Abstract:
A bonding tool for bonding a wire to a substrate. The bonding tool has a body portion, a working tip coupled to one end of the body portion, an orifice extending along a longitudinal axis of the body and the working tip, and a polymer coating disposed over at least a portion of a surface of the orifice.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to a bonding tools used for fine wire bonding, and more particularly to a capillary bonding tool having a polymer coating disposed over the surface of the capillary. 
     BACKGROUND OF THE INVENTION 
     Wire bonding is, at this time, the dominant method used for the interconnection of semiconductor-based devices. In the wire bonding industry, tool failure leads to the destruction of the semiconductor device being interconnected, which can significantly lower yields. To prevent tool failures, tools are exchanged regularly prior to the predicted time of failure. 
     The wire bonding process is defined by the “Process Window,” which is the range of parameters that the bonding machine is qualified to operate within. To maintain bond quality, adjustments must be made to the bonding parameters. Bonding parameters, however, are not allowed to be outside of the approved process window. This leads to a loss of productivity as the bonding process must be stopped and tools exchanged. Furthermore, additional expense is incurred as a larger number of tools than necessary are used. 
     Contamination build-up and accelerated wear limit the useable time for conventional ceramic-based tools (e.g. Alumina, Zirconia toughened Alumina, Zirconia, cemented Tungsten Carbide, etc.) in wire bonding applications. Adhesion of organic-derived carbonaceous material and the subsequent embedding in the carbonaceous material of metallic particles from the bonding wire causes unsymmetrical loading of the tool and eventually leads to mechanical failure of the bonding tool by flexural bending. The carbonaceous material is produced by the action of the Electric Flame Off (EFO, for ball formation) on the petroleum and stearate-based materials typically placed on the wire surface to improve de-spooling of the wire. Enhanced surface wear is also caused due to the localization of bonding forces to a point load on the tool face. 
     U.S. Pat. Nos. 5,871,141, 5,558,270, and 5,421,503 assigned to the same assignee as the present invention, describe various conventional bonding tools for producing wire bonds on semiconductor devices and are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     In view of the shortcomings of the prior art, the present invention relates to a bonding tool having an orifice extending along a longitudinal axis of the tool body, a working tip coupled to one end of the tool body, and a polymer coating disposed over at least a portion of a surface of the orifice. 
     The present invention also relates to a method of manufacturing a bonding tool for bonding a fine wire to a substrate, comprising the steps of forming a cylindrical body, forming a taper at a first end of the body, forming an orifice extending along a longitudinal axis of the body, and coating the body with a polymer. 
     According to another aspect of the present invention, a first coating is disposed along the surface of the orifice and a second coating is disposed along an exterior surface of the bonding tool. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following Figures: 
     FIG. 1 is a cross sectional view of a conventional bonding tool; 
     FIGS. 2A-2B are cross sectional views of a bonding tool according to a first exemplary embodiment of the present invention; 
     FIGS. 3A-3B are cross sectional views of a bonding tool according to a second exemplary embodiment of the present invention; 
     FIG. 4 is a graph plotting the effect the exemplary embodiment of the present invention has on bonding force; 
     FIG. 5 is a graph plotting the effect the exemplary embodiment of the present invention has on bond power; 
     FIG. 6 is a graph plotting the effect the exemplary embodiment of the present invention has on bonding ball size; 
     FIG. 7 is a graph plotting the effect the exemplary embodiment of the present invention has on bonding ball size ratio; 
     FIG. 8 is a graph plotting the effect the exemplary embodiment of the present invention has on bond performance (i.e., shear force/unit area); and 
     FIG. 9 is a flow chart illustrating a coating process according to an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To overcome the aforementioned deficiencies, a polymer coating, such as parylene is deposited on the surface of a bonding tool. This surface coating is found to reduce the adhesion of contaminants, such as Carbon, Gold, Copper, Chlorine, Calcium, Iron, Sulfur, Beryllium, Oxides, Nitrides, etc., to the surface of the bonding tool, thereby increasing the time to failure and stabilizing the wire bonding process window. It is contemplated that the coating will be used mostly on newly produced bonding tool although the coating may also be applied to previously produced bonding tools if desired. 
     FIG. 1A is a cross sectional view of a conventional bonding tool  100  and FIG. 1B is an enlarged cross sectional view of working tip  102 . In FIG. 1A, tool  100  has an orifice  104  that extends from the end  106  of tool  100  to working tip  102 . A bonding wire (not shown) is inserted into orifice  104  and exits through working tip  102 . As the bonding wire is pulled through orifice  104 , it contacts surfaces  108  and  110  (shown in FIG. 1B) depositing contaminants thereon and wears away the unprotected surfaces of tool  100 . 
     FIG. 2A is a cross sectional view of a bonding tool according to a first exemplary embodiment of the present invention. In FIG. 2A, bonding tool  200  has coating  220 ,  222  deposited on the exterior and interior surfaces, respectively, of bonding tool  200 . The coating  220 ,  222  may be the same or different as discussed in detail below. In an exemplary embodiment, the coating  220 ,  222  may be a polymer, such as parylene or polycorbofluoro for example. In an exemplary embodiment, the coating may have an average thickness of up to about 2.0 microns. In a preferred embodiment, the thickness of coating  220 ,  222  is about 0.1 microns. The invention is not so limited, however, in that another thickness of coating  220 ,  222  may be used based on the particular application. As shown in greater detail in FIG. 2B, coating  220 ,  222  also protects the surfaces  226 ,  224 , respectively, where the bonding wire (not shown) has the greatest contact. Furthermore, since the polymer coating  220 ,  222  has an electrical isolation properties, it may also be used as an insulator for conductive capillaries. In addition, the inclusion of a polymer coating  222  on the interior surface of bonding tool  200  may reduce the drag force of the bonding wire as it passes through the bonding tool  200 . 
     Generally, capillaries and bonding tools have a cylindrical shape and are produced by either direct ceramic die pressing followed by sintering or injection molding followed by sintering, and are machined to final shape by grinding or Electro Discharge Machining operations (EDM). 
     FIG. 9 is a flow chart illustrating the coating process according to the exemplary embodiment of the present invention. At Step  900 , the surface of the bonding tool is treated. Exemplary treatments include plasma, heat, and chemical treatments. At Step  902 , the treated bonding tool is placed in a vacuum chamber. At 
     Step  904 , the tool is subjected to vaporization at about 150° C. and 1.0 torr with a di-Para-Xylylene dimer ([2,2] paracyclophane), for example. At Step,  906 , the tool is then subjected to pyrolesis at about 690° and 0.5 torr to produce a precursor monomer of para-xylylene for example. At Step  908 , deposition of coating  220 ,  222  such as Poly-Para-Xylylene for example, is performed at about ambient temperature (25° C. for example) and about 0.1 torr of chamber pressure. This exemplary process deposits a substantially uniform, continuous coating  220 ,  222  on tool  200  having a thickness of about 0.1 micron. It is understood by those of skill in the art that deposition of the coating may be accomplished by various methods of vapor deposition including chemical vapor deposition (CVD), physical vapor deposition (PVD) and their variants. Additionally, the bonding tool may be coated by immersing the bonding tool in a coating material. 
     FIG. 3A is a cross sectional view of a bonding tool according to a second exemplary embodiment of the present invention. In FIG. 3A, bonding tool  300  has coating  320  deposited on a portion of the interior surface of bonding tool  300 . As shown in greater detail in FIG. 3B, coating  320  is placed on the surfaces where the bonding wire (not shown) has the greatest contact. It is also contemplated that the coating may be placed on any combination of interior and/or exterior surfaces of the bonding tool as desired. 
     It is further contemplated that the coating may be either a soft or hard coating in any combination, such as a hard coating  220  on a exterior of the bonding tool and a soft coating  222  on an interior of the bonding tool, or visa versa. Use of a hard coating  220  on the exterior of the bonding tool  200  may increase the wear resistance of the bonding tool, especially along portion  226 , which comes in contact with the surface of the bonding pad (not shown) during the bonding process. Whereas, using a soft coating  222  on the interior surfaces of the bonding tool  200  tends to reduce the drag force of the wire as it passes through the bonding tool. Examples of such hard coatings are Amorphous Alumina and Si 3 N 4 . Examples of such soft coatings may be Polymers (i.e., Polyolefines, Polyparaxylylenes, Floropolymers, etc.). 
     The efficacy of the exemplary polymer coating in improving the reliability and stability of the wire bonding process is illustrated in FIGS. 4-8. Referring to FIG. 4, graph  400  is illustrated. In FIG. 4, graph  400  plots the effect of the polymer coating on the bonding force. In FIG. 4, the ordinate is the life span of the bonding tool with respect to the number of bonded wires and the abscissa is the bonding force required to form the bonds. Plot  402  is a linear representation of points  403  plotted for a conventional bonding tool. As shown, plot  402  increases significantly as the number of bonds increase. Plot  402  also shows that the bonding tool fails (that is, falls outside of the process window discussed above, requiring replacement) after about 800,000 bonds. By contrast, plot  404  is a linear representation of points  405  plotted for a bonding tool according to the present invention. As shown, plot  404  shows a negligible increase in bonding force as the number of bonds increase. Plot  404  also shows that the life of the exemplary bonding tool is almost twice that of the conventional bonding tool. 
     In FIG. 5, graph  500  plots the effect of the polymer coating on the bonding power. In FIG. 5, the ordinate is the life span of the bonding tool with respect to the number of bonded wires and the abscissa is the bonding power required to form the bonds. Plot  502  is a linear representation of points  503  plotted for the conventional bonding tool. As shown, plot  502  increased significantly as the number of bonds increase. By contrast, plot  504  is a linear representation of points  505  plotted for a bonding tool according to the present invention. As shown, plot  504  shows a negligible increase in bonding power as the number of bonds increase. 
     In FIG. 6, graph  600  plots the effect of the polymer coating on the bonding ball size. In FIG. 6, the ordinate is the life span of the bonding tool with respect to the number of bonded wires and the abscissa is the average bonding ball diameter. Plot  602  is a linear representation of points  603  plotted for a conventional bonding tool and plot  604  is a linear representation of points  605  plotted for a bonding tool according to the present invention. As shown, not only is the average ball diameter for the conventional bonding tool initially greater that that of the exemplary bonding tool, the rate of increase in average diameter is greater that the exemplary tool. This increased diameter may effect the bonding process as the density requirements of the industry increase. 
     In FIG. 7, graph  700  plots the effect of the polymer coating on the bonding ball size ratio. In FIG. 7, the ordinate is the life span of the bonding tool with respect to the number of bonded wires and the abscissa is the bonding ball size ratio. Plot  702  is a linear representation of points  703  plotted for the conventional bonding tool. As shown, plot  702  decreases significantly as the number of bonds increase. By contrast, plot  704  is a linear representation of points  705  plotted for a bonding tool according to the present invention. As shown, plot  704  shows a negligible change in the ball size ratio as the number of bonds increase. Thus, by using the exemplary bonding tool, greater uniformity of bonding balls may be achieved. 
     In FIG. 8, graph  800  plots the effect of the polymer coating on the shear force per unit area required to break a bond. In FIG. 8, the ordinate is the life span of the bonding tool with respect to the number of bonded wires and the abscissa is the shear force. Plot  802  is a linear representation of points  803  plotted for the conventional bonding tool. As shown, plot  802  decrease as the number of bonds increase, thereby indicating that the bonds become weaker as the bonding tool is used. By contrast, plot  804  is a linear representation of points  805  plotted for a bonding tool according to the present invention. As shown, plot  804  shows a sight increase in shear force required to break a bond as the number of bonds increase. 
     Although the invention has been described with reference to exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the true spirit and scope of the present invention.