Abstract:
Systems and methods for scribing a semiconductor wafer with reduced or no damage or debris to or on individual integrated circuits caused by the scribing process. The semiconductor wafer is scribed from a back side thereof. In one embodiment, the back side of the wafer is scribed following a back side grinding process but prior to removal of back side grinding tape. Thus, debris generated from the scribing process is prevented from being deposited on a top surface of the wafer. To determine the location of dicing lanes or streets relative to the back side of the wafer, the top side of the wafer is illuminated with a light configured to pass through the grinding tape and the wafer. The light is detected from the back side of the wafer, and the streets are mapped relative to the back side. The back side of the wafer is then cut with a saw or laser.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a division of U.S. application Ser. No. 11/441,453, filed May 25, 2006. 
     
    
     TECHNICAL FIELD 
       [0002]    This application relates to processing a wafer and, in particular, to a method and/or system for dicing a wafer from the back side thereof. 
       BACKGROUND INFORMATION 
       [0003]    Integrated circuits (ICs) are generally fabricated in an array on or in a semiconductor substrate. For example,  FIG. 1  is a perspective view of a typical semiconductor wafer  100  having a plurality of ICs  110  formed thereon. The ICs  110  are separated by dicing lines or streets  112  that form a lattice pattern on a top surface of the semiconductor wafer  100 . 
         [0004]    The ICs  110  are singulated by mounting the back side of the semiconductor wafer  100  on a tape frame (not shown) and cutting along the streets  112  formed on the top surface of the semiconductor wafer  100 . Cutting is generally carried out by a cutting machine called a dicer that includes a chuck table for holding the semiconductor wafer  100  and a mechanical saw or laser for cutting the semiconductor wafer  100 . Mechanical saws generally include a rotary spindle and a cutting blade mounted on the spindle. The cutting blade may include, for example, a disk-like base and an annular cutting edge fitted to the outer peripheral portion of the side surface of the base. The cutting edge generally includes diamond abrasive grains. 
         [0005]    The streets  112  are generally visible from the top side of the semiconductor wafer  100 . Thus, from the top side of the semiconductor wafer  100 , the mechanical saw or laser may be guided along the streets  112  to cut the semiconductor wafer  100  into individual ICs. The tape frame, also referred to as dicing tape, holds the ICs  110  in place during and after the dicing process. However, the top surface of the ICs  110  are left unprotected during the dicing process and may be damaged by the mechanical saw or laser. For example, metals, low-k dielectrics, or other materials formed in the streets  112  on the top surface of the semiconductor wafer  100  can damage the ICs  110  and/or the mechanical saw. 
         [0006]      FIG. 2  is an enlarged top view of the semiconductor wafer  100  shown in  FIG. 1  illustrating metal features  210 ,  212  formed in the streets  112  on the top surface of the wafer. The metal features  210 ,  212  may include, for example, coupons or test circuits used during the manufacturing process and sacrificed when the semiconductor wafer  100  is diced. A test circuit may include, for example, a metal pattern called a test element group (Teg) applied over the semiconductor substrate  100 . The metal features  210 ,  212  tend to clog or otherwise damage diamond impregnated saws typically used in the dicing process. Mechanical sawing produces burrs because a Teg, for example, is generally made of a soft metal such as copper or the like. In addition, as thinner wafers are produced, mechanical saws cause more edge chipping. Thus, yield (e.g., the number of functioning ICs produced from the wafer) decreases. 
         [0007]    Laser dicing can also damage the ICs  110  and reduce yield. Instead of using a traditional saw blade, a laser beam is focused onto the top surface of the semiconductor wafer  100  to thereby “cut” the semiconductor wafer  100  into the individual ICs  110 . The process of laser dicing generates excessive heat and debris. The heat can cause heat affected zones and recast oxide layers. Cracks may form in the heat affected zones and reduce the die break strength. Further, the debris produced by lasers is molten in one state and can be very difficult to remove. Sacrificial coatings can be used to protect the top surface of the ICs  110  from debris during laser dicing. The sacrificial coating must then be removed after the dicing process. Another process uses a water jet in conjunction with the laser. The water jet washes the debris away during the dicing process. However, sacrificial coatings and water jet or other cleaning processes add time and expense to the overall dicing process. 
         [0008]    Therefore, a method of dicing finished semiconductor wafers that increases throughput and yield is desirable. 
       SUMMARY OF THE DISCLOSURE 
       [0009]    The embodiments disclosed herein provide systems and methods for scribing a semiconductor wafer with reduced or no damage or debris to a top surface of the semiconductor wafer caused by the scribing process. The semiconductor wafer is scribed from a back side thereof. In one embodiment, the back side of the wafer is scribed following a back side grinding process but prior to removal of back side grinding tape. Thus, debris generated from the scribing process is prevented from being deposited on a top surface of the wafer. 
         [0010]    Currently, wafers are diced from the top side primarily because the dicing lanes or streets are visible only from the top surface. Thus, a cutting tool (e.g., a laser or a saw) must be aligned to these cutting lanes or streets from the top side of the wafer. In one embodiment, an infrared light source is used to illuminate the wafer from the front side thereof to allow alignment of the cutting tool to the dicing lanes or streets from the back side of the wafer. Advantageously, dicing from the back side prevents or reduces chipping, cracking, and/or laser generated debris from depositing on the top surface of the wafer. 
         [0011]    According to the foregoing, in one embodiment, a method is provided for cutting a semiconductor wafer having a plurality of integrated circuits formed on or in a top surface thereof. The integrated circuits are separated by one or more streets visible from the top surface of the semiconductor wafer. The method includes illuminating the top surface of the semiconductor wafer with a light. A portion of the light passes through the one or more streets to a bottom surface of the semiconductor wafer. The method also includes imaging the portion of the light passing from the bottom surface of the semiconductor wafer so as to determine a location of the one or more streets relative to the bottom surface of the semiconductor wafer. After the location of the streets are determined, a portion of the bottom surface of the semiconductor wafer is cut corresponding to the location of the one or more streets. 
         [0012]    In another embodiment, a method of manufacturing integrated circuits is provided. The method includes forming multiple electronic circuit components on or in a top surface of a semiconductor wafer. The electronic circuit components are separated by one or more streets. The method also includes covering the electronic circuit components with a protective layer and removing a portion of the semiconductor wafer from a first bottom surface thereof to form a second bottom surface of the semiconductor wafer. The second bottom surface of the semiconductor wafer is then imaged to determine a location of the one or more streets relative to the second bottom surface. Then, a portion of the second bottom surface is cut corresponding to the location of the one or more streets. 
         [0013]    In another embodiment, a system is provided for cutting integrated circuits. The system includes a light source configured to illuminate a top surface of a wafer, an imaging device configured to generate image data corresponding to light from the light source that passes from the top surface of the wafer to a bottom surface of the wafer, and a processor configured to process the image data so as to map a location of a cutting lane on the top surface of the wafer to the bottom surface of the wafer. 
         [0014]    In another embodiment, a system is provided for cutting integrated circuits. The system includes means for mapping a location of a cutting lane along a surface of the wafer, the cutting lane not visible from the surface of the wafer, and means for cutting the surface of the substrate along a path corresponding to the cutting lane. 
         [0015]    Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a perspective view of a typical semiconductor wafer having a plurality of ICs formed thereon. 
           [0017]      FIG. 2  is an enlarged top view of the semiconductor wafer shown in  FIG. 1  illustrating metal features formed in the streets on the top surface of the wafer. 
           [0018]      FIG. 3  is a flowchart illustrating a process for manufacturing ICs according to an embodiment of the invention. 
           [0019]      FIGS. 4A-4G  are side view schematics of a portion of an exemplary semiconductor work piece that is thinned and cut according to the process shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]    To avoid cutting metal features (e.g., test circuits) or other materials such as low-k dielectrics formed on a top surface of a semiconductor wafer, certain embodiments disclosed herein provide systems and methods for cutting the back side of the semiconductor wafer. The back side of the semiconductor surface is also referred to herein as the “bottom” or “bottom surface” and is generally on an opposite surface from the front or top surface where ICs and dicing lines or streets are formed. 
         [0021]    For convenience, the term cutting may be used generically to include trenching (cutting that does not penetrate the full depth of a target work piece) and throughcutting, which includes slicing (often associated with wafer row separation) or dicing (often associated with part singulation from wafer rows). Slicing and dicing may be used interchangeably in the context of this disclosure. 
         [0022]    As discussed above, the streets are generally visible from the top surface of the semiconductor wafer. However, the streets are not visible from the back side of the semiconductor wafer. Thus, according to one embodiment, the top surface of the semiconductor wafer is illuminated with a wavelength of light that pass through the semiconductor wafer and be detected from the back side thereof. As discussed in detail below, in one embodiment, the light is provided by a diffuse infrared (IR) light source. The detected light is used to map the streets relative to the back side. The back side is then cut at locations corresponding to the mapped streets. 
         [0023]    In one embodiment, the back side of the semiconductor wafer is cut as part of a wafer thinning process. To reduce the thickness of ICs, semiconductor wafers are thinned after device fabrication and before dicing for individual packaging. Back side grinding is a conventional method for reducing silicon wafers from their original thickness to a diminished thickness suitable for final packaging. Grinding the back surface of the semiconductor wafer is fast and produces good total thickness variation and surface finish. Reducing the thickness of the semiconductor wafer generally improves cooling of the device after packaging. 
         [0024]    The process of thinning wafers is performed after the ICs have been formed on the top surface of the semiconductor wafer. Back side grinding tape is applied to the top surface of the semiconductor wafer to protect the ICs. The semiconductor wafer then goes into a grinding machine and the back surface is ground away until a desired thickness is achieved. In conventional processes, the back side grinding tape is then removed and the back surface of the semiconductor wafer is mounted on a tape frame. Thus, the top surface of the semiconductor wafer is exposed and cut using a mechanical saw or laser. 
         [0025]    However, according to certain embodiments disclosed herein, the back side grinding tape is left in place over the top surface of the semiconductor wafer during the cutting process. Thus, after grinding the back side, the grinding tape acts as a dicing tape to protect and hold the ICs in place while the back side of the semiconductor surface is cut. After dicing, the individual ICs can then be peeled from the grinding tape for packaging. An artisan will recognize from the disclosure herein that an individual die picked from the grinding tape may need to be flipped before being placed for packaging. Because the grinding tape is not removed from the top surface and a dicing tape is not applied to the back side of the wafer before dicing, fewer steps are used in the overall IC fabrication process. Thus, throughput is increased. Further, cutting the back side of the semiconductor wafer reduces damage to the ICs and increases yield. In one embodiment, however, the back side of the semiconductor wafer is placed on a tape frame after cutting the backside of the semiconductor wafer. The grinding tape is then removed from the top surface of the semiconductor and the tape in the tape frame is stretched to allow picking and placing of an individual die. In such an embodiment, the individual die does not need to be flipped after being picked from the tape and before being placed for packaging. 
         [0026]    Reference is now made to the figures in which like reference numerals refer to like elements. For clarity, the first digit of a reference numeral indicates the figure number in which the corresponding element is first used. In the following description, numerous specific details are provided for a thorough understanding of the embodiments of the invention. However, those skilled in the art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, or materials. Further, in some cases, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the invention. Furthermore, the described features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0027]      FIG. 3  is a flowchart illustrating a process  300  for manufacturing ICs according to one embodiment. At a step  310 , the process  300  includes forming ICs on a top surface of a semiconductor wafer. The ICs include one or more layers formed using known semiconductor IC manufacturing processes. As discussed above with respect to  FIG. 1 , the ICs are separated by streets that form a lattice pattern on the top surface of the wafer. 
         [0028]    At a step  312 , a protective layer is formed over the ICs and the top surface of the wafer. In one embodiment, the protective layer includes conventional back side grinding tape. The protective layer is configured to protect the ICs during subsequent processing steps and to hold the ICs in place after the wafer is cut. At a step  314 , the wafer is thinned by grinding its bottom surface using, for example, a grinding machine. 
         [0029]    After the bottom surface is ground away to a desired thickness, the protective layer, the ICs and the top surface of the wafer are illuminated, at a step  314 , with light configured to pass through the protective layer and the semiconductor wafer. Thus, the protective layer and the semiconductor wafer are at least partially transparent to the wavelength of the light. The wavelength is selected so as to provide the desired transparency while still providing sufficient resolution to detect the streets. The light is diffuse so as to flood a portion of the top surface within the field of view of an imaging device (discussed below). Further, the intensity of the light is configured to provide a sufficient number of photons in the flooded area so as to satisfy the sensitivity threshold of the imaging device. 
         [0030]    At a step  318 , the light passing through the bottom surface of the wafer is detected using an imaging device. The imaging device may include, for example, a CCD or CMOS imager configured to detect the wavelength of the light. The field of view of the imaging device is set so as to provide sufficient resolution so as to detect the streets. In one embodiment, the field of view of the imaging device is substantially matched to the area illuminated by the light. To achieve the desired resolution, the field of view of the imaging device may be substantially less than the area of the bottom surface of the wafer. Thus, the imaging device is scanned across the bottom surface of the wafer and the scanned images are combined to generate the overall image. 
         [0031]    At a step  320 , the detected light is used to map the street locations relative to the bottom surface of the wafer. In one embodiment, the street locations are mapped by imaging the bottom surface and using pattern recognition to determine which portions of the overall image correspond to an image of a street. In one embodiment, the pattern recognition includes techniques used to recognize patterns in aerial photographs. For example, a Hough transform technique may be used that determines whether line segments in different portions of the image are part of a longer straight line forming a street. In another embodiment, a user maps the street locations by visually locating the street locations on an image created by the imaging device and entering the street locations into a computer. The entered street locations are then used to generate a scribe map for subsequent cutting of the bottom surface of the wafer. 
         [0032]    In a step  322 , the process includes cutting portions of the bottom surface of the wafer corresponding to the mapped street locations. The bottom surface may be cut with a mechanical saw or a laser. In certain embodiments, the semiconductor wafer is scribed without cutting all the way through the wafer so as to avoid contact between a mechanical saw blade and any test devices or other structures above the top surface of the wafer. Thus, damage to the ICs and/or the saw is eliminated or reduced during the dicing process. After scribing the back side of the wafer, the wafer can be broken or otherwise diced along the scribed lines and individual ICs can be removed from the back side grinding tape for packaging. An artisan will recognize from the disclosure herein that an individual die picked from the grinding tape may need to be flipped before being placed for packaging. 
         [0033]    In one embodiment, the back side of the semiconductor wafer is placed on a tape frame after cutting the backside of the semiconductor wafer. The grinding tape is then removed from the top surface of the semiconductor and the tape in the tape frame is stretched to allow picking and placing of an individual die. In such an embodiment, the individual die does not need to be flipped after being picked from the tape and before being placed for packaging. 
         [0034]    By way of example,  FIGS. 4A-4G  are side view schematics of a portion of an exemplary semiconductor work piece  400  that is thinned and cut according to the process  300  shown in  FIG. 3 . The work piece  400  includes a silicon wafer  410  having a top surface  412  and a first bottom surface  414 . A plurality of layers  416 ,  418  are formed on the top surface  412 . As an artisan will recognize, the layers  416 ,  418  may include interconnect layers and insulation layers that form electronic circuitry. For example, the layers  416 ,  418  may include materials such as Cu, Al, SiO 2 , SiN, fluorosilicated glass (FSG), organosilicated glass (OSG), SiOC, SiOCN, and other materials used in IC manufacture. For illustrative purposes, two layers  416 ,  418  are shown. However, an artisan will recognize that fewer or more layers can be used for particular ICs. 
         [0035]    As represented by dashed lines in  FIG. 4A , in this example, a first IC area  420  and a second IC area  422  are formed in the layers  416 ,  418 . The first IC area  420  and the second IC area  422  are separated from one another by a street  424 . Although not shown, metallic test structures, low-k dielectrics, or other materials may be formed in the street  424 . In one embodiment, the width of the street  424  (e.g., the distance between the first IC area  420  and the second IC area  422 ) is in a range between approximately 8 μm and approximately 12 μm. However, an artisan will recognize that the street  424  may have other widths. For example, in other embodiments, the width of the street  424  is in a range between approximately 12 μm and approximately 50 μm. 
         [0036]    In the manufacturing stage shown in  FIG. 4A , the silicon wafer  410  has a thickness (e.g., the distance between the top surface  412  and the first bottom surface  414 ) in a range between approximately 250 μm and approximately 1000 μm. As discussed above, to thin the silicon wafer  410 , the bottom surface  414  is ground until a desired thickness is reached. 
         [0037]    As shown in  FIG. 4B , grinding tape  426  is applied over the top surface  412  and the layers  416 ,  418  The grinding tape  426  protects the first IC area  420  and the second IC area  422  while the silicon wafer is thinned by grinding the first bottom surface  414  using, for example, a grinding machine. The grinding tape  426  is transparent to infrared light. Suitable back side grinding tape is available from, for example, Furukawa Electric Co., LTD. of Tokyo, Japan, Lintec Corp. Advanced Materials Div. Of Tokyo, Japan, and Toyo Adtec Co., LTD. of Kamakura, Japan. In one embodiment, the grinding tape  426  is substantially transparent to infrared light with wavelengths ranging between approximately 1.2 μm and approximately 1.3 μm. As discussed below, the silicon wafer  410  is also substantially transparent to these wavelengths. 
         [0038]    The first bottom surface  414  is ground until a desired reduced thickness (represented by dashed line  428  in  FIG. 4B ) is achieved. As shown in  FIGS. 4C-4G , the grinding process produces a second bottom surface  430 . After grinding, the silicon wafer has a reduced thickness in a range between approximately 50 μm and approximately 200 μm. If the top surface  412  and the second bottom surface  430  are substantially smooth, it is easier to image the street locations using infrared light. Thus, in certain embodiments, the grinding process is followed by additional processes known in the art to smooth the second bottom surface  430 . 
         [0039]    After grinding the silicon wafer  410  to the desired thickness, the location of the street  424  is mapped with respect to the second bottom surface  430  so that the second bottom surface  430  can be scribed along the street  424 . As shown in  FIG. 4C , an infrared light source  432  is configured to flood a portion of the grinding tape  426 , the layers  416 ,  418  and the silicon wafer  410  with diffuse infrared light  434 . The light source  432  may include, for example, an infrared light-emitting diode (LED) configured to generate infrared light in a desired band. Although not shown, a filter may also be used to reduce the range of wavelengths produced by the light source  432  to the desired band. 
         [0040]    The wavelength of the infrared light  434  is selected so as to provide sufficient transparency through the grinding tape  426  and the silicon wafer  410 . The silicon wafer  410  tends to absorb shorter wavelengths. However, longer wavelengths may reduce the resolution such that the street  424  is not detectable from the back side of the silicon wafer  410 . In one embodiment, the wavelength is in a range between approximately 1.2 μm and approximately 1.3 μm. 
         [0041]    The intensity of the infrared light  434  is sufficient to be detectable by an imaging device  436  positioned on the opposite side of the silicon wafer  410 . The imaging device  436  may include a CCD or CMOS camera configured to detect the infrared light after it passes through the grinding tape  426  and the silicon wafer  430 . For example, the imaging device  436  may include a Germanium or InGaAs CCD. In one embodiment, the imaging device  436  comprises an Alpha NIR camera available from FLIR Systems, Indigo Operations of Goleta, Calif. 
         [0042]    As shown in  FIG. 4C , the imaging device  436  is in communication with a processor  438 , such as a microprocessor, a digital signal processor, or the like. The processor  438  controls the operation of the imaging device  436 . The processor  438  executes software stored in a storage device  442  so as to perform various tasks discussed herein such as scanning the second bottom surface  430 , generating image data, and processing the image data so as to map a location of the street  424 . The processor  438  may be in communication with a communication device  440  so as to send and/or receive image data and/or mapped street locations. 
         [0043]    The field of view of the imaging device  436  is selected so as to provide sufficient resolution to detect the street and is matched to the area flooded by the infrared light  434 . In one embodiment, the imaging device  436  has a field of view in a range between approximately 450 μm and approximately 550 μm. As discussed above, the infrared light source  432  and the imaging device  436  may be scanned over the work piece  400  and the scanned images can be combined to generate an overall image of the infrared light  434  passing through the second bottom surface  430 . The overall image is used to map the location of the street  424  with respect to the second bottom surface  430 . As discussed above, pattern recognition such as a Hough transforms or another image processing technique is used in one embodiment to determine whether line segments in various portions of the overall image are part of a longer straight line of the street  424 . In another embodiment, a user maps the street locations by visually locating the street locations on an image created by the imaging device and entering the street locations into the processor  438  for storage in the storage device  442 . The stored street locations are then used to generate a scribe map for subsequent cutting of the bottom surface of the wafer. 
         [0044]    After the location of the street  424  has been mapped with respect to the second bottom surface  430 , the second bottom surface  430  is cut along the street  424 .  FIG. 4D  illustrates a mechanical saw blade  444  cutting a kerf into the second bottom surface  430  in a location corresponding to the street  424 . An artisan will also recognize that a laser can be used instead of a saw to ablate the second bottom surface  430  of the silicon wafer  410 . However, if the laser cutting is performed while imaging is also performed, the image of the second bottom surface  430  may include speckles produced by the laser that may make it more difficult to determine the location of the street  424 . Thus, in applications where part of a wafer is imaged while another part of the wafer is cut, it is preferable to use a dicing saw. However, an artisan will recognize that image processing techniques can be used to remove speckles from the image to facilitate concurrent imaging and laser cutting. Further, in one embodiment, the location of the street  424  is mapped before the laser is activated. Thus, the image of the second bottom surface  430  does not include speckles produced by the laser. Once the street  424  is mapped, the location information is stored and the light source  432  is turned off. The second bottom surface  430  can then be cut by the laser along the street  424 . 
         [0045]    During the cutting process, the grinding tape  426  remains over the layers  416 ,  418  and the top surface  412  of the silicon wafer  410  so as to protect the IC areas  420 ,  422  from debris. The grinding tape  426  also holds the work piece  400  in place during and after the cutting. 
         [0046]    As shown in  FIGS. 4D-4F , the second bottom surface  430  is scribed in certain embodiments so as to avoid contact between the saw blade  444  and any metallic test structures, low-k dielectric material, or other materials that may be located above the top surface  412  of the silicon wafer  410  in the area of the street  424 . Thus, damage to the IC areas  420 ,  422  and/or the saw blade  444  is eliminated or reduced during the scribing process. The second bottom surface  430  may be scribed to different depths, depending on the application. For example,  FIG. 4E  illustrates a scribed portion  446  extending approximately half way between the second bottom surface  430  and the top surface  412 . As another example,  FIG. 4F  illustrates a scribed portion  448  extending substantially to the top surface  412 . 
         [0047]    As shown in  FIG. 4G , the first IC area  420  and the second IC area  422  can then be completely diced and removed from the grinding tape  426  for individual packaging. After scribing, the first IC area  420  and the second IC area  422  may be diced, for example, by breaking the silicon wafer  410  along the scribed lines. Thus, by detecting street locations with respect to the back side of the wafer  410  and cutting the back side of the wafer  410  along the street locations, debris and chipping in the first IC area  420  and the second IC area  422  can be reduced or eliminated. 
         [0048]    It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. For example, the back side of the wafer can be cut without first thinning the wafer. As another example, other wafer materials besides silicon and other light sources with different wavelengths can be used. However, substrates such as ceramics or the like are generally non-homogeneous, making it more difficult to image light passing through them. Thus, while non-homogeneous wafer materials can be used, substantially homogenous materials such as silicon are preferred. 
         [0049]    As yet another example, the grinding tape is not transparent to the light used for imaging. Rather, according to one embodiment, the grinding tape includes cut outs or transparent windows corresponding to the streets and/or street intersections. In such embodiments, illumination through the cut outs or windows is detected from the backside of the wafer to indicate the street locations. Thus, a cutting tool can then be aligned to the streets from the back side of the wafer. 
         [0050]    The scope of the present invention should be determined only by the following claims.