Abstract:
A method for bonding an IC chip to a substrate where the method comprises the steps of providing an IC chip with a plurality of bumps each having a buffer layer and a conductive layer, providing a substrate having a plurality of conductive elements arranged corresponding to the plurality of bumps, placing a non-conductive film between the plurality of conductive devices and their corresponding bumps, and pressing and heating the IC chip and the substrate so that the plurality of bumps are in contact with the plurality of conductive elements respectively. The bonding structure is formed between a first and second substrate where the structure has a buffer layer having an opening and formed on the first substrate, a conductive layer formed on the buffer layer, and a non-conductive film formed between the conductive layer and the second substrate as a bonding medium for the bonding structure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a structure and method for bonding an IC chip. More particularly, the present invention relates to a structure and method for bonding an IC chip using Non-conductive film (NCF).  
         [0003]     2. Description of the Related Art  
         [0004]     Liquid crystal displays (LCDs) have been widely used in place of cathode ray tubes (CRT) and have become the mainstream of the market nowadays. The manufacturing of the LCDs involves many processes, among which the bonding of IC chips to the LCD panel is one of the most important. Of all the methods used for the process, tape automated bonding (TAB) and chip-on-glass (COG) technologies are the most commonly seen. They may also be used to bond other chips to printed circuit boards (PCBs) or leadframes.  
         [0005]     To bond the IC chips onto a glass substrate, manufacturers often use anisotropic conductive film (ACF) as the adhesive medium, because ACF has the characteristic of having anisotropic conductivity. In general practices, the IC chip is bonded by ACF to the glass substrate first through bumps each corresponding to a pin of the IC chip (i.e. the bumps of the IC chip pins are bonded by ACF onto the glass substrate, respectively). However, the use of this material faces considerable difficulties when a product requires that the LCD panel be designed with higher density of pins or bumps on the IC chip. Firstly, the conductive particles within ACF adjacent to bumps have a tendency to bridge the neighboring bumps electrically, thus causing short circuit. Secondly, the horizontal insulation of the ACF is contingent on the pitch of pins or bumps on the IC chip, the density of conductive particles in the ACF, the diameter, as well as the coating, of the conductive particles. Although fine-pitch ACF may be used to alleviate the foregoing problems, a high manufacture cost will be needed.  
         [0006]     To solve this problem, some manufacturers use non-conductive film (NCF) to replace ACF. However, experimental evidences show that NCF cannot be used successfully for Au bumps. Attempts have been made to solve the above dilemma. U.S. Pat. No. 6,537,854 discloses a multi-layered bump structure, in which the multi-layered bumps are of a corrugated or serrated shape, and a method of bonding the multi-layered bumps to a substrate to form an ohmic contact. The multi-layered bumps are each so formed with the corrugated or serrated shape that the adhesive material is arranged to present a conductive interface so that the desired contact resistance can be obtained. However, the conductive metal layers formed by photolithography typically has a total thickness of only a few thousand angstroms, which, during the flip chip bonding process, will cause the base film of the conductive metal layers to deform under stress as a result of its low Young&#39;s modulus. The conductive layers are thus easy to crack and the resistance and reliability of the contact points of the bumps are insufficient for products. Furthermore, as the base film of each of the multi-layered bumps is made of polymer, its ability to penetrate the oxide layer is lower than that of conventional Au bump, and thus may suffer from the problem of excessively high electrical resistance at the contact points.  
         [0007]     Therefore, an improved structure and method for bonding an IC chip are highly desired for current and future LCD products that require high density of IC chip pins bonded onto LCD panel or PCBs.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to eliminate the problem of short circuiting or hight cost that comes with conventional bonding technology using ACF or fine pitch ACF as the adhesive material.  
         [0009]     It is another object of the present invention to address the problem of the conductive layer being susceptible to cracking and having too high contact point resistance that come with a prior art bonding technolgy using NCF as the adhesive material.  
         [0010]     To achieve the above objects, the present invention discloses a method and a structure for bonding an IC chip. The bonding method according to the present invention comprises the steps of providing an IC chip having a plurality of bumps each having a buffer layer and a conductive layer, providing a substrate having a plurality of conductive elements arranged corresponding to the plurality of bumps, placing a non-conductive film between the plurality of conductive devices and their corrsponding bumps, and pressing and heating the IC chip and the substrate so that the plurality of bumps are in contact with the plurality of conductive elements respectively.  
         [0011]     The bonding structure according to the present invention is formed between a first substrate and a second substrate; the structure comprises a buffer layer having an opening and formed on the first substrate, a conductive layer formed on the buffer layer, and a non-conductive film formed between the conductive layer and the second substrate as a bonding medium for the bonding structure.  
         [0012]     In addtion, a recess formed on the top of said bonding structure has a depth of at least 2 μm for improving the problem of having remnant adhesive material left on the conductive connection interface. And the top of the bonding structure further comprises at least one trough in order to channeling out excessive said adhesive material effectively.  
         [0013]     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations, and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated into, and constitute a part of, this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. The figures are not to scale. In the drawings:  
         [0015]      FIG. 1  is a cross-sectional view of a bonding structure according to the present invention;  
         [0016]      FIG. 2  is a perspective view of a smart bump according to the present invention;  
         [0017]      FIG. 3A  is a cross-sectional view of the buffer layer of the smart bump according to the present invention;  
         [0018]      FIG. 3B  is a top view of the buffer layer of the smart bump according to the present invention;  
         [0019]      FIG. 3C  is a cross-sectional view of the buffer layer of the smart bump of another embodiment according to the present invention;  
         [0020]      FIG. 3D  is a top view of the buffer layer of the smart bump of yet another embodiment according to the present invention;  
         [0021]      FIG. 4A  is a cross-sectional view of the smart bump of type I of the bonding structure of the invention shown in  FIG. 2 ;  
         [0022]      FIG. 4B  is a cross-sectional view of the smart bump of type II of the bonding structure of the invention shown in  FIG. 2 ;  
         [0023]      FIG. 4C  is a cross-sectional view of the smart bump of type III of the bonding structure of the invention shown in  FIG. 2 ;  
         [0024]      FIG. 4D  is a cross-sectional view of the smart bump of type IV of the bonding structure of the invention shown in  FIG. 2 ;  
         [0025]      FIG. 4E  is a cross-sectional view of the buffer layer of the foregoing smart bumps according to the present invention;  
         [0026]      FIG. 4F  illustrates stress-strain curves with respect to the conventional Au bump, smart bumps according to the present invention;  
         [0027]      FIG. 4G  illustrates resistance curves of the conventional Au bump and the smart bump after heating cycle test of COG NCF process;  
         [0028]      FIG. 5  is a top view of the smart bump according to the present invention;  
         [0029]      FIG. 5A  through  FIG. 5C  are cross-sectional views of the smart bump according to the present invention; and  
         [0030]      FIG. 6A  through  FIG. 6C  are top views of the smart bump according to another embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]      FIG. 1  shows a cross-sectional view of a bonding structure according to the present invention. The bonding structure, placed between a first substrate  11  and a second substrate  12 , comprises a buffer layer  13 , a conductive layer  14  and an adhesive layer  15 . The first substrate  11  is an IC substrate having conductive pads  16  surrounded by an insulating passivation layer  21 . The second substrate  12  is a glass substrate having a plurality of conductive elements  17 . The conductive layer  14  is formed partially on the buffer layer  13  and conductive pad  16 . The adhesive layer  15  is formed between the conductive layer  14  and the second substrate  12  to function as a connection medium to finish the bonding structure. The adhesive layer  15  may be a non-conductive film made of epoxy resin or acrylic resin or any adhesive material.  
         [0032]     A bonding method used in conjunction with the bonding structure involves the following steps: First, placing the adhesive layer  15  between the conductive elements  17  and the conductive layer  14 . Then, the first and second substrates  11 ,  12  are pressed and heated to make the conductive layer  14  be in contact with the conductive elements  17 . By the bonding structure and method, the conductive pads  16  are electrically connected with the conductive elements  17 .  
         [0033]      FIG. 2  shows a perspective view of a smart bump of the bonding strcuture acccording to an embodiment of the present invention. As shown in the diagram, on an IC substrate  11  is placed a plurality of condcutive pads  16 . The IC substrate  11  may be a substrate supporting a plurality of ICs for driving a LCD and the conductive pads  16  thereon is used for external connection. The conductive pads  16  may be metal pads including aluminum, tungsten, copper and so on, and each pad may be surrounded by a passivation layer  21 . The passivation layer  21  may be made of dielectric materials such as silicon nitride or silicon oxide. A buffer layer  13  is formed on the IC substrate  11  and may be made of polymers, such as polyimide. The buffer layer  13  is used mainly to reduce Young&#39;s modulus of the bump and further shield the passivation layer  21  against the bonding pressure required for the bonding process as to protect it from cracking. A conductive layer  14  is partially formed on the buffer layer  13  and conductive pad  16  and may be made of metals or metal alloys and formed by electroplating or sputtering. A recess area  19  with a depth about 2 micrometers or larger is formed on the top surface of the conductive layer  14 .  
         [0034]      FIG. 3A  shows a cross-sectional view of the buffer layer of the smart bump of the bonding strcuture of the present invention.  FIG. 3B  shows a top view of the buffer layer of the smart bump of the bonding strcuture of the present invention. As shown in the figures, the buffer layer  13  has an opening  31 , which may be formed by spin coating, lithography and etching. The opening  31  is formed to communicate with the conductive layer (not shown) thereabove and the conductive pad  16  therebelow. The opening  31  may be shaped like a square  32 , a rectangle, a semi-circle, a circle or a polygonal. Besides, at least an opening is formed on the buffer layer  13  to form a better electric connection.  
         [0035]      FIG. 3C  shows a cross-sectional view of the buffer layer of the smart bump of another embodiment of the present invention.  FIG. 3D  shows a top view of the buffer layer of the smart bump of another embodiment of the present invention. The buffer layer  13  has an opening  33  formed by a photolithography such as a half-tone process. Viewed from top, the opening  33  is shaped like a frame  34 .  
         [0036]      FIG. 4A  shows a cross-sectional view of the smart bump of a particular type (hereinafter termed Type I) of the bonding structure according to the present invention. In the bump  40   a , a conductive layer  14  is formed on a buffer layer  13  with at least one opening by electroplating or sputtering. And then the conductive layer  14  has a similar profile of said buffer layer and a recess  19  located on the opening of the buffer layer. The conductive layer  14  has a thickness H 3  around its recess  19 , which is greater than a thickness H 2  of the buffer  13  (i.e. H3&gt;H2). And to obtain a better Young&#39;s modulus of the bump  40   a , the thickness H 2  of the buffer layer  13  is at least one-third a thickness H 1  of the bump  40   a  (i.e. H2/H1&gt;=⅓).  
         [0037]      FIG. 4B  shows a cross-sectional view of the smart bump of another particular type (hereinafter termed Type II) of the bonding structure of the present invention. In the bump  40   b , a conductive layer  14  is formed on a buffer layer  13  with at least one opening by sputtering or electroplating. And then the conductive layer  14  has a similar profile of said buffer layer and a recess  19  located on the opening of the buffer layer. The conductive layer  14  has a thickness H 3  around its recess  19 , which is less than the thickness H 2  of the buffer layer  13  (i.e. H3&lt;H2), and substantially equal to a thickness H 4  of the conductive layer  14  on the buffer layer  13 . Preferably, the thickness H 3  is larger than 1 μm so that the conductive layer  14  may not crack during the bonding process. In addition, to let the bump  40   b  have a better Young&#39;s modulus, the thickness H 2  is made in proportion to the thickness H 1  (i.e. H2/H1&gt;=⅓).  
         [0038]      FIG. 4C  shows a cross-sectional view of the smart bump of yet another particular type (hereinafter termed Type III) of the bonding structure of the present invention. In the bump  40   c , the conductive layer  14  is formed partially on the buffer layer  13  by sputtering or electroplating. The buffer layer  13  has two openings in such as a manner that the conductive layer  14  is shaped like a Π form.  
         [0039]      FIG. 4D  shows a cross-sectional view of the smart bump of the other particular type (hereinafter termed Type IV) of the bonding structure of the present invention. In the bump  40   d , the conductive layer  14  is formed on the buffer layer  13  with at least one opening by electroplating or sputtering. The conductive layer  14  has a thickness H 3  on the opening of the buffer layer, which is equal to a thickness H 1  of the smart bump  40   d  and greater than a thickness H 2  of the buffer layer  13  (i.e. H3=H1 and H3&gt;H2). To obtain a better Young&#39;s modulus of the bump  40   d , the thickness H 2  of the buffer layer  13  is at least one-third the thickness H 1  of the bump  40   d  (i.e. H2/H1&gt;=⅓).  
         [0040]     Referring to  FIG. 4E , a different kind of the buffer layer&#39;s structure of the foregoing smart bumps of the present invention is depicted therein. In this structure, any two neighboring bumps have a common buffer layer  13  over a common passivation layer  21 . In other words, the buffer layer  13  of a first bump  401  and a second bump  402  is not separated during the spin coating, micrography and etching processes.  
         [0041]     The foregoing bumps  40   a ,  40   b ,  40   c ,  40   d  further comprise a multi-layered metal structure  41  located between the buffer layer  13  and the conductive layer  14  and may be made of Al, Ni, Cu, Ag, Au, or a combination of the above including an alloy or a stack. Take a stack multi-layered metal structure for example, it could be of a Ni base coating and Au top coating. And it could also be the stack layers comprising the adhesion film, the wetting film and the conductive film. The main purpose of the adhesion film is to allow the bumps to adhere well to the buffer film  13  and the conductive pad  16 . The adhesion film is made from a material such as tungsten, titanium or chromium. The wetting film is made from a material such as nickel or copper. The conductive film such as gold is then formed over the wetting film. The multi-layered metal structure  41  is formed by, for example, sputtering or evaporation. With the multi-layered metal structure  41 , a combination structure among the elements of the bumps  40   a ,  40   b ,  40   c  and  40   d  may be mechanically enhanced.  
         [0042]     Referring to  FIG. 4F , stress-strain curves with respect to the conventional Au bump and the smart bump of the invnetion are shown therein. The curve  421  in  FIG. 4F  indicates the measured coefficients of strain -stress of the smart bump according to the present invention, and the curve  422  indicates the measured coefficients of strain-stress of the conventional Au bump. By this drawing, it may be known that the smart bump under an identical pressure of 100 MPa, has a strain amount of 60% while the conventional Au bump has a strain amount of approximately 15%. The smart bump has at least four times strained amount as compared to that of the conventional Au bump. That is to say that the smart bump has a better compressive resistance compared to the conventional Au bump. Thus the thickness of the smart bump is reduced to 9 um, it is still enough for compensation of height difference caused from the bumping process and thus promotes a yield of the COG bonding process.  
         [0043]     Referring to  FIG. 4G , results of a heating cycles test with respect to the conventional Au bump and the smart bump after the COG NCF process. Under the different heating temperature, the resistance of the conventional Au bump is shown in the curve  423  and the resistance of the smart bump is shown in curve  424 . Through this drawing, it may be known that an unstable contact resistance and even an open circuit occur when the temperature is greater than 80° C. in the conventional Au bump owing to the mismatched thermal expansion coefficients between the Au bump and the ACF. On the other hand, contact resistance of the smart bump is stable even when the temperature is rapidly increased from 20° C. to 100° C.  
         [0044]      FIG. 5  is a top view of the smart bump of another embodiment according to the present invention. For the sake of keeping overflowing adhesive material from remaining in the conductive interface of the bonding structure, which may causes poor contact of the conductive interface, a recess  51 , which preferrably have a depth of about 2 μm or larger, is provided to the conductive layer  14  as shown in  FIG. 5A . In addition, at least a trough  52  is formed on top of the smart bump for channeling excessive said adhesive material out effectively. In one option, a trough  52  may be formed on the conductive layer  14  as shown in  FIG. 5B . As yet another option, a trough  52  may be formed on the conductive layer  14  and the buffer layer  13  as shown in  FIG. 5C . The depths of the recess  51  and the troughs  52 ,  52 ′ may vary according to necessity and are processed by etching.  
         [0045]      FIG. 6A through 6C  show cross-sectional views of the smart bump according to another embodiments of the present invention. As shown in  FIG. 6A , a bump  61  is provided with a circular recess  62  and two troughs  63   a ,  63   b . As shown in  FIG. 6B , a bump  64  is provided with a square recess  65  and four troughs  66   a - 66   d , wherein each of the troughs  66   a - 66   d  extends perpendicuclarly to one side of the square recess  65 . As shown in  FIG. 6C , a bump  67  is provided with a square recess  68  and four troughs  69   a - 69   d , wherein each of the troughs  69   a - 69   d  extends along diagonals of the square recess  68  and outside of the square recess  68 . Based on the above-mentioned structures which channeling out the adhesive material, the recess  68  may be shaped like a square, a rectangle, a circle, a polygonal, a frame. And in accordance with the situation, the smart bump has one or more troughs which may be designed in direction, position and shape.  
         [0046]     Since NCF may be used for the bonding structures and the bonding method of the invention, the problems of short circuit and increased cost due to the use of ACF in conventional bonding structure have been eliminated. In addition, since the bumps of the invention have been designed to optimize Young&#39;s modulus and the thickness of conductive layer compared to that of conventional Au bump, both the problems of conductive layer cracking during the bonding process and excessively resistance at the contact points that have been plaguing conventional bonding structures are avoided. Furthermore, since a recess connected with and without troughs has been provided on top of the bump structures of present invention, the problem of overflowing NCF material to cause excessively high contact resistance at the bonding interfaces has been avoided.  
         [0047]     While this invention has thus far been described in connection with the preferred embodiments thereof, it will readily be possible for those skilled in the art to put this invention into practice in various other manners without departing from the scope set forth in the appended claims.