材料與材料分析(Materials and Materials Analysis): C-2 X-光能量散佈能譜儀-1/7 X-光能量散佈能譜儀簡介

C-2-1 X-光能量散佈能譜儀簡介

X-光能量散佈能譜儀(X-ray Energy Dispersive Spectroscope)是一種能譜儀，收集高能電子(電子束)撞擊試片後產生的X-光訊號，形成一如圖C-11的能譜，橫軸為能量(KeV)，縱軸為訊號強度(counts)，一般簡稱EDS。依能峰所在的能量位置，鑑定元素的種類；再從各能峰的強度，算出組成元素的比例。這種成份分析技術，稱之為能量散佈能譜學 (X-ray Energy Dispersive Spectroscopy, EDS or EDX or XEDS)。

X-ray energy dispersive spectroscope (EDS) is an spectroscope of energy type, collecting X-ray signals generated from a specimen after being hit by high energy electrons to form a spectrum as shown in Figure C-11, the x-axis is energy axis with unit KeV and y-axis is intensity with counts. Elements are identified by their corresponding energy peaks, and composition ratio is calculated by calculating intensity of energy peaks. This kind of analytical technique is called X-ray energy dispersive spectroscopy (EDS or EDX or XEDS)

圖C-11 典型能量散佈能譜。橫軸是能量單位，KeV，最常顯示範圍為0 ~ 10 KeV；縱軸是訊號強度，Counts。

由於操作簡單，EDS成為目前電子顯微鏡系統(SEM, TEM, STEM)中最廣泛被使用的成份分析附屬設備。舊型的EDS使用鋰漂移矽偵測器(lithium drifted silicon detectors, Si(Li)-detectors)將X-光訊號轉換成電子訊號，由於鋰原子遠小於矽原子，鋰漂移矽晶體內摻雜的鋰原子在工作狀態下的電場，會被逐漸推出晶體。因此，EDS必須在攝氏零下一百度以下運作，才能將偵測晶體內的鋰原子凍在晶格位置上。因為從室溫降溫至工作溫度的時間長(將近4小時)，為了隨時可以使用EDS，必須持續保持液態氮不間斷。因為液態氮桶容積的限制，每4 ~ 5天必須添加一次液態氮，是使用該設備麻煩的地方。新型的EDS使用矽漂移偵測器(Si drift detector, SDD)將X-光訊號轉換成電子訊號，矽漂移晶體內沒有摻雜異質原子，同時改善後接於偵測晶體的場效電晶體(FET)的設計，使用冷卻器(chiller)冷卻至攝氏零下廿度即可。冷卻器開機10分鐘後就可達到穩定的工作狀態，因此，新型EDS不使用狀態下是關機的。SDD EDS訊號接收率提升至每秒750000。更優的性能和更方便的操作，使得SDD EDS逐漸全面汰換Li-drift EDS。

Because of simple operation, EDS has become the most popular analytic instruments for electron microscopes, including SEM, TEM, and STEM. Traditional EDS used a Li drift silicon detector (Si(Li)-Detector) to transfer X-ray signals to electronic signals. Since the volume of the lithium atom is much smaller than that of the Si atom, these doped Li atoms will be driven out of the crystal under an applied electrical field at room temperature. The EDS detector thus has to work at temperatures below -100 ^oC to freeze Li atoms at lattice sites. It takes about 4 hours to stabilize the cooling procedure from room temperature down to liquid nitrogen temperature and let EDS work normally. Thus, we always keep liquid nitrogen in the tank and let the EDS be turn-on condition. It is an annoying job to fill out liquid nitrogen every 4 ~ 5 days. New EDS uses a Si drift detector (SDD), without extrinsic atoms in the silicon crystal, to transfer X-ray signals to electronic signals, and new designed field effect transistor (FET) which can process signals more than 750000 cps. The cooling temperature now is only – 20 ^oC, and it takes only about 10 minutes to reach the stable working condition. SDDs have largely displayed Si(Li) detectors based on better performance and more convenient operation.

高能電子撞擊試片後，產生的X-光有兩大類：連續X-光和特性X-光。連續X-光是因入射的高能電子受試片原子的原子核庫倫電場減速而損失動能，損失的能量以X-光的形式輻射出來，如圖C-12所示，某一高能電子從位置1移到位置2時，動能受原子核庫倫電場減速由E1降至E2。連續X-光又名剎車X-光，是由入射電子發出的，能量是連續的，在EDS能譜內形成背景，在0.5 ~ 3.0 KeV範圍內特別明顯，同樣的電鏡操作條件下，重元素試片有較高的背景強度。特性X-光則是入射的高能電子撞擊並游離試片內原子的內層電子，外層電子填補內層電子空位時，多餘的能量以X-光釋出，如圖C-13所示。特性X-光是由樣品發出的，能量是量子化的，且有特定的能量，可以用來鑑定元素。EDS能譜是由特性X-光和連續X-光加總而成，如圖C-14解析圖C-11中的特性X-光和連續X-光。

Two kinds of X-rays, continuum X-ray and characteristic X-ray, generated from the specimen after being hit by high energy electrons. Continuum X-ray is released when the incident electron is decelerated by the Coulomb field of the nucleus, as shown in Figure C-12. The high energy electron travel from position 1 to position 2, its corresponding kinetic energy drops from E1 to E2. Continuum X-ray also known as Bremsstrahlung X-ray is emitted from the incident electron, and can be any amount of energy below the energy of primary beam. It forms the background of an EDS spectrum, and significant in the range of 0.5 ~ 3.0 KeV. For same electron microscope conditions, the background intensity of specimens of heavy elements is higher than that of light elements. When a high energy electron penetrates the outer electron shells and knocks out of one inner shell electron, a hole will be left in the inner shell, and the atom is in an excited state. One of outer shell electron will jump to fill the hole in the inner shell to return to the lowest energy state of the atom, and emits an X-ray of specified energy equaling to the difference of the outer shell and the inner shell, as shown in Figure C-13. The X-ray of this type is called characteristic X-ray, emits from the specimen, is quantization, and can be used to characterize elements in a specimen. An EDS spectrum is the sum of characteristic X-ray superimposing on continuum X-ray, as displayed in Figure C-14 which resolves the EDS spectrum shown in Figure C-11.