Abstract:
This invention discloses a semiconductor substrate supports a semiconductor power device. The semiconductor substrate includes a plurality of polysilicon segments disposed over a gate oxide layer including two outermost segments and inner segments wherein each of the inner segments functioning as a gate and the two outermost segments functioning as a field pate and an equal potential ring separated by an oxide-plug gap having an aspect ratio greater or equal to 0.5. Each of the inner segments functioning as a gate having a side wall spacer surrounding edges of the inner segments, and the oxide plug gap being filled with an oxide plug for separating the field plate from the equal potential ring. A plurality of power transistor cells disposed in the substrate for each of the gates covered by an overlying insulation layer having a plurality of contact openings defined therein. A plurality of metal segments covering the overlying insulation layer and being in electric contact with the power transistor cells through the contact openings. A plurality of deep-and-narrow gaps between the metal segments wherein each gap having an aspect ratio equal or greater than 0.5. A passivation layer disposed in the deep-and-narrow gaps between the metal segments having a thickness substantially the same as the metal segments for blocking mobile ions from entering into the power transistor cells.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the structure and fabrication process of MOSFET power devices. More particularly, this invention relates to a novel and improved MOSFET device structure and fabrication process wherein reduced number of masks are employed such that MOSFET power devices can be manufactured with simplified method at lower cost while the device reliability is improved. 
     2. Description of the Prior Art 
     The goal of reducing the production cost of the MOSFET power device cannot be easily achieved. This is particularly true when the power MOSFET devices become more complicate both in cell structure and in device topology. It causes the fabrication processes to become more complex which typically requires application of increased number of masks. Longer manufacture time cycles are required which leads to higher production costs. Increased number of masks employed in the fabrication processes introduces further concerns. As more masks and processing steps are applied, more uncertainties of production yield and product reliability are introduced. The production costs are further impacted due to these undesirable factors. For these reasons, many technical improvements are attempted to reduce the number of masks employed for MOSFET fabrication. 
     In U.S. Pat. No. 5,404,040 entitled “Structure and Fabrication of Power MOSFETS including Termination Structures” (issued on Apr. 4, 1994), Hshieh etl al. disclose a power MOSFET, as that shown in FIG.  1 . The MOSFET is manufactured by a five mask process on a semiconductor body  2000  and  2001 . A first insulating layer  2002  lies over the active and termination areas. A main polysilicon portion,  2003 C and  2003 B, lies over the first insulating layer largely above the active area. Also a first and second peripheral polysilicon segments  2003 C 1  and  2003 C 2  lie over the first insulating layer above the termination area which are etched as two separated segments with a separating gap  2013 E. A gate electrode  2016  contacts the main polysilicon potion. A source electrode  2015 A and  2015 B, is formed to contact the active area, the termination area and the first polysilicon segment  2003 C 1  through an opening in the second insulating layer  2012 . The second polysilicon segment  2003 C 2  extends over a scribe line section of the termination area where the semiconductor is cut into separate dice. In this termination area, a metal portion is formed to contact this second polysilicon segment During a dicing process, the second polysilicon segment and the metal portion are electrically shorted to the semiconductor body. The metal portion in combination of the second polysilicon segment are useful to equalize the potential at the outer peripheral of the MOSFET and reduces the likelihood of device malfunction. 
     The MOSFET as that shown in FIG. 1 presents several difficulties in the fabrication processes. Specifically, it is difficult to remove a silicon segment to form the gap  2013 E for separating the first polysilicon segment  2003 C 1  from the second polysilicon segment  2013 C 2 . If the gap  2013 E is a small gap, then a wet etch process is not suitable due to its difficulties in controlling the etching dimensions. On the other hand, if a dry etch is applied in order to make the gap  2013 E with a small gap-width, then the opening surface may be damaged as a result of dry etch process. In addition to the difficulties in manufacture, the structure in the termination area presents further difficulties and limitations. Due to the opening of this gap  2013 E, a passivation layer is required to prevent mobile ions from entering into the device. As will be further discussed below, a requirement of applying a pad mask to define the passivation layer is necessary which results in more complicate manufacture processes and higher MOSFET production cost Additionally, this configuration in the termination area causes a walk out phenomenon of the breakdown voltage. A more detail technical description will be provided below when a novel structural feature of this invention is disclosed to improve the termination configuration in order to resolve the walkout problems. 
     The number of masks required in DMOS fabrication generally is closely related to the structure of a MOSFET transistor, and particularly the requirement to apply a pad mask is related to its requirement to have a passivation layer. Please refer to FIGS. 2A and 2B respectively for a cross sectional view of a conventional planar and trenched device structure for a DMOS transistor  10 . The DMOS transistor  10  is supported on a N+ substrate  15  and an N− epi-taxial layer  20  formed on its top. The cell  10  includes a p-body region  25  surrounding a source region  30  wherein the source region  30  and the p-body region  25  formed in the substrate and partially covered under a gate  40 . The body-region  25  and the source region  30  are insulated from the gate  40  by a gate oxide layer  35 . The DMOS cell  10  is then covered with a PSG or BPSG protection layer  45 . A contact mask is then applied to open contact areas. The metal layer  50  is deposited on top of the device which is then etched by applying a metal mask to define the source metal  50 - 1 , the gate metal  50 - 2 , the field plate  50 - 3  and an equal protection ring (EQR)  50 - 4 . After defining the metal segments  50 - 1  to  50 - 4 , due to the requirement to prevent mobile ions from entering into the device between the gaps of these metal contacts, e.g., gap-A, gap-B, and gap-C as that shown in FIG. 1, a passivation layer typical comprising a PSG, a silicon nitride or an oxynitride layer has to be formed. The passivation layer  60  is then deposited and etched by the use of a pad mask to expose the areas above the source metal  50 - 1  and gate metal  50 - 2 . The gaps between the metal segments, i.e., gap-A, gap-B, and gap-C, are now covered by the passivation layer  60 . The metal ions are blocked by either the metal segments  50 - 1  to  50 - 4 , or by the passivation layer and prevented from entering into the device. 
     Disadvantages of the foregoing process is that it requires additional manufacture processes and time due to the application of a pad mask for removing the passivation layer  60  from the areas above the source metal  50 - 1  and the gate pad  50 - 2 . Furthermore, the passivation layer typically formed with PSG, silicon nitride, or oxynitride, having a thickness ranging from 0.5 to 1.5 micrometers. Under a very heavy contamination situation, the thickness of the passivation layer may not be sufficient to block the mobile ions from entering into the transistor cells. As these metal segments  50 - 1  to  50 - 4  are defined usually by employing a wet etching process, the gaps between the metal segments typically have a large lateral distance of approximately 15-20 micrometers because of the undercut. With such large gaps between the metal segments, the passivation layer in the gaps can only be formed in conformity with the layer profile and thus having the same thickness as the passivation layer deposited in other areas. The thickness of the passivation layer covering the gaps between the metal segments is thus mostly limited to be about the same as passivation layer formed else where. With a thickness limitation described above and the fact that the passivation layer cannot reliably protect the DMOS device from invasion of mobile ions, the reliability of a DMOS device cannot be assured. The traditional wet etching process typically performed for patterning the metal layer to produce large lateral gaps between the metal segments in a conventional DMOS device thus leads to this technical difficulty. 
     Therefore, there is still a need in the art of power device fabrication, particularly for DMOS design and fabrication, to provide a structure and fabrication process that would resolve these difficulties. 
     SUMMARY OF THE PRESENT INVENTION 
     It is therefore an object of the present invention to provide a new MOSFET fabrication process and a new device structure to enable those of ordinary skill in the art of DMOS fabrication to reduce the number of masks and to improve the device reliability for mobile ion protection such that aforementioned limitations and difficulties as encountered in the prior art can be overcome. 
     Specifically, it is an object of the present invention to provide an improved MOSFET structure and fabrication process wherein the number of masks required for manufacturing a MOSFET power device is reduced to three masks by taking advantage of the improved structural features and by applying modern manufacture technology such that the production costs of the MOSFET can be significantly reduced. 
     Another object of the present invention is to provide a novel MOSFET structure and fabrication process wherein improved structure in the termination area is provided with an improved configuration of field plate such that a thick initial oxide layer is no longer needed and the requirement of applying a separate active mask specifically for defining the active area by etching away a thick initial oxide layer is eliminated such that the number of masks required to fabricate a MOSFET transistor can be reduced. 
     Another object of the present invention is to provide a novel MOSFET structure and fabrication process wherein improved structure in the termination area is provided with an improved configuration of field plate such that a breakdown walkout problem is resolved and the requirement of applying a separate active mask specifically for defining the active area by etching away a thick initial oxide layer is eliminated such that the number of masks required to fabricate a MOSFET transistor can be reduced while the performance of the device is improved. 
     Another object of the present invention is to provide an improved MOSFET fabrication structure and process wherein the requirement of applying a separate source blocking mask specifically for defining the source regions by carrying out a source implant is eliminated while the contact resistance for the source metal is reduced by removing a top portion of the substrate by a dry etch process such that the number of masks required to fabricate a MOSFET transistor can be reduced and the resistance between the source regions and the source metal can be improved. 
     Another object of the present invention is to provide an improved MOSFET fabrication structure and process wherein the requirement of applying a separate pad masks specifically for defining the passivation layer to expose the areas above the source and gate metal segments are eliminated while a mobile ion blocking layer of greater thickness is provided such that the number of masks required to fabricate a MOSFET transistor can be reduced and the device reliability can be improved. 
     Another object of the present invention is to provide an improved MOSFET fabrication structure and process wherein a dry etch process is applied to etch the metal layer for defining various metal segments with gaps of reduced widths filled with mobile ion blocking material such that these material will remain in the gaps without being etched away during a process of etching the passivation layer thus a requirement of pad mask is eliminated. 
     Another object of the present invention is to provide an improved DMOS fabrication process wherein the passivation layer are formed in the a narrow and deep gaps between metal segments formed by applying a dry etch process such that thickness of the passivation layer is substantially approximate to that of metal layer and the mobile ions are effectively blocked by the passivation layer with greater thickness whereby the device reliability is improved with significantly reduced likelihood of mobile ions contamination. 
     Briefly, in a preferred embodiment, the present invention includes a method for fabricating a MOSFET device supported on a substrate. The method includes the steps of (a) growing an oxide layer on the substrate followed by depositing a polysilicon layer and applying a gate mask as a first mask for forming a plurality of polysilicon gates; (b) depositing a NSG layer overlying the top surface followed by applying an anisotropic dry etch for removing the NSG layer, and forming an oxide plug between the field plate and the equal potential ring (EQR) polysilicon segments and a plurality of side wall spacers around the gates;(c) implanting a body dopant followed by a body diffusion for forming body regions; (d)implanting a source dopant to form a plurality of source regions; (e)forming an overlying insulation layer covering the MOSFET device followed by applying a dry oxide etch with a contact mask as a second mask to open a plurality of contact openings there-through; (f)performing a dry silicon etch to remove a top portion of source dopant area from a central portion of each of the source regions followed by performing a wet etch to open a plurality of lateral source contact areas above the source regions; (g)performing a low energy body dopant implant and a high energy body dopant implant to form a shallow high concentration body dopant region and a deep high concentration body dopant region in the body regions then removing the contact mask; (h) performing a high temperature reflow process for the overlying insulation layer and for driving the source regions and the shallow and deep high concentration body dopant regions into designed junction depths; (i) depositing a metal layer followed by applying a metal mask as a third mask for patterning the metal layer to define a plurality of metal segments by employing an anisotropic dry etch thus defining a plurality of deep-and-narrow gaps between the metal segments wherein each gap having an aspect ratio equal or greater than 0.5; (j) depositing a passivation layer over an entire top surface and filling the deep-and-narrow gaps between the metal segments; and (k) etching away the passivation layer over the entire top surface without applying a mask while leaving the passivation layer inside the deep-and-narrow gaps substantially intact for serving a function of blocking mobile ions from entering into the MOSFET device whereby the MOSFET device is manufactured with a three-mask process. 
     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 
     FIG. 1A is cross sectional view of a prior art MOSFET device manufactured with a five mask process with an improved termination structure; 
     FIGS. 2A and 2B are a cross-sectional views of a prior art planar and trenched DMOS device structure respectively; 
     FIGS. 3A to  3 G show the processing steps for manufacturing a planar DMOS device of the present invention; 
     FIG. 4 the filling of a deep and narrow gap when an aspect ratio is equal or greater than 0.5; and 
     FIGS. 5A and 5B show the cross sectional views of two alternate preferred embodiment showing respectively a planar and a trenched MOSFET devices of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A novel MOSFET fabrication process is disclosed in this invention wherein a three-mask process is employed to significantly simplify the manufacture processes. Referring to FIGS. 3A to  3 G for the processing steps in manufacturing the MOSFET device  100 . As shown in FIG. 3A, the processing steps begins by first growing a N −  epitaxial layer  110  with a resistivity ranging from 0.1 to 10 ohm-cm on top of a N+ substrate  105 . The substrate has a resistivity of 0.001 to 0.02 ohm-cm. The thickness and the resistivity of the epitaxial layer  110  depend on the requirements for the on-resistance and breakdown voltage. In a preferred embodiment, the thickness of the epi-layer  110  is about six to eight microns. A gate oxide layer  120  of thickness in the range of 100 to 1000 Angstroms is grown. A polysilicon layer  125  having a thickness of approximately 3000 to 10,000 Angstroms is then deposited. A POCL 3  doping process is carried out followed by an arsenic (As) implant process with an ion beam of energy at 60-80 Kev with a flux density in the range between 5 to 8×10 15 /cm 2 . A polysilicon mask, i.e.; the first mask, is applied to carry out the an anisotropic etching process to define the polysilicon gate  125 . It should also be noticed that a segment  125 ′ and  125 ″ of the polysilicon on the peripheral portion provided to form the field plate and equal potential ring (EQR) are defined to have a narrow gap  126  of about 0.5 to 2.0 micrometers in gap width. Referring to FIG. 3B, a NSG layer  128  of 0.5 to 1.0 micrometer in thickness is deposited. For the narrow gap  126  between the polysilicon segments  125 ′ and  125 ″, since the aspect ratio, i.e., the ratio of the width to the depth of the gap  126 , is equal or greater than 0.5, the NSG layer  128  fills the gap  126  without a significant dip caused by the variations of height across gap  126  and the top surface is approximately a flat surface. 
     Referring to FIG. 3C, an an-isotropic dry etch process is performed to remove the entire NSG layer  128  except the side-wall spacers  128 ′ around the edges of the polysilicon gates  125 . Also, the NSG layer  128  filling the gap  126  functioning as an oxide plug  128 ″ remains substantially intact except a very thin layer is removed from the top of the gap  126 . Thus the oxide plug  128 ″ filling the gap  126  basically has a same thickness as the polysilicon layer  125 ′ and  125 ″. The oxide plug  128 ″ formed in the gap  126  is used to block subsequent p-body implant from entering into the silicon layer underneath the oxide plug. A potential problem arising from a low breakdown voltage when additional body dopant region is formed between the field plate and the equal protection ring (EQR) is therefore eliminated. The oxide plug  128 ″ formed in the gap  126  therefore provides the advantage that an initial oxide layer commonly used to prevent a body dopant region from being implanted between the field plate and the EQR is no longer required and thus removing a mask requirement for defining the initial oxide areas. A p-body implant at 30-80 Kev with an ion beam of 3×10 13  to 3×10 14 /cm 2  flux density is applied to implant the p-body regions  130 . The oxide plug in the gap  126  between the field plate and the EQR thus blocks the p-body dopant from entering the substrate. A p-body diffusion process is then carried out at an elevated temperature of 1,000-1,200° C. for ten minutes to three hours to increase the depth of the p-body region  130  to 1.0-6.0 μm. 
     Referring to FIG. 3D, a source implant is carried out with a source ion beam of either a phosphorus ions at an energy of 60-100 Kev and ion flux density of 5×10 15  to 1×10 16 /cm 2  or an arsenic ions at an energy of 60-150 Kev and ion flux density of 1×10 15  to 1×10 16 /cm 2  to form the source regions  140 . In FIG. 3E, a BPSG or PSG is deposited to form an insulation layer  145  of approximately 5000-15,000 Å in thickness. A contact mask  148  is applied to first perform a dry etch to etch the insulation layer  145  to define a plurality of contact windows  150 . A silicon etch is performed to remove a top layer of the silicon doped with source implant ions. A wet etch is then performed to open the lateral n+ contact areas  150 . A thin layer of about 500-1000 Angstroms from the top layer of the silicon is removed by the silicon etch. A shallow body implant is performed to form a shallow high concentration body region  160  with either a low energy boron implant with an ion flux of 1×10 14  to 2×10 15 /cm 2  at about 20 to 60 Kev or a high energy BF 2  implant with an ion flux of 1×10 14 /cm 2  to 2×10 15  at about 100-240 Kev. Then a high energy body implant is carried out by either skipping a step of growing an implant oxide layer or implanting with an implant angle smaller than seven degree (7°), e.g., at zero degree relative to the perpendicular direction to the top surface of the substrate, to form a deep high concentration body region  165  with  175 - 1  to  175 - 3 . These deep and narrow gaps have an aspect ratio equal or greater than 0.5 wherein the aspect ratio is defined as: 
     
       
         Gap Aspect Ratio=(Depth of Gap)/(Gap Lateral Width)  (1) 
       
     
     For gaps  175 - 1  to  175 - 3 : 
     
       
         Aspect Ratio of Gaps≧0.5  (2) 
       
     
     The gap width of about one to four micrometers and a depth of about three to five micrometers. The depth of the gaps  175 - 1  to  175 - 3  is essentially the same as the thickness of the metal contact layer  170 . A passivation layer composed of mobile ion blocking materials such as PSG, nitride or oxyin nitride (Si x N y O z ) or combination of PSG and nitride, is deposited over the entire top surface of the device. With the gap aspect ratio equal or greater than 0.5, the mobile ion blocking material also fill up the deep and narrow gaps  175 - 1  to  175 - 3  between the metal segments. 
     Referring to FIG. 3G, a dry etching process is performed without applying a pad mask to remove the entire layer of the passivation material from the top surface. In dry etching the top layer away, only a small portion from the top of the mobile ion blocking material filled in the between-the-contact gaps  175 - 1  to  175 - 3  is removed while the major portions of the filling material in these deep and narrow gaps  175 - 1  to  175 - 3  composed of mobile ion blocking material are kept intact. 
     FIG. 4 is a cross section view for one of the deep and narrow gaps  175 - 1  to  175 - 3  with aspect ratio equal to 0.5. When a passivation layer with a layer thickness T is deposited over the top surface, the gap which has an aspect ratio of 0.5, i.e., has a lateral gap width of 2T, is filled up with the passivation layer because the layer has a thickness T and the gap width is 2T. By controlling the aspect ratio of the gaps  175 - 1  to  175 - 3 , a thick passivation layer which is almost as thick as the metal layer can be formed. 
     Referring to FIG. 5A for a planar MOSFET power device  100 ′ with structural features manufactured by applying the processing steps described above. The MOSFET power device  100 ′ further provides a special structural feature in the termination area to avoid a breakdown walkout problem. Specifically, when applying a polysilicon mask to etch the polysilicon layer, the inner segment  125 ′ is formed to have a segment D poly  where the segment width is less than the lateral diffusion length of the body dopant D L , i.e., 
     
       
         D poly &lt;D L   (3) 
       
     
     By making the segment width D poly  less than the lateral diffusion length of the body dopant D L , the p-body region  130  would then extend beyond the outer edge of the segment  125 ′. The distance which the p-body extends beyond the polysilicon segment  125 ′ is represented by δ and 
     
       
         δ&gt;0  (4) 
       
     
     In this preferred embodiment, the gap between the field plate  125 ′ and the EQR  125 ″ now filled with an oxide plug  128 ″ has a special width of approximately 0.5 to 2.0 micrometers, the segment width of the segment  125 ′ is about 0.5-1.0 micrometer. Compared to prior art process, the fabrication process of this invention has the advantage the dimension of the gap width can be controlled with high precision when the polysilicon mask is applied. In contrast, in the prior art, the width of this segment cannot be easily controlled due to a greater alignment imprecision over thick layer of metal for etching and removing the gap, i.e., gap  2013 E of Hshieh&#39;s prior art device. 
     According to FIGS. 3A to  5  and the above description, this invention discloses a method for fabricating a MOSFET device supported on a substrate  105 . The method includes the steps of (a) growing an oxide layer  120  on the substrate followed by depositing a polysilicon layer  125  and applying a gate mask as a first mask for forming a plurality of polysilicon gates  125 ; (b) depositing a NSG layer  128  overlying the top surface followed by applying an anisotropic dry etch for removing the NSG layer  128  and forming a plurality of side wall spacers  128 ′ around the gates  125  and forming an oxide plug  128 ″ filling the gap between the field plate  125 ′ and the EQR ring  125 ″; (c) implanting a body dopant followed by a body diffusion for forming body regions  130 ; (d)implanting a source dopant to form a plurality of source regions  140 ; (e) forming an overlying insulation layer  145  covering the MOSFET device followed by applying a dry etch with a contact mask  148  as a second mask to open a plurality of contact openings therethrough; (f) performing a silicon etch to remove a top portion of a source dopant area from a central portion of each of the source regions  140  followed by performing a wet etch to open a plurality of lateral source contact areas  150  above the source regions; (g)performing a low energy body dopant implant and a high energy body dopant implant to form a shallow high concentration body dopant region  160  and a deep high concentration body dopant region  165  in the body regions  130  then removing the contact mask  148 ; (h) performing a high temperature reflow process for the overlying insulation layer  145  and for driving the source regions  140  and the shallow and deep high concentration body dopant regions  160  and  165  into designed junction depths; (i) depositing a metal layer  170  followed by applying a metal mask as a third mask for patterning the metal layer to define a plurality of metal segments  170 - 1  to  170 - 4  by employing an anisotropic dry etch thus defining a plurality of deep-and-narrow gaps  175 - 1  to  175 - 3  between the metal segments  170 - 1  to  170 - 4  wherein each gap having an aspect ratio equal or greater than 0.5; (j) depositing a passivation layer  175 ′ over an entire top surface and filling the deep-and-narrow gaps between the metal segments; and (k) etching away the passivation layer over the entire top surface without applying a mask while leaving the passivation layer inside the deep-and-narrow gaps  175 - 1  to  175 - 3  substantially intact for serving a function of blocking mobile ions from entering into the MOSFET device whereby the MOSFET device is manufactured with a three-mask process. 
     In a preferred embodiment, the step (a) of applying a gate mask for forming a plurality of polysilicon gates includes a step of etching the polysilicon layer in a termination area into an inner segment and an outer segments wherein the inner segment having a width less than a lateral diffusion of the body dopant; and the step of forming the inner segment and the outer segment is a step of etching a polysilicon gap between the inner and the outer segments having an aspect ratio equal or greater than 0.5. In another preferred embodiment, the step (b) of depositing a NSG layer  148  overlying the top surface is a step of filling the polysilicon gap  126 ; and the step of applying an anisotropic dry etch for removing the NSG layer is a step of removing the NSG layer above the polysilicon gap while leaving the NSG layer as an oxide plug  128 ″ in the polysilicon gap  126  substantially intact. In another preferred embodiment, the step (a) of etching the polysilicon gap between the inner and outer segments  125 ′ and  125 ″ is a step of dry etching the polysilicon gap  126  with width substantially between 0.5 to 4.0 micrometers. In yet another preferred embodiment, the step (i) of employing an anisotropic dry etch for defining a plurality of deep-and-narrow gaps between the metal segments is a step of etching the deep-and-narrow gaps between the metal segments substantially having a width of approximately one to four micrometers. In another preferred embodiment, the step (j) of depositing a passivation layer  175  over an entire top surface and filing the deep-and-narrow gaps between the metal segments is a step of depositing a mobile ion blocking layer of PSG over the top surface and filling the deep-and-narrow gaps. In another preferred embodiment, the step (i) of depositing a passivation layer over an entire top surface and filling the deep-and-narrow gaps between the metal segments is a step of depositing a mobile ion blocking layer of silicon nitride over the top surface and filling the deep-and-narrow gaps. In another preferred embodiment, the step (i) of depositing a passivation layer over an entire top surface and filling the deep-and-narrow gaps between the metal segments is a step of depositing a mobile ion blocking layer of oxynitride over the top surface and filling the deep-and-narrow gaps. In another preferred embodiment, the step (i) of depositing a metal layer to form electric contacts through the contact openings further comprising a step of controlling a thickness of the metal layer for controlling a depth of the deep-and-narrow gaps between the metal segments. 
     Referring to FIG. 5B for a trenched MOSFET power device  200  with structural features manufactured by applying the processing steps similar to that employed for a planar device  100 ′ described above. The trenched MOSFET power device  200  also includes an oxide plug  228  filling the gap between the field plate  225 ′ and the EQR ring  225 ″ such that an initial oxide mask for defining the active area is not required. The device  200  also provides a similar structural feature in the termination area to avoid a breakdown walkout problem. Specifically, when applying a polysilicon mask to etch the polysilicon layer, the inner segment  225 ′ is formed to have a segment D poly  where the segment width is less than the lateral diffusion length of the body dopant D L , as that shown in Equation (3). By making the segment width D poly  less than the lateral diffusion length of the body dopant D L , the p-body region  230  would then extend beyond the outer edge of the segment  225 ′. The distance which the p-body extends beyond the polysilicon segment  225 ′ is represented by δ and δ&gt;0 according to Equation (4). The gap between the field plate  225 ′ and the EQR  225 ″ now filled with an oxide plug  228 ″ has a width of approximately 0.5 to 2.0 micrometers, the segment width of the segment  225 ′ is about 0.5-1.0 micrometer. 
     Therefore, the present invention provides a new MOSFET fabrication process and a new device structure to enable those of ordinary skill in the art of MOSFET fabrication to reduce the number of masks and to improve the device reliability for mobile ion protection such that the limitations and difficulties as encountered in the prior art can be overcome. Specifically, the present invention provides an improved MOSFET structure and fabrication process to reduce the number of masks required for manufacturing a MOSFET power device to three masks by taking advantage of the improved structural features and by applying modern manufacture technology such that the production costs of the MOSFET can be significantly reduced. An improved structure in the termination area is provided with an improved field plate with an oxide plug filling the gap between the filed plate and the EQR ring such that a thick initial oxide layer is no longer needed and the requirement of applying a separate active mask specifically for defining the active area by etching away a thick initial oxide layer is eliminated such that the number of masks required to fabricate a MOSFET transistor can be reduced. The improved structure in the termination area with an improved equal potential ring also provide a solution to a breakdown voltage walkout problem. Also, the requirement of applying a separate source blocking mask specifically for defining the source regions by carrying out a source implant is eliminated while the contact resistance for the source metal is reduced by removing a top portion of the substrate by a wet etch process such that the number of masks required to fabricate a MOSFET transistor can be reduced without a source blocking mask and the resistance between the source regions and the source metal can be improved. Additionally, a requirement of applying a separate pad masks specifically for defining the passivation layer to expose the areas above the source and gate metal segments are eliminated while a mobile ion blocking layer of greater thickness is provided such that the number of masks required to fabricate a MOSFET transistor can be reduced and the device reliability can be improved. 
     Although the present invention has been described in terms of the presently preferred embodiment, 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, wit 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.