Abstract:
A semiconductor wafer having multiple dies has a partially metallized backside. After wafer dicing, each of the multiple dies has, on its backside, a metallized area surrounded by a peripheral non-metallization ring. The non-metallization ring allows for easier optical inspection of the dies for determining the extent of any backside chipping caused by the wafer dicing. The peripheral non-metallization rings are generated by not metalizing the areas flanking the saw streets of the wafer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to integrated circuit device assembly and, more particularly, to wafer dicing or wafer saw singulation. 
         [0002]    Typical integrated circuit (IC) fabrication is a multi-step process in which rectangular dies are formed together on a single round semiconductor, e.g., silicon, wafer. Once the deposition and removal steps of component manufacture are completed, the wafer undergoes singulation, where the wafer is diced to separate the individual dies. The singulated dies may then be mounted on corresponding lead frames or substrates and encapsulated with a molding compound to generate individual packaged chips that are ready for mounting on circuit boards. Note that IC device assembly generally includes a second singulation step since, typically, multiple singulated dies are mounted on a metal sheet comprising a corresponding plurality of connected lead frames (or substrates), where the multiple dies and corresponding lead frames are batch-processed during encapsulation to generate a plurality of packaged ICs. The packaged ICs are then singulated or separated from each other, for example using a metal punch that cuts the outer portions of the lead frames to generate individual packaged ICs. 
         [0003]    A wafer that is ready for wafer dicing typically has the die&#39;s active components on a top side of the wafer and the semiconductor substrate on a bottom side. Wafer dicing may include a preliminary step of wafer thinning, sometimes called backside grinding, where the bottom side is ground down in order to make thinner, lighter, and smaller dies. Wafer dicing may also include a preliminary backside metallization step where the entire bottom side is coated with metal after thinning and before dicing. 
         [0004]    Backside metallization may be used to create an electrically conductive contact and/or a better heat-conductive contact for the dies. Backside metallization is common in power devices, such as power quad flat no-lead (PQFN) devices, since metals are highly thermo-conductive and particularly helpful in dissipating heat. As suggested by their name, PQFN devices do not have externally projecting leads. Instead, the devices are mounted on, and connected to, printed circuit boards through metal pads on the bottom and/or sides of the package, typically using solder balls. 
         [0005]    Before dicing, an adhesive tape is attached to the wafer backside in order to keep the dies in place during wafer dicing. A wafer saw is typically used to cut up the wafer. A wafer saw is a narrow circular grinding saw, which includes embedded abrasive grit, adapted for cutting wafers, along its cutting edge. The wafer saw cuts along a grid pattern of saw streets, also known as scribe lines, which separate the dies on the wafer. The cutting action of the wafer saw includes both grinding and downward pressure from the top side. By the time the wafer saw cuts through most of the wafer and approaches the bottom side, the thickness of the remaining wafer in the saw street is greatly reduced and this, combined with the downward pressure of the wafer saw, may result in cracking and the breakage of larger pieces of substrate on the bottom side than on the top side. In other words, the severity of backside chipping is often worse than that of topside chipping. The breakage from backside chipping may later cause die cracking and, consequently, device failure if the die is later sufficiently stressed. 
         [0006]    Typically, a visual or other optical inspection is performed to determine the extent of backside chipping in any particular die. However, in wafers with backside metallization, it is difficult to visually determine the extent of backside chipping since, although it may be relatively easy to see a crack line from viewing the side of the die, it is often difficult to see the depth of the crack in a bottom view of the die. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Other aspects, features, and advantages of the invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. Note that elements in the figures are not drawn to scale. 
           [0008]      FIG. 1  shows a bottom view of a wafer having a metallized backside in accordance with one embodiment of the invention; 
           [0009]      FIG. 2  shows a detail area of  FIG. 1 , which includes portions of a cut path and adjoining dies; 
           [0010]      FIG. 3  shows a bottom view of a wafer having a metallized backside, in accordance with another embodiment of the invention; 
           [0011]      FIG. 4  shows a detail area of  FIG. 3 ; 
           [0012]      FIG. 5  shows a bottom view of a wafer having a metallized backside, in accordance with yet another embodiment of the invention; 
           [0013]      FIG. 6  shows a detail area of  FIG. 5 , which includes portions of a cut path, a full die, and a partial die; and 
           [0014]      FIG. 7  shows a bottom view of a singulated die of  FIG. 1 , after wafer dicing. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. Embodiments of the present invention may be embodied in many alternative forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. 
         [0016]    As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “has,” “having,” “includes,” and/or “including” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that, in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. 
         [0017]    In one embodiment, a wafer backside is metallized such that a peripheral no-metal ring along each die edge surrounds a central metallized area of each die backside. The no-metal ring allows for easier visual inspection of the singulated die for the extent of backside chipping, which will allow for better quality control by flagging dies deemed too-severely chipped. Dies flagged as too-severely chipped may then be discarded or otherwise handled. In addition, the no-metal-ring backside metallization may also reduce the level of backside chipping that occurs during wafer dicing. 
         [0018]    Referring now to  FIG. 1 , a simplified bottom view of an exemplary wafer  100  having a metallized backside  101  in accordance with one embodiment of the invention is shown. Note that, for simplicity of description, the wafer  100  is shown having an array of eight large full dies and several peripheral partial dies. As would be appreciated by a person of ordinary skill in the art, a typical wafer comprises an array of scores or hundreds of full dies. 
         [0019]    The metallization on the backside  101  is indicated by a diagonal-line pattern. The metallized backside  101  comprises a regular rectangular grid of cut paths, including exemplary cut path  102 , which goes between, among others, exemplary adjoining dies  103  and  104 . The wafer  100  also includes partial dies along its circumference, such as exemplary partial die  105 , which are typically discarded after singulation. Partial dies are also separated from adjoining dies, whether partial or whole, by cut paths such as cut path  102 . Note that in some alternative implementations, the wafer  100  has only full dies and no partial dies. All of the cut paths include no-metal zones, which are not metallized and comprise, instead, exposed substrate. The no-metal zones form peripheral, rectangular rings around the full die backsides. In other words, the backside  101  of the wafer  100  has an array of non-contiguous metallization zones, which are separated by no-metal zones. 
         [0020]    A detail area  106 , shown as a dashed circle, which covers a section of the cut path  102  between dies  103  and  104 , is described in more detail below. The no-metal zones may be, for example, (1) zones that are metallized along with the rest of backside  101  and from which the metallization is subsequently removed to expose the substrate or (2) zones which do not get metallized, while other portions of backside  101  are metallized to generate metallized backside  101 . Backside metallization may be achieved by, for example, vapor deposition or sputtering. 
         [0021]      FIG. 2  shows detail area  106  of  FIG. 1 , which includes portions of cut path  102  and dies  103  and  104 . The cut path  102  comprises no-metal zone  201 , which extends from metallization edge  202  under die  103  to metallization edge  203  under die  104 . In other words, no-metal zone  201 , which is exposed wafer substrate material, is co-extensive with the cut path  102 . Center line  204  is at the center of the cut path  102  and marks the pre-dicing border between the adjoining dies  103  and  104 . Saw street  205 , which flanks the center line  204  on both sides and is indicated by a diagonal-line pattern, indicates the section of the wafer  100  that will be removed through grinding in the dicing step by the wafer saw (not shown). The width of the saw street  205  is substantially equal to the width of the wafer saw. 
         [0022]    Die edge  206  indicates the edge of the device-component region on the topside of die  103 . Die edge  206  is further away from center line  204  than the nearest edge of saw street  205  since device components of the dies should not be removed in the wafer dicing process; only unused border segments should be affected by the dicing. Metallization edge  202  does not extend all the way out to die edge  206 . In one implementation, die edge  206  extends about 2 mils+/−1 mils beyond metallization edge  202 . Note that a mil is one thousandth of an inch, or about 0.025 millimeters. 
         [0023]    Similarly, die edge  207  of die  104  extends about 2 mils beyond metallization edge  203 . Note that there may be a narrow buffer area between the edges of saw street  205  and die edges  206  and  207  of dies  103  and  104 , respectively, to reduce the risk of inadvertently harming device components of the dies. However, note that die edges  206  and  207  of dies  103  and  104  may extend all the way out to the corresponding edges of saw street  205 . 
         [0024]    The other cut paths of wafer  100  are substantially similar to cut path  102 . This means that each full die of wafer  100  has, on its backside, a no-metal border around a metallized central portion, where the no-metal border—or peripheral ring—extends at least a mil inside from the die edge. 
         [0025]      FIG. 3  shows a bottom view of wafer  300  having metallized backside  301 , in accordance with another embodiment of the invention. The wafer  300  is substantially similar to the wafer  100  of  FIG. 1 , with the exception of the layout of the cut paths, including exemplary cut path  302 , as described below. Dies  303  and  304  are substantially similar to the corresponding dies of wafer  100 . Detail area  306 , covering a section of cut path  302  between dies  303  and  304 , is described in more detail below. 
         [0026]      FIG. 4  shows detail area  306  of  FIG. 3 . Most of the elements of  FIG. 4  are substantially similar to the corresponding elements of  FIG. 2  and are similarly labeled, but with a different prefix. Cut path  302  has two narrow no-metal zones  401  and  409  flanking die edges  406  and  407 , respectively, rather than the single no-metal zone  201  of  FIG. 3 . Cut path  302  includes metallization zone  408 —indicated by a diagonal-line pattern—flanking center line  404  of cut path  302  and located between no-metal zones  401  and  409 . Metallization zone  408  is no wider than saw street  405  and preferably slightly narrower. 
         [0027]    No-metal zone  401  extends from a first edge of metallization zone  408  to metallization edge  402  of die  303 . Die edge  406  of die  303  is between metallization zone  408  and metallization edge  402 . In one implementation, the distance between metallization edge  402  and die edge  406  is 2+/−1 mils. 
         [0028]    Similarly, no-metal zone  409  extends from the other edge of metallization zone  408  to metallization edge  403  of die  304 . Die edge  407  of die  304  is between metallization zone  408  and metallization edge  403 . In one implementation, the distance between metallization edge  403  and die edge  407  is 2+/−1 mils. 
         [0029]      FIG. 5  shows a bottom view of wafer  500  having metallized backside  501 , in accordance with yet another embodiment of the invention. Wafer  500  is substantially similar to wafer  100  of  FIG. 1 , with the exception of the backsides of the partial dies, such as partial dies  505  and  510 . The backsides of the partial dies are not metallized. The full dies, such as exemplary dies  503  and  504 , are separated by cut paths, such as exemplary cut path  502 —which are substantially similar to cut path  102  of  FIG. 2 . The partial dies, such as dies  505  and  510  are separated from other dies by cut paths such as cut path  511 , described in further detail below in reference to  FIG. 6 , which shows detail area  506  of  FIG. 5 . 
         [0030]    In other words, backside  501  is selectively metallized so that the full dies—such as dies  503  and  504  have metallized sections centered within their respective backsides, where the outer edges of metallized sections are inside of the die edges, preferably at least 1 mil inside. Thus, the die backsides have peripheral no-metal rings surrounding metallization zones. The peripheral rings are preferably at least 1 mil wide. The other sections of backside  501 —such as the backsides of the partial dies—are not metallized. This may help reduce the amount of metal used in the fabrication of wafer  500 . Note that, although backside metallization is typically performed after device component fabrication, backside metallization—selective or otherwise—may be applied prior to device component fabrication. 
         [0031]      FIG. 6  shows detail area  506  of  FIG. 5 , which includes portions of cut path  511 , full die  504 , and partial die  510 . Cut path  511  comprises saw street  512  flanking center line  604 , where center line  604  marks the pre-dicing border between dies  504  and  510 . Saw street  512  is indicated by a diagonal-line pattern. 
         [0032]    Saw street  512  may extend out to die edge  606  of die  504 , though a narrow buffer zone between them is preferable. Die edge  606  indicates the edge of the device-component region on the topside of die  504 . The metallized backside area of die  504  extends to metallization edge  602  of die  504 . In one implementation, the distance from metallization edge  602  of die  504  to die edge  606  is 2+/−1 mils. 
         [0033]    Note that, typically, the width of the peripheral no-metal-zone ring of a die backside is substantially uniform. However, in some implementations different sides of the peripheral rings may have different widths. In addition, in some implementations, the width of any particular side of a peripheral ring may vary and not stay uniform along the entirety of the particular side. 
         [0034]      FIG. 7  shows a bottom view of singulated die  103  of  FIG. 1 , after wafer dicing. Note that other singulated full dies—such as, for example, die  104  of  FIG. 1 , dies  303  and  304  of  FIG. 3 , and dies  503  and  504  of FIG.  5 —would appear substantially similar. Cut border  701  corresponds to the portions of the saw streets—such as saw street  205  of FIG.  2 —that adjoined die  103  prior to singulation. Die border  702  corresponds to the portions of the die edges—such as die edge  206  of FIG.  2 —that adjoined die  103  prior to singulation. 
         [0035]    Metallization zone  703  is the metallized portion of the backside of die  103 . Metallization zone  703  is inside rectangular metallization border  704 . Metallization border  704  corresponds to the portions of the metallization edges—such as metallization edge  202  of FIG.  2 —that adjoined die  103  prior to singulation. The no-metal zone between metallization border  704  and cut border  701  is peripheral no-metallization ring  705 . In other words, the backside of die  103  forms a first rectangle and metallization zone  703  forms a second, smaller, rectangle concentric with the first rectangle—that is to say, the center of the backside of die  103  substantially coincides with the center of metallization zone  703 . In one implementation, the width of peripheral no-metallization ring  705  is at least 2 mils. 
         [0036]    Embodiments of the invention have been described where the metallized backside area of a die—such as metallization zone  703  of FIG.  7 —is a uniformly metallized rectangle within a peripheral no-metallization ring. In alternative embodiments, the metallized area is in a shape other than a rectangle. For example, the metallized area may be in the shape of an oval, a non-rectangular polygon, or a polygon having cut outs. Note that a non-rectangular backside metallization area of a die remains within the corresponding peripheral no-metallization ring of the die. 
         [0037]    Embodiments of the invention have been described where the no-metal zones of the metallized backside are exposed wafer substrate. In alternative embodiments, the no-metal zones comprise a non-metallic material covering the wafer substrate. 
         [0038]    Implementations of the no-metal-zone edge have been described where the metallization-zone edges are from 1 to 3 mils inward from the corresponding die edge. In alternative implementations, the metallization-zone edges are further in or out from the corresponding die edge. 
         [0039]    Embodiments of the invention have been described having particular dimensions defined. In alternative embodiments, the dimensions may vary beyond the described dimension ranges. 
         [0040]    It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. 
         [0041]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0042]    Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. As used in this application, unless otherwise explicitly indicated, the term “connected” is intended to cover both direct and indirect connections between elements. 
         [0043]    The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as limiting the scope of those claims to the embodiments shown in the corresponding figures. 
         [0044]    The embodiments covered by the claims in this application are limited to embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non-enabled embodiments and embodiments that correspond to non-statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims. 
         [0045]    Although the steps in the following method claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those steps, those steps are not necessarily intended to be limited to being implemented in that particular sequence.