Measurement Principle / Measurement Mode

SPM is a generic term for all microscopes used to observe sample surfaces three-dimensionally by moving a tiny needle (probe), with a tip radius of only a few dozen nanometers, close to the sample surface. This is done to detect the mechanical and electromagnetic interactions between the probe and sample as it scans the sample surface (see the figure below). These microscopes are sometimes referred to by various names, such as scanning probe microscopes, probe microscopes, or SPMs. One of the most basic types of scanning probe microscope is the atomic force microscope (AFM). Another type of SPM is the scanning tunneling microscope (STM), which functions by detecting the tunneling current that flows between the probe and sample. (A tunneling current exists even without contact, due to the tunneling effect, which acts at extremely close distances.)

STM is capable of even atomic resolution, particularly in ultra-high vacuum environments, but they also have various limitations. For example, STMs are not able to observe nonconductive samples, and in atmospheric conditions, they are extremely sensitive to sample surface contamination. In contrast, atomic force microscopes detect the forces acting between the sample and probe (which are sometimes attractive and sometimes repulsive, but difficult to accurately understand). Therefore, AFMs can be used to observe samples of insulation materials. They can also observe samples easily even in atmospheric air. Consequently, AFMs are widely used, and most commercially marketed SPM systems are based on AFM principles. AFM systems can be generally categorized as using contact mode or dynamic mode. Furthermore, by simultaneously detecting other interactions between the sample and probe, some systems are now able to not only visualize the three-dimensional contours of samples, but also map various other properties. All of these types of systems are referred to as SPMs. In light of the many possible types of SPM systems, an acronym of the form "SXM" is often used, where the "X" indicates the physical quantity detected.

Invented in the 1980s, SPMs are relatively new in history. The STM was invented in 1981 and the AFM in 1986. The invention of the STM enabled clear observation of an actual spatial image of the structure of 7 x 7 Si(111) in an ultra-high vacuum. For their achievement, the inventors, Gerd Binnig and Heinrich Rohrer (at the IBM Zurich Research Laboratory), were awarded the Nobel Prize and became instantly famous, something still fresh in our minds. The SPM is different from a typical microscope, which uses beams and lenses to increase or reduce magnification, such as optical or electron microscopes. However, given certain conditions and samples, it is able to distinguish between atoms and molecules, and its magnification capacity rivals transmission electron microscopes. Another big advantage is that it can observe samples in air or liquids. Therefore, as a microscope essential for nanotechnology research, applications for SPM systems are expected to expand even further in future.

Basic Configuration of the Scanning Probe Microscope

There are many derivatives of the SPM, such as the magnetic force microscope (MFM), surface potential microscope (or Kelvin probe force microscope, KFM), or lateral force microscope (LFM), but their most fundamental principles are illustrated by the atomic force microscope (AFM).

The AFM uses a cantilever (a beam anchored at only one end) with a needle-shaped tip to detect force. The tiny forces acting between the needle on the tip of the cantilever and the sample (atomic forces) cause the cantilever to vary in how it bends and vibrates. These variations are detected with high sensitivity using laser light reflected off the back side of the cantilever. At the same time, either the cantilever or sample is precisely scanned and controlled three-dimensionally by a scanner with a piezoelectric element. In general, the distance from the sample (Z-height) is fed-back and controlled so that either the amount of bending is kept constant (contact mode), or the amount of vibration is kept constant (dynamic mode), as the cantilever is scanning the sample surface (XY-plane).
A 3D contour image of the sample surface (an image used to observe the shape of the sample surface) is obtained by loading the Z-axis feedback value (output voltage to the scanner) for each scanned position (X and Y coordinates) into a computer that reconstructs the data as a 3D image. The contour image can be further processed (using offline software) to analyze any cross section of interest, by shading or false color-coding, or displaying the section in bird’s eye view mode for example, or to analyze the surface roughness. The basic system configuration is shown above.

In addition to basic AFM contour images, Shimadzu SPM models also makes it possible to use various SPM techniques to obtain images based on information from signals indicating the physical properties of sample surfaces, such as electrical current, electrical potential, hardness, or viscoelasticity. (Some of these features are optional.)

Atomic Image of Mica (Using disk-shaped air-spring isolator)

In contact mode, static atomic forces are detected based on the degree of cantilever bending, as indicated in the basic operating principles described in Q1-2. When the cantilever is moved close to the sample surface, the cantilever bends slightly due to tiny repulsive forces. Feedback control is used to stabilize the repulsive force, or in other words, to keep the degree of cantilever bending constant. The feedback values are then input to a computer to render a visual image of the surface contour. Because the repulsive forces are kept constant, this mode is also referred to as constant force mode. Due to the simple operating principle and ease of use, this has conventionally been the most common AFM mode. Due to the high resolution, this mode was also generally used for atomic and molecular-level observations of substances such as the mica sample shown below.
When observing samples using contact mode in atmospheric air, the probe scans the surface layer of the sample that has adsorbed water (contamination layer). Therefore, in addition to the repulsive forces from the sample, the cantilever can sometimes also be affected by adhesion forces (meniscus forces), which can cause noise that resemble lateral drag marks in images. Consequently, this mode is not suited to visualizing samples that tend to move, or soft surfaces.

In dynamic mode, a vertical excitation force is applied to the cantilever so that it vibrates at close to its resonant frequency. When the vibrating cantilever tip is moved close to the sample, the vibration amplitude varies. Feedback control is then used to keep the vibration amplitude constant. Since there is little chance of the probe catching on the sample during scanning, this mode is well suited to samples that tend to move, or adsorptive samples. In addition, dynamic mode cantilevers are stiffer, with a higher spring constant than contact mode cantilevers, in order to reduce the effects of static electricity. Another feature is the use of phase mode to obtain a phase signal while observing the surface contour. In recent years, dynamic mode has become a standard AFM technique.
The following is a comparison of both modes.

In dynamic mode, the system detects how far the phase of the detected sinusoidal cantilever vibration signal lags behind the signal of the original vibration source. These signals can then be used to create a visual image of differences in sample surface properties. The phase lag in cantilever vibration in response to sample surface properties is similar to feet sticking when walking in mud. Therefore, determining surface properties is like determining the properties of the ground based on the degree to which the feet are sticking

In this mode, the system vibrates the sample vertically at a constant frequency while scanning in contact mode; presses the probe against the sample; and then detects the response in terms of cantilever amplitude and phase lag. These signals can then be used to visualize differences in the viscosity and elastic properties of the sample surface.

In this mode, the system applies a bias voltage between the probe and sample while scanning in contact mode, and detects the current flow between the probe and sample. It then simultaneously displays an image of the in-plane distribution of the current flow detected, along with the surface contour. This provides an image based on the localized resistance levels of the sample.

In this mode, the sample is scanned with a magnetized probe tip, which is vibrating near the resonant frequency, and is kept at a constant distance from the sample. This causes the cantilever amplitude and phase to vary due to repulsive and attractive forces on the probe, due to magnetic leakage from the sample surface. By detecting the amount of variation, magnetic information about the sample surface can then be visualized. This observation method is referred to as magnetic force microscopy (MFM).

In this mode, the system measures the electric potential of the sample surface by applying an alternating current to a conductive cantilever, and then detecting the static electric force acting between the sample surface and the cantilever. This observation method is generally referred to as Kelvin probe force microscopy (KFM), or electric force microscopy (EFM).

The sample is scanned in a direction perpendicular to the cantilever axis in contact mode, and the torsion on the cantilever is detected in order to visualize the lateral forces (friction) acting between the probe and sample. This observation method is generally referred to as lateral force microscopy (LFM), or friction force microscopy (FFM).

In this mode, the system measures and graphs the force acting on the probe (how far the cantilever bends) as the distance between the probe and sample is varied, which is referred to as force curve measurement. This mode is used to measure the adsorption and relaxation forces on the sample. It is also used for detailed cantilever selection, and to verify properties.

In this mode, the probe is used to mark sample surfaces with letters or graphics. Examples include mechanical methods that carve the surface, and electric methods that anodize the surface. This process is also referred to as nanolithography.

(3) Image Resolution
In reality, what is referred to as atomic resolution for SPM systems is often based on data obtained from the cleavage face of lamellar crystals (such as mica and HOPG) using contact mode. Cleaving samples into layers results in atomically-smooth clean surfaces, which makes it much easier to obtain an image of lattice periodicity at the atomic scale. A so-called atomic image of the cleavage face of mica is shown below. It is referred to as an atomic image because the lattice size (5.2 Å) is consistent with the lattice constant obtained using X-ray diffraction or other methods. This image can be interpreted as a periodic signal generated from a kind of moire striping or stick-slip phenomenon, resulting from the arrangement of atoms in the clean sample face, and the arrangement of atoms in the probe tip. Note that this does not mean that the probe tip is able to enter the indentations of the periodicity; rather, it simply means that it recognizes the periodicity of the arrangement. Research aimed at achieving true atomic resolution has been continuing in recent years, with efforts that are pushing the limits of sensitivity using frequency modulation AFM (FM-AFM).