US3702263A - Process for electrolessly plating magnetic thin films - Google Patents

Abstract

A PROCESS FOR ELECTROLESSLY PLATING SUBSTRATES WITH THIN FILMS OF MAGNETIC MATERIAL WHICH IS CARRIED OUT IN A TEMPERATURE RANGE WHICH INCLUDES ROOM TEMPERATURE. THE PROCESS INCLUDES THE IMMERSION OF SUBSTRATE IN AN AQUEOUS SOLUTION WHICH CONTAINS IRON IONS AND AT LEAST ANOTHER METAL ION SUCH AS NICKEL, WHICH UPON DEPOSITION FORM THE MAGNETIC MATERIAL. A REDUCING AGENT SUCH AS SODIUM HYPOPHOSPHITE IS PROVIDED IN THE BATH IN AN AMOUNT SUFFICIENT TO CAUSE DEPOSITION OF THE METALS IN SMALL GRAIN SIZES. SUFFICIENT HYDROXYL IONS ARE ALSO PROVIDED TO MAINTAIN THE BATH IN CONDITIONS WHICH RANGE FROM SLIGHTLY ACIDIC TO HIGHLY ALKALINE. THE BATH IS THEN MAINTAINED IN A TEMPERATURE RANGE WHICH INCLUDES ROOM TEMPERATURE OVER WHICH RANGE THIN FILMS HAVING A CHARACTERISTIC SILVERY FINISH INDICATIVE OF SMALL GRAIN SIZE ARE PRODUCED. THE PROCESS ALSO INCLUDES THE STEP OF AGING A FERROUS BATH FOR A PERIOD OF TIME SUFFICIENT TO CONVERT THE FERROUS IONS TO STABILIZE THE BATH. THE ABOVE-MENTIONED CONSTITUENTS AND OTHER COMPLEXING AND BUFFERING AGENTS WHICH PREVENT THE PRECIPITATION OF THE METALS FROM SOLUTION ARE PRESENT IN THE SOLUTION IN SELECTED CONCENTRATIONS WHICH PERMIT THE PLATING OF SMALL GRAIN SIZE MAGNETIC FILMS AT LOW TEMPERATURE.

Description

Nov. 7, 1972 p. w. HALL ETAL 3,7G2,263

PROCESS FOR ELECTROLESSLY PLATING MAGNETIC THIN FILMS Filed Feb. 20, 1970 FEGW OERSTEDS g:

THICKNESS A (iii) Fe AT65C Fe Fe (LL) Fe AGED 2 HOURS INVENTORS DAVID w. HALL JOHN A. LINDHOLM 10- LUBOMYR T. ROMANKIW O' ARNOLD F. SCHMECKENBECHER l I 1 5,000 10,000 0 15,000 20,000 Y THICKNESS A 671m H63 ATTO NEY United States Patent O 3,702,263 PROCESS FOR ELECTROLESSLY PLATING MAGNETIC THIN FILMS David W. Hall, Poughkeepsie, John A. Lindholm, Newburgh, Lubomyr T. Romankiw, Millwood, and Arnold F. Schmeckenbecher, Poughkeepsie, N.Y., assignors to giternational Business Machines Corporation, Armonk,

.Y. Continuation-impart of abandoned application Ser. No. 634,531, Apr. 28, 1967. This application Feb. 20, 1970, Ser. No. 13,233

Int. Cl. H01f /00 US. Cl. 117-240 2 Claims ABSTRACT OF THE DISCLOSURE A process for electrolessly plating substrates with thin films of magnetic material which is carried out in a temperature range which includes room temperature. The process includes the immersion of a substrate in an aqueous solution which contains iron ions and at least another metal ion such as nickel, which upon deposition form the magnetic material. A reducing agent such as sodium hypophosphite is provided in the bath in an amount sufficient to cause deposition of the metals in small grain sizes. 'Sufficient hydroxyl ions are also provided to maintain the bath in conditions which range from slightly acidic to highly alkaline. The bath is then maintained in a temperature range which includes room temperature over which range thin films having a characteristic silvery finish indicative of small grain size are produced. The process also includes the step of aging a ferrous bath for a period of time sufiicient to convert the ferrous ions to stabilize the bath. The above-mentioned constituents and other complexing and buffering agents which prevent the precipitation of the metals from solution are present in the solution in selected concentrations which permit the plating of small grain size magnetic films at low temperature.

CROSS-REFERENCE TO OTHER APPLICATIONS This application is a continuation-in-part of application Ser. No. 634,531, filed Apr. 28, 1967, entitled Process for Electrolessly Plating Magnetic Thin Films, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to a process for plating magnetic materials on substrates. More specifically, it relates to a process, operable in a low temperature range which includes room temperature, for plating magnetic thin films having isotropic or anisotropic characteristics. The magnetic thin films resulting from the process of this invention have values of coercivities which are very low and which heretofore were only obtainable by electroplating, vacuum evaporation or sputtering. Such low coercivity films have application as fiux closure paths between double layered film memory elements which are presently coming into wide use.

Description of the prior art The electroless deposition of films having magnetic properties has been accomplished by the prior art within the recent past and there are many techniques which are capable of producing such films. The majority of these electroless plating techniques, however, are designed to operate at temperatures which exceed 40 C. UJS. Pat. 3,178,311, issued Apr. 13, 1965, in the name of L. Cann and assigned to the Bunker-Rama Corporation, Stamford,

Conn., deals with an electroless plating process for depositing a metallic phosphide coating on a base member at room temperature.

The above patent shows an electroless plating bath which incorporates very high sodium hypophosphite concentrations (-100 g./l.) in addition to E.D.T.A. and formaldehyde. The E.D.T.A. is utilized as a complexing agent while the formaldehyde and hypophosphite combined are utilized as reducing agents. The bath provided by 11.5. Pat. 3,178,311 is not known to provide films which have magnetic properties which are suitable for use in the film memory environment but rather it is directed to producing coatings which have high conductivity coupled with good characteristics of adherence and hardness.

Other baths and processes, however, are known which are specifically directed to producing metallic films which have magnetic characteristics which permit their use as memory elements in the chain-like memory environment. One such process is the subject of US. Pat. 3,385,725, Ser. No. 353,849, filed Mar. 23, 1964, to Arnold F. Schmeckenbecher and assigned to the same assignee as the present application. In this patent, Permalloy type films are grown by a chemical reduction process employing an electroless solution, wherein the hypophosphite ion concentration is maintained below 7.00 grams/ liter and the pH of the solution is adjusted to at least 8. Also, the plating bath is operated at a temperature in the range of 50-9'5 C. The patent indicates that where the pH is lower than 8, that very little iron is deposited in the film notwithstanding the relatively high concentration of iron in the solution. Optimum results are attained using the bath of the Schmeckenbecher patent when the initial pH is adjusted to 10 or above.

Another bath and process specifically directed to producing metallic films having magnetic characteristics is the subject of US. Pat. 3,372,037, Ser. No. 468,355, filed June 30, 1965, to Richard S. McGrath and Norman W. Silcox. In this patent, the electroless solution is formed from iron salts, which upon ionization yield only ferric ions. The ferric ions in conjunction with selected concentrations of nickel ions, hypophosphite ions, tartrate ions and ammonium molecules, provide a magnetic film which is capable of withstanding stress to a much higher level than that observed with conventionally obtained films. The films are obtained in a temperature range of 65 -75 C. The magnetic thin films of this patent appear as an agglomeration of spheres with a diameter of the order of about 1000 A. or larger.

In addition to the above-mentioned patents, there is a plurality of issued patents all of which are directed to achieving electroless plating of films with desired magnetic properties. These patents, like the above-mentioned patents, all operate at temperatures in excess of 40 C., or like the Cann patent mentioned hereinabove use baths with large number of ingredients which require ranges of concentrations which do not result in films with good magnetic properties suitable for operation in the memory environment. Magnetic properties such as low coercivity, heretofore obtainable only by electroplating, vacuum evaporation or sputtering, are not attainable in electroless baths which are operated at temperatures in excess of 40 C. Further, by operating at temperatures which do not exceed 40 C., a variety of substrates formerly not usable because they were subject to deformation at high temperatures are available.

SUMMARY OF THE INVENTION The method of the present invention in its broadest aspect, comprises the steps of formulating an aqueous electroless plating bath containing at least a metallic salt selected from the group consisting of nickel salts and ferric salts in concentrations sufficient to plate a single metal or combination of metals on a substrate; a reducing agent such as sodium hypophosphite in concentrations suificient to reduce the metal or metals at a rate which provides depositions of small grain size, a source of hydroxyl ions such as ammonium hydroxide to provide hydroxyl ions in amounts sufiicient to maintain the bath in conditions which range from slightly acidic to highly alkaline, maintaining the bath in temperature range in which a silvery finish indicative of small grain size and lack of agglomeration in the deposited metal film is obtained; and immersing a substrate upon which a film is to be deposited in the bath for a time sufiicient to provide a desired film thickness.

In accordance with a more particular aspect of the invention, the process utilizes an electroless plating bath useful in plating with a metal or a combination of metals having magnetic characteristic which consists of an aqueous solution containing nickel (Ni++) ions in concentrations which range from 0.020 to 0.35 mole per liter, ferric (Fe+++) ions as ammonium sulfate salts in concentrations of 0.011 to 0.35 mole per liter, hypophosphite ions in concentrations of 0.105 to 1.0 mole per liter, tartrate ions in concentration of 0.060 to 0.70 mole per liter and ammonia is added to the solution in amounts to adjust the pH of the bath in a range of 7-13. The temperature of the bath is regulated to be in the range of 10 to substantially 35 C. Using the above ranges of constituents, a substrate may be immersed in the bath for periods varying between 5 minutes and 400 minutes to produce film thicknesses which vary from 300 to 20,000 A. or for longer periods of time to produce greater thicknesses. Where both nickel and iron ions are used in the bath, magnetic films having 80% nickel to iron compositions are obtainable. Films of this composition are known to have the characteristic of zero magnetostriction. The resulting films can have both isotropic and anisotropic characteristics. The latter characteristics are obtainable by plating the substrate in the presence of a magnetic orienting field. Utilizing the above outlined ranges of constituents and regulating the temperature in the range defined, metal films are obtainable which have coercivities which heretofore were only obtainable using electroplating, vacuum evaporation or sputtering techniques. In addition, the process includes the step of aging a ferrous bath of the same composition as the ferric bath outlined above for a time sufiicient to cause the conversion of the ferrous ions of the ferrous bath to the ferric species. Aging times of 12-60 hours have been found appropriate. Heating the ferrous bath speeds the conversion and reduces the aging time.

The above films further contain larger amounts of phosphorus compared, for example, to the .25 to 2% by weight phosphorus present in the films of the McGrath patent cited above. The phosphorus contents may be as high as 8% by weight. The films, however, have much lower coercivity (H much better anisotropy, easy axis loop squareness, and much higher (B /B ratio. Compared to McGraths films, the hard axis loop is completely closed, while the easy axis loop shows a perfect square. The easy axis dispersion of the films is much lower than in the high temperature films of the type described by Mc- Grath and is comparable with the easy axis dispersion in electroplated, evaporated, or sputtered films. In addition, the films can be produced in a magnetically inverted state (H being greater than H with the easy axis dispersion remaining very low. Such films cannot be obtained according to the processes taught by McGrath et al. or by Schmeckenbecher.

It is, therefore, an object of this invention to provide an improved electroless plating process.

Another object is to provide an improved electroless plating process which operates in a temperature range which includes room temperature.

Another object is to provide an improved electroless 4 plating process the use of which provides films having magnetic characteristics.

Still another object is to provide an improved electroless plating process which provides depositions which have coercivities and magnetic easy axis dispersion heretofore not obtainable by electroless methods.

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

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric cross-sectional drawing showing an elongated substrate with thin magnetic films plated on opposing flat surfaces thereof and showing flux closing films joining the films at their edges.

FIG. 2 shows a graph depicting the variation of coercive force with film thickness for electroplated films, for films electrolessly plated from baths using ferric (Fe+++) ions in accordance with the teaching of the present invention, and for films electrolessly plated from a ferrous bath at temperatures in excess of 40 C.

FIG. 3 shows the variation in composition with thickness of a deposited film using aged ferrous (Fe++) and ferric (Fe+++) ions in an electroless plating bath of the present invention at low temperatures and a prior art ferrous (Fe++) ion containing bath at 65 C.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with a preferred embodiment of the process of this invention, thin magnetic films can be applied to substrates by following the detailed steps outlined hereinbelow. The process of the present invention is based upon the controlled autocatalytic reduction of the nickel and iron by a reducing agent, the hypophosphite anions. Nickel-iron-phosphorus alloys are chemically deposited from the electroless solution by placing into contact therewith substrates which are composed of copper, nickel, cobalt, iron, steel, aluminum, zinc, palladium, platinum, brass, manganese, chromium, molybdenum, tungsten, titanium, tin, gold, silver, carbon or graphite or alloys containing combinations thereof. The catalytic properties of these materials which are inherent or activated by techniques hereafter discussed, brings about a reduction of the nickel and iron to the nickel-ironphosphorus alloys by the hypophosphite. Of course, it will be realized by those versed in the art that noncatalytic surfaces such as non-metallic materials are amenable to the process provided the surface of the noncatalytic material is sensitized or activated, such as by forming a film of one of the catalytic ions on the surface thereof. This is accomplished by a variety of techniques which are well known to those skilled in the art.

When electrolessly plating nickel and iron in the alkaline solution, the presence of compound-forming water soluble nickel complexes is necessary in order to prevent the precipitation of the nickel as a hydroxide or hypophosphite. The precipitation is avoided with the addition of sufiicient ammonia molecules in the form of ammonia salts which form the nickel hexamine complex ion. To arrest the precipitation of the ferric ion and ferrous ions, tartrate ions are added to the solution.

It will be recognized by those versed in the art that other complexing or sequestering agents are usable in place of the ammonia and tartrate ions. These include organic complex-forming agents containing one or more of the following functional groups: primary amino group (NH secondary amino group NH), tertiary amino group N-), imino group (=NH), carboxy group (COOH), and hydroxy group (OH). The preferred agents are rochelle salt, seignette salt, tartaric acid, ammonia, ammonium hydroxide, and ammonium chloride. Related polyamines and N-carboxy methyl derivatives thereof are also usable in the process. However, cyanides, on the other hand, are deleterious in that the plating process does not function in their presence. The nickel and iron are added in the form of any water soluble salt, the criterion being that the salt is not antigonistic to the plating solution and that it furnishes nickelous cations and ferric cations.

In carrying out the electroless plating process, the article that is to undergo plating, that is, the catalytic surface, is properly prepared by mechanical cleaning and degreasing techniques in accordance with standard practice in the industry. If the surface that is to receive the electroless deposit is formed of copper or copper alloy, the article is then further cleaned by immersing the same in a 10% solution of hydrochloric acid for about 30 seconds at room temperature. It is then immersed in a 0.1% to 0.01% palladium chloride solution for about to 15 seconds at room temperature. As a result of the exchange reaction palladium is deposited on the catalytic surface. In the next step the surface is dipped in a NaH PO solution free of metal ions and is allowed to remain in it until hydrogen bubbling occurs. Palladium acts as a catalyst to initiate the reduction of the nickel and iron by the reductant in the metal plating solution which is the hypophosphite.

The catealytic surfacethat terminology is hereafter used to include those materials which contain surfaces which are inherently catalytic in the electroless solution, or are made to behave as such by well-known techniques in the artis exposed to the electroless plating solution for a sufficient period of time to permit the formation of a nickel-iron-phosphorus alloy on the surface thereof. To induce a direction of easy magnetization or preferred anisotropy, the electroless plating is conducted in the presence of a magnetic field. Isotropic properties, that is, magnetic properties that are the same in every direction along the surface of the film, are also available with the electroless plating technique in accordance with the invention. As those versed in the art will recognize, the external field is not applied when isotropic properties are sought.

Referring now to FIG. 1 there is shown a thin film memory element 1 having upper and lower films 2, 3, respectively which have been plated onto the upper and lower surfaces of copper substrate 4. For purposes of this description, it is assumed that films 2, 3 have been electrolessly plated or electroplated in an orienting magnetic field to give them desired anisotropic characteristics and all that remains is to provide a flux path closure between films 2, 3 by means of fiux closure films 5, 6. Flux closure films 5, 6 should have high permeability and as low a coercivity as possible. Under the usual circumstances where a plurality of elements 1 are disposed on a substrate in close spaced relationship, it is not possible to obtain the required low coercivity film by vacuum evaporation or sputtering because the deposition of a uniform thickness film is extremely difficult if not impossible where the deposition surface is not flat. Films of the desired low coercivity can be attained by an electroplating technique but only Where the length of the memory element is short. Where the memory element is long so that the resistance along its length exceeds several ohms, variations in the composition will be obtained along the length of the memory element due to potential drop. The thickness or the composition of the resulting film is not affected in the above described way in an electroless plating bath. The c0- ercivity of the resulting film obtained in an electroless plating bath, however, does appear to be temperature dependent and will be shown heerinbelow, values of coercivity below 3 oersteds cannot be attained by high temperature (above 40 C.) electroless plating baths for film thickness in the 1000-5000 A. range.

After appropriate cleaning and preparation well known to those skilled in the plating arts, memory element 1 is immersed in a plating bath for a time suflicient to provide a desired thickness offilm. ' Fihns 2, 3 are shielded from further plating by coating them with a coating of photoresist or other material which cannot be plated upon so that only the edges of element 1 are available for plating. The composition of the electroless solution contains chemical species and constituents in the concentrations shown in the following Table I.

It should be appreciated that the examples shown in Table I are given by way of illustration and explanation only and are not intended in any way to limit the inventive contribution to the specific examples set forth. The preferred minimum and maximum concentrations for each chemical specie or ion constituent is given in Table I.

As brought out in the examples above, the nickel to iron ratio [Ni++/Fe+++] varies from about 5/1 to about 1/1; the hypophosphite anion has a lower limit of about 10- moles per liter and an upper limit of about l000 10- moles per liter; the pH may vary from 7.0 to essentially 13. It is also to be noted that the table, includes as complexing agents ammonia, ammonium and tartaric salts. In this regard, the complexing agent need only be capable of forming a stable water-soluble complex with nickel and iron and is preferably selected from the group consisting of ammonia and organic complexforming compounds having at least one functional group selected from the group consisting of amino, imino, carboxy, and hydroxy radicals in concentrations ranging fromabout 60X l0 moles per liter to about 70(l 10- moles per liter. The electroless deposition reaction is conducted at temperatures varying from about 10 C. to about 35 C. but desirably at a temperature between 20 C. to 25 C. and preferably at about 23 C. Lastly note that the nickel ions vary from about 20.0 10' moles per liter to about 350 10 moles per liter while the iron ions vary from about 11.0 10- moles per liter as a minimum to about 350 10- moles per liter but preferably the former is maintained at a concentration of about 59.0)(10 moles per liter and the latter at about 12.0 10- moles per liter.

After immersion in the temperature range of 1035 C. for a length of time to provide a deposition of a desired thickness, flux closure films 5, 6 couple the edges of films 2 and 3 together to form a closed magnetic circuit. Films 5, 6 may have either isotropic or anisotropic characteristics depending on whether or not a magnetic field is applied during plating. In the present application, an external field is preferably applied to establish an axis of magnetization. This is the easy axis and, when no external field is present, the magnetization lies along the easy axis. In a storage element, the so-called hard axis is orthogonal to the easy axis.

Flux closure film 5, 6 as will be shown hereinbelow for film thicknesses in the vicinity of 1,500 A. and above have coercivities of 2 oersteds and less where ferric (Fe+++) ions are introduced into the plating bath when plated on a very smooth surface. For the same film thicknesses similar values of coercivity can be obtained by introducing ferrous (Feions into the bath and aging the bath. The hypophosphite reducing agent is added either before aging and then after aging is supplemented with an additional quantity of the reducing agent. In other alternatives the reducing agent is added only after aging the bath. The low coercivities are obtainable using the formulation of the present invention by regulating the bath temperature in a range of 10-35 C.

Referring now to FIG. 2, there is shown a log-lo g graph ferric ammonium sulfate or by aging a bath containing ferrous ions for a period of time prior to use. Also shown is a plot of thickness vs. percent iron deposited for a ferrous ion bath operated at 65 C. The type of bath of coercive force (H in oersteds vs. film thickness in utilized was similar to those shown in the above-menangstroms A. for (A) Ni-Fe-P films electrolessly plated at tioned Schmeckenbecher patent.

65 C., from a Ferrous bath, (B) a Ni-Fe-P film elec- Before discussing FIG. 3, it should be appreciated that trolessly plated at 25 C. in a bath according to the present bath stability has been recognized as a problem in the invention using ferric (Fe+++) ions, (C) Ni-Fe films electroless plating art in general and, in particular, in electroplated in accordance with a prior art teaching and, the ctroless plating of magnetic materials where com- (D) Ni-Fe-P films electroplated in accordance with a position of the films must be accurately controlled. The prior art t hi prior art went to great lengths to prevent the oxidation Th plot of curve (A r lt d f a film l t d in of ferrous to ferric believing the latter ion to be deleteriaccordance with the teaching of the above-mentioned 0118 to P p hath Operation Recently, however, ferric Schmeckenbecher patent at a temperature of 65 c. A ions the above-mentioned silccx-MeGrath p t) consideration f curve (A i FIG, 2 l l i di have been successfully incorporated into electroless platthat coercivities in the vicinity of 4 oersteds are obtaining baths to Produce magnetic thin filInS- However, the able only at film thicknesses approaching 10,000 A. Con- Prior art has not recognized or taught that there is y sidering now curves C and D which were obtained using Particular advantage to aging a ferrous hathone immedielectroplated films, it is seen that lower values of coercivity ate advantage is that composition Changes fih'ns are attainable in much thinner films than are attainable ing a ne s belOW 5,000 are materially redneedin films electrolessly plated at 65 C. The values for Another advantage is that aging PermitS the bath to undercurve (C) were obtained from an article by I. W. Wolf s all its inherent changes due to chemical reactions, in the Journal of the Electro Chemical Society, vol. 108, stable complex ion formation, etc prior to a Process No. 10, pp. 959-964, October 1961. The values for curve Which even a fresh ferric bath must g Thus, aging (D) w btai d f an ti l b W, O, F it r 1, the bath in and of itself contributes to stability of the in the Journal of the Electro Chemical Society, vol. 111, bath in y that eannet be accounted for Y p y No. 1, pp. 35-39, January 1964. troducing the ferric species directly into the bath.

A comparison of curve (B) with curves (A), (C) and FIG. 3, shows the effect on composition of the resulting (D) clearly shows that by electrolessly plating at 25 C., film for (i) a film produced from the directly introduced that values of coercivity heretofore only obtainable with ferric sPetiteS and a film Produced y the aged other techniques such as electroplating are now obtainable ferrous i011 P Ci Curves (i), (ii) were plotted from using the electroless plating technique of the present infilms Plated at a temperature below Cnrve vention. Electrolessly plating at higher temperatures as Was Plotted from a film Plated at to indicate that demonstrated by curve (A) does not provide coercivities While b h stability is a function of temperature, it is f th d ir di v l clearly not the only parameter affecting stability, since The mechanism which provides the low coercivit data taken from two films deposited at below C. values of curve (B) is not clearly understood, but is be- Where the y difierenee Was in the manner of Producing liev d t depend o th in size f h deposited t the ferric species, does not provide the same result. Curve rial. At temperatures in the vicinity of 40 C. and above, 40 also Shows that composition varies sharply with a deposited film exhibits a characteristic golden brown t i k ss n the 3 g While enl've color, which is indicative of large surface roughness, large Shows little variation in composition in the same thickgrain size, and relates directly to the agglomeration of ness range. Curve (ii) shows that a ferrous bath aged large spheres on the surface of the deposit. Between apfor only 2 hours affects the iron content rather differently proximately 35 C. and 39 C., the color of a deposited and more closely approaches the 80-20 composition defilm dramatically changes to a shiny silvery color. This is sired for most magnetic films. Aging a bath which incorbelieved to be due to a change in grain size which results porates iron in the ferrous specie certainly converts the from the complex interaction of the bath constituents in ferrous specie to the ferric specie but, in addition, appears the 35 C. to 39 C- ternpefatnre region- The g YP to eliminate complex chemical activity which can be exphosphite concentrations utilized in the bath of the prespected in any freshly made bath. The compositions of ent invention are believed to affect the rate of deposition h present invention, if made with the ferrous specie, in such a Way that a material having smaller grain Size exhibit high stability after aging for approximately 24-60 lower coercivity: better anisotrfjpyt Improved hours, preferably the latter. This time can be shortened easy ax s loop squareness (higher B /B ratio), and lower by aging at high temperatures easy axls (.llsperslon To further illustrate the practice of the present inven- In addmon to coerclvity i of at} elec' tion, further examples are given in Table H wherein the trolessly plated. l f Wlth thickness (-iependmgon salt from which the cations and anions are derived is whether a ferric 1011 1s introduced dlrectly into the bath s eclfically indicated and the concentrations given 111 or whether a ferrlc 10m 1s provided by aglng the bath for p T M I h l a period of time prior to use. FIG. 3 is a plot of the moles/mars as Wlth F examples of a e L; 6 Va ues variation in composition (percent iron deposited) with in the chart are multlphed factor of Accordthickness of a deposited film using ferric ions provided g y actual t l concentratf? are g from the by introducing them directly from a ferric salt such as table by multiplying the value indicated by 10- TABLE II constituent Moles/literXlO (as ions) hiritt t raoiiijii32:13::::""'"i:::::::::::: it? 18% r32 133 13% 3 15% o,H, 120 240 120 120 80 NH4OH (28-30% NHa) specific gravity 0 9, ml ll 40 60 so so 60 so smnqzmo 12 12 12 1 8 44 5 12 '11enqperature C 23 23 2 23 23 23 23 Plating thickness, A./mln -2,500 a0 -1,50% 0 w oa/2 -2,00% 0 -2, coo/ 1 33 Macon/ g -5,000 /5t Percent Fe plated The last two examples represent ferrous baths which have been aged for 16 hours and for 2 hours respectively.

The Ni(++) and Fe(+++) ions are supplied in the form of any water soluble salts such as chlorides, sulfates, acetates, sulfamates and mixtures thereof as long as the anions do not interfere with the deposition. More particularly, the ferric ion is furnished in the form of ferric ammonium sulfate, ferric chloride, ferric sulfate, and ferric nitrate. Any water soluble iron salt is usable that yields ferric ions in solution, provided the salt is compatible with the other ion species present. Similarly, the hypophosphite ions are furnished in the form of water soluble salts of various bases such as sodium hypophosphite, potassium hypophosphite, hypophosphorus acid, and mixtures thereof.

Although it is preferred to use complexing and sequestering agents such as ammonia and sodium potassium tartrate, organic reagents which contain one or more of the following functional groups in concentrations that range from 60.0 moles per liter to 700x10- moles per liter and preferably at about 200x10" moles per liter: primary amino group (-NH secondary amino group N-), imino group (=NH), carboxyl group and hydroxy group (-OH). The preferred agents include rochelle salt, seignette salt, ammonia, ammonia hydroxide and ammonium chloride.

Similarly, various alkalizing agents may be added which include all the complexing agents heretofore listed, which in aqueous solution have a basic reaction and in addition, all water soluble bases such as sodium potassium, and lithium hydroxide, and the like.

Surface active substances may be added such as sodium lauryl sulfate, as long as the substances do not interfere with the plating reaction. Exaltants also may be added to increase the rate of deposition by activating the hypophosphite anions such as succinic acid, adipic anions, alkali fluorides and other exaltants which are known to those in the art. Stabilizers may be added in minute concentrations such as 10 parts per billion. These may be stabilizers such as thiourea, sodium ethylxanthate, lead sulfate and the like. Also, pH regulators and buffers such as boric acid, disodium phosphate and others may be included in the solution.

Other metal ions may be added to the electroless solution in their lowest oxidation states, such as cobalt molybdenum, chromium, and the like to adjust the coercive force of the resulting films.

Until this point, no mention has been made of the fact, that the composition and process of the present invention are useful in electrolessly plating nickel alone over a temperature range which includes room temperature. To accomplish nickel plating alone, the examples of Table I and Table II need only be modified by removing references to the Fe+++ ion or the FeNH (SO .l2H O and the related amounts of these constituents. The resulting baths with nickel as the sole metallic ion electrolessly plate nickel over a temperature range of 1035 C. and preferably at 23 C.

With respect to the composition of the magnetic materials resulting from plating in the baths of Tables I, II, it should be understood that the preferred embodiments recited therein provide films containing to 30% Fe and include also the composition 80% Ni-% Fe which makes the films ideal for use as memory elements. Compositions ranging from 2-3% iron to in excess of 50% iron may be produced by following the teaching of the present invention.

With respect to the sensitizing of the substrate prior to plating, it should be appreciated that the condition of the substrate (roughness) has an effect on the anisotropy and coercivity of the resulting film. It has been found that smoother surfaces can be produced by simply initiating plating by touching the substrate with palladium or any 10 other catalytic metal. In this manner, the starting surface is substantially smoother than when a coating of palladium is electrolessly plated on the surface of the substrate.

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

What is claimed is: 1. A process for electrolessly plating a magnetic mate rial on a substrate comprising the steps of:

preparing an initial solution including a nickel salt in a concentration sufiicient to provide nickel ions in concentrations ranging from 20x10" moles/liter to 350 10 moles/liter, hypophosphite ions in concentrations ranging from 105 l0- moles/liter to 190 10- moles/liter, tartrate ions in concentrations ranging from 60 10- moles/liter to 240x10- moles/liter introduced into said solution as sodium potassium tartrate, and ammonium ions in concentrations ranging from 40 milliliters/liter to milliliters/liter to form the nickel hexamine complex ion in said solution; maintaining the pH of said solution in the range of 7 maintaining the temperature of said solution in the range of 20 to 25 C.;

immersing said substrate in said solution to plate a nickel coating thereon having a substantially small grain size and a silvery finish.

2. A process for electrolessly plating a magnetic material on a substrate comprising the steps of:

preparing an initial solution including a nickel salt and a ferrous salt in concentrations suflicient to provide nickel and ferrous ions in concentrations ranging from 20 l0 moles/liter to 350 10- moles/liter and 11 l0- moles/liter to 350 10 moles/liter, respectively, hypophosphite ions in concentrations ranging from 10 moles/liter to x10" moles/liter, tartrate ions in concentrations ranging from 60 10- moles/liter to 240x10 moles/liter introduced into said solution as sodium potassium tartrate, and ammonium ions in concentrations ranging from 40 milliliters/liter to S0 milliliters/liter to form the nickel hexamine complex ion in said solution;

maintaining the pH of said solution in the range of 7 to 13; maintaining the temperature of said solution in the range of 20 C. to 25 C.;

aging said solution in the range of 24-60 hours to convert said ferrous ions to ferric ions, and;

immersing said substrate in said solution to plate a nickel-iron coating thereon having a substantially small grain size and a silvery finish.

References Cited UNITED STATES PATENTS 3,372,037 3/1968 McGrath et a1. 106-1 3,483,029 12/ 1969 Koretzky et al. 117-236 3,327,297 6/1967 Croll 340-174 3,305,327 2/1967 Schmeckenbecher 29-195 3,549,417 12/1970 Judge et a1 117-240 3,532,541 10/1970 Koretzky et a1. 117-240 3,178,311 4/1965 Cann 117-160 WILLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant Examiner U.S. Cl. X.R.

Images