Back-scattered detection provides atomic-scale localization precision, stability, and registration in 3D

Abstract

State-of-the-art microscopy techniques (e.g., atomic force microscopy, scanning-tunneling microscopy, and optical tweezers) are sensitive to atomic-scale (100 pm) displacements. Yet, sample drift limits the ultimate potential of many of these techniques. We demonstrate a general solution for sample control in 3D using back-scattered detection (BSD) in both air and water. BSD off a silicon disk fabricated on a cover slip enabled 19 pm lateral localization precision (Deltaf = 0.1-50 Hz) with low crosstalk between axes (</=3%). We achieved atomic-scale stabilization (88, 79, and 98 pm, in x, y, and z, respectively; Deltaf = 0.1-50 Hz) and registration ( approximately 50 pm (rms), N = 14, Deltat = 90 s) of a sample in 3D that allows for stabilized scanning with uniform steps using low laser power (1 mW). Thus, BSD provides a precise method to locally measure and thereby actively control sample position for diverse applications, especially those with limited optical access such as scanning probe microscopy, and magnetic tweezers.

Ultrastable atomic force microscopy: Improved force and positional stability

“…Such stabilization of the tip and the sample only yields improved tip-sample stability -the desired goal -if the two detection lasers are stable with respect to each other. Key to our success is that we achieved a differential pointing stability of 0.2 Å [67]. Excellent differential stability is achieved by minimizing the beam path and the number of non-common optical elements [67,68].…”

“…Key to our success is that we achieved a differential pointing stability of 0.2 Å [67]. Excellent differential stability is achieved by minimizing the beam path and the number of non-common optical elements [67,68]. Moreover, the effects of thermal expansion are minimized by measuring the motion of the sample very close ($10 lm) to the object under study.…”

“…Laser-based detection of bead motion in optical traps has detected bead position in 3D with 1-Å precision [66]. To leverage this technology, we integrated these concepts into a package that was compatible with AFM [67] and effectively deconvolved the complex optical signal inherent in scattering light off of asymmetric AFM tips [68]. The combination of these advancements let us actively stabilize the tip-sample position of an AFM in 3D, originally at ambient conditions [26] and now in liquid.…”

“…Improved stability requires a reference mark that is firmly attached to the surface. We currently use an array of silicon posts ($800-nm dia., 50-nm tall [67]) that are fabricated onto a cover slip using a shadow mask, a thin membrane with holes in it. There are a number of ways to make these masks from Fig.…”

“…Second, US-AFM requires separate and independent control of the tip and the sample, which is not available in current commercial AFMs. Therefore, a potential user must build a custom AFM incorporating two independent three axis stages [26] and a pair of actively stabilized lasers [67,69]. As the mica issue is resolved and demand for ultrastable AFM capabilities grows, we see no fundamental barrier to commercialization.…”