Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/18—Electroplating using modulated, pulsed or reversing current
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/08—Electroplating with moving electrolyte e.g. jet electroplating
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/12—Semiconductors
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/40—Forming printed elements for providing electric connections to or between printed circuits
  • H05K3/42—Plated through-holes or plated via connections
  • H05K3/423—Plated through-holes or plated via connections characterised by electroplating method
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
  • H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
  • H01L21/288—Deposition of conductive or insulating materials for electrodes conducting electric current from a liquid, e.g. electrolytic deposition
  • H01L21/2885—Deposition of conductive or insulating materials for electrodes conducting electric current from a liquid, e.g. electrolytic deposition using an external electrical current, i.e. electro-deposition
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
  • H05K2203/14—Related to the order of processing steps
  • H05K2203/1492—Periodical treatments, e.g. pulse plating of through-holes
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
  • H05K2203/15—Position of the PCB during processing
  • H05K2203/1572—Processing both sides of a PCB by the same process; Providing a similar arrangement of components on both sides; Making interlayer connections from two sides

Definitions

• the production of high-aspect ratio printed circuit boards poses well-known problems for good quality electrolytic copper metallization.
• the panels can be from 3 mm and up to 10 mm thick with aspect ratios of typically 10 : 1. However, there is a current trend requiring even thicker panels and with an aspect ratio up to 15 : 1. Such panels typically can be larger than "normal" production panels which gives added problems in handling due to their weight.
• One of the limiting factors in copper deposition is the mass transport of ions into the high-aspect ratio holes. Achieving the required copper thickness in the hole without over-plating the surface causing resist over-plating with pattern plate or poor line definition with panel plate are the main problems in the production of high-aspect ratio panels.
• a further factor with back panels is the difficulty of component mounting using the press-fit technique when the copper deposit distribution is poor.
• low electroplating current densities have been used which obviously have a negative impact on productivity.
• reverse pulse plating can allow the use of higher current densities with improved surface distribution and throwing power in the through-holes as described in DE 42 25 961 C2 and DE 27 39 427 A1.
• Kruse describes in Galvanotechnik (3/2002, p.680) a method for reverse pulse plating of e.g. printed circuit boards, in which the sum of the duration of two forward pulses intermitted by an off pulse has been set to 5 - 250 ms, and the duration of reverse pulses has been set to 0 - 5 ms.
• an electrodeposition (cathodic) pulse may range from about 500 - 3000 ms, while that for an electrodissolution (anodic) pulse may range from about 1 - 300 ms.
• the duration of forward pulses often has been set to 10 - 80 ms, and duration of reverse pulses has been set to 0.5 - 6 ms. This has resulted in a frequency range of about 12 to about 95 Hz.
• printed circuit boards have been to be produced which were 2 mm thick and which contained through holes with an aspect ratio of 10 : 1 acceptable throwing power of copper deposition in the through holes has been achieved at a current density in the range of 1 - 10 A/dm 2 for the forward pulses and at a current density in the range of 10 - 40 A/dm 2 for the reverse pulses.
• the current densities must be decreased in order to achieve an acceptable result in throwing power.
• the object of the present invention is therefore to fulfill the above requirements and more specifically to achieve sufficient metal plating thickness in high-aspect ratio printed circuit boards. Another object of the present invention is also to ensure that electroplating efficiency is as high as possible which implies that metal plating current density at the printed circuit boards must be as high as possible.
• a suitable average plating current density is held to be at least 1.7 A/dm 2 , more preferably at least 2 A/dm 2 and most preferably at least 3 A/dm 2 .
• the method according to the present invention serves to electroplate a workpiece which is preferably plate-shaped such as a printed circuit board and which has high-aspect ratio holes.
• the method comprises the following method steps: a. The workpiece is brought into contact with a metal plating electrolyte and at least one anode. b. A voltage is applied between the workpiece and the anodes, to the effect that a current flow is provided to the workpiece.
• the current flow generated is a pulse reverse current flow.
• the pulse reverse current has a frequency of at most about 6 Hertz, preferably at most 4 Hertz and more preferably at most 2.5 Hertz. In each cycle time of the pulse reverse current flow at least one forward current pulse and at least one reverse current pulse are provided.
• one forward current pulse and one reverse current pulse are provided.
• the ratio of the duration of the forward current pulses to the duration of the reverse current pulses of one cycle is set to at least 5, more preferably to at least 15 and still more preferably to at least 18. This ratio may be set to at most 75 and more preferably to at most 50. The ratio may most preferably be set to about 20.
• the duration of the forward current pulses of one cycle may preferably be set to at least 100 ms, more preferably to at least 160 ms and most preferably to at least 240 ms.
• the duration of the reverse current pulses of one cycle may preferably be set to at least 0.5 ms, more preferably to at least 8 ms and most preferably to at least 12 ms.
• the peak current density at the workpiece of the forward current pulses may be set to at least 3 A/dm 2 . It may be set to at most 15 A/dm 2 . Most preferably the peak current density at the workpiece of the forward current pulses may be about 5.5 A/dm 2 .
• the peak current density at the workpiece of the reverse current pulses may especially be set to at least 10 A/dm 2 . It may be set to at most 60 A/dm 2 . Most preferably the peak current density at the workpiece of the reverse current pulses may be in the range of from about 16 to about 20 A/dm 2 .
• the ratio of the peak current density of the forward current pulses to the peak current density of the reverse current pulses may be set to at least 1 , more preferably to at least 2 and still more preferably to at least 3. This ratio may be set to at most 15 and more preferably to at most 4. The ratio may most preferably be set to about 3.
• the rise times of the forward and reverse current pulses may be adjusted depending on the technical objective pursued.
• the workpiece is preferably plate-shaped. It may more preferably be a printed circuit board or any other plate-shaped electrical circuit carrier, such as a semiconductor wafer (integrated circuit) or any hybrid (IC-) chip carrier like a multi-chip module.
• a semiconductor wafer integrated circuit
• IC- hybrid
• a first voltage is applied to between a first side of the workpiece and at least one first anode, to the effect that a first pulse reverse current flow is provided to the first side of the workpiece, said first pulse reverse current flow having at least one first forward current pulse and at least one first reverse current pulse flowing in each cycle time.
• a second voltage is applied to between a second side of the workpiece and at least one second anode, to the effect that a second pulse reverse current flow is provided to the second side of the workpiece, said second pulse reverse current flow having at least one second forward current pulse and at least one second reverse current pulse flowing in each cycle time.
• the first forward and reverse current pulses of one cycle may be offset relative to the second forward and reverse current pulses of one cycle, respectively. In a more preferred embodiment of the present invention this offset between the first current pulses and the second current pulses is approximately 180°.
• the current flow may comprise, in each cycle time, two forward current pulses with one zero current break between the two forward current pulses and one reverse current pulse.
• the current flow may comprise, in each cycle time, one forward current pulse followed by one reverse current pulse and after that one zero current break.
• the current flow may comprise, in each cycle time, one forward current pulse followed by one reverse current pulse without any zero current break in this cycle.
• the current flow may comprise, in each cycle time, one forward current pulse, followed by one zero current break and after that one reverse current pulse.
• an average current density l av can be specified in relation to the cycle time.
• the average current density can be calculated by the following equation:
• liw forward current density
• Irv reverse current density
• t f w forward pulse duration
• trv reverse pulse duration
• i,j integers >1 , which denote individual forward and reverse pulses, respectively, in each pulse cycle
• t ct cycle time, wherein a cycle time can additionally comprise the time of zero current break pulses if such zero current break pulses are used.
• the average current density may be set in a range of 1 to 10 A/dm 2 , more preferably 2 to 6 A/dm 2 and most preferably 3 to 5 A/dm 2 .
• the average current density is set to a value of about 4 A/dm 2 .
• At least one parameter of the pulse reverse current flow selected from the group comprising the ratio of the duration of the forward current pulses to the duration of the reverse current pulses of one cycle and the ratio of the peak current density of the forward current pulses to the peak current density of the reverse current pulses of one cycle, may be varied. More specifically it turns out to be advantageous to increase, in the course of metal plating the workpiece, the ratio of the peak current density of the forward current pulses to the peak current density of the reverse current pulses and/or to decrease the ratio of the duration of the forward current pulses to the duration of the reverse current pulses.
• Another improvement of the invention comprises bringing the workpiece into contact with the metal plating electrolyte by delivering the metal plating electrolyte towards the surface of the workpiece at an electrolyte flow velocity relative to the surface of the workpiece.
• the metal plating electrolyte is preferably forced under agitation towards the workpiece.
• the electrolyte flow velocity at the surface of the workpiece comprises a velocity component normal to the surface of the workpiece being at least 1 m/sec.
• the velocity may be set to at least about 1.4 m/sec, more preferably to at least about 7.2 m/sec. It may be set to be at most about 11.5 m/sec.
• the method comprises providing at least one anode being inert and dimensionally stable.
• Anodes are preferably used which contain titanium or tantalum as the basic material, which is preferably coated with noble metals or oxides of the noble metals. Platinum, iridium or ruthenium, as well as the oxides or mixed oxides of these metals, are used, for example, as the coating. Besides platinum, iridium and ruthenium, rhodium, palladium, osmium, silver and gold, or respectively the oxides and mixed oxides thereof, may also basically be used for the coating.
• a particularly high resistance to the electrolysis conditions could be observed, for example, on a titanium anode having an iridium oxide surface, which was irradiated with fine particles, spherical bodies for example, and thereby compressed in a pore-free manner.
• anodes may also be used, which are formed from noble metals, for example platinum, gold or rhodium or alloys of these metals.
• Other inert, electrically conductive materials, such as carbon (graphite) may also basically be used. These anodes are provided in order to reduce the excessive polarisation voltage and to keep the anodes electrically conductive and also, at the same time, to protect the anodes from electrolytic sputtering.
• the insoluble anode can be made from an expanded metal sheet of titanium, which was activated with noble metal (e.g. platinum).
• noble metal e.g. platinum
• a rod-like anode can extend into tubular cathodes.
• the cathodes may be formed from a tubular expanded metal which, at the same time, renders possible a very good exchange of electrolyte as a consequence of the lattice structure.
• the metal plating electrolyte may be a copper plating electrolyte.
• copper ions may be replenished to the electrolyte by dissolving copper metal.
• the copper plating electrolyte may contain at least one compound capable of oxidizing copper metal to copper ions.
• oxidizing compound may be for example a ferric compound, such as ferric ion, ferric sulphate more specifically.
• iron-(ll) sulphate heptahydrate to the electrolyte after a short time the effective iron-(ll)/ iron-(III) redox system is formed, wherein iron-(ll) sulphate heptahydrate is excellently suited for aqueous acid copper baths.
• iron compounds with anions which lead in the copper electrolyte to undesired secondary reactions such as for example chloride or nitrate may also not be used.
• an ion generator which contains parts of copper.
• the generator is separated from the electroplating chamber containing the anode.
• the electrolyte which is weakened by a consumption of copper ions, containing the compounds as e.g. iron-(ll) sulphate, is guided past the anodes, whereby iron-(lll) compounds are formed from the iron-(ll) compounds.
• the electrolyte is subsequently conducted through the copper ion generator and thereby brought into contact with the copper parts.
• the iron-(lll) compounds thereby react with the copper parts to form copper ions, i.e. the copper parts dissolve.
• the iron-(lll) compounds are simultaneously converted into the iron- (II) compounds.
• the total concentration of the copper ions contained in the electrolyte is kept constant.
• the electrolyte passes from the copper ion generator back again into the electroplating chamber in which it comes in contact with the workpiece and the anodes.
• iron-(ll) and iron-(lll) compounds are used as the electrochemically reversible redox system.
• redox systems of the following elements: titanium, cerium, vanadium, manganese and chromium. They can be added to the copper deposition solution for example in the form of titanyl sulphuric acid, cerium(IV) sulphate, sodium metavanadate, manganese(lll) sulphate or sodium chromate. Combined systems can be advantageous for special applications.
• the concentrations of the compounds of the redox system must be set in such a way that, through the resolution of the metal parts, a constant concentration of the metal ions in the deposition solution can be maintained. This guarantees that the insoluble anodes, coated with noble metals or oxides of the noble metals, are not damaged.
• the oxidizing compound may also be oxygen.
• Oxygen, contained in the air, is constantly introduced into the electrolytic fluid by electrolyte movements so that oxygen dissolves in the fluid.
• This oxygen is also capable of dissolving copper by oxidizing the copper parts in the ion generator, wherein oxygen ions are formed.
• the influencing parameters were held constant as far as possible, and the - artificial convection by means of forced flooding was investigated.
• a specially designed multilayer printed circuit board with electrochemical flow sensor was used as part of these investigations.
• a schematic of one test hole on the test board is shown in fig. 1.
• This test board comprises a micro electrode array.
• test board was placed in a test chamber which allowed the variation of key parameters as follows:
• the test chamber is shown in fig. 2. This test chamber is being used for hydrodynamic studies.
• the test chamber comprises a housing 1 , which emcompasses an adjustable disc 2. On this disc 2 the test printed circuit board 3 is arranged in a vertical arrangement.
• the item with numeral 4 is a stopper.
• the electrochemical cell also comprises a counter electrode 5 and a reference electrode 6, which both are also schematically displayed in fig. 1.
• a nozzle 7 serves to impinge metal plating electrolyte to the surface of the printed circuit board 5 at an angle a which is defined as the angle between the axis of the nozzle 7 and the upper right hand part of the test printed circuit board 3 as shown in this fig.
• Finally there is a lateral nozzle adjustment means 8 which allows fine tuning of the point of impingement of the metal plating electrolyte at the test printed circuit board.
• Fig. 3 shows a microsection through one test coupon having a hole with a diameter of 0.2 mm showing the inner layer electrode connections, the results from experiments with this test coupon being given in fig. 4.
• FIG. 5 shows the experimental set-up (particle image velocimetry apparatus) used to carry out the tests.
• a dynamic system is illuminated by two laser beams, and the resulting interference pattern information is recorded on a camera.
• the fluid flow velocity of electrolyte solution shall be selected such that it has a velocity component normal to the surface of the workpiece of at least 1 m/sec, preferably of at least 5 m/sec and most preferred of at least 10 m/sec.
• the standard Inpulse module for horizontal processing of printed circuit boards (in which boards are conveyed in a horizontal path and in a horizontal plane of transport for processing same) but may also be conveyed in a vertical or any other plane of transport has a spray bar to cathode (workpiece) separation of 95 mm and an anode to cathode separation of 75 mm.
• both the spray bar and the anode are set much closer to the cathode at 15 mm and 8 mm for the anode. This enables a more intense electrolyte flow towards the panel and also has an added advantage making the use of anode shielding unnecessary whilst retaining excellent surface distribution.
• the metal plating electrolyte may vary to some extent. Throwing power may be efficiently improved if the electrical conductivity of the metal plating electrolyte is increased. This may be affected by increasing the acid concentration for example.
• the additive concentrations are more typical of electrolytes adjusted to produce high-aspect ratio panels. In particular the copper concentration is 15 - 20 g/l lower than in a standard Inpulse ® electrolyte.
• the pulse plating parameters were varied from DC plating conditions at 4 A/dm 2 to pulse plating with forwards 250 ms and reverse 25 ms.
• a selection of the parameters used together with the throwing power achieved is shown in Table 2.
• phase shift in pulse parameter of 180° was used. This means that the reverse pulse was applied to the anodes on one side of the test panel at the same time that the forward pulse was applied to the anodes on the other side.
• the pulse wave form schematic in fig. 7 (current as a function of time) illustrates this setting showing phase shift between top and bottom anodes (top curve: current at the top side of the cathode, bottom curve: current at the bottom side of the cathode).
• a throwing power of only 30 % would be achieved at 3 A/dm 2 with horizontal DC.
• a throwing power of 55 % is achieved under vertical conditions in DC.
• a throwing power of 90 % is achieved, but this is at an average current density of 2 A/dm 2 .
• Using forced agitation improved throwing power is possible as discussed hereinbelow. But even this is not at such a high-current density.
• metal plating electrolytes may be employed which have the same composition as the metal plating electrolytes described above for horizontal processing.
• vertical plating pulse plating may be performed under the same conditions as in horizontal processing. Therefore as to these plating conditions in vertical plating reference is made to the above description.
• electrolyte agitation is usually made with a combination of air agitation in the electrolyte itself and a mechanical agitation of the circuit board being plated. This mechanical agitation must ensure that the panels are moved evenly and remain vertical in the electrolyte. Otherwise solution flow will not be uniform through all the holes in the panel. To ensure this cathode movement, systems are used which clamp the panel securely and which are also used to supply current to the panel. These agitation systems, air in the electrolyte and movement of the panel, can lead to uneven fluid transport due to non-defined air agitation and due to the movement of the panel through the agitation bubbles.
• Eductors spray nozzles which use the Venturi Principle, i.e. drawing of additional liquid through the nozzle is affected by the spray created, so that high-volume flow is achieved
• Eductors using the Venturi Principle allow small pumps to circulate larger volumes of liquid. The kinetic energy of one solution will cause the flow of another.
• Eductors can give a 4 - 6 times increase in volume of solution movement when compared to the volume pumped. This increased volume is however at a lower pressure than the directly pumped solution.
• Fig. 9 shows two sizes of commonly used Eductors in electrolytic copper plating systems. The smaller Eductor shown will pump a lower volume, but will allow more Eductors to be placed on one pipe, so giving a more even electrolyte flow.
• This installation is with two pipes placed one on each side below the cathode with the Eductors adjustable pointing upwards towards or away from the cathode.
• the disadvantages associated with this set-up are that the electrolyte flow uniformity depends on the positioning of the Eductors and also of the distance between the nozzle and the panel.
• the Eductors can be positioned between the anodes in the plating cell pointing directly towards the cathode.
• This set-up has the advantage of giving a more direct flow of electrolyte towards the panel and is shown in fig. 11 as a view from the top of the installation to the side thereof.
• the Eductors 9 are shown to be disposed at the sides of the tank in front of the anodes 11.
• the disadvantage of all Eductor installations is that the solution flow can never be completely uniform over the panel surface. A compromise must be made between the number of Eductors installed and flow uniformity.
• To overcome the limitations of flow uniformity by the use of Eductors a moving spray system has been developed and is being tested in a trial tank in laboratory conditions.
• the system consists of a spray head which moves regularly over the surface of the cathode and produces an intensive forced flooding of the panel and the through holes at the point of spraying.
• the head moves in a plane between anode and cathode and delivers the electrolyte in the direction towards the panel. It is so dimensioned that it does not interfere with the electrodeposition process.
• Results with high-aspect ratio panels have shown a significant improvement in throwing power when compared to standard air agitation and a more uniform deposit in comparison to Eductor agitated equipment on the same scale.
• Fig. 12 shows plating results from a 3.0 mm panel with a 0.3 mm hole (aspect ratio: 10 : 1) using the moveable spray system. Average current density for electroplating was 2 A/dm 2 . Throwing power was found to be 90 - 95 %. Reinforcement plating was performed by DC plating in horizontal plating equipment at a current density of 5 A/dm 2 .
• Fig. 13 shows plating results from a 5.0 mm panel with a 0.5 mm hole (aspect ratio 10 : 1 ) using a modified pulse plating sequence together with the moveable spray system to give optimal electrolyte exchange.
• the average current density applied is 1.7 A/dm 2 .
• the throwing power was found to be 95 - 100 %.

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