Patent Document

PRIORITY CLAIM 
     The present application claims priority to Italian Patent Application No. TO2008A000999, filed Dec. 29, 2008, which application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     An embodiment of the present invention relates to a semiconductor power device of a multi-drain type and to a corresponding edge-termination structure. 
     BACKGROUND 
     In the last few years, a wide range of solutions have been developed for improving the efficiency of semiconductor power devices, and in particular for obtaining an increase of the breakdown voltage and a decrease of the output resistance. 
     For example, U.S. Pat. Nos. 6,228,719, 6,300,171, 6,404,010, and 6,586,798, which are incorporated by reference, describe vertical-conduction semiconductor power devices of the multi-drain (MD) type, wherein, within an epitaxial layer forming part of a drain region having a given type of conductivity, charge-balance columnar structures are provided, having opposite conductivity. These structures have a dopant concentration substantially equal and opposite to the dopant concentration of the epitaxial layer in such a way as to provide a substantial charge balance. Charge balance enables high breakdown voltages to be obtained, and moreover the high concentration that the epitaxial layer can consequently assume enables a low output resistance (and reduced losses in conduction) to be achieved. 
     In order to manufacture the columnar structures, a sequence of steps of growth of epitaxial layers of a first conductivity, for example of an N type, is envisaged, each step being followed by an implantation of dopant of the second conductivity, in the example of a P type. The implanted regions are stacked and subjected to a subsequent process of diffusion of the dopant atoms so as to give rise to uniform columnar structures. 
     Next, body regions of the power device are formed in contact with the columnar structures, in the active region, in such a way that the columnar structures constitute an extension of the body regions within the drain region. 
     The evolution of this technology has shown a progressive increase in the density of the elementary strips forming the devices in order to increase further the charge concentration of the epitaxial layer and obtain devices that, given the same breakdown voltage (substantially linked to the height of the columnar structures), would present a lower output resistance. On the other hand, however, the increase in the density of the elementary strips has led to a reduction of the thermal budget of the devices and a corresponding increase in the number of steps of epitaxial growth, and hence an increase in the manufacturing costs and time and in the defectiveness intrinsically linked to the epitaxial growth. 
     Alternative techniques have hence been developed in order to obtain the charge-balance structures, which envisage, for example, formation of trenches within the epitaxial layer and subsequent filling of the same trenches with semiconductor material appropriately doped to achieve the charge balance. For example, in patent applications WO 2007/116420 and WO 2007/122646, which are incorporated by reference, techniques are described for obtaining charge-balance structures in electronic power devices, which envisage the formation of trenches and filling thereof without any residual defectiveness via a particular technique of non-selective epitaxial growth performed in the same trenches. 
     It is also known that the formation of efficient edge-termination structures may be a key point for ensuring proper operation of the power devices. In fact, it is at the edge areas (i.e., the areas surrounding the active area in which the electronic components are provided) that the highest number of breakdown phenomena occurs on account of the thickening of the electric field lines due to the presence of discontinuities, such as sharp edges or the curvature of the doped regions. Edge terminations have the function of reducing the intensity of the electric field locally so as to prevent peaks of intensity at the edges. 
       FIGS. 1-4  (which are not drawn to scale, as the subsequent figures are also not to scale) show an example of an edge-termination structure of a known type, for a vertical-conduction charge-balance power device. In particular,  FIG. 1  shows a schematic and simplified top plan view, whilst  FIGS. 2 ,  3  and  4  show cross sections taken along lines II-II, III-III and IV-IV indicated in  FIG. 1 , respectively. 
     The power device, designated by  1 , is formed in a die  2  of semiconductor material, for example silicon. The die  2  has, in top plan view, a generically rectangular or square shape; the borders and edges of the die  2  correspond to the so-called “scribe lines” (designated by LT), at which the starting wafer of semiconductor material has been cut. In the die  2  it is possible to define a peripheral portion  2   a , adjacent to the scribe lines, and a central portion  2   b , in which the power device  1  is physically provided. 
     The die  2  comprises a substrate  3  having a first type of conductivity, for example of an N ++  type, and an epitaxial layer  4 , formed on the substrate  3 , also having the first type of conductivity, in the example of an N type. Within the epitaxial layer  4  it is possible to distinguish an active area  4   a , designed to house elementary electronic components  50  (in the example, MOS transistors) of the power device  1 , and an edge area  4   b , designed to house an edge-termination structure of the device and adjoining the peripheral portion  2   a  of the die  2 . In particular, the epitaxial layer  4  constitutes a common drain surface region for the plurality of elementary electronic components  50  (the MOS transistors) forming the power device  1 . 
     The edge-termination structure comprises a ring region  5 , in particular a region doped with a second type of conductivity, of a P type, with low concentration, for example, lower than 10 16  at/cm 3 , formed in a surface portion of the epitaxial layer  4 . The ring region  5  is provided within the edge area  4   b , surrounds the active area  4   a  completely (forming a ring around it), and has an area of superposition with a peripheral portion of the same active area  4   a . In particular, the ring region  5  has a rounded-off and curved profile in such a way as to reduce local concentrations of the field lines. 
     Charge-balance structures  7  (which have in cross section a column conformation, see in particular  FIGS. 2-4 ) traverse the epitaxial layer  4  substantially throughout its thickness, stopping at a certain distance from the substrate  3 , both at the active area  4   a  and at the ring region  5  in the edge area  4   b . The charge-balance structures  7  are, for example, obtained through successive steps of epitaxial growth and implantation of dopant atoms in order to obtain stacked doped regions, and through a final step of diffusion of the dopant atoms. 
     The charge-balance structures  7  follow the layout of the regions in which they are formed, and are constituted by doped regions having the second type of conductivity (P) and a doping level such as to create a substantial charge balance. In particular, in the active area  4   a  the charge-balance structures  7  are constituted (in plan view, see  FIG. 1 ) by first strips  7   a , having a substantially rectilinear extension, parallel to one another and to a first side of the die  2  (and to a first axis x), which repeat periodically and at substantially the same distance in a direction parallel to a second side of the die  2  (and to a second axis y, orthogonal to the first axis x). Instead, in the edge area  4   b  the charge-balance structures  7  follow the pattern and the profile of the ring region  5 , within which they are housed, and are made up of second strips  7   b , once again parallel to and set at substantially the same distance from one another, each of which is constituted by: a first rectilinear portion parallel to the first side of the die  2  (and to the first axis x); a second rectilinear portion parallel to the second side of the die  2  (and to the second axis y); and a curved connecting portion between the first rectilinear portion and the second rectilinear portion (in particular having substantially the same radius of curvature as the ring region  5 ). 
     In particular, given their columnar extension in the thickness of the epitaxial layer  4 , the charge-balance structures  7  constitute vertical walls or diaphragms extending in strips within the same epitaxial layer  4 . In addition, in current design rules, the number of the second strips  7   b  that occupy the ring region  5  is determined by the dimension of the ring and by the pitch (in terms of spacing and size) of the first strips  7   a  in the active area  4   a.    
     Body wells  9  are present within the active area  4   a , having the second type of conductivity (P) and contacting each first strip  7   a  of the charge-balance structures  7 , at the surface portion of the epitaxial layer  4 . In particular, the first strips  7   a  constitute extensions of the body wells  9  within the drain region in the epitaxial layer  4 . Source regions  10 , having the first type of conductivity (N), are provided inside each body well  9 . In particular, in the area of superposition between the active area  4   a  and the ring region  5 , the outermost body wells  9  join the same ring region  5 . In addition, in the edge area  4   b , the second strips  7   b  of the charge-balance structures  7  are joined to one another by the ring region  5 . 
     The power device  1  further comprises, on the surface of the epitaxial layer  4 , a first dielectric region (for example, made of silicon oxide)  12 , having a greater thickness at the edge area  4   b  and a smaller thickness in the active area  4   a , where it provides the gate-oxide regions of the elementary electronic components  50 . A gate region (made of polysilicon or other conductive material)  14  is provided on the first dielectric region  12 ; on the gate-oxide regions, the gate region  14  provides the gate structures of the elementary electronic components  50 . 
     In addition, a second dielectric region (for example, made of field oxide)  15  covers the first dielectric region  12  and the gate region  14 . The second dielectric region  15  is traversed, at the edge area  4   b , by a gate metal contact  18 , designed to contact the gate region  14 . In addition, the second dielectric region  15 , the first dielectric region  12 , and the gate region  14  are traversed, in the active area  4   a , by a source metal contact  16 , extending to contact and short-circuit the source regions  10  and the body wells  9 . At the periphery of the edge area  4   b  (adjacent to the peripheral portion  2   a  of the die  2 ), the surface of the epitaxial layer  4  is left exposed so as to enable an equipotential-ring (EQR) metal contact  19  to contact a doped region  20 , in particular a doped region having the first type of conductivity (N), provided in the surface portion of the epitaxial layer  4 . The doped region  20  is set at a distance from the ring region  5 , and has the same ring layout as the latter, surrounding it completely. The contact region  20  has the function of bringing to the surface the drain potential so as to limit horizontally the electric field lines in reverse biasing. 
     In analysing the cross sections of  FIGS. 2-4 , it is to be noted in particular that the cross section of  FIG. 2  is taken in a direction transverse to the direction of extension of the charge-balance structures  7 , and that the cross sections of  FIGS. 2 and 3  are both taken along the direction of extension of the charge-balance structures  7 , but on the outside and on the inside, respectively, of a first strip  7   a.    
     It has been shown that power devices of the type described, although having considerable advantages as compared to traditional solutions, may be subject to phenomena of early breakdown that can jeopardize their performance or, in the worst case, prevent their subsequent use (i.e., destroy them). 
     SUMMARY 
     An embodiment of the present invention overcomes the above drawbacks and further improves a charge-balance power device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present disclosure, one or more embodiments are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a schematic and simplified top plan view of a portion of a power device of a known type, in particular corresponding to an edge-termination structure; 
         FIGS. 2 ,  3 , and  4  show cross sections of the device of  FIG. 1 , taken along lines II-II, III-III, and IV-IV of  FIG. 1 , respectively; 
         FIG. 5  shows an IN plot representing the reverse breakdown characteristic of the device of  FIG. 1 ; 
         FIG. 6  shows a microscope photograph of the result of an emission-microscopy (EMMI) analysis on the portion of the power device of  FIG. 1 ; 
         FIG. 7   a  is a schematic and simplified top plan view of a power device according to an embodiment of the present invention; 
         FIG. 7   b  is a top plan view of a portion of the device of  FIG. 7   a , in particular regarding an edge-termination structure thereof; 
         FIGS. 8 ,  9  and  10  show cross sections of the device of  FIGS. 7   a  and  7   b , taken along lines VIII-VIII, IX-IX, and X-X of  FIG. 7   b , respectively; 
         FIGS. 11-12  show plots representing the breakdown characteristics of the device of  FIGS. 7   a  and  7   b;    
         FIG. 13  shows a microscope photograph of the result of an EMMI analysis on the portion of the power device of  FIG. 7   b;    
         FIGS. 14 and 15  show top plan views similar to that of  FIG. 7   b , regarding further embodiments of the present invention; 
         FIG. 16  shows a cross section of the power device shown in  FIG. 15 , taken along the line XVI-XVI; and 
         FIGS. 17 and 18  show top plan views similar to that of  FIG. 7   b , regarding further variant embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     It has been discovered and verified experimentally that the critical features of early breakdown afflicting the multi-drain power devices of the type previously described derive principally from the presence of discontinuities in the charge balance occurring within the edge area  4   b , and specifically in the ring region  5 . 
     In particular (see again  FIG. 1 ), these discontinuities occur at the joining points between the charge-balance structures  7  in the active area  4   a  and those in the edge area  4   b , and more particularly in the contact points between the first strips  7   a  and the second, innermost, strip  7   b  (i.e., the one closest to the active area  4   a ), at which the charge may not be balanced and an excess of charge occurs. In detail, two types of intersection zones with marked charge unbalancing are generated: areas that in top plan view can be defined as “cuspidal”, designated by P C  in  FIG. 1 , at the intersection between a first set of first strips  7   a  and the curved portion of the aforesaid second, innermost, strip  7   b ; and areas that in top plan view are T-shaped, designated by P T  in  FIG. 1 , at the intersection between a second set of first strips  7   a  and the second rectilinear portion of the second, innermost, strip  7   b.    
     These areas of excess of charge are a source of early triggering of breakdown having a triangular reverse-biasing characteristic, as shown in  FIG. 5 , whatever the number of the second strips  7   b  that are present in the ring region  5 . 
     The early triggering phenomena are moreover highlighted by the EMMI analysis shown in  FIG. 6 , regarding the same portion of device of  FIG. 1 , in which evident light emission phenomena are visible at the areas with higher charge unbalancing (cuspidal areas P C ). 
     Furthermore, it is noted (reference may be made to the cross sections of  FIGS. 2-4 ) that the sections of the edge area  4   b , in a direction orthogonal or parallel to the direction of extension of the first strips  7   a , are different from one another, due to the asymmetry introduced in the structure by the arrangement of the second strips  7   b  in the ring region  5 , in particular in terms of the local charge balance that it is possible to obtain and of the number of charge-balance structures  7  joined to the ring region  5 . 
     In order to overcome these critical features, an embodiment of the present invention envisages a different configuration of the charge-balance structures, such that the charge-balance structures occupy substantially uniformly and without any substantial discontinuity the entire area of the power device. 
     In detail, and as is shown in  FIGS. 7   a  and  7   b , where the same reference numbers are used for representing elements similar to others described previously, the charge-balance structures, here designated by  7 ′, of the power device, here designated by  1 ′, comprise strips extending parallel to one another over the entire area of the die  2 , and in particular throughout the active area  4   a  (first strips  30   a ) and throughout the edge area  4   b  (second strips  30   b ), without any mutual intersection. In addition, the strips  30   a ,  30   b  extend also to the peripheral portion  2   b  of the die. The strips  30   a ,  30   b  extend, for example, parallel to the first side of the die  2  and to the first axis x, repeating periodically along the second axis y, at a substantially uniform distance of separation. In particular, the charge-balance structures  7 ′ do not follow in this case the layout of the ring region  5 . It is to be noted that the first strips  30   a  extend from the active area  4   a  into the edge area  4   b , and also that the first, outermost, strip  30   a  (i.e., the one closest to the edge area  4   b ) is parallel to the adjacent second, innermost, strip  30   b  (i.e., the one closest to the active area  4   a ), without intersecting it throughout its extension. 
     This arrangement makes it possible to avoid discontinuity areas in the charge balance (in particular the cuspidal areas within the ring region  5 ), that have been shown to be at the origin of the points of local charge unbalancing in the epitaxial layer  4 . In particular, the fact that all the charge-balance structures  7 ′ extend in a parallel way means that high electric field is always sustained by parallel “walls” of opposite charge, without intersecting structures that may locally induce an increase in the electric field. 
     The sections of the edge area  4   b , in a direction orthogonal or parallel to the direction of extension of the strips  30   a ,  30   b , as shown in  FIGS. 8-10 , are in this case clearly equivalent in terms of the local charge balance that it is possible to obtain given that all the strips of the charge-balance structures  7 ′ extend parallel to one another, and in particular demonstrate how charge balance is obtained in the edge area  4   b  in a way substantially similar to the active area  4   a.    
     It has been shown experimentally that the structure described, with the consequent elimination of local singularities, makes it possible to inhibit triggering of early breakdown. In this regard,  FIG. 11  shows the resulting IN characteristic of the power device  1 ′, whilst  FIG. 12  shows the comparison between the evolutions of the normalized breakdown voltage BVdss (breakdown voltage between drain and source with the gate set at ground potential) as a function of the net charge φ n  (charge of a P type minus charge of an N type) given in arbitrary units (a.u.), in the power device  1 ′ according to an embodiment of the present invention (solid line) and in a traditional device (dashed line). The shift observed experimentally of the peak of BVdss (that should ideally correspond to the charge balance, φ n =0), indicates that the local unbalancing in the structure may induce clamping of the maximum breakdown value. 
     An optimization of the layout in the edge area  4   b  may enable elimination of this structural clamping in the breakdown voltage. 
     Also the EMMI analysis in breakdown conditions ( FIG. 13 ) confirms the previous results, highlighting the presence of a substantially uniform emission between the strips of the charge-balance structures  7 ′. 
     As is shown in  FIG. 14 , a different embodiment of the present invention envisages that the strips  30   a ,  30   b  of the charge-balance structures  7 ′ do not extend throughout the entire surface of the die  2 , but stop inside the edge area  4   b , before reaching the peripheral portion  2   a  of the die  2 . For example, the strips  30   a ,  30   b  interrupt beyond the ring region  5 , in an intermediate point between the ring region  5  itself and the EQR metal contact  19  and the corresponding doped region  20 . In particular, beyond the intermediate point there is no presence either of prolongations of the first strips  30   a  or prolongations of the second strips  30   b , or of the same second strips  30   b.    
     A further embodiment of the present invention, shown in  FIG. 15  (in top plan view) and in  FIG. 16  (in a section transverse to the direction of extension of the strips  30   a ,  30   b ), further envisages the possibility that the edge-termination structure of the power device  1 ′ does not comprise the ring region  5  in the edge area  4   b . In the example shown, the strips  30   a ,  30   b  extend again parallel to one another and in a uniform way, over the entire area of the die  2 , in the active area  4   a  and in the edge area  4   b  of the epitaxial layer  4 . This embodiment may solve the problem of “charge unbalancing” induced by the ring region  5 , in the cases where it is possible to verify that the accumulation of charges due to the presence of points with high electric field (which are no longer eliminated by the ring region) does not lead to a significant decrease in reliability. In fact, the ring region  5  introduces a surface charge at the edge area  4   b , which adds (with its sign) to the charge present in the charge-balance structures, consequently introducing in the proximity of the surface a certain local lack of uniformity in the charge balance. 
     Advantages of the semiconductor power device and of the corresponding edge-termination provided according to one or more embodiments of the present invention are clear from the foregoing description. 
     In particular, an embodiment removes, in multi-drain power devices basing their operation on charge balance in the epitaxial layer operating as extension of the drain, any discontinuity in the charge balance and consequently early breakdown phenomena, rendering the breakdown characteristic hard. The periodic and uniform structure of the charge-balance structures is maintained also in the edge region of the power device, providing a substantial charge balance in the drain region not only in the active area but also at the edge, and preventing the edge-termination structure from introducing significant singularities and significant local excesses of charge. In particular, the improvement of the performance in reverse biasing enables improvement of the industrialization of the process and the quality and reliability of the final devices produced. 
     Thanks to charge balancing, it is also possible to obtain values of resistivity of the epitaxial layer lower than 2 Ω·cm, and values of inhibition voltage comprised between 100 and 1500 V, as the thickness of the epitaxial layer varies. 
     Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     In particular, as is shown in  FIG. 17  in an embodiment, a first spacing d 1  may be provided in the active area  4   a  between the first strips  30   a , and a second spacing d 2 , different from the first, between the second strips  30   b  in the edge area  4   b . In the example shown, the distance between the first strips  30   a  is greater than the distance between the second strips  30   b  (but also the opposite solution may be adopted, d 2 &gt;d 1 ). 
     As is shown in  FIG. 18 , in an embodiment the strips  30   a ,  30   b  may have a different size (in particular, a different width L in a direction transverse to their main direction of extension) in the active area  4   a  with respect to the edge area  4   b . For example, the first strips  30   a  may have a transverse dimension L 1 , and the second strips  30   b  a second transverse dimension L 2 , smaller than the first one (once again, the opposite solution may be alternatively adopted, L 2 &gt;L 1 ). 
     In particular, the aforesaid alternative embodiments may make it possible to afford greater margins of freedom in redefining the “design rules” in order to offset the charge unbalancing induced by the ring region  5  (in the case where it is not possible or desirable to eliminate it in order not to risk reliability problems), optimizing the size and spacing of the strips of the charge-balance structures in the edge area  4   b.    
     Furthermore, in an embodiment, the charge-balance structures  7 ′, instead of extending only within the epitaxial layer  4 , may also reach the substrate  3  and terminate within the same substrate  3 . 
     Clearly, different techniques may be used for obtaining the charge-balance columnar structures. For example, as described in patent application No. WO 2007/006503, which is incorporated by reference, the successive steps of epitaxial growth and implantation for creating superimposed implanted regions, instead of being designed to form the columnar structures, may be designed to form regions which are to define, between one another, the charge-balance columnar structures. Alternatively, as described in patent application No. EP-A1-1911075, which is incorporated by reference, it is possible to envisage successive superimposed implantations (in corresponding regions of epitaxial growth) to provide both the charge-balance columnar structures and the regions of opposite conductivity set between the same columnar structures. Alternatively, it may be possible to use the non-selective technique of epitaxial growth within trenches, substantially as described in the aforesaid patent applications Nos. WO 2007/116420 and WO 2007/122646, which are incorporated by reference. 
     One or more of the embodiments described may also be used, with the appropriate small modifications, to obtain a generic charge-balance power device (for example, a bipolar diode, a Schottky diode, a BJT, an IGBT, etc.) and the corresponding edge-termination structure. In particular, it is clear that, in the case, for example, of a diode power device, the epitaxial region constitutes an extension of a cathode terminal (even though usually also this power device is identified by the term “multi-drain”). Furthermore, it is evident that it is possible to obtain dual structures in which the charge balance is provided by means of formation of columnar structures with N doping in an epitaxial layer with P doping. 
     From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.

Technology Category: 5