Abstract:
A planarizing method performed on a non-planar wafer involves forming electrically conductive posts extending through a removable material, each of the posts having a length such that a top of each post is located above a plane defining a point of maximum deviation for the wafer, concurrently smoothing the material and posts so as to form a substantially planar surface, and removing the material. An apparatus includes a non planar wafer having contacts thereon, the wafer having a deviation from planar by an amount that is greater than a height of at least one contact on the wafer, and a set of electrically conductive posts extending away from a surface of the wafer, the posts each having a distal end, the distal ends of the posts collectively defining a substantially flat plane.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to wafer processing and, more particularly, to wafer processing for electrical connections. 
       BACKGROUND 
       [0002]    Semiconductor wafers are typically highly polished with very smooth surfaces (i.e. deviations of less than 1 nm). However, they are not necessarily uniformly flat across the extent of the wafer. The same is true for wafers of ceramic or other materials. Flatness variation, called “wafer bow,” may be a result of the wafer manufacturing process itself or processing of the wafer (e.g. through depositing of metal or dielectric onto the wafer) and can be on the order of 25 μm or more on the concave and/or convex side. If the polished side is concave, the wafer is often referred to as “dished” whereas if it is convex the wafer is called “bowed.” Note however, that an individual wafer can concurrently have both types of non-planarities (i.e. one portion is bowed whereas another portion is dished. 
         [0003]    For simplicity herein, the terms “dished,” “bowed” and “non-planar” are interchangeably used herein to generically refer to a non-flat wafer of, for example, semiconductor or ceramic, irrespective of whether it would formally be called dished or bowed.  FIG. 1  illustrates, in simplified form, a conventional non-planar wafer  100 . As shown, the wafer  100  is between 500 μm and 750 μm thick and has a maximum deviation “δ” at the edges of 25 μm from flat. As a result, in the example of  FIG. 1 , the deviation from highest to lowest point across both sides is 40 μm. In most cases, with conventional processes for forming chips and interconnecting them to other chips, this amount of bow is sufficiently small relative to the size of typical connections that it can be disregarded. However, such variations can render a wafer unsuitable for use where the pitch and/or height of the individual contacts is less than or equal to 25 μm, unless further expensive polishing operations are performed to reduce the bow to an acceptable level, if it is possible to do so at all. Moreover, if the same types of connections will be used but the chip will be stacked with another chip, the bowing would be on the order of about 50 μm (i.e. taking into account the maximum deviation of 25 μm each for both chips and/or on both sides). 
         [0004]    Thus, there is a need for a way to make use of individual wafers that have bow on a side with contacts that are less in height than the bow or on a pitch where such bow could make it impossible to connect to them. 
       SUMMARY OF THE INVENTION 
       [0005]    We have devised a way to overcome the above problem, rendering wafers that are bowed by up to 20 μm each suitable for use with small pitch and/or height contacts and suitable for stacking despite their bowed nature. 
         [0006]    One aspect of the invention involves a planarizing method performed on a non-planar wafer. The method involves forming electrically conductive posts extending through a removable material, each of the posts having a length such that a top of each post is located above a plane defining a point of maximum deviation for the wafer, concurrently smoothing the material and posts so as to form a substantially planar surface, and removing the material. 
         [0007]    Another aspect of the invention involves an apparatus. The apparatus includes a non planar wafer having contacts thereon. The wafer has a deviation from planar by an amount that is greater than a height of at least one contact on the wafer. A set of electrically conductive posts extends away from a surface of the wafer. The distal ends of the posts collectively define a substantially flat plane. 
         [0008]    Through use of the approaches described herein, bowed wafers can be used with various techniques that allow for via densities, pitch and placement and involve forming small, deep vias in, and electrical contacts for, the wafers—on a chip, die or wafer scale, even though the heights or densities of the contacts thereon are small relative to wafer bow. 
         [0009]    The advantages and features described herein are a few of the many advantages and features available from representative embodiments and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages are mutually contradictory, in that they cannot be simultaneously present in a single embodiment. Similarly, some advantages are applicable to one aspect of the invention, and inapplicable to others. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates, in simplified form, a conventional non-flat wafer; 
           [0011]      FIGS. 2 through 6  illustrate, in simplified from, use of our approach on a bowed wafer that is considered “dished;” 
           [0012]      FIG. 7  illustrates, in simplified form, the wafer of  FIG. 6  after a set of chips have been connected to it; 
           [0013]      FIG. 8  through  FIG. 15  illustrate, in simplified from, use of our approach on a bowed wafer  100  that is considered “bowed;” 
           [0014]      FIG. 16  illustrates, in simplified form, the wafer of  FIG. 15  after a set of chips have been connected to it using the planarizing posts formed using the process; 
           [0015]      FIG. 17  illustrates, in simplified form, a pair of dished wafers that have been planarized according to the approach described herein and joined to each other; and 
           [0016]      FIG. 18  illustrates, in simplified form, a pair of bowed wafers that have been planarized according to the approach described herein and joined to each other. 
       
    
    
       [0017]    Note that all of the FIGS. are grossly distorted and out of scale for simplicity of presentation. 
       DETAILED DESCRIPTION 
       [0018]    U.S. patent applications, Ser. Nos. 11/329,481, 11/329,506, 11/329,539, 11/329,540, 11/329,556, 11/329,557, 11/329,558, 11/329,574, 11/329,575, 11/329,576, 11/329,873, 11/329,874, 11/329,875, 11/329,883, 11/329,885, 11/329,886, 11/329,887, 11/329,952, 11/329,953, 11/329,955, 11/330,011 and 11/422,551, incorporated herein by reference, describe various techniques for forming small, deep vias in, and electrical contacts for, semiconductor wafers. The techniques allow for via densities, pitch and placement that was previously unachievable and can be performed on a chip, die or wafer scale. In some cases, it is desirable to perform the techniques described therein on a wafer but the contact heights or densities are small relative to wafer bow. Advantageously, we have developed a way to do so.  FIGS. 2 through 6  illustrate, in simplified from, use of our approach on a bowed wafer  100  that is considered “dished.” The process is as follows: 
         [0019]    First, as shown in  FIG. 2 , a material  200  is applied to the dished side  202  of the wafer  100  to a thickness that is at least equal to, and typically more than, the maximum deflection δ on that side (as indicated by the dashed line  204 ). 
         [0020]    Depending upon the particular implementation, the material  200  could be a flowable material or fairly solid material. In general, to reduce the number of processing steps, the material will be a photoresist or photosensitive dielectric, so that it can be patterned. Alternatively, a machine-able or moldable material could be used. In the case of a substantially solid material, example suitable materials include photoresists from the Riston® dry film line of photoresist, commercially available from E. I. du Pont de Nemours &amp; Co. Specifically, The Riston® PlateMaster, EtchMaster and TentMaster lines of photoresist are suitable and at, respectively, about 38 μm, 33 μm and 30 μm in thickness, are all more than sufficient o handle the deviations at issue. 
         [0021]    In the case of a device bearing wafer, using a material  200  that can be patterned makes it easier to match and create openings over the locations of the contacts or device pads on the wafer  100 . In addition, if a substantially solid material  200  is used, the wafer can also contain unfilled vias or features extending into the wafer and there is little to no risk of those vias becoming filled by the material  200 —indeed it can protect them from becoming filled by subsequent steps if desired. 
         [0022]      FIG. 3  illustrates, in simplified form, the wafer  100  after the material has been patterned to form openings  300 - 1 ,  300 - 2 ,  300 - 3 ,  300 - 4 ,  300 - 5 ,  300 - 6 ,  300 - 7 ,  300 - 8 ,  300 - 9 ,  300 - 10  in the wafer over pre-formed connection points. 
         [0023]    Thereafter, the openings are filled with electrically conductive material, typically a metal, using any suitable process including, for example in the case of metal, deposition or plating (electro- or electroless) or some combination thereof 
         [0024]      FIG. 4  illustrates, in simplified form, the wafer  100  of  FIG. 3  after the openings  300 - 1 ,  300 - 2 ,  300 - 3 ,  300 - 4 ,  300 - 5 ,  300 - 6 ,  300 - 7 ,  300 - 8 ,  300 - 9 ,  300 - 10  have each been filled with the electrically conductive material  402 . 
         [0025]    Next, the surface  400  of the wafer  100  is polished smooth using a conventional polishing or other smooth finishing method that will result in as small a deviation as possible, with maximum deviation of less than the contact height, typically from ±0 μm to about 10 μm. However, in some implementations where a post and penetration connection will be used, that approach can allow for greater deviations due to the inherent flexibility that such connections provide. 
         [0026]      FIG. 5  illustrates, in simplified form, the wafer  100  after the polishing operation has been completed. 
         [0027]    Next, as shown in  FIG. 6 , after the material  200  has been removed, using a process appropriate to the selected material  200 , a series of elevated, conductive “posts”  600 ,  602 ,  604   606 ,  608 ,  610 ,  612 ,  614 ,  616 ,  618  will remain and, although the posts  600 ,  602 ,  604   606 ,  608 ,  610 ,  612 ,  614 ,  616 ,  618  may be of differing heights, their upper surfaces will be substantially flat (i.e. within the maximum deviation of the polishing or smooth finishing method). As a result, the connection points on the wafer  100  can now be connected to, or another chip, die or wafer can be stacked without encountering the problems of the prior art noted above. 
         [0028]      FIG. 7  illustrates, in simplified form, the wafer  100  of  FIG. 6  after a set of chips  702 ,  704 ,  706 ,  708  have been connected to it using the planarizing posts  600 ,  602 ,  604   606 ,  608 ,  610 ,  612 ,  614 ,  616 ,  618  formed using the process. 
         [0029]      FIG. 8  illustrates, in simplified form, a wafer  800  that is considered “bowed.” 
         [0030]      FIG. 9  through  FIG. 15  illustrate, in simplified from, use of our approach on the bowed wafer  800  of  FIG. 8  “bowed.” The process is as follows: 
         [0031]    First, as shown in  FIG. 9 , as with  FIG. 2 , a material  200  such as described in connection with  FIG. 2 , is applied to the wafer  800 , although, in this case, it is applied to the bowed side  802  of the wafer  100 . 
         [0032]    As illustrated in  FIG. 10 , the material  200  is again applied to a thickness that is at least equal to, and typically more than, the maximum deflection δ on that side (as indicated by the dashed line  1002 ). 
         [0033]      FIG. 11  illustrates, in simplified form, the wafer  800  after the material has been patterned to form openings  1100 - 1 ,  1100 - 2 ,  1100 - 3 ,  1100 - 4 ,  1100 - 5 ,  1100 - 6 ,  1100 - 7 ,  1100 - 8 ,  1100 - 9 ,  1100 - 10  in the wafer over pre-formed connection points. 
         [0034]    Thereafter, as above, the openings are filled with an electrically conductive material, typically metal, using any suitable process including, for example, deposition or plating (electro- or electroless) or some combination thereof 
         [0035]      FIG. 12  illustrates, in simplified form, the wafer  800  of  FIG. 11  after the openings have been filled. 
         [0036]    Next, as shown in  FIG. 13 , the wafer  800  will be polished smooth, in this case down to a level indicated by the dashed line  1300 , using a conventional polishing or other smooth finishing method that will result in it being substantially flat (i.e. having a deviation from a commercially creatable “perfectly flat” of between 0 μm and no more than about 10 μm). 
         [0037]      FIG. 14  illustrates, in simplified form, the wafer  800  after the polishing operation has been completed. 
         [0038]    Next, as shown in  FIG. 15 , after the material  200  has been removed, using a process appropriate to the selected material  200 , a series of elevated, electrically conductive “posts”  1500 ,  1502 ,  1504   1506 ,  1508 ,  1510 ,  1512 ,  1514 ,  1516 ,  1518  will remain and, although the posts  1500 ,  1502 ,  1504   1506 ,  1508 ,  1510 ,  1512 ,  1514 ,  1516 ,  1518  may be of differing heights, their upper surfaces will be substantially planar (within the maximum deviation of the polishing or smooth finishing method). As a result, the connection points on the wafer  800  can now be connected to, or another chip, die or wafer can be stacked without encountering the problems of the prior art noted above. 
         [0039]      FIG. 16  illustrates, in simplified form, the wafer  800  of  FIG. 15  after a set of chips  1602 ,  1604 ,  1606  have been connected to it using the planarizing posts  1500 ,  1502 ,  1504   1506 ,  1508 ,  1510 ,  1512 ,  1514 ,  1516 ,  1518  formed using the process. 
         [0040]    Thus, should now be appreciated that the approaches described above will allow one to readily connect, on a wafer basis, a pair of wafers that are at a maximum bowed deviation irrespective of whether they are dished or bowed in configuration. 
         [0041]      FIG. 17  illustrates, in simplified form, a pair of dished wafers  1700 ,  1702  that have been planarized according to the approach described herein and joined to each other. 
         [0042]      FIG. 18  illustrates, in simplified form, a pair of bowed wafers  1700 ,  1702  that have been planarized according to the approach described herein and joined to each other. 
         [0043]    Of course, the same approach could be used to connect a dished to a bowed or a bowed to a dished wafer in the same manner. 
         [0044]    It should thus be understood that this description (including the figures) is only representative of some illustrative embodiments. For the convenience of the reader, the above description has focused on a representative sample of all possible embodiments, a sample that teaches the principles of the invention. The description has not attempted to exhaustively enumerate all possible variations. That alternate embodiments may not have been presented for a specific portion of the invention, or that further undescribed alternate embodiments may be available for a portion, is not to be considered a disclaimer of those alternate embodiments. One of ordinary skill will appreciate that many of those undescribed embodiments incorporate the same principles of the invention and others are equivalent.