Abstract:
In view of the problems in all existing bonding methods, such as high temperature, high pressure, and high surface modification cost, etc., the present invention provides a bonding method and a bonding structure formed using the same, which can overcome such drawbacks and also achieve wafer-level bonding under a condition of normal temperature and low pressure. The bonding method comprises: generating bonding structures capable of being mutually mechanical interlocked, wherein, the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked is higher than the bonding energy therebetween; and, utilizing the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked to bond the bonding structures capable of being mutually mechanical interlocked.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of semiconductor manufacturing, in particular to a bonding method and a bonding structure formed with the bonding method. 
       BACKGROUND 
       [0002]    Currently, methods for Cu—Cu bonding mainly include thermosonic bonding, thermocompression bonding, and surface modification bonding, etc., all of these bonding methods have drawbacks such as high bonding temperature, high bonding pressure, or high surface modification cost, etc. Owing to the excessively high bonding temperature and pressure, it is unable to carry out bonding of wafer with semiconductor devices or bonding of thin chips in the semiconductor manufacturing process. 
       SUMMARY 
       [0003]    To overcome the above-mentioned drawbacks in the Cu—Cu bonding, the present invention provides a bonding method and a bonding structure formed with the bonding method. 
         [0004]    The present invention provides a bonding method, comprising: 
         [0005]    generating bonding structures capable of being mutually mechanical interlocked, wherein, the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked is higher than the bonding energy therebetween; and 
         [0006]    utilizing the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked to bond the bonding structures capable of being mutually mechanical interlocked. 
         [0007]    The present invention further provides a bonding structure formed with the above-mentioned bonding method. 
         [0008]    Since the bonding method and the bonding structure according to the present invention utilize the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked which is higher than the bonding energy therebetween to accomplish bonding, the bonding can be conducted at normal temperature and low pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The accompanying drawings are provided here to facilitate further understanding on the present invention, and constitute a part of this document. They are used in conjunction with the following embodiments to explain the present invention, but shall not be comprehended as constituting any limitation to the present invention, wherein: 
           [0010]      FIG. 1  is a flow chart of the bonding method according to the present invention; 
           [0011]      FIGS. 2 a , 3 a , 4 a , 5 a , and 6 a    are top views of illustrative bonding structures capable of being mutually mechanical interlocked; 
           [0012]      FIGS. 2 b , 3 b , 4 b , 5 b , and 6 b    are sectional views of the structures shown in  FIGS. 2 a , 3 a , 4 a , 5 a , and 6 a    when viewed along the direction of the bold arrow in each figures; 
           [0013]      FIGS. 7 a -7 g    are illustrative flow charts of generating bonding structures capable of being mutually mechanical interlocked; and 
           [0014]      FIG. 8  shows illustrative bonding structures capable of being mutually bonded, with beveled bonding sections. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Hereafter the embodiments of the present invention will be detailed, with reference to the accompanying drawings. It should be appreciated that the embodiments described here are only provided to describe and explain the present invention, but shall not be deemed as constituting any limitation to the present invention. 
         [0016]    As shown in  FIG. 1 , the bonding method according to the present invention can comprise the following steps: 
         [0017]    S 11 . generating bonding structures capable of being mutually mechanical interlocked, wherein, the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked is higher than the bonding energy therebetween. 
         [0018]    The types of bonding structures capable of being mutually mechanical interlocked can be those shown in  FIGS. 2-6 , wherein,  FIGS. 2 a , 3 a , 4 a , 5 a , and 6 a    are top views of bonding structures capable of being mutually mechanical interlocked, while  FIGS. 2 b , 3 b , 4 b , 5 b , and 6 b    are sectional views of the structures shown in  FIGS. 2 a , 3 a , 4 a , 5 a , and 6 a    when viewed along the direction of the bold arrow in each figure. However, those skilled in the art should appreciate that other bonding structures capable of being mutually mechanical interlocked are also possible in actual applications, and  FIGS. 2-6  are only illustrative. Namely, there is no particular limitation on the shape and size of the bonding structures and the shape and size of the bumps on the bonding structures, and various bonding structures belong to the scope of the present invention as long as they can be mutually mechanical interlocked and the bonding is realized under a principle that the frictional heat generated by the bonding structures is higher than the bonding energy therebetween. Moreover, the bonding structures capable of being mutually mechanical interlocked with beveled bonding sections as shown in  FIG. 8  are also in the scope of the present invention. Such beveled bonding sections may be formed in consideration of the specific process, and, of course, they can be made artificially and intentionally. In addition, such bonding structures with beveled bonding sections make the bonding easier. 
         [0019]    S 12 . utilizing the frictional heat generated by the bonding structures capable of being mutually mechanical interlocked to bond the bonding structures capable of being mutually mechanical interlocked. 
         [0020]    Preferably, in the step S 11 , the procedure of generating bonding structures capable of being mutually mechanical interlocked may comprise: forming a first under-bump metal (UBM) layer pattern on a substrate; forming a second UBM layer pattern on the first UBM layer pattern; and, forming a bump on the second UBM layer pattern, so as to form the bonding structures capable of being mutually mechanical interlocked, wherein, the bump can be bonded with the first UBM layer pattern when the bonding structures capable of being mutually mechanical interlocked are bonded. 
         [0021]    Preferably, the bump is bonded with the first UBM layer pattern by pressurized thermal annealing. Preferably, the first UBM layer pattern is composed of an adhesion layer on the substrate and a seed layer on the adhesion layer. Preferably, the adhesion layer is formed by a metal material that can adhere to the substrate (e.g., silicon substrate, silicon dioxide substrate, etc.), for example, the metal material can be at least one of TiN, TiW, Ti, Cr, Ta, Mo, and Co, etc. Preferably, the seed layer is also formed by a metal material, such as Cu, Au, or Ni, etc. 
         [0022]    In addition, the bump may also be formed by a metal material. 
         [0023]      FIGS. 7 a -7 g    show illustrative flow charts of generating bonding structures capable of being mutually mechanical interlocked. However, those skilled in the art should appreciate that those flow charts are only illustrative, and any method or process that can be used to form bonding structures capable of being mutually mechanical interlocked in the semiconductor field can be used, including any processing method in semiconductor CMOS manufacture process and bipolar manufacture process. 
         [0024]    First, as shown in  FIG. 7 a   , a first adhesion layer  2  and a first seed layer  3  are formed as an under-bump metal (UBM) layer on a substrate  1  by sputtering, and the first adhesion layer  2  and the first seed layer  3  are photoetched with a masking technique, so as to form the pattern shown in  FIG. 7 a   , wherein, the first adhesion layer  2  is usually formed by at least one of TiN, TiW and Ti, and the first seed layer  3  is usually formed by Cu. 
         [0025]    Then, as shown in  FIG. 7 b   , a second UBM layer is formed by sputtering, and the second UBM layer can be composed of a second adhesion layer  4  and a second seed layer  5 . 
         [0026]    Next, as shown in  FIG. 7 c   , a mask layer  6  is formed on the second UBM layer, and is photoetched to form an area where a bump is to be formed subsequently. 
         [0027]    Next, as shown in  FIG. 7 d   , a bump  7  is formed by electroplating or sputtering, etc. Next, as shown in  FIG. 7 e   , the mask layer  6  is removed. Next, the second UBM layer is photoetched, to obtain the pattern shown in  FIG. 7 f   . Finally, bonding is accomplished on a flip-chip bonding apparatus, as shown in  FIG. 7   g.    
         [0028]    Thus, by utilizing the frictional heat generated by the bumps which is higher than the bonding energy between the bumps, the bonding at the bump-to-bump interface can be realized by applying very low force (as long as the force is higher than the friction force between the bumps) in the bonding process. To relieve the stress and enhance the strength of the bonding interface, in the step shown in  FIG. 7 g    the bonding structures are annealed for 30 min. at 100° C. after the bonding is completed mainly utilizing the frictional heat generated by the side walls of the bumps and mechanical interlock. In addition, the top of the bump  7  can not be bonded if the first adhesion layer  2  and the first seed layer  3  are not formed. Therefore, to increase reliability, preferably the bonding of the top of the bump  7  is accomplished by thermocompression bonding after the first adhesion layer  2  and the first seed layer  3  are formed and the bonding of the side walls of the bump  7  is completed, i.e., pressurized thermal annealing is carried out after the bonding of the side walls of the bump  7  is completed, to facilitate the bonding between the top of the bump  7  and the first seed layer  3 . However, according to method disclosed in the present invention, forming the first seed layer  3  and the first adhesion layer  2  is not an indispensable procedure. 
         [0029]    The bonding principle of the bonding method according to the present invention is as follows: 
         [0030]    According to the formula of maximum static frictional force: f=μ×N, where, μ is maximum static friction coefficient (for example, in the case that the bump  7  is made of copper, the Cu—Cu static friction coefficient is 1.6 (please see the Table 3.1 in Vol. I “Mechanics” of “Berkeley Physics Tutorial” by C. Kittel, et al), N is positive pressure, and N=τS, where, r is the maximum critical shearing stress of the bump (in the case of a bump made of copper, the theoretical maximum critical shearing stress of copper is 1500 MPa, but the practical value is much lower than that value), S is the lateral stressed area of the bump. The frictional heat is W=f×H=μ×N×H=μ×τ×S×H, where, W is frictional heat, f is maximum static frictional force, and H is the height of the bump. For example, for the bonding structures capable of being mutually mechanical interlocked as shown in  FIGS. 2 a  and 2 b   , the frictional heat per unit area can be expressed as: 
         [0000]    
       
         
           
             
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         [0000]    If the frictional heat per unit area is higher than the bonding energy between the bumps, bonding can be accomplished without external heat. In the case that the bump  7  is made of copper, since the bonding energy of copper is 3 J/m2, bonding can be realized as long as the L1 is at the order of micrometer, and the required pressure only has to be higher than the maximum static frictional force. The calculated frictional force is much lower than IN when the L of the bump  7  is 6 μm, which means the required pressure is very low; in other words, the bonding can be accomplished by applying very low pressure only. Hence, the bonding method disclosed in the present invention can be used to accomplish wafer-level bump bonding at normal temperature and low pressure. 
         [0031]    The present invention further provides a bonding structure formed with the above-mentioned bonding method. Since illustrative bonding structures capable of being mutually mechanical interlocked have been described above in detail in conjunction with the bonding method according to the present invention, the bonding structure will not be further detailed here. In addition, the bonding method according to the present invention can not only be applied for wafer-level bonding but also for chip-level and system-level bonding. 
         [0032]    While the present invention is disclosed as above in some embodiments, the embodiments shall not be deemed as constituting any limitation to the present invention. Those skilled in the art can easily make various alternations and modifications to the embodiments without departing from the spirit and scope of the present invention. Therefore, the protected domain of the present invention shall be only confined by the claims.