1372 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 20, NO. 6, DECEMBER 2011 Q Control of an Atomic Force Microscope Microcantilever: A Sensorless Approach Matthew W. Fairbairn, Student Member, IEEE, S. O. Reza Moheimani, Fellow, IEEE, and Andrew J. Fleming, Member, IEEE Abstract—The scan rate and image resolution of the atomic force microscope (AFM) operating in tapping-mode may be im- proved by modifying the quality (Q) factor of the AFM micro- cantilever according to the sample type and imaging environment. Piezoelectric shunt control is a new method of controlling the Q factor of a piezoelectric self-actuating AFM microcantilever. The mechanical damping of the microcantilever is controlled by an electrical impedance placed in series with the tip oscillation circuit. A synthetic impedance was designed to allow easy modification of the control parameters which may vary with environmental condi- tions. The proposed techniques are experimentally demonstrated to reduce the Q factor of an AFM microcantilever from 297.6 to 35.5. AFM images obtained using this method show significant improvement in both scan rate and image quality. [2011-0123] Index Terms—Atomic force microscope (AFM), AFM probe, microcantilevers, microsensors, piezoelectric cantilever, piezoelec- tric shunt control, synthetic impedance, tapping-mode AFM. I. I NTRODUCTION T HE ATOMIC force microscope (AFM) [1] senses inter- atomic forces occurring between a sharp probe tip and a sample surface to produce images of sample surfaces such as ceramic materials, biological membranes, metals, polymers, and semiconductors with subnanometer resolution [2]–[6]. The images produced are 3-D with resolution on the order of 0.1 to 1 nm. The AFM uses a microcantilever, with a sharp probe tip on its lower surface, which is scanned over a sample surface. Deflection of the cantilever, due to interatomic forces between the probe tip and the sample, at each scan point is representative of the sample height. By plotting the sample height versus the horizontal position of the probe, a 3-D image of the surface can be obtained. The high image resolution of the AFM is due to the size of the probe tip, which may be only a few atoms wide. This gives the AFM an advantage over optical microscopes, which are limited by the wavelength of visible light, which is approximately 400–700 nm. One of the main advantages of the AFM over other types of nonoptical microscopy is that it can image samples under Manuscript received April 25, 2011; revised August 9, 2011; accepted August 29, 2011. Date of publication October 14, 2011; date of current version December 2, 2011. Subject Editor C. Mastrangelo. The authors are with the School of Electrical Engineering and Computer Science, The University of Newcastle, Callaghan, NSW 2308, Australia (e-mail: Reza.Moheimani@newcastle.edu.au). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2011.2168809 Fig. 1. Schematic of the instrumentation of an AFM operating in tapping mode. natural conditions (e.g., in air or liquid). There is no need to place the sample in vacuum, coat it with metal, or dry it, which may cause damage to a living sample, making the AFM particularly useful for biological investigations [7], [8]. One drawback of the AFM compared to an optical microscope is that it takes some time to obtain an image, whereas the optical microscope can produce images in real time. Most commonly, the probe tip is dragged across the sample in constant contact, which is referred to as contact-mode imaging. Continuous lateral force on the sample from the probe tip may cause damage to soft fragile samples. Tapping mode [9], [10] was developed to reduce lateral forces on such samples. A schematic showing the typical instrumentation of an AFM operating in tapping mode is shown in Fig. 1. When operating in tapping mode, the cantilever probe is oscillated at one of its resonance frequencies, tapping the sample once every oscillation cycle while scanning. A piezoelectric stack actuator located at the base of the cantilever is typically used to oscillate the cantilever. New methods of actuation, such as electrostatic actuation [11] and coating the cantilever with piezoelectric material to act as a bimorph actuator, are being implemented to reduce the size of the AFM. The magnitude of the cantilever oscillations in free air (A 0 ) is determined by the driving signal amplitude, the cantilever spring constant, and the quality (Q) factor of the cantilever’s resonance. The magnitude of cantilever oscillations when tap- ping the sample [A(t)] is typically measured using the optical lever method which involves reflecting a laser beam off the can- tilever onto a photodiode sensor. Any change in position of the reflected laser spot on the sensor represents tip displacement. The ac signal obtained is then converted to a dc value using an rms-to-dc converter. This dc signal is sent to the Z -axis feedback controller (refer to Fig. 2) which controls the vertical 1057-7157/$26.00 © 2011 IEEE