An integrated hinged dual-probe for co-target fast switching imaging.

The diversity of functional applications of atomic force microscopes is the key to the development of nanotechnology. However, the single probe configuration of the traditional atomic force microscope restricts the realization of different application requirements for the same target area of a single sample, and the replacement of the working probe will lead to the loss of the target area. Here, the design, simulation, fabrication, and application of a unique atomic force microscope dual-probe are presented, which consists of a pair of parallel cantilevers with a narrow gap and a U-shaped hinged probe base. The Integrated Hinged Dual-Probe (IHDP) is developed specifically for fast switching of probes working in limited space and independent and precise manipulation of each probe. The deflection signal sensing of two cantilevers is achieved simultaneously by a single laser beam, and the decoupled independent cantilever deflection signals do not interfere with each other. The switching of the working probe is achieved by a piezoelectric ceramic with a 2 µm stroke and U-shaped hinge structure, which is fast and does not require tedious and repetitive spatial position calibration. By measuring standard grid samples, IHDP exhibits excellent measurement and characterization capabilities. Finally, a working probe switching imaging experiment was conducted on solidified rat cardiomyocytes, and the experimental process and imaging results demonstrated the superiority of IHDP in switching probe scanning imaging of the same target area of a single sample. The two probes of IHDP can undergo arbitrary functionalization modifications, which helps achieve multidimensional information acquisition for a single target.

Large Range Atomic Force Microscopy with High Aspect Ratio Micropipette Probe for Deep Trench Imaging.

Atomic force microscopy (AFM) has been adopted in both industry and academia for high-fidelity, full-profile topographic characterization. Typically, the tiny tip of the cantilever and the limited traveling range of the scanner restrict AFM measurement to relatively flat samples (recommend 1 µm). The primary objective of this work is to address these limitations using a large-range AFM (measuring height >10 µm) system consisting of a novel repairable high aspect ratio probe (HARP) with a nested-proportional-integral-derivative (nested-PID) AFM system. The HARP is fabricated using a reliable, cost-efficient bench-top process. The tip is then fused by pulling the end of the micropipette cantilever with a length up to hundreds of micrometers and a tip diameter of 30 nm. The design, simulation, fabrication, and performance of the HARP are described herein. This instrument is then tested using polymer trenches which reveals superior image fidelity compared to standard silicon tips. Finally, a nested-PID system is developed and employed to facilitate 3D characterization of 50-µm-step samples. The results demonstrate the efficacy of the proposed bench-top technique for the fabrication of low-cost, simple HAR AFM probes that facilitate the imaging of samples with deep trenches.

Speeding up the Topography Imaging of Atomic Force Microscopy by Convolutional Neural Network.

Atomic force microscopy (AFM) provides unprecedented insight into surface topography research with ultrahigh spatial resolution at the subnanometer level. However, a slow scanning rate has to be employed to ensure the image quality, which will largely increase the accumulated sample drift, thereby, resulting in the low fidelity of the AFM image. In this paper, we propose a fast imaging method which performs a complete fast Raster scanning and a slow µ-path subsampling together with a deep learning algorithm to rapidly produce an AFM image with high quality and small drift. A supervised convolutional neural network (CNN) model is trained with the slow µ-path subsampled data and its counterpart acquired with fast Raster scan. The fast speed acquired AFM image is then inputted to the well-trained CNN model to output the high quality one. We validate the reliability of this method using a silicon grids sample and further apply it to the fast imaging of a vanadium dioxide thin film. The results demonstrate that this method can largely improve the imaging speed up to 10.3 times with state-of-the-art imaging quality, and reduce the sample drift by 8.9 times in the multiframe AFM imaging of the same area. Furthermore, we prove that this method is also applicable to other scanning imaging techniques such as scanning electrochemical microscopy.

Correlative AFM and Scanning Microlens Microscopy for Time-Efficient Multiscale Imaging.

With the rapid evolution of microelectronics and nanofabrication technologies, the feature sizes of large-scale integrated circuits continue to move toward the nanoscale. There is a strong need to improve the quality and efficiency of integrated circuit inspection, but it remains a great challenge to provide both rapid imaging and circuit node-level high-resolution images simultaneously using a conventional microscope. This paper proposes a nondestructive, high-throughput, multiscale correlation imaging method that combines atomic force microscopy (AFM) with microlens-based scanning optical microscopy. In this method, a microlens is coupled to the end of the AFM cantilever and the sample-facing side of the microlens contains a focused ion beam deposited tip which serves as the AFM scanning probe. The introduction of a microlens improves the imaging resolution of the AFM optical system, providing a 3-4× increase in optical imaging magnification while the scanning imaging throughput is improved ≈8×. The proposed method bridges the resolution gap between traditional optical imaging and AFM, achieves cross-scale rapid imaging with micrometer to nanometer resolution, and improves the efficiency of AFM-based large-scale imaging and detection. Simultaneously, nanoscale-level correlation between the acquired optical image and structure information is enabled by the method, providing a powerful tool for semiconductor device inspection.

Rapid broadband discrete nanomechanical mapping of soft samples on atomic force microscope.

In this paper, an approach to achieve rapid broadband discrete nanomechanical mapping of soft samples using an atomic force microscope is developed. Nanomechanical mapping (NM) is needed to investigate, for example, dynamic evolution of the nanomechanical distribution of the sample-provided that the mapping is fast enough. The throughput of conventional NM methods, however, is inherently limited by the continuous scanning involved where the probe visits each sampling location continuously. Thus, we propose to significantly reduce the number of measurements through discrete mapping where only discrete sampling locations of interests are visited and measured. An online-searching learning-based technique is utilized to achieve rapid probe engagement and withdrawal with the interaction force minimized at each sampling location. Then, a control-based nanoindentation measurement technique is used to quickly acquire the nanomechanical property at each location, over frequencies that can be chosen arbitrarily in a broad range. Finally, a decomposition-based learning approach is explored to achieve rapid probe transitions between the sampling locations. The proposed technique is demonstrated through experiments using a Polydimethylsiloxane (PDMS) sample and a PDMS-epoxy sample as examples.

Nanoscale mechanics by tomographic contact resonance atomic force microscopy.

We report on quantifiable depth-dependent contact resonance AFM (CR-AFM) measurements over polystyrene-polypropylene (PS-PP) blends to detail surface and sub-surface features in terms of elastic modulus and mechanical dissipation. The depth-dependences of the measured parameters were analyzed to generate cross-sectional images of tomographic reconstructions. Through a suitable normalization of the measured contact stiffness and indentation depth, the depth-dependence of the contact stiffness was analyzed by linear fits to obtain the elastic moduli of the materials probed. Besides elastic moduli, the contributions of adhesive forces (short-range versus long-range) to contact on each material were determined without a priori assumptions. The adhesion analysis was complemented by an unambiguous identification of distinct viscous responses during adhesion and in-contact deformation from the dissipated power during indentation.

Mechanical mapping of single membrane proteins at submolecular resolution.

The capacity of proteins to carry out different functions is related to their ability to undergo conformation changes, which depends on the flexibility of protein structures. In this work, we applied a novel imaging mode based on indentation force spectroscopy to map quantitatively the flexibility of individual membrane proteins in their native, folded state at unprecedented submolecular resolution. Our results enabled us to correlate protein flexibility with crystal structure and showed that α-helices are stiff structures that may contribute importantly to the mechanical stability of membrane proteins, while interhelical loops appeared more flexible, allowing conformational changes related to function.

Quantitative measurement of cantilever spring constant using heterodyne interferometer.

Using thermal fluctuation induced oscillation, spring constant of different types of reference cantilevers have been measured by heterodyne laser interferometer with traceable displacement at a resolution of 2.6 x 10(-15) m/(Hz)1/2. This method provides precise measurements of the scanning probe cantilevers with spring constants ranging from mN/m to kN/m. The accuracy of the spring constant was verified by geometric calculation of the reference levers and measurement by electrostatic force balance. Factors that affect the accuracy impact of thermal tune background have been thoroughly investigated.

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).

An atomic force microscope tip designed to measure time-varying nanomechanical forces.

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.

Radial compression elasticity of single DNA molecules studied by vibrating scanning polarization force microscopy.

The radial compression properties of single DNA molecules have been studied using vibrating scanning polarization force microscopy. By imaging DNA molecules at different vibration amplitude set-point values, we obtain the correlations between radially applied force and DNA compression, from which the radial compressive elasticity can be deduced. The estimated elastic modulus is approximately 20-70 MPa under small external forces (<0.4 nN) and increases to approximately 100-200 MPa for large loads.

Direct measurement of tapping force with a cantilever deflection force sensor.

An experimental set up dedicated to the measurement of atomic force microscope tapping force was developed. In the set-up, a standard TappingMode probe cantilever was used to tap another cantilever equipped with its own low noise and high sensitivity deflection detection system for force measurement. The amplitude and phase change of the tapping lever as well as the deflection of the sensing lever were simultaneously recorded as a function of tip/surface separation. Since the deflection of the sensing cantilever reflects the average force over one interaction cycle, we measured the total average force quantitatively after calibrating the spring constant and deflection sensitivity of the sensing lever. Considerable effort was made to achieve the same force curve in the tapping force measurement as occur during imaging of conventional samples such that the detected tapping force reflects the same interaction of the imaging process.

A torsional resonance mode AFM for in-plane tip surface interactions.

Changing the method of tip/sample interaction leads to contact, tapping and other dynamic imaging modes in atomic force microscopy (AFM) feedback controls. A common characteristic of these feedback controls is that the primary control signals are based on flexural deflection of the cantilever probes, statically or dynamically. We introduce a new AFM mode using the torsional resonance amplitude (or phase) to control the feedback loop and maintain the tip/surface relative position through lateral interaction. The torsional resonance mode (TRmode ) provides complementary information to tapping mode for surface imaging and studies. The nature of tip/surface interaction of the TRmode facilitates phase measurements to resolve the in-plane anisotropy of materials as well as measurements of dynamic friction at nanometer scale. TRmode can image surfaces interleaved with TappingMode with the same probe and in the same area. In this way we are able to probe samples dynamically in both vertical and lateral dimensions with high sensitivity to local mechanical and tribological properties. The benefit of TRmode has been proven in studies of water adsorption on HOPG surface steps. TR phase data yields approximately 20 times stronger contrast than tapping phase at step edges, revealing detailed structures that cannot be resolved in tapping mode imaging. The effect of sample rotation relative to the torsional oscillation axis of the cantilever on TR phase contrast has been observed. Tip wear studies of TRmode demonstrated that the interaction forces between tip and sample could be controlled for minimum tip damage by the feedback loop.

Studies of tip wear processes in tapping mode atomic force microscopy.

Tip integrity is crucial to atomic force microscope image quality. Tip wear not only compromises image resolution but also introduces artifacts. However, the factors that govern wearing have not been systematically studied. The results presented here of tip wearing on a rough titanium surface were determined by monitoring changes in tip shape and the evolution of histograms of complex surface curvatures under different control parameters. In contrast with the common assumption that operating at a low set point (the ratio of tapping amplitude to free oscillation amplitude) wears the tip quickly, we observed that a low set point actually minimizes tip wear on a hard surface regardless of the free amplitude. The results can be interpreted qualitatively with theoretical calculations based on momentum exchange at tapping impact. Operating at a low set point allows more robust scanning than with a high set point (tapping near free amplitude), providing a method to slow down tip wear. Another advantage of a low set point is that amplitude error grows faster than with a high set point by nearly an order of magnitude, permitting an increase in scanning speed.