C-2 X-光能量散佈能譜儀 - 2/7 EDS偵測器晶體的靈敏係數

C-2-2 EDS偵測器晶體的靈敏係數

EDS偵測器前端的分隔窗，晶體前端的金屬電極與dead layer，對通過的X-光有衰減的作用，而且對愈低能量的X-光衰減作用愈大。所以EDS偵測器對所有X-光的靈敏度並非單一定植，而是如圖C-15(a)和(b)所示，低於4.0 KeV後，靈敏係數開始緩緩下降，在低於1.74 KeV的低能量區，靈敏係數會迅速下降，對於鈹窗、高分子超薄窗、超薄窗、無窗等型式的EDS偵測器分別在0.7, 0.32, 0.24, 0.16 KeV時降至零，在4.0 ~ 19.0 KeV之間靈敏係數保持0.95定值，超過19.0 KeV後靈敏係數又逐漸緩緩下降(圖C-15(a))。EDS通常用的能量區間是0 ~ 10.0 KeV。

Three components, including the window at the front end of the EDS detector, the 20 nm thick gold electrode and the dead layer ahead at the front side of the Si crystal, as shown in Figure C-15c, will decay X-rays when they pass through them. The sensitivity curve of the Si(Li) crystal is shown in Figure C-15a and b. The sensitive factor starts to drop slowly when the energy is lower than 4.0 KeV, then drops quickly when the energy is lower than 1.74 KeV. The sensitive factor drops zero at 0.7, 0.32, 0.24, and 0.16 KeV for Be-window, polymer UTW, UTV, and windowless respectively. The sensitive factor is a constant value, 0.95, at the energy range 4.0 ~ 19.0 KeV. The sensitive factor drops slowly again when the energy is higher than 19.0 KeV (Figure C15(a)). The most used energy region for EDS spectra is 0 ~ 10 KeV.

圖C-15 EDS偵測器的靈敏係數曲線。(a)0 ~ 30 KeV範圍的EDS偵測器的靈敏係數曲線；(b)低能量區 0 ~ 4 KeV範圍的EDS偵測器的靈敏係數曲線；(c) EDS偵測器結構示意圖，矽晶體本質區前面三層結構都會衰減通過的X-光。

EDS能譜中的能峰高度並不完全直接對應元素的濃度，而是元素的原始能峰高度和靈敏係數曲線的卷積(convolution)結果。由於EDS靈敏係數曲線在小於1.74 KeV的低能量區急速下降的特性，所以碳、氮、氧等常見輕元素的能峰高度遠低於它們應有的高度。圖C-16(a)中虛線能峰代表從三氧化二鋁試片產生的EDS訊號應有的能峰高度和靈敏係數曲線，二者卷積後的實際SEM/EDS能譜如圖C-16(b)所示，即使氧的濃度為鋁的1.5倍，氧的能峰仍遠低於鋁的能峰。類似的情形常見於其他含有輕元素的EDS能譜，例如圖C-17中的氮化矽和氧化矽。所以要簡略地從能峰高度判斷二元素的相對濃度時，必須先將能峰高度除以對應的靈敏係數。

The heights of energy peaks in EDS spectra are not completely corresponding to the concentration of elements directly. All peaks shown in an EDS spectrum are a convolution of original peaks with the sensitivity curve. Because the sensitivity of EDS drops quickly in energy regions less than 1.74 KeV, the peak height of typical light elements, such as C, N, and O, is always much lower than it should be. Figure C-16(a) illustrates ideal energy peaks of oxygen and aluminum from an Al₂O₃ specimen and the sensitivity curve, their convolution is shown in Figure C-16(b). The O peak height is much lower than that of Al, even the intensity ratio of O to Al should be 1.5. Similar situations are commonly observed in EDS spectra with light elements, for examples, EDS spectra of Si₃N₄ and SiO₂ shown in Figure C-17. Thus, to simply estimate the relative concentration of two elements, we have to divide the peak height by its corresponding sensitivity factor first.

圖C-16 三氧化二鋁的EDS能譜。(a)氧和鋁理論上能峰高度示意圖以及靈敏係數曲線；(b)三氧化二鋁真正的SEM/EDS能譜。

圖C-17 含輕元素的EDS能譜。(a)氮化矽的TEM/EDS能譜；(b)氧化矽的TEM/EDS能譜。