Abstract:
A method of etching a semiconductor structure, comprises contacting an under bump metallization (UBM) with an etching composition. The UBM includes an underlying layer comprising titanium and an overlying layer comprising a second metal. The etching composition is a liquid comprising at least 0.1 wt % hydrofluoric acid and at least 0.1 wt % phosphoric acid.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for etching semiconductor structures and to an etching composition for use in such a method. 
     2. Description of Related Art 
     Under bump metallization is being increasingly used in preference to wire bonding, to connect a semiconductor device to a packaging substrate. This metallization is not limited to the outer perimeter of the device, but may also be deposited on the top metal level of the device at a final stage of wafer processing. 
     In a typical under bump metallization structure, a bump or pillar of solder is connected to an underlying contact pad, which is typically aluminum, via an under bump metallization (UBM) made up of two or more layers. For example a lower layer in contact with the contact pad may promote adhesion of the bump to the contact pad and also functions as a diffusion barrier to protect the underlying integrated circuit, whereas an upper layer in contact with the solder bump is formed from a material that is wettable by the solder so as to strengthen the bond between the bump and the contact pad. 
     One conventional UBM structure includes a lower layer comprising titanium and an upper layer comprising copper. When such a structure is etched to form its final shape, the underlying titanium layer is etched to a greater extent than the overlying copper layer, resulting in undercut. This undercut results in less contact area to the metallic contact pad, as well as stress in the bump. 
     Conventional etching techniques using dilute hydrofluoric acid (dHF) produce an undercut not less than about 1 μm, whereas benchmarks for future generations of technology call for an undercut of 0.5 μm or less. Alkaline etching chemistries and hydrogen peroxide blends are also unsatisfactory, owing to their slow etch rates and high temperature requirements. 
     SUMMARY OF THE INVENTION 
     Thus, in one aspect, the present invention relates to a method of etching a semiconductor structure, comprising contacting an under bump metallization (UBM) with an etching composition. The UBM includes an underlying layer comprising titanium and an overlying layer comprising a second metal. The etching composition is a liquid comprising at least 0.1 wt % hydrofluoric acid and at least 0.1 wt % phosphoric acid. 
     In preferred embodiments of the method according to the present invention, the second metal is copper. 
     In preferred embodiments of the method according to the present invention, the contacting is performed for a time sufficient to over etch the underlying layer by at least 100% after nominal removal of the underlying layer. 
     In preferred embodiments of the method according to the present invention, the underlying layer is a composite material comprising aluminum sandwiched between top and bottom layers of titanium second metal is copper. 
     In preferred embodiments of the method according to the present invention, the underlying layer overlies a contact pad of a semiconductor device. 
     In preferred embodiments of the method according to the present invention, the contacting is performed until the contact pad is exposed by removal of at least a part of said underlying layer. 
     In preferred embodiments of the method according to the present invention, the contact pad is formed in a semiconductor substrate. 
     In preferred embodiments of the method according to the present invention, the contact pad is formed in an interlayer insulating film. 
     In preferred embodiments of the method according to the present invention, the contact pad comprises aluminum. 
     In preferred embodiments of the method according to the present invention, upon completion of said contacting an undercut of the underlying layer relative to the overlying layer is less than or equal to 0.5 μm. 
     In preferred embodiments of the method according to the present invention, the undercut is less than or equal to 0.3 μm. 
     In preferred embodiments of the method according to the present invention, the etching composition comprises from 0.5 wt % to 20 wt % hydrofluoric acid, from 0.5 wt % to 20 wt % phosphoric acid and at least 60 wt % deionized water. 
     In preferred embodiments of the method according to the present invention, the contacting is performed at a temperature from 15° C. to 30° C. 
     In preferred embodiments of the method according to the present invention, the contacting is performed at a temperature from 20° C. to 25° C. 
     In another aspect, the present invention relates to an etching composition, wherein the etching composition is a liquid comprising at least 0.1 wt % hydrofluoric acid, at least 0.1 wt % phosphoric acid and greater than 50% deionized water. 
     In preferred embodiments of the etching composition according to the present invention, the composition comprises from 0.5 wt % to 20 wt % hydrofluoric acid, from 0.5 wt % to 20 wt % phosphoric acid and at least 60 wt % deionized water. 
     In preferred embodiments of the etching composition according to the present invention, the composition consists essentially of from 0.5 wt % to 20 wt % hydrofluoric acid, from 0.5 wt % to 20 wt % phosphoric acid, balance deionized water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a semiconductor device on which layers for a UBM and a solder bump have been formed, but which has not yet been wet etched; 
         FIG. 2  is a cross-sectional view of the semiconductor device of  FIG. 1 , after wet etching has been performed to delimit the UBM structures; 
         FIG. 3  is a close-up of the detail III in  FIG. 2 , illustrating the underetch phenomenon that occurs when using conventional techniques and compositions; and 
         FIG. 4  is a close-up of the detail IV in  FIG. 2 , illustrating the underetch phenomenon that occurs when using the methods and etching compositions according to preferred embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , a semiconductor substrate  101  such as a silicon wafer has any of various semiconductor devices formed thereon. A contact pad  103 , sometimes referred to as a bonding pad or landing pad, is formed overlying and in contact with the semiconductor substrate  101 . The contact pad  103  is preferably formed of aluminum, but may instead be formed of other conductive materials such as copper. 
     Surrounding the contact pad  103  is a passivation layer  105 . The passivation layer  105  may serve to shield the substrate  101  from contaminants and moisture, as well as to prevent corrosion or other damage to the substrate and components formed thereon. Passivation layer  105  may comprise silicon dioxide, silicon nitride, or an organic material such as polyimide. 
     In the present embodiment, contact pad  103  is formed in the semiconductor substrate  101 ; alternatively, contact pad  103  could be formed in an interlayer insulating film (not shown). 
     An opening in the passivation layer  105  exposes contact pad  103 . 
     UBM structure  102  overlies the passivation layer  105  in this embodiment, and fills the opening through which the contact pad  103  is exposed, such that UBM  102  contacts the contact pad  103 . 
     More particularly, UBM  102  is made up of a lower layer  104  that comprises titanium, and an upper layer  106  that comprises copper. Each of the layers  104  and  106  may in turn be composed of two or more sublayers, and it will be appreciated by those skilled in this art that the titanium of lower layer  104  may be present in fewer than all of the sublayers of layer  104 . Similarly, the copper of upper layer  106  may be present in fewer than all of the sublayers of layer  106 . 
     For example, layer  104  could be a composite material comprising aluminum sandwiched between top and bottom layers of titanium. 
     A layer of patterned photoresist  107  covers the UBM  102  in selected regions, while exposing it in others. A solder bump  109  contacts the UBM  102  through an opening in the photoresist layer  107 . 
     In the processing that generates the structure shown in  FIG. 2  from the structure shown in  FIG. 1 , typically the photoresist layer  107  is first stripped away by any suitable conventional technique. 
     Then, the UBM undergoes a wet etching process so as to remove the layers  104  and  106  in the regions not covered by the solder bump  109 , to produce the structure shown in  FIG. 2 . However, when a conventional etching composition is used, such as dilute hydrofluoric acid (dHF), the layer  104  is heavily undercut, as shown in  FIG. 3 . In particular, the extent of undercut d indicated in  FIG. 3  is typically 1.0 μm or more. 
     Experiments conducted by the present inventors have shown that, as soon as the lower layer  104  is reached by the etchant, removal of titanium occurs very quickly. This, and the galvanic corrosion due to the titanium of layer  104  being in contact with the copper of layer  106  in this embodiment, leads to severe undercutting of layer  104  below layer  106 . 
     On the other hand, the present inventors have surprisingly discovered that a mixture of phosphoric acid (H 3 PO 4 ) and hydrofluoric acid (HF), inclusive of mixtures of the foregoing components with deionized water, reduces the extent d of undercut to less than 0.5 μm, as shown in  FIG. 4 . Furthermore, the use of such an etching composition also reduces the extent of corrosion of contact pad  103  when the same is made of aluminum. 
     Without wishing to be held to any particular theory, it is considered that adding H 3 PO 4  to dHF reduces the undercut by reducing the galvanic corrosion. Addition of H 3 PO 4  may alter the surfaces of Cu and Ti in such a way that the dissolution of Ti is suppressed. 
     The etching composition preferably comprises at least 0.1 wt % hydrofluoric acid, at least 0.1 wt % phosphoric acid and greater than 50% deionized water. More preferably, it comprises from 0.5 wt % to 20 wt % hydrofluoric acid, from 0.5 wt % to 20 wt % phosphoric acid and at least 60 wt % deionized water. The contacting is preferably performed at a temperature from 15° C. to 30° C., and more preferably from 20° C. to 25° C. 
     The following table shows various wet etching compositions that were tested by the present inventors, including the associated process temperatures, the extent of titanium over etch after nominal removal of titanium, and the resulting magnitude of undercut d. 
                                                             Ti over etch                       after nominal               Etchant   Temperature   removal of Ti   Ti undercut                           DI:HF   RT    80%   0.3 μm-0.5 μm           100:1                       DI:H 3 PO 4 :HF   RT   200%   &lt;0.2 μm           100:1:1                       H 3 PO 4 :HF   RT   200%   &lt;0.2 μm           100:1                       H 3 PO 4 :HF   RT   &gt;150%       0.4 μm           1:100                       H 3 PO 4 :HF:DI   40° C.   150%     0.2 μm           3:1:100                       H 3 PO 4 :HF:DI   RT   200%   &lt;0.2 μm           3:1:100                        
As shown in the foregoing table, the combined use of phosphoric acid and hydrofluoric acid in the wet etching composition resulted in substantially less titanium undercut than the use of conventional dilute hydrofluoric acid alone. Furthermore, the above table shows that, when using mixtures of phosphoric acid and hydrofluoric acid, the extent of undercut could be limited even when over-etching the titanium layer  104  to a much greater extent. Such overetching can be important to ensure complete removal of layer  104  following detection of an endpoint signal indicating nominal removal of the layer  104 , as when the layer  104  is relatively uneven and significant portions of layer  104  remain even after the layer  105  has been partly exposed.
 
     While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided merely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.