Nodule-free electroless nip plating

Details Nodule-free electroless nip plating Nodule-free electroless nip plating

2000-02-22

2004-02-03

Barr, Michael (Department: 1762)

Coating processes

Immersion or partial immersion

Metal base

C427S129000, C427S131000, C427S132000, C427S405000, C427S443100

Reexamination Certificate

active

06685990

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for depositing a nodule-free coating layer on a substrate surface by means of an electroless plating process. The invention has particular utility in depositing amorphous nickel-phosphorus (NiP) layers on suitably-shaped substrates, e.g., disk-shaped substrates, utilized in the manufacture of longitudinal magnetic recording media.
BACKGROUND OF THE INVENTION
Magnetic recording media are widely used in various applications, particularly in the computer industry. A conventional longitudinal magnetic recording disk medium 1 used in computer-related applications is schematically depicted in cross-sectional view in FIG.
1
and comprises a non-magnetic substrate
10
selected from metals, metal alloys, polymers, polymer-based materials, glass, ceramics, metal-ceramic composite materials, and glass-ceramic composite materials, typically an aluminum (Al)-based alloy, such as an aluminum-magnesium (Al—Mg) alloy, having sequentially deposited on at least one surface
10
A thereof: a “seed” or plating layer
11
, typically of an amorphous nickel-phosphorus material, such as NiP and Ni
3
P; a polycrystalline underlayer
12
, typically of chromium (Cr) or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy; a protective overcoat layer
14
, typically comprised of diamond-like carbon (DLC); and a lubricant topcoat layer
15
, typically comprising a perfluoropolyether compound.
According to conventional automated manufacturing methodology for fabricating such type magnetic recording media, each of the polycrystalline underlayer
12
, magnetic recording layer
13
, and protective overcoat layer
14
is deposited on, e.g., an amorphous NiP- or Ni
3
P-plated substrate, by a suitable physical vapor deposition (PVD) or chemical vapor deposition (CVD) technique, typically cathode sputtering. When utilized with relatively soft substrates, such as Al—Mg alloy substrates
10
, the NiP plating layer
11
is typically deposited by an electroless plating process to form a layer having a thickness of about 15 &mgr;m, in order to increase the hardness of the substrate surface, thereby providing a suitable surface for subsequent polishing and/or texturing. The presence of amorphous NiP or Ni
3
P “seed” or plating layer
11
is also necessary for ensuring proper polycrystallinity of the Cr-based underlayer
12
, which, in turn, is required for facilitating proper epitaxial growth thereover of a suitably polycrystalline magnetic recording layer
13
. For example, an amorphous NiP or Ni
3
P “seed” layer
11
induces a Cr-based underlayer
12
deposited thereon to exhibit a (
200
) crystallographic orientation, which, in turn, causes the magnetic recording layer
13
deposited and epitaxially grown thereon to exhibit an advantageous bi-crystal cluster microstructure, as disclosed in U.S. Pat. No. 5,733,370, the entire disclosure of which is incorporated herein by reference.
In some instances, the “seed” layer
11
is provided with a textured or roughened surface to facilitate preferential alignment of the Cr-based underlayer
12
to exhibit the (
200
) crystallographic orientation or to reduce “stiction” between the transducer head and the recording medium when in use. In other instances, a requirement for substrates with high track-per-inch (“TPI”) and low track mis-registration (“TMR”) necessitates formation of NiP or Ni
3
P “seed” or plating layers
11
with defect-free surfaces after plating and/or polishing, with an attendant requirement for a high degree of planarity.
Suitable baths and procedures for electroless plating of non-magnetic nickel-phosphorus (NiP) amorphous “seed” or plating layers, wherein the formula “NiP” is taken to include all ratios of nickel-to-phosphorus, are disclosed in U.S. Pat. Nos. 3,531,322 and 4,659,605, the entire disclosures of which are incorporated herein by reference. By way of illustration only, a suitable electroless plating bath for deposition of amorphous NiP “seed” or plating layers consistent with the requirements of the present invention, includes a source of nickel ions (e.g., NiCl
2
or NiSO
4
), a source of hypophosphite ions (e.g., NaH
2
PO
2
), a buffering agent, e.g., a carboxylic acid, boric acid or borate, and certain ester complexes, e.g., an ester complex of glucoheptonic acid, and stabilizing agents, etc. Another suitable NiP electroless plating bath includes a source of nickel ions, an unsaturated carboxylic acid, and a source of hypophosphite ions. In addition to these, electroless NiP plating baths usable within the context of the invention include, inter alia, Enthone 6450 (Enthone-OMI, New Haven, Conn.), Fidelity 4355 (OMG Fidelity Products Corp., Newark, N.J.), and U
1
C SHDX (Uyemura Int'l Corp., Ontario, Calif.).
NiP electroless plating baths, such as described above, can provide non- magnetic, amorphous NiP deposits, with a phosphorus (P) content within the range of from about 8 to about 12% and a corresponding nickel (Ni) content of from about 92 to 88%. Further, these baths are typically operated at an acidic pH, i.e., below about 5, and at an elevated temperature, i.e., above about 140° F., typically about 180-200° F., to provide a practically useful plating rate of about 3 to about &mgr; inches/min. while still providing a non-magnetic deposit which does not become magnetic with age.
An essential requirement of the above-described NiP electroless plating process is that the thus-plated NiP layer be characterized by an unusually smooth surface which is free of imperfections such as nodules and pits. Prevention of formation of such imperfections is particularly important in the manufacture of rigid magnetic media, such as, for example, hard disks, since irregularities of any kind in excess of one-millionth of an inch can cause head crash or defective recording.
In practice, however, the requisite freedom from formation of surface irregularities during electroless plating of non-magnetic, amorphous NiP “seed” layers, such as the above-mentioned nodules and pits, frequently is not achieved in continuous manufacturing processing for the fabrication of magnetic recording media, e.g., hard disks, leading to increased substrate rejection rates. For example, abnormal nodule growth is frequently observed when less costly, more readily-available materials, e.g., polymeric or polymer-based materials, are utilized as components of the NiP electroless plating line in order to reduce or minimize equipment expense. Such abnormal nodule growth can result in the presence of residual “bumps” after post-deposition polishing of the NiP layer or add to the manufacturing cost by necessitating a two-step polishing process to ensure complete nodule removal. Moreover, in instances where a leveling agent is added to the NiP electroless plating bath to produce smooth layers, the effect of any abnormal nodule growth will be exacerbated.
Accordingly, there exists a need for an improved electroless plating process suitable for forming defect-free “seed” or plating layers for use in the manufacture of high density magnetic recording media, which “seed” or plating layers are substantially free of abnormal nodule growth and require little or no post-deposition polishing prior to subsequent layer deposition thereon. In addition, there exists a need for an improved electroless processing methodology for manufacturing substrates for high-density magnetic recording media which is simple, cost-effective, and fully compatible with the productivity and throughput requirements of automated manufacturing technology.
The present invention fully addresses and solves the above-described problems attendant upon the formation of substrates utilized in the manufacture of high-density magnetic recording media, while maintaining full compatibility with all chemical and mechanical aspects of conventional recording media manufacturing technology.
DISCLOSURE OF THE INVENTION
An advantage of the present invention is an improved method of electroless plating of substrates.
Another advantage of

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