Abstract:
A Merged P-i-N Schottky device in which the oppositely doped diffusions extend to a depth and have been spaced apart such that the device is capable of absorbing a reverse avalanche energy comparable to a Fast Recovery Epitaxial Diode having a comparatively deeper oppositely doped diffusion region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of the co-pending, commonly-owned U.S. patent application Ser. No. 11/402,039, filed on Apr. 11, 2006, by Chiola et al., and titled “MERGED P-i-N SCHOTTKY STRUCTURE,” which is a divisional of U.S. patent application Ser. No. 10/766,466, filed on Jan. 27, 2004, by Chiola et al., and titled “MERGED P-i-N SCHOTTKY STRUCTURE,” the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is related to semiconductor devices and more particularly to a Merged P-i-N Schottky (MPS) diode. 
     BACKGROUND OF THE INVENTION 
       FIG. 1A  shows the cross-section of a Fast Recovery Epitaxial Diode (FRED)  10  according to the prior art. FRED  10  comprises a lightly-doped N silicon epitaxial layer  12  which is formed on a highly doped N+ silicon substrate  14 . A p+ doped diffusion well  16  is formed on a portion of the upper region of epitaxial layer  12 . FRED  10  includes first major electrode  18  that is in surface-to-surface contact with diffusion well  16  and silicon dioxide layer  20  which surrounds and is partially in contact with the outer periphery of diffusion well  16 . FRED  10  also includes second major electrode  22  which is disposed on a surface of silicon substrate  14  opposing first major electrode  18  of FRED  10 . 
     Diffusion well  16  of FRED  10  is relatively shallow and may range between 3 μm to 6 μm for 200-600 volt devices. It has been found that 3 μm-6 μm deep diffusion well  16  provides a good tradeoff between performance and manufacturing complexity. However, devices having shallow diffusion wells do not have the capability to absorb reverse avalanche energy well. 
     The difference between the bulk breakdown voltage (BV), which represents the ideal breakdown voltage for a planar junction, and the actual BV for a FRED has been used to isolate the reason for the inability of FRED  10  to satisfactorily absorb reverse avalanche energy. 
     Referring, for example, to Table 1, FRED  10  which has a 6 μm deep diffusion well  16  can have an actual device BV that may be between 36-70 volts lower than bulk BV. It should be noted that although the thickness of the epitaxial layer  12  contributes to the difference between the bulk BV and the actual device By, the peak electric field is found near the corners of diffusion well  16  at breakdown due to the crowding of the electric field lines. It is believed that these localized regions of high electric field, which are near the small-radius curvature of diffusion well  16 , generate “hot spots” that lead to avalanche failure. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Standard Epi Profile 
               
             
          
           
               
                 Epi  
                 Epi res.  
                 Junction 
                 Device  
                 One-d BV 
                 Delta  
               
               
                 thick [μm] 
                 (Ohm-cm) 
                 Depth (Xj) 
                 BV[V] 
                 (“bulk” BV) [V] 
                 BV [V] 
               
               
                   
               
               
                 30 
                 12 
                 6 
                 344 
                 383 
                 39 
               
               
                 34 
                 12 
                 6 
                 356 
                 426 
                 70 
               
               
                 30 
                 14 
                 6 
                 350 
                 386 
                 36 
               
               
                 34 
                 14 
                 6 
                 377 
                 442 
                 65 
               
               
                   
               
             
          
         
       
     
     It has also been found that FRED  10  having a linearly graded or double-profiled epitaxial layer  12  still has a device BV that is lower than an ideal bulk BV. 
     Referring to Table 2, for example, FRED  10  having a linearly graded epitaxial layer  12 , and a 6 μm diffusion well  16  exhibits an actual device BV that is between 25-32V lower than the ideal bulk By. Epitaxial layer  12  of FRED  10  of Table 2 has a linearly graded tail (Epi 2) with a concentration of dopants that is varied during the epitaxial growth and is kept constant during the final growth of the second layer (Epi I).  FIG. 1B  shows a linearly graded epitaxial layer  12  graphically. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Graded Profile 
               
             
          
           
               
                 Epi 1  
                 Epi 1 
                 Epi 2  
                 Epi 2 
                   
                 Device 
                 One-d BV 
                 Delta  
               
               
                 thick  
                 Res 
                 Thick 
                 Res  
                 Xj 
                 BV  
                 (“bulk”  
                 BV 
               
               
                 [μm] 
                 (Ohm-cm) 
                 [μm] 
                 (Ohm-cm) 
                 (μm) 
                 [V] 
                 BV) [V] 
                 [V] 
               
               
                   
               
               
                 15 
                 20 
                 15 
                 20-1 
                 6 
                 300 
                 325 
                 25 
               
               
                 15 
                 20 
                 15 
                 20-4 
                 6 
                 352 
                 384 
                 32 
               
               
                 15 
                 20 
                 15 
                 20-6 
                 6 
                 365 
                 392 
                 27 
               
               
                   
               
             
          
         
       
     
     Referring to Table 3, as yet another example, FRED  10  having a double-profiled epitaxial layer  12 , and a 6 μm diffusion well  16  can have an actual device BV which is 27-28 volts less than the ideal bulk By. Epitaxial layer  12  of FRED  10  of Table 3 has a first layer (Epi 2) of constant concentration and a second layer (Epi 1) of constant concentration.  FIG. 1C  shows a double-profiled epitaxial layer graphically. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Double Profile 
               
             
          
           
               
                 Epi 1 
                 Epi 1 
                 Epi 2  
                 Epi 2 
                   
                 Device 
                 One-d BV 
                 Delta  
               
               
                 thick 
                 Res 
                 Thick  
                 Res 
                 Xj  
                 BV  
                 (“bulk” 
                 BV 
               
               
                 [μm] 
                 (Ohm-cm) 
                 [μm] 
                 (Ohm-cm) 
                 (μm) 
                 [V] 
                 BV) [V] 
                 [V] 
               
               
                   
               
             
          
           
               
                 15 
                 20 
                 15 
                 3.5 
                 6 
                 315 
                 343 
                 28 
               
               
                 15 
                 20 
                 15 
                 8 
                 6 
                 354 
                 381 
                 27 
               
               
                   
               
             
          
         
       
     
     Comparison of the data in Table 1, Table 2 and Table 3 indicates that by grading the profile of epitaxial layer  12 , the difference between actual device BV and the ideal bulk BV can be reduced. However, the difference between the actual and the ideal breakdown voltages remains high for FRED  10  having a shallow 6 μm diffusion well. Moreover, the crowding of the electric field lines near the corners of diffusion well  16  is still observed in FRED  10  of Table 2 and Table 3. Thus, profile grading does not appear to strengthen the ability of FRED  10  to absorb the reverse avalanche energy. 
     Referring now to Table 4, diffusion well  16  of FRED  10  of Table 2 having a linearly graded profile was extended from 6 μm to 10 μm. In order to achieve a total bulk thickness of 30 μm, the Epi layer was thickened by 4 μm. As shown in Table 4, the increase in the depth of diffusion well  16  by 4 μm reduced the difference between the actual device BV and the ideal bulk BV. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Graded Profile 
               
             
          
           
               
                 Epi 1 
                 Epi 1 
                 Epi 2  
                 Epi 2 
                   
                 Device 
                 One-d BV 
                 Delta  
               
               
                 thick 
                 Res 
                 Thick 
                 Res 
                 Xj  
                 BV 
                 (“bulk” 
                 BV 
               
               
                 [μm] 
                 (Ohm-cm) 
                 [μm] 
                 (Ohm-cm) 
                 (μm) 
                 [V] 
                 BV) [V] 
                 [V] 
               
               
                   
               
               
                 19 
                 20 
                 15 
                 20-1 
                 10 
                 365 
                 398 
                 32 
               
               
                 19 
                 20 
                 15 
                 20-4 
                 10 
                 321 
                 340 
                 19 
               
               
                 19 
                 20 
                 15 
                 20-6 
                 10 
                 265 
                 277 
                 12 
               
               
                   
               
             
          
         
       
     
     Further increases in the depth of diffusion well  16  from 15 μm to 20 μm in the epitaxial layer  12  of the device of Table 4 showed further reduction in the difference between actual device BV and ideal bulk BV. While this reduction between actual and ideal breakdown voltages is partly due to the thinning of the bulk thickness caused by the deepening of diffusion well  16 , the deepening of diffusion well  16  has a substantial reducing effect on the difference between the actual and the ideal breakdown voltages. This reduction is believed to be due to the relaxation of the electric field lines as the radius of curvature near the corners of diffusion well  16  is increased, as well as, the spreading of the field lines toward the main portion of the PN junction (the junction between the diffusion well  16  and epitaxial layer  12 ), which helps to distribute the reverse avalanche energy over a wider area. 
       FIG. 2  shows FRED  24  having a 20 μm deep diffusion well  16  and a graded epitaxial layer  12  of very low doping level (approximately 1×10 4  cm 3 ). FRED  24  has an actual device BV which is only about 2.9 volts less than the ideal Bulk BV at 25° C. and about only 8.3 volts at 125° C. for 100 μA. Depending on the doping of epitaxial layer  12 , FRED  10  ( FIG. 1A ) can have an avalanche voltage that changes by 25-40 volts when the temperature of the PN junction is raised from 25° C. to 125° C. It should be noted that corners of diffusion well  16  of FRED  24  are flatter and thus have a larger radius, which, it is believed, contribute to the capability of FRED  24  to absorb the reverse avalanche energy and increase the actual device BV of FRED  24 . 
     To obtain a deep diffusion well  16 , such as the one shown in  FIG. 2 , diffusion of dopants must be conducted at relatively high temperatures which may be in the order of 1250° C. or higher, and typically for a long drive-in time. In contrast, shallower diffusion well  16 , such as the one shown by  FIG. 1A  may be obtained at considerably lower temperatures, which maybe in the order of about 1100° C., and for a shorter drive-in time. Given that many fabrication laboratories do not have the capability for deep diffusion at high temperatures, it is desirable to have an alternative device, which does not require a high temperature diffusion step, that is capable of absorbing the reverse avalanche energy of a FRED having a deep diffusion well  16 , such as FRED  24  of  FIG. 2 . 
     SUMMARY OF THE INVENTION 
     A semiconductor device according to the present invention is an MPS capable of absorbing the reverse avalanche energy absorbed by a FRED  24  (e.g.  FIG. 2 ) having a deep diffusion well  16 . 
     MPS devices are known. U.S. Pat. No. 4,862,229 shows an MPS type device in which oppositely doped diffusions are integrated with a Schottky structure. The conventional thinking in the design of prior art MPS devices is to space the diffusions as close as possible so that under the reverse bias condition the depletion layers around the diffusions link up quickly to improve the breakdown voltage of the device. It has been found, however, that contrary to conventional thinking, in an MPS having shallow diffusion stripes, spacing the diffusions wider apart improves the absorption of reverse avalanche energy and thus improves the ability of the device to withstand breakdown under reverse bias conditions. Specifically, it has been found that by appropriate adjustment of the distance between the diffusion stripes in an MPS, shallower diffusions can be used to achieve the same ability to withstand breakdown as a prior art device with a deeper diffusion well. Thus, with lower temperature processing (and thus lower cost) a device can be obtained that exhibits the same or comparable characteristics as a device manufactured by a higher temperature process (and thus higher cost). 
     An MPS according to the present invention include an array of P+ diffusion stripes each spaced from at least one other adjacently disposed stripe. According to the present invention, the distance between each stripe has been set so that the capability of the MPS device to withstand breakdown under reverse bias condition is adapted to be close to that of a FRED with a deeper diffusion well. 
     The stripes in an MPS according to the present invention may be diffused to a depth of about 5 μm at a relatively low temperature of about 1100° C., and spaced apart up to 19 μm above which the practical advantages of spacing the stripes farther from each other are diminished or lost. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view of the active region of a prior art FRED. 
         FIG. 1B  is a graphical illustration of the doping profile in a linearly graded epitaxial layer. 
         FIG. 1C  is a graphical illustration of the doping profile in a double-profiled epitaxial layer. 
         FIG. 2  is a cross-sectional view of the active region of another prior art FRED. 
         FIG. 3  is a top view of an MPS according to the present invention with the top contact removed from the view for better illustration. 
         FIG. 4  is a cross-sectional view of  FIG. 3  along line  4 - 4  viewed in the direction of the arrows. 
         FIGS. 5 and 6  illustrate processing steps for manufacturing an MPS according to the present invention. 
         FIG. 7  shows a graphical illustration of the reverse avalanche capability of several embodiments of the present invention. 
         FIG. 8  shows a graphical illustration of the relationship between the spacing of the diffusion regions in an MPS according to the present invention and reverse avalanche energy. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 3 and 4 , where like numerals indicate like features, MIPS  26  according to a preferred embodiment of the present invention includes a plurality of P+ doped diffusion stripes  28  formed in N− doped silicon substrate  14 . Diffusion stripes  28  are spaced from one another by a distance “d” which may be increased to increase the ability of MPS  26  to absorb the reverse avalanche energy, and may be only 5 μm deep. First major electrode  18  forms a Schottky contact with epitaxial layer  12  that is exposed between stripes  28 , thereby forming an MIPS structure. 
     Referring now to  FIGS. 5-6 , MPS  26  ( FIG. 4 ) may be manufactured by first epitaxially growing an N− doped silicon layer  12  over an N+ doped silicon substrate  14 . Next, oxide layer  20  is either grown or deposited on N− doped epitaxial layer  12 . Multiple windows  19  are then opened in oxide layer  20  in a photolithographic step, and P+ doped diffusion stripes  28  are formed in the top surface of N doped epitaxial layer  12  by implanting dopants through windows  19 . Next, the oxide over P+ doped diffusions  28  is removed. 
     Subsequently, first major electrode  18  is deposited over the opening in oxide layer  20  to make contact with diffusion stripes  28 , epitaxial layer  12  in the spaces between diffusion stripes  28  and at least portions of silicon dioxide layer  20 . Second major electrode  22  is also deposited on silicon substrate  14  opposite to first major electrode  18  to obtain MIPS  26  as shown in  FIG. 4 . 
     According to an aspect of the present invention, diffusion stripes  28  may be formed at a relatively low temperature of about 1100° C. for a short time of about 6 hours, to a relatively shallow depth of about 5 μm, and spaced by a distance “d”. The distance “d” between diffusion stripes  28  may be increased as desired to improve the ability of the device to absorb the reverse avalanche energy in MPS  26 . 
     In the preferred embodiment, edges of the opening in oxide layer  20  may lie over the outermost stripes  28 . 
     Also, the thickness and the doping concentration of epitaxial layer  12 , as well as, the distance between diffusion stripes  28  may be varied to obtain various embodiments of MPS  26  according to the present invention. 
     MIPS  26 , according to the first embodiment, includes diffusion stripes  28  spaced 8 μm apart, the second embodiment includes diffusion stripes  28  spaced 12 μm apart and the third embodiment includes diffusion stripes  28  that are spaced 18 μm apart. Each embodiment may have an epitaxial layer  12  which is 30 μm thick and is doped to have a resistivity of about 12 ohms/cm or an epitaxial layer  12  which is 30 μm thick and is doped to have a resistivity of about 11 ohms/cm. 
     Referring to  FIG. 7 , it is shown that as distance “d” between diffusion stripes  28  is increased MPS  26  becomes more capable of absorbing reverse avalanche energy. For example, as distance “d” is increased from about 8 μm to about 18 μm, reverse avalanche energy of MIPS  26  is increased from about 7.5 mJ to about 37.5 mJ. This is a comparable avalanche energy to prior art devices with deep p-well ( FIG. 2 ) which may exhibit an avalanche energy of about 50 mJ (n12A). 
     Referring to  FIG. 8 , it is shown that as distance “d” between diffusion stripes  28  is increased MPS  26  can absorb a larger avalanche current and thus a larger reverse avalanche energy. 
     Other experiments have shown that a device according to the present invention exhibits a slightly higher leakage current than a comparable prior art device. The increase in the leakage current was found, however, to be within acceptable design limits. Also, a device according to the present invention was found to exhibit a breakdown voltage capability similar to prior art devices, reverse recovery comparable to prior art devices, and a forward voltage drop comparable to prior art devices. However, a device according to the present invention was advantageously manufactured to have diffusion stripes that extend to a lower depth than the diffusion well in a prior art device ( FIG. 2 ) of comparable characteristics. 
     Thus, a device according to the present invention can be manufactured using lower diffusion temperature and/or lower drive-in time for boron (P+ type diffusion) drive-in at a lower cost, compared to a functionally comparable prior art device. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.