Abstract:
Method for forming a through semiconductor via (TSV) in a semiconductor wafer comprising: etching an annular recess into a front side of the semiconductor wafer, the annular recess surrounding a pillar of the semiconductor material; filling the annular recess with an insulative material to form an insulative annulus; etching a recess into the front side in the pillar of the semiconductor material; filling the recess in the portion of the semiconductor material with a metal to form a through semiconductor via (TSV); thinning the semiconductor wafer from a backside of the semiconductor wafer and stopping on the insulative annulus to expose the pillar of the semiconductor material; recessing the pillar of the semiconductor material from the back side to form a recess that exposes an end of the TSV; and filling the recess with a metal to a level at least even with a level of the insulative annulus.

Description:
BACKGROUND 
       [0001]    The present invention relates to a semiconductor structure, and more specifically, to a protected through silicon via (TSV) for providing vertical interconnection in a semiconductor structure. 
         [0002]    In semiconductor technology, a through-silicon via (TSV) is a vertical electrical connection that passes through a silicon wafer, for example. TSV technology is important in creating 3D packages and 3D integrated circuits. A 3D package may contain two or more semiconductor devices stacked vertically. 
         [0003]    The through-silicon via technique may form holes in the silicon wafer by etching, for example, and then fill the holes with conductive materials, such as copper, polysilicon or tungsten to form vias or conductive channels. The wafer may be then thinned to be stacked or bonded together to form a 3D stack of semiconductor devices. 
         [0004]    Semiconductor wafers are most commonly silicon. It should be noted however that TSVs may be utilized to pass through semiconductor materials other than silicon such as gallium arsenide. In this case, the TSVs may be referred to more generally as through semiconductor vias, still denoted as TSVs. 
       BRIEF SUMMARY 
       [0005]    The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to one aspect of the exemplary embodiments, there is provided a method for forming a through semiconductor via (TSV) comprising: obtaining a semiconductor wafer having a front side and a back side; etching an annular recess into the front side so as to extend only partially through the semiconductor wafer, the annular recess surrounding a pillar of the semiconductor material; filling the annular recess with an insulative material to form an insulative annulus; etching a recess into the front side in the pillar of the semiconductor material, the recess extending to a depth less than a depth of the insulative annulus in the semiconductor wafer; filling the recess in the portion of the semiconductor material with a metal to form a through silicon via (TSV); thinning the semiconductor wafer from the backside and stopping on the insulative annulus to expose the pillar of the semiconductor material and stopping the thinning before exposing the TSV in the pillar of the semiconductor material; recessing the pillar of the semiconductor material from the back side to form a recess that exposes an end and a side of the TSV; and filling the recess with a metal to a level at least even with a level of the insulative annulus. 
         [0006]    According to another aspect of the exemplary embodiments, there is provided a method for forming a through semiconductor via (TSV) in a semiconductor wafer comprising semiconductor material, the method comprising: etching an annular recess into a front side of the semiconductor wafer, the annular recess surrounding a pillar of the semiconductor material; filling the annular recess with an insulative material to form an insulative annulus; etching a recess into the front side in the pillar of the semiconductor material; filling the recess in the portion of the semiconductor material with a metal to form a through semiconductor via (TSV); thinning the semiconductor wafer from a backside of the semiconductor wafer and stopping on the insulative annulus to expose the pillar of the semiconductor material; recessing the pillar of the semiconductor material from the back side to form a recess that exposes an end of the TSV; and filling the recess with a metal to a level at least even with a level of the insulative annulus. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
           [0008]      FIGS. 1, 2A to 5A, 2B to 5B and 6 to 11  illustrate a first exemplary process for forming a protected through semiconductor via wherein: 
           [0009]      FIG. 1  is a cross sectional view of a semiconductor wafer in which a front end of the line (FEOL) layer and a back end of the line (BEOL) layer have been formed on a front side of the semiconductor wafer; 
           [0010]      FIG. 2A  is a plan view of a semiconductor wafer and  FIG. 2B  is a cross sectional view of the semiconductor wafer in the direction of arrows B-B in which an annular recess is formed; 
           [0011]      FIG. 3A  is a plan view of the semiconductor wafer and  FIG. 3B  is a cross sectional view of the semiconductor wafer in the direction of arrows B-B in which the annular recess is filled with an insulation material to form an insulative annulus; 
           [0012]      FIG. 4A  is a plan view of the semiconductor wafer and  FIG. 4B  is a cross sectional view of the semiconductor wafer in the direction of arrows B-B in which a pillar of semiconductor material in the center of the insulative annulus is etched through the BEOL layer and FEOL layer to form a recess; 
           [0013]      FIG. 5A  is a plan view of the semiconductor wafer and  FIG. 5B  is a cross sectional view of the semiconductor wafer in the direction of arrows B-B in which the recess is filled with a metal to form a through semiconductor via (TSV); 
           [0014]      FIG. 6  is a cross sectional view of the structure of  FIG. 5B  in which the semiconductor wafer has undergone a thinning operation from the back side of the semiconductor wafer; 
           [0015]      FIG. 7  is a cross sectional view of the structure of  FIG. 6  in which a capping layer and a photoresist layer have been formed on the back side of the semiconductor wafer; 
           [0016]      FIG. 8  is a cross sectional view of the structure of  FIG. 7  in which the photoresist layer and capping layer have been opened to exposed the pillar of semiconductor material; 
           [0017]      FIG. 9  is a cross sectional view of the structure of  FIG. 8  in which the pillar of semiconductor material has been recessed; 
           [0018]      FIG. 10  is a cross sectional view of the structure of  FIG. 9  in which a diffusion barrier has been deposited; 
           [0019]      FIG. 11  is a cross sectional view of the structure of  FIG. 10  in which a capping metal has been formed in the recess. 
           [0020]      FIGS. 12 to 17  illustrate a second exemplary process for forming a protected through semiconductor via wherein: 
           [0021]      FIG. 12  is a cross sectional view of the semiconductor wafer similar to  FIG. 5B  in which the recess is filled with a diffusion barrier and a metal to form a through semiconductor via (TSV); 
           [0022]      FIG. 13  is a cross sectional view of the structure of  FIG. 12  in which the semiconductor wafer has undergone a thinning operation from the back side of the semiconductor wafer; 
           [0023]      FIG. 14  is a cross sectional view of the structure of  FIG. 13  in which a capping layer and a photoresist layer have been formed on the back side of the semiconductor wafer; 
           [0024]      FIG. 15  is a cross sectional view of the structure of  FIG. 14  in which the photoresist layer and capping layer have been opened to exposed the pillar of semiconductor material; 
           [0025]      FIG. 16  is a cross sectional view of the structure of  FIG. 15  in which the pillar of semiconductor material has been recessed; 
           [0026]      FIG. 17  is a cross sectional view of the structure of  FIG. 16  in which a diffusion barrier and a capping metal have been formed in the recess. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Referring to the Figures in more detail and particularly referring to  FIG. 1 , there is shown a cross sectional view of a semiconductor wafer  10 . The semiconductor wafer  10  has a front “active” side  12  and a back “inactive” side  14 . Device components such as transistors, capacitors, etc. may be fabricated on the front side  12  while wafer thinning operations may be performed on the back side  14 . 
         [0028]    The semiconductor wafer  10  may be any semiconductor wafer that is presently known or may exist in the future. For example, the semiconductor wafer may comprise any semiconductor material including but not limited to group IV semiconductors such as silicon, silicon germanium or germanium, a III-V compound semiconductor, or a II-VI compound semiconductor. 
         [0029]    Front end of the line (FEOL) components, such as transistors and the like, may be conventionally added to the front side  12  of the semiconductor wafer  10  to form FEOL layer  30 . Modern day semiconductor wafers usually have a back end of the line (BEOL) wiring layer  32 , consisting of several wiring sublayers, in which the various FEOL components may be connected. The individual FEOL components in the FEOL layer  30  and the various wiring sublayers in BEOL wiring layer  32  are not shown for clarity. 
         [0030]    Forming the FEOL layer and BEOL layer prior to forming the through semiconductor via is a preferred exemplary embodiment. It should be understood that the FEOL layer and BEOL layer may be formed after the forming of the through semiconductor via and then the through semiconductor via may be extended through the FEOL layer and BEOL layer if desired. 
         [0031]    Referring now to  FIGS. 2A to 5A and 2B to 5B , the “A” Figure is a plan view of a semiconductor wafer  10  and the “B” Figure is a cross sectional view of a portion of a semiconductor wafer  10  in the direction of arrows B-B in the “A” Figure. 
         [0032]    As shown in  FIGS. 2A and 2B , an annular recess  16  has been formed in semiconductor wafer  10 , including through BEOL layer  32  and FEOL layer  30 . The annular recess  16  through the BEOL layer  32  may be formed by a conventional reactive ion etching process typically used during a process to etch through an insulator. The annular recess through the FEOL layer  30  and the body of the semiconductor wafer  10  may be formed by a conventional dry silicon etching process such as a Bosch etch. A Bosch etch is an alternating etch and passivation dry etch to obtain a vertical, or at least near vertical, etch. In actual practice, the etching process to etch through the BEOL layer  32  may also etch through the FEOL layer  30 . However, the conventional reactive ion etch to etch through the BEOL layer  32  may form a tapered recess so it is preferred to switch to the Bosch etch to form the vertical or at least near vertical walls of the recess  16  through the silicon of the silicon wafer  10 . 
         [0033]    Mathematically, an “annulus” is a ring-shaped object, especially a region bounded by two concentric circles. “Annular” is used to refer to an object that is an annulus, as is the annular recess  16  in the exemplary embodiments. In the exemplary embodiments, the outer ring  18  and the inner ring  20  bound the annular recess. 
         [0034]    The annular recess  16  extends only part way into the semiconductor wafer  10  from the front side  12 . For purposes of illustration and not limitation, semiconductor wafer  10  may have a thickness of about 775 μm (micrometers) and the annular recess  16  may have a depth of about 75 μm. For purposes of illustration and not limitation, the depth of the annular recess  16  has been exaggerated with respect to the thickness of the semiconductor wafer  10 . In the center of the annular recess  16  is a pillar  23  of the BEOL layer and a pillar  22  of semiconductor material which is actually a portion of the semiconductor wafer  10  which has not been etched during the formation of the annular recess  16 . The pillar  22  of semiconductor material may also include the portion of the FEOL layer directly above the pillar  22 . 
         [0035]    The inner ring  20  (equivalent to the diameter of the pillar  23  of the BEOL layer and the pillar  22  of semiconductor material) may have a diameter of about 8 μm while the diameter of the outside ring  18  may be about 20 μm. These dimensions are for purposes of illustration and not limitation and may change as the design of the semiconductor wafer  10  may change. 
         [0036]    Referring now to  FIGS. 3A and 3B , insulator material has been conventionally deposited into the annular recess  16  to form an insulative annulus  24 . The insulator material deposited into the annular recess may be, for example, an oxide of silicon. In one exemplary embodiment, the recess  16  may also be lined with silicon nitride before deposition of the oxide of silicon. In a further exemplary embodiment, the recess  16  may be entirely filled with silicon nitride as the insulator material. Any overburden of the insulator material may be removed by a process such as chemical mechanical polishing (CMP). 
         [0037]    Normally, a lithographic mask would be provided through which the semiconductor wafer would be etched to form annular recess  16 . That lithographic mask may be removed prior to deposition of the insulator material to form the insulative annulus  24 . Such well known lithographic processing need not be shown here as it is not germane to the exemplary embodiments. 
         [0038]    In  FIGS. 4A and 4B , the pillar  23  of the BEOL layer and the pillar  22  of semiconductor material have been conventionally etched by processing similar to that of forming recess  16  to now form a recess  26  in the pillar  23  of the BEOL layer and the pillar  22  of semiconductor material. The parameters to etch the recess  26  may be somewhat different than the parameters to etch the annular recess  16  described previously, as would be known by a person skilled in the art, because of the smaller diameter of the recess  26 . In one exemplary embodiment, the recess  26  may have a diameter of about 6 μm but the actual diameter of the recess may vary from about 4 μm to 8 μm. Further, the actual diameter should be slightly less than the inner diameter of the annulus which in this exemplary embodiment is 8 μm, so that a portion of the pillar  22  of semiconductor material remains in the final structure. The recess  26  should have a depth that is slightly less than the depth of the insulative annulus  24 . For purposes of illustration and not limitation, if the insulative annulus  24  has a depth of 75 μm, the depth of recess  26  may be about 60 μm. As will become apparent hereafter, the insulative annulus  24  protects during backside grinding a via that will be formed in recess  26  by a process to be described hereafter. Thus, the recess  26  should have a depth less (i.e., extend less into the semiconductor wafer  10 ) than the insulative annulus  24 . 
         [0039]    Referring now to  FIGS. 5A and 5B , a metal, preferably copper, has been deposited into recess  26  by conventional means, for example such as electro chemical deposition (ECD). While copper is preferred, in some exemplary embodiments tungsten may also be used to fill recess  26 . Any overburden may be removed by a conventional CMP process. Prior to deposition of the metal to form the via  28 , a diffusion barrier (not shown) may be conventionally deposited on the sides and bottom of the recess  26 . Such a diffusion barrier may include, for example, combinations of tantalum nitride/tantalum and titanium nitride/tantalum. The via  28  is surrounded by the unetched portion of pillar  23  of the BEOL layer and the unetched portion of pillar  22  of semiconductor material. 
         [0040]    Normally, a lithographic mask would be provided through which the pillar  23  of the BEOL layer and the pillar  22  of semiconductor material may be etched to form recess  26 . That lithographic mask may be removed prior to deposition of the metal to form the via  28 . Such well known lithographic processing need not be shown here as it is not germane to the exemplary embodiments. 
         [0041]    In one exemplary embodiment, the semiconductor wafer  10  may be flipped over for thinning of the semiconductor wafer  10 . Thinning of the semiconductor wafer  10  may be by a conventional grinding process. Referring now to  FIG. 6 , the semiconductor wafer  10  has been thinned by a grinding process which stops on the insulative annulus  24 . 
         [0042]    As noted previously, the via  28  has a smaller depth than insulative annulus  24 . This difference in depth is important for two reasons. The first reason is that the difference in depth allows for some process variation without adversely affecting the via  28 . The second reason is that during the backside grinding process, the via  28  is protected from contact during the backside grinding process. The via  28  in one exemplary embodiment may have a diameter of about 4 to 8 μm which may be susceptible to breaking off during the backside grinding process so protecting the via  28  during the backside grinding process is very important. 
         [0043]    In a next process, referring now to  FIG. 7 , a capping layer  34  may be blanket deposited on the back side  14  of the semiconductor wafer  10  followed by a layer of photoresist  36 . The capping layer  34  may comprise an oxide or a nitride and may have a thickness of about 25 to 100 nanometers. 
         [0044]    Thereafter, the photoresist  36  may be exposed and developed to create an opening  38  through which the capping layer  34  may be conventionally etched to expose the pillar  22  of semiconductor material within the insulative annulus  24  as shown in  FIG. 8 . The pillar  22  of semiconductor material while exposed to the opening  38  also covers an end  40  of the via  28 . 
         [0045]    In order for the via  28  to be electrically connected, the end  40  of the via  28  may need to be exposed. Accordingly, as shown in  FIG. 9 , the pillar  22  of semiconductor material may be etched by a silicon reactive ion etching process to pull back the pillar  22  of semiconductor material to form recess  42 . It is noted that the end  40  of the via  28  is now exposed. Etching of the pillar  22  of semiconductor material is continued to make sure that enough of the end  40  of the via  28  is exposed. About 5 μm of the end  40  of the via  28  may be exposed after etching of the pillar  22  of semiconductor material. 
         [0046]    After the formation of the recess  42  shown in  FIG. 9 , the photoresist  36  may be conventionally stripped. Referring now to  FIG. 10 , a conductive diffusion barrier  43 , such as tantalum nitride/tantalum, may be deposited within recess  42  and over capping layer  34 . Then, as shown in  FIG. 11 , a capping metal  44 , preferably copper, but could also be tungsten or nickel, may be blanket deposited over capping layer  34  and diffusion barrier  43  and into recess  42 . Any overburden of the capping metal  44  and diffusion barrier  43  may be conventionally removed by a CMP process, stopping on the capping layer  34  to result in the structure shown in  FIG. 11 . 
         [0047]    Further processing may continue to form other redistribution wiring sublayers, to form passive circuits such as inductors or to form pads for C-4 connections. 
         [0048]    The via  28  may be subsequently connected to other semiconductor chips (not shown) or semiconductor wafers (not shown) to form 3D integrated circuit chips and/or 3D integrated circuit packages. 
         [0049]    As noted previously, the via  28  in one exemplary embodiment may have a diffusion barrier. Referring to  FIGS. 12 to 17 , there is illustrated a second exemplary embodiment in which the semiconductor wafer  100  may have a diffusion barrier formed on via  28 . Referring first to  FIG. 12 , which is a view similar to  FIG. 5B , an insulative annulus  24  and via  28  have been formed in semiconductor wafer  100 . In this second exemplary embodiment, a diffusion barrier  46 , for example tantalum nitride/tanlalum or titanium nitride/tantalum as described previously, has been formed on the sidewalls of the recess  26  (recess  26  as shown in  FIG. 4B ) prior to deposition of the metal fill. The via  28  now includes the diffusion barrier  46 . 
         [0050]    As shown in  FIG. 13 , the semiconductor wafer  100  has been thinned from the backside  14  stopping on the insulative annulus  24 . 
         [0051]    Referring now to  FIG. 14 , the capping layer  34  and photoresist  36  have been formed on the backside of the semiconductor wafer  100  and in  FIG. 15 , the photoresist  36  and capping layer  34  have been opened to expose the pillar  22  of semiconductor material through opening  38 . 
         [0052]    In  FIG. 16 , the pillar  22  of semiconductor material has been etched to pull back the pillar of semiconductor to form recess  42 . The end  40  of the via  28  is now exposed. The diffusion barrier  46  on the via  28 , which preferably comprises copper, facilitates the reactive ion etching of the pillar  22  of semiconductor material. 
         [0053]    Referring now to  FIG. 17 , the photoresist  36  has been stripped followed by deposition of a conductive diffusion barrier  43  and a capping metal  44 , preferably copper, over capping layer  34  and into recess  42 . Any overburden of the capping metal  44  and diffusion barrier  43  may be conventionally removed by a CMP process, stopping on the capping layer  34  to result in the structure shown in  FIG. 17 . 
         [0054]    It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.