Method and apparatus for producing electroplated magnetic memory disk, and the like

A process and apparatus for producing magnetic memory disks, and the like, of the type in which a thin layer of fine grain nickel/phosphorous paramagnetic material is first deposited on a large grain electrically conductive substrate, and a main magnetic layer is then electroplated over the nickel/phosphorous paramagnetic layer. In the practice of the method of the invention, the paramagnetic layer is electroplated onto the substrate through one or more openings in one or more rotating masks to provide directly a smooth fine grain layer of uniform density and thickness so as to obviate any necessity for time consuming and expensive polishing and burnishing operations of the paramagnetic film prior to the electroplating thereon of the main magnetic film; and the magnetic layer is also electroplated onto the paramagnetic layer through openings in one or more masks to provide a magnetic layer of controlled thickness for uniform magnetic response over the entire surface of the disk.

BACKGROUND OF THE INVENTION 
In present-day data processing systems, it is the usual practice to employ 
magnetic memory disks for storing binary bits representing digital data. 
The memory disks usually comprise a magnetic disc which is scanned by a 
magnetic transducer head. The magnetic head is capable of inducing flux 
reversals in the magnetic domains of the disk and, in turn, of reading a 
pattern of magnetic orientations on the disk, and translating changes in 
the magnetic orientation into a series of digitally encoded binary bits. 
Several types of magnetic head/magnetic memory disk interfaces are used in 
present-day data processing systems. For example, magnetic tape memories 
and floppy magnetic disk memories include magnetic heads which are in 
intimate contact with the magnetic memory. Another type of magnetic memory 
is known as the Winchester type which uses rigid magnetic disks. The 
Winchester magnetic disk memory provides maximum reliability and minimum 
error generation by eliminating physical contact between the magnetic head 
and the magnetic disk. This is achieved by means of a flying magnetic head 
which does not actually contact the surface of the magnetic disk. 
It is evident that for maximum efficiency it is essential that the actual 
displacement of the head from the surface of the magnetic disk be kept at 
a minimum. Present-day systems are available in which the displacement is 
of the order of 10-14 microns. Accordingly, for satisfactory operation of 
the Winchester system it is essential that the surface of the magnetic 
disk be extremely flat and uniform. 
The magnetic disk for the Winchester system is currently prepared from a 
slurry of gamma ferric oxide mixed in a matrix of an organic material 
capable of forming a thin uniform magnetic film. A rigid disk was used and 
the magnetic film deposited on the disk was burnished to provide the 
uniform surface characteristics required in that type of drive. 
U.S. Pat. No. 3,634,047 discloses a method and apparatus for electroplating 
the magnetic film on the disk substrate so as to provide a magnetic memory 
disc suitable for use in the Winchester system. However, prior to the 
electroplating of the main magnetic film, the practice is to provide a 
fine grain paramagnetic film. This is usually achieved by electroless 
deposition techniques of a film of paramagnetic nickel/phosphorous 
material. However, prior to the electroplating of the main film, it is 
necessary for the paramagnetic nickel/phosphorous film in accordance with 
the prior art techniques to be burnished and polished so as to remove some 
of the nodules that result from the electroless deposition process. 
U.S. Pat. No. 3,634,209 describes a process for producing magnetic memory 
devices in which the nickel/phosphorous fine grain paramagnetic film is 
deposited on the substrate by electroplating means, and in which the main 
magnetic film is then electroplated over the paramagnetic film. However, 
again, in order to achieve the uniform density required for the 
Winchester-type of system, the paramagnetic nickel/phosphorous film must 
be polished and burnished prior to electroplating the main magnetic film. 
The requirement for burnishing and polishing in the prior art methods is 
primarily due to the difficulty of maintaining constant current densities 
over the entire plating surfaces of the disk during electroplating. In 
particular, since the thickness of an electrodeposit at any point on a 
plateable surface is proportional to the time integral of the current at 
that point, and since the magnetic properties of a deposit varies somewhat 
with the current density developed during electroplating, the lack of 
close control over current density in conventional electroplating 
apparatus has made it very difficult to plate magnetic surfaces capable of 
high density recording. 
Accordingly, the prior art approach to provide a magnetic memory disk 
capable of high density recording and suitable for use in a Winchester 
system usually involves the following steps: 
(1) An aluminum substrate is prepared by stamping a plate into the proper 
pre-defined dimensions. Standards have been defined by the American 
Society for testing materials for disks of fourteen inch, eight inch, five 
and one-quarter inch and 3.1 inch outer diameters. 
(2) The substrates are then machined and stress relieved to obtain the 
finest tolerances possible. 
(3) The substrates are then diamond turned and/or polished to an extremely 
fine finish. 
(4) The polished substrates are then subjected to a series of plating 
operations to place a thin film of fine grain paramagnetic 
nickel/phosphorous material over the surface of the substrate. This film 
may be of the order of 0.0002 inches thick. The film may be deposited on 
the polished substrates either by electroless deposition techniques, or by 
electroplating as described in the Wolf patent. 
(5) The coated substrates are then polished again in an effort to remove 
some of the nodules that result from the deposition process. 
(6) After the polishing operation, the disks are reracked and subjected to 
an electroplating operation, for example, such as described in the 
Faulkner patent, so that the main magnetic film may be deposited over the 
paramagnetic film with the required overall degree of uniformity. 
(7) A protective barrier coating may then be formed over the surfaces of 
the plated disk. 
The prior art methods, as described above, are relatively expensive, 
especially in the requirements of the polishing and burnishing operations. 
These operations are usually performed manually, and are the leading 
causes for product failure. 
Additional problems occur when the paramagnetic nickel/phosphorous film is 
deposited by electroless methods due to slight variations in the 
characteristics of the film over the surface of the disk. These variations 
result in major changes in the signal response during read/write 
operations. 
An important objective of the present invention is to provide a method and 
process by which the paramagnetic nickel/phosphorous film may be deposited 
on the substrate by electroplating techniques, so as to obviate the 
problems encountered when electroless deposition is used, and by which the 
paramagnetic film is provided with a high degree of uniformity so as to 
eliminate any need for the time consuming and expensive manual polishing 
and burnishing operations. 
A detailed description may be found in the prior art with regard to the 
inducement of the growth of grains in an electroplated metal film and the 
nucleation on the film surface. Once a stable nucleus is established, the 
surrounding atomic steps (actually lattice grains) are energetically 
favored sites of high binding energy for further atoms to deposit. Growth 
can thus occur at levels well below the absolute barrier energy of the 
surface until the layer is complete. Since an abundance of surface flaws 
exists, providing the necessary critical energies for lattice formation, 
the initiation of growth on the surface by electrochemical reaction can 
proceed without undue duress. This activity is demonstrated in any 
electroplating operation. 
The growth of the metallic electroplated film is fractal with random 
curdling in a plane grid. Each cascade stage replaces a curd with a 
certain number of subcurds. Not only are the positions of the subcurds 
random, but their numbers and their resulting distributions involve 
classic birth and death processes. At each stage, each curd edge can be 
viewed as having acquired a random offspring made of the subcurd edges. 
The classical results on birth and death processes show that the number 
N(m) of the offspring subcurds present after the m'th generation are 
determined by the following alternative: 
When &lt;N.sub.1 &gt;.ltoreq.1, that is D.ltoreq.2, it is almost certain that the 
offspring will eventually die out, meaning that the edge will eventually 
become empty, and hence of zero dimension. On the other hand when &lt;N.sub.1 
&gt;&gt;1, that is, D&gt;2, then the offspring subcurd will have a less than one 
probability of dying off and a non-zero probability of expanding in 
numbers forever. D represents a similarity dimension in describing factal 
interactions. 
Accordingly, the following asymptotic relationship holds true: 
##EQU1## 
The foregoing relationship suggests a generalized similarity dimension D-2. 
The two-dimensional eddy traces obey an obvious modification of the same 
analysis, after replacing N.sub.1 by a random N.sub.2 such that &lt;N.sub.2 
&gt;=Nr. When (N.sub.2).ltoreq.1, that is, D.ltoreq.1, each eddy face will 
eventually become empty. When (N.sub.2)&gt;1, that is D&gt;1, it can be 
demonstrated that: 
##EQU2## 
Hence the actual action of lattice growth in the electrodeposited 
paramagnetic nickel/phosphorous film is a competitive rate between a 
non-zero random curd fractal and the lattice deposition equilibrium of the 
structure. By observation, the competitive reaction results in an 
irregular film. Therefore, the biasing of the curdling in favor of the 
lattice energy results in a truly uniform paramagnetic film. 
The paramagnetic film is composed of fine grain magnetic material and it 
consists of 92-88% nickel and 8-12% phosphorous. To achieve a truly 
uniform paramagnetic nickel/phosphorous film by electroplating, the 
electroplating process must be controlled so as to provide nucleation 
energy for electro-deposition on the flawed lattice areas, and by then 
providing for a non-zero curdling growth for a predetermined period, 
followed by a forced curdling death, leaving enough randomly oriented 
nucleation sites for the next growth cycle. In the practice of the present 
invention, the foregoing is achieved by providing an apertured rotating 
mask in the electroplating apparatus between the anode and the target 
electrode so that the periodicity of the uniform current flow in the 
electrolyte is maintained. The rate of deposition is a function of the 
cycling period of the mask, and the average grain size and distribution of 
the grain size can be readily controlled. 
The use of the apertured mask obviates any need for burnishing or polishing 
operations and reduces the entire manufacturing process from several hours 
to about 20 minutes. The aperture in the rotating mask insofar as the 
paramagnetic layer is concerned is oriented to follow the radius line of 
the target electrode, and it provides a uniform current density across the 
surface of the target electrode as it rotates because each increment of 
the surface area sees the same current. A second apertured mask may be 
used for electroplating the magnetic layer over the paramagnetic layer in 
which the aperture has parallel sides so as to cause the magnetic layer to 
be slightly thicker towards the center of the disk for uniform magnetic 
response over the entire disk surface. Also, as will be described, two 
masks may be used and multiple apertures for improved results.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
The apparatus of FIG. 1 includes a container 10 for an appropriate 
electrolyte for carrying out the electrodeposition process. The container 
may be composed of polypropylene or any other appropriate non-corrosive 
material. 
A target electrode 12 is mounted in the container 10, and it may take the 
form, for example, of an aluminum disk which is intended to form a 
magnetic disk memory. 
The disk 12 may be formed as an aluminum substrate prepared by stamping a 
plate into proper pre-defined dimensions, and which is then machined and 
stress relieved to obtain a smooth surface. The substrate is then diamond 
turned and polished to an extremely fine finish. A layer 14 of zinc oxide 
is formed on the disk to assist in the plating process. 
An anode 16 is also mounted in the container 10, and is held in place by an 
appropriate substrate 18. The anode is formed, for example, of nickel, of 
the order of 99.99% purity. The substrate 18, line container 10, may be 
formed of polypropylene, or other appropriate plastic. 
In accordance with the invention, one or more masks 20 are rotatably 
mounted in container 10 between the anode 16 and the target electrode 12 
in the illustrated embodiment. Two such masks designated "A" and "B" are 
shown, one displaced 200 mils from the target and the other displaced 200 
mils from the anode. Each mask 20 has a pair of apertures 22, (FIG. 2) 
each with a radial length corresponding to the radius of the area of 
target 12 to be electroplated with the paramagnetic nickel/phosphorous 
layer, and each with radially extending sides. The masks "A" and "B" are 
rotated in unison by an electric drive motor 25 with their apertures 
aligned. 
In a constructed embodiment, the target electrode 12 was placed in the 
container 10 in spaced and facing relationship with the anode 16, and 
spaced from the anode by a distance d=1/2"-11/2". The masks 20 may be 
formed, for example, of polypropylene, Delrin, or an epoxy glass, or any 
other appropriate material. 
As shown in FIG. 2, each mask 20 may have a disk shape, and may, for 
example, have a diameter d.sub.1 equal to 11". Each aperture 22 may have 
the shape shown in FIG. 2, and may have the following placement and 
dimensions: 
r.sub.1 =2.550 inches 
r.sub.2 =0.750 inches 
.alpha.=22.degree. 
In order to deposit the magnetic layer of the paramagnetic layer, masks 20 
of FIG. 1 are replaced by masks 21 of FIG. 3. Masks 21 each have six 
apertures 23 in the illustrated embodiment which are shaped to have 
parallel sides, as shown, to control the thickness of the magnetic layer, 
as explained above. Both discs 21 are rotated in unison with their 
apertures 23 aligned. 
The anode 16, as shown in FIG. 4, may be mounted on substrate 18 by a 
number of screws 28. As mentioned above, the anode may be formed of nickel 
of, for example, 99.99% purity. 
A source 30 of direct current voltage is connected across the anode 16 and 
the target electrode 12 (FIG. 1). 
The nickel/phosphorous layer may be electrodeposited on the target 
electrode 12, for example, from any appropriate known electrolyte, such as 
a sulfate electrolyte. The following table sets forth the composition of a 
nickel/sulfate chloride electrolyte by which the nickel/phosphorous layer 
may be deposited on the surface of the target electrode. 
______________________________________ 
NiSO.sub.4 6H.sub.2 O 
70 g./l. 
NaBO.sub.3 15 g./l. 
NaH.sub.2 PO.sub.2 H.sub.2 O 
3 g./l. 
Saccharin 8 g./l. 
Sodium Formate (Na COOH) 
10 g./l. 
______________________________________ 
Plating may be carried out at current densities of 25-200 milliamps/square 
inch. 
In the practice of the process, optimum results can be obtained when the 
resistivity of the electrolyte is of the order of 25 ohms per cc. The 
resistivity can be established to a desired value by adding propylene 
glycol to the electrolyte 
The curve of FIG. 5 illustrates the manner in which the paramagnetic film 
is deposited on the target electrode by the apparatus so as to achieve a 
truly uniform film. This is achieved by the rotating mask in the apparatus 
of the invention which provides nucleation energy for electrodeposition on 
the flawed lattice areas, as shown by the curve of FIG. 4 (first period); 
and by then providing for a non-zero curdling growth for a predetermined 
period (second period), followed by a forced curdling death (third 
period), leaving enough randomly oriented nucleation sites for the next 
growth. 
It will be appreciated that while a particular embodiment of the invention 
has been shown and described, modifications may be made. It is intended in 
the claims to cover all modifications which come within the true spirit 
and scope of the invention.