Patent Publication Number: US-2020291525-A1

Title: Process and apparatus for selective passivation of electroless nickel activation or nucleation sites

Description:
RELATED APPLICATIONS 
     This application claims the benefit of, and priority to, United States Provisional Patent Application Ser. No. 62/816,504 filed on Mar. 11, 2019, entitled “Process and Apparatus for Selective Passivation of Electroless Nickel Activation or Nucleation Sites”, the entire disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     This disclosure is directed generally to electroless plating in electronics manufacturing, and more specifically to processes and apparatus that provide selective passivation of electroless nickel nucleation sites to control formation and growth of undesired nickel plating between traces or other unwanted surface without inhibiting plating on the required features. 
     BACKGROUND 
     Electroless plating is a poorly understood phenomena that is an active subject for industrial and academic research. For example, during the plating process unwanted nucleation sites of the metal to be plated often form resulting in superfluous or extra plating on regions where plating is not desired. Although reaction mechanisms have been studied and published results have recently become available, these publications do not provide insight into how such unwanted nucleation sites or superfluous plating forms, or how to control such unwanted formation during plating. One such publication is found in the  Journal of The Electrochemical Society,  158, (9) D585-D589 (2011) entitled “Density Functional Theory Analysis of Reaction Mechanism of Hypophosphite Ions on Metal Surfaces”, by Masahiro Kunimoto, et al., hereby incorporated by reference. This is a mechanistic approach that does not understand the mechanisms of nodule formation and therefore provides no guidance for controlling a process for nodule formation. 
     Further complicating the process is that nucleation site formation is a desired feature on many surfaces. It provides for the actual plating to occur. Further, it creates surface roughness and increases the surface area for adhesion of the metal containing surface to another surface. Both of these physical features are aids in bonding another surface to the metal coated surface. However, and contrary to desired metal plating, superfluous or extra plating occurs where the metal plating is not wanted and the formation cannot be controlled by voltage or applied current. In electroless plating, the units to be plated are electrically isolated and therefore nucleation of the metal formation cannot be controlled by voltage. There is insufficient knowledge of the phenomena of nucleation site formation, and its root causes in plating, to permit control of its formation. 
     The inventors began the study of control of nucleation formation by initially looking at the chemistry of the plating bath. Electrochemical baths for electroless nickel plating generally contain a liquid nickel metal source plus a trace of cobalt, and typically, a reducing agent, such as a boron based reducing agent (borohydride) or phosphorous based (hypophosphite). There is usually a complexing agent, such as succinic acid. A stabilizer, such as bismuth, lead, or antimony, can also be included at parts per billion (“ppb”) concentration. pH control agents, such as sulfuric acid and sodium hydroxide are sometimes present. Reaction byproducts (sometimes referred to as metal turnover) will tend to build up in the bath. Activation solution will not work in electroless nickel plating without palladium (“Pd”). A low palladium concentration or Pd elimination from the solution will not allow nickel plating on electrically isolated copper. 
     Inter-process interactions were also studied. Problems first appeared when a conventional argon only ion-gun treatment was used for substrate feature cleaning. The conventional ion-gun sputters off iron particles onto/into polyimide. The iron particles can self-activate and plate. Iron is autocatalytic and doesn&#39;t require any palladium in the bath in order to plate. A vacuum deposited seed layer is provided in argon gas. Also, when using argon, the argon converts polymers into conductive carbon which conductive carbon then covers solid surfaces. This mechanism either induces water (hydrogen and oxygen) out of the polymer, or otherwise induces other atoms out of the polymer, leaving a conductive carbon behind. This conductive carbon will tend to plate with nickel. Thus, faced with all this information, the problem was where to break the chain in order to solve the problem of extra or superfluous plating. Accordingly, significant development is needed to address these problems. 
     SUMMARY 
     Embodiments of the present application disclose processes and apparatus that enable selective passivation of electroless nickel nucleation sites to control formation and growth of undesired nickel plating between traces without inhibiting plating on the required features. The inventors discovered the role of fluid mechanics on plating mechanisms, and a basic finite element model has been built to ascertain if the patterns of nucleation could be controlled by fluid flow interactions with the physical structure of traces (such as copper traces). The inventors discovered from results of the model, that bismuth (Bi) would be depleted at the dielectric interface to the trace wall more so than any other component of the plating bath. Based on this discovery the inventors have developed processes and apparatus to enable selectivity of the passivating agent. 
     In some embodiments, a process of controlling nucleation site formation during which a metal is plated upon a substrate in a plating bath is provided, comprising: providing a plating bath in which a passivating agent is included in the bath; and increasing activity of the passivating agent on unwanted nucleation sites by increasing activity of the passivating agent on the unwanted nucleation sites. 
     Of particular advantage, the step of increasing activity of the passivating agent on unwanted nucleation sites, and thus selective passivation, may be accomplished in a variety of ways according to the broad teaching and discovery of the inventors. For example, in one aspect, embodiments of the present disclosure utilize mass-transfer control to increase activity of the passivating agent. In one example, mass-transfer control is enabled by increasing agitation to quickly passivate small particles. In a further embodiment, one or more fluid eductors at point of web entry into the plating bath are employed. In a still further embodiment, a baffle is added to contain a high mass-transfer zone within the plating bath. In a still further embodiment, fluid mechanics are used, such as for example creating low velocity flow for bulk of dwell time to provide normal material properties and build-up. 
     In another aspect, a process of selective passivation during which a metal is plated upon a substrate in a plating bath is disclosed, comprising: providing a plating bath in which a passivating agent is included in the bath, and increasing activity of the passivating agent on nucleation sites in selective regions by increasing convection and/or diffusion of the passivating agent on the nucleation sites in the selective regions. In some embodiments, the plating bath is a bath for electroless plating of nickel and the passivating agent is at least one selected from the group consisting of bismuth, lead and antimony. In some embodiments activity of passivating agent is increased by increasing mass-transfer of the passivating agent, for example by increasing fluid velocity of the passivating agent in a limited zone within the bath. 
     In another aspect, an apparatus for selective passivation during an electroless nickel plating process is disclosed. In general, the apparatus comprising: a plating cell for confining a plating bath therein, and including at least one eductor for increasing the fluid velocity or flow rate of the plating bath, and a baffle to confine the increased fluid flow of the plating bath to only a portion of the plating bath. In some embodiments the eductor comprises a series of opening which act as nozzles to increase the fluid velocity or flow rate of the plating bath. In some embodiments the eductor is positioned in the plating cell near the entry point of a web into the plating cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG. 1  is the schematic representation of the hypophosphite reaction chain. 
         FIG. 2  is a photomicrograph of superfluous or extra plating formation between traces. 
         FIG. 3  is a photomicrograph illustrating nickel plating on traces without formation of superfluous or extra plating according to embodiments of the present disclosure. 
         FIG. 4A  is a high magnification photograph of mass-transfer effects to assist chemistry in controlling formation of superfluous or extra plating in undesired regions according to embodiments of the present disclosure. 
         FIG. 4B  illustrates enhanced mass-transfer of the passivating agent (Bismuth in this example) early in the plating process according to embodiments of the present disclosure. 
         FIG. 4C  is a graphic representation of the enhanced mass-transfer of the passivating agent (bismuth in this example) early in the plating process and reduction for the bulk of plating according to embodiments of the present disclosure. 
         FIG. 5A  is a schematic, perspective view of the details of one example of a mechanical implementation of a plating bath apparatus according to embodiments of the present disclosure. 
         FIG. 5B  is a perspective view of  FIG. 5A  taken orthogonal to the view of  FIG. 5A . 
         FIG. 6A  is an alternative embodiment of a plating bath apparatus to that shown in 
         FIGS. 5A and 5B  according to embodiments of the present disclosure. 
         FIG. 6B  is another alternative embodiment of a plating bath apparatus where activity of the passivation agent is increased by increasing concentration of the passivation agent, according to embodiments of the present disclosure. 
         FIG. 7  is a bottom view of one of the eductors of  FIG. 5A . 
         FIG. 8  is a schematic plan view of the running direction of a web beneath an educator in a plating bath apparatus according to embodiments of the present disclosure. 
         FIG. 9  is a high magnification photograph showing plating on traces and little plating in between the traces achieved according to embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The inventors have discovered that understanding the chemical mechanism during plating is a key aspect to controlling formation and growth of undesired metal (such as nickel) plating between traces without inhibiting plating on the desired features. While the figures and description below discuss a plating system using bismuth as the passivation agent, the invention is not limited to the specific examples described herein, and those of ordinary skill in the art will understand that the discoveries, teachings and principles of the invention can apply to plating systems utilizing other chemical constituents. 
     Turning to the figures,  FIG. 1  illustrates a hypophosphite reaction chain present in one example of a plating system. Hypophosphite (“H 2 PO 2 ”) needs a step edge of plane of at least 25 atoms square (5×5 atoms) to adsorb. OH −  needs to adsorb next to it (rate limiting step). H 0  plus e −  shed. OH −  joins phosphorous and desorbs. The e −  can reduce an adsorbed Ni 2+  or another adsorbed H 2 PO 2  to P. 2 H 0  combine to make the observed hydrogen gas. Breaking this chain is how applicant controls and prevents excessive Ni nodules formation. 
     Bismuth (“Bi”) is a key interaction component with the above reaction chain.  FIG. 2  shows unwanted superfluous or extra plating formation when Bi is not controlled. Bi is essential for the stability of the electroless nickel plating bath extending its useful life beyond an hour, or even beyond a day. However, Bi is an anti-catalyst and while it will immersion plate to copper, it will also adsorb onto palladium (“Pd”). Bi also poisons H 2 PO 2  adsorption for 25 atom radius. Low density of adsorption sites allows for disruption by Bi. However, one must get enough Bi to the nucleation sites before metal nucleation grows too large. Metal nucleation formation in the initial state can be controlled by the amount of Bi. But too much Bi causes areas needing plating to passivate. The isolated features, such as copper traces carried on a web, upon which plating is desired, must still activate and plate. Thus, the need is to supply sufficient Bi in the bath to achieve bath stability but not excessive Bi to overly passivate the desired plating areas. 
     Applicants discovered that the solution is not to attempt to control these variables strictly by the Bi concentration in the bath, but by being able bring sufficient Bi to the nucleation sites early in the plating process to confine or control metal nucleation formation. This is accomplished by controlling the activity of the passivating agent at the metal nucleation sites to achieve desired, selective passivation. As discussed in more detail below, controlling the activity of the passivating agent can be accomplished by a variety of techniques, such as for example without limitation using mass-transfer techniques, such as agitation, rather than relying solely on the concentration of Bi in bath. Reduced Bi concentration is critical such that no excessive Bi passivation occurs while still providing stability to the bath. 
     For example, based on the broad teaching of the present invention, convection and/or diffusion of the passivating agent may be selectively controlled. Convection and/or diffusion may be modeled using the convection-diffusion equation, as shown below: 
     
       
         
           
             
               
                 
                   
                     
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     where
 
c is the variable of interest (such as species concentration for mass transfer, temperature for heat transfer),
 
D is the diffusivity (also referred to as diffusion coefficient), such as mass diffusivity for particle motion or thermal diffusivity for heat transport,
 
{umlaut over ({acute over (υ)})} is the velocity field that the quantity is moving with,
 
R describes “sources” or “sinks” of the quantity c. For example, for a chemical species, R
 
&gt;0 means that a chemical reaction is creating more of the species, and R&lt;0 means that a chemical reaction is destroying the species, and
 
▾ represents gradient and ▾∘ represents divergence. In this equation, c represents concentration gradient.
 
     By tightly controlling the chemistry, OH −  coordination is controlled as shown in  FIG. 3 . Too many OH −  ions can dramatically increase nodules. Too little Bi (such as for example 500 parts per billion (“ppb”) or less) in the bath fails to passivate. On the other hand, too much Bi passivates everything. Unwanted extra or superfluous plating in the spaces  2 ,  4  between traces  1 ,  3  and  5 , can clearly be seen in  FIG. 3 . 
     As shown in  FIGS. 4A-4C , mass-transfer of Bi must initially be high or increasing, but reduced for the bulk of plating cycle. In the present disclosure, mass-transfer of Bi is aided by vigorous agitation during the beginning of the plating process, but then reduced for the bulk of the plating process. The fluid velocity or flow rate of the bath, as shown in  FIG. 4B  brings the Bi atoms into contact with the nucleation sites and has a greater effect on mass-transfer o f the Bi than the Bi concentration of the bath itself. Thus, a lower Bi concentration in the bath can still be effective if aided by the use of agitation early in the plating process and mass-transfer control of the Bi to passivate the nucleation sites.  FIG. 4C  is a line graphic representation of Bi concentration (ppm) plotted versus space (in mm).  FIG. 4B  shows arrow surface velocity field versus concentration of Bi (in ppm).  FIG. 4A  is a photograph at magnification of 3.51 KX taken by photomultiplier at extra high tension (“EHT”) of 5.00 kV. 
     In order to mechanically implement our findings, we have constructed an electroless nickel plating cell  10  as shown in  FIGS. 5A and 5B , where activity of the passivating agent is controlled by fluid mechanics wherein at least one eductor (the illustrative embodiment employs two fluid eductors  12 ,  14 ), is placed at point of web  8  entry to aid in nucleation site control by increasing the velocity or flow rate of the Bi-containing bath within the control zone  18  bounded by the baffle  16 . The baffle  16  limits the increased velocity flow within the control zone  18  to control the residence time of the high mass transfer zone of Bi passivation. The web  8 , carrying the traces or other substrates to be plated, is advanced through the plating cell  10  by roller  28 . Activation sensors  20 ,  22  move to activate plating. Increased instrumentation in the form of flow meters (not shown) complete the cell  10 . The pH is controlled between pH&lt;4.5 but greater than 4.35. Most preferred pH is about 4.42, but it is difficult to precisely maintain pH because plating stops. Bi concentration and pH each contribute to inhibiting nucleation sites. However, too low a Bi concentration does not inhibit plating of OH − , Ni ++ , and HPO 2   −  plating. 
     In a bottom view of one of the eductors  12 ,  14  as shown in  FIG. 7 , a plurality of openings  30 ,  31 ,  32 ,  33 , etc. act as nozzles which increase the velocity or flow rate of the fluid passing through the openings to create agitation of the bath and increase mass transfer of Bi to passivate selective nucleation sites. By positioning the eductors  12 ,  14 , transverse to the machine direction of travel of the web through cell  10  as shown by Arrow MD, the openings in the eductors effectively increase the velocity of the Bi across the entire width of the web. By positioning eductors  12 ,  14 , near the entry of the web  8  into the cell  10 , passivation and thus control is effected while nucleation formation is in an early stage. The early stage formation presents the most effective time to control nucleation formation. The bottom of eductors  12 ,  14  containing the openings  30 ,  31 ,  32 ,  33 , etc. is submerged below the bath surface but at a distance of about 1 inch above the web  8 , with the holes directed vertically towards the web  8 . The schematic representation of this arrangement of the openings and the web is illustrated schematically in  FIG. 8 . The openings should be immersed at least 5 mm below the surface of the bath. In order to contain the agitation of the bath to cause the Bi to passivate the nucleation sites effectively, even at low concentration of Bi, a baffle  16  is introduced into the bath to contain the agitation to a predetermined zone at the beginning of the passage of web  8  through the bath. 
     While we have illustrated two eductors  12 ,  14 , in  FIGS. 5A and 5B , the embodiments are not limited to only two eductors. One eductor, or any suitable number of eductors, may be used. In the alternative embodiment of  FIG. 6A , three eductors  12 ,  13 ,  14 , are used to create the mass-transfer of Bi by creating increased velocity or flow rate of fluid leading to sufficient agitation to passivate the nucleation sites. It should be understood that the bottom of each educator  12 ,  13  and  14 , is provided with the series of openings shown in  FIG. 7 . However, the number, arrangement and size of the opening may be varied, just as the number of eductors can vary. It is not the precise number, pattern, or distribution of the eductors or the openings in each which is as important as achieving the mass-transfer of Bi to effect passivation of the nucleation sites. Depending on the size or volume of the plating cell, the number and geometry of the sites carried by the web where plating is desired, the number of eductors, and the number of opening in the eductors may be less or more than we have illustrated in our embodiments. 
       FIG. 6B  illustrates another alternative embodiment of a plating bath apparatus where in this instance activity of the passivation agent is increased by increasing concentration of the passivation agent. In this embodiment, no eductors are used, and increased activity of the passivation agent is achieved by increased concentration instead of increased fluid flow or velocity. 
     More specifically, as shown in  FIG. 6B , the apparatus is comprised generally of a plating cell  40  having at least one high passivating agent mass-transfer rate plating bath  42 , and at least one standard (low) mass-transfer rate plating bath  44 . Web  46  carrying a flexible substrate travels through both of the plating baths  42 ,  44  via rollers  48 . 
     To increase the activity of the passivating agent, at least one high passivating agent concentration bath  42  is provided. This bath  42  is independent of the standard plating bath  44 . When Bi is the passivating agent, in one example plating bath  42  contains Bi at a concentration range of 600 ppb to 1500 ppb. To selectively passivate, the web  46  first travels through the high passivating agent concentration plating bath  42 . The time of exposure to the passivating agent may vary, and is generally in the range of 15 to 20 seconds. Once the web  46  exits the high passivating agent mass-transfer rate plating bath  42 , the web then enters the standard (low) mass-transfer rate plating bath  44 , where plating continues. 
     While a continuous moving web carrying traces or other flexible substrate is shown and described in the above embodiments, it should be understood that the present invention may also be used for plating of discrete panels. When plating discrete panels, increased activity of the passivating agent may be accomplished by moving the panels, or by keeping the panels stationary and turning pumps on and off to deliver increased fluid flow of the passivating agent. 
       FIG. 9  is a high magnification of 1.50 KX taken by photomultiplier at an EHT of 5.00 kV of two traces taken at different times. Related to aspect ratio, the trace height/space is about 1:1 whereas  FIG. 2  was approximately 2:3. 
     A number of manufacturing advantages are provided with implementation of the present disclosure including reduced down time, which minimizes waiting time for the resource and reduces delay in work-in-progress. The need for a second plater to handle the down time and roll backup can be avoided. Additionally, less material is wasted since unwanted plating or scrap is reduced or eliminated. 
     It is to be expressly understood that the embodiments shown herein are illustrative only and should not be viewed as limiting the scope of the embodiments by which this disclosure can be implemented. Those skilled in the art, to whom this disclosure is directed, will, upon reading this disclosure, envision modification to the disclosed embodiments and other embodiments not expressly disclosed, without the exercise of their own invention.