Abstract:
A method of fabricating a N+/P+ zener diode where the reverse breakdown occurs in a controlled, and uniform manner leading to improved speed of operation and increase in current handling capability.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to an N+/P+ zener diode where the implanted regions are designed to steer the current flow away from the sidewalls of the diode and more toward the bottom walls in order to induce uniform reverse breakdown leading to improved speed of operation and increase in current handling capability. 
       BACKGROUND OF THE INVENTION 
       [0002]    MOS devices are susceptible to damage from electrostatic discharge, or ESD. While numerous techniques have been developed to protect MOS devices, there has been a need for an ESD-protection device and method which could be fabricated through simple semiconductor manufacturing techniques. 
         [0003]    One such known ESD-protection device is illustrated in  FIGS. 1 and 2 . As shown in  FIG. 1 , a conventional P type substrate  100  is provided, with an epitaxial layer  110  of P-material formed thereon in a conventional manner. Using CMOS fabrication techniques, a P type implant  120  is formed into and through the epitaxial layer  110  until the implanted region electrically contacts the substrate  100 . Using conventional techniques, an N+ deposition  130  is formed within the P-implanted region  120 . By reverse biasing the junction of the N+ implant  130  and the P implanted region, a depletion layer  140  is formed, which is represented electrically as a capacitor  210  in parallel with the N+ IP zener diode  220  in  FIG. 2 . The composite structure protects the internal circuitry from ESD discharge by providing a low resistance path to ground during an ESD event. 
         [0004]    While the device of  FIGS. 1 and 2  has advantages, when breakdown of the zener diode  220  occurs, the current is distributed over the entire interface of the N+ implant  130  and the sinker region  120 , as shown by the arrows in  FIG. 1 , and breakdown typically begins at the sidewalls of the N+ implant  130 , and not the bottom of the N+ implant  130 . As such, with this breakdown profile, it takes longer for the zener device  220  to completely turn on, and does not provide a low resistance path to ground. 
         [0005]    In U.S. Pat. No. 4,758,537, there exists a P− region  11  that will prevent lateral breakdown over an upper sidewall portion of the N++ region  11  as shown in  FIG. 2  of that patent, but the P− region  11  will not prevent lateral breakdown of a lower sidewall portion of the N++ region  11 , and as a result substantial lateral breakdown occurs. 
         [0006]    The present invention attempts to provide a zener diode where the breakdown current is steered uniformly through the bottom wall of the diode in order to provider higher current handling and improved speed of operation. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention relates to an N+/P+ zener diode where the implanted regions are designed to steer the current flow away from the sidewalls of the diode and towards the bottom walls in order to induce uniform reverse breakdown, thereby leading to improved speed of operation and increase in current handling capability. 
         [0008]    In one aspect, the present invention provides a method of operating a zener diode by initiating vertical breakdown of the zener diode between an implant region of one conductivity type and an implant region of an opposite conductivity type; and during the step of initiating vertical breakdown, inhibiting lateral breakdown of the zener diode between a sidewall of the implant region and an adjacent region. 
         [0009]    In another aspect, the present invention provides a zener diode that has a substrate of one conductivity type; a sinker dopant region of the same conductivity type as the substrate, disposed above and electrically connected to the substrate; a dopant region disposed above the sinker dopant region, the dopant region having an opposite conductivity type as the substrate and the sinker dopant region, the dopant region further having sidewalls and a bottom, with the bottom contacting the sinker dopant region; and an epitaxial region, the epitaxial region surrounding the dopant region, thereby being adjacent to all sidewalls of the dopant region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0011]      FIG. 1  illustrates in cross-sectional side view a prior art design and its electrical representation. 
           [0012]      FIG. 2  illustrates a circuit diagram of the  FIG. 1  design. 
           [0013]      FIG. 3  illustrates in cross-sectional view of an embodiment of the present invention. 
           [0014]      FIGS. 4(   a )-( e ) illustrate a flow diagram of the fabrication steps for forming the structure of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring first to  FIG. 3 , a first implementation of the invention is illustrated. The substrate  300  is formed of P+ doped material. Although this first implementation is described with respect to a P+ substrate, and layers corresponding thereto above this P+ substrate  300 , it will be understood that the present invention can be implemented with an N+ substrate, and corresponding layers above, as is known in the art. A P− epitaxial layer  330  is grown over the P+ substrate. Within the epitaxial layer, a P+ sinker region  310  is created. Over a central region  310 A of the sinker layer there is an N+ implant region  320 . Surrounding the N+ implant  320 , and extending below a bottom surface  322  of the N+ implant  320 , is the P− epitaxial region  330  As shown, the portion of the P− epitaxial region adjacent to the N+ implant region has a width of X+Y, where the values of X and Y are determined based on the implant conditions, total thermal out diffusion, and photolithographic mask bias. In a typical arrangement the value of X can range from 0 to 5 um and the value of Y can range between 2 um to 20 um. 
         [0016]    Contacts are made to the N+ and the P+ regions using standard semiconductor processing methods consisting of deposition and patterning of a dielectric film followed by etch, metal deposition and patterning. The metal  340  contact to the N+ layer serves as the anode of the device. The metal  350  contact to the P+ layer serves as the cathode of the device. 
         [0017]    Due to the existence of the P+ sinker  310 A and the P− epitaxy  330  that surrounds the N+ implant  320 , the reverse breakdown will occur vertically, and only from the bottom surface of the N+ implant  320  that interfaces with a top surface of the central region  310 A of the P+ sinker  310 . This is schematically illustrated by the vertical arrows. 
         [0018]      FIGS. 4(   a )- 4   e ) illustrate fabrication steps for the device illustrated in  FIG. 3 . It is understood that the overall process steps are described, and that one of ordinary skill will understand certain specific steps needed in order to execute them. 
         [0019]      FIG. 4(   a ) illustrates a starting point, in which a P− epitaxial layer  330  has already been grown over a P+ substrate  300 . Next, in  FIG. 4(   b ), there is shown a mask  520  that is used so that a P+ sinker regions  310  can be implanted into the P− epitaxial layer  330 . After implantation and thermal drive-in the resulting structure is shown in  FIG. 4(   c ). The P+ sinker regions  310  and the P+ substrate  300  out diffuse and connect to each other, leaving the P− epitaxial region  330 , which surrounds the central sinker region  310 A. 
         [0020]      FIG. 4(   d ) illustrates formation of a mask layer  540 , which is then used to allow for the selective implantation of N+ region  320 , which through annealing is then driven to the appropriate depth, so that the bottom of the N+ region  320  contacts the P+ sinker region  310 A. 
         [0021]      FIG. 4(   e ) illustrates the formation of the electrical connections, with the N+ and P+ regions forming the anode and the cathode 
         [0022]    Thicknesses and doping of various layers described above can vary, as well as temperature and times for the annealing processes. In a specific embodiment that has been found advantageous, the P+ substrate is 8 to 15 mohm-cm in resistivity, the P-epi layer is 4 to 14 um thick with a typical resistivity of 10 ohm-cm. The concentration of the boron in the P+ layer is approximately between 1E18/cm3 to 7E18/cm3. The corresponding peak doping of the dopants in the N+ region is in between 1E19/cm3 to 1E20/cm3. 
         [0023]    It will be appreciated from the foregoing that the structure of  FIG. 3  can be fabricated using a simple and inexpensive process sequence, making the fabrication of the invention attractive for numerous applications 
         [0024]    The breakdown voltage of the Zener diode, can be modified by adjusting the concentration of the N+ region  320  and the P+ type sinker  310 A. By providing low series resistance, the device can sink high currents during an ESD event, thus protecting the circuit connected to this device. 
         [0025]    Having fully described a preferred embodiment of the invention and various alternatives, those skilled in the art will recognize, given the teachings herein, that numerous alternatives and equivalents exist which do not depart from the invention. It is therefore intended that the invention not be limited by the foregoing description, but only by the appended claims.