Abstract:
Methods for forming die that have minimal edge and surface damage are provided. Die formed by these methods are less susceptible to cracking and breakage. Thus, yield and performance of devices fabricated with die formed by these methods are advantageously improved. To form the die, trenches are formed in a wafer around the peripheral edge of the die by processes that cause only minimal damage to the edges of the die. The wafer is cut through the trenches into sections containing the die without contacting the edge of the die. The sections are then mounted onto a holder and thinned to produce the die.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to thinning and dicing of wafers. In particular, the present invention relates to a method of thinning and dicing wafers that reduces edge defects. 
     2. Description of the Background 
     Solid materials having semiconductor or opto-electronic properties are used in applications ranging from the well-known integrated circuits of computer technology to sensors and optical devices used in advanced communications. For these applications, the materials must often be cut, thinned, and polished to a precise size, to meet the dimensional tolerances and surface characteristics required by the application. 
     Bulk materials are usually first cut into sheets or wafers for subsequent processing, dicing, and thinning procedures. To create smaller pieces or sections of the material, the sheet or wafer is diced. Dicing is a term from the semiconductor industry and refers to cutting a semiconductor wafer into squares or rectangles known as “die”. In semiconductor applications, circuitry has usually been fabricated into each die before it is cut. A semiconductor-dicing saw is used to saw the sheet or wafer into individual die. 
     Thinning is traditionally performed by mounting or bonding the material onto a substrate device and then using lapping, polishing, or diamond point turning systems to reduce the thickness of the material. In the manufacture of semiconductors, thinning is performed on a wafer or large piece of crystal material in which semiconductor processing may or may not have been performed. In other applications, the material may be thinned after the wafer or sheet is cut into die. 
     A problem with dicing and thinning is that these processes can damage the material. FIG. 1A illustrates a saw blade  100  cutting through a wafer  105  and producing a cut  110  in the material. As the saw blade  100  moves through wafer  105  it removes crystal material producing the cut. As illustrated in FIG. 1B, the saw also produces edge damage  120 , in which chips, jagged edges, microcracks, fractures and other defects occur in the wafer material as the saw removes the material. Edge damage  120  occurs along sidewalls  125  of the cut  110  and surfaces  123  of wafer  105  near cut  110 . Edge damage  120  also extends into the wafer  105  in the form of microcracks and fractures. 
     The die resulting from the dicing process may then be thinned. However, edge damage  120  is not removed by the thinning process. FIG. 2A illustrates an assembly  200  used for thinning die  210 . Die  210  is cut from wafer  105  and mounted to a substrate  215 , which may be an inactive mechanical support or an active electronic component, depending on the application of die  210 . An adhesion material  218  such as epoxy or wax can be used to mount the die  210  to substrate  215 . The location of the edge damage  120  on die  210  created by the dicing process is illustrated in the sectional view of FIG. 2B, taken along line I—I of FIG.  2 A. Die  210  has an initial thickness T i , illustrated in the expanded view of FIG.  2 C. 
     The mounted die  210  is thinned from top surface  230 . FIGS. 2D and 2E illustrate die  210  thinned to a final thickness T f , which is less than the initial thickness T i . The edge damage  120  remains in die  210  after thinning, and is thus present in the final application of die. 
     Edge damage  120  reduces the strength of the material of die  210 , leaving it susceptible to cracking, which reduces the yield and performance of the device to be fabricated from the die. For example, when an integrated circuit chip is handled and packaged the silicon crystal is subjected to mechanical and temperature-induced stresses. 
     These stresses can cause the cracks in the damaged edge region to propagate into the active chip areas, damaging the chip circuitry and reducing the reliability of the integrated circuit. 
     Another application in which damage to the edge and surface of a die reduces yields and device performance is in infrared detection sensors, known as infrared focal plane arrays. In these sensors, two crystals, a detector die and a readout circuit chip, are bonded together to form the sensor. The detector die is typically composed of a material such as Si, Ge, InSb, HgCdTe, and InGaAs having an array of photodiodes formed therein. The readout circuit chip is typically a conventional silicon integrated circuit having the necessary circuitry for picking up the signals detected by the detector array, and amplifying and processing those signals for the specific monitoring application. 
     These sensors are particularly susceptible to cracking because they are operated at low temperature and the two crystals that form the sensor, the detector die and the readout circuit chip, have different thermal expansion coefficients. Therefore, when the sensor is cooled, the size of one of the crystals changes more than the other, introducing strain into the device and causing it to crack and fail. Most often it is the thinner and weaker detector crystal that cracks. 
     This problem has been addressed by designing sensors to minimize the strain between the two crystals as they cool, as described in U.S. Pat. No. 5,264,699 to Barton et al. and U.S. Pat. No. 5,308,980 to Barton. However, these methods do not directly address the defects that reduce the strength of the detector material. As with the integrated circuit chips described above, microcracks and other damage at the edges and surface can propagate into the crystal as it is strained when the device is cooled. 
     SUMMARY 
     In accordance with the embodiments of this invention, methods for forming die are provided which produce die that have minimal edge and surface damage. A trench is formed in a first surface of a wafer from which the die will be produced. The trench surrounds the die such that at least one sidewall of the trench forms the peripheral edge of the die. The depth of the sidewalls of the trench is at least as large as the thickness of the edge of the die after thinning. The trench is made by a method that causes minimal damage to the die. 
     Methods that can be used to form the trench with only minimal damage to the die include, for example, partial sawing, wet chemical etching, reactive ion etching, and ion milling. A combination of these methods can be used to create die having the desired sidewall straightness and edge damage. For example, partial sawing can form trenches with straight sidewalls and can be followed with a wet chemical etch to remove any damage created by the partial sawing. A protective coating is used to protect the first surface of the die and any circuitry or other structure that may be formed thereon. 
     The wafer is cut into sections that contain the die. A through-cut is produced that extends from the bottom surface of the trench to a second surface of the wafer that is opposite the first surface of the wafer to form the section. The through-cut is displaced from the sidewall of the trench by a length so that a shelf is created on the section between the sidewall, which forms a peripheral edge of the die, and the through-cut. 
     The section is mounted onto a holder, which may be a substrate with an adhesion material between the substrate and the section. The section is mounted so that the first surface faces into the holder. The section is thinned from the second surface until the desired thickness is reached. 
     The substrate may be an integrated circuit readout chip, the adhesion material an epoxy, and the die an infrared sensitive crystal, thus forming an infrared sensor device. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A is a perspective view of a wafer and saw blade. The wafer has been cut by the saw blade. 
     FIG. 1B is a cross-sectional view of a wafer that has been cut by a saw and the resulting damaged region. 
     FIG. 2A is a perspective view of a die mounted on a substrate for thinning. 
     FIG. 2B is a cross-sectional view of the mounted die and substrate of FIG. 2A taken along line I—I of FIG.  2 A. 
     FIG. 2C is an enlarged view of section A of FIG. 2B, illustrating the location of the edge and surface damage to the die caused by sawing. 
     FIG. 2D is the cross-sectional view of FIG. 2B after the die has been thinned. 
     FIG. 2E is an enlarged view of section B of FIG. 2D, illustrating the location of the edge and surface damage to the die after thinning. 
     FIG. 3A is a perspective view of a wafer with die to be cut and thinned from the wafer indicated by dashed lines. 
     FIG. 3B is a sectional view of the wafer of FIG. 3A taken along line II—II. 
     FIG. 4 is a sectional view of the wafer of FIG. 3B having trenches formed therein. 
     FIG. 5A is a perspective view and FIG. 5B a sectional view of the wafer of FIGS. 3A and 3B with a protective coating over the die. 
     FIG. 5C is a sectional view of the wafer of FIG. 5B with trenches formed around the die and coating material. 
     FIG. 5D is an enlarged view of section C of FIG. 5C showing the damage along the bottom and sidewalls of the trench. 
     FIG. 5E is a sectional view showing the additional removal of material in the trenches after a wet chemical etching process. 
     FIG. 5F is an enlarged view of section E of FIG. 5E showing the trench after the wet etch and the removal of edge damage. 
     FIG. 6A is a side view of the wafer after being cut through each trench into sections, and the edge and surface damage caused by the through-cut. 
     FIGS. 6B and 6C illustrate an embodiment in which a protective coating is applied to cover the surface of the die and the trenches before the wafer is cut into sections through each trench. 
     FIGS. 7A and 7B illustrate a perspective and sectional view, respectively, of each section of the wafer after the protective coating has been removed. 
     FIG. 8 illustrates another method of forming sections. 
     FIG. 9A is a sectional side view of a section of the wafer containing the die in a holder for thinning. 
     FIG. 9B is an enlarged view of section G of FIG. 9A showing the location of edge damage on the wafer section relative to the holder position. 
     FIG. 9C is a sectional side view of the die in the holder after thinning. 
     FIG. 9D is an enlarged view of section H of FIG.  9 C. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 3A and 3B illustrate a wafer  300  of material to be cut and thinned into die  310 , which are outlined with dashed lines. The term “die” commonly refers to a rectangular or square section of semiconductor material containing integrated circuitry. As used herein, however, the term “die” encompasses any planar section of material that has been cut and thinned from a larger piece of that material. Die  310  need not be square or rectangular, and may or may not contain processed semiconductor devices or other structures, such as solder or indium bumps, formed thereon. Die  310  will have the dimensional and surface characteristics necessary for their intended application. 
     Wafer  300  may be any material that can be formed into die  310  in accordance with the embodiments of the invention. For example, wafer  300  may be a crystalline material, such as Si or Ge, used for semiconductor devices, or InSb, HgCdTe, CdTe, or InGaAs, which are used in infrared detectors, or other crystalline materials, e.g., saphire. Wafer  300  may be a layered or composite material, a ceramic such as alumina, beryllia, or zirconia, or a non-crystalline material such as glass. 
     Wafer  300  is typically a sheet of material that has been sliced from a larger piece of the material. Wafer  300  may be lapped and/or etched to create a uniform finish on the wafer surfaces  305 ,  306 . Wafer  300  typically has a thickness T wafer  of between 300 μm (micron) and 1,000 μm. 
     In the embodiments of the invention, material is removed from the top surface  305  of wafer  300  to create trenches  400  around each die  310 , as illustrated in FIG.  4 . The sidewalls  405  of the trenches  400  form the peripheral edges  315  of the die  310 . The material is removed from upper surface  305  without creating defects, or creating only a very few defects, in the material. Thus, minimal damage is done to die  310  as the trenches  400  are formed, leaving damage-free, or nearly damage-free peripheral edges  315  for each die. 
     Material is removed so that the depth D of sidewalls  405  is slightly beyond the required final thickness T f  of edge  315  of die  310 . The required final thickness T f  of edge  315  will depend on the intended application of die  310 . For example, in infrared detectors, the thickness T j  of die  310  is typically in the range of between 3 μm to 50 μm, and more typically in the range of between 10 μm to 30 μm. 
     Suitable methods for removing material from surface  305  include, for example, etching processes, such as ion etching of the surface by wet chemistry, an ion milling of the surface, and reactive ion etching. Mechanical removal processes, such as partial sawing of the wafer material, may also be used. As defined herein, “partial sawing” refers to a method in which a dicing saw cuts only partially into the wafer, leaving a trench or groove in the wafer, without cutting all the way through the wafer. When partial sawing is used, typically only a shallow cut is made with the dicing saw, causing much less damage to the wafer material than a through-cut such as cut  110  of FIG.  1 B. The choice of method used to form trench  400  depends on factors such as the material of wafer  300 , the desired thickness of edge  315 , the degree of damage the method causes to the sidewall, and the straightness of the sidewall produced by the method. These factors will, in turn, depend on the intended application of die  310 . 
     Of the methods listed above, reactive ion etching methods may be advantageous because these methods use low ion energies, which reduces damage to the edges  315  of die  310 , while providing a high etch rate, which allows trench  400  to be formed quickly. Reactive ion etching also creates relatively straight sidewalls. In terms of damage produced, wet chemical etching generally leaves the fewest defects in the surface, followed by reactive ion etching, ion milling, and partial sawing. A problem, however, with wet chemical etching, particularly when an isotropic etching method is used, is that straight sidewalls are not produced. Wet etching methods that preferentially remove material in one direction of the crystal produce somewhat straighter sidewalls than isotropic etching, while still leaving the surface relatively damage-free. Ion milling and partial sawing generally create the most damage, but produce the straightest sidewalls. To produce the die  310  having straight sidewalls and minimally damaged edges, a process in which these methods are combined, such as the exemplary process sequence illustrated in FIGS. 5A-5F, can be used. 
     As illustrated in FIGS. 5A and 5B, a protective coating  520  is deposited or coated onto the surface of wafer  300  over die  310 . When etching processes are used to remove material from surface  305 , coating  520  serves as a mask for the regions that are not etched, to protect die  310  from removal of material and surface damage. When mechanical material removal processes are to be used, coating  520  protects any structures or circuitry that may have been formed on die  310  from debris, such as saw debris. An exemplary coating  520  may be a photoresist material, for example Shipley 1845 (Shipley Company, Marlborough, Mass.), which is applied using conventional methods. Those skilled in the art will recognize that a variety of other protective coatings, such as patterned silicon nitride or silicon dioxide, may be used. 
     As illustrated in FIGS. 5C and 5D, trench  400  is then created by a method, for example, partial sawing, which creates straight sidewalls. In partial sawing, a shallow cut is made with a dicing saw. Unlike a cut which goes all the way through the wafer  300 , such as cut  110  illustrated in FIGS. 1A and 1B, such a shallow cut produces only a very small amount of damage to the edges of die  310 . 
     To minimize any damage that may be caused by partial sawing, the depth D of sidewalls  405  made by partial sawing should be as little beyond the final thickness T f  of edge  315  as possible, because, in general, the deeper the saw cut the more damage produced in the material. The direction of rotation of the saw blade should also be down into the wafer, as illustrated by arrow  107  of FIG. 1A, to reduce damage. Higher blade rotation rates, for example 20,000 rpm or greater on a 2 inch (5.1 cm) diameter blade, and slower feed rates, for example 10 mm/minute, also minimize damage. The saw blade is typically a resin-bonded blade having a width of, for example, 0.05 to 0.2 mm, typically 0.1 mm, with embedded diamond grit having a grit size of, for example, 3 μm to 9 μm. Smaller grit generally produces less damage to the crystal, but the diamond grit must be large enough to produce a cut. Such a blade can be used in, for example, an MTI NSX-250 dicing saw (Manufacturing Technology, Inc., Ventura, Calif.). 
     Other material removal methods that produce straight sidewalls, such as reactive ion etching or ion milling can also be used to produce trench  400  of FIGS. 5C and 5D. When trench  400  is to be formed by these method, the choice of chemicals and procedures used will depend on the material of wafer  300  and the desired depth of trench  400 , as understood by one of skill in the art. 
     FIG. 5D illustrates some limited damage  525  that may have been done to the edges  315  in forming trench  400  with straight sidewalls. This limited damage  525  may be acceptable, depending on the intended application of die  310  and the particular material of wafer  300 , in which case wafer  300  is directly cut into sections, as described below in reference to FIGS. 6A-6C. 
     To remove any damage  525  from the sidewalls  405  of trench  400  and produce a damage-free surface, a wet chemical etch may be performed. The results of such a wet chemical etch are shown in FIGS. 5E and 5F. The wet etch process removes a small amount of material, for example, 2 μm to 10 μm, from the sidewalls and bottom of trench  400 , and also cleans the surface of the trench. The resulting sidewalls  405  of trench  400  are as defect free as possible, but some undercutting  413  and rounding  414  may result. The wet chemical etch is performed using the procedure and chemicals appropriate for the material being etched, as understood by those of skill in the art. 
     Once trenches  400  are completed, wafer  300  is ready to be cut and divided into the individual sections from which each die  310  will be formed. As illustrated in FIG. 6A, a through-cut  610  is made within the trench  400  through wafer  300 , such that the dicing saw does not contact the sidewall  405  of the trench. Trench  400  is made and cut so that a region of the trench, shelf  616 , isolates the edge  315  from through-cut  610  and from damage due to the dicing saw. Shelf  616  extends from the sidewall  405  of trench  400  to the sidewall  615  of the through-cut  610 , and should be long enough so that damage  620  caused by the dicing saw does not reach die  310 . Typically, this length is at least the width of a saw blade, for example, 0.1 mm, used cut trench  400 . Larger shelf lengths provide more protection but sacrifice more material. In general, a shelf  616  length of up to, e.g., 2.5 mm can be used. Thus, sidewall  405 , which forms the peripheral edge  315  of die  310 , is protected from damage. 
     In FIG. 6A, the protective coating  520  from the previous processing is illustrated as remaining on wafer  300  during cutting of through-cut  610 . However, if additional protection for sidewalls  405  is desired, the coating  520  may be removed, and, as illustrated in FIGS. 6B and 6C, another layer of coating  630  that covers the sidewalls  405  may be applied to wafer  300 . The wafer is then cut, with the dicing saw moving through both the wafer  300  and coating material  630 , to produce cut  610 . 
     As illustrated in FIGS. 7A and 7B, after wafer  300  is cut, any protective coating  520  or  630  on the wafer  300  is removed, leaving sections  700 . Each section  700  contains a partially formed die  310  on an upper portion of wafer  300 . The shelf  616  forms a buffer region which allows each section  700  to be handled at sidewall  615  without touching the edge  315 , further protecting edge  315  from damage that may result from handling sections  700 . 
     It should also be noted that cutting the wafer  300  into sections  700  may be done before the wet chemical etch illustrated in FIGS. 5E and 5F is performed. In this case, the wet etch is performed after through-cut  610  is made and before coating  520  is removed. The wet chemical etch will remove any defects that may have been introduced by the cutting and will clean the surface of any saw debris. 
     Other methods or combinations of methods can be used to obtain sections  700  having damage-free, or nearly damage-free edges  315  on die  310 . For example, FIG. 8 illustrates a process in which nearly damage-free edges of die  310  are formed by using shallow, partial saw cuts, as discussed above, to create trenches  400 . In the exemplary process illustrated in FIG. 8, more than one trench  400  is cut between each die  310 , and only one sidewall  805  of each trench forms an edge  315  of the die  310 . Depending on the final application of die  310 , these shallow saw cuts may be followed with a wet chemical etch, as described in reference to FIG. 5D above. FIG. 8 illustrates trenches  400  formed without a protective coating, such as coating  520  of FIGS. 5A and 5B. However, a protective coating may be used for the partial sawing and typically is used if a wet chemical etch is to be performed. 
     As shown in FIG. 8, a portion  809  of the wafer  300  is left between trenches  400  after trenches  400  are made. Portion  809  is typically removed when cutting wafer  300  into sections  700 . In one embodiment, the with W of portion  809  is less than the width of the saw used to dice the wafer  300 , and therefore all material in portion  809  is removed in the cut. Alternatively, two cuts through wafer  300  may be made along, for example, lines  822  and  821 , so that the saw does not contact the edge  315  and a shelf  616  remains on each section. 
     To finish forming the die  310  from sections  700 , the back surface  306  of each section  700  is thinned until the back surface  940  of die  310  is reached. As illustrated in FIG. 9A, section  700  is mounted onto a holder  905  with the top surface  305  facing into holder  905  and bottom surface  306  exposed. In one embodiment, holder  905  is made of a substrate  910  and an adhesion material  912 , which is used between the substrate  910  and the section  700 . 
     Any thinning process, for example, polishing, lapping, etching, grinding, or diamond point turning, may be used to remove material from surface  306  to thin the wafer. As illustrated in FIGS. 9A and 9B, the damage  620  created by through-cut  610  is on the portion of section  700  to be removed by thinning, and, hence, damage  620  will be removed. 
     As the thinning process continues, shelf  616  is removed. If the depth D of sidewall  405  was made just slightly larger than the desired thickness T f  of the die, section  700  can be thinned until shelf  616  is removed to provide die  310  having the correct thickness. If viewed normal to surface  306 , removal of shelf  616  provides a visual reference guide to the thickness of the section  700  being thinned, because as shelf  616  is removed, the area of surface  306  becomes smaller, until surface  940  is reached. As illustrated in FIGS. 9C and 9D, the resulting die  310  is free of edge and surface damage. 
     The particular substrate  910  and adhesion material  912  used for holder  905  depends on the intended application of die  310 . In one example, die  310  forms part of an infrared sensor, such as the infrared sensors described in U.S. Pat. No. 5,308,980 to Barton and U.S. Pat. No. 5,264,699 to Barton et al., incorporated herein by reference. In such an infrared sensor, die  310  may be an infrared sensitive crystal such as InSb, and substrate  910  is a silicon integrated circuit that functions as a readout chip for the infrared sensitive crystal. Typically, the IR sensor will also contain solder or indium bump bonds between die  310  and substrate  910 , to electrically connect the infrared sensitive crystal to the readout chip. The die  310  and substrate  910  remain bonded together in the completed sensor, and therefore the adhesion material  912  is typically an epoxy capable of holding die  310  and substrate  910  together. 
     Examples of suitable epoxies for adhesion material  912  used in such a sensor include, for example, Bondline 7247 and 6460 (Bondline Electronic Adhesives, San Jose, Calif.). These epoxies are chosen to be both chemically and physically compatible with all Processing steps used to make the sensor and with conditions under which the completed sensor is operated. For example, the epoxies can withstand the temperature changes encountered when detector is in use. 
     In another embodiment, the substrate  910  is simply a mechanical support for die  310  and adhesion material  912  is a material, for example a parafin or bees wax, that allows die  310  to be easily removed from the substrate when the thinning process is complete. 
     The embodiments described above are intended to be illustrative only, and not limiting. Many variations and modifications in accordance with the invention will be evident to those of skill in the art. Therefore, the appended claims are to encompass all such changes and modifications as falling within the scope of this invention.