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).