Dynamic magnetic information storage or retrieval – Head – Hall effect
Reexamination Certificate
1999-08-30
2001-02-27
Mills, Gregory (Department: 1763)
Dynamic magnetic information storage or retrieval
Head
Hall effect
C360S119050, C360S121000, C257S422000, C029S603150, C216S022000
Reexamination Certificate
active
06195229
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a merged thin film magnetic head incorporating an inductive head for writing data and a magnetoresistive (MR) sensor to read recorded data and in particular to a method of trimming to align the top and bottom poles at write gap of the write inductive head at wafer level with minimized impact on read sensor performance.
DESCRIPTION OF THE PRIOR ART
Personal computers store data on hard disk drives which consist of one or more magnetic disks that store data. Data are written to and read from the disks by read/write heads, one on each side of a single disk. These read/write heads determine the density of data that can be stored on a given size disk. The heads are more difficult and costly to manufacture than the disks, involving a variety of rigorous thin film deposition and patterning steps. The trend in the computer industry is toward higher densities with an increasing number of bits per square inch. Concurrently with this trend is the trend toward a high data rate.
In a disk drive, bits are stored magnetically, with a binary one or zero being determined by the direction of a magnetic field recorded on the surface of the disk. The read/write head writes data to the disk by switching the magnetic field in a given area, and it reads the data by sensing the direction of the recorded magnetic field. Conventional read/write heads perform both the write function and the read function with a single inductive head. An electrically conductive coil is used to induce a magnetic field at a transducing gap to write information on a magnetic disk. The same coil is used during the read mode to sense the magnetic field recorded on the disk.
A problem with inductive heads is that a large number of coil turns is required to sense read signals. The high coil turns increase head response time and therefore significantly reduce the data rate. As the area in which a bit is stored gets narrower, necessitated by the desired higher write densities and smaller disk sizes, the read pulse signals are narrower, and experience undesirable noise and therefore are more difficult to process.
Presently, magnetoresistive (MR) sensor elements are used to read recorded magnetic signals. The MR sensor measures changes in resistance when a magnetic field is applied to change the magnetization of the MR element. The MR element may operate using either the anisotropic MR (AMR) effect, the spin-valve (SV) effect or the giant magnetoresistive (GMR) effect. By measuring the resistance change, MR heads can be used for reading data, but the writing of data must still be performed with inductive heads. Therefore, a “merged” head design is employed wherein read/write heads have both an inductive write head and an MR read head.
In the structure of a merged AMR head, the AMR element consists of three layers, an MR sensor layer composed of Permalloy (NiFe), a tantalum spacer layer and a NiFe alloy soft adjacent layer (SAL). The SAL has a higher resistance than the low resistance MR sensor layer and provides an external bias which improves the linearity of the response. The MR sensor is coupled to external read circuits by interconnect leads, for example a tri-layer sandwich of Ta/Au/Ta, and the distance across the active MR sensing region defines the read track width.
NiFe shields are provided around the MR element to prevent stray magnetic flux and flux from adjacent tracks from affecting the MR sensor. Between shields, the flux is guided to the MR sensor. The shields are separated from the MR element and the interconnects by a dielectric thin film, such as aluminum oxide, 300-2000 Angstroms thick depending on linear recording density.
A write head consisting of copper coils, a write gap and a magnetic yoke structure is fabricated on top of the read head. The second shield of the MR element also functions as the bottom pole (P1) of the write head.
Specifically, thin film transducers are fabricated with a bottom pole layer P1 and a top pole layer P2, made of Permalloy or other high moment soft magnetic materials. The pole layers are connected at a back closure to complete a magnetic flux path. Opposite the back closure, a nonmagnetic transducing gap is formed at pole tips which are extensions of the bottom pole layer P1 and top pole layer P2. An electrical coil of one or more layers separated by insulation is fabricated between the two pole layers. Changes in electrical current supplied to the coil cause magnetic flux changes in the magnetic yoke (P1/P2) at the transducing gap which cause magnetization change representing data bits to be registered on an adjacent moving magnetic disk. Conversely, flux changes representing data bits on an adjacent magnetic disk may be read by the MR element and processed by read circuitry.
Inductive write head performance is determined in part by the precision of the alignment between the top pole tip (P1) and the bottom pole tip (P2). This alignment defines the characteristics of the fringe field at the transducing gap, such as magnetic field strength and gradient. It is important that the pole tips have the same width so that flux leakage is minimized. The alignment of pole tips has in the past been attempted by a pole trimming process during fabrication of the inductive thin film head.
U.S. Pat. No. 5,578,342 describes a process for producing a conventional thin film magnetic head which uses the top magnetic pole as a self-aligning mask for partially trimming the bottom magnetic pole. The yoke and pole tip regions to be trimmed are processed by separate and distinct photolithographic steps, attempting to achieve noncritical alignment in the yoke area, while maintaining critical alignment in the pole tip region which includes the transducing gap.
The bottom pole P1 is first deposited on a substrate, P1 being wider than is desired in the final product. Next an insulating layer is deposited over P1 which forms the transducing gap. After deposition of a coil assembly surrounded by insulation, a top pole P2 is deposited over an insulating layer. The nonaligned pole tip structure is aligned by using a material removal process such as ion milling. Ion milling is a process in which a surface is bombarded by high energy ions to remove the nonaligned portions of the pole tip. A protective photoresist mask shields the top pole and a portion of the insulating layer at the transducing gap. The result desired is an aligned pole tip structure. In practice, however, the self-masking of the top pole P1 during ion milling limits the accuracy of the final pole tip structure. Attempts at more precise masking have improved the process but it still falls short, especially as head designs have become smaller.
As described previously, a merged MR structure combines a magnetoresistive (MR) read head and a separate write head. The MR layer is sandwiched between a bottom shield layer S1 and a top shield layer S2. In this structure, the top shield layer S2 of the MR head is used as the bottom pole P1 of the write head. A problem with present MR heads of this type is that the write head generates significant side-fringe fields during writing, caused by flux leakage from the top pole P2 to parts of the bottom pole P1 that extend beyond the desired alignment. Side fringing fields limit track density by limiting the minimum track width possible. When a track written by such a write head is read by the MR element of the read head, off-track performance is poor because of interference with adjacent tracks.
In U.S. Pat. No. 5,438,747 a merged MR head is provided which has vertically aligned side walls to minimize side-fringing and improve off-track performance. The bottom pole piece P1, which comprises the second shield layer S2 of the read head, has a pedestal pole tip with a short length dimension. A pedestal pole tip with a length as short as two times the length of the gap layer G optimally minimizes the side writing and improves off-track performance. The bottom pole tip structure of the write head is constructed by ion beam milling using the top pole tip structure as a m
Higa-Baral Bertha
Shen Yong
Wang Lien-Chang
Kallman Nathan N.
Mills Gregory
Powell Alva C
Read-Rite Corporation
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