材料分析 Part C-1 表面分析儀 – 1/2 歐傑電子能譜儀(AES)

C-1表面分析儀

      高能粒子(電子，離子，中子，光子)撞擊試片後，在試片內部產生許多訊號。這些在試片內部產生的訊號，部分訊號逸出試片，部分訊號在試片內部穿梭中被吸收，最終成為熱能。用適當的偵測器收集這些訊號逸出試片的訊號就可以做成份分析。表面分析儀偵測的幾種訊號有一共通的特性，它們在固態試片內的自由平均行程都很短，只有在試片上表面起往試片內部5奈米內的深度內產生的訊號才有機會逸出試片；10奈米深度後，逸出試片的機率幾乎為零。因此物理學上將這些儀器劃分為表面分析儀器，一般材料分析實驗室常用的有歐傑電子能譜儀(AES)，X射線光電子能譜儀(XPS)，二次離子質譜儀(SIMS)。

High energy particles, such as electrons, ions, neutrons, and photons, will generate signals from a specimen after striking the specimen. Parts of these generated signals can escape from the specimen, parts of them will be absorbed during traveling in the specimen and be transferred to heat finally. When those escaped signals are collected by suitable detectors, information of composition of the specimen can be extracted. Signals collected by surface analyzers have a common characteristics: their mean free paths in the solid sample are very short, less than 10 nm, 5 nm averagely. So, only those generated in the depth less than 5 nm have chance to escape from the specimen. There is rarely signal from depth below 10 nm. That is the reason why we call instruments of this sort to be surface analyzers. Auger electron microscopes (AES), X-ray photon spectroscopes (XPS), and Secondary ion mass spectroscopes (SIMS) are three most used surface analyzers in materials analysis laboratories.

表面分析儀的工作腔必須是超高真空，也就是說其真空度優於 1x10^-9 torr。一般電子顯微鏡，例如TEM，的工作腔真空度為1x10^-6 torr，在此真空狀態下，約100秒的時間，樣品表面就蓋滿5奈米厚的碳原子；5分鐘以後，表面分析所需要的訊號就完全無法逸出。

The vacuum of working chamber of surface analyzer must be ultra-high vacuum, i.e. < 1x10^-9 torr. Electron microscopes, for example TEM, the vacuum of their working chambers is around 1x10^-6 torr. It takes only about 100s to deposit 5 nm thick carbon atoms on the top surface of the sample under such vacuum condition. After 5 minutes, all signals used for surface analysis are not able to escape from the sample. 

C-1-1 歐傑電子能譜儀(AES)

在超高真空的環境下，利用一能量1 ~ 10 KeV的電子束激發試片表面，造成表面原子發射Auger電子，藉由量測Auger電子的特性動能，可研判試片表面元素的種類、含量、和化態(Chemical state)。Auger電子的產生機構如圖C-2所示，一K軌域的電子被入射的高能電子撞擊出成為二次電子，在K軌域留下一空缺；下一瞬間，某一L₁軌域的電子躍下填補此K軌域空缺，放出一(E_K – E_L1) 能量的X-射線；此X-射線逸出過程恰好撞擊到另一L_2,3軌域的電子，L_2,3軌域的電子吸收此X-射線能量，轉化為動能，脫離原子的束縛。經過此連續機構而逸出原子的電子被命名為歐傑電子，紀念1924年發現該電子的法國科學家皮爾歐傑(Pierre Auger)。

An electron beam of 1 to 10 KeV is used to excite Auger electrons from a specimen surface in an ultra-high vacuum environment. Kinds of elements, their concentration, and chemical bonding state can be obtained by analyzing the characteristics of kinetic energy of Auger electrons. The mechanism of Auger electron generation is shown schematically in Fig. C-2. An electron in the K shell is knocked out to be a secondary electron by a high energy incident electron, and a vacancy is left in the K shell. An electron in the L₁ shell jumps into the K shell vacancy and emits an X-ray with an energy of E_K – E_L1. The emitted X-ray hit an electron in L_2,3 shell before escaping out of the atom. The L_2,3 shell electron absorbs the X-ray and transfers the radiation energy to kinetic energy, then escapes from the atom. The electron generated by these continuous mechanisms is named Auger electron in memory of Dr. Pierre Auger, a French physicist discovered this sort of electron in 1924. 

圖C-2 歐傑電子產生程序的示意圖。(a)高能電子撞擊K軌域電子；(b)某一個L軌域的電子躍下填補K軌域的空缺，多餘的能量以X-射線的方式釋出，並射向L軌域另一個電子；(c) L軌域電子吸收X-射線的能量，轉化為動能，逸出原子。

歐傑電子的能量約為50 ~ 2000 eV，屬低能量範圍，所以對輕元素較靈敏。歐傑的原始能譜(raw spectrum)有很高的背景值和很小的峰背比(P/B)，傳統上很少人直接使用原始能譜，而是使用微分能譜(differential spectrum)。典型的歐傑微分能譜中，背景訊號被拉成近乎水平線，如圖C-3所示，各元素能峰則相對放大，其能量解析度約為7 eV，比原始能譜稍差。AES微分能譜中，元素的能峰有許多特性微細結構。這些特性微細結構源自於(1)主量子軌域含有不同能量的的副軌域，例如：L軌域有2s, 2p；M軌域有3s, 3p, 3d等。(2)電子軌域的自旋角動量(spin angular momentum)和軌道角動量(orbit angular momentum)，例如：3p^1/2, 3p^3/2；  3d^1/2, 3d^3/2, 3d^5/2，利用這些特性微細結構，可以分辨元素的化態。新世代的歐傑能譜儀增加訊號偵測器的個數，提升P/B值，因此愈來愈多AES能譜直接用原始能譜。

The energy of Auger electrons falls in the range of 50 ~ 2000 eV. They are sensitive to light elements due to their low energy electrons. The background of AES raw spectra is high, and peak-to-background ratio (P/B) is small. So traditionally, differential spectra were usually used instead of raw spectra. A typical AES differential spectrum with nearly horizontal background is shown in Fig. C-3, peaks for elements are enlarged with an energy resolution about 7 eV, a little degrade from their original spectra. There are many fine structures, resulting from (1) there are sub-orbitals for each principle quantum orbital, such as 2s and 2p for L shell, 3s, 3p, 3d for M shell, (2) spin angular momentum and orbit angular momentum for shells, such as 3p^1/2, 3p^3/2, 3d^1/2, 3d^3/2, 3d^5/2 , …etc., in these AES differential spectra. Chemical bonding state can be characterized from these fine structures. More detectors are built in new generation Auger spectroscopes. This improves the P/B of raw AES spectra, and raw AES spectra are used more and more now.

圖C-3 典型AES能譜。(a)原始能譜；(b)微分能譜。。

目前常用的歐傑電子能譜儀有二：球扇電子能量分析器(SSA)和筒鏡能量分析器(CMA)，其基本結構示意圖分別如圖C-4和(圖C-5所示。在歐傑電子能譜儀內加裝離子鎗，用一定的速率濺射試片表面，每間隔一定的時間再以電子束做歐傑電子分析，可對樣品做縱深分析，甚至成像3D成份映像。由於在Z方向的解析度高(可達1 nm)，使得歐傑電子非常適用於多層薄膜結構的分析。一般使用氬離子(Ar⁺)做為入射離子源，調整氬離子的入射量、入射角度、濺射間隔時間、和入射能量，可以控制剝離表層原子的速率，調整縱深分析的解析度。總濺射時間，視試片的總厚度決定。縱深分析時，在注入氬離子階段，分析室的真空度不得超過1 x 10^-6 torr。

There two main types of AES, spherical sector analyzer (SSA) and cylindrical mirror analyzer (CMA), their basic configurations are schematically shown in Fig. C-4 and Fig. C-5 respectively. When an ion gun is included, the specimen can be sputtered in a controlled rate, a depth profile of composition as well as 3D mapping of the specimen can thus be obtained. The AES is a good analytical technique for multilayer materials system due to its good resolution in z direction. Generally, Ar⁺ source is used for the ion gun. The sputtering rate can be controlled by modulating Ar⁺ dose, incident angle, energy, and sputtering time, the resolution of the depth profile can thus be regulated. The total sputtering time for a depth profile depends on the total specimen thickness. The vacuum must be controlled to be better than 1 x 10^-6 torr during Ar⁺ sputtering.

圖C-4 球扇電子能量分析器(SSA)型歐傑電子能譜儀基本結構示意圖。

圖C-5 筒鏡能量分析器(CMA)型歐傑電子能譜儀基本結構示意圖。