Abstract:
A power device includes a semiconductor substrate having a plurality of alternately arranged pillars of first and second conductivity types. At least one of the plurality of pillars of second conductivity type includes a first trench epitaxial layer of the second conductivity type disposed on a trench sidewall of the second trench and a trench bottom surface of the second trench, a second trench epitaxial layer of the second conductivity type disposed on the first trench epitaxial layer of the second conductivity type, and an insulating material layer disposed on the second trench epitaxial layer of the second conductivity type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure incorporates by reference the commonly assigned U.S. patent application Ser. No. 12/234,549 filed Sep. 19, 2008, entitled “Superjunction Structures for Power Devices and Methods of Manufacture,” as if set forth in full in this document, for all purposes. 
     BACKGROUND 
     The present invention relates in general to semiconductor technology and in particular to power semiconductor devices such as transistors and diodes and their methods of manufacture. 
     The key component in power electronic applications is the solid-state switch. From ignition control in automotive applications to battery-operated consumer electronic devices, to power converters in industrial applications, there is a need for a power switch that optimally meets the demands of the particular application. Solid-state switches including, for example, the power metal-oxide-semiconductor field effect transistor (power MOSFET), the insulated-gate bipolar transistor (IGBT) and various types of thyristors and rectifiers have continued to evolve to meet this demand. In the case of the power MOSFET, for example, double-diffused structures (DMOS) with lateral channel (e.g., U.S. Pat. No. 4,682,405 to Blanchard et al.), trenched gate structures (e.g., U.S. Pat. No. 6,429,481 to Mo et al.), and various techniques for charge balancing in the transistor drift region (e.g., U.S. Pat. No. 4,941,026 to Temple, U.S. Pat. No. 5,216,275 to Chen, and U.S. Pat. No. 6,081,009 to Neilson) have been developed, among many other technologies, to address the differing and often competing performance requirements. 
     Some of the defining performance characteristics for the power switch are its on-resistance, breakdown voltage and switching speed. Depending on the requirements of a particular application, a different emphasis is placed on each of these performance criteria. For example, for power applications greater than about 300-400 volts, the IGBT exhibits an inherently lower on-resistance as compared to the power MOSFET, but its switching speed is lower due to its slower turn off characteristics. Therefore, for applications greater than 400 volts with low switching frequencies requiring low on-resistance, the IGBT is the preferred switch while the power MOSFET is often the device of choice for relatively higher frequency applications. If the frequency requirements of a given application dictate the type of switch that is used, the voltage requirements determine the structural makeup of the particular switch. For example, in the case of the power MOSFET, because of the proportional relationship between the drain-to-source on-resistance Rds-on and the breakdown voltage, improving the voltage performance of the transistor while maintaining a low Rds-on poses a challenge. Various charge balancing structures in the transistor drift region have been developed to address this challenge with differing degrees of success. 
     Device performance parameters are also impacted by the fabrication process. Attempts have been made to address some of these challenges by developing a variety of improved processing techniques. 
     Whether it is in ultra-portable consumer electronic devices or routers and hubs in communication systems, the varieties of applications for the power switch continue to grow with the expansion of the electronic industry. The power switch therefore remains a semiconductor device with high development potential. 
     BRIEF SUMMARY 
     In accordance with one aspect of the invention, a power device includes a semiconductor region which in turn includes a plurality of alternately arranged pillars of first and second conductivity type. Each of the plurality of pillars of second conductivity type further includes a plurality of implant regions of the second conductivity type arranged on top of one another along the depth of pillars of second conductivity type, and a trench portion filled with semiconductor material of the second conductivity type directly above the plurality of implant regions of second conductivity type. 
     In accordance with another aspect of the invention, a power device includes: an active region and a termination region surrounding the active region, and a plurality of pillars of first and second conductivity type alternately arranged in each of the active and termination regions, each of the plurality of pillars of second conductivity type in the active and termination regions further including a plurality of implant regions of the second conductivity type arranged on top of one another along the depth of the pillars of second conductivity type, and a trench portion filled with semiconductor material of the second conductivity type directly above the plurality of implant regions of second conductivity type. 
     In accordance with another aspect of the invention, a method for forming pillars of alternating conductivity type in a power device includes: forming a plurality of epitaxial layers of a first conductivity type over a substrate; forming a plurality of implant regions of a second conductivity type in each of a predetermined number of the plurality of epitaxial layers; forming trenches extending into the upper-most one of the plurality of epitaxial layers; and filling the trenches with semiconductor material of the second conductivity type, wherein the plurality of implant regions of second conductivity type in the predetermined number of the plurality of epitaxial layers are vertically aligned with corresponding ones of the trenches so that the semiconductor material filling the trenches together with the plurality of implant regions of second conductivity type in the predetermined number of the plurality of epitaxial layers form a plurality of pillars of second conductivity type, and those portions of the plurality of epitaxial layers separating the plurality of pillars of second conductivity type from one another form a plurality of pillars of first conductivity type. 
     In accordance with another aspect of the invention, a method for forming pillars of alternating conductivity type in a power device includes: forming a first epitaxial layer of a first conductivity type over a substrate; forming a lower portion of a plurality of deep trenches in the first epitaxial layer; filling the lower portion of the plurality of deep trenches with semiconductor material of a second conductivity type; forming a second epitaxial layer of first conductivity type over the first epitaxial layer; forming an upper portion of the plurality of deep trenches in the second epitaxial layer directly over the lower portion of the plurality of deep trenches so that each lower portion and a corresponding upper portion of the plurality of deep trenches together form one of the plurality of deep trenches; and filling the upper portion of the plurality of deep trenches with semiconductor material of second conductivity type, wherein the semiconductor material filling the lower and upper portions of the plurality of deep trenches form a plurality of pillars of second conductivity type, and those portions of the first and second epitaxial layers separating the plurality of pillars of second conductivity type from one another form a plurality of pillars of first conductivity type. 
     In accordance with another aspect of the invention, a method for forming a power field effect transistor includes: forming an N-type epitaxial layer over a substrate; forming one or more P-type epitaxial layers over the N-type epitaxial layer; forming a plurality of trenches extending through the one or more P-type epitaxial layers; filling the plurality of trenches with N-type semiconductor material; forming P-type body regions in the one or more P-type epitaxial layers; forming N-type source regions in the P-type body regions; and forming gate electrodes adjacent to but insulated from the P-type body regions and the N-type semiconductor material, the gate electrodes overlapping with the N-type source regions, wherein the plurality of trenches filled with N-type semiconductor material form N-pillars, and those portions of the one or more P-type epitaxial layers separating the N-pillars form P-pillars. 
     In accordance with another aspect of the invention, a power field effect transistor (FET) includes: an N-type epitaxial layer over a substrate; one or more P-type epitaxial layers over the N-type epitaxial layer; a plurality of trenches extending through the one or more P-type epitaxial layers, the plurality of trenches being filled with N-type semiconductor material; P-type body regions in the one or more P-type epitaxial layers; N-type source regions in the P-type body regions; and gate electrodes adjacent to but insulated from the P-type body regions and the N-type semiconductor material, the gate electrodes overlapping with the N-type source regions, wherein the plurality of trenches filled with N-type semiconductor material form N-pillars, and those portions of the one or more P-type epitaxial layers separating the N-pillars form P-pillars. 
     In accordance with another aspect of the invention, a method for forming a power field effect transistor includes: forming one or more epitaxial layers of a first conductivity type over a substrate; forming a plurality of lower trenches extending through the one or more epitaxial layers; filling the plurality of lower trenches with semiconductor material of a second conductivity type; forming one or more epitaxial layers of the second conductivity type over the one or more epitaxial layers of first conductivity type; forming a plurality of upper trenches extending through the one or more epitaxial layers of the second conductivity type; filling the plurality of upper trenches with semiconductor material of the second conductivity type, wherein the plurality of lower trenches and the plurality of upper trenches are off-set from one another along the lateral dimension. 
     In accordance with another aspect of the invention, a power field effect transistor (FET) includes a semiconductor region including a plurality of alternately arranged pillars of first and second conductivity type, wherein each of the pillars of the first conductivity type has a middle section that is wider than its upper and lower sections, and each of the pillars of the second conductivity type has a middle section that is narrower than its upper and lower sections. 
     In accordance with another aspect of the invention, a method for forming a super-junction structure in a power device includes: forming one or more epitaxial layers of a first conductivity type over a substrate; forming a plurality of trenches extending in the one or more epitaxial layers; lining the sidewalls and bottom of the trenches with a an epitaxial layer of a second conductivity type; forming a dielectric layer in the plurality of trenches over the epitaxial layer of second conductivity type; and filling the plurality of trenches with conformal material. 
     In accordance with another aspect of the invention, a method for forming a super-junction structure in a power device includes: forming one or more epitaxial layers of a first conductivity type over a substrate; forming a plurality of trenches extending in the one or more epitaxial layers; filling each trench with an epitaxial layer of a second conductivity type such that only a center portion of each trench along the top of the trench remains unfilled; and filling the center portion of each trench along the top of the trench with a dielectric material. 
     In accordance with another aspect of the invention, a method for forming a super-junction structure in a power device includes: forming one or more epitaxial layers of a first conductivity type over a substrate; forming a plurality of trenches extending in the one or more epitaxial layers; lining sidewalls and bottom of the plurality of trenches with a first epitaxial layer of a second conductivity type; filling each trench with a second epitaxial layer of the second conductivity type such that only a center portion of each trench along the top of the trench remains unfilled; and filling the center portion of each trench along the top of the trench with a dielectric material. 
     In accordance with another aspect of the invention, a power device includes a plurality of trenches extending in one or more epitaxial layers of a first conductivity type, the plurality of trenches being filled with a first epitaxial layer of a second conductivity type, a second epitaxial layer of the second conductivity type, and a layer of insulating material, the first epitaxial layer lining the trench sidewalls and bottom, the second epitaxial layer extending over and being in direct contact with the first epitaxial layer, and the layer of insulating material extending over and being in direct contact with the second epitaxial layer, the first epitaxial layer, the second epitaxial layer, and the third layer of insulating material in each trench forming a pillar of second conductivity type, and those portions of the one or more epitaxial layers separating the pillars of second conductivity type forming pillars of first conductivity type such that the pillars of first and second conductivity type form pillars of alternating conductivity type. 
     In accordance with another aspect of the invention, a power device includes at least first and second N-type epitaxial layers extending over a substrate, and a plurality of trenches extending in the second N-type epitaxial layer, the plurality of trenches being filled with a first epitaxial layer, a second P-type epitaxial layer, and a third layer of conductive material, the first epitaxial layer lining the trench sidewalls and bottom, the second P-type epitaxial layer extending over and being in direct contact with the first epitaxial layer, and the third layer of conductive material extending over and being in direct contact with the second P-type epitaxial layer, the first epitaxial layer, the second P-type epitaxial layer, and the third layer of conductivity type in each trench forming a P-pillar, and those portions of the at least first and second N-type epitaxial layers separating the P-pillars forming N-pillars such that the P-pillars and the N-pillars form pillars of alternating conductivity type. 
     In accordance with another aspect of the invention, a method for forming pillars of alternating conductivity type in a power device includes: forming at least first and second N-type epitaxial layers over a substrate; forming a plurality of trenches extending in the second epitaxial layer; and filling the plurality of trenches with a first epitaxial layer, second P-type epitaxial layer, and a third layer of conductive material, the first epitaxial layer lining the trench sidewalls and bottom, the second P-type epitaxial layer extending over and being in direct contact with the first epitaxial layer, and the third layer of conductive material extending over and being in direct contact with the second P-type epitaxial layer, the first epitaxial layer, the second P-type epitaxial layer and the third layer of conductive material in each trench forming a P-pillar, those portions of the at least first and second N-type epitaxial layers separating the P-pillars forming N-pillars such that the P-pillars and the N-pillars form pillars of alternating conductivity type. 
     In accordance with another aspect of the invention, a power device includes: one or more N-type epitaxial layers extending over a substrate; a plurality of trenches extending into the one or more N-type epitaxial layers, the plurality of trenches being filled with P-type silicon material, the P-type silicon material in the plurality of trenches forming P-pillars, those portions of the one or more N-type epitaxial layers separating the P-pillars forming N-pillars such that the N-pillars and the P-pillars form alternating P-N-pillars; and an active region and a termination region surrounding the active region, wherein the alternating P-N-pillars are disposed in both the active region and the termination region, the termination region includes a predetermined number of floating P-pillars, and each N-pillar located between two adjacent ones of the predetermined number of floating P-pillars includes a N-type surface region along its upper surface, the N-type surface region having a lower doping concentration than the rest of the N-pillar in which it is formed. 
     In accordance with another aspect of the invention, a power device includes: one or more N-type epitaxial layers extending over a substrate; a plurality of trenches extending into the one or more N-type epitaxial layers, the plurality of trenches being filled with P-type silicon material, the P-type silicon material in the plurality of trenches forming P-pillars, those portions of the one or more N-type epitaxial layers separating the P-pillars forming N-pillars such that the N-pillars and the P-pillars form alternating P-N-pillars; an active region and a termination region surrounding the active region, wherein the alternating P-N-pillars are disposed in both the active region and the termination region, the alternating P-N-pillars in the termination region surround the active region in a concentric fashion and include a predetermined number of floating P-pillars, each floating P-pillar includes a P-type ring along its top; a plurality of field plates disposed in the termination region over but insulated from the one or more N-type epitaxial layers, the plurality of field plates surround the active region in a concentric fashion; and a plurality of contacts configured so that each of the plurality of contacts makes contact between one of the plurality of field plates and one or more of the P-type rings, the plurality of contacts being disposed directly above a corresponding one of the predetermined number of floating P-pillars. 
     In accordance with another aspect of the invention, a power device includes an active region surrounded by a termination region, and a plurality of trenches extending into one or more epitaxial layers of a first conductivity type, the plurality of trenches being filled with silicon material of a second conductivity type, the silicon material of a second conductivity type in the plurality of trenches together with portions of the one or more epitaxial layers separating the plurality of trenches from one another forming a plurality of concentric octagon-shaped pillars of alternating conductivity type extending through the active region and the termination region, wherein four of the eight legs of each of the plurality of concentric octagon-shaped pillars have a different length than the other four legs, and sidewalls of the plurality of trenches along all eight legs of the plurality of concentric octagon-shaped pillars have the same plane direction. 
     In accordance with another aspect of the invention, a method of forming a power device having an active region surrounded by a termination region, the method comprising: forming a plurality of trenches in one or more epitaxial layers of a first conductivity type; and filling the plurality of trenches with silicon material of a second conductivity type, the silicon material of a second conductivity type in the plurality of trenches together with portions of the one or more epitaxial layers separating the plurality of trenches from one another forming a plurality of concentric octagon-shaped pillars of alternating conductivity type extending through the active region and the termination region, wherein four of the eight legs of each of the plurality of concentric octagon-shaped pillars have a different length than the other four legs, and sidewalls of the plurality of trenches along all eight legs of the plurality of concentric octagon-shaped pillars have the same plane direction. 
     In accordance with another aspect of the invention, a power device includes an active region surrounded by a termination region, a plurality of stripe-shaped pillars of alternating conductivity type extending through the active region, and a plurality of octagon-shaped pillars of alternating conductivity type extending through the termination region in a concentric fashion, surrounding the active region. 
     In accordance with another aspect of the invention, a power device includes: an active region surrounded by a termination region; a plurality of pillars of alternating conductivity type arranged in a concentric fashion in the active and termination regions; a plurality of polysilicon gates arranged in concentric fashion in the active region; an outer metal gate runner extending along an outer perimeter of the termination region in a concentric fashion, the outer metal gate runner being connected to a gate pad; and a plurality of supplementary metal gate runners directly connected to the outer metal gate runner, and extending from the outer metal gate runner toward a center of the active region but terminating before reaching the center of the active region, wherein a first group of the plurality of the polysilicon gates directly connects to all of the plurality of supplementary metal gate runners, and a second group of the plurality of the polysilicon gates directly contact only two of the plurality of supplementary metal gate runners. 
     In accordance with another aspect of the invention, a power device includes: an active region and a termination region surrounding the active region; a plurality of pillars of alternating conductivity type arranged in a concentric fashion in both the active and the termination regions; a plurality of polysilicon gate stripes extending through the active and termination regions; and a gate runner metal extending along an outer perimeter of the termination region, the plurality of polysilicon stripes connecting to the gate runner metal along their opposite ends. 
     In accordance with another aspect of the invention, a method for forming pillars of alternating conductivity type in a power device, the method comprising: forming one or more N-type epitaxial layers over a substrate; forming P-type body regions in the one or more N-type epitaxial layers; forming gate electrodes extending adjacent to but being insulated from the one or more N-type epitaxial layers by a gate dielectric; after forming the P-type body regions and the gate electrodes, forming a plurality of deep trenches extending in the one or more N-type epitaxial layers; and filling the plurality of deep trenches with P-type silicon to form a plurality of P-pillars, those portions of the one or more N-type epitaxial layers separating the plurality of P-pillars forming N-pillars such that the P-pillars and the N-pillars form pillars of alternating conductivity type. 
     In accordance with another aspect of the invention, a high voltage device includes: one or more N-type epitaxial layers extending over a substrate; a plurality of trenches extending into the one or more N-type epitaxial layers, the plurality of trenches being filled with P-type silicon material, the P-type silicon material in the plurality of trenches forming P-pillars, those portions of the one or more N-type epitaxial layers separating the P-pillars forming N-pillars such that the N-pillars and the P-pillars form alternating P-N-pillars; a plurality of P-wells each formed in an upper portion of one of the P-pillars; and an anode terminal comprising a Schottky barrier metal directly contacting a top surface of the N-pillars to form a Schottky contact therebetween, the Schottky barrier metal further directly contacting the P-wells. 
     In accordance with another aspect of the invention, a high voltage device includes: one or more N-type epitaxial layers extending over a substrate; a plurality of trenches extending into the one or more N-type epitaxial layers, the plurality of trenches being filled with P-type silicon material, the P-type silicon material in the plurality of trenches forming P-pillars, those portions of the one or more N-type epitaxial layers separating the P-pillars forming N-pillars such that the N-pillars and the P-pillars form alternating P-N pillars; an N-type epitaxial layer extending over the alternating P-N Pillars; and an anode terminal comprising a Schottky barrier metal directly contacting a top surface of the N-type epitaxial layer to form a Schottky contact therebetween, the N-type epitaxial layer separating the Schottky barrier metal from the P-pillars so that the P-pillars float. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  show three different layout configurations for a superjunction power devices; 
         FIG. 2  shows a simplified cross section view of a superjunction FET; 
         FIG. 3  shows a simplified cross section view along a portion of a die where the active region transitions to termination region through transition region, in accordance with an embodiment of the invention; 
         FIGS. 4A-4H  are simplified cross section views depicting various steps in an exemplary process for forming the pillar structure shown in  FIG. 3 ; 
         FIGS. 5A-5G  are simplified cross section views showing various embodiments in which the flexibility provided by combining the trench-fill process with the multiple Epi and implant process is advantageously exploited to obtain performance improvements; 
         FIGS. 6A-6B  are simplified cross section views showing a process for forming P-pillars using a two-step pillar process; 
         FIGS. 7A-7D ,  8 A- 8 D,  9 A- 9 B and  10 A- 10 B are simulation results illustrating the impact of various process parameters on the degree of electric field concentration along the trench depth; 
         FIGS. 11A and 11B  show a simplified cross section view where N-enrichment regions are formed at the bottom of N-filled trenches to create a local charge imbalance, and the corresponding electric field curve, respectively; 
         FIGS. 12A and 12B  show another simplified cross section view with N-enrichment regions at the bottom of N-pillars to create a local charge imbalance, and the corresponding electric field curve, respectively; 
         FIGS. 13A-13B  are simplified cross section views showing a three step trench fill process used in forming a super junction device that is free of voids; 
         FIGS. 14 and 15  show simplified cross section views of superjunction power devices which minimizes formation of voids in the trench fill process; 
         FIGS. 16A-16C ,  17  and  18  are simplified cross section views showing various embodiments of super-junction devices with pillars of alternating conductivity type; 
         FIGS. 19A-19L  are simplified cross section views showing various stages of a process for forming a super junction trench-gate MOSFET; 
         FIG. 20  is simplified cross section view used to illustrate the process for forming the planar-gate variation of the trench-gate MOSFET in  FIGS. 19A-19L ; 
         FIGS. 21 and 22  are simplified cross section views along a portion of the die where the active region transitions into the termination region; 
         FIG. 23  is a simplified cross section view of a superjunction power device where field plates are electrically connected to their corresponding floating P-pillars; 
         FIG. 24A  is a top layout view of a corner region of a super junction power device; 
         FIG. 24B  is an expanded view of the corner of the termination region of the top layout view in  FIG. 24A ; 
         FIGS. 25A-25D  are simplified cross section views showing various embodiments of super-junction high voltage merged PiN Schottky rectifiers; 
         FIGS. 26A-26B  are simplified cross section views showing two additional embodiments of super junction high voltage Schottky rectifiers; 
         FIGS. 27A and 27B  are respectively top view of a die layout diagram and top view of a wafer where the wafer flat extends parallel to the laterally extending pillars in  FIG. 27A ; 
         FIG. 28  is a top view of a die layout diagram illustrating rotation of the die for the purpose of eliminating the non-uniform epi filling due to variations in plane direction; 
         FIG. 29  is a top layout view of a corner of a super-junction power MOSFET with striped active P-pillars surrounded by concentric termination P-pillars; 
         FIG. 30  is a top view of a fully concentric layout design where a supplementary metal gate runner is bussed through a center portion of the die to provide metal connection to the concentric polysilicon gates; 
         FIG. 31A  shows a top layout view of a fully concentric design with a gate runner design that provides a more balanced gate propagation delay throughout the die; 
         FIG. 31B  is an expanded view of an inner portion of the top view in  FIG. 31A ; 
         FIG. 31C  is an expanded view of the upper right quadrant of the die shown in  FIG. 31A ; and 
         FIG. 32  shows a top layout view of a fully concentric pillar design with stripe polysilicon gates. 
     
    
    
     DETAILED DESCRIPTION 
     The power switch can be implemented by any one of power MOSFET, IGBT, various types of thyristors and rectifiers and the like. Many of the novel techniques presented herein are described in the context of the power MOSFET and Schottky rectifiers for illustrative purposes. It is to be understood however that the various embodiments of the invention described herein are not limited to the power MOSFET and Schottky rectifiers and can apply to many of the other types of power switch technologies, including but not limited to, for example, IGBTs and other types of bipolar switches and various types of thyristors and rectifiers. Further, for the purposes of illustration, the various embodiments of the invention are shown to include specific P and N type regions (e.g., for an n-channel MOSFET). It is understood by those skilled in the art that the teachings herein are equally applicable to devices in which the conductivities of the various regions are reversed. 
     In the super junction technology, the alternating P/N-pillars in the active and termination regions may be arranged in a number of different layout configurations.  FIGS. 1A-1C  show three such layout configurations. In  FIG. 1A , P/N-pillars  102  and  104  in both active region  108  and termination region  106  are arranged in a concentric configuration (hereinafter referred to as “full concentric” configuration); in  FIG. 1B , P/N-pillars  112  and  114  in both active region  118  and termination region  116  are arranged in a parallel (or striped) configuration (hereinafter referred to as “full parallel” configuration); and in  FIG. 1C , P/N-pillars  122  and  124  in active region  128  are arranged in a parallel (or striped) configuration, and P/N-pillars  122  and  124  in termination region  126  are arranged in a concentric configuration (hereinafter referred to as “parallel-concentric” configuration). Each of these layout configurations has its own merits and drawbacks. Some of the inventions and embodiments described herein address various drawbacks of these layout configurations. 
     The full concentric configuration in  FIG. 1A  enjoys uniform charge balance throughout active region  108  and termination region  106 , but the active channel area may be reduced because the gate feeds must extend into the interior of active area  108  to feed the concentric active polysilicon gates. The channel may need to be removed at the corners to eliminate areas of lower threshold voltage and prevent parasitic NPN turn-on. Thus, as the die size is reduced, the penalty in on-resistance (Rds-on) attributed to these corners in the active area may become greater. 
     The full parallel configuration in  FIG. 1B  also enjoys uniform charge balance throughout the active and termination regions but without the Rds-on penalty of the full concentric configuration. However, the P/N-pillar design in the full parallel configuration may be limited to an N-rich balance condition to insure that the P-pillars extending out into termination area  116  from active area  118  become fully depleted somewhere along their length. By using concentric pillars for the termination, as in  FIG. 1C , the electric field can be distributed across the termination region without full pillar depletion. 
     In the design where pillars (e.g., P-pillars) are formed using a trench etch and fill process, corners of the concentric pillars may be difficult to etch and fill resulting in voids in the epi fill that cause charge imbalance. These corners may thus become areas of high electric field stress. If they are shorted to source potential, either of the  FIG. 1A  and  FIG. 1C  layout configurations may have a lower breakdown voltage at these corners. In the parallel-concentric configuration shown in  FIG. 1C , these corners may be moved outside active area  128  where they can float and are thus not fixed at source potential thereby minimizing or eliminating them as a source of localized lower breakdown voltage. Also, the active channel area can be maximized and gate feeds used that are more conventional only requiring a perimeter gate runner to make connection to the active polysilicon gates. 
     In order to achieve good Unclamped Inductive Switching (UIS) characteristics, it is desirable to design the device so that breakdown first occurs in the active region as opposed to any other region of the device including the termination region. One way to achieve this is to make sure that all regions of the device have sufficiently higher breakdown voltage than the active area by locally modifying the charge balance in these regions.  FIG. 2  shows an embodiment where this is achieved. In  FIG. 2 , P-pillars  230 ,  236  in both active region  204  and termination region  202  may have the same width W 3  and similar doping profiles. N-type mesa regions  232 ,  234  (alternatively referred to as N-pillars in this disclosure) in active region  204  and termination region  202  may be grown with the same epitaxial layer or layers. 
     Using known techniques, mesa width W 1  and P-pillar width W 3  as well as the doping profiles in P-pillars  230 ,  236  and N-type mesas  232 ,  234  may be designed to achieve a charge balance condition resulting in termination region  202  having a higher breakdown voltage than active region  204 . In contrast, mesa width W 2  in active region  204  may be adjusted to obtain a different charge balance condition that results in a lower breakdown voltage than other areas of the device including termination region  202 . In one embodiment, mesa width W 2  in active region  204  may be made smaller than mesa width W 1  in termination region  202  so that active region  204  is more P-rich. In another embodiment, mesa width W 2  in active region  204  may be made greater than mesa width W 1  in termination region  202  so that active region  204  is more N-rich. These techniques ensure that breakdown occurs in active region  204  first thus resulting in a more stable breakdown characteristic and a more uniformly distributed current flow during a UIS event. Accordingly, both the breakdown and UIS characteristics of the device are improved. Note that an N-rich active region may result in a improved (lower) Rds-on at the expense of UIS performance, and a P-rich active region may provide a better UIS performance at the expense of Rds-on. Depending on the design goals, one approach may be preferred to the other. A number of techniques for achieving various performance improvements are described next. 
       FIG. 3  shows a cross section view along a portion of a die where active region  301  transitions to termination region  302  through transition region  304 . In this exemplary embodiment, transition P-pillars  329 A are bridged to the first contacted P-pillar  329 B in active region  301  through a diffusion region  342  marked as PIso. This bridging diffusion may extend over N-type mesa regions  333 A. In this and other embodiments disclosed herein, the N-type mesa regions separating the P-pillars may also be referred to as “N-pillars.” When N-type mesa regions  333 A in transition region  304  have the same or smaller width than active N-pillars  333 B, an increase in P charge in transition region  304  occurs. This increase in P charge can reduce the breakdown voltage below that of active area  301 . To compensate for this increase in P charge, the width of N-pillars  333 A in transition region  304  may be made greater than the width of N-pillars  333 B in the active region. This can ensure that the breakdown voltage in transition region  304  remains higher than in active area  301 . In the embodiment shown in  FIG. 3 , transition region  304  is defined by the span of the bridging diffusion  342 . 
     As with the  FIG. 2  embodiment, the width of all P-type pillars  329 A,  329 B,  329 C in all regions (the termination, transition and active regions) may be substantially the same, and the width of termination mesa regions  333 C may be greater than the width of the active mesa regions  333 B. However, the width of termination mesa regions  333 C may be greater than, the same as, or smaller than the width of transition mesa regions  333 A. In active region  301 , in one embodiment, P-pillars  329 B may have the same width and may be spaced from one another by the same distance. However, in another embodiment, the width of P-pillars  329 B in active region  301  may be smaller than the spacing between them, thus providing a N-rich condition in the active region. In one embodiment, the active and transition N-pillars and P-pillars may be stripe-shaped with termination N-pillars and P-pillars surrounding the active and transition regions in a concentric fashion similar to the layout configuration shown in  FIG. 1C . In yet another embodiment, the active, transition, and termination N-pillars may be concentric similar to the layout configuration shown in  FIG. 1A . 
     In the designs where the pillars (e.g., P-pillars) are formed by etching deep trenches and filling them with silicon, as for example in high voltage super junctions designs, process reliability may be directly related to the trench depth to width ratio (i.e., the trench aspect ratio). For higher trench aspect ratios, epi filling of the trenches becomes more difficult.  FIG. 3  shows a technique whereby P-pillars that extend deep into the drift region are formed without requiring deep trenches. 
     In  FIG. 3 , a multi-epi process with multiple aligned implantations is combined with a trench process to form all P-pillars  329 A,  329 B,  329 C. As can be seen, each P-pillar includes three P-implant regions  335 A,  335 B,  335 C stacked on top of each other, as well as a trench-filled portion  337  along the upper portion of the P-pillar. Along each P-pillar, the three P-implant regions  335 A,  335 B,  335 C and the upper trench-filled portion  337  correspond to separate N-epi layers. That is, in the exemplary embodiment shown in  FIG. 3 , four N-epi layers are used to form the P-pillars. More or fewer than four epi layers may be used depending on the design goals. 
     The technique exemplified by the  FIG. 3  embodiment provides a number of advantages. First, the trench etch depth is substantially reduced thus allowing for reduced trench CD and easier trench filling. Also, the cell pitch can be reduced due to the trench etch angle. That is, the trenches are etched with a taper from bottom to top, thus resulting in a wider trench width at the top. This allows the trenches to be fully filled without the risk of pinch off at the top creating voids in the pillar. Having a shallower trench etch reduces the width of the trench at the top which is a function of the tangent of the etch angle. Consequently, the width of the pillar at the top is smaller and the shallower trenches can be made with a smaller etch CD because they are easier to fill. Thus a smaller cell pitch and a lower Rds-on can be obtained. Further, as is described more fully in connection with  FIGS. 5A through 5G  below, this technique advantageously: (1) allows use of different P implant CDs for active and termination P-pillars to insure that break down occurs in the active area first, and (2) allows adjusting the P implant CDs in the active region to insure that the avalanche breakdown occurs well below the junction formed by the P-type body regions and the N-type drift region. 
     Before turning to  FIGS. 5A to 5G , an exemplary process for forming the pillar structure shown in  FIG. 3  will be described with references to  FIGS. 4A to 4H .  FIG. 4A  shows N+ starting substrate  404 . In  FIG. 4B , a first N-epi layer  407 A may be grown using conventional techniques. In  FIG. 4C , a P-implant may be carried out to form P-implant regions  401 A. A conventional masking and implant process may be used to form P-implant regions  401 . In  FIG. 4D , the steps corresponding to  FIG. 4C  are repeated two more times to form second and third N-epi layers  407 B,  407 C and the corresponding P-implant regions  401 B,  401 C, followed by formation of a fourth and thicker N-epi layer  407 D. The second to fourth N-epi layers may be formed using conventional techniques. The second to fourth N-epi layers may be formed with a uniform, stepped, or graded doping concentration. As will be discussed further below, different implant doping concentration and/or energy may be selected in forming each P-implant region  401 A,  401 B,  401 C in order to obtain the desired charged imbalance condition along the pillar length and/or depth. 
     In  FIG. 4E , trenches  403  may be patterned and etched deep enough to reach the uppermost P-implant regions  401 C. Backside alignment techniques may be used to ensure alignment of trenches  403  with P-implant regions  401 C. In  FIG. 4F , trenches  403  may be filled with P-epi  425  using known techniques. In  FIG. 4G , P-epi  425  may be planarized using, for example, a conventional chemical mechanical polishing (CMP) process. In  FIG. 4H , P-type body region  438 , N+ source regions  418 , the P+ heavy body regions as well as the gate structure and its overlying layers may be formed using known techniques. While in the process depicted by  FIGS. 4A-4G , a separate N-type buffer layer is not incorporated between the substrate and the first N-epi layer  407 A (as is done in  FIG. 3 ), such buffer layer can be incorporated by forming a suitable N-epi layer before forming N-epi layer  407 A in  FIG. 4C . Alternatively, N-epi layer  407 A can be made thicker to account for the buffer layer. 
     As can be seen, this process yields a super junction device with P-pillars that are formed from a combination of multiple P-implant regions  401 A,  401 B,  401 C and a relatively shallow trench-filled portion  403 . Accordingly, the trench etch depth is substantially reduced thus allowing for reduced trench CD and easier trench filling. This technique also provides a number of other advantages over conventional techniques, some of which will be discussed with reference to the embodiments shown in  FIGS. 5A-5G . 
       FIGS. 5A-5G  show various embodiments in which the flexibility provided by combining the trench-fill process with the multiple Epi and implant process is advantageously exploited to obtain performance improvements.  FIG. 5A  is a cross section view that is similar to  FIG. 3  except that a larger P-implant CD is used in forming the bottom-most P-implant region  535 A of active P-pillars  529 B than that used in forming the bottom-most P-implant regions of transition P-pillars  529 A and termination P-pillars  529 C. In this manner, the charge imbalance at the bottom of the active P-pillars is made greater than the charge imbalance at the bottom of the termination and transition P-pillars, thus ensuring that the break down first occurs in the active region. 
       FIG. 5B  shows a variation of the  FIG. 3  embodiment where P-implant regions  555 A,  555 B along P-pillars in the active, transition and termination regions are floating (i.e., P-implant regions  555 A,  555 B do not merge). This can be achieved by carefully controlling the P-implant dose and energy in the process steps corresponding to  FIGS. 4C and 4D . Floating the pillar regions allows for reduced charge balance sensitivity because these regions can assume a potential similar to the floating pillar regions in the termination.  FIG. 5C  shows another embodiment that is similar to that in  FIG. 5B  except that a larger P-implant CD is used in forming the bottom-most P-implant regions  555 A in the active P-pillars as compared to that used in forming the bottom-most P-implant regions  555 A in the transition and termination P-pillars. Similar to the  FIG. 5A  embodiment, the greater charge imbalance in the active region due to the larger P-implant regions  502  forces the break down to occur in the active region. 
     In the  FIGS. 5A and 5C  embodiments, the larger P-implant regions may be formed for every other active P-pillar as opposed to all active P-pillars. This technique is advantageous in that the impact of the larger P-implant regions on current flow (e.g., pinching off the current path at the bottom of the P-pillars) is reduced thus improving Rds-on. The larger P-implant regions could also be formed at the bottom of every third active P-pillar or every fourth active P-pillar or some other pattern depending on the avalanche current and as long as breakdown occurs in a uniform manner in the active region. 
       FIG. 5D  shows a cross section view of yet another embodiment which is similar to that shown in  FIG. 3  except that multiple N-implant regions  572 A,  572 B,  572 C are formed in between the P-implant regions  535 A,  535 B,  535 C. These N-implant regions enable better control of charge balance and imbalance as well as make the pillars N rich at the bottom. Having the N rich imbalance at the bottom is another method of moving the avalanche location deeper in the silicon without penalizing Rds-on as a result of pillar pinch off. Referring to  FIGS. 4C and 4D , N-implant regions  572 A,  572 B, and  572 C can be formed immediately before or immediately after forming the corresponding P-implant regions using known techniques. Referring back to  FIG. 5D , while the N-implant regions  572 A,  572 B,  572 C and the P-implant regions  535 A,  535 B,  535 C are formed in a manner that provides similar charge balance characteristics in the active, transition and termination regions, in the embodiment shown in the  FIG. 5E , a smaller N-implant CD is used in forming the lower-most N-implant regions  572 A in the active region as compared to the lower-most N-implant regions  572 A in the transition and termination regions, thereby forcing break down in the active region. The smaller N-implant regions  572 A in the active region may alternatively be formed at the bottom of every other active N-pillar (instead of each and every active N-pillar as shown in  FIG. 5E ). In yet another embodiment shown in  FIG. 5F , the bottom-most N-implant region may be eliminated altogether in the active region by blocking the N-implant in the active region only. Alternatively, the bottom-most N-implant region may be eliminated from every other active N-pillar. 
       FIG. 5G  shows yet another embodiment in which a larger P-implant CD is used for the lower-most P-implant regions  535 A in the active P-pillars than that used in the termination and transition regions, and the bottom-most N-implant regions are eliminated only in the active region. Of the various embodiments, this embodiment provides the most P-rich condition at the bottom of the active P-pillars. Other permutations can be envisioned in view of the various embodiments disclosed herein. For example, the larger P-implant regions in  FIG. 5G  can be formed at the bottom of every other active P-pillar to obtain better Rds-on, or in  FIG. 5G , instead of fully eliminating the bottom-most N-implant region, smaller N-implant regions (such as regions  572 A in  5 E) can be used to improve Rds-on. Further, the P-implant regions and/or N-implant regions, or select groups of them, shown in the various embodiments may be formed so that they do not extend along the full length of the P-pillar (e.g., into the page). Also, while the cross section views in FIGS.  3  and  5 A- 5 G show the mesa widths in the active region to be different than the mesa widths in the transition and termination regions, in one embodiment, all mesa widths in the active, transition and termination regions are the same, and the bottom-most P-implant regions in the active region and/or the bottom-most N-implant regions in the active region are manipulated as outlined in the various embodiments described above to insure that the active region has a lower break down voltage than the termination region. It is also noted that the various embodiments described above may be implemented using any of the three layout configurations shown in  FIGS. 1A-1C . 
     While the embodiments described with reference to  FIGS. 3 ,  4 A- 4 H and  5 A- 5 G disclose techniques whereby the trench etch depth is substantially reduced thus allowing for easier trench filling, another technique for achieving the same is shown in  FIGS. 6A-6B . 
       FIGS. 6A-6B  are cross section views showing a process for forming P-pillars using a two-step pillar process. In  FIG. 6A , a first N-epi layer  604  is grown over a highly doped substrate  602  using known techniques. A bottom portion  606 A of a deep trench  606  is formed in first epi layer  604 , and then filled with P-type silicon material  608 A using conventional methods. A planarization process (e.g., chemical mechanical polish) may be carried out to planarize the top surface of the silicon before the next process step of forming the second N-epi layer  609  is carried out. Second epi layer  609  is formed over first epitaxial layer  604  using conventional techniques. A top portion  606 B of deep trench  606  is then formed in second epi layer  609 , and then filled with P-type silicon material  608 B using conventional methods. Back-side alignment techniques may be used to ensure that top and bottom trenches  606 A,  606 B are properly aligned. 
     Next, A planarization process may be carried out to planarize the top surface of the silicon before the subsequent steps. A post bake (e.g., at temperature of 1200 degree C. and 60 minutes) may be carried out to anneal out the defects in the two epi layers. Further processing may be carried out next to form the surface structures. For example, in the case of a MOSFET, as shown in  FIG. 6B , P-type body region  610 , N+ source regions  614 , P+ heavy body regions  612  as well as the gate structure and its overlying layers may be formed using known techniques. The two-step pillar process depicted by  FIGS. 6A-6B  may be expanded to include additional epi, trench etch and trench fill steps, for example, for higher voltage devices in which the trenched pillars extend even deeper. 
     This technique provides a number of advantages. By forming and filling the deep trench in multiple steps, filling the deep trench is made easier. Also, this technique can easily be scaled to accommodate higher voltage devices. That is, depending on the target breakdown voltage and the corresponding trench depth, the number of epi layers and corresponding trench etch and fill steps can be increased. Additionally, this technique provides significant flexibility by allowing independent selection of thicknesses and doping profiles for the two N-epi layers, as well as independent selection of doping profiles for the two trench fills. This flexibility enables more precise control of the electric field characteristics along the depth of deep trenches  600 . For example, the degree of field concentration, as well as the electric field profile (e.g., flat or double peak) and the position of the peak electric field, can be controlled by adjusting the thicknesses of the epi layers and the doping profiles of the epi layers and the trench fill material. 
     For example, the simulation results in  FIGS. 7A-7D  illustrate the impact of the doping concentration of the trench fills on the degree of electric field concentration along the trench depth (in these simulations, the thicknesses and doping concentration of the two N-epi layers are not changed). In these figures, t 1  and t 2  indicate the thicknesses of the bottom epi  604  ( FIG. 6A ) and top epi  609  ( FIG. 6 ), respectively; N 1  and N 2  indicate the doping concentrations of the bottom epi  604  and top epi  609 , respectively; and P 1  and P 2  indicate the doping concentration of the P-type silicon material  608 A in the bottom portion  606 A of deep trench  606  and the P-type silicon material  608 B in the top portion  606 B of deep trench  606 , respectively. As another example, the simulation results in  FIGS. 8A-8D  illustrate the impact of the doping concentration of the trench fills on the location of the peak electric field (in these simulations, similar to those in  FIGS. 7A-7D , the thicknesses and doping concentration of the two N-epi layers are not changed). The simulation results in  FIGS. 9A-9B  illustrate yet another example where the various parameters and physical dimensions are manipulated to create a double peak in the electric field, thus increasing the break down voltage. The simulation results in  FIGS. 10A-10B  illustrate still another example where an even higher breakdown voltage can be obtained by using the appropriate thicknesses for the epi layers and doping concentrations for the trench fill material (it is noted that this particular simulation result shows that, as compared to standard 900V MOSFETs, the parameters and dimensions identified in the table in  FIG. 10B  yield a substantially lower Rds-on—a factor of 10—for the same break down voltage). Thus, the location of peak impact ionization and the avalanche point along the depth of the pillar can be controlled so as to be below the body junction. 
     As discussed above, it is advantageous to induce the onset of avalanche breakdown along the bottom half of the pillars away from the body-drift junction.  FIG. 11A  shows a cross section view in accordance with an exemplary embodiment of the invention where N-enrichment regions  1105  may be formed at the bottom of N-filled trenches  1107  to create a local charge imbalance thereby inducing the onset of avalanche breakdown at the pillar bottoms. This characteristic of the  FIG. 11A  structure can be seen from the electric field curve shown in  FIG. 11B . The electric field curve shows the electric field profile along the depth of the cross section view in  FIG. 11A . As can be seen, N-enrichment regions  1105  cause the electric field peak to occur near the bottom of the pillars. N-enrichment regions  1105  preferably have a higher doping concentration than N-pillars  1108  to create the charge imbalance. Since the electric field profile and thus the breakdown voltage is a function of the total N-type and P-type charge in both pillars, the N-pillars can be more heavily doped and made narrower thus reducing the pitch. This benefit is realized because the impact of the lateral diffused compensation from the P-pillar doping into the N-pillar is minimized. 
     The structure in  FIG. 11A  may be formed as follows. An N-epi layer  1104  may be grown over N+ substrate  1102  using conventional techniques. An N-enrichment implant may be carried out to form N-enrichment regions  1105  where bottoms of N-pillars  1108  will terminate. A conventional masking and implant process may be used to form N-enrichment regions  1105 . The implant doping concentration and energy may be set in accordance with the target charge imbalance condition at the pillar bottoms. An alternate method for forming N enrichment regions  1105  is to grow an N-type epi layer, etch the pillar trenches and then implant the bottom of the trenches with N enrichment dopants. 
     One or more P-epi layers  1106  may be grown over N-epi layer  1104  using conventional techniques. The one or more P-epi layers  1106  may be formed with a uniform, stepped, or graded doping concentration. Trenches  1107  may be patterned and etched deep enough to reach N-enrichment regions  1105 . Backside alignment techniques may be used to ensure alignment of trenches  1107  with N-enrichment regions  1105 . Trenches  1107  may be filled with N-epi using known techniques. The N-epi used to fill the trenches need not be fully planarized since any portions of N-epi remaining over the P-epi  1106  can be used in forming the top structure of the power device. Alternatively, the N-epi used to fill the trenches may be planarized using, for example, a conventional chemical mechanical polishing (CMP) process. P-body regions  1110 , N+ source regions  1114 , P+ heavy body regions  1112  as well as gate structure  1116  and its overlying layers (not shown) may be formed using known techniques. In one implementation, P-body regions  1110 , source regions  1114  and heavy body regions  1112  are all formed after gate structure  1116  is formed. 
     In another embodiment shown in  FIG. 12A , one or more P-epi layers  1206   a  may be grown over N-epi layer  1204  using convention techniques. The one or more P-epi layers  1206   a  may have a uniform, stepped, or graded doping concentration. Lower trenches  1207  may be patterned and etched deep enough to reach N-enrichment regions  1205 . Lower trenches  1207  may be filled with N-epi  1208   a  using know techniques. One or more N-epi layers  1208   b  is then grown over the one or more P-epi layers  1206   a  and the N-epi filled lower trenches  1207 , using conventional techniques. The one or more N-epi layers  1208   b  may have a uniform, stepped, or graded doping concentration. Upper trenches  1209  may be patterned and etched deep enough to reach the one or more P-epi layers  1206   a . Upper trenches  1209  may be filled with P-epi  1206   b  using know techniques. The upper trenches may be aligned to the lower trenches using backside alignment techniques or can be run perpendicular to the lower trenches. 
     As can be seen, N-pillars  1208  have a reverse taper along their upper half. P-epi  1206   b  used to fill upper trenches  1209  may be planarized using, for example, a conventional CMP process. P-body regions  1210 , N+ source regions  1214 , P+ heavy body regions  1212  as well as gate structure  1216  and its overlying layers (not shown) may be formed using known techniques. In one implementation, P-body regions  1210 , source regions  1214  and heavy body regions  1212  are all formed after gate structure  1216  is formed. An advantage of this structure is that the wider P-pillar at the surface pushes the avalanche point down so that avalanche occurs below the surface P-body region and the increased electric field at the bottom due the N-enrichment causes the peak electric field to occur at the bottom. This characteristic of the  FIG. 12A  structure can be seen from the electric field curve shown in  FIG. 12B . The electric field curve shows the electric field profile along the depth of the cross section view in  FIG. 12A . The Wider N-Pillar with the N-enrichment at the bottom increases the peak current at which pinch-off occurs due to JFET effects and lowers Rds-on. 
     The N-enrichment regions  1105  in  FIG. 11A and 1205  in  FIG. 12A  advantageously induce avalanche breakdown at the bottom of the pillars and thus result in a device with improved UIS capability. The N-enrichments may be used in other advantageous ways as well. 
     As discussed above, device ruggedness can be improved in trench epi fill charge balance devices by initiating breakdown in the active area and having the breakdown voltage be substantially lower than other areas, such as termination regions, gate runner areas, and other areas that are likely to be a potential source of charge imbalance. In one embodiment, N-enrichment regions may be formed at the bottom of N-pillars in the active region only. In another embodiment, N-enrichment regions may be formed at the bottom of every other active N-pillar. In other embodiments, N-enrichment regions may be wider than or narrower than the active N-pillars or distributed along length of a N-pillar in an uniform or non-uniform pattern. In yet another variation, the distribution of N-enrichment may not need to be the same for every P-pillar. In yet another embodiment, the N-enrichment regions may be formed in a blanket manner (i.e., adjacent N-enrichment regions merge together) in the active region. Alternatively, the N-enrichment regions may be formed in a blanket manner across both the active and termination regions. 
     In accordance with other variations of the  FIGS. 11A and 12A  technique, regions of P-enrichment (not shown) are formed at the bottom of the P-pillars or at the bottom of the N-pillars to disrupt charge balance and thereby create a location of lower breakdown voltage so that avalanche initiates at this localized area. The P-enrichment regions may be implemented in both the active and termination regions, thus ensuring that breakdown occurs near the bottom of the pillars and is far from the silicon surface. Alternatively, the P-enrichment regions may be implemented in the active region only, so that the charge balance is disrupted in the active region to ensure breakdown in the active region. The P-enrichment regions may be wider or narrower than the P-pillars or distributed along a length of a P-pillar in a uniform or non-uniform pattern. In another variation, the P-enrichment regions may be formed at the bottom of every other P-pillar in the active region. In yet another variation, the distribution of P-enrichment may not need to be the same for every P-pillar. 
     The technique depicted in  FIGS. 11A ,  12 A and its various embodiments described above may be implemented using any of the three layout configurations shown in  FIGS. 1A-1C . 
     In the super junction approach where P-pillars are formed using deep trenches, it is difficult to prevent formation of voids in the epi material that is typically used to fill the deep trenches. Such voids are undesirable because they can lead to reliability and leakage problems.  FIGS. 13A-13B  are cross section views showing a three step trench fill process used in forming a super-junction device that is free of voids. 
     In  FIG. 13A , one or more N-type epitaxial layers, marked in the figure as N epi  1302 , is (are) formed over a suitable substrate (not shown). Trenches  1304  are etched in N epi  1302  using known techniques. P-type epitaxial layer  1306  lining the trench sidewalls and bottom is formed using conventional methods. Next, dielectric layer  1308  (e.g., grown or deposited oxide) is formed over P-epi layer  1306  in trenches  1304  using known methods. Finally, the central remaining portion of trenches  1304  is filled with a conformal material such as amorphous silicon or polysilicon that is doped or undoped using known methods. This technique eliminates formation of voids since epi is used to only partially fill the trench, and a conformal material, such as polysilicon, is used to fill the difficult-to-fill central portion of deep trenches. The void-free filling of deep trenches is achieved while preserving the ability to carefully set the P-charge for charge balance purposes via P-liner  1306 . That is, using conformal material  1310  to fill the central portion of the trenches eliminates formation of voids while P epi liner  1306  enables accurate control of the P-charge. Dielectric layer  1308  in turn, advantageously, serves as a cap layer preventing out-diffusion of P-dopants from P-epi liner  1306  to poly fill  1310 . Thus charge balance becomes independent of trench width. 
     In  FIG. 13B , the surface structure of the device, in this case a planar gate MOSFET, is formed. Incorporating the three step trench fill process in other MOSFET structures (e.g., trench gate variation) or other types of super junction devices (e.g., planar gate or trench gate IGBTs) would be obvious in view of this disclosure. In  FIG. 13B , P-type body regions  1312 , N+ source regions  1314  and P+ heavy body regions  1316  are formed in N epi  1302  using conventional techniques. The gate structure, including gate dielectric  1318  and gate electrodes  1320  are formed using known processes. Source metal  1322 , contacting source regions  1314 , heavy body regions  1316  and poly fill  1310 , is formed using known techniques. A back-side drain metal (not shown) contacting the substrate (not shown) is formed in accordance with conventional methods. 
     As stated earlier, it is difficult to fill deep trenches with epi without formation of voids. Voids can lead to reliability and leakage issues and are thus undesirable.  FIG. 14  shows a simplified cross section view of a superjunction power device which minimizes formation of voids in the trench fill process. Trenches  1408  (one of which is shown in  FIG. 14 ) are formed in one or more layers of N-epi  1402 . Those portions of the one or more N-epi layers  1402  that separate adjacent trenches  1408  from one another, form N-pillars. A thick P-epi liner  1404  is grown in trenches  1408 . P-epi liner  1404  sets the P charge for the P-pillars. Most of trench  1408  is filled by P-epi liner  1404 , and its thickness is selected so that the top of trenches  1408  remain open after the epi growth process. A dielectric material  1406  is then used to fill the rest of the trench. In one embodiment, dielectric material  1406  comprises high quality thermal oxide. The thermal oxide fill helps improve reliability. Also, the P charge can be easily controlled by P-epi liner  1404 . The remaining structural details of the power device (not shown), and the manner of forming them, may be similar to those shown in  FIG. 13B . 
     Another embodiment directed to eliminating formation of voids in deep trenches is shown in  FIG. 15 . This embodiment is similar to the  FIG. 14  embodiment except that the fill process includes forming an additional epi layer in the trenches. In  FIG. 15 , trenches  1508  (one of which is shown in  FIG. 15 ) are formed in one or more layers of N-epi  1502 . Those portions of the one or more N-epi layers  1502  that separate adjacent trenches  1508  from one another, form N-pillars. A first P-epi liner  1510  lining the trench sidewalls and bottom is formed in trenches  1508 . Trenches  1508  are then mostly filled by another lighter doped P-epi layer  1504 . The thicknesses of P-epi layers  1510  and  1504  are selected so that the top of trenches  1408  remain open after the epi fill process. A dielectric material  1506  is then used to fill the rest of the trench. In one embodiment, dielectric material  1506  comprises high quality thermal oxide. The remaining structural details of the power device (not shown), and the manner of forming them, may be similar to those shown in  FIG. 13B . 
       FIG. 16A  shows a cross section view of yet another super junction device with pillars of alternating conductivity type. In this embodiment, the P-pillars include 2 differently doped P epi layers: an outer P− epi layer  1610 A, and a center P epi layer  1610 C. These two epi layers may be formed by successively filling the trench with P epi layers of increasing doping concentration. An advantage of such P-pillar structure is that the width of the N-pillars is kept intact since the outer P epi layer  1610 A is lightly doped and thus the lateral diffusion of P-type dopant into adjacent N-pillars has minimal or no impact on the increase in the N-pillar resistivity. Keeping the width and resistivity of the N-pillars intact improves Rds-on. 
     The  FIG. 16A  structure further includes a retro-graded N-pillar. As can be seen, the N-pillar includes, from bottom to top, N+ region  1618 , N epi layer  1606  and N− epi layer  1608 . Such retro-graded N-pillar advantageously reduces the peak electric at the P-N junction formed by the P-body and N-drift regions, and lowers the Rds-on. The N+ region  1618  of the N-pillar can be formed as follows. Prior to forming N− epi layer  1604  over substrate  1602 , a heavy dose of arsenic is implanted into region  1604  of substrate  1602  using a mask. During subsequent heat cycles, the arsenic in heavily implanted region  1604  up-diffuses into N− epi layer  1604 , thus forming N+ region  1618 . 
     The remaining portions of the  FIG. 16A  structure may be formed as follows. N epi layer  1606  and N− epi layer  1608  are formed over N− epi layer  1604  using conventional techniques. Deep trenches extending into N− epi layer  1608  and N epi layer  1606  are formed using known methods. The deep trenches terminate in or prior to reaching N− epi layer  1604 . The deep trenches are then filled with P epi layers  1610 A and  1610 C using conventional epi processes. The remaining surface structures of the super junction device, in this case a trench gate MOSFET, are formed using known techniques. The gate trenches housing gate electrodes  1612  are formed in N− epi  1608 , followed by forming a thick bottom oxide along the trench bottom. Gate dielectric  1616  is formed along the trench sidewalls followed by forming recessed gate electrodes  1612  in the gate trenches. Source regions  1614  are formed adjacent the gate trenches, and a source metal contacting source regions  1614  and P epi layers  1610 A, and  1610 C is formed along the top surface of the structure. A drain metal (not shown) contacting substrate  1602  is formed along the backside. 
       FIG. 16B  shows a cross section view of another super-junction device with pillars of alternating conductivity type. In this embodiment, the P-pillars include 2 differently doped epi layers: an outer P+ epi layer  1611 A and a center P− epi layer  1611 C. An advantage of such P-pillar structure is that the P-pillar charge is set by the outer P+ epi layer  1611 A which lines the trench. Thus, the effect of the charge variation of the P-pillar at the top and the bottom remains consistent and is independent of the trench angle which makes the trench wider at the top. Keeping the P-pillar charge the same from top to bottom of the trench reduces the variation in charge imbalance resulting from the trench angle, which in turn results in less variation of breakdown voltage. 
       FIG. 16C  shows a cross section view of yet another super junction device with pillars of alternating conductivity type. In this embodiment, the P-pillars include 3 differently doped P epi layers: outer P− epi layer  1613 A, center P− epi layer  1613 C and middle P epi layer  1613 B sandwiched by outer epi layer  1613 A and center epi layer  1613 C. These three epi layers may be formed by successively filling the trench with P epi layers of different doping concentrations. Such P-pillar structure provides a compromise between  FIGS. 16A and 16B . This structure offers the advantage of the  FIG. 16A  structure, namely, the lateral diffusion of P-type dopants into the N-pillars is substantially reduced thus minimizing or eliminating the impact on the N-pillar resistivity. The  FIG. 16C  structure also offers the advantage of the  FIG. 16B  structure in that the effect of the charge variation of the P-pillar at the top and the bottom remains consistent and is independent of the trench angle because the P-pillar charge is set by the middle P+ epi layer  1613 B. Thus, the  FIG. 16C  structure has the advantages of reduced breakdown voltage variation and reduced Rds-on. 
       FIG. 17  shows a cross section view of yet another super junction device with pillars of alternating conductivity type. In this embodiment, the P-pillars include 3 differently doped P epi layers: outer P− epi layer  1710 A, center P+ epi layer  1710 C and middle P epi layer  1710 B sandwiched by outer epi layer  1710 A and center epi layer  1710 C. These three epi layers may be formed by successively filling the trench with P epi layers of increasing doping concentration. An advantage of such P-pillar structure is that the width of the N-pillars is kept intact since the outermost P epi layer  1710 A is lightly doped and thus out-diffuses minimally. Keeping the width of the N-pillars intact improves Rds-on. 
     In another embodiment shown in  FIG. 18 , P-enrichment region  1822  is formed at the bottom of each P-pillar using similar techniques to those described above for forming N-enrichment regions  1105  in  FIG. 11 . Such P-enrichment regions advantageously compensate up-diffusion of dopants from N− epi layer  1604 . Many of the same variations discussed above in connection with N-enrichment regions  1105  in  FIG. 11  may be applied to the  FIG. 18  embodiment. Also, enrichment region  1822  can be formed along the bottom of the P pillars in the embodiments shown in  FIGS. 16A-16C  and  17 . 
     Although epi layer  1610 A in  FIG. 16A , epi layer  1611 C in  FIG. 16B , epi layers  1613 A,  1613 C in  FIG. 16C , epi layer  1710 A in  FIG. 17 , and epi layers  1810 A and  1810 C in  FIG. 18  are all shown as P− epi, they could be lightly doped N− or intrinsic epi silicon. Further, center layer  1611 C in  FIG. 16B , center layer  1613 C in  FIG. 16C  and center layer  1810 C in  FIG. 18  embodiments need not be from epi, and can instead be from another material such as polysilicon. 
       FIGS. 19A-19L  are cross section views showing various stages of a process for forming a super-junction trench-gate MOSFET. These figures show a portion of the die where active region  1903  transitions into termination region  1905 . In  FIG. 19A , N-epi  1902  is formed over a suitable substrate (not shown) using conventional techniques. N-epi  1902  may comprise multiple epi layers with different doping concentrations, or may be a single epi layer with graded or uniform doping concentration. Using masking layer  1904  (e.g., comprising photoresist), a boron implant process  1906  is then carried out to form P-Iso region  1908  in N-epi layer  1902 . P-iso region  1908  extends in the transition and termination regions, and is incorporated for reasons stated above in connection with  FIG. 3 , and is formed in a similar manner to that in  FIG. 3 . 
     In  FIG. 19B , P-iso drive and trench hard mask oxidation are carried out in one process step. Hard mask  1910  (comprising oxide) is then formed over the top surface. Trenches  1912  are defined using photoresist  1911 . In  FIG. 19C , trenches  1912  are formed in N-epi layer  1902  using known techniques. Gate dielectric  1914  is then formed along trench sidewalls and bottom using conventional techniques (e.g., gate oxidation). Prior to forming gate dielectric  1914 , a thick bottom dielectric may optionally be formed along trench bottoms to reduce the gate to drain capacitance. In  FIG. 19D , conventional polysilicon deposition and etch back steps are carried out to form recessed gate electrodes  1916  in trenches  1914 . 
     In  FIG. 19E , P-type body regions  1918  are formed in N-epi  1902  using conventional implant and drive processes. In  FIG. 19F , source regions  1920  are formed in the active region adjacent active trenches using conventional implant and drive processes. In  FIG. 19G , an oxide-nitride-oxide (ONO) composite layer  1922  is formed using known techniques. In one embodiment, the ONO comprises from bottom to top: pad oxide  1922 A, nitride  1922 B, and thick LTO  1922 C. Pad oxide  1922 A serves as a nitride etch stop in a later process step. In  FIG. 19H , deep trenches  1924  are defined and etched into N-epi  1902  using techniques described in this disclosure, or using known techniques. 
     In  FIG. 19I , known techniques are used to fill trenches  1924  with P-type silicon  1926 . As can be seen, vertically extending portions  1902 A of N-epi  1902  between adjacent P-filled pillars  1926 , form N-pillars. N-pillars  1902 A and P-pillars  1926  thus form the super-junction structure, namely, the alternating pillars of opposite conductivity type. In  FIG. 19J , the top oxide layer in ONO composite layer  1922  is removed, defining edge LTO  1922 A, and BPSG  1930  is formed over the surface of the device using conventional processes and then annealed. In  FIG. 19K , BPSG  1930  is defined and etched using masking layer  1932  (e.g., comprising photoresist) to form contact openings in BPSG  1930 . The remaining portions of BPSG  1930  cover the gate electrodes and extend over source regions  1920 . A conventional heavy body implant is carried out to form P+ heavy body regions  1934  through the contact openings. Alternatively, dimples may be etched in body regions  1918 , and P-type dopants may then be implanted along the bottom of the dimples to form P+ heavy body regions in body regions  1918 . The dimples also expose sidewalls of the source regions to which contact can be made by the later formed source metal. In  FIG. 19L , a conventional BPSG reflow is carried out to round the corners of the dielectric, followed by formation of the various metal layers (e.g., source metal layer  1936  and gate metal layer  19368 ). A backside metal (e.g., drain metal, not shown) contacting the substrate on the backside of the die is formed using known techniques. 
     In termination region  1905 , the P-Iso region  1908 , which is electrically connected to source metal  1936 , connects a number of the P-pillars together and thus biases these P-pillars along their tops to the source potential. All termination P-pillars located to the right of P-Iso region  1908  float, and are marked in  FIG. 19L  as “floating P-pillars  1940 .” 
     As can be seen, P-pillars  1926  are formed relatively late in the process as compared to conventional trenched pillar processes where the P-pillars are formed early in the process. Because the P-pillars are formed late in the process after most of the thermal budget has been completed, the out-diffusion of the P-pillar dopants is advantageously minimized. This enables use of tighter pitch for P-pillars and results in lower Rds-on without compromising breakdown voltage. 
     The process embodiment depicted by  FIGS. 19A-19L  is directed to a trench gate MOSFET however, this process can be modified to implement a planar gate MOSFET with the same advantages, as described next. This process will be described with reference to  FIG. 20 , which is a cross section view of a planar gate MOSFET that is formed using the process that is described next. One or more N-type epitaxial layers  2027 A,  2027 B are formed over a suitable substrate  2024  using know techniques. P-type enrichment regions  2021  may optionally be formed in epi layer  2027 A prior to forming epi layer  2027 B. As described in connection with other embodiments, P-enrichment regions  2021  advantageously create more of a charge imbalance at the bottom of the P-pillars, thereby inducing the onset of avalanche breakdown at the bottom of the P-pillars and away from the body-drift junction. 
     The gate dielectric and the overlying planar gate electrodes  2014  are defined and formed over N-epi  2027 B using known techniques. P-type dopants are then implanted with planar gate electrodes  2014  serving as a blocking layer, followed by a drive step, thus forming P-type body regions  2038  in N-epi  2027 B. After the drive step, the P-type body regions laterally extend under the gate electrodes. Using known techniques, source regions  2018  are then defined and formed in body regions  2038  adjacent each edge of gate electrodes  2014 . A conformal layer, e.g., a nitride layer, (not shown) extending over the gate electrodes and stepping down between adjacent gate electrodes over the body and source regions is formed using known techniques. A thick layer of LTO (not shown) is then formed over the nitride layer. The LTO is then defined and etched to expose silicon surfaces between adjacent gate electrodes where deep trenches  2003  are to be formed. The LTO would cover the source regions  2018 . A conventional silicon etch is then carried out to form deep trenches  2003 . 
     Next, the trenches are filled with P-type silicon using conventional techniques. The LTO is removed using the conformal (e.g., nitride) layer as an etch stop. The conformal layer may then be removed or may be left intact and used as a self alignment spacer to space off a high energy P+ implant from the poly gate edges. Dielectric cap  2051  (e.g., comprising BPSG) covering gate electrodes  2014  is formed using conventional techniques. Dielectric cap  2051  form contact openings between adjacent gate electrodes. A heavy body implant is carried out through the contact openings to form the P+ heavy body regions in body regions  2038  between adjacent source regions  2018 . The remaining process steps would be similar to those shown in  FIG. 18L  and thus will not be described. As can be seen, in this planar gate process, as in the trench gate process of  FIGS. 18A-18L , the P-pillars are formed late in the process after much of the thermal budget is used, thus minimizing out-diffusion of P-pillar dopants. 
     In the process technology where pillars are formed by etching trenches and filling them with silicon (rather than using the multi-epi process), varying the mesa width is undesirable as it results in non-uniform trench etch and filling. Therefore, center-to-center pillar spacing needs to be maintained constant to the extent possible. However, with a constant pillar spacing other provisions need to be made to obtain the desired surface electric field profile.  FIGS. 21 and 22  are cross section views showing exemplary embodiments where the desired surface electric profile is obtained using surface N regions and/or surface P regions. 
       FIG. 21  shows a cross section view along a portion of the die where the active region transitions into the termination region. The cross section view in  FIG. 21  is similar to that shown in  FIG. 19L  except N− surface regions  2142  are formed along the top of those N-pillars  2102 A located between floating P-pillars  2140  in termination region  2105 . N− surface regions  2142  advantageously serve to spread the surface electric field thus improving the break down voltage in the termination region. All other structural features in the  FIG. 21  cross section view are similar to those in  FIG. 19L  and thus will not be described. 
     The process steps for forming N− surface regions  2142  can be incorporated in the process sequence depicted by  FIGS. 19A-19L  as follows. In  FIG. 19A , prior to forming any of the regions along the top surface, N− surface region  2142  may be formed by growing a lightly doped N− epitaxial layer over N-epi  1902 . The N− epitaxial layer  2142  would have a lower doping concentration than N-epi  1902 . Alternatively, N− surface region  2142  may be formed by performing a blanket compensation P-implant into N-epi  1902  so that a top layer of N-epi  1902  is converted to a lighter doped N− layer. The process sequence as depicted in  FIGS. 19A-19L  would then be carried out to form the MOSFET. 
     In  FIG. 21 , the top regions of floating P-pillars  2140  in termination region  2105  together with their adjacent N− surface regions  2142  make an already P-rich condition along the pillar tops even more P-rich, thus potentially lowering the break down voltage in termination region  2105 . The surface regions of floating P-pillars  2140  can be compensated to improve the charge balance along the pillar tops. In one embodiment, after forming the P-pillars as depicted by the process steps corresponding to  FIG. 19I , and before the process steps corresponding to  FIG. 19J  (i.e., while the top surfaces of all P-pillars are exposed), a blanket compensation N implant is carried out to compensate the top regions of all P-pillars in the active and termination regions. Thus, the blanket compensation N implant into the top regions of all P-pillars ensures that the termination region has a higher break down voltage than the active region. 
     While  FIG. 21  shows implementation of the N− surface regions in a trench gate MOSFET, the N− surface regions may also be incorporated in the planar gate variation. In the planar gate embodiment however, a JFET implant carried out only in the active region may be necessary to increase the doping concentration in the JFET regions which would otherwise be made up of the lightly doped N− surface region. The JFET implant enhances the transistor Rds-on which would otherwise be adversely impacted by the N− surface region. In the process described above in connection with the planar gate embodiment shown in  FIG. 20 , the JFET implant may be incorporated after forming the N− surface region but before forming the gate structure. 
     In the  FIG. 22  embodiment, surface P-enrichment regions  2244  are formed along top surfaces of the P-pillars in termination region  2205 . The active P-body regions  2218  may extend deeper than P-enrichment regions  2244 , and may have a higher doping concentration than P-enrichment regions  2205 . The doping and depth of P-enrichment regions  2244  as well as the doping and depth of N− surface regions  2242  may be designed to obtain a charge balance state resulting in a high breakdown voltage with a low peak electric field and substantially evenly distributed electric field across the termination region. In one embodiment, it is desirable to have the charge of P-enrichment regions  2244  higher than the charge of the N surface regions to minimize the peak surface electric field. 
     In any of the above embodiments, the P-pillars in both the active and termination regions may be equally spaced from one another so that all N-pillars have the same width. 
     Conductive field plates are used to spread the electric field more uniformly in the termination region. It is desirable to electrically connect the field plates to the underlying pillars so that they can assume the potential of their corresponding pillar.  FIG. 23  shows a cross section view of a super junction power device where field plates  2330  are electrically connected to their corresponding floating P-pillars  2304  through surface well regions (also referred to as “P rings”) formed along tops of P-pillars  2304 . These electrical connections are made by forming contacts between field plates  2330  and corresponding well regions through one or more dielectric layers  2332 . However, as cell pitch is reduced, forming a contact between the field plates and their underlying pillars becomes more difficult.  FIG. 24A  is a top layout view of a corner region of a super-junction power device showing a technique for contacting field plates to the underlying pillars for small cell pitches.  FIG. 24B  is an expanded view of the corner the termination region of the top layout view in  FIG. 24A . 
     In  FIG. 24A , active region  2404  and termination region  2402  are delineated. The pillar configuration is similar to the parallel-concentric configuration shown in  FIG. 1C . That is, the P-pillars in active region  2404  extend parallel to each other, while the P-pillars in termination region  2402  extend in a concentric fashion around the active region. As shown, contact structures  2406  for electrically connecting the field plates to the underlying floating P-pillars are advantageously located directly over the P-pillars where the P-pillars make a 90 degrees turn.  FIG. 24B  will be used to show and describe contact structures  2406  in more detail. 
     In  FIG. 24B , P-pillars  2412  are spaced equally from one another in the active and termination regions. P-rings  2408  are formed along the tops of corresponding P-pillars, and extend in a concentric fashion around the active region similar to the floating P-pillars in which they are formed. In one embodiment, the width of P-rings  2408  gradually decreases in the direction away from the active region. Field plates  2410  (e.g., comprising polysilicon) extend over but are off-set relative to corresponding P-pillars and P-rings so that each of them extends partially over a corresponding P-pillar and partially over an adjacent mesa region (or N-pillar). Field plates  2410  surround the active region in a concentric fashion similar to P-rings  2408 . While the P-rings are shown in  FIGS. 23 ,  24 A and  24 B to be centered about the corresponding P-pillars, the P-rings may be off-set to the right or the left relative to the corresponding P-pillar. The offset P-rings can be advantageously used to merge two adjacent P-pillars along the surface. 
     Contacts  2406  are formed directly above corresponding P-pillars  2412  in the corner regions where the P-pillars make a 90 degree turn. The corner regions provide additional space in which contacts  2406  can be formed. A conductive or semiconductive material  2414  (e.g., a metal) is used to make the connection between each P-pillar  2412  and the corresponding polysilicon field plate  2410  through contacts  2406 . 
     In one embodiment, two or more P-rings  2408  physically touch thereby merging the corresponding P-pillars at the surface. This advantageously provides a larger surface area for forming the contacts. This technique is particularly useful in designs with tighter cell pitches where forming one contact per P-pillar can be difficult. In another embodiment, the termination N-pillars located between the floating P-pillars include a lightly doped N− region along their tops similar to those shown in  FIGS. 21 and 22 . In yet another embodiment, highly doped P+ regions are formed in the P-rings to ensure a more robust contact between the field plates and the corresponding P-rings. The P+ regions may be formed at the same time the P+ heavy body regions are formed in the active area of device. 
     The technique described in connection with  FIGS. 23 and 24A ,  24 B may be used in the design of edge termination for any superjunction or non-superjunction power semiconductor device (e.g., MOSFET, IGBT, diode). 
       FIGS. 25A-25D  are cross section views showing various embodiments of super-junction high voltage merged P-N Schottky rectifiers. These figures show a portion of the die where the active region transitions to the termination region. In  FIG. 25A , active region  2501  includes alternating P-N pillars  2530 ,  2532 . P-pillars  2530  are trench-filled pillars, similar to various other embodiments described herein. P-wells  2538  extend along a top side of corresponding P-pillars  2530 . P-wells  2538  are wider than P-pillars  2530  and extend to a predetermined depth from the top silicon surface. A Schottky barrier metal  2571  together with an overlying metal layer  2572  (e.g., comprising aluminum) extend along the top surface of the top epitaxial layer. Schottky barrier metal  2571  and metal layer  2572  together form the anode terminal of the Schottky rectifier. As can be seen, the anode terminal contacts both the top surface of N-pillars  2532  and the top surface of P-wells  2538 . Where the anode terminal makes contact with N-pillars  2532 , Schottky contacts are formed. 
     During operation, the alternating P-N pillar structure keeps the high electric field away from the Schottky surface area, thus reducing the reverse leakage. Additionally, the P-N pillars can support high voltages thus allowing use of low resistivity N pillars (where the current flows), thereby reducing the series resistance component of the diode forward voltage. Moreover, P-wells  2538  serve to pinch off the active N-pillars near the surface at a lower voltage, which helps to further reduce the reverse leakage current. Thus, a high voltage Schottky rectifier with low forward voltage and low reverse leakage is obtained. 
     In one embodiment, the active and transition P-N pillars may be stripe-shaped with termination P-N pillars surrounding the active and transition regions in a concentric fashion similar to the layout configuration shown in  FIG. 1C . In yet another embodiment, the active, transition, and termination P-N pillars may be concentric similar to the layout configuration shown in  FIG. 1A . In the latter layout configuration, the gate feed issues associated with MOSFETs is not present since there are no gate structures in the Schottky rectifier. 
       FIG. 25B  shows a variation of the  FIG. 25A  embodiment in which P+ contact regions  2506  are formed in P-wells  2538 . The anode terminal forms an ohmic contact with P-wells  2538  through P+ contact regions  2506 . This allows the diode to operate at high current density with lower conduction voltage due to the parallel conduction of the PN diode when the forward voltage exceeds the built in potential. 
       FIG. 25C  shows a variation of the  FIG. 25B  embodiment where P-wells  2538  are not included, but P+ contact regions  2506  are included so that the anode terminal makes ohmic contact with P-pillars  2530  along their tops through the P+ contact regions  2538 . This variant trades off lower forward voltage for increased leakage resulting from the higher electric field at the barrier. 
       FIG. 25D  shows a variation of the  FIG. 25A  embodiment where P-wells  2538 A have approximately the same width as the P-pillars in which they are formed. With the width of P-well regions reduced as compared to the  FIG. 25A  embodiment, the mesa width (N-pillar width) near the surface is increased thus lowering the forward voltage of the Schottky rectifier. 
     The MOSFET process described above can be modified in simple ways to form the Schottky rectifiers in  FIGS. 25A-25D . For example, the  FIG. 25A  embodiment may be obtained by eliminating the source implant from the MOSFET process. The P+ regions in  FIGS. 25B and 25C  correspond to the heavy body region of the MOSFET. Because of the compatibility between the processes for the MOSFET and the Schottky rectifier, MOSFET (e.g.,  FIG. 3 ) and Schottky rectifier (e.g.,  FIG. 25A ) can be easily integrated in a single die to thereby obtain a SynchFET. 
       FIGS. 26A and 26B  are cross section views showing two additional embodiments of super junction high voltage Schottky rectifiers. In  FIG. 26A , after trench-filled P-pillars  2630  are formed, an N-type epitaxial layer  2682  is formed over the P-N pillar structure, after which the anode terminal comprising Schottky barrier metal  2671  and an overlying metal layer  2672  (e.g., comprising aluminum) is formed over and directly contacts N-type epitaxial layer  2682 . In this embodiment, P-pillars  2630  are not connected to the anode terminal and thus float. This embodiment advantageously lowers the forward voltage of the Schottky rectifier by increasing the Schottky contact area, and reduces leakage current by using the floating pillars to deplete the active N-pillars at a low voltage. 
       FIG. 26B  shows a variation of the  FIG. 26A  embodiment where instead of N-epi layer  2682 , an N-implant is performed to thereby form N-implant region  2684  along the top surface of the P-N pillars. As shown, the anode terminal comprising Schottky barrier metal  2671  and an overlying metal layer  2672  (e.g., comprising aluminum) directly contacts N-implant region  2684 . 
     In  FIGS. 25A-25D  and  26 A- 26 B, while the P pillars are shown as being of the trench filled variety, the pillars can have a lower portion that is made up of multiple bubble-shaped P-regions, and an upper portion that is trench filled, similar to P-pillars  329 A,  329 B,  329 C shown in  FIG. 3 . Furthermore, the transition region  2504 ,  2604  and termination regions  2502 ,  2602  in  FIGS. 25A-25D  and  26 A- 26 B are similar to the corresponding regions in  FIG. 3 , and thus will not be described. The same advantages obtained in integrating these regions with the active region of the MOSFET in  FIG. 3  are realized with the Schottky structure of  FIGS. 25A-25D  and  26 A- 26 B. 
     A challenge in filling trenches having a high aspect ratio is avoiding formation of voids in the trench or preventing premature epi closure along the top of the trench due to localized growth near the top corners of the trench. In superjunction layout configurations where trench sidewalls have different plane directions, filling trenches with epi becomes even more difficult because the epi filling process is sensitive to silicon crystal plane direction. For example, the rate at which epitaxial silicon grows along the &lt;100&gt; plane direction is different than the rate at which epitaxial silicon grows along the &lt;110&gt; plane direction. This is more clearly illustrated in  FIGS. 27A and 27B .  FIG. 27A  shows a top view of a die layout diagram, and  FIG. 27B  shows a top view of a wafer where the wafer flat extends parallel to the laterally extending pillars in  FIG. 27A . 
       FIG. 27A  shows a full concentric octagon layout configuration. That is, P-pillars  2710  and N-pillars  2706  in both active region  2712  and termination region  2714  are concentric octagons. P-pillars  2710  are formed in trenches using techniques disclosed herein. While the sidewalls of vertically and horizontally extending trenches have the same plane direction, namely, &lt;110&gt;, the diagonally extending trenches have the &lt;100&gt; plane (this assumes that the die shown in  FIG. 27A  is positioned on the wafer so that the horizontally extending trenches run parallel to the wafer flat, as depicted in  FIGS. 27A and 27B ). This variation in plane direction will result in non-uniform filling of the trenches. 
     In accordance with one embodiment, the non-uniform epi filling due to variations in plane direction can be eliminated by rotating the wafer 22.5 degrees. This is illustrated in  FIG. 28 . As can be seen, by rotating the wafer and thus the dies on the wafer during photolithography processing, the dies can all be oriented so that all trench sidewalls line up along the &lt;(tan 22.5)10&gt; plane direction. In  FIG. 28 , similar to  FIG. 27A , P-pillars  2810  and N-pillars  2806  in both active region  2812  and termination region  2814  are concentric octagons, and P-pillars  2810  are formed in trenches using techniques disclosed herein. Alternatively, N-pillars  2806  may be formed in trenches using techniques disclosed herein. In one embodiment, the wafer is maintained in the 22.5 degree rotated position throughout the process sequence. The structural details of the power device (e.g., MOSFET, IGBT or rectifier) housed in the die shown in  FIG. 28  are not shown, but such details can be found in other embodiments disclosed herein. 
     In the  FIG. 1C  layout design where the striped active P-pillars are surrounded by termination P-pillars, the sharp corners of the concentric rectangular or square shaped termination pillars are difficult to etch and fill and may result in formation of voids in the epi fill. Such voids can cause charge imbalance resulting in localized breakdown voltage. These voids can also result in areas in the termination region having high electric field stress that become a source of electron injection into the oxide and a magnet for positive charges, resulting in reliability failures. 
       FIG. 29  is a top layout view of a corner of a super-junction power MOSFET with striped active P-pillars  2904  surrounded by concentric termination P-pillars  2904 . In this layout design the sharp corners of the concentric termination P-pillars in  FIG. 1C  are eliminated by forming octagon shaped termination P-pillars. By eliminating the sharp corners, the concentric termination trenches can be filled without formation of voids in the fill material. As can be seen, the corner edges of the octagon shaped termination P-pillars gradually increase in length in the direction away from the active region. Also, as compared to the fully concentric octagon design with equal length legs, the  FIG. 29  layout configuration provides better packing and thus a more efficient use of silicon. 
     In  FIG. 29 , gap region  2908  is formed between an end of active P-pillars  2904  and the first concentric P-pillar in termination region  2910 . Two full floating mesa regions (or N-pillars)  2914  are inserted to provide isolation between gap  2708  and corner areas and termination  2910 . These features reduce the sensitivity to breakdown voltage due to charge imbalance in this area. In one embodiment, the trench fill process described in connection with the  FIG. 28  embodiment is used to further eliminate the possibility of void formation due to non-uniform epi filling caused by differing plane directions along the termination trench sidewalls. That is, prior to filling the trenches, the wafer is rotated 22.5 degrees so that all trench sidewalls line up along the &lt;(tan 22.5)10&gt; plane direction. In one embodiment, the wafer is maintained in the 22.5 degree rotated position (relative to wafer flat) throughout the process sequence. The structural details of the power device (e.g., MOSFET, IGBT or rectifier) housed in the die shown in  FIG. 28  are not shown, but such details can be found in other embodiments disclosed herein 
       FIG. 30  is a top view of a fully concentric layout design where a supplementary metal gate runner  3002  is bussed through a center portion of the die to provide metal connection to all the concentric polysilicon gates  3008 . A cross section view along line A-A′ is also included in  FIG. 30 . Metal gate runner  3004  extends around the perimeter of the die and connects to a gate pad (not shown), and to supplementary metal gate runner  3002 . P-pillars  3010 , N-pillars  3006  and planar polysilicon gates  3008  all extend in a concentric fashion in active region  3012  and termination region  3014 . 
     Supplementary metal gate runner  3002  extends far enough inside the die to contact the inner most concentric polysilicon gate. A drawback of this gate runner design is that it does not provide a balanced gate propagation delay. As can be seen, the inner most concentric polysilicon gates travel a shorter distance before contacting supplementary metal gate runner  3002  than do the outer most concentric polysilicon gates. The inner concentric polysilicon gates thus exhibit a lower propagation delay than the outer concentric polysilicon gates. This imbalance in propagation delays makes it difficult to obtain good switching speeds without dynamic failure of the device. 
       FIG. 31A  shows a top layout view of a fully concentric design with a gate runner design that provides a more balanced gate propagation delay throughout the die. While  FIG. 31A  does not show the concentric P and N pillars and the concentric polysilicon gates, these regions of the die in  FIG. 31A  are similar to those in  FIG. 30 . Outer metal gate bus  3114  extends around the outer perimeter of the die and contacts gate pad  3116 . Four supplementary metal gate runners  3118 A,  3118 B,  3118 C and  3118 D extend from outer metal gate bus  3114  toward the center of the die, but terminate before reaching the die center. In this design, while a number of the outer concentric polysilicon gates directly tie into the four supplementary metal gate runners, a number of the inner-most concentric polysilicon gates are not directly connected to all of the supplementary metal gate runners. This can be seen more clearly in  FIG. 31B  which is an expanded view of an inner portion of the top view in  FIG. 31A . 
     In  FIG. 31B , the concentric poly gates  3108  as well as the end portions of supplementary metal gate runners  3118 A and  3118 B in the active region can be seen. Concentric polysilicon gates  3108  encircle a center portion of the die through which polysilicon feed  3120  extends. Polysilicon feed  3120  extends laterally to make electrical contact with supplementary metal gate runner  3118 A to the left of the die center, and to supplementary metal gate runner  3118 C (not shown in  FIG. 31B ) to the right of the die center. As can be seen, none of the supplementary metal gate runners extend all the way to the center of the die. Instead, the lengths to which these four supplementary metal gate runners extend are carefully selected in order to obtain a more balanced gate propagation delay among the concentric poly gates  3108 . As a result, a number of the inner-most concentric polysilicon gates do not directly tie into all of the supplementary metal gate runners. 
       FIG. 31C  is an expanded view of the upper right quadrant of the die shown in  FIG. 31A .  FIG. 31C  will be used to illustrate how the particular lengths selected for the supplementary metal gate runners for this embodiment result in a more balanced propagation delay through the concentric poly gates. Two measurements are shown. The inner measurement, marked by reference numeral  3122 , measures one half the length of a concentric gate poly that extends between two supplementary metal gate runner  3118 C to just below the bottom end of supplementary metal gate runner  3118 B (i.e., does not tie into gate runner  3118 B), ties into supplementary metal gate runner  3118 A (not shown in  FIG. 31C ), and circles back around to tie into metal gate runner  3118 C. That is, the measured concentric gate poly ties into gate runners  3118 A and  3118 C, but not  3118 B and  3118 D. On the other hand, the outer measurement, marked by reference numeral  3124 , measures a length of the outermost concentric gate poly connected between two supplementary gate runners as it circles around the die. The measurements illustrate that the portion of the measured inner poly gate that extends between supplementary gate runner  3118 C and  3118 A (i.e., the closest two gate runner to which it is directly connected) is substantially equal to the portion of the measured outer poly gate that extends between supplementary gate runners  3118 B and  3118 C (i.e., the closest two gate runners to which it is directly connected). Also, those concentric poly gates closer to the center of the die that do not contact any of the four supplementary metal gate runners have short enough length that the propagation delay through them would not be very different from the outer concentric poly gates that are tied to two or all four of the supplementary gate runners. 
     As can be seen, while this technique does not obtain perfectly equal propagations delays through all concentric poly gates, it significantly improves the imbalance in the gate poly propagation delay that is present in the  FIG. 30  gate runner design. As  FIG. 31A  illustrates, while supplementary metal gate runners  3118 A,  3118 B,  3118 C,  3118 D extend into the middle of the die, ample surface area remains for bonding to the source metal. The technique illustrated in  FIGS. 31A-31C  can be implemented in the planar gate variation or the trench gate variation. 
       FIG. 32  shows a top layout view of a fully concentric pillar design with stripe polysilicon gates. Metal gate runner  3204  extends around the perimeter of the die and connects to a gate pad (not shown). P-pillars  3210  and N-pillars  3206  extend in a concentric fashion in active region  3212  and in termination region  3214 . Polysilicon gates  3208  (which can be of the planar gate or trench gate varieties) are stripe shaped and extend through both active and termination regions, and contact metal gate runner  3204  along their opposite ends. 
     This configuration advantageously eliminates the need for the supplementary gate runner(s) that is needed in the concentric gate poly design, thus resulting in area savings. While some channel area is lost where gate poly stripes  3208  cross over P-pillars  3210 , the impact on Rds-on due to channel resistance increase is small in high voltage devices. 
     While the above provides a complete description of specific embodiments of the present invention, various modifications, alternatives and equivalents are possible. For example, while some embodiments of the invention are illustrated in the context of planar gate MOSFETs, the same techniques could easily be applied to other planar-gate structures such as planar gate IGBTs by merely reversing the polarity of the substrate from those shown in the figures. Similarly, some of the structures and process sequences are described in the context of N-channel FETs, however, modifying these structures and process sequences to form P-channel FETs would be obvious to one skilled in the art in view of this disclosure. Further, the various techniques disclosed herein are not limited to planar gate structures and may be implemented in trench gate MOSFETs, trench gate IGBTs (which have trench gates), shielded gate MOSFETs or IGBTs (which have trenched gates with underlying shield electrode(s)), and rectifiers (including Schottky rectifiers, TMBS rectifiers, etc.). 
     Additionally, while not specifically called out for each embodiment, the various embodiments including many of the termination designs and charge balance techniques may be implemented in any of the three layout configurations shown in  FIGS. 1A-1C . Similarly, many of the embodiments disclosed herein including many of the termination designs and charge balance techniques are not limited in implementation to the trench epi fill charge balance process technology, and may also be implemented in the multi-epi layer pillar process technology. For this and other reasons, therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.