Patent Publication Number: US-2006003491-A1

Title: Apparatus for ejecting relatively thin IC chip from semiconductor wafer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This U.S. non-provisional application claims priority under 35 U.S.C. §119 from Korean Patent Application No. 2004-51957, which was filed in the Korean Intellectual Property Office on Jul. 5, 2004, the contents of which are incorporated by reference herein in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to electronic packaging technology and, more particularly, to an apparatus for ejecting relatively thin IC chips from a semiconductor wafer in a chip separation process.  
      2. Description of the Related Art  
      Integrated circuits (IC) are reaching the limit of minimization, and further, two-dimensional wafers restrict the rise in degree of integration of IC chips. Three-dimensional (3-D) stacking technology is therefore being studied to provide higher integration density.  
      A great number of IC chips are simultaneously fabricated on the wafer during the wafer fabrication process. Each individual IC chip is then separated from the wafer and vertically stacked in a bare form or a package form. Such 3-D stacking is inevitably confronted with an increase in thickness of stack. So, 3-D stacking often requires a relatively thin IC chip. The thin IC chip is, however, very susceptible to mechanical shocks during separation or handling.  
       FIG. 1  shows, in a cross-sectional view, a conventional apparatus  10  for ejecting the thin IC chip  22  from the wafer  20  during separation of the IC chip  22 .  
      Referring to  FIG. 1 , after the wafer fabrication process is completed, the wafer  20  undergoes an electrical die sorting (EDS) process and wafer sawing process. In the wafer sawing process, the wafer  20  is sawed along scribe lines and thereby the individual IC chips  22  are divided from each other. The divided individual IC chips  22  are temporarily supported by an adhesive tape  23  attached in advance to the bottom face of the wafer  20 . Typically, the adhesive tape  23  is a UV-sensitive tape that has high adhesive strength and is highly expandable. When exposed to UV rays, the UV-sensitive tape  23  can lose adhesive strength.  
      To completely separate the individual IC chip  22  from the wafer  20 , the chip-ejecting apparatus  10  is used together with UV exposure. The chip-ejecting apparatus  10  is located under the wafer  20 , and a vacuum holder  11  supports the UV-sensitive tape  23  by applying a vacuum force. Then ejecting pins  13  move upward by the ascent of a pin-driving plate  14  and push up on the IC chip  22 . In addition, a chip-transferring tool  30  pulls the IC chip  22  by vacuum force, keeping in contact with the top face of the IC chip  22 .  
      Although the UV-sensitive tape  23  is exposed to UV rays so as to reduce the adhesive strength, the UV-sensitive tape  23  still has some residual adhesive strength. Such residual adhesion is not a problem in the case of a normal, relatively thick IC chip. However, adhesion becomes an issue in the case of the relatively thin IC chip  22  having a thickness of about 801 μm and less. While the ejecting pins  13  still exert a force enough to detach the IC chip  22  from the UV-sensitive tape  23 , such a force exerted by the ejecting pins  13  may be too strong for the thin IC chip  22 . Since the IC chip  22  is mainly made of silicon which is inherently brittle, the thin IC chip  22  may often succumb to the mechanical stress caused by the chip-ejecting force and, therefore, may be cracked or broken.  
      The thin IC chip  22  may further encounter another problem during separation. The above-discussed residual adhesion of the UV-sensitive tape  23  may also unfavorably affect the planarity of the thin IC chip  22  and therefore warp the IC chip  22 . Such warpage may invite a poor placement of the thin IC chip  22  on a next-level substrate, and may also lead to defective wire bonding between the IC chip and the substrate.  
      Some attempts to relieve mechanical stress applied to the thin IC chip have been disclosed in Korean Utility Model No. 194288 and Korean Patent Nos. 142152 and 206911, for example. According to these disclosures, the ejecting pins are vested with elasticity to reduce mechanical stress. However, the foregoing problem relating to chip warpage is still unsettled.  
     SUMMARY OF THE INVENTION  
      Embodiments of the present invention provide an apparatus and method for ejecting a thin IC chip from a UV-sensitive tape that is attached to a bottom face of a semiconductor wafer.  
      In one embodiment, the chip ejecting apparatus includes a vacuum holder, an ejecting block, and a plurality of ejecting pins. The vacuum holder is adapted to support the UV-sensitive tape by applying a vacuum force to the UV-sensitive tape. The ejecting block is positioned in the vacuum holder and configured to move vertically relative to the vacuum holder. The plurality of ejecting pins are inserted in a corresponding plurality of pin holes located in the ejecting block and are configured to move vertically and elastically relative to the ejecting block. The ejecting pins are adapted to move upward from the ejecting block to position the ejecting pins against the UV-sensitive tape below the IC chip to be ejected. The ejecting block is adapted to then move upward from the vacuum holder to position the ejecting block across a bottom surface of the IC chip.  
      Another embodiment provides a method of separating an IC chip from a semiconductor wafer. The sawed IC chip is supported on a UV-sensitive tape. A vacuum force is applied to a bottom surface of the UV-sensitive tape in an area below the chip. A vacuum force is applied to a top surface of the IC chip with a chip transferring tool. The adhesion of the UV-sensitive tape is reduced by exposing the tape to UV rays. The IC chip is pushed upward from the wafer by applying a plurality of point forces to the bottom surface of the UV-sensitive tape with a plurality of ejecting pins. The IC chip is then pressed against the chip transferring tool to eliminate or reduce warping of the chip by applying a surface pressure across a bottom surface of the IC chip with an ejecting block moving upward to the bottom surface of the IC chip. The vacuum force is removed from the bottom surface of the UV-sensitive tape, and the IC chip is transferred to a next place by the chip transferring tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of a conventional apparatus for ejecting a thin IC chip from a wafer.  
       FIG. 2  is a top plan view of an apparatus for ejecting a thin IC chip from a wafer according to an embodiment of the present invention.  
       FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2 .  
       FIG. 4  is a cross-sectional view of an ejecting pin of the chip-ejecting apparatus  50  of  FIG. 2 .  
       FIG. 5  is a top plan view showing a wafer loaded on a wafer table for chip separation;  
       FIG. 6  is a cross-sectional view showing a vacuum holder partly supporting UV-sensitive tape;  
       FIG. 7  is a cross-sectional view showing ejecting pins pushing up a thin IC chip;  
       FIG. 8  is a cross-sectional view showing that an ejecting block applying a pressure to the bottom face of the thin IC chip; and  
       FIG. 9  is a cross-sectional view showing a chip-transferring tool transferring the thin IC chip.  
    
    
     DETAILED DESCRIPTION  
      Exemplary, non-limiting embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, the disclosed embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The principles and feature of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.  
      In this disclosure, well-known structures and processes are not described or illustrated in detail to avoid obscuring the present invention. Furthermore, the figures are not drawn to scale in the drawings. Rather, for simplicity and clarity of illustration, the dimensions of some of the elements are exaggerated relative to other elements. Like reference numerals are used for like and corresponding parts of the various drawings.  
       FIG. 2  is a top plan view of an apparatus  50  for ejecting a thin IC chip from a wafer according to an embodiment of the present invention.  FIG. 3  is a cross-sectional view taken along the line III-III of  FIG. 2 . In addition,  FIG. 4  is a cross-sectional view of an ejecting pin  53  of the chip-ejecting apparatus of  FIG. 2 .  
      Referring to FIGS.  2  to  4 , the thin IC chip ejecting apparatus  50  includes a vacuum holder  51  and a dual ejecting unit  52 . The vacuum holder  51  supports, by applying vacuum force, parts of a UV-sensitive tape around the targeted IC chip to be ejected. The dual ejecting unit  52 , which is placed at the central region of the vacuum holder  51 , ejects the individual thin IC chip from the UV-sensitive tape. The dual ejecting unit  52  has several ejecting pins  53 , a pin-driving plate  54 , a vacuum hose  55 , a block-driving shaft  56 , and an ejecting block  57 .  
      The ejecting block  57  is movably inserted in the central region of the vacuum holder  51  and configured to move vertically. Further, the ejecting block  57  is connected to the block-driving shaft  56 , which is located under the ejecting block  57  and supplies motive force to the ejecting block  57 . As will be described later, the ejecting block  57  is a tool for applying a pressure to the bottom face of the IC chip so as to compensate the warpage of the IC chip. The ejecting block  57  has a number of vacuum holes  58  and pin holes  59 , which are regularly arranged. The ejecting block  57  may have a size, in a plan view, substantially equal to or smaller than that of the IC chip.  
      The vacuum holes  58  can communicate with the vacuum hose  55  directly or through the block-driving shaft  56 . The vacuum hose  55  is configured such that the pin-driving plate  54  does not interfere with the vacuum hose  55 . Each individual pin hole  59  contains an ejecting pin  53 . The ejecting pins  53  are movably inserted in the pin holes  59 , respectively and configured to move vertically and elastically. Further, the ejecting pins  53  are connected to the pin-driving plate  54 , which is located under the ejecting pins  53  and supplies motive force to the ejecting pins  53 . As will be described later, the ejecting pins  53  are tools for pushing up an IC chip to detach the IC chip from the UV-sensitive tape.  
      Referring to  FIG. 4 , each individual ejecting pin  53  has a pin holder  53   a , an elastic member  53   b , and a pin head  53   c . The pin holder  53   a  has a cylindrical shape with a hollow area  53   d  inside. The elastic member  53   b  and a lower part of the pin head  53   c  are located in the hollow area  53   d  of the pin holder  53   a . The elastic member  53   b , such as a spring, underlies the pin head  53   c  and provides an upward elastic force to the pin head  53   c . An upper part of the pin head  53   c  protrudes from the pin holder  53   a  by elastic force of the elastic member  53   b . The pin head  53   c  can move downward, compressing the elastic member  53   b . Such elastic movement of the pin head  53   c  can reduce the mechanical stress applied to the thin IC chip.  
      FIGS.  5  to  9  show in sequence a method for ejecting the thin IC chip  22  from the UV-sensitive tape  23  by using the above-discussed ejecting apparatus  50 .  
       FIG. 5  shows, in a top plan view, the wafer  20  loaded on a wafer table  70  for chip separation. As shown in  FIG. 5 , the wafer  20 , after completing the wafer sawing process, is positioned on the wafer table  70 . The wafer  20  is surrounded with and temporarily supported on a wafer ring  21  by UV-sensitive tape  23 . The wafer  20  has a number of the IC chips  22  divided from each other by sawing, and the individual IC chips  22  remain on the Uv-sensitive tape  23 . The wafer  20 , i.e., the IC chips  22 , has a relatively thin thickness of about 80 μm or less. The UV-sensitive tape  23  has high adhesive strength and is highly expandable, but loses adhesive strength when exposed to UV rays.  
      The wafer table  70  includes a hollow circular center  72 . The wafer  20  loaded on the wafer table  70  is located over the hollow circular center  72 , and further, the above-discussed ejecting apparatus  50  is located within the hollow circular center  72  under the wafer  20 .  
      After the wafer  20  is loaded on the wafer table  70 , the vacuum holder  51  of the ejecting apparatus  50  partly supports the UV-sensitive tape  23 , as shown in  FIG. 6 . Referring to  FIG. 6 , the ejecting apparatus  50  moves under a targeted one of the individual IC chips  22 , and the vacuum holder  51  supports parts of the UV-sensitive tape  23  around the targeted IC chip  22  to be ejected by applying a vacuum force to the UV-sensitive tape  23 . Further, the ejecting block  57  in the vacuum holder  51  also supports the UV-sensitive tape  23  underneath the targeted IC chip  22  though the vacuum holes  58  (see  FIG. 2 ).  
      In addition, a chip-transferring tool  60  located above the wafer  20  moves onto the targeted IC chip  22  and is placed in contact with the top face of the IC chip  22 . By doing so, the chip-transferring tool  60  can prevent an undesirable shift in position of the IC chip  22  or an accidental fall of the IC chip  22  when the IC chip  22  is ejected from the UV-sensitive tape  23 .  
      After the UV-sensitive tape  23  is supported, the ejecting pins  53  push up the IC chip  22 , as shown in  FIG. 7 . Referring to  FIG. 7 , the ejecting pins  53  inserted in the ejecting block  57  move upward, pushed by the pin-driving plate  54 , applying point forces to the underside of the IC chip  22 . The IC chip  22  is, therefore, detached from the LW-sensitive tape  23 .  
      As discussed above, the ejecting pin  53  can move elastically. Referring to  FIGS. 4 and 7 , the pin head  53   c  protruding from the pin holder  53   a  meets the UV-sensitive tape  23 . Thereafter, when the pin-driving plate  54  moves upward, the ejecting pin  53  is pushed upward from the ejecting block  57 . At this time, the pin head  53   c  resists moving upward due to the UV-sensitive tape  23  compressing the elastic member  53   b . Such elastic movement of the pin head  53   c  reduces mechanical stress applied to the thin IC chip  22 . While the ejecting pins  53  push up the bottom face of the IC chip  22 , the chip-transferring tool  60  also moves upward while still keeping in contact with the top face of the IC chip  22 . In the meantime, the ejected IC chip  22  may be unfavorably warped due to the residual adhesion of the UV-sensitive tape  23 , as discussed above.  
      Subsequently, the ejecting block  57  moves upward and applies a pressure to the bottom face of the IC chip  22 , as shown in  FIG. 8 . Referring to  FIG. 8 , the block-driving shaft  56  pushes up the ejecting block  57 , so the ejecting block  57  moves upward from the vacuum holder  51 . Then the ejecting block  57  applies a pressure to the bottom face of the IC chip  22 , and further, the chip-transferring tool  60  applies a pressure to the top face of the IC chip  22 . By doing so, a warped IC chip  22  returns to a flat state. Moreover, since the ejecting block  57  keeps in contact with the entire bottom face of the IC chip  22 , the ejecting block  57  can disperse mechanical stress potentially remaining in the IC chip  22  throughout the entire bottom face.  
      After the ejecting block  57  moves upward, the chip-transferring tool  60  carries the ejected IC chip  22 , as shown in  FIG. 9 . Referring to  FIG. 9 , the vacuum hose  55  stops supplying vacuum force to vacuum holes  58 , and the chip-transferring tool  60  supports the IC chip  22  by applying vacuum force. Then the chip-transferring tool  60  transfers the IC chip  22  to a next place for a chip-attaching process. Meanwhile, the ejecting pins  53  and the ejecting block  57  return to their original position.  
      While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.