Abstract:
A trench MOSFET structure having improved avalanche capability is disclosed, wherein the source region is formed by performing source Ion Implantation through contact open region of a thick contact interlayer, and further diffused to optimize a trade-off between Rds and the avalanche capability. Thus, only three masks are needed in fabrication process, which are trench mask, contact mask and metal mask. Furthermore, said source region has a doping concentration along channel region lower than along contact trench region, and source junction depth along channel region shallower than along contact trench, and source doping profile along surface of epitaxial layer has Gaussian-distribution from trenched source-body contact to channel region.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the cell structure and device configuration of semiconductor devices. More particularly, this invention relates to an improved trenched MOSFET configuration having improved avalanche capability by three masks process. 
     BACKGROUND OF THE INVENTION 
     Please refer to  FIG. 1A  for a conventional N-channel trench MOSFET structure of prior art (U.S. Pat. No. 6,888,196) with an N+ source regions having same surface doping concentration and junction depth along trenched source-body contact and channel region. The disclosed N-channel trench MOSFET cell is formed in an N epitaxial layer  102  supported on an N+ substrate  100 . Near the top surface of a P body region  103 , which is formed within said epitaxial layer  102 , N+ source region  104  is implanted around the top portion of trenched gates  105  and adjacent to the sidewalls of trenched source-body contact  106 . As mentioned above, said N+ source region  104  has a same surface doping concentration and a same junction depth (Ds, as illustrated in  FIG. 1A ) along epitaxial surface, which is related to the formation process of said N+ source regions  104 . 
       FIG. 1B  shows the fabrication method of said N+ source regions  104 . After the formation of the P body region  103  and its diffusion, said N+ source regions  104  is formed by performing source Ion Implantation through a source mask (not shown). The top surface of said P body region  103  suffered the same source Ion Implantation and the same N+ dopant diffusion step, therefore said N+ source regions has same doping concentration and same junction depth (Ds, as shown in  FIG. 1A ) along epitaxial surface. 
     This uniform distribution of doping concentration and junction depth of said N+ source regions may lead to a hazardous failure during UIS (Unclamped Inductance Switching) test, please refer to  FIG. 1C  for a top view of said N+ source region  104  and said trenched source-body contact  106  shown in  FIG. 1A . As illustrated, R bc  is the base resistance from said trenched source-body contact  106  to the cell corner, R be  is the base resistance from said trenched source-body contact  106  to the cell edge. Obviously, R bc  is greater than R be  because the distance from said trenched source-body contact  106  to the cell corner is longer than that from said trenched source-body contact  106  to the cell edge, resulting in said UIS failure occurring at the trench corner and a poor avalanche capability for closed cell at cell corners as the parasitic NPN bipolar transistor is easily turned on. 
     Accordingly, it would be desirable to provide a new and improved device configuration to avoid the UIS failure occurred at the trench corner in a trench MOSFET while having a better avalanche capability. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide new and improved device configuration to solve the problems discussed above. 
     One aspect of the present invention is that, the source region is formed by performing source Ion Implantation through open region of a thick contact interlayer covering the epitaxial layer, which means source region is implanted after the formation of the thick contact interlayer, as shown in  FIG. 2A . Using this method, the doping concentration of said source region along the epitaxial surface is Gaussian-distributed from the contact window to channel region, and the junction depth of said source region is shallower in channel region than that in contact open region, resulting in a lower base resistance than prior art. 
     Another aspect of the present invention is that, in a preferred embodiments, the dopant of source region is diffused to just reach cell edge, please refer to  FIG. 2B  for a top view of an N-channel trench MOSFET structure, the dash-dotted line illustrate the area of N+ source region with a doping concentration no less than 1×10 19  cm −3 . At cell corners, the N region has a lower doping concentration due to the Gaussian-distribution, which is less than 1×10 19  cm −3 . Therefore, a Source Ballast Resistance (SBR) of said N region exists at cell corners, which reduces the Emitter injection efficiency of the parasitic NPN bipolar transistor, thus rendering it difficult to turn on, avoiding the UIS failure issue and improving the avalanche capability. 
     Another aspect of the present invention is that, in a preferred embodiment, the dopant of source region is diffused further after reaching the cell edge to optimize trade-off between R ds  (resistance between drain and source) and avalanche capability, please refer to  FIG. 2C  for another top view of an N-channel trench MOSFET structure. At the cell edge, the N+ source region is adjacent to the gate oxide, while at the cell corner, the area of lower doped N regions is smaller than that in  FIG. 2B . It seems that the source resistance is reduced at cell corner, breaching the desire of enhancing the avalanche capability, however, as the R ds  is the same important, and it is reduced by shortening the distance of highly doped region to the cell edge, therefore, a trade-off is achieved between the avalanche capability and the R ds , optimizing the device to a better performance. 
     Another aspect of the present invention is that, the semiconductor power device has a thick contact interlayer comprising a layer of un-doped SRO (Silicon Rich Oxide) and a layer of BPSG (Boron Phosphorus Silicon Glass) or PSG (Phosphorus Silicon Glass) whereon. When forming the trenched source-body contact, the contact width within said BPSG or PSG layer is wider than that in un-doped SRO layer because during etching process, the BPSG or PSG has about 5˜10 times etch rate of un-doped SRO if dilute HF chemical is used, resulting in a reduction of contact resistance between the contact filling-in tungsten plug and source metal. 
     Another aspect of the present invention is that, the semiconductor power device has vertical trenched source-body contact penetrating said thick contact interlayer, said source region, and into said body region. Especially, the contact width within said BPSG or PSG layer is wider than that in other portion. 
     Another aspect of the present invention is that, the semiconductor power device has a trenched source-body contact with vertical sidewalls in said thick contact interlayer and said source region, and has slope sidewalls in body region to enlarge heavily-doped body contact resistance reduction area wrapping said slope trench contact and the bottom to further improve device avalanche capability. 
     Another aspect of the present invention is that, in an embodiment of the present invention, the trenched source-body contact is filled with source metal instead of tungsten plug to enhance the metal contact performance and reduce the fabrication cost. 
     Another aspect of the present invention is that, in a preferred embodiment, the semiconductor power device has a termination area with trenched floating rings for saving body mask during fabrication process. 
     Another aspect of the present invention is that, in a preferred embodiment, the semiconductor power device has source ion implantation through contact mask for saving source mask during fabrication process. 
     Another aspect of the present invention is that, the fabrication cost is reduced as only three masks (trenched gates mask, trenched contacts mask and metal mask) are needed during fabrication process. 
     Briefly, in a preferred embodiment, as shown in  FIG. 3A , which is also the X 1 -X 1 ′ cross section view of  FIG. 2B  and  FIG. 2C , the present invention discloses a trench MOSFET formed on a substrate heavily doped with a first semiconductor doping type. Onto said substrate, an epitaxial layer of a first semiconductor doping type is grown with a doping concentration lower than that of said substrate. A plurality of trenches are etched within said epitaxial layer and filled with doped poly padded with a gate oxide to form trenched gates. Body region of a second semiconductor doping type is extending between every two adjacent trenched gates. Onto the top surface of said epitaxial layer, a thick contact interlayer comprising of a layer of un-doped SRO and a layer of BPSG or PSG whereon. Near the top surface of said body region, source region heavily doped with a first semiconductor doping type is implanted through the open region of said thick contact interlayer to form source region with Gaussian-distribution from said open region to channel region. Trenched source-body contact with vertical sidewalls, which is filled with tungsten plugs padded with barrier layer of Ti/TiN or Co/TiN, is penetrating through said thick contact interlayer, said source region and into said body region to electrically contact said source region and said body region with source metal. Especially, the contact width in top BPSG or PSG layer is wider than that in lower SRO layer for the reduction of contact resistance between said tungsten plugs and said source metal. Around each bottom of said trenched source-body contact, a body contact resistance reduction area heavily doped with a second semiconductor doping type is formed to further reduce the contact resistance. Source metal is deposited onto a resistance-reduction layer of Ti or TiN to electrically contact with said tungsten plug. Drain metal is deposited onto the rear side of said substrate after grinding. 
     Briefly, in another preferred embodiment, as shown in  FIG. 4 , the present invention discloses a trench MOSFET similar to that in  FIG. 3A  except that, the trenched source-body contact in  FIG. 4 , which is filled with tungsten plugs padded with barrier layer of Ti/TiN, has vertical contact trench sidewalls within said thick contact interlayer and said source region, and has slope contact trench sidewalls within said body region. 
     Briefly, in another preferred embodiment, as shown in  FIG. 5 , the present invention discloses a trench MOSFET similar to that in  FIG. 3A  except that, said trenched source-body contact in this embodiment is filled with source metal instead of tungsten plugs, to further enhance the contact performance and reduce the fabrication cost. 
     Briefly, in another preferred embodiment, as shown in  FIG. 6 , the present invention discloses a trench MOSFET similar to that in  FIG. 4  except that, said trenched source-body contact in this embodiment is filled with source metal instead of tungsten plugs, to further enhance the contact performance and reduce the fabrication cost. 
     Briefly, in another preferred embodiment, as shown in  FIG. 8 , the present invention discloses a trench MOSFET with termination area having trenched floating rings. In active area, the structure can be same as that in  FIG. 3A , in termination area, a plurality of trenched floating rings are formed from top surface of said body region and extending into said epitaxial layer. By employing said trenched floating rings, the body mask is saved, and on the other hand, the breakdown voltage of device is enhanced. Between said active area and said termination area, a wider trenched gate for gate connection is formed to electrically contact said trenched gate with gate metal via a trenched gate contact. Especially, the width of said trenched gate contact in top BPSG or PSG layer is wider than that in lower portion. 
     This invention further disclosed a method of manufacturing a semiconductor power device with three masks. The method includes a step of forming a plurality of trench MOSFET cells having source region with Gaussian-distribution from the contact opening area of a thick contact interlayer covering the epitaxial layer to channel region. The method further includes a step of forming a trenched source-body contact with different contact width. In an exemplary embodiment, the step of forming said source region with Gaussian-distribution further comprises applying a contact mask onto said thick contact interlayer to etch the contact trench through which the source ion implantation is carried out and diffused to form Gaussian-distribution from the contact trench opening to channel region. In another exemplary embodiment, the step of forming said trenched source-body contact with different width further comprises wet etching in dilute HF of said thick contact interlayer which comprising a BPSG or PSG layer and an un-doped SRO layer underneath, as the BPSG or PSG layer has a etch rate 5˜10 times of that in un-doped SRO layer, the width of contact trench within said BPSG or PSG layer is wider than that within said un-doped SRO layer. In another exemplary embodiment, the method further comprises a step of forming said trenched source-body contact with vertical or slope sidewalls (not shown). In another exemplary embodiment, the method further comprises a step of forming said trenched source-body contact with vertical sidewalls in said thick contact interlayer and said source region, and has a slop sidewalls within the body region. In another exemplary embodiment, the method further comprises a step of forming said trenched source-body contact filled with tungsten plug padded with a layer of Ti/TiN or Co/TiN as barrier layer. In another exemplary embodiment, the method further comprises a step of forming said trenched source-body contact filled with source metal which is Al alloys onto a barrier layer of Ti/TiN or Co/TiN. 
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
         FIG. 1A  is a side cross-sectional view of a prior art. 
         FIG. 1B  is a side cross-sectional view or prior art for showing the formation method of source region in prior art. 
         FIG. 1C  is a top view of prior art for showing the disadvantage of prior art. 
         FIG. 2A  is a side cross-sectional view for showing the formation method of source region according to the present invention. 
         FIG. 2B  is a top view for showing a source region diffusion method according to the present invention. 
         FIG. 2C  is a top view for showing another source region diffusion method according to the present invention. 
         FIG. 3A  is a side cross-sectional view of an N-channel trench MOSFET showing a preferred embodiment according to the present invention, which is also the X 1 -X 1 ′ cross section in  FIG. 2A . 
         FIG. 3B  is the doping profiles for showing the relationship between depth from epitaxial surface and doping concentration in trench contact and channel region, respectively. 
         FIG. 3C  is the side cross-sectional view of the preferred embodiment shown in  FIG. 3A  for showing the X 2 -X 2 ′ cross section in  FIG. 2A . 
         FIG. 4  is the side cross-sectional view of an N-channel trench MOSFET showing another preferred embodiment according to the present invention. 
         FIG. 5  is the side cross-sectional view of an N-channel trench MOSFET showing another preferred embodiment according to the present invention. 
         FIG. 6  is the side cross-sectional view of an N-channel trench MOSFET showing another preferred embodiment according to the present invention. 
         FIG. 7A  is the top view of trench MOSFET with closed cell according to the present invention. 
         FIG. 7B  is the top view of trench MOSFET with stripe cell according to the present invention. 
         FIG. 8  is the side cross-sectional view of another preferred embodiment with termination area having trench floating rings. 
         FIG. 9A to 9D  are a serial of side cross-sectional views for showing the processing steps for fabricating the trench MOSFET as shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Please refer to  FIG. 3A  for a preferred embodiment of the present invention, which also is the X 1 -X 1 ′ cross section of  FIG. 2B  and  FIG. 2C , where an N-channel trench MOSFET is formed on a heavily N+ doped substrate  300  coated with back metal  390  on rear side as drain electrode. Onto said substrate  300 , a lighter N doped epitaxial layer  301  is grown, and a plurality of trenches is etched wherein. Doped poly is filled into these trenches padded with a gate oxide layer  320  to serve as trenched gates  311 . P body region  304  is extending between two adjacent trenched gates  311 , and near the top surface of said P body region, N+ source regions  308  is formed with Gaussian-distribution from the open region of a thick contact interlayer  330  to channel region. Said thick contact interlayer is composed of a layer of un-doped SRO  330 - 1  and a layer of BPSG or PSG  330 - 2 . Trenched source-body contact filled with tungsten plug  314  which is padded with a barrier layer  313  of Ti/TiN or Co/TiN, are formed by penetrating through said thick contact interlayer  330 , said source region  308  and extending into said P body region  304  with vertical sidewalls or slope sidewalls (not shown). Especially, the width of said trenched source-body contact in the BPSG or PSG layer  330 - 2  is wider than that in other portion. Underneath the bottom of said trenched source-body contact, a p+ body contact resistance reduction area  312  is implanted to further reduce the contact resistance between said tungsten plug  314  and said body region  304 . Onto a resistance-reduction layer  318  of Ti or Ti/TiN, source metal  340  composed of Al alloys is deposited to electrically contact with tungsten plug  314 . 
     In order to further make clear,  FIG. 3B  illustrates the doping profiles along said trenched source-body contact and the channel region from said epitaxial surface in N-channel trench MOSFET shown in  FIG. 3A . In  FIG. 3B , N+ represents said N+ source region  308 , P represents said P body region  304 , p+ represents said p+ contact area  312 .  FIG. 3C  shows the X 2 -X 2 ′ cross section of  FIG. 2B  and  FIG. 2C . In cell corners, N region  328  has a lower doping concentration than said N+ source region  308 , resulting in a lower source resistance to further enhance avalanche capability. 
     Please refer to  FIG. 4  for another preferred embodiment of the present invention which also shows the X 1 -X 1 ′ cross section of  FIG. 2B  and  FIG. 2C . In  FIG. 4 , the N-channel trench MOSFET is similar to that in  FIG. 3A  except that, said trenched source-body contact has vertical sidewalls within said thick contact interlayer  330  and said N+ source region  308 , and has slope sidewalls within said P body region  304 . By employing this structure, p+ body contact resistance reduction area  312  is enlarged to wrap the slope sidewalls and the bottom of said trenched source-body contact to further enhance avalanche capability. 
     Please refer to  FIG. 5  for another preferred embodiment of the present invention which also shown the X 1 -X 1 ′ cross section of  FIG. 2B  and  FIG. 2C . In  FIG. 5 , the N-channel trench MOSFET is similar to that in  FIG. 3A  except that, said trenched source-body contact is not filled with tungsten plug, but the source metal  340 ′ over said barrier layer  313 . By employing this structure, the contact performance between said N+ source region and said P body region with said source metal is improved. 
     Please refer to  FIG. 6  for another preferred embodiment of the present invention which also shown the X 1 -X 1 ′ cross section of  FIG. 2B  and  FIG. 2C . In  FIG. 6 , the N-channel trench MOSFET is similar to that in  FIG. 4  except that, said trenched source-body contact is not filled with tungsten plug, but the source metal  340 ′ over said barrier layer  313 . By employing this structure, the contact performance between said N+ source region and said P body region with said source metal is improved. 
       FIG. 8  shows the cross section view of an N-channel trench MOSFET with termination area having trench floating rings  311 - 2 , which also is the A-B-C-D cross section of  FIGS. 7A and 7B , wherein said N-channel trench MOSFET is the same structure as  FIG. 2A . Between said N-channel MOSFET and said termination area, a wider trenched gate  311 - 1  is formed to contact with gate metal  340 - 1  via trenched gate contact which is filled with tungsten plug  315 . 
       FIGS. 9A to 9D  are a serial of exemplary steps that are performed to form the preferred N-channel trench MOSFET in  FIG. 8 . In  FIG. 9A , an N doped epitaxial layer  301  is grown on an N+ substrate  300 . A trench mask (not shown) is employed to define a plurality of gate trenches for trench MOSFET, a wider gate trench for gate connection and a plurality of floating rings for termination area. Then, these trenches are dry Si etched to a certain depth. After that, a sacrificial oxide layer is grown and then removed to eliminate the plasma damage may introduced during etching process. Next, a first insulation layer is deposited overlying the inner surface of all trenches to serve as gate oxide  320  onto which doped poly is deposited within those trenches and then etched back or CMP (Chemical Mechanical Polishing) to form trenches gates  311  for trench MOSFET, a wider trench gate  311 - 1  for gate connection, and trenches rings  311 - 2  for termination area. Then, over the entire top surface, a step of P dopant Ion Implantation is carried out for the formation of P body regions  304 , and then followed by a step of diffusion for P body drive-in. 
     In  FIG. 9B , a layer of un-doped SRO  330 - 1  and a layer of BPSG or PSG  330 - 2  are successively deposited onto top surface of said epitaxial layer. Then, a contact mask (not shown) is applied to define the trenches for trenched source-body contact and trenched gate contact. After the removal of the contact mask, a screen oxide  380 , which is about 300 A, is deposited along the contact areas and surface of said BPSG or PSG layer  330 - 2 . Then, a step of N+ Source Ion Implantation is carried out over entire top surface for the formation of said N+ source region, and followed by a step of diffusion for N+ source region drive-in. 
     In  FIG. 9C , the screen oxide is first removed by dry or wet oxide etching. Then, a step of dry Si etch is carried out to etch said contact trenches into said source region  308 , said body region  304  and said wider trenched gate  311 - 1 , respectively. After that, BF2 Ion is implanted over entire top surface to form p+ body contact resistance reduction area  312  underneath each bottom of said trenched source-body contact and followed by a step of RTA (Rapid Thermal Annealing) to activate BF2. 
     In  FIG. 9D , wet etching in dilute HF is carried out to enlarge the contact width in PSG or BPSG and a barrier layer of Ti/TiN or Co/TiN and contact trench filling-in material tungsten is successively deposited and then etched back or CMP to form tungsten plugs  314  and  315  filled in trenched source-body contact and trenched gate contact, respectively. Then, a metal layer of Al/Cu is deposited after Ti or Co silicide formation by RTA, over a resistance reduction layer of Ti or TiN and patterned by a metal mask (not shown) to form source metal  340  and gate metal  340 - 1  by metal etching. Last, after the backside grinding, backside metal  390  is deposited onto the rear side of said substrate. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.