Etching a substrate: processes – Forming or treating article containing magnetically...
Reexamination Certificate
1999-08-25
2002-05-21
Gulakowski, Randy (Department: 1746)
Etching a substrate: processes
Forming or treating article containing magnetically...
C216S100000, C216S109000, C029S603110
Reexamination Certificate
active
06391212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium.
2. Background of the Invention
Magnetic TFH transducers are known in the prior-art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719 for inductive devices and U.S. Pat. Nos. 4,190,871 and 4,315,291 for magnetoresistive (MR) devices.
In the operation of a typical inductive TFH device, a moving magnetic storage medium is placed near the exposed pole-tips of the TFH transducer. During the read operation, the changing magnetic flux of the moving storage medium induces changing magnetic flux upon the pole-tips and gap between them. The magnetic flux is carried through the pole-tips and back-portion core around spiralling conductor coil winding turns located between the core arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole-tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium. Magneto-resistive (MR) TFH devices can only operate in the read mode. The electrical resistance of an MR element varies with its magnetization orientation. Magnetic flux from the moving magnetic storage medium induces changes in this orientation. As a result, the resistance of the MR element to a sensing electric current changes accordingly. The varying voltage signal is representative of the magnetic pattern stored on the magnetic medium.
Prior-art magnetic recording inductive thin film heads include top and bottom magnetic core pole layers, usually of the alloy Ni—Fe (permalloy), connected through a via in the back-portion area, and separated by a thin gap layer between the pole-tips in the front of the device. The bottom pole-tip is usually designed to be wider than the top pole-tip in order to prevent “wraparound” due to misregistration or misalignment, as taught by R. E. Jones in U.S. Pat. No. 4,219,855. Alternatively, one or both pole-tips are trimmed by ion-milling or by reactive ion etching (RIE) to ensure similar width and proper alignment. Such a technique is disclosed, for example, by U. Cohen et al. in U.S. Pat. No. 5,141,623. As the track width decreases in order to increase the recording density, the write head pole-tips must be very narrow. P. K. Wang et al. describe elaborate schemes to obtain pole-tips for writing very narrow track width, in IEEE Transactions on Magnetics, Vol. 27, No. 6, pp. 4710-4712, November 1991.
One of the problems associated with the prior-art pole-tip designs is that during write operations, substantial noise is introduced along the track-edges (on the magnetic storage medium), which adds to the noise generated by the medium during read operations. During the write operations, significant portions of the intense magnetic flux lines, emanating from the corners and side-edges of the pole-tips, deviate from a direction parallel to the track's length. The non-parallel magnetic field magnetizes the medium in the wrong directions, giving rise to noise along the track-edges. This noise is usually characterized as “track-edge fringing noise” and is a major obstacle to increasing the track density. According to a paper by J. L. Su and K. Ju in IEEE Transactions on Magnetics, Vol. 25, No. 5, pp 3384-3386, September 1989, the track-edge noise extends about 2.5 &mgr;m on each side of the written track. As track density increases, the track width decreases along with the strength of the read-back signal. If the track-edge fringing noise remains the same, the signal to noise ratio (SNR) is directly proportional to the track width, and deteriorates rapidly as the latter decreases. The current state-of-the-art magnetic thin film media can support lineal density of about 40,000-60,000 flux changes per inch (FCI), corresponding to domain length of about 0.4-0.7 &mgr;m. Yet, the track width is at least an order of magnitude larger, about 8-12 &mgr;m. There is no apparent reason why the media could not support much narrower tracks, if not for the rapid deterioration of the SNR. By eliminating most of the track-edge fringing noise, the useful track width could be decreased to about 1.0 &mgr;m, or less. This represents an increase of recording density by about an order of magnitude.
In addition to the medium's noise, there is also the head's noise. A significant portion of the head's noise is due to edge-closure domains in the pole-tips. This noise contribution becomes more dominant as the width of the pole-tips decreases. This problem was described, for example, by D. A. Herman in Paper No. 299, “Laminated Soft magnetic Materials”, The Electrochemical Society Confererice, Hollywood, Fla., October 1989.
SUMMARY OF THE INVENTION
The present invention provides an inductive “pinched-gap” thin film head (TFH) device having pole-tips that are in substantial contact along their side-edges, thereby enclosing a gap segment. Since no magnetic flux lines emanate from the corners and side-edges, the write magnetic field is precisely confined to across the pinched-gap segment in a direction parallel to the track's length. As a result, the usual noise-producing non-parallel magnetic field from the pole-tips' corners and side-edges is virtually eliminated. The written (medium) track width is precisely defined by the width of the pinched-gap segment. It incorporates a high degree of magnetization coherency and is substantially free of the track-edge noise.
An effective technique to confine the gap includes photolithographic definition and etching gap-vias through the gap to the bottom pole-tip sides or side-edges, followed by deposition of the top magnetic pole. The distance between the gap-vias defines the width of the pinched-gap segment, which in turn accurately and consistently determines the width of the written track. The total width of the pole-tips is not as crucial as in prior-art devices. Either one or both pole-tips may be deposited wider than their final dimension and, following the top pole deposition, trimmed by ion-milling or by reactive ion etching (RIE) to their final width. Depositing wider pole-tips improves the composition and thickness uniformities of the device.
Alternatively, intentional wraparound on both sides of the bottom pole-tip also produces a confined gap segment with substantial contact of the pole-tips along their side-edges. Depositing a top pole-tip that is wider than the bottom one readily produces such a wraparound. The width of the pinched-gap segment (as well as the track width) is determined by the width of the bottom pole-tip. Incomplete step coverage by the gap layer at the side-edges of the bottom pole-tip provides substantial contact there. To ensure contact along the upper corners of the bottom pole-tip, they can be exposed by ion-milling (with a thin mask) or by etching gap-vias prior to the deposition of the top pole. Excess width of the top pole-tip on both sides of the bottom pole-tip can be trimmed by ion-milling, RIE, or by chemical etching.
Although the pinched-gap transducer's efficiency is not adversely affected in the write operation, it may be impaired in the read mode due to the partial shorting of the pole-tips. A dual-element or tandem TFH device, combining a pinched-gap TFH device as a write element, and a separate TFH device as a read element may be advantageous. The pinched-gap TFH device is particularly suitable for use as a write element in combination with a separate read element, such as a magnetoresistive (MR) element, or another i
Ahmed Shamim
Gulakowski Randy
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