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.