Nowadays, high-resolution transmission electron microscope (TEM) and scanning electron microscope (SEM) are increasingly used in material analysis and research, and they have become an indispensable research on crystal structure and chemical composition in modern laboratories. Comprehensive instrument. Transmission electron microscope is often used to observe the fine material structure that ordinary microscopes cannot distinguish. Scanning electron microscope is mainly used to observe the morphology of solid surfaces. Sometimes, combining the two organically can get a more comprehensive material analysis result. The following will compare from many aspects, the difference between scanning electron microscope and transmission electron microscope.
1. Analyze the signal
scanning electron microscope
Scanning electron microscope manufacturing is based on the interaction of electrons and substances. When a beam of high-energy incident electrons bombard the material surface, the excited region will produce secondary electrons, Auger electrons, characteristic X-rays and continuum X-rays, backscattered electrons, transmission electrons, and visible, ultraviolet, and infrared light. Electromagnetic radiation generated by the area. At the same time, electron-hole pairs, lattice vibration (phonons), and electron oscillations (plasma) can also be generated. In principle, the interaction between electrons and matter can be used to obtain information about various physical and chemical properties of the measured sample itself, such as morphology, composition, crystal structure, electronic structure, and internal electric or magnetic fields. Scanning electron microscope is based on the above-mentioned different information generation mechanism, using different information detectors, so that selective detection can be realized. For example, the collection of secondary electrons and backscattered electrons can obtain information about the microscopic morphology of the material; the collection of X-rays can obtain information about the chemical composition of the material.
Transmission Electron Microscope
According to De Broglie (De Broglie, a French scientist in the 20th century) proposed that moving microscopic particles have wave-particle duality, the electron beam is also volatile, and the wavelength of the electron wave is shorter than that of visible light. (For example, the wavelength of the electron wave is 0.00251 nanometers at an accelerating voltage of 200 kV). Obviously, a microscope made with an electron beam as a light source will have a much higher resolution than an optical microscope. More importantly, because electrons are moved by electric field force in an electric field, and moving electrons are deflected by Lorentz force in a magnetic field, it is possible to use scientific methods to focus and image the electron beam.
2. Structure
Scanning Electron Microscope
1. The lens barrel
lens barrel includes electron gun, condenser lens, objective lens and scanning system. Its function is to generate a very thin electron beam (about a few nm in diameter), and scan the electron beam on the surface of the sample while exciting various signals.
2. Electronic signal collection and processing system
In the sample chamber, the scanning electron beam interacts with the sample to generate a variety of signals, including secondary electrons, backscattered electrons, X-rays, absorbed electrons, Auger electrons, etc. . Among the above signals, the most important is the secondary electrons, which are the outer electrons in the sample atoms excited by the incident electrons, which are generated in the region from a few nm to tens of nm below the sample surface. The generation rate is mainly determined by The morphology and composition of the sample. Generally speaking, scanning electron image refers to the secondary electron image, which is the most useful electronic signal for studying the topography of the sample surface. The probe of the detector for detecting secondary electrons is a scintillator. When electrons hit the scintillator, light is generated in the scintillator. This light is transmitted to the photomultiplier tube by the light pipe, and the light signal is converted into a current signal. After pre-amplification and video amplification, the current signal is converted into a voltage signal, which is finally sent to the grid of the picture tube.
3. Electronic signal display and recording system
scanning electron microscope images are displayed on the cathode ray tube (picture tube), and are photographed and recorded by a camera. There are two picture tubes. One is used for observation with a lower resolution and is a long afterglow tube; the other is used for photographic recording and has a higher resolution and is a short afterglow tube
4. Vacuum system and power supply system
The vacuum system of the scanning electron microscope consists of a mechanical pump and oilIt is composed of diffusion pump, and its function is to make the vacuum degree of 10 in the lens barrel. The
power system supplies specific power required by each component.
Transmission Electron Microscope
1. Electron optics part
The entire electronic optics part is completely placed in the lens barrel, and the electron gun, condenser lens, sample chamber, objective lens, intermediate lens, projection lens, observation chamber, fluorescent screen, camera mechanism and other devices are arranged in order from top to bottom. According to the different functions of these devices, the electronic optics can be divided into lighting system, sample chamber, imaging system and image observation and recording system.
(1) Illumination system The
illumination system consists of an electron gun, a condenser and corresponding translation, centering and tilt adjustment devices. Its function is to provide an illumination source with high brightness and good coherence for the imaging system. In order to meet the needs of dark field imaging, the illumination electron beam can be tilted in the range of 2-3 degrees.
① The electron gun
consists of a cathode, a grid and an anode. After being energized and heated in a vacuum, the electrons emitted from the cathode obtain higher kinetic energy to form a directional high-speed electron flow.
②Condenser
The function of the condenser is to converge the electron beams emitted from the electron gun, and control the illumination aperture angle, current density and spot size.
(2) sample chamber
The sample chamber has a sample rod, a sample cup and a sample stage.
(3) imaging system The
imaging system is generally composed of an objective lens, an intermediate lens and a projection lens. The resolving power of the objective lens determines the resolving power of the electron microscope, and the function of the intermediate mirror and the projection lens is to further enlarge the image from the objective lens.
(4) Image observation and recording system
This system consists of fluorescent screen, camera, data display and so on.
2. Vacuum system
vacuum system consists of mechanical pump, oil diffusion pump, reversing valve, vacuum measuring instrument and vacuum pipeline. Its function is to remove the gas in the lens barrel, so that the vacuum degree of the lens barrel must be at least above Torr. If the vacuum is low, the collision between electrons and gas molecules will cause scattering and affect the contrast. It will also cause the high voltage ionization between the electron grid and the anode to cause inter-electrode discharge. The residual gas will also corrode the filament and contaminate the sample.
3. The unstable acceleration voltage and lens magnetic current of the power supply control system
will cause serious chromatic aberration and reduce the resolution of the electron microscope. Therefore, the stability of the acceleration voltage and lens current is an important criterion for measuring the performance of the electron microscope. The circuit of
TEM is mainly composed of high voltage DC power supply, lens excitation power supply, deflector coil power supply, electron gun filament heating power supply, and vacuum system control circuit, vacuum pump power supply, camera drive device and automatic exposure circuit. In addition, many high-performance electron microscopes are also equipped with scanning accessories, energy spectroscopy, and electron energy loss spectroscopy.
3. Function
Scanning Electron Microscope
1. Scanning Electron Microscope pursues high-resolution morphology and morphological image of solid matter (Secondary Electron Detector SEI)-Topography Analysis (Surface Geometry, Shape, Size)
2, Space for Displaying Chemical Composition Change, phase identification based on chemical composition---chemical composition image distribution, micro-area chemical composition analysis
1) Use X-ray energy spectrometer or spectroscopy (EDS or WDS) to collect characteristic X-ray signals, and generate corresponding samples , Element surface distribution map or qualitative and quantitative analysis of fixed-point chemical composition, phase identification.
2) Using backscattered electrons (BSE) based on the contrast of the average atomic number (generally related to relative density) to generate a distribution image of the chemical composition phase;
3) using cathodoluminescence, based on certain trace elements (such as transition metal elements, rare earth elements) Etc.) The image of the trace element distribution generated by the contrast of the light intensity excited by the electron beam.
4) Using the sample current, based on the average atomic number contrast, the generated chemical composition phase distribution image, the image is opposite to the backscattered electron image..
5) Use Auger electrons to analyze the qualitative theorem of chemical element distribution on the 1nm surface layer of the sample material.
3. Special applications in semiconductor device (IC) research:
1) Imaging using electron beam induced current EBIC, which can be used to locate and damage the pn junction in integrated circuits
2) Using sample current imaging, the results can be displayed The open and short circuit of the metal layer in the circuit, so the resistance contrast image is often used to check the metal wiring layer, polycrystalline wiring layer, metal-to-silicon test pattern and the conductive form of the thin film resistor.
3) Use the secondary electron potential contrast image to reflect the potential on the surface of the sample. From it, you can see the level and distribution of the potential on the surface of the sample. It is especially convenient to determine the hidden open circuit or hidden short circuit of the device.
4. Use backscattered electron diffraction signals to conduct crystal structure (arrangement of atoms in the crystal) of the sample material, analyze the crystal orientation distribution, and identify the phase based on the crystal structure.
Transmission Electron Microscope
The early transmission electron microscope function was mainly to observe the morphology of the sample, and later developed to be able to analyze the crystal structure of the sample in situ by electron diffraction. The two functions that can observe the morphology and crystal structure in situ are not available in other structural analysis instruments (such as light microscopes and X-ray diffractometers).
After the addition of accessories to the transmission electron microscope, its function can be developed from the original sample internal structure observation (TEM), in-situ electron diffraction analysis (Diff), to in-situ component analysis (energy spectrometer EDS, Characteristic energy loss spectrum (EELS), surface topography observation (secondary electron image SED, backscattered electron image BED) and transmission scanning image (STEM).
combined with the sample stage is designed into a high temperature stage, a low temperature stage and a stretching stage. The transmission electron microscope can also observe the dynamic structure and composition changes of the sample in the heating state, low temperature cooling state and stretching state, making the transmission electron microscope function Further broaden. The broadening of the functions of
transmission electron microscope means that one instrument can perform a variety of analyses without changing the sample, especially for the comprehensive analysis of the morphology, crystal structure and composition (valence) of the same micro-area.
Scanning Electron Microscope Transmission Electron Microscope
Four, the principle of contrast
scanning electron microscope
1, the main source of the contrast of the thickness of the amorphous sample. Different microdomains of the sample are formed by differences in atomic number and thickness. Originating from incoherent scattering of electrons, the higher the Z, the greater the proportion of scattering; the increase of d, the more scattering will occur. The difference between the different micro-zones Z and d makes the scattered electrons I entering the aperture of the objective lens and focusing on the image plane different, forming the contrast of the image. Areas with higher Z and thicker samples are displayed as darker areas on the screen. The contrast change on the image reflects the change of the atomic number and thickness of the corresponding area of the sample. The quality and thickness contrast is affected by the objective diaphragm aperture and acceleration V. Choose a large aperture (more scattered electrons participate in imaging), the image brightness increases, and the contrast between the scattered and non-scattered areas decreases. Choose a low voltage (more electrons are scattered outside the diaphragm aperture), the contrast is improved and the brightness is reduced. Supports membrane method and extraction replication, and the quality and thick contrast images are more intuitive.
2, diffraction contrast
The diffraction contrast is derived from the differences in the Bragg reflection conditions and the structural amplitudes of each part of the crystal sample. For example, when the voltage is constant, the intensity of the incident beam is constant, false is L, and the intensity of the diffracted beam is ID. In the case of ignoring absorption, the transmitted beam is L-ID. In this way, if only the transmitted beam is imaged through the objective lens diaphragm, then due to strong or weak diffraction or no diffraction of each crystal plane in the sample, the corresponding intensity of the transmitted beam will change, thereby forming a contrast on the phosphor screen. In the process of forming the contrast, the diffraction of the electron beam by the crystal plays a decisive role. The crystal structure of
transmission electron microscope
can be studied by high resolution transmission electron microscope,This technique is also called phase contrast microscopy. When using a field emission electron source, the observation image is reconstructed from the difference in the phase of the electron wave caused by the interaction of the electron with the sample. However, because the image also depends on the number of electrons shot on the screen, the recognition of phase contrast images is more complicated.
The contrast of the transmission electron micrograph of the amorphous sample is formed by the difference in atomic number or thickness in different micro-regions of the sample, that is, the mass thickness contrast (the mass thickness is defined as the mass thickness in the cylinder above the unit area of the lower surface of the sample). The quality), also called the contrast of quality. The thickness contrast is suitable for explaining the electronic image of the composite film sample. The larger the mass thickness, the strong absorption and scattering effect on electrons, so that more electrons are scattered outside the light barrier, corresponding to a relatively safe contrast. A small mass thickness corresponds to a brighter contrast.
5. Requirements for the sample
scanning electron microscope
SEM sample preparation has no special requirements for the thickness of the sample, and the specific section can be presented by cutting, grinding, polishing or cleaving methods to transform it into an observable surface. If such a surface is directly observed, only surface processing damage can be seen. Generally, different chemical solutions are used for preferential corrosion to produce contrast that is conducive to observation. However, corrosion will cause the sample to lose part of the true condition of the original structure, and at the same time introduce some human interference. For the thin layer of the sample, the error is greater.
Transmission Electron Microscope
Because the quality of the microscopic image obtained by TEM strongly depends on the thickness of the sample, the observation part of the sample should be very thin. For example, the TEM sample of memory device can only have a thickness of 10-100nm, which is for TEM sample preparation. Brings great difficulty. In the process of sample preparation, beginners use manual or mechanical control to polish the finished product rate is not high, once excessive grinding will make the sample scrap. Another problem of TEM sample preparation is the positioning of the observation point. General sample preparation can only obtain a thin observation range of the order of 10 mm. When precise positioning and analysis are required, the target often falls outside the observation range. The current ideal solution is to perform fine processing through focused ion beam etching (FIB).
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