Abstract:
A fast recovery rectifier structure with the combination of Schottky structure to relief the minority carriers during the forward bias condition for the further reduction of the reverse recovery time during switching in addition to the lifetime killer such as Pt, Au, and/or irradiation. This fast recovery rectifier uses unpolished substrates and thick impurity diffusion for low cost production. A reduced p-n junction structure with a heavily doped film is provided to terminate and shorten the p-n junction space charge region. This reduced p-n junction with less total charge in the p-n junction to further improve the reverse recovery time. This reduced p-n junction can be used alone, with the traditional lifetime killer method, with the Schottky structure and/or with the epitaxial substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/799,252 filed May 10, 2006 and entitled “Fast Recovery Rectifiers”. The provisional application is herein incorporated by reference. 
     
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to the structures of the fast recovery rectifiers. This invention disclosed the method of reducing the reverse recovery time from the conventional fast recovery rectifiers by either introducing the Schottky structure at the front and/or the back of the substrate or introducing the junction termination layer to reduce the total space charge of the p-n junction. Furthermore, this invention also provides the low cost manufacturing of the very fast recovery rectifiers. 
         [0004]    2. Description of Related Art 
         [0005]      FIG. 1  shows the cross section of the conventional fast recovery rectifier. The doped substrate  100  is made by either n type or p type material. The doped substrate  100  used is a wafer with sawed or unpolished rough surface treatment for cost savings. The polish of the wafer surface is not required in this case. Doping of wafer can be done by either film method or by standard deep diffusion method. The p-n junction  103  be formed on the doped substrate  100 , and the depth of the p-n junction  103  is usually from less than 10 microns to over 20 microns so that the depth of the p-n junction  103  is deeper than the surface damaged region of the rough surface. Then the lifetime killer such as Pt, Au, etc. is added to the wafer with thermal treatment. The reverse recovery time is depending on doping concentration, the thermal treatment temperature, and time of the lifetime killer species. The other method for the lifetime reduction can also be done by irradiation of electrons or other species. After the irradiation treatment, the substrate is being processed by annealing. 
         [0006]    The diffusion to the doped substrate  100  is generally using opposite polarity species at the p-n junction  103 . The depth of this p-n junction  103  must be deeper than the surface damage region for the reduction of the leakage current. The diffusion to the doped substrate  100  is generally using the same polarity at the backside layer  101 . The passivation layer  104  can be formed after the formation of the p-n junction  103  as well as the life time killer process. The irradiation of the electrons and or other species can be done after the process of the wafer or even in the packaged parts. The passivation layer  104  can be done by conventional glass passivation process or multiple CVD process. The top metal layer  105  is, deposited, evaporated, sputtered, or plated alone with the top of the p-n junction  103 . The bottom layer  105 B metallization is to be done by similar metallization process or by nickel plating. After the completion of the process, the wafer is diced into chips for assembly. 
         [0007]      FIG. 2  shows the cross section of epitaxial based fast recovery rectifier in prior art. The epitaxial layer  102  of the same polarity is grown on the heavily doped substrate  100 . The doping concentration and the thickness of the epitaxial layer  102  are determined by the breakdown voltage. The epitaxial layer  102  can be made by single or multiple layers. The p-n junction  103  anode diffusion can be done by either ion implantation or diffusion method of the opposite polarity of the epitaxial layer  102 . After the formation of the p-n junction  103  anode diffusion, the life time killer such as Pt, Au or other species with proper thermal treatment can be added to the wafer. After the p-n junction diffusion, the deep etched structure is to be done by wet etch prior to the passivation process. The passivation layer  104  is done either by the conventional glass passivation or multiple CVD layers method. The top metal layer  105  is then opened for the metallization. The top metal layer  105  can be done by the contact metallization by either Ti—TiN—Al, TiNiAg or Nickel plating for either wire bond or soldering. The backside layer  101  can be done by the implantation of similar polarity of the silicon doped substrate  100  or omitted if the doped substrate  100  is heavily doped. The bottom layer  105 B metallation is done either by Ti—Ni—Ag or Cr—Au or by Ni plating. After the completion of the process, the wafer is then ready for the dicing and assembly. 
         [0008]    U.S. Pat. No. 6,261,874, Francis and Ng disclosed the fast recover diode structure that conjunction of Beam Radiation defects and He implanted defects to reduce the reverse recovery time. With this structure, the soft recovery time can be resulted. U.S. Pat. No. 6,486,524 Ahmed disclosed the complicated structure using p-n junction as well as the Al or Pd Schottky for the fast reverse recovery time. U.S. Pat. No. 6,603,153, Francis and Ng disclosed the fast recovery rectifier structure that is similar U.S. Pat. No. 6,261,875, U.S. Pat. No. 6,699,755 Bol disclosed a fast recovery diode structure similar to U.S. Pat. No. 6,486,524. U.S. Pat. No. 6,870,199 Yoshikawa et al disclosed the multiple life time control region for the improvement of di/dt caused breakdown. U.S. Pat. No. 6,927,141 Andoh et al disclosed the termination structure by using equal metal ring. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the present invention to provide a low cost and the improvement of the process and device structures. By using the common fast recovery rectifier using glass and/or CVD process for the termination structure is the basic structure to begin with. The substrate is an as sawed or non-polished substrate instead of polished wafers for the reduction of the cost. Deep diffusion of opposite polarity to the substrate in one side and the same polarity in the other side are performed. The depth of the diffusion is from less than 10 micron to over 20 microns depending on the surface damage region of the unpolished wafer. This invention added an etched region with either wet etches or wet/dry etches for the provision of the Schottky contact. This etched region can be a round shape, hexagon, stripe or other shapes. The metallization is then provided for the ohmic contact for the heavily doped rough surface and Schottky contact of the etched smooth region. The purpose of the Schottky contact is to absorb the minority carrier injections at the forward bias, thus the reverse recovery time is reduced. The similar structure at the other side can also be added for the reduction of the minority carriers. Thus the reduction of the reverse recovery time can be further reduced. In order to improve the reverse recovery time, the conventional method is using the epitaxial wafers and then using life time killing method. The heavily doped substrate is used to decrease of the forward voltage drop due to series resistance and in the mean time to reduce the lifetime by reducing the thickness of the active lightly doped epitaxial region. The second objective of this invention is to form a reduced p-n junction space charge region by using a thin and very high doped film of the same polarity as the top junction layer which is opposite polarity of the base silicon substrate. This early termination of the junction charge region reduces the total space charge of the p-n junction thus the smaller reverse recovery time can be achieved. This method can be used in conjunction with the life time killers and/or with the Schottky structures. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows the cross section of the conventional fast recovery rectifier. 
           [0011]      FIG. 2  shows the cross section of epitaxial based fast recovery rectifier in prior art. 
           [0012]      FIG. 3  is a cross-sectional view of the present invention fast recovery rectifier. 
           [0013]      FIG. 4  disclosed a cross section similar to  FIG. 3  except the back side of the wafer is also etched like the top surface. 
           [0014]      FIG. 5  disclosed a cross section of epitaxial based fast recovery rectifier. 
           [0015]      FIG. 6  shows a normal charge diagram of an n-p junction. 
           [0016]      FIG. 7  is smaller than the built-in potential of  FIG. 6 . 
           [0017]      FIG. 8  disclosed a cross section of epitaxial based fast recovery rectifier by using guard rings as the termination structure. 
           [0018]      FIG. 9  disclosed a cross Section of epitaxial based fast recovery rectifier by using guard rings as the termination structure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiment One 
       [0019]      FIG. 3  is a cross-sectional view of the present invention fast recovery rectifier that difference with the  FIG. 1  is in the etched structure  103 A. In addition, the etched structure  103 A of  FIG. 3  can be etched (preferred by wet etch) either in round, hexagon, strip or other shapes. The diameter or the width of the etched structure is generally larger than the twice of the p-n junction  103  depth. In general, it is over 20 microns. 
         [0020]    The p-n junction  103  is deposited, evaporated, sputtered, or plated alone with the top metal layer  105 . This layer  105  can be finished by contact metal and/or barrier metal and/or top metal for soldering or wire bonding. The backside layer  105 B is to be done by similar metallization process or by nickel plating. After the completion of the process, the wafer is diced into chips for assembly. 
       Embodiment Two 
       [0021]      FIG. 4  disclosed a cross section similar to  FIG. 3  except the back side of the wafer is also etched like the top surface. The purpose for this structure is to get the addition of the Schottky surface to absorb the minority carrier at the backside of the chip. The location of the etched structure  103 B is not necessary to be aligned with the top surface structure  103 A. The shape of the etched structure  103 B can be round, hexagon, stripe or other structures depending on the requirement. Both  FIG. 3  and  FIG. 4  of the present invention offer the low cost version of the fast recovery rectifiers. 
       Embodiment Three 
       [0022]      FIG. 5  disclosed a cross section of epitaxial based fast recovery. The epitaxial layer  102  of the same polarity is grown on the heavily doped substrate  100 . The doping concentration and the thickness of the epitaxial layer  102  are determined by the breakdown voltage. The epitaxial layer  102  can be made by single or multiple layers. The p-n junction anode diffusion  103  can be done by either ion implantation or diffusion method of the opposite polarity of the epitaxial layer  102  using silicon dioxide as the mask. For the ion implantation, the photo resist can be used in addition to the oxide layer as the mask. The schottky contact region  107  is the masked region of region. This masked schottky contact region  107  is used for the Schottky contact. 
         [0023]    In this embodiment, after the formation of the p-n junction anode layer, the life time killer such as Pt, Au or other species with proper thermal treatment can be added to the wafer. On the other way, after the top layer diffusion, the deep etched structure is to be done by etching, preferred by wet etch prior to the passivation process. The passivation layer  104  is done either by the conventional glass passivation or multiple CVD layers method. 
         [0024]    Next, the top metal layer  105  is then opened for the metallization. The top metal layer  105  can be done by the Schottky contact metallization by either Pt, Au, Ni, Mo, W, Cr, Ti etc. followed by the barrier metal layer such as TiW, TiN, and the top metal such as Al, Ag, etc. for either wire bond or soldering. The backside layer  101  can be done by the implantation of similar polarity of the silicon substrate or omitted if the substrate is heavily doped. The bottom layer  105 B is done either by Ti—Ni—Ag or Cr—Au or by Ni plating. After the completion of the process, the wafer is then ready for the dicing and assembly. 
         [0025]      FIG. 6  shows a normal charge diagram of an n-p junction. The total area of qNa×xp 1  at p region is equal to qNd×xn 1  at the n region. Where Na is the doping concentration of the p region and xp  1  is the charge distance of the p region. Using a heavily doped p layer to move the distance xp 2  of p region smaller, the total area of qNa×xp 2  is smaller than qNa×xp 1  with same doping level of Na. In order to balance the total charge, qNd×xn 2  is the same as qNa×xp 2 . Where Nd is the doping concentration of n region and xn is the charge distance of the n region. Thus the built-in potential of  FIG. 7  is smaller than the built-in potential of  FIG. 6 . In order to get good ohmic contact, this very thin layer of P++at xp 2  must be presented; the same is true for n-p junction. Reduced p-n junction is made by the following conditions: a) the doping concentration of p region of said reduced p-n junction is in the same magnitude from 2 to 10 times of the n region, this can be accomplished by lightly doped implant with low energy and dose, the energy of the implant is from 500 ev to 50 Kev with implant dose from 1.0E12 to 1.0E15 per cm 2 , light implant anneal is done by either RTA or furnace in inert ambient, or the very shallow p++ region is placed on the top of the p region and the implant dose is from 1.0E12 to 1.0E15 per cm 2  with the energy from 100V to 35 Kev, then implant anneal is done by RTA; the heavily doping concentration but very shallow p type region be used to terminate said p-n junction; b) used the heavily doping concentration but very shallow p type region, the heavily doping concentration but very shallow p type region may use low temperature p++type diffusion with the temperature from 700 deg. C. to 1100 deg C. from 60 seconds to one hour with furnace or RTA diffusion. said reduced p-n junction ranged from 0.5 ev. to 0.9 ev. 
         [0026]    The planner termination structure for high voltage guard ring either is using single or multiple guard rings for the epitaxial wafer, the epitaxial layer or multiple epitaxial layers is deposited on the similar polarity substrate, the implant dose of the opposite polarity region ranging from less than 1E10 per cm 2  to 1E16 per cm 2  with the implant energy from 100v to over 100 Kev and time 10 seconds to one hour with the temperature from 600 to 1100 deg C to form the p-n junction or the Schottky region by blocking the implant species, and the implant condition for the guard ring can be the same as the opposite polarity structure. 
       Embodiment Four 
       [0027]      FIG. 8  disclosed a cross section of epitaxial based fast recovery rectifier by using guard rings as the termination structure. The insulating layer  104 ′ is a thick oxide from 200A to over 2 microns. This insulating layer  104 ′ can be formed either by oxidation or CVD layers or both. The guard ring  106  structure is either by single guard ring or multiple guarding rings depending on the requirement of the reverse blocking voltage. The guard ring  106  can be formed either by diffusion or implant. The implant dose for the guard ring  106  is from less than 1E10 per cm 2  to over 1E15 per cm 2  and the implant energy from less than 100V to over 100 Kev depending on the design requirement. The implant dose of the diode regions  103 ′ is done from less than 1E10 per cm 2  to over 1E15 per cm 2  and the implant energy from less than 100V to over 100 Kev. This implant of diode regions  103 ′ and guard ring  106  is in the opposite polarity as the base material of epitaxial layer  102 . The diode termination layer  103 A′ is a very heavy doped region with the same polarity of diode region  103 ′. This diode termination layer  103 A′ can be formed either by the implant energy from 100V to 50 Kev and the dose from 1E11 per cm 2  to over 1E15 per cm 2  or by diffusion with temperature from 700 deg C to over 1100 deg C and the time from over one hour to less than 30 seconds. 
         [0028]    In this embodiment, the purpose of diode termination layer  103 A′ is to terminate the p region into the narrower space thus the total charge will be smaller. The lifetime killer such as Pt, Au, or can be added before or after the process. The top metal layer  105  can be either formed by Au, Pt, W, Mo Cr, Ni, Ti and other metal or by forming the silicide. 
         [0029]    Next, the diffusion barrier such as TiW, TiN or other layer before the contact layer of the 105 is deposited. The contact layer of the 105 can be either Al for wire bonding or Ag or Au for soldering. The backside layer  101  is implanted or diffused with the similar polarity of the doped substrate  100 . If the resistivity of the doped substrate  100  is lower enough, this layer  101  can be eliminated. The bottom layer  105 B can be used for the similar metallization process as the top metal layer  105  or by using convention method such as Cr—Au, Ti—Ni—Ag or even with Nickel plating. 
       Embodiment Five 
       [0030]      FIG. 9  disclosed a cross Section of epitaxial based fast recovery rectifier by using guard rings as the termination structure. The insulation layer  104 ′ is a thick oxide from 200A to over 2 microns. The guard ring  106  structure is either by single guard ring or multiple guarding rings depending on the requirement of the reverse blocking voltage. The guard ring  106  can be formed either by diffusion or implant. The implant dose for the guard ring  106  is from less than 1E10 per cm 2  to over 1E15 per cm 2  and the implant energy from less than 100V to over 100 Kev depending on the design requirement. The implant species is in the opposite polarity as the epitaxial layer  102 . The Schottky contact region  107  can be either in round, hexagon, stripe or other shapes. The Schottky contact region  107  is formed by blocking of implant during the formation of region  103  by either oxide and/or photoresist as the blocking layer. The size of 107 is designed by the needs of reverse recovery time. The top metal layer  105  can be either formed by Au, Pt, W, Mo Cr, Ni, Ti and other metal or by forming the silicide, and then the diffusion barrier such as TiW, TiN or other layer before the contact layer is deposited. The contact layer can be either Al for wire bonding or Ag or Au for soldering. The backside layer  101  is a heavily doped region with the same polarity as the doped substrate  100  and the epitaxial layer  102 . This layer can be done either by implantation or diffusion. If the resistivity of the substrate  100  is lower enough, this backside layer  101  can be eliminated. The bottom layer  105 B uses for the similar metallization process as the top metal layer  105  or by using convention method such as Cr—Au, Ti—Ni—Ag or even with Nickel plating. The lifetime killer such as Pt, Au or radiations can be added before or after the process. 
         [0031]    The fast recovery rectifier of this invention is accomplished by one or more following condition; 
         [0032]    Schottky structure is accomplished with the standard fast recovery rectifier by using Pt, Au and/or radiation lifetime killer. 
         [0033]    Reduced p-n junction is used with the standard fast recovery rectifier with Pt, Au, and/or radiation lifetime killer. 
         [0034]    Reduced p-n junction is used with Schottky structure and standard fast recovery rectifier with Pt, Au and/or radiation lifetime killer. 
         [0035]    Reduced p-n junction alone. 
         [0036]    Reduced p-n junction is used with Schottky structure. 
         [0037]    Reduced p-n junction is used with Schottky structure and standard fast recovery rectifier with Pt, Au and/or radiation lifetime killer with epitaxial substrates. 
         [0038]    The fast recovery rectifier of this invention uses the Pt, Au and/or radiation lifetime killer, and combine with Schottky structure to reduce the minority carriers at the forward bias and in the rectifying process condition. The fast recovery rectifier use unpolished rough surface doped substrate and rough diffusion to provide a low cost. This invention is to form a reduced p-n junction space charge region by using a thin and very high doped film of the same polarity as the top junction layer which is opposite polarity of the base silicon substrate. This early termination of the junction charge region reduces the total space charge of the p-n junction thus the smaller reverse recovery time can be achieved. This method can be used in conjunction with the life time killers and/or with the Schottky structures. 
         [0039]    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.