Abstract:
A method of dividing a semiconductor wafer into dice, wherein the semiconductor wafer has a circuit side, an underside, and at least one street index that defines the dice, is disclosed. The method includes placing the underside of the wafer on a support having a surface and at least one recess in the surface corresponding to the at least one street index of the wafer, aligning the street index of the semiconductor wafer with the at least one recess in the surface, and dividing the semiconductor wafer along the at least one street index.

Description:
CROSS-REFERENCE TO RELARED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/316,893, filed on May 21, 1999 and now issued as U.S. Pat. No. 6,112,740, which is a continuation of patent application Ser. No. 08/946,626 filed Oct. 7, 1997 and now issued as U.S. Pat. No. 5,950,613, which is a division of patent application Ser. No. 08/755,832 filed Nov. 26, 1996, now issued as U.S. Pat. No. 5,809,987. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed generally to a wafer cutting chuck used in conjunction with a wafer cutting blade for cutting a semiconductor wafer into dice and, more particularly, to a chuck which reduces wear and damage to a cutting blade, and an associated method. 
     2. Description of the Background 
     Integrated circuits have touched almost every aspect of society, such as children&#39;s games and toys, engine computers in automobiles, personal computers in homes and offices, and controllers in industrial processes. Better ways to fabricate integrated circuits are constantly being sought. 
     Integrated circuits are fabricated on semiconductor wafers, and the each wafer typically contains between 50 and 1,000 individual integrated circuits. Between the integrated circuits are spaces, known as “street indices”, which separate the individual integrated circuits on the wafer. Street indices are as small as possible, and are typically 4 mil to 6 mil wide. In a process known as “dicing”, wafers are cut along the street indices to form separate integrated circuits, known as “dice”. A street index which has been cut is known as a “street”. When the dicing process is completed, the streets form a grid which defines the dice cut from the wafer. 
     The dicing process is performed with wafer spindle and blade assemblies having circular cutting blades. The design and use of wafer spindle and blade assemblies and cutting blades are well known in the prior art, and such devices may be obtained from Disco Hi Tec America, Inc., located in Santa Clara, Calif. The cutting blades are about one mil thick and spin at speeds between 30,000 and 60,000 revolutions per minute. Cutting blades are often nickel-plated with a diamond grit cutting edge to insure smooth, clean cuts, with minimal fraying and splintering. 
     Wafers are placed on a smooth, level surface, known as a “cutting chuck”, where they are diced by a cutting blade. During the dicing process, a cutting blade will occasionally protrude below a wafer and into the underlying cutting chuck. The contact between the cutting blade and cutting chuck accelerates the wear on the cutting blade, and often breaks the cutting blade and results in damage to the cutting chuck. 
     It is well known in the prior art to use a wafer frame and adhesive tape to maintain dice in place during the dicing process. The wafer frame is generally flat and defines an opening which is larger than the wafer. The adhesive tape is attached to the wafer frame and stretched across the opening. A wafer is secured to the adhesive tape within the opening, and the frame is secured, for example by a vacuum, to the cutting chuck for dicing. After the dice have been cut, the frame, along with the adhesive tape and the dice, are removed from the cutting chuck. The dice are separated from the adhesive tape, the adhesive tape is removed from the frame, and the frame is reused. The adhesive tape is known as “sticky back” and is usually a polymer-based film, such as poly-vinyl chloride (“PVC”), with an adhesive coating on one side. The adhesive tape is usually about 3 mils thick. The dice stick to the adhesive, so that when the wafer is cut the dice remain in place on the cutting chuck and are not scattered. Because a cutting blade extends slightly below the wafer, the cutting blade is exposed to the adhesive tape. Unfortunately, the adhesive binds to the cutting blade, causing accelerated blade wear and “gumming-up” the cutting blade. The gumming-up of the cutting blade reduces the effectiveness of the blade, increases friction between the cutting blade and the wafer resulting in increased heat build up on the blade, and causes binding of the cutting blade, potentially breaking it. Those factors reduce the rate at which the cutting blade can be moved across a wafer, thereby increasing the amount of time required to dice a wafer. 
     Unfortunately, the accelerated wear and damage caused to cutting blades from impinging upon the chuck and exposure to the adhesive requires that they be replaced after dicing only about five or six wafers. Worn cutting blades lack the sharpness to cleanly cut a wafer, and cutting blades exposed to adhesives have rough sides and an irregular cutting surface formed from hardened adhesive picked up during previous cuts of a wafer. The continued use of a worn cutting blade may result in damaged or destroyed wafers caused by the cutting blade sailing catastrophically and spraying debris across the wafer. Replacing cutting blades is expensive, however, not only in terms of the costs of the cutting blade, but also in terms of down time of the dicing process and interruption of the fabrication process while an old cutting blade is being removed and a new cutting blade is being installed. 
     Thus, the need exists for an improved cutting chuck which reduces the amount of wear and damage to a cutting blade. In particular, the need exists for a cutting chuck which does not interfere with a cutting blade during dicing, and which reduces or prevents contact between a cutting blade and adhesives used to secure a wafer onto the cutting chuck. 
     SUMMARY OF THE INVENTION 
     The present invention is directed generally to a wafer cutting chuck used in conjunction with a wafer cutting blade for cutting a semiconductor wafer into dice. The chuck of the present invention reduces wear and damage to a cutting blade. The chuck has a surface for supporting a wafer. The chuck also has a plurality of recesses in its surface for accommodating a cutting blade of a wafer spindle and blade assembly. The recesses are at least as wide as the cutting blade and they correspond to street indices on the wafer. 
     Preferably, the chuck is constructed of a metal, a ceramic, or silicon. In a most preferred embodiment of the present invention, the recesses include ports which are connected to a vacuum pump. The ports allow a vacuum, created by the vacuum pump, to pull an adhesive tape from the wafer, so that the cutting blade of the wafer spindle and blade assembly does not contact the adhesive tape. 
     A spacer may also be used in conjunction with a wafer cutting blade and a conventional chuck for cutting a semiconductor wafer into dice. The spacer is located on the chuck and has a surface for supporting a wafer. The spacer also has a plurality of recesses in its surface for accommodating a cutting blade of a wafer spindle and blade assembly. The recesses are at least as wide as the cutting blade and they correspond to street indices on the wafer. 
     Preferably, the spacer is constructed of silicon. In a most preferred embodiment of the present invention, the recesses include ports which are connected to a vacuum pump. The ports allow a vacuum, created by the vacuum pump, to pull an adhesive tape from the wafer, so that the cutting blade of the wafer spindle and blade assembly does not contact the adhesive tape. 
     The present invention is also directed to a method of practicing the invention. The method includes applying an adhesive to a bottom side of the wafer, placing the wafer on the chuck, aligning the street indices on the wafer with the recesses in the chuck, and dicing the wafer along the street indices. 
     In a preferred method of practicing the invention, a step of applying a vacuum to the adhesive is performed prior to the dicing step. The vacuum is sufficient to displace adhesive tape from the wafer along the street indices of the wafer so that when the wafer is diced the cutting blade does not contact the adhesive tape. Following the dicing step, the vacuum is released and the adhesive tape resumes its original position. 
     The invention solves the above-mentioned shortcomings in the prior art by providing recesses in the chuck so that the cutting blade of the wafer spindle and blade assembly does not contact the chuck, thereby reducing wear on the cutting blade. Furthermore, the preferred embodiment of the invention prevents the cutting blade from contacting the adhesive tape, further reducing wear on the cutting blade. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the present invention to be clearly understood and readily practiced, the present invention will be described in conjunction with the following figures, wherein: 
     FIG. 1 is a top plan view of a wafer dicing machine constructed in accordance with the present invention; 
     FIG. 2 is a cross-sectional view, taken along line II—II of FIG. 1, of a wafer dicing machine chuck constructed in accordance with the present invention; 
     FIG. 3 is a cross-sectional view of a wafer dicing machine in operation and constructed in accordance with the present invention; 
     FIG. 4 is a cross-sectional view of a portion of an alternative embodiment of a chuck constructed in accordance with the present invention; 
     FIG. 5 is a cross-sectional view of a portion of an alternative embodiment of a wafer dicing machine constructed in accordance with the present invention; and 
     FIG. 6 is a cross-sectional view of a portion of an alternative embodiment of a wafer dicing machine constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is to be understood that the Figures have been simplified and some elements have been drawn out of proportion to illustrate those aspects of a wafer dicing machine relevant for a clear understanding of the invention, while eliminating, for the purpose of clarity, many of the elements found in a typical wafer dicing machine. Those of ordinary skill in the art will recognize that other elements are required to produce an operational wafer dicing machine. However, because such elements are well known in the art, and because they do not further aid in the understanding of the present invention, a discussion of such elements is not provided herein. 
     FIG. 1 is a top plan view of a wafer dicing machine  10  constructed in accordance with the present invention. The machine  10  includes a chuck  12  on which a wafer frame  14  is secured. The wafer frame  14  has an opening  16 , which is spanned by adhesive tape  18 . The adhesive tape  18  secures a wafer  20  within the opening  16  of the frame  14 . The wafer  20  includes a number of individual integrated circuits  22  separated by street indices  24 . The street indices  24  form a pattern on the wafer  20  which defines the individual integrated circuits  22 . Recesses  34 , described below, are formed in the chuck  12  and correspond to the street indices  24  on the wafer  20 . Preferably, the recesses  34  extend beyond the edge of the wafer  20 . Also shown in FIG. 1 is a wafer spindle and blade assembly  26 . The wafer spindle and blade assembly  26  is movable relative to the chuck  12  and is used to cut the wafer  20  along the street indices  24 , so as to separate the individual integrated circuits  22  into dice, as is well known in the prior art. Although the wafer spindle and blade assembly  26  is described as moving relative to the chuck  12 , the dicing machine  10  may also be operated with a wafer spindle and blade assembly  26  having a fixed position and with the chuck  12  being moved relative to the wafer spindle and blade assembly  26 . 
     FIG. 2 is a cross-sectional view of the wafer dicing machine  10  along lines II—II of FIG.  1 . The wafer spindle and blade assembly  26  includes a motor  28 , a shaft  30 , and a cutting blade  32 . The wafer  20  is fastened to adhesive tape  18  and the wafer  20  is supported on the top surface  36  of the chuck  12 . The chuck  12  has a number of recesses  34  formed in its top surface  36 , and there is one recess  34  corresponding to each street index  24  of the wafer  20 . As a result, the number of recesses  34 , and their spacing, will vary depending on the size of the wafer  20  being diced and the pattern of street indices  24 . The recesses  34  are at least as wide as the cutting blade  32 , and are at least as deep as the cutting blade  32  can be reasonably expected to protrude below the top surface of the chuck  12 . Preferably, the recesses  34  are between approximately three and eight mils wide, and between approximately ten and fifty mils deep. 
     The recesses  34  in a chuck  12  may correspond to the street indices  24  of one size wafer  20  having one pattern of street indices  24 , so that there is a one-to-one correspondence between the recesses  34  in the chuck  12  and the street indices  24  of the wafer  20 . In that embodiment, a different chuck  12  is used for each different size of wafer  20  and each different street index  24  pattern. Alternatively, a chuck  12  may contain recesses  34  which correspond to several different street index  24  patterns, so that one chuck  12  may be used with several wafers  20  having different sizes and street index  24  patterns. In that embodiment, there is not a one-to-one correspondence between the recesses  34  in the chuck  12  and the street indices  24  of a wafer  20 , because there are more recesses  34  in the chuck  12  than there are street indices  24  in any one wafer  20 . As a result, when a wafer  20  is diced, not all of the recesses  34  are used. A chuck  12  having recesses  34  corresponding to several street index  24  patterns has the advantage of reducing the number of times that a chuck  12  needs to be changed when wafers  20  of varying sizes and street index  24  patterns are being diced. 
     The recesses  34  are preferably formed by either a cutting process or an etch process. Forming recesses  34  through a cutting process can be done simply and easily with a cutting device, such as a wafer spindle and blade assembly, by cutting the recesses  34  into the chuck  12 . Forming the recesses  34  with an etch process can be done in several ways. Preferably, however, a nitride mask having openings where the recesses  34  are to be formed is deposited on the chuck  12 . If the chuck  12  is made of silicon, a potassium hydroxide etch (KOH) is used to etch silicon at a rate of about 6-7 microns per hour at 52° C. The nitride mask can then be removed, leaving only the recesses  34 . 
     The recesses  34  may be formed in many cross-sectional shapes. For example, recesses  34  may have cross-sectional shapes that are squared, “v”-shaped, semi-circular, semi-elliptical, and semi-trapezoidal, to suit the cutting blade  32  of the wafer spindle and blade assembly  26 . When the recesses  34  are formed by a cutting process, the shape of a recess  34  is easily controlled by selecting an appropriately shaped blade. The shape of a recess  34  can be controlled in an etch process with the proper choice of isotropic and anisotropic etches, as is well known in the art of semiconductor etching. 
     The recesses  34  preferably extend approximately 0.250 inches beyond the edge of the wafer  20  in order to allow for the cutting blade  32  to completely cut a street in a wafer  20 . The recesses  34 , of course, may extend mostly or entirely across the chuck  12 , so as to eliminate any risk of the cutting blade  32  hitting the end of a recess  34 . 
     The chuck  12  is preferably formed from either metal, a ceramic, or silicon, although other materials may be used. Silicon is preferred because the etching of silicon is well understood, particularly by manufacturers of semiconductor products. On the other hand, metals, such as aluminum, can be easily machined to contain the desired number and shape of recesses. The use of ceramics, of course, will provide a very flat and very hard surface. 
     The wafer  20  is held in place and the dice are held together by adhesive tape  18 . Preferably, the adhesive tape  18  is only sticky on the side adjacent to the wafer  20 . The other side of the adhesive tape  18 , the side adjacent to the chuck  12 , is not sticky. The adhesive tape  18  is secured to the wafer frame  14  where its sticky side contacts the wafer frame  14 . The wafer frame  14 , in turn, is secured to the chuck  12  by a vacuum generated by a vacuum pump  38 . Conduits  40  in and around the chuck  12  channel the vacuum from the vacuum pump  38 , through the chuck  12 , and to vacuum openings  42  on the top surface of the chuck  12 . The vacuum openings  42  correspond with the location of the wafer frame  14  in order to hold the wafer frame  14  against the chuck  12 . The vacuum openings  42  are shown holding the wafer frame  14  by engaging the adhesive tape  18 , which is fastened to the wafer frame  14 . Alternatively, however, the wafer frame  14  may be held by the vacuum openings  42  directly by providing holes in the adhesive tape  18 , or by the adhesive tape  18  stopping short of the vacuum openings  42 . The number of vacuum openings  42  may vary, as is known in the prior art. For example, a plurality of closely-spaced openings  42  may be provided. Alternatively, one or a small number of elongated openings  42  may exist on the top surface of the chuck  12  for engagement of the wafer frame  14 . In addition, a control valve  44  is preferably provided between the vacuum pump  38  and the vacuum openings  42  to connect and disconnect the vacuum pump  38  with the vacuum openings  42 . Alternatively, the control valve  44  may be omitted and the vacuum pump  38  may simply be turned on and off when needed. A pressure release valve  46  may also be provided to release the vacuum within the conduit  40  and allow the frame  14  to be removed. As an alternative to the pressure release valve  46 , the vacuum pump  38  may be run in reverse to repressurize the vacuum openings  42 . 
     The recesses  34  in the chuck  12  allow the wafer  20  to be diced without any risk of the cutting blade  32  contacting the chuck  12 . As a result, the chuck  12  shown in FIG. 2 substantially reduces wear on the cutting blade  32 , thereby extending the cutting blade&#39;s  32  useful life. 
     According to the invention illustrated in FIG.  2  and described above, a method of dicing a wafer  20  is also disclosed. An adhesive, such as a one-sided adhesive tape  18 , is applied to a wafer  20 . The wafer  20  is placed on a chuck  12  with the non-sticky side of the adhesive tape  18  adjacent to the surface  36  of the chuck  12 . The street indices  24  of the wafer  20  are aligned with the recesses  34  of the chuck  12 . Finally, the wafer  20  is diced along the street indices  24 . When the wafer  20  is diced the cutting blade  32  does not contact the chuck  12  because the recesses  34  correspond to the street indices  24 , and the subsequent streets  51 , of the wafer  20 . 
     FIG. 3 is a cross-sectional view of a wafer dicing machine  10  in operation. The machine  10  includes a chuck  12  constructed according to a most preferred embodiment of the invention. A wafer  20  is secured to adhesive tape  18 , and both the wafer  20  and the adhesive tape  18  are located on a top surface  36  of the chuck  12  with a wafer frame  14 . A plurality of recesses  34  are located in the chuck  12  and correspond with street indices  24  on the wafer  20 . A vacuum pump  38  is connected to each of the recesses  34  via conduits  40  in and around the chuck  12  and ports  48  in the recesses  34 . The ports  48  are evenly spaced and exist throughout the recesses  34  to form a generally uniform vacuum throughout. Each port  48  preferably is a three to eight mil opening in the recess  34 , and each opening is spaced approximately 0.5 inches apart. Elongated openings, different sized openings, and different spacing of the openings are also contemplated. 
     As shown in FIG. 4, a port may also be formed by a porous material  50 , such as a porous ceramic, adjacent to the recess  34 . In that embodiment, the conduit  40  terminates short of the recess  34  and a vacuum is formed in the recess  34  via the porous material  50 . 
     Referring back to FIG. 3, the vacuum pump  38  creates a pressure drop within the recesses  34  beneath the adhesive tape  18 , causing the adhesive tape  18  to be pulled away from the wafer  20 . When the adhesive tape  18  is pulled away from the wafer  20 , it is out of the way of the cutting blade  32 . As a result, the problems caused to cutting blades  32  by adhesive tape  18 , such as wearing on the cutting blade, gumming up of the cutting blade, binding up of the cutting blade, and breakage of the cutting blade, are eliminated. A pressure drop between approximately eighteen and twenty inches of mercury relative to the ambient pressure is usually sufficient to pull the adhesive tape  18  from the wafer  20 . A valve  52 , such as a solenoid-controlled valve, may be used to connect and disconnect a recess  34  to the vacuum pump  38 . One valve is preferably provided for each recess  34 , or portion of the recess  34 , so that the use of the vacuum can be confined to the recess  34 , or portion of the recess  34 , through which the cutting blade  32  is currently passing. When the dicing process has finished, pressure is returned to the recesses  34 , allowing the adhesive tape  18  to regain its original shape against the street  51  cut in the wafer  20 . 
     According to the invention illustrated in FIG.  3  and described above, a method of dicing a wafer  20  is also disclosed. An adhesive, such as a one-sided adhesive tape  18 , is applied to a wafer  20 . The wafer  20  is placed on a chuck  12  with the non-sticky side of the adhesive tape  18  adjacent to the surface  36  of the chuck  12 . The street indices  24  on the wafer i  20  are aligned with the recesses  34  in the chuck  12 . A vacuum is applied to the adhesive tape  18  to pull the adhesive tape  18  from the wafer  20 . The wafer  20  is diced along the street indices  24 . Finally, when the dicing is finished, the vacuum is removed from the recess  34 , such as through a pressure release valve  46 , and the adhesive tape  18  returns to its original shape against the wafer  20 . Since the wafer  20  is diced while the adhesive tape  18  is pulled from the wafer  20 , the cutting blade  32  does not contact the adhesive tape  18 . 
     The present invention may be easily modified for use with existing wafer dicing machines  10 . FIG. 5 shows a cross-sectional view of an alternative embodiment of the invention adapted for use with a conventional wafer dicing machine. The alternative embodiment may be constructed of the same materials and in the same manner as the chuck  12  described above, with the exception of the differences described below. The conventional machine includes a conventional chuck  53  which is fitted to the machine. A spacer  60 , embodying the invention and containing recesses  34  corresponding to the street indices  24  on a particular wafer to be diced, is secured to the conventional chuck  53 . The spacer  60  is held in place, for example, by a vacuum provided to vacuum openings  42  by a vacuum pump  38  and conduits  40  normally used to secure a wafer frame  14 . A wafer  20  may be secured to the spacer  60  in a number of ways. For example, double-sided adhesive tape  18  may be applied to the spacer  60 , and the wafer  20  applied to the double-sided adhesive tape  18 . Alternatively, an adhesive, without a carrying medium such as tape, may be applied directly to either the wafer  20  or the spacer  60 , and used to secure the wafer  20  to the spacer  60 . 
     The spacer  60  may be constructed in the same manner as the chuck  12  described above with respect to FIGS. 1-4. For example, the spacer  60  may be made from metal, ceramic, silicon, plastic, or a plastic-like material, such as a liquid crystal polymer, and the recesses  34  may be formed with a cutting process or an etching process. Preferably, the spacer  60  is constructed of silicon and the recesses  34  are formed by a cutting process. The spacer  60  is preferably a silicon wafer, for example a wafer which has been damaged or is in some way unsuitable for forming integrated circuits thereon. Such pieces of silicon are abundant in semiconductor processing facilities. The thickness of the spacer  60  is preferably between eighteen mils and twenty-nine mils, although almost any thickness greater than fifteen mils is generally suitable. 
     After the wafer spindle and blade assembly  26  has cut the wafer  20  into dice, the dice may be removed from the spacer  60  while the spacer  60  is being held in place by the vacuum. Alternatively, the spacer  60  and the dice may be removed from the conventional chuck  53 , and the dice and the spacer  60  separated by mechanical means or with the use of a chemical solvent. By providing a spacer  60  embodying the invention and secured to a conventional chuck  53  via a vacuum, a conventional wafer dicing machine can realize the benefits of the present invention without modification. The embodiment illustrated in FIG. 5 eliminates damage to the cutting blade  32  caused by impingement of the cutting blade  32  on the chuck  53  or the spacer  60 . 
     FIG. 6 shows a cross-sectional view of an alternative embodiment of the invention shown in FIG.  5 . The alternative embodiment illustrated in FIG. 6 may be constructed of the same materials and in the same manner as embodiments described above, with the exception of the differences described below. The embodiment illustrated in FIG. 6 is more complex and has more advantages than the embodiment illustrated in FIG.  5 . As in the embodiment illustrated in FIG. 5, recesses  34  in a spacer  60  correspond to the street indices  24  on a wafer  20  to be diced. In FIG. 6, some of the vacuum openings  42  in the conventional chuck  52  are used to secure the conventional chuck  53  and the spacer  60  together. Other vacuum openings  54  in the conventional chuck  53 , however, connect with vacuum conduits  56  in the spacer  60  which are used to secure a wafer frame  14  to the spacer  60  via vacuum openings  58 . Furthermore, the conduits  56  provide a vacuum within recesses  34  in the spacer  60  via ports  48 . As a result, one-sided adhesive tape  18  may be used to secure the wafer  20  to the frame  14 , and the vacuum in the recesses  34  will separate the adhesive tape  18  from the wafer  20 . As discussed above, the vacuum to the recesses  34  may be controlled individually with valves  52  to connect and disconnect the recesses  34  to the vacuum pump  38 . The spacer  60  is preferably between approximately 0.25 inches and 0.5 inches thick, although almost any thickness greater than 100 mils is generally suitable. The embodiment illustrated in FIG. 6 eliminates damage to the cutting blade  32  caused by impingement of the cutting blade  32  on either the chuck  53  or the spacer  60 , as well as impingement of the cutting blade  32  on adhesive tape  18 . 
     Those with ordinary skill in the art will recognize that many modifications and variations of the present invention may be implemented. The foregoing description and the following claims are intended to cover all such modifications and variations.