MEMS-BASED INTEGRATED-CIRCUIT MASS-STORAGE SYSTEMS

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MEMS-BASEDINTEGRATED-CIRCUITMASS-STORAGE SYSTEMS

• L. R. Carley, G. R. Ganger, D. F. Nagle
• Carnegie-Mellon University

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Paper highlights

• Discusses a new secondary storage technology that
  could revolutionize computer architecture
• Faster than hard drives
• Lower entry cost
• Lower weight and volume
• Lower power consumption
• Paper emphasis is on physical description of
  device

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DISK DRIVE LIMITATIONS

• Disk drive capacities double every year
• Better than the 60 per year growth rate of
  semiconductor memories
• Two major limitations of disk drives are
• Access times decreases have been minimal
• Minimum entry cost remains too high for many
  applications

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Stating the problem

• We need a type of new mass storage that can
  break both barriers of
• Access times
• Minimum entry cost
• New mass storage should also be significantly
  cheaper than non-volatile RAM
• 100 now buys 1 GB of flash memory

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MEMS

• Microelectromechanical systems (MEMS) use
• Same parallel wafer-fabrication process as
  semiconductor memories
• Keeps the prices low
• Same mechanical positioning of R/W heads as disk
  drives
• Data can be stored using higher density thin-film
  technology

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Main advantages of MEMS (I)

• Potential for dramatic decreases in
• Entry cost
• Access time
• Volume
• Mass
• Power dissipation
• Failure rate
• Shock sensitivity

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Main advantages of MEMS (II)

• Integrate storage with computation
• Complete systems-on-a-chip integrating
• Processing unit
• RAM
• Non-volatile storage
• Many many new portable applications

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THE CMU MEMS PROTOTYPE

• Like a disk drive, it has
• recording heads
• a moving magnetic recording medium
• Major departures from disk drive architecture are
• MEMS recording headsprobe tipsare fabricated in
  a parallel wafer-level manufacturing process
• Media surface does not rotate

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How the media surface moves

• Media surfaces that rotate require ball bearings
• Very small ball bearings have striction
  problems that prevent accurate positioning
• Elements would move by sticking and slipping
• Best solution is to have media sled moving inX-Y
  directions
• Sled moves in Y-direction for data access
• Sled is suspended by springs

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Conceptual view
Sled suspension is omitted from drawing
Sled with magnetic coating on bottom
Fixed part with tip array
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The media sled

• Size is 8mm x 8mm x 500 mm
• Held over the probe tip array by a network of
  springs
• Motion applied through electrostatic actuators
• Motion limited to 10 or less of
  suspension/actuator length
• Each probe tip can only sweep 1 of the media sled

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The probe tip array

• Includes a large number of probe tips for
• Being able to access whole media sled(in
  combination with X-Y motions of sled)
• Improving data throughput
• Increasing system reliability

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Probe tip positioning (I)

• Most MEMS include some form of tip height control
  because
• Media surface is not perfectly flat
• Probe tip heights can vary
• CMU prototype places each probe tip on a
  separate cantilever
• Cantilever is electrostatically actuated to a
  fixed distance from the media surface

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Probe tip positioning (II)

• IBM Millipede
• Uses a 32 x 32 array of probe tips
• Each tip is placed at the end of aflexible
  cantilever
• Cantilever bends when tip touches surface
• HP design places media surface and probe tips
  sufficiently apart
• No need to control probe tips

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Probe tip positioning (III)

• CMU solution is most complex of three
• Must control individual heights of 6,400 probe
  tips
• Required by recording technology

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Probe tip fabrication

• Major challenge is fabricating read/write probe
  tips in a way that is compatible with the
  underlying CMOS circuitry
• This includes
• thermal compatibility
• geometrical compatibility
• chemical compatibility
• ...

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Media positioning

• Systems current target is to have each probe tip
  in the middle of a 100 mm square
• Media actuator must be able to move at least 50
  mm in each direction
• Can be achieved with an actuation voltage of 120V
• Well above CMOS rated voltage

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Storing, reading and writing bits

• CMU prototype uses same magnetic recording
  technology as current disk drives
• Minimum mark size is around 80mm x 80mm
• Other solutions include
• Melting pits in a polymer (IBM Millipede)
• Raises tip wear issues
• Phase change media (HP prototype)
• Same technology as CD-ROM

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PROTOTYPE PERFORMANCE (I)

• All data were obtained through simulation
• Average service time around 0.52 ms
• Disk drive service time is 10.1 ms
• Key factor for service time is X-seek time
• I/O bandwidth depends on
• number of simultaneously active tips
• per-tip data rate

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PROTOTYPE PERFORMANCE (II)

• Sustainable data rate is not a linear function of
  access data rate
• Track switching time now depends on access
  velocity
• Faster sled means higher turn around time
• Maximum sustainable data rate ofsingle tip
  varies from 1.4 to 1.8 Mb/s
• Reached for peak data rate of 2 to 3 MB/s

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Application performance

• PostMark benchmark
• Models file activity in Internet servers
• Prototype is 3.4 times faster than current
  drives
• Much faster metadata updates
• TPC-D benchmark
• Models transaction processing
• Prototype is 3.9 times faster despite extensive
  caching in competing disk drive

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POTENTIAL APPLICATIONS

• Lighter and less shock sensitive than disk drives
• Great for notebook PCs, PDAs and video
  camcorders
• Lower cost than disk drives in 1 to 10 GB range
• Will open many new applications
• High areal densities
• Great for storing huge amounts of data
• Can combine computing and storage on a single chip

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MY OVERALL OPINION

• Technology has a bright future if and when
  production kinks get solved
• We should remain somewhat skeptical
• Not the first gap-filling technology to be
  tried
• Bubble memories were hot in the 70s
• Lower RAM prices killed them in the early 80s
• Watch prices of non-volatile RAM