材料與材料分析(Materials and Materials Analysis): 材料分析Part B-2-3 SEM影像對比機構

SEM影像主要分成二大類：二次電子影像和反射電子影像。圖B-9指出二次電子影像來自較淺層的區域，約從上表面起到下方100奈米的深度，有較佳的解析度；反射電子則來自上表面至下方數微米的區域，因為電子束擴展效應(beam broadening)，產生訊號的直徑可能10倍電子束大小，所以在同樣的加速電壓條件下解析度比二次電子影像差。應用這種來自不同深層的影像特性，在半導體元件的某些類型失效分析上有其獨特的效果。半導體元件的失效如果發生在某一層的金屬線路上，在分析過程中，其上層的金屬層必須先磨除，但是失效金屬層上方的金屬間介電層(IMD)卻必須保留。此時BEI影像才能看清楚失效金屬層因製程缺陷造成的損傷。

There two types of SEM images, SEI and BEI, used routinely. As shown in Fig. B-9, for an incident electron beam, SEI is formed by SE emitting from shallow region less than100 nm below the top surface and has better spatial resolution, while BEI is formed by BE emitting as deep as several micrometer , and from a volume with a diameter ten times larger than the electron beam due to the effect of beam broadening. From time to time, it is useful to use signals emitting from different depth in failure analysis of semiconductor devices. For example, if the failure occurred in a metal layer, all metal layers above it must be removed, while the IMD layer above it must be kept. BEI works well when deep feature is the point.

圖B-9. 高能電子射入塊材試片後產生二次電子和反射電子的示意圖。

二次電子的能量落在0 ~ 50電子伏特範圍，只能從較淺的表面層逸出，所以表面型態會影響二次電子的產生率，進而產生明暗對對比。二次電子影像的對比機構是表面型態對比(topography contrast)，以圖B-10(a)解說，較易瞭解。對於同材質的試片，粗糙不平的表面，在二次電子影像中，會比光滑的表面亮。從黑白灰階影像明暗代表的意義，告訴我們粗糙不平的表面產生較多的二次電子。圖B-11解釋此結果的緣由，雖然整個水滴型的體積內都產生二次電子，但是只有在距離表面一定深度以內的二次電子才能逸出試片，也就是圖B-11中藍色區域的二次電子才能跑出試片成為有效的訊號。從圖B-11示意圖中明顯看出在不同幾何面的二次電子的產生率，九十度轉角的區域產生的二次電子最多。圖B-12是一典型含有九十度轉角結構的二次電子影像，每個九十度轉角結構都有一條亮線(紅色箭頭指處)。

Since SE can emit from a depth less than 100 nm below the top surface, the morphology of the specimen surface will affect the yield of SE, then induce contrast in the image. Topography contrast dominate in SEI as shown in Fig. B-10(a). For the same material, its rough surface will yield more SE and show bright in the SEM image, while the smooth surface yield less SE and shows dark in the same image. It is easy to explain this phenomenon by schematic in Fig. B-11. For smooth surfaces which are normal to the incident electron beam, only SE in the blue volume can emit out the specimen. For rough surfaces with lots of inclined surfaces as shown in Fig. B-11(b) have larger blue volumes emitting SE. Regions with 90-degree corners have the largest blue volume, Fig. B-11(c). Fig. B-12 shows an SEI having several corners along with white lines as marked by red arrows.

圖B-10. (a)二次電子影像的表面型態對比，粗糙面在二次電子影像中比光滑面亮；(b)反射電子影像的原子序對比，對同樣的光滑表面，重元素組成的區域在反射電子影像中較亮。

當高能電子入射試片，被重元素反射的機率比被輕元素反射的機率高。因此元素的原子序愈大，反射電子的生成率就愈高，所以如圖B-10(b)所示，對於同樣光滑度表面的試片，含重元素的區域，在反射電子影像有較高的亮度，所以反射電子影像中的對比機構以原子序對比(atomic number contrast)為主。圖B-13是一反射電子影像實例，含鉍(z = 83)的相最亮，氧化鋅(z = 30)晶粒最暗，而氧化銻(z = 51)的亮度介於二者之間。

Phases consisted of heavy elements have higher ability to backscatter incident electrons and will show bright contrast in the BEI. So, for a polished surface, regions consisted of heavy elements are brighter than those consisted of light elements, as illustrated in Fig. B-10(b). Atomic number contrast dominate in BEIs. Fig. B13 shows a BEI with three phases inside, the white phase is Bi (z = 83) rich phase, the gray phase is Sb₂O₃ (Sb, z = 51), and the dark gray phase is ZnO grains (Zn, z = 30).