材料分析 Part B: 顯微鏡影像分析技術

對應材料分析儀器的功能分類，所得到的訊息也分為三大類：影像、能譜、繞射圖案，本節先介紹影像。影像的主要功能是顯示分析物的形貌(morphology)和大小(dimension)。提供最高解析度的顯微鏡是TEM，目前台灣TEM分析需求量最大的用戶是半導體業界，在半導體業界的TEM分析中約百分之九十是影像分析。

Typical, data of materials analysis are summed up into three groups: images, spectra, and patterns. We will introduce images in this paragraph, and others later. Images offer information of morphology and dimension of materials analyzed. For microscopes family, TEM has the best resolution for routine analysis. Semiconductor is the number one TEM user in Taiwan. More than 90% TEM analyses are image analysis in semiconductor industry.

影像之所以能判別分析物的形貌與大小，主要在於光源通過試片時會產生散射、繞射、反射等交互作用，同劑量密度的光源經過不同材質或同材質但不同晶向，經歷不同程度的散射、繞射、反射，最後到達影像偵測器的像素時劑量不同。在黑白影像中，劑量高的像素呈亮區，劑量低的像素則呈暗區，這種影像中的明暗變化稱之為對比(contrast)。

When homogenous beams pass through the specimen, interactions, such as scattering, diffraction, and reflection, of light with the specimen occurs, the final dose reach the image detector vary from pixels to pixels. In black and white images, they show bright in high dose pixels and dark in low dose pixels. This change in intensity is named contrast.

對比是影像的第一要素。影像因為有對比才能區別不同材質或不同晶向的晶粒。在不同種類，尤其是不同光源，的顯微鏡中，影像對比機構不同。例如，同材質但不同粗糙度(roughness)的樣品表面，例如圖10(a)，在光學顯微鏡影像中，如圖10(b)所示，平滑區是亮區；而在SEM二次電子影像，如圖10(c)所示，粗糙區是亮區。工程師必須先瞭解影像的對比機構，才能從影像中正確解讀試片內的結構。

Contrast is the most important factor of an image. It is the contrast that can tell the difference in materials and crystals of different orientation. The mechanism of image contrast is different for different type of microscope, especially with different light source. For example, a specimen of single phase has two part of surfaces, smooth and rough, as shown in Fig. 9(a). The smooth region will be bright in an optical microscope image, as shown in Fig. 9(b), but will be dark in a SEM SE image, Fig. 9(c). An engineer can read the structure of the analyzed material from images when he/she understand the mechanism of the image contrast, otherwise those images are just black and white images.

影像第二項要素是解析度，隨著影像倍率提高，通常解析度也會跟著提高。但是到達儀器的解析度極限之後，倍率的提高只會犧牲影像視野，不會繼續提升解析度。影像第三項要素是訊雜比(S/N)，訊雜比必須高過某值後，影像才會有一定的視覺品質。通常訊號強度必須超過影像偵測器飽和強度的一半，才有較佳的訊雜比。

The second factor for image is resolution. The resolution power of a microscope usually increases with the magnification until it reach the resolution limit of the microscope. Further increase in magnification does not raise resolution power, while reduces the view only. The third factor for images is signal-to-noise ratio (S/N). An image looks quality only when its S/N is over certain value. Usually, the maximum intensity of an image has to be higher than the half of saturated intensity of the detector, then the S/N reaches a satisfied value.

圖B-1. 試片表面形貌與對應顯微鏡影像的示意圖。(a)試片表面形貌；(b)光學顯微鏡影像，(c)SEM二次電子影像。。