Tutorial 9
Derek Wright
Wednesday, March 16th, 2005
Mass Storage Devices
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Storage Principles
Hard Disk Drives
Magneto-Optical Discs
Compact and Digital Versatile Discs
AFM Based Mass Storage
Storage Principles
• Classified by type:
– Prewritten
– Write once, read many (WORM)
– Read/Write
• Classified by operating principle:
– Mechanical
– Optical
– Magnetic Field
– Electric Field
Early Storage
• Mechanical only
– Punch cards
– Phonographs
Hard Disk Drives
• Use ferroelectric material to store
information as dipole directions
• Current aerial density = ~80 Gb/in2
– Bit size is tens of nm
• Data is stored in concentric “tracks”
• Data is written and read with a movable
electromagnetic head
Hard Disk Drives
Hard Disk Drives
• Magnetic disks are Al or glass substrates
with thin films of sputtered magnetic layers
• Disks spin at 5400 – 15000 RPM
• Read/Write head can move from the
center of the disk outwards on a “slider”
• Head is about 10 nm away from the disk
• One side of a disk is used for positioning
information and is hard-coded at the
factory
Hard Disk Drives
Inductive Write Heads
• Saturation:
– How magnetized something stays after you
apply and remove an external magnetic field
• High Coercivity:
– Hard to flip a dipole
– Hard magnetic material
• Low Coercivity:
– Easy to flip a dipole
– Soft magnetic material
Inductive Write Heads
• Write Head Requirements:
– High saturation
– Soft magnetic material
– Thermal, mechanical, and chemical stability
• High Saturation:
– To achieve high density, the magnetic material
on the disk must have high coercivity
– A high saturation in the write head makes
flipping the dipoles on the disk possible
Inductive Write Heads
• Soft Magnet Material:
– It takes energy to flip the dipole in the write
head (hysteresis loss)
– Using a soft magnetic material minimizes this
loss
• Thermal Stability:
– High process temperatures required
• Mechanical, Chemical Stability:
– Must be stable once it’s used in the hard drive
Inductive Write Heads
• Use Giant Magneto-Resistance (GMR)
• The resistivity of a GMR material greatly
changes under influence of external
magnetic field
• http://www.research.ibm.com/research/de
mos/gmr/index.html
– The Flash animation makes it really easy to
understand
Giant Magneto-Resistance
Magnetic Disks
• NiP:
– Creates smooth surface
• Cr:
– Controls microstructure and magnetic
properties of magnetic layer
• B doped CoPtCr:
– Magnetic layer
• C + Lubricant:
– Protects magnetic layer and head
Magnetic Disks
How Good Is A Hard Drive Disk?
• The quality of a disk can be measured by how
abruptly one domain changes to the next
• Measured using “transition parameter”
• a = [Mr(d + 0.5)/Hc]1/2
Future Trends
• Perpendicular dipole
orientation
– Allows higher density
because the bits can
go deep into the disk
reducing thermal
switching
– Similar to how DRAMs
went from using interlayer plate capacitors
to trench capacitors
Future Trends
Magneto-Optical Discs
• Use a slightly different setup than more
common optical disks
• Data measured as intensity changes of
reflected laser beam
• When writing, a magnetic layer’s dipoles
are flipped when
– A B-field is applied
– The laser heats the magnetic material
Magneto-Optical Discs
• When reading, a laser is reflected off the
material
• The laser’s plane of polarization is slightly
rotated
– The direction of rotation depends on the
direction of the dipole the laser is hitting
• The reflected beam passes through a
polarizer
– Converts plane rotation into intensity change
Why MO?
• Disk is not deformed in the process of
writing
– Less wear
• Rewritable
• Solid industry standards
• Current capacities up to 9.1 GB on a 51/2”
disk
Some Magneto-Optical Disks
A Magneto-Optical Setup
Compact and Digital Versatile
Discs
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Standard 120 mm diameter, 1.5 mm thick
Single track of pits molded into polycarbonate
Track begins at the center and spirals outward
CD:
– Track Spacing = 1.6 m
– Minimum Pit Length = 0.83 m
• DVD:
– Track Spacing = 0.74 m
– Minimum Pit Length = 0.4 m
CDs and DVDs
CDs and DVDs
• Laser beam (~800 m) is focused on the
bottom of the disc
• The polycarbonate further focuses the
beam (~1.7 m)
– Increases tolerance to disc surface defects
• The laser beam is reflected of the metal
with the pits in it
• A beam splitter and photodetector are
used to measure the reflected light
CDs and DVDs
• http://www.ee.washington.edu/conselec/C
E/kuhn/cdaudio/95x6.htm
Destructive Interference
• The depth of a pit is chosen to
be ¼  of the laser in the
polycarbonate
– Laser  = ~500nm
– ¼  = pit depth = 125 nm
• The laser spot is bigger than
the area of the pit
• When the beam hits a pit, the
light from the pit destructively
interferes with the light from
outside the pit
– Lower intensity reflected!
CDs vs. DVDs
• DVDs use reduced pit and track
dimensions, and thus need a shorter
wavelength laser
• DVDs can have multiple sides and layers
– The layers are semitransparent
– The laser can focus on only one of the layers
CDs vs. DVDs
• http://www.crutchfieldadvisor.com/ISEOrgbtcspd/learningcenter/home/dvd_closerlook.html
Rewritable CDs and DVDs
Rewriteable CDs and DVDs
• Writing  Create amorphous mark
– Use a laser to heat the active layer above its
melting point
– Turn off the laser and the active layer will cool
very fast and become frozen in the
amorphous phase
• Erasing  Re-crystallize the marks
– Use intermediate laser power
– Just enough energy to move back into
crystalline state, but not to melt
Rewriteable CDs and DVDs
AFM Based Mass Storage
• An AFM tip can be heated and pressed into a
polymer to make a dent
– Dent = 1, No Dent = 0
• Then the tip can use thermal flux to read the bit
– Tip in dent = high flux, Tip not in dent = low flux
• The tip or recording medium can be moved
around to read and write tracks
• Problems: Low data rate
• Can be fixed by using many tips in parallel
AFM Millipede
AFM Millipede
AFM Millipede
AFM Millipede
Thank You!
• This presentation will be available on the
web.