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.

   

圖C-12 連續X-光產生機構的示意圖。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

圖C-13 特性X-光產生機構的示意圖。(a)基態原子狀態；(b)一內層電子被入射電子撞擊出原子，在內層軌域留下一電洞；(c) 一外層電子躍下填補電洞，並將多餘的能量以X-光的形式釋出。

圖C-14 圖C-11中的EDS能譜分解成特性X-光和連續X-光。

材料與材料分析(Materials and Materials Analysis): C-1 表面分析儀 – 2/2 X射線光電子能譜儀 & 二次離子質譜儀

材料與材料分析(Materials and Materials Analysis): C-1 表面分析儀 – 2/2 X射線光電子能譜儀 & 二次離子質譜儀: C-1-2  X 射線光電子能譜儀 (XPS) X 射線光電子能譜儀 (X-ray Photoelectron Spectroscope, XPS) 又稱化學分析電子能譜儀 (Electron Spectroscope for Chemical Analysis, ESCA...

C-1 表面分析儀 – 2/2 X射線光電子能譜儀 & 二次離子質譜儀

C-1-2  X射線光電子能譜儀(XPS)

X射線光電子能譜儀(X-ray Photoelectron Spectroscope, XPS)又稱化學分析電子能譜儀(Electron Spectroscope for Chemical Analysis, ESCA)，也是必須在超高真空的環境下操作的表面分析儀。由於能量解析度考量，XPS最常用的X射線光源是Mg Kα和Al Kα。光電子的能量範圍和和部分歐傑電子的能量範圍重疊，因此，XPS和AES可以共用球扇電子能量分析器。XPS分析有二種模式：全能譜快速分析和局部能譜慢速分析。全能譜快速分析主要用於定性分析和半定量分析，鑑定試片的組成元素種類，典型的XPS全能譜如圖C-6所示，一般能量範圍從0至800或1000 eV，最高元素訊號能峰值將近二萬。全能譜經常會包含一、二個歐傑能峰，如圖C-6的Na KLL能峰。局部能譜慢速分析則主要用於定量分析和化學鍵結分析，只針對包含特定能峰的10 ~ 30 eV做慢速掃描，得到十幾萬以上的訊號強度，然後根據程式資料庫內儲存的資料做曲線配湊(Curve fitting)運算，在算出最佳配湊後，可以決定某特定元素各種化態離子的比值，如圖C-7所示。

X-ray Photoelectron Spectroscope (XPS) is also known as Electron Spectroscope for Chemical Analysis (ESCA)。XPS also works in an ultra-high vacuum environment. Mg Kα and Al Kα X-rays are generally used in XPS, because of their high energy resolution. Energy of X-ray photoelectrons overlaps with part of Auger electrons, so some Auger peaks show up in XPS spectra. There are two analysis mode for XPS, full spectrum with fast scan and partial spectrum with slow scan. The full spectrum is for qualitative and semi-quantitative analyses, a typical XPS spectrum of this mode is shown in Fig. C-6, the energy range is from 0 to 800 or 1000 eV with a maximum peak intensity close to 20000 counts, and a Na KLL Auger peak included. Composition of the specimen can be quickly identified. The partial spectrum is for quantitative and chemical bonding shift analyses. Only 10 to 30 eV energy range is scanned, the speed of energy scan is slow, and high intensity, over 100K, is collected. Curve fitting is then performed by repeated calculation using database built in the program. All chemical bonding states of a specified element can be determined after a best curve fitting is obtained, as shown in Fig. C-7.

XPS的缺點是空間解析度差，也就是探束(probe)很大，舊型的通常是直徑3 ~ 5釐米，產生訊號的體積很大，訊號強度很高，可達十幾萬，比EDS的數千高百倍，所以相對上定量分析的結果較準確。近代科學工程利用光纖聚焦，目前XPS的光源可縮小至7微米。當然，訊號強度也下降。XPS的另一優點是試片不必是導體。加裝適當的離子束後，XPS也可以做成分份縱深分析。

The disadvantage of XPS is its poor spatial resolution (big probe size). The probe is about 3 ~ 5 mm in diameter for old generation XPS. However, the volume excited is large and the intensity of the signal can be over 100 K counts, which is much higher than that of EDS. This makes XPS be more accurate in composition quantitative analysis. Current science and engineering use optical fibers to focus X-ray down to 7 um in diameter. Of course, the signal intensity drops with probe size inevitably. Another advantage of XPS is that no conduct specimen is required. XPS can perform composition depth profile when a suitable ion gun is available. 

圖C-6 典型XPS全能譜，能譜中包含有歐傑電子的能峰。

圖C-7 典型XPS局部能譜與曲線配湊。

C-1-3  二次離子質譜儀(SIMS)

二次離子質譜儀(Secondary Ion Mass Spectrometer)簡稱SIMS，係利用1 ~ 20 keV的主離子(通常是O₂⁺或者Cs⁺)撞擊固態試片的表面，靠近試片表面1 ~ 2 nm內的原子或分子得到足夠的能量，脫離試片表面形成濺射粒子(sputtered particles)，濺射粒子以中性粒子為主和一小部份二次離子(secondary ions)，如圖C-8所示。將二次離子收集至質譜儀(mass spectrometer)後，經質譜儀分析離子的質荷比(m/q)，而達到分析試片表面元素的目的(圖C-9)。由於主離子束對試片表面有濺射的作用，所以重複間隔一定時間收集暨分析二次離子，可以得到成份縱深分布曲線(圖C-10)，這是半導體界最常做的分析，用來分析離子佈植的深度。如果將首次離子聚焦成一離子探束(ion probe)並掃瞄試片表面，則可作二維成份影像分析，再搭電腦軟體控制與訊號收集和重建，則可進一步獲得3-D成份映像圖。

When a solid sample is sputtered by primary ions (O₂⁺ or Cs⁺ generally) of 1 ~ 20 keV, a fraction of the sputtered particles emitted from the target is ionized, as shown in Fig. C-8, from the surface to a depth of 1 to 2 nm. SIMS collects all ions into the mass spectrometer and identify elements by measuring their ratio of mass to charge (m/q). The primary ion beam itself can sputter the specimen, thus when secondary ions are collected and analyzed repeatedly at a constant rate, a depth profile can be obtained. An elemental map can be obtained by focusing the primary ions to scan the specimen. Further, a 3D map can be obtained by ions collection and reconstruction process using a program.   

圖C-8 二次離子的生成機制示意圖。

圖C-9 二次離子的靜態能譜圖。(黃悉雅博士提供(1999))

圖C-10 SIMS縱深分析圖。(黃悉雅博士提供(1999))

SIMS的有如下的特性

(1)可偵測週期表上所有的元素，而且可鑑別同位素。

(2)高偵測零敏度，可達ppm，甚至可達ppb (1 x 10¹² to 10¹⁶ atom/cm³ )。

(3)偵測濃度範圍(Dynamic range)可達10⁶，是常用材料分析儀器中最大動態範圍者。

(4)屬破壞性分析，樣品只有一次分析的機會。

(5)搭配標準試片可精準定量分析。

Characteristics of SIMS include 

(1) Capability of analyzing all elements, including their isotopes, in the periodic table. 

(2) High sensitivity, concentration as low as ppm, even ppb (1 x 10¹² to 10¹⁶ atom/cm³ ) can be detected.

(3) High dynamic range, ~ 10⁶, the highest one among routinely used materials analysis instruments. 

(4) A type of destructive analysis, the feature is gone once analyzed.

(5) Accurate quantitative analysis is possible with the use of standards.

目前常用的二次離子質譜儀有三種型式。(1)四極質譜儀：特點為對低質量元素靈敏，適用於超淺接面分析。(2)磁場質譜儀：特點為質量解析度高。(3)飛行式質譜儀：特點為偵測速率快，高靈敏度，入射主離子流小。

There are three main types of SIMSs. (1) Quadrupole mass spectrometer: sensitive to light elements, suitable for ultra-shallow junctions. (2) Magnetic mass spectrometer: high mass resolution. (3) Time of flight (TOF) mass spectrometer: high detection rate, high sensitivity, and small primary ion current.

參考文獻:

1] 凌永健，“二次離子質譜儀分析”，材料科學叢書2-材料分析，第二版，汪建民主編(2014)。

2] Secondary ion mass spectrometry – Wikipedia, https://en.wikipedia.org/wiki/Secondary_ ion_mass_ spectrometry (2020)