TEM影像本質上屬科學與工程類的影像，影像內容的主要重點是有沒有包含工程需要的訊息。要充分萃取影像內的訊息，必須先瞭解影像的成像對比機構，然後才能解讀無誤。例如3月25日發布的一文中，圖B-1所示，同樣的樣品表面在OM和SEM的影像中明暗對比恰好相反，如果用OM影像的觀念解釋SEM影像必定誤判樣品表面狀態。以下針對前一章節(B-4-2)談論到的幾種影像，簡單敘述其影像的成像對比機構。
TEM images are essentially images of science and engineering, the information inside images is the key point of these images. It is necessary to understand the mechanism of imaging to fully and correctly extract information form an TEM image. For example, the corresponding brightness of the sample shown in Fig. B-1 in the text issued at March 25 reversed in OM and SEM image. If we use the contrast mechanism of OM to explain the SEM image, we will misjudge the condition of the sample surface. Below, I will discuss the mechanism of imaging for types of images mentioned in the last section (B-4-2).
TEM BF影像
TEM明場像中的影像對比機構有二個: 原子序對比和繞射對比。以下用圖B-28說明。圖B-28(a) 和圖B-28(b)中是一半導體元件中的MOS(metal-oxide-silicon)結構，從下往上依序為矽基板，金屬矽化物，氧化層，多晶矽+襯墊(氮化矽+氧化矽)，氮化矽覆蓋層。在氮化矽覆蓋層上方為氧化矽介電層。
(1) 原子序對比：
黑白的TEM明場像中有效原子序愈大(或密度愈大)的相，顏色愈暗。因為原子序愈大的原子將入射電子散射到高角度的能力愈強。物鏡光圈置入後，通過原子序大的的區域的電子被擋掉的比例愈大，最後成像的電子劑量就愈少，因此影像呈暗色。由圖B-28(a)的灰階顯示各相的密度排列順序為金屬矽化物 > 矽 ~ 氮化矽 > 氧化矽。
(2) 繞射對比：
只存在於晶體相，非晶質相沒有繞射對比。所以圖B-28(a)和圖B-28(b)中，矽基板，金屬矽化物，和多晶矽三相本身的明暗度會有明顯變化。矽基板是單晶，所以在閘極下方較暗的半圓形代表應力場的存在，使該環帶區域的晶格方向和其他區域略有不同，因此和入射電子束的夾角不同，形成繞射對比。金屬矽化物和多晶矽是多晶相，每一晶粒和入射電子的夾角不同，自然產生繞射對比。氮化矽和氧化矽都是非晶質，沒有規則性的晶格，相內各處的電子束繞射情況都一樣，所以整個相的明暗度均勻，而且不會隨試片的傾轉而有所變化。所有晶體在試片傾轉過程中，其明暗度都一直在改變，因為晶格面和入射電子束的夾角一直在變動，繞射情況也一直在更改。
There are two kinds of image contrast mechanisms in TEM BF images, atomic number contrast (z-contrast) and diffraction contrast. Let me illustrate them by means of Fig. B-28. The feature in Fig. B-28(a) & (b) are MOS(metal-oxide-silicon) structure of semiconductor devices. They are constituted of Si substrate, metal silicide, oxide layer, poly, spacer (nitride and oxide), and capped nitride. Above capped nitride is a silicon dioxide dielectric layer.
(1) Atomic number contrast:
In black and white TEM images, phases with higher atomic number are darker because they elastically scatter more incident electrons to high angles. The inserted objective aperture blocks electrons scattered to high angles. Electrons finally reach the image detector are less for phases consisted of high atomic number elements. Thus, their corresponding images are dark. From the grey level, the density of these phases are metal silicide > Si ~ silicon nitride > silicon oxide.
(2) Diffraction contrast:
This mechanism exists in crystalline phases only not in amorphous phases. In Fig.B-28, the brightness of Si substrate, silicide, and poly vary from place to place. Si substrate is a single crystal, thus the light-dark half circle band under the gate indicates that there is a strain field which distorts the lattice locally and makes the diffraction condition different from other regions. Metal silicide and poly silicon are polycrystal, each crystal has its own diffraction condition and is different from other crystals of same composition. Both nitride and oxide are amorphous phases without any defined crystal lattice, thus the diffraction condition is same everywhere inside each phase, and each phase is monochromatic even the specimen being tilted. The brightness of a crystalline phase varies when the specimen is tilt because the angles between the incident beam and crystal planes change.
圖B-28. TEM明場像。(a)矽基板在任意晶向；(b)矽基板在[011]正極軸晶向。
圖28(a)的矽基板與金屬矽化物都和入射電子束成任意角度，所以通過的二相的入射電子主要匯流到透射電子束。金屬矽化物的有效原子序較大，對入射電子有較強的散射作用，因此等量的入射電子通過二相後，通過金屬矽化物的電子被散射到高角度的比例較大。當物鏡光圈置入後，散射到高角度的電子被物鏡光圈擋住，沒有貢獻到影像，也就是說，矽基板成像的電子劑量較多，所以矽基板的對應影像比金屬矽化物的影像亮。當傾轉矽基板使其[011]極軸和入射電子束平行時，通過矽基板的入射電子形成強烈的繞射，很大比例的電子分流到繞射電子束，被物鏡光圈擋住而沒有貢獻到影像，此時矽基板成像的電子劑量和金屬矽化物成像的電子劑量接近，形成的明場像就如圖28 (b)所示，矽基板和金屬矽化物明暗度幾乎相同，人類肉眼無法辨識。
In Fig.B-28(a), the orientations of the Si substrate and the metal silicide are arbitrary related to the incident beam, and most of electrons passing through the specimen converge to the transmitted beam. The effective atomic number of the metal silicide is larger than that of Si and has larger ability for elastically scattering. When same electron dose passes these two phases, the ratio of electrons scattered to high angles is higher for those going through the metal silicide. The inserted objective aperture blocks electron scattered to high angles. This makes the corresponding image of Si substrate has more electrons and shows bright contrast. Electrons passing Si substrate are in strong diffraction condition when the Si substrate is tilted to [011] zone axis. More than half of electrons are now divided into diffracted beams and are blocked by the objective aperture. The net electron dose for Si substrate and metal silicide is then almost same, and the corresponding images have same gray level, as shown in Fig.B-28(b). It is hard for human eyes to distinguish these two phases in this image.
有的TEM教科書將原子序對比分成質量對比(mass contrast)和厚度對比(thickness contrast)。從電子散射的角度來說是同一件事，原子序的原子質量也大，對入射電子的散射能力較強，同一組成的試片隨著厚度增加，對入射電子的散射比例也增加。因此David B. Williams和C. Barry Carter在Transmission Electron Microscopy[1, 2]一書中將二者合併為一，稱為質厚度比(mass-thickness contrast)。
Some TEM textbooks used the terminology of mass contrast and thickness contrast instead of atomic number contrast. They are same if we consider the behavior of electron scattering, atoms with higher atomic number have larger mass and then larger ability for electron scattering, the probability for incident electrons being elastically scattered to high angles increases with the specimen thickness. David B. Williams and C. Barry Carter used mass-thickness contrast in their book, Transmission Electron Microscopy [1, 2].
References:
1] David B. Williams and C. Barry Carter “Transmission Electron Microscopy”, Plenum Press, New York (1996)
2] David B. Williams and C. Barry Carter, “Transmission Electron Microscopy, Microscopy”, 2nd edition, Plenum Press, New York (2009)
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