Abstract:
A method and apparatus are disclosed for mounting a wafer on a mount and thinning the wafer. The wafer includes a front surface having bumps with an adhesive tape having a backing attached thereto and a back surface. The front surface of the wafer is mounted facedown on a suction surface with the backing of the adhesive tape abutting the surface. The wafer is then suctioned, after which the back surface of the wafer undergoes a grinding process to thin the wafer. Since the backing attached to the bumps on the wafer is substantially planar and sits substantially flat on the suction surface of the wafer mount, the force exerted on the wafer from the thinning process does not overcome the suction force holding the wafer on the wafer mount. Thus, the bumped wafer may be thinned without damaging the bumps and the active surface of the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for mounting and thinning a wafer. In particular, the present invention relates to a method and apparatus for mounting a bumped wafer to a wafer mounting chuck and thinning the wafer to a predetermined thickness. 
     2. State of the Art 
     Typically, in a manufacturing process, a plurality of integrated circuits is simultaneously patterned and defined on the front surface of a single silicon wafer. The circuits are generally aligned in rows and columns in an orthogonal format. After the integrated circuits are fully defined, the wafer is diced by a singulation machine along lines between the rows and columns, separating the wafer into a plurality of individual integrated circuit dice. The integrated circuit dice can then be secured within individual packages and/or incorporated into electronic devices. 
     In the typical manufacturing process, the silicon wafer is sliced from a generally cylindrical ingot. The wafer is at first sliced sufficiently thick so as not to warp or break during the various manufacturing processes. However, in some instances, the desired thickness for the finished dice is less than the initial thickness of the sliced wafer. Therefore, after the integrated circuit patterns are formed on the wafer, it has been necessary to grind the back surface of the wafer to reduce its thickness as desired for the individual integrated circuit die. 
     Grinding machines for grinding down the back surfaces of silicon wafers are known in the art. The known machines have chuck tables for securing a plurality of wafers in position to be ground by one or more grinding wheels. Examples of such grinding machines are illustrated in U.S. Pat. No. 5,679,060 (Leonard), U.S. Pat. No. 4,753,049 (Mori), U.S. Pat. No. 5,632,667 (Earl), and U.S. Pat. No. 5,035,087 (Nishiguchi). 
     Currently available wafer processing systems are unsatisfactory, particularly for grinding wafers after the contact pads of the integrated circuits thereon are bumped, known as bumped wafers. Recently, the market demands the thinning of wafers to about 6 mils or less for chips utilized in ultra-compact applications such as in cell phones. For example, see U.S. Pat. No. 5,476,566 (Cavasin), which discloses a method for thinning wafers by adhesively attaching the wafers to a supporting substrate, but does not disclose thinning wafers after being bumped. Also, U.S. Pat. No. 6,162,703 (Muntifering et al.), assigned to the assignee of the present invention, discloses a method for thinning and singulating dice from an unbumped wafer by adhesively attaching the unbumped wafer to a table and precutting notches in the unbumped wafer prior to the thinning thereof. However, for bumped wafers, it is necessary to thin the wafer after bumping because, currently, the wafer must be at least 12 mils thick to undergo the bumping process without the likelihood of damage thereto. Further, it is important that the wafer be held tightly in place during the thinning process, typically with a vacuum chuck. 
     Vacuum chucks include a series of apertures in the surface of the chuck to which a vacuum source is connected. The suction created between the surface of the chuck and the bottom of the wafer securely holds the wafer in place. For example, see U.S. Pat. No. 6,120,360, assigned to the assignee of the present invention, which discloses a vacuum chuck made for securing to the planar face surface of a wafer. However, the vacuum chuck is segmented into quarters and also requires the wafer to be quartered, resulting in additional process steps and potential for error in handling four times the number of wafer parts per wafer. 
     Although vacuum chucks perform very well for wafers having a planar face surface through which air cannot pass, such vacuum chucks will not work well for a bumped wafer. Specifically, the required suction force between the surface of the chuck and the active surface of the wafer cannot be achieved since the suctioned air will pass through the gap provided by the bumps formed on the bond pads of the integrated circuits formed on the surface of the wafer. To overcome such problems, vacuum chucks for bumped wafers are typically made to provide the suction on the active surface&#39;s periphery where there are no bumps. However, such vacuum chucks do not provide the necessary suction at the wafer&#39;s periphery for effectively holding a bumped wafer for the thinning thereof because there is not enough surface area proximate the wafer&#39;s periphery without the integrated circuits and bumps thereon. As a result, it has been suggested to increase the area proximate the wafer&#39;s periphery without the integrated circuits and bumps formed on the bond pads thereof to provide greater suction on the wafer. However, this would unacceptably limit the number of bumped dice per wafer, thereby resulting in a reduction of yield. 
     Therefore, it would be advantageous to provide a method and apparatus for thinning bumped wafers that provide the necessary area for suction without limiting the number of bumped dice on the wafer. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for mounting a bumped wafer. The present invention further relates to a method and apparatus for mounting a bumped wafer to a wafer mounting chuck and thinning the wafer. 
     In a preferred embodiment of the present invention, the wafer includes a front surface and a back surface, the front surface including conductive bumps on the bond pads of the integrated circuits located thereon. The present invention includes an adhesive tape having an adhesive and a backing, the adhesive of the adhesive tape attaching the tape to the front surface of the wafer and, particularly, to the bumps on the bond pads of the integrated circuits located on the front surface of the wafer. According to the present invention, the adhesive and the tape attaches to the bumps so that an outer surface of the backing of the tape is substantially planar. 
     With the adhesive tape attached to the front surface of the wafer, the wafer is mounted, facedown, to a wafer mounting chuck. The wafer mounting chuck includes a suction surface with apertures therein which communicate a suction force to the wafer. The suction surface is configured to hold the wafer by the suction force applied thereto and, particularly, to hold the outer surface of the adhesive tape which is adhesively attached to the wafer using the suction force applied thereto. Thus, the outer surface of the adhesive tape provides a large surface area for holding the wafer via the suction force. 
     Once the wafer is suctioned facedown to the wafer mounting chuck, the wafer is ready for a thinning process. In particular, the wafer is thinned by removing material from the back surface of the wafer by grinding or chemical mechanical polishing. In this manner, bumped wafers may be thinned to less than 12 mils and, preferably, between about 6 mils and about 12 mils. After the thinning process, a wafer mount tape is applied to the back surface of the wafer. The adhesive tape is then removed from the active surface of the wafer with the aid of de-tape. The de-tape has a stronger adhesive than that of the adhesive tape so that the de-tape may be applied to an end portion of the adhesive tape for peeling the adhesive tape from the front surface of the wafer. The wafer may then undergo singulation or, rather, the wafer may be segmented into separate integrated circuit dice and/or a plurality of integrated circuit dice. 
     In an aspect of the present invention, the adhesive tape overlying the bumps on the bond pads of the integrated circuits and the front surface of the wafer provides an outer surface that is substantially planer so that-the outer surface of the tape is suctionable. Further, the suction force is applied to substantially the whole outer surface of the backing in the desired areas so that the force exerted on the wafer from the thinning process does not overcome the suction force holding the wafer on the wafer mounting chuck. In this manner, the bumped wafer may be thinned to a desired level or an ultra thin level without damaging the bumps on the bond pads and the integrated circuits formed on the front surface of the wafer. 
     Another aspect of the invention provides that the bumped wafer be thinned to less than 12 mils thick. Since wafers being bumped are currently required to be at least 12 mils thick, it is necessary for the bumps to be formed on the wafer before thinning the wafer to the desired thickness between the preferred range of about 6 mils to about 12 mils. 
     Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention may be ascertained from the following description of the invention when read in conjunction with the accompanying drawings. 
         FIGS. 1 through 9  illustrate a method and apparatus for mounting a wafer having bumps to a wafer mounting chuck and then thinning the wafer, in which: 
         FIG. 1  is a simplified top plan view of an active surface of a wafer according to the present invention; 
         FIG. 2  is a simplified and enlarged partial cross-sectional view of the wafer depicted in  FIG. 1  along line  2 , according to the present invention; 
         FIG. 3  is a simplified and enlarged partial cross-sectional view of a wafer and an adhesive tape facing each other in an unattached position according to the present invention; 
         FIG. 4  is a simplified and enlarged partial cross-sectional view of a wafer and an adhesive tape facing each other in an attached position according to the present invention; 
         FIG. 5  is partially a simplified cross-sectional view of a wafer facing a wafer mounting chuck in an unmounted position and partially a diagram of a mounting apparatus and a vacuum integrated with the wafer mounting chuck, according to the present invention; 
         FIG. 6  is partially a simplified cross-sectional view of a wafer facing a wafer mounting chuck in a mounted position and partially a diagram of a mounting apparatus and a vacuum integrated with the wafer mounting chuck, according to the present invention; 
         FIG. 7  is a simplified cross-sectional view of a wafer positioned on a wafer mounting chuck with a wafer mount tape being applied on the back surface of the wafer; 
         FIG. 8  is a simplified cross-sectional view of the adhesive tape being removed from the front surface of the wafer with the wafer mount tape maintained on the back surface of the wafer, and 
         FIG. 9  is a simplified cross-sectional view of a wafer having the wafer mount tape on the back surface of the wafer and a dicing apparatus for singulating the wafer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will be hereinafter described with reference to the accompanying drawings. It should be understood that the illustrations are not meant to be actual views of any particular apparatus and/or method, but are merely idealized representations which are employed to more clearly and fully depict the present invention than would otherwise be possible. Additionally, elements and features common between the figures retain the same numerical designation. 
     Depicted in drawing  FIGS. 1 through 7  are a method and apparatus for mounting a bumped wafer and then thinning the bumped wafer. Turning to drawing  FIG. 1 , there is illustrated a top plan view of a wafer  110 . The wafer  110  includes a front surface  112  and a back surface  114  (see FIG.  2 ). The front surface  112  of the wafer  110  includes individual integrated circuits separated by street indices or streets  118 . The street indices  118  are arranged in horizontal rows and vertical columns and define individual integrated circuit dice  116  in the wafer  110 . The wafer  110  preferably is made of silicon or gallium arsenide, although any semiconductor material may be used such as germanium, lead sulfide and silicon carbide. 
     Depicted in drawing  FIG. 2  is a partial cross-sectional view of the wafer  110  taken along line  2  in drawing FIG.  1 . On the front surface  112  of the wafer  110  there are conductive bumps  120  on the bond pads of the integrated circuits made to ultimately provide external interconnections for the integrated circuits in each of the individual integrated circuit dice  116 . The conductive bumps  120  are preferably ball shaped, but may be shaped as columns and/or studs. The conductive bumps  120  may be formed of any known conductive material or alloy thereof, such as solder, lead, tin, copper, silver and/or gold and conductive polymers and/or conductive composites. The conductive bumps  120  are typically bonded to the wafer  110  through a reflow process at a predetermined temperature dependent upon the material properties of the conductive bumps  120 . Currently, in order for the wafer  110  to successfully undergo the process steps of bonding the conductive bumps  120  thereto, the wafer should be at least 12 mils thick. Therefore, according to the present invention, it is necessary for the bumps to be formed on the wafer before thinning the wafer to the desired thickness, currently, such as between about 6 mils and about 12 mils, although the wafer may thinned to any desired thickness less than 6 mils. 
     Depicted in drawing  FIG. 3  is the wafer  110  and an adhesive tape  130 , such as backgrind tape, prior to being in an attached position. The adhesive tape  130  includes an adhesive  132  with an adhesive surface  134  and a backing  136  with an outer surface  138 . The outer surface  138  of the backing  136  is nonadhesive. The adhesive  132  used for the adhesive tape  130  may be, but is not limited to, a pressure sensitive silicone adhesive, acrylic adhesive, UV curable adhesive, and/or any adhesive that allows the tape to be easily removed without damaging the wafer  110 . It is also desirable for the adhesive  132  on the adhesive tape  130  to leave a nonconductive ash when it oxidizes or bums to prevent any potential problems of electrical connections with any portion of the wafer  110  and the individual integrated circuit dice  116 . The backing  136  for the adhesive tape  130  may be of a polymer material or paper or the like. As such, the backing  136  may be rigid or flexible so long as the backing  136  is substantially planar for mounting the wafer  110  (discussed further below). Further, the backing  136  should be of sufficient strength so that it will not easily tear. 
     Referring to drawing  FIG. 4 , the adhesive tape  130  is adhesively placed and attached to the conductive bumps  120  to overlie the front surface  112  of the wafer  110 . The adhesive tape  130  is preferably substantially the size of the wafer  110  so that it overlies each of the conductive bumps  120 . The adhesive tape  130  may also overlie portions of the wafer  110  without the conductive bumps  120  thereon, namely a periphery of the front surface  112 , to provide protection of the front surface  112 . Such positioning of the adhesive tape  130  may be accomplished manually and/or by machinery. 
     As depicted in drawing  FIG. 4 , the adhesive  132  attached to the conductive bumps  120  may conform to and/or about the conductive bumps  120  so that the adhesive  132  attaches between about 10% and about 60% of the bumps&#39; surface area. The range of necessary surface area for sufficient attaching depends on the type of adhesive  132  employed, as known in the art. As such, it is desired that the adhesive  132  has sufficient strength to withstand a grinding process (discussed further below). Further, an important feature of the present invention is that the adhesive tape  130  conforms to the conductive bumps  120  in a manner that allows the outer surface  138  of the backing  136  to be substantially planar. 
     Referring to drawing  FIG. 5 , there is shown a cross-sectional view of the wafer  110  and a wafer mounting chuck  150  prior to the wafer  110  being mounted thereon. As shown, the wafer  110  is inverted with its front surface  112  facedown so that the substantially planar outer surface  138  of the backing  136  of the adhesive tape  130  is facing the wafer mounting chuck  150 . The wafer mounting chuck  150  includes a suction surface  152  on which the substantially planar outer surface  138  is to be attached or mounted. The suction surface  152  includes apertures  154  that communicate with the chamber  156  in the wafer mounting chuck  150 . The chamber  156  in turn communicates with a vacuum  160  which provides suction at the suction surface  152 . The vacuum  160  is integrated with a mounting apparatus  162  to which the wafer mounting chuck  150  is connected. The number of apertures  154  in the suction surface  152  may vary depending on the required suction involved, which may be determined by one of ordinary skill in the art. For example, a plurality of closely spaced, minuscule apertures  154  having small diameters may be provided. Alternatively, the apertures  154  may be larger and more spread out. 
     As shown in drawing  FIG. 6 , the wafer  110  with its front surface  112  facedown is placed on the wafer mounting chuck  150  to be suctioned thereto. In particular, the substantially planar outer surface  138  sits flat against the suction surface  152  of the wafer mounting chuck  150  so that the wafer  110  may be suctioned to the mounting apparatus  162  via the vacuum  160 . In this manner, the planarity of the outer surface  138  of the adhesive tape  130  allows the vacuum  160  to provide a suction force  166  through the apertures  154  that sufficiently secures the wafer  110  to the suction surface  152  without substantial leakage affecting the suction force  166 . In the suctioned position, the back surface  114  of the wafer  110  faces upward in an exposed position. 
     The back surface  114  of the wafer  110  is then processed through a normal back-grind or back-lap process to thin the wafer  110  to a desired thickness by a grinder  164 . The grinder  164 , as depicted in drawing  FIG. 6 , is only intended to represent a generic wafer back-grinding tool. In the grinding operation, the wafer  110  may be moved to successive grinding stations with grinding wheels of decreasing grain size and abrasiveness so that the roughness of the back surface  114  is successively decreased. As such, the wafer  110  is thinned to a predetermined thickness  168  ( FIG. 7 ) of less than about 12 mils and, preferably, the wafer is thinned to between about 6 mils and about 12 mils, although the wafer may be thinned to any desired thickness, such as less than 6 mils. 
     According to the present invention, it is well appreciated that the planarity of the outer surface  138  of the adhesive tape  130  provides sufficient suction force to be applied on the suction surface  152  of the wafer mounting chuck  150  and on the wafer  110  to undergo grinding without damaging the wafer  110  or without wafer movement. Further, the increased application of a suction force that the adhesive tape  130  provides allows thinning of the wafer  10  to the predetermined thickness  168  after being bumped. 
     After backgrinding the wafer  110 , the wafer  110  may remain on the wafer mounting chuck  150  or be moved to another type of wafer mount chuck  170 , such as a chuck  170  with vacuum ports  174  about a chuck periphery  172  and an air gap  176  at a center portion of the chuck  170  (as shown in drawing FIG.  7 ). As such, the wafer  110  is suctioned to the wafer mount chuck  170  via the vacuum ports  174  with the back surface  114  of the wafer  110  exposed. A wafer mount tape  180  having an adhesive surface  182  is then applied to the back surface  114  of the wafers  110  and to a film frame  184 . A lamination roller  186  may be provided to aid in the adhesive attachment of the wafer mount tape  180  to the back surface  114  of the wafer  110  by simply rolling the lamination roller  186  thereon. In the case of the wafer  110  being placed on the chuck  170  having an air gap  176 , air pressure is provided in the air gap  176  to prevent the lamination roller  186  from cracking, breaking or causing fatigue to the wafer  110 . Any excess wafer mount tape  180  may then be removed using a tape blade  188  or any well-known removing device used in the art. 
     As illustrated in drawing  FIG. 8 , the wafer  110  is removed from the wafer mount chuck  170  in preparation for removing the adhesive tape  130 . Removing the adhesive tape  130  may be accomplished using “de-tape”  192 , which has a stronger adhesive than that of the adhesive tape  130 . As such, the de-tape  192  may be attached to an end portion of the adhesive tape  130  to peel the adhesive tape  130  from the front surface  112  of the wafer  110 . As previously set forth, after removing the adhesive tape  130 , it is desirable for the adhesive  132  on the adhesive tape  130  to leave a nonconductive ash through oxidation or burning to prevent any potential problems of the electrical connections with any portion of the wafer  110  and the individual integrated circuit dice  116 . 
     The wafer  110  with the wafer mount tape  180  on its back surface  114  is then prepared for dicing or a singulating process. As illustrated in drawing  FIG. 9 , the wafer  110  is sitting with its bumps exposed to the dicing apparatus  196 . As such, the wafer  110  is diced along the street indices or streets  118  (see  FIG. 1 ) into individual integrated circuit dice  116  by the dicing apparatus  196 . After dicing, the wafer mount tape  180  on the back surface  114  of each of the segment intergrated circuit dice  116  may be removed therefrom by suitable pick and place equipment (not shown) in preperation for further processing of the integrated circuit dice  116 . 
     The above descriptions and drawings are only illustrative of preferred embodiments which achieve the objects, features and advantages of the present invention, and it is no intended that the present invention be limited thereto. Any modification of the present invention which comes within the spirit and scope of the following claims is considered part of the present invention.