Patent Application: US-80484104-A

Abstract:
a method and apparatus for establishing more uniform deposition across one or more faces of a workpiece in an electroplating process . the apparatus employs eductors in conjunction with a flow dampener member and other measures to provide a more uniform current distribution and a more uniform metal deposit distribution as reflected in a coefficient of variability that is lower than conventional processes .

Description:
in the following detailed description , reference is made to the accompanying drawings which form a part hereof , and in which are illustrated specific embodiments in which the invention may be practiced . those skilled in the art will recognize that the invention is not limited to the specific embodiments illustrated in these drawings . in the drawings , the following parts have been identified by the following numbers . 100 . plating cell 102 . workpiece 104 . arrow indicating electrolyte flow 106 . air bubbles 108 . pipe 110 . rack 112 . anode 114 . rail 116 . eductor 118 . impingement point 120 . fluid flow profile 122 . jet centerline 124 . velocity profile 126 . anode chamber 128 . porous fiber cloth 130 . non - conducting shielding 132 . pump 134 . manifold 136 . guide 138 . baffle 140 . arrow indicating electrolyte flow 142 . arrow indicating electrolyte flow 144 . hole 146 . baffle 148 . side chamber 150 . outlet hole 152 . arrow indicating vertical vibration 154 . arrow indicating oscillation 156 . copper foil 158 . measuring point fig1 to 12 show one embodiment of the present invention in a series of cross - sectional views . the following detailed - description of the preferred embodiments refers to these figures . the plating cell ( 100 ) was designed with a range of attributes for enhancing the uniformity of deposition over the workpiece ( 102 ). the attributes are variable high velocity eductor - induced agitation , lateral oscillation of the workpiece perpendicular its face , use of an anode chamber , variable anode to workpiece distance , variable frequency vertical vibration of the workpiece , and non - conducting shielding of the anodes within the anode chamber . in the plating cell ( 100 ), the workpiece ( 102 ) serves as the cathode for metal deposition . the plating cell ( 100 ), which , in one embodiment , holds 1700 liters of bath electrolyte is capable of accommodating one rack or workpiece holder ( 110 ) which holds one workpiece ( 102 ). in this embodiment , the workpiece is 18 inches by 24 inches high . the size of the plating cell ( 100 ), the number of workpieces ( 102 ), dimensions of the workpiece ( 102 ) and other specific details given here relate to a particular embodiment that was evaluated experimentally and are not limiting . sets of anodes ( 112 ) are hung on rails ( 114 , fig1 and 11 ) on each side of the rack ( 110 ) and facing the workpiece ( 102 ), and may be encased in an anode chamber 126 ( fig1 and 11 ). these anodes may be plates or , more typically , they are a panel of metallic balls . the anode chamber ( 126 ) may have a porous fiber cloth ( 128 ) between the anodes ( 112 ) and workpiece ( 102 ). the cloth may be formed from a polymeric material . this cloth ( 128 ) spreads the current distribution between the anodes ( 112 ) and the workpiece ( 102 ) such that the anode chamber ( 126 ) acts as a virtual anode . one cloth was obtained from crosible filtration , located in moravia , n . y . 13118 . it was specifically a 100 % polypropylene filter material . the reported porosity was 2 - 4 cubic feet per minute . other filter cloth available has a porosity of 20 - 30 cubic feet per minute . a wide variety of filter cloths would be acceptable provided they have pores small enough that for the given distance between the cloth and the workpiece the cloth serves as a virtual anode . non - conducting shielding ( 130 ) at the top of the anode chamber ( 126 ) prevents edge effects from affecting the uniformity of copper deposition on the workpiece ( 102 ). the distance from the anode chamber ( 126 ) to the workpiece ( 102 ) is adjustable , with a range varying from about 165 to 300 mm , and preferably from about 210 mm to 250 mm , and more preferably from about 210 to 220 mm . in one embodiment two 300 l / min pumps ( 132 ) are used to circulate electrolyte through manifolds ( 134 ) on either side of the plating cell ( 100 ) and through eductors such as ½ in eductors ( 116 ) located horizontally under the anode chambers ( 126 ). in fig1 - 12 , three eductors ( 116 ) are shown on each side of the plating cell ( 100 ). in one embodiment , the eductors are spaced on 6 inch centers , but the number of eductors ( 116 ) and the spacing may change from those cited and is not limiting . the number and placement of eductors ( 116 ) should be chosen so as to facilitate uniform flow of electrolyte across the entire surface of the workpiece ( 102 ) as described herein . electrolyte flowing out of the eductors ( 116 ) is directed vertically past the workpiece ( 102 ) by a solution flow velocity dampening member ( 136 ), whereby the variations in electrolyte solution are suppressed . in one embodiment of the invention , the solution flow velocity dampener is a series of shaped guides ( 136 ) located below the workpiece ( 102 ). the use of the shaped guides ( 136 ) directs the solution flow parallel the surface of the workpiece thereby dampening the variations in solution flow velocity described above in the prior art , reducing the glancing effect , and resulting in more uniform flow across the surface of the workpiece ( 102 ). the solution flow velocity dampening members that are useful herein may have a variety of shapes . for example , curved panel sections with various radii of curvature relative to the surface of the workpiece and flat ramps with various incline angles relative to the surface of the workpiece . as taught herein , the optimum configuration for the shaped dampening member is easily determined without undue experimentation by those of ordinary skill in the art . the radius of curvature utilized for one embodiment was 8 . 25 inches . a useful range may be about 6 to 12 inches for a plating cell in which the distance between the bottom of the shaped guide and the workpiece is approximately 10 . 5 inches . baffles ( 138 , fig1 ) below each anode chamber ( 126 ) prevent solution from flowing back to the other side of the anode chamber ( 126 ). the velocity of the electrolyte flowing past the workpiece ( 102 ) can be changed by 1 ) changing the pump ( 132 ) settings and 2 ) moving the anode chambers ( 126 ) closer to the workpiece ( 102 ). the electrolyte flows vertically up ( indicated by arrows 104 ) past the workpiece ( 102 ) and then across ( indicated by arrows 140 ) the top of the plating cell ( 100 ) and out ( indicated by arrow 142 ) through a hole ( 144 ) in a baffle ( 146 ) in the plating cell ( 100 ) to a side chamber ( 148 ). solution is suctioned through outlet holes ( 150 ) from the side chamber ( 148 ) through the pump ( s ) ( 132 ) and back through the manifolds ( 134 ) and out through the eductors ( 116 ). the side chamber ( 148 ) with its enclosed electrolyte and in conjunction with pump ( s ) ( 132 ) and manifold ( 134 ) serves as an electrolyte supply system . in one embodiment , as electrolyte is pumped through the eductors ( 116 ), electrolyte in the plating cell ( 100 ) is pulled into the eductors ( 116 ) in about a 4 : 1 ratio ( 4 parts electrolyte pulled into the eductors ( 116 ) from the plating cell ( 100 ) to 1 part electrolyte pumped through the eductors ( 116 )) to increase the flow of electrolyte past the workpiece ( 102 ). a filter ( not shown ) in the side chamber ( 148 ) can be used to maintain cleanliness of the electrolyte . in some cases , uniformity of metal distribution over the workpiece ( 102 ) can be improved by vibration of the workpiece ( 102 ). vibration is in the vertical direction as shown by the double - ended arrow ( 152 ) adjacent to the rack ( 110 ) in fig1 . vibration may be - particularly important for workpieces with interconnect features such as fine pitch surface tracks , through - holes , vias and the like . vibration of the workpiece ( 102 ) is accomplished by two horizontally mounted rotary eccentrically weighted devices powered by variable speed motors ( not shown ) and mounted to each end of the load bar ( not shown ) to which the rack ( 110 ) is attached . those skilled in the art understand that other means for accomplishing vibration include , but are not limited to ; pneumatic rotary ball device , pneumatic rotary turbine device , electromagnetic linear motion device , pneumatic sliding piston device , and ultrasonic electromagnetic device . the frequency of vibration available using this configuration typically ranges from about 0 to 3570 cycles per minute . oscillation of the workpiece ( 102 ) perpendicular to the anodes ( 112 ), as shown by the double - ended arrow ( 154 ) above the rack ( 110 ) in fig1 , or oscillation of the anodes ( 112 ) perpendicular to the workpiece ( 102 ) or oscillation of both anodes ( 112 ) and workpiece ( 102 ) with respect to each other results in the flow of electrolyte through the holes in the workpiece ( 102 ), improving the current distribution and therefore plated metal distribution on the workpiece ( 102 ). oscillation may be particularly important for workpieces with interconnect features such as fine pitch surface tracks , through - holes , vias and the like . in one embodiment , oscillation of the workpiece ( 102 ) is produced by a positive drive from a variable speed motor - reducer with crank arm and linkage ( not shown ). those skilled in the art understand that other means for accomplishing oscillation include , but are not limited to reversing rack and pinion device , off axis side crank device , grooved cam traverse mechanism device , yoke strap eccentric circular cam mechanism device , reversible worm screw jack device , electromechanical linear drive device , and reversible pneumatic or hydraulic cylinder device . the frequency of oscillation can shift from about 6 to 63 cycles per minute with a stroke of about 25 mm , although this range is not limited . this is the method of oscillation employed in this plating cell ( 100 ), although the invention is not limited to this method . thus in one embodiment , the workpiece ( 102 ) is moved parallel to and / or perpendicular to the anodes ( 112 ). in accordance with certain embodiments of the invention , uniformity of metal distribution over the workpiece ( 102 ) can be improved by changing the distance between the anodes ( 112 ) and the workpiece ( 102 ). the distance from the anode chamber ( 126 ) to the workpiece 102 may vary from about 165 to 300 mm , preferably from about 210 mm to 250 mm , and more preferably from about 210 to 220 mm . uniformity of metal distribution over the workpiece ( 102 ) can also be improved by placing non - conducting shielding ( 130 ) at the top of the anode chamber ( 126 ) to reduce edge effects . uniformity of metal distribution over the workpiece ( 102 ) can be further improved by placing a baffle ( 138 ) at the bottom of the anode chamber ( 126 ). the invention is particularly useful in plating circuit boards having features such as througholes and vias . because more uniform deposition is available in accordance with the invention , good plating of the features can be achieved independently of the location of the feature on the workpiece . thus workpiece having more demanding features to plate can be successfully processed substantially independently of the location of the feature on the workpiece . problems associated with uneven deposition due to uneven boundary layer due to uneven plating solution flow are minimized and a robust plating technique is provided . the invention will be illustrated by the following examples , which are intended to be illustrative and not limiting . this example illustrates the use of the plating cell ( 100 ) with air sparging to deposit copper onto a workpiece ( 102 ), to demonstrate the prior art . the experiments were conducted in the plating cell ( 100 ) shown in fig2 . an acid copper sulfate electrolyte containing ˜ 97 g / l cuso 4 , 210 - 215 g / l of concentrated h 2 so 4 , ˜ 63 ppm cl − , and 350 ppm polyethylene glycol ( peg ) was used as the copper electroplating bath . as known by those skilled in the art , the chloride / peg acts as a suppressor and is not difficult to control . the plating bath does not contain difficult - to - monitor / control additives such as brighteners and / or levelers and hence the bath is considered “ additive - free .” the plating bath temperature was maintained in the range of 22 to 25 ° c . the initial experiments for plating cell ( 100 ) characterization were conducted on a stainless steel panel ( 450 mm × 600 mm ), as a workpiece ( 102 ). the copper plating process was controlled by dc current at 25 a / ft 2 ( provided by a pe86 dual output rectifier ) to obtain a copper film with a thickness of about 25 micrometers on both surfaces of the stainless steel panel after each test , copper foils ( 156 ) that plated on both sides of the stainless steel panel workpiece ( 102 ) were peeled off to analyze the copper thickness distribution . fig1 illustrates the position of each measuring point ( 158 ) on the copper foil ( 156 ). there were thirty - six equi - spaced measuring points on the foil ( 156 ) and the edge points were 38 mm away from the foil ( 156 ) side . the uniformity of copper deposits on the steel panel workpiece ( 102 ) surface was defined by the ratio of the standard deviation to the average copper thickness , expressed on a percent basis (( σ / ā )× 100 %), that is , the coefficient of variation ( cov )). the quantity σ is the standard deviation based on the measuring points ; and ā is the mean thickness that is given by : ā = σh i / n where n is the number of measuring points and h i is the copper thickness at each measuring point . for these experiments , n = 36 . the smaller the value of the cov , the more uniformly is current distributed over the steel panel workpiece ( 102 ) surface , and the more uniformly is metal distributed over the steel panel workpiece ( 102 ) surface . the value of cov for a conventional workpiece with dimensions of about 450 mm × 600 mm in the electronics industry is about 10 % to 12 % although more typical values may be about 15 %. in this example the cov value determined from analysis of the copper foil was 13 . 99 %. the thickness of the copper deposit was measured with a micrometer . this example illustrates the use of the plating cell ( 100 ) to deposit copper uniformly onto a workpiece ( 102 ), to demonstrate the effects of the various attributes of the plating cell ( 100 ) of the present invention , such as flow rate of electrolyte through the eductors ( 116 ), anodes ( 112 ) to workpiece ( 102 ) distance , oscillation ( 154 ) of the workpiece ( 102 ), and vibration ( 152 ) of the workpiece ( 102 ). the experiments were conducted in the plating cell ( 100 ) shown in fig1 to 12 . an acid copper sulfate electrolyte containing ˜ 97 g / l of cuso 4 , 210 - 215 g / l of concentrated h 2 so 4 , ˜ 63 ppm cl − , and 350 ppm polyethylene glycol ( peg ) was used as the copper electroplating bath for all experiments . the chloride / peg is termed a suppressor and is not difficult to control . the plating bath does not contain difficult - to - monitor / control additives such as brighteners and / or levelers and hence we consider the bath as “ additive - free .” the plating bath temperature was maintained in the range of 22 to 25 ° c . the initial experiments for plating cell ( 100 ) characterization were conducted on a stainless steel panel ( 450 mm × 600 mm ), as a workpiece ( 102 ). the cell operating parameters , which were eductor ( 116 ) flow rate ( low flow designates flow with a pump setting about one - half the maximum ( high ) flow ), oscillation ( 154 ) frequency , vibration ( 152 ) frequency , and anode ( 112 ) to steel panel workpiece ( 102 ) distance , were selected as factors to evaluate the effect of plating cell ( 100 ) configuration on the current distribution over the panel workpiece ( 102 ) surface . the copper plating process was controlled by dc current at 25 a / ft 2 ( provided by a pe86 dual output rectifier ) to obtain a copper film with a thickness of about 25 micrometers . in all experiments , the anode chamber ( 126 ) was used , as was the porous fiber cloth ( 128 ), and 152 mm of anode non - conducting shielding ( 130 ). after each test , copper foils ( 156 ) that plated on both sides of the stainless steel panel workpiece ( 102 ) were peeled off to analyze the copper thickness distribution . fig1 illustrates the position of each measuring point ( 158 ) on the copper foil ( 156 ). there were - thirty - six equi - spaced measuring points on the foil ( 156 ) and the edge points were 38 mm away from the foil ( 156 ) side . the uniformity of copper deposits on the steel panel workpiece ( 102 ) surface was defined as described in example 1 above , with n = 36 in this example also . the desired percentage value of cov for the cell conditions in the electronics industry and more particularly printed circuit board industry for panels of approximately this size is less than 10 %. the experimental matrix , designed using a full factorial method , is listed in table 1 . minitab software was used to design the factorial method , although other methods could be used . the target performance criterion for the initial cell experimental study was to plate approximately 25 micrometers of copper over the steel panel workpiece ( 102 ) surface and evaluate the uniformity of copper thickness distribution . as shown in table 1 , a cov of less than 10 % was achieved under the plating cell operating conditions of test 5 to test 12 , and the lowest cov value was achieved in test 5 . fig1 shows a graph of the data from the factorial matrix . the graph plots the cov versus the changes in each of the operating parameters and shows which operating parameter has the strongest influence on the uniformity of copper thickness across the surface of the stainless steel panel workpiece ( 102 ). fig1 shows that the distance between the anodes ( 112 ), which controls the anode ( 112 ) to steel panel workpiece ( 102 ) distance , has the strongest influence on the uniformity of copper distribution over the steel panel workpiece ( 102 ), compared to the other parameters . however , one skilled in the art would recognize that oscillation and vibration may be important when the workpiece incorporates interconnects with fine pitch lines , through - holes , vias and the like . these data would also indicate that even closer anode ( 112 ) to steel panel workpiece ( 102 ) spacing may offer further improvements in copper uniformity . these observations are confirmed by the data in table 1 which show that a more uniform copper thickness distribution ( low cov ) can be obtained by using a closer distance between the anode chamber ( 126 ) and the stainless steel panel workpiece ( 102 ). the test 5 plating cell configuration gave the most uniform copper thickness distribution over the steel panel workpiece ( 102 ) surface , with the closest anode ( 112 ) to steel panel workpiece ( 102 ) distance , at a high flow rate , high oscillation frequency and high vibration frequency . based on the test results shown in fig1 , the effect of oscillation ( 154 ) and vibration ( 152 ) are unclear , although they suggest that higher vibration ( 152 ) and oscillation ( 154 ) frequencies will improve the uniformity of metal on the steel panel workpiece ( 102 ). the effects of oscillation ( 154 ) and vibration ( 152 ) might be seen more clearly on a patterned workpiece which has interconnect features such as fine pitch lines , through - holes , and vias and the like . this example illustrates the use of the plating cell ( 100 ) to deposit copper uniformly onto a workpiece ( 102 ), to demonstrate further effects of the various attributes of the plating cell ( 100 ), such as flow rate of electrolyte through the eductors ( 116 ), anodes ( 112 ) to workpiece ( 102 ) distance , use of an anode chamber ( 126 ), use of a porous fiber cloth ( 128 ), use of additional non - conducting shielding ( 130 ), and use of a baffle ( 138 ) under the anode chamber , on the current distribution over the panel workpiece ( 102 ) surface . the experiments were conducted in the plating cell ( 100 ) shown in fig1 to 12 . an acid copper sulfate electrolyte containing ˜ 97 g / l of cuso 4 , 210 - 215 g / l of concentrated h 2 so 4 , ˜ 63 ppm cl − , and 350 ppm polyethylene glycol ( peg ) was used as the copper electroplating bath for all experiments . the chloride / peg acts as a suppressor and is not difficult to control . the plating bath does not contain difficult - to - monitor / control additives such as brighteners and / or levelers and hence we consider the bath as “ additive - free .” the plating bath temperature was maintained in the range of 22 to 25 ° c . the initial experiments for cell characterization were conducted on a stainless steel panel ( 450 mm × 600 mm ), as a workpiece ( 102 ). the copper plating process was controlled by dc current at 25 a / ft 2 ( provided by a pe86 dual output rectifier ) to obtain a copper film with a thickness of about 25 micrometers . after each test , copper foils ( 156 ) that plated on both sides of the stainless steel panel workpiece ( 102 ) were peeled off to analyze the copper thickness distribution . fig1 illustrates the position of each measuring point ( 158 ) on the copper foil ( 156 ). there were thirty - six equi - spaced measuring points on the foil ( 156 ) and the edge points were 38 mm away from the foil ( 156 ) side . the uniformity of copper deposits on the steel panel workpiece ( 102 ) surface was defined as described in example 1 above , with n = 36 in this example also . the desired percentage value of cov for the cell conditions in the electronics and more particularly printed circuit board industry is less than 10 %. the experimental matrix and results are listed in table 2 . the target performance criterion for the experimental study was to plate approximately 25 micrometers of copper over the steel panel workpiece ( 102 ) surface and evaluate the uniformity of copper thickness distribution . table 2 shows the effect of each plating cell attribute . comparing test 5 with 5c and test 11 with 11c shows that decreasing the distance between the anode ( 112 ) and panel workpiece ( 102 ) from 213 to 203 mm decreased the uniformity ( increased the cov ) of metal deposition across the steel panel workpiece ( 102 ). comparing test 5 with 5d shows that increasing the non - conducting shielding ( 130 ) at the top of the anode chamber ( 126 ) from 152 to 191 mm improved the uniformity ( decreased the cov ) of metal deposition across the steel panel workpiece ( 102 ). comparing test 5 with 5e and test 11 with 11e shows that removing the anode chambers ( 126 ) from the cell decreased the uniformity ( increased the cov ) of metal deposition across the steel panel workpiece ( 102 ). comparing test 5 with 5f shows that removing the porous fiber cloth ( 128 ) from the anode chamber ( 126 ) decreased the uniformity ( increased the cov ) of metal deposition across the steel panel workpiece ( 102 ). comparing test 5c with 5cg and test 5d with 5dh shows that adding a baffle ( 138 ) under the bottom of each anode chamber ( 126 ) improved the uniformity ( decreased the cov ) of metal deposition across the steel panel workpiece ( 102 ). comparing test 5d with 5di and 5dj shows that changing the porous fiber cloth ( 128 ) to that of a different manufacturer decreased the uniformity ( increased the cov ) of metal deposition across the steel panel workpiece ( 102 ). in summary , the best result was achieved in test 5dh , which ran at high flow , 26 cycles / min oscillation , 1400 cycles / min vibration , 213 mm distance between anode ( 112 ) and steel panel workpiece ( 102 ), used an anode chamber ( 126 ) with a porous fiber cloth ( 128 ), had 191 mm non - conducting shielding ( 130 ) on top of the anode chamber ( 126 ), and had a baffle ( 138 ) attached below both anode chambers ( 126 ). the invention having now been fully described , it should be understood that it might be embodied in other specific forms or variations without departing from its spirit or essential characteristics . accordingly , the embodiments described above are to be considered in all respects as illustrative and not restrictive , the scope of the invention being indicated by the appended claims rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein . 1 m . paunovic and m . schlesinger ( 2000 ), modern electroplating , wiley inc . ny . 2 m . paunovic and m . schlesinger ( 1998 ), fundamentals of electrochemical deposition , wiley inc . ny . 3 ward , m ., d . r . gabe and j . n . crosby ( 1999a ), proc . european pcb convention , munich germany , november . 5 weber , a . 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