Frequently Asked Questions About Atomic Force Microscopy

Atomic force microscopy (AFM) is a type of scanning probe microscopy (SPM) that uses a very sharp probe that is raster-scanned to produce a true 3D topographical map of the surface of a sample with nanoscale resolution.

More details can be found in “How does AFM work?”

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Atomic force microscopy is a type of scanning probe microscopy (SPM), but it is not the only type. Atomic force microscopy and scanning tunnelling microscopy (STM) are the two most common types of SPM.

All types of scanning probe microscopes use a physical probe touching the surface of a sample to scan the surface and collect data. However, their measurement mechanisms are very different:

• STM uses an electrical current between the microscope’s scanning tip and the sample surface; samples must be conductive or semi-conductive.
• AFM measures the cantilever deflection (bending) caused by forces between the tip and the sample surface; AFM can be used to measure a much wider range of materials than STM.

See how AFM compares with other materials characterization methods in "What makes AFM a good/ideal technology for studying samples at the nanoscale?"

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The basic working principle of an atomic force microscope is:

1. A sharp tip at the free end of a cantilever is raster-scanned over a small area of sample.
2. As the tip passes over the surface, variations in height cause the cantilever to bend.
3. This bending, or change in deflection, is detected through movement of a laser or super luminescent diode (SLD) that is reflected off the cantilever into a position-sensitive photodetector (PSPD).

Throughout this process, piezo actuators operate within an electronic feedback loop to move the tip or sample closer or further away from each other to maintain the relative tip-sample distance and a constant setpoint. 

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Atomic force microscopes are classified as either ‘"tip-scanning," in which the AFM probe is scanned over a stationary sample, or as "sample-scanning," in which the sample is actively scanned under a stationary AFM probe. A tip-scanning configuration provides significant advantages with regards to system versatility and sample size. As the sample is not moved, tip-scanning systems can accommodate much larger, heavier samples or multiple smaller samples, and are more easily modified to correlate/integrate additional techniques.

Bruker’s Large-Sample AFMs are tip-scanning systems. The resulting open-access platform delivers maximum flexibility to accommodate the widest variety of experiments, modes, techniques, and semi-automated measurements without compromising performance.

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Contact Mode

Contact mode was the first AFM imaging mode and is the basis for all other AFM techniques in which the probe tip is in constant physical contact with the sample surface.

How does contact mode AFM work?

• As the AFM probe scans along the top of the sample, height variations in the sample surface induce a vertical deflection of the cantilever.
• A feedback loop maintains the vertical deflection at a preset load force and uses the feedback response to generate a topographic image.

Though reasonably easy to operate, contact mode has the inherent drawback that the lateral forces exerted on the sample during scanning can be quite high. These forces can result in sample distortion/damage or the movement/displacement of relatively loosely attached objects.

Tapping Mode

TappingMode AFM was developed to address the high lateral forces associated with contact mode.

How does tapping mode AFM work?

• The AFM cantilever is oscillated close to its resonant frequency while perpendicular to the sample surface. As the tip intermittently “taps” along the surface, height variations cause changes in the amplitude of the oscillation.
• The feedback loop increases/decreases the tip-sample distance to maintain a constant amplitude and uses this movement to produce a topography image.

The development of TappingMode enabled researchers to acquire high-resolution topographical images of samples too fragile to withstand the lateral forces of contact mode.

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While the introduction of TappingMode enabled AFM imaging for a much wider range of samples, its one drawback compared to contact mode is that it cannot directly measure forces. Bruker’s PeakForce Tapping, however, solved this issue by performing a force curve at every pixel position on the sample surface.

How it works:

• PeakForce Tapping is a non-resonant imaging mode, where the tip-sample distance is modulated in a sinusoidal motion at amplitudes that are typically less than 100 nm and at typical frequencies of 1-2 kHz.
• When the AFM probe is brought into contact with the sample surface, the tip-sample interaction is controlled by holding the maximum force, or “peak force,” between the tip and the sample constant.

An advantage that PeakForce Tapping has over other force-distance curve-based imaging modes is that it utilizes a continuous feedback loop to adjust the relative tip-sample position. As such, the imaging force control benefits from the results of the previous tip-sample interactions. PeakForce Tapping also uses sinusoidal ramping rather than linear ramping so that as the tip moves closer to the surface, its velocity approaches zero.

Together, these features of PeakForce Tapping enable direct and precise control of the tip-sample interaction force, facilitating imaging at forces down to ~10 pN. This superior force control maintains tip shape and sample integrity, leading to consistently accurate and high-resolution imaging of even the smallest structures, such as atomic defects and DNA double helix.

As a force-distance curve-based mode, perhaps the greatest power of PeakForce Tapping technology comes from its ability to simultaneously enable and enhance other correlative and quantitative mapping techniques, delivering new possibilities in an ever-expanding set of topographical, mechanical, biological, electrical and chemical applications at the nanoscale.

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The following table compares methods commonly used to characterize materials at the nanoscale and can be used to understand:

• What is the difference between AFM and SEM?
• What is the difference between AFM and STM?
• What is the difference between AFM and TEM? and
• What is the difference between AFM and confocal microscopy?

Each technique has a great deal of nuance, and values given are approximations for common values achievable. The green marker indicates “definitely yes," the yellow marker indicates “sometimes yes, with additional considerations," and the red marker indicates “definitely no."

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Atomic force microscopy is used to collect information about the nanoscale structure and properties of almost any type of sample.

What kinds of observations can be made with atomic force microscopy?

While atomic force microscopy is best known for its ability to resolve surface structure, it provides valuable information about other material properties at the nanoscale. These include mechanical, electrical, electrochemical, piezoelectric, magnetic, thermal, and optical properties. AFM can also be used to manipulate a sample (push, pull, or write) in nanolithography and intra-/inter-molecular (un)binding studies.

With a large variety of measurement types and high degree of sample flexibility, AFM has become a fundamental nano-characterization technique in both academic research and industry for a range of applications, including:

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There are very few limitations on what kinds of samples can be analyzed using AFM. AFM has been used to probe the surfaces of crystalline and non-crystalline materials, microelectronic devices, live cells, dental materials, and so much more. For each sample type, sample preparation, experimental design, and probe choice should be considered carefully.

See the documents below for select examples of AFM used for a variety of materials:

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AFM is a highly accurate technique, and additionally has a high resolution and repeatability when following well-defined standard procedures and maintenance. Accuracy is fundamentally tied to resolution, which can be affected by the probe tip, system noise, and the sample itself. Additionally, accuracy for quantitative techniques depends on the accuracy of all other variables in the system. Bruker takes extreme care to enable AFM users to get the most accurate, high-resolution, and highly repeatable measurements possible.

To ensure accuracy in quantitative techniques like PeakForce QNM and nDMA, Bruker offers:

• Pre-calibrated probes with a QR code that can easily be scanned for quick integration into the analysis workflow.
• Accessible maintenance with a variety of service packages available
• Easy scanner calibration
  

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When considering most standard AFM systems, the scan speed used to obtain optimal quality images strongly depends on several variables, including the sample, probe, imaging mode, scan size, tip-sample interaction forces, and the number of lines collected per image. 

How long does AFM imaging take?

For most AFM systems, image acquisition time is on the order of several minutes. While any AFM system can simply scan faster, there is an inherent trade-off between imaging speed and tip-sample force. As such, imaging at higher scan speeds will typically result in lower quality images.

Dedicated high-speed AFM systems, such as Bruker’s Dimension FastScan, are specifically designed to minimize this trade-off such that high-resolution images can be acquired in seconds or several frames per second. By optimizing critical components involved in the system feedback loop, including the XYZ-scanners, laser optics, system electronics, and by using small cantilevers, a substantial improvement in system bandwidth is achieved that allows optimal force control to be maintained at significantly faster imaging speeds.

High-speed AFM imaging greatly improves time to data and overall productivity while enabling direct observation of dynamic processes. Applications that would most benefit from high-speed imaging can be broadly categorized into 3 main areas:

• Survey: the efficient exploration of an unknown, heterogeneous sample, to understand the different morphologies that best represent the surface, and to capture a representative set of images at high resolution.

• Screening: the quantitative characterization of a surface property (roughness, number of phases, particle size and shape, stiffness, etc.) on a large number of samples of the same class. In this case, the AFM images are only an intermediary; the end product is a comparison graph that quantitatively represents the measured property for each of the samples.

• Dynamics: the observation of sample changes over time, at sufficient speed to resolve the observed process (crystallization, dissolution, self-assembly, aging phenomena, etc.).

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AFM is generally non-destructive, though some modes may drag the tip along the sample surface, affecting soft or fragile samples. Other AFM modes are conducted with a goal of indenting or scratching the sample. With any AFM mode, there is no damage to the bulk of larger samples. With tapping and PeakForce Tapping modes, there is little to no damage to the sample surface. A comparison to other characterization methods can be found in "What makes atomic force microscopy an ideal technique for studying samples at the nanoscale."

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AFM images are always collected as 3 dimensional datasets but can be presented as either 2D contour maps or as 3D plots. These representations often have color scales where darker colors are shorter features and lighter colors are taller features.  

Refer to the above table of nanoscale characterization methods for materials to see a comparison to other common techniques.

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AFM does not require vacuum, unlike traditional scanning tunneling microscopy (STM). A comparison to other characterization methods can be found in "What makes atomic force microscopy an ideal technique for studying samples at the nanoscale."

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Bruker AFMs are advantageous because they can be used to conduct studies in a wide variety of different sample environments. There are available capabilities to enable measurements in liquid, temperature control, and humidity control. AFM is a highly adaptable technique, and if your sample requires additional considerations, please do not hesitate to contact our experts. 

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An atomic force microscope uses a probe to measure tip-sample forces as the tip presses against the sample. The AFM probe is a consumable that consists of a sharp tip on a free-moving cantilever mounted on a chip. One or several cantilevers can be mounted on a comparatively large silicon chip (Figure 1).

The geometry and material of both cantilever and tip can be optimized for a given application, resulting in a large library of probe options.

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Most AFM probes are made from silicon or silicon nitride. Silicon probes enable stiffer cantilevers and sharper tips, while silicon nitride probes are more durable and flexible. Bruker’s TappingMode probes are generally made from silicon, and contact mode and ScanAsyst probes are generally made from silicon nitride. There is a considerable amount of functionality and flexibility when modifying these common AFM probe materials with coatings or replacing them with individual molecules of interest. The geometry and material of both cantilever and tip can be optimized for a given application, resulting in a large library of probe options on www.BrukerAFMprobes.com.

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The elements of a “good” AFM probe are application-dependent. As such, choosing the right AFM probe relies on balancing parameters of interest including stiffness, spring constant, resonant frequency, and quality factor while considering the needs of your application and the AFM mode used.

Watch our 2023 webinar "The Fundamentals of AFM Probe Selection" to learn about the functional and experimental considerations for selecting a probe.

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Bruker is the only major AFM equipment manufacturer that also owns and operates a probes nanofabrication facility, and our broad experience enables us to design and fabricate a wide range of probe types to directly address the evolving needs of AFM users. For more information and to place an order, visit www.BrukerAFMprobes.com.

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Bruker offers a versatile range of AFM instruments that can be equipped with many scanning modes, offering solutions for every application.

Additionally, the Dimension XR family of AFMs offers turnkey system solutions that deliver unique, first-and-only modes and capabilities for the utmost performance in advanced materials research. These optimized AFM packages deliver the highest performance to address nanomechanical, nanoelectrical, and nanoelectrochemical applications.

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Bruker offers a versatile range of AFM instruments capable of imaging a broad range of sample sizes. We are happy to recommend specific instrumentation, operational modes, and configurations to enable highest quality measurements on large samples in your specific application.

The Dimension Icon is the world’s most-proven, highest-performance, and most-versatile large-sample AFM platform. With wide-open tip and sample access, an unrivalled suite of operating modes and accessories, and numerous ease-of-use features, the Dimension Icon delivers the most complete AFM solution for the widest range of applications.

The Dimension Edge provides the highest levels of AFM performance, functionality, and accessibility in its class. Designed to deliver expert-level results easily and consistently, the Dimension Edge provides large-sample AFM capabilities and techniques to researchers at price points well below expectations for such performance.

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Bruker offers a versatile range of AFM instruments capable of imaging a broad range of sample sizes. We are happy to recommend specific instrumentation, operational modes, and configurations to enable highest quality measurements on small samples in your specific application.

As the benchmark advanced-research AFM, the MultiMode’s long history of success is based on its combination of market-leading highest-resolution, superior performance, unparalleled versatility, and proven record of productivity and reliability. With the latest features and accessories, the MultiMode 8-HR takes this legendary AFM platform to even higher levels of performance, speed, and ease of use.

The compact Innova AFM delivers routine high-resolution imaging and application flexibility for a wide range of materials science research. With its highly customizable feature set, Innova offers the utmost value in a research AFM at a moderate cost.

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Bruker Materials Research AFMs offer industry-leading performance and accuracy in semiconductor applications. We are happy to recommend specific instrumentation, operational modes, and configurations to enable highest quality measurements on semiconductor materials and devices for your specific application.

The Dimension FastScan Pro delivers the highest metrology-level speed and performance of any industrial AFM. The system enables automated or semi-automated measurements for full coverage of 200 mm wafers, or multiple samples within a 200 mm area, with optional chucks for 300 mm wafers. Ensuring the utmost in ease-of-use and reliability, the Dimension Pro delivers the lowest cost per measurement for quality control, quality assurance, and failure analysis.

The IconIR300 large-sample nanoscale infrared (IR) spectroscopy system provides high-speed, high-accuracy nanoscale characterization for semiconductor applications. Through its combination of proprietary photothermal IR spectroscopy for chemical ID and nanoscale AFM property mapping capabilities, the IconIR300 enables automated wafer inspection and defect identification on the widest range of wafer (up to 300 mm) and photomask samples.

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Bruker Materials Research AFMs achieve high speeds without compromising measurement quality. We are happy to recommend specific instrumentation, operational modes, and configurations to enable highest quality high-speed measurements for your specific application.

The Dimension FastScan is the first-and-only high-speed tip-scanning system that achieves frames per second (fps) scan rates without compromising resolution or system performance – independent of sample size. Whether collecting survey scans at >125 Hz to find specific regions of interest or capturing the dynamic behavior of a sample at rates of 1 fps in air or fluid, FastScan redefines the AFM experience.

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The Dimension IconIR system combines nanoscale infrared (IR) spectroscopy and AFM on a single platform to deliver the most advanced spectroscopy, imaging, and property mapping capabilities available for materials research. The IconIR enables correlative microscopy and chemical imaging with sub-10 nm resolution and monolayer sensitivity on a large-sample platform to provide the ultimate sample flexibility for the broadest range of applications.

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All of Bruker’s Materials Research AFMs are capable of operating in fluid and offer environmental control options for studies on biological samples. This provides the flexibility often required for AFM systems used in multi-user facilities or multi-focused research groups.

Investigators who are mainly focused on biological research and/or AFM integration with light microscopy techniques should refer to Bruker’s BioAFM systems to best address their research requirements.

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