Abstract:
A trench-gated power MOSFET contains a highly doped region in the body region which forms a PN junction diode with the drain at the center of the MOSFET cell. This diode has an avalanche breakdown voltage which is lower than the breakdown voltage of the drain-body junction near to the wall of the trench. Thus the MOSFET breaks down in the center of the cell avoiding the generation of hot carriers that could damage the gate oxide layer. The drain-body junction is located at a level which is above the bottom of the trench, thereby avoiding any deep diffusion that would increase the cell width and reduce the cell packing density. This compact structure is achieved by limiting the thermal budget to which the device is exposed after the body region is implanted. As a result, the body and its highly doped region do not diffuse significantly, and dopant from the highly doped region does not get into the channel region of the device so as to increase its threshold voltage.

Description:
FIELD OF THE INVENTION 
     This invention relates to power MOSFETs and in particular to a power MOSFET which has a greater cell packing density and therefore a lower on-resistance but without sacrificing the ability of the MOSFET to resist punchthrough breakdown. 
     BACKGROUND OF THE INVENTION 
     The design of a power MOSFET entails a number of objectives, many of which are in conflict. Several important objectives are: to minimize the resistance of the device when it is turned on, frequently referred to as the “on-resistance”; with trench-gated MOSFETs, to protect the corners of the trench against high electric fields that can generate hot carriers and damage the gate oxide layer; to minimize the threshold voltage necessary to turn the MOSFET on; and to maximize the resistance of the device to punchthrough breakdown across its channel region. 
     U.S. Pat. No. 5,072,266 to Bulucea et al. teaches the formation of a deep body diffusion in the center of the MOSFET cell to protect the corners of the trench against high electric fields and hot carrier generation. An example of such a MOSFET is shown in FIG. 1, which shows a trench-gated MOSFET  10  including a gate  11 , an N+ source region  12 , a P-body  13 , and a drain  14  which includes an N+ substrate  15  and an N-epitaxial layer  16 . N+ source region is contacted by a metal layer  17  and drain  14  is contacted by a metal layer  18 . In accordance with the teachings of the Bulucea et al. patent, a deep P+ diffusion  19  is formed in the center of the MOSFET cell. Deep P+ diffusion  19  is formed by implanting P-type dopant through the surface of the epitaxial layer  16  and heating the device to cause the dopant to diffuse downward to a level below the floor of the gate trench. The presence of the deep P+ diffusion causes the device to break down in the bulk silicon at the center of the MOSFET cell. 
     While the deep P+ diffusion does help to prevent hot carrier generation near the gate, it tends to limit the width W of the MOSFET cell and therefore the number of cells that can be formed within a given area of the chip This in turn limits the on-resistance of the device, because the total cell perimeter available to conduct current generally increases with the cell density. Conversely, if W is made too small, an excessive amount of P-type dopant gets into the channel region  13 A during the diffusion process, and this increases the threshold voltage of the device. 
     Accordingly, there is a need for a power MOSFET which is not vulnerable to hot carrier injection and yet allows a greater packing density so as to reduce its on-resistance. 
     SUMMARY OF THE INVENTION 
     A power MOSFET in accordance with this invention includes a semiconductor substrate of a first conductivity type and an epitaxial layer formed on a surface of the substrate, the epitaxial layer including a portion in contact with the substrate. The doping concentration of the portion of the epitaxial layer is lighter than the doping concentration of the Substrate. A trench is formed at a surface of the epitaxial layer, the trench extending, into the epitaxial layer and having a plurality of section s which define a MOSFET cell. A gate is formed in the trench. 
     A source region of the first conductivity type is located adjacent a sidewall of the trench at a surface of the epitaxial layer in the MOSFET cell. A body of a second conductivity type adjoins the source region in the MOSFET cell, the body comprising a channel region adjacent the sidewall of the trench. A drain of the first conductivity type forms a first PN junction with the body, the body being located above the first PN junction, the drain being located below the first PN junction. The entire first PN junction is located at a level above a bottom of the trench, a portion of the first PN junction near the sidewall of the trench having a first breakdown voltage. 
     A heavily-doped region of the second conductivity type is formed within the body at a central region of the MOSFET cell, the dopant concentration of the heavily-doped region being greater than the doping concentration of the body, such that a diode comprising a portion of a second PN junction at the central region of the MOSFET has a second breakdown voltage, the second breakdown voltage being lower than the first breakdown voltage. As a result, avalanche breakdown takes place at the central region of the MOSFET cell rather than near the sidewall of the trench where hot carriers could cause damage to the gate oxide layer. The lower boundary of the heavily-doped region preferably extends to a level that is below the bottom junction of the body region but above the bottom of the trench. 
     Another aspect of the invention includes a method of fabricating a power MOSFET. The method comprises providing a semiconductor substrate of a first conductivity type; growing an epitaxial layer of the first conductivity on a surface of the substrate; forming a trench in the epitaxial layer, the trench defining a cell of the MOSFET; forming a first insulating layer on a surface of the trench; forming a gate in the trench, the gate being separated from the epitaxial layer by the insulating layer; implanting dopant of a second conductivity type into the epitaxial layer to form a body, a lower boundary of the body forming a first PN junction with a portion of the epitaxial layer of the first conductivity type; implanting dopant of the first conductivity type into the epitaxial layer to form a source region; depositing a second insulating layer over the epitaxial layer; forming an opening in the second insulating layer to expose at least a portion of the source region; implanting additional dopant of the second conductivity type into the epitaxial layer to form a heavily doped region, the heavily doped region being spaced apart from a wall of the trench and forming a second PN junction with the portion of the epitaxial layer of the first conductivity type; depositing a metal layer over the second insulating layer such that the metal layer contacts the portion of the source region; and limiting the thermal energy to which the power MOSFET is exposed following the implantation of the body such that the body does not diffuse substantially. As a result, the body remains quite compact and a high cell packing density can be obtained. 
     In one embodiment, the thermal budget following the formation of the first insulating layer through the deposition of the metal layer is less than or equal to the equivalent of 950° C. for 60 minutes. In another embodiment, the thermal budget following the formation of the first insulating layer through the deposition of the metal layer is less than or equal to the equivalent of 900° C. for 50 minutes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional view of a trench-gated MOSFET in accordance with U.S. Pat. No. 5,072,266 to Bulucea et al. 
     FIG. 2A shows a cross-sectional view of a trench-gated MOSFET in accordance with this invention, including a heavily doped region at the center of the MOSFET cell. 
     FIG. 2B shows a detailed view of the body region of the MOSFET shown in FIG. 2A wherein the deepest portion of the P+ region coincides with the body-drain junction. 
     FIG. 2C shows a detailed view of the body region of a MOSFET according to the invention wherein the deepest portion of the P+ region is located below the body-drain junction. 
     FIG. 2D shows a detailed view of the body region of a MOSFET according to the invention wherein the deepest portion of the P+ region is located above the body-drain junction. 
     FIGS. 3A-3G illustrate the steps of a process that can be used to fabricate a MOSFET in accordance with this invention. 
     FIGS. 4A and 4B are graphs showing the doping profile in a vertical cross-section through the mesa of a MOSFET fabricated in accordance with this invention. 
     FIGS. 4C and 4D are graphs showing the doping profile in a vertical cross-section through the mesa of a MOSFET fabricated by a conventional process. 
    
    
     DESCRIPTION OF THE INVENTION 
     A cross-sectional view of a trench-gated power MOSFET in accordance with this invention is shown in FIG.  2 A. MOSFET  20  is formed in an N-type epitaxial (“epi”) layer  202 , which is grown on an N+ substrate  204 . A polysilicon gate  206  is located in a trench  208  and is isolated from the N-epi layer  202  by a gate oxide layer  210 . A single MOSFET cell  212  is located between opposing segments of gate  206 , it being understood that a typical power MOSFET would include a large number of MOSFET cells arrayed in a lattice-like arrangement. The individual cells could be closed geometric figures such as squares or hexagons or could be longitudinal stripes. The sections of the trench  208  define a mesa  209  which is typically from 1 μm to 4 μm wide. French  208  is typically from 0.7 μm to 1.6 μm deep. 
     The N-epi layer  202  and N+ substrate  204  together form a drain region  214  of MOSFET  20 , which is contacted by a metal layer (not shown) on the bottom of the N+ substrate  204 . MOSFET cell  212  includes an N+ source region  216  and a P-body  218 . N+ source region  216  is contacted by a metal layer  220  through openings in a borophosphosilicate glass (BPSG) layer  221 . P-body  218  is contacted in a region outside the plane of the drawing. N+ source region  216  and P-body  218  are normally shorted together to prevent the parasitic bipolar transistor represented by drain region  214 , P-body  218  and N+ source region  216  from becoming active. 
     Also shown in FIG. 2A is a termination region  240 , which includes a polysilicon field plate  242  formed over an oxide layer  244 . Termination region  240  also includes a P-region  246 . As shown, field plate  242  is contacted by metal layer  220  through an opening in BPSG layer  221 . 
     In accordance with this invention, P-body  218  includes a relatively heavily doped P+ region  222 . In the embodiments shown in FIGS. 2A and 2B, P+ region  222  extends all the way to meet a substantially planar horizontal PN junction  217  between P-body  218  and N-epi layer  202 . In other embodiments, P+ region  292  extends downward to a level below PN junction  217  (as shown in FIG.  2 C), but in most embodiments P+ region  218  does not extend to a level below the bottom of the trench  208 . In some embodiments, as shown in FIG. 2D, the lower extremity of P+ region may be at a level above PN junction. In all embodiments, P+ region  222  is doped to a level such that a diode is formed in the center of MOSFET cell  212 , the anode of the diode being represented by P+ region  222  and the cathode of the diode being represented by the N-epi layer  202 . The central diode in MOSFET cell  212  has a breakdown voltage which is lower that the breakdown voltage of the PN junction  217  nearer the walls of the trench  208 . Thus avalanche breakdown occurs in MOSFET  20  at the center of MOSFET cell  212 , at a location where damage to the gate oxide layer  210  is avoided. At the same time, there is no deep diffusion in MOSFET cell  212  which limits the width W of MOSFET cell  212   
     Body region  218  is shown in detail in FIG.  2 B. Because of the concentration of dopant in P+ region  222 , the breakdown voltage of diode D 1  at the center of MOSFET cell  212  is lower than the breakdown voltage of diode D 2  near the wall of trench  208 . Thus diode D 1  breaks down before diode D 2  and prevents the generation of hot carriers near trench  208  and gate oxide layer  210 . 
     FIGS. 3A-3G illustrate the steps of a process for forming MOSFET  20 . 
     The process begins with the formation of N-epi layer  202  on a surface of N+ substrate  204 , as shown in FIG.  3 A. This step is performed by a known process of epitaxial growth. N-epi layer  202  may be, for example 4 μm thick and may be doped with N-type impurity to a concentration of 3.5×10 16  cm −3  in the case of a 30V device. 
     Oxide layer  244  is grown on the top surface of N-epi layer  202  and, by a known photolithographic process employing a first mask, a hole  304  is formed in oxide layer  244 . A P-type impurity such as boron is implanted into N-epi layer  202  through hole  304  to form P region  246 . This implant may be carried out, for example, at a dose of 1×10 14  cm −2  and an energy of 60 keV. 
     P region  246  then driven-in at, for example, a temperature of 950° C. for 180 minutes in a wet atmosphere. P region  246  diffuses both laterally and vertically as shown in FIG.  3 B. 
     A second photolithographic mask (not shown) is then formed, exposing what is to be the active area of the device, and oxide layer  244  is etched as shown in FIG. 3C. A third, trench mask  304  is then formed, with gaps  306  where the trench  208  is to be located, also as shown in FIG.  3 C. 
     The trench  208  is etched in N-epi layer  202  through mask  304 , using a reactive ion etch (RIE) process. Trench  208  can be 0.8 μm wide and 0.9 μm deep, for example. A sacrificial oxide layer (not shown) can be grown on the exposed surfaces of N-epi layer  202  to repair the damage caused by the RIE etch. The sacrificial oxide layer is removed, and gate oxide layer  306  is grown to a thickness of, for example, 600 Angstroms on the exposed surfaces of N-epi layer  202 , including in particular the sidewalls and floor of trench  208 , as shown in FIG.  3 D. 
     A polysilicon layer  308 , which can be 1 μm thick, is deposited over the top surface of the structure, and a fourth mask (not shown) is formed on the surface of polysilicon layer  308 . Polysilicon layer  308  is then etched, leaving gate  206  and field plate  242 , as shown in FIG.  3 E. The top surface of gate  206  is roughly coplanar with the top surface of N-epi layer  202 . 
     As shown in FIG. 3F, P-body  218  is formed by implanting P-type impurity, for example boron, into N-epi layer  202 . This is preferably accomplished using two implant steps: for example, the first at a dose of 6×10 12  cm −2  and an energy of 80 keV, and the second at a dose of 6×10 12  cm −2  and an energy of 150 keV. As shown in FIG. 3F, because the implantation goes into the entire region between the portions of trench  208 , a PN junction  217  between P-body  218  and N-epi layer is substantially horizontal and planar. 
     Next a fifth photolithographic mask (not shown) is formed, blocking the areas where the P-body is to be contacted in the third dimension with respect to the plane of FIGS. 3A-3G. An N-type dopant such as arsenic is implanted, for example at a dose of 8×10 15  cm −2  and an energy of 80 keV, to form N+ source region  216 . The fifth photolithographic mask is then removed. 
     BPSG layer  221  is then deposited and allowed to flow at, for example, 875° C. for 25 minutes. A sixth, contact mask (not shown) is formed on BPSG layer  221  with holes where contact will be made to N+ source region  216 , field plate  242 , and P-body  218 . BPSG layer  221  and gate oxide layer  306  are etched through the holes in the sixth mask, forming, an opening  310  to N+ source region, an opening  312  to field plate  242 , and an opening (not shown) to P-body  218 . Openings  310  and  312  are shown in dashed lines in FIG.  3 G. 
     A P-type impurity such as boron is then implanted through these openings to form P+ region  222 . This can be done in a two-stage process, the first stage being, performed at a dose of 1×10 15  cm −2  and an energy of 60 keV and the second stage being performed at a dose of 1×10 15  cm −2  and an energy of 120 keV. Alternatively, the first stage can be performed at a dose of 2×10 15  cm −2  and an energy of 60 keV and the second stage can be performed at a dose of 3×10 14  cm −2  and an energy of 120 keV, or the boron can be implanted in a one-stage process at a dose of 2×10 15  cm −2  and an energy of 100 keV. Because BPSG layer  221  overlaps the edges of N+ source region  216  near the sidewalls of trench  208 , the P-type impurity is confined to a central region of the MOSFET cell  212 . The energy of the implant can be adjusted to insure that the deepest portion of P+ region  222 , coincides with the planar PN junction  217  between P-body region  218  and N-epi layer  202  (as shown in FIGS.  2 A and  2 B), is located below the PN junction  217  (as shown in FIG.  2 C), or is located above the PN junction (as shown in FIG.  2 D). 
     Next, BPSG layer  221  is reflowed at, for example, 900° C. for 30 minutes and metal layer  220 , typically aluminum, is deposited to a thickness of, for example, 2.8 μm. A seventh, metal mask is formed over the surface of metal layer  220 , and metal layer  220  is etched in a conventional manner to form a source-body bus. 
     The result of the process is MOSFET  20  shown in FIGS. 2A-2D. 
     It is important that, following the implantation of P-body  218 , the “thermal budget” of the process be limited such that P-body  218  does not diffuse appreciably but instead remains localized in the “mesa” between the sections of trench  208 . After the implantation of P-body  218 , the junction of P-body  218  and N-epi layer  202  should not move more than about 0.3 μm, and preferably not more than about 0.1 μm. For example, the total “thermal budget” to which the structure is exposed following the implantation of P-body  218  should be limited to an amount that is less than or equal to the equivalent of 1050° C. for 30 minutes and preferably less than 950° C. for 60 minutes. Alternatively, the structure could be given a rapid thermal anneal (RTA) at about 1100° C. for about 120 seconds. This thermal budget includes the heating required to flow and reflow BPSG layer  221 , as described above. 
     FIGS. 4A and 4B show dopant profiles taken at a vertical cross-section of the mesa in a device fabricated in accordance with this invention. FIG. 4A shows the dopant profile immediately following the implantation of the P-body. FIG. 4B shows the dopant profile after the source has been implanted and both implanted dopants have been made electrically active during the BPSG flow and reflow anneals. Note the twin peaks in FIG. 4A from the two-stage implant. As indicated by a comparison of FIGS. 4A and 4B the junction between the P-body and the N-type drain moved from a level about 0.55 μm below the top surface of the mesa to a level about 0.65 μm below the surface of the mesa, i.e., about 0.1 μm. The peak doping concentration in the P-body was about 1×10 17  cm −3 . By comparison, FIGS. 4C and 4D show the dopant profiles in a conventional MOSFET after implantation of the P-body and in the finished device, respectively. Owing to the conventional diffusion of the body, the body-drain junction moved almost 1.0 μm deeper into the mesa, and the peak dopant concentration in the body was about 5×10 16  cm −3 . 
     The process described above is suitable for fabricating a 30V MOSFET. MOSFETs with different voltage ratings can be fabricated by varying the parameters of the process, in particular the thickness of N-epi layer  202  and gate oxide layer  210  and the doping concentration of P-body  218 . Table 1 gives typical process parameters for 20V, 30V and 60V devices, including the gate oxide thickness T ox , the N-epi thickness T epi , the N-epi doping concentration N epi , and the implant dose and energy required to create the P-body and avoid punchthrough breakdown. In each case the P-body is doped in a two-stage process to implant enough charge into the P-body region. The implant doses (D 1 ,D 2 ) and energies (E 1 ,E 2 ) for both of the stages is given. 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 N epi   
                 Dose 1   
                 E 1   
                 Dose 2   
                 E 2   
               
               
                 Rating 
                 T ox   
                 T epi   
                 (cm −3 ) 
                 (cm −2 ) 
                 (keV) 
                 (cm −2 ) 
                 (keV) 
               
               
                   
               
             
             
               
                 20 V 
                 500 A 
                 3.5 μm 
                   8E16 
                 1E13 
                 80 
                 1E13 
                 150 
               
               
                 30 V 
                 600 A 
                   4 μm 
                 3.5E16 
                 6E12 
                 80 
                 6E12 
                 150 
               
               
                 60 V 
                 700 A 
                 5.5 μm 
                   9E15 
                 4E12 
                 80 
                 4E12 
                 150 
               
               
                   
               
             
          
         
       
     
     As noted above, a diode is formed at the center of the MOSFET cell. Dopant is implanted into the body to ensure that the central diode (D 1 ) has an avalanche breakdown voltage lower than the avalanche breakdown voltage of the body-drain junction near the sidewall of the trench. In the example described above, the source contact opening in the BPSG layer is used for implanting the dopant but this need not be the case. Other masks or layers may be used to localize the dopant in the central area of the MOSFET cell. 
     The breakdown voltage of diode D 1  is a function of the doping concentration and gradient of P+ region  222  at its junction with N-epi layer  202  at the center of MOSFET cell  212 . Similarly, the breakdown voltage of diode D 2  is a function of the doping concentration and gradient of the P-body  218  at its junction with N-epi layer  202  near gate trench  208  and includes the effects of the trench corners. These relationships are well known and are available from many sources, such as Sze,  Physics of semiconductor Devices , 2nd Ed., Wiley Interscience (1981), p. 101 (FIG.  26 ), which is incorporated herein by reference. The depth of the trench  208  should not exceed by too much the depth of the P-body junction. In a preferred embodiment, the trench  208  is deeper than the P-body  218  by about 0.3 μm. 
     The embodiment described above is illustrative only and not limiting. Many other embodiments in accordance with this invention will be apparent to those skilled in the art from the description above.