Abstract:
A semiconductor device is disclosed and includes a drain region of a first conductivity type, having a first major surface. Diffused into the drain region is a body region of a second conductivity type. A source region is diffused in the body region and it has a general polygonal shape when viewed at the first major surface with two notches directed towards the center of the source region from opposite sides. The body region is accessible through the notches. An oxide layer covers the source and body regions except for a contact opening position over the source region between the two notches exposing only that portion of the source region that is between the two notches and at least a portion of the accessible body region in each of the two notches to facilitate a source contact.

Description:
PRIORITY STATEMENT UNDER 35 U.S.C. 119(E) AND 37 C.F.R. 1.78.  
       [0001]    This non-provisional application claims priority based upon the prior U.S. Provisional Patent Application No. 60/134999, filed May 20, 1999 entitled “A Minimum Size Cellular MOS-Gated Device Geometry” in the name of Richard A. Blanchard. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to semiconductor devices and methods of manufacturing the same, and more particularly to devices such as Metal Oxide Semiconductor (MOS) transistors, Insulated Gate Bipolar Transistors (IGBTs) and MOS-gated conductivity modulated devices including MOS-controlled thyristors (MCTs).  
           [0003]    The designers of MOS transistors are often faced with the dilemma of improving the on-resistance of an MOS semiconductor device, while at the same time preventing the device from latching up from the conduction of a parasitic bipolar transistor formed by the source, body and drain regions of the semiconductor device.  
           [0004]    This problem was discussed in U.S. Pat. No. 4,860,072 entitled Monolithic Semiconductor Device and Method of Manufacturing Same. The patent disclosed a monolithic semiconductor device for use in various applications such as lateral and vertical MOS transistors, insulated gate conductivity modulated devices and the like, as well as a method of manufacturing same. This device includes source, body and drain regions, with the body region including a channel section which is disposed adjacent an insulated gate formed on the surface of the device. The source region includes a central contact area flanked by a pair of body segments which extend up through the source region and which create a resistive path between the source contact area and the channel section. A voltage is developed across the resistive path which tends to maintain a parasitic bipolar transistor that is formed by the source, body and drain regions in a non-conducting state. A source metallization bridges the two body segments and the intermediate source contact thereby shorting the body region to the source. The geometry of the device is reduced since the contact area need not be increased to ensure that the source metallization contacts the entire source as well as both body segments.  
           [0005]    Similarly, in U.S. Pat. No. 4,639,754, a Vertical MOSFET with Diminished Bipolar Effects disclosed an IGFET device that includes a semiconductor wafer having a first conductivity type drain region contiguous with a wafer surface. A second conductivity type body region extends into the wafer from the wafer surface so as to form a body-drain PN junction having an intercept at the surface; the body region further including a body-contact portion of relatively high conductivity disposed at the surface. A first conductivity type source region extends into the wafer so as to form a source-body PN junction which has first and second intercepts at the surface. The first intercept is spaced from the body-drain intercept so as to define a channel region in the body region at the surface, and the second intercept is contiguous with the body contact portion. The second intercept is relatively narrowly spaced from the first intercept along most of the length of the first intercept and is relatively widely spaced from the first intercept at one or more predetermined portions. A source electrode contacts both the body-contact portion and the source region at the wafer surface.  
           [0006]    In U.S. Pat. No. 4,495,513 entitled Bipolar Transistor Controlled by Field Effect by Means of an Isolated Gate there is disclosed a bipolar semiconductor structure in which the conductive and blocked states are controlled by an isolated gate. The structure comprises a P+ type substrate constituting the emitter of a bipolar transistor, an N type epitaxial layer constituting the base, a P+ type area having a large surface, constituting a collector, covered with a collector contract and surrounded by an area wherein the epitaxial N type layer is exposed, an N+ type source area included in the collector area and extending along the border of the same so as to define an interval which constitutes the control gate of the structure, a resistive source access zone connected, on the one hand, to the source, and on the other hand, to the collector contact, the resistance of this zone being sufficient for preventing the structure from being rendered conductive in an irreversible manner.  
           [0007]    However, none of the devices achieves a interdigitated and/or cellular geometry that allows the optimization of both device on-resistance and ruggedness.  
         SUMMARY OF INVENTION  
         [0008]    A semiconductor device is disclosed and includes a drain region of a first conductivity type, having a first major surface. Diffused into the drain region is a body region of a second conductivity type. A source region is diffused in the body region and it has a general polygonal shape when viewed at the first major surface with two notches directed towards the center of the source region from opposite sides. The body region is accessible through the notches. An oxide layer covers the source and body regions except for a contact opening position over the source region between the two notches exposing only that portion of the source region that is between the two notches and at least a portion of the accessible body region in each of the two notches to facilitate a source contact.  
           [0009]    The polygonal geometry shape with opposing notches in the source and the location of the contact to the source increases the resistance of the source between the channel region and the source contact. This structure lends itself to both cellular and interdigitated configurations. The reduced size of the source contact in conjunction with the two notches in the source allows for a smaller cell size, which in turn allows greater cell density and lowers the overall RDS(on) of the device.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0010]    Objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, wherein like references characters are used throughout to designate like parts:  
         [0011]    [0011]FIG. 1 is a single cell top view of a MOS-gated power device showing a square or a rectangular layout according to the invention;  
         [0012]    [0012]FIG. 2 is a cross-sectional view as seen from section line II-II of FIG. 1;  
         [0013]    [0013]FIG. 3 is a cross-sectional view as seen from section line III-III of FIG. 1;  
         [0014]    [0014]FIG. 4 is a cross-sectional view of FIG. 1 as seen from section line IV-IV;  
         [0015]    [0015]FIG. 5 is a top down of a MOS-gated power device showing a first octagonal cell layout according to the invention;  
         [0016]    [0016]FIG. 6 illustrates a second octagonal cell layout according to the invention;  
         [0017]    [0017]FIGS. 7 a  through  7   d  provide a cross section of the process steps for manufacturing the device according to the invention;  
         [0018]    [0018]FIG. 8 is a top down view showing the gate of a square layout and the gate connection to a semiconductor device according to the invention;  
         [0019]    [0019]FIG. 9 is an illustration of a portion of an interdigitated layout of a MOS-gated power device according to the invention;  
         [0020]    [0020]FIG. 10 is a cross-sectional view of the power device of FIG. 9 as seen along sectional line X-X;  
         [0021]    [0021]FIG. 11 is a top view of an interdigitated MOS-gated power device according to the invention; and  
         [0022]    [0022]FIG. 12 is a top view of an extended cell layout of a MOS-gated power device according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0023]    [0023]FIG. 1 is a top view of a cell  20  that includes the N+ source region  1  and a P body region  23 . P-type body region  23  usually consists of a combination of two P-type diffusions that do not totally overlap on the surface of the device. Dashed lines represent the contact area  25  for the source and body regions. The N+ source region  1  has a square shape with two opposing notches  30 . A contact area  32  for just the source is centrally located between the outer edges of the two opposing notches  30 . The source contact area  32  is positioned to be a maximum distance from the channel area beneath the edges  34  of the source region  1 .  
         [0024]    [0024]FIG. 2 is a cross-sectional view of FIG. 1 as seen by section lines  11 - 11 . The P body region  23  is diffused into an N type drain region  27  that also includes an N− region  37  and an N+ region  39 . Typically, the N+ region  39  is connected to a drain contact not shown. The source region  1  is connected to the source metal  29  at contact area  25 . The source metal  29  overlaps the source region  1 . The drain region  27  includes an N+ region  39  and N− region  37  where the P-body region  23  extends into the N− region  27 . Traditionally, the deep P+ portion of region  23   b  extends approximately two to ten microns into the N− region  37 . However, with the invention of U.S. Pat. No. 5,216,275, which is incorporated herein by reference, the P body region can extend way into the N− region  37  substantially more than the two to ten microns. In fact, it can approach to very nearly touching the N+ region  39  or can, with the help of a thin dielectric layer between the P+ region and the N+ region  39 , extend to the N+ region. Alternatively, by making the features of the cell  20  small and with the added resistance with the layout of FIG. 1, the deep P+ diffused portion of P-type region  23  may be eliminated. The bottom N+ region  39  is usually connected to a metal layer that serves as the drain contact.  
         [0025]    A gate polysilicon layer  33  is shown extending over the P body region  23  but from section lines II-II a channel region such as channel region  11  of FIG. 3 is not present under the gate layer  23 . An gate oxide layer  35  insulates the gate polysilicon layer  33  from the source region  1  and the P body region  23 . A second dieletric layer, often of silicon dioxide, separates gate poly layer from source metal  29 . The source contact metal  29  contacts the P body region  23  in the opposing grooves  30  at contact area  53 .  
         [0026]    [0026]FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 as seen from section lines III-III and illustrates the N+ region  39 , the N region  37 , the body region  23  and the source region  1 . The source metal contact  29  is shown over the source region  1  between the opposing grooves  30 . The channel region  11  is located beneath the edges  34  of the source region  1 . The conductivity of the P body region  23  in the channel region  11  changes from a P type to an N type conductivity when a positive potential of the proper magnitude is applied to the gate polysilicon  33 . This change in conductivity facilitates the conduction of carriers between the source region  1  and the N+ region  39  (the drain).  
         [0027]    [0027]FIG. 4 is a cross-sectional view of the embodiment of FIG. 1 as seen from dimension lines IV-IV and illustrates a first prong  61  and a second prong  63  of the source region  1 . As can be seen at area  59 , neither the first prong  61  nor the second prong  63  contacts the source metal  29 .  
         [0028]    [0028]FIG. 5 illustrates an alternate shape for each cell from the embodiment of FIG. 1. Each cell  20  has an octagon shape. Each has two notches  67  and  71  in the source region  1 , in which a portion of the P body region  23  is available to contact with the source metal layer  29 . This shape provides for a minimum size for each cell when a plurality of cells are utilized. However, any polygonal shape may be utilized such as hexagonal, square, round and rectangular so long as there are two opposing slots to expose the P+ body diffusion  23 .  
         [0029]    In FIG. 5, each cell  20  may be smaller than other previous cells. Therefore, more cells may be connected in parallel per unit area resulting in the reduction in RDS(on), the on resistance.  
         [0030]    In FIG. 6 the notches  67  and  71  of each cell  27  are perpendicular to the two notches  67  and  71  of each cell  26 . Each cell  27  is orientated such that its notches  57  and  71  are not in alignment with the notches of any adjacent cell  26 . However, the notches  67  and  71  of each cell  26  are in alignment with the notches of any other cell  26  and similarly for each cell  27 . The embodiment of FIG. 6 provides for more efficient carrier injection into the channel regions over the embodiment of FIG. 5.  
         [0031]    [0031]FIG. 7 illustrate the steps of manufacturing a semiconductor device according to the invention. Referring to FIG. 7 a , a wafer  51  having an N+ conductivity has an epitaxial layer  37  created either by growth or deposition. A field oxide layer  32  is then grown, masked and etched to create the active layer  50 .  
         [0032]    On the active layer  50 , as shown in FIG. 7 a , a gate oxide  35  is grown on which the gate polysilicon  33  is deposited and doped. A second masked and etch step is performed on the gate polysilicon  33  and the gate oxide  35  is also etch.  
         [0033]    In FIG. 7 b , the deep P+ is implanted through a masked followed by the P− doping with no mask present. The P-type body dopent is introduced through the opening in the gate oxide. The source region  1  is then masked for the N+ implant following which there is a diffusion step and a oxide deposition  31 , (FIG. 7 c ).  
         [0034]    In FIG. 7 d  for the source metal  29 , the active region  51  is masked, etched and a metal deposition is performed. The deposited metal is then masked and etched to create the semiconductor device.  
         [0035]    [0035]FIG. 8 illustrates the gate contact to each of the cells  20  or  26  and  27  depending on which embodiment is being practiced. The gate covers the majority of the surface of the device, and, when properly biased, allows carriers to flow from the source of the device to the drain of the device through channel region  11 .  
         [0036]    In FIG. 9, there is illustrated a top down view of an interdigitated layout having three source regions  1  in a group  10 . Each source region  1  includes an N+ doping area having the shape of the capital letter “I”. The widest area, T 1  of the “I” is represented by dimension lines  5 . The thinnest area, W C  of the N+ source region has a dimension represented by dimension lines  73 . The separation between each source region  1 , SI, is represented by dimension lines  91 . The height of the top of the letter “I” from the bottom of the letter “I” is T L , represented by dimension line  96 . The internal dimension S C , is represented by dimension line  93 . There is a contact region identified by dashed lines  13  and  15  having a width “W” represented by dimension line  95 . The contact region width W is less than S C . Therefore, the series resistance between the contact area  85  to the ends of the letter “I”, the top end  19  and the bottom end  97  provides a current path so that the current will flow from the contact area  85  to the end of the source region  1 . This length adds a series resistance in the source region  1  which enables the devices to have a more rugged performance. Ruggedness is the ability of a semiconductor device to avoid latch up.  
         [0037]    [0037]FIG. 10 is a cross sectional view of the embodiment of FIG. 9 as seen from section lines X-X. There is a deep P+ region  2  beneath the P region  23 . A source region  1  extends over a channel region  11  as with the cellular embodiment. There is an oxide layer  31  covering both the P region  23  and the source region  1  except for an opening through which the contact metal  29  passes. A layer of oxide  35  insulates the top of the gate  33  from the source metal layer  29 . Dashed line  9  illustrates the position of the gate polysilicon layer  33  just over the edge of the source region  1  and the channel  11 .  
         [0038]    The top view of an interdigitated MOS-gated power device  100  is illustrated in FIG. 11 where the embodiment of FIG. 9 has been extended to form a multiple source region  10  device. The configuration of the device  100  is that of multiple parallel rows  101  that are joined together by connections  103 . The source regions  20  in connection  103  may be in line with the source regions  20  of the parallel rows  101  or perpendicular to them as shown at  105 .  
         [0039]    In FIG. 12 the source regions  20  are arranged in a number of extended cells  300 . The extended cells may be arrange in the embodiment of FIGS.  5  are  6  with many of the same advantages being provided.  
         [0040]    The above embodiments were shown illustrating a particular conductivity type. However, as is well know in the art the conductivity may be switch from N to P and P to N.