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).