  15  functions as MOSFET channels. n+ type source layer  16  is formed on p type base layer  15  and disposed on either side of the trenches. n+ type source layer  16  can have, for example, a degree of impurity concentration of 1×10 20  [atoms/cm]. p+ type contact layer  17  is formed on p type base layer  15 . p+ type contact layer  17  is adjacent to n+ type base layer  16  between trenches  14 , such that n+ type source layer is disposed between p+ contact layer  17  and the adjacent trench  14 . P+ type contact layer  17  has a higher impurity concentration than that of p type base layer  15 . For example, it can have a degree of impurity concentration of 1×10 20  [atoms/cm 3 ]. 
         [0020]    In  FIG. 1B , the trenches  14  of the cell unit are lined and capped with an insulating layer  18 , having a gate electrode layer  19  formed and enclosed within the insulating layer  18  on either side of the trenches  14 , and disposed generally adjacent to the n+ source layers  16  and p type base layers  15 , a trench source electrode extending inwardly of the trench and generally filling the bounds of the insulating layer  18  within the trench  14 , and a source electrode layer  21  overlying trenches  14 . The insulating layer  18  is formed along inner walls of each trench  14  by using, for example, Silicon oxide (SiO 2 ). The gate electrode layer  19  is provided within the insulating layer  18  and adjacent to a side surface of p type base layer  15  through the insulating layer  18 . The gate electrode layer  19  functions as a MOSFET gate. The gate electrode layer  19  is composed of polysilicon, for example. The trench source electrode layer  20  is formed within the trenches within the insulating layer  18 . The upper face of trench source electrode layer  20  is covered or capped by the insulating layer  18 . The trench source electrode layer  20  is composed of polysilicon, for example. The source electrode  21  contacts the upper face of n+ type source layer  16  and the upper face of p+ type contact layer  17 . The source electrode  21  is electrically connected to trench source electrode layer  20  through a connection (not shown). More precisely, the trench source electrode layer  20  is at the same potential as the source electrode  21 . Thanks to this, the electric field concentration is relaxed and the breakdown voltage of the cell unit can be improved. 
         [0021]    Next, the termination unit will be described. As shown in  FIG. 1A , in the termination unit, the trenches  14  which were arranged consecutively in the n− layer  13  terminate in a final trench  14 F. The termination unit includes n+ type semiconductor substrate  12  having an n− type epitaxial layer  13  formed thereon, and a drain electrode  11  formed on the underside of the substrate  12  as in the unit cell region of  FIG. 1B . Note that in the termination unit, on top of p type base layer  15 F which is located intermediate of the final two trenches  14 ,  14 F, the n+ type source layer  16  is not formed but the p+ contact layer is formed intermediate of, but spaced by the p layer from, the trenches  14 . Additionally, gate electrode  19  is provided only on the side of the final trench  14 F facing the adjacent trench  14  which is on the outermost side of the termination unit. 
         [0022]    The insulating layer  18  within and capping the trenches in the cell units is extended, in the termination unit, over the n− type epitaxial layer  13  in a direction away from the last unit cell. The source electrode  21  is formed thereover, and likewise extends over the insulating layer in the direction away from the unit cells. 
         [0023]      FIGS. 2A and 2B  are graphs showing n type impurity concentration along the lines A-A′ and B-B′ in the termination unit and the cell unit of the first comparative example shown in  FIGS. 1A and 1B . The vertical axis of  FIGS. 2A and 2B  show the impurity concentration and the horizontal axis shows the position of direction Y shown in  FIGS. 1A and 1B . As shown in  FIGS. 2A and 2B , n+ type semiconductor substrate  12  in the termination unit and in the cell unit can have, an n type impurity concentration of 1×10 20 [atoms/cm 3 ] and n− type epitaxial layer  13  can have an n type impurity concentration of 1×10 15 [atoms/cm 3 ]. However, the impurity concentration curves showing n type impurity concentration in the termination unit and in the cell unit are substantially the same. 
         [0024]    As one of performances required when using this semiconductor device as a switching element, avalanche resistance is required. This avalanche resistance can be improved by structural design in order to make the breakdown voltage of the termination unit higher than the breakdown voltage of the cell unit. According to the first comparative example, in order to make the breakdown voltage of termination unit higher than that of the cell unit, it is necessary to lower the concentration of n− type epitaxial layer  13 , but in that case, as on-resistance increases, the performance of the semiconductor device will be lowered. 
       Comparative Example 2 
       [0025]    Now referring to  FIGS. 3A and 3B , we are going to explain the semiconductor device by referring to a second prior art comparative example. As shown in  FIG. 3A  and  FIG. 3B , the semiconductor device according to the second comparative example also includes the cell unit which functions as a MOSFET and the termination unit which is provided on the periphery of the cell unit. It should be noted that in the second comparative example, shown in  FIGS. 3A and 3B , the parts that have the same structure as the first comparative example and duplicate descriptions denoted by the same reference numerals, have be omitted. 
         [0026]    The primary difference in the semiconductor device in the second comparative example and the semiconductor device in first comparative example, is that the n− type epitaxial layer  13  of the cell unit and the termination unit is provided in a two-layer structure which has high concentration n− type epitaxial layer  13 A and low concentration n− type epitaxial layer  13 B. The low concentration n− type epitaxial layer  13 B has the same degree of impurity concentration as n− type epitaxial layer  13  in the first comparative example, for example, it has a degree of impurity concentration of 1×10 15 [atoms/cm 3 ]. Then, high concentration n− type epitaxial layer  13 A has a large impurity concentration with regard to low concentration n− type epitaxial layer  13 B, for example, the degree of its impurity concentration is 1×10 16 [atoms/cm 3 ]. In this prior art device, the trenches  14  extend into, but do not extend through, the high impurity concentration n− layer  13 A, and thus are not in direct contact with the underlying low impurity concentration n-layer  13 B This difference in impurity concentration between high concentration n− type epitaxial layer  13 A and low concentration n− type epitaxial layer  13 B is realized by repeating the growth of epitaxial layer in different conditions on top of n+ type semiconductor substrate  12  or changing implant conditions of n− type impurities to form the epitaxial layer or the like. By using a bi-layer having different concentrations for the n− type impurity, it is possible to reduce the on-resistance of the device. 
         [0027]      FIGS. 4A and 4B  are graphs showing n type impurity concentration along the lines A-A′ and B-B′ on the termination unit and the final cell unit of the second comparative example as shown in  FIGS. 3A and 3B . The vertical axis of  FIGS. 4A and 4B  show impurity concentrations and the horizontal axis show the position of direction Y shown in  FIGS. 3A and 3B . As shown in  FIGS. 4A and 4B , n+ type semiconductor substrate  12  in the termination unit and the cell unit has an n− type impurity concentration on the order of 1×10 2 ° atoms/cm 3 . Low concentration n− type epitaxial layer  13 B has an n type impurity concentration of 1×10 15  atoms/cm 2  and high concentration n-type epitaxial layer  13 A has an n type impurity concentration of 1×10 16  atoms/cm 3 , for example. The impurity concentration curves showing n type impurity concentration of the termination unit and the cell unit are substantially the same. 
         [0028]    In the semiconductor device in the second comparative example, where the n− type epitaxial layer  13  is divided into two layers which are a high concentration n− type epitaxial layer  13 A and a low concentration n− type epitaxial layer  13 B., on-resistance is reduced because a high concentration n− type epitaxial layer  13 A extends and is positioned immediately below trenches  14 . However, using this architecture for the n− type epitaxial layer, the breakdown voltage of the termination unit has a lower field plate effect than the cell unit, which is also lower than the voltage of the cell unit, and avalanche resistance of the termination unit is thereby reduced. 
       Embodiment 1 
       [0029]    Referring now to  FIGS. 5A and 5B , a first embodiment of the semiconductor device hereof is described. The semiconductor device of  FIG. 5A  and  FIG. 5B  includes the cell unit which functions as a MOSFET and the termination unit provided on the periphery or end of the cell unit. It should be noted that, in the first embodiment shown in  FIGS. 5A and 5B , the parts that have the same structure as the first and the second comparative examples and duplicate descriptions denoted by the same-reference numerals, will be omitted. 
         [0030]    In the semiconductor device according to the first embodiment, n− type epitaxial layer  13  in the cell unit is provided in a two-layer structure including high impurity concentration n− type epitaxial layer  13 A and low impurity concentration n− type epitaxial layer  13 B. In the semiconductor device according to the first embodiment, in contrast to the semiconductor device of the second comparative, the two-layer structure of high concentration n− type epitaxial layer  13 A and low concentration n− type epitaxial layer  13 B does not extend to surround the termination unit, and this bi-layer structure terminates at the termination unit such that at least a portion of the termination trench  14 F is in contact with n-low layer  13 B. 
         [0031]    Low concentration n− type epitaxial layer  13 B, in the same way as n− type epitaxial layer  13  in the second comparative example, has in this example a degree of impurity concentration of 1×10 15  [atoms/cm 3 ]. High concentration n− type epitaxial layer  13 A has a higher or larger large impurity concentration as compared to that of low concentration n− type epitaxial layer  13 B, in this example an impurity concentration on the order of 1×10 16  [atoms/cm 3 ]. 
         [0032]      FIGS. 6A and 6B  are graphs showing n type impurity concentration along the lines A—A′ and B-B′ in the termination unit and the cell unit of the first embodiment shown in  FIGS. 5A and 5B . The vertical axes of  FIGS. 6A and 6B  show impurity concentrations and the horizontal axes show the positions of direction Y shown in  FIGS. 5A and 5B . As shown in  FIGS. 6A and 6B , n+ type semiconductor substrate  12  in the termination unit and the cell unit can have an n type impurity concentration of 1×10 2 ° [atoms/cm 3 ]. Low concentration n− type epitaxial layer  13 B in the cell unit has an n type impurity concentration of 1×10 15  [atoms/cm 3 ], and high concentration n− type epitaxial layer  13 A has an n type impurity concentration of 1×10 16  [atoms/cm 3 ]. n− type epitaxial layer  13  in the termination unit, for example, is an extension of low impurity concentration n− layer  13 B and thus has the same impurity concentration of 1×10 15  [atoms/cm 3 ]. 
         [0033]    In the semiconductor device in the first embodiment, n-type epitaxial layer  13  in cell unit is divided into two layers which are high impurity concentration n− type epitaxial layer  13 A and low impurity concentration n− type epitaxial layer  13 B. This results in reduced on-resistance because high concentration n− type epitaxial layer  13 A is formed up to immediately below trenches  14  of the cell unit. Alternatively, high concentration n− type epitaxial layer  13 A is not formed in the termination unit. As a result, the breakdown voltage of the termination unit is not lower than the breakdown voltage of the cell unit, and the inherent reduction in avalanche resistance in prior art devices which occurred as a result of reducing on resistance is be prevented. 
         [0034]    It should be noted that the impurity concentration of high concentration n− type epitaxial layer  13 A in the cell unit can be arbitrarily set in a range such as 1×10 15  to 1×10 17  [atoms/cm 3 ] to reduce the on-resistance. The impurity concentration of low concentration n− type epitaxial layer  13 B in the cell unit or n− type epitaxial layer  13  in the termination unit can be arbitrarily set in a range such as 1×10 14 -1×10 16  [atoms/cm 3 ], but lower than the impurity concentration in layer  13 A, to improve the avalanche resistance where the on resistance has been lowered with the high concentration over low concentration n− bi-layer  13 . 
       Embodiment 2 
       [0035]    Referring now to  FIGS. 7A and 7B , an additional embodiment of the reduced on-resistance but sufficient avalanche resistance structure is shown. As shown in  FIG. 7A  and  FIG. 7B , the semiconductor device according to the second embodiment also includes a cell unit which functions as a MOSFET and a terminal unit provided on the periphery of the cell unit. It should be noted that in the second embodiment shown in  FIGS. 7A and 7B , the parts that have the same structure as the first and the second comparative examples and duplicate descriptions denoted by the same reference numerals, will be omitted. 
         [0036]    As shown in  FIGS. 7A and 7B , the second embodiment is different from the first embodiment because of the structure of the termination unit. In the second embodiment, on the outer non-unit cell or termination side of trench  14 F, p− type diffusion layer  22  is formed. This p− type diffusion layer  22  is formed over n− type epitaxial layer  13  only to the non-cell side of the termination cell  14 F, and thus the base and unit cell side of termination trench  14 F is in contact with the same n− layer which extends under, and in contact with portions of, the unit cells, and impurity concentration of about 1×10 15  to 1×10 16 [atoms/cm 3 ]. The p− type diffusion layer  22  may be formed by ion implantation of p type impurities into the n-layer  13  and subsequent annealing. 
         [0037]      FIGS. 8A and 8B  are graphs showing n type impurity concentration along the lines A-A′ and B-B′ in the termination unit and the cell unit of the second embodiment shown in  FIGS. 7A and 7B . The vertical axes of  FIGS. 8A and 8B  show impurity concentrations and the horizontal axes show the position of direction Y shown in  FIGS. 7A and 7B . As shown in  FIGS. 8A and 8B , n+ type semiconductor substrate  12  in termination unit and cell unit can have a degree of n type impurity concentration of 1×10 2 ° [atoms/cm]. Low concentration n− type epitaxial layer  13 B in the cell unit can have a degree of n type impurity concentration of 1×10 15  [atoms/cm 3 ], for example, and high concentration n− type epitaxial layer  13 A can have a degree of n type impurity concentration of 1×10 16  [atoms/cm 3 ], for example. 
         [0038]    In the semiconductor device in this embodiment, p− type diffusion layer  22  is provided on n− type epitaxial layer  13  in the termination unit. The curve of n type impurity concentration in the terminal unit and the curve of the p type impurity concentration are represented by a dashed line and the curve of effective impurity concentration is represented in a solid line. n− type epitaxial layer  13  in the termination unit can have the degree of n type impurity concentration of 1×10 15  [atoms/cm 3 ], for example, and p− type diffusion layer  22  can have a degree of p type impurity concentration of 1×10 15  to 1×10 16  [atoms/cm 3 ], for example. In this case, p− type diffusion layer  22  will either become a low concentration p-type layer by offsetting the effect of the n− type impurity in the n− layer  13  from which it is formed. The p type impurity concentration of p− type diffusion layer  22  is set so as to have the effective n type impurity inside p-type diffusion layer  22  in the range of 1×10 13  to 1×10 15  [atoms/cm 3 ]. 
         [0039]    In the semiconductor device in the second embodiment, n− type epitaxial layer  13  in the cell unit is divided into two layers which are high concentration n− type epitaxial layer  13 A and low concentration n− type epitaxial layer  13 B. Due to this, on-resistance is reduced in comparison to an n− layer of a single impurity concentration, because high concentration n− type epitaxial layer  13 A is formed up to immediately below trenches  14  in the cell unit. However, at the termination unit, on n− type epitaxial layer  13 , p− type diffusion layer  22  is formed. Therefore, the breakdown voltage of the termination unit is further improved than in the first embodiment, and avalanche resistance can be improved as compared to having an n− bi-layer extend past the termination unit  14 F. 
         [0040]    While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

Technology Category: 5