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).