Decoding the Nanoworld: Pushing the Envelope in Atomic Force Microscopy

Figure 1 - The Cypher AFM offers the highest open-loop and closed-loop resolution available.

Over the past decade, atomic force microscopy/scanning probe microscopy (AFM/SPM) has emerged as the leading tool for investigations at the nanoscale—doing everything from imaging, to compositional differentiation, to explorations of molecular forces. Aside from some interesting tweaks, add-ons, and repackaging, however, the field has seen no fundamentally new instruments for several years. For the extremely high-resolution AFM/SPMs, there has literally been no completely new microscope for well over a decade.

Enter the new Cypher™ AFM (Asylum Research, Santa Barbara, CA, Figure 1), designed from the ground up with a host of features that are described below.

Resolution

Figure 2 - Open-loop atomic resolution scanning tunneling microscopy (STM) on the cleavage plane of highly oriented pyrolitic graphite (HOPG), 5-nm scan. Inset: Closed-loop atomic resolution STM on HOPG, 6-nm scan.

AFM/SPM has become such a dominant tool in nanoscience and nanotechnology because of three-dimensional imaging and resolution. While Cypher exceeds the open-loop resolution of other commercially available AFM/SPMs (Figure 2), its closed-loop resolution is the key differentiator (Figure 2 inset).

Almost all commercial AFMs use piezo-electric crystals (piezos) as the transducers that generate the fine scale motion used in scanning the probe across the surface. Piezos are crystals that expand very slightly (typically one part in a thousand) when a voltage is applied to them. These devices can generate motion that is a small fraction of the size of a single atom. Their downside is that at larger scan sizes they exhibit nonlinearity (their expansion is no longer proportional to voltage) and hysteresis, which prevent accurate control of the probe position.

The differences between open-loop and closed-loop systems involve how these piezos are controlled. In simple terms, “open loop” refers to operation of the AFM/SPM without positional feedback to compensate for inaccuracies and distortions caused by the piezos in the scanning mechanism. Open-loop images can be striking and reveal a great deal of information, but the actual dimensions of features will carry those inaccuracies. Further, open-loop systems cannot accurately return the probe to a specific point in an image; thus it is not possible to make measurements at specific points of interest in the image. In “closed-loop” systems, the probe position is constantly monitored by additional sensors, and the voltage to the piezos is corrected to keep the probe at its desired position. The result is that the images are more accurate and reflect the true dimensions of the sample features being scanned. The probe can also be precisely returned to points in the image after acquisition, opening a multitude of measurement possibilities, e.g., force, stiffness, and electrical measurements.

In the past several years, because of the advantages mentioned above, “closed loop” has become a standard feature on many commercial AFMs, but has always come with a major tradeoff. In closed-loop operation, the system uses data from the sensors to correct the probe position; therefore any noise in the sensors directly affects the probe position. Even with sensors with subnanometer noise levels, this still means that sensor noise is a significant issue for scan sizes below a few hundred nanometers. Typically, users are forced to switch to open-loop operation at these small scan sizes to avoid this noise issue. By pushing sensor noise levels well below the Å (0.1-nm) level, the Cypher AFM has effectively eliminated this issue. The system uses patented sensor technology that is capable of atomic resolution in all three axes. With positioning accuracies better than 60 pm (0.06 nm) in X, Y, and Z, these Nanopositioning Sensors (NPS™) are the quietest available on an AFM today by a factor of 5 to 10×. Thus, users no longer have to choose between the accuracy and control of closed-loop and the low noise of open-loop systems. The end result is that Cypher, with the highest resolution commercially available in an AFM, produces the most accurate images, measurements, positioning, and nanomanipulation possible, all at atomic resolution.

Immunity to vibration and drift

To further improve image and measurement resolution and accuracy, Cypher’s system-level mechanical design is inherently more immune to normal environmental vibration. The reason is simple: The design includes an extremely short and rigid mechanical path from the sample to the scanning probe—the shorter this mechanical path, the less susceptible the AFM/SPM is to the introduction of noise and thermal drift. The system design is also integrated with additional features to reduce noise and drift, including temperature control within the integrated system enclosure, which allows experiments to be conducted from minutes to hours with minimal thermal drift of the probe position relative to the sample. The enclosure (Figure 1) also provides additional shielding from acoustic noise such that, for most laboratory environments, the system requires no additional vibration or acoustic noise protection, while still routinely providing atomic resolution.

Ease of use

Figure 3 - With the Cypher AFM’s SpotOn™ automated laser alignment, the user simply clicks on the desired laser spot location, and motorized stages automatically align the laser and photodetector.

In addition to its high resolution and accuracy, Cypher breaks new ground in ease of use. Specifically, the process of probe alignment with the laser and photodetector is simplified to a single mouse-click (Figure 3). The system also incorporates diffraction-limited optics for submicron-resolution sample and tip viewing with Kohler illumination to further improve operability and ease of use.

Sub-picoNewton force measurements

One of the most important nonimaging uses of AFMs is the measurement of molecular forces. These measurements are often made in liquid and have contributed greatly to the understanding of forces in molecular biology. For example, the AFM can measure the forces required to mechanically unfold a single protein molecule tethered between the tip of the probe and a surface. In liquids, the ultimate limits on force sensitivity are set by thermal noise. Water molecules continually bumping into the cantilever cause a background vibration that can mask extremely small forces. Smaller cantilevers fundamentally reduce this background noise and improve the force sensitivity of the AFM. With its compatibility with extremely small cantilevers, the Cypher AFM pushes the force noise floor into the sub-picoNewton range, achieving sensitivity heretofore unique to optical tweezers. However, Cypher achieves this force sensitivity at much higher spring constants and with much sharper probes than optical tweezers, enabling measurements to be made in the higher force gradients found near surfaces where the optical tweezer probe would become stuck to the surface.

>10× Faster scanning

Figure 4 - Closed-loop ac-mode height image of the {001} cleavage plane of muscovite mica after etching in hydrofluoric acid for 3 hr. Single atomic steps are clearly resolved. This 512 × 512 pixel image was taken using a 10-μm cantilever with a resonance frequency of 4.6 MHz and a response time of 33 μsec. With a line rate of 40 Hz, the image was acquired in under 13 sec.

Finally, Cypher supports interchangeable laser spots as small as 3 μm, permitting the use of small cantilevers (less than 10 μm in length), which, in turn, improve sensitivity and allow extremely high-speed imaging while still maintaining feedback. While fast imaging has been an important research topic in AFM/SPM recently, most high-speed imaging is done in fairly specialized situations. The most common imaging mode in which AFMs are operated is ac mode in air (also known as tapping mode, intermittent contact, or dynamic mode). In this mode, the vibrating cantilever itself is the slowest element in the Z feedback loop and it sets the total response speed of the instrument. Current commercial cantilevers with lengths of about 100 μm have response times that are a good fraction of a millisecond. This limits imaging speeds to at most a few lines per second. The small cantilevers enabled by Cypher have response times that are 10–20 times faster and enable a commensurate increase in imaging speeds. The image in Figure 4, for example, was generated in just 13 sec (as compared to many minutes required for older systems) with clearly resolved atomic steps on hydrofluoric acid-etched mica.

Other modes of operation

Cypher supports all major scanning and force measurement modes, as well as piezoresponse force microscopy (PFM) for high sensitivity, high bias, and cross-talk-free measurements of piezoelectrics, ferroelectrics, and other materials. The system also offers built-in nanolithography and nanomanipulation with the highest resolution available.

The authors are with Asylum Research, 6310 Hollister Ave., Santa Barbara, CA 93117, U.S.A.; tel.: 805-696-6466; e-mail: monte@asylumresearch.com.

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