DEVELOPMENT OF SCANNING METHODS IN AFM IMAGING

Size: px

Start display at page:

Download "DEVELOPMENT OF SCANNING METHODS IN AFM IMAGING"

Gavin Bell
6 years ago
Views:

1 DEVELOPMENT OF SCANNING METHODS IN AFM IMAGING M. S. Rana Department of Electrical and Electronic Engineering,Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh H. R. Pota School of Engineering and Information Technology,The University of New South Wales, Canberra, ACT-2600, Australia I.R. Petersen School of Engineering and Information Technology, The University of New South Wales, Canberra, ACT-2600, Australia Abstract An atomic force microscope (AFM) is a scientific instrument capable of investigating, controlling, and manipulating matter on a nanoscale. It is a fundamental part of research in the field of nanotechnology because of its capability to obtain threedimensional images of specimens in the areas of life sciences and material science. However, the imaging performances of currently available AFMs are influenced by their conventional zigzag raster pattern scanning method. This is due to the presence of high frequency harmonics of the triangular signal which excites the tube resonance at high scanning speeds. Over the last two decades, several works have attempted to overcome this issue in order to meet current demands. This article presents an overview of the developments in scanning techniques for high-speed AFM imaging. Keywords Nanotechnology, atomic force microscope, raster scan, spiral scan, cycloid scan, Lissajous scan. I. INTRODUCTION One of the major techniques in nanotechnology that relies on nanopositioning is scanning probe microscopy (SPM). It uses a physical probe to scan back-and-forth over the surface of a sample. SPMs are different from optical microscopes because the user does not see the surface directly. Instead, these tools feel the surface and create an image that represents it. The first SPM, the scanning tunneling microscope (STM), was invented in 1981 by the Swiss scientists Gerd Binnig and Heinrich Rohrer [1]. It can only be used to measure the topography of surfaces which are electrically conductive to some degree. Its use on surfaces which are non-conductive in nature is limited. To overcome this, in 1986, Binnig et al. [2] invented the atomic force microscope (AFM) which is a powerful imaging tool for inspecting materials and is capable of scanning both conductive and nonconductive samples regardless of the environment in which it is applied. A schematic view of an AFM is shown in Fig. 1. It is now a fundamental research tool in a broad range of disciplines and enables the precise control, manipulation, and interrogation of matter at the nanoscale level [3] [5]. Of the available microscopy techniques, it has almost all the capabilities required to make it a strongly preferred imaging tool. It can generate t h r e e - 97

low-frequency resonant modes, inherent hysteresis and creep nonlinearities; (ii) the crosscoupling effect in the piezoelectric tube scanner (PTS); (iii) the limited bandwidth of the probe; (iv) the

2 d i m e n s i o n a l images of material surfaces (as shown in Fig. 2 of a TGQ1 sample) at an extremely high resolution rate. Existing AFMs have the following limitations which prevent from achieving high scanning speeds: (i) the complex behavior of their piezo materials, such as vibrations due to the lightly damped low-frequency resonant modes, inherent hysteresis and creep nonlinearities; (ii) the crosscoupling effect in the piezoelectric tube scanner (PTS); (iii) the limited bandwidth of the probe; (iv) the limited bandwidth of the proportional-integral (PI) controllers; (v) the offset, noise and limited sensitivity of the position sensors; (vi) the limited sampling rate of the measurement unit; and (vii) the limitations of the conventional raster scanning method which uses a triangular reference signal. Over the last two decades, researchers have tried to resolve these issues. This paper focuses on the development of scanning techniques for high-speed AFM imaging. The rest of this paper is organized as follows: Section II discusses the problems of the conventional scanning method. Section III provides details of current developments in scanning techniques for high-speed AFM imaging. Finally, a conclusion and brief remarks are presented in Section IV. II. LIMITATIONS OF CONVENTIONAL SCANNING METHODS down to the atomic level (10-10 m). It is extensively used in areas such as nanolithiography, DNA nanotechnology, optics, microelectronics, material science, and nanofabrication [6] [19]. As conventional AFMs take minutes to acquire an image whereas many biological and chemical processes occur in less than a minute, a great deal could be gained by them attaining a faster scanning 98 Currently, conventional AFMs use the raster pattern scanning technique which is a key limitation for high-speed imaging. In raster pattern scanning, as shown in Fig. 3 (c), the PTS moves along the x axis (fast axis) in the forward and reverse directions (line scan), and then along the y axis (slow axis) in small steps to reach the next scan line. These movements are accomplished by applying a triangular wave signal to the x axis and a slowly increasing staircase signal to the y axis of the scanner, as illustrated in Figs. 3 (a) and (b), respectively. The triangular signal contains odd harmonics of the fundamental frequency which excite the resonance of the PTS. The amplitudes of these signal harmonics attenuate as 1=n2, with n being the harmonic number. If a fast triangular waveform is applied to the scanner, it inevitably excites the scanner s mechanical resonance, causing the scanner to vibrate and trace a distorted triangular waveform which can significantly distort

3 the generated AFM image. To avoid this problem, the scanning speed of an AFM is often limited to 1% [20] of its PTS s first resonance frequency. For most AFMs, the resonance frequency is approximately 1 khz which means that the scanning speed is limited to about 10 Hz. where the pitch (P) is the distance between two consecutive intersections of the spiral curve with a line passing through its origin and is calculated as: III. NEW SCANNING PATTERNS To overcome the limitations of the conventional scanning method, three general approaches proposed in the literature spiral, cycloid, and Lissajous scanning, are briefly discussed in the following subsections. A. Spiral Scanning Patterns To overcome the problems of a triangular reference signal, a non-raster scanning method, i.e., spiral scanning, is proposed in [21] [23]. This section presents details about the generation of input signals that force the scanner to move in a spiral pattern as shown in Fig. 4. To generate a spiral pattern as shown in Fig. 5 (b), the following signals are applied in the x and y axis, respectively, as shown in Fig. 5(a) [5], [24] [26]: Vx(t) = r sinwt; (1) Vy(t) = r coswt; (2) where r is the instantaneous radius at a time t. The area of a sample to be scanned in a spiral trajectory of pitch P at a linear velocity (v) with an instantaneous radius (r) and angular velocity (w) (rad/s) at any time (t) is: where rend is the final value of the spiral radius. The image shown in Fig. 5(c) is obtained using the spiral scanning method at a 30 Hz scanning speed. A track-follow linear quadratic Gaussian (LQG) controller presented in [27] is applied for high-speed nanopositioning along Archimedean spiral trajectories, where it achieves a very high-speed operation at scanning frequencies near the controller s bandwidth. A spiral technique with an H controller, which exploits the spiralwise narrow-band frequency content of the reference signal to enable very high-speeds and accurate positioning, is proposed in [28]. The effectiveness of the spiral trajectory nanopositioning scheme compared with that of the conventional raster positioning pattern is examined in [29] by applying it to a MEMS-based scanningprobe data-storage setup for thermo-mechanical storage on a polymer medium. A spiral scanning method with an improved multi-input multi-output (MIMO) model predictive control (MPC) scheme is applied to the PTS in [5]. By using this controller, the AFM is able to scan a 6 mm radius image within 2.04 s with a quality better than that obtained using the conventional raster pattern scanning method. However, this method s initial scanning speed is slow. 99

Fig. 3. In order to force the scanner to trace a raster pattern (c) in the x y plane, a triangular signal (a) is applied to the fast-axis, and a staircase signal (b) to the slow-axis. B.

4 Fig. 3. In order to force the scanner to trace a raster pattern (c) in the x y plane, a triangular signal (a) is applied to the fast-axis, and a staircase signal (b) to the slow-axis. B. Cycloid Scanning Patterns To overcome the problems of the spiral scanning method, a new sinusoidal scanning technique, i.e., cycloid scanning, is introduced in [30] and shown in Fig. 6. In a cycloid scan trajectory, to cover the entire scanning area, a series of circular paths is followed, with their centers continuously shifted. To generate the cycloid pattern, the following signals are applied to the x and y axes, respectively, as shown in Fig. 6(b): 100

5 where w =2p f and f is the scan frequency, r is the amplitude of the input waveforms and a is the ramp rate of the x input signal. The image shown in Fig. 6(c) is obtained using the cycloid scanning method at a 30 Hz scanning speed. The significance of this method is that it does not require specialized apparatus to develop high-quality images at very high scanning speeds and works quite satisfactorily without the need to dampen the vibratory modes of the scanner which is necessity in highspeed raster scanning AFMs [31]. Although the advantages scanning are its largely uniform spatial resolution and speed, it has the problem of scanning the same area twice. C. Lissajous Scanning Patterns In [32] an alternative non-raster scanning method based on the Lissajous pattern, which allows much faster operation than ordinary scanning patterns, is introduced. In it, a twodimensional Lissajous pattern is created by the interferenc 101

[33], with an ability that cannot be achieved by a conventional scan trajectory known as multiresolution imaging.

6 of two single tone, constant amplitude, constant frequency waveforms in a two-dimensional space. Besides an extremely narrow frequency spectrum, the Lissajous scanning trajectory possesses some unique properties that make it particularly well suited for high-speed imaging applications [33], with an ability that cannot be achieved by a conventional scan trajectory known as multiresolution imaging. This pattern is achieved by applying the following signals to the X and Y piezos of the scanner, respectively:. IV. CONCLUSION AFMs have found extensive applications in the field of nanotechnology which, over time, have become more demanding. At present, they are a fundamental part of research in this field. To achieve fast operation of an AFM according to the demands 102

scanning methods presented in Table I. However, the spiral scanning method has a slow initial scanning speed while the cycloid one scans the same area twice.

7 of modern science, it is necessary to address the issues which affect its imaging performance. To overcome the limitations of the conventional scanning method, research conducted over the past two decades has resulted in development of the spiral, cycloid and Lissajous scanning methods presented in Table I. However, the spiral scanning method has a slow initial scanning speed while the cycloid one scans the same area twice. On the other hand, the analysis and design of Lissajous scan trajectories are challenging owing to a nonlinear relationship between their harmonic actuation frequencies and resultant durations and shapes. Therefore, there is plenty of scope to undertake further research in this area. 103

8 ACKNOWLEDGMENT This work has supported by the Australian Research Council (ARC) under the grant of FL REFERENCES [1] G. Binnig and D. P. E. Smith, Single tube three-dimensional scanner for scanning tunneling microscopy, Review of Scientific Instruments, vol. 57, no. 8, pp , Aug [2] G. K. Binnig, C. F. Quate, and C. Gerber, Atomic force microscope, Physical Review Letters, vol. 56, no. 9, pp , Mar [3] M. S. Rana, High Performance Control of an Atomic Force Microscope for Faster Image Scanning, Ph.D. dissertation, University of New South Wales, Australia, [4] C.-L. Chen, J.-W. Wu, Y.-T. Lin, L.-C. Fu, and M.-Y. Chen, Precision sinusoidal local scan for large-range atomic force microscopy with auxiliary optical microscopy, IEEE/ASME Transactions on Mechatronics, vol. 20, no. 1, pp , Apr [5] M. S. Rana, H. R. Pota, and I. R. Petersen, Spiral scanning with improved control for faster imaging of AFM, IEEE Transactions on Nanotechnology, vol. 13, no. 3, pp , May [6] T. Ando, High speed atomic force microscopy coming of age, Nanotechnology, vol. 23, no. 6, p , [7] Z. Sun, B. Song, N. Xi, R. Yang, L. Hao, and L. Chen, Compensating asymmetric hysteresis for nanorobot motion control, in IEEE International Conference on Robotics and Automation (ICRA), May 2015, pp [8] A. Sebastian and S. M. Salapaka, Design methodologies for robust nano-positioning, IEEE Transactions on Control Systems Technology, vol. 13, no. 6, pp , Nov [9] S.-K. Hung and L.-C. Fu, Novel threedimensional beam tracking system for stationarysample-type atomic force microscopy, IEEE Transactions on Instrumentation and Measurement, vol. 55, no. 5, pp , [10] Y. K. Yong, B. Ahmed, and S. O. R. Moheimani, Atomic force microscopy with a 12- electrode piezoelectric tube scanner, Review of Scientific Instruments, vol. 81, no. 3, pp , Mar [11] E. Meyer, H. J. Hug, and R. Bennewitz, Scanning Probe Microscopy: The Lab on a Tip. Berlin Heidelberg New York: Springer-Verlag, [12] J. A. Vicary and M. J. Miles, Pushing the boundaries of local oxidation nanolithography: short timescales and high speeds, Ultramicroscopy, vol. 108, no. 10, pp , Sep [13] G. Kwon, S. H. Kim, M. Jeong, S. H. Han, C. S. Choi, S. J. Han, J. Hong, and H. Lee, Highspeed atomic force microscope lithography using a piezo tube scanner driven by a sinusoidal waveform, Ultramicroscopy, vol. 109, no. 8, pp , July [14] Y.-Z. Peng, J.-W. Wu, K.-C. Huang, J.-J. Chen, M.-Y. Chen, and L.- C. Fu, Design and implementation of an atomic force microscope with adaptive sliding mode controller for large image scanning, in Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC), Orlando, USA, Dec [15] M. S. Rana, H. R. Pota, and I. R. Petersen, The design of model predictive control for an AFM and its impact on piezo nonlinearities, European Journal of Control, vol. 20, no. 4, pp , Apr [16] P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lantz, H. E. Rothuizen, R. Stutz, and G. K. 104

9 Binnig, The millipede - nanotechnology entering data storage, IEEE Transactions on Nanotechnology, vol. 1, no. 1, pp , Mar [17] A. Sebastian, A. Pantazi, S. O. R. Moheimani, H. Pozidis, and E. Eleftheriou, Achieving subnanometer precision in a MEMS-based storage device during self-servo write process, IEEE Transactions on Nanotechnology, vol. 7, no. 5, pp , Sep [18] Habibullah, I. Petersen, H. Pota, and M. Rana, LQG controller with sinusoidal reference signal modeling for spiral scanning of atomic force microscope, in Industrial Electronics and Applications (ICIEA), th IEEE Conference on, Jun. 2013, pp [19] M. S. Rana, H. R. Pota, I. R. Petersen, and H. Habibullah, Effect of improved tracking for atomic force microscope on piezo nonlinear behavior, Asian Journal of Control, vol. 17, no. 3, pp. 1 15, July [20] I. A. Mahmood and S. O. R. Moheimani, Making a commercial atomic force microscope more accurate and faster using positive position feedback control, Review of Scientific Instruments, vol. 80, no. 6, pp , Jun [21] I. A. Mahmood, S. O. R. Moheimani, and B. Bhikkaji, A new scanning method for fast atomic force microscopy, IEEE Transactions on Nanotechnology, vol. 10, no. 2, pp , Mar [22] I. A. Mahmood and S. O. R. Moheimani, Fast spiral-scan atomic force microscopy, Nanotechnology, vol. 20, no. 36, pp , Aug [23] S. K. Hung, Spiral scanning method for atomic force microscopy, J. Nanosci. Nanotechnol., vol. 10, no. 7, pp , Jul [24] C. B. Ahn, J. H. Kim, and Z. H. Cho, Highspeed spiral-scan echo planar NMR imaging-i, IEEE Transactions on Medical Imaging, vol. 5, no. 1, pp. 2 7, Mar [25] M. S. Rana, H. R. Pota, and I. R. Petersen, Performance of sinusoidal scanning with MPC in AFM imaging, IEEE/ASME Transactions on Mechatronics, vol. 20, no. 1, pp , Feb [26] S. Ljunggren, A simple graphical representation of fourier-based imaging method, Journal of Magnetic Resonance, vol. 54, no. 2, pp , Sep [27] A. Kotsopoulos, A. Pantazi, and T. Antonakopoulos, Control for highspeed archimedean spiral nanopositioning, in Proceedings of the 17 th IEEE International Conference on Electronics, Circuits, and Systems (ICECS), Athens, Greece, Dec. 2010, pp [28] A. Kotsopoulos, A. Pantazi, A. Sebastian, and T. Antonakopoulos, High-speed spiral nanopositioning, in 18th IFAC World Congress, 2011, pp [29] A. G. Kotsopoulos and T. A. Antonakopoulos, Nanopositioning using the spiral of archimedes: The probe-based storage case, Mechatronics, vol. 20, no. 2, pp , Mar [30] Y. K. Yong, S. O. R. Moheimani, and I. R. Petersen, High-speed cycloid-scan atomic force microscopy, Nanotechnology, vol. 21, no. 36, pp , Sep [31] B. Bhikkaji, M. Ratnam, A. J. Fleming, and S. O. R. Moheimani, Highperformance control of piezoelectric tube scanners, IEEE Transactions on Control Systems Technology, vol. 15, no. 5, pp , Sep [32] A. Bazaei, Y. K. Yong, and S. O. R. Moheimani, High-speed lissajousscan atomic force microscopy: Scan pattern planning and control design issues, Review of Scientific 105

10 Instruments, vol. 83, no. 6, pp , Jun [33] T. Tuma, A. Sebastian, J. Lygeros, and A. Pantazi, The four pillars of nanopositioning for scanning probe microscopy: The position sensor, the scanning device, the feedback controller, and the reference trajectory, IEEE Control Systems, vol. 33, no. 6, pp , Dec [34] S. Moon, S.-W. Lee, M. Rubinstein, B. J. F. Wong, and Z. Chen, Semi-resonant operation of a fiber-cantilever piezotube scanner for stable optical coherence tomography endoscope imaging, IEEE/ASME Transactions on Mechatronics, vol. 18, no. 20, pp , Sep [35] Y. K. Yong, A. Bazaei, and S. O. R. Moheimani, Video-rate lissajousscan atomic force microscopy, IEEE Transactions on Nanotechnology, vol. 13, no. 1, pp , Jan

Vibration compensation for high speed scanning tunneling microscopy

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 12 DECEMBER 1999 Vibration compensation for high speed scanning tunneling microscopy D. Croft and S. Devasia a) University of Utah, Department of Mechanical