Dynamic magnetic information storage or retrieval – Head – Core
Reexamination Certificate
2002-01-08
2004-06-15
Ometz, David (Department: 2653)
Dynamic magnetic information storage or retrieval
Head
Core
Reexamination Certificate
active
06751055
ABSTRACT:
BACKGROUND
The present invention relates to electromagnetic transducers, which may for example be employed in information storage systems or measurement and testing systems.
Conventional heads for reading or writing information on a media such as a disk or tape are formed in multiple thin film layers on a wafer substrate that is then divided into thousands of individual heads. An inductive transducer for such a head includes electrically conductive a coil sections encircled by a magnetic core including first and second pole layers, the core forming a magnetic circuit. Portions of the pole layers adjacent the media are termed pole tips. The magnetic core is interrupted by a submicron nonmagnetic gap disposed between the pole tips, so that the media bit closest to the gap becomes part of the magnetic circuit of the core and communicates magnetic flux between the pole tips and the media. To write to the media electric current is flowed through the coil, which produces magnetic flux in the core encircling the coil windings, part of the magnetic flux fringing across the nonmagnetic gap adjacent to the media so as to write bits of magnetic field information in tracks on the moving media.
A magnetoresistive (MR) sensor may be formed prior to the inductive transducer, the sensor sandwiched between soft magnetic shield layers. A first soft magnetic shield layer is conventionally formed on an alumina (Al
2
O
3
) undercoat that has been formed on an Al
2
O
3
TiC wafer. The second shield layer may also serve as the first pole layer for a combined MR and inductive transducer that may be termed a merged head. A structure in which a second shield layer is separated from an adjacent first pole layer may be called a piggyback head.
Typically the first pole layer is substantially flat and the second pole layer is curved, as a part of the second pole layer is formed over the coil windings and surrounding insulation, while another part of the second pole layer nearly adjoins the first pole layer adjacent the gap. The second pole layer may also diverge from a flat plane by curving to meet the first pole layer in a region distal to the media-facing surface, sometimes termed the back gap region, although typically a nonmagnetic gap in the core does not exist at this location.
The throat height is the distance along the pole tips from the media-facing surface at which the first and second pole layers begin to diverge and are separated by more than the submicron nonmagnetic gap. The point at which the pole layers begin to diverge is called the zero throat height. Because less magnetic flux crosses the gap as the pole layers are further separated, a short throat height is desirable in obtaining a fringing field for writing to the media that is a significant fraction of the total flux crossing the gap. Typically the throat height is determined by the curve of the second pole layer away from the gap in an area termed the apex region. An angle at which the second pole layer diverges from the first at the zero throat height is termed the apex angle.
To form the curves in the second pole layer, an organic photoresist is deposited on and about the coil sections and then the wafer is cured to create sloping sides upon which the second pole layer is electroplated. Photoresist is typically employed at this stage due to the difficulty in uniformly filling regions between the coil sections and forming sloping sides in the apex region. Curing photoresist at an elevated temperature, which changes its consistency from gel to solid and can create such sloping sides, forms hardbaked photoresist. Hardbaked photoresist has a coefficient of thermal expansion that is higher than that of other materials used to form the head, and so resistive heating in the coil sections can cause the area within the pole layers to expand, resulting in protrusion of the pole tips.
Most of the soft magnetic material in a conventional head is formed of permalloy (Ni
0.8
Fe
0.2
) and most of the dielectric material, aside from the baked photoresist around the coils, is formed of alumina. Alumina, as well as the AlTiC wafer that is conventionally employed for making heads, may have been selected for use with permalloy due to substantially similar thermal expansion coefficients. Having matching thermal expansion coefficients reduces problems such as strain and cracks between layers that expand or contract by different amounts.
Current commercially available disk drive heads “fly” at a separation of less than a microinch (about 25 nanometers) from a rigid disk that may be spinning at 10,000 revolutions per minute. Thus, even a small protrusion caused by the resistive heating of the coil could result in a crash that destroys the head and/or disk and renders irretrievable any data stored on the disk. Even without a crash, contact with the disk could move the head off track, causing data errors. Alternatively, avoiding a crash or data errors may require increasing the separation of the sensor from the disk, substantially decreasing the resolution.
SUMMARY
In accordance with the present invention, an inductive transducer is disclosed having inorganic nonferromagnetic material disposed in an apex region adjacent to a submicron nonferromagnetic gap in the core. The inorganic nonferromagnetic material has a much lower coefficient of thermal expansion than that of hardbaked photoresist, and significantly reduces pole tip protrusion even for the situation in which a much larger amount of insulation surrounding the coil sections within the core is made of hardbaked photoresist. Alternatively, the entire insulation surrounding the coil sections within the core, in addition to the apex region, can be formed of inorganic nonferromagnetic material, further reducing pole tip protrusion.
Also in accordance with the present invention, a transducer is disclosed having silicon dioxide (SiO
2
) rather than alumina in an undercoat layer that joins the wafer substrate and the thin film layers of the transducer. It has been discovered that thermal expansion of the undercoat may be magnified at the pole tips by the distance between the undercoat and the pole tips. Silicon dioxide has lower coefficient of thermal expansion than that of alumina and so reducing the thermal expansion of the undercoat can provide a magnified reduction in pole tip protrusion. Silicon dioxide may also replace alumina in other areas, for example in a piggyback layer joining an inductive transducer and a magnetoresistive transducer.
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Alfoqaha Arshad
Krounbi Mohamad T.
Sankar Sandrawattie
Seagle David J.
Young Kenneth R.
Lauer Mark
Ometz David
Silicon Edge Law Group LLP
Western Digital (Fremont) Inc.
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