C-2 X-光能量散佈能譜- 5/7 EDS能量解析度與能峰重疊

C-2-4 EDS能量解析度與能峰重疊

C-2-4-1  能量解析度

EDS能量解析度由能峰的半高寬定義，目前多數EDS能譜儀的能量解析度落於130 ~ 140 eV之間，最佳值為121 eV。整個EDS能譜的能量解析度並非定值，而是如圖C-22所示，在低能量區的能量解析度較佳，愈高能量區的能量解析度較差。傳統上，大家通用錳(Mn) Kα (5.898 KeV)能峰的半高寬定義該EDS能譜儀的能量解析度，如圖C-23(a)。實務上，如果一下子找不到錳金屬做試片，用鉻(Cr)或鎳(Ni)也可以替代。圖C-23(b)為一用鉻(Cr) Kα (5.414 KeV)量出的EDS能量解析度。

The energy resolution of EDS is defined by the full width of half maximum (FWHM) of energy peaks in EDS spectra. It falls the range of 130 to 140 eV, and the best resolution reported is 121 eV [1]. As shown in Figure C-22, peaks in a EDS spectrum have not the same energy resolution, peaks of higher energy have worse energy resolution. Traditionally, the EDS energy resolution is defined by the FWHM of Mn Kα (5.898 KeV), as shown in Figure C-23(a). Practically, if Mn is not available, Cr and Ni can be used in place. Figure C-23(b) shows an energy resolution 135 eV measured from a Cr Kα (5.414 KeV).

圖C-22 相同積分強度的EDS能峰隨能量的高度與寬度變化情況示意圖。

圖C-23 EDS能譜儀的能量解析度。(a) SDD EDS，錳(Mn) Kα (5.898 KeV)半高寬= 124 eV [1]； (b) Si(Li) EDS，鉻(Cr) Kα (5.414 KeV)半高寬= 135 eV。

EDS能量儀的能量解析度是可以調變的，就好像腳踏車可以換檔至省力模式或快速模式一樣。調整能譜儀的訊號脈衝處理器的時間常數( 1 ~ 50 us)可以在某範圍內調整能譜儀的能量解析度，大的時間常數有較佳的能量解析度但訊號接收率低，小的時間常數能量解析度較差，但單位時間內可接收較多的訊號。訊號脈衝處理器的時間常數在往昔是屬於設備商等級才能調整的儀器參數，隨著儀器性能與操控性的提升，目前已經看到使用者可以透過視窗介面直接更改設定的版本，因此使用者可以依實驗特性的需求，選擇要較佳的能量解析度或較高的訊號強度，做適當的調整。

The energy resolution of an EDS is not fixed, it can be modulated by changing its time constant, 1 to 50 us, of the pulse processor. Large time constant will give a better energy resolution but lower count rate, and smaller time constant gives higher count rate but low energy resolution. The time constant used to be aligned by the manufacture or agent only, now TEM/EDS users can change it through the operation window whenever the object of the EDS analysis changes.

C-2-4-2  能峰重疊

在各種電子顯微鏡的微區成份分析技術中，EDS的操作最簡單，儀器最經濟，所以被廣泛使用。但是EDS的能量解析度只有約為130 eV，和WDS的5 eV，AES的7 eV，EELS的< 1.0 eV等相比，相差甚多。低能量解析度導致EDS能譜中常有能峰重疊的現象。

EDS is the easiest one to be operated and the most economical one among all analytical techniques of electron microscopes. However, the energy resolution is only about 130 eV, very poor compared with 5 eV for WDS, 7 eV for AES, and less than 1.0 eV for EELS. This shortage results in serious problem of pathological overlap. 

矽基半導體材料系統最常見的能峰重疊有二組。第一組是Si Kα (1.740 KeV)-Ta Mα  (1.710 KeV)和Si Kα-W Mα (1.775 KeV)，能量差分別為30 eV和35 eV，如圖C-24。很明顯的，在同一EDS能譜中很難區分Si Kα-Ta Mα-W Mα 三能峰。這類EDS能譜分析另外有一個問題，以圖C-24(a)下半部的EDS能譜為例，依教科書的準則，高能量的Ta Lα能峰存在，證實位於1.7 KeV的能峰有Ta Mα能峰，可是此能峰究竟是純Ta Mα能峰?還是Si Kα + Ta Mα能峰? 運用材料學的知識，資料庫的比對，和細用現代EDS軟體的功能等技術都可解決此問題。

There are two sets of energy peak overlap observed commonly in semiconductor material system. The first set is Si Kα (1.740 KeV) - Ta Mα (1.710 KeV) and Si Kα -W Mα (1.775 KeV), as shown in Figure C-24. Their energy difference is 30 eV and 35 eV respectively. Obviously, it is hard to distinguish energy peaks of Si Kα, Ta Mα, and W Mα, when they show up in the same EDS spectrum. There is another problem in these EDS spectra. Let us take the low EDS spectrum in Figure C-24(a) for example. As taught in textbooks, we know Ta Mα must be there since Ta Lα is observed. However, we do not know that the peak locating at 1.7 KeV is a pure Ta Mα peak or a combination Si Kα and Ta Mα peaks. This problem can be solved by using knowledge of the materials system, comparison with database, and using the EDS software deeply.

圖C-24 EDS能峰重疊問題。(a) 上: 矽的EDS能譜，下: 二氧化矽和钽的EDS能譜；(b) 上: 矽的EDS能譜，下: 二氧化矽、氮化鈦、鈦、鎢的EDS能譜。Si Kα (1.740 KeV), Ta Mα (1.710 KeV), W Mα (1.775 KeV)，從貫穿上下二個EDS能譜的紅色虛線指出Si Kα能峰-Ta Mα能峰 和Si Kα能峰-W Mα能峰 在同一EDS能譜時完全重疊的情形。

第二組是N Kα (0.392 KeV)和Ti Lα (0.452 KeV)，能量差為60 eV。圖C-25(a)是金屬鈦的EDS能譜，此時Ti Lα能峰和Ti Kα能峰的強度比約為0.14。圖C-25(b)是二氧化矽和氮化鈦的EDS能譜，此時Ti Lα能峰和Ti Kα能峰的強度比約為0.51，遠高於0.14，此資料顯示這裡的Ti Lα能峰不是純Ti Lα能峰而是N Kα 加Ti Lα能峰。從圖C-25(b)中的低能量區，可以看出氧能峰(0.525 KeV)和鈦L (0.452 KeV)能峰是可分離的，二者的能量差為73 eV。從此結果推估，在0.5 KeV處的能量解析度約為65 eV。

The second set is N Kα (0.392 KeV) and Ti Lα (0.452 KeV), their difference is 60 eV. Figure C-25(a) is an EDS spectrum of metal Ti, the peak intensity ratio of Ti Lα to Ti Kα is 0.14. Figure C-25(b) is an EDS spectrum of SiO₂/TiN, the peak intensity ratio of Ti Lα to Ti Kα is 0.51, significantly higher than 0.14, so the peak at 0.45 KeV is a combined peak of N K and Ti Lα rather than a pure peak of Ti Lα. From the low energy region in Figure C-25(b), energy peaks of O K (0.525 KeV) and Ti Lα (0.452 KeV) are distinguishable. This indicates that the energy resolution at 0.5 KeV is around 65 eV.

圖C-25 EDS能峰重疊問題。(a) 金屬鈦的EDS能譜；(b)二氧化矽和氮化鈦的EDS能譜。