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

• 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

• 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

• 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

• 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

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

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

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

• 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 in
X-Y directions
– Sled moves in Y-direction for data access
– Sled is suspended by springs

Sled suspension is omitted from drawing
Sled with magnetic coating on bottom
Fixed part with tip array

• Size is 8mm x 8mm x 500 m m
• 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

• 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

• 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

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

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

• 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
– ...

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

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

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

• 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 of
single tip varies from 1.4 to 1.8 Mb/s
– Reached for peak data rate of 2 to 3 MB/s

• 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

• Lighter and less shock sensitive than disk drives
– Great for notebook PC’s, PDA’s 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

• 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 70’s
– Lower RAM prices killed them in the early 80’s
• Watch prices of non-volatile RAM