Modeling a MEMS probe-based storage device

Maria Varsamou1, Angeliki Pantazi2 and Theodore Antonakopoulos1

1Department of Electrical and Computer Engineering, University of Patras, Greece

e-mail: mtvars, antonako@upatras.gr

Probe-based Storage Device

System Simulator

Conclusions and References

Movement

Parameters

External

Disturbance

Stored Data

Scanner

Movement

Model

Channel

Model 1

Channel

Model 2

Channel

Model Ν

Movement in

X,Y axis

...

Multiplexer

Read-back

Signal

Ν Storage Fields

User Data

Data DetectionChannel

Statistics

Decoding/

Error Correction

Probe-based Storage System Simulator

Error Statistics

Read-back Signal

Evaluation GUI

Readback Signal

1

ScopePositioning System

Line Offset

x-disturbance

y -disturbance

X

Y

LineScan

Line _Offset

[ShockTime ShockSigx ]

[ShockTime ShockSigy ]

16 Data Fields

Y

X

ReadOut1x16

External

DisturbanceScanner Movement

Read-back Signal

Generation

Stored Data

0 1020

0

20

40-1

-0.5

0

0.5

B 01

010

200

20

40-1

-0.5

0

0.5

Α00

Electronics and

Media Noise

05

1015

20

0

20

40-1

-0.5

0

0.5

05

1015

20

0

20

40-1

-0.5

0

0.5

05

1015

20

0

20

40-1

-0.5

0

0.5

1

00 01 10

A B C

1 1 1 1 10 00 0 0 0 0 0 0 00

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 3000

10

20

30

40

B C A A A B C B C B C A A B C

X

Y

•Read procedure of a single line assuming no errors during recording•Microscanner velocity: 2.5nm/μsec•Symbol distance: 14nm•Electronics + Media NoiseSNR: 12 dB

All sectors are corrected successfully

Errors in one storage field

Read-back signal from one storage field

External Disturbance

Reference Movement

Ultra-high density storage device based on AFM techniques (> 1Τbit/in2) [1].

Thermo-mechanical recording in thin polymer films.

Parallel operation of multiple probes to compensate for low data rate of individual probes.

The medium is moved underneath the probes on Χ/Υ axes via an electromechanical microscanner.

Tip radius ~ 3-5nm

– Movement using two voice-coil actuators, one for each direction X,Y – Movement area: 120 x 120 μm2

– Mass balancing for disturbance rejection -> 100 times acceleration reduction.

– Two pairs of thermal sensors provide X/Y position information of the microscanner to the servo controller – Sensitivity 1 - 2 nm

Scan TableCoilMagnet

Thermal Position Sensors

A preamble is used at the beginning of each line for synchronization purposes.

Dedicated servo fields with predefined indentations sequences are used for generating a medium-derived positioning error signal (PES).

During write/read operation the microscanner is moved with a constant velocity.

Write pulse: 1ms – 5msResistive heater temperature: 350oC –500oC Tip temperature ~ 200oC –300oC Write Force: 50nN – 300nN (Electrostatic force pulse ~ 3V – 10V)

Positioning system

polymer

Resistiveheater

substratesubstrate substrate

currentwrite

“1”Writing “0” does not alter the polymer surface

Scan direction

WRITE

Sensing current

Less cooling by substrate

More cooling by substrate

=> T => R

Resistive heater temperature : 100oC –200oC

ΔR/R ~ 10-4

per nm

READ

010

2030

40

010

2030

40-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

Απόσταση (nm)Απόσταση (nm)

Κα

νο

νικ

οπ

οιη

μέν

ο Β

άθ

ος

Distance (nm) Distance (nm)

X-AxisY-Axis

Norm

aliz

ed D

epth

Experimental Simulator

(a) (b)

2IBM Research - Zurich, 8803 Rüschlikon, Switzerland

e-mail: agp@zurich.ibm.com

2D cantilever chip arrayMultiplexer

Storage medium on X/Y scanner

...

...

...

... ... ... ...

Servo field

Data field

Thermal position sensor

Preamble Data

Υ-Axis

Χ-Axis

Movement in a storage field

Seek

Initial

position

Field size: 100μm x 100 μmMicroscanner velocity: 2.5nm/μsecSymbol distance: 20nm

Χ-Axis

Υ-Axis

1 μm

All-‘1’ sequence Sync Track ID Multiple Sectors Data

Χ-Axis reference movement

Y-Axis reference movementTime (sec)

Time (sec)

Symbol distance Density

27 nm 1.2 Tbit/in2

18 nm 3.0 Tbit/in2

15 nm 4.0 Tbit/in2

Storage fields layout Each probe performs write/read/erase operations on a dedicated storage field

~ 100μm x 100μm.

The data are stored on constant symbol distance on X-axis, forming sequences of indentations which are stored on constant line distance on Y-axis.

Data Controller Architecture

CRC ECC Interleaving

(1,k) Line CodingDemux

(1,k) Line Coding

Mux

AFE

AFECRC ECC De-interleaving

(1,k) LineDecoding

Mux

Detection

Demux

UserData(Sector)

UserData(Sector)

(RS Encoder)

(RS Decoder)

(1,k) LineDecoding

Detection

Simulation Results

Exact simulator that enables the reliability study of a probe-based MEMS storage device even under extreme noise conditions and various kinds of external disturbances.

The simulator incorporates all system functionalities, i.e. the microscannermovement and the sensing capabilities, the read-back signal of multiple storage fields and the complete data mapping and coding scheme.

Based on Matlab/Simulink standard and custom functions/models.

The reference movement signal is generated according to the line offset from the beginning of the storage field.

External disturbances in the form of acceleration measurements over time can be applied as an input.

Exact models of the microscanner and the thermal position sensors based on measurements on a actual prototype are included.

The exact LQG algorithms that control the microscanner movement on both X,Y axes are implemented.

The medium-derived PES that assists the control algorithms is generated.

Thermo-mechanical write/read

Storage field representation

Distinct models to generate the read-back signal from each storage field –Can be parameterized for any number of fields.

A 3D indentation model (b) produced by experimental data regarding the actual indentation (a) is used.

Normally, 3D huge arrays with depth values at nanometer-level accuracy for all stored lines for every storage field would be necessary for the read-back signal generation.

Due to the (1,k)-constrained code, there are only three allowable combinations of successive symbols -> (00) , (01), (10)

Three 3D patterns (A,B,C) can be used along with the data sequences of ‘1’ and ‘0’ stored on the device.

Introduction

Example sequence of A,B,C patterns

Read-back signal generation system

A) Simulated vs. Experimental read-back signal

B) Complete read operation simulation when the system is affected by a specific external disturbance

For every (X,Y) value of the probe movement, the current line and the current symbol inside the line is calculated. Depending on the next stored symbol, the A,B,C pattern is decided. The (X,Y) depth value in the pattern gives the read-back sample.

The model also includes the various noise sources that affect the read-back signal [2], such as the electronics noise and the media noise (due to the anomalies on the polymer surface), based on measurements on an actual prototype.

Dataflow Graphical User Interface

Based on the read-back signals, the procedures of symbol detection, data decoding and error correction can be perfomed to recover the initial user data.

Statistics regarding the bit errors that appear in each storage field, the symbol errors that affect the ECC codewords, the total number of codewordsthat cannot be decoded are produced.

The simulator is a flexible tool that can be used to determine the reliability of a probe-based storage device under various noise conditions, evaluate new microscanner technologies and control architectures, as well as other parameters that affect the device functionality and performance.

Although it is based on the thermo-mechanical recording mechanism, it can be easily modified to simulate any probe-storage technology.

[1] A. Pantazi, A.Sebastian, et al, “Probe-based ultrahigh-density data storage technology,” IBM J. Res. and Dev., vol. 52, no. 4/5, pp. 493–511, 2008.

[2] A.Sebastian, A. Pantazi, H. Pozidis, and E. Eleftheriou, “Nanopositioning for Probe-Based Storage Device,” IEEE Control Systems Magazine, pp. 26–35, August 2008.

Atomic Force Microscopy (AFM) techniques use nanometer-sharp tips for imaging the surface of materials down to the nanometer scale. Such tips are exploited for creating storage devices capable of storing information with much higher density than conventional devices. This work presents a very accurate simulator of such a device and verifies its accuracy using experimentaldata of a prototype platform.

Write Operation

Read Operation

Storage