US3525677A - Electrodeposition of constant-composition thin films - Google Patents

Description

ELECTRODEPOSITION OF CONSTANT-COMPOSITION THIN FILMS Filed March 24, 1969 H. E. AUSTEN Aug. 25, 1970 3 Sheets-Sheet 2 FIG. 3

FIG.4

iNVENTOR HERMAN E.

AUSTEN BY 4; w 4% GMwBms HIS" ATTORNEYS,

I Aug. 25, 1970 H. E. AUSTEN 3,525,677

ELECTRODEPOSITION OF CONSTANT-COMPOSITION THIN FILMS Filed March 24, 1969 FIG. 5

3 Sheets-Sheet, 5

INVENTOR HERMAN E. AUSTEN BY GXMQ QMJMM (5 MWBMQ H15 ATTORNEYS United States Patent O U.S. Cl. 204-43 1 Claim ABSTRACT OF THE DISCLOSURE Electrodeposition utilizing a plating current that varies in a predetermined manner as a function of time so as to compensate for variation in the composition of electrodeposited thin magnetic nickel-iron films is disclosed. A batch fabrication circuit embodiment and several continuous plating embodiments for supplying the plating current of the present invention are also disclosed.

BACKGROUND OF THE INVENTION It is well known that thin filmsfor example, thin magnetic nickel-iron films-which are electrodeposited from an aqueous electrolytic solution vary in composition as the thickness of the electrodeposited thin magnetic nickel-iron film increases. The present invention provides a plating current which has a large initial magnitude and a significantly smaller final magnitude during the plating of the initial portion of the films to a particular thickness, and which is substantially constant at the smaller magnitude during the plating of films of a greater thickness, so as to ,provide constant-composition electrodeposited thin films over a range of thicknesses. The present invention also provides thin magnetic film storage elements, including nickel-iron thin film storage elements, having constant magnetic coercivity properties and a thickness of 1,000 angstroms or less.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of a circuit embodiment which is used for batch fabrication of thin films.

FIG. 2 is a diagram of a continuous plating embodiment which has an anode of a particular shape.

FIG. 3 is a diagram of a continuous plating embodiment which has an anode of another shape.

FIG. 4 is a diagram of a continuous plating embodiment which shows a tapered insulator positioned between the anode and the substrate.

FIG. 5 is a diagram of a continuous plating embodiment that has an anode with multiple windings which are each supplied with an individually controlled current.

DESCRIPTION OF SEVERAL EMBODIMENTS The percentage composition of the elements of electrodeposited thin films (that is, films having a thickness of 10,000 angstroms or less) is known to vary to a noticeable extent as a function of the thickness of the films, especially in the first 1,000 angstroms of thickness. For example, if the instantaneous percentage of iron in a typical electrodeposited nickel-iron or nickel-iron-molybdenum film is plotted versus the thickness of the film up 3,525,677 Patented Aug. 25, 1970 to several thousand angstroms, the resulting curve approximates an exponential curve of decreasing magnitude.

A constant percentage composition of the elements in an electrodeposited thin film is obtained by the present invention with a plating current which has a large initial magnitude and which, as the thickness of the electrodeposited thin film increases, continually decreases to a smaller final magnitude, so as to compensate for variation in the composition of the elements of the electrodeposited thin film which would otherwise result. If the compositional variation of a particular thin film is known, a predetermined variation of the magnitude of the plating current as a function of time that exactly corresponds to the variation in composition of the electrodeposited thin film as a function of thickness may be provided, by conventional means known in the art, within the scope of the present invention. However, for the described deposition of a thin nickel-iron magnetic film storage element, an exponentially decreasing plating current will substantially eliminate compositional variations in the initial portion of the film.

Constant values for H the coercive force, and H the anisotropy field, are obtained, regardless of thickness, when a thin nickel-iron magnetic film storage element is electrodeposited according to the present invention. On the other hand, variable H and B values are obtained when a plating current of a constant magnitude is employed to deposit a variable-composition thin nickel-iron magnetic film storage element from the same electrolytic plating solution. The difference between the two plating methods is especially pronounced when magnetic thin nickel-iron films having a thickness of 200 angstroms, or less, are produced.

Although the present invention is not limited to the electrodeposition of any particular thin film, one representative nickel-iron plating solution which has been successfully employed has the following composition:

Grams per liter Sodium saccharin Tiisodium salt of naphthalene 1,3,6 trisulfonic acid 1.75

This electrolytic plating solution is adjusted to a pH of approximately 2.25, and the temperature of this solution is maintained at approximately 25 degrees centigrade. When this plating solution is employed, the plating current density may exponentially decrease from approximately 50 milliamperes per square centimeter of plated area to approximately 5 milliamperes per square centimeter of plated area in four or five seconds to produce constant-composition thin nickel-iron films.

The electrolytic plating solution is contained in the plating cell 60 of FIG. 1, which has a conventional plating anode electrode 64 and a conventional plating cathode electrode 66. A permanent orienting magnet (not. shown) is positioned around the plating cell 60, in the manner known in the electrodeposition art, so as to provide an orienting magnetic field when thin anisotropic magnetic storage films are produced. The anode electrode 64 is connected to the monitoring resistor 70,

which is connected across the input terminals of the oscilloscope 72, which is used to monitor exponential plating currents, and the cathode electrode 66 is connected to the positive terminal of the ammeter 74, which is used to monitor constant plating currents. The negative terminal of the ammeter 74 is connected to the anodes of the the diodes 76 and 78, and the cathode of the diode 76 is connected to the variable resistor 80, which is connected to the fixed resistor 82. The resistance of the variable resistor 80 and the resistance of the fixed resistor 82 substantially determine the rate at which the exponentially decreasing plating current is supplied to the plating cell 60 by the capacitor 84.

When the four-stage gang-coupled switch 86 is in the Bias/Charge position, the capacitor 84 is charged to the voltage of the variable power supply 90 through the current-limiting resistor 88 and sections 86b and 86c of the switch 86. When the switch 86 is in the Plate position, the capacitor 84 is discharged through the plating cell 60, sections 86b, 86c, and 86d of the switch 86, and the resistors 80 and 82. If the four-stage gang-coupled switch 92 is in the Capacitor Only position, thin alloy films with a thickness of only 100 angstroms, or even less, may be electrodeposited if the plating current is interrupted, when the desired thickness of the thin alloy film is obtained, by switching the switch 86 to the Bias/ Charge position. When the switch 86 is in the Bias/ Charge position, a DC. bias voltage is supplied to the anode electrode 64 through the current-limiting resistor 96 and section 86a of the switch 86 to prevent etching of the surface of the cathode electrode 66 due to any reverse electromotive force that may exist between the anode electrode 64 and the cathode electrode 66.

The base 104 of the NPN transistor 102 is connected to the bias resistor 98 and to the Zener diode 100. The Zener diode is connected to ground through section 920 of the switch 92 when it is in the Capacitor/Transistor position, and the Zener diode then maintains the base of the transistor 102 at a substantially constant voltage level. The emitter 106 of the transistor 102 is connected to the resistor 110, which is connetced in series with the variable resistor 112.

When the switch 92 is in the Capacitor/Transistor position and the switch 86 is in the Plate position, the negative plate of the capacitor 84 is connected to the cathode of the diode 76 through section 86b of the switch 86, and the anodes of the diodes 76 and 78 are then at an initial voltage level that is determined by the potential of the negative plate of the capacitor 84. The positive plate of the capacitor 84 is then connected to the power supply 94 through section 860 of the switch 86 and section 92b of the switch 92. The power supply 94 will now supply a constant positive voltage to the anode electrode 64 of the plating cell 60 through section 92b of the switch 92 and section 86d of the switch 86.

The initial plating current that flows through the plating cell 60 will be relatively large in magnitude, and the collector 108 of the transistor 102, which is connected to the cathode of the diode 78 through section 92d of the switch 92, will initially be at a voltage level which will not sustain current through the transistor 102. As the capacitor 84 discharges, the voltage level at the anodes of the diodes 76 and 78 will become more positive; however, the transistor 102 will remain in a nonconductive state until a predetermined bias point is reached, as determined by the adjustment of the resistor 112, at which point the transistor 102 will begin to conduct and will maintain a constant plating current through the plating cell 60 as the thickness of the electrodeposited constant-composition alloy film increases.

The conductive wire 120 of FIG. 2for example, a copper or beryllium copper wire-acts as the cathode electrode in a continuous plating process. Electrical connection is made to the conductive wire 120 by the conductive bar 122, Which is held in contact with the conductive wire and with the conductive connection ring 121 by the spring 124. The plating solution is contained in the plating cell 126, and the conventional seals 127 and 128 prevent leakage of the solution from the plating cell 126 as the conductive wire 120 passes through it. The anode electrode coil 129 is so wound that its windings are more closely spaced to each other and to the conductive wire 120 as it approaches the entrance 130 of the plating cell 126. As the conductive wire 120 passes through the plating cell 126 of FIG. 2, it will be subjected to a predetermined decreasing plating current density, due to the manner in which the anode electrode coil 120 is wound, which compensates for variation in the composition of the thin alloy film which is electrodeposited onto the conductive wire 120.

The conductive wire 120 serves as a cathode electrode in the embodiment of FIG. 3, and the windings of the anode electrode coil 131 of this embodiment are also more closely spaced to each other in the vicinity of the entrance 130 of the plating cell 126 than they are elsewhere. In addition, an electrically-insulating material 132for example, glass or plastic-is positioned between the conductive wire 120 and the anode electrode coil 131 near the entrance 130. The electrically-insulating material is so constructed that it has a tapered surface between the points A and B, which position the electrically-insulating material 132 closer to the conductive wire 120 at the point A than it is at the point B, the taper of the electrically-insulating material 132 between the points A and B being determinative of the desired plating current density.

The conductive wire 120 of FIG. 4 again serves as a cathode in the manner previously described, and in this embodiment the anode electrode coil 134 is constructed of a number of electrically conductive members 135, which are so formed that, as they approach the entrance 130 of the plating cell 126, they taper towards the conductive wire 120. The conductive members 135 are connected together at one end by the electrically conductive ring 138 and at the other end by the electrically conductive ring 140. The taper of the conductive members 135 between the points A and B is again determinative of the desired plating current density.

The conductive wire 120 of FIG. 5 also is a cathode electrode, and the separate anode electrode coils 152 through 158 of this embodiment are each coupled to one of the variable resistors 142 through 148. The variable resistors 142 through 148 are connected to the resistor 160, which is connected to the terminal 162, which in turn is coupled to a voltage supply. By controlling the current that flows through the anode electrode coils 152 through 158, the predetermined plating current density that is required to produce a constant-composition electrodeposited thin alloy film on the conductive wire 120 is obtained.

What is claimed is:

1. The method of electroplating onto a substrate cathode electrode a thin magnetic iron-nickel alloy film that has a substantially uniform constant composition comprising the steps of:

connecting the substrate cathode electrode into an electroplating circuit including a condenser element and an electroplating cell containing a plating solution having iron and nickel therein;

charging the condenser element;

and then discharging the condenser element through the substrate cathode electrode and the plating solution to cause a continuously decreasing electric plating potential to be applied across the electrodes of the plating cell and a continuously decreasing electric plating current to flow in the electroplating circuit,

thereby causing the plating of an initial portion of the thin magnetic iron-nickel film having a thickness of 1,000 angstroms or less,

wherein the initial portion would be of variable com- OTHER REFERENCES PQ if it W Plated fromfhe Plating Solution Tarapada Baneriee et al.: Trans. Faraday Soc., vol. 44, with an electric plating potentlal and current that have constant magnitudes' W. O. Freitag et al.: Iour. Electrochemical Soc., vol.

5 112, No. 1, pp. 6467, January 1965. References cued G. H. Cock'ett et al.: Jour. Electrochemical Soc., vol.

UNITED STATES PATENTS 10s, No. 9, p 52 906-908, September 1961.

"""""""" GERALD L. KAPLAN, Primary Examiner FOREIGN PATENTS E US. Cl. X.R. 1,397,417 3/1965 France.

1,188,409 3/1965 Germany.

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