What is an X-ray microscope?
An X-ray microscope is a microscopy technique that uses X-rays as a light source to observe the structure of an object.
X-ray microscope mainly uses transmission (absorption) X-rays and fluorescent X-rays, which are highly penetrating X-rays that can produce contrasting images by taking advantage of the attenuation reactions inherent to the internal structure, thickness, and composition of the material.
In addition, by rotating the specimen and constructing a 3D image from the continuously acquired images, it is possible to obtain a tomographic image (so-called CT). In general, spatial resolution in microscopy using electromagnetic waves depends on the wavelength of the electromagnetic waves. Since the wavelength of X-ray is 100 to 10,000 times shorter than that of visible light, it is possible to obtain a high-resolution image.
Uses of X-ray microscope
X-ray microscope is mainly used for research and development in the industrial field as well as for inspections at manufacturing sites. It can also be used to evaluate the structure of rocks and other materials to obtain parameters for characterization as new raw materials.
In the semiconductor manufacturing field, it is increasingly used to characterize products that have undergone ultrafine processing. When observing biological samples that contain a large amount of water, images with high contrast can be obtained by using the X-ray wavelength range where water absorption is low.
Principle of X-ray microscope
X-ray microscope uses X-rays to irradiate a sample and obtain images and perform component analysis by using the transmission (absorption) X-rays and fluorescent X-ray signals obtained from the material. The wavelengths of the X-rays used are often called soft X-rays (1-10 nm). In particular, the region from 2.3 to 4.3 nm is called the “water window” because water absorption is extremely low, and is used for observation of biological samples.
X-ray microscopies are classified into two types: those that use X-ray transmittance as a contrast to acquire images, and those that detect fluorescent X-rays generated by X-ray irradiation. X-ray fluorescence is a signal produced by the emission of X-rays corresponding to the energy difference between the inner and outer shells when outer-shell electrons relax into holes created by the excitation of inner-shell electrons in a material by X-ray irradiation.
Since fluorescent X-rays have wavelengths unique to each atom, they can be applied to elemental analysis. X-ray microscopes can also be broadly classified into two types, depending on the optical system and the presence or absence of optical elements. X-ray microscope without optical elements uses the projection magnification method and the contact method for observation.
Since the X-ray image cannot be magnified by using a lens, the sample is physically separated from the imaging surface and the image is magnified and projected. The imaging method using optical elements is realized by using zone plates with light commentary or mirrors that utilize total or multilayer reflection.
X-ray microscope
1. The difference between X-ray microscope and Electron microscopy
X-ray microscopes use X-rays as their light source, whereas electron microscopy uses electron beams to illuminate the specimen and magnify the image. An electron beam is a fast stream of electrons. An atom is composed of a nucleus made up of protons and neutrons, with electrons orbiting it. When protons, neutrons, and electrons are accelerated to very high speeds in a device called an accelerator, they become radiation in the form of proton, neutron, and electron beams.
Unlike X-rays, electron beams are particle beams and therefore have limited penetrating power. The penetration power of an electron beam is determined by the acceleration voltage: the higher the acceleration voltage, the deeper the electrons can penetrate, and the lower the density of the irradiated object, the deeper it can penetrate.
Transmission Electron Microscope (TEM)
A thin-film sample is irradiated with electron beams, and the electron beams transmitted through the sample are passed through an electron lens to produce an enlarged image on a fluorescent plate illuminated by the electron beams. The electron lens bends the electron beam by means of an electric or magnetic field to form an image.
Scanning Electron Microscope (SEM)
A narrowly focused electron beam is irradiated in a vacuum to scan the surface of a sample to detect secondary electrons and reflected electrons emitted from the sample. Secondary electrons are those emitted by the irradiated electron beam that knocks out other electrons in the sample, while reflected electrons are those emitted by the irradiated electrons that are reflected from the surface of the sample.
When an X-ray detector is attached to a scanning electron microscope, it can be used as an X-ray analyzer to determine the type and amount of elements contained in a sample.
2. Scanning X-ray microscope
This is a type of X-ray microscope that uses hard X-rays as a probe. Hard X-rays have a short wavelength of around 0.1 nm, and in principle, high resolution is possible. In addition to transmission (absorption), refraction, and reflection, interactions with materials include photoelectrons, fluorescent X-rays, elastic scattering, inelastic scattering, magnetic absorption and scattering, and many others.
Furthermore, its high transmissivity allows for non-destructive observation and is used for atmospheric measurements. Scanning X-ray microscopes consist of a focused X-ray beam, a stage for scanning the sample, and a detector. While scanning the sample, X-ray analysis (transmitted X-ray, fluorescent X-ray, scattered X-ray, etc.) is performed to visualize various types of information.