材料分析Part B-4 穿透式電子顯微鏡(TEM) -B-4-1 TEM簡介

將圖B-9中試片的厚度降至150奈米以下，再將入射電子的加速電壓增至100 KV以上，則絕大多數的入射電子會穿過試片。這些穿過試片的電子可以初步劃分為「未被未被散射電子」，「彈性散射電子」，「非彈性散射電子」三大類，如圖B-20所示。穿透式電子顯微鏡的訊號偵測器接收這些電子後，形成繞射圖案、影像、電子能量損失能譜等資料，再加上背向的X光訊號，組成一套可同時分析奈米微區的結構、組成、晶相等的材料分析技術。

If the specimen thickness in Fig. B-9 is reduced to thinner than 150 nm, and the acceleration voltage is increased up to 100 KV and higher, most of incident electrons will penetrate the specimen. Electrons through the specimen are divided into three groups: un-scattered electrons, elastically scattered electron, and inelastically scattered, as shown in Fig. B-20. All these signals are then acquired by detectors in TEM to be data of diffraction patterns, images, electron energy loss spectra as well as characteristic X-ray emitting from the other side. This is a materials analysis technology being able to resolve the microstructure/composition/crystallography of a volume of nano scale simultaneously. 

圖B-20. 高能電子射入薄片試片後產生各種電子訊號的示意圖。

圖B-21展示一傳統型式TEM/STEM的外形和基本結構示意圖，以及上述各種訊號形成的相對位置。電子束穿過薄片試片後，先在物鏡的後聚焦面處形成電子繞射圖案，然後在第一成像面形成倒立放大實像，電子能量損失能譜儀(EELS)安裝在傳統相機室的下方，接收多數穿透過試片的電子，並依能量損失量線性排列成電子能量損失能譜，能量散佈能譜儀(EDS)則安裝在試片斜上方，接收入射電子撞擊試片後產生的特性X光。

A traditional TEM/STEM and its schematic configuration are shown in Fig. B21. Locations of typical TEM/STEM data generated are pointed out too. After penetrating the foil specimen, high-energy electrons form a diffraction pattern at the objective back focal plane, a magnified image at the first image plane. An EELS is installed at the bottom of the TEM column to collect most electrons through the specimen. An EDS is equipped at diagonally above the specimen to collect characteristic X-ray emitting from the specimen.

圖B-21. 傳統TEM的外形、基本結構、典型訊息及其產生的位置。

以前TEM依其性能區分成三大類型：傳統TEM(CTEM)，高分辨TEM(HRTEM)，分析式TEM(AEM)。CTEM的最佳影像分辨率(解析度)略大於0.2 nm，其物鏡間隙較大，試片的傾轉角度可達45度，可以從數個特定晶向分析同一個晶體；HRTEM的最佳分辨率(解析度)小於0.2 nm，但其物鏡間隙較小，試片的傾轉角度約只有15度；AEM則是有STEM模式，電子束一般可達2 nm，使用場效電子鎗的STEM可達1 nm，加裝EDS或EELS或二者都有以便進行成份分析。三種TEM機型對應的主要操作模式如圖B-22所示。1995年後，因為日漸精密的機械加工與電腦輔助系統的引入，三類TEM之間的疇界逐漸被打破。現在的TEM都是使用場效電子鎗，影像最佳分辨率(解析度)達到0.16 nm，同時有STEM模式，而且STEM的影像解析度可達0.2 nm。二種模式的切換只是按鈕動作，加上一些微調即可。圖B-23顯示現代TEM機台擁有的功能，其中新型空錐影像最早於1992年出現在Zeiss 912 TEM，Zeiss退出TEM市場後，Philip的Tecnai系列繼承此成像功能，而後FEI-Philip到目前的Thermo-FEI的所有TEM/STEM都有此成像功能。另外上有些TEM使用特別設計的物鏡和試片承載台，可進行臨場(in-situ)實驗。而加上球面像差矯正器的TEM/STEM，影像分辨率和解析度都可優於0.1 nm，只是價格相對高許多。

TEMs were used to divided into three groups: conventional TEM (CTEM), high-resolution TEM (HRTEM), and analytic TEM (AEM). The gap of the pole piece of CTEMs is large enough to tilt the specimen up to 45 degree, so a special crystal can be analyzed from several low index zone axes. The image resolution of CTEMs is a little larger than 0.2 nm. HRTEMs had a smaller pole-piece gap, tilting angle is usually limited to be 15 degree, and has an image resolution approaching to 0.18 nm. AEMs were used for composition analysis by EDS or EELS or both in STEM mode. Its probe size was about 2 nm for a LaB6 emitter and 1 nm for FEG type. After 1995, most of TEMs are FEG type with image resolution better than 0.2 nm and coupled with STEM mode with probe size smaller than 0.2 nm due to the progress in precision machining and the aid of personal computer in operation system. Now, one TEM/STEM has functions shown in Fig. B-23. New type hollow cone showed up in Zeiss 912 in 1992, and Philips Tecnai series (then FEI-Philips and Thermo-FEI TEM/STEM) after Zeiss withdrawing from the TEM market. Some TEM/STEM can do in-situ experiment with special a designed objective chamber and specimen holders. The image resolution as well as electron probe size can be improved to beyond 0.1 nm when an objective and a C2 spherical correctors are equipped. However, the price goes up very much.

圖B-22. 傳統TEM機型的分類，與其主要的操作模式。

圖B-23. 現代TEM/STEM經常性操作模式。