本章節所要討論的特徵影像並非試片本身待分析的結構，但是卻經常出現各種TEM試片。不是每個TEM試片都會有這些特徵影像，但是經常有些TEM試片會出現一或二種下列提到的特徵影像。
What are going to discuss in this paragraph are some feature images which are not characteristic structures in the specimen but observed in many TEM specimens from time to time. It is not for all TEM specimen to have these feature images, however sometimes some TEM specimens have one or two of them
厚度條紋(thickness fringes)
此種特徵條紋如圖B-36所示，黑白相間的條紋和試片的邊緣平行，條紋的間距會因繞射狀態而改變。此類條紋在機械研磨的TEM試片的邊緣經常看到，目前半導體元件的TEM都是用FIB製備，試片厚度大致均勻，所以很少看到此類條紋，但是當TEM試片剛好切在某些特殊的位置時，仍可以看到厚度條紋。厚度條紋形成的原因類似牛頓環的原理，在一玻璃材質的契形試片邊緣，當試片厚度等於四分之一波長，四分之三波長…等位置，入射光波從試片上表面和下表面個別反射的波剛好反相，二者干涉後呈暗線；在試片厚度等於二分之一波長，一個波長…等位置，從試片上表面和下表面各別反射的波剛好同相，二者干涉後呈亮線。在晶體TEM試片中，除了入射電子波的波長外，繞射狀態也會影響條紋間距。
Typical thickness is shown in Fig. 36, alternative black and white fringes run parallel to the specimen edge, and their spacings vary with diffraction conditions. This kind of fringes is common in TEM specimens prepared by mechanical grinding and polishing. They are hardly to be observe in the field of semiconductor industry since all TEM samples are prepared by FIB and their thicknesses are nearly constant through the specimen. However, thickness fringes can still be visible when TEM specimens are cut from some special positions. The principle of forming thickness fringes is similar to that of Newton’s rings. When light incidents a wedge specimen made of glass, dark lines are observed at positions of specimen thickness equaling to 1/4 λ, 3/4 λ, .., etc., where the phases of the reflected waves from top surface and bottom surface are reverse. White lines are observed at positions of specimen thickness equaling to 2/4 λ, λ, .., etc., where the phases of the reflected waves from top surface and bottom surface are in phase. For crystalline TEM specimen, the spacing of thickness fringes is affected by both the wavelength and the diffraction condition.
圖B-36 TEM明場像。機械研磨的矽試片，試片邊緣呈現厚度條紋。(a) [0 0 1]正極軸；(b) [4 0 0] 雙束條件。[1]
彎曲條紋(bend contour)
為了拍攝清晰的高分辨影像，TEM試片厚度常會減薄至50奈米以下，當試片本身的物理結構無法支撐它本身的重量時，在試片薄區就會產生局部性的彎曲。彎曲的晶體將造成入射電子束和同一族(h k l)晶面的夾角連續改變，如圖B-37(a)示意圖上半部所示，因此在同一晶體內，繞射狀態卻一直在改變，造成對應影像的強度也一直在改變，如示意圖B-37(a)示意圖下半部所示。圖B-37(b)為一金屬試片的彎曲條紋，圖B-37(c)是圖B-37(b)中紅色框區域的放大圖，圖中沿著紅線的影像強度變化和圖B-37(a)所示的明場影像強度與繞射狀態的變化吻合。圖B-37(c)中黑色帶狀區域相當於圖B-37(a)中間偏離參數(deviation parameter) s小於零的地帶，該地帶內明場像和暗場像的強度都降至最低。
To obtain clear HRTEM images, the thickness of the TEM specimen is usually reduced to be less than 50 nm. If the structure is not able to support its weight itself, the specimen bends locally. The bending results in that the angle between the incident electron beam and the same family (h k l) crystal planes varies from place to place, as shown schematically in Fig. B-37(a). The diffraction condition then changes correspondingly, so does the image intensity. It is a typical bend contour in a metal specimen in Fig. B-37(b), and Fig. B-37(c) is the magnified image of the area in the red rectangle in Fig. B-37(b). The variation of the image intensity along the red line across the black band meet the variation of BF image intensity as well as diffraction condition shown in Fig. B-37(a). The diffraction condition of the black band is thus in s < 0 conditions which has minimum BF and DF intensity.
圖B-37彎曲條紋。(a)彎曲的晶體和對應繞射狀態與影像強度變化的示意圖[2]；(b)金屬晶體內的彎曲條紋；(c) (b)中紅框區域的放大影像，紅色線條畫過區域的影像強度變化對應(a)中的繞射狀態與影像強度曲線。
彎曲條紋常見於延性的金屬材料試片，尤其是大於數十微米薄區的金屬TEM試片中。彎曲條紋的形狀和繞射狀態有關。圖B-38(a)整組彎曲條紋的形狀類似該晶體的[0 1 1]菊池線圖案，數條彎曲條紋的交會點是面心立方晶金屬正[0 1 1]極軸的位置，每一條彎曲條紋的帶狀區域內都對應一組雙束繞射條件。當脆性晶體材料的TEM試片中有薄又寬的區域時，也會產生彎曲條紋，如圖B-38(b)的右下方的矽單晶基板內。此時要拍攝良好的矽基板-氧化層-多晶矽HRTEM影像，必須傾轉試片，使彎曲條紋的交會點中心移到試片薄區又恰好位於某個MOS結構的正下方。
Bend contours are frequently observed in ductile materials, such as metals, especially when a metal TEM specimen with thin enough areas more than several ten micrometers wide. The shape of bend contour is closely related to the local diffraction condition. In Fig, B-38(a), the appearance of this set of contours looks like the [0 1 1] Kikuchi patterns of fcc crystals, the center of intersection of all bend contours is where the exact [0 1 1] zone axis of the fcc metal crystal locates, and the diffraction condition in each bend contour is two beam condition. Bend contours show up in brittle crystalline materials too when the specimen is thin and wide, as shown in Si substrate in Fig. B-28(b). If we want to take good HRTEM images of Si sub\oxide\poly Si, we have to tilt the specimen to make the center of the bend contours locate at a MOS structure where is thin enough for HRTEM images.
圖B-38彎曲條紋。(a)金屬試片[0 1 1]；(b) 矽基板[0 1 1]。
參考文獻
1] 鮑忠興和劉思謙，近代電子顯微鏡實務，第二版，滄海書局，台中 (2012)。
2] Practical Electron Microscopy in Materials Science, edited by J. W. Edington, p.113, Van Nostrand Reinhold Company, (1976).
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