Abstract:
Edge termination structures for semiconductor devices are provided including a plurality of spaced apart concentric floating guard rings in a semiconductor layer that at least partially surround a semiconductor junction. The spaced apart concentric floating guard rings have a highly doped portion and a lightly doped portion. Related methods of fabricating devices are also provided herein.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates microelectronic devices and more particularly to edge termination for silicon carbide devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    High voltage silicon carbide (SiC) Schottky diodes, which can handle voltages between, for example, about 600V and about 2.5 kV, are expected to compete with silicon PIN diodes fabricated with similar voltage ratings. Such diodes may handle as much as about 100 amps or more of current, depending on their active area. High voltage Schottky diodes have a number of important applications, particularly in the field of power conditioning, distribution and control. 
         [0003]    An important characteristic of a SiC Schottky diode in such applications is its switching speed. Silicon-based PIN devices typically exhibit relatively poor switching speeds. A silicon PIN diode may have a maximum switching speed of approximately 20 kHz, depending on its voltage rating. In contrast, silicon carbide-based devices are theoretically capable of much higher switching speeds, for example, in excess of about 100 times better than silicon. In addition, silicon carbide devices may be capable of handling a higher current density than silicon devices. 
         [0004]    A conventional SiC Schottky diode structure has an n-type SiC substrate on which an n −  epitaxial layer, which functions as a drift region, is formed. The device typically includes a Schottky contact formed directly on the n −  layer. Surrounding the Schottky contact is a p-type JTE (junction termination extension) region that is typically formed by ion implantation. The implants may be aluminum, boron, or any other suitable p-type dopant. The purpose of the JTE region is to reduce or prevent the electric field crowding at the edges, and to reduce or prevent the depletion region from interacting with the surface of the device. Surface effects may cause the depletion region to spread unevenly, which may adversely affect the breakdown voltage of the device. Other termination techniques include guard rings and floating field rings that may be more strongly influenced by surface effects. A channel stop region may also be formed by implantation of n-type dopants such as Nitrogen or Phosphorus in order to prevent the depletion region from extending to the edge of the device. 
         [0005]    Additional conventional terminations of SiC Schottky diodes are described in “Planar Terminations in 4H—SiC Schottky Diodes With Low Leakage And High Yields” by Singh et al., ISPSD &#39;97, pp. 157-160. A p-type epitaxy guard ring termination for a SiC Schottky Barrier Diode is described in “The Guard-Ring Termination for High-Voltage SiC Schottky Barrier Diodes” by Ueno et al., IEEE Electron Device Letters, Vol. 16, No. 7, July, 1995, pp. 331-332. Additionally, other termination techniques are described in published PCT Application No. WO 97/08754 entitled “SiC Semiconductor Device Comprising A PN Junction With A Voltage Absorbing Edge.” 
         [0006]    As briefly discussed above, junction termination extension (JTE), multiple floating guard rings (MFGR) and field plates (FP) are commonly used termination schemes in high voltage silicon carbide devices. JTE may be a very effective edge termination, however JTE may also require tight control of the product of the active doping concentration and junction depth. Furthermore, additional fabrication costs may be incurred as a result of added photolithography and implantation steps. 
         [0007]    FP is also a conventional technique for edge termination of a device and may be cost-effective. In conventional FP devices, high fields are supported by the oxide layer under the metal field plate. This technique performs well for silicon devices where the highest field in the semiconductor is relatively low. However, in SiC devices the electric fields in the blocking state may be very high (˜2 MV/cm) which multiplies by a factor of 2.5 at the oxide-semiconductor interface. This leads to very high oxide fields and may result in long-term reliability problems. Thus, FP may be unsuitable for use in SiC devices. 
         [0008]    Multiple floating guard rings in addition to JTE has been proposed as a technique for reducing the sensitivity of the JTE to implant dose variation. See Kinoshita et al., “Guard Ring Assisted RESURF: A New Termination Structure Providing Stable and High Breakdown Voltage for SiC Power Devices,”  Tech. Digest of ISPSD &#39; 02, pp. 253-256. Kinoshita et al. reported that such techniques reduced the sensitivity to implant dose variation. However, the area utilized for termination was increased to almost three times the area of JTE alone as the guard rings are added to both the inner edge of the JTE and the outside of the JTE. 
         [0009]    MFGR may also be a cost-effective method of edge termination because it may use fewer fabrication steps than JTE. However, MFGR may be very sensitive to surface charges in the oxide-semiconductor interface. The ideal electric field profile of an ideal Multiple Floating Guard Rings (MFGR) termination is shown in  FIGS. 1A through 1D .  FIG. 1A  illustrates a conventional MFGR device where the spacing between the p-type SiC guard rings is illustrated as constant for simplicity. At the blocking state, the depletion region starts at the main junction and expands both laterally and vertically. Once the depletion region punches through to the first guard ring, the potential of the first guard ring gets pinned to that of the main junction. At this point, the punch-through side of the guard ring injects a small amount of holes into the n-region. This lost charge is replaced by the depletion of the n charge from the outer edge of the guard ring. This punch-through and charge injection continues on until the depletion region reaches the final guard ring. Since the amounts of n-charge depleted between the guard rings are the same (constant spacing MFGR&#39;s), the peak x-field each guard ring sees is the same for all guard rings, as shown in  FIG. 1B . However, as seen in  FIG. 1C , the peak y-field is different for all guard rings because the amount of n-charge depletion is different for all guard rings. The highest y-field value is present at the main junction and successive guard rings have reduced levels of y-field. The vector sum of the x and the y fields is illustrated in  FIG. 1D , and shows the highest electric field at the bottom corner of the main junction (circled in  FIG. 1A ). Therefore, breakdown is expected to happen at the circled bottom edge of the main junction if equally spaced MFGR termination is used. If it is desired that each floating guard ring supports the same electric fields, the spacing between the guard rings may vary. The spacing between the main junction and the inner-most guard ring may be the smallest, and the spacing at the outer-most guarding may be the largest. 
         [0010]    One potentially critical issue with the MFGR termination scheme is that it is very sensitive to the charge at the oxide-semiconductor interface. The net charge at metal-oxide-semiconductor (MOS) gate regions of MOS transistors can be very low. However, field oxides often typically have lower quality when compared to thermally grown gate oxides and plasma processing steps may result in higher oxide charges. When a large amount of positive charge is present at the oxide-semiconductor interface, the surface of the lightly doped n-layer turns into n +  regions, which compresses the equi-potential lines. This results in a very high field at the oxide-semiconductor interface and, therefore, reduces the effectiveness of the floating guard rings that may result in a reduction of blocking voltage for the devices. In addition, this charge, mostly positive, can move towards or away from the oxide-semiconductor interface, causing time dependent breakdown voltage, or breakdown walk-out. Breakdown walk-out refers to a phenomenon where the breakdown voltage starts at a first value and increases with time and bias. This problem may be even greater in silicon carbide devices because the field oxides are generally deposited. Deposited oxides, typically, have inferior characteristics to those of thermally grown layers, and the oxide-semiconductor interface in a silicon carbide device has much greater charge density compared to that of a silicon device. 
         [0011]    Putting Offset Field Plates on each guard ring was suggested in Yilmaz, “Optimization and Surface Charge Sensitivity of High Voltage Blocking Structures with Shallow Junctions,”  IEEE Transactions on Electron Devices,  Vol. 38, No. 3, July 1991, pp. 1666-1675. Such a structure is illustrated in  FIG. 2 . As seen in  FIG. 2 , an n-type semiconductor layer  10  has a main junction  12  and a series of floating guard rings  14  formed therein. An oxide layer  16  is provided on the semiconductor layer  10  and openings are provided in the oxide layer  16 . The offset field plates  18  are provided in the openings to contact the floating guard rings  14  and to extend onto the oxide layer  16 . 
         [0012]    Yilmaz demonstrated that the voltage that each guard ring supports can be distributed evenly and the sensitivity to parasitic charges can be reduced by spreading the equipotential lines near the interface. This technique can be implemented in silicon devices relatively easily because the doping densities of the drift layer in silicon devices are generally low, and guard rings can have reasonably large spacing between them. However, in silicon carbide devices, the doping densities in the drift layer can be up to 100 times or more than that of a silicon device with the same blocking capability and the electric field each guard ring supports may be up to 10 times or more greater than that of a silicon device. Therefore, the guard rings may need to be placed much closer to each other compared to a silicon device, and the field oxide thickness that may be needed may be much thicker than that used in silicon devices. Such requirements may be difficult to achieve with conventional fabrication techniques, such as photolithography, for silicon carbide devices because the Offset Field Plate-Floating Guard Ring structure has each field plate contacting each guard ring separately and the edge of the guard ring should not overlap with the edge of the next guard ring. To meet these requirements, each guard ring may need to be enlarged, and the alignment tolerance of the guard rings should be less than 0.25 μm. Such alignment requirements may be difficult, if not impossible, to achieve with conventional contact aligners for SiC. Step coverage may also be another issue with the Offset Field Plate-Floating Guard Ring structure because the thickness of the oxide that may be needed. Additionally, in field plate designs the quality of the oxide may be important in achieving acceptable results as it is the oxide that supports the field or voltages. Oxides in silicon carbide devices, generally have lower quality than that available in silicon devices. Accordingly, the Offset Field Plate-Floating Guard Ring structure may not be practical for silicon carbide devices. 
         [0013]    Guard ring edge termination structures suitable for use in silicon carbide devices are discussed in commonly assigned U.S. Pat. No. 7,026,650 issued on Apr. 11, 2006 to Ryu et al. entitled Multiple floating guard ring edge termination for silicon carbide devices, the disclosure of which is hereby incorporated herein by reference in its entirety. As discussed therein, a surface charge compensation layer, such as a thin p-type layer, is provided in addition to the multiple floating guard rings. The surface charge compensation layer may be used to at least partially neutralize the effects of charges at oxide-semiconductor interfaces in the silicon carbide devices. 
       SUMMARY OF THE INVENTION 
       [0014]    Some embodiments of the present invention provide edge termination structures for semiconductor devices including a plurality of spaced apart concentric floating guard rings in a semiconductor layer that at least partially surround a semiconductor junction. The spaced apart concentric floating guard rings have a highly doped portion and a lightly doped portion. In certain embodiments, the semiconductor device may be a silicon carbide semiconductor device, the semiconductor layer may be a silicon carbide layer and the semiconductor junction may be a silicon carbide-based semiconductor junction. 
         [0015]    In further embodiments of the present invention, the highly doped portion of the floating guard rings extend a first distance into the silicon carbide layer and the lightly doped portion of the floating guard rings extend a second distance into the silicon carbide layer. 
         [0016]    In still further embodiments of the present invention, the first and second distances may be the same. 
         [0017]    In some embodiments of the present invention, the first distance may be less than the second distance. The first distance may be about 0.5 μm and the second distance is about 0.8 μm. The lightly doped portion of the floating guard rings may have a first doping concentration in a portion adjacent the highly doped portion of the floating guard rings and a second doping concentration, greater than the first doping concentration, beneath the highly doped portion of the floating guard rings. The first doping concentration may be about 1.0×10 17  and the second doping concentration may be about 1.4×10 17 . The second doping concentration may be retrograde profile toward the SiO 2 /SiC interface. 
         [0018]    In further embodiments of the present invention, the highly doped portion of the floating guard rings may have a dopant concentration of from about 5.0×10 18  cm −3  to about 1.0×10 20  cm −3  and the lightly doped portions of the floating guard rings may have a dopant concentration of from about 5.0×10 16  cm −3  to about 5.0×10 17  cm −3 . 
         [0019]    In still further embodiments of the present invention, the dopant concentrations may decrease from a main junction of the device to a periphery of the device. In particular embodiments, the dopant concentration of the lightly doped portion of the guard rings may decrease from the main junction of the device to the periphery of the device providing a gradient in the lightly doped portions of the guard rings. 
         [0020]    In some embodiments of the present invention, the highly doped portions and the lightly doped portions extend a distance of from about 0.3 μm to about 0.8 μm into the silicon carbide layer. 
         [0021]    In further embodiments of the present invention, the floating guard rings may be uniformly spaced, non-uniformly spaced and/or combinations of uniformly and non-uniformly spaced. 
         [0022]    In still further embodiments of the present invention, the plurality of floating guard rings may include from about 2 to about 100 guard rings. 
         [0023]    In some embodiments of the present invention, the silicon carbide layer may be an n-type silicon carbide layer and the plurality of spaced apart guard rings may be p-type silicon carbide. 
         [0024]    In further embodiments of the present invention, the silicon carbide layer may be a p-type silicon carbide layer and the plurality of spaced apart guard rings may be n-type silicon carbide. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a diagram of a conventional MFGR structure and the ideal field profile of that structure. 
           [0026]      FIG. 2  is a diagram of a conventional MFGR structure with offset field plates. 
           [0027]      FIG. 3  is a cross section of an edge termination structure according to some embodiments of the present invention. 
           [0028]      FIGS. 4A through 4D  are cross sections illustrating processing steps in the fabrication of edge termination structures according to some embodiments of the present invention. 
           [0029]      FIG. 5  is a plan view of a double guard ring mask for ion implantation according to some embodiments of the present invention. 
           [0030]      FIG. 6  is a blocking histogram for diodes with robust guard ring termination (“robust GR termination”) and double guard ring termination (“double GR termination”) according to some embodiments of the present invention. 
           [0031]      FIG. 7  is a graph illustrating representative reverse IV curves for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention. 
           [0032]      FIG. 8  is a graph illustrating simulations of SiC JBS diode blocking characteristics for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention. 
           [0033]      FIGS. 9 and 10  are graphs illustrating potential distribution of JBS with double guard ring termination and robust guard ring termination at 680V, respectively, according to some embodiments of the present invention. 
           [0034]      FIGS. 11 and 12  are graphs illustrating electrical field distribution comparisons underneath termination junctions, and at the SiO 2 /SiC interface, respectively, according to some embodiments of the present invention. 
           [0035]      FIG. 13  is a cross section illustrating an edge termination structure according to some embodiments of the present invention. 
           [0036]      FIG. 14  is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in  FIG. 13 . 
           [0037]      FIG. 15  is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The invention will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein. Rather, the disclosed embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. Like numbers refer to like elements throughout. 
         [0039]    It will be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to” or “responsive to” (and/or variants thereof) another element, it can be directly on or directly connected, coupled or responsive to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” “directly coupled to” or “directly responsive to” (and/or variants thereof) another element, there are no intervening elements present. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. 
         [0040]    It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
         [0041]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising” (and/or variants thereof), when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. In contrast, the term “consisting of” (and/or variants thereof) when used in this specification, specifies the stated number of features, integers, steps, operations, elements, and/or components, and precludes additional features, integers, steps, operations, elements, and/or components. 
         [0042]    Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to another elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompasses both an orientation of “lower” and “upper,” depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. 
         [0043]    Example embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, may be expected. Thus, the disclosed example embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein unless expressly so defined herein, but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention, unless expressly so defined herein. 
         [0044]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present application, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0045]    As is described in more detail below, embodiments of the present invention may provide improved edge termination of semiconductor devices, such as P—N, Schottky, PiN or other such semiconductor devices. Particular embodiments of the present invention provide edge termination for silicon carbide (SiC) devices. For example, embodiments of the present invention may be utilized as edge termination for SiC Schottky diodes, junction barrier Schottky (JBS) diodes, PiN diodes, thyristors, transistors, or other such SiC devices without departing from the scope of the present invention. 
         [0046]    In particular, some embodiments of the present invention provide an improved edge termination for high voltage silicon carbide devices using double guard ring termination (“double GR termination”) as will be discussed in detail below with respect to  FIGS. 3 through 15 . Commonly assigned U.S. Pat. No. 7,026,650 to Ryu et al. (hereinafter “the ′650 patent), which has been incorporated herein by reference above, discusses robust guard ring termination (“robust GR termination”) used in conjuction with SiC power devices. As discussed therein, a surface charge compensation layer, such as a thin p-type layer, is provided in addition to the multiple floating guard rings. The guard ring termination structure discussed in the &#39;650 patent has shown higher blocking capabilities than the conventional guard ring termination, including junction termination extension (JTE). However, the maximum breakdown voltage may still be less than the value predicted in theory. 
         [0047]    Accordingly, some embodiments of the present invention provide double guard ring (DGR) termination structures for highly doped and lightly doped implantations, respectively. In particular, the highly doped portion of the guard ring may be achieved by Aluminum (Al) implants, while the lightly doped portion of the guard ring may be achieved by Boron (B) implant. Providing both highly doped and lightly doped implants in accordance with some embodiments of the present invention may provide a doping gradient from the main junction to the termination edge such that the electrical field can be further reduced in devices according to some embodiments of the present invention. In other words, the mean doping concentration of the guard ring is greater at the main junction of the device and decreases toward the periphery of the device. The doping gradient in accordance with some embodiments of the present invention is especially useful for Boron in SiC, which can diffuse during a high temperature activation process. Details with respect to some embodiments of the present invention will be discussed further herein with respect to  FIGS. 3 through 15 . 
         [0048]    Referring now to  FIG. 3 , a cross section of a silicon carbide semiconductor device  20  illustrating some embodiments of the present invention will be discussed. As illustrated in  FIG. 3 , a silicon carbide layer  30 , such as a lightly doped n-type silicon carbide layer, has formed therein a main junction  32 , for example, of p-type silicon carbide, and a plurality of double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4 , such as p-type silicon carbide floating guard rings, in accordance with some embodiments of the present invention. Although four double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  are illustrated in  FIG. 3 , embodiments of the present invention are not limited to this configuration. Any number of double guard rings may be included without departing from the scope of the present invention. 
         [0049]    As further illustrated in  FIG. 3 , an insulating layer  26 , such as an oxide layer, is provided on the silicon carbide layer  30 . The insulating layer  26  may be a deposited or grown oxide and may be fabricated utilizing techniques known to those of skill in the art. In particular embodiments of the present invention, the insulating layer  26  may be an oxide, such as SiO 2 , a nitride, such as Si 3 N 4 , an oxide-nitride-oxide structure and/or an oxynitride or organic films such as a polyimide layer. 
         [0050]    As is further illustrated in  FIG. 3 , the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  each include first and second portions  34  and  36 , respectively. In some embodiments of the present invention, the first and second portions  34  and  36  of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  may be p-type silicon carbide. Although the first and second guard rings DGR 1  and DGR 2  are illustrated as having a distance D 1  between them equal to zero, embodiments of the present invention are not limited to this configuration. The distance between the first and second guard rings DGR 1  and DGR 2  can be greater than zero without departing from the scope of the present application. Further illustrated are the distances between the second DGR 2  and third guard DGR 3  rings and the third DGR 3  and fourth DGR 4  guard rings D 2  and D 3 , respectively. The distances D 1 , D 2  and D 3  may all be different distances without departing from the scope of the present invention. 
         [0051]    For the structure illustrated in  FIG. 3  where p-type silicon carbide double guard rings are provided in an n-type silicon carbide layer, the dose charge (concentration×depth=dose) of the highly doped portion (p + )  34  of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  may be from about 5.0×10 18  to about 1.0×10 20  cm −3 . The lightly doped portion (p − )  36  may be from about 5.0×10 16  to about 5.0×10 17  cm −3 . As used herein, “n + ” or “p + ” refer to regions that are defined by higher carrier concentrations than are present in adjacent or other regions of the same or another layer or substrate. The highly doped portions  34  and the lightly doped portions  36  of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  may extend a distance D 4  from about 0.3 to about 0.8 μm into the silicon carbide layer  30 . Although  FIG. 1  illustrates that the highly doped portion  34  and the lightly doped portion  36  extend into the substrate a same distance D 4 , embodiments of the present invention are not limited to this configuration. Alternative embodiments will be discussed further below with respect to  FIG. 13 . 
         [0052]    The highly doped  34  and/or lightly doped  36  portions of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  may have higher doping concentrations closer to the main junction  32  of the device  20  and lower doping concentrations at the periphery of the device  20 . In some embodiments of the present invention, Aluminum ions are implanted to achieve the highly doped portions  34  and Boron ions are implanted to achieve the lightly doped portions  36 . Providing the lightly doped portions  36  in a second guard ring adjacent the highly doped guard ring allows a doping gradient from the main junction  32  to the termination edge to be provided, especially for Boron in SiC which can diffuse during high temperature activation process. Thus, the electric field may be further reduced in accordance with some embodiments of the present invention. 
         [0053]    The oxide-semiconductor interface is expected to have from about 1.0×10 12  to about 2.0×10 12  cm −3  of positive charge. The surface of the surface low does portions  36  of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  will, typically, be depleted by the positive surface charges, and the negative charges in the depletion region low does portions  36  of the double guard rings DGR 1 , DGR 2 , DGR 3  and DGR 4  will reduce the E-field lines originating from the oxide interface charges, and possibly neutralize the negative effects of the positive interface charges. 
         [0054]    As discussed above, in some embodiments of the present invention, the double guard rings  34 / 36  may be uniformly spaced, non-uniformly spaced or combinations of uniformly and non-uniformly spaced. In other words, the lengths of D 1 , D 2 , D 3  and the like may vary and may not be constant. Additionally, in certain embodiments of the present invention, from about 1 to about 100 guard rings  34 / 36  may be provided. The guard rings  34 / 36  may extend a distance of from about 10 μm to about 1000 μm from the main junction of the device. 
         [0055]    In fabricating devices according to certain embodiments of the present invention, the lightly doped portions  36  may be formed before or after formation of the highly doped portions  34 . Both the highly doped portion  34  and the lightly doped portion  36  may be provided by, for example, ion implantation, or other techniques known to those of skill in the art. Alternatively, the lightly doped portion  34  or the highly doped portion  36  may be an epitaxially grown layer of SiC or deposited layer of SiC that is formed on the layer  30  and, in the case of the regions, patterned to provide the desired surface charge compensation regions and/or layers. In such case, the guard rings may be formed prior to formation of the SiC layer or after formation of the SiC layer. 
         [0056]    While embodiments of the present invention have been illustrated with reference to a P—N main junction, as will be appreciated by those of skill in the art in light of the present disclosure, edge termination techniques according to embodiments of the present invention may be utilized with other devices and/or junction types, such as Schottky junctions. 
         [0057]    As discussed above with respect to  FIG. 3 , some embodiments of the present invention provide a novel edge termination structure including double guard rings for the high and low implant doses to further reduce the electrical field. In particular, as discussed above, some embodiments of the present invention provide a gradient of the lightly doped portion  34  of the guard ring, which can further improve the blocking capability of power devices. Furthermore, in accordance with some embodiments of the present invention, the lightly doped portion  34  may have a wide range of tolerance in processing such as misalignment, opening definition, and the like. Double guard ring termination structures according to some embodiments of the present invention may be processed using existing processes and, therefore, may not increase the processing steps and difficulty of processing these devices. A higher blocking capability provided by devices according to some embodiments of the present invention may result in the improvement in other parameters of power devices, such as reducing on-resistance by a thinner drift layer. A thinner drift layer may reduce the power device die size further to achieve a higher die yield. 
         [0058]    Methods of fabricating double guard ring edge termination structures according to some embodiments of the present invention will now be discussed with respect to  FIGS. 4A through 4D . As seen in  FIG. 4A , a silicon carbide layer  30  has formed in it a junction  32  and spaced apart highly doped portions  34  of the double guard ring structure. Such regions may be formed, for example, by ion implantation into a silicon carbide substrate and/or epitaxial layer. For example, aluminum ions having a doping concentration of from about 5.0×10 18  to about 1.0×10 20  cm −3  may be implanted into the silicon carbide layer  30  to provide the highly doped portions  34  illustrated in  FIG. 4A . 
         [0059]    Referring now to  FIG. 4B , a mask layer  100  may be formed and patterned on the silicon carbide layer  30  and may correspond to the junction  32  and highly doped portions of the guard ring  34 . The mask layer  100  may be made of conventional mask materials and may, for example, be patterned using conventional photolithography or other such techniques known to those of skill in the art. The mask layer  100  opens windows adjacent the junction  32  and the highly doped portions of the guard rings  34 . 
         [0060]    As illustrated in  FIG. 4C , the lightly doped portions  36  of the guard rings may be formed through ion implantation using the mask layer  100  as an ion implantation mask. For example, boron ions having a doping concentration of from about 5.0×10 16  to about 5.0×10 17  cm −3  may be implanted into the silicon carbide layer  30  to provide the lightly doped portions  34  illustrated in  FIG. 4C . The mask layer  100  may then be removed and the insulating layer  26  may be formed on the resulting structure as illustrated in  FIG. 4D . The insulating layer  26  may, for example, be formed by thermal oxidation and/or depositing an oxide on the resulting structure. 
         [0061]    While embodiments of the present invention have been described with reference to particular operations in fabrication, particular mask patterns and the like, as will be appreciated by one of skill in the art in light of the present disclosure, other operations, sequences of operations, mask patterns and the likes may be utilized while still benefiting from the teachings of the present invention. For example, a different sequence of implantation of guard rings and surface charge compensation regions may be provided. Furthermore, the particular operations in fabrication of the device may depend on the device being fabricated. Thus, for example, the fabrication of a transistor may have different fabrication steps than the fabrication of a diode. Accordingly, embodiments of the present invention should not be construed as limited to particular operations in fabrication but may encompass and fabrication operations that provide edge termination structures as described herein. 
         [0062]    Referring now to  FIG. 5 , a mask used to form double guard ring termination structures in accordance with some embodiments of the present invention will be discussed. As illustrated in  FIG. 5 , the mask  500  has been patterned to form the lightly doped portion  34 , for example, boron, of the double guard rings and the highly doped portion  36 , for example, aluminum, of the double guard rings. The portions of the mask  500  used to implant the highly doped portions  36  are set out in  FIG. 5 . Furthermore, the portions of the mask  500  used to implant the lightly doped portions  34  are set out in  FIG. 5 . Accordingly, double guard ring termination in accordance with some embodiments of the present invention may allow the dose of the lightly doped guard ring (B) to be gradually reduced from the main junction of the device to the periphery using one time implantation. 
         [0063]    Referring now to  FIG. 6 , a blocking histogram for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention will be discussed. As illustrated by the histogram, a blocking voltage of about 130 V higher was achieved using the double guard ring termination structure in accordance with some embodiments of the present invention. 
         [0064]      FIG. 7  is a graph illustrating representative reverse IV curves for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention.  FIG. 8  is a graph illustrating simulations of SiC JBS diode blocking characteristics for diodes with robust guard ring termination and double guard ring termination according to some embodiments of the present invention. The devices were fabricated using the same wafer. 
         [0065]      FIGS. 9 and 10  are graphs illustrating potential distribution of JBS with double guard ring termination and robust guard ring termination at 680V, respectively, according to some embodiments of the present invention.  FIGS. 11 and 12  are graphs illustrating electrical field distribution comparisons underneath termination junctions, and at the SiO 2 /SiC interface, respectively, according to some embodiments of the present invention. It will be understood that a lower electric field yields a lower leakage current. 
         [0066]    Referring now to  FIG. 13 , a cross section illustrating an edge termination structure  20 ′ according to some embodiments of the present invention. Like reference numerals refer to like elements throughout, accordingly details with respect to like numbered elements discussed above with respect to  FIG. 3  will not be repeated herein in the interest of brevity. As illustrated in  FIG. 13 , the lightly doped portion of the double guard ring structure has first and second portions  46  and  47 . As illustrated in  FIG. 13 , the first portion of the lightly doped portion  46  may be a p − −  layer and may have a doping concentration of about 1.0×10 17  cm −3 . The first portion of the lightly doped portion may extend about 0.5 μm into the semiconductor layer  30 . Furthermore, the second portion of the lightly doped portion  47  may be a p −  layer and have a doping concentration of about 1.4×10 17  cm −3 . The second portion of the lightly dopes portion  47  may extend about 0.8 μm into the semiconductor layer  30  and may extend beneath the highly doped portion  36 . Embodiments of the present invention illustrated in  FIG. 13  may exhibit improvement in electric field characteristics as illustrated in  FIGS. 14 and 15  discussed below. 
         [0067]      FIG. 14  is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in  FIG. 13 .  FIG. 15  is a graph illustrating electric field characteristics in accordance with some embodiments of the present invention illustrated in  FIG. 13 . In particular,  FIG. 15  is a graph illustrating robust GR termination and double guard ring termination with a retrograde profile of the second doping concentration. 
         [0068]    Although embodiments of the present invention are discussed above primarily with respect to silicon carbide semiconductor devices, embodiments of the present invention are not limited to silicon carbide devices. For example, devices according to some embodiments of the present invention may be silicon (Si), gallium nitride (GaN) or gallium arsenide (GaAs) without departing from the scope of the present invention. 
         [0069]    In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.