High speed atomic force microscopy enabled by a sample profile estimator

Abstract

In this paper, an estimation scheme for imaging in Atomic Force Microscopy (AFM) is presented which yields imaging rates well beyond the bandwidth of the vertical positioner and allows for high-speed AFM on a typical commercial instrument. The estimator can be applied to existing instruments with little to no hardware modification other than that needed to sample the cantilever signal. Experiments on a calibration sample as well as lambda DNA are performed to illustrate the effectiveness of this method. These show a greater than an order-of-magnitude improvement in the imaging rate. In atomic force microscopy (AFM), 1 a controller is used to tightly regulate the tip-sample interaction. Topological information is typically derived from the corresponding motion of the vertical (z) actuator. 2 As a result, the rate of taking a single measurement is limited by the bandwidth of the z-piezo loop and a high overall frame rate comes at a cost in terms of the scanning size and the resolution. 3 Advanced hardware and specialized controller designs have been developed and integrated in a variety of commercial and research AFMs 4-6 to create high-speed AFM (HS-AFM) instruments that approach video-rate imaging.Algorithmic approaches to HS-AFM seek to work with existing hardware and improve imaging rates by alternative approaches to selecting and acquiring measurements or through signal processing techniques to extract more information from poor signals. Because they require little to no hardware modification, they can be applied to existing AFMs to improve imaging rates as well as to HS-AFM instruments for even further gains. One example is our previous work for high-speed imaging of biopolymers and other string-like samples. 7 In this local raster scan, imaging time is reduced by using feedback to focus the measurements in the region of interest, namely, close to the biopolymer. Its effectiveness is due in part to the structure of the sample, and the approach is therefore limited to string-like samples. To allow for more general samples, in this work we present a fast estimation scheme that determines the sample profile at rates independent of the z-actuator loop. As a result, high-speed imaging can be achieved by integrating this signal with the conventional raster motion executed by any commercial AFM. It is also compatible with novel scanning trajectories, such as spiral scanning, 8 cycloids, 9 Lissajous figures, 10 and our local raster scan. 7 Driven by the requirements of imaging soft samples, we focus on intermittent (tapping) mode AFM.There are a few existing estimation-based approaches to AFM imaging. One relies on building an advanced controller based on robust control theory for the z-piezo. 11 By taking advantage of the structure of the controller, a signal was derived with a transfer function of 1 to the sample profile, implying an infinite bandwidth in the estimator. It is, however, only applicable to a particular instrument and relies on the specific controller. Given the fact that most commercial AFMs are equ...

A compressed sensing measurement matrix for atomic force microscopy

“…1234845 The fourth approach for improving the temporal resolution of AFM is to gather information in a fundamentally different way. These methods include estimator-based schemes such as transient mode AFM [12], high speed sample profile estimation [13], and local raster scanning [14] in which measurements are used online to steer the tip.…”

High-speed dynamic-mode atomic force microscopy imaging of polymers: an adaptive multiloop-mode approach

“…This is because the tapping amplitude is sensitive and highly nonlinear with respect to the tip–sample distance [ 10 – 11 ]. The speed of TM-imaging might be increased through either hardware [ 12 – 14 ] or software (algorithms) improvement [ 6 , 8 , 15 – 16 ]. However, the existing hardware improvements via the use of high-bandwidth piezo actuators and cantilever are only applicable for small-size imaging (less than 30% of the imaging size of regular AFMs [ 14 ]), and the existing algorithm improvement based on advanced control techniques may lead to a potential sample deformation and damage due to the lack of control of the tip–sample interaction force [ 7 – 8 16 ].…”

“…The presented AMLM imaging approach can achieve high-speed TM imaging for both large- and small-size imaging while maintaining the image quality and keeping the average tip–sample interaction force at the minimum level for stable cantilever tapping. AMLM imaging is fundamentally different from current efforts to high-speed TM imaging [ 6 , 8 , 15 – 16 ] insofar through the introduction of the control of the averaged cantilever deflection (the TM deflection) – in addition to the transitional RMS amplitude feedback control, along with an online iterative feedforward control to track the sample topography. Although this AMLM technique has been proposed recently [ 1 ], imaging results of only one polymer sample at large scanning size (50 μm) were obtained and presented.…”

A specimen-tracking controller for the transverse dynamic force microscope in non-contact mode

“…To resolve these issues in AFMs, control engineers worked on various controllers to track specimens in contact and noncontact mode for accurate AFM imaging performance (e.g. [10], [11], [12]).…”