Abstract:
A method for cutting a semiconductor wafer into semiconductor chips that reduces defects at the semiconductor chip corners. The method includes a pre-cutting processing step of trimming the semiconductor chip corners so that mechanical stress is reduced at the corners. The method includes dicing channels on a semiconductor wafer thereby defining the geometrical shape of one of the semiconductor chips, modifying the corners of the one of the semiconductor chips, and cutting the semiconductor wafer to separate the one of the semiconductor chips from other semiconductor chips.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to semiconductor devices, and more particularly to a semiconductor chip shape modification and the apparatus for making such modification. 
       BACKGROUND 
       [0002]    Semiconductor wafers are generally produced in relatively large dimension in the form of large diameter disks. The semiconductor wafers are subsequently diced and cut into semiconductor chips of much smaller size for use in the production of integrated circuits. The geometric shape of the peripheral area of a semiconductor chip has a profound impact on the physical integrity of the chip. Traditionally, semiconductor chips were formed by cutting a processed wafer into predetermined pieces using a straight cutting device, such as a mechanical saw or a laser-cutting device. These mechanical straight-cutting tools create semiconductor chips that have square or rectangular shapes. Because of the ninety-degree corners in the square and rectangular shapes, there is always a tremendous amount of stress occurring at the corners of these square and rectangular shaped semiconductor chips. This additional mechanical stress causes physical defects, such as chipping and/or cracking at the edges of the semiconductor chips. Such physical defects often occur during the subsequent packaging process steps when the chips undergo additional stress. 
         [0003]      FIG. 1  depicts a prior art semiconductor wafer  100  and chip  110 . In this case, the semiconductor wafer  100  is cut using traditional straightedge mechanical saw  120  that follows a perpendicular dicing pattern  101 . Once the semiconductor wafer  100  is cut using the mechanical saw  120 , numerous chips (or dies)  110  are formed. As a result of using the mechanical saw  120  in a straight-line fashion, the resulting dies  110  have square or rectangular shapes. 
         [0004]      FIG. 2  illustrates the problem associated with this prior art semiconductor chips  110  having a square or rectangular shape as a result of using this traditional cutting method. Specifically, as depicted in  FIG. 2 , there is tremendous amount of mechanical stress built-up at the ninety-degree corners  130  of the square and rectangular semiconductor chips  110 . Such stress makes the semiconductor chip  110  prone to physical defects. For instance, when additional layers are being pressed onto the semiconductor chip  110  in a subsequent packaging process, the corners  130  of the semiconductor chip  110  may experience such as delamination or chipping. 
         [0005]    When delamination occurs, for example, the physical defect would propagate inward, towards the active area of the semiconductor chip  110 . Once the physical defect caused by delamination, such as a crack, reaches the active area, the semiconductor chip  110  would no longer be able to function properly and therefore fails. Some prior art semiconductor chips  110  include physical barriers to prevent propagation of cracks, such as introduction of a crack stop, these crack stops are largely ineffective as semiconductor technology advances and different materials are being used to make the chips. These measures attempt to fix the symptom rather than solving the root cause of the problem. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0007]      FIGS. 1 and 2  are a perspective view of a prior art wafer and a mechanical saw and a top view of a prior art chip that illustrate the problems associated with the prior art semiconductor chip cutting method; 
           [0008]      FIG. 3  is a perspective view of a wafer and a trimming device in accordance with one embodiment of the present disclosure; 
           [0009]      FIG. 4  is a close-up view of the wafer and the trimming device in accordance with one embodiment of the present disclosure; 
           [0010]      FIG. 5  is another close-up view of the wafer and the trimming device in accordance with one embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The making and using of one or more embodiments in accordance with the present disclosure are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are illustrative and do not limit the scope of the invention. Moreover, in the figures, various aspects of the structures and apparatuses have been depicted and schematically represented in a simplified manner to more clearly describe and illustrate the disclosure. 
         [0012]    By way of overview and introduction, the present disclosure is directed to a method for creating a semiconductor chip without straightedge corners, i.e., corners that are not ninety-degree angles. 
         [0013]      FIG. 1  is an illustrative perspective view showing a processed semiconductor wafer  100  before the wafer  100  has been diced into smaller, individual semiconductor chips. The semiconductor wafer  100  is typically aligned on a vacuum chuck or other holding device with reference to one or more distinct regions provided on the circumference that has a flat surface. A resin layer is placed underneath the processed semiconductor wafer  100 . In this embodiment, on the semiconductor wafer  100 , cut grooves  101  are formed in two directions perpendicular to each other, and the cut groves  101  parallel to one another are equidistantly positioned. The cut groves are in positions along the scribe lines. 
         [0014]    As  FIG. 2  shows, in the first step, a first protection layer, which is a tape having a first adhesion layer, is provided. Then, in the next step, after a semiconductor wafer  100  has been manufactured, the semiconductor wafer  100 , on which a plurality of semiconductor chips is formed, is provided. The semiconductor wafer  100  has a top surface and a bottom surface, and the bottom surface of the wafer  100  is placed on the first protection layer and is adhere to the first protection layer by the adhesive layer. 
         [0015]    In this embodiment, plurality of predefined vertical and cut grooves (or “saw streets”)  101  are formed on the semiconductor wafer  100  to define the individual semiconductor chips and to indicate how the wafer  100  would be subsequently separated into separate semiconductor chips. 
         [0016]    The semiconductor wafer  100  is adhesively bonded to a backing sheet prior to and during the dicing step. Once the wafer is pattern diced to produce a plurality of semiconductor chips, each chip must be removed from the backing sheet for further process. Generally speaking, adhesives such as acrylate adhesives are used to bond the semiconductor wafer to the backing sheet. 
         [0017]    In generally a pressure sensitive adhesive tape, which is designed to exhibit the temporary bonding desirable for use in the semiconductor wafer dicing process, is used. The adhesion level can be tailored to provide sufficient bonding strength so that the wafer can be securely held in position during dicing and cleaning of the semiconductor chip. Typical adhesion levels of such tapes as characterized by 180 degree peel on stainless steel can range as low as, but not limited to 0.5 oz/in, and as high as, but not limited to, 90 oz/in. When the pressure sensitive tape of the present invention is subjected to an elevated temperature of at least 50 degree Celsius, the adhesive becomes detackified and loses its pressure sensitive adhesive properties. Upon heat treatment, the typical peel adhesion of the tape can be decreased sufficiently low to permit removal of the diced chip from the tape. The detackification of the adhesive is irreversible. 
         [0018]    In the current disclosure, the pressure sensitive adhesive tape typically comprises a backing film, a pressure sensitive adhesive layer and a release liner to protect the adhesive coating. The backing film is typically a polymeric material, or a blend of polymeric materials. Such materials include but are not limited to polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polyester, polyamide, polyurethane, polyether, polycarbonate, polysolfone, polyketone, polyetherketone, polyimide, copolymers of styrene-diene, copolymer of butylenes terephthalate-ether, and natural or synthetic rubbers. Alternative backing materials that can be used include foam, metal foil, and paper. In generally, expandable films which exhibit good heat resistance are preferred. The backing film generally has a thickness of from 0.1 to 5 millimeters, preferably from 0.5 to 1.0 millimeter. 
         [0019]    The pressure sensitive adhesive layer of the pressure sensitive adhesive tape comprise a pressure sensitive adhesive, at least one multifunctional monomeric or oligomeric component, and/or at least one free radical initiator, and optionally, a crosslinking agent. 
         [0020]    The pressure sensitive adhesive or adhesive blend may comprise, for example, tackified natural rubbers, synthetic rubbers, tackified styrene block copolymers, polyvinyl ethers, acrylic adhesives, poly-alpha-olefins and silicone adhesives, as well as blends thereof. In some embodiments, the acrylic adhesives are polymers or copolymers of acrylic acid, t-bytylmethacrylate, butyl acrylate, 2-ethyl-hexylacrylate, glycidyl methacrylate, hydroxyethylacrylate, N-methylol acrylamide, isobornyl methacrylate, N-vinylpyrrolidone or vinyl acetate. 
         [0021]    The multifunctional vinyl monomeric or oligomeric component includes, but is not limited to, vinyl ethers, styrenic monomers, diene monomers, acrylates and methacrylates. Before cutting the semiconductor wafer  100  into to smaller dies using the traditional methods such as mechanical saws or laser cutting device, however, an additional process step is performed. 
         [0022]    In this embodiment, as  FIG. 3  shows, before cutting the finished semiconductor wafer  100  into separate dies using mechanical saws, a separate trimming device is applied to the semiconductor wafer  100 . In this embodiment, a trimming device comprises a cylindrically shaped trimming part  300 . This cylindrical-shaped trimming part  300  acts as an abrasive drill. The trimming part  300  operates by rotating in a clock-wise or a counter clock-wise direction. The axis of the rotation for the trimming part  300  is along the same direction as the length of the cylinder. The speed of the rotation, or the operating RPM, of the trimming part  300  can be varied. 
         [0023]    An important aspect of the present disclosure is the location of the trimming part  300  when being applied to the semiconductor wafer  300 . The primary purpose of applying the trimming device to the semiconductor wafer is to remove some chip material from the sharp, ninety-degree corners from the semiconductor chips. The goal is to change the sharp edge corners into round shaped edges. Therefore, the positioning of the trimming part  300  of the trimming device is controlled that, when the trimming part  300  rotates, the trimming part  300  removes the appropriate amount of the semiconductor material from the chip corners. 
         [0024]    In this embodiment, as  FIG. 4  shows, the trimming part  300  is positioned at the center of the “intersection” of a horizontal and a vertical cutting groove. In this embodiment, when the trimming part  300  is lowered onto the semiconductor wafer  100 , the trimming part  300  begins at a fixed position moves laterally during its operation. 
         [0025]    The diameter of the trimming part  300  is set depending on the amount of the semiconductor chip corner one wants to remove. The diameter setting of the trimming part  300  is when, as in this embodiment, the trimming part  300  moves in a relatively small circular path. Therefore the diameter of the trimming part  300  in this embodiment is set to be large enough so that when the trimming part  300  rotates along its vertical axis, the trimming portion will be in contact with all four corners of the four semiconductor chips. 
         [0026]    The physical elements of the trimming part  300  (i.e., the abrasive drill) will now be described. A component of the trimming part  300  of the trimming device is the grit  310  ( FIG. 4 ) on the surface of the drill. The trimming part  300  is used as an abrasive. In this embodiment, the grit  310  can be formed of diamond or CBN (cubic boon nitride), or other suitable materials that is sufficiently hard and can be used as a cutting tool. In this embodiment, the grit sizes can vary from 2 to 8 um. 
         [0027]    As  FIG. 3  shows, in this embodiment, the angular shape of the grit is preferred than blocky shape for better cutting quality. In particular, the grit should be sharpened and the sharpened edge should be exposed. 
         [0028]    Also, the size of the grit also affects both the life expectancy of the drill and cutting quality. With the bigger grit size, the drill can remove more silicon material with the same spindle speed. If the drilling surface contains larger size grit, the life expectancy of the drill is prolonged. However, large grit size would decrease the cutting quality. Therefore, the grit size selection needs to be balanced between cutting capability and manufacturing cost. 
         [0029]    Also, the grit concentration affects the quality and life expectancy of the drill. In general, higher concentration of the grit can extend the life expectancy of the drill and reduce die top chipping. On the other hand, lower concentration of the grit can lower the life expectancy of the drill but increase die top chipping. Hard bonding material can hold the grit more tightly on the drill, thus increasing the drill life expectancy. 
         [0030]    However, soft bonding material, while contribute to lower drill life expectancy, can accelerate the self-sharpening effect of the drill. 
         [0031]      FIG. 4  is a close-up view of the wafer and the trimming device in accordance with one embodiment of the present disclosure. In this embodiment, an abrasive drill  300  is placed in the center of the intersection of a vertical and a horizontal cutting grove on a processed wafer  100 . As such, the abrasive drill  300  is equidistant from the corners of four semiconductor dies  111 ,  112 ,  113 , and  114 . 
         [0032]    In the current embodiment, the diameter of the trimming part  300  is pre-selected such that when the trimming part  300  rotates, the grit on the surface of the trimming part  300  come into contact with any of the corners of the four semiconductor dies  111 ,  112 ,  113 , and  114 . As a result, the four corners of the semiconductor chips  111 ,  112 ,  113 , and  114  are being trimmed by the abrasive drill  300  at the same time. Note that in this embodiment, as discussed above, once the abrasive drill  300  is being placed into the pre-drilling position, the abrasive drill  300  is in a fixed position and does not move when the trimming commences. 
         [0033]    As discussed before, the amount of semiconductor material to be trimmed is also determined by the duration of the trimming operation and the width of the trimming part diameter, both of which can be controlled and preset. Also, the vertical distance between the trimming part and the semiconductor wafer  100  can also be controlled. In this embodiment, the trimming part  300  is placed onto the semiconductor wafer  100 , so that the base of the trimming part  300  touches the surface of the groove channel. As a result, the entire vertical portions of the semiconductor chip corners are in contact with the trimming part  300 . 
         [0034]    After the corners of the semiconductor chips  101 ,  102 ,  103 , and  104  have been trimmed and achieved an intended rounded shape, a cutting device will then be used to dice the semiconductor wafer  100  into semiconductor chips. A cutting device, such as a mechanical saw or a laser cutting apparatus, is used to cut the semiconductor wafer into a plurality of individual semiconductor chips along the channels. 
         [0035]      FIG. 5  is another a close-up view of the wafer and the trimming device in accordance with one embodiment of the present disclosure.  FIG. 5  provides a more detailed illustration with respect to how the trimming part  300  operates to alter the geometric shape of the semiconductor chip corners. In this embodiment, as  FIG. 5  shows, the trimming part  300 , when being applied to the semiconductor wafer  100 , does not stay at a fixed position. Rather, the trimming part  300  moves laterally in a pre-programmed path to the targeted positions. 
         [0036]    In this embodiment, the semiconductor wafer  100  is first aligned relative to the trimming device. Data corresponding to the arrangement and configuration of the semiconductor chips including the shape of the semiconductor chips and position may then be recognized or identified using a system for reading the surface of the semiconductor wafer  100  using, for example, a computer vision system or a scanning system. The trimming device having the trimming part  300  may then be activated and moved relative to the semiconductor wafer  100  based on the recognized data to trim the desired semiconductor material from the ninety-degree corners of the semiconductor chips. 
         [0037]    In this embodiment, the trimming part  300  is first lowered on the semiconductor wafer  100  and positioned at the center of an intersection formed by a horizontal cutting groove and a vertical cutting groove. At this point, the trimming device is not in physical contact with the corners of any semiconductor chips  111 ,  112 ,  113 , and  114 . 
         [0038]    Then, the trimming part  300  moves laterally in a pre-programmed circular path. Specifically, the trimming part  300  first moves away from its starting position towards the corner  121  of the semiconductor chip  111 . At the same time, before the trimming part  300  reaches corner  121  of the semiconductor chip  111 , the trimming part  300  begins to rotate along its longitudinal axis at a predetermined RPM. When the trimming part  300  reaches the corner  121  of the semiconductor chip  111 , the side wall of the cylindrical trimming part  300 , which is covered with a plurality of grinding grit, being in physical contact with the semiconductor chips&#39; corners. As a result, the cylindrical trimming part  300  removes portions of semiconductor material from the corner  121  of the semiconductor chip  111  (i.e., trimming corner  121  of the semiconductor chip  111 ). 
         [0039]    Next, after the trimming part  300  has removed the pre-determined amount of material from the corner  121  of the semiconductor chip  111 , the trimming part  300  moves laterally in a clock-wise direction toward corner  122  of the semiconductor chip  112 . Again, the trimming part  300 , while still rotating along its longitudinal axis, reaches the corner  122  of the semiconductor chip  112 , removes portions of semiconductor material from the corner  122  of the semiconductor chip  112 . 
         [0040]    Thereafter, the trimming part  300  moves again laterally in a clock-wise direction, trimming corners  123  and  124  of the semiconductor chips  113  and  114 , respectively. This constitutes one trimming cycle in accordance with the current disclosure. 
         [0041]    Of course, the pre-programmed path of movement for the trimming part  300  does not have to be a circular shape. In fact, the movement path for the trimming part  300  can be programmed to form any path the user desires, based on the amount of the semiconductor material the user desires to remove from the corners of the semiconductor chips. For example, the trimming part  300  can start with trimming corner  121  of the semiconductor chip  111 , and then move diagonally and trim corner  123  of the semiconductor chip  113  next. 
         [0042]    Also, at the end, the semiconductor chips may have a variety of desired shapes including, for example a circular shape, a rounded edge shape, or any other contoured geometric shape. The movement path for the trimming part  300 , therefore, can be modified to achieve the desired geometric shape for the semiconductor chips. In particular, after trimming by the trimming part  300 , the four corners of the semiconductor chip are in an octagonal shape or a round shape. After the trimming step, the angle between one side of the semiconductor chip and the side that has been trimmed is greater than ninety degrees. 
         [0043]    The embodiments discussed above solve the aforementioned problems associated with prior semiconductor chips. More specifically, the embodiments discussed above eliminate any corner with a ninety-degree angle in a semiconductor chip. 
         [0044]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.