Optics: measuring and testing – Position or displacement
Reexamination Certificate
1998-12-14
2002-04-09
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Position or displacement
C356S507000, C073S105000
Reexamination Certificate
active
06369900
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to glide heads for evaluating glide quality of a disc surface, and to related methods. In particular, the invention relates to glide heads with modified air bearing surfaces for improved optical fly height measurements.
Disc drives are used for storing information, typically as magnetically encoded data, and more recently as optically encoded data, on a disc surface. In general, in optical storage systems, data is in the form of marks carried on the surface of the disc which are detected using the reflected laser light. There are a number of different optical disc technologies which are known in the industry.
To read from and/or write to a magnetic or optical disc, a transducing head is positioned over a surface of the disc while the disc rotates at high speed. The head takes the form of a slider which is designed to “fly” just over the surface of the rotating disc.
Increased storage density is becoming increasingly important. One technique known to increase storage density is to decrease the “fly height” of the head. Fly height of a read/write head is defined as the distance between the disc surface and the transducing element located at the trailing edge of the slider's air bearing surface. A reduced fly height allows information to be written or read more precisely and such information can be stored in a smaller area (i.e., at a higher density). Lower fly heights of read/write heads impose narrower restrictions on acceptable heights of asperities on a disc surface since higher defects would result in contact between the read/write head and the disc. Contact between a read/write head and a disc generally results in damage to the head and/or to the disc.
To ensure reliability, glide tests are used to evaluate the defect distribution on a surface of a disc. In particular, glide tests are used by computer disc manufacturers to control and assure the quality of the disc media. Generally, all hard drive discs are tested before shipment. During a glide test, the glide head or slider flies over a disc surface generally at a predetermined clearance from the disc surface, known as the glide height or fly height. The glide height or fly height of a glide head/slider is the distance between the disc surface and a close point on the glide head air bearing surface. If contact occurs between the glide head and a disc defect or asperity, the slider vibrates and deforms. Interactions between the glide head and the disc can be measured with a transducer, such as a piezoelectric transducer or a thermal transducer.
The glide heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. Each slider body includes an air bearing surface (ABS). As the disc rotates, the disc drags air beneath the ABS, which develops a lifting force that causes the glide head to lift and fly above the disc surface. Glide heads generally are designed to have a fly height that is sensitive to the linear velocity of the disc surface relative to the glide head. For example, to detect smaller defects on a disc surface, the disc velocity can be decreased to decrease the fly height of the glide head.
The glide/fly height used in a glide test generally is substantially lower than the fly height of the read/write head to be used with the disc to account for variability in the fly height of a given read/write head given production variability and differences in operating conditions of the disc drive during use. Also, there is variability in the glide height of a specific glide head due to production variability.
Various techniques have been used in the art to measure the fly height of a glide head. One technique to measure fly height is by measuring electrical capacitance between the head and the disc. Another common technique to measure fly height is using optical methods such as interferometry in which a transparent test disc is used to fly the glide head. Light is shined through the disc onto the glide head from a source on the other side of the disc. Using known techniques, the reflected light can be examined to determine fly height.
Glide heads generally have been made from aluminum oxide-titanium carbide AlTiC composite ceramic. AlTiC is a two phase material with discrete grain boundaries between particles of the two materials. While AlTiC is a convenient material, the material can introduce errors during optical fly height measurements. In particular, interferometry with the head and a transparent disc is often used to measure fly height. When optical properties are not uniform across the head, the optical measurement may depend on the optical properties of the material at the optical focal point on the glide head. The lack of uniformity leads to errors in fly height measurement.
SUMMARY OF THE INVENTION
In a first aspect, the invention pertains to a glide head including an air bearing surface and an optical mark on the air bearing surface. The optical mark has different optical properties from the portions of the air bearing surface not covered with an optical mark. The glide head can be part of an asperity detection apparatus, where the glide head further includes a suitable transducer.
In another aspect, the invention pertains to a method of evaluating fly height, the method including focusing light onto an optical mark on an air bearing surface of a glide head. The optical mark has different optical properties from the portions of the air bearing surface not covered with an optical mark.
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Sundaram Ramesh
Yao Wei H.
Pham Hoa Q.
Seagate Technology LLC
Westman Champlin & Kelly P.A.
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