材料分析Part B-2-4 加速電壓對SEM影像的影響

二次電子和反射電子穿入試片的深度(圖B-9中的h_S和h_B)和SEM的加速電壓與試片的組成元素有關。同樣的材質，加速電壓愈大，訊號來自愈深層，如圖B-14所示；同樣的加速電壓，試片組成元素的原子序愈大，電子束穿透能力愈低，訊號來自較淺層的區域。圖B-15顯示一組不同加速電壓的雲母片二次電子影像。圖B-15 (a)的加速電壓為15 KV，圖B-15 (b)的加速電壓為3 KV。在15 KV的加速電壓下，雲母片呈半透明狀態；而在3 KV的加速電壓下，雲母片表面的微粒清晰可見。改變加速電壓，探索試片表面至深層的不同結構是SEM操作者必備的進階技術。

The real depth of SE and BE emitting from the specimen depends on the accelerated voltage and the composition of the specimen. Signals are from deeper regions when higher acceleration voltage is used for a specified specimen, as shown in Fig. B-14. For a specimen consisted of heavy elements (high atomic number atoms), the ability of penetration of the high energy electrons becomes smaller and signals emit from shallower regions. Fig. B-15 shows two SEM SEI of mica slices by 15 KV and 3 KV acceleration voltages respectively. Mica slices look semitransparent in the SEI of 15 KV, and particles on mica surfaces are clearly visible in the SEI of 3 KV. A skill SEM operator knows how to acquire the structure of different level by using different acceleration voltage.

圖B-14. 加速電壓對電子束穿透力與訊號產生範圍的影響。(Ref: Practical Scanning Electron Microscopy, edited by J. I. Goldstein, etc., 3rd edition, New York  (1977).)

圖B-15. 雲母片SEM二次電子影像。(a)V_acc = 15 KV；(b) V_acc = 3 KV。(感謝工研院工材所陳世昌先生提供，1996)