Method for determining an outer diameter rolloff in a process for making magnetic disks

A method is shown for testing a magnetic disk to be used in a disk drive. The method includes the steps of measuring a height profile of an outer radial edge of the disk, using the profile to determine a slope value for each of a pair of radial segments of the disk, calculating a difference value between the slope values of the pair, comparing the difference value to a preselected threshold difference value and indicating when the difference value is equal to or less than the threshold difference value. The threshold value indicates an outer most diameter of the disk where fly height operation for a head is still stable.

FIELD OF THE INVENTION 
The present invention is directed to disk drives. More particularly, the 
present invention provides an efficient and accurate method for 
determining an outer diameter rolloff for each magnetic disk to be used in 
a disk drive. The present invention can be implemented during a process of 
magnetic disk manufacture to assure that only disks having at least a 
preselected maximum radius for a data track band, are made available for 
assembly into disk drives. 
BACKGROUND OF THE INVENTION 
Disk drives are commonly used in workstations, personal computers, laptops 
and other computer systems to store large amounts of data that are readily 
available to a user. In general, a disk drive comprises a magnetic disk 
that is rotated by a spindle motor. The surface of the disk is divided 
into a series of data tracks. The data tracks are spaced radially from one 
another across a band having an inner diameter and an outer diameter. As 
should be understood, to maximize the amount of data that can be stored on 
a disk surface, the inner and outer diameters of the data track band 
should be as close as possible to the inner and outer diameters of the 
disk itself. 
Each of the data tracks extends generally circumferentially around the disk 
and can store data in the form of magnetic transitions within the radial 
extent of the track on the disk surface. An interactive element, such as a 
magnetic transducer, is used to sense the magnetic transitions to read 
data, or to generate an electric current that causes a magnetic transition 
on the disk surface, to write data. The magnetic transducer includes a 
read/write gap that contains the active elements of the transducer at a 
position suitable for interaction with the magnetic surface of the disk. 
As known in the art, the magnetic transducer is mounted by a head structure 
to a rotary actuator and is selectively positioned by the actuator over a 
preselected data track of the disk to either read data from or write data 
to the preselected data track of the disk, as the disk rotates below the 
transducer. The head structure includes a slider having an air bearing 
surface that causes the transducer to fly above the data tracks of the 
disk surface due to fluid currents caused by rotation of the disk. The air 
bearing surface of the slider has a leading edge and a trailing edge. 
Typically, in currently used heads, such as, e.g., Transverse Pressure 
Contour (TPC) heads, two spaced rails are arranged to extend 
longitudinally along the lateral sides of the air bearing surface, one 
adjacent each lateral side, from the leading edge to the trailing edge of 
the surface. The rails provide various pressure effects to cause head 
flying operation. 
Thus, the transducer does not physically contact the disk surface during 
normal operation of the disk drive. The amount of distance that the 
transducer flies above the disk surface is referred to as the "fly 
height". It is a design goal to maintain the fly height of the head at an 
even level regardless of the radial position of the head. 
In modern disk drives, a relatively rigid or hard disk is used as the 
magnetic medium. The disk comprises a hard substrate such as aluminum. 
Layers of various materials are applied to the surface of the aluminum 
substrate by, e.g., a sputtering process to provide layers that are 
substantially smooth and flat. The surfaces obtained from the sputtering 
process are designed to facilitate an even fly height for the head. The 
layered materials include a layer of magnetic material to provide the 
recording medium for the magnetic transitions representing data. 
Typically, the outer diameter of the substrate is slopped at the radial 
outer end of the disk shape. This is referred to as the rolloff of the 
disk. Thus, at the outer diameter of the disk, the disk surface is no 
longer flat and usable to sustain a stable fly height of the air bearing 
surface of the head. Indeed, the flying behavior of the air bearing 
surface can become unstable if the head moves too far into the rolloff 
region of the disk, which can result in contact between the head and the 
disk surface. Any contact between the head and the disk surface may result 
in damage to the disk or head leading to early disk drive mechanical 
failure. 
Accordingly, it is important to design the disk drive such that the outer 
diameter of the data track band is spaced suitably inward from any portion 
of the disk rolloff region where fly height degradation can occur when 
reading data from or writing data to. data tracks arranged at the outer 
diameter of the data track band. However, it is desirable that each disk 
used in a disk drive have a maximum radius relevant to the rolloff region 
that is equal to or greater than a preselected threshold radius so as to 
not impact the radial extent of the data track band beyond an acceptable 
amount. 
To that end, during the manufacture of magnetic disks that are to be used 
in a disk drive, a check should be made of the rolloff radius of each disk 
as it moves through the manufacturing process, so as to reject any disk 
having a rolloff radius less than the preselected threshold value. In this 
manner, each disk made available for assembly into a disk drive will be 
able to accommodate a maximum data track band width for a maximized data 
capacity for the drive, without undesirable fly height instability at the 
data tracks near the outer diameter of the data track band. 
At present, the rolloff radius for a disk used in a disk drive is 
determined by reference to a "dub-off" value. The dub-off value is defined 
as the maximum height undulation between two radii of the disk at the 
outer diameter. However, it has been determined that the dub-off value 
does not provide adequate information regarding fly height stability for a 
head positioned at a data track near or at the outer diameter of the data 
track band. In fact, there is a poor correlation between the dub-off value 
and fly height performance. Accordingly, the presently known disk 
measurement procedures do not provide an adequate system or process for 
achieving a reliable quality control for disks relevant to maximizing data 
capacity by assuring compliance by each disk with a maximum data band 
width having fly height stability at the outer diameter of the band. 
SUMMARY OF THE INVENTION 
The present invention provides an efficient and accurate method for 
determining an outer diameter roll-off value for a magnetic disk that 
provides information relevant to fly height stability. The present 
invention can be implemented in a disk manufacturing process to insure 
that each disk passing through the manufacturing process has an acceptable 
outer diameter roll-off value that accommodates stable fly height at the 
outer data tracks of a maximized data track band width. 
According to the present invention, profile information relating to the 
outer edge of each disk is obtained by using a profilometer or metrology 
tool. The profile information is analyzed by a processor to calculate the 
slope of the disk surface at the rolloff region of the disk, at each of a 
sequence of radial segments of the disk. The sequence of radial segments 
begins at a starting point near the outermost radius of the disk, and 
continues to the outermost radius of the disk. The radial extent of each 
segment covers a distance approximately equal to a width dimension of a 
head to be used with the disk. For example, the slope is taken over a 
radial segment approximately equal to the width of an air bearing surface 
of the head. 
Various pairs of points within the rolloff region of the disk surface are 
then selected, with each point of a pair being located in a different 
radial segment, e.g., at the center point of each respective segment. The 
two points of each pair are spaced from each other by a dimension also 
relevant to a width dimension of the respective head, e.g., when the head 
has a rail arrangement, as discussed above, the distance between the radii 
defined by the rails can be used. The change in slope between the points 
of each pair is plotted. The pairs of points are selected to provide slope 
change information from the beginning of the rolloff region to and 
including the outermost radius of the disk. 
According to the present invention, the rolloff point is defined as the 
maximum radius at which the absolute value of the slope change between 
points of a pair, including the maximum radius at the outermost point of 
the pair, is less than or equal to a preselected threshold value. The 
threshold value is determined empirically, and is selected to be a value 
of the slope change at which fly height performance of a head to be used 
with the disk, is stable. The maximum radius for each disk, as determined 
by the method of the present invention, is then compared to a specified 
maximum radius desired for disks to be assembled into disk drive products, 
to determine if the disk is acceptable for assembly into a disk drive. 
The rolloff method according to the present invention is based upon the 
geometry of a head to be used with the disk and the method determines a 
value for a maximum radius that is closely related to fly height 
performance for the head. When used as a quality control step in a process 
for manufacturing disks, the system and method of the present invention 
assures assembly of high quality disks in respect of fly height stability 
of a head at the outer diameter of a data track band designed to maximize 
the storage capacity of the disk.

DETAILED DESCRIPTION 
Referring now to the drawings, and initially to FIG. 1, there is 
illustrated an exemplary disk drive designated generally by the reference 
numeral 20. The disk drive includes a plurality of storage disks 22a-d and 
a plurality of read/write heads 24a-h. Each of the storage disks 22a-d is 
provided with a plurality of data tracks to store user data. As 
illustrated in FIG. 1, one head is provided for each surface of each of 
the disks 22a-d such that data can be read from or written to the data 
tracks of all of the storage disks. 
The storage disks 22a-d are mounted for rotation by a spindle motor 
arrangement 29, as is known in the art. Moreover, the read/write heads 
24a-h are supported by respective actuator arms 28a-h for controlled 
positioning over preselected radii of the storage disks 22a-d to enable 
the reading and writing of data from and to the data tracks. To that end, 
the actuator arms 28a-h are pivotally mounted on a pin 30 by a voice coil 
motor 32 operable to controllably rotate the actuator arms 28a-h radially 
across the disk surfaces. 
Each of the read/write heads comprises a magnetic transducer 25 mounted to 
a slider 26 having an air bearing surface. As typically utilized in disk 
drive systems, the sliders 26 cause the magnetic transducers 25 of the 
read/write heads 24a-h to "fly" above the surfaces of the respective 
storage disks 22a-d for non-contact operation of the disk drive system, as 
discussed above. When not in use, the voice coil motor 32 rotates the 
actuator arms 28a-h to position each of the read/write heads 24a-h over a 
respective landing zone 58, where the read/write heads 24a-h come to rest 
on the storage disk surfaces. 
A printed circuit board (PCB) 34 is provided to mount control electronics 
for controlled operation of the spindle motor 29 and the voice coil motor 
32. The PCB 34 also incudes read/write channel circuitry coupled to the 
read/write heads 24a-h, to control the transfer of data to and from the 
data tracks of the storage disks 22a-d. The manner for coupling the PCB 34 
to the various components of the disk drive is well known in the art. 
Referring to FIG. 2, the data tracks extend across each surface of the 
storage disks 22a-d within a band having an inner diameter 40 and an outer 
diameter 42. The actuator arms 28a-h are controlled by the control 
electronics on the PCB 34, during read/write operations, to position the 
respective heads 24a-h over preselected data tracks within the bands 
defined by the diameters 40, 42. As should be understood, it is desirable 
for the outer diameter 42 of each disk surface to be as close to the outer 
diameter of the disk 22a-d, as possible, to provide a maximum radial width 
for storing data on the disk surfaces. 
Referring now to FIGS. 3a and 3b, there is illustrated an exploded end view 
of each of two types of disk ends commonly found in disk drives. In FIG. 
3a, the slope of the surface of the disk 22 first moves upward, before 
turning downward at the outermost diameter of the disk 22. This is 
referred to as a "ski jump" type disk. In FIG. 3b, the surface of the disk 
22 gradually tapers from a flat surface to a curved surface at the 
outermost diameter of the disk 22. In each of FIGS. 3a and 3b, there is 
also shown a head 24, including air bearing surfaces comprising rails 46 
and 48. The rails 46, 48 cause the head 22 to fly above the surface as 
shown in the drawing. 
As known in the art, the fly height of the head becomes unstable when the 
rails 46, 48, and particularly outer rail 48, approaches the curved 
portions of the outer diameter of the disk 22. Thus, the outer diameter 42 
of the data track band is placed at a suitable distance from the curved 
rolloff region to maintain an acceptable and stable fly height of the head 
22 during read/write operations at the outer diameter 42. Due to 
manufacturing tolerances, the precise curved configuration for each 
particular disk will vary. Accordingly, it is desirable that the curved 
configuration, as shown in either FIGS. 3a and 3b, for any particular disk 
22 assembled into the drive 20 not impact fly height stability within a 
preselected maximum radius for the outer diameter 42. 
FIG. 4 shows, in block diagram form, an exemplary quality control test 
system according to the present invention for screening each disk 22a-d, 
prior to assembly into the disk drive 20, to make certain that fly height 
stability is acceptable at the selected value for the outer diameter 42 of 
the data track band. To advantage, the testing according to the present 
invention can be performed by the quality control system on a substrate 
prior to sputtering to make a magnetic disk. In this manner, the 
suitability of a disk is determined at an early stage of a manufacturing 
process, and the sputtering process to make magnetic disks is performed 
using substrates that are already shown acceptable in respect of fly 
height stability. 
A profilometer or metrology tool 50, such as a production mechanical 
metrology tool sold commercially by Tencor Instruments of Mountain View, 
California or optical profilometers, as sold by Zygo of Middlefield, Conn. 
and Wyco of Tucson, Ariz., can be used to obtain disk end height profile 
information corresponding to the disk end shapes shown in FIGS. 3a and 3b, 
for each disk 22a-d. The height profile information can, as discussed 
above, be performed on substrates that will be used to make magnetic 
disks. The profilometer is coupled to a processor 52 to transmit the disk 
end height profile information for processing according to the present 
invention. 
As shown conceptually in each of FIGS. 3a and 3b, the processor 52 divides 
the surface of a disk or substrate being tested into radial segments 54. 
The radial extent of each segment is approximately equal to the width of 
one of the air bearing rails 46, 48 of the head 24 that will be used in 
the disk drive 20 to read or write data from or to the disk being tested. 
The processor 52 then calculates the slope of each segment 54. 
Thereafter, the processor 52 selects pairs of segments 54, with the 
segments 54 of each pair being separated by a distance equal to the radial 
spacing between the rails 46, 48 of the head 24, as shown for one pair by 
the darkened segments in FIGS. 3a and 3b. The absolute value of the 
difference or change between the slopes of the segments 54 of each pair is 
then calculated by the processor 52. 
According to the exemplary embodiment of the present invention, the maximum 
radius for stable fly height operation, i.e. the rolloff point, is the 
radius at the outer segment 54 of a pair wherein the slope change is equal 
to or less than a predetermined value. The predetermined value is set 
empirically, as, e.g., via simulation of disk operation, to provide a 
value for slope change at which fly height operation of a head is still 
stable. In one operation of a test system according to the invention, the 
predetermined value for absolute slope change was found to be 0.0001. 
The rolloff point for each disk or substrate tested, as determined by the 
processor 52, is then compared to a desired value for the outer diameter 
42 for the data track band. The desired value for the outer diameter 42 
is, of course, selected to obtain a maximum radial extent for the data 
track band, to be able to store as much data as possible on the disk. The 
disk tested will be assembled into the disk drive 20 only if the rolloff 
point calculated by the processor 52 is equal to or greater than the 
desired value to make certain that only disks having a stable fly height 
operation at the desired outer diameter 42 are used in the disk drive 20. 
In the case of testing of a substrate, the substrate will only be used to 
make a magnetic disk if the rolloff point calculated by the processor 52 
for the substrate is equal to or greater than the desired value. The 
processor 52 has an accept/reject output to indicate whether a disk or 
substrate being tested is suitable for assembly into the disk drive 20. 
The quality control test method of the present invention determines a 
rolloff point using profile information that is closely related to head 
geometry, and, accordingly, provides a value for a maximum radius having a 
close correlation to fly height stability. 
FIG. 5 illustrates in graph form, the height profile for each of four 
representative disks labeled A, B, C and D. The height profile is 
typically determined by a disk manufacturer to show the geometry, in .mu. 
inches, of the end of the disk, relative to a reference for the disk 
surface. As can be seen in FIG. 5, each of the disks A, B, C and D has a 
nearly zero change from the reference value, until the outer diameters are 
reached, at approximately between 44 and 46.5 mm. At that radius, the 
deviations for the disks A, B, C and D, from the surface reference value, 
begin to rise and fall rapidly, indicating curved surface configurations 
of the types shown in FIGS. 3a and 3b. Disks A and B show the typical 
rolloff profile as shown in FIG. 3b, while disks C and D have a ski-jump 
type of rolloff profile, illustrated in FIG. 3a. 
FIG. 6 illustrates in graph form, the slope change, as measured by the 
present invention, for each of the four representative disks, again 
labeled A, B, C and D on the graph. As can be seen in FIG. 6, the slope 
change values begin to change rapidly from a zero value at the outer radii 
of the disks A, B, C and D, in approximately the same region (46 mm) as 
shown by the height profile to be the end of the respective disk. 
FIG. 7 provides additional indications of slope change effects upon a head. 
In a test conducted with the four disks A, B, C and D, a PZT device was 
attached to a head operating, in turn, over each of the disks A, B, C and 
D. The PZT device generates an electric signal when vibrated mechanically, 
as, e.g., when the head contacts the disk surface. As can be seen in FIG. 
7, the PZT signal, measured in volts, jumps at radii of between 46 and 
46.5 mm, indicating head/disk contact at the rolloff regions of the disks, 
as shown in FIGS. 5 and 6. 
The table of FIG. 8 shows the close correlation between the rolloff point 
determined for each of disks A, B, C and D, using the present invention, 
and the radius at which PZT voltage value becomes greater than 0.05 volts, 
indicating head/disk contact. As can be seen from the table, the PZT 
voltage values are at radii approximately equal to the rolloff point for 
the respective disk, as determined by the present invention. The table 
also shows prior art dub-off values for the disks (designated as OD 
Rolloff Value in .mu.m), each of which shows a large jump in value at 47 
mm, beyond the actual rolloff value where head/disk contact occurs, as 
indicated by the PZT signal strength. Thus, the presently used disk 
rolloff measurement does not accurately correlate rolloff information to 
fly height stability. 
FIG. 9 provides additional information for the disks A, B, C and D, on 
changes in fly height (glide height in this instance), versus slope change 
of the type measured by the method of the present invention. The glide 
shown of FIG. 9 is measured from a disk surface. As can be seen, glide 
height degradation is above -0.2.mu. inches at a slope change value of 
0.0001, and between -0.3 and -0.4.mu. inches at a slope change value of 
-0.0001. These values have been found to correlate to acceptable fly 
height stability. 
FIG. 10 is a graph showing changes in each of gap fly height and minimum 
fly height for a TPC head at correct skew angle, each as a function of 
slope change. This graph also shows acceptable changes in fly height for 
each of the read/write gap of the transducer, and the minimum fly height 
for the head itself, at absolute values for slope change of 0.0001. FIG. 
10 shows the skew effect on the head, as it relates to fly height 
degradation.