Measuring and testing – Instrument proving or calibrating – Roughness or hardness
Reexamination Certificate
2000-08-07
2002-06-25
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Roughness or hardness
Reexamination Certificate
active
06408677
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a calibration disk that can be used for calibrating a glide head, used in the process of manufacturing hard memory disks.
2. Description of the Related Art
The key components of a hard disk drive are a magnetic disk and a magnetic head, which is typically separated from the magnetic disk by a small gap. The gap is created by the magnetic head remaining relatively stationary while the magnetic disk rotates on a spindle. The rotation of the magnetic disk generates a thin film of air known as an “air bearing” over the surface of the disk that supports the magnetic head, which is essentially flying over the surface of the magnetic disk.
The permitted recording density of the magnetic disk is strongly influenced by the gap or “fly height” between the disk and the magnetic head. Because decreasing the fly height of the magnetic head allows the recording density of the magnetic disk to be increased, the magnetic disk should have a very smooth surface so the magnetic head can fly very close to the surface. Magnetic disks with protrusions related to defects or contamination that exceed the fly height of the magnetic head should not be used in production hard disk drives. This is because an impact between the magnetic head and a protrusion can cause undesirable effects, such as a hard drive crash, formation of wear debris, unusable recording area, a thermal spike in a magnetoresistive head and the like. In order to ensure acceptable surface conditions of the magnetic disk, glide tests are widely employed by the hard disk industry for purposes of quality control.
The basic operation of the glide test is to fly a test head, i.e., a glide head, at a height related to the fly height and margin requirements of the magnetic head, and to sense any contact between the glide head and any defects on the surface of the magnetic disk. If the glide head contacts a defect, the disk is rejected. The term “glide head” as used herein indicates a head used in a magnetic disk testing system, as distinguished from the term “magnetic head,” which is used in general to refer to a read-write head.
The contact detection is typically accomplished with a piezoelectric (PZT) sensor or an acoustic emission (AE) sensor. The PZT sensor, as is well known to those skilled in the art, uses a piezoelectric crystal to convert mechanical energy into an electrical signal. Thus, a PZT sensor converts the mechanical energy generated by the glide head contacting a defect into an electrical signal that can be used to indicate the size and location of the defect.
An AE sensor uses a sensing technique similar to a PZT sensor. The difference is the mounting position and configuration of the sensor. The sensing material in an AE sensor is typically a PZT (PbZrO
3
-PbTiO
3
) ceramic that has a piezoelectric effect and which is housed in a metal container and mounted close to the head/slider suspension. Both the PZT and AE sensors give electrical signals excited by acoustic vibration. For more information related to the aforementioned sensing technology see U.S. Pat. Nos. 5,423,207 and 4,532,802.
The magnetic disk can also be tested for defects using non-contact methods such as a magneto-resistive (MR) head, a laser, or an optical tester. For more information related to MR technology see U.S. Pat. No. 5,527,110, and see U.S. Pat. No 5,550,696 for a method to calculate a protrusion height based on a diffracted laser beam detected by a linear photo-detector array. An optical tester optically scans the magnetic disk for defects. The detection is usually performed by comparing the light reflected from a defect with the light reflected from an area of the disk that does not have defects. The optical tester is calibrated in such a way that a rejection of a magnetic disk occurs when the height of a defect is above a desired threshold.
Another important parameter of the magnetic disk is the minimum height at which a head can fly without contacting the disk surface, known as the avalanche height, which occurs at the avalanche point on an avalanche curve (described subsequently). It will be noted that the avalanche height can differ from the fly height of the glide head. While the fly height is usually determined by potential extrinsic defects, such as contamination, the avalanche height is determined by intrinsic surface topology. The avalanche height is defined as the fly height at which the lowest part of the head starts to contact the disk surface. For example, the landing zone of a magnetic disk, which is usually heavily textured to prevent excessive friction, has a relatively large avalanche height due to the additional surface roughness created by the heavy texturing. The data zone, however, has a smoother surface because there is no need to reduce friction. Consequently, a glide head can fly lower over the data zone than the landing zone, and thus, the data zone has a relatively lower avalanche height. The avalanche height is a useful indication of the surface finish and gives an absolute fly height below which flying is not possible without contacting the disk.
While magnetic disks are ideally flat and smooth, in practice there is typically an amount of disk waviness and runout. Disk waviness causes the effective height of a disk's surface to vary relative to the mean disk surface. If the waviness of the disk surface has a wavelength that is less than a longitudinal dimension of the glide head, the glide head cannot follow the disk surface. Consequently, the amplitude of the waviness needs to be accounted for when determining fly height. A typical amplitude of the waviness of a disk is 20 to 60 nm (nanometers) and has a wavelength that is typically smaller than the length of a conventional glide head.
Disk runout is a deviation from a level surface and is caused by improper clamping of the disk, for example. The runout effect typically creates a variation from a level surface over an area of the disk that is much greater than the length of the glide head. Disk runout causes acceleration of the glide head which induces fly height fluctuations. A typical disk runout is approximately 2 to 10 &mgr;m (micrometers).
To accurately test a magnetic disk with a glide head, it is important for the glide head to be calibrated so that the fly height at which the test is carried out is known. Calibration ensures that the threshold for defects is set to an appropriate limit (i.e., the height above which a surface defect becomes unacceptable). A conventional method of calibration is performed by flying a glide head over a glass disk on a fly height tester. The fly height tester operates by passing a beam of light through the glass disk. The interference pattern of light reflected off the glide head and light reflected off the surface of the glass disk is used to determine the distance between the disk surface and the glide head. This procedure is performed for a number of different linear velocities of the glass disk to establish the relationship between linear velocity and fly height for that particular glide head.
The linear velocity versus fly height relationship is then used to determine the linear velocity at which to fly the glide head over production disks on a glide tester. Thus, a linear velocity can be selected that achieves the desired glide height, in order to test for defects on a production disk that are higher than the glide height. This procedure is performed for each individual glide head because each glide head has different flying characteristics.
There are several drawbacks to the use of a fly height tester for calibration of a glide head. For example, the fly height tester uses a glass disk, which may have different characteristics than a production memory disk, including differences in waviness, runout and the like. Changes in surface topology will cause a change in the flying characteristics of the glide head and, thus, the fly height of the glide head may be different when the glide head flies over a production disk. The difference in
Komag Corporation
Raevis Robert
Skjerven Morrill LLP
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