Abstract:
In one embodiment, relatively thin but wide metal bus strips overlying a high power FET are formed to conduct current to the source and drain narrow metal strips. A passivation layer is formed over the surface of the FET, and the passivation layer is etched to expose almost the entire top surface of the bus strips. A copper seed layer is then formed over the surface of the wafer, and a mask is formed to expose only the seed layer over the bus strips. The seed layer over the bus strips is then copper or gold electroplated to deposit a very thick metal layer, which effectively merges with the underlaying metal layer, to reduce on-resistance. The plating metal does not need to be passivated due to its thickness and wide line/space. Other techniques may also be used for depositing a thick metal over the exposed bus strips.

Description:
FIELD OF THE INVENTION 
   This invention relates to high power field effect transistors (FETs) and, in particular, to a technique for reducing the on-resistance of such FETs using an additional metal layer. 
   BACKGROUND 
   One type of conventional high power FET is formed by creating long rows of alternating source and drain regions separated by a channel region. A gate overlies the channel region. The gate width is thus very large to create a high current FET. The source and drain regions are the same conductivity type, and a threshold voltage on the gate creates a conductive channel between the source and the drain to conduct current. Such a transistor is a lateral FET. Narrow metal strips contact and interconnect the source regions, and other metal strips contact and interconnect the drain regions. 
   Another type of high power lateral FET forms cells of source and drain regions separated by a channel, where a gate overlies the channel. Each adjacent cell pair forms a single transistor, and the cells are all connected in parallel by narrow metal strips to conduct a high current. 
   High power FETs may also be vertical transistors, using double diffusion, where the doped regions formed in the silicon surface are all source and channel regions, and a voltage on the gate inverts the channel region. The current path is then vertical, where the drain is the bottom surface of the silicon substrate. A metal layer contacts all the source regions. Another type of vertical FET uses a vertical gate. 
   Various types of high power FETs are described in U.S. Pat. No. 5,355,008, co-invented by the present inventor and incorporated herein by reference. 
   In very high power FETs, such as those conducting one amp or more, the high current creates a significant voltage drop across the metal overlying and contacting the doped regions, due to the inherent resistivity of the metal layer. Although the on-resistance can be lowered by making the metal thicker, problems arise in masking, etching, and passivating such thick metal. Additionally, the same metal layer may be patterned for interconnecting components of low power circuitry formed on the same chip as the high power FET, and a thick metal layer would undesirably increase the minimum possible line width. 
   One solution for lowering the on-resistance is creating an insulating layer over the first-layer metal strips (formed by a first metal layer) and forming vias (small holes) in the insulating layer over portions of the first-layer metal strips. A wide metal source bus strip (formed by a second metal layer), perpendicular to the narrow source metal strips, is then formed over the insulating layer and in the vias to provide a low resistance coupling to each of the first-layer source metal strips. A similar wide drain metal bus strip may be formed to contact the first-layer drain metal strips. 
   Although the wide bus strips reduce on-resistance, there are significant drawbacks in making the bus strips thick. Firstly, depositing, masking, and etching a thick metal is time consuming. Secondly, the second metal layer may be patterned for interconnecting other circuits, and a thick metal layer increases the minimum possible line width. Thirdly, it is difficult to thoroughly passivate a thick metal layer due to the high step. 
   Forming additional overlying metal layers, connected to the underlying metal with vias through an insulating layer, for further reducing on-resistance similarly require substantial time to deposit, mask, and etch the metal. Additionally, the small vias incur some voltage drop due to their small cross-sectional area. 
   What is needed is a simple technique to form thick metal conductors overlying a high power FET for conducting source or drain current. 
   SUMMARY 
   In one embodiment, relatively thin but wide metal bus strips overlying a high power FET are formed to conduct current to the source and drain narrow metal strips. A passivation layer is formed over the surface of the FET, and the passivation layer is etched to expose almost the entire top surface of the bus strips. The exposed bus strips are then coated with a very thin suitable seed layer, such as copper in the case of copper electroplating. After a single masking step, a very thick metal (e.g., copper) layer is then plated over the exposed bus strips. The thin metal bus and the thick metal plating are considered to be merged. If only vias were formed in the passivation layer, such a plating process would not adequately reduce on-resistance, since the thin bus would only be contacted at points and therefore still carry the transistor current. 
   The plating metal does not need to be passivated, since the underlying structure is already protected by passivation, the metal is very wide, and the spacings between neighboring metal portions are also very wide. 
   The plating process is performed on a wafer scale prior to dicing of the chips. Multiple wafers may be plated at the same time. 
   An additional benefit of this technique is that the plating process may be performed by an outside vendor without any contamination of the wafer during the transportation of the wafers, since the wafers are already passivated and do not need to go back into the fab which built the wafers. 
   Other techniques may also be used for depositing a thick metal over the exposed bus strips, such as sputtering, flash evaporation, or other well known techniques. Such deposition is greatly simplified by the wide lines and spaces (e.g., no problems with alignment or voids) and the fact that this last metal layer does not need to be passivated. 
   This technique may also be applied to a metal layer conducting FET current other than the metal bus. This technique may be used to form a thick top metal in any type of FET. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified front view of a lateral high power FET formed in accordance with one embodiment of the invention. 
       FIG. 2  is a partial cross-sectional view of the FET of  FIG. 1  along line  2 - 2 . 
       FIG. 3  schematically illustrates the wafer-level plating to form a thick metal layer over the source and drain metal busses for reducing on-resistance. 
       FIG. 4  is a front view of another embodiment of a metal pattern for the merged-thick metal layer over the FET. 
       FIG. 5  is a flowchart illustrating certain steps in one embodiment of the invention. 
   

   Elements in the various figures labeled with the same numeral may be identical. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates a simplified high power lateral FET  10 . In an actual lateral FET, there would be many more rows of alternating source and drain strips.  FIG. 2  is a partial cross-section along line  2 - 2  in  FIG. 1 . The invention may apply to any type of FET, and the particular example of an n-channel lateral FET is not intended to be limiting. For example, the invention may be applied to N-channel or P-channel cellular FETs and vertical FETs, such as described in U.S. Pat. No. 5,355,008, assigned to the present assignee and incorporated herein by reference. 
   N-type source regions  12  ( FIG. 2 ) and n-type drain regions  14  are formed in a p-type layer  16  (or p-type tub) of a silicon substrate. The various conductivity types may be reversed for a p-channel FET. Gate oxide is grown over the channel regions, and doped polysilicon gates  18  are formed. The gates  18  are connected to a voltage source (not shown) for turning on and off the transistor. 
   A first insulating layer  20 , such as BPSG, is then formed over the surface, and contact holes  22  are etched in the insulating layer  20 . 
   A first metal layer (metal  1 ) is deposited over the surface and etched to form the source metal  24  and the drain metal  26 . The first metal layer also fills the contact holes  22  to contact areas of the source and drain regions  12  and  14 . A thin barrier metal, such as a tungsten alloy, may first be formed over the exposed silicon as a barrier layer between the first metal layer (e.g., AlSiCu) and the silicon. 
   The metal layer  1  is thin (e.g., less than 1.0 micron) to simplify processing steps, such as depositing, fine-line masking, and etching the metal, as well as enabling the formation of a quality insulation layer over the structure. The metal layer  1  may also be patterned for use in low power circuitry  27  on the same chip, and an increased thickness reduces the minimum line width possible. Without any additional metal layers, the inherent resistivity of the long, narrow, and thin source and drain metal  24  and  26  would create a substantial voltage drop at high currents since the source and drain metal strips are only electrically contacted at one end. 
   A second insulating layer  28  of oxide is deposited on the surface and etched to form vias  30  over various points along the source and drain metal  24  and  26 . For simplicity in  FIG. 1 , only one via  30  is shown for each source and drain metal strip. 
   Next, a second metal layer (metal  2 ) is deposited and etched to form relatively wide source and drain busses  32  and  34  perpendicular to the source and drain metal  24  and  26  ( FIG. 2  is cut along the source bus  32 ). The metal  2  also fills the vias  30  to contact the source and drain metal  24  and  26 . Due to their width, the busses provide a relatively low resistance current path to the source and drain metal  24  and  26 . The second metal layer  32  must still be relatively thin (e.g., less than 3.0 micron) to simplify processing steps, such as depositing, masking, and etching the metal, as well as enabling the formation of a quality insulation layer over the structure. The metal layer  2  may also be patterned for use in low power circuitry on the same chip, and the increased thickness reduces the minimum line/width that is useable in digital/analog areas. 
   The on-resistance of the busses  32  and  34  is decreased using the following inventive process. A third insulating layer  38 , such as nitride (a protective passivation layer), is formed over the surface of the entire FET (and other circuits if any), and a large area over each of the busses  32  and  34  is exposed by etching the third insulating layer  38 . A thick third metal  40  (e.g., AlCu or AlSiCu) is then deposited onto the exposed busses  32  and  34  to create a very low resistance conductor even at the highest currents for FETs. The thickness of the third metal  40  may be considerably greater than 3.0 microns without any drawbacks, since the third metal layer is not used for any other circuits on the chip. If the third metal  40  is copper based, its resistivity is much less than the Al based metal used for the underlying metal layers. The combination of the thickness and material of the third metal  40  greatly reduce the voltage drop of the FET. 
   Since the third metal  40  is thick and wide, and since there are large spaces between conductors, the third metal  40  does not require passivation, and passivation is optional.  FIG. 1  shows the metal  40  in dashed outline, generally corresponding to the opening in the nitride passivation layer  38  ( FIG. 2 ). 
   The third metal  40  can be formed by plating, sputtering, flash evaporation, or any other deposition technique. Gold may also be a suitable metal  40 . 
   Suitable bonding pads  42  are then formed on the metal  40  for wire bonding, if necessary. The wire bonding can be anywhere along the length of the thick metal  40 . If it is desired to not have the wire bonding pads over the thick metal  40 , the mask used for defining the metal  40  can cover the areas of the underlying metal layer (e.g., aluminum) where the pads are to be. After the busses are copper-plated, the pad area is then exposed, and gold wires may then be easily bonded to the aluminum. This process is advantageous since it is difficult to bond gold wires to copper, and additional processes need to be performed to form Au/Ni or Pd/Ni bonding pads over the copper. By not plating areas of the aluminum busses and later using those areas as bond pads, several processing steps are avoided. 
   In one embodiment, the fabrication facility fabricating the chip is a clean room. The facility forms the wide openings over the busses  32  and  34  and blanket-deposits the thin layer of copper over the exposed busses  32  and  24  and elsewhere over the wafer in a conventional copper sputter process. This thin copper layer acts as a seed layer in a subsequent plating step. The wafer may then be taken out of the clean room and transported to an outside vendor for performing a single masking step to expose the thin copper layer only over the top of the busses  32  and  24  and then plating a very thick copper layer (e.g., 2.5 microns-40 microns) over the exposed copper layer. The mask and thin copper not over the busses  32  and  24  are then removed in a conventional process. 
   In another embodiment, the masking step before the plating is not performed, and the entire surface of the wafer is plated over the seed layer. The wafer is then subjected to a chemical-mechanical planarization (CMP) step that removes all the copper except the copper within the trenches formed in the third insulating layer  38  over the busses  32  and  34 . The third insulating layer  38  must be made thick in such a case. When using this CMP method, it is preferred that the wafer surface also be planarized after forming the second insulating layer  28  over the busses  32  and  34 . 
   Methods for copper plating integrated circuits are well known. Certain copper plating methods are described in U.S. Pat. Nos. 6,432,753, 6,610,600 and 6,037,258, incorporated herein by reference. The seed and plating metal may also be Au. 
     FIG. 3  illustrates a simplified plating process for forming metal  40 . 
   The wafer  46 , along with other similar wafers, is placed in an electrolyte solution  50 . A copper electrode  52  is placed in the solution and biased so that the copper atoms travel through the solution and plate the exposed wafer surface. Details of copper electroplating are well known and need not be described in detail. 
   After the metal  40  is formed and wire bonding pads created (if necessary), the dies are separated, and the chips are packaged (e.g., chips mounted on lead frames and wires bonded to pads  42  and package leads, etc.). No further passivation is needed since the metal  40  is so robust. 
     FIG. 4  illustrates a pattern of metal  40  and busses that is different from the simple horizontal strips shown in  FIG. 1 .  FIG. 4  is a top down view of a high power FET chip  58  formed using the present invention. The thick metal  40  is on top, with pads  42  for wire bonding to the package terminals (the gate pad is not shown for simplification). The source and drain metal  24  and  26  strips ( FIG. 1 ) are in the vertical direction under the metal  40  in  FIG. 4 . The serpentine merged-metal pattern, made up of serpentine source and drain busses with the overlying thick metal  40 , electrically contacts most source and drain metal strips at two areas of the strips for reduced on-resistance. In one embodiment, the chip in  FIG. 4  may be on the order of 30×30 mils. 
   In one embodiment, the bonding pads  42  are Au/Ni or Pd/Ni pads formed over the metal  40  (e.g., copper). In another embodiment, pads  42  are end portions of the aluminum busses that have not been plated with metal  40 . 
   The optimal metal  40  pattern may be different for different types of FETs, such as cellular, lateral, vertical, etc. In a vertical FET, the thick merged-metal  40  would only contact source regions on the top surface of the chip. Gates in a vertical FET may be vertical or lateral. 
   In a preferred embodiment, the thick metal  40  is merged with at least 25 percent of the top surface area of the underlying metal (e.g., busses  32  and  34 ), determined by the large continuous opening in the insulating layer exposing at least 25 percent of the underlying metal. In a more preferred embodiment, at least 50 percent of the underlying metal is covered by the thick metal  40 . In another embodiment, at least 75 percent of the underlying metal is covered by the thick metal  40 . The thickness of the metal  40 , relative to the thickness of the underlying metal, causes the metal  40  to carry a majority of the FET current to reduce on-resistance. In one embodiment, the metal  40  is at least twice the thickness of the underlying metal layer (e.g., bus  32 ). 
     FIG. 5  is a self-explanatory flowchart that summarizes the process described herein in steps  61 - 68 . 
   Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit and inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.