C-2-3-2 定點分析/線掃描/成分映像
從成份分析的形式來說，EDS成份分析有三種：定點分析、線掃描、成分映像。定點分析通常先將特徵物移到螢光屏中心，然後將電子束精確地微移到待分析特徵物上，即可開始分析。定點分析可以在TEM模式或在STEM模式下進行。在STEM模式下進行只能用聚焦的電子束，在TEM模式下進行則可以用聚焦電子束，也可以用非聚焦電子束。如果待分析的特徵物夠大，個人建議使用TEM模式非聚焦電子束進行分析，一來取樣空間較大，二來減小輻射損傷和局部積碳效應。線掃描和成分映像則必須在STEM模式下進行，才能自動化逐點等距收集訊號。
From forms of analysis, EDS analysis can be divided into three types: position resolved, line scan, and mapping. For position resolved analysis, we usually move the feature to the screen center, accurately position the electron beam on the feature, and start to analyze it. Position resolved analysis can be performed either in TEM mode or in STEM mode. In STEM mode, only a focused electron beam is used. Either a focused electron beam or a non-focused electron beam can be used in TEM mode. It is suggested to use TEM mode and a non-focused electron if the feature is large enough. Advantages of this method include large sampling space, less electron beam radiation damage, and less local carbon contamination. Both line scan and mapping must be performed in STEM mode to keep positions and distances accurately and automatically.
由於現代TEM都兼備STEM功能，而且非常容易切換，因此演變成新TEM機台的EDS分析都在STEM模式用能譜影像(spectrum image)技術進行。能譜影像技術意指對一特徵區域掃描，在攝取一張STEM影像的同時，每一像素內包含一個對應的EDS能譜。能譜影像除了提供組成元素的成分映像圖外，也可從中拉出直線成份分佈圖(line profiles)，如同線掃描一般；也可以選定一微區，萃取出該微區的EDS能譜，如同定點分析一般。能譜影像成份分析上有許多便利的地方，使TEM/EDS成份分析變得容易許多，唯一的缺點是訊號強度比起線掃描和定點分析弱了許多。如圖C-20所示，對於四層薄膜材料的定點分析，如果每點收集20秒，得到的EDS能譜，其中最大能峰的尖峰強度達到8000，總收集時間為80秒。改用線掃描通過此四層薄膜，假設共設定100點，每點訊號收集時間為2秒，則每點的EDS能譜中，最大能峰的尖峰強度約為800，總收集時間為200秒。這樣的分析方法會花費較長的時間，而且每個EDS能譜的訊號強度降低，好處是可以看到所有元素通過界面的變化。改用成分映像法掃描一包含此四層薄膜的區域，假設共設定3000 (100 x 30)點，每點訊號收集時間只能為0.2秒，則每點的EDS能譜中，最大能峰的尖峰強度約為80，總收集時間為600秒。這樣的分析方法會花費更長的時間，每個EDS能譜的訊號強度更低，好處是可以看到所有元素的二維分佈，如圖C-21(c) ~ (e)所示。
Currently, all TEMs have STEM functions, and switch between two modes is only a finger job. All new TEM users are used to performing EDS analysis by using STEM spectrum image. Spectrum image means once an area including the interested feature is scanned, EDS spectra for all pixels as well as the STEM image are stored. Besides elemental maps, line profiles and EDS spectra of any specific area can all be extracted from a set of data file. STEM/EDS spectrum image makes the composition analysis job easier and more powerful. However, the intensity extracted one pixel or a line of pixel is somehow less than that obtained from position resolved or line scan analysis. As illustrated in Figure C-20, if we use position resolved method to analyze the composition of each thin film layer and set the dwell time for each point to be 20s. Four EDS spectra are obtained and the maximum peak intensity can reach 8000 counts. The total acquisition time is 80s. When line scan is used, 100 points in the line crossing four layers of thin films are set and the dwell time for each point is 2s. The total acquisition time becomes 200s, and the maximum peak intensity is around 800 counts. We can see the composition change cross these films with the scarify of time and intensity. When EDS mapping is used. 3000 points are set to scan an area covering four layers of thin films. The dwell time for each point can be only 0.2s, the total acquisition time is 600s. The maximum peak intensity is now 80 counts only. However, the distribution of elements in two dimensions can be observed, as shown in Figure C-21.
圖C-20 (a)定點分析，(b)線掃描，(c)成分映像 三種EDS分析法的示意圖。比較在訊號收集時間和能峰訊號強度方面的差異。
圖C-21 III-V半導體元件STEM/EDS分析。(a)STEM HAADF影像；(b)從能譜影像中萃取出來的EDS直線成份分佈圖(line profiles)，萃取位置是由(a)中紅色粗線箭頭所示，而加總平均的側向範圍則如紅色細線矩形所示；(c)從能譜影像中萃取出來的Al map；(d)從能譜影像中萃取出來的Ga map；(e)從能譜影像中萃取出來的As map。
如前段所述，能譜影像中單一像素對應的EDS能譜訊號強度太弱。改善的方法是將同相材料內的像素加總，例如:將圖-C20(c)第二層薄膜中10像素的EDS訊號加總，則最高能峰的尖峰高度就可達到800；若將該層薄膜中100像素的EDS訊號加總，則最高能峰的尖峰高度就可達到8000。同樣地，在萃取EDS直線成份分佈曲線時，也不限於只用單一行像素的訊號，側向增加10 ~ 50行的訊號做加總平均，才能得到平滑的直線成份分佈曲線。如圖C-21(b)所示，這組EDS直線成份分佈曲線的萃取位置是如圖C-21(a)中紅色粗線箭頭所示，而加總平均的側向像素數則如紅色細線矩形所示。
As stated in the last paragraph, the intensity of the EDS spectrum corresponding to each pixel in the spectrum image is very weak. Improvement can be reached by summing the intensity of several pixels, for examples, when EDS spectra of 10 pixels in the #2 layer thin film, the peak intensity of the maximum peak can reach 800 counts, and 8000 counts by summing 100 pixels. Similarly, the EDS line profiles of one line of pixels is very rough, then becomes smother and smother when the width of the line becomes wider and wider. Usually, lateral 10 to 50 pixels are integrated to be one value in the line. As shown in Figure C-21(b), this set of EDS line profiles are extracted from the position indicated by the red thick straight arrow and the width illustrated by the red thin rectangle.
早期的能譜影像只能一次掃描，此時每一點的停留時間(dwell time, τ)約為10 ~ 100毫秒，進階的能譜影像技術具有快速掃描的功能，每一點的停留時間可降至10 微秒，多重掃描後再將訊號加總。快速掃描可降低試片的電子輻射損傷，積碳速率，和電子束擴展等效應。
Old versions of spectrum image can scan the interested area one time only, the dwell time for each pixel is set to be about 10 to 100 ms. Gradually, some spectrum image techniques developed fast scan mode and reduced the dwell time for each pixel to be 10 us. For each pixel, multi-scan occurs and signals for all scanning are summed. Advantages of fast include less radiation damage, less carbon contamination, and less beam broadening effect.
無論是一次掃描還是快速掃描，一次能譜影像的資料蒐集都是10分鐘以上的工作，此時試片漂移問題就會很明顯，因此能譜影像技術都附有漂移修正的功能，可設定每隔一定的時間，快速掃描一下試片，和初始影像做比對，如果發現有試片漂移，會自動偏折電子束掃描區域，修正後續掃描區域和初始掃描區域吻合。
It usually takes more than ten minutes to acquire a set of spectrum image data, no matter fast scan or slow scan methods. The specimen drift becomes significant, and functions of drift correction is necessary for spectrum image techniques. The program fast scans the specimen and compares the scanned image with the initial scanned image in a period of time as set, a compensate current is sent to the deflection coils to make the following scanning as matching the initial area as possible if any specimen drift is detected.
現在的能譜影像軟體可賦予成份映像圖各種顏色，但是各種顏色的映像圖中的明暗度只代表該元素的濃度的高低，無法看出元素濃度的真正值，例如圖C-21(d) and (e)。而且成份映像圖中的明暗度是可以調整的，因此二張成份映像圖中的明暗度無法明確顯示此二種元素的濃度。要知道元素間濃度值必須從EDS直線成份分佈圖，圖C-21(b)，才能看出。
Current spectrum image software can assign various colors to elemental maps, but the brightness of these colorful maps indicates high or low concentration only, not concentration value, as shown in Figure C-21(d) and (e). The brightness can be tuned manually, so the difference in brightness of two maps is not corresponding to the difference in concentration. A set of line profiles, as shown in Figure C-21(b), are extracted to show variation in concentration through layers of thin films.
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