Title: Detection of defects embedded in servo pattern on stamper by using scattered light
Abstract: Defects of a hard disk drive servo pattern stamper are detected by comparing a scattered light beam pattern against the known servo pattern. A magnetic field is applied to stamper and the beam is linearly polarized. Variations in the physical offset of the beam, its scatter, are indicative of physical defects. Variations in the Kerr rotation of scattered beam are indicative of magnetic defects.
Patent Number: 6,903,888 Issued on 06/07/2005 to Leigh, et al.
| Inventors:
|
Leigh; Joseph (Campbell, CA);
Kurataka; Nobuo (Campbell, CA)
|
| Assignee:
|
Seagate Technology LLC (Scotts Valley, CA)
|
| Appl. No.:
|
346504 |
| Filed:
|
January 15, 2003 |
| Current U.S. Class: |
360/31; 356/237.1; 356/237.2; 356/237.3; 356/237.4; 356/237.5; 360/16; 360/17; 369/53.1; 369/53.15; 369/53.16; 369/53.17; 369/53.2 |
| Intern'l Class: |
G11B 027/36; G11B020/18 |
| Field of Search: |
360/31,53,16,17,781.4,770.5,770.8
324/210-212
356/243.8,243.6,2371-2374
369/533.3,533.5,5315-5317,5327-5328,532.2,531-532
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Tran; Sinh
Assistant Examiner: Figueroa; Natalia
Attorney, Agent or Firm: Minisandram; Raghunath S., Castillo; Jesus Del
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims benefit of U.S. provisional patent application Ser.
No. 60/392,788, filed on Jun. 28, 2002, which is hereby incorporated by reference.
Claims
1. A method of detecting defects in a magnetic hard disk servo pattern stamper, comprising:
scanning a servo pattern on a stamper with an incident beam;
detecting a reflected beam from the stamper to form a detected pattern signal,
the reflected beam scattered according to the pattern on the stamper; and
comparing the detected pattern against a reference servo pattern signal to identify
defects.
2. The method according to claim 1 wherein a physical offset of scattered beam
depends on the presence or absence of a servo pattern or debris.
3. A method of detecting defects in a magnetic hard disk servo pattern stamper, comprising:
scanning a servo pattern on a stamper with an incident beam;
detecting a reflected beam from the stamper to form a detected pattern signal,
the reflected beam scattered according to the pattern on the stamper, wherein said
detecting further comprises:
applying a magnetic field to the stamper;
detecting a Kerr rotation of the reflected scattered beam to form a Kerr rotation
signal; the Kerr rotation signal having a pattern; and
comparing the detected pattern against a reference servo pattern signal to identify
defects by comparing the Kerr rotation signal pattern against a reference Kerr
rotation servo pattern signal to identify magnetic defects.
4. A method of detecting defects in a magnetic hard disk servo pattern stamper, comprising:
scanning a servo pattern on a stamper with an incident beam;
detecting a reflected beam from the stamper to form a detected pattern signal,
the reflected beam scattered according to the pattern on the stamper, wherein said
detecting further comprises:
applying a magnetic field to the stamper;
detecting a Kerr rotation of the reflected scattered beam to form a Kerr rotation
signal; the Kerr rotation signal having a pattern;
comparing the detected pattern against a reference servo pattern signal to identify
defects by comparing the Kerr rotation signal pattern against a reference Kerr
rotation servo pattern signal to identify magnetic defects; and
wherein a physical offset of scattered beam depends on the presence or absence
of a servo pattern or debris.
5. The method according to claim 3 wherein the incident beam is linearly polarized light.
6. The method according to claim 3 wherein the magnetic field is applied by an electromagnet.
7. A method of detecting defects in a magnetic hard disk servo pattern stamper, comprising:
scanning a servo pattern on a stamper with an incident beam;
detecting a reflected beam from the stamper to form a detected pattern signal,
the reflected beam scattered according to the pattern on the stamper;
comparing the detected pattern against a reference servo pattern signal to identify
defects; and
wherein the reference servo pattern signal is determined by
scanning a known good stamper servo pattern with an incident beam to form a reflected
beam scattered by the stamper servo pattern;
detecting the amplitude of the reflected beam to form an detected amplitude signal;
and
forming a reference servo pattern signal comprising a statistical representation
of the detected amplitude signal.
8. The method according to claim 7 wherein the statistical representation is
a Kurtosis value calculated from the amplitude signal detected from at least one
servo spoke.
9. The method according to claim 7 wherein the statistical representation is
a Kurtosis value calculated from the amplitude signal detected from substantially
all servo spokes on a track.
10. Apparatus for detecting defects in a magnetic hard disk servo pattern stamper, comprising:
means for scanning a servo pattern on a stamper with an incident beam;
means for detecting a reflected beam from the stamper to form a detected pattern,
the reflected beam scattered according to the pattern on the stamper; and
means for comparing the detected pattern against a reference servo pattern to
identify defects.
11. Apparatus according to claim 10 wherein a physical offset of scattered beam
depends on the presence or absence of a servo pattern or debris.
12. Apparatus for detecting defects in a magnetic hard disk servo pattern stamper, comprising:
means for scanning a servo pattern on a stamper with an incident beam;
means for detecting a reflected beam from the stamper to form a detected pattern,
the reflected beam scattered according to the pattern on the stamper, wherein said
detecting further comprises:
means for applying a magnetic field to the stamper;
means for detecting a Kerr rotation of the reflected scattered beam to form a
Kerr rotation signal; the Kerr rotation signal having a pattern; and
means for comparing the detected pattern against a reference servo pattern to
identify defects comprising a means for comparing the Kerr rotation signal pattern
against a reference Kerr rotation servo pattern signal to identify magnetic defects.
13. Apparatus for detecting defects in a magnetic hard disk servo pattern stamper, comprising:
means for scanning a servo pattern on a stamper with an incident beam;
means for detecting a reflected beam from the stamper to form a detected pattern,
the reflected beam scattered according to the pattern on the stamper, wherein said
detecting further comprises:
means for applying a magnetic field to the stamper;
means for detecting a Kerr rotation of the reflected scattered beam to form a
Kerr rotation signal; the Kerr rotation signal having a pattern;
means for comparing the detected pattern against a reference servo pattern to
identify defects comprising a means for comparing the Kerr rotation signal pattern
against a reference Kerr rotation servo pattern signal to identify magnetic defeats;
and
wherein a physical offset of scattered beam depends on the presence or absence
of a servo pattern or debris.
14. Apparatus according to claim 12 wherein the incident beam is linearly polarized light.
15. Apparatus according to claim 12 wherein the magnetic field is applied by
an electromagnetic means.
16. Apparatus for detecting defects in a magnetic hard disk servo pattern stamper, comprising:
means for scanning a servo pattern on a stamper with an incident beam;
means for detecting a reflected beam from the stamper to form a detected pattern,
the reflected beam scattered according to the pattern on the stamper;
means for comparing the detected pattern against a reference servo pattern to
identify defects; and
wherein the reference servo pattern signal is determined by
means for scanning a known good stamper servo pattern with an incident beam to
form a reflected beam scattered by the stamper servo pattern;
means for detecting the amplitude of the reflected beam to form an detected amplitude
signal; and
means for forming a reference servo pattern signal comprising a statistical representation
of the detected amplitude signal.
17. Apparatus according to claim 16 wherein the statistical representation is
a Kurtosis value calculated from the amplitude signal detected from at least one
servo spoke.
18. Apparatus according to claim 16 wherein the statistical representation is
a Kurtosis value calculated from the amplitude signal detected from substantially
all servo spokes on a track.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to field of disk drives and more particularly to methods
for writing servo tracks on magnetic hard disks.
2. Description of Related Art
Hard disk drives provide prerecorded tracking servo information on the data
recording surfaces of their magnetic hard disks. This servo information typically
comprises servo bursts spaced evenly along tracks. Data is recorded between the
servo bursts. In most cases, servo bursts are approximately radially aligned, describing
a small arc from the disk's ID to its OD. This radial alignment makes them look
like arced spokes of the wheel. They are made to form and arc because the servo
data is read by a rotary actuator that describes the same arc because the traverses
between a disk's ID and its OD.
FIG. 1 illustrates a disk
10 having a number of servo data spokes
12.
While there are eight illustrated in the figure, a typical disk drive disk will
typically have hundreds of such servo data spokes spaced it even angles around
disk. The number of such servo data spokes depends upon the track density. As a
general rule, the greater the number of spokes, the higher the track density that
can be employed in the disk drive. In many disk drives today, the servo data takes
up approximately 11 percent of the total disk drive recording surface.
The servo bursts may be written onto a disk's surface using a variety of techniques.
The most common method is to write the servo onto the disk using the disk drive's
own magnetic head controlled typically by an externally introduced picker that
grasps the drive's rotary actuator arm upon which the read/write head is mounted.
An external mechanism incrementally moves the arm while other circuits command
the disk drive to write the servo bursts.
Another common servo-writing method comprises writing servo bursts onto the
disks already assembled onto the disk drive spindle but prior to the disk drive
spindle/disk combination, also known as a hub/disk assembly ("HDA"), being assembled
into the disk drive itself.
A newer approach employs a stamper to "print" the servo patterns on the disk
using
a high permeability stamper, as illustrated in FIG. 2, to impose a pattern on media.
As illustrated in the top leftmost portion of the figure, the disk is first DC
erased. For example, an externally applied field, the large arrows H in the figure,
causes all the magnetic domains
14 of the media to switch in an uniform
direction as illustrated. Next, a high permeability stamper
16, having a
desired pattern
17, is pressed against the disk
10. An externally
applied field of opposite polarity, illustrated by the now downward arrows, is
now applied to the disk through the stamper
16. This causes the disk areas
in contact with the stamper
16 to switch their magnetic direction to be
aligned with the externally applied field. The areas not in contact with the stamper
are shielded by the stamper. The shielded areas do not changed their magnetic orientation.
This causes the disk to assume a reversed magnetic orientation
15 in the
pattern
17 of the stamper
16.
While FIG. 2 illustrates vertically oriented magnetic domains
14 and
15 which are useful in perpendicular recording, the same technique may be
employed using horizontally oriented magnetic fields to encode horizontally oriented
magnetic domains.
The stamper
16 appears identical to FIG. 1 when viewed from a plan view.
In other words, the stamper
16 would encode the images of the servo bursts
radially aligned in arced spokes as illustrated in FIG.
1.
A problem that occurs in writing servo onto a disk regardless of the technique
used is that the disk drives can tolerate only so much servo error before servo
must be rewritten or the disk scrapped. Most drives cannot, for example, tolerate
two bad servo bursts in a row.
Today's disk drive manufacturing processed, therefore, typically check the
quality stampers servo data patterns before the stamper is used to print servo
data onto a disk.
There are three conventional methods for inspecting stampers for servo defects:
- 1. manual visual inspection;
- 2. atomic force microscopy ("AFM"); and
- 3. optical microscopy.
The problem with the first method is that it is to manually labor-intensive.
The problem with us the last two methods are that they are too time-consuming.
Better and faster methods to test stampers are needed.
SUMMARY OF THE INVENTION
The invention comprises detecting defects embedded in servo patterns on a magnetic
hard disk servo pattern stamper using scattered light. The defects may be physical
defects, magnetic defects or both. A beam is scanned across a servo pattern on
a stamper. The scattered beam is detected and then compared to the servo pattern
to identify defects. Physical defects cause offsets in the scattered beam. Magnetic
defects cause unexpected Kerr rotation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of the arced spoke patterns of servo data on a magnetic
hard disk.
FIG. 2. illustrates a method for recording magnetic marks onto the disk
using a high permeability stamper.
FIG. 3 illustrate the detection of physical stamper defects using scattered light.
FIG. 4 illustrates the detection of magnetic stamper defects using scattered
light and Kerr rotation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 illustrates an apparatus for detecting physical defects on a hard disk
servo pattern stamper. In the Fig., stamper
16 has embossed on at least
one surface thereof a servo pattern consisting of raised surfaces
17. The
servo pattern may have physical defects. A first kind of physical defect is a dropout
34. A dropout consists of a missing embossed pattern, i.e., an empty space
appears where the servo pattern would call for stamper to have an embossed raised
feature. This is illustrated by the dotted line
34 in the figure indicating
the absence of embossed feature. This is a dropout.
The second kind of physical defect is debris
36. Debris
36 may
be lodged onto the stamper
16 in area of the servo pattern
17. Debris
not only causes the false recording of a servo "pattern" on the magnetic hard disk,
the debris may cause physical damage to the disk.
Referring again to FIG. 3, a light source
30 emits a beam of light
31 which reflected by the stamper at point
33, for example, to a
reflected beam
35, which is then turned detected by light detector
32,
which may also conduct the comparison of the received pattern to the expected pattern,
or may pass the data onto a computer (not shown) for this purpose. The beam may
be of any kind of light of sufficiently small wavelength so that it does not have
a significant amount of diffractive interference with the servo pattern. The light
source is preferably a laser.
The angle of reflection of the reflected beam
35 is equal to the angle
of incidence of the incident beam
31. Thus the reflected beam will have
offset
37 at the detector
32 depending and whether the beam strikes
the surface
18 of the stamper or one of the embossed features
17
of the servo pattern. The dashed line
38 indicates the path of the reflected
beam had it struck an embossed feature
34, which is also drawn as a dotted
line to indicate a dropout.
The offset
37 may also occur when the beam
30 strikes debris
36
instead of the stamper surface
18 or embossed features
17.
In order to detect physical defects in the servo pattern, the beam must be scanned
across the servo pattern on the stamper while the optical detector
32 detects
the reflected beam, which can either be the beam
35 reflected from the surface
18 of the stamper or the beam
38 reflected from an embossed feature
17 of the stamper (or debris). By comparing the scanned beam against a known
servo pattern, the differences between patterns will be indicative of the presence
of physical defects in the pattern. The locations of these defects can then be
mapped for subsequent verification by the use of AFM or optical microscopy, for example.
A scan may comprise any of the following: rotating a stamper on a spinstand (optional)
under the optical detector (
30,
32) in combination with indexing
the detector radially, using an X-Y positioner (not shown), scanning the beam
30
itself, or any combination of these. If the beam itself is scanned, the angle of
incidence will correspondingly change. This must be compensated for in the detection algorithm.
FIG. 3 illustrates a beam size much smaller than the size of servo patterns
being observed. If, however, the beam size (spot size) is much larger, for example,
larger even then the size of the servo patterns being detected, the physical offsets
in the reflected beam may be detected by the optical detector
32 in the
form of amplitude variations. The amplitude variations may be so small that individual
servo patterns
17,
18 or defects
34 or
36 cannot be
detected. Under these circumstances, defects in the servo patterns may be detected
by using pattern recognition techniques. For example, the a known good stamper
servo pattern may be scanned one or more times to form a reference pattern of the
amplitude density of the detected beam. The pattern detected from a scan of a stamper
under test may then be compared to this reference pattern. Statistically significant
variations between the two patterns indicate the presence of defects.
A preferred statistical technique for detecting these differences is to use a
statistical
measurement known as Kurtosis. Kurtosis is a measure of peakedness of an amplitude
density curve. This measurement is highly sensitive to peaks and valleys because
it employs the fourth power of deviations from a mean or baseline signal. The statistical
formula for Kurtosis is
##EQU1##
Z is the distance from a baseline signal for a sample j.
For example, if a beam spot size is on the order of six microns and it is used
to scan a servo burst or spoke 50 microns in length, the scanning system may take
samples at 5 micron increments. The entire spoke may be scanned with only nine
samples. Each of these samples would have an amplitude distance Z from an amplitude
baseline signal. The Kurtosis value for the servo burst or spoke could be calculated
and then compared to a reference Kurtosis value computed from signal generated
by a known good servo pattern. If the deviation, for example, exceeded a predetermined
value indicative of the presence or absence of a defect, the particular bad servo
burst or spoke may be flagged for later inspection by more sensitive equipment
such as atomic force microscopes.
Alternatively, the Kurtosis value may be calculated for every servo
burst or spoke on a track. If the deviation, for example, exceeded a predetermined
value indicative of the presence or absence of a defect somewhere on a track, the
particular bad track may be flagged for later inspection by more sensitive equipment.
Referring to FIG. 4, magnetic defects in the stamper may be detected by
the use of Kerr rotation. Magnetic defects comprise areas of the stamper having
reduced or no permeability such that that even if the stamper does not have a physical
defect, it still would not print the desired servo pattern because of its failure
to conduct enough of the external magnetic field to the magnetic hard disk. Alternatively,
the small particles can adhere to the servo pattern. If the particles are non magnetic,
they will reduced the effective permeability at the location of the particle. If
they are magnetic, they will cause increased magnetization at the location of the
particle. In either case, they may be detected through Kerr rotation.
In the figure, a source of magnetic field such as electromagnetic
48 applies
a magnetic field to the stamper
16 from one side thereof. A Kerr rotation
optical detector (
40,
42,
47 and
49) is mounted facing
the other side of the stamper
16. The Kerr rotation optical scanner consists
of a light source
40, preferably a laser beam
41. This beam passes
through a polarizer
47, which preferably may be a linear polarizer. This
beam
41 strikes the stamper at a point
43 where it is reflected through
a polarization analyzer
49 and is then detected by and optical detector
42. When the beam strikes the surface of the stamper
16, it's polarization
is rotated slightly by a phenomenon known as Kerr rotation. The polarizing analyzer
49 transmits the beam
45 depending upon it's polarization. Thus the
level of transmission will depend upon the polarization of the reflected beam
45.
This difference in beam intensity may be detected by the optical detector
42,
which may also conduct the comparison of the received pattern with the expected
pattern, or pass the data onto a computer (not shown) for this purpose.
As discussed above the connection with FIG. 3, the magnetic defects in the servo
pattern may be determined by scanning the beam across the servo patterns, detecting
reflected beam intensity and comparing it is to the pattern expected. The differences
between the intensity of the reflected beam and the expected intensity are caused
by magnetic defects. In particular, a statistical representation of a the Kerr
rotation signals measured from a good pattern may be formed into a reference pattern.
A preferred statistical representation is the Kurtosis of at least one servo spoke
on a track. Alternatively, the statistical representation may be the Kurtosis of
all servo spokes on a track.
As before, the scan may be affected by rotating the stamper and radially indexing
the optical detector, by moving stamper on and X-Y positioner, by scanning the
beam itself, or by any combination of these.
While either of the aforementioned techniques may be used independently of
each other, they are preferably combined. Thus the detector is preferably adapted
to detect both offsets in the reflected beam indicative of physical defects and
also is adapted to detect variations in the Kerr rotation of the reflected beam.
The apparatus for doing so will look very much like the apparatus depicted in FIG.
4. The optical detector
40 would also be additionally sensitive to
the beam offsets
37 from FIG.
3. With a combined apparatus, a single
scan of the stamper will detect both physical and magnetic defects.
The description of the preferred apparatus may be varied by those ordinary skill
as appropriate and should not be taken as a limitation on the scope of the pending claims.
*
