Electroplating Cell Including Means To Agitate The Electrolyte In Laminar Flow

Abstract

An electroplating cell is constructed to prevent current spreading in the electrolyte during the plating of a metal or metal alloy onto a substrate. The cell is constructed such that the cross-sectional area of current path is substantially the same as the cross-sectional area of a pair of electrodes spaced apart in the cell. This is accomplished by placing the electrodes in the cell such that their edges are substantially in contact with the dielectric or insulating walls of the cell. The cell also contains electrolyte agitating means to provide uniform laminar flow of the electrolyte across the surface of one of the electrode. Metal alloy films deposited with the use of this cell exhibit uniform thicknesses on rather large surface areas. Where magnetic metal alloys are plated, the films not only exhibit uniform thicknesses laterally on the whole cathode but uniform composition and magnetic properties throughout as well.

Parent Case Text

This application is a continuation-in-part of copending U.S. Pat. application Ser. No. 693,375, filed Dec. 26, 1967, now abandoned.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved electroplating cell, from which metal films having uniform thicknesses and uniform compositions can be deposited.

2. Description of the Prior Art

Electroplating, because of its inherent simplicity, is used as a manufacturing technique for the fabrication of metal and metal alloy films. One of the severe problems in plating metal films arises from the fact that when a plating current is applied, the current tends to spread in the electrolyte on its path from the anode to the cathode. This current spreading leads to nonuniform local current density distribution on the cathode. Thus, the film is deposited in a nonuniform fashion, i.e., the thickness of the film varies in direct proportionality with the current density variation at the cathode. Additionally, where metal alloy films are deposited, for example, magnetic film compositions of nickel and iron or nickel, iron and copper, this nonuniform current density distribution causes a variation in the compositional makeup of the alloy film. In those cases where plating is done for decorating purposes, or even in cases of plating for corrosion protection purposes, the thickness uniformity and compositional uniformity are not of extreme importance.

When plating is used for the purpose of making thin film electronic components such as resistors, capacitors, conductors, magnetic devices or other, where both thickness and alloy composition determine the operation of the device, the uniformity of thickness and alloy composition are very important and critical. In connection with this, one distinguishes between the variation in composition of the alloy through the thickness of the film and between the variation of composition and/or thickness from spot to spot laterally over the entire plated area (cathode).

When the films are to be used in computer memories, which demand constant magnetic characteristics across the entire film compositional makeup variation results in deviations of magnetostriction (.lambda.). This deviation becomes a severe problem in magnetic films. This is especially so in terms of the magnetostriction of the deposited film, since zero magnetostriction is achieved with alloys including approximately 80 percent nickel and 20 percent iron. When the alloy varies by any considerable degree from these proportions, it does not exhibit a zero magnetostriction. Thus, local composition and thickness of a film are key factors in determining local magnetic properties of the film. The film properties may vary through the thickness of the film and may also vary along the lateral profile of the film from point to point over a large area of the plated film. From the point of view of magnetic memories, it is very important to have the least amount of variation in magnetic properties, both through the thickness and from place to place on rather large surface areas.

Considerable progress has been accomplished in the area of obtaining compositional uniformity through the thickness of the films by the controlling of bath makeup and operating conditions. For example, in copending U.S. Pat. application Ser. No. 573,417, filed Aug. 18, 1966, now U.S. Pat. No. 3,480,522, to James M. Brownlow and assigned to the same assignee as is this application, there is disclosed a method of controlling plating conditions by pulse plating in combination with a dilute plating bath.

In contrast to the above solution for obtaining uniformity through the film's thickness, very little progress has been made in obtaining lateral uniformity in the plated film. In this area, use of aids to improve current distribution has been the main approach to solving the problem. Normally, current balancing has been accomplished by the use of auxiliary cathodes or auxiliary anodes, conducting shells, bipolar conductors, and shields. A detailed discussion of the above various current balancing aids is provided in a publication of Robert H. Rousselot in Metal Finishing Journal at pages 57-63, Mar. 1961. Although these various aids are helpful in current controlling, they have several drawbacks. For example, when these aids are used, only an average current density can be calculated at the cathode. The true current density varies from point to point. The true current density at each point on a cathode is therefore unknown. Further, the cathode area plated represents only a small portion of the useful area of the cathode, i.e., only the central area of the plated cathode will exhibit uniformity of thickness, the remaining areas will be discarded. Thus, inefficient plating is obtained. The current used to plate on the various aids is wasted. Chemicals necessary to plate on various plating aids are also wasted. Additionally, in the anode position, its size and shape has to be optimized for each configuration of the cell and for each configuration of the specific aid used to obtain the best possible film thickness and compositional uniformity in the small cathode area of interest. Further, every time the ionic strength of the plating solution is changed, the geometry of the electrodes has to be optimized in order to maintain uniformity of thickness in the deposited film. All of the above problems are further compounded when it is desired to scale up the plating apparatus from laboratory to production scale. The geometry of the cell of all the electrodes and aids, as far as the shape and size of the electrodes and of the auxiliary equipment are concerned, has to be optimized again. The scale up cannot be accomplished by simple dimensional scale up of the cells and electrodes. Thus, a great deal of experimentation, of the trial and error variety, must be done every time the geometry or size of the electroplating cell is changed or when the size, shape, and spacing of the electrodes are changed.

In an article entitled, "Current and Metal Distribution in Electrodeposition," in the publication Plating, Vol. 39, No. 2, ages 165-170 (1952), there is presented a theoretical discourse on idealized plating bath containers from which metals can be plated having uniformity of thickness. The theory disclosed is based primarily upon Ohm's law, neglecting pertinent parameters such as polarization. The article admits that while desired results can be obtained theoretically the ideal case, at that time, had not been realized. The article further infers that in order to obtain the ideal case, one may choose either of two arrangements for a rectangular container, viz. a viz., "A rectangular tank infinitely long in one direction with an anode infinitely far from the cathode," or "Two very large flat plates immersed in a very large tank so as to be parallel to another." It is readily seen that neither of the two above arrangements are practical. Further, it should be noted that the article attempts to control primary current distribution only, based on its idealization of Ohm's law; consequently the article neglects secondary current distribution problems, which are critical in alloy plating as in the present invention. Thus, while it is indicated that the unobtainable ideal case of the article may provide uniformity of metal thickness, it nowhere suggests that at the same time uniformity of composition can equally be obtained when plating an alloy such as in the case of the present invention.

Further, an article entitled, "Engineering Design of Electrochemical Systems," J. Newman, Industrial & Engineering Chemistry, Vol. 60, No. 4, pages 12-27 (April 1968) discusses the unimportance of Ohm's Law, contrary to above article, and clearly demonstrates the importance of the secondary current distribution due to proper uniform agitation conditions.

SUMMARY OF THE INVENTION

According to one aspect of the invention, an improved electroplating cell is provides. This cell, as is illustrated by the embodiments disclosed herein, includes spaced apart electrodes having their edges substantially in contact with a suitable dielectric material, which may constitute the walls of the cell. The cell so provided requires that the cross-sectional area of the current path across the electrodes is substantially the same as the cross-sectional area along the length of the electrodes. This results in substantially equipotential lines which are parallel to both electrodes, and a constant current density throughout the whole cathode area. Current density is uniform, well defined, and well known for each point on the cathode. Where constant current plating is used, the anode can be placed at any reasonable distance from the cathode and the plating results are completely reproducible from one plating to the other and show excellent lateral uniformity in composition, thickness and magnetic properties.

In scaling up the cell from laboratory to production scale, a simple dimensional scale up of the cell dimensions results in reproducible plating conditions. Whereas in conventional electrochemical apparatus, dimensional scale up normally cannot be used. Further changes in ionic strength of the bath in no way affect the plating operation or uniformity of thickness, composition or magnetic properties.

The electroplating cell of this invention can be used for the electrodeposition of any metal and from any plating bath composition where uniformity of thickness and composition is desired.

Again, in accordance with the principles of the present invention, an improved electroplating method of fabricating magnetic thin film devices is realized. In the practice of this method, a relatively dilute aqueous bath including nickel-iron-copper is used. The film is plated on a smooth planar copper substrate cathode in the bath. Plating is accomplished in the cells of this invention.

Therefore, it is an object of the present invention to provide an improved electroplating cell.

It is a more specific object to provide an improved electroplating cell in which the cross-sectional area of the current path is substantially the same as the cross-sectional area along the length of the electrodes.

A further and equally important object is to provide an improved electroplating cell in which metal films having uniformity of thickness, composition and magnetic properties can be deposited.

Still another object of this invention is to provide an improved electroplating apparatus in which metal films may be plated with uniformity of thickness and composition without the use of current balancing aids.

It is still another object of this invention to provide an improved electroplating apparatus which may be scaled up from laboratory to production size without the need for undue experimentation to optimize the parameters of the apparatus.

And yet another object of this invention is to provide a method of electroplating magnetic films having uniformity of thickness, composition and magnetic properties.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the electroplating bath cell of this invention.

FIG. 2 is a side sectional view of the structure of FIG. 1 showing the path of current.

FIG. 3 is a side sectional view of a prior art electroplating bath cell showing the current path generated therein.

FIG. 4 is a plot comparing uniformity of the thickness of a plated film in the apparatus of this invention with a like film prepared in a prior art apparatus.

FIG. 5 is a plot comparing the uniformity of composition of a film containing Ni, Fe and Cu prepared in the apparatus of this invention with a like film prepared in a prior art apparatus.

FIG. 6 is a plot comparing the magnetic properties, coercive force and anisotropy field of the films used in the plot of FIG. 5.

FIG. 7 is a plot comparing anisotropy dispersion and magnetic skew in the films used in the plot of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the perspective views of FIGS. 1 and 2 which show the structure used in one mode of practicing the present invention, the bath container is designated 10. The walls of the container 10 are made from any suitable dielectric material such as glass or a plastic material, e.g., polymethacrylate. On either side of container 10 there is mounted Helmholtz coils 12 which can be energized during the plating of a magnetic film so that the fabricated film structure exhibits uniaxial anisotropy. The coils provide a magnetic field of about 40 oe or more. A cathode 14 is in the form of an insulating board on which there is affixed a conductive sheet or coating. The upper surface of the conductive sheet is very smooth. Cathode substrate materials which have been found to be satisfactory are rolled copper sheets, evaporated copper, evaporated silver, silver, sputtered gold, said electrolessly deposited silver, copper, nickel or cobalt. Cathode 14 is mounted on support block 16 and is in intimate contact therewith. Support block 16 is supported on a dielectric material base 18 on the bottom of container 10. Support block 16 is a conductive material which is adapted to receive cathode 14 and which extends to and is in contact with the walls of container 10. This arrangement of cathode 14 and support block 16 effectively extends the edges of cathode 14 to the walls of container 10. Cathode 14 is placed in support block 16 and is substantially flush or level therewith. Alternately, cathode 14 can be made to extend the full width and length of container 10 without the aid of support block 16 by simply making the cathode the size of the perimeter of container 10. Contact is made to the cathode 14 through support post 20, the outside surface of which is insulated since it is immersed in the electroplating bath. This post 20 is connected at a terminal 22 to an electrical current source, not shown. Mounted on a shoulder of post 20 is anode 24. Anode 24 extends substantially the full width and length of container 10, so that like cathode 14, its edges are substantially in contact with the insulating walls of container 10. Anode 24 is supported in a recess 11 fashioned in the walls of container 10. The anode 24 can be prepared from a conductive material such as, molybdenum, nickel, platinum and the like. Wound around anode 24 is a wire winding 26 of the same material from which the anode 24 is prepared. Winding 26 is provided to increase the surface area of anode 24 to at least twice that of the cathode. This increased surface area on anode 24 lowers the current density at the anode thus preventing anodic oxidation deposits which may from time to time fall onto the cathode and interfere with metal plating thereon. Alternately, the surface area of anode 24 can be increased by corrugating or by grooving the solid anode metal. The electrical connection to the anode is supplied by a wire connection 28 which leads to current supply source, not shown.

The bath level during plating is indicated by line 30 with anode 24 being in contact or immersed in the bath during the plating operation. The bath is agitated during the plating operation by a motor 32 which is connected to carrier 36 by linkage designated 34, or any other suitable linkage. The linkage 34 is designed to conform to the recess 13 of opposing walls of container 10. Anode 24 is substantially the same size as cathode 14. Similarly, linkage 34 is thinly made so that anode 24 remains substantially in contact with the walls thereat. When motor 32 is energized, the carrier 36 moves the base portion 35 continuously at a substantially uniform rate in a path back and forth along the length of the cathode 14 and just above the surface of cathode 14. As a result, a homogenization of the bath solution occurs on the surface of cathode 14. The agitating means comprising linkages 36, 34 and the base portion 35 is adapted to cause a uniform laminar flow of the bath across surface of cathode 14 without causing any measurable turbulence thereat. The agitating means can be fashioned from any nonconductive material such as plastics and the like Turbulence must be avoided since such turbulence cause local non-uniform polarization, thus negates compositional homogeneity. To avoid such local turbulence, i.e., below recess 13, agitating means is provided with sharp edges (the lower portion of linkage 34) so as to provide minimal resistance to the flow of the bath. The base portion 35 is similarly designed to provide minimal resistance to flow. It is triangular in form with its blunted apex at an angle which permits flow thereover with minimal turbulence, while its base is flat. In operation, the agitating causes the bath to flow over the base, and to effect mixing with bulk of the bath at the apex of said base 35 by convection. As the mixture passes the apex, the laminar flow is restored.

Referring again to FIG. 2, the current path, indicated by dash lines 46, is seen to have a cross-sectional area substantially equal to the cross-sectional area of cathode 14 and anode 24, i.e., the current across the electrodes 14 and 24 is confined to the boundaries thereof and is not allowed to diverge or spread in its path between said electrodes 14 and 24. As a result, the current density is relatively constant throughout the whole cathode 14 area. The current density is found to be relatively uniform and well defined; and the current density value can be predicted at any point on the cathode 14, since they are the same at any given point thereon. Consequently, films produced in the electroplating cell of this invention are uniformly thick throughout, and where metal alloys are being plated the metal compositions will also be uniform throughout the film's thickness.

In contrast, FIG. 3 depicts a prior art electroplating cell, generally designated as numeral 110, in which current balance aids (guard rings) 480 are used to improve current distribution across the cathode 140 and anode 240. It is seen that current (lines) across the electrodes 14 and 24 travels in an arcuate path at the edges of the electrodes and gradually begins to travel in parallel lines 460 toward the central portion of the cathode 140. Thus, only at the central portions of cathode 140 is there a uniformity of current density. Films prepared in this cell will have uniformity of thickness at the central portions thereof; consequently, efficient use of the film cannot be had because the outer portions are nonuniform in thickness and must be discarded. This is especially true where magnetic films are plated, since nonuniformity in the films' thickness and/or film composition also results in nonuniformity of magnetic properties in the film.

To better illustrate the invention, several magnetic films were plated onto the cathode in the apparatus of this invention and compared with magnetic films similarly plated in the prior art apparatus shown in FIG. 3. The --------------------------------------------------------------------------- films were plated from baths having the following compositions:

Bath Number 1 2 3 4 g./1 g./1 g./1 g./1 __________________________________________________________________________ Triton x- 100 0.8 0.6 -- 0.9 1.0 1.0 Sulfamic Acid -- -- Saccharin 1.0 -- 0.4 -- KNaC.sub.4 H.sub.4 O.sub.6.sup.. 4 H.sub.2 O 7.5 7.5 -- -- NiSO.sub.4.sup.. 6 H.sub.2 O 15.0 15.0 -- -- FeSO.sub. . 7 H.sub.2 O 2.0 2.5 -- -- CuSO.sub.4.sup.. 5 H.sub.2 O 1.25 -- -- -- NiCl.sub.2.sup.. 6 H.sub.2 O -- -- 109 -- FeCl.sub. 2.sup.. 4 H.sub.2 O -- -- 3.88 -- H.sub.3 BO.sub.3 -- -- 12.5 -- Na Lauryl Sulfate -- -- 0.2 -- Cu(NO.sub.3).sub.2 -- -- -- 100 Sulfuric acid (conc.) -- -- -- 30 ml./1 Formic acid (conc.) -- -- -- 40 ml./1 Acetic acid (anhydrous) -- -- -- 20 ml./ 1 __________________________________________________________________________

Platings from baths 1 and 2 were made using the "pulse plating" technique described in U.S. Pat. application Ser. No. 573,417, now U.S. Pat. No. 3,480,522 to James M. Brownlow, and having the same assignee as this application. The description of the pulse plating technique described in the above stated application is incorporated herein.

The films are plated at constant current without agitation for 10 to 15 second intervals. They can also be plated with shaped current pulses. After the plating intervals are completed, the bath is agitated and is allowed to come to rest for about 15 to 60 seconds. This sequence of steps is repeated until the desired film thickness is attained. All platings were performed in a magnetic field of 40 oersteds and a bath temperature of 20.degree. C. The films were plated on cathodes having an area of 3 .times. 3 inches. Films having thicknesses of from 1,000 A to 1,800 A were plated.

Plating from baths 3 and 4 were made using well-known continuous plating techniques, as opposed to the above described pulse plating technique. The solution was agitated continuously during the plating process.

At the completion of the plating operation, the magnetic films were tested for uniformity of composition, thickness and magnetic properties. Films from baths 1 and 2 were plated on 800 A of silver evaporated on 15 mil thick 3 .times. 3 inch glass substrates. Measurements of thickness, composition and of magnetic properties coercive force, anisotropy field, anisotropy dispersion, and magnetic skew were made on the 3 inches by 3 inches plates along the diagonal from corner to corner at convenient intervals. Exactly the same spot was used to take all the measurements. At least 10 spots were examined along the diagonal.

The composition and thickness were evaluated using 1 mm. .times. 3 mm. spot in connection with the X-ray fluorescence technique. Characteristic K radiation of Ni, Fe and Cu was monitored from which composition and thickness were determined.

Magnetic properties, H.sub.o, H.sub.k, .alpha..sub.90 and .beta., were measured using the Kerr magneto-optic technique which is well known in the art. A spot 3 mm. .times. 3 mm. in size was examined in each case. The spot selected was always the same spot which was previously examined using the x-ray fluorescence technique for film thickness and alloy composition.

Illustrative of uniformity of film thickness obtained from this invention is the plot of film thickness as measured along the diagonal of the film shown in FIG. 4. It is seen that the profile of the thickness along the diagonal of the film plated in the cell of this invention (represented by the heavily drawn line) is relatively uniform throughout the film, while the thickness of films obtained from the prior art cell (shown by the curve labeled prior art cell) is nonuniform in character.

Referring to FIG. 5, there is shown a comparison plot of the Ni, Fe and Cu composition measurements taken along the diagonals the plated films prepared by this invention and by the prior art. The plot is representative of measurements made on a large number of films. The heavily drawn lines are indicative of the relatively uniform compositions obtainable only in this invention. The more finely drawn lines are indicative of the nonuniform compositions obtained by the prior art.

Magnetic properties of the films prepared by this invention and by the prior art are shown in FIGS. 6 and 7. The heavily drawn lines are again representative of the uniformity of magnetic properties of films plated by this invention, and the finely drawn lines show the nonuniformity exhibited by films prepared by the prior art.

In summary, an apparatus for plating metal films having highly uniform thickness, composition and magnetic properties throughout the film has been devised. The apparatus is characterized by having an anode and a cathode arranged in a bath container such that the edges of the anode and cathode are substantially in contact with a dielectric material. The electrodes so arranged prohibit spreading of the current in the electrolyte along its path across the plates. Therefore, equipotential lines are formed parallel to both electrodes, current density is uniform and constant throughout the whole cathode area.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be under understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. An improved apparatus for electrodepositing metal films having substantially uniformity of thickness and composition having in combination a plating bath container to contain a plating bath disposed therein, having all four of its sides and base, fashioned from a dielectric material;

A pair of electrode means spaced apart in said bath container and in substantial contact with the walls thereof, being mounted within said container and said plating bath, one of said electrode means being a conductive block supported on the dielectric base of said container and being adaptable to receive a conductive substrate member for completing a current path across said electrode means;

non-conductive agitating means disposed in said container for providing uniform laminar flow of said bath across the surface of one of said electrode means;

said non-conductive agitating means being constructed so as to provide minimal resistance to the flow of said bath, thereby preventing turbulence therein; and

means for applying a current across said electrode means whereby a current path having a cross-sectional area which is substantially the same as the cross-sectional area along the length of said electrode means is provided.

2. An apparatus according to claim 1 wherein there is added support means for maintaining said electrodes in spaced apart relation.

3. An apparatus for electroplating metal films according to claim 1 wherein the sides of said non-conductive agitation means have sharp edges and the base portion thereof is triangular in shape so as to provide minimal resistance to the flow of said bath during agitation thereof.

4. An apparatus for electroplating metal films according to claim 1 wherein opposite walls of said container are recessed so as to support one of said electrodes.

5. An apparatus for electroplating metal films according to claim 1 including magnetic field generating means disposed outside of said container to provide a magnetic field of about 40 oe to thereby establish plane orientation in an electrodeposited magnetic film.