Height drift correction in non-raster atomic force microscopy

2007 American Control Conference, 2007

Images in atomic force microscopy (AFM) are built pixel-by-pixel through a raster scan process and can take on the order of minutes to obtain. The problem of imaging a sample can be characterized as using a short-range or point-like sensor to obtain information about a system over a region and is common across a broad range of fields in science and engineering. In many cases, as in most AFM images, the region to be scanned consists primarily of empty or uninteresting space. In this situation raster-scanning, while easy to implement, is extremely inefficient. It can be viewed as an open-loop scheme because no use is made of data being acquired by the sensor. In this paper, we survey results from the literature describing alternative scanning and sampling approaches. These algorithms often use prior information about the system being measured as well as real-time feedback from previously measured points to keep the sensor in the regions of interest.

Measurement Science and Technology, 2015

It is a significant challenge to reduce the scanning time in atomic force microscopy while retaining imaging quality. In this paper, a novel non-raster scanning method for high-speed imaging is presented. The method proposed here is developed for a specimen with the simple shape of a cell. The image is obtained by scanning the boundary of the specimen at successively increasing heights, creating a set of contours. The scanning speed is increased by employing a combined prediction algorithm, using a weighted prediction from the contours scanned earlier, and from the currently scanned contour. In addition, an adaptive change in the height step after each contour scan is suggested. A rigorous simulation test bed recreates the x-y specimen stage dynamics and the cantilever height control dynamics, so that a detailed parametric comparison of the scanning algorithms is possible. The data from different scanning algorithms are compared after the application of an image interpolation algorithm (the Delaunay interpolation algorithm), which can also run on-line.

The Journal of Strain Analysis for Engineering Design, 2011

From two AFM images, scanned in two orthogonal directions, a method is proposed to reconstruct a reference image that is deprived of the slow-scan drifts of each image, and hence that benefits from the fast-scan accuracy of both images. This method, extending that initially proposed by [1], is formulated as a global Digital Image Correlation problem. The analysis provides not only a reference image but also a residual map, allowing to check for the validity of the correction, and the slow-drift corrections. The algorithm is applied to AFM images of spherulites obtained in tapping mode. It is shown that artificial strains as large as 16% can be corrected from this procedure.

Review of Scientific Instruments, 2002

The atomic force microscope measures surface topography by maintaining a certain cantilever deflection or vibration amplitude as the cantilever is scanned over a sample surface. The desired cantilever deflection or amplitude is referred to as the setpoint, and is maintained by moving the sample toward or away from the cantilever. The signal from the cantilever deflection detector has a real component, due to cantilever deflection, and a drift component due to various sources of drift. We present a method of eliminating the drift component by sensing and correcting it in real time. Our method involves automatically changing the setpoint so as to maintain a certain set difference in the relative feature richness of two traces taken with slightly offset setpoints. We show how the system maintains a setpoint only 70 mV above minimum, perturb it with a gentle blow of air that causes 200 mV of detector drift, and observe its recovery within 13ϩϪ6 s.

Journal of Molecular Recognition, 2012

Atomic force microscopy (AFM) image acquisition is performed by raster-scanning a faint tip with respect to the sample by the use of a piezoelectric stage that is guided by a feedback system. This process implies that the resulting images feature particularities that distinguish them from images acquired by other techniques, such as the drift of the piezoelectric elements, the unequal image contrast along the fast-and the slow-scan axes, the physical contact between the tip of nondefinable geometry and the sample, and the feedback parameters. Recently, high-speed AFM (HS-AFM) has been introduced, which allows image acquisition about three orders of magnitude faster (500-100 ms frame rate) than conventional AFM (500 s to 100 s frame rate). HS-AFM produces image sequences, large data sets, which report biological sample dynamics. To analyze these movies, we have developed a software package that (i) adjusts individual scan lines and images to a common contrast and z-scale, (ii) filters specifically those scan lines where increased or insufficient force was applied, (iii) corrects for piezo-scanner drift, (iv) defines particle localization and angular orientation, and (v) performs particle tracking to analyze the lateral and rotation displacement of single molecules.

Mechatronics, 2006

The atomic force microscope (AFM) system has evolved into a useful tool for direct measurements of intermolecular forces with atomic-resolution characterization that can be employed in a broad spectrum of applications. The distance between cantilever tip and sample surface in non-contact AFM is a time-varying parameter even for a fixed sample height, and typically difficult to identify. A remedy to this problem is to directly identify the sample height in order to generate high-precision atomic-resolution images. For this, the microcantilever (which forms the basis for the operation of AFM) is modeled as a single mode approximation and the interaction between the sample and cantilever is derived from a van der Waals potential. Since in most practical applications only the microcantilever deflection is accessible, we will use merely this measurement to identify the sample height. In most non-contact AFMs, cantilevers with high-quality factors are employed essentially for acquiring high-resolution images. However, due to high-quality factor, the settling time is relatively large and the required time to achieve a periodic motion is long. As a result, identification methods based on amplitude and phase measurements cannot be efficiently utilized. The proposed method overcomes this shortfall by using a small fraction of the transient motion for parameter identification, so the scanning speed can be increased significantly. Furthermore, for acquiring atomic-scale images of atomically flat samples, the need for feedback loop to achieve setpoint amplitude is basically eliminated. On the other hand, for acquiring atomic-scale images of highly uneven samples, a simple PI controller is designed to track the desired constant sample height. Simulation results are provided to demonstrate the feasibility of the approach for both sample height identification and tracking the desired sample height.

Physical Review B, 2014

It has been demonstrated that atomic force microscopy imaging with CO-functionalized tips provides unprecedented resolution, yet it is subject to strong image distortions. Here we propose a method to correct for these distortions. The lateral force acting on the tip apex is calculated from three-dimensional maps of the frequency shift signal. Assuming a linear relationship between lateral distortion and force, atomic force microscopy images could be deskewed for different substrate systems.

IEEE/ASME Transactions on Mechatronics, 2017

We propose a new scan waveform ideally suited for high-speed Atomic Force Microscopy. It is an optimization of the Archimedean spiral scan path with respect to the X,Y scanner bandwidth and scan speed. The resulting waveform uses a constant angular velocity spiral in the center and transitions to constant linear velocity towards the periphery of the scan. We compare it with other scan paths and demonstrate that our novel spiral best satisfies the requirements of high-speed Atomic Force Microscopy by utilizing the scan time most efficiently with excellent data density and data distribution. For accurate X,Y, and Z positioning our proposed scan pattern has low angular frequency and low linear velocities that respect the instruments mechanical limits. Using Sensor Inpainting we show artifact free high resolution images taken at two frames per second with a 2.2 m scan size on a moderately large scanner capable of 40 m scans.

Dynamic Systems and Control, Parts A and B, 2005

The atomic force microscope (AFM) system has evolved into a useful tool for direct measurements of intermolecular forces with atomic-resolution characterization that can be employed in a broad spectrum of applications. The distance between cantilever tip and sample surface in non-contact AFM is a time-varying parameter even for a fixed sample height, and typically difficult to identify. A remedy to this problem is to directly identify the sample height in order to generate high-precision atomic-resolution images. For this, the microcantilever (which forms the basis for the operation of AFM) is modeled as a single mode approximation and the interaction between the sample and cantilever is derived from a van der Waals potential. Since in most practical applications only the microcantilever deflection is accessible, we will use merely this measurement to identify the sample height. In most non-contact AFMs, cantilevers with high-quality factors are employed essentially for acquiring high-resolution images. However, due to high-quality factor, the settling time is relatively large and the required time to achieve a periodic motion is long. As a result, identification methods based on amplitude and phase measurements cannot be efficiently utilized. The proposed method overcomes this shortfall by using a small fraction of the transient motion for parameter identification, so the scanning speed can be increased significantly. Furthermore, for acquiring atomic-scale images of atomically flat samples, the need for feedback loop to achieve setpoint amplitude is basically eliminated. On the other hand, for acquiring atomic-scale images of highly uneven samples, a simple PI controller is designed to track the desired constant sample height. Simulation results are provided to demonstrate the feasibility of the approach for both sample height identification and tracking the desired sample height.

IEEE Transactions on Nanotechnology, 2011

In recent years, the Atomic Force Microscope (AFM) has become an important tool in nanotechnology research. It was first conceived to generate 3D images of conducting as well as nonconducting surfaces with a high degree of accuracy. Presently, it is also being used in applications that involve manipulation of material surfaces at a nanoscale. In this paper we describe a new scanning method for fast atomic force microscopy. In this technique, the sample is scanned in a spiral pattern instead of the well established raster pattern. A Constant Angular Velocity (CAV) spiral scan can be produced by applying single frequency cosine and sine signals with slowly varying amplitudes to the x-axis and y-axis of AFM nanopositioner respectively. The use of single frequency input signals allows the scanner to move at high speeds without exciting the mechanical resonance of the device. Alternatively, the frequency of the sinusoidal setpoints can be varied to maintain a constant linear velocity while a spiral trajectory is being traced. Thus, producing a Constant Linear Velocity (CLV) spiral. These scan methods can be incorporated into most modern AFMs with minimal effort since they can be implemented in software using the existing hardware. Experimental results obtained by implementing the method on a commercial AFM indicate that high-quality images can be generated at scan frequencies well beyond the raster scans.