Abstract:
A method and apparatus for enhancing the microthrowing power in a plating bath. The method involves the use of ultrasonic vibration of an electrochemical solution to increase the uniformity of copper deposition for blind hole vias. The apparatus includes a series of ultrasonic transducers positioned between anodes for vibration of the solution being electro deposited on the cathode.

Description:
FIELD OF THE INVENTION 
     The present invention relates to vias metallization of printed circuit boards and more particularly, the present invention relates to enhancing the throwing power in the electroplating of the vias. 
     BACKGROUND OF THE INVENTION 
     In view of the continuous advancements in semiconductor performance together with rapid expansion of the demand for sophisticated electronic devices, particularly in mobile and portable applications, the need for fabricating circuit feature of a small size and interconnection substrates is substantially increasing. Multi-layered printed circuit boards are now using high aspect ratio through hole vias and blind vias openings for high density interconnections. Uniform plating distribution inside these vias represents a main issue for PCB reliability. 
     New ways to improve mass transport and new electrolyte additives have increased the uniformity of electrodeposition inside blind vias. 
     Blind vias having a diameter (d) of 150 microns or less and an aspect ratio (AR) (see equation [1]) greater than 1 are difficult to plate properly using conventional techniques. Currently, in order to enhance copper deposition inside blind vias, the technique of reverse pulse plating or the use of complex chemical solutions have been proposed and used. These processes are not without their limitations, despite the fact that they are useful. As is known, industrial plating solutions can be extremely complex and can contain up to four organic additives. Additive concentrations require constant monitoring and are usually adjusted because many of these additives are destroyed or sacrificed during the plating process. Another limitation is that the solution, subsequent to use is environmentally unfriendly and can result in expensive disposal costs. 
     Regarding a pulse step position, this process also employs complex chemical solutions and involves a significant capital investment since the method does not employ the same current rectifiers typically associated with conventional DC plating. One of the other limitations to this process is that health problems could be an issue for the operators since reverse pulse systems emit strong magnetic fields. 
     As is known in fluid dynamics, ultrasonic agitation enhances mass transfer and this technique can be applied to electrochemistry. This was proposed by Walker in, Chemistry in Britain, 1990, pp. 251-254. 
     Although there have been advances in the electroplating of the circuit board vias, these methods remain complex to control and run. There is a need in the industry to have a method which is easier to operate and which provides for a similar or more efficient electrodeposition. The present invention satisfies these needs. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an improved system and method for enhancing the throwing power in an electroplating cell. 
     The method is particularly well adapted for industrial applications of PCB plating for high production levels with uniform application of the plating material. 
     According to a further object of one embodiment, there is provided a method for electroplating blind vias or through holes in a printed circuit comprising the steps of: providing a printed circuit board having blind vias or through holes therein; providing a plating cell containing solution for plating in the vias of the printed circuit board, the plating cell further including anodes; providing ultrasonic vibration means for vibrating the plating solution during electrodeposition; and vibrating the solution to electroplate the blind vias or through holes. 
     It has been found that ultrasonic agitation in accordance with the present invention substantially increased the microthrowing power improvement for small interconnection blind vias. 
     The ultrasonic treatment may occur using transducers operating in the range of 20 kHz to 60 kHz suitably positioned within the plating bath. For purposes of the instant application, copper electrodeposition was employed and to this end the transducers were positioned within titanium hollow containers in view of the fact that the containers are chemically inert, under certain conditions, to the plating bath and do not interfere with the electroplating procedure. It will be appreciated by those skilled in the art that the container may comprise any suitable material and this will depend on the environment in which the transducers are employed and the nature of the compounds in the solution. 
     It is envisioned that the ultrasonic transducers are positioned directly within the cell at a suitable location to induce hydrodynamic cavitation within the cell and thus increase the uniformity of deposition within the blind vias. To augment the electrodeposition efficiency, chemical additives may be used in combination with the ultrasonic agitation. Suitable additives are known to those skilled in the art. 
     Other known methods may be combined with the ultrasonic treatment of the solution such as agitation of the PCB board or substrate to be treated in addition to the ultrasonic treatment of the solution. Further, it is clearly envisioned that other forms of treatment including reverse pulse deposition could also be used in combination with the ultrasound treatment. 
     Another object of one embodiment of the invention is to provide a method of plating blind vias in integrated circuits, comprising the steps of: providing a printed circuit board to be plated; providing a plating cell containing solution for plating in the blind vias or through holes of the printed circuit board, the cell further including anodes; providing ultrasonic vibration means for vibrating the plating solution during electrodeposition; introducing a gas adjacent the printed circuit board for localized agitation of the plating solution around the printed circuit board; and vibrating the solution to electroplate the blind vias or through holes. 
     According to a further object of one embodiment of the present invention, there is provided a system for electroplating vias in a printed circuit board, the system comprising: an electroplating cell having a pair of anodes; means for supplying power to the cell; an electrochemical solution; a substrate for receiving material to be electroplated; and ultrasonic vibration means for vibrating the solution. 
     Having thus described the invention, reference will now be made to the accompanying drawings illustrating preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the plating bath system in accordance with one embodiment of the present invention; 
     FIG. 2 is a schematic cross-sectional illustration of a blind via feature; 
     FIG. 3 is a graphical illustration of the variation in mean microthrowing parameters as a function of plating conditions for different vias sizes; 
     FIG. 4 is a graphical illustration of the variation of the mean deposit quality parameters as a function of plating conditions for different vias sizes; 
     FIG. 5 is a graphical illustration similar to FIG. 3 for further vias sizes; 
     FIG. 6 is a graphical illustration similar to FIG. 4 for further vias sizes; 
     FIG. 7 is a graphical illustration similar to FIGS. 3 and 5 for different vias; and 
     FIG. 8 is a graphical illustration similar to FIGS.  4  and  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, shown is a schematic illustration of the plating bath according to one embodiment, with the cell being globally referenced by numeral  10 . The cell includes a reaction vessel  12  within which an electrochemical solution is known to be included, the solution not being shown. The cell includes a plurality of anodes  14  which are suitably connected to a bus bar  16  with the bus being connected to suitable source of power (not shown). The cathode, shown in the example as the substrate  18  is disposed in the cell  10  as indicated in FIG.  1 . In this example, the substrate comprises a PCB having a blind vias openings (not shown) and other small features. 
     In the embodiment of FIG. 1, the ultrasonic transducers  20  (dashed lines) are positioned within hollow containers  22  which, in the example, comprise polygonal titanium containers. Since the plating bath comprises a conventional DC copper bath, the titanium container was selected in view of the chemical inertness in this system. Other variations for the material of which the container is made will depend on the nature of the solution and the overall cell. To augment deposition, an apertured air hose  24  is connected to a source of pressurized gas (not shown) such as air, nitrogen, noble gases etc. The gas is bubbled in the solution to cause localized agitation of the solution at the cathode  18 . Further, the cathode  18  may be moved relative to vessel  12  laterally in the direction of arrow A to further assist in deposition. This may be moved manually or mechanically. 
     The titanium containers include a plurality of ultrasonic transducers  20  as indicated with the total power for a single container comprising 500 watts at between 20 kHz and 60 kHz and preferably 40 kHz operating frequency. The two cans employed were inserted between and behind two pairs of anodes  14  as illustrated in FIG. 1 in 600 L of copper plating bath. The cathode consisted of a blind vias drilled test panel of a printed circuit board. 
     With reference to FIG. 2, shown is a schematic cross-sectional illustration blind via feature. The feature is denoted by numeral  26  and includes a metal clad layer  28 , a dielectric layer  30 , a second metal clad layer  32  positioned beneath layer  30  and a plated metal layer broadly denoted by  34 . With respect to the symbology used in FIG. 2, the following is representative of the physical meaning and value/units of the symbols used in FIG.  2 : 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Symbol 
                 Physical Meaning 
                 Value and/or Units 
               
               
                   
               
             
             
               
                 AR 
                 Blind via aspect ratio 
                 — 
               
               
                 d 
                 Blind via diameter 
                 μm 
               
               
                 h 
                 Blind via depth 
                 μm 
               
               
                 I b   
                 Copper blind via bottom thickness 
                 μm 
               
               
                 I min   
                 Minimum copper blind vias thickness 
                 μm 
               
               
                 I t   1  and I t   2   
                 Surface copper thickness 
                 μm 
               
               
                 I w   1  and I w   2   
                 Copper blind via wall thickness 
                 μm 
               
               
                 P 1   
                 Mean microthrowing power 
                 % 
               
               
                   
                 parameter 
               
               
                 P 2   
                 Mean deposit quality parameter 
                 % 
               
               
                   
               
             
          
         
       
     
     In order to calculate the points for the graphical illustrations to be discussed hereinafter, the following formula were used:              [   1   ]           AR   =     h   d                 [   2   ]             P   1     =       2   3                       (       l   w   1     +     l   w   2     +     l   b       )       (       l   t   1     +     l   t   2       )       ×   100                 [   3   ]             P   2     =         3                   l   min         (       l   w   1     +     l   w   2     +     l   b       )       ×   100                                  
     Regarding FIGS. 3 through 8, Table 1 represents the experimental conditions used to generate the data points. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Ultrasonic Agitation 
                 Current Density 
               
               
                 Experiment 
                 Air Agitation 
                 (W.cm −2 ) 
                 (A.dm −2 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 Yes 
                 No 
                 2.2 
               
               
                 B 
                 No 
                 0.093 
                 2.2 
               
               
                 C 
                 No 
                 0.19  
                 2.2 
               
               
                 D 
                 Yes 
                 0.093 
                 2.2 
               
               
                 E 
                 Yes 
                 0.19  
                 2.2 
               
               
                 F 
                 Yes 
                 No 
                 1.65 
               
               
                 G 
                 Yes 
                 0.045 
                 1.65 
               
               
                 H 
                 Yes 
                 0.093 
                 1.65 
               
               
                 I 
                 Yes 
                 0.19  
                 1.65 
               
               
                 J 
                 Yes 
                 0.045 
                 2.2 
               
               
                   
               
             
          
         
       
     
     For the data in FIGS. 3 through 8, a plating time corresponding to a 25 micron deposit thickness and a side-to-side motion of the PCB were used. These two conditions together with air agitation are representative of conventional conditions used in the PCB plating industry. All of the lengths (I x ) were evaluated using cross-sectional samples taken at different locations on the PCB. High P 1  values are indicative of uniformity in the deposit while high P 2  are representative of the absence of defects in the deposits. 
     The results shown in FIGS. 3 and 4 demonstrate that the combination of air and ultrasonic agitation (condition D and E) were crucial and yielded high P 1  and P 2  relative to conditions A through C. It was determined that ultrasonic agitation in the absence of air agitation was not sufficient. 
     With respect to FIGS. 5 through 8, experimentation involved the combined effect of air and ultrasonic agitation with the exception of condition F (air agitation only). 
     From an analysis of FIGS. 3 through 8 significant improvements in both P 1  and P 2  values were obtained when using the combination of air and ultrasonic agitation relative to those results from condition F. This was found particularly valid when small apertures with high aspect ratios were plated. High aspect ratio data is provided in FIGS. 5 and 6. 
     With reference to the combination of FIGS. 3,  5  and  7 , P 1  were noted to approach and in some instances exceed the 100% level therefore demonstrating the efficiency of the instant process. 
     Although embodiments of the invention have been described above, it is not limited thereto and it will be apparent to those skilled in the art that numerous modifications form part of the present invention insofar as they do not depart from the spirit, nature and scope of the claimed and described invention.