MEMS-BASED INTEGRATED-CIRCUIT MASS-STORAGE SYSTEMS

1. MEMS-BASEDINTEGRATED-CIRCUITMASS-STORAGE SYSTEMS L. R. Carley, G. R. Ganger, D. F. Nagle Carnegie-Mellon University

2. 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

3. 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

4. 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

5. 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

6. Main advantages of MEMS (I) • Potential for dramatic decreases in • Entry cost • Access time • Volume • Mass • Power dissipation • Failure rate • Shock sensitivity

7. 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

8. 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 heads—probe tips—are fabricated in a parallel wafer-level manufacturing process • Media surface does not rotate

9. 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

10. Conceptual view Sled suspension is omitted from drawing Sled with magnetic coating on bottom Fixed part with tip array

11. 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

12. 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

13. 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

14. 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

15. Probe tip positioning (III) • CMU solution is most complex of three • Must control individual heights of 6,400 probe tips • Required by recording technology

16. 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 • ...

17. Media positioning • System’s 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

18. 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

19. 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

20. 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

21. 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

22. POTENTIAL APPLICATIONS • 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

23. 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 70’s • Lower RAM prices killed them in the early 80’s • Watch prices of non-volatile RAM