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).

C-2 X-光能量散佈能譜- 7B/7 積碳與輻射損傷(B)-輻射損傷

 C-2-7 輻射損傷(Radiation damage) 

在發展出楔形研磨拋光法之前，用手工研磨拋光法製備TEM試片的過程，包含局部磨薄的渦穴研磨，然後氬離子減薄。如果渦穴研磨不足，離子減薄的時間過長，氬離子會損傷TEM試片，如圖C-32所示。圖C-32(a)用渦穴研磨拋光至小於10微米的厚度，氬離子減薄時間小於一小時，6H-SiC/Ti界面完好清晰。圖C-32(b)用渦穴研磨拋光至大於30微米的厚度，氬離子減薄時間約為八小時，在試片最薄處，接近6H-SiC/Ti界面的鈦磊晶層已被非晶質化。這是TEM試片的第一階段輻射損傷，是為離子輻射損傷，主要是離子轟擊過程中，動量轉移造成原子脫離原來的晶格位置，使晶體變成非晶質。

Before wedge polish method being developed, the process of manual grinding and polishing TEM specimens includes dimple grinding for local thinning and argon ion milling. Samples will be damaged if dimpling is insufficient and ion milling time is too long, as shown in Figure C-32. The HRTEM image shown in Figure C-32(a) was from a TEM sample well dimpled to be thinner than 10 um, and the corresponding ion milling time was less than 1 hour. The BF image shown in Figure C-32(b) was from a TEM sample dimpled to be about 30 um, and the corresponding ion milling time was about 8 hours. The mechanism of first stage TEM specimen radiation damage is momentum transfer that causes atoms shifting from their lattice positions and amorphization during ion bombardment. 

圖C-32 6H-SiC/Ti TEM試片。手工研磨拋光+渦穴研磨後，離子減薄。(a)離子減薄時間小於1小時；(b)離子減薄時間約8小時；(c) (b)中方塊區域的放大圖。

TEM試片的第二階段輻射損傷發生在TEM分析中，如果將電子束集中照射局部區域過久，也會造成輻射損傷，此時是電子輻射損傷，電子的質量很小，所以電子輻射損傷主要是當少數電子由動能轉換成熱能造成的。固態無機材料的界面和晶界，是TEM試片中相對上較容易遭受輻射損傷的區域。半導體TEM試片分析中，低介電係數材料和光阻層最容易遭受此加熱式的輻射損傷。圖C-33顯示在長時間用電子束集中連續照射後，造成6H-SiC/Ti界面逐漸非晶質化，非晶質層從試片邊緣逐漸延伸入試片，非晶質層厚度逐漸變厚。

The second radiation occurs during TEM analysis. The radiation damage is caused by converging the electron beam to illuminate the specimen locally. The mass of electrons is small, so the damage mechanism in this stage is a transformation from kinetic energy to heat when electrons are trapped in the specimen. Grain boundaries and interfaces are positions where have higher energy and prone to being damaged by heat for solid state inorganic materials. For TEM specimens of semiconductor devices, layers of low K materials and PR are phases easy to be radiation damaged by thermal energy. Images shown in Figure C-33 display that the interface of 6H-SiC/Ti were amorphized by continuous electron beam illumination. The amorphous layer extended from the edge into the specimen, and was getting thickness gradually.

圖C-33  高溫熱處理後的6H-SiC/Ti TEM試片經電子束集中連續照射後，界面非晶質化的變化。(a) t = 0；(b) t = 6 分鐘；(c) t = 10 分鐘。

C-2 X-光能量散佈能譜- 7A/7 積碳與輻射損傷(A)-積碳

 C-2-6 積碳 (Carbon contamination) 

電子顯微鏡分析中，試片進行分析的局部區域表面很容易沈積一層以碳為主的物質，此現象稱之為積碳。積碳在目前SEM和TEM使用的真空系統中是必然的現象。聚焦的電子束又比平行電子束更容易造成積碳。目前的TEM/EDS能譜攝取都在STEM模式進行，攝取的時間又比影像攝取的時間長多數十倍，因此STEM/EDS分析後必然造成明顯的積碳，嚴重影響後續的TEM分析，所以EDS分析通常放在TEM分析項目的最後。

Carbon contamination, the surface of local region analyzed being prone to deposition of a layer of carbon contained material, is common during analysis in electron microscopes. It is inevitable for carbon contamination in SEMs and TEMs, considering the current vacuum systems used. Focal electron beams make carbon contamination more serious than parallel electron beam do. Unfortunately, all current TEM/EDS analyses are performed in STEM mode and the acquisition times for EDS analysis are much longer than those used for imaging. There is always an obvious carbon contaminated region after EDS analysis and it will cause some troubles in following TEM analysis. So, EDS analysis is usually the last item to be analyzed.    

高能電子打斷碳氫氧之間的鍵結後，氣態的氫分子和氧分子被真空系統抽走；而碳原子被電子撞擊黏附在試片表面，在電子束照射下，試片的局部區域會被加熱，黏附在試片表面的碳原子獲得足夠動能沿溫度梯度線移動，最後沉積在電子束邊緣冷熱交界處。當電子束直徑大於100奈米時，積碳區形成一甜甜圈的形狀；當電子束直徑小於50奈米時，積碳區趨向形成一錐桶的形狀。如圖C-28和圖C-29所示。在成份映像圖的攝取中，映像圖左側邊緣是積碳最嚴重的地方，而且積碳基本上在試片二面有，如圖C-30所示。

Chemical bonds of C-H-O are broken by high energy electrons under the illumination of high energy electron. Molecules of hydrogen and oxygen are pumped, while C atoms are adhered on the surface of the specimen. Carbon atoms receive energy from electron bombardment, move along the traces of temperature gradient, and stop at the boundaries of the electron beam. When the beam size is large (> 100 nm), the shape of carbon contamination is like a donut. When the beam size is small (< 50 nm), the shape of carbon contamination is cone like, as shown in Figure C-28 and Figure C-29. The left side of the elemental map is where most serious carbon contamination occurs during mapping, as shown in Figure C-30. Carbon contamination happens at both surfaces of the TEM specimen.

圖C-28 甜甜圈形貌的積碳點。(a)TEM 明場像；(b)TEM EELS 碳成分影像，影像中的雙箭頭的長度代表187奈米。

圖C-29 錐桶形貌的積碳點。(a) STEM 明場像，影像中的黑色雙箭頭的長度代表69奈米；(b) SEM 二次電子影像，圖中白色物體為積碳形成的錐桶。

圖C-30 STEM/EDS成份映像圖攝取後的TEM試片積碳情形。(a) TEM明場像，黃色虛線矩形內明顯暗色區域是成份映像圖左側的積碳情形，紅色虛線矩形內略微暗色區域是成份映像圖其他區域的積碳情形。(b)SEM二次電子影像，顯示TEM試片上方(高能電子入射面)積碳情形，白色箭頭指處，是成份映像圖左側位置。(c)SEM二次電子影像，顯示TEM試片下方積碳情形，白色箭頭指處，是成份映像圖左側位置。

目前大多數的TEM工程師都習慣用電漿清洗的方式降低TEM試片的積碳速率。基本上，電漿清洗是例行TEM分析工作中降低積碳速率的好方法。如果積碳是整個TEM分析工作極重要的問題，最佳減少積碳的方法應該是使用低溫試片座(cold stage)^[1]。圖C-31顯示一典型的Gatan低溫試片座。筆者在2003年在聯華電子工作時，曾經在FEI Tecnai F20 TEM證實使用一般試片座時，將電子束聚焦在矽晶片30秒鐘，就產生明顯的黑點；相同條件下，使用低溫試片座，將電子束聚焦在矽晶片4分鐘，仍然看不到任何黑點。低溫試片座使用液態氮冷卻試片，試片表面的碳原子在液態氮冷卻下完全被凍在原位，因此不會有積碳問題。台灣半導體業界，TEM工程師不用低溫試片座的原因在於耗時，倒入液態氮後要等約90分鐘，試片座和試片之間才會達到溫度平衡，使用後移出試片座之前又要加溫約40分鐘，使試片回到室溫。一件原來只要一小時的STEM/EDS分析工作，卻要花費二小時以上的等待，顯然浪費時間。一般而言，台灣半導體業界的TEM工程師無法領略在先進TEM分析工作中，靜候至平衡穩定的價值。

Now, most of TEM engineers are used to employ plasma cleaning to knock out dangling bonds of surface atoms to reduce the carbon deposition rate. It is a good method in routine TEM jobs. The best way to prevent carbon contamination is to use a cold stage^[1] to replace the regular specimen stage when carbon contamination is seriously concerned. A typical Gatan cold state is shown in Figure C-31. I had tested it in a FEI Tecnai F20 TEM when I worked in UMC TEM Lab. in 2003. A black spot was clearly observed after focusing the electron beam on the silicon substrate for 30 seconds with a regular specimen stage. There was no mark observed after focusing the electron beam on the silicon substrate for 4 minutes when a cold stage was used. The liquid nitrogen temperature freezes all carbon atoms on the specimen surfaces, so no carbon atom is able to move and stack locally. The reason why semiconductor TEM engineers do not like to use cold stages to prevent carbon contamination is the problem of time consuming. It takes about one and half hours to reach temperature equilibrium between the holder and the specimen after pouring liquid nitrogen into the cold stage. It will take another 40 minutes to warm up the specimen to room temperature before removing the cold stage out of the microscope. When semiconductor TEM engineers think that they can finish a STEM/EDS analysis in one hour but need to wait for extra two hours, they do not use cold stages. TEM engineers in Taiwan semiconductor industry usually do not catch the value of waiting for stable condition in advanced TEM analysis.

圖C-31 Gatan TEM低溫試片座(Gatan cold stage)，右端圓槽用於盛裝液態氮。

C-2 X-光能量散佈能譜- 6B/7 EDS能譜儀(B)

 

圖C-26 EDS能量散佈能譜儀的基本結構示意圖。準直器，隔絕窗，晶體前後電極，矽偵測晶體，場效電晶體(初階放大器)，主放大器，堆積排除器，複頻分析器，腔體。

(5) 場效電晶體：接在矽偵測晶體之後，作為初步放大器，將從矽晶體傳來的脈衝電流放大約千倍，並將其轉換成脈衝電壓，再傳送至主放大器。因為此階段的訊號強度很低，類比電子元件本身因熱擾動造成的電子雜訊可能和訊號相同等級，因此場效電晶體必須用液態氮冷卻至絕對溫度140K以抑制電子元件本身熱擾動引起的電子雜訊。新型SDD 晶體，場效電晶體直接製作在矽晶體上，大幅減少線路電子雜訊的生成。因此冷卻至-20^oC就足夠^[1]。

(5)Field effect transistor: It is connected to the Si crystal, acts as a preamplifier to magnify the pulse current about 1000 times and converts the signal from pulse current to pulse voltage, then sends them to the main amplifier. The intensity of signals from Si crystal to FET is so weak, the background signals generated by thermal excitation in this analog device may be same level with those real signals. The EFT was thus cooled to 140K to minimize thermally induced electronic noise. The new EFT is directly manufactured on the SDD crystal to eliminate noise from the conduction lines, and the cooling temperature, -20^oC, is enough.^[1]  

(6) 主放大器：將從場效電晶體放出的毫伏特等級的電壓放大至約10伏特等級，然後後送至複頻分析器(MCA)。EDS偵測器儀器操作參數的設定中，時間常數(time constant)的設定即設定主放大器的放大運算時間。理論上，時間常數愈大算出來的放大電壓對應值愈準確，但是大的時間常數有時會造成後一個脈衝電壓跨在前一個脈衝電壓訊號的尾巴上，造成電壓值失真。所以大的時間常數設定有較佳的能量解析度，但是接收計量數會降低。

(6) Main amplifier: The function of the main amplifier is to magnify those pulse voltages of meV to about 10 volts, then sends them to multichannel analyzer (MCA). The time to magnify pulse voltages is the time constant that is set for the EDS detector at installation. Theoretically, large time constant will give a more precise magnified voltage for each pulse voltage from the FET. However, it will give a long tail for the voltage signal too, a following pulse voltage superimpose on this tail will give a wrong value for this signal. It means that large time constant gives a better energy resolution with a scarify of count rate.  

(7) 堆積排除器：附設於主放大器的一組迴路，其功能在於排除太快進來的訊號，使其無法進入主放大器，避免訊號失真。從場效電晶體產生的脈衝電壓傳至主放大器後，主放大器將其放大約一萬倍後輸出，然後放電回到基態，再處理下一個脈衝電壓。如果進入偵測器的訊號速率過高，在主放大器尚未完全處理完正在處理的訊號，則從場效電晶體傳來的脈衝電壓會經由堆積排除器導開並移除。

(7) Pile-up rejector: It is a set of circuits in the main amplifier to prevent a pulse voltage entering the main amplifier before the previous pulse voltage being processed completely. It starts to work when the count rate is too high to be able to be processed by the main amplifier. The function of the pile-up rejector is to make sure that the main amplifier processes the signal one by one precisely without signal distortion.

(8) 複頻分析器：將從主放大器送入的脈衝電壓類比訊號，轉換成數位訊號，依序放入對應的通道中，然後輸出至顯示器。最早的複頻分析器有1024通道，當散佈量(dispersion)設定為10 eV時，每一通道對應10 eV，能譜的顯示範圍為0 ~ 10 KeV。目前新型複頻分析器已增至4096通道，散佈量可設定5.0 eV或10.0 eV。圖C-27顯示散佈量設定對EDS能譜的影響。在電子束和攝取時間都一樣的條件下，對應散佈量10 eV的EDS能譜的強度是散佈量5 eV的EDS能譜的二倍。所以，調整複頻分析器的散佈量設定可以在略為損失能量解析度的條件下，提升EDS訊號強度，而不用增加攝取時間，此點對於容易電子輻射損傷的試片非常有用。

(8) Multi-channel analyzer (MCA): MCA convert those analog signals sent by the main amplifier to digital signals and puts them into corresponding channels, then output them to a display. Traditional MCAs have 1024 channels. The EDS spectrum displays 0 ~ 10 KeV. New MCAs have been increased to 4096 channels, and its dispersion can be set to be 5 eV/ch or 10 eV/ch. Results of different setting are shown in Figure C-27. The intensity of 10 eV/ch dispersion is twice of that of 5 eV/ch dispersion, while conditions of electron beam and acquisition time are same. This is very useful for specimens prone to electron damage. The EDS intensity can be increased without increasing acquisition time.

(9)EDS 腔體。

(9) EDS column.

圖C-27 同一電子束和取樣時間條件下攝取的EDS能譜。(a) 5 eV/ch；(b) 10 eV/ch。

參考文獻

1] Keith Thompson, “Silicon Drift Detectors”, www.thermoscientific.com (2012).

C-2 X-光能量散佈能譜- 6A/7 EDS能譜儀(A)

C-2-5 EDS能譜儀 

C-2-5-1  EDS能譜儀結構

X-光能量散佈能譜儀的基本結構示意圖如圖C-26所示。包含下列幾個主要的結構：

The basic configuration of EDS spectrometer is shown in Figure C-26. It is consisted of several main modules.

圖C-26 EDS能量散佈能譜儀的基本結構示意圖。準直器，隔絕窗，晶體前後電極，矽偵測晶體，場效電晶體(初階放大器)，主放大器，堆積排除器，複頻分析器，腔體。

(1) 準直器：其功能在於排除來自高角度的系統X-光訊號。2005年後的EDS這方面的設計都大幅改善。圖C-19中的EDS能譜是使用2005年前的EDS能譜儀攝取的，在矽[110]正極軸的繞射條件下，部分高角度繞射電子打到下半部的物鏡，因此有顯著的對應來自物鏡的鐵和鈷X-光訊號的能峰。目前的半導體試片的EDS分析都是在正極軸的繞射條件下執行，但是看不到鐵和鈷的能峰，因為都被設計良好的準直器擋住排除^[1]。

(1) Collimator: The function of collimator is to block X-rays coming from high angles, which are usually generated from the TEM system rather than from the specimen. Collimators of EDS detectors manufactured after 2005 have been improved a lot. The EDS spectrum shown in Figure C-19 was acquired by using a TEM/EDS system manufactured before 2005, and it has significant energy peaks of Fe and Co which were generated by diffracted electron beams hitting the pole piece since the EDS spectrum was acquired at the Si [110] exact zone condition. Today, almost all EDS analyses of specimens of semiconductor are acquired at [110] exact zone conditions. Either Co or Fe peak is hardly observed in these EDS spectra. These X-rays emitted from high angle sites (related to the entrance of the EDS detector) are blocked by collimators of well designed.^[1] 

(2) 隔絕窗：隔絕窗的功能在於保持腔體的真空狀態下，讓X-光訊號盡可能的進入偵測晶體。最早期用的是用7.5 ~ 8.0微米的鈹窗^[2]，此時能量低於0.7 KeV的X-光訊號無法穿透，所以偵測不到包含O以下的元素。進入使用約2 ~ 6微米的有機膜超薄窗世代後，EDS可以偵測到碳；而無窗的EDS偵測器可偵測到錋(z = 5)。

(2) Window: the function of window is to keep the chamber in high vacuum condition and let as many as X-rays into the chamber at the same time. The window was first made of 7.5 ~ 8.0 um thick Be.^[2] X-rays with energy less than 0.7 KeV could not go through the window, so elements with atomic number smaller than 9 could not be detected by Be window EDS. When 2 ~ 6 um thick polymer ultrathin window is used, elements with atomic number larger than 6 (included) can be detected by EDS. Windowless EDS can detect B (z = 5).

(3) 電極：奈米金薄膜蒸鍍在矽晶體的前後，形成歐姆接觸電極。在前端的電極需較薄，約20奈米，盡量減小對進入X-光訊號的衰減；後端的電極約50 ~ 200奈米。

(3) electrodes: Gold is coated on the front and back sides of the Si crystal to form ohmic contact electrodes. The thickness of the front electrode is about 20 nm to minimized attenuation for X-rays, and that of the back electrode is 50 ~ 200 nm. 

 

(4) 矽晶體：為一p-n二極體結構。工作狀態下，外接一逆向偏壓，所以沒有X-光進入時，此p-n二極體為絕緣體，沒有電流通過。進入矽晶體的X-光，將矽原子價帶內的電子激發到導電帶，在價帶留下一電洞，構成一電子電洞對。形成一電子電洞對的平均能量為3.8 eV。在外加電場下，被激發到導電帶的電子移向正極，形成一電流脈衝流入接於後面的場效電晶體。過去，矽晶體的純度不足，偏向p型晶體，必須摻雜入鋰原子，鋰原子容易放出一個電子，使其為電中性，是為矽鋰晶體(Si(Li)晶體)。這種EDS偵測器，稱為鋰漂移矽偵測器。因為鋰離子直徑為0.12 nm^[2]，明顯小於矽原子間距，在電場驅動下容易漂移。為防止鋰離子在電場的驅動下逐漸移向負極，必須用液態氮將矽晶體冷卻絕對溫度90K，將鋰離子凍結在晶格位置上。現在新型的矽漂移(SDD)晶體的純度足夠，不用摻雜鋰原子，因此零下20^oC的冷卻溫度就已足夠。熱電致冷式冷卻器取代液態氮，EDS的冷卻時間從4小時降至10分鐘。

(4) Silicon crystal: The Si crystal is a p-n junction diode device. The Si crystal is reverse biased in working status, so it is an insulator, and there is no current without any X-ray entering. Entering X-rays transfer their energies to electrons in the valence bands and excite them to the conduction bands, create electron-hole pairs. Electrons in conduction bands are then driven to positive electrode by the applied bias and form a pulse of current into the FET transistor. The Si crystal used for many years is a p-type silicon, an intrinsic region is formed by doping Li atoms. This kind of EDS detector is called Li-drifted detector, and the crystal is called Si(Li) (pronounced “silly”) crystal。Because the diameter of lithium (0.12 nm)^[1] is much smaller than the spacing between silicon atoms, lithium ions will be pushed to negative electrode under an applied electrical field. Li-drifted detectors have to be cooled to 90K by liquid nitrogen to freeze those lithium ions at lattice sites. Now the silicon crystal is pure enough, it is intrinsic itself without doping lithium, so -20^oC is enough to cool the EDS detector. So, liquid nitrogen is replaced by a Peltier chiller. It takes only 10 minutes to cool down the EDS detector to be ready to use.