Abstract:
A method of electrically conductive via formation in a fully processed wafer involves defining at least one trench area on a backside of the fully processed wafer, forming at least one trench within the trench area to an overall depth that will allow for a via formed within the trench to be seeded over its full length, forming the via within the trench into the fully processed wafer to a predetermined depth, depositing a seed layer over the full length of the via, and plating the seed layer to fill the via with an electrically conductive metal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority, under 35 USC 119(e)(1), of U.S. Provisional Patent Application Ser. No. 60/829,772, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     When working with semiconductor wafers, it is desirable to handle thick wafers to ensure they do not break during processing. However, the deeper the via, the harder it is to get a seed layer into it using currently available deposition techniques and equipment, particularly if cost is a consideration. Typically, applying a seed layer to a depth of more than 150 um in a narrow via is generally difficult and cost-ineffective. Thus, it is difficult to use the vias of the aforementioned patent applications on a wafer while maintaining the typically about 500 to 750 um (or thicker) thickness that is common to most commercially available wafers, without first thinning them, if a plating approach to filling the narrow width vias will be used. However, thinning such wafers to the appropriate thickness can render the wafers flexible but, because they are brittle, significantly likely to break during handling. While this may be an acceptable risk for a pre-processed wafer, where the cost is minimal, it is not acceptable for a fully processed wafer (i.e. one that already has undergone at least its front end processing, and in many cases its backend processing as well) because the cost is significantly higher. 
     SUMMARY OF THE INVENTION 
     We have devised a way to overcome the above problem in a manner that allows the above-referenced vias to be used on fully processed wafers without having to thin the wafer down to fragile thicknesses. Moreover, our approach does not require the use of extremely high precision or high cost techniques to do so. 
     In our approach, the thickness of only select portions of the wafer will be reduced. In this manner, within the reduced areas, the thickness is such that the above-referenced via approaches can be used and an appropriate seed layer can be deposited. Moreover, because the only portions of the area are reduced, the overall structural rigidity of the wafer can be retained to the extent necessary to allow for the necessary handling. Still further, regions where the thickness is to be reduced can be limited to areas where vias are to be located and can be etched in large fashion using coarse etching techniques if desired because the boundaries are not critical. In fact, in some cases, post via-formation thinning will remove the boundaries entirely. 
     One implementation of the approach for electrically conductive via formation in a fully processed wafer involves defining at least one trench area on a backside of the fully processed wafer, forming at least one trench within the trench area to an overall depth that will allow for a via formed within the trench to be seeded over its full length, forming the via within the trench into the fully processed wafer to a predetermined depth, depositing a seed layer over the full length of the via, and plating the seed layer to fill the via with an electrically conductive metal. 
     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 or limitations on equivalents. 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. Additional features and advantages will become apparent in the following description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A through 1H  illustrate, in simplified form, one example approach of the “trench” technique; 
         FIG. 2  illustrates, in simplified form, a wafer using one example approach as described herein; 
         FIGS. 3A through 3L  illustrate a variant of the aforementioned approach involving two or more “stacked” trenches; and 
         FIG. 4  is a photograph of a portion of a wafer created using one of the approaches described herein. 
     
    
    
     DETAILED DESCRIPTION 
     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, describe various techniques for forming small, deep vias in semiconductor wafers. The contents of those applications is incorporated herein by reference as if fully set forth herein. 
     Our techniques described in those applications allow for via densities 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 on a wafer that already has had its devices formed, and its backend processing (i.e. formation of inter-device, interconnect metalization layers) completed, or on a wafer that has not been pre-thinned. Typical semiconductor wafers are typically about 500 um to 750 um or greater in thickness. 
     However, as noted above, with wafers of that thickness, it is extremely difficult to create numerous, small densely packed vias that can extend down to device contacts or one or more of the backend metalization layers and can be seeded so that they can be metalized. The approaches described in the above-referenced patent applications generally avoid the issue by pre-thinning the wafer before via formation, if a wafer scale approach is to be used. However, as noted above, thinning the full wafer beyond a certain amount causes it to become too fragile to be handled using routine wafer processing approaches. 
     Our approach is a “trench” technique that involves first etching regions of the backside of the fully formed wafer to allow the wafer to be thin in those regions where it needs to be for deep via formation while allowing proper seed deposition, while a large portion of the wafer is maintained either at full thickness or some pre-thinned thickness that still maintains the wafer at a thickness and overall rigidity that will allow it to be further handled in a routine manner. 
     Depending upon the particular implementation, the thinned regions can:
         i) cover an area equal to or exceeding the area of an individual chip as it will exist once diced;   ii) cover smaller areas where groups of vias will be formed; or   iii) cover only those individual areas where individual vias will be formed.       

     For purposes of consistency, such a reduced area will be referred to herein as a “trench” with the understanding that the term is not intended to incorporate any particular shape, aspect (i.e. depth to width) relationship or other geometric configuration. 
     Moreover, these trenches can be created so that they can assist in some later-performed step as will be described in greater detail below. 
     Irrespective of the which of the above three approaches are used, the total area of the trenches relative as a fraction of the overall area of the wafer should typically be under 75% and typically 50% or less to ensure the structural integrity of the wafer and ability for it to be handled in a conventional manner is maintained. 
     Advantageously, trench formation can be done with the same etch processes used for forming the vias. Alternatively, the trench(es) can be formed using a lower-precision process like a wet-etch process. 
     Still further, the trench depth can be greater or lesser depending upon the via diameter and depth. Notably, the trench depth needed is one that will allow for a via that is formed into the wafer within the trench to be seeded to its complete depth. For example, a wafer with an overall thickness of 750 um, would only require a trench that would render the wafer 400 um thick in the required area if 50 um diameter vias were to be created. or a trench to render the wafer 150 um thick if 10 um diameter vias were to be made. As noted above, deeper trenches (i.e. thinner wafer regions) can also be accommodated, although the thinner the wafer within the trench, the greater the remaining area of wafer that must be left. 
     In some cases, typically those with the narrowest diameter vias, a multi-stepped approach can be used in which two or more “stacked trenches” are used to bring the wafer down to the necessary reduced thickness while maintaining the overall structural integrity. 
     Finally, while the approach is illustratively described with respect to vias such as described in the above referenced, commonly assigned, patent applications, the same approach can be used with other via formation techniques that have similar issues. 
     Turning now to the figures,  FIGS. 1A through 1G  illustrate, in simplified form, one example approach of the “trench” technique. 
       FIG. 1A  illustrates, in simplified form, a part of a semiconductor wafer  100 . As shown, the semiconductor is full thickness and ready for dicing, in that both front end and backend processing are complete. However, for purposes of this example, we presume that the wafer will also be processed to add vias from the back side down to some portion of one of the layers of metalization placed as part of the backend processing. 
     The process proceeds as follows. 
     First, as shown in  FIG. 1B , a trench  102  is formed over the area where the deep metal filled vias will be, but the wafer is too thick to allow for either via etching down to the depth required, seed deposition or both. The trench area  104  is defined and formed using, for example, a conventional dry etch or wet etch approach down to a depth  106  sufficient to define a new outer surface  108 , of sufficient distance from (in this example) the metalization point  114  that will be connected to, and to meet the criteria necessary for via creation and seed deposition down to that metalization point  114 . 
     Note that the surface shape of the trench  102  is limited only by the ability to define it. Thus, depending upon the particular application, any shape from a simple quadrilateral or circular shape to a highly complex geometric shape can be used. 
     Next, the via is formed in the desired manner. As shown in  FIG. 1C  through  FIG. 1G , for purposes of illustration, an annular via approach such as described in the above-referenced patent applications is used. 
     For purposes of illustration and simplicity, two vias  110 ,  112  within the trench  102  are shown. For purposes of understanding, the via  110  on the left will extend to an intermediate point  110  in the metalization layers, whereas the via  112  on the right will only extend to the first metalization layer  116 . However, it should be understood that, because the vias  110 ,  112  are of different depths, they would not be formed at the same time, but rather, vias of a common depth would be created at the same time if they had the same diameter. In addition, it should be understood that the annular vias  110 ,  112  are not in any way shown to scale and, in fact all proportions are grossly exaggerated and out of scale. 
     Returning to the Figures, as shown in  FIG. 1C , an annular ring shaped via trench  118  is formed. As shown in  FIG. 1D , the via trench  118  is filled with an insulator  120 . As shown in  FIG. 1E , the inner island of semiconductor material  122  within the insulator  120  is removed. As shown in  FIG. 1F , a seed layer  124  is deposited and the via filled with metal  126  using, in this example, an electroplating process. Thereafter, any additional desired processing steps, such as formation of device pads or other acts not relevant to understanding the process, are performed. 
     Finally, as shown in  FIGS. 1G and 1H , depending upon the extent of the trench used (i.e. if it was a single trench exceeding the boundaries of the chip or not) the wafer will either simply be sawn or diced (for the former case) or it can now be thinned to provide access to the contacts  128 ,  130  formed by the metal-filled vias and then diced or sawn. In either case, depending upon the thinning approach, in some implementation variants, the newly formed vias can be used as posts  132 ,  134 , in other implementation variants, the vias can have contact pads  128 ,  130  attached to them. 
     Note that, although the approach was illustrated for a pair of vias, the approach would be the same for a single via or for multiple vias (from two to literally hundreds or more), the only difference being the size or shape of the trench that would be used. 
       FIG. 2  illustrates, in simplified form, a wafer  200  using one example approach as described herein, both from the trench side ( FIG. 2A ) and, in cross section taken at A-A ( FIG. 2B ). With this approach, the trenches  202  are sized and shaped so as to be aligned with, and slightly larger than, the individual chips  204  formed on the wafer  200 . 
       FIGS. 3A through 3L  illustrate a variant of the aforementioned approach involving two or more “stacked” trenches to accommodate, for example, extremely narrow vias in a thick wafer or specific contact formation requirements. 
     As with the above approach, this approach begins with a fully formed wafer  300 , a simplified portion of which is shown in  FIG. 3A . 
     As shown in  FIG. 3B , a trench  302  is formed in the back side  304  of the wafer  300  down to a specified depth  306 , again using, for example, a wet or dry etch approach. 
     Next, as shown in  FIG. 3C , a pair of smaller trenches  308 ,  310  are formed in the bottom surface  312  of the first trench  302  in the same way as the first trench  302  was formed. To avoid confusion, note that the “same way” merely means that the lower surface  312  of the first trench  302  is treated as a starting surface (i.e. like the original surface  314  of the wafer  300  before the first trench  302  was formed). It is not intended to imply that the identical approach used to form the first trench must be used—the same or a different approach could be used. 
     After formation of the secondary depth trenches  308 ,  310 , the distance between the bottom surface  316  of the secondary trenches  308 ,  310  and the desired connection points  318 ,  320  will be within the necessary range to allow for seed deposit with the intended via sizing. 
     Now, the actual via formation process can begin and, for purposes of example and simplicity, the basic annular via approach is used (bearing in mind that, as above, dimensions are not to scale). Thus, as shown in  FIG. 3D , an annular via  322 ,  324  is created that extends from the bottom  316  of the secondary trench  308 ,  310  to the respective desired connection points  318 ,  320 , here again, a respective point in the metalization layers. 
     At this point it should be noted that, even though two different trenches  302 ,  308  were used to establish the appropriate depth, on a wafer basis, the thickness of a significant portion of the wafer remains at the original surface  314  to connection point  318  thickness N. Moreover, an even greater portion of the wafer  300  remains at a thickness of N−Z and only a small portion of the wafer is at a thickness of N−(Y+Z). Still further, using this multi-depth (or multiple stepped) approach, a flexibility in selecting the depths Y and Z is available without significant risk of weakening the wafer. 
       FIG. 3E  illustrates a view of the portion of the wafer  300  taken from the trench side.  FIG. 3E , therefore, provides an alternate view of surfaces “a”,“b”, and “c” and the annular trench “d”. 
       FIG. 3F  shows the wafer  300  after the annular vias  322 ,  324  have been filled with an insulator  326 .  FIG. 3G  shows the wafer  300  after the island  328  of semiconductor material within the annular insulator  326  has been removed down to the desired metalization connection point  318 ,  320 .  FIG. 3H  shows the wafer  300  after the void has been seeded  322  and filled with metal  324 . 
     At this point, with this approach, two new options become available. With one option, as shown in  FIG. 3I , the wafer  300  can be diced and the chips, can be thinned to expose the newly formed contacts  326 ,  328 , or the wafer  300  can be thinned before dicing—in either case followed by, for example, creation of contact pads  330 ,  332  if desired or necessary ( FIG. 3J ). With the other option, if, for example, the reason for the need for very narrow vias was related to the point to be connected to as opposed to the overall via pitch and the secondary trenches were appropriately sized and spaced, the wafer or chip (if diced) could be thinned down simply to the secondary vias ( FIG. 3K ). Thereafter, the secondary vias could themselves be filled with a conductor  334 , such as shown in  FIG. 3L , before, or as part of, a contact pad formation process. In this manner, greater wafer or chip thickness can be maintained. 
       FIG. 4  is a photograph of a portion of a wafer created using one of the above approaches. As can be seen, in the approach of  FIG. 4 , multiple trenches are used within the boundaries of a single chip, in this case on an individual via basis. 
     Finally, it is worth noting that, in some variants, the trenches can be formed so as to serve other purposes as well. For example, in various implementations, the trenches can be designed to help channel insulator into an annular trench or to confine metal that will become or otherwise form a rerouting trace. Thus, depending upon the particular implementation and configuration, the trenches can advantageously serve multiple purposes beyond merely addressing the via depth problem noted above. 
     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.