Nanopositioning for Probe-Based Data Storage - IEEE Control ...

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APPLICATIONS OF CONTROL

Nanopositioning for Probe-Based Data Storage ABU SEBASTIAN, ANGELIKI PANTAZI, HARIS POZIDIS, and EVANGELOS ELEFTHERIOU

moelectric topography sensor. Experimental results using single cantilevers have shown that data can be recorded at a density of 641 Gb/in2 and read back with raw error rates better than 10−4 [3]. Furthermore, a feasibility study shows that densities of 4 Tb/in2 can be achieved by using an advanced polymer medium [4]. One of the main drawbacks of thermomechanical probe-based data storage is the relatively low data rate per microcantilever (approximately 100 kb/s at acceptable power levels). Hence multiple microcantilevers must be operated in parallel to achieve data rates comparable to those of alternative data storage devices. A small-scale prototype system based on the thermomechanical probe storage concept has been developed at IBM (see Figure 1). This system consists of an array of microcantilevers with integrated read and write micro-heaters [5]. Each microcantilever performs write/read/erase operations in an individual storage field with an area of approximately 100 μm × 100 μm. To position the storage medium relative to the read/write probes, a microelectromechanical (MEMS) microscanner with X/Y motion capability is used [6] with the polymer medium spin-coated onto the scan table. To sense the motion of the microscanner in both scan directions, thermal position sensors are employed [7], [8]. Besides the thermal position sensors, medium-derived positional information is also used [9]. Data is stored in the storage fields along straight lines parallel to the X-axis, called tracks. A key challenge in probe-based data storage is maintaining the 2D Cantilever position of the read/write probes Array Chip on the target track as the probes are scanned along the length of the track during a read/write operation. Each track is scanned in the Y horizontal or X-direction with constant velocity, while maintainX ing the fine positioning in the Storage Medium on a MEMS Microscanner cross-track or Y-direction in the (a) (b) presence of disturbances and noise. At the end of each track a FIGURE 1 Scanning-probe-based storage device. (a) A schematic of the storage device com- short seek operation is employed prising the two-dimensional microcantilever array chip and the storage medium on a MEMS to move into the next track if necmicroscanner. (b) A photograph of the probe-based storage device prototype. essary. Precise positioning and navigation of the read/write Digital Object Identifier 10.1109/MCS.2008.924795

oday’s wide variety of data-storage applications creates an increasing demand for high-capacity, smallform-factor, and low-power memory devices. The areal densities that conventional storage technologies can achieve are expected to eventually reach fundamental physical limits such as those due to the super-paramagnetic effect in magnetic storage and the scaling limits imposed by lithography in flash memories. Therefore, new technologies are being investigated to address future needs of data-storage applications. Probe-based data storage is considered one such alternative. Data-storage concepts based on local probe techniques are derived from scanning-probe microscopy, where nanometer-sharp tips are used to interrogate and manipulate matter down to the atomic scale. Among the various probe-based data-storage concepts, thermomechanical recording and retrieval of data encoded as nanometer-scale indentations in thin polymer films is arguably the most advanced [1], [2]. The presence or absence of indentations corresponds to logical ones or zeros, respectively. Thermomechanical writing is achieved by applying a force through the microcantilever tip to the polymer layer and simultaneously heating the polymer layer locally. A microheater integrated into the silicon microcantilever on top of the tip is used for local heating. To read the written information, an additional microheater integrated into the same microcantilever serves as a ther-

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transducers on the track centerlines are critical in all types of storage devices. However, these requirements are even more stringent in the probe-based data storage device where accuracies on the order of a nanometer are needed to achieve reliable storage and retrieval of data. In subsequent sections we provide a brief overview of the various positioning components and feedback control schemes that enable this level of positioning accuracy.

POSITIONING SYSTEM The positioning system for the scanning-probe-based datastorage device prototype consists primarily of a microscanner that positions the storage medium relative to the cantilever array, the thermal position sensors that provide global position information, and the servo fields that generate medium-derived position information.

on the medium. In the probe-storage device prototype, certain storage fields and the associated microcantilevers are reserved for the generation of position information (see Figure 4). These storage fields are known as servo fields. Prewritten servo patterns in the servo fields can provide an alternative position signal. This signal measures the deviation of the microcantilever tip from the track center during the read/write operation. The medium-derived position-error signal (PES) is based on sequences of indentations (bursts), which are mutually displaced along the Y direction, arranged in such a way as to produce two signals in quadrature, which can be combined to provide a PES [2], [9]. Medium-derived PES can be generated using the servo burst configuration illustrated in Figure 4(b), where the circles represent written

Microscanner A photograph of the microscanner is shown in Figure 2. The microscanner has an X/Y-displacement range of about 120 μm, which is approximately 20% larger than the pitch between adjacent microcantilevers in the array. The scanner consists of a scan table and a pair of voicecoiltype actuators. The mechanical components are fabricated from a 400-μm-thick silicon wafer. Each voicecoil actuator consists of a pair of permanent magnets with a miniature coil mounted between them on the base plate. Actuation is achieved by applying a current to the coil, which generates a force on the magnets and induces a displacement of the actuator. This motion is coupled to the scan table by means of a mass-balancing scheme that makes the scanner robust against external shocks and vibrations.

Thermal Position Sensors Two pairs of thermal position sensors are used to provide information on the X/Y position of the microscanner. These sensors are relatively large microheaters fabricated on the cantilever-array chip and positioned directly above the scan table such that they partially overlap the scan table (see Figure 3). Displacement of the scan table results in a change of the overlapping area, which in turn results in a change of the temperature of the sensors. The change in temperature translates into a change in the electrical resistance that is sensed as a change in current. The thermal sensors operate over the entire travel range of the microscanner and hence are capable of providing global position information.

Voicecoil

Scan Table

FIGURE 2 Photograph of the microscanner. The microscanner consists of a scan table, which carries the storage medium, and two voicecoil-type actuators, which are used to move the scan table in the X/Y-plane.

Scan Table

Thermal Position Sensors

Medium-Derived Position-Error Signal Unlike nanopositioning applications such as scanningprobe microscopy, which require relative positioning, data-storage devices require absolute positioning. Moreover, this absolute positioning needs to be provided across the entire area of the storage field with nanometer-scale precision. Hence, position information must be inscribed

Magnet

FIGURE 3 Microscanner and thermal position sensors. Two pairs of thermal position sensors are used to provide positional information for the microscanner. These sensors are fabricated on the cantilever-array chip, and are positioned directly above the scan table.

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that the Q-signal exhibits zero crossings at points where the I-sigA comprehensive model of the microscanner is used nal has local extrema, as shown in Figure 4(c). A signal that has zero for control design and simulation purposes. crossings at all track center locations and a linear range between −TP/2 and TP/2 can be generated indentations recorded on four different storage fields. The by combining signals I and Q. The medium-derived PES servo bursts are recorded prior to the regular operation of provides Y-positional information around each track centhe device in the servo fields and are written with extreme- terline, and therefore has a maximum range of TP. ly high precision using a scheme known as self-servo writing. Servo bursts labeled A and B are used to create the Modeling and Identification in-phase signal (I), while servo bursts labeled C and D are A comprehensive model of the microscanner is used for used to create the quadrature signal (Q). The I-signal is control design and simulation purposes. The motion of obtained by subtracting the readback signal of the B field the microscanner along the X and Y-scan directions is from that of the A field, while the Q-signal is obtained by modeled by the transfer functions Pxx and Pyy , respecsubtracting the readback signal of the D field from that of tively. Specifically, the transfer function Pxx relates the the C field. The cross-track distance between indentation output displacement x with the coil current ux , while the centers of the same burst is equal to the track pitch (TP), transfer function Pyy relates the output displacement y whereas the distance between indentation centers in bursts with the coil current uy . Some cross-coupling between A and B (or C and D) is TP/2. The distance between the A the two axes is present. and C centers is TP/4. During regular operation of the The models Pxx and Pyy are identified using the thermal device, the data are recorded in their respective storage position sensors. The dynamics of the thermal position fields aligned with the sequence of indentations in burst C. sensors are primarily governed by the thermal dynamics In other words, the track centerlines of the data fields coin- of the micro-heaters that constitute these sensors. These cide with the corresponding centerlines of burst C. Note are adequately modeled by first-order transfer functions

Layout of Storage Fields A

B

C

D

A Field

B Field Track:0 Track:1 Track:2 Track:3

TP Track Center Line

Y

C Field

D Field

X

Servo Fields Data Fields

Y X

(a)

(b)

Amplitude

I-Signal Q-Signal PES

Y

Track:1

Track:2 (c)

FIGURE 4 Medium-derived position-error signal. A few microcantilevers and their respective storage fields are dedicated to the generation of the medium-derived position-error signal. The sequence of indentations written in these storage fields is displaced vertically. The mediumderived position-error signal (PES) is generated by combining the readback signals from these storage fields.

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PSD (nm2/Hz)

Phase (°)

Magnitude (dB)

with a bandwidth of around 4 kHz. The dynamics of the arbitrary initial position of the scan table, the target track thermal position sensors can be measured independently has to be located, which is achieved by the seek-and-settle from those of the microscanner using electrical means, and procedure. Because actuation distances are smaller and the can be accounted for while identifying Pxx and Pyy . The actuation components are lighter than those of storage frequency responses of the microscanner along the X and devices such as hard disk drives, MEMS-based storage Y-scan directions and the corresponding model responses devices such as the one described here are expected to have are shown in Figure 5(a). As can be seen, the dynamics are shorter seek times. Experimental results show seek times of dominated by the first mode, which can be accurately captured by a simple mass-spring-damper second-order model. 25 For the X and Y axes, the primary resonances are at 176.4 Hz and 0 156.5 Hz, respectively. The quality X Frequency Response −25 factor in both cases is around ten. X Transfer Function Fit The second-order response, how−50 Y Frequency Response ever, does not fully describe the Y Transfer Function Fit −75 behavior of the microscanner in the 101 102 103 104 higher frequency regime, where Frequency (Hz) the microscanner exhibits higher 0 order resonance modes. These higher order resonances change −50 with the position on the X/Y plane, −100 which further complicates exact −150 modeling of the microscanner. Sec−200 ond-order approximations to the −250 models Pxx and Pyy are used for the control design as described below. 102 103 104 101 The noise characteristics of Frequency (Hz) both the thermal position sensors (a) and the medium-derived PES are 10−1 shown in Figure 5(b). The resoluThermal Position Sensor Noise tion of the thermal position senPES Noise sor is approximately 1 nm over 10−2 its sensing bandwidth. The predominant noise sources are Johnson noise and the 1/f -noise of the silicon resistor. Although the 10−3 thermal position sensors have high resolution, these sensors suffer from drift and sensitivity 10−4 variations. The primary noise sources in the case of mediumderived PES are medium and 10−5 channel noise, which affect the readback signals from the servo fields. The resolution of PES is 10−6 similar to that of the thermal 100 101 102 104 103 position sensors but exhibits Frequency (Hz) good low-frequency fidelity. (b)

CONTROL DESIGN During regular operation of the device, the servo system has two main functions. First, from an

FIGURE 5 Experimental identification and modeling. (a) The frequency responses of the microscanner along the X- and Y-scan directions along with corresponding transfer function fit. (b) The noise spectral characteristics of the thermal position sensors and the medium-derived position-error signal (PES).

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approximately 1 ms for the worst-case seek operation [10]. The second function of the servo system is to maintain the position of the read/write probes on the center of the target track during the scanning process along the length of this track, which is achieved by the track-follow procedure. Dur-

ing track-follow, each track is scanned in the horizontal direction with constant velocity, while maintaining the fine positioning in the cross-track direction in the presence of disturbances and noise. In this article we describe the control architectures for the track-follow procedure.

Control Architecture for the X-Scan Direction

Feedforward Reference

Tracking Controller

−

Notch Filter

−

Regulator

Kalman Estimator

X Position Measurement FIGURE 6 X-control architecture. This schematic shows the control architecture for the X-scan direction, which uses positional information from the thermal position sensor. The primary components are a Kalman estimator, a linear-quadratic regulator, and a tracking controller.

Reference

Feedforward

Medium-Derived PES Tracking Controller II

Reference

Notch Filter

Tracking Controller I

−

2x1 Controller

Regulator

Kalman Estimator

Y Position Measurement (a) Reference Medium-Derived PES

Reference

Feedforward

2x1 H∞ Controller

Notch Filter

−

Y Position Measurement (b) FIGURE 7 Y-control architectures. The Y-control architecture in (a) is similar to that of the Xscan direction, but with two tracking controllers. Hence the Y-control architecture has one tracking controller for each of the two sensors, that is, the thermal position sensor and the medium-derived position-error signal (PES). An alternative two-sensor-based multi-input, single-output control architecture for the Y-scan direction is shown in (b).

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Controllers have to be designed to control the microscanner along both scan directions. However, the objectives for control design are slightly different for the X and Y-scan directions. Along the X-scan direction, a constant scan velocity must be maintained. The scan velocity is determined by the data rate of the storage device. Deviations from a uniform scan velocity while reading and writing result in timing jitter in the readback signals. This phenomenon has a significant impact on the performance of the storage device, as described below. Low-frequency components of the positioning error, on the other hand, are more tolerable owing to timing-recovery circuits that form part of the read channel. This fact implies that the X control architecture can rely on the thermal position sensors alone, even though these sensors are susceptible to drift and sensitivity variations. A schematic of the control architecture is shown in Figure 6. The main components are a Kalman estimator, a linear quadratic regulator, and a tracking controller. The first mode of Pxx dynamics and the thermal position sensor’s sensing transfer function are used to design the estimator. To account for the higher order resonant modes, notch filters are employed. An integral controller is chosen as the tracking controller. The control architecture also encompasses a feedforward controller.

Control Architecture for the Y-Scan Direction While scanning along the X direction, the scanner has to be maintained at a constant position in the Y direction such that the probes are close to the track centerlines. Any deviation from

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Kff r

Ktr

−

n

d −

u

NF

Pxx

x

xm

Sx

∧

P

Kr

FIGURE 8 Block diagram representation of the control architecture for the X-scan direction. The system models, the components of the controller, and the various signals of interest are depicted.

10.18

Y Measured Position ( m)

10.16 10.14 10.12 10.1 10.08 10.06 10.04 10.02 10 9.98

−45 −35 −25 −15

−5 5 15 25 X Measured Position ( m) (a)

35

45

30 20 10 0 Magnitude (dB)

the track center would lead to a loss in the signal strength during reading. In the X-scan direction, the timingrecovery circuits allow for some tolerance to low-frequency errors. However, in the Y-scan direction, both high- and low-frequency errors have to be minimized. Hence, control in the Y direction cannot be based solely on thermal position sensors because of the low-frequency noise, drift, and sensitivity variations that are present. In fact, it is almost impossible to achieve absolute positioning in a 100-μm × 100-μm storage field with nanometer-scale accuracy over the lifetime of a device without some form of medium-derived position information. The medium-derived PES described above is utilized for this purpose. However, it is advantageous to also use the thermal position sensors because of their large range and good high-frequency noise characteristics. Hence, for the Y-scan direction, a control architecture based on both sensors is preferable. In the two-sensor based control architecture, frequency separation is introduced to take advantage of the better noise characteristics of the thermal position sensors at higher frequencies and the low-frequency fidelity provided by the medium-derived PES [see Figure 5(b)]. In one approach, an architecture similar to that of the X direction is chosen but with two tracking controllers, one for each of the two sensors. The tracking controllers are designed in such a way as to force the frequency separation. An alternative approach is to design an H∞ multi-input, single-output controller that takes into account the noise spectral characteristics of the two sensors [9], [11]. Schematics of these control architectures are presented in Figure 7.

−10 −20 −30 −40

CLOSED-LOOP PERFORMANCE EVALUATION

−50

Tracking capability, disturbance rejection, and sensitivity to noise are the three main performance measures associated with a nanopositioning system [12]. For probebased data storage, where the data rate per probe is relatively low, a linear scan velocity of a few millimeters per second can suffice. Hence, the requirement on tracking bandwidth is rather modest. In contrast, disturbance rejection is required, but its

−70

Txn: Experimental Txn: Theoretical Txd: Experimental Txd: Theoretical

−60 −80 101

102

103

104

Frequency (Hz) (b) FIGURE 9 Closed-loop performance. (a) A 100-μm scan operation along the X-scan direction is performed in closed-loop while stepping in the Y-scan direction by 40 nm. These measurements are obtained from the thermal position sensors. (b) These closed-loop transfer functions depict the sensitivity of the closed-loop system to output disturbances and sensing noise.

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Positioning accuracy is primarily determined by the sensing noise. extent depends on the application of the probe-based storage device. If used in mobile applications, it is necessary to have good disturbance rejection capability up to about 500 Hz. The higher the requirement on disturbance rejection, the higher the sensitivity of the system to measurement noise. This fundamental tradeoff needs to be addressed when designing controllers for these devices.

Closed-Loop Transfer Functions Closed-loop transfer functions serve as the main tools for assessing the performance of the closed-loop system. The closed-loop transfer functions can be evaluated a posteriori from the models for the system and the various other components of the control system. A block diagram representation of the control architecture for the X-scan direction is shown in Figure 8. Here Sx denotes the sensor dynamics, Pˆ denotes the estimator, K r denotes the regulator, NF denotes the notch filter, K f f denotes the feedforward controller, and K tr denotes the tracking controller. The signals x, r, d, and n denote the X position, reference, output disturbance, and sensing noise, respectively. The transfer function relating the scanner position with the reference signal Txr , the transfer function relating the position with disturbance Txd , and the transfer function relating the position with sensing noise Txn serve as performance measures of the control system. Of the three, the closed-loop transfer functions Txd and Txn are critical to probe-based data storage. These transfer functions can be

evaluated using available models of the actuators, the sensors, and the designed controllers. Controllers are designed in a such a way that they satisfy the performance measures imposed in the form of these closed-loop transfer functions. The scan performance and the closed-loop frequency responses for a controller designed for the Xscan direction are presented in Figure 9. The analytically obtained closed-loop transfer functions are further validated by comparing their frequency responses with those measured experimentally. It can be seen that Txd has a −3dB bandwidth of approximately 300 Hz, which is sufficient for the operation of a prototype. For a probe-based storage device employed as a mobile storage device, a significantly higher disturbance rejection capability, for example, −3-dB bandwidth above 1000 Hz, is required.

Positioning Accuracy

PSD (nm2/Hz)

In the absence of external disturbances, positioning accuracy is primarily determined by the sensing noise. As no independent measure is available, the positioning accuracy has to be estimated using closed-loop transfer functions and the spectral characteristics of the sensing noise. For the Y-scan direction, we have two closed-loop transfer functions relating sensing noise to the position. One transfer function relates the position to the thermal position sensor noise, while the other transfer function relates the position to the medium-derived PES noise. An example of the characterization of Y positioning accuracy is shown in Figure 10. In this example it is estimated that the thermal position sensors contribute a 1σ positioning error of 0.67 nm, while the medium-derived PES contributes a 1σ positioning error of 0.29 nm. The net positioning error standard deviation is estimated to be 0.73 nm. Positioning inaccuracies due to deterministic components such as cross-coupling must be added to the stochastic component to obtain the net positioning error. 10−2 It is possible to estimate the positioning accuracy while scanning in the X direction in 10−3 a manner similar to that described above. In this case, only the thermal position sensor noise must be accounted for. The closed-loop 10−4 transfer function relating the X position to the −5 X sensing noise together with the spectral 10 characteristics of the X sensing noise are used in this estimation. In the example presented in 10−6 Figure 11(b), the 1σ positioning accuracy is estimated to be 0.71 nm. 10−7 In a typical nanopositioning system, it is difInduced by Thermal Position Sensor Noise Induced by PES Noise ficult to independently verify the positioning 10−8 1 2 3 4 accuracies. However, the nanoscale manipula10 10 10 10 tion and interrogation capabilities of the Frequency (Hz) read/write probes can be used to verify the FIGURE 10 Characterization of Y-positioning accuracy. The Y-positioning error is positioning performance of the closed-loop sysquantified using the closed-loop transfer functions. Both the thermal position sensor noise and medium-derived position-error-signal (PES) noise contribute to tem. The approach involves writing a sequence of nanoscale indents on the polymer medium the Y-positioning error.

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and then reading the indents back using the same control scheme. Owing to the microscanner perturbations arising from positioning inaccuracies, the indentation centers deviate from their intended locations. These deviations are measured by a careful analysis of the readback signals [see Figure 11(a)]. Assuming that the positioning errors incurred while reading and writing are independent, the measured deviations of the indentation centers provide a reliable estimate of the positioning error. The spectrum of the estimated positioning error using this approach is compared with that obtained using the closed-loop transfer functions. The agreement between the two spectra validates the estimates based on the closed-loop transfer functions [see Figure 11(b)]. The medium-derived estimate for the X positioning accuracy is 0.89 nm.

Probe-based data-storage devices are being considered as an ultra-highdensity, small-form-factor alternative to conventional data storage. leads to deviations from the optimal sampling instants and hence higher probability of error. Simulations of the probebased data-storage read channel show that there is a direct link between the X-positioning error and the probability of error for a fixed SNR [13]. In particular, the probability of error increases as the X-positioning error increases.

Impact of Positioning Error on Device Performance 400

Readback Signal Expected Indentation-Center Actual Indentation-Center

Readback Signal (mV)

300 200 100 0 −100 −200 −300 −400

0

20

40

60

80

100 120 140 160 180 200 nm (a)

10−2 10−3 PSD (nm2/Hz)

The positioning errors in both scan axes influence the performance of the storage device significantly. However, the X- and Y-positioning errors impact the system performance in markedly different ways. The probability of error incurred while reading back recorded information is the main performance measure for a storage device. The nanometer-scale microscanner perturbations while reading and writing introduce distortions in the readback signal and hence affect this probability. Even in the absence of any positioning inaccuracies, the probability of error is a function of the available signal-to-noise ratio (SNR) in the readback signals. A typical relationship between the probability of error and SNR is shown in Figure 12. Compared with the frequency content of the readback signals, the microscanner perturbations arising from Y-positioning errors are low frequency in nature. Hence, with a high probability, these microscanner perturbations along the Y-scan direction cannot distort a readback signal within the time frame of a few symbol periods. Both the stochastic and the deterministic component of the Y-positioning error can thus be treated as off-track error, resulting in a reduction in the SNR and hence an increase in the probability of error. Even a Y-positioning error down to very low frequencies leads to a deterioration of device performance. On the other hand, the low-frequency components of the X-positioning error are corrected by the timing-recovery circuits that form part of the probe-based data-storage read channel. The higher frequency components, however, lead to timing jitter in the readback signal. This timing jitter

10−4 10−5 10−6 10−7 10−8 101

Estimate from Txn Medium-Derived Estimated 102

103

104

Frequency (Hz) (b) FIGURE 11 Characterization of the X-positioning accuracy. (a) The readback signal shows the deviation of the indentation centers from their intended locations due to the X-positioning error. (b) The medium-derived estimate of the Xpositioning error is compared with the estimate obtained using the closed-loop transfer function.

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The X and Y positioning inaccuracies both result in an increased probability of error. The requirements for positioning accuracy in the probe storage device are set by the SNR margin given by the difference between SNRACT and SNRMIN .

READ/WRITE DEMONSTRATION

Probability of Error

The positioning capability of the storage-device prototype is further demonstrated by writing and reading data from multiple tracks in parallel using multiple levers. An ASCII file is transformed into a symbol sequence that is subse-

quently stored in four data fields using four cantilevers. The file occupies three data tracks in each field. The track size spans 90 μm in the X direction, and the distance between adjacent tracks is 40 nm in the Y direction. The symbols are spaced 20 nm apart in the X direction. Owing to the reduced information rate of the modulation code, the resulting storage density is 540 Gb/in2 . During the recording and readback of the stored information, the control architectures described above are utilized. The linear velocity is 0.25 nm/μs, which corresponds to a data rate of 12.5 kb/s. Figure 13(a) shows the readback signals from four cantilevers as they read from three tracks. Each row represents a different data lever, and the three tracks are shown as columns in the readback signals. A closeup of the readback signal is shown in Figure 13(b). From the readback signals, the symbols of each track are detected with a raw error rate of approximately 10−4 , thus demonstrating the positioning capability of the system.

X-Positioning Error

P(E)max

X-Error = 0 SNRMIN

SNRACT

Y-Positioning Error

SNR (dB)

Y-Error = 0

FIGURE 12 Impact of the X- and Y-positioning errors on device performance. The Y-positioning error results in a reduced signal-to-noise ratio (SNR) and hence a higher probability of error. The X-positioning error results in timing jitter and hence a subsequent increase in the probability of error for a fixed SNR. SNRACT denotes the available SNR in the absence of positioning errors, P(E )max denotes the maximum tolerable probability of error, and SNRMIN denotes the minimum SNR required to achieve a probability of error less than P(E )max .

CONCLUSIONS Probe-based data-storage devices are being considered as an ultra-high-density, smallform-factor alternative to conventional data storage. The probe-based data-storage concept is derived from scanning-probe microscopy, where nanometer-sharp tips are used to interrogate and manipulate matter down to the atomic scale. One implementation of this concept is based on a thermomechanical principle for storing and retrieving data encoded as nanometer-scale indentations in thin polymer films. Ultra-high densities of more than 1 Tb/in 2 have been achieved with this scheme. A small-scale

0.4

V

0.2 0 −0.2 Readback Signal (V)

0.3

V

0.2 0 −0.2

V

0.2 0 −0.2 0.2 0 −0.2

0.2 0.1 0 −0.1 −0.2

V

−0.3 0.5

1

1.5

2

2.5 3 Samples (a)

3.5

4

4.5

5 x 104

−0.4 1,700 1,800 1,900 2,000 2,100 2,200 2,300 2,400 2,500 Samples (b)

FIGURE 13 Readback signals. (a) Readback signals from four cantilevers while reading from three different tracks. (b) A closeup of the readback signal from one of the levers.

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prototype system comprising all of the elements of a probebased data storage device has been developed. One of the key challenges is the positioning of the storage medium relative to the read/write probes with nanometer-scale accuracy. A microscanner with X/Y motion capability is used to position the storage medium. Position information along both scan directions is provided by a pair of thermal position sensors. In addition, medium-derived position information provides a measure of the cross-track deviations in the Y-scan direction. Control architectures for both scan directions are presented. The X control architecture relies on thermal position sensors alone, whereas the Y control architecture relies on both the thermal position sensors and the medium-derived PES. Nanometer-scale positioning accuracies are achieved over a bandwidth of a few hundred hertz. Read/write demonstrations with sufficiently low error rates demonstrate the efficacy of the nanopositioning schemes employed.

ACKNOWLEDGMENTS We thank our colleagues of the Probe Storage Group at the IBM Zurich Research Laboratory. Special thanks go to M. Despont, U. Drechsler, M. Lantz, and H. Rothuizen for designing, modeling, and fabricating MEMS parts; to D. Jubin for characterizing and assembling the MEMS components; and to P. Bächtold for designing the electronics used in this work. We also thank C. Bolliger for assisting with the preparation of this manuscript.

REFERENCES [1] P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Dürig, B. Gotsmann, W. Häberle, M. Lantz, H. Rothuizen, R. Stutz, and G. Binnig, “The ‘millipede’—Nanotechnology entering data storage,” IEEE Trans. Nanotechnol., vol. 1, pp. 39–55, 2002. [2] E. Eleftheriou, T. Antonakopoulos, G.K. Binnig, G. Cherubini, M. Despont, A. Dholakia, U. Dürig, M.A. Lantz, H. Pozidis, H.E. Rothuizen, and P. Vettiger, “Millipede—A MEMS based scanning-probe data storage system,” IEEE Trans. Magn., vol. 39, no. 2, pp. 938–945, 2003. [3] H. Pozidis, W. Häberle, D.W. Wiesmann, U. Drechsler, M. Despont, T. Albrecht, and E. Eleftheriou, “Demonstration of thermomechanical recording at 641 Gbit/in2 ,” IEEE Trans. Magn., vol. 40, no. 4, pp. 2531–2536, 2004. [4] D. Wiesmann, U. Dürig, B. Gotsmann, A. Knoll, H. Pozidis, F. Porro, and R. Vecchione, “Ultra-high storage densities with thermo-mechanical probes and polymer media,” in Proc. Innovative Mass Storage Technologies Workshop, Enschede, The Netherlands, (2007) pp. 19–20. [Online]. Available: http://imst2007.ewi.utwente.nl/imst_program.pdf [5] M. Despont, U. Drechsler, R.R. Yu, B.H. Pogge, and P. Vettiger, “Waferscale microdevice transfer/interconnect: From a new integration method to its application in an AFM-based data-storage system,” J. Microelectromechan. Syst., vol. 13, no. 6, pp. 895–901, 2004. [6] M.A. Lantz, H. Rothuizen, U. Drechsler, W. Haeberle, and M. Despont, “A vibration resistant nanopositioner for mobile parallel-probe storage applications,” J. Microelectromechan. Syst., vol. 16, no. 1, pp. 130–139, Feb. 2007. [7] M.A. Lantz, G.K. Binnig, M. Despont, and U. Drechsler, “A micromechanical thermal displacement sensor with nanometer resolution,” Nanotechnol., vol. 16, pp. 1089–1094, May 2005. [8] S. Devasia, E. Eleftheriou, and R. Moheimani, “A survey of control issues in nanopositioning,” IEEE Trans. Contr. Syst. Technol., vol. 15, no. 5, pp. 802–823, Sept. 2007. [9] A. Pantazi, A. Sebastian, G. Cherubini, M. Lantz, H. Pozidis, H. Rothuizen, and E. Eleftheriou, “Control of MEMS-based scanning-probe data-storage devices,” IEEE Trans. Contr. Syst. Technol., vol. 15, no. 5, pp. 824–841, Sept. 2007.

[10] A. Sebastian, A. Pantazi, G. Cherubini, M. Lantz, H. Rothuizen, H. Pozidis, and E. Eleftheriou, “Towards faster data access: Seek operations in MEMS-based storage devices,” in Proc. IEEE Conf. Control Applications, Munich, Germany, Oct. 2006, pp. 283–288. [11] A. Pantazi, A. Sebastian, H. Pozidis, and E. Eleftheriou, “Two-sensor-based H∞ control for nanopositioning in probe storage,” in Proc. IEEE Conf. Decision and Control, Seville, Spain, Dec. 2005, pp. 1174–1179. [12] A. Sebastian and S. Salapaka, “Design methodologies for robust nano-positioning,” IEEE Trans. Contr. Syst. Technol., vol. 13, no. 6, pp. 868–876, Nov. 2005. [13] A. Sebastian, A. Pantazi, and H. Pozidis, “Jitter investigation and performance evaluation of a small-scale probe storage device prototype,” in Proc. 50th Annu. IEEE Global Telecommunications Conf. (GLOBECOM 2007), Washington, D.C., Nov. 2007, pp. 288–293.

AUTHOR INFORMATION Abu Sebastian received the B.E. (honors) in electrical and electronics engineering from Birla Institute of Technology and Science, Pilani, India, in 1998 and the M.S. and Ph.D. in electrical engineering from Iowa State University in 1999 and 2004, respectively. He is a research staff member at IBM’s Zurich Research Laboratory in Rüschlikon, Switzerland. His research is focused on the application of control theory to nanoscale devices. He has worked on the analysis of atomic force microscope dynamics and the development of novel modes of operation as well as probe-based data storage. Angeliki Pantazi received the Diploma and Ph.D. in electrical engineering and computer technology from the University of Patras, Greece, in 1996 and 2005, respectively. She is a research staff member at the IBM Zurich Research Laboratory in Rüschlikon. Her research focuses on probebased technologies with emphasis on nanopositioning and system-level design for probe-based data-storage devices. Haralampos Pozidis received the Diploma in computer engineering and informatics from the University of Patras, Greece, in 1994 and the M.Sc. and Ph.D. in electrical engineering from Drexel University, Philadelphia, Pennsylvania, in 1997 and 1998, respectively. From 1998 until 2001, he was with Philips Research (Eindhoven, The Netherlands), where he worked on read channel design for optical storage devices. Since 2001 he has been with the IBM Zurich Research Laboratory, working on basic recording technology, signal processing, and overall system design for MEMSbased scanning-probe data-storage devices. Evangelos Eleftheriou received a Diploma in electrical engineering from the University of Patras, Greece, in 1979 and the M.Eng. and Ph.D. in electrical engineering from Carleton University, Ottawa, Canada, in 1981 and 1985, respectively. He joined the IBM Zurich Research Laboratory in 1986, where he has worked on transmission technology, magnetic recording for HDD and tape drives, and MEMS-based probe storage. He currently manages the laboratory’s advanced storage technologies group. He was a corecipient of the 2003 IEEE Communications Society Leonard G. Abraham Prize Paper Award. In 2005, he was a corecipient of the Eduard Rhein Technology Award. Also in 2005, he became an IBM Fellow and was elected to the IBM Academy of Technology.

AUGUST 2008

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IEEE CONTROL SYSTEMS MAGAZINE 35

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