Abstract:
A semiconductor device, such as a LDMOS device, comprising: a semiconductor substrate; a drain region in the semiconductor substrate; a source region in the semiconductor substrate laterally spaced from the drain region; and a drift region in the semiconductor substrate between the drain region and the source region. A gate is operatively coupled to the source region and is located offset from the drain region on a side of the source region opposite from the drain region. When the device is in an on state, current tends to flow deeper into the drift region to the offset gate, rather than near the device surface. The drift region preferably includes at least first and second stacked JFETs. The first and second stacked JFETs include first, second and third layers of a first conductivity type, a fourth layer intermediate the first and second layers including alternating pillars of the first conductivity type and of a second conductivity type extending between the source and drain regions; and a fifth layer intermediate the second and third layers, including alternating pillars of the first and second conductivity types extending between the source and drain regions.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates in general to semiconductor devices and more particularly to an improved high voltage Lateral Double-Diffused Metal Oxide Semiconductor (LDMOS) device and method of making such a device. 
       BACKGROUND OF THE INVENTION 
       [0002]    In general, high-voltage integrated circuits in which at least one high-voltage transistor is arranged on the same chip together with low voltage circuits are widely used in a variety of electrical applications. In these circuits, a LDMOS transistor is an important high voltage device.  FIG. 1  is a diagrammatic view of a typical LDMOS device showing a flux tube and impact ionization zones of the device. As shown, LDMOS device  10  includes a P substrate  12 , an nwell layer  14 , n+ drain  16 , p body  18 , n+ source  20  diffused in p body  18 , P+ tap  19 , source contact  22 , and drain contact  24 . A gate  26  overlies source  20  and p body  18  and is located between source  20  and drain  16 . A flux tube  28  extends between drain  16  and source  20  and forms a surface channel under gate  26 . Impact ionization zones  30  and  32  are located along tube  28  at drain  16  and at p body  18  at areas  34  and  36 , respectively, of the drift region.  FIG. 2  is a graphical illustration of the surface E-field for the device of  FIG. 1 . The surface E-field  38  has peaks  40  and  42  of drift region ND and a plateau  44  between peaks  40 ,  42 . 
         [0003]    In order improve a high voltage LDMOS device, it is desirable to cut the surface E-field peaks and to lower the surface E-field. As an example, in order to meet the global requirements of a high voltage device where VRMS can be 110V to 277V, the Vpeak can be 186V to 470v and a voltage spike can be 336V to 620V, the device desirably should have a breakdown voltage of 700V. 
         [0004]    U.S. Pat. No. 6,097,063, issued Apr. 1, 2000, inventor Fujihira, is of interest and discloses a semiconductor device which has a drift region in which current flows if it is in the on mode and which is depleted if it is in the off mode. The drift region is formed as a structure having a plurality of first conductive type drift regions and a plurality of a second conductive type compartment regions in which each of the compartment regions is positioned among the adjacent drift regions in parallel to make p-n junctions respectively. The disclosed device is disadvantageous in the number of process steps required to make the device. 
         [0005]    There is thus a need for a high voltage LDMOS device which has reduced surface E-field and E-field peaks, a reduced on resistance, reduced device size, and a simplified process for making the device. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, there is provided a fulfillment of the needs and a solution to the problems discussed above. 
         [0007]    According to a feature of the present invention, there is provided: 
         [0008]    a semiconductor device comprising: 
         [0009]    a semiconductor substrate; 
         [0010]    a drain region in said semiconductor substrate; 
         [0011]    a source region in said semiconductor substrate laterally spaced from said drain region; 
         [0012]    a drift region in said semiconductor substrate between said drain region and said source region; and 
         [0013]    a gate operatively coupled to said source region located offset from said drain region on a side of said source region opposite from said drain region; 
         [0014]    wherein, when said device is in an on state, current tends to flow deeper into said drift region to said offset gate, rather than near the device surface. 
         [0015]    According to another feature of the present invention there is provided: 
         [0016]    a semiconductor device comprising: 
         [0017]    a semiconductor substrate; 
         [0018]    a drain region in said semiconductor substrate; 
         [0019]    a source region in said semiconductor substrate laterally spaced from said drain region; 
         [0020]    a drift region in said semiconductor substrate between said drain region and said source region; 
         [0021]    wherein said drift region includes at least first and second stacked junction field effect transistors (JFETs). 
         [0022]    According to a further feature of the present invention there is provided 
         [0023]    a semiconductor device comprising: 
         [0024]    a semiconductor substrate; 
         [0025]    a drift region in said semiconductor substrate; 
         [0026]    wherein said drift region includes at least first and second stacked junction field effect transistors (JFETs). 
         [0027]    According to a still further feature of the present invention there is provided: 
         [0028]    a method of making a semiconductor device having a drift region comprising: 
         [0029]    providing a semiconductor substrate of a first conductivity type; 
         [0030]    producing a buried well of a second conductivity type in said semiconductor substrate; 
         [0031]    producing a shallow buried layer of a first conductivity type inside said buried well; 
         [0032]    producing an epitaxial layer of a second conductivity type on said buried well; 
         [0033]    etching spaced, parallel trenches in said epitaxial layer and buried well; wherein the width of said trenches is equal or less than the width of the width of the regions between said etched trenches; 
         [0034]    filling said trenches by segment epitaxial refill with material of said first conductivity type, resulting in alternating pillars of said first and second conductivity type; and 
         [0035]    producing a top layer of a first conductivity type on said epitaxial layer. 
         [0036]    According to another feature of the present invention there is provided 
         [0037]    a method of making a semiconductor device having a drift region comprising: 
         [0038]    providing a semiconductor substrate of a first conductivity type; 
         [0039]    producing a buried well of a second conductivity type in said semiconductor substrate; 
         [0040]    producing a shallow buried layer of a first conductivity type inside said buried well. 
         [0041]    producing an epitaxial layer of a second conductivity type on said buried well; 
         [0042]    etching spaced, parallel trenches in said epitaxial layer and buried well; wherein the width of said trenches is equal or less than the width of the width of the regions between said etched trenches; 
         [0043]    filling said trenches by oxide/Si refill in which a trench side-wall thermal oxide layer has a Si of said first conductivity type refill; and 
         [0044]    producing a top layer of a first conductivity type on said epitaxial layer. 
         [0045]    According to another feature of the present invention there is provided: 
         [0046]    a method of making a semiconductor device having a drift region, comprising: 
         [0047]    providing a semiconductor substrate of a first conductivity type; 
         [0048]    producing a buried well of a second conductivity type in said semiconductor substrate; 
         [0049]    producing a shallow buried layer of a first conductivity type inside said buried well; 
         [0050]    producing an epitaxial layer of a first conductivity type on said buried well; 
         [0051]    producing two wells of a second conductivity type in said epitaxial layer located in source and drain areas; 
         [0052]    etching spaced, parallel trenches in said epitaxial layer and buried well, wherein the width of said trenches is equal or larger than the width of the width of the regions between said etched trenches; 
         [0053]    filling said trenches by segment epitaxial refill with material of said second conductivity type, resulting in alternating pillars of said first and second conductivity type; wherein the alternating pillars of said first and second conductivity type are linked with the two wells of said second conductivity type located in said source and drain areas; and 
         [0054]    producing a top layer of a first conductivity type on said epitaxial layer. 
         [0055]    According to another feature of the present invention there is provided: 
         [0056]    a method of making a semiconductor device having a drift region, comprising: 
         [0057]    providing a semiconductor substrate of a first conductivity type; 
         [0058]    producing a buried well of a second conductivity type in said semiconductor substrate; 
         [0059]    producing a shallow buried layer of a first conductivity type inside said buried well; 
         [0060]    producing an epitaxial layer of a first conductivity type on said buried well; 
         [0061]    producing two wells of a second conductivity type in said epitaxial layer located in source and drain areas; 
         [0062]    etching spaced, parallel trenches in said epitaxial layer and buried well, wherein the width of said trenches is equal or larger than the width of the width of the regions between said etched trenches; 
         [0063]    filling said trenches by oxide/Si refill in which a trench sidewall thermal oxide layer has a Si of said second conductivity type refill, and the alternating pillars of said first and second conductivity types are linked with the two wells of said second conductivity type located in said source and drain areas. 
     
     
       DESCRIPTION OF THE DRAWINGS 
         [0064]      FIG. 1  is a diagrammatic view showing a known high voltage LDMOS device. 
           [0065]      FIG. 2  is a graphical view of the surface E-field of the device of  FIG. 1 . 
           [0066]      FIG. 3  is a graphical, diagrammatic view useful in explaining the present invention. 
           [0067]      FIGS. 4A ,  4 B, and  4 C are diagrammatic views respectively of a known gate structure, and of two gate structures according to embodiments of the present invention. 
           [0068]      FIG. 5  is a perspective, diagrammatic view of an embodiment of the present invention. 
           [0069]      FIG. 6  is a top, sectional, plan, diagrammatic view of a portion of the embodiment of  FIG. 5 . 
           [0070]      FIG. 7  is a front, sectional, elevational, diagrammatic view of a portion of the embodiment of  FIG. 5 . 
           [0071]      FIG. 8  is a side, elevational, sectional, diagrammatic view on a larger scale of the embodiment of  FIG. 5 . 
           [0072]      FIG. 9  is a perspective, diagrammatic view of another embodiment of the present invention. 
           [0073]      FIGS. 10A-10D  are schematic views illustrating a method for forming n/p pillars according to the present invention. 
           [0074]      FIGS. 11A and 11B  are respective diagrammatic views of other embodiments of the present invention. 
           [0075]      FIG. 12  is a diagrammatic view of another embodiment of the present invention. 
           [0076]      FIGS. 13A and 13B  are respective diagrammatic views of other embodiments of the present invention. 
           [0077]      FIGS. 14A and 14B  are respective diagrammatic views of other embodiments of the present invention. 
           [0078]      FIGS. 15A-15E  are respective diagrammatic views of other embodiments of the present invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0079]    According to the present invention, an improved high voltage LDMOS device is provided in which the surface E-field peaks and the surface E-field of the device are reduced.  FIG. 3  is a diagrammatic view illustrating these effects. As shown,  FIG. 3  is similar to  FIG. 2 , but peaks  40  and  42  have been crossed out at  40 ′ and  42 ′, and the E-field plateau has been lowered at  44 ′ or  44 ″. These E-field effects can be attained by removing the surface channel or by removing the surface channel facing to the drain side at peak  40  and by increasing the junction depletion width to drop the E-field and by making the drift region easily depleted at the off-state high voltage. As a consequence, the breakdown voltage will also increase. 
         [0080]    According to one aspect of the invention, the gate of the high voltage LDMOS device is offset from a location between the source and the drain to a location on the other side of the source away from the drain. This aspect is illustrated in  FIGS. 4A-4C . As illustrated in  FIG. 4A , a standard plane gate  50  is located between source  52  and drain  54 . The surface channel edge  56  on the drain side is a weak point for the LDMOS device due to a surface E-field peak. This is especially true for a device having a thin gate oxide with n/p pillar super junction located in the gate oxide area due to the rough surface topography.  FIG. 4B  shows one embodiment of the present invention where the plane gate  60  has been offset away from drain  62  to the other side of source  64 . The complicated high voltage LDMOS design of  FIG. 4A  has been changed to a relatively simple HV PIN diode design at area  66 . In this embodiment, the channel region current flow has been controlled to make the drain current flow deeply into the semiconductor bulk, thus reducing surface electric field. The surface E-field is reduced because there is no surface channel and surface gate oxide facing the n+ drain  62 . A large device SOA (Safe Operating Area) is also obtained because the P-body (P-well)  68  facing n+ drain  62  effectively collects hole current when the device is in the on state. 
         [0081]      FIG. 4C  shows another embodiment of the present invention wherein offset trench gate  70  is located on the side of source  72  away from that facing drain  74 . As with the embodiment of  FIG. 4B , the high field region of the device of  FIG. 4A , has been changed to a p-n junction area  76  and the drain current flows deeply into the semiconductor bulk. As will be described later, the small width of Pwell  68  of the embodiment of  FIG. 4B  and of Pwell  78  of the embodiment of  FIG. 4C  can handle a high breakdown voltage based on a 4-D stacked JFET control drift region  69 ,  79 . 
         [0082]    Referring now to  FIGS. 5-8 , there is shown in greater detail an embodiment of the present invention. As shown in  FIG. 5 , high voltage LDMOS semiconductor device  100  includes a P substrate  102 , stacked JFETs (Junction Field Effect Transistors)  104  and  106 , n+ drain region  108 , n-well  112 , p-well  114 , n+ source  118 , p+ tap  116 , offset plane gate  120 , thin oxide layer  122 , drain contact  124 , source contact  126 , and gate contact  128 .  FIGS. 6 and 7  show in greater detail portions of the structure of the stacked JFETs with 4-direction extension (4D) JFET control which results in high breakdown voltage (BV) and low on resistance Rdson for the device.  FIG. 7  is a cross-sectional view of a portion of the stacked JFETs which include first, second, and third layers  150 ,  152 , and  154 , respectively, of p-type conductivity material. A fourth layer  156  is provided between layers  150  and  152  and includes pillars  158  of p-type conductivity material alternating with pillars  160  of n-type conductivity material. A fifth layer  162  is provided between layers  152  and  154  that also includes alternating pillars  158  and  160 .  FIG. 6  is a top, plan sectional view showing a portion of the pillar structure of  FIG. 6 . As shown, pillars  158 ,  160  extend between n+ well region  112 /n+ drain region  108  and p-body or well  114  with source  116 .  FIG. 8  shows the JFETs extending between HV NWELLs  171  and  172 . 
         [0083]    According to another aspect of the invention, an unbalanced doping concentration is provided with the p-type pillars having a relatively high doping concentration and the n-type pillars having a relatively low doping concentration (but still much greater than normal n-drift doping) in order to increase the n-drift depletion region width based on JFET control. Additionally, the p pillars are made relatively narrower and the n pillars relatively wider. Thus, the wider n pillars have reduced sensitivity for pillar width variation due to process variation. 
         [0084]    Because the n/p pillars are not located at the thin gate oxide region, the gate oxide sensitivity for the top topography of the super junction areas will be reduced. In addition, with the offset gate structure, the n+ source is screened by the p-well resulting in a large device SOA area. 
         [0085]      FIG. 7  illustrates the 4-D depletion extensions by JFET control in both the lateral and vertical directions at the dashed line region  170 .  FIG. 6  illustrates the depletion region extensions in the lateral direction in more detail at the dashed line regions  180  at the source ends of p pillars  158  and at the dashed line regions  182  at the drain ends of n pillars  160 . In the off state of the device, a reverse-biased voltage across the source/drain causes the n-drift and p-drift regions to be depleted in both the vertical and lateral directions under JFET control; a depletion region pinch-off happens in the drift region, which supports the device breakdown, and the large depletion region width t supports high drain to source breakdown voltage. In the on-state of the device: the increase in n-drift doping reduces the on-state resistance of the device, therefore improving the device current handling capability; the on-state resistance can be adjusted by different kinds of n-drift and p-drift layout configurations; and the on-state resistance can be continuously reduced by the 4-D stacked drift. 
         [0086]    Referring now to  FIG. 9 , there is shown an offset trench gate embodiment of the present invention. As shown, trench gate LDMOS device  200  includes a trench gate  202  which is offset from n+ drain region  204  by its location on the side of n+ source region  206  opposite from drain region  204 . Trench gate  202  can be a polysilicon or other appropriate conductive material. Trench gate device  200  is otherwise similar in construction to the embodiment of  FIG. 5  and includes stacked JFETs  208  and  210  with alternating n/p pillars  212 ,  214  sandwiched between top p layer  216 , intermediate p layer  218 , and substrate p layer  220 . N+ drain region  204  is diffused in n-well  222 , and n+ source region  206  is diffused in p-well  224 , which also has p+ tap region  226 . A thin oxide layer  228  separates trench gate  202  from n+ source  206  and buried n-well  230 . 
         [0087]    Referring now to  FIGS. 10A-10D , there will be described a method of forming the n/p pillar structure of the JFET region according to another aspect of the present invention. In general, the method comprises: providing a bulk p− silicon layer  300 , producing a buried nwell layer  301 , making a p− layer  320  in layer  301  and growing an n− epitaxial layer (p− epitaxial layer is an option)  302  on layer  301  ( FIG. 10A ); etching spaced parallel trenches  304  in layer 2   301  and  302 , the width of the trenches  304  being equal or less than the width of the regions  306  between trenches  304  (the width of the trenches  304  being equal or larger than the width of the regions  306  between trenches  304  for p-epitaxial option) ( FIG. 10B ); filling trenches  304  by segment epitaxial refill with p material  308  (n material corresponding to p− epitaxial as an option) ( FIG. 10C ); resulting in alternating n pillars  310  and p pillars  312  ( FIG. 10D ). A top layer  322  of p type is then produced. Segment epitaxial refill is carried out by intrinsic epitaxial refill and in-situ doped epitaxial refill, or different level in-situ doped epitaxial refills. The in-situ doped epitaxial layer with needed doping concentration is provided for p-n pillars charge balance. The intrinsic epitaxial layer with un-doped material is provided for process adjustment and also to support some doped layer lateral diffusion for higher breakdown voltage. Trenches  304  can also be filled with oxide/Si refill in which a trench side-wall thermal oxide layer has a Si refill. The sidewall oxide is used to prevent lateral diffusion between p pillars and n pillars in the device fabrication. In this case, there is an electric field through the trench side wall oxide by a capacitor (n pillars/oxide/p pillars), instead of the diode (n pillars/p pillars), to make the n pillars depleted in the device off-state for high device breakdown voltage. The refill materials could be Si, polysilicon, SiC and also high electron mobility materials, such as SiGe. 
         [0088]    Referring now to  FIGS. 11A-15E , other embodiments of the present invention are shown. 
         [0089]    Dual n-type wells are shown in  FIG. 11A , one for n-epi (without HVNWELLS  400 ,  402 ) or p-epi (with HVNWELLS  400 ,  402 ) and the other for buried Nwell  404 . Step n-type wells are shown in  FIG. 11B , one for n-epi (without HVNWELLS  400 ,  402 ) or p-epi (with HVNWELLS  400 ,  402 ) and the other for buried Nwell  404 . The buried Nwell  404  is undercut  406  in the drift area near the source side. In this way, the n/p pillars near the source side and n-epi. (or HVNWELL) in source area are easily pitched off for high breakdown voltage. However, in the on-state, due to no buried Nwell near the source region, the drift area near the source side becomes small, which will induce a slightly higher on-state resistance. 
         [0090]      FIG. 12  shows a p-type layer  500  on top of the dual n-type wells and an adjustment p-type layer  502  between the top/buried n-type wells. 
         [0091]      FIG. 13A  shows a stacked JFET region linked by an optional n-type well—i. e., HVNWELLS  600  and  602 .  FIG. 13B  shows p-epi regions  610 ,  612  built on top of buried Nwell regions  614  and  616 . Because there is no link-up between n-type drain to n/p pillar area and no link-up between n/p pillar area to the channel region, HVNWELL regions are added to the p-epi regions grown on the top of the buried Nwell regions. 
         [0092]      FIG. 14A  shows a device with shallow trenches (such as 8 um-12 um) in which there is no JFET 2  and JFET 1   700  is built on the top of buried Nwell  702 .  FIG. 14B  shows a device with deep trenches (such as 12 um-20 um) in which JFET 1   800  is stacked with JFET 2   802 . 
         [0093]      FIGS. 15A-15E  show various arrangements without and with various type adjustment layers.  FIG. 15A  shows no adjustment layer  900  where JFET 1   902  and JFET 2   904  merge together into a single big JFET.  FIG. 15B  shows a relatively high p-type adjustment layer  910  with shallow trench option where JFET 1   912  is located on the top of buried Nwell  914 .  FIG. 15C  shows a relatively medium p-type adjustment layer  920  with deep trench option where JFET 1   922  is stacked with JFET 2   924 .  FIG. 15D  shows a relatively low p-type adjustment layer  930  (the p-type doping concentration will be compensated by the top of the buried Nwell  932 ) with shallow trench option. The top of the buried Nwell  932  doping concentration is reduced due to the compensation, therefore the n pillars on the bottom of JFET 1   934  is easily depleted for high breakdown voltage.  FIG. 15E  shows a relatively low p type adjustment layer  940  (the p-type doping concentration will be compensated by the top of the buried Nwell) with deep trench option. The top of the buried Nwell doping concentration is reduced due to the compensation, and, therefore, the middle area of the n pillars in the big JFET (JFET 1  merge to JFET 2 ) is easily depleted for high breakdown voltage. 
         [0094]    The invention has the following advantages, among others. 
         [0095]    1. The surface electric field of a high voltage LDMOS device can be significantly reduced by the offset gate for high device breakdown voltage. The percentage of channel/source area for high voltage (such as 700V) is very small, especially in advanced technology, with minimum effect for channel/source resistance. 
         [0096]    2. A complicated high voltage LDMOS design is changed to a relatively simple high voltage PIN diode design. 
         [0097]    3. Resurf (reduced surface E-field) not only comes from the drift region, but also comes from the channel region location in the device of the invention due to drain current flow deeply into the semiconductor bulk, therefore reducing surface electric field in the LDMOS. 
         [0098]    4. The off-set gate with p-body (p-well) facing the drain effectively collects hole current at the device on-state for large device safe operating area (SOA). 
         [0099]    5. The off-set gate oxide is not located at the p/n pillar areas, therefore, there is no sensitivity for the top topography of the superjunction areas and the gate oxide can be very thin and compatible with advanced technology trend. 
         [0100]    6. The stacked JFET drift region with 4-depletion (4-D) extensions results in high breakdown voltage of the device. 
         [0101]    7. There is a high doping concentration in the (p/n pillar) drift region for super junction purpose with dual current paths for low on resistance. 
         [0102]    8. A trench pillar 4-D stacked JFET drift off-set gate high voltage LDMOS can be easily integrated to an advanced technology platform. 
         [0103]    Although the invention has been described in detail with particular reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention. Thus, the n-drift region of the device can have more or less than two stacked JFETs. Although the invention has been described relating to nmos devices, it is equally applicable to pmos devices.