材料分析Part B-3 聚焦離子束(FIB)

聚焦離子束(focus ion beam, FIB)原來用於半導體元件的線路修補，利用鎵離子束轟擊功能有誤的半導體元件的局部線路，將其切割，再利用鎵離子束加強化學氣相沈積法(ion beam enhanced CVD)，在該局部區域沈積介電層(通常使用二氧化矽)，最後仍用鎵離子束加強化學氣相沈積法跨接金屬線形成新迴路。

Focus ion beam (FIB) was designed to do circuit repair. Parts of metal lines in a semiconductor device with improper function are cut by FIB with its Ga⁺ ion bombardment. These removed parts are then filled with dielectric material (SiO₂ usually) by using ion beam enhanced CVD (chemical vapor deposition). A new circuit is then re-built by connecting some metal lines by ion beam enhanced CVD metal.

由於FIB具有精準缺割的功能，半導體元件的橫截面分析可直接臨場進行，不用取出研磨拋光後再放回SEM進行分析，節省許多樣品製備的時間。雖然鎵離子掃描樣品時可產生二次電子，形成二次電子影像供分析樣品結構，但是由於高能離子的轟擊作用，二次電子影像看到的顯微結構已經是過去式的顯微結構。為了降低輻射損傷，盡量看到並同時保留樣品的現時真實結構，新一代的FIB加裝電子鎗，用電子束掃描成像。這種同時有離子鎗和電子鎗的FIB稱為雙束聚焦離子束(dual beam FIB)，其結構示意圖如圖B-16所示。離子束和試片表面的法線平行，電子束則和法線傾斜一個角度，目前通常是52度。

Due to its accurate cutting function, FIB can be used to analyze the cross-section structure of semiconductor devices by collecting secondary electrons excited by Ga⁺ ions to form SEIs. However, the structure is damaged after ion beam scanning, and the secondary electron image shows the past structure instead of current structure. An electron gun emitting electron beam to scan the sample was then added to the new generation FIB. This type of FIB is called dual beam FIB, schematically shown in Fig. B-16. Ion beam is parallel to the normal of the specimen, and electron beam usually tilts 52 degree away from the normal.

圖B-16. 雙束聚焦離子束結構示意圖。(Ref : DM of FEI)

現在新型的雙束FIB電子束分析，除了電子束不垂直試片表面外，其他和影像和SEM幾乎完全一樣，也有二次電子和反射電子二種模式。在FIB的SEIs或BEIs上水平距離(x)的量測也可以用影像上的標尺比對，但是垂直距離(y)則必須在FIB內，直接用影像軟體量測，或者要經過角度投影換算，如圖B-17所示。典型半導體元件的雙束FIB二次電子影像如圖B-18所示。

Current dual beam FIB can perform materials analysis by using SEI and BEI as SEMs do. The only difference is that the electron is parallel to the normal of the specimen surface in SEM but it tilts 52 degree away from the normal in FIB. Under such condition, only the horizontal distance can be measured by using the micro bar in the image, any vertical distance has to be measured by the built-in program or multiply a factor as illustrated in Fig. B-17. A typical FIB secondary electron image of a semiconductor device is shown in Fig. B-18.

圖B-17. 雙束聚焦離子束中電子影像y方向距離的修正。

圖B-18. 典型半導體元件的雙束FIB二次電子影像。

雙束FIB除可用二次電子和反射電子做影像分析外，也可加裝EDS做成份分析，加上本身具有臨場切割的能力，已取代一部分的SEM市場。雙束FIB另一大市場是半導體元件的TEM試片製備，在未來的章節再詳細討論。

Besides image analysis by using SE and BE, FIB is also able to perform composition analysis by attaching an EDS system. SEM has been replaced by FIB in some laboratories in many ways, because the latter has the ability of in-situ cutting as well as materials analysis. Another major application of FIB is to make TEM specimens of semiconductor devices. It will be discussed in detail later.

FIB 切割實心樣品做橫截面分析時，除了因離子轟擊造成的表面損傷外，沒有其他明顯的問體。但是如果切有中空的樣品時，經常會因再沈積的作用，在中空處上緣多出一層材料出來，如圖B-19示意圖所示。

There is no obvious problem in cutting solid samples, excluding surface damage due to ion beam bombardment. But there will be a significant extra layer at top and sides for a hollow structure as shown in Fig. B-19.

圖B-19. 中空結構的元件在FIB切割過程中，會在空孔的上緣和側邊多出一層”薄膜”。(a)without FIB cutting，(b) with FIB cutting。