Discriminating between protrusion defects and recess defects on a disk recording medium

#Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.

#A defect scan may be performed on a number of disks as part of a quality control procedure when manufacturing the disks. If an excessive number of defects are detected, a disk may be discarded as unusable, or the manufacturing process of the disks may be modified in an attempt to reduce the number of defects. However, protrusion defects (e.g., due to an asperity on the disk) are typically considered more significant than recess defects (e.g., a pit on the disk) because a protrusion defect may damage the head due to a collision. It is therefore important to discriminate between protrusion defects and recess defects to avoid unnecessary reduction in yield or unnecessary modifications to the disk manufacturing process.

#A defect scan may also be performed after installing a disk into a disk drive as part of a manufacturing procedure for the disk drive. If a defect is detected during the defect scan, the area associated with the defect may be mapped out as unusable. If the defect is caused by a protrusion on the disk (asperity), the area mapped out as defective may include a margin which prevents any part of the head from colliding with the protrusion. If a large number of protrusion defects are detected, the overall capacity of the disk may be significantly reduced. It is therefore desirable to discriminate between true protrusion defects and recess defects which may generate a signature similar to protrusion defects.

#FIG. 1A shows an apparatus comprising a thermal sensor for detecting at least one protrusion defect and at least one recess defect on a disk according to an embodiment of the present invention.

#FIG. 1B is a flow diagram according to an embodiment of the present invention wherein a protrusion defect is discriminated from a recess defect based on an attack time of a sensor signal.

#FIG. 1C shows an attack time of a pulse in a sensor signal for a protrusion defect and a recess defect according to an embodiment of the present invention.

#FIG. 1A shows an apparatus according to an embodiment of the present invention comprising a thermal sensor 2 operable to detect at least one protrusion defect (x) 4 and at least one recess defect (o) 6 on a disk 8, the thermal sensor 2 operable to generate a sensor signal 10. The apparatus further comprises control circuitry 12 operable to execute the flow diagram of FIG. 1B, wherein an attack time of the sensor signal is measured (step 16 and FIG. 1C) when the thermal sensor passes over one of the protrusion defect (x) and the recess defect (o) (step 14), and the control circuitry discriminates between the protrusion defect and the recess defect based on the attack time (step 18).

#The embodiments of the present invention may be implemented by any suitable apparatus. In one embodiment, the apparatus of FIG. 1A comprises a spin stand for evaluating disks during development and/or manufacturing of the disks. The spin stand may analyze the disks using the defect scan in order to modify a manufacturing process, and/or to qualify the disks prior to being installed into a production disk drive. In an alternative embodiment, the apparatus of FIG. 1A may comprise a production disk drive that may perform the defect scan as part of a manufacturing procedure or while the disk drive is deployed in the field. For example, a production disk drive may analyze the disks using the defect scan in order to map out areas of the disk as unusable, and/or to control the fly height of the head so as to avoid colliding with protrusion defects. In one embodiment, a production disk drive may accumulate the defect scan data for later analysis by the disk drive manufacturer in order to modify a manufacturing process of the disks and/or the disk drives.

#Any suitable thermal sensor 2 may be employed in the embodiments of the present invention. In one embodiment, the thermal sensor 2 comprises a magnetoresistive (MR) element which may be the read element of a read head, or a separate MR element designated for defect scanning. In one embodiment, a resistivity of the thermal sensor 2 changes with temperature (e.g., increases with temperature) wherein the protrusion defect 4 and the recess defect 6 cause the temperature (and resistivity) of the thermal sensor 2 to increase at different rates. Accordingly in this embodiment, the sensor signal 10 may reflect the change in the resistivity of the thermal sensor 2, such as the change in a voltage across the thermal sensor 2 when applying a current to the thermal sensor 2.