Modeling a MEMS probe-based storage device Maria 1 Varsamou , 1Department 2 Pantazi Angeliki of Electrical and Computer Engineering, University of Patras, Greece e-mail: mtvars, [email protected] 2IBM Research - Zurich, 8803 Rüschlikon, Switzerland e-mail: [email protected] Introduction 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. Probe-based Storage System Simulator Movement Parameters Read-back Signal Evaluation GUI Channel Model 1 Movement in X,Y axis Scanner Movement Model External Disturbance Probe-based Storage Device Read-back Signal Stored Data Channel Model 2 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. Channel Statistics Data Detection Α 0 -0.5 -1 40 ... Magnet Stored Data 20 0 0 Decoding/ Error Correction Channel Model Ν 10 20 Error Statistics Electronics and Media Noise Multiplexer Ν Storage Fields Coil User Data Thermal Position Sensors – 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. Tip radius ~ 3-5nm The simulator incorporates all system functionalities, i.e. the microscanner movement 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. – Two pairs of thermal sensors provide X/Y position information of the microscanner to the servo controller – Sensitivity 1 - 2 nm Thermo-mechanical write/read Line _Offset Line Offset X [ShockTime ShockSigx ] x-disturbance Y [ShockTime ShockSigy ] y -disturbance LineScan Write pulse: 1ms – 5ms Resistive heater temperature: 350oC –500oC Tip temperature ~ 200oC –300oC Write Force: 50nN – 300nN (Electrostatic force pulse ~ 3V – 10V) External Disturbance ReadOut1x16 Resistive heater 1 Readback Signal X substrate substrate Writing “0” does not alter the polymer surface Sensing current Positioning system 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. More cooling by substrate => T => R Less cooling by substrate 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. Field size: 100μm x 100 μm Microscanner velocity: 2.5nm/μsec Symbol distance: 20nm Χ-Axis reference movement Servo field Data field Thermal position sensor 10 20 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. A) Simulated vs. Experimental read-back signal 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. Experimental B) Complete read operation simulation when the system is affected by a specific external disturbance •Read procedure of a single line assuming no errors during recording •Microscanner velocity: 2.5nm/μsec •Symbol distance: 14nm •Electronics + Media NoiseSNR: 12 dB Movement in a storage field ... 0 0 Simulation Results Read-back Signal Generation ΔR/R ~ 10 per nm ... 20 16 Data Fields -4 ... ... ... -1 40 “1” Resistive heater temperature : 100oC –200oC ... ... -0.5 substrate polymer READ 0 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 codewords that cannot be decoded are produced. Scanner Movement Y write current 01 B 0.5 Dataflow Graphical User Interface Scope Positioning System Scan direction Υ-Axis 00 0.5 Scan Table Multiplexer Storage medium on X/Y scanner WRITE Read-back signal generation system System Simulator 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 experimental data of a prototype platform. 2D cantilever chip array and Theodore 1 Antonakopoulos Simulator Seek Time (sec) Density 1.2 Tbit/in2 3.0 Tbit/in2 4.0 Tbit/in2 Preamble Data 0.4 Normalized Depth Βάθος Κανονικοποιημένο Χ-Axis Symbol distance 27 nm 18 nm 15 nm Y-Axis reference movement Initial position Time (sec) Χ-Axis 0.2 0 Reference Movement -0.2 -0.4 -0.6 -0.8 -1 40 30 Y-Axis 10 0 0 Απόσταση (nm) 1 μm All-‘1’ sequence Sync Track ID 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. Data Controller Architecture Demux ECC External Disturbance (b) 00 Mux 01 1 AFE Interleaving Read-back signal from one storage field X-Axis 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. (1,k) Line Coding CRC 40 30 Distance (nm) (a) Multiple Sectors Data 20 10 Απόσταση (nm) Distance (nm) Υ-Axis User Data (Sector) 20 0.5 0 0 -0.5 -0.5 -1 40 -1 40 0.5 0 (1,k) Line Coding (RS Encoder) -0.5 Write Operation -1 40 20 0 Read Operation Mux User Data (Sector) CRC ECC (1,k) Line Decoding 5 0 10 0 A 10 5 0 15 20 20 0 B 0 5 20 15 10 C Detection 40 0 1 0 0 0 0 20 40 60 80 0 1 0 1 0 1 0 0 0 1 240 260 280 0 30 AFE 20 (RS Decoder) (1,k) Line Decoding 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. Example sequence of A,B,C patterns Demux De-interleaving 20 20 15 All sectors are corrected successfully Conclusions and References 10 0.5 Errors in one storage field Detection 10 Y Storage 0 B X C A A A 100 B 120 C 140 160 B C 180 B 200 C 220 A A B C 300 [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.