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Large storage capacity, removable,
magnetic disk technology
Introduction
Hard Disk Drives (HDDs) continue to be the primary
storage sub-system in most computing platforms. The
technology employed offers the best performance,
reliability and cost per mega-byte of all the known
storage techniques. However, HDD designs marry the
storage medium with the recording mechanism, making the
two inseparable, and creating a fixed capacity storage
unit. Other products such as the Floppy and Optical disk
drives satisfy the un-met needs, namely, program-load,
data backup and data portability. The use of multiple
storage mechanisms in the same computing platform
increases its cost, complexity and size. Consequently,
there is a need for an economical device that can
satisfy the storage requirements of the portable,
multi-functional wireless platform, offering data
portability, security along with reliability and high
performance.
HDD recording component technologies continue to
improve at a very rapid pace, and the storage capacity
is increasing at the rate of 60% per annum. It would be
appropriate then to consider a recording mechanism that
would allow the disk in an HDD to be removable. A number
of companies have attempted such projects, unfortunately
the products exhibited poor reliability, and none have
qualified as mainstream units.
High capacity floppy disk drives have successfully
attained large market share, but the migration of
recording technology has been slow, with the result,
storage capacity has lagged application requirements.
Optical disk drives, such as the CD and DVD, have
achieved data capacities of 650MB and 4.7GB
respectively, but the large mass of the optics installed
in the accessing mechanism causes these devices to be
slow performers and thus relegated to sequential data
transfer functions, and secondary storage.
US patents 4974106, 5968627, 6023393 and 6113753
describe a technology that utilizes HDD recording heads
and magnetic films with a flexible, metal substrate to
attain high capacity, reliable, removable disk storage
products with performance similar to the HDD. In this
paper some key features of this technology are presented
along with comparisons, where appropriate, with HDD and
Floppy technology.
Areal Density
Areal density, expressed as billions of bits per
square inch of disk surface area (Gbits/in²) is the
product of linear density, bits of information per inch
recorded along the circumferential of the data track,
multiplied by track density, the tracks per inch created
along a radial line on the disk. Improvements in areal
density have been the dominant reason for the larger
storage capacity of HDD products. Floppy disk drives
utilize a plastic base film that is anisotropic, namely,
its expansion properties are significantly different
along the axis the film was first stretched compared to
an orthogonal axis. This results in Floppy drives having
a lower track density utilizing the same servo control
techniques. Figure 1 shows the areal density migration
for these two magnetic recording technologies.
Figure
1
It can be seen from the figure the slope or rate of
change in areal density for HDD products increased
markedly after the year 1997. This was a direct result
of the successful commercializing of GMR (Giant
Magneto-resistive) head technology. GMR transducers
utilize materials that change resistance in the present
of a magnetic field. The signal output is larger than
prior generation heads, and allows HDDs to operate with
very small, written track widths. Furthermore, the
transducer is mounted to a head structure that develops
both negative and positive air pressures, thereby,
maintaining the recording element in close proximity to
the magnetic film for increased linear recording
density.
HDD head/disk interface
Figure 2 illustrates a typical HDD slider with a GMR
recording transducer.
Figure
2
The slider is mounted to a flexural suspension (not
shown) and supported against the hard disk surface with
a normal force of about 3 grams. In the design
illustrated, the GMR transducer is located at the
trailing edge of the top rail. The red colored surfaces
on the head form the air bearing rails. The leading edge
of the head marked "X" has a taper of about
0.5 degree, shown by the lighter colored region, this
blends into the rails with the darker red color. The
area depicted in green is a recessed area that is about
1 to 4 microns in depth. During operation air in the
vicinity of the disk surface will move at the tangential
velocity of the disk. This air gets entrained under the
air bearing creating pressures as shown in Figure 3,
Figure 3
The figure depicts a section made along line
"X-Y" of the head shown in Figure 2. The
flexural force is marked "F" and the disk
velocity as "U". The head section is colored
dark blue while the 1 to 4 micron recess area is
illustrated in a lighter blue color. Positive pressure
is developed under the rails, the red zones of Figure 2,
while negative pressure is created due to expansion of
air in the recessed green area. The negative and
positive pressures attain equilibrium with the applied
force "F". The magnitude of these air
pressures are related to the tangential velocity
"U", but variations of velocity, such as from
an inner track to the outer track, have little effect on
the separation distance between the GMR transducer and
the magnetic medium. This occurs because changes in the
positive and negative pressures, due to velocity change,
tend to offset each other. The blue regions in Figure 2
are large depth relief's on the slider that remain at
atmospheric conditions at all times.
To understand the need for good cleanliness in HDD
products, one should consider the performance of this
head to (a) anomalies or protrusions on the disk
surface, and (b) to airborne contaminants that enter the
head region
(a) Anomalies or protrusions on
disk surfaces:
As mentioned in the earlier
discussion the air bearing has both positive
pressures and suction effects and due to these it is quite stiff, furthermore the disk
is thick, greater than 0.63 mm (0.025 inch),
thus any anomaly on the disk surface with a
height greater than or equal to the lowest
flying height would result in a hard contact and
friction, generating heat in the area of contact
or the GMR transducer if that is where the
contact occurred. These hard contacts appear as
thermal asperities in the read-back signal,
reducing system error rate performance.
Consequently, hard disks are polished to an
"Ra" (a roughness parameter) value
significantly smaller than the minimum head
flying height, and the head and disk elements
are contained in a sealed enclosure, the
Head-Disk Assembly (HDA).
(b) Air borne contaminants:
Airborne contaminants can enter
the air bearing region at the slider leading
edge "A", as illustrated in Fig. 3.
These contaminants are swept through this air
bearing by the relative motion between the
slider and the disk. The path taken by these
particles is dependent on the topography of the
slider surface, and on the pressures created in
the air bearing. The 1 to 4 micron recessed
cavity of Figure 3 contributes to fast take-off
of the slider during spin-up. However, it also
tends to attract and hold solid particles as
they travel through the head/disk interface.
Large quantities of such particles in the
recessed cavity can cause the air bearing to
fail and the slider to crash into the disk,
creating damage and loss of recorded data. Thus
HDD heads operate in a sealed environment, and
this eliminates the entrainment of solid
contaminants from the environment into the HDA.
The passage of large particles through the
head/disk interface can create hard contacts
between the recording element and the hard disk,
which contribute to friction, heat, and thermal
asperities and degraded performance.
Compliant Head/Disk interface
Figure 4 illustrates the head/disk arrangement in the
StorCard product. The StorReader has a rotor to which is
mounted a flexural member. The recording head is
attached to the flexural member at the end furthest from
the rotor pivot. The head is loaded against the disk
with a force of about 3 grams. The disk is a thin sheet
of 316 Stainless Steel or Titanium, 0.038mm (0.0015
inch) thick, polished and sputter coated with a magnetic
film. It is centered on a spindle motor (not shown) that
rotates at speeds of about 3600 RPM. Opposing the
recording head is a fixed rail that allows the disk to
be sandwiched between it and the recording head, shown
in a section view made at the trailing edge of the
slider in Figure 5, similar to a Floppy head
arrangement.
The disk, in Figure 5, is moving out of the page. The
solid black area is the location of the GMR transducer.
The flexural force "F" urges the disk towards
the fixed rail (bottom element in Figure 5). Air bearing
forces develop in this film. Air bearing forces also
develop in the opposing film between the recording head
and the disk. The recessed slots in the slider remain at
atmospheric conditions. The fixed rail has a larger
surface area than the rails on the head, thus it
develops greater air bearing force causing the disk to
flex locally, and take on a preferred shape as shown.
The slider rail geometry is designed to cause the disk
to attain this configuration, whereby, a small spacing
only occurs at the transducer element and larger spacing
at other points on the head. The head has a pitch angle,
with the leading edge operating at larger spacing than
the trailing edge, similar to the HDD interface.
Figure 4
Figure 5
Figure 6 illustrate the plan view of the slider with
external dimensions similar to the HDD head. The air
bearing rails have a leading edge taper, the lighter red
colored region, that blends into the darker red region.
The edges of the slider have generous blends. The GMR
transducer is mounted at the trailing edge of the middle
rail. The longitudinal regions colored blue, are deep
slots, greater than 8 microns in depth created to define
the air bearing geometry.
Figure
6
Figure
7
Figure 7 illustrates the air bearing pressures
developed under this head. Positive pressures develop
under the rails, while the slots remain at atmospheric
conditions. There is no negative pressure or suction
effect over the entire slider geometry. This head disk
arrangement has a large air film on one side and a small
air film opposing it as shown in Figure 5. To understand
how the interface can operate in an unsealed
environment, one can analyze its performance with, (a)
anomalies or protrusions on the disk surface, and (b)
air borne contaminants that enter the head region.
(a) Anomalies or protrusions on
disk surfaces:
The
head/disk interface is designed to produce a
large clearance, or flying height, on the
non-data side of the disk. This flying height is
an order of magnitude larger than that provided
on the data side of the disk specifically at the
recording transducer. Therefore exposure to
anomalies or protrusions is minimized on this
side. The slot and rail configuration in the
slider is designed to give a shape to the disk
and attain a low clearance at the recording
element, and higher clearances everywhere else
on the slider. If disk surface anomalies or
protrusions become lodged between the head and
the disk in the neighborhood of the recording
element, a soft contact occurs, and the disk
moves away from the contact, resulting in less
friction and less heat generation. This is
possible because the disk floats on air films
between the two heads, and the non-data head has
a thicker air film that has a lower stiffness
than the opposed thinner air film. This is very
different than the head/disk interface in an HDD.
(b) Air borne contaminants:
The
non-data side of the disk presents no problem
with regard to the passage of tiny wear
particles and other airborne contaminants. These
particles enter the head/disk interface at the
leading edge, are convected by disk motion
through the thick air film, and exit at the
slider trailing edge. The passage of wear
particles and other airborne contaminants
through the air bearing requires a closer look.
The interface has been designed using the
hydrodynamic pressure from the non-data side of
the disk, the stiffness and flexibility of the
recording disk, an etched air bearing surface in
the recording head slider, and the applied
gimbal force "F". Longitudinal slots
are fabricated in the slider, creating narrow
rails or "sub-rails" surrounded by
ambient pressure boundaries. The rails are
shaped so as to produce the required low flying
height in the immediate vicinity of the
recording element, which is located at the
trailing edge of the slider. The flying height
elsewhere is higher, increasing progressively
from the transducer location. This fact alone
causes the air bearing interface of the data
head to have less exposure to head/disk contact
caused by particles and other airborne
contaminants. However, there is another
significant benefit that is discussed below.
Understanding of the motion of tiny particles in the
thin air film of the head/disk interface requires the
consideration of low Reynolds number fluid dynamics. The
characteristics of viscous flow are classified according
to the magnitude of the non-dimensional Reynolds number. Because the
particle diameter is so small, the resulting Reynolds
number has a magnitude of less than one. This classifies
the motion of the particle to be a low Reynolds number
flow or "creeping" flow. In this case, the
drag force acting on the particle is proportional to the
particle diameter to the first power. However, the
particle mass and weight are each proportional to the
particle diameter raised to the third power. For
realistic particle sizes, such as with a diameter of 25
nm; as this particle enters the air bearing, it almost
immediately takes on the local air velocity, regardless
of its own initial velocity. The particle then is
convected through the air bearing interface, moving with
the speed and direction of the adjacent air molecules.
The narrow rails of the slotted air bearing surface have
sharp pressure variations, pressure gradients, between
the central portion of the rail and the slot edge. This
pressure gradient causes air to be bled off from the
rail into the slot. It also causes a significant
fraction of wear particles and other airborne
contaminants to be convected into the slots. Once in a
slot, a particle is convected longitudinally along the
slider length and flushed out of the air bearing at the
trailing edge. The ambient pressure in the slot and the
higher air pressure over the rails adjacent to the slot
keep the particle within the slot boundaries. Particles
that avoid being captured by the ambient slots travel
the full slider length. Those that arrive in the
vicinity of the magnetic transducer may contact either
the slider or disk, but this would be a soft contact
since the light, flexible disk can move away from a
contact in the direction of larger flying height. Such
contact will have little or no adverse effect on the
recording head and its transducer.
Exposure to the environment
The StorCard product has a shutter design that is
locked when the card is not inserted into the reader
mechanism. The shutter is designed to maintain a small
spacing with the covers over a finite distance. This
space acts as a labyrinth seal restricting particles
from entering the disk housing. The StorReader has a
flexural seal that restricts particles from entering the
reader when a StorCard is not insert into the
mechanism. This seal is designed to move in a plane
perpendicular to the mechanism, and upon insertion of
the StorCard a seal is established between the reader
and the card surfaces restricting entry of environmental
contaminants. The GMR transducer in the recording head
has a thin layer of Carbon deposited on it to prevent
corrosion. The disk is also coated with a layer of
Carbon and a topical lubricant, similar to the HDD.
During operation liners attached to the StorCard walls,
similar in construction to the Floppy diskette, wipe the
disk. Finally, the flexible metal disk is orders of
magnitude harder than the Mylar disk and will not
scratch easily. It is also harder than the Aluminum hard
disk.
Summary
US patents 4974106, 5968627, 6023393 and 6113753
describe a flexible medium recording technology that
creates a compliant head disk interface. The recording
head and the magnetic films are similar to the HDD, and
products that utilize this technology can benefit from
the research and development directed towards increasing
areal recording density in HDD products. Furthermore, in
removable media products the recording medium can be
removed and a new one inserted to provide unlimited data
storage capacity. The key opportunity is to identify a
capacity point that can enable a variety of
applications. The product can then have long service
life and manufacturing efficiencies would reduce product
cost to attain an attractively priced data storage
mechanism.
