高能電子與試片的交互作用(Interaction of High Energy Electrons with the Specimen)

由於待測樣品(材料或組件)的顯微結構不是肉眼或簡單的放大鏡/光學顯微鏡(OM)可觀察的，因此這些顯微結構對於人類的眼睛來說相當於一個「黑箱」。我們必須藉由一些比材料顯微結構尺寸上小許多的高能粒子(離子、電子、光子等)撞擊樣品，交互作用後產生一些可偵測訊號，然後用適當的偵測器 “看”這些材料顯微結構，如圖7所示。

The details of those structure are too small to be observed by either a magnified lens or an optical microscope, thus they are black boxes for human being eyes. To see them, we have to use some tiny and high energy particles, such as ions, electrons, and photons, to knock them, and acquire signals generated from the interactions by suitable detectors, as stated in Fig. 7.

圖7. 用高能粒子或光子做材料分析示意圖。

以高能電子束為例。當高能電子入射試片後，撞擊原子後產生反射電子、二次電子、歐傑電子、特性X…等諸多訊號，如圖8所示。唯有那些逸出試片並進入偵測器的訊號才是有效的訊號，其他在試片內被吸收(absorbed)的訊號和逸出試片但沒進入偵測器的訊號都是無效的訊號。從塊材(bulk)試片的角度來看，以上諸多訊號中，X光的穿透力最強，所以訊號來自最深層到表面都有，同時範圍也最廣闊(空間解析度最差)；反射電子能量最大，穿透力小於X光，但大於其他電子訊號；二次電子來自100奈米內的淺層區域；歐傑電子來自10奈米內的表面層，如圖9所示。

Let’s consider high energy electron beam only, many kinds of signals, such as backscattered electrons (BE), secondary electrons (SE), Auger electrons, and characteristic X-ray, are induced when high energy electrons strike a specimen and hit electrons in atoms, as shown in Fig. 8. Signals are effective signals only when they escape out of the specimen and enter the detector, those absorbed in the specimen or not entering the detector are ineffective signals. In a bulk specimen, X-ray comes out from the deepest and widest regions due to its powerful ability in penetration. Backscattered electrons have the highest energy among all electron signals, so does its penetration. Secondary electrons can be emitted from regions less than 100 nm in thickness. Auger electrons escape only from surface regions less than 10 nm below the top surface, as shown in Fig. 9.

圖8. 高能電子/光子和原子交互作用，產生訊號示意圖。

圖9. 塊材試片中被高能電子激發後產生的訊號和深度示意圖。