材料分析B-4-5 TEM明場像的化妝師-物鏡光圈(Objective aperture, the make up artist of TEM BF images)

TEM明場像的化妝師-物鏡光圈(Objective aperture, the make up artist of TEM BF images)

 TEM是固態微奈米材料分析的終極武器之一。TEM影像的分辨率很高，明場影像約0.4奈米，無球差的高分辨影像達0.18奈米，球差修正的高分辨影像達0.05奈米，可以解析目前半導體元件中的各層奈米薄膜結構。由於台灣本身沒有生產TEM，所以學界或工業界，對於操作上如此高精密度、高複雜性的材料分析儀器都謹慎管理，對初階(甚至中階)的使用者有許多的限制，不能調動C1光圈，不能調動C2光圈，….等等，但是一定要學會正確操作物鏡光圈。在TEM模式下，物鏡光圈是調整影像對比的樞紐。要精確地量出一、二奈米的薄膜厚度，除了TEM本身的高分辨率(或解析能力)外，各層奈米薄膜之間也要有足夠的對比。

TEM is one of the ultimate instruments for solid state micro-nano materials analysis. The resolution power of TEM image is high, BF images about 0.4 nm, HRTEM images without Cs corrector about 0.18 nm, and HRTEM images with Cs corrector 0.05 nm, enough to resolve nano thin film structures in semiconductor devices. Since there is no TEM manufacturer in Taiwan, all Taiwan TEM laboratories, academic and industry, are very carefully to manage this high precision and high complexity MA instrument, put many limit rules on junior (some even middle level) TEM users, such as do not touch the C1 aperture, do not change the C2 aperture, … etc. But, every TEM user must know how to operate the objective aperture correctly. The objective aperture is the hinge to adjust the image contrast in TEM mode. Obviously, besides the resolution power of the instrument, sufficient image contrast is another key to measure thin film thickness of 1 to 2 nano meters accurately.

TEM明場影像對比機構主要有二種：原子序對比和繞射對比。這二種影像對比機構源自入射的高能電子和試片之間的散射與繞射作用。當一束高能電子撞擊到一群原子時，入射的高能電子會被原子核散射，其分布的機率如圖B4-5-1(a)示意圖所描述。通過試片輕元素材料區域的高能電子，被散射的狀態如綠色曲線所示，集中在以光軸為中心小角度範圍內。通過試片重元素材料區域的高能電子，被散射到高角度的比例增加，如紫色曲線所示。置入物鏡光圈後，被散射到高角度的入射電子被物鏡光圈擋住，無法繼續前進成像。從圖B4-5-1(b)和(c)看出，置入物鏡光圈後，穿過輕元素材料區域和穿過重元素材料區域的高能電子被擋住的比例不同，通過輕元素材料區域的高能電子被擋的較少，成像的劑量較多，在黑白影像中呈亮區；反之亦然，重元素材料區域呈暗區。因此，置入物鏡光圈後，原子序對比提升，物鏡光圈愈小，原子序對比愈強烈。

There are two main image contrast mechanisms in TEM bright-field (BF) images: atomic contrast (or called z contrast) and diffraction contrast. These two mechanisms are caused by scattering and diffraction between incident high energy electrons and the specimen. When an electron beam strikes a bunch of atoms, the distribution of elastically scattered incident electrons is shown schematically in Figure B4-5-1(a). The green curve describes the distribution of elastically scattered high energy electrons passing through regions of light elements, centralizes around the optical axis in a small angle range. And the purple curve describes the distribution of elastically scattered high energy electrons passing through regions of heavy elements. Electrons scattered to high angles will be blocked off to contributed to the final image when an objective aperture is inserted. As shown in Figure B4-5-1(b) and (c), the number of electrons blocked by the objective aperture is different for regions consisted atoms of different atomic number. Electrons passing through regions consisted of light element will be less blocked, the corresponding pixels have high dose and show bright contrast, and vice versa, pixels corresponding to regions of heavy elements show dark contrast. The smaller objective aperture gives more atomic number contrast in TEM BF images. 

圖B4-5-1 入射高能電子被原子核散射分佈示意圖，橫軸為徑向角度，縱軸為強度。(a)輕元素和重元素散射的差異，曲線下的面積相等；(b)置入中尺寸的物鏡光圈，部分通過重元素區域的散射電子被擋掉；(c)置入小尺寸的物鏡光圈，部分通過輕元素區域的散射電子被擋住，大部分通過重元素區域的散射電子被擋掉。

當晶體試片的某個極軸和透射電子束平行時，此時該晶體在強烈繞射條件。圖B4-5-1(a)中的入射高能電子分佈狀態變成如圖B4-5-2(a)所示，電子分佈變成局部集中的狀態。類似前述的情形，圖B4-5-2(b)和B4-5-2(c)顯示，置入物鏡光圈後，部分繞射電子束的電子被擋住，無法繼續前進成像，此晶體最後成像的電子劑量因此相對低，在TEM明場影像中呈暗色。

When a zone axis (h₁ k₁ l₁) of a crystal is tilted to be parallel to the incident electron beam, the crystal is in a strong diffraction condition. The distribution of electrons passing through this crystal will change from Figure B4-5-1(a) to B4-5-2(a), electrons locate locally at some points. As described in last paragraph, parts of electrons are blocked away from the final image by the inserted objective aperture, as shown in Figure B4-5-2(b) and Figure B4-5-2(c). Thus, this crystal shows dark contrast in TEM BF images due to low electron dose.

圖B4-5-2 入射高能電子和晶體產生繞射後電子分佈示意圖，橫軸為徑向角度，縱軸為強度。(a)電子集中在幾個局部的區域；(b)置入中尺寸的物鏡光圈，小部分繞射電子被擋掉；(c)置入小尺寸的物鏡光圈，繞射電子都被擋掉。

除了調整影像的對比外，物鏡光圈也可以降低球面像差效應造成的疊影。圖B4-5-3顯示一組TEM明場像，分別為(a)沒有物鏡光圈，(b)100微米的物鏡光圈，(b)30微米的物鏡光圈。很明顯的，使用30微米以下的物鏡光圈，可幾乎完全消除球面像差效應造成的疊影，避免TEM明場像內含有可能造成誤解的訊息。

Besides adjusting the image contrast, the objective aperture can minimize the shadow caused by the spherical aberration. As shown in Figure B4-5-3, (a)TEM BF image without objective aperture, (b)TEM BF image with an objective aperture of 100 um, (c)TEM BF image with an objective aperture of 30 um. Obviously, an objective aperture of 30 um is enough to eliminate any visible shadow caused by the spherical aberration.

圖B4-5-3 TEM明場像。(a)無物鏡光圈，試片內有多餘的亮影(紅色箭頭)，試片邊緣有多餘的輪廓亮影(藍色箭頭)；(b)100 um的物鏡光圈，試片內仍有多餘的亮影(紅色箭頭)，試片邊緣仍有多餘的輪廓亮影(藍色箭頭)；(c)30 um的物鏡光圈，無多餘的亮影。