Abstract:
A power MOSFET layout according to one embodiment of the invention comprises a substrate and a plurality of cells. Each of the cells includes a base portion, a plurality of protruding portions extending from the base portion, and a plurality of photo-resist regions. Each of the cells is geometrically configured with the base portion and the plurality of protruding portions defining a closed cell boundary enclosing each of said cells. The cells are formed over the substrate, and the closed cell boundaries of the cells are arranged regularly with each other with no overlapping among the cells. The base portions are disposed in a matrix arrangement having rows and columns. The base portions are oriented from end to end in a direction of the columns and the protruding portions extend from the base portions along a direction of the rows. The photo-resist regions cover the base portions on the same column. None of the protruding portions are disposed between the base portions on the same column. The cells are doped with N type dopants by using the photo-resist regions as masks.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims priority from R.O.C. Patent Application No. 091132834, filed Nov. 7, 2002, the entire disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention is related to a power MOSFET (metal oxide semiconductor field effect transistor) layout, and more particularly to a power MOSFET layout with protruding portions. 
     FIG. 1  illustrates the conventional layout for a power MOSFET. The power MOSFET is a trenched field effect transistor and has a plurality of cells. Each cell has a cell body contact region  11 , a source region  12 , and an insulation layer  13 , surrounded by a trenched gate  10 .  FIG. 2  illustrates the cross-sectional view taken along line A-A′ of FIG.  1 . The power MOSFET is formed in the substrate  17 . The trenched gate  10  is composed of conductive material isolated from the substrate by a coat of insulation  13 . For example, the N type power MOSFET includes a source region  12  formed of N type material, a body  14  formed of P type material, an epitaxial layer  15  formed of lightly doped N type material (N − ), and a drain contact layer  16  formed of heavily doped N type material (N+). 
   The manufacturing of the power MOSFET is first to form the trench by the photolithography etching process, and then to form the insulation layer  13  and the gate electrode  10  in turn. Then, p type dopants are implanted to form the P-body  14 . A photo-resist profile is then formed on the substrate  17 . The photo-resist profile covers the region  11 . Then N type dopants are implanted to form the source region  12 . After that the cell body contact region  11  is formed. 
   However, the photo-resist profile covering the cell body contact region  11  has a plurality of independent islands, which may peel off during the manufacturing process. Accordingly the manufacturing is difficult to perform, and small scale process technology is also difficult to achieve. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the present invention are directed to a power MOSFET layout configured for easy manufacturing, especially for small scale process technology. The power MOSFET layout includes a plurality of cells arranged in a matrix having rows and columns. Each cell includes a base portion and a plurality of protruding portions. The base portion is a cell body contact region, and the protruding portions are source regions. The cell may include other cell body contact regions. The cell body contact regions are arranged along the columns with no source regions disposed on the columns. The photo-resist regions can extend along the columns of cell body contact regions without covering the source regions. This increases the coverage of the photo-resist regions over the conventional layout having relatively small, discrete cell body contact regions, and thus the possibility of photo-resist peel-off is reduced. 
   In one embodiment, a power MOSFET layout comprises a substrate and a plurality of cells. Each of the cells includes a base portion, a plurality of protruding portions extending from the base portion, and a plurality of photo-resist regions. Each of the cells is geometrically configured with the base portion and the plurality of protruding portions defining a closed cell boundary enclosing each of said cells. The cells are formed over the substrate, and the closed cell boundaries of the cells are arranged regularly with each other with no overlapping among the cells. The base portions are disposed in a matrix arrangement having rows and columns. The base portions are oriented from end to end in a direction of the columns and the protruding portions extend from the base portions along a direction of the rows. The photo-resist regions cover the base portions on the same column. None of the protruding portions are disposed between the base portions on the same column. The cells are doped with N type dopants by using the photo-resist regions as masks. 
   In another embodiment, a power MOSFET layout comprises a substrate and a plurality of cells. Each of the cells includes a base portion and a plurality of protruding portions extending from the base portion. Each of the cells is geometrically configured with the base portion and the plurality of protruding portions defining a closed cell boundary enclosing each of said cells. The cells are formed over the substrate, and the closed cell boundaries of the cells being geometrically arranged so that one cell does not overlay another cell on the substrate. The base portions are disposed in a matrix arrangement having rows and columns. The base portions are oriented from end to end in a direction of the columns and the protruding portions extend from the base portions along a direction of the rows. The base portions on the same column are configured to be covered by a photo-resist region. None of the protruding portions are disposed between the base portions on the same column. The cells are processed by using the photo-resist regions as masks. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional layout for a power MOSFET. 
       FIG. 2  illustrates the cross-sectional view taken along line A-A′ of FIG.  1 . 
       FIG. 3A  is a Z type layout  3 - 1  of a power MOSFET according to an embodiment of the invention. 
       FIG. 3B  is an S type layout  3 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 4A  is the second Z type layout  4 - 1  of a power MOSFET according to invention. 
       FIG. 4B  is the second S type layout  4 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 5A  is the third Z type layout  5 - 1  of a power MOSFET according to another embodiment of the invention. 
       FIG. 5B  is the third S type layout  5 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 6A  is a multi-protruding-portion layout  6 - 1  of a power MOSFET according to another embodiment of the invention. 
       FIG. 6B  is the second multi-protruding-portion layout  6 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 7A  is a U type layout  7 - 1  of a power MOSFET according to another embodiment of the invention. 
       FIG. 7B  is the second U type layout  7 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 7C  is the third U type layout  7 - 3  of a power MOSFET according to another embodiment of the invention. 
       FIG. 8A  is a multi-protruding-portion layout  8 - 1  of a power MOSFET according to another embodiment of the invention. 
       FIG. 8B  is the second multi-protruding-portion layout  8 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 9A  is an interlaced type layout  9 - 1  of a power MOSFET according to another embodiment of the invention. 
       FIG. 9B  is the second interlaced type layout  9 - 2  of a power MOSFET according to another embodiment of the invention. 
       FIG. 9C  is the third interlaced type layout  9 - 3  of a power MOSFET according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention are directed to a power MOSFET which includes a substrate and a plurality of MOSFET cells. Each MOSFET cell is geometrically configured with a base portion of a longitudinal shape and at least one protruding portion extending from the sides of the base portion, such that the base portion and the protruding portions define a closed cell boundary enclosing the MOSFET cells. MOSFET cells are formed in the substrate, and all closed cell boundaries are arranged regularly with each other, i.e., with no overlapping among the cells. In specific embodiments, the protruding portions of each cell are proximally extended into the space separated by the protruding portions of other adjacent cells. 
     FIG. 3A  is a Z type layout  3 - 1  of a power MOSFET according to an embodiment of the invention. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell has a base portion of a longitudinal shape, which is the cell body contact region  31 , and two protruding portions, which are the source regions  32 . The protruding portions extend from the sides of the base portion, and are located at opposite ends of the base portion. The power MOSFETs further include trenched gates  30 , an insulation layer for insulating the gates  30 , and the substrate. 
   In the process of forming gates  30 , the cell body contact regions  31 , and the source regions  32 , the gates  30  are formed first. Then a first photo-resist profile is formed on the substrate. Therefore, the region of the substrate not covered by the photo-resist is doped with P type dopants. Then a second photo-resist profile is formed on the substrate, and the region covered by the second photo-resist profile is the photo-resist region shown as dashed lines. The photo-resist region includes a plurality of belt-like photo-resist regions  33 . The regions not covered by the second photo-resist are doped with N type dopants. Then the cell body contact regions  31  and the source region  32  are formed. Each of the belt-like photo-resist regions  33  covers the cell body contact regions  31  of the same column. The covered area of each of the photo-resist regions is larger than that of the conventional layout, and thus the possibility of photo-resist peel-off is reduced. 
   The present invention is of a protruding type layout. The invention can use belt-like photo-resist regions to cover a plurality of cell body contact regions  31  during the formation of the photo-resist profiles. Particularly, there is no protruding portion between the base portions covered by belt-like photo-resist regions  33 . The base portions are cell body contact regions  31 . Thus, the photo-resist-covered area is enlarged through the belt-like photo-resist regions  33 . There are several variant types of layout according to different embodiments of the invention, which will be described in detail in the following paragraphs. 
     FIG. 3B  is an S type layout  3 - 2  of a power MOSFET according to an embodiment of the invention. The difference between the S type layout  3 - 2  and the Z type layout  3 - 1  is that, for layout  3 - 2 , the relative positions of the base portion and protruding portions are different. The layout  3 - 2  uses belt like photo-resist regions to cover the cell body contact regions of the same column, for increasing the covered area and reducing the possibility of photo-resist peel-off. Power MOSFETs include a substrate and many MOSFET cells. Each MOSFET cell includes the base portion of a longitudinal shape, which is the cell body contact region  37 , and two protruding portions, which are source regions  38 , extending from the sides of the base portion. Further, the power MOSFETs include trenched gates  36 , an insulation layer isolating the gates  36 , and the substrate. 
   The photo-resist profile for forming the source regions  38  includes the belt like photo-resist region  39 , which is indicated in dashed lines. The possibility of the photo-resist peel-off is reduced because each of the photo-resist regions covering the base portions of the same column increases the photo-resist profile covered area. 
     FIG. 4A , is a Z type layout  4 - 1  of a power MOSFET according to the invention. The difference between layout  4 - 1  and layout  3 - 1  (in  FIG. 3A ) is that, for layout  4 - 1 , the end part of protruding portions of the layout  4 - 1  further includes an end portion, that is, a cell body contact region  44 , for better grounding. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape, which is the cell body contact regions  41 , and two protruding portions extending through the sides of the base portion. One end of the protruding portion connecting to the base portion is a source region  42 , and the other end is the end portion, that is, the cell body contact region  44 . Further, the power MOSFETs include trenched gates  40 , an insulation layer isolating the gates  40 , and the substrate. 
   Each photo-resist region  43 , which is illustrated in dashed line, having a photo-resist profile for forming the source region and the cell body contact region, further comprises a plurality of protruding portions  43 - 1 . The protruding portion covers each end portion, extending to the sides from a belt-like photo-resist region located at the base portion. This produces a configuration in which the belt-like photo-resist region covers a plurality of the cell body contact regions of the same column and protruding portions at the same time. The possibility of photo-resist peel-off is reduced because the covered area of each photo-resist region  43  is larger than that in the conventional layout. 
     FIG. 4B  is an S type layout  4 - 2  of a power MOSFET according to an embodiment of the invention. The difference between layout  4 - 2  and layout  3 - 2  (in  FIG. 3B ) is that, for layout  4 - 2 , the end part of a protruding portion further comprises a cell body contact region. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape, which is a cell body contact region  47 , and two protruding portions extending from the sides of the base portion. One end of each protruding portion connecting to the base portion is a source region  48 , and the other end is a end portion, which is a cell body contact region  45 . Further, the power MOSFETs include trenched gates  46 , an insulation layer isolating the gates  46 , and the substrate. 
   Each photo-resist region  49 , which is illustrated in dashed line, having a photo-resist profile for forming the source region  48  and the cell body contact regions  45 , 47 , further comprises a plurality of protruding portions  49 - 1 . The protruding portion covers each end portion, extending to the sides from the belt-like photo-resist region located at the base portion. This produces a configuration in which the belt-like photo-resist region covers a plurality of the cell body contact regions of the same column and protruding portions at the same time. The covered area of the photo-resist region  49  is relatively large and thus the possibility of photo-resist peel-off is reduced. 
     FIG. 5A  is a Z type layout  5 - 1  of a power MOSFET according to an embodiment of the invention. The difference between layout  5 - 1  and layout  3 - 1  (in  FIG. 3A ) is that, for layout  5 - 1 , each base portion is wider for increasing the periphery length of source regions. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape and two protruding portions. The base portion is divided into three belt-like zones, which are a first side zone, source region  52 - 1 , a second side zone, source region  52 - 2 , and a third middle zone, a cell body contact region  51 . The protruding portions, which are the source region  52 , extend from the sides of the base portion. Further, the power MOSFETs include trenched gates  50 , the substrate, and an insulation layer isolating the gates  50 . 
   The power MOSFET of layout  5 - 1  features a wider base portion than layout  3 - 1  does, so the periphery length for this power MOSFET&#39;s source regions is longer. One way to judge the layout quality is to reference the parameter PD, wherein PD=W/D, W is the periphery length of the cell&#39;s source electrode, and D equals to the cell area. When parameter PD is larger, it implies the power MOSFET has better properties. Therefore, the power MOSFET layout  5 - 1  is of better quality compared to layout  3 - 1 . Each belt-like photo-resistant region  53 , indicated by the dashed line, covers the cell body contact regions on the third middle zone  51  of the same column for reducing the possibility of photo-resist peel-off. 
     FIG. 5B  is an S type layout  5 - 2  of a power MOSFET according to an embodiment of this invention. The difference between layout  5 - 2  and layout  3 - 2  (in  FIG. 3B ) is that, for layout  5 - 2 , the base portion of layout  5 - 2  is wider than those of layout  3 - 2  in order to increase the periphery length of the source regions. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each of the MOSFET cells includes a base portion of a longitudinal shape and two protruding portions. The base portion is divided into three belt-like zones, which are a first side zone, source region  58 - 1 , a second side zone  58 - 2 , and a third middle zone, a cell body contact region  57 . The protruding portions, which are source regions  58 , extend from the sides of the base portion. Further, the power MOSFETs include trenched gates  56 , the substrate, and an insulation layer isolating the gates  56 . 
   The parameter PD of power MOSFET layout  5 - 2  is larger than that of layout  3 - 2 . The possibility of photo-resist peel-off is reduced because each belt-like photo-resist region  59 , indicated by the dashed line, at the same time covers the cell body contact regions on the third middle region  57  of the same column. Therefore, a goal of the present invention is reached. 
     FIG. 6A  is a multi-protruding-portion layout  6 - 1  of a power MOSFET. The layout  6 - 1  is composed of two MOSFET cells of layout  5 - 1 . The periphery length of each source region is increased, and the parameter PD is also increased. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each of the MOSFET cells includes a base portion of a longitudinal shape and four protruding portions. The base portion is divided into three belt-like zones, which are a first zone, source regions  62 - 1 , a second zone  62 - 2 , and a third middle zone, a cell body contact region  61 . The protruding portions extend from the sides of the base portion to be the source region  62 . Further, the power MOSFETs of layout  6 - 1  include trenched gates  60 , the substrate, and an insulation layer isolating the gates  60 . The belt-like photo-resist region  63 , indicated by the dashed line, covers larger area than that shown in the prior art so as to reduce the possibility of photo-resist peel-off. 
     FIG. 6B  is a multi-protruding-portion layout  6 - 2  of a power MOSFET. The layout  6 - 2  is composed of two MOSFET cells of layout  5 - 2 . The periphery length of each source region is increased, and so is the parameter PD. Power MOSFETs of layout  6 - 2  include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape and four protruding portions. The base portion is divided into three belt-like zones, which are a first zone, source region  68 - 1 , a second zone, source region  68 - 2 , and a third middle zone, a cell body contact region  67 . The protruding portions extend from the sides of the base portion. Further, the power MOSFETs of layout  6 - 1  include trenched gates  66 , the substrate, and an insulation layer isolating the gates  66 . The belt-like photo-resist region  69 , indicated by the dashed line, covers a larger area than that of the prior art so as to reduce the possibility of photo-resist peel-off. 
     FIG. 7A  is a U type layout  7 - 1  of a power MOSFET. The difference between layout  7 - 1  and the Z type layout  3 - 1  is that, for layout  7 - 1 , the protruding portions are on the same side of the base portion. Thus, a plurality of cell body contact regions can connect with each other so as to enlarge the photo-resist covered area to reduce the possibility of photo-resist peel-off. Power MOSFETs of layout  7 - 1  include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape, which is a cell body contact region  701 , and two protruding portions, which are a source regions  702 , extending from the same sides of the base portion. The power MOSFETs of layout  7 - 1  further include trenched gates  700 , an insulation layer isolating the gates  700 , and the substrate. The belt-like photo-resist region  703 , having a photo-resist profile for forming the source region and the cell body contact region, covers cell body contact region  701  of the same column. The covered area of this photo-resist region is larger than that of the prior art in order to reduce the possibility of photo-resist peel-off. 
     FIG. 7B  is a U type layout  7 - 2  of a power MOSFET. The difference between layout  7 - 2  and the layout  7 - 1  is that, for layout  7 - 2 , two adjacent MOSFETs stay back to back. Power MOSFETs of layout  7 - 2  include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape, which is a cell body contact region  707 , and two protruding portions, which are a source regions  708 , extending from the sides of the base portion. The power MOSFETs of layout  7 - 2  further include trenched gates  706 , an insulation layer isolating the gates  706 , and the substrate. The base portions of the adjacent two MOSFET cells of the same row and are grouped into one set. The belt-like photo-resist region  709 , having a photo-resist profile for forming the source region and the cell body contact region, covers cell body contact region  707  of the same column. The covered area of this photo-resist region is larger than that of the prior art in order to reduce the possibility of photo-resist peel-off. 
     FIG. 7C  is a U type layout  7 - 3  of a power MOSFET. The difference between layout  7 - 3  and layout  7 - 2  is that, for layout  7 - 3 , the base portions of the two adjacent cells are connected together for reducing the cell area. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape, which is a cell body contact region  711 , and two protruding portions, which are a source regions  712 , extending from the same sides of the base portion. Further, the power MOSFETs of layout  7 - 3  include a trenched gates  710 , an insulation layer isolating the gates  710 , and the substrate. The belt-like photo-resist region  713 , having a photo-resist profile for forming the source region and the cell body contact region, covers cell body contact region of the same column. The covered area of this photo-resist region is larger than that of the prior art in order to reduce the possibility of photo-resist peel-off. 
     FIG. 8A  is a multi-protruding-portion layout  8 - 1  of a power MOSFET. Layout  8 - 1  features a wider base portion so as to increase the periphery length of the source region. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape and a plurality of protruding portions. The base portion is divided into three zones, the first two zones are source regions  82 - 1  and  82 - 2 , and the third middle zone is cell body contact region  81 . The protruding portions extend from the sides of the base portion and are the source regions  82 . Further, the power MOSFETs include trenched gates  80 , the substrate and an insulation layer isolating the gates  80 . The principal feature of power MOSFET layout  8 - 1  is that the periphery length of the source region of this transistor is longer, such that the parameter PD=W/D larger, which indicates better properties. The belt-like photo-resist region  83 , shown as the dashed lines, covers a plurality of the cell body contact regions  81  of the third middle zone of the same column to cover a larger area than that shown in the prior art so as to reduce the possibility of photo-resist peel-off. 
     FIG. 8B  is a multi-protruding-portion layout  8 - 2  of a power MOSFET. The difference between layout  8 - 2  and layout  8 - 1  is that, for layout  8 - 2 , the other end from the joint part of the protruding portion and the base portion includes a cell body contact region  85  for a better ground connection. Power MOSFETs include a substrate and a plurality of MOSFET cells. Each MOSFET cell has a base portion and a plurality of protruding portions. The base portion is divided into three zones, the first two zones are source regions  88 - 1  and  88 - 2 , and the third middle zone is the cell body contact region  87 . The protruding portions extend from the sides of the base portion. The connected part of the protruding portion is the source regions  88  and the end part of the protruding portion is the cell body contact region  85 . Further, the power MOSFETs include trenched gates  86 , the substrate and an insulation layer isolating the gates  86 . The ends of adjacent MOSFET cells of the same row are an end set of every two ends. Each belt-like photo-resist region  89 - 1 , shown as dashed lines, covers the cell body contact region of a plurality of end sets of the same column at the same time. Each photo-resist region  89  covers the cell body contact region in the third zones of the base portions of the same column. Compared to the conventional layout, the covered area of the photo-resist regions  89  and  89 - 1  is relatively large to reduce the possibility of photo-resist peel-off. 
     FIG. 9A  is an interlaced type layout  9 - 1  of a power MOSFET. The difference between layout  9 - 1  and layout  8 - 1  is that, for layout  9 - 1 , each of the protruding portions of the cells in layout  9 - 1  extends into the space between the protruding portions of the adjacent cells. Power MOSFETs of layout  9 - 1  include a substrate and several MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape and several protruding portions. The base is divided into three zones. The first zone is source region  902 - 1 , the second zone is source region  902 - 2 , and the third middle zone is cell body contact region  901 . The protruding portions, extending from the sides of the base portion, are the source regions  902 . Further, the power MOSFETs include trenched gates  900 , the substrate, and the insulation layer isolating the gates  900 . The belt-like photo-resist region  903 , indicated by dashed lines, covers a plurality of the cell body contact regions in the third zones of the base portions of the same column so as to reduce the possibility of photo-resist peel-off. 
     FIG. 9B  is an interlaced type layout  9 - 2  of a power MOSFET. The difference between layout  9 - 2  and layout  9 - 1  is that, for layout  9 - 2 , the other end of the protruding portion, connecting to the base portion, of MOSFETs has a cell body contact region  905  to improve grounding. Power MOSFETs of layout  9 - 2  includes a substrate and a plurality of MOSFET cells. Each of the MOSFET cells includes a base portion and a plurality of protruding portions. The base portion includes a cell body contact region  907  and a part of a source region  908 . The protruding portions extend from the sides of the base portion. One end of the protruding portion, connecting to the base portion, is the source region  908 , and the other end is the end part of the protruding portion, that is, the cell body contact region  905 . Further, the power MOSFETs include trenched gates  906 , the substrate, and the insulation layer isolating the gates  906 . Each of the belt-like photo-resist region  909 , indicated by dashed lines, includes a plurality of protruding portions  909 - 1 . Each protruding portion extends from the belt-like photo-resist region in the third middle zone to the sides so as to cover the end part of the protruding portion and, therefore, covers a plurality of the cell body contact regions in the third middle zones of the base portions of the same column and protruding portions. The photo-resist region  909  covers a larger area than that in the prior art so as to reduce the possibility of photo-resist peel-off. 
     FIG. 9C  is an interlaced type layout  9 - 3  of a power MOSFET. The difference between layout  9 - 3  and layout  9 - 2  is that, for layout  9 - 3 , the cell body contact regions on the protruding portions are located in the middle of each protruding portion. Power MOSFET cells include a substrate and a plurality of MOSFET cells. Each MOSFET cell includes a base portion of a longitudinal shape and a plurality of protruding portions. The base portion is divided into three zones, the first zone is a source region  912 - 1 , the second zone is a source region  912 - 2 , and the third middle zone is cell body contact region  911 . The protruding portions extend from the sides of the base portion. One end of the protruding portion, connected with the base portion, is source region  912 , the other end of the protruding portion is source region  913 , and the middle of the protruding portion between the source region  912  and source region  913  is a cell body contact region  916 . Further, the power MOSFETs include trenched gates  910 , the substrate, and an insulation layer isolating the gates  910 . The belt-like photo-resist region  914 , indicated by dashed lines, covers a plurality of cell body contact regions on the third middle zones  911  of the base portions of the same column. The belt-like photo-resist region  915 , indicated by dashed lines, covers the cell body region  916 , located between the protruding portions, of the same column. The area covered by the belt-like photo-resist region  914  or  915  is so large as to reduce the possibility of photo-resist peel-off. 
   The above-described arrangements of apparatus and methods are merely illustrative of applications of the principles of this invention and many other embodiments and modifications may be made without departing from the spirit and scope of the invention as defined in the claims. For instance, the sizes and shapes of the various regions of the cells may vary. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.