Abstract:
A conductive structure for a semiconductor integrated circuit and method for forming the conductive structure are provided. The semiconductor integrated circuit has a pad and a passivation layer partially covering the pad to define a first opening portion having a first lateral size. The conductive structure electrically connects to the pad via the first opening portion. The conductive structure comprises a support layer defining a second opening portion. A conductor is formed in the second opening portion to serve as a bump having a planar top surface.

Description:
[0001]    This application claims priority to Taiwan Patent Application No. 096105681 filed on Feb. 15, 2007, the disclosures of which are incorporated herein by reference in their entirety. 
       CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to a semiconductor structure. More particularly, the present invention relates to a semiconductor structure having a flattened surface and a method for forming the same. 
         [0005]    2. Descriptions of the Related Art 
         [0006]    Bump electroplating techniques are used in microelectronics and micro system fields, such as in connections between flat panel displays (FPD) and drivers IC, in conductive lines and air bridges techniques in GaAs chips, and in the manufacturing of X-ray masks in LIGA techniques. The aforementioned techniques apply bump electroplating techniques at different stages throughout the manufacturing process. 
         [0007]    For example, in a IC chip and circuit board setup, the IC chip is connected to the circuit board in many available ways, and the main packaging method is related to the gold bump electroplating technique. The pad of the IC chip is thereby electrically connected to the circuit board by the use of this technique. The aforementioned techniques not only greatly shrink dimensions of the chip, but also affix the chip on the circuit board, thus reducing the size, decreasing the sensitivity, and improving heat dissipation of the chip and the circuit board. Additionally, electroplating techniques are low cost, an obvious benefit in the development and manufacturing of these chips. 
         [0008]    A typical bump electroplating process, such as a gold bump electroplating process, requires forming an under bump metal (UBM) on the pad first. The UBM is applied as an adhesion layer between the bump and the pad, as well as connected to a conductive layer that is jointly used as media during electroplating. The conductive layer is substantially formed either simultaneously with, or separately from, the UBM. Meanwhile, the conductive layer and the UBM may be formed by the same process and made of the same materials. Therefore, the bump can be formed successfully on the UBM and be electrically connected to the pad via the UBM. Before the electroplating process, a plurality of conductive layers, or conductive UBM, are formed on the surface of the chip, except the pad. After the bump is formed in the electroplating process, the conductive layers or UBM are then removed by etching. 
         [0009]    The bump is larger in lateral dimension than the pad so that after the electroplating process, the bottom of the bump provides sufficient support when the bump is being connected to the circuit board, thus preventing cracks or deformations from compression. Referring to  FIG. 1 , the UBM  12  is naturally formed with two corner portions  12   a  when the UBM  12  is formed on the opening portion defined by the pad  13  and the passivation layer  14 . When the bump  10  is isotropically accumulated along the upward direction from the UBM  12 , for example by the electroplating process, the bump  10  would naturally cover the corner portions  12   a . Therefore, the top surface  11  of the bump  10  would be rugged due to the rugged surface of the UBM  12  and present jutting corners  101  and  102 . 
         [0010]    The aforementioned jutting corners  101  and  102  can cause contacting surface damage or a bad connection when the bump  10  is being connected to the circuit board and can result bad electric conductivity. Therefore, extra processes such as polishing are needed to eliminate the jutting corners. The extra processes are not convenient and are difficult during the stages of quality control. 
         [0011]    Further, since the surface of the chip may be rugged, the UBM may suffer break point and hinder electric conductivity when the UBM is formed on the surface of the chip. Additionally, the UBM may be not uniform in thickness, thus resulting in increased electric resistance of the UBM. To solve the aforementioned issues, prior arts are provided with a thicker UBM. However a thicker UBM would have higher resistance. Since the main objective of the UBM is to be the adhesion layer between the bump and the pad, and the UBM naturally has higher resistance compared to the pad and bump, a thicker UBM will result in a significant increase in electric resistance between the bump and the pad, thus harming electric conductivity between the chip and the circuit board. The aforementioned situations influence the electroplating process and decrease the yield of electroplating the bump, resulting in either the need for post-processing procedures, or an unusable chip altogether. 
         [0012]    Accordingly, a solution of providing a conductive structure for a semiconductor integrated circuit is highly desired in semiconductor technology. 
       SUMMARY OF THE INVENTION 
       [0013]    The primary objective of this invention is to provide a conductive structure for a semiconductor integrated circuit. The semiconductor integrated circuit comprises a pad and a passivation layer that partially covers the pad to define a first opening portion. The conductive structure is then adapted to electrically connect with the pad through the first opening portion. The conductive structure comprises a supporting layer with a second opening portion in which a conductor with a flattened top surface is formed as a bump. 
         [0014]    Another objective of this invention is to provide a conductive structure for a semiconductor integrated circuit. The semiconductor integrated circuit comprises a pad and a passivation layer. The conductive structure is configured to ensure no brake point on the passivation layer and ensure a conductive layer with steady resistance. 
         [0015]    To achieve the aforementioned objective, the present invention discloses a conductive structure comprising a supporting layer and a conductor. The supporting layer covers the edge of the first opening portion and defines a second opening portion with a second lateral dimension. The conductor is formed in the second opening portion. The second lateral dimension is not substantially larger than the first lateral dimension. 
         [0016]    The present invention further discloses a method for forming the aforementioned conductive structure for a semiconductor integrated circuit, in which the semiconductor integrated circuit comprises a pad and a passivation layer partially covering the pad to define a first opening portion with a first lateral dimension. The aforementioned method comprises the following steps of: (a) forming a supporting layer to define a second opening portion with a second lateral dimension, in which the second lateral dimension is not substantially larger than the first lateral dimension: (b) forming a conductor in the second opening portion, in which the conductor is adapted to electrically connect with the pad through the first opening portion. 
         [0017]    The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a profile diagram of a semiconductor integrated circuit of the prior art; 
           [0019]      FIGS. 2(   a )- 2 ( e ) show process flow, as well as structure diagrams, of a preferred embodiment of the present invention; 
           [0020]      FIG. 3  is a modified embodiment of the embodiment seen in  FIG. 2(   b ); 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]      FIG. 2(   a ) to  FIG. 2(   e ) collectively illustrate the preferred embodiment of the present invention, in which the manufacture process of a semiconductor integrated circuit  20  is shown. 
         [0022]    First, as shown in  FIG. 2(   a ), the semiconductor integrated circuit  20  comprises a pad  21  and a protection layer  22 . In the embodiment, the pad  21  is made of aluminum, and the protection layer  22  partially overlaps the pad  21  to expose a portion of the pad  21  to form a first opening portion as a contact window, and later, a bump. The first opening portion has a first lateral dimension W 1 . Since the protection layer  22  partially overlaps the edges of the pad  21 , the lateral dimension W 1  of the first opening portion is smaller than the lateral dimension of the pad  21 . 
         [0023]    Then, a conductor, such as a titanium/tungsten alloy conductor  23 , is formed for conducting current in the electroplate process so that the bump can be formed. In the present invention, the titanium/tungsten alloy conductor  23  comprises a central portion  231  and a periphery portion  232 , wherein the central portion  231  overlaps the first opening portion, and the periphery portion  232  extends onto the protection layer  22  as shown in  FIG. 2(   b ). The conductor can also comprise a partially thicker portion as shown in  FIG. 3 . In  FIG. 3 , the central portion comprises a first vertical dimension H 1 , while one portion of the periphery portion comprises a second vertical dimension H 2 . The second vertical dimension H 2  is not smaller than the first vertical dimension H 1 . Detailed description of the  FIG. 3  is described hereinafter. 
         [0024]    A supporting layer made of a nonconductor, such as a Ployimide (PI) layer  24 , is then formed on the titanium/tungsten alloy conductor  23 . The portion of the PI layer  24  that overlaps the first opening portion is cleaned by a certain method similar to etching to expose the titanium/tungsten alloy conductor  23  and to define a second opening portion with a second lateral dimension W 2 , as shown in  FIG. 2(   c ). The titanium/tungsten alloy conductor  23  exposed by the second opening portion is configured to be a bottom surface for forming the bump in the electroplate process. To prevent the disadvantages induced by rugged bottom surface of the prior art, the bottom surface has to be a substantially smoother surface. Thus, the second lateral dimension is not greater than the first lateral dimension. So that the portion of the titanium/tungsten alloy conductor  23  exposed by the second opening portion has an expected smoother surface. The portion of the PI layer  24  around the second opening portion has a third vertical dimension H 3 . The supporting layer is made of BCB materials. 
         [0025]    The conductor is then made in the second opening portion by the electroplate process as a bump  25 . The bump  25  can be made of gold and with a fourth vertical dimension H 4 . Since the supporting layer is made of nonconductor, the fourth vertical dimension H 4  of the bump  25  has to be continuously formed until it is larger than the third vertical dimension H 3  of the supporting layer.  FIG. 2(   d ) shows the fourth vertical dimension H 4  which is greater than the third vertical dimension H 3 , in which the bump  25  with a flattened top surface  251  can be cleanly electrically connected to the circuitry board as shown in  FIG. 2(   d ). 
         [0026]    Finally, as shown in  FIG. 2  ( e ), the unnecessary parts are cleaned by a certain method similar to etching. Only the bump  25  and a specified ratio of the supporting layer  24  as well as the structure therebelow remain. It can be understood that although the bump  25  shown in  FIG. 2(   e ) has a smaller lateral dimension compared to the prior art, the supporting layer  24  surrounding the sidewall of the bump  25  ensures that the bump  25  has a flattened top surface for a clean electrical connection as well as a more stable conductive structure. 
         [0027]      FIG. 3  shows a modified embodiment of the present invention. The figure is a drawing of the partial manufacturing result of a semiconductor integrated circuit  30 . The rest of the manufacturing processes of the present embodiment are the same as the previous embodiment, and thus the redundant details are omitted hereinafter. The conductive structure of the present embodiment comprises a titanium/tungsten alloy conductor  33  with a partially thicker portion. More particularly, the titanium/tungsten alloy conductor  33  has a central portion  331  and a periphery portion  332 , wherein the central portion  331  has a first vertical dimension H 1  as well as a first portion  333  of the periphery portion  332  and the second portion of the periphery portion  332  has a second vertical dimension H 2 . The second vertical dimension H 2  is not smaller than the first vertical dimension H 1 .  FIG. 3  shows that the second vertical dimension H 2  is greater than the first vertical dimension H 1 . 
         [0028]    Since the chip may have rough surface, if the titanium/tungsten alloy conductor  33  has insufficient thickness in the periphery portion  332 , the titanium/tungsten alloy conductor  33  may suffer break points on the rough surface which will lead to an increased resistance or a non-uniform resistance, ultimately influencing the electroplating process. However, if the central portion  331  of the titanium/tungsten alloy conductor  33  is much thicker, the titanium/tungsten alloy results in a higher resistance, which influences the electric conductivity between the bump and the pad. Therefore a titanium/tungsten alloy conductor  33  with various thicknesses is brought up with the second vertical dimension H 2  greater than the first vertical dimension H 1 . After the bump is formed, the second portion of the periphery portion  332  is cleaned by a certain method similar to etching. Only the first portion  333  remains. Thus the eventual conductive structure of the bump is not influenced by the thicker second portion of the periphery portion  332 . 
         [0029]    In the aforementioned embodiments, the conductor is made of under bump metal, UBM, and not limited to titanium/tungsten alloy. For example, the conductor can alternatively be made of titanium. 
         [0030]    By the aforementioned description, it can be understood that the conductive structure of the present invention allows for a design for forming a supporting layer on the passivation layer that ensures a flattened top surface of the bump when it is formed for electrical connection to the circuitry board. Also, by forming a conductor with various thicknesses, the conductor is free from having break points in the electroplating process. Meanwhile, the portion of the conductor that connects to the pad has a thinner thickness to ensure a lower resistance between the bump and the pad. 
         [0031]    The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.