Chanmin Su

Topography and phase imaging using the torsional resonance mode

The torsional resonance mode (TR mode) is an innovative technique recently introduced for scanning probe microscopes (SPMs). In the TR mode, a cantilever tip vibrates laterally as compared to vibrating vertically in the tapping mode (TM). The tip in the TR mode remains at an almost constant height and interacts aggressively with the surface and/or the near surface because of very high torsional stiffness. In this paper, a comparative study of TM and TR modes is presented for further understanding of the mechanism of the phase angle data produced during the different tip?surface interaction modes. Topography and phase angle measurements were made on a self-assembled monolayer (SAM) sample with a two-phase structure and metal particle (MP) tapes. It is found that although surface topography images are similar using both techniques, the TR mode phase angle image provides more detailed contrast than that obtained using the TM phase technique. The experimental evidence showed that viscoelastic deformation was primarily responsible for the phase contrast and not the friction process. Next, statistical analysis of the phase angle on the MP tapes suggests that the technique can be used for evaluating the particle concentration and the uniformity of viscoelasticity and thus screening of the magnetic tapes. The reasons for the improved contrast in the phase angle imaging in the TR mode are also discussed in this paper.

A control approach to cross-coupling compensation of piezotube scanners in tapping-mode atomic force microscope imaging

In this article, an approach based on the recently developed inversion-based iterative control (IIC) to cancel the cross-axis coupling effect of piezoelectric tube scanners (piezoscanners) in tapping-mode atomic force microscope (AFM) imaging is proposed. Cross-axis coupling effect generally exists in piezoscanners used for three-dimensional (x-y-z axes) nanopositioning in applications such as AFM, where the vertical z-axis movement can be generated by the lateral x-y axes scanning. Such x/y-to-z cross-coupling becomes pronounced when the scanning is at large range and/or at high speed. In AFM applications, the coupling-caused position errors, when large, can generate various adverse effects, including large imaging and topography distortions, and damage of the cantilever probe and/or the sample. This paper utilizes the IIC technique to obtain the control input to precisely track the coupling-caused x/y-to-z displacement (with sign-flipped). Then the obtained input is augmented as a feedforward control to the existing feedback control in tapping-mode imaging, resulting in the cancellation of the coupling effect. The proposed approach is illustrated through two exemplary applications in industry, the pole-tip recession examination, and the nanoasperity measurement on hard-disk drive. Experimental results show that the x/y-to-z coupling effect in large-range (20 and 45 microm) tapping-mode imaging at both low to high scan rates (2, 12.2 to 24.4 Hz) can be effectively removed.

An integrated approach to piezoactuator positioning in high-speed atomic force microscope imaging

In this paper, an integrated approach to achieve high-speed atomic force microscope (AFM) imaging of large-size samples is proposed, which combines the enhanced inversion-based iterative control technique to drive the piezotube actuator control for lateral x-y axis positioning with the use of a dual-stage piezoactuator for vertical z-axis positioning. High-speed, large-size AFM imaging is challenging because in high-speed lateral scanning of the AFM imaging at large size, large positioning error of the AFM probe relative to the sample can be generated due to the adverse effects--the nonlinear hysteresis and the vibrational dynamics of the piezotube actuator. In addition, vertical precision positioning of the AFM probe is even more challenging (than the lateral scanning) because the desired trajectory (i.e., the sample topography profile) is unknown in general, and the probe positioning is also effected by and sensitive to the probe-sample interaction. The main contribution of this article is the development of an integrated approach that combines advanced control algorithm with an advanced hardware platform. The proposed approach is demonstrated in experiments by imaging a large-size (50 microm) calibration sample at high-speed (50 Hz scan rate).

A New Approach to Scan- Trajectory Design and Track: AFM Force Measurement Example

In this article, two practical issues encountered in the design and track of scan trajectories are studied: One issue is the large output oscillations occurring during the scanning, and the other one is the effect of modeling errors on trajectory tracking. Output oscillations need to be small in scanning operations, particularly for lightly damped systems, such as the piezoelectric actuators and the flexible structures. Moreover, modeling errors are ubiquitous in practical applications. The proposed approach extends the recently developed optimal scan-trajectory design and control method by introducing the prefilter design to reduce the output oscillations. Furthermore, a novel enhanced inversion-based iterative control (EIIC) algorithm is proposed. The EIIC algorithm is then integrated with the optimal scan-trajectory design method to compensate for the effect of modeling errors on the scanning. The convergence of the iterative control law is discussed, and the frequency range of the convergence is quantified. The proposed approach is illustrated by implementing it to the high-speed adhesion-force measurements using atomic force microscope. Simulation and experimental work are presented and discussed to demonstrate the efficacy of the proposed approach. The experimental results show that compared to the conventional DC-gain method, the proposed approach can reduce the tracking error by over 25 times during the force-curve measurements. DOI: 10.1115/1.2936841

Parametrization of atomic force microscopy tip shape models for quantitative nanomechanical measurements

The uncertainty of the shape of the tip is a significant source of error in atomic force microscopy (AFM) based quantitative nanomechanical measurements. Using transmission electron microscopy, scanning electron microscopy, or tip reconstruction images, it is possible to parametrize the models of real AFM tips, which can be used in quantitative nanomechanical measurements. These measurements use algorithms described in this article that extend classical elastic, plastic, and adhesive models of contact mechanics. Algorithms are applicable to the tips of arbitrary axisymmetric shapes. Several models of AFM tip have been utilized. The goal of tip model parameterization is to develop AFM tip-independent quantitative mechanical measurements at the nanometer scale. Experimental results demonstrate independence of the AFM measurements from tips and their closeness to bulk measurements where available. In this article the authors show the correspondence between microtensile, nanoindentation, and AFM based indenta...

A current cycle feedback iterative learning control approach to AFM imaging

In this article, we proposed a novel current cycle feedback (CCF) iterative learning control (ILC) approach to achieve high-speed imaging on atomic force microscope (AFM). AFM-imaging requires precision positioning of the AFM probe relative to the sample in 3-D (x-y-z) dimension. It has been demonstrated that with advanced control techniques such as the inversion-based iterative-control (IIC) technique, precision positioning of the AFM probe in the lateral (x-y) direction can be successfully achieved. Additional challenges, however, must be overcome to achieve precision positioning of the AFM-probe in the vertical direction. The main contribution of this article is the development of the CCF-ILC approach to the AFM z-axis control. Particularly, the proposed CCF-ILC controller design utilizes the developed robust-inversion technique to minimize the model uncertainty effect on the feedforward control, and remove the causality constraints existing in other CCF-ILC approaches. Experimental results for AFM imaging are presented and discussed to illustrate the proposed method.

Mechanical Property Mapping at the Nanoscale Using PeakForce QNM Scanning Probe Technique

Development of PeakForce QNM® a new, powerful scanning probe microscopy (SPM) method for high resolution, nanoscale quantitative mapping of mechanical properties is described. Material properties such as elastic modulus, dissipation, adhesion, and deformation are mapped simultaneously with topography at real imaging speeds with nanoscale resolution. PeakForce QNM has several distinct advantages over other SPM based methods for nanomechanical characterization including ease of use, unambiguous and quantitative material information, non-destructive to both tip and sample, and fast acquisition times. This chapter discusses the theory and operating principles of PeakForce QNM and applications to measure mechanical properties of a variety of materials ranging from polymer blends and films to single crystals and even cement paste.

Theoretical modelling and implementation of elastic modulus measurement at the nanoscale using atomic force microscope

Quantitative studies of mechanical behaviour and primarily elastic modulus are essential for material science at the nanometer scale. AFM nanoindentation is the most promising approach to address the problem. In our study we perform AFM-based nanoindentation (deflection-versus-distance curves) on a set of polymer materials with microscopic moduli ranging from 1 MPa to 10 GPa. The measurements were done with probes of different tip shapes and force levels from 100 nN to 3 μN. The tip geometry was evaluated from TEM and SEM micrographs and piecewise linearly interpolated for the use of analysis software; probe spring constant was determined from thermal tune data. The comparative analysis of nanoindentation data was carried out using models of Sneddon and Oliver-Pharr. We derived Sneddons integrals in closed form for any practical tip shape using a piecewise linear interpolation. Oliver-Pharrs method to account for plasticity for the unloading curve was adapted for Sneddons integrals. An interactive software implementation with both models was developed and applied.

Iteration-based Scan-Trajectory Design and Control with Output-Oscillation Minimization: Atomic Force Microscope Example

In this article, two practical issues occurring in the design and control of scan trajectories are studied: one, the reduction of the output oscillations during the scanning, and two, the effect of the modeling errors on the output precision during the scanning. Output oscillations need to be minimized in scanning operations of systems such as the piezo actuator and the flexible structure, while modeling errors widely exists in practical applications. The proposed approach extends the developed optimal scan trajectory design and control method, by introducing the design of a prefilter into the framework to reduce the output oscillations. Furthermore, a novel inversion-based iterative control algorithm is proposed and integrated with the optimal scan method to compensate for the modeling error effect on the scanning. The convergence of the iterative control law is discussed, and the convergence range is quantified. The proposed approach is illustrated by implementing it on the high-speed adhesion force measurement using AFM. Simulation and experimental results are presented and discussed to demonstrate the efficacy of the proposed approach.

High-speed atomic force microscopy for patterned defect review

This paper reports recent progress in using Atomic Force Microscopy as a defect review tool for patterned wafers. The key developments in the AFM technology are substantial scan speed improvements and the ability to reach feature bottom-CDs in a narrow trench. The latter is accomplished by controlling the tip-sample interaction via the short-range interaction force. Narrow trenches with vertical side wall angles comparable to current FinFET dimensions were imaged using the AFM, where imaging speeds for this sample reached about 0.2 frames per second, providing quantified topographic data for key features of the trenches. The sub-10 nm resolution data of high speed AFM demonstrates the technology as a viable solution for defect review.