Abstract:
In one embodiment of the present invention, a method for fabricating semiconductor devices comprises forming an active region about a front-side of a substrate. A plurality of trenches are then formed about a back-side of the substrate. A grid of banks separates the trenches. A conductive material is then applied to the back-side of the substrate. The trenches and the conductive material act to reduce the on-state resistance of the substrate and enhance thermal conductivity, while the grid of banks maintains the structural strength of the wafer.

Description:
FIELD OF THE INVENTION 
     Embodiments of the present invention relate to semiconductor device fabrication, and more particularly to reduced substrate resistance characteristics. 
     BACKGROUND OF THE INVENTION 
     For power metal oxide semiconductor field effect transistors (MOSFET), substrate resistance contributes to the total on-state resistance as a parasitic component. A vertical semiconductor device like a vertical double-diffused MOSFET (VDMOSFET), a trench MOSFET (TMOSFET), an insulated gate bipolar transistor (IGBT), or the like, represent a power switch. The resistance in the on-state of the switch is composed of a series connection of resistive elements. 
       FIG. 1A  shows a cross sectional view of an VDMOSFET device employed in the conventional art. While  FIG. 1B  shows a corresponding series connection of resistive elements modeling the conductive path in the device in the on-state employed in the conventional art. The on-state resistance (R ds-on ) comprises metal film resistance (Rm)  110 ,  120 , junction between metal film and semiconductor (i.e. drain and source) resistance (Rj)  130 ,  140 , channel resistance (Rc)  150 , drift region resistance (Rd)  160 , and substrate region resistance (Rs)  170 . In the case of current low voltage power MOSFETs the substrate resistance (Rs)  170  contribution can be at least 40% of the total on-state resistance (Rds-on). Reducing the on-state resistance is beneficial to making the switch more efficient. 
     As semiconductor technology progresses, reduced on-state resistance becomes critical. In the conventional art, thinning the substrate by a method such as back-lapping and polishing the wafer can achieve some reduced on-state resistance. Wafer thickness is currently about 200 μm. Thinner wafers are possible. However, thin wafers can easily break during handling thereby resulting in lower manufacturing yields. It would be desirable to reduce the on-state resistance of the power switch without resulting in the poor manufacturing yields attributed to very thin wafers. 
     SUMMARY OF THE INVENTION 
     Thus, there is a need for a semiconductor device having improved on-state resistance. What is needed further is a means for reducing on-state resistance of a semiconductor power switch without reducing manufacturing yields, e.g., without making the wafer overly thin. 
     In one embodiment of the present invention, a method for fabricating semiconductor devices comprises forming an active region about a front-side of a substrate. A plurality of trenches is then formed about a back-side of the substrate. A grid of banks separates the trenches. The trenches act to reduce the on-state resistance, while the grid of banks maintains the structural strength of the wafer. 
     In another embodiment of the present invention, a method for fabricating semiconductor devices comprises forming an active region about a front-side of a substrate. A plurality of trenches is then formed about a back-side of the substrate. A grid of banks separates the trenches. A conductive layer is then deposited on the back-side of the substrate. The trenches act to reduce the on-state resistance, while the grid of banks maintains the structural strength of the wafer. The conductive layer further improves the resistance of the substrate during the on-state of the device. 
     In another embodiment of the present invention, a method for fabricating semiconductor devices comprises forming an active region about a front-side of a substrate. A plurality of trenches is then formed about a back-side of the substrate. A grid of banks separates the trenches. A conductive material is then deposited on the back-side of the substrate and fills the trenches. The trenches act to reduce the on-state resistance, while the grid of banks maintains the structural strength of the wafer. The conductive material and trenches further increases thermal and electrical conductivity of the substrate during the on-state of the device. 
     Embodiments of the invention reduce substrate resistance measured across the wafer without any negative impact on yield of the device during manufacturing. The solution according to the invention is to modify the substrate by etching trenches from the back-side and filling them with a conductive material. In one embodiment, the trenches being randomly distributed across the wafer collect the majority of the current flowing between the device contacts on the front-side of the substrate and its back-side metallization. In another embodiment, the trenches being aligned to the layout of the devices at the front-side of the wafer collect the majority of current flowing between the device contacts on the front-side of the substrate and its back-side metallization. 
     In one embodiment, the trenches are etched after the completion of the front-side manufacturing of the device. The front-side of the processed wafer is covered with an insulating film called a passivation layer. The wafers leave the clean room and are grinned from the back-side to a predefined thickness, e.g. 200 μm. 
     In one example, the back-side of the substrate is covered with a photoresist which is used as a mask to etch the trenches. The mask openings are not aligned to the devices located at the front-side of the substrate. The opening dimensions are less than one quarter (¼) of the edge dimension of the device die in one example, and the opening locations are random with respect to the device dies. The depth of the trenches may exceed half the thickness of the finished wafer. The photoresist mask is removed and a standard back-side metal can be deposited. In an exemplary embodiment, the back-side metal may be solderable, e.g., it can consist of a tri-metal layer of tin, nickel, and silver (Ti/Ni/Ag). 
     In one embodiment, a photoresist mask is applied to the front side of the wafer and pad openings may be etched in the passivation layer to define the location of the electric contacts to the devices. The wafer is sawed to singulate the dies and the trenches from the back-side are filled with a conductive material, e.g. it can be a solder paste or a conductive epoxy. 
     As another embodiment of the present invention, the trenches are filled with the conductive material after the deposition of the back-side metal and before the singulation of the dies. 
     Accordingly, embodiments of the present invention provide semiconductor devices having trenches and a conducting layer on the back-side of the substrate, adapted to reduce substrate resistance. Embodiments of the present invention also provide semiconductor devices having banks on the back-side of the substrate, adapted to assure mechanical stiffness of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1A  shows a cross sectional view of a VDMOSFET device employed in the conventional art. 
         FIG. 1B  shows a corresponding series connection of resistive elements modeling the conductive path in a VDMOSFET ( FIG. 1A ) device employed in the conventional art. 
         FIG. 2  shows a flow diagram of a process for reducing substrate resistance in accordance with one embodiment of the present invention. 
         FIGS. 3A ,  3 B,  3 C,  3 D,  3 E, and  3 F show side-sectional views of a wafer being processed in accordance with one embodiment of the present invention. 
         FIG. 4  shows a back-side plain view of a patterned photoresist layer on a wafer in accordance with one embodiment of the present invention. 
         FIG. 5  shows a back-side plain view of a wafer having a grid of trenches etched therein in accordance with one embodiment of the present invention. 
         FIG. 6  shows a back-side plain view of a wafer having a grid of trenches etched therein in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Embodiments of the present invention reduce substrate resistance by etching trenches into the substrate from the back-side and filling the trenches with a conductive material. Embodiments of the present invention also maintain the structural strength of the substrate. Thus, embodiments of the present invention advantageously reduce on-state resistance without reducing manufacturing yield. 
     Referring now to  FIG. 2 , a flow diagram of a process for reducing substrate resistance in accordance with one embodiment of the present invention is shown. As depicted in  FIG. 2 , device fabrication begins with various well known device fabrication processes, at steps  210 . The various device fabrication processes may include deposition, implanting, diffusion, etching, masking, photolithography, and the like. The various device fabrication processes result in formation of an active layer on the front-side of the substrate. 
     The active layer is comprised of a plurality of die regions. Each die region is comprised of one or more components, interconnects, and the like. The plurality of die regions is laid out in a pattern on the front-side of the wafer. Each die region is separated from another by scribing borders. The scribing borders allow for separation of the die regions at the end of processing, so that they may be packaged as individual devices. 
     A passivation layer is then formed on the front-side of the substrate, at step  215 . The passivation layer may be formed by any well-known method, such as deposition, evaporation, sputtering, chemical vapor deposition (CVD), or the like. The passivation layer provides electrical component isolation. The passivation layer also provides physical protection for the active region, during subsequent handling and fabrication processes. 
     The substrate is then thinned, at step  220 . The substrate may be thinned by any well-known method, such as back-lapping and polishing or the like. The back-lapping and polishing process is performed on the back-side of the wafer. In an exemplary configuration, the resulting substrate has a final thickness of 150-300 μm. 
     A photoresist is then coated on the back-side of the wafer, at step  225 . The photoresist is then patterned at step  230 . The patterning may de done by any well-known method, such photolithography, or the like. 
     Photolithography typically utilizes a photomask and ultraviolet light, wherein the photoresist is selectively exposed to form a desired pattern of areas covered with photoresist and uncovered areas. In one implementation, the shape, size and location of the mask openings may be of any configuration. In another implementation, the mask provides a pattern of openings not aligned to the die regions on the front-side of the substrate. The opening locations are random with respect to the die regions. In another implementation, the mask opening dimensions are less than one-quarter (¼) of the edge dimension of the die region. Thus, there are a few openings placed on the back-side, corresponding to the area of a die region. In another implementation, the openings are in the shape of small squares forming a modified chessboard pattern. In another implementation, the openings are arranged so that areas on the back-side of the wafer, which correspond with the scribing borders on the front-side of the wafer, do not contain openings. Aligning the pattern of die regions on the front of the wafer and the openings on the back of the wafer can be accomplished with any well-known means. 
     The back-side of the substrate is then etched at step  235 . The etching may be done by any well-known method, such as wet etching, dry etching or the like. The etching process forms a pattern of one or more trenches and banks on the back-side of the substrate, as defined by the patterned photoresist. 
     In one implementation, the etching process may have a significant isotropic component. Thus, the openings of the resulting trenches are greater than the opening in the patterned photoresist. In another implementation, the isotropic etching process results in trench walls with sloped sides. In an exemplary configuration, the slope of the trench walls is less than 85°. 
     In another implementation, the banks form a common grid assure mechanical stiffness of the wafer. In another implementation, the banks do not form any straight lines corresponding to the pattern of the die regions. In another implementation, areas on the back-side of the wafer, which correspond with the scribing borders on the front-side of the wafer, do not contain trenches. 
     The photoresist is then removed from the back-side of the wafer, at step  240 . The photoresist removal process may be done by any well-known method, such as chemical resist strip, resist ashing, or the like. 
     The back-side of the wafer is then subjected to a metallization process, at step  245 . The metallization process can be performed by any well-known means, such as evaporation, sputtering, or the like. In one implementation, the resulting conductive layer conformal coats the back-side of the substrate, including the banks, trench side-walls, and trench floors. 
     In one implementation of the present invention, the conductive layer is formed by a tri-metal multi-layer, which provides good electric contact to the substrate and a solderable surface. In an exemplary configuration the tri-metal multi-layer consists of tin, nickel and silver (Ti/Ni/Ag). In another implementation, the conductive layer is a conductive material, which planarizes the back-side of the wafer. The planarizing conductive material fills the trenches. In an exemplary configuration, the planarizing conductive material is a solder paste. In another exemplary configuration, the planarizing conductive material is a conductive epoxy. 
     Device fabrication then continues with various device fabrication processes, at steps  250 . The various device fabrication process steps may include forming contact pad regions, singulation of the wafer into individual die, die attachment, wire bonding, and the like. 
     In an alternative embodiment of the present invention, the trenches are filled with a conductive material after the deposition of the metal layer on the back-side of the wafer and before the singulation of the dies. The conductive material planarizes the back-side of the wafer prior to singulation. 
     The methods of the present embodiments provide the advantage of reducing substrate resistance. The methods of the present embodiments also provide the advantage of collecting the majority of current flowing between the device contacts, by the back-side metallization. The methods of the present embodiments also provide the advantage of maintaining structural strength of the substrate. 
     Referring now to  FIGS. 3A-3F , side-sectional views of a wafer  305  being processed in accordance with one embodiment of the present invention are shown. As depicted  FIG. 3A , the wafer  305  initially comprises a substrate  310  with an active region formed thereon  315  (i.e. front-side). A passivation layer  320  covers the active region. In one exemplary configuration, the wafer  305  has a thickness of 150-300 μm. 
     As depicted in  FIG. 3B , a photoresist layer  325  is deposited on the substrate  310  (e.g. back-side). The photoresist is patterned to form a plurality of openings  330 . Trenches  335  are formed in the substrate  310  under the plurality of openings  330  in the photoresist layer  325 . Banks  337  of substrate  310  remain where the photoresist layer  325  is present. 
     In one implementation of the present invention, the depth of the trenches  335  exceeds half the thickness of the substrate  310  and does not extend to the active region  315 . In an exemplary configuration, the depth of the trenches  335  is approximately 100-200 μm. In another implementation, the sides  340  of the trenches  335  are sloped. In an exemplary configuration, the sloped sides  340  of the trenches  335  have an angle α, which is less than 85°. In another implementation, the size, shape and spacing of the trenches  335  may be of any configuration. In another implementation, the trenches  335  are randomly spaced with respect to corresponding die regions on the front-side of the wafer  305 . In another implementation, the trenches  335  are not located corresponding to scribing borders between the die regions on the front-side of the wafer  305 . In another implementation, the plurality of trenches  335  are located in a region corresponding to each die region on the front-side of the wafer  305 . 
     In another implementation, the width and/or length of the trenches  335  is a fraction of the edge length and/or width of the die regions. In an exemplary configuration, the width and/or length of the trenches  335  is approximately one quarter (¼) of the edge length and/or width of the die regions. In another implementation, the shape of the trenches  335  are square and arranged to form a modified chessboard pattern with a grid of banks separating the trenches. In one implementation, the trenches  335  should not be of sufficient length and/or width to form a straight groove, susceptible to cleaving, in the pattern of the grid of banks  337 . Thus, the banks  337  form a common grid assuring mechanical stiffness of the wafer  305 . 
     As depicted in  FIG. 3C , the photoresist layer  325  is stripped and a conformal conductive layer  345  is deposited on the substrate  310  (e.g. back-side). 
     In one implementation of the present invention, the conductive layer  345  is a tri-metal multi-layer, which provides good electric contact to the substrate, and a solderable surface. In an exemplary configuration the tri-metal multi-layer consisting of Ti/Ni/Ag. 
     As depicted in  FIG. 3D , the passivation layer  320  is patterned to define contact pad openings  350 . 
     As depicted in  FIG. 3E , the wafer  305  is separated, along the scribing borders, into individual die  355  (a single die is shown for illustrative purposes). Furthermore, the trenches are filled with a conductive material  360 , which planarizes the back-side of the substrate  310 . In an exemplary configuration, the planarizing conductive material  360  is a solder paste. In another exemplary configuration, the planarizing conductive material  360  is a conductive epoxy. 
     As depicted in  FIG. 3F , the die  355  is packaged on a frame  365 . The die and frame form a subassembly of the final device packaging, such as TO-type package, small outline integrated circuit (SOIC), or the like. 
     In an alternative embodiment of the present invention, the trenches  335  are filled with a conductive material  360  after the deposition of the back metal layer  345  and before singulation of the dies. 
     The present embodiments provide the advantage of reducing substrate  310  resistance. The present embodiments also provide the advantage of collecting the majority of current flowing between the device contacts, by the back-side metallization  345 . The present embodiments also provide the advantage of maintaining structural strength of the substrate  310 . 
     The present embodiments provide the advantage of reducing substrate  310  resistance, both thermal and electrical. The substrate  310  is thinner and the back-side conductive layer  345  and/or conductive material  360  is closer to the active region  315 . Therefore, the thinner substrate  310  provides for reduced electrical resistance. Furthermore, the back-side conductive layer  345  and/or conductive material  360  readily removes thermal heat generated in the substrate  310 . While, the back-side conductive layer  345  and/or conductive material  360  also collects the majority of current flowing between device contacts, such as front-side source contacts and back-side drain contact. Thus, the present embodiments also provide the advantage of increasing device reliability by reducing thermal and electrical resistance. The present embodiments also provide the advantage of maintaining structural strength of the wafer  305 . 
     Referring now to  FIG. 4 , a back-side plain view of a patterned photoresist layer  410  on a wafer  405  in accordance with one embodiment of the present invention is shown. As depicted in  FIG. 4 , the back-side of the wafer  405  is substantially covered with a photoresist  410 . Sections of the photoresist  410  have been removed to define a pattern of openings  415 . 
     In one implementation, the size, shape and spacing of the openings  415  may be of any configuration. In another implementation, the openings  415  are randomly spaced with respect to corresponding die regions on the front-side of the wafer  405 . In another implementation, the width and/or length of the openings  415  is a fraction of the edge length and/or width of the die regions. In an exemplary configuration, the width and/or length of the openings  415  is less than one quarter (¼) of the edge length and/or width of the die regions. In another implementation, the openings  415  are arranged so that the areas on the back-side of the wafer  405 , which correspond with scribing borders on the front-side of the wafer  405 , do not contain openings  415 . In another implementation, a plurality of openings  415  are located corresponding to each die region on the front-side of the wafer  405 . In another implementation, the shape of the openings  415  are square and arranged to form a modified chessboard pattern with a grid of photoresist  410  separating the openings  415 . In another implementation, the openings  415  should not be of sufficient length and/or width to form a straight groove in the pattern of the grid of photoresist  410 . 
     Referring now to  FIG. 5 , a back-side plain view of a wafer  505  in accordance with one embodiment of the present invention is shown. As depicted in  FIG. 5 , the back-side of the wafer  505  has a plurality of trenches  515  etched into a substrate  510 . Banks  520  separate the trenches  515 . 
     In one implementation of the present invention, the depth of the trenches  515  exceeds half the thickness of the substrate  510 . In an exemplary configuration, the depth of the trenches  515  is approximately 100-200 μm, for a wafer  505  of approximately 150-300 μm thick. In another implementation, the sides of the trenches  515  are sloped. In an exemplary configuration, the sloped sides of the trenches  515  have an angle α, which is less than 85°. In another implementation, the size, shape and spacing of the trenches  515  may be of any configuration. In another implementation, the trenches  515  are randomly spaced with respect to corresponding die regions on the front-side of the wafer  505 . In another implementation, the width and/or length of the trenches  515  is a fraction of the edge length and/or width of die regions on the front-side of the wafer  505 . In an exemplary configuration, the width and/or length of the trenches  515  is approximately one quarter (¼) of the edge length and/or width of the die regions. In another implementation, areas on the back-side of the wafer  505 , which correspond with scribing borders on the front-side of the wafer  505 , do not contain trenches  515 . In another implementation, the shape of the trenches  515  are square and arranged to form a modified chessboard pattern with a grid of banks  520  separating the trenches  515 . In another implementation, the trenches  515  should not be of sufficient length and/or width to form a straight groove, susceptible to cleaving, in the pattern of the grid of banks  520 . Thus, the banks  520  form a common grid assuring mechanical stiffness of the wafer  505 . 
     The present embodiment provides the advantage of reducing substrate  510  resistance, both thermal and electrical. The present embodiment also provides the advantage of maintaining structural strength of the wafer  505 . 
     Referring now to  FIG. 6 , a back-side plain view of a wafer  605  having a grid of trenches  615  etched therein in accordance with another embodiment of the present invention is shown. As depicted in  FIG. 6 , the back-side of the wafer  605  has a plurality of trenches  615  etched into a substrate  610 . Banks  620  separate the trenches  615 . 
     In one implementation of the present invention, the depth of the trenches  615  exceeds half the thickness of the substrate  610 . In an exemplary configuration, the depth of the trenches  615  is approximately 100-200 μm, for a wafer  605  of approximately 150-300 μm thick. In another implementation, the sides of the trenches  615  are sloped. In an exemplary configuration, the sloped sides of the trenches  615  have an angle α, which is less than 85°. In another implementation, the size, shape and spacing of the trenches  615  may be of any configuration. In another implementation, the trenches  615  are randomly spaced with respect to corresponding die regions on the front-side of the wafer  605 . In another implementation, the width and/or length of each trench  615  is a fraction of the edge length and/or width of the die regions. In an exemplary configuration, the width and/or length of each trench  615  is approximately one quarter (¼) of the edge length and/or width of the die regions. In another implementation, areas on the back-side of the wafer  605 , which correspond with scribing borders on the front-side of the wafer  605 , do not contain trenches  615 . 
     In another implementation, a plurality of trenches  615  are placed in a region corresponding to each die region on the front-side of the wafer  605 . In another implementation, the shape of the trenches  615  are various sized squares and arranged to form a modified chessboard pattern with a grid of banks  620  separating the trenches  615 . In another implementation, the trenches  615  should not be of sufficient length and/or width to form a straight groove, susceptible to cleaving, in the pattern of the grid of banks  620 . Thus, the banks  620  form a common grid assuring mechanical stiffness of the wafer  605 . 
     The present embodiment provides the advantage of reducing substrate  610  resistance, both thermal and electrical. The present embodiment also provides the advantage of maintaining structural strength of the wafer  605 . 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.