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)