WO2002086196A1

WO2002086196A1 - Copper acid baths, system and method for electroplating high aspect ratio substrates

Info

Publication number: WO2002086196A1
Application number: PCT/US2002/012355
Authority: WO
Inventors: Douglas W. Mckesson; Joseph Cole; Rudolf P. Sedlak
Original assignee: Rd Chemical Company
Priority date: 2001-04-19
Filing date: 2002-04-18
Publication date: 2002-10-31

Abstract

Electroplating baths, system and method for electroplating copper on a substrate (240) having high aspect ratio confirmations thereon and a photoresist deposited on portions of the substrate are disclosed. The substrate (240) is immersed in a copper electrode (230) plating system (200). The system (200) comprises a container (220) adapted to hold an electrolytic bath (210) having acidic pH. The electroplating baths, systems and method offer enhanced throwing power through employing an acidic bath and plating additives disclosed herein.

Description

COPPER ELECTROPLATING SYSTEMS AND METHODS OF HIGH ASPECT

RATIO ELECTROPLATING

Field Of The Invention The present invention relates generally to the field of electroplating, and more particularly to electroplating copper in an acidic bath to achieve improved plating uniformity.

Background Of The Invention Traditionally, a popular technology for the electroplating of copper has involved the use of a cyanide containing bath. The use of cyanide salts in copper plating electrolytes has become environmentally disfavored because of ecological considerations. Another technology that was commonly used for electroplating copper in the electronics industry is based on the use of potassium pyrophosphate as an electrolyte. This methodology worked well, but it was not a chemically stable system, so in a production mode, it was difficult to control speed, thickness, and quality of the copper electroplated.

Accordingly, a variety of stable, non-cyanide electrolytes for various metals have heretofore been proposed for use as replacements for the well-known and conventional commercially employed cyanide counterparts. One such example of a replacement is disclosed in U.S. Patent No. 4,469,569, which uses an organo-phosphate chelating agent that complexes with cupric ions and an alkali carbonate as a bath stabilizing and buffering agent. Although this type of system provides adequate results for the decorative plating industry, it is unusable in many electronics applications.

The bath described above and disclosed in U.S. Patent No. 4,469,569 is alkaline. Many electronics applications require an acid bath, since a printed circuit board to be electroplated will generally be partially coated with a photoresist material that dissolves, delaminates, or is otherwise rendered nonfunctional in alkaline baths. Because of this major drawback, an acid bath is required for copper electroplating of electronic components which have photoresist on a portion which is not to be electroplated. Current technology in electroplating in electronics, and especially in the printed circuit board manufacturing industry, and in the semiconductor manufacturing industry involves the use of a copper sulfate/sulfuric acid electrolyte electroplating bath that although stable, lacks the capacity to electroplate uniformly over widely varying topologies with high aspect ratios. The term "aspect ratio", usually applied to a through-hole, or a blind (not completely drilled through) hole, also known as a "via", refers to the ratio of the height, or depth of the hole to the diameter of the hole. Currently any aspect ratio higher than 8 or 10 is referred to as "high". An exemplary copper plating bath comprises sulfuric acid, copper sulfate, 50 ppm of chloride (a cuprous ligand), organic additives for leveling and brightening, and water.

As printed circuit boards and semiconductors have become more complex, the circuits and interconnects involved have become denser, requiring circuit boards to have ever increasing numbers of layers which must electrically interconnect with one another. In the case of printed circuit boards, the multiple layer constructions are electrically connected by drilling holes through the board, and the holes are then plated with an electrically conducting material such as copper. Similarly, many semiconductor devices contain interconnects, such as vias, that must be conductively plated. The recent trend is to replace aluminum with copper as the metal of choice in producing semiconductor devices, and, unlike aluminum, copper cannot be sputtered onto a device, but must be electroplated on. In plating the tlirough holes of a printed circuit board, initially, an electroless copper plating procedure is generally used to establish conductivity by plating having a thickness of about 50 millionths of an inch to make the hole well conductive. Other replacement technologies for electroless copper are currently being used to make the hole walls conductive, and may be used with this new technology. Then a copper electroplating procedure is used, which is a much faster process and is much less expensive than electroless plating. The trend for such holes has been to reduce the diameter of the hole as well as to increase the length of them, because of the increasing layers on the board and the higher density of interconnects (e.g., more holes per square inch of printed circuit board). Semiconductor devices have been faced with this problem for a long time now, but it was easily addressed when aluminum was being used, since aluminum could be sputter deposited uniformly on the features of interest. With the change to copper and the requirement of electroplating the copper, the semiconductor industry is suddenly faced with the same problem that has become a gradually increasing problem in the printed circuit board industry over at least the last ten years. In the case of tlirough holes or vias, known electroplating processes suffer from nonuniform coating of copper, as more copper ends up being deposited at the entrance and exit portions of the hole than toward the center. This lack of "throwing power" or high aspect ratio plating uniformity, dictates that improved systems are required in order to adequately plate the central portions of the holes without overplating the entrance and exit portions of the holes/vias. Efforts to improve the known sulfuric acid based baths have been made by changing the sulfuric acid /copper sulfate ratio and/or varying the amounts and types of organic brightener/leveler used in the bath. However, none of these efforts have significantly improved the ability to adequately plate regions having high aspect ratios. Fig. 1 is a schematic representation showing the inconsistent plating of copper that occurs along a through hole of a printed circuit board when plated by using current plating baths known in the art, in this case a sulfuric acid bath as described above. Because a higher voltage differential is needed to deliver the copper ions to the walls in the center region 112 of the hole 110, copper 50 deposits at a greater rate on the walls in the regions of the ends 114,116 (entrance and exit portions of the hole, respectively) and therefor accumulates more in these regions compared with the center portion. In order to deposit an adequate thickness of copper at the central region 112, it may require the electroplating process to continue until the accumulations at the end of the holes are unacceptably thick, sometimes to the degree where the hole can actually be partially or totally closed by the deposits. Additionally, it is known to those skilled in the art that slowing the electroplating process gives a more uniform deposit, and allows improved throwing power. This has extended the processing time required to get .001" of plating thickness at the center of the hole 112 from a norm in 1990 of about 35 minutes, to typically 75 minutes, and in some cases up to 180 minutes, and even as long as 240 minutes. These time requirements are cost prohibitive and cause a bottleneck in the stream of production.

Another approach toward achieving a more uniform deposition of copper on high aspect ratio conformations in an acidic bath is what is known in the art as "pulse plating". In a pulse plating procedure, a known acid bath is set up according to current methods and the copper is initially plated at low voltage. After a predetermined time, when a greater thickness of copper has accumulated at the openings/surfaces of the high aspect ratio conformations than at the centers of the conformations, the current in the system is reversed and a deplating procedure is carried out at relatively high voltage, which has the effect of removing copper plating from the thicker areas (i.e., openings) at a higher rate than the removal rate from the centers. After a predetermined time, the current is again reversed and applied at relatively low voltage to plate copper again. By repeating this cycle, a greater accumulation/plating thickness of copper at the center regions is built up, relative to the deposits at the openings/surfaces. However, this process is time consuming, thereby still causing a bottle neck in a production line. Further, the pattern of plating and deplating changes for each different substrate to be plated, causing set up to be expensive and complicated. Still further, special organic brighteners are required in this process, because the brighteners currently used in the art will decompose during the current switching cycles. Also, rectifiers for driving the cycling are expensive compared to those currently used.

Thus, there remains a need for improved electroplating systems and/or techniques for adequately plating high aspect ratio conformations that have photoresist present thereon.

Summary Of The Invention

The present invention is directed to improved electrolyte baths for the electroplating of copper, and systems and methods using such baths for electroplating copper with sufficient throwing power to effectively electroplate high aspect ratio conformations on substrates having photoresist on a portion thereof. Such a bath is disclosed to include copper ions and at least one ligand capable of binding copper ions in the electrolyte bath, wherein the electrolyte bath has a pH less than about 6. The ligand of choice will have a very high metal chelate stability constant, and if the ligand is an acid, it will have a pKa low enough that there is enough free, non-protonated ligand available to react with the copper present to keep the free, un-complexed copper levels down to a small fraction (<50%, preferably less than 20% and more preferably less than 1%) of the total Copper in the bath.

The electrolyte baths preferably have a pH of about 0 to 4, more preferably about 0.

The copper ions may comprise cuprous ions or cupric ions, more preferably cuprous ions.

In a cupric ion system, a ligand used may be hydrochloric acid, potassium chloride, or ammonium chloride.

For use with cuprous ions, bipyridyl and cuprous methane sulfonate may be employed, or chloride ions, for example. Also, a step is preferably taken to prevent cuprous ions in the bath solution from being oxidized to cupric ions by atmospheric oxygen. Preferably such step is accomplished by bubbling nitrogen through the bath.

One example of an electrolyte bath according to the present invention includes methane sulfonic acid, bipyridyl, cuprous methane sulfonate, and deionized water.

Another example of an electrolyte bath according to the present invention includes hydrochloric acid; cuprous chloride, and deionized water.

Another example of an electrolyte bath according to the present invention includes

1.0 Molar cupric chloride, 1.5 Molar hydrochloric acid, (1 gram/liter of polyethylene glycol may be added to give a smoother deposit facilitate measurement of the deposit thicknesses), and deionized water. An example of a cupric ion electrolyte bath includes hydrochloric acid, cupric chloride, and deionized water.

A cupric ion bath may include hydrochloric acid and cupric chloride at a ratio of about 1.5M:1.0M, respectively. A method of electroplating copper with sufficient throwing power to effectively electroplate high aspect ratio conformations on substrates having photoresist on a portion thereof is described as providing an electrolyte bath having cuprous ions in solution and a pH less than about 6, providing a copper anode, submerging the copper anode and the substrate in the electrolyte bath, electrically connecting the copper anode and the substrate with an electric current source; and applying current to plate copper on the substrate.

Brief Descriptions of The Drawings

The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail. In the accompanying drawings:

Fig. 1 is a schematic sectional representation of a hole in a printed circuit board having been copper plated by a prior art technique.

Fig. 2 is a schematic representation of a cutaway of an electroplating system.

Fig. 3 is a schematic sectional representation of a hole in a printed circuit board illustrating the potential differential between the openings of the hole and the central wall portions of the hole.

Fig. 4 is a simplified schematic representation of an overhead view of an electroplating system.

Fig. 5 is a graph showing the plating thickness distribution over various cathode plates, one of which was plated in a HCl/ cupric chloride electrolyte bath according to the present invention, and one of which was plated in a known copper sulfate/sulfuric acid electrolyte bath.

Fig. 6 is a graph showing the plating thickness distribution over various cathode plates, two of which were plated in a methane sulfonic acid/bipyridyl electrolyte bath according to the present invention, and one of which was plated in a known copper sulfate/sulfuric acid electrolyte bath.

Fig. 7 is a schematic sectional representation of a hole in a printed circuit board showing an ideal, uniform copper plating thereon. Detailed Description Of The Invention

Before the present systems and techniques for electroplating are described, it is to be understood that this invention is not limited to particular bath or process described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either both of those included limits are also included in the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

It must be noted that as used herein and in the appended claims, the singular forms "a", "and", and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a brightener" includes a plurality of such brighteners and reference to "the hole" includes reference to one or more holes and equivalents thereof known to those skilled in the art, and so forth.

The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed. Definitions

The term "throwing power" is used to describe the extent to which an electroplating system can effectively plate regions within crevices, cracks, holes, etc.

The term "aspect ratio" refers to the ratio of the depth of a hole to the diameter of the hole. The term "high aspect ratio plating" refers to the extent to which an electroplating system can effectively plate regions within holes and on other high aspect ratio features that occur in printed circuit boards and semiconductor devices. Thus "high aspect ratio plating" requires the system to have good throwing power.

The term "cuprous" refers to the monovalent anion of copper, i.e., Cu⁺. The term "cupric" refers to the divalent anion of copper, i.e., Cu^"1".

The term "current efficiency" refers to the percentage of current in the system that is used in the plating reaction, rather than to produce hydrogen gas at the cathode, or oxygen gas at the anode.

Referring to Fig. 2, a simple schematic of a copper electrode plating system 200 is shown. An electrolyte bath 210 is provided in a container 220. Copper anodes 230 and a substrate 240 to be electroplated are submerged in the bath 210. Anodes 230 are electrically connected to one pole (i.e., negative pole) of a rectifier 250 and the substrate 240 is electrically connected to the opposite pole. Electric power is supplied to the system by the rectifier, by which copper ions enter the bath 210. The portion of the substrate 240 which is electrically conductive and connected to the rectifier 250 functions as a cathode in the system, where copper ions are reduced, with copper plating resulting on the substrate. In a simple system using copper sulfate and sulfuric acid in the bath, for example, the sulfate has no ability to complex the copper ions in solution and therefor there is a great availability of copper ions for reaction at the cathode. Such an arrangement lacks sufficient throwing power for adequate high aspect ratio plating, since the available copper ions plate readily to the areas of lower voltage drop, respectively of the substrate 240 to be plated.

Fig. 3 illustrates the effective voltages that result along a through hole 260 or other high aspect ratio feature to be electroplated. The voltage at locations 262,266, the openings of the hole, is greater than the voltage at location 264, near the center portion of the hole, because of the greater distance through the electrolyte that the ions must travel, which presents a greater resistance and, hence, a greater voltage drop. Additionally, there is a direct relationship between aspect ratio and voltage drop; thus, the higher the aspect ratio, the greater is the voltage drop between a surface/opening of the high aspect ratio conformation and the center or feature to be plated which is furthest from the surface/opening. Because the ions can more easily travel to the surface of the substrate and openings of the hole, and because the voltage is relatively greater there, a greater rate of electroplating occurs in these locations, relative to the center of the hole 264, which results in a situation such as the example shown in Fig. 1. To effectively combat this phenomenon, a greater throwing power is needed. The present invention addresses several aspects to achieving a greater throwing power in order to effectively electroplate high aspect ratio features with copper. One such aspect is to effectively reduce the ready availability of copper ions in the bath so as to slow the electroplating process down at the entrances of the holes, thereby allowing plating in the centers of the holes to progress and develop more evenly with the openings. Thus, the copper must be complexed in solution with the electrolyte, such as with a ligand or chelating agent. However, the electrolyte must be acidic so as not to disrupt any photoresist 60 that may be present on the substrate to be electroplated. A pH of 6 or less is needed to prevent damage to the photoresist, with the systems preferably using a bath having a pH in the range of about 0 to 4, and more preferably about 0.

An effective ligand must also be very soluble in the bath solution, i.e., the electrolyte producing the ligand must be strongly acidic so as to provide an adequate supply of the ligand, since both copper ions and hydrogen ions will be competing to react with the ligand in solution. In one example, chloride is used as the ligand. The reaction that occurs during the electroplating at the surface of the substrate is:

Cu⁺⁺ + 2e → Cu (1)

At the same time, a side reaction occurs, as follows:

2H₂O + e -> H₂ + 2OH (2) To minimize the effects of differential voltage between the center of a high aspect ratio (or the location furthest from the surface of the substrate) and the opening of the high aspect ratio feature, the total potential between the anode and cathode should be maximized. The higher the total potential, the less that the differential potential is significant during the electroplating process. For example, if N_surface is the amount of voltage effecting plating at the surface of the substrate (i.e., at the entrance 262 of the hole 260), then V_T is the amount of voltage required to electroplate at the center portion 264 of the hole walls, where Nx =

N_surface + ΔN, and ΔN represents the voltage drop between the opening 262 and the center

264. By increasing the voltage V_surface as much as practically possible, the differential plating rates between locations on or near the surface of the substrate (e.g., 262 and 266) and locations furthest from the surface in a high aspect ratio conformation (e.g., center portion 264) can be minimized, because ΔN remains substantially constant and becomes a less significant component of N_τ as Nsurface is made larger, i.e., . ΔN/ N_sur_face + ΔN becomes smaller as Nsurface increases. As the voltage applied to the system is increased, the side reaction (2) becomes more prevalent, and, at some level can even become the dominant reaction. This greatly reduces the current efficiency, as most of the power of the operation is being directed toward producing hydrogen at this point and not for electroplating copper. Accordingly, a balance must be struck between the amount of voltage increase that is inputted to the system and the desired current efficiency. The amount of voltage used is a function of many variables, including the specific electrolyte being used, the distance from the anode to the cathode, as well as the temperature of the bath. The absolute voltage used in any given application may be determined by those of ordinary skill in the art, given the above parameters.

In addition to increasing the voltage and the free copper content, the present invention may decrease the current efficiency on the upper, more available sites, but it is not a uniform effect. The current efficiency at the sites of electroplating is decreased by the present invention, by binding up the copper ions to make them less available for reduction at the cathode (i.e., substrate where the electroplating is taking place). An electrolyte used must contain a ligand that will not only bind copper, but will also provide a ready supply of non-protonated species at the acidity level that the electroplating is being performed at. If the ligand does not sufficiently bind the copper, the electroplating will proceed with the same disadvantages inherent in the known sulfuric acid bath systems, i.e., the ready availability of copper ions at the surface of the substrate will result in a rapid electroplating process with insufficient throwing power. An electroplating system that has sufficient throwing power to effectively electroplate high aspect ratio conformations on substrates having photoresist on a portion thereof, will therefore be an acidic system, having a pH of less than about 6, and usually around 0-4, more usually about 0, and is characterized by a slower electrodeposition rate, relative to the known sulfuric acid electroplating systems described above, at the surface of the substrate, or in the locations of highest current density, while exhibiting a faster deposition rate, relative to the known sulfuric acid electroplating systems described above, at the center of a hole or location on the high aspect ratio conformation that is furthest from the surface or highest current density location, and thus has the lowest relative current density, or, at least the deposition at the center relative to the openings will be greater than that of known systems.

Testing of these characteristics was performed in a Larry Cell3 (a specialized model of a Hull Cell, model no. NC467, produced by Larry King Corp, Rosedale, NY.), a top view of which is illustrated in Fig. 4. The Hull Cell or Larry Cell 300 is a four sided chamber adapted to contain an anode 310, cathode 320 and electrolytic bath 330. Parallel sides 302 and 304 join non-parallel sides 306 and 308. Anode 310 (i.e., a copper plate) was mounted against side 306, which is perpendicular to sides 302 and 304. Cathode 320 (e.g., a conducting plate which was, in this example, a brass plate, although other conductive materials can be used) was mounted against wall 308, which joins walls 302 and 304 and angles away from wall 306. This configuration sets up a current density gradient along cathode 320 when anode 310 and cathode 320 are connected to a rectifier and electricity is provided to the system to begin the electroplating process. That is, the highest current density occurring on cathode 320 is that location that is closest to the anode, i.e., end 320a of cathode 320 in this example. The current density decreases along the length of cathode 320, with the decreasing gradient being a function of the angle of wall 308 with respect to wall 306, which causes a gradual increase in the distance of the cathode 320 from the anode 310 in travelling from end 320a to 320b. Thus, the lowest relative current density occurs at the furthest location from the anode 310, i.e., end 320b of anode 320. Cell 300 is configured to hold 267 ml of electrolyte when the anode and cathode plates 310,320 are in position. Each electrolyte bath is generally operated at room temperature during the tests, although it may prove preferable to heat the electrolyte bath in a production environment. A DC rectifier having no greater than about 5%, and preferably less than about 1% variation (ripple) in output amperage is used to provide 1 amp of electrical energy to the system. The plating tests were generally run at one ampere for about 83 minutes for a divalent species (the time to theoretically plate the cathode with 1 mil (.001") of copper uniformly, in a system with sufficient throwing power to uniformly plate the cathode in the cell 300), 41.5 minutes for a monovalent species. Running these tests at one ampere total is typical, and results in the high current density end of the test panel receiving about 40 amps per square foot current density, with the low current density end of the panel receiving about 0.5 Amps/square foot. Tests were performed without organic brightener added, since this varies with each specific application. An important goal of the present invention is to achieve more plating at lower current density (relative increase in plating rate) and/or to relatively decrease the plating rate at higher current density, relative to the standard sulfuric acid model. The fact that tests were performed without brightener is not to be considered a limitation, as this was done merely to confirm the theory of this technology. It is understood that an organic brightener system will be added in commercial applications of the present invention. After running the electroplating process for the prescribed period, the rectifier was shut down and the cathode 320 was removed from the system. The thickness of the copper plating that was electroplated on the cathode 320 was then measured, using a micrometer to plot the variation in thickness of the plating deposited as a function of distance from the anode 310, thereby measuring the throwing power of the system. Although providing superior throwing power to known sulfuric acid/copper sulfate electrolyte baths, a cupric bath comprising 1.0 Molar cupric chloride, 1.5M HCl, 0.1% Polyethylene Glycol 400 q.s. and deionized water did not exhibit a desirable bath life before the composition of the bath needed to be adjusted, e.g., about 20 minutes. This phenomenon was caused by the fact that the copper ions being plated at the cathode were cupric ions having a valence of +2, while the ions coming off the anode and going into the electrolyte solution were cuprous ions having a valence of +1. To extend the life of the bath to at least 12 hours minimum and preferably for a period of years, systems starting with cuprous ions in the electrolyte solution were developed. A problem with such systems is that cuprous ions are readily oxidized to cupric ions in atmospheric oxygen. To control this undesirable reaction, the bath must be protected from atmospheric oxygen. One way of protecting the bath (although other methods, such as enclosing the bath for example, could be used) is to bubble nitrogen through the bath, at least during electroplating, and preferably continuously. A flow rate of about 0.2 liters per minute of nitrogen is preferably used in the Hull cell testing. Other gases, such as inert gases, like argon or neon could alternatively be used, as could carbon dioxide, or any other medium that would be non interfering with the electroplating process, safe, and would ensure an oxygen free environment for the bath.

Fig. 5 shows the plating results using a HCl/cuprous chloride electrolyte bath according to the present invention, compared with the results using a known copper sulfate/sulfuric acid electrolyte bath. Plot 410 is a measurement of the thicknesses of copper deposited on cathode 320, according to the distance from the highest current density edge 320a, using the copper sulfate/sulfuric acid electrolyte bath. This bath was made up of 1 M sulfuric acid, 1 M copper sulfate, 50 ppm chloride ions supplied in the form of HCl and a balance of water (preferably deionized water) to make up the total volume of 267 ml. Plot 400 is a measurement of the thicknesses of copper deposited on cathode 320, according to the distance from the highest current density edge 320a, using an HCl/CuCl electrolyte bath. This bath was made up of 3 M HCl (hydrochloric acid), 0.1 M CuCl,(cuprous chloride) and a balance of water (preferably deionized water) to make up the total volume of 267 ml. Nitrogen was bubbled into the bath at a rate of about 0.2 liters per minute.

After running each of the above tests for about 83 minutes and measurement of the copper plating along each of the cathodes 320, a comparison of the plots 400,410 reveals that the thickness of the copper plating nearest the highest current density edge 320a for the lαiown copper sulfate/sulfuric acid electrolyte bath 410 is greater than the thickness of the copper plating in corresponding locations on the cathode 320, for the HCl/CuCl electrolyte bath according to the present invention. More specifically, up to about 1.9" (i.e. from 0.0" up to about 1.9") from the highest current density edge 320a, the copper plating using the known bath 410 is thicker than the copper plating using the HCL/CuCl bath 400. Conversely, the thickness of the copper plating at the distant end portion (nearest end 320b) is substantially thicker on the cathode subjected to the HCL/CuCl bath 400, compared to the same locations on the cathode subjected to the known copper sulfate/sulfuric acid electrolyte bath 410, indicating the superior throwing power of the HCl/CuCl bath 400.

Fig. 6 shows the plating results of a run 500 using a methane sulfonic acid/ bipyridyl electrolyte bath according to the present invention, compared with the results using a known copper sulfate/sulfuric acid electrolyte bath 520. Plot 500 indicates the measures thicknesses of copper deposited on cathode 320 in the methane sulfonic acid/bipyridyl runs 500, according to the distance from the highest current density edge 320a. In run 500, the electrolyte bath was made up of 1 M (Normal) methane sulfonic acid, 0.12 M bipyridyl, 0.05 M cuprous methane sulfonate, 0.2% polyethylene glycol 200 (used to enhance the smoothness of the plating deposition, simply to facilitate the measurement of thickness of plating deposits), and a balance of water (preferably deionized water) to make up the total volume of 267 ml. Nitrogen was bubbled into the bath at a rate of 0.2 liters per minute. Plot 520 is a measurement of the thicknesses of copper deposited on cathode 320, according to the distance from the highest current density edge 320a, using a copper sulfate/sulfuric acid electrolyte bath. This bath was made up of 1 M sulfuric acid, 1 M copper sulfate, 50 ppm chloride ions supplied in the form of HCl, 0.2% polyethylene glycol

200 and a balance of water (preferably deionized water) to make up the total volume of 267 ml. After running each of the above tests for about 83 minutes (83 minutes for cupric and sulfuric baths; 41.5 minutes for cuprous baths) and measurement of the copper plating along each of the cathodes 320, a comparison of the plots 500 and 520 reveals that the thickness of the copper plating nearest the highest current density edge 320a for the known copper sulfate/sulfuric acid electrolyte bath 520 is greater than the thickness of the copper plating in corresponding locations on the cathodes 320, for the methane sulfonic acid/bipyridyl run 500, according to the present invention. More specifically, up to about 3.2" from the highest current density edge 320a, the copper plating using the known bath 520 is thicker than the copper plating using the methane sulfonic acid/bipyridyl bath 500. Conversely, the thickness of the copper plating at the distant end portion (nearest end 320b) is thicker on the cathode subjected to the methane sulfonic acid/bipyridyl bath 500, compared to the same locations on the cathode subjected to the known copper sulfate/sulfuric acid electrolyte bath 520, indicating the superior throwing power of the methane sulfonic acid/bipyridyl bath 500. An even more important indicator of throwing power is the slope of each of the plots. Theoretically, a system exhibiting sufficient throwing power to plate a cathode 320 in the cell 300 with a completely uniform copper plating would have a slope of zero (i.e., the plot would be completely flat and horizontal). Thus, the lower the slope of the plot from these results, the better is the throwing power. As can be seen in the plots 500,520 of Fig. 6, the plot 500 clearly has a significantly lower slope (flatter, gentler slope) than that of the plot 520, which is fairly steep.

Fig. 7 is a schematic representation showing an ideal, consistent plating of copper along a through hole of a printed circuit board. This is a distribution of copper that would result using a system that would be characterized on any of the graphs in Figs. 5-6 by a perfectly horizontal line. As can be seen, the thickness of the copper plating 50 on the walls in the center region 112 of the hole 110 is equal to the thickness of the copper plating 50 in the regions of the ends 114,116 (entrance and exit portions of the hole, respectively) and also the thickness of the plating 50 adjacent the photoresist 60.

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. For example, bromide may be combined in a cuprous system, or an electrolyte that complexes cupric and doesn't stabilize cuprous may be used in a cupric system that does not utilize cuprous ions. Bromide-bipyridyl systems may be used with cuprous electrolyte baths, similar to chloride-bipyridyl cuprous baths. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

That which is claimed is:

I . An electrolyte bath for use in electroplating copper with sufficient throwing power to effectively electroplate high aspect ratio conformations on substrates having photoresist on a portion thereof, said bath comprising: copper ions; and a ligand capable of binding copper ions in the electrolyte bath, wherein said electrolyte bath has a pH less than about 6.

2.The bath of claim 1, having a pH of about 0 to 4.

3.The bath of claim 2 having a pH of about 0.

4.The bath of claim 1, wherein said copper ions comprise cuprous ions.

5. The bath of claim 1, wherein said copper ions comprise cupric ions.

6.The bath of claim 5, wherein said ligand is selected from the group consisting of hydrochloric acid, potassium chloride and ammonium chloride.

7.The bath of claim 4, wherein said ligand comprises bipyridyl and cuprous methane sulfonate.

8.The bath of claim 4, wherein said ligand comprises chloride.

9.The bath of claim 4, further comprising nitrogen in solution.

10. The bath of claim 4, further comprising an inert gas in solution.

I I. The bath of claim 4, further comprising carbon dioxide in solution.

12. An electrolyte bath for use in electroplating copper, said bath comprising: methane sulfonic acid; bipyridyl cuprous methane sulfonate; and deionized water.

13. The bath of claim 12, wherein said methane sulfonic acid, bipyridyl and cuprous methane sulfonate are combined at a ratio of 1M:0.12M:0.05M, respectively.

14.The bath of claim 12, further comprising nitrogen in solution in said bath.

15. The bath of claim 12, further comprising an inert gas in solution in said bath.

16. . The bath of claim 12, further comprising carbon dioxide in solution in said bath.

17. An electrolyte bath for use in electroplating copper, said bath comprising: hydrochloric acid; cuprous chloride; and deionized water.

18. The bath of claim 17, wherein said hydrochloric acid and said cuprous chloride are combined at a ratio of about 3M:0.1M, respectively.

19. The bath of claim 17, further comprising nitrogen in solution in said bath.

20. The bath of claim 17, further comprising an inert gas in solution in said bath.

21. The bath of claim 17, further comprising carbon dioxide in solution in said bath.

22. An electrolyte bath for use in electroplating copper, said bath comprising: hydrochloric acid; cupric chloride; and deionized water.

23. The bah of claim 22, wherein said hydrochloric acid and said cupric chloride are combined at a ratio of about 1.5M:1.0M, respectively.

24. A method of electroplating copper with sufficient throwing power to effectively electroplate high aspect ratio conformations on substrates having photoresist on a portion thereof, said method comprising the steps of: providing an electrolyte bath having cuprous ions in solution and a pH less than about 6: providing a copper anode; submerging the copper anode and the substrate in the electrolyte bath; electrically connecting the copper anode and the substrate with an electric current source; and applying current to plate copper on the substrate.

25. The method of claim 24, further comprising the step of : bubbling nitrogen through the bath during the step of applying the current.

26.The method of claim 24, further comprising the step of : bubbling an inert gas through the bath during the step of applying the current.

27. The method of claim 24, further comprising the step of : bubbling carbon dioxide through the bath during the step of applying the current.

28.The method of claim 24, wherein the electrolyte bath comprises methane sulfonic acid, bipyridyl, cuprous methane sulfonate, and deionized water.

29.The method of claim 24, wherein the electrolyte bath comprises hydrochloric acid, cuprous chloride, and deionized water.