SPM Q&A

Q1:
Please give me an overview of the scanning probe microscope.
Q2:
Please explain the basic principle of the scanning probe microscope.
Q3:
What is the cantilever like?
Q4:
What is the difference between contact mode and dynamic mode?
Q5:
What samples can be observed by SPM?
Q6:
What is the range of SPM observations and what is the magnification?

The Scanning Probe Microscope (SPM) is a generic name for a microscope that moves a microscopic probe close to the sample surface. It scans the surface while detecting the forces and electromagnetism acting between the sample and probe to provide three-dimensional observations of the sample surface. (See Fig. 1.) These devices are commonly known as Scanning Probe Microscopes, SPM, or AFM. The Atomic Force Microscope (AFM) is the most basic type of scanning probe microscope (SPM).

The scanning probe microscope was discovered in the 1980s and has a relatively short history. SPM are rather strange microscopes that are different from conventional optical microscopes and electron microscopes. They offer magnification equivalent to transmission electron microscopes and can identify the arrangements of atoms or molecules for certain samples under specific conditions. Despite the superb performance, the scanning probe microscope offers the advantage of being able to measure samples in the atmosphere or a liquid, without the need for a vacuum. Recently, its range of applications is expected to expand from simple 3D topographic observations to the visualization of various properties of the sample surface, making the SPM an essential microscopic instrument for nanotechnology research.

A number of microscopes are derived from the scanning probe microscope, including the Magnetic Force Microscope (MFM), Kelvin Force Microscope (KFM), and Lateral Force Microscope (LFM). The microscope with the most basic operating principle is the Atomic Force Microscope (AFM).

The AFM generally uses a cantilevered probe. The minute interatomic forces acting between the probe and the sample cause the cantilever to curve, and this curvature is detected with high sensitivity using the reflections of a laser beam shone onto the back of the cantilever. Simultaneously, the scanning of the cantilever or the sample itself is controlled extremely accurately in three dimensions by a scanner using a piezoelectric element. Generally, the cantilever scans across the sample surface (XY plane), while feedback control is applied to the distance from the sample (Z height) to maintain constant curvature of the probe (contact mode). The amount of feedback (output voltage to scanner) corresponding to each scanned XY position is input to a computer and processed to produce a 3D topographic image of the sample surface (observations of the sample surface shape).
The topographic image can be displayed as a grayscale display, pseudo-color display, or three-dimensional bird's-eye view. Offline image analysis software allows the analysis of any desired cross-sectional shape and the analysis of surface roughness. Fig. 2 shows the basic configuration of the SPM.

In addition to the basic AFM topographic images, Shimadzu scanning probe microscopes can obtain images of signals reflecting the sample surface properties, such as current and potential, hardness, and viscoelasticity (in some cases, options are required), and can be applied for a variety of SPM techniques.

Fig. 3 shows examples of cantilevers and Fig. 4 shows scanning electron micrograph of the probe at the tip. The probe is integrally formed at the tip of the soft and minute cantilever using semiconductor processing technology. Commercial versions are available made from SiN (silicon nitride) and Si (silicon). Typically, the cantilever is 200 µm long and 1 µm thick. The probe at the tip is 3 µm long with 20 nm tip radius of curvature. A variety of tip shapes is available for different applications. The probe may be coated with a magnetic material, metal film, or diamond-like carbon.

The cantilever is extremely small and is embedded in a base approximately 1.5 mm × 2.5 mm in size. The base is held in tweezers when replacing the cantilever.

SiN cantilevers for use in the contact mode are available in two lengths: 200 µm and 100 µm. The spring constant is different in each case, although the probe shapes are identical. Novices prefer the long cantilever, due to the simple optical axis adjustment. However, some users prefer the short cantilever because of the properties resulting from the high lever ratio of the cantilever.

Many types of cantilevers are available. New technical research and development is being actively pursued into softer cantilevers, for example. The Shimadzu SPM-9700 is designed to accept a variety of cantilever types.

A variety of AFM operating modes is available. They can be broadly categorized into contact mode (DC mode) and dynamic mode (resonant mode, AC mode).
The contact mode detects static interatomic forces. The basic principle is explained in Q2. As the cantilever approaches the sample surface, it deflects due to minute repulsive forces. Feedback control maintains a constant repulsive force (that is, a constant cantilever deflection). The amount of feedback is input to a computer to produce a 3D topographic image of the sample surface. As the repulsive force is held constant, this mode is also called the "constant force mode." This is the most standard mode for conventional AFM, due to its easy operating principle and ease of use. Due to its high resolution, this mode is generally used for observations at the atom or molecule level, such as the image of mica atoms in Fig. 5. For observations in the contact mode, the probe scans the sample surface immersed in the adsorbed water layer (contamination layer). Consequently, the cantilever is subjected to adhesion forces (meniscus forces) as well as repulsive forces from the sample, which can result in noise in the images like sideways drag lines. The technique is unsuited to samples that can easily move and to soft samples.

In a dynamic mode, vertical vibrations are applied to the cantilever, causing it to vibrate at close to its resonant frequency. As the vibrating cantilever tip approaches the sample, the amplitude changes. Feedback control is applied to this phenomenon, to maintain a constant vibration amplitude. As the probe rarely contacts the sample during scanning, this technique is suitable for easily-moved samples and adsorptive samples. As cantilevers for the dynamic mode are harder (with larger spring constant) than those for the contact mode, they are relatively unaffected by static electricity. As using a phase mode offers phase signals at the same time as surface topography observations, the dynamic mode has recently become the standard mode.
Table 1 shows a comparison of the two modes.

The scanning probe microscope offers observations of sample surfaces in the atmosphere, a liquid, or a gas environment. However, it does not permit observations of rapidly changing surfaces, unrestrained surfaces, surfaces that can float up or sample surfaces with a high aspect ratio, and care is required with interpretation of the images for some samples. The scanning probe microscope performance (resolution, reproducibility, ability to acquire non-topographic signals) is strongly affected by the sample characteristics, surface treatment, surface retention method, atmosphere around the sample, and type and individual differences of cantilever. It is clearly an exaggeration to say that anything can be observed using an SPM. A thorough understanding of the SPM principle and characteristics is important. The adjustment of the sample conditions to maximize the instrument performance is an important subject for research. The same points apply to other analytical instruments.

It depends on the scanner used.
Four types of scanner can be used with the SPM-9600, including options. The scanning range (= observation range) differs according to the scanner used. The maximum scanning range of the standard scanner is 30 µm × 30 µm in the horizontal (X, Y) field of view and 5 µm in the perpendicular (Z) direction (height). The maximum X, Y values using the wide scanner unit are 125 µm. The maximum Z value with the deep scanner unit is 13 µm. However, as the length of the probe at the cantilever tip is normally several µm and the shape thickens toward the base, SPM is unsuited to observe the bottom of holes more than several µm deep, regardless of the Z value.

The magnification is calculated as the apparent length on the displayed image divided by the length on the actual sample. For example, 125 µm that appears 125 mm on a 17-inch monitor or printout represents 1,000× magnification. This is the minimum magnification for normal use. Reducing the scanning range (= observation range) is equivalent to increasing the magnification. The scanning range can be set up to 0.1 nm in the software, which calculates to 125 mm/0.1 nm = 1, 250,000,000× magnification, but this is really just playing with numbers. Apart from specialists in the field of atom and molecule research, a minimum scanning range of about 100 nm is normally practical. This equates to a magnification of about 1,000,000×.