REVIEW OF SCIENTIFIC INSTRUMENTS 85, 073706 (2014)

High-speed adaptive contact-mode atomic force microscopy imaging with near-minimum-force Juan Ren and Qingze Zoua) Department of Mechanical and Aerospace Engineering, Rutgers University, 98 Brett Rd, Piscataway, New Jersey 08854, USA

(Received 8 April 2014; accepted 8 June 2014; published online 16 July 2014) In this paper, an adaptive contact-mode imaging approach is proposed to replace the traditional contact-mode imaging by addressing the major concerns in both the speed and the force exerted to the sample. The speed of the traditional contact-mode imaging is largely limited by the need to maintain precision tracking of the sample topography over the entire imaged sample surface, while large image distortion and excessive probe-sample interaction force occur during high-speed imaging. In this work, first, the image distortion caused by the topography tracking error is accounted for in the topography quantification. Second, the quantified sample topography is utilized in a gradientbased optimization method to adjust the cantilever deflection set-point for each scanline closely around the minimal level needed for maintaining stable probe-sample contact, and a data-driven iterative feedforward control that utilizes a prediction of the next-line topography is integrated to the topography feeedback loop to enhance the sample topography tracking. The proposed approach is demonstrated and evaluated through imaging a calibration sample of square pitches at both high speeds (e.g., scan rate of 75 Hz and 130 Hz) and large sizes (e.g., scan size of 30 μm and 80 μm). The experimental results show that compared to the traditional constant-force contact-mode imaging, the imaging speed can be increased by over 30 folds (with the scanning speed at 13 mm/s), and the probe-sample interaction force can be reduced by more than 15% while maintaining the same image quality. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884343] I. INTRODUCTION

In this paper, we present an approach that addresses major concerns in large-size (e.g., lateral scan size over 25 μm) contact-mode imaging using atomic force microscope (AFM) in both the speed and the force exerted to the sample. Contactmode (CM) imaging finds its applications in quantitatively measuring nanoscale sample topography of a wide variety of materials,1, 2 ranging from hard materials such as silicon2 to relatively soft materials such as polymers.3–5 Current CMimaging, however, is largely limited in both the imaging speed and the magnitude of the normal force (i.e., the probe-sample interaction force). High-speed CM-imaging is challenging as the dynamics of the piezoelectric actuator (along with the probe fixture) can be excited as the scanning speed increases, resulting in large positioning error of the probe relative to the sample, particularly when the imaging size is large and the hysteresis effect of the piezo actuator is pronounced.6–8 Although efforts in both hardware improvements1, 7, 9 and software/control algorithms6, 10, 11 have been proposed to compensate for such dynamics-hysteresis caused probe positioning errors, substantial fluctuation of the cantilever deflection can still exist, resulting in significant image distortions and/or large normal force (exerted to the sample).6, 7, 12 Control of the normal force, although crucial, remains as a challenge when imaging at high speed and/or over large size. The proposed approach aims to achieve large-size, high speed a) Author to whom correspondence should be addressed. Electronic mail:

[email protected].

0034-6748/2014/85(7)/073706/10/$30.00

CM-imaging while keeping the normal force around the minimal level (for maintaining a stable probe-sample interaction). The speed of constant-force CM (CF-CM) imaging mode1, 2, 13 —the most commonly used CM-imaging—is constrained by the increasingly stringent precision positioning of the probe relative to the sample as the imaging speed increases. In CF-CM imaging, the x-y-z 3D positioning of the probe relative to the sample is needed to maintain the normal force at the set-point value so that the sample topography can be measured as the z-axis piezo displacement.1, 2 As the scanning speed and the imaging size increase, the dynamics and the hysteresis effect of the piezo actuators can be excited,8, 14, 15 resulting in large probe-sample positioning error—the sample topography cannot be measured as the displacement of the z-piezo actuator anymore. The imaging speed can be increased by using high bandwidth piezo actuators.16, 17 The samples that can be imaged, however, are rather small (

High-speed adaptive contact-mode atomic force microscopy imaging with near-minimum-force.

In this paper, an adaptive contact-mode imaging approach is proposed to replace the traditional contact-mode imaging by addressing the major concerns ...
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