US3489660A - Electroplating bath and method - Google Patents

Description

United States Patent Oflice 3,489,660 Patented Jan. 13, 1970 3,489,660 ELECTROPLATING BATH AND METHOD Peter P. Semienko, Roslindale, and Emil Toledo,

Brighton, Mass., assignors to Honeywell Inc., Minneapolis, Minn., a corporation of Delaware No Drawing. Filed Jan. 3, 1966, Ser. No. 517,944 Int. Cl. C23b 5/08, 5/04, 3/06 US. Cl. 204-43 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates to methods of electroplating nickelous thin magnetic films and more particularly to a novel method for electroplating thin non-magnetostrictive nickel-iron films from a novel type electrolyte comprised of nickel ammonium sulfate and nickel sulfamate and ferrous ammonium sulfate ingredients arranged to provide a novel control over magnetic properties as well as improving convenience in selection of source ion materials and providing improved bath stability.

Thin magnetic films of nickel-rich alloys are commonly electroplated, especially for use in data processing apparatus. The magnetic properties of such films are commonly difiicult to control, especially for films which must have controlled magnetostriction. Such films are presently plated in electrolytes which are relatively unstable; as well as being somewhat expensive and inconvenient to prepare. The present invention is directed to providing an electroplating bath, for such nickelous magnetic films, which alleviates these problems.

Some magnetic nickelous film, especially Permalloy (nickel-iron; usually in -82 proportions) present a kind of dilemma as to selection of the source-ion ingredients. That is, simple compounds, such as simple sulfates, are relatively convenient to use as such ingredients being available as high purity reagent grade solids and quite stable during plating life, unfortunately yield a plated film, the micro-structure of which exhibits a great deal of undesirable stress; conversely, most complex sourceion compounds present relatively the opposite benefits and drawbacks. For instance, in plating Permalloy, one may, for convenience, use sulfates of nickel and iron which are readily available in reliable forms (AR grade purity, etc.) and are stable in solution, but which characteristically provide a high stress film. One the other hand, one might use nickel and iron sulfamate to provide a low stress film, but will find that these ingredients are relatively expensive and inconvenient to obtain as solids at a reliable quality and also are very apt to be unstable in solution. Iron sulfamate is especially unstable; for instance the free iron therefrom oxidizes all too readily, especially at higher bath pH and temperature. The invention provides a solution to this dilemma by prescribing nickel ammonium sulfate source-ion ingredients for electroplating nickel alloy films; especially in combination with ferrous ammonium sulfate for plating magnetic Permalloy. This complex (double) salt is unusual in that it is conveniently available in high grade lots and provides a very stable electrolyte.

In addition to the above-mentioned problems of convenience, stability andlow stress, plating thin magnetic films also presents some especially serious problems in controlling magnetic properties, such as coercivity, squareness, zero-magnetostriction, and the like. These characteristics are, in part, commonly related to the microstructure of the plated film (e.g. how the nucleation sites are started, how they build up during plating, etc.) and are often affected by small, otherwise trivial, changes in plating conditions, such as a shift in bath temperature, pH, specific gravity and the like. Since the novel nickel ammonium sulfate ba-se electrolyte mentioned above exhibits some such problems, it has been found that a nickel sulfamate additive thereto greatly improves control over magnetic properties without impairing stability. Therefore, it is preferred, according to the invention, to replace some nickel ammonium sulfate in the novel electrolyte with a major portion of nickel sulfamate. Nickel sulfamate can be so added in amounts which keep the plated alloy composition unchanged, but yet surprisingly provide unique control over the micro-structure and magnetic properties of the plated film. For instance, it has been found that varying the ratio of nickel sulfamate to nickel ammonium sulfate can have a pronounced control over squareness in a plated film, yet without changing the alloy composition. This rather surprising effect will be recognized as highly advantageous by those skilled in the art of plating magnetic films for computer memories. More particularly, replacing some of the nickel ammonium sulfate in a Permalloy plating bath with nickel sulfamate, according to a prescribed range of ratios, can yield a plated non-magnetostrictive alloy with high squareness where otherwise the same alloy would be markedly non-square-a critical change for certain applications.

An added advantage associated with replacing some nickel ammonium sulfate with nickel sulfamate is improved bath stability. The use of nickel sulfamate with nickel ammonium sulfate provides an advantageous means of including the desirable sulfamate ion in an electrolyte to achieve magnetic control advantages (e.g. over a purely sulfate bath) and the like, without the drawbacks of other prior art sulfamate sources. For instance, nickel sulfamate is cheaper and more readily available in reliably pure grades than iron sulfamate and, most important, is much more stable since it will not oxidize as readily; Whereas prior art sulfamate baths would typically have a pH of about 3.0-3.6 and thus readily oxidize free iron (such as from ferrous sulfamate, ferrous ammonium sulfate, etc.) complex sulfamated baths according to the invention can be kept at about 2.22.4 pH and thus are more stable. For instance, it has been found that with such sulfamated nickel ammonium sulfate baths oxidation of ferrous ammonium sulfate therein is negligible over an extended bath life. Nickel sulfamate also improves low temperature solubility of nickel in the novel ammonium sulfate bath. Thus, it has been found advantageous according to the invention to replace some nickel ammonium sulfate with a major portion of nickel sulfamate to improve stability and low temperature solubility; still keeping enough nickel ammonium sulfate present to prevent the bath from becoming unstable. It is preferred to keep enough nickel ammonium sulfate present to prevent bath pH from arising above about 3 pH, since iron ammonium sulfate begins to oxidize at a higher pH.

The above objects and features of novelty, as well as others described below and apparent to those skilled in the art, may be derived, according to one form of the invention, by providing a novel electrolyte for electroplating thin magnetic Permalloy films. This electrolyte consists essentially of an aqueous solution of nickel ammonium sulfate and nickel sulfamate in major amounts and in a prescribed ratio, according to the magnetic properties, stability and solubility required, and including ferrous ammonium sulfate in a minor amount, sufiicient to derive the prescribed nickel-iron alloy at the prescribed plating conditions; plus other customary electroplating constituents.

To provide the above-mentioned novel features and advantages according to the present invention, an electroplating process and novel electrolyte therefor will now be described for electroplating a thin magnetic nickeliron film onto a moving Wire substrate continuously. More particularly a small copper coated Wire will be drawn at a prescribed speed through the novel Permalloy plating electrolyte under prescribed plating conditions, first having preferably been electropolished, preferably in a novel sulfamic acid bath. A plated wire may be so devised having controlled magnetostriction, squareness, etc. to be applicable for use in magnetic memories, such as associated with data processing machines.

Prior to introduction, continuously, into the plating electrolyte, the wire substrate will have been prepared to exhibit prescribed dimensional, metallurgical and surface characteristics, such as having a coating of at least one-half mil copper polished to a prescribed smoothness of about #12. (:2) (STM rating). A 8 mil wire (diameter) having a beryllium-copper core has been found satisfactory for this, being introduced at the beginning of a continuous magnetic plating line by unwinding from a spool and drawn through each treating station successively at about 5 inches per minute. More preferably, the wire is first electrolytically drawn to remove imperfections on the surface of the raw beryllium-copper wire according to a novel treatment described in the co-pending commonly assigned application Ser. No. 518,013 to P. Semien-ko and E. Toledo entitled Metal Treatment incorporated by reference herein. It is also preferred that this drawn wire then be provided with the aforementioned copper coating according to a novel copper plating process as described in a co-pending commonly assigned application Ser. No. 518,184 entitled Improved Copper Coating by Seminko et al., the details of which are incorporated herein. Subsequent to such wire forming and copper plating treatments the wire is polished to a prescribed smoothness, such as about #12 to provide the proper magnetic characteristics when plated with a thin film. Preferably, this is effected by elcctro-polishing in a sulfamic acid bath as described below relative to Table IV. Subsequent to this electro-polishing the wire may be further continually advanced through a clean water rinse and thence to a magnetic plating station Where it is advanced through the aforementioned novel electrolyte to provide a thin magnetic film of a few microns thereon.

The wire is thus continually advanced, as stated, through the magnetic film electroplating station, being drawn through the novel electrolyte therein as described below, such as using the bath and plating conditions recited in Table III. It will be presumed that, besides the plating described below, the plating will be otherwise conducted as known by those skilled in the art, for instance using soluble anode means and, preferably, noniretallic fluid distributing means to distribute electrolyte homogeneously about the wire substrate. A preferred form of such distributing means accommodates high agitation, high current density conditions.

It was found that the novel electroplating electrolyte may take a number of forms. For instance, as noted in Table I, a number of nickel-rich alloys (over 75% nickel) may be electroplated with improved results from novel ammonium sulfate-containing electrolytes. For instance, as noted for baths A, B, C, and D, ammonium sulfate source ingredients may conveniently provide the indicated plated films comprised of nickel, nickel-iron, nickel-cobalt and nickel-iron-cobalt, respectively. Films were plated from baths A, B and C in batch processes and from bath D continuously on 5 mil wire. Several improvements were noted as regards these plating baths and the results derived therefrom. The baths were found to be stable over a long period of time, for instance, maintaining a prescribed accurate composition over periods of upwards from about nine months, without requiring addition agents to adjust the composition, pH, etc. thereof.

TABLE I Bath Parameter Code A B C I) (1) Nickel ammonium sulfate, gm. 250 250 250 250 (2) Nickel sulfamate (82 oz./gal.), L.- (3) Ferrous ammonium sulfate, gm- 0 20 0 8 (4) Cobalt ammonium sulfate, gm--- 0 0 100 8 (5) Saccharin, gm 0.4 .4 4 4 (6) Sodium lauryl sulfate, gm. 0. 1 1 1 1 (7) Boric acid, gm 6O 60 (8) Water, L To 1.0 1. l) 1. 0 1. 0 L, (9) Bath temperature, 0-- 90 90 90 (10) pH 2. 2 2. 2 2. 3 2. 3 (11) Current density (ma/cm exemplar -400 80-400 80-400 80-400 Batch Batch Batch Continuous Plated alloy Ni 50 N1, 50 Fe 80 Co, 20 Ni 78 Ni, 18 Fe, 4 0

TABLE II.(PARAMETER CODE AS TABLE I) 1 2 3 4 5 6 T1 T2 T3 T4 340 360 430 450 750 800 300 135 75 1. 8 1. 6 1. 7 1. 8 1. 6 1. 6 0 300 56 53 53 54 52 45 8. 5 7. 0 7. 0 8. 5 20 20 23 23 22 30 4 4 4 4 1.6 1.6 1.8 1.8 2.0 0.5 .1 .1 .1 .1 288 320 330 330 405 450 70 45 to 60 60 6O 6.4 6.4 6.4 6.4 6.4 6.4 1.0 1.0 1.0 1.0

Max. 85 0., Pref. to 69 C. 85 65-70 C.

2.0-2.8 (Pref; 2.2-2.4) 2.0 2. 2 2. 2 2. 3 11 400-500 350 350 350 350 Control of magnetostriction Average Better Best Best For plating the above-mentioned 8020 nickel-iron thin magnetic films, however, it is preferred to use modifications of the general bath indicated in Table I, namely baths 1 through 6 indicated schematically in Table II wherein a portion of the nickel ammonium sulfate source ion ingredients are replaced by a prescribed major amount of nickel sulfamate as aforementioned. The plating con ditions obtaining in Table II will be presumed to be those above-mentioned for continuously electroplating a cOpper-surfaced wire of about 58 mils, advanced continuously at about 5 inches per minute etc. Baths 1 through 6 are preferred, bath #6 being the most preferred and bath #1 the least preferred, while baths T1 through T4 are somewhat more experimental. Baths 1 through 6 exhibited an extremely high degree of stability requiring minimal adjustments thereof over a 9 month period. for instance, one parameter of stability is oxidation of iron ions detected by analysis for free Fe for instance. Fe++ was found free and unoxidized even after several months use of these baths, as opposed to what has been noted with prior art sulfamate baths, wherein the Fe++ would become oxidized after just a few days use. More particularly, bath #1 required somewhat more frequent adjustments (e.g. because a Slight positive or negative magnetostriction was noted on the plated film). Such minor stability shifts may be caused, for example by slight evaporation or by plating out of the source ion constituents, thus modifying the operating (plating) range of the bath. However, all of these baths can tolerate relatively wide changes in concentration of iron and nickel ions and are also considerably less sensitive to changes in temperature, specific gravity (evaporation) and ratio of iron to nickel than prior art baths. Using baths 1 to 6, relatively-stress-free magnetic thin films of about 81 nickel/ 19 iron(:0.1%) and having practically zero magnetostriction were derived. A feature of these sulfamate-ammonium sulfate baths was that they permitted platin gat a relatively cool bath temperature (vs. pure ammonium sulfate baths), namely about 69 C. and also plated relatively quickly, using reasonable high current density levels. The low optimum pH range thereof (2.22.4) also improved stability and prevented any significant oxidation of ferrous source-ions. For baths 1 through 6 the substrate wire was 5 to 6 mils diameter (5 mil were electrolytically drawn to about 4.0 mils and having about 2 microns copper electroplated thereon); the magnetic film being plated thereon to thicknesses from 0.75 to 1.55 microns (115%).

The preferred conditions for plating thin Permalloy magnetic films as indicated above, using the af rementioned nickel ammonium sulfate/nickel sulfamate electrolytes are summarized in Table III below including the ranges and preferred conditions therefor.

Table III-Bath ranges Nickel ammonium sulfate concentration-50-300 gm./ 1.

Nickel sulfamate concentrationsufficient for plated alloy at plating condit. up to stability limit, e.g.: 100-300 gm./l. at 6570 C. 05 gm./l. at 85 C.

Nickel sulfamate for Perm no at Nickel Ammonium Sulfate- 8 N a y es-wof- 19 (i0 1%) 1 260 ml. 0 mat about 0.

Ratio 1 4 2 Pref. e.g..for

Bath temperature (C.)-6570 for low magnetostriction pH about 2.0-2.8 (pref. 2.2-2.4)max. 3.4 Plated thickness (microns)-0.52.0 (exemplary) 78-82 Ni 81 Ni Plated alloys m prefer. T9178 (i0.1%)

With a controlled bath temperature (preferably close to 65 C.) a prescribed range of ratios of nickel sulfamate/nickel ammonium sulfate will be required to control the alloy composition, such as to the zero magnetostrictive alloy (81 nickel/19 iron). However, within this range these ratios may be adjusted to produce variations in microstructure, magnetic properties and the like, surprisingly with no change in alloy composition. For instance, While a 1/4 ratio can yield this alloy, a 3/ 1 ratio can improve the fine smooth grain structure of the same alloy and also radically alter its squareness for magnetic purposes. As a rule of thumb, the amount of nickel ammonium sulfate used may be established for the solubility thereof at a prescribed bath temperature range, with nickel sulfamate added thereto to derive the required structure and dependent magnetic properties. And, of course, once the nickel ion concentration has been selected this will dictate the amount of ferrous ammonium sulfate (or other source ion ingredients) sufficient to provide iron of the proper amount in the plated alloy under the prescribed plating conditions, as known by those skilled in the art. Ferrous ammonium sulfate is preferred because it is conveniently available in high grade, inexpensive forms. Ferrous ammonium sulfate is also more stable than other such ingredients, for instance, being much more stable and less subject to oxidation than ferrous sulfamate. This is especially true at higher pH, such as at about 3, although some oxidation will occur even with ferrous ammonium sulfate above 3.4 pH. Those skilled in the art will also appreciate the advantages of operating at the low indicated pH and low bath temperatures, while still maintaining a reasonable plating efficiency.

As before mentioned it is preferred that the copper covered wire substrate be electro-polished in a sulfamic acid bath prior to introduction to the electroplating station. Thus, for instance, the copper plated wire aforementioned may be continually advanced through an electropolishing bath where the copper finish may be finally smoothed between prescribed min/max. limits and also be sensitized for subsequent magnetic plating in the aforementioned novel sulfate/sulfamate electrolyte. This electro-polish may be performed preferably by a smoothing electrolyte such as indicated in Table IV (Examples I, II, and III thereof) below. A novel sulfamic electro-polishing bath is provided according to the invention, both to polish more smoothly and efiiciently, and also to reduce contamination of the substrate for subsequent plating. For instance, eliminating the sulfamic acid constituent from a phosphoric acid polish bath has been found to induce the formation of undesirable oxidation sites which will prevent plating thereon causing dropouts. Similarly, using sulfuric acid alone corrodes the copper layer catastrophically, leaving intolerable discontinuities therein. The sulfamic type baths act to reduce the activity of the polishing bath and inhibit post-copper-plating oxidation (which degrades subsequent magnetic plating). Thus, the sulfamic polishing baths provide the best control over plate-able surface finishing at a minimum loss of plated copper thickness. For instance, they can produce a reproducible surface leveling of from 1 to 300 micro-inch RMS for dropout free magnetic plating. The preferred electro-polishin'g conditions are indicated for Examples I-III below wherein it will be presumed that the abovementioned on-line wire treating conditions apply, such as advancing the wire at about five-inches per minute and wherein the cell used is understood to include a cylindrical lead polishing cathode, as known in the art.

(a) saccharin in a concentration of 0.4 to 4.0 g./l., (b) sodium lauryl sulfate in a concentration of 0.1 to

TABLE IV Examples I II (Pref) III Range Bath:

Orthophosphoric acid 100 300 400 200-400 1111. Water 300 200 100 100-300 ml. Sulfamic acid 1-15 (Pref. 6) 1-10 (Pref. 4) 1-5 (Pref. 2) 1-15 gm. (Pref. 2-6 gm.).

(B ath at room temperature, current density/time immersed, 2-50 Ina/cm. (Pref. 12); for about 50 sec.)

1 Under about 50 gin/L. water.

Sulfamic acid may be used up to the solubility limits of concentration to maintain smoothness, but about gm. sulfamic acid per liter water is preferred.

The above polishing steps have achieved a surpiising smoothness when used with the copper plated wire aforementioned, reducing roughness a predetermined controlled amount for instance from #40 (STM smoothness; micro-inches, peak-to-peak) to as little as #1. Any desired smoothness on the order of up to 3% of a typical plated thickness (about one micron, i.e. 40 microinchcs) has been achieved. For instance, with Example III above, a current density of 50 rna./cm. will level a 4 micron copper coating on 5 to 8 mil wire to about #4 STM roughness, reducing its thickness only about 1 micron.

The acid concentrations and other polishing conditions may be varied as understood by those skilled in the art. This electro-polishing step may also be applied to other metal (coated) substrates from the copper family, such as copper alloys, silver alloys etc. It is not applicable for metals like nickel, iron and their alloys, however.

The copper-plated, electro-polished plated wire is now ready for use, e.g. to be provided with a magnetic thin film of controllable properties, according to the invention. For instance, the wire may be further continually ad vanced thrOllgh a following clean water rinse and thence to the above magnetic plating station for providing a thin magnetic film of a few microns, by electroplating a nickeliron magnetic film from the above sulfamate electrolyte.

It will be apparent to those skilled in the art that the principles of the present invention may be applied to different embodiments from that shown; for instance to other types of substrates for improving the magnetic properties of films plated thereon. Likewise, the described electro-polishing step may be used to smooth other similar types of substrates. While in accordance with the provisions of the statutes, there have been illustrated and described the best forms of the invention known, it will be apparent to those skilled in the art that changes may be made in the condition of the processes disclosed without departing from the spirit of the invention as set forth in the appended claims and that, in some cases, certain features of the invention may be used to advantage without a corresponding use of other features.

Having now described the invention, what is claimed as new and for which it is desired to secure Letters Patent 1. An aqueous bath for the electrodeposition of an 8020 nickel iron magnetic information-storing alloy film, said bath (a) comprising nickel source material consisting essentially of 1) nickel ammonium sulfate in a concentration of 50 to 300 g./l., and

(2) nickel sulfamate in a concentration of 100 to 300 g./l., with the weight ratio of said nickel source materials being between about 1 to 4 and 4 to 1,

(b) comprising iron source material consisting essentially of ferrous ammonium sulfate in a concentration of 7 to 9 g./l., and

(0) having a pH substantially between 2.0 and 2.8.

2. An aqueous bath as defineed in claim 1 further comprising 0.4 g./l., and

(c) boric acid in a concentration of 30 to 70 g./l.

3. A method of electroplating a non-magnetic substrate with a substantially -20 nickel-alloy magnetic infformation-storing film, said method comprising the steps 0 (A) preparing an aqueous electrolyte from iron source material and nickel source material in amounts to plate said '8020 nickel-iron film, with said iron source material consisting essentially of ferrous ammonium sulfate and with said nickel source material consisting essentially of nickel ammonium sulfate and nickel sulfamate, providing said nickel ammonium sulfate in an amount to provide said bath with a pH not substantially above 3, and

(B) plating said film onto said substrate from said electrolyte.

4. A method as defined in claim 3 comprising the further step of maintaining said electrolyte at a temperature around 65 C. during said plating step.

5. A method as defined in claim 3 comprising the further step of maintaining said electrolyte at a temperature substantially between 65 C. and C. during said plating step.

6. A method as defined in claim 3 in which said electrolyte is prepared with said nickel ammonium sulfate being present in an amount to maintain the bath pH substantially between 2.2 and 2.4.

7. A method as defined in claim 3 in which said electrolyte is prepared with the weight ratio of said nickel ammonium sulfate to said nickel sulfamate being between about 4 to 1 and 1 to 4.

8. A method as defined in claim 3 further characterized in that the film is plated onto said substrate from said electrolyte with a current density substantially between 300 and 600 milliamperes per square centimeter.

9. A method of preparing a plated wire memory element having an 80-20 nickel-iron magnetic film plated onto a conductive copper bearing surface, said method comprising the steps of (A) preparing an aqueous bath for electroplating said nickel-iron film from nickel source material and iron source material in amounts to plate said 80-20 nickel-iron alloy therefrom, with said iron source material consisting essentially of ferrous ammonium sulfate and with said nickel source material consisting essentially of nickel sulfamate and nickel ammonium sulfate with said nickel ammonium sulfate being present in an amount to maintain the pH of said bath substantially between 2.0 and 2.8 and with the Weight ratio of said nickel ammonium sulfate to said nickel sulfamate being substantially between 4 to 1 and 1 to 4, and

(B) electroplating said film onto said substrate from said electrolyte.

10. The method defined in claim 9 (A) comprising the further step of maintaining said electrolyte at a temperature substantially between 65 C. and 85 C. during said plating, and

(B) in which said film is plated onto said substrate with a current density between 300 and 600 milliamperes per square centimeter.

11. The method defined in claim 9 in which said bathpreparing step includes (A) introducing said nickel ammonium sulfate in a concentration of 50 to 300 g./ 1., (B) introducing said nickel sulfamate in a concentration of 100 to 300 g./l., and (C) introducing said ferrous ammonium sulfate in a concentration of 7 to 9 g./l. 12. The method defined in claim 11 in which said bathpreparing step further includes (A) introducing saccharin in a concentration of 0.4

to 4.0 g./l., (B) introducing sodium lauryl sulfate in a concentration of 0.1 to 0.4 g./l., and (C) introducing boric acid in a concentration of 30 to References Cited UNITED STATES PATENTS 116,579 7/1871 Farmer 204-49 XR 1,531,140 3/ 1925 Schneider 204-43 XR 2,254,161 8/1941 Waite et a1. 204-49 OTHER REFERENCES Metal Finishing, p. 30, July 1962. Bartelson, B. I. et al.: Electrodeposition of Ni-Fe 10 Films, IBM Tech. Disclosure Bulletin, vol. 3, No. 2, p.

63, July 1960.

Berger, P.: Electrolytic Polishing of Brass Pressing, pp. 72-77, Metal Finishing, December 1948.

The Metal Industry, p. 5, Jan. 1, 1943.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R.