材料分析Part B-2-2 SEM影像的形成

SEM影像既不是光學物理上稱的實像，也不是虛像，而是由一聚焦電子束掃描物體表面產生訊號後，被影像偵測器同步接收訊號後產生的對應數位影像，如圖B-6所示，同步產生器控制物鏡上的掃描線圈和影像偵測器同步。因此，在試片上產生訊號多的點，在影像偵測器對應的像素呈亮點；在試片上產生訊號少的點，在影像偵測器對應的像素呈灰點或暗點。這種用電子束掃描物體形成的影像，是電子束和物體的卷積(convolution)，和經過透鏡形成的倒立實像不同。以圖B-7做簡單的說明。物體是一簡單的黑白二相結構，二相界面的寬度在結構上是原子等級的(atomic sharp)。經由電子束掃描所得的影像，在界面形成一有灰階變化的薄層。電子束聚焦的愈小，薄層的寬度愈窄，但是不可能為零。所以，瞭解SEM影像形成機構的工程師在看SEM影像時，在腦中要自動將某些“薄層”的厚度歸零。

Physically, an SEM image is neither a real image nor a virtual image. It is an image formed by scanning a focused electron beam and then collecting signals generated synchronously, as shown in Fig. B-6. The scan generator controls the deflection coils in the objective lenses and the image detector synchronously, so a bright pixel in the image detector is corresponding to a point with strong intensity on the specimen. The image is a convolution of the electron probe and the object, not the upside down projection of the object through the lens directly. Let us give a simple explanation by using the figure B-6, the object is consisted of black and white two parts. The interface of these two parts is atomic sharp structurally. There will be a thin gradient layer at the interface in an SEM image which is formed by scanning. The width of this thin gradient layer becomes smaller when a smaller electron probe is used, but not to be zero. Thus, engineers who had learned basic SEM should know what are thin layers and what are lines in some SEM images automatically.

圖B-6. SEM同步掃描試片和接收訊號形成影像。

圖B-7. 電子束大小對掃描影像的影響。電子束愈小，掃描影像愈接近物體的真實結構。

由於SEM影像由電子束掃描試片表面形成，所以電子束的形狀直接決定影像的品質，唯有圓形的電子束才不會產生方向性變形的影像。當電子束在聚焦狀態時偏離圓形而呈橢圓形時，我們稱此時的電子束有散光的存在。當電子束有散光時，在很低倍率的影像中，試片內的結構會模糊，甚至變形，而稍微提高倍率，SEM就無法得到足夠清晰的影像，如圖B-8所示。圖B-8(a)和(b)的影像倍率只有600X，而(c)和(d)也只有2000X，對SEM而言都是低倍率。圖B-8(a)和(c)是電子束散光調整良好的影像，可清楚看見金屬線路圖案。圖B-8(b)和(d)是電子束有相當程度的散光的影像，圖B-8(b)中的金屬線路圖案明顯扭曲變形，而圖B-8 (d)的金屬線路圖案完全無法清晰成像。要得到高品質的SEM影像，首先必須將SEM的電子束散光調好。

The quality of SEM images depends strongly on the shape of the electron beam. There will be distortion in SEM images even at low magnification if there is astigmatism in the electron beam, as shown in Fig. B-8. The magnification of Fig. B-8 (a) & (b) is 600X, and that of Fig. B-8 (c) & (d) is 2000X, all are low magnification in SEM images. There is almost astigmatism free in Fig. B-8(a) and (c), but a certain level of astigmatism in Fig. B-8(b) and (d). So, those metal patterns are clearly visible in Fig. B-8(a) and (c), but distorted and blurred in Fig. 8-(b) and (d). High quality SEM images can only be available when the astigmatism of the electron beam is well aligned. 

圖B-8. 電子束散光調整對SEM影像的影響。(a) & (b) 影像倍率= 600X；(c) & (d) 影像倍率= 2000X。(a) & (c) 散光調整良好；(b) & (d) 散光調整不良。