Current atomic force microscopes (AFMs) support single-probe operation, in which one nanosized tip enables versatile operations such as surface imaging, nanomanipulation, and nanomanufacturing to be performed, although one at a time. Some AFM operations involve switching between imaging and the operational mode, which is cumbersome, challenging, and limiting, particularly when different probe geometries are preferred for each mode. This paper presents a new dual-probe atomic force microscope (DP-AFM) that has two independent probes operating in a common workspace. Such a setup enables two AFM operations to be carried out simultaneously. For instance, one probe can be used to image, while the other probe performs one of the many tip-based processes. The hardware and software design involved in developing the DP-AFM is discussed in detail. Furthermore, to demonstrate the capability of dual-probe arrangement, a controller is developed for real-time plowing depth control, where one probe is used to plow the surface, while the other is used to image the plow profile, thus enabling real-time feedback control of the AFM plow process. Experimental results show that the plow depth can be regulated with nanometer-level accuracy.
Repetitive controllers use delayed feedback for periodic operations to provide a feedforward-like control action capable of high bandwidth operation. However, the signal to be tracked/rejected must be perfectly periodic, with known time-period, to obtain asymptotic convergence. Any deviation from periodicity can severely degrade the tracking performance of the controller. This paper explores the idea of employing a variation of repetitive controller to expand the domain of signals that can be tracked with the repetitive framework. The signals intended to be tracked belong to a class of quasiperiodic signals that can be represented as an algebraic sum of periodic and polynomial signals. Many of the commonly occurring physiologic signals, speech, and vibration signals belong to this signal class. Moreover, periodic signals that drift can also be modeled as this type of quasiperiodic signals. The derived quasi-repetitive controller guarantees asymptotic convergence with a plug-in architecture that can be added to an existing feedback design. One practical application of this controller occurs in atomic force microscopy (AFM), where imaging a sloped or non-flat sample surface induces quasiperiodic disturbances in the control loop. Experimental results demonstrate accurate and high speed imaging can be performed using the prescribed controller.
This article presents a novel method to improve the measurement sensitivity and reduce impact forces in tapping-mode atomic force microscopy by reshaping the tip trajectory. A tapping drive signal composed of two harmonics is used to generate an oscillating trajectory with a broader valley compared to the typical sinusoidal trajectory. The wide broad valley reduces the velocity of the tip in the vicinity of the sample and allots a greater portion of each period in the vicinity of the sample. Numerical simulations show that this results in decreased impact force and increased sensitivity of the cantilever oscillation to changes in tip-sample offset. Experimental results demonstrate an increase in image sharpness and decrease in tip wear using the bi-harmonic driving signal.
The tapping mode (TM) is a popularly used imaging mode in atomic force microscopy (AFM). A feedback loop regulates the amplitude of the tapping cantilever by adjusting the offset between the probe and sample; the image is generated from the control action. This paper explores the role of the trajectory of the tapping cantilever in the accuracy of the acquired image. This paper demonstrates that reshaping the cantilever trajectory alters the amplitude response to changes in surface topography, effectively altering the mechanical sensitivity of the instrument. Trajectory dynamics are analyzed to determine the effect on mechanical sensitivity and analysis of the feedback loop is used to determine the effect on image accuracy. Experimental results validate the analysis, demonstrating better than 30% improvement in mechanical sensitivity using certain trajectories. Images obtained using these trajectories exhibit improved sharpness and surface tracking, especially at high scan speeds.
This paper presents a method and cantilever design for improving the mechanical measurement sensitivity in the atomic force microscopy (AFM) tapping mode. The method uses two harmonics in the drive signal to generate a bi-harmonic tapping trajectory. Mathematical analysis demonstrates that the wide-valley bi-harmonic tapping trajectory is as much as 70% more sensitive to changes in the sample topography than the standard single-harmonic trajectory typically used. Although standard AFM cantilevers can be driven in the bi-harmonic tapping trajectory, they require large forcing at the second harmonic. A design is presented for a bi-harmonic cantilever that has a second resonant mode at twice its first resonant mode, thereby capable of generating bi-harmonic trajectories with small forcing signals. Bi-harmonic cantilevers are fabricated by milling a small cantilever on the interior of a standard cantilever probe using a focused ion beam. Bi-harmonic drive signals are derived for standard cantilevers and bi-harmonic cantilevers. Experimental results demonstrate better than 30% improvement in measurement sensitivity using the bi-harmonic cantilever. Images obtained through bi-harmonic tapping exhibit improved sharpness and surface tracking, especially at high scan speeds and low force fields.
