材料分析B-4-5 TEM明場像的化妝師-物鏡光圈(Objective aperture, the make up artist of TEM BF images)

TEM明場像的化妝師-物鏡光圈(Objective aperture, the make up artist of TEM BF images)

 TEM是固態微奈米材料分析的終極武器之一。TEM影像的分辨率很高，明場影像約0.4奈米，無球差的高分辨影像達0.18奈米，球差修正的高分辨影像達0.05奈米，可以解析目前半導體元件中的各層奈米薄膜結構。由於台灣本身沒有生產TEM，所以學界或工業界，對於操作上如此高精密度、高複雜性的材料分析儀器都謹慎管理，對初階(甚至中階)的使用者有許多的限制，不能調動C1光圈，不能調動C2光圈，….等等，但是一定要學會正確操作物鏡光圈。在TEM模式下，物鏡光圈是調整影像對比的樞紐。要精確地量出一、二奈米的薄膜厚度，除了TEM本身的高分辨率(或解析能力)外，各層奈米薄膜之間也要有足夠的對比。

TEM is one of the ultimate instruments for solid state micro-nano materials analysis. The resolution power of TEM image is high, BF images about 0.4 nm, HRTEM images without Cs corrector about 0.18 nm, and HRTEM images with Cs corrector 0.05 nm, enough to resolve nano thin film structures in semiconductor devices. Since there is no TEM manufacturer in Taiwan, all Taiwan TEM laboratories, academic and industry, are very carefully to manage this high precision and high complexity MA instrument, put many limit rules on junior (some even middle level) TEM users, such as do not touch the C1 aperture, do not change the C2 aperture, … etc. But, every TEM user must know how to operate the objective aperture correctly. The objective aperture is the hinge to adjust the image contrast in TEM mode. Obviously, besides the resolution power of the instrument, sufficient image contrast is another key to measure thin film thickness of 1 to 2 nano meters accurately.

TEM明場影像對比機構主要有二種：原子序對比和繞射對比。這二種影像對比機構源自入射的高能電子和試片之間的散射與繞射作用。當一束高能電子撞擊到一群原子時，入射的高能電子會被原子核散射，其分布的機率如圖B4-5-1(a)示意圖所描述。通過試片輕元素材料區域的高能電子，被散射的狀態如綠色曲線所示，集中在以光軸為中心小角度範圍內。通過試片重元素材料區域的高能電子，被散射到高角度的比例增加，如紫色曲線所示。置入物鏡光圈後，被散射到高角度的入射電子被物鏡光圈擋住，無法繼續前進成像。從圖B4-5-1(b)和(c)看出，置入物鏡光圈後，穿過輕元素材料區域和穿過重元素材料區域的高能電子被擋住的比例不同，通過輕元素材料區域的高能電子被擋的較少，成像的劑量較多，在黑白影像中呈亮區；反之亦然，重元素材料區域呈暗區。因此，置入物鏡光圈後，原子序對比提升，物鏡光圈愈小，原子序對比愈強烈。

There are two main image contrast mechanisms in TEM bright-field (BF) images: atomic contrast (or called z contrast) and diffraction contrast. These two mechanisms are caused by scattering and diffraction between incident high energy electrons and the specimen. When an electron beam strikes a bunch of atoms, the distribution of elastically scattered incident electrons is shown schematically in Figure B4-5-1(a). The green curve describes the distribution of elastically scattered high energy electrons passing through regions of light elements, centralizes around the optical axis in a small angle range. And the purple curve describes the distribution of elastically scattered high energy electrons passing through regions of heavy elements. Electrons scattered to high angles will be blocked off to contributed to the final image when an objective aperture is inserted. As shown in Figure B4-5-1(b) and (c), the number of electrons blocked by the objective aperture is different for regions consisted atoms of different atomic number. Electrons passing through regions consisted of light element will be less blocked, the corresponding pixels have high dose and show bright contrast, and vice versa, pixels corresponding to regions of heavy elements show dark contrast. The smaller objective aperture gives more atomic number contrast in TEM BF images. 

圖B4-5-1 入射高能電子被原子核散射分佈示意圖，橫軸為徑向角度，縱軸為強度。(a)輕元素和重元素散射的差異，曲線下的面積相等；(b)置入中尺寸的物鏡光圈，部分通過重元素區域的散射電子被擋掉；(c)置入小尺寸的物鏡光圈，部分通過輕元素區域的散射電子被擋住，大部分通過重元素區域的散射電子被擋掉。

當晶體試片的某個極軸和透射電子束平行時，此時該晶體在強烈繞射條件。圖B4-5-1(a)中的入射高能電子分佈狀態變成如圖B4-5-2(a)所示，電子分佈變成局部集中的狀態。類似前述的情形，圖B4-5-2(b)和B4-5-2(c)顯示，置入物鏡光圈後，部分繞射電子束的電子被擋住，無法繼續前進成像，此晶體最後成像的電子劑量因此相對低，在TEM明場影像中呈暗色。

When a zone axis (h₁ k₁ l₁) of a crystal is tilted to be parallel to the incident electron beam, the crystal is in a strong diffraction condition. The distribution of electrons passing through this crystal will change from Figure B4-5-1(a) to B4-5-2(a), electrons locate locally at some points. As described in last paragraph, parts of electrons are blocked away from the final image by the inserted objective aperture, as shown in Figure B4-5-2(b) and Figure B4-5-2(c). Thus, this crystal shows dark contrast in TEM BF images due to low electron dose.

圖B4-5-2 入射高能電子和晶體產生繞射後電子分佈示意圖，橫軸為徑向角度，縱軸為強度。(a)電子集中在幾個局部的區域；(b)置入中尺寸的物鏡光圈，小部分繞射電子被擋掉；(c)置入小尺寸的物鏡光圈，繞射電子都被擋掉。

除了調整影像的對比外，物鏡光圈也可以降低球面像差效應造成的疊影。圖B4-5-3顯示一組TEM明場像，分別為(a)沒有物鏡光圈，(b)100微米的物鏡光圈，(b)30微米的物鏡光圈。很明顯的，使用30微米以下的物鏡光圈，可幾乎完全消除球面像差效應造成的疊影，避免TEM明場像內含有可能造成誤解的訊息。

Besides adjusting the image contrast, the objective aperture can minimize the shadow caused by the spherical aberration. As shown in Figure B4-5-3, (a)TEM BF image without objective aperture, (b)TEM BF image with an objective aperture of 100 um, (c)TEM BF image with an objective aperture of 30 um. Obviously, an objective aperture of 30 um is enough to eliminate any visible shadow caused by the spherical aberration.

圖B4-5-3 TEM明場像。(a)無物鏡光圈，試片內有多餘的亮影(紅色箭頭)，試片邊緣有多餘的輪廓亮影(藍色箭頭)；(b)100 um的物鏡光圈，試片內仍有多餘的亮影(紅色箭頭)，試片邊緣仍有多餘的輪廓亮影(藍色箭頭)；(c)30 um的物鏡光圈，無多餘的亮影。

EDS與EELS的競爭與比較

 C-3-7 EDS與EELS的競爭與比較

由於幾何位置的關係，傳統的TEM/EDS系統中，從試片發出的特性X-光，其中只有4%被EDS偵測器接收。也就是說EDS的偵測效率很差；另外EDS能量解析度約為130電子伏特(eV)。相對地，EELS的偵測效率可以高達90%以上，而且在使用場效電子鎗穿透式電鏡的條件下，能量解析度優於1.0電子伏特(eV)。因此在1980年代中期，EELS由SEELS演進到PEELS後，許多TEM使用者預測EDS在TEM應用領域將會逐漸被EELS取代[1]。三十幾年過去，EDS不但沒有被EELS取代，反而隨著超薄窗型和無窗型的偵測器問世，逐漸蠶食EELS分析碳、氮、氧的領域。當然，EELS能譜儀最大製作公司- Gatan也不會坐視TEM成份分析的生意大餅被吞食，近幾年開發出新型的EELS能譜儀，大幅提升在重元素的偵測能力，也就是能量損失大於1000電子伏特能區的收集能力。

Due to the geometric relationship of signals and the detector, only about 4% of characteristic X-rays emitted from the specimen can enter the traditional EDS detector. It means that the collection efficiency of the EDS detector is quite low. Besides, the energy resolution of EDS is about 130 eV. On the contrary, the collection efficiency of EELS can be over 90%, and its energy resolution can be better than 1.0 eV when a FEG TEM is used. Thus, in the mid 1980s, many TEM users predicted that EDS was going to be phased out of the TEM applications [1]. However, more than 30 years passed, EDS is not replaced by EELS in the TEM market, but blooms more and more. Currently, most of TEM users have used STEM/EDS of ultra-thin-window type detectors or windowless type detectors to map C, N, O contained phases widely. Gatan, the main manufacture of EELS spectrometers, of course, is not able to stand for the loss in the market of TEM composition analysis. The developed new generation EELS spectrometers which are more efficiently in collecting electrons suffering large energy loss, more than 1000 eV, by striking atoms of heavy elements.

圖C3-15比較三組同一個MOS結構的氮和氧的成份映像圖。(a)使用單一超薄窗EDS偵測器STEM/EDS系統，訊號收集時間90分鐘；(b)使用四無窗EDS偵測器STEM/EDS系統，訊號收集時間30分鐘；(c)使用TEM/GIF系統，攝像時間約3分鐘。明顯地，從訊號收集時間和訊號強度來說，EELS對輕元素的成份映像能力還是遠優於EDS。但是EDS的價格優勢遠大於EELS，加上EELS的操作程序遠比EDS複雜。因此，目前台灣裝設在TEM上的EDS的數目遠大於EELS。

Three sets of N and O elemental maps are shown in Figure C3-15, (a)an STEM/EDS system with mono ultra-thin-window EDS detector, collection time is 90 minutes, (b)an STEM/EDS system with four windowless EDS detectors, collection time is 30 minutes, (c) a TEM/GIF system, total imaging time is about 3 minutes. Obviously, EELS is better than EDS when collection time and the intensity of signal are considered only. However, the price of EDS is much lower than that of EELS, and the operation of EELS is much complicate compared with EDS. Therefore, the number of TEM equipped with EDS overwhelms that of TEM equipped with EELS. 

圖C3-15 MOS的EDS和EELS氮和氧成分映像圖。(a)單一超薄窗EDS偵測器 STEM/EDS系統；(b) 四無窗EDS偵測器 STEM/EDS系統；(c)TEM/GIF系統。訊號收集時間分別為90分鐘，60分鐘，3分鐘。

參考文獻

1] E. Van Cappellen, “Energy Dispersive X-ray Microanalysis in Scanning and Conventional Transmission Electron Microscopy”, in the book “X-ray Spectrometry: Recent Technological Advances”, edited by Kouichi Ysuji, Jasna Injuk, and René Van Grieken, published by John Wiley & Sons Ltd. (2004).

C-2 X-光能量散佈能譜- STEM/EDS分析上的假訊 (Artifacts in STEM/EDS Analysis)

 C-2-8  STEM/EDS分析上的假訊 (Artifacts in STEM/EDS Analysis)

由於STEM/EDS硬體的性能與軟體的功能大幅提升，近幾年來，在半導體元件的TEM分析中，EDS分析已成不可或缺的資料。目前所有的EDS分佈都在STEM模式下，用能譜影像(spectrum image)技術分析，先做出一組成份映像圖(elemental maps)，如圖C2-34所示。成份映像圖顯示元素的二維分佈狀況，每一元素映像圖內的訊號明暗度，可以顯示元素濃度的相對高低，但無法告知絕對的濃度值，而不同元素映像圖內的明暗度不能代表濃度的相對高低。元素絕對的濃度值必須選取局部區域運算，或從直線成份分佈圖，才能讀出線上各點的濃度值。

Due to significant improvements in the performance of hardware and functions of software, EDS analysis has become a necessary and routine item in TEM analysis of semiconductor devices recently. An EDS analysis is performed by spectrum image technique in STEM mode. A set of elemental maps are then extracted from this spectrum image, as shown in Figure C2-34. EDS elemental maps tell the distribution of elements in two-dimension, and relative concentration by brightness in the same elemental map, but no values of concentration, and the difference in brightness in different elemental map does not means high or low in concentration. Values of concentration of elements are available by extracting EDS spectra from local regions, or from EDS line profiles from this spectrum image. 

  

圖C2-34 STEM HAADF image and elemental maps of nano particles extracted from a spectrum image 

製程工程師往往希望拿到的EDS成份直線分佈圖，圖中元素曲線非常平滑好看。但是由於EDS訊號收集效率和能譜影像像素數目的因素，能譜影像中每一像素內的EDS訊號強度不高，從奈米層次的相拉出的EDS原子百分比成份直線分佈圖中的元素曲線通常是上下震盪的。為了滿足客戶的要求，TEM分析實驗室常對原始數據做一些後續的平滑處理，如下面各圖所示。圖(a) 是EDS原始強度line profiles，在標示 I 的區域內，各元素的訊號強度為背景值，物理上是沒有試片的區域。但是將其換算成圖(b)的EDS at%成份直線分佈圖後，雖然氮和氧的曲線震盪幅度很大，代表氮和氧的濃度變化很大，因此圖(b)圖面上的意義顯示區域I 為一含氮和氧的區域。雖然數學上，7/10和700/1000值相等，但是物理上(或統計學上或統計學上)，二者的意義完全不同。將類似區域I的訊號放入EDS at% line profiles或EDS wt% line profiles運算中，除了造成可能誤導的訊息外，物理上完全沒有意義的是。將圖(b)平滑處理成圖(c)或圖(d)之後，這些曲線平滑處理造成的假象，更容易誤導許多不知整個分析歷史的看圖者，導致工程師製程調整方向錯誤。

Process engineers always like elemental curves in EDS line profiles are smooth. However, due to the limit of EDS collection efficiency and large pixel number in spectrum images, the signal intensity is low in each pixel in spectrum images. EDS at% line profiles of phases in nano scale are usually fuzzy. To meet requirements of customers, many TEM service laboratories do some off-line data process to make EDS line profiles smooth. Some artifacts in these processed EDS line profiles may mislead engineers in process tuning, as plots shown below. Region I in EDS intensity line profiles, Figure (a), is a region with no physical specimen. But, when the y axis is transferred to at% as shown in Figure (b), it looks region I is a region consisting of N and O with large variation. Both 7/10 and 700/1000 are identical mathematically, but significantly different in physics (or statistically). It is meaningless to put region I into EDS at% line profiles or EDS wt% line profiles calculation physically, except to result in misleading information. Those fuzzy elemental curves, especially N and O in region I, in Figure (b) can be improved by data smoothing process. They are changed to Figure (c) and Figure (d) by 3-points and 5-points smoothing respectively. Those elemental curves look better compared with them in Figure (b). This will mislead people who do not know the history of these data in detail, and even guide process engineers to tune processes in wrong directions.

圖C2-24 EDS line profiles. (a) EDS intensity line profiles, (b) EDS at% line profiles, (c) EDS at% line profiles, 3-points smoothing, (c) EDS at% line profiles, 5-points smoothing.

除了引起前述的錯誤外，數據平滑處理對於只有幾奈米厚度薄膜的濃度也頗有影響。從圖(b)量得的鉭/氮化鉭薄膜約為9.2奈米，最高濃度約為71.4 at%；經三點平滑處理後，鉭訊號曲線從非對稱變成高斯對稱，半高寬為8.3奈米，最高濃度降為58.4 at%；經五點平滑處理後，鉭訊號曲線也是成高斯對稱，半高寬為10奈米，最高濃度降為48.6 at%。而IV區的銅的最高濃度始終保持在77 at%左右，平均濃度也保持在70 ~ 75 at% 之間。顯見平滑處理對於大尺寸的物體，元素曲線確實可以變得較好看，真實濃度也變化不大。但是對奈米尺寸的物體，其組成的濃度和濃度分佈都會明顯失真。

Besides errors induced as stated before, smoothing process will affect the concentration of thin films of several nano meters in thickness. The thickness of Ta/TaN is about 9.2 nm, maximum Ta concentration is 71.4 at% in Figure (b). The distribution of Ta becomes Gaussian symmetry after 3-points smoothing, the full width of half maximum is about 8.3 nm, and its maximum concentration drops to 58.4 at%. The maximum Ta concentration becomes to 48.6 at% and the FWHM is about 10 nm if a 5-points smoothing is processed. Obviously, smoothing EDS line profiles is good for objects of large scale but may cause misleading errors in objects of several nano meters. 

C-3 電子能量損失能譜(EELS) – 同素異形體與試片厚度效應

C-3-6 成份映像 – 同素異形體與試片厚度效應

C-3-4節中，圖C-41顯示EELS能譜可以區分矽的矽元素和二氧化矽的矽元素，二者L邊刃的起始能量和近邊刃微細結構明顯不同。在半導體元件的顯微結構中有多處純矽與二氧化矽相鄰的結構，EELS的成份映像技術是否可以區分它們? 圖C-46顯示純元素矽和二氧化矽的矽的分佈在EELS成份映像是可以分離的，只要三個攝像的能窗位置和寬度設置適當。

Figure C-41 in paragraph C-3-4 shows that the Si L_2,3 edges of element Si and the oxidated Si can be clearly distinguished in EELS spectra. They are different in both threshold energy and near edge fine structure. There are many sites where Si and SiO₂ are next to each other in semiconductor devices. Can they be distinguished by EELS mapping? Figure C-46 states that the distribution of Si/Si and Si/SiO₂ can be mapped separately by adequately setting those three energy windows.

圖C-46 矽的EELS成分映像圖。(a)明場像；(b)氧元素成份映像圖；(c)單晶矽和多晶矽的矽元素成份映像圖；(d)二氧化矽的矽元素成份映像圖。Ref[1]

做EELS分析的TEM試片相對上要偏薄，盡量避免入射電子產生多重散射，造成近邊刃微細結構失真。多重散射也會影響EELS成分映像圖的品質，如圖C-47所示。圖C-47(b)明場像中的鎢栓中間的縫清晰可見，而圖C-47(a)明場中的鎢栓中間的縫則只隱約可見，經驗上得知，對應圖C-47(a)的試片比對應圖C-47(b)的試片厚。因此在鈦元素成份映像圖中，薄試片(圖C-47(d))中的TiN層和Ti層的對比清晰許多。

The thickness of TEM specimen for EELS analysis needs to be thin enough to avoid multi scattering for incident electrons. Multi scattering will smear out near edge fine structure of characteristic edges of elements. Quality of EELS elemental maps will be decreased too when multi scattering occurs, as shown in Figure C-47. The seam in Figure C-47(a) is not as clear as that in Figure C-47(b). This indicates that the specimen thickness of Figure C-47(b) is thinner than that of Figure C-47(a) by experience. The contrast between the Ti layer and the TiN layer is higher in the elemental map of the thin specimen (Figure C-47(d)) than those in the thick specimen (Figure C-47(c)).

圖C-47 EELS成分映像圖的試片厚度效應。(a)厚試片的明場像；(b)薄試片的明場像；(c)厚試片的鈦元素成份映像圖；(d)薄試片的鈦元素成份映像圖。

參考文獻

1] J. S. Bow, W. T. Chang, Y. M. Tsou, H. S. Chou, and C. Chiou, Proc. ISTFA, 101-105 (2002).

 

C-3 電子能量損失能譜(EELS) - 成份映像

 C-3-5 成份映像

成份映像圖顯示某特定區域內組成元素的二維分佈情形。在TEM/STEM的分析技術中，主要獲取成份映像圖的技術有EDS成份映像圖和EELS成份映像圖二種。EDS能譜中，元素能峰和背景的強度相差甚多，亦即P/B很高，所以在EDS元素映像圖中，只要設定包含能峰的適當能窗，即使不扣除背景，就能清楚地顯示某特定元素的分布。但是在EELS能譜中，元素特性邊刃座落在一高強度的背景上，如果只設定單一能窗，則獲取的成份映像圖中可能有超過一半的訊號是背景訊號，而不是真正的元素訊號，如圖C-44的邊刃後能窗(能窗III)所蘊含的訊號。所以獲取EELS成份映像圖，除了設定包含特性邊刃的邊刃後能窗外，必須同時設定二個邊刃前的能窗，如圖C-44中的能窗I和能窗II。由二個邊刃前能窗的訊號，推算出邊刃後能窗的訊號中的背景訊號(能窗B)，扣除後，才是真正的元素訊號(能窗IV)。

Elemental maps display two dimensional distributions of elements in local interested regions. EDS mapping and EELS mapping are two main analysis techniques in TEM/STEM systems. Peaks of elements, especially major elements, in EDS spectra have high P/B ratio, elemental maps show the distributions of elements clearly once proper energy windows are set, with or without background subtraction. On the contrary, all characteristic edges fall on high background in EELS spectra, the map obtained from an energy window including part of characteristic edge will include background noise as well as true element signal, as shown in the post edge window (window III) in Figure C-44. It is necessary to set two more pre-edge windows to calculate out the background (window B) in the post edge window to obtain the true signal (window IV). 

圖C-44 EELS能譜示意圖顯示運算成份映像圖需要設定三能窗，二個邊刃前能窗: 能窗I和能窗II，和一個邊刃後能窗(能窗III)。能窗III包含背景訊號(能窗B)和真正元素訊號(能窗IV)。

圖C-45顯示一組典型TEM模式下拍攝的EELS三能窗影像和演算後的成份映像圖。圖C-45(a)為TEM明場像顯示一分析區域，此區域的組成元素包含Ti, Ni, Zr, Sb等四元素。圖C-45(b)相當於圖44中能窗IV的影像，顯示富鈦相呈一近橫躺的T字形。圖C-45(e)的邊刃後能窗影像相當於圖44中能窗III的影像，和圖C-45(c)和圖C-45(d)相比，雖然隱約顯示富鈦相的區域比周圍他相稍亮，但影像的亮度變化，會被誤認為鈦的分佈幾乎涵蓋整個區域，而且都有相當的濃度。透過二個邊刃前能窗影像算出背景影像(能窗B)，再從能窗III影像中扣除後，才能得到圖C45(b)的鈦成份映像圖。

A typical set of energy-selected images acquired in TEM mode and the final processed elemental map are shown in Figure C-45. Figure 45(a) is a TEM BF image showing an interested region being consisted of Ti, Ni, Zr, and Sb. Figure C-45(b) is the elemental map corresponding the image of energy IV in Figure 44 and indicates that the shape of the Ti-rich phase look like a lying down T. Figure C-45(e) is the Ti post-edge image corresponding to the image of window III in Figure C-44. The region of Ti-rich phase in this image is a little brighter compared with corresponding regions in Figure C-45(c) and Figure C-45(d), but is not distinguishable. A true Ti map, as shown in Figure C-45, is only obtained after background subtracted from the post-edge image.

圖C-45 EELS成分映像圖的演算。(a)明場像；(b)鈦元素成份映像圖；(c) pre-edge 1影像；(d) pre-edge 2影像；(e) post-edge影像。

C-3 電子能量損失能譜(EELS)-元素鍵結化態

 C-3-4 元素鍵結化態

原子的鍵結能和在鍵結方向的電子分布密度會因周圍原子的不同而改變。元素鍵結能的改變大概在0 ~ 7 eV的範圍內，常見的固態材料分析技術中，能解析鍵結能位移的有歐傑(AES)、電子能量損失譜(EELS)、X光吸收光譜(XAS)和X射線光電子能譜(XPS)四種。其中AES、EELS、XAS三種分析技術能同時解析電子分布密度的改變。其中，EELS和XAS的能量解析度可小於1.0 eV，而EELS的特點在於空間解析度高，可以解析小於1奈米的微區。

Both chemical bonding energy and the density of states of electrons along the bond of the atoms change with the surrounding atoms. The change in bonding energy of atoms falls in the range of 0 ~ 7 eV. There are four typical material analysis techniques for solid state materials can resolve the shift in bonding energy, they are Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), and X-ray photon spectroscopy (XPS). AES, EELS, and XAS also can resolve the change in the density of states of electrons. The energy resolution of both EELS and XAS is better than 1.0 eV. Besides high energy resolution, the spatial resolution of EELS can be smaller than 1.0 nm.

圖C-41顯示單晶矽、碳化矽、二氧化矽三者物質中的矽的扣除背景後的EELS特性邊刃。圖C-41(b)是圖C-41(a)的低能量區域的局部放大圖，清楚顯示三種矽L特性邊刃的起始能量的不同，元素態的矽是共價鍵，鍵結能為99 eV；碳化矽中的矽和碳接近共價鍵，矽鍵結能增強為101 eV；二氧化矽中的矽為離子鍵，其鍵結能增強為103 eV。二氧化矽中矽的近邊刃微細結構明顯和其他二者不同，顯示Si-O鍵明顯和Si-Si與Si-C鍵不同。圖C-42顯示金屬鋁、氮化鋁、三氧化二鋁，和藍寶石的鋁的扣除背景後的EELS特性邊刃，有著類似圖C-41中的變化。所以只要有足夠的資料庫，從EELS扣除背景後的元素特性邊刃，即可判斷該元素的鍵結化態(chemical bonding state)。目前最常用的EELS 特性邊刃的資料庫是Gatan 建立的EELS Atalas^[1]。

Figure C-41 shows three kinds of background subtracted Si L edges, Si of Si, Si of SiC, and Si of SiO₂. Figure C-41(b), magnification of the low energy region of Figure C-41(a), shows the different threshold energy of Si L edges, 99 eV for Si/Si, 101 eV for Si/SiC, and 103 eV for Si/SiO₂. The near edge fine structure of Si of SiO₂ is obviously different from the other two, indicating that Si-O bonds is significantly different from Si-Si and Si-C. Figure C-42 shows four kinds of background subtracted Al L edges, Al of Al, Al of Al₂O₃, Al of AlN, and Al of sapphire. All these Al L edges show similar variation with those Si L edges in Figure C-41. The chemical bonding state of any element can be identified from its background subtracted characteristic edge once data base of all elements is established. The EELS Atlas^[1] edited by Gatan is most popular at present.

圖C-41 正常化後的三種Si L特性邊刃。紅線是元素Si的Si，藍線是元素6H-SiC的Si，綠線是元素SiO₂的Si，

圖C-42 正常化後的三種Si L特性邊刃。紅線是元素Si的Si，藍線是元素6H-SiC的Si，綠線是元素SiO₂的Si，

TEM電子源能量解析度愈高，EELS的能量解析度愈高，近邊刃微細結構也會愈清楚，對應的材料電子物理特性也被解析地愈透徹。圖C-43顯示鈷用不同能量解析度的電子源解析出來的L特性邊刃微細結構。

The energy resolution of EELS increases with the energy resolution of the TEM electron beam. The near edge fine structure resolved by TEM/EELS system with better energy resolution will tell electronic properties more detail and exact. Figure C-43 shows the near edge fine structure of Co L₂₃ edge from TEM/EELS system with different energy resolution.

圖C-43 CoO內Co L₂₃的近邊刃微細結構和TEM能量分辨率的關係。(a)能量分辨率 ~ 0.8 eV；(b)能量分辨率 ~ 0.5 eV；(c)能量分辨率 ~ 0.2 eV。ref. [2] (Courtesy of FEI Dr. Bert Freitag and Dr. Peter Tiemeijer)

參考文獻

1] EELS Atlas, edited by C. C. Ahn, O. L. Krivanek. Gatan, 1983.

2] 鮑忠興和劉思謙，近代穿透式電子顯微鏡實務，第18頁，第二版，台中 (2012).

C-3 電子能量損失能譜(EELS) - EELS能譜背景扣除

 C-3-3 EELS能譜背景扣除[2021/06/08更新]

C-3-1節中述及EELS在儀器操作和後續資料處理的複雜度都遠大於EEDS，C-3-2節已闡明EELS儀器操作上的複雜性，本章節將簡介如何後續處理EELS能譜。

In section C-3-1, we mentioned that the complexity of EELS in both operation and data process is much more than that of EDS. The complexity of operation has been discussed in section C-3-2, and this section will introduce how to process acquired EELS spectra. 

在材料科學與工程領域的成份分析應用，主要使用EELS的核損失區域。圖C3-3-1為一典型的核損失區域EELS能譜，此能譜內包含一小段邊刃前背景，邊刃起始點(threshold)，近邊刃微細結構區(NEFS or ELNES)，邊刃延伸結構區(EXELFS)等，各有其物理意義和用途。邊刃前背景主要用於做曲線契合，找出特性邊刃下的背景訊號。邊刃起始點代表此元素的鍵結能，用來判別特性邊刃對應的元素。近邊刃微細結構區內能量強度的變化特性，代表此被分析元素的化學鍵結型態，元素的每一種化態都有其如指紋般唯一對應的微細結構。邊刃延伸結構區內能量強度的變化特性受元素化態影響較小，用來做定量分析。

Core loss regions in EELS spectra are mainly used for applications of composition analysis for the field of materials of science and engineer. Some features, including pre-edge background, threshold, near-edge-fine-structure (NESF) or energy-loss near-edge structure (ELNES), and extended energy-loss fine structure (EXELFS), in a typical EELS spectrum, as shown in Figure C3-3-1, have their own uses and physical meanings. The pre-edge background is used for fitting the background under the characteristic edge. The element can be identified from the threshold energy which stands for the bonding energy of the element. The variation of intensity in the energy range of 50 eV behind the threshold energy is called near edge fine structure which indicates the chemical bonding state of the element analyzed and is finger-print unique. The intensity variation of EXELFS is little affected by neighbor atoms and used for quantitative analysis.

圖C3-3-1 典型核損失區域的EELS能譜。包含一小段邊刃前背景，邊刃起始點(threshold)，近邊刃微細結構區(NEFS)，邊刃延伸結構區(EXELFS)。

EELS能譜中，元素的特性邊刃座落在一高強度的背景訊號上，唯有將背景訊號扣除後，才能看到元素特性邊刃的真正形貌，尤其是近邊刃微細結構。從累積的EELS能譜分析結果中，發現背景訊號的變化近似一指數函數，y = ax^b。由於EELS能譜的橫軸是能量損失，而且訊號強度隨能量損失的增加而降低，所以背景訊號強度可以下面的式子近似:

I = A E^-r   ------------------------- (C 3-3-1)

Elemental characteristic edges mount on a high intensity background in EELS spectra. The true shape of an elemental characteristic edge, especially the ELNES is only visible after its corresponding background is removed. The intensity variation of background was found to approximate an exponential function, y = ax^b. The x-axis of EELS spectra is energy loss, and the intensity drops with increasing energy loss, so the background intensity can be approximated by the equation below:

I = A E^-r   ------------------------- (C 3-3-1)

C3-3-1式二邊取對數後，變成一直線方程式 y = a + bx的形式

ln(I) = ln(A) – r ln(E) ------------------------- (C 3-3-2)

其中A和r二個常數在EELS能譜中都並非是固定單一值，隨著試片厚度，收集角度(由TEM相機長度和EELS能譜儀入口光圈決定)，和損失能量的大小而變化。常數r的值大概落在 2 ~ 5之間，而常數A的值則落在10 ~ 30之間，而且每一組A，r值只適用在某能量範圍內^[1]。每個元素特性邊刃下背景訊號對應的A，r值都不同，因此無法像EDS一樣，一次將全能譜的背景契合出來，EELS能譜中每個元素對應的背景都需個別契合運算。

Equation C3-3-1 becomes a linear equation (y = a + bx) as shown below, after logarithm for both sides being taken.

ln(I) = ln(A) – r ln(E) ------------------------- (C 3-3-2)

Values of both constants, A and r, are not unique for all EELS spectra, they vary with specimen thickness, collection angles (depending on the cameral length and the spectrometer entrance aperture), and energy loss. The value of r falls in the range of 2 to 5, while A in the range of 10 to 30, and each set of r and A is only valid over a specified energy range^[1]. Unlike EDS spectra which one set of background is fitted for the whole spectrum, the background of each characteristic edge in any EELS spectrum must be fitted seperately.

圖C3-3-2解說傳統上如何處理EELS能譜。先對EELS能譜取對數，找到最契合背景訊號的直線，然後從EELS能譜中將背景扣除後。除了是TEM數位相機的主要生產公司外，Gatan也是生產柱體後形式EELS能譜儀的最主要公司，因此其影像控制與處理軟體DigitalMicrograph，也是控制能譜儀和處理EELS能譜與影像的軟體。在DigitalMicrograph中，EELS能譜背景扣除法有三個選項，一般以冪函數為主要方法。如圖C3-3-3所示，先在特性邊刃前設置一10 ～ 60 eV的能窗，然後前後移動，找出最佳的背景契合曲線。

Figure C3-3-2 shows how to process an EELS spectrum traditionally, including taking logarithm, linear fitting, background subtraction. Gatan is the main manufacture for post-columnar EELS spectrometers as well as TEM digital cameras, its image process program, DigitalMicrograph, can control EELS spectrometers and process EELS spectra too. There are three models to fit the background in DigitalMicrograph EELS module, and power law is the one most used. As shown in Figure C3-3-3, a pre-edge window of 10 to 60 eV is set and moved forward and backward to find the best background fitting.    

圖C3-3-2 EELS能譜扣除背景運算。(a)原始EELS能譜；(b)取對數後的EELS能譜；(c)找出各元素的背景契合直線；(d)去除背景後的Si特性邊刃；(e)去除背景後的C特性邊刃。

圖C3-3-3 Gatan DigitalMicrograph對EELS能譜扣除背景運算。Ref [2]

扣除背景訊號後的EELS特性邊刃才能顯示出其真正的近邊刃微細結構。前段提及特性邊刃的微細結構是唯一對應，所以被分析物的化學鍵結狀態，可以通過和資料庫內已儲存的能譜做比對而鑑定。圖C3-3-4中顯示一典型的例子，圖C3-3-4(a)從半導體元件中的缺陷區得到的碳特性邊刃，圖C3-3-4(b)和圖C3-3-4(c)則分別為銅環碳膜和low k介電材料中的碳特性邊刃。比對之後，可以推斷此缺陷區域的碳應是low k介電材料。

The true NEFS of a characteristic edge can only be viewed after background subtraction. Since the NEFS is unique, it can be used to identify the chemical bonding state of an analyzed material by comparison with corresponding characteristic edges in database. A typical example is shown in Figure C3-3-4. The characteristic C edge shown in Figure C3-3-4(a) is obtained from a defect in a semiconductor device, while Figure C3-3-4(b) and Figure C3-3-4(c) are characteristic C edges of carbon film of Cu grid and the low k dielectric respectively. The carbon in the defect can thus be deduced to be the low k material by comparing the spectra in Figure C3-3-4(a) with those in Figure C3-3-4(c) and Figure C3-3-4(c).

圖C3-3-4 扣除背景後的碳特性邊刃。(a)來自試片的缺陷區域；(b)來自銅環碳膜；(a)來自low k材料。

參考文獻

1] David B. Williams and C. Barry Carter, Transmission Electron Microscopy, Microscopy, vol.1 Spectroscopy, chapter 35, Plenum Press, New York (2009).

2] Handout of Gatan EELS school (2007).

C-3 電子能量損失能譜(EELS) - 3-2 電子能量損失能譜攝取

 C-3-2 電子能量損失能譜攝取 [2021/06/03更新] 

        前面章節提到EELS的操作的複雜度遠大於EDS的。以定點分析為例，當電子束已定位待分析區域後，EDS分析只要按下EDS控制軟體中的“開始”按鈕，待訊號足夠後，再按下“停止”按鈕即可，或預設收集時間，時間到自動停止。但是EELS分析，卻必須先完成一套調整與測試，才能按下EELS控制軟體中的“開始”按鈕。

        I mentioned that the operation of EELS is much more complicate than that of EDS in last paragraph. Let us take the position analysis for an example. For EDS analysis, we only have to press the “start” button in the EDS control software, wait for enough intensity acquired, then press the “stop” button, or set a live time and wait for automatic stop. For EELS analysis, a set of tuning and test must be performed before pressing the “start” button in the EELS control software.

        要攝取良好的EELS能譜，總共有6個重要的調整和設定步驟: (1)切換至繞射模式；(2)能譜歸零；(3)選定散佈值；(4)設定適當的能譜偏移；(5)設定適當的單次能譜攝取時間；(6)設定加總攝取次數。當TEM工程師充分瞭解每一步驟的物理意義，並靈活運用時，才能攝取正確與良好的EELS能譜。

        There are six important steps of tuning and setting to acquire good EELS spectra: (1) switch to diffraction, (2) zero set the zero loss peak, (3) select an adequate dispersion, (4) set an adequate energy offset, (5) set an adequate acquisition time for a single EELS spectrum, (6) to set the accumulation number. Correct and good EELS spectra can only be acquired when TEM engineers fully understand the physical mechanisms in steps and operate them flexibly.

繞射模式：影像模式時，穿過試片後不同能量損失的電子，通過投射透鏡後聚焦位置有所不同，    

                    如圖C3-2-1所示，造成進入EELS能譜儀的訊號的比例和試片發出的訊號比例不同，引

                    起定量分析上很大的誤差。

Diffraction mode: In image mode, high energy electrons suffering energy losses after penetrating the 

                             specimen will be focused at different height after the projector lenses, as shown in 

                             Figure C3-2-1. This makes the ratio of signals into the EELS spectroscope be different 

                             from that emitted from the specimen, which will cause big error in quantitative analysis.

能譜歸零：調整零損失峰落在能譜零點的位置，以確定其他特性邊刃在能譜上的能量位移是源自

                    化學鍵結，而不是能譜偏移造成的。

Zero set: Tuning the zero loss peak right at the “zero” channel in the spectrum to make sure that any shift 

                in a characteristic edge is due to chemical bonding shift instead of spectrum shift.

 

圖C3-2-1 不同能量損失的電子通過投射透鏡後聚焦在不同的平面，噵致後續進入EELS能譜儀的

                比例改變。

散佈值：能譜的散佈值相當於影像的倍率。散佈值為1.0 eV/ch時，倍率最小(最小能量解析度)；散

                佈值為0.05 eV/ch時倍率最大(最大能量解析度)。一般成份分析選擇1.0 eV/ch，分析化學

                鍵結則選擇0.2或0.1 eV/ch，測量EELS能譜儀的能量解析度則用0.05 eV/ch。

Dispersion: The dispersion to spectra is like magnification to images. The minimum magnification (the 

                    smallest energy resolution) is dispersion equaling 1.0 eV/ch, and the maximum magnification 

                    is dispersion equaling 0.05 eV/ch (the highest energy resolution). We use 1.0 eV/ch dispersion 

                    for composition analysis, 0.1 or 0.2 eV/ch for chemical bonding analysis, 0.05 eV/ch for 

                    measuring the energy resolution of the EELS system.

能譜偏移：將高劑量的零損失峰，低損失能峰，和部分低能量損失區域等移出訊號偵測器(閃爍器)

                    範圍，以避免過高劑量的電子損傷閃爍器，同時可讓最低能量損失特性邊刃的單次能

                    譜攝取時間盡量提高。

Energy offset: This is to move high dose parts, including zero loss peak, low loss region, and some parts 

                        of the core loss region, of the spectrum out of the scintillator to protect it from high dose 

                        electron beam damage. This offset can also raise the acquisition time of a single spectrum 

                        of the interested region.

單次能譜攝取時間：適當的單次能譜攝取時間()並非唯一，而是一個範圍。在此時間範圍內，最

                                    低能量損失特性邊刃與其前面的背景區不會過飽和，而高能量損失特性邊刃

                                    也能被有效偵測。

Acquisition time of a single spectrum: 

        The acquisition time of a single spectrum (t) is an optimum range instead of a single value. The 

        intensity of the edge of the lowest energy loss and its pre-background is not saturated, and the edge of 

        the highest energy loss is visible in this time range.

加總攝取次數：最長的單次能譜攝取時間受限於最低能量損失特性邊刃的能量位置，可能導致高

                            能量損失特性邊刃的訊號不足。藉由多次攝取能譜後加總，可以補強此問題，同

                            時提高整個能譜的訊號強度。總訊號收集時間等於加總次數(N)乘以t，N大小的

                            限制以總訊號收集時間後，不造成明顯的試片飄移和試片輻射損傷為原則。

Accumulation number: The characteristic edge of high energy loss in the spectrum may be weak due to 

                                       the limit in acquisition time for the edge of low energy loss. This can be 

                                       compensated by summing several spectra from multi-acquisition. Total collection 

                                      time is N x t, where N is the accumulation number. There should not be 

                                      detectable specimen shift and electron beam damage in total collection time.

        對一理想做EELS分析的TEM試片，得到的EELS能譜強度分佈如圖C3-2-2示意圖所示。設零損失峰的強度是Io，低損失能峰的強度約為0.01Io，矽特性邊刃的最高點強度約為1 x 10-3 Io，碳特性邊刃的最高點強度約為0.1 x 10-3Io，而氧特性邊刃的最高點強度約為0.04 x 10-3Io。隨著能量損失的增加，能譜訊號強度迅速大幅降低。此EELS能譜強度變化的特性使EELS的攝取無法像EDS那樣的簡單。

        For a thin enough TEM specimen for EELS analysis, a schematic full EELS spectrum is shown in Figure C3-2-2. Let the intensity of zero loss peak to be Io, then the intensity of low loss peak is about one percent of Io, the maximum of Si characteristic edge is about 1 x 10-3 Io, the maximum of C characteristic edge is about 0.1 x 10-3 Io, and the maximum of O characteristic edge is about 0.04 x 10-3 Io. The intensity drops quickly with increasing energy loss. This characteristic of variation in intensity with energy loss makes the job of EELS spectrum acquisition more complicate than EDS does.

        用數毫秒的攝取時間，可以攝取到完整的零損失峰，但是低損失能峰和特性損失能峰邊刃的強度則不足，泯沒於背景訊號中。增長攝取時間使矽特性邊刃有足夠的訊號強度，此時零損失峰和低損失能峰則會過飽和，而氧特性邊刃的強度則尚稍嫌不足。因此做EELS分析，在正式攝取能譜之前必須先做一些測試，根據測試結果設定適當的單次能譜攝取時間，和對應的能譜偏移使過飽和的訊號移出偵測器的範圍，避免過高劑量損傷偵測器。

        A zero loss peak can be acquired with an acquisition time of several milliseconds, but the intensity of low loss peak and characteristic edges of elements are not distinguishable with this short acquisition time. When the intensity of Si characteristic edge is adequate by aligning a suitable acquisition time, the intensity of zero loss peak and low loss peak will be saturated, while characteristic edge of oxygen are weak. So, some tests before formal acquiring must be performed to evaluate an adequate acquisition time of a single EELS spectrum, and an adequate energy offset to shift signals with oversaturated intensity out of the detector, which protects the detector from high dose damage.

 

圖C3-2-2 典型EELS能譜訊號強度變化示意圖。

        單次攝取EELS能譜時間是攝取EELS能譜實驗中一非常重要的設定。圖C3-2-3顯示一組攝取矽的EELS能譜。攝取零損失峰時，所需要的單次攝取時間很短，通常為0.01秒，也可以降至0.004秒，仍可以攝取到訊號，因為單次攝取時間很短，加總次數就可以很多次。圖C3-2-3 (a)顯示50次加總的結果，總有效收集時間為0.2秒。因為單次攝取時間太短，只有零損失峰可見，在100 eV處並沒有看到Si L特性邊刃。將單次能譜攝取時間提到0.2秒，即可看到明顯的Si L2,3特性邊刃，如圖C3-2-3 (b)所示。當單次能譜攝取時間超過元素特性邊刃的臨界攝取時間後，再經由多次攝取加總後，訊號強度就可以線性增加，如圖C3-2-3 (c)所示，經10次加總後，Si L2,3的強度接近80000。同時，Si L2,3的能譜輪廓線也明顯變得較平滑。

        The acquisition time for a single EELS spectrum is an important setting in EELS spectrum acquisition. A set of EELS spectra of Si in Figure C3-2-3 explains this. The acquisition time is usually about 0.01s when zero loss peak is included, but also can be as short as 0.004s, and the accumulation number can be 50 for this short time acquisition. Figure C3-2-3 (a) shows that there is only zero loss peak visible, Si L2,3 edges at around 100 eV are not visible even the total acquisition 0.2s. When the acquisition for a single EELS spectrum is 0.2s, the Si L2,3 edge is clear visible, as shown in C3-2-3 (b). The maximum intensity of Si L2,3 is ten times when 10 spectra are accumulated, as shown in C3-2-3 (c), and the Si L2,3 spectrum is much smooth.

圖C3-2-3  攝取時間對EELS能譜訊號的影響。(a)攝取時間= 0.004s，加總次數= 50；

                  (b) 攝取時間= 0.2s，加總次數= 1；(c) 攝取時間= 0.2s，加總次數= 10。

        為了能進行特性邊刃的背景扣除，攝取EELS能譜時，通常會在特性邊刃前預留一小段能量區，通常為50 eV，最小為30 eV。待攝取的EELS能譜的最小損失能量愈大，所需要的單次攝取時間就愈大。例如: 前述的Si L特性邊刃(99 eV)對應的單次攝取時間為0.2秒，同樣的機台狀況和試片厚度條件下，則攝取C K特性邊刃(284 eV)對應的單次攝取時間約需1.0秒；攝取O K特性邊刃(532 eV)對應的單次攝取時間約需3.0秒。

        The acquisition time becomes longer when the minimum loss energy of the specified spectrum goes higher. For example, under the same condition of TEM and specimen thickness of the previous Si L2,3 edge (99 eV), the acquisition times for C K edge (284 eV) and O K edge (532 eV) are about 1.0s and 3.0s respectively.

C-3 電子能量損失能譜(EELS) - 電子能量損失能譜簡介

C-3-1 電子能量損失能譜簡介 [2021/05/31更新，2021/06/03第二次更新]

    Eel一字在英文意指鰻魚，其複數形為eels，恰和接下來要討論的TEM成份分析技術的縮寫相同。因此在西方TEM領域的幽默笑話中，常將EELS和鰻魚群連結一起。

    Eel is a fish with snake-like shape, its plural form is eels just same to the abbreviation of a TEM composition analysis technique. So, many TEM researchers are fond of making jokes with EELS by eels.

 

圖C3-1-1 典型EELS和鰻魚連結的笑話。

    電子能量損失能譜儀(Electron Energy Loss spectroscope)，簡稱EELS，是除EDS之外，另一種加裝在TEM的成份分析附屬設備。相對EDS的130 eV能量解析度，EELS有很高的能量解析度，一般使用熱場效電子鎗當電子源的TEM，能量解析度可達到於1.0 eV以下；使用冷場效電子鎗當電子源的TEM，能量解析度可達到於0.6eV以下；如果再加上單光分光器，則能量解析度可達到於0.2eV以下。除能量解析度外，EELS的空間解析度也大幅優於EDS，其原因如圖C3-1-2所示，由於接受的是穿透式片的訊號，所以大角度散射的訊號並沒有進入訊號偵測器中。對於一試片厚度約為50奈米的TEM試片而言，如果電子束的大小是dp， EDS的空間解析度約為2 ~ 6 dp，而EELS則約為1.1 ~ 1.2 dp。由於攝取的是TEM中的穿透訊號，EELS的訊號強度也數倍大於EDS。雖然EELS的性能遠優於EDS的性能，但是由於現場操作和後續資料處理的複雜度遠大於EDS，因此在TEM/STEM的成份分析領域，未能如EDS一般地被廣泛使用。

    Besides EDS, EELS (electron energy loss spectroscopy) is another attachment on TEM for chemical analysis. The energy resolution of EELS is very high compared with that of EDS, 130 eV. It can be better than 1.0 eV, 0.5 eV, and 0.2 eV with a thermal FEG emitter, a cold FEG emitter, a cold FEG coupled with a monochromator respectively. Apart from energy resolution, EELS has a much better spatial resolution too. The mechanism is shown in Figure C3-1-2, those signals generated by high angle scattered electron do not go into the EELS detector. If the electron probe size is dp, the EDS spatial resolution is about 2 ~ 6 dp, while the EELS spatial resolution is about 1.1 ~ 1.2 dp, for a TEM specimen of 50 nm thick. Because EELS collect transmitted signals, its intensity is several times of that of EDS. The complexity of EELS in both operation and data process makes it not as popular as EDS does in TEM/STEM composition analysis. 

圖C3-1-2 示意圖顯示電子束進入試片後作用體積擴大的情形，和EDS與EELS空間解析度間的關係。Ref[1]

    圖C3-1-3比較典型的EDS能譜和EELS能譜，二者之間有三點明顯的差異。(1)訊號能峰與背景比值(P/B)的差異。EDS能譜中，P/B比值高；而EELS能譜中，特性邊刃座落在一很高的背景上，所以P/B比值低。(2)能量解析度能的差異。能譜的橫坐標都是能量值，EDS能譜的單位是KeV，而EELS能譜的單位是eV，顯示EELS的能量解析度高許多。高能量解析度的優點是可以解析出元素化態的差異，缺點則是無法一次檢測出所有元素。(3)訊號強度的差異。定點EELS的攝取時間往往是定點EDS的十分之一，但是訊號強度卻往往是10倍以上。

    Comparing the EDS spectrum and the EELS spectrum in Figure C3-1-3, we find three remarkable differences between them. (1) Peak to background ratio (P/B). All main energy peaks in EDS spectra are much high than the backgrounds right below them, while all EELS characteristic edges mount on high backgrounds. (2) Energy resolution. The unit of x-axes of both kinds of spectra is energy, KeV for EDS spectra and eV for EELS spectra. This indicates that EELS has much better energy resolution. (3) Intensity. The acquisition time of position resolved EELS spectra is usually about 10 percent of that of EDS, but the corresponding intensity of EELS spectra is about ten times of that of EDS. 

 

圖C3-1-3 典型EDS能譜和EELS能譜比較。(a)EDS能譜，高P/B比值，低能量解析度。(b)EELS能譜，高能量解析度，低P/B比值。

整個EELS能譜分為三個區域，零損失峰，低損失區域，和核損失區域，如圖C3-1-4所示。零損失峰是訊號最強的區域，在能譜上-5 ~ 5 eV(或0 ~ 5 eV)的區域，包含未被散射的電子和彈性散射電子。低損失區域又稱電漿子區域在能譜上5 ~ 50 eV的區域，最高訊號強度約為零損失峰強度的百分之一，由撞擊到外層電子的非彈性散射電子組成。EELS能譜中大於50 eV的區域延伸至2000 eV是核損失區域，由撞擊到內層電子的非彈性散射電子組成，能譜中訊號強度最弱的區域，因此在圖C3-1-4示意圖中，Y軸必須放大約200倍，才能同時顯示出來。

The whole EELS spectrum is divided into three parts, zero loss, low loss region, and core loss region, as shown in Figure C3-1-4. Zero loss peak is the region of -5 ~ 5 eV (or 0 ~ 5 eV) in the EELS spectrum, consisted of unscattered and elastically scattered electrons. Low loss region (plasmon region) is the region of 5 ~ 50 eV, consisted of electrons inelastically scattered by out-shell electrons, and its peak intensity is about one percent of that of zero loss peak. Core loss region is the region behind 50 eV and to 2000 eV, consisted of electrons inelastically scattered by inner-shell electrons. The intensity keeps going down with increasing energy loss, core loss region needs to be magnified about 200 times to be visible in the schematic diagram, Figure C3-1-4. 

 

圖C3-1-4 EELS全能譜示意圖。共分零損失峰，低損失區域，和核損失區域三個區域。

EELS 於1970年代問世。第一代的EELS能譜儀主結構示意圖如圖C3-1-5(a)所示，從燈絲發出的高能電子穿過TEM試片後，經物鏡、中間透鏡、投射透鏡等電磁透鏡，每經過一個透鏡都會形成一交叉點，交叉點可看做是前個一透鏡的像(image)和後個一透鏡的物(object)。投射透鏡的交叉點相當於進入EELS能譜儀磁稜鏡的物，通過磁稜鏡後的像是一組依能量高低線性排列的高能電子。一個可移動的狹縫掃描這組線性排列的高能電子，每一段時間內，某一特定能量範圍的高能電子通過狹縫，撞擊閃爍器激發出光電子，光電倍增管接收閃爍器發出的光電子，並將其轉換成數位電子訊號，由控制電腦讀取。此種EELS訊號攝取是藉由狹縫-閃爍器-光電倍增管模組一步一步移動收集訊號，稱之為序列式EELS(serial EELS)，SEELS中狹縫的寬度決定能量解析度。SEELS收集完一整組能譜的時間太長，因此1980年代中期發展出平行式EELS能譜儀(PEELS, parallel EELS)，其主結構示意圖如圖C3-1-5(b)所示，通過磁稜鏡後依能量線性排列的高能電子，經由四組四極電磁透鏡放大後，同時撞擊一長條狀閃爍器激發光電子，光電子經由光纖傳送到一1024通道的光二極體陣列偵測器(後來改用陣列式電耦合元件(CCD)，目前已發展至4096通道)，再轉換成數位電子訊號，然後由電腦逐一讀取CCD上各像素的電子訊號。由於電腦讀取的速度很快，相當於一瞬間內同時完成讀取所有的訊號，故稱為PEELS。

EELS was developed in 1970s. The schematic diagram of the basic configuration of the first generation of EELS is shown in Figure C3-1-5(a). High energy electrons emitted from the filament go through the thin foil specimen, pass objective lenses, intermediate lenses, and projector lenses. These high energy electrons cross over whenever they pass one set of magnetic lenses. The cross-over can be treated as the image of the previous lenses and the object of the following lenses. The prism of the EELS spectrometer takes the cross-over behind the projector lenses as its object and forms a spectrum of electrons line-up in energy loss. A slit scan those line up high energy electrons, lets a specified energy range of electrons pass through and hit the scintillator behind at a specified period of time. A photomultiplier receives photons emitted from the scintillator, transfers to digitized electron signal, and sends them to the computer. This is so called serial EELS (SEELS). The main shortage of SEELS is time consuming. In the middle 1980s the parallel electron energy loss spectrometer (PEELS) was developed to replace SEELS. The schematic diagram of the basic configuration of the PEELS is shown in Figure C3-1-5(b), those line-up high energy electrons are magnified by four sets of quadruple electro-magnetic lenses and hit an yttrium-aluminum garnet (YAG) scintillator to generate photons. A fiber-optic window coupled with the scintillator conducts photons onto a 1024 photodiode array (replaced by CCD later, which had been expanded to be 4096 channels). Electrons transferred from photons in the diode array are then read by the computer. The reading speed is so fast that the whole EELS spectrum seems to be read simultaneously, that is why this spectrometer is called PEELS [2]. 

圖C3-1-5 EELS能譜儀主結構示意圖。(a)序列式能譜儀(SEELS)。(b) 並列式能譜儀(PEELS)。Ref [2] 

PEELS的高能量解析度和高訊號接收率，使得當時許多TEM使用人預測EDS將逐漸退出TEM成份分析領域[3]。可是將近40年過去，EDS卻仍是TEM最多的成份分析附屬設備，比EELS多十倍以上。最主要的問題在於EELS的操作比EDS困難許多，後續資料處理也複雜許多，不是一般TEM工程師能夠處理的，尤其是在台灣。

In the mid of 1980s, many TEM people thought that EDS will phase out and will be replaced by PEELS due to its high energy resolution and high efficiency in signal collection [3]. However, nearly 40 years passed, EDS is still the most popular attachment in TEM for chemical analysis, and more than 10 times of EELS in number. The key point is that both the operation and data process of EELS are too complicate to be operated by engineers without being well trained, especially in Taiwan.    

參考文獻

1] David B. Williams and C. B. Carter, Transmission Electron Microscopy, page 666, Springer, (2009)

2] Handout of Gatan EELS school (2007 & 2015)

3] E. Van Cappellen, “Energy Dispersive X-ray Microanalysis in Scanning and Conventional Transmission Electron Microscopy”, in X-ray Spectrometry: Recent Technological Advances, Edited by Kouichi Tsuji, Jasna Injuk, and Rene Van Grieken (2004).