Title：
TRANSMISSION ELECTRON MICROSCOPE (TEM)
報告題名：穿透式電子顯微鏡
Author(s): TSAI-CHEN HSIEH,
Class: 3rd year of Department of Material Science and Engineering
Student ID: D0513682
Course: Surface Analysis
Instructor: Dr. Wei-Chun Lin
Department: Department of Materials Science and Engineering Academic Year: Semester 1, 2018
Abstract
In this paper, we introduce the transmission electron microscope (TEM). Including the component of TEM and the limitations of TEM. By knowing the principle and the limitation of TEM, we can use it for observed the morphology of our specimen.
Keyword：
Aberration Aperture Astigmatism Application Characterization Contrast Electron Gun Electromagnetic Lens中文摘要
多數材料的改良和研發都藉由微觀結構的改善，因此表面分析技術就更顯得重 要。表面分析領域的持續發展為提供科技進步的基礎。其領域中包含的技術有 許多種：有進行元素分析的 XPS/ESCA(X-射線光電子光譜/化學分析電子光 譜)，結構分析的 XRD (X-射線繞射分析)和觀察樣品表徵的 TEM(穿透式電子顯 微鏡)和 SEM (掃描式電子顯微鏡)等等。而 TEM 的分辨率又高於 SEM，雖然不如 SEM 為 3D 圖示，但也為一非常重要的分析儀器。此篇報告主要著重在穿透式電 子顯微鏡(TEM)的介紹，大致分為三部分作介紹，分別為其工作原理、缺點(限 制)和應用。在各部分再分割成各自的小部分來講解：工作原理從內部的構造配 件開始介紹，缺點以儀器本身所造成的影像失真為主，應用範圍則為現今市場 上常介的應用：例如，LED 產業、半導體產業等。以淺顯的介紹希望大家可以 能大致了解穿透式電子顯微鏡的作用。
關鍵字：
材料分析 穿透式電子顯微鏡 穿透式電子顯微鏡工作原理 穿透式電子顯微鏡構造 穿透式電子顯微鏡影像 結構分析 像差Table of Content
1. Introduction 1.1History (p.4) 1.2 Component of TEM (pp.4-6) 1.2.1 Electron Gun 1.2.2 Electromagnetic Lens 1.2.3 Aperture 1.2.4 Specimen Holder 1.2.5 Fluoroscopic Screen 1.3 Sample Preparation (pp.6-7) 1.4 Imaging Methods (p.7) 1.5Calibrating the TEM (p.7) 2. Limitations 2.1 Resolution (p.8) 2.2 Aberration (pp.8-9) 2.2.1 Spherical Aberration 2.2.1 Chromatic Aberration 2.2.3 Astigmatism 2.3 Contrast (pp.9-10)2.3.1 Mass Thickness Contrast 2.3.2 Diffraction Contrast 2.4 Component analysis (p.10) 3. Application of TEM (pp.10-11)
List of Figures
1.1 Component of TEM
1.3 Three common specimen formats 1.4 Spherical aberration
1.5 Spherical aberration vs. No aberration 1.6 Chromatic aberration
1.7 Astigmatism 1.8 Contrast
1.9 TEM image of LED 2.1 TEM image of DRAM
List of Tables
1.1 History
In 1897, J.J. Thomson discovered the electron and it will be deflected by magnetic field.
In 1927, Davisson-Germer discovered electron diffraction.
In 1934, Ernst August Friedrich Ruska designed the first electron microscope. 1.2 Component of TEM
1.2.1 Electron Gun
The function of the electron gun is to generate an electron beam, providing a high-energy, sufficiently bright and stable light source. And there are some normal types of electron gun: Thermionic Emission, Field Emission Gun (FEG) and Schottky FEG. The comparison of electron gun:
1.2.2 Electromagnetic Lens
There are four main form of lenes: Condenser lenses, Objective lenses, Intermediate lenses and Projector lenses.
The condenser lenses is used to adjust the electron beam. The purpose of the condenser lens is to focus the light onto the specimen. This lens is used to form the beam and limit the amount of current in the beam and it is also used to control the diameter of the electron beam.[1]
The objective lenses is the main lenses form of TEM. And it also is the place putting the specimen. The structure of the objective lenses has divided into two parts, the specimen is put in the gap between them.
The "intermediate lens" is placed between the objective lens and the projector lens. The intermediate lens works in the following way. The intermediate lens changes focusing position either on a diffraction pattern or a TEM image produced by the objective lens by adjusting its excitation, and forms the magnified pattern or image on the object plane of the projector lens. Normally, the intermediate lens consists of three parts: The first part mainly selects focusing position, the second part magnifies the focused pattern or image, and the third part mainly achieves rotation-free condition.[2] Magnification in the electron microscope can be varied from hundreds to several hundred, thousands of times. This is done by varying the strength of the projector and intermediate lenses.
1.2.3 Aperture
The aperture may be fixed in size or variable in size. They can be inserted into the electron beam path or removed, or moved in a plane perpendicular to the electron beam path. Generally TEM has four sets of apertures.
The aperture placed behind the first concentrating lens mainly changes the size of the electron beam and filters out electrons that are too far from the optical axis to increase the quality of the electron beam---enhance coherence and reduce the energy dispersion range.
The aperture (C2 aperture) placed behind the second condenser can change the beam size and the far electrons, and the convergence angle of the specimen.
The aperture placed on the focal plane behind the objective lens, called the objective aperture, is the most important aperture.
The functions of the objective aperture include: (1) filtering electrons that are too far from the optical axis of the objective to improve the focus of the image; (2) improving
the image contrast (3) selecting a direct electron beam or a single diffracted electron beam to form a bright field image or a dark field image.
The aperture placed on the first imaging plane is called the selection aperture, and the selection aperture is used to limit the analysis area.
The two apertures closest to the specimen are the most susceptible to contamination and have a life time of one to two years.
1.2.4 Specimen Holder
The function of Specimen Holder is to load the specimen, insert it into the TEM, and provide translation and tilting functions to move the analytically valuable area to the center of the fluorescent screen.
1.2.5 fluoroscopic screen
The fluorescent screen is coated with a fluorescent substance (ZnS). When the electron beam hits the fluorescent substance, the fluorescent substance is excited to emit visible light, so that the electronic image or the diffraction pattern amplified by the projector lens can be converted into a visible light image. In a TEM equipped with a digital camera, the function of the fluorescent screen is to monitor whether or not the maximum brightness is obtained during the adjustment.
1.3 Sample preparation
The TEM specimen is 3 mm. The following figure shows three common three specimen formats:
1.4 Imaging methods
The basic imaging principle of an electromagnetic lens is the same as that of an optical convex lens, and can be expressed by the following formula without considering the rotation of electrons in a magnetic field:
+𝑓𝑓1 𝑖𝑖 = 1 𝑓𝑓 𝑓𝑓0: 𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜 𝑑𝑑𝑑𝑑𝑑𝑑𝑜𝑜𝑑𝑑𝑑𝑑𝑜𝑜𝑜𝑜 𝑓𝑓𝑖𝑖: 𝑑𝑑𝑖𝑖𝑑𝑑𝑖𝑖𝑜𝑜 𝑑𝑑𝑑𝑑𝑑𝑑𝑜𝑜𝑑𝑑𝑑𝑑𝑜𝑜𝑜𝑜 𝑓𝑓: 𝑓𝑓𝑜𝑜𝑜𝑜𝑑𝑑𝑓𝑓 𝑑𝑑𝑑𝑑𝑑𝑑𝑜𝑜𝑑𝑑𝑑𝑑𝑜𝑜𝑜𝑜 Definition of Magnification: 𝑀𝑀 = 𝑓𝑓𝑖𝑖 ∕ 𝑓𝑓0
1.5 Calibrating the TEM
An accurate calibration of magnification, diffraction patterns and the relative rotation between the two is critical in quantitative transmission electron microscopy (TEM). Before attempting quantitative TEM analysis, a series of calibrations of the individual TEU should be undertaken. For the magnification calibration of a TEM, the general procedure is to take micrographs of standard samples with known spacings at all magnification values accessible by that standard, Most standards are only suitable for a specific magnification range. New types of calibration samples can now be made with the technology of the semiconductor industry. One major advantage of this type of calibration sample is that the sample can be self-calibrated to accuracies well beyond the resolution of a TEM.[3]
2.1 Resolution
Due to the existence of the diffraction phenomenon, the image formed by the point source passing through the lens is not a point but a disc. The resolution is the smallest distance between the two discs that can be resolved. The smaller the distance, the higher the resolution. The limit that the human eye can distinguish is that the center overlap strength is 81% of the center strength of the disc. In TEM, the resolution is related to the objective lens. The smaller the objective lens, the better the resolution, but too small will make the Airy disc larger due to the diffusion effect.
𝑑𝑑 = 2𝛼𝛼𝑓𝑓 𝛼𝛼 = 0.77𝜆𝜆14𝐶𝐶𝑠𝑠 1 4 𝑟𝑟𝑚𝑚𝑖𝑖𝑛𝑛 = 0.91𝜆𝜆 3 4𝑜𝑜𝑠𝑠 1 4 2.2 Aberration
An electron microscope, like an optical microscope, cannot have an ideal lens. There are some main types of lens defects are listed below.
2.2.1 Spherical aberration
The high-energy electrons passing through the edge of the lens are deflected at a greater angle to focus closer to the lens. Therefore, after an ideal point source passes through the lens, its image is no longer an ideal source, but a halo of the disc. In the past, spherical aberration greatly affected the resolution. However, since 1998, TEM has added a spherical aberration corrector. The spherical aberration is significantly improved. So that the resolution of today's TEM can be as low as 1 angstrom.
2
2.2.1 Chromatic Aberration
Through high-energy electrons of the same lens radius, the lower energy is
deflected at a larger angle, so it is concentrated closer to the lens. This effect is caused by the aperture. The smaller the aperture chromatic aberration, the lower the spherical aberration. A balance needs to be found among the three. The following formula is the minimum radius of an optical disc that is refracted by electrons of different energies: 𝑟𝑟𝐶𝐶 = 𝐶𝐶𝐶𝐶𝛼𝛼 �𝛥𝛥𝛥𝛥𝛥𝛥�
3
2.2.3 Astigmatism
Focusing on different distances on different planes through high-energy electrons of the same radius of the lens. This is a defect that must be adjusted to make
high-resolution images. It has now been improved with astigmator and computer programs.
2.3 Contrast
Image contrast is an important key to judging information.
2.3.1 Mass thickness Contrast
The contrast of the thickness is formed by the scattering of electrons. The larger the atomic order, the smaller the proportion of direct electrons at a small angle and the darker the image. High angle scattered electrons increase as the thickness of the sheet increases.
2.3.2 Diffraction Contrast
Diffraction contrast: When the lattice plane and the electron beam are at an angle of Bragg will occur a strong diffraction, and the number of electrons that generate strong diffraction is relatively reduced, forming a darker region in the image.
2.4 Component analysis
Let’s talk about the three main types of component analysis methods: fixed-point analysis, line profile , and elemental mapping.
Fixed point can be use in both STEM and TEM methods. And the line profile which is used with the STEM method is much better than the TEM does.
Due to TEM is unable to detect its component signals by image, and must rely on other ancillary equipment such as energy disperser (EDS) and electronic energy loss spectrometer(EELS).
3.Application Of TEM Due to the advancement of TEM technology, TEM has been used in various
industries. For example, the LED industry, dynamic random access memory (DRAM) and flash memory (NAND Flash Memory), etc. It can even overcome the unstable state of the liquid under vacuum and detect the liquid sample.[4]
4.TEM image of LED
5.TEM image of DRAM
References
[1] C.Wang, “Lenses and Apertures of A TEM,” EMA 6518 Course Present. [2] “Keywords "intermediate lens" | Keywords | Glossary of TEM
Terms | JEOL.” [Online]. Available:
https://www.jeol.co.jp/en/words/emterms/search_result.html?keyword=inte rmediate lens. [Accessed: 23-Dec-2018].
[3] J. P.Mccaffrey, “Calibrating the tem,” 1976.
[4] T.Xu andL.Sun, “Investigation on material behavior in liquid by in situ TEM,”
Superlattices Microstruct., vol. 99, pp. 24–34, 2016.
Figure
1.近代穿透式電子顯微鏡實務
2 http://highscope.ch.ntu.edu.tw/wordpress/wp-content/uploads/2009/09/12.jpg
3 https://photographylife.com/what-is-chromatic-aberration
4 TEM image of LED http://www.ma-tek.com/zh-tw/services/index/material/Pro_20
5 TEM image of DRAM https://semimd.com/chipworks/2014/02/07/intels-e-dram-shows-up-in-the-wild/
6 TEM image of NAND flash memory
https://www.google.com/search?q=TEM+NAND&rlz=1C1JZAP_zh-TWTW696TW696&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiFvrnijMPfAhWJ y7wKHfx7DtcQ_AUIDigB&biw=1366&bih=577#imgrc=ZND46tZNNvB9hM:
