TEM DF影像
TEM暗場像中的影像對比機構只有一個：繞射對比，因為TEM暗場像通常只用單一的繞射電子束成像。這類影像目前常用的有三大類型。
The only image contrast mechanism in TEM dark-field (DF) images is diffraction contrast, since TEM DF images usually use only single diffraction beam to form images. There are three kinds of DF images frequently used today.
第一類型暗場像用於分析晶體缺陷，影像中暗色背景區域和亮色特徵物屬同一晶粒。圖B-29(b)顯示一典型例子，差排造成晶體局部晶格扭曲，使該區域的繞射條件和晶粒其他地方略微不同，形成明暗對比。在明場像中，差排以暗色呈現，而在中央暗場像中，差排以亮色呈現，而且更清晰。此類型TEM暗場像，必須先將待分析的晶體傾轉至某些特定的雙束條件(two beam conditions)。
The first type of dark field images is used to analyze crystal defects in crystals. Bright features and dark background are in the same crystal. Fig. B-29(b) shows one typical example. Lattices around dislocations are distorted locally and cause corresponding diffraction conditions to be different from other regions of the crystal. Dislocations are black in BF images and white in CDF images. CDF images always reveal more detail structure of dislocations than BF ones do. The selected crystal has to be tilted to some specified two beam conditions for this type of DF images.
圖B-29. 差排TEM影像。(a)明場像；(b)中央暗場像。
第二類型暗場像如圖B-30所示，用於分析奈米多晶材料，影像中暗的晶粒和亮的晶粒屬同一晶相材質，但是晶粒的方向不同，對應的繞射狀態不同。當晶粒尺寸比試片厚度小數倍時，由於重疊的問題，在明場像中，很難看清單一晶粒的輪廓。選用一小部分繞射點形成中央暗場像，雖然只能看到部分晶粒，但可以量測晶粒的大小。這一類型的暗場像如果採用空錐影像(hollow cone)技術攝取，則對於適當的多晶材料系統，可以一次同時將同組{h k l}，例如: {2 0 0}, {1 1 1}, {2 2 0}等，的所有繞射電子束拍攝在同一張暗場像中。
The second type of dark field images is used to analyze nano polycrystal materials. Bright and dark grains are same phase with different diffraction condition due to different orientation related to the incident beam. Crystals are overlapped and hard to be distinguishable in the BF images when their grain size is much smaller than the thickness of the specimen, as shown in Fig.B-30. Only parts of grains are in bright contrast when a few of diffracted beams are selected to form the CDF image, while their size can be measured easily. If hollow cone image is used for this kind of analysis, a special family of {h k l}, such as {2 0 0}, {1 1 1}, {2 2 0}, …etc., diffraction beams can be imaged simultaneously in single DF image for some polycrystal materials.
圖B-30. 奈米多晶材料的TEM影像。左上角是明場像；左下角是選區繞射圖案，圖中黃色圓圈代表物鏡光圈；其他四張為中央暗場像。
第三類型暗場像如圖B-31(c)所示，用於相鑑定，影像中亮色特徵物和暗色區域為不同相的晶體。圖B-31(b)明場像用圖B-31(a)的選區繞射圖案中大黃色圓圈內的電子束成像，包括透射電子束、316不鏽鋼的繞射電子束、M23C6析出物的繞射電子束。而圖B-31(c)中央暗場像只用單一M23C6析出物的繞射電子束成像。
The third type of dark field images is used for phase identification. The bright features and dark background are not same phase. Fig.B-31(b) is a BF image using a large objective aperture indicated in Fig.B-31(a) to include the transmitted beam and diffracted beams from the matrix of 316 stainless steel and M23C6 precipitates to form the image. Fig.B-31(c) is a CDF image using a small objective aperture indicated in Fig.B-31(a) to include one diffracted beam of M23C6 precipitates to form the image.
圖B-31. 316不鏽鋼基材與M23C6析出物的TEM影像。(a)選區繞射圖案；(b)明場像(使用如大黃色圓圈所示的物鏡光圈)；(c)中央暗場像(使用如小黃色圓圈所示的物鏡光圈)。
STEM ADF和HAADF影像
如前一章節圖B-27所示，在STEM影像模式時，當相機長度減小後，環狀影像偵測器排除透射電子束的訊號，所形成的影像為STEM暗場像。當環狀影像偵測器的內收集角在0.3 ~ 1.0度之間時，STEM暗場像的對比機構以繞射對比為主，在1.0 ~ 2.0度之間時，原子序對比的比例逐漸提高，這些STEM暗場像稱為環狀暗場(ADF)像。當環狀影像偵測器的內收集角大於2.0度後，繞射電子束的訊號非常微弱，收集到的訊號幾乎都是被彈性散到高角度的入射電子，這些STEM暗場像稱為高角度環狀暗場(HAADF)像，此時影像對比機構為原子序對比。這些STEM暗場像特徵的變化如圖B-32所示。
As shown in Fig.B-27 in the last paragraph, the annual detector will exclude the transmitted beam when small cameral lengths are used. These STEM images are then dark field type. Diffraction contrast dominates in these STEM DF images when the corresponding inner collection angles fall in the range of 0.3 ~ 1.0 degrees, but weight of atomic number contrast increases gradually when the inner collection angle is larger than 1.0 degree and smaller than 2.0 degree. We call STEM DF images in this range to be ADF images. When the inner collection angle is larger than 2.0 degree, the contribution of the diffraction beams can be neglected, electrons scattered elastically to high angles are signals to form these images, and we call these images HAADF images. The image contrast mechanism in HAADF images is atomic number contrast only. Characteristics of these images are shown in Fig.B-32.
圖B-32. 半導體元件的STEM影像。(a)對應的TEM明場像；(b)STEM明場像；(c)STEM ADF像；(d)STEM HAADF像。
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