TEM暗場像中的影像對比機構只有一個：繞射對比，因為TEM暗場像通常只用單一的繞射電子束成像。這類影像目前常用的有三大類型。

The only image contrast mechanism in TEM dark-field (DF) images is diffraction contrast, since TEM DF images usually use only single diffraction beam to form images. There are three kinds of DF images frequently used today.

 

第一類型暗場像用於分析晶體缺陷，影像中暗色背景區域和亮色特徵物屬同一晶粒。圖B-29(b)顯示一典型例子，差排造成晶體局部晶格扭曲，使該區域的繞射條件和晶粒其他地方略微不同，形成明暗對比。在明場像中，差排以暗色呈現，而在中央暗場像中，差排以亮色呈現，而且更清晰。此類型TEM暗場像，必須先將待分析的晶體傾轉至某些特定的雙束條件(two beam conditions)。

The first type of dark field images is used to analyze crystal defects in crystals. Bright features and dark background are in the same crystal. Fig. B-29(b) shows one typical example. Lattices around dislocations are distorted locally and cause corresponding diffraction conditions to be different from other regions of the crystal. Dislocations are black in BF images and white in CDF images. CDF images always reveal more detail structure of dislocations than BF ones do. The selected crystal has to be tilted to some specified two beam conditions for this type of DF images. 

 

 

 

圖B-29. 差排TEM影像。(a)明場像；(b)中央暗場像。

 

 

第二類型暗場像如圖B-30所示，用於分析奈米多晶材料，影像中暗的晶粒和亮的晶粒屬同一晶相材質，但是晶粒的方向不同，對應的繞射狀態不同。當晶粒尺寸比試片厚度小數倍時，由於重疊的問題，在明場像中，很難看清單一晶粒的輪廓。選用一小部分繞射點形成中央暗場像，雖然只能看到部分晶粒，但可以量測晶粒的大小。這一類型的暗場像如果採用空錐影像(hollow cone)技術攝取，則對於適當的多晶材料系統，可以一次同時將同組{h k l}，例如: {2 0 0}, {1 1 1}, {2 2 0}等，的所有繞射電子束拍攝在同一張暗場像中。

The second type of dark field images is used to analyze nano polycrystal materials. Bright and dark grains are same phase with different diffraction condition due to different orientation related to the incident beam. Crystals are overlapped and hard to be distinguishable in the BF images when their grain size is much smaller than the thickness of the specimen, as shown in Fig.B-30. Only parts of grains are in bright contrast when a few of diffracted beams are selected to form the CDF image, while their size can be measured easily. If hollow cone image is used for this kind of analysis, a special family of {h k l}, such as {2 0 0}, {1 1 1}, {2 2 0}, …etc., diffraction beams can be imaged simultaneously in single DF image for some polycrystal materials. 

 

 

圖B-30. 奈米多晶材料的TEM影像。左上角是明場像；左下角是選區繞射圖案，圖中黃色圓圈代表物鏡光圈；其他四張為中央暗場像。

 

 

第三類型暗場像如圖B-31(c)所示，用於相鑑定，影像中亮色特徵物和暗色區域為不同相的晶體。圖B-31(b)明場像用圖B-31(a)的選區繞射圖案中大黃色圓圈內的電子束成像，包括透射電子束、316不鏽鋼的繞射電子束、M₂₃C₆析出物的繞射電子束。而圖B-31(c)中央暗場像只用單一M₂₃C₆析出物的繞射電子束成像。

The third type of dark field images is used for phase identification. The bright features and dark background are not same phase. Fig.B-31(b) is a BF image using a large objective aperture indicated in Fig.B-31(a) to include the transmitted beam and diffracted beams from the matrix of 316 stainless steel and M₂₃C₆ precipitates to form the image. Fig.B-31(c) is a CDF image using a small objective aperture indicated in Fig.B-31(a) to include one diffracted beam of M₂₃C₆ precipitates to form the image.

 

 

 

圖B-31. 316不鏽鋼基材與M₂₃C₆析出物的TEM影像。(a)選區繞射圖案；(b)明場像(使用如大黃色圓圈所示的物鏡光圈)；(c)中央暗場像(使用如小黃色圓圈所示的物鏡光圈)。

 

 

如前一章節圖B-27所示，在STEM影像模式時，當相機長度減小後，環狀影像偵測器排除透射電子束的訊號，所形成的影像為STEM暗場像。當環狀影像偵測器的內收集角在0.3 ~ 1.0度之間時，STEM暗場像的對比機構以繞射對比為主，在1.0 ~ 2.0度之間時，原子序對比的比例逐漸提高，這些STEM暗場像稱為環狀暗場(ADF)像。當環狀影像偵測器的內收集角大於2.0度後，繞射電子束的訊號非常微弱，收集到的訊號幾乎都是被彈性散到高角度的入射電子，這些STEM暗場像稱為高角度環狀暗場(HAADF)像，此時影像對比機構為原子序對比。這些STEM暗場像特徵的變化如圖B-32所示。

As shown in Fig.B-27 in the last paragraph, the annual detector will exclude the transmitted beam when small cameral lengths are used. These STEM images are then dark field type. Diffraction contrast dominates in these STEM DF images when the corresponding inner collection angles fall in the range of 0.3 ~ 1.0 degrees, but weight of atomic number contrast increases gradually when the inner collection angle is larger than 1.0 degree and smaller than 2.0 degree. We call STEM DF images in this range to be ADF images. When the inner collection angle is larger than 2.0 degree, the contribution of the diffraction beams can be neglected, electrons scattered elastically to high angles are signals to form these images, and we call these images HAADF images. The image contrast mechanism in HAADF images is atomic number contrast only. Characteristics of these images are shown in Fig.B-32. 

 

 

 

圖B-32. 半導體元件的STEM影像。(a)對應的TEM明場像；(b)STEM明場像；(c)STEM ADF像；(d)STEM HAADF像。

 

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)

 

現代TEM/STEM系統有二大影像模式：TEM影像和STEM影像。TEM影像如圖B-24所示，包含明場(BF)像，暗場(DF)像，和高分辨(HRTEM)影像三種。前二種的分辨率在0.3 ~ 0.4 nm之間，倍率範圍從1000X 到100KX之間；HRTEM影像的分辨率可達到約0.16 nm，倍率範圍從100KX 到1000KX之間。STEM影像有明場像，環形暗場(ADF)像，高角度環形暗場(HAADF)像，高分辨(HRSTEM)影像(通常倍率大於5MX)等幾種。STEM影像的好處是影像可以自由旋轉，對於半導體元件的影像，很容易將基板和薄膜的界面調成水平方向。2015年後的TEM如果安裝4K x 4K的CCD或CMOS數位相機，只擷取2K x 2K的影像，也有同樣的功能。

There two main types of images, TEM images and STEM images, for current TEM/STEM systems. TEM images include bright-field (BF) images, dark-field (DF) images, and high resolution TEM (HRTEM) images, as shown in Fig. B-24. The routine magnification for both BF/DF images falls in the range of 1000X to 100KX, the best resolution of BF/DF images is about 0.3 to 0.4 nm. The magnification used for HRTEM images is from 100KX to 1000KX, and the resolution approaches 0.16 nm. STEM images include bright-field images, annual dark field (ADF) images, and high angle annual dark field (HAADF) images, high resolution STEM (HRSTEM) images (> 5 MX). The advantage of STEM images is free to rotate images, so all STEM images of semiconductor devices can easily be set all interface running horizontally. For TEMs manufactured after 2015 and equipped with a 4K x 4K CCD (or CMOS) camera, all TEM images can do the same rotation if only images of 2K x 2K taken.

 

 

圖B-24. TEM模式下的典型影像。(a)明場像；(b)中央暗場像；(c)高分辨影像；(d)和局部放大高分辨影像。

 

嚴格定義的TEM明場像是使用最小的物鏡光圈，只讓透射電子束通過成像，但是此種影像對比過於強烈，影像非黑即白缺少灰階變化，所以除非分析晶體缺陷，很少使用此類型的明場像，而是用大一點的物鏡光圈，包含部分第一階的繞射電子束，緩和影像對比，在影像中增加一些灰階層次。在這類型的TEM明場像，物鏡光圈除了調整對比外，也有減少物鏡球面像差的功能，如圖B-25所示，在無物鏡光圈時，某些黑色顆粒的旁邊會有一形狀大小相同的“亮”顆粒，放入100 um的物鏡光圈後，大部分“亮”顆粒消失，只剩下一，二顆微粒的側緣有亮線，這是物鏡球面像差造成的影像，所以在使用30 um物鏡光圈的影像內這些假像完全消失。根據經驗，分析奈米材料時，必須使用60微米以下的物鏡光圈，才能得到清晰且沒有亮點假象的奈米相TEM明場像。

The strict BF image means those images using the smallest objective aperture to block all diffracted beams and allowing only the transmitted beam to pass to form the image. The contrast of this kind of BF images is too high, absent of grey levels. We use this kind of BF image to analyze crystal defects only. We usually use a larger objective aperture to include several diffracted beams as well as the transmitted beam to form BF images with tones of grey. Besides changing the contrast of images, the objective aperture can reduce spherical aberration, as illustrated in Fig. B-25. Some particles in the BF image without objective aperture are coupled with a “bright shadow”, all of these “bright shadow” disappear when a 100um objective aperture is used, butl a few of particles with bright lines aside are still observed. There is no bright line observed at any particle when a 30um objective aperture is used. According to experience, it is better to use objective aperture not larger than 60um to minimize the effect of spherical aberration to obtain clear BF images of nano phases, especially nano wires.

 

圖B-25. TEM明場像。(a)無物鏡光圈；(b)物鏡光圈 = 100 um；(c)物鏡光圈 = 30 um。

一般的TEM暗場像一定使用小於30 um的物鏡光圈擋住透射電子束和其他的繞射電子束，只讓某一特定的繞射電子束通過成像。而且此繞射電子束必須傾轉到光軸上，形成中央暗場(CDF)像，暗場像上的的球面像差效應最小，影像才會清楚，如圖B-26所示，只有在中央暗場像才看得到鎢矽化物內的疊差，而偏離光軸的暗場像因球面像差之故，分辨率不足，無法解析細微的結構。

Generally, a TEM DF image uses an objective aperture < 30um to block transmitted beam and other diffracted beams, only allow one specific diffraction beam to pass through to form a DF image. This selected diffracted beam should be tilted to go along the optical axis to for a center dark field (CDF) image that minimize the effect of spherical aberration in the DF image, as shown in Fig. B-26. The tiny structure, such as a stacking fault in the tungsten silicide, can only be observed in the CDF image, but not in the DF image that has significant spherical aberration resulted from using an off-axis diffracted beam.

 

 

圖B-26. TEM模式下的明場像(a)，對應的暗場像(b)，與中央暗場像(c)。

 

 

可以得到前述二種影像的試片和機台條件，不一定可以得到HRTEM影像。要得到足夠清晰的HRTEM影像，下列幾個條件必須同時滿足。(1)試片必須是晶體而且夠薄，而且因試片製備造成的表面損傷層要更薄；(2)晶體的低指數晶軸(low index zone axis)要能夠被傾轉至和電子束平行；(3)TEM物鏡散光必須調整至接近零的狀態；(4)適當的物鏡欠焦(under focus)。

It is easy to acquire both BF and DF images when a TEM specimen is inserted, but it requires some special conditions to get good HRTEM images. First, the TEM specimen must be a crystal and thin enough with a very thin damaged layers on the surfaces. Second, the low index zone axis of the crystal has to be tilted parallel to the TEM optical axis. Third, TEM must be well aligned especially the astigmatism of the objective lens. Fourth, optimum under focus has to be used.

 

STEM明場像可用STEM BF影像偵測器攝取，也可用環狀影像偵測器搭配大於3米的相機長度攝取。整體來說，STEM明場像和TEM明場像很類似，但是因試片彎曲造成的條紋，在STEM明場像中可以被緩和許多。當相機長度逐漸減小時，環狀影像偵測器內收集角逐漸變大，透射電子束直接穿過中間中空區域，只有繞射電子束形成影像，是為環形暗場像。當相機長度變得很小時，環狀影像偵測器內收集角變得很大，環狀影像偵測器只收集到被彈性散射到高角度的彈性散射電子，此時攝取到的影像是為高角度環形暗場像。相機長度，內收集角，和收集訊號種類的關係說明於圖B-27中。

STEM BF images can be acquired by an annual dark filed (ADF) detector as well as a STEM BF detector. When a ADF detector is used to acquire STEM BF images, a large camera length (CL) has to be used. STEM BF images look similarly to corresponding TEM BF images. However, many bend contour fringes in TEM BF images can be eliminated in STEM BF images. The inner collection angle of the ADF detector increases gradually when the CL becomes small and small. The ADF detector then collected signals in diffracted beams only, so we call this kind of images to be annual dark field images. The inner collection angle of the ADF becomes very large when the CL becomes very small. Only those electrons elastically scattered to high angles are collected at this moment, STEM images at these conditions are called high angle annual dark field (HAADF) images. Relationships among CLs, inner collection angles, and signals are described in Fig. B-27.  

 

 

圖B-27. STEM模式下相機長度(CL)，內收集角(θ_in)，和收集訊號種類的關係。

 

 

將圖B-9中試片的厚度降至150奈米以下，再將入射電子的加速電壓增至100 KV以上，則絕大多數的入射電子會穿過試片。這些穿過試片的電子可以初步劃分為「未被未被散射電子」，「彈性散射電子」，「非彈性散射電子」三大類，如圖B-20所示。穿透式電子顯微鏡的訊號偵測器接收這些電子後，形成繞射圖案、影像、電子能量損失能譜等資料，再加上背向的X光訊號，組成一套可同時分析奈米微區的結構、組成、晶相等的材料分析技術。

If the specimen thickness in Fig. B-9 is reduced to thinner than 150 nm, and the acceleration voltage is increased up to 100 KV and higher, most of incident electrons will penetrate the specimen. Electrons through the specimen are divided into three groups: un-scattered electrons, elastically scattered electron, and inelastically scattered, as shown in Fig. B-20. All these signals are then acquired by detectors in TEM to be data of diffraction patterns, images, electron energy loss spectra as well as characteristic X-ray emitting from the other side. This is a materials analysis technology being able to resolve the microstructure/composition/crystallography of a volume of nano scale simultaneously. 

 

 

圖B-20. 高能電子射入薄片試片後產生各種電子訊號的示意圖。

 

 

圖B-21展示一傳統型式TEM/STEM的外形和基本結構示意圖，以及上述各種訊號形成的相對位置。電子束穿過薄片試片後，先在物鏡的後聚焦面處形成電子繞射圖案，然後在第一成像面形成倒立放大實像，電子能量損失能譜儀(EELS)安裝在傳統相機室的下方，接收多數穿透過試片的電子，並依能量損失量線性排列成電子能量損失能譜，能量散佈能譜儀(EDS)則安裝在試片斜上方，接收入射電子撞擊試片後產生的特性X光。

A traditional TEM/STEM and its schematic configuration are shown in Fig. B21. Locations of typical TEM/STEM data generated are pointed out too. After penetrating the foil specimen, high-energy electrons form a diffraction pattern at the objective back focal plane, a magnified image at the first image plane. An EELS is installed at the bottom of the TEM column to collect most electrons through the specimen. An EDS is equipped at diagonally above the specimen to collect characteristic X-ray emitting from the specimen.

 

 

圖B-21. 傳統TEM的外形、基本結構、典型訊息及其產生的位置。

 

 

以前TEM依其性能區分成三大類型：傳統TEM(CTEM)，高分辨TEM(HRTEM)，分析式TEM(AEM)。CTEM的最佳影像分辨率(解析度)略大於0.2 nm，其物鏡間隙較大，試片的傾轉角度可達45度，可以從數個特定晶向分析同一個晶體；HRTEM的最佳分辨率(解析度)小於0.2 nm，但其物鏡間隙較小，試片的傾轉角度約只有15度；AEM則是有STEM模式，電子束一般可達2 nm，使用場效電子鎗的STEM可達1 nm，加裝EDS或EELS或二者都有以便進行成份分析。三種TEM機型對應的主要操作模式如圖B-22所示。1995年後，因為日漸精密的機械加工與電腦輔助系統的引入，三類TEM之間的疇界逐漸被打破。現在的TEM都是使用場效電子鎗，影像最佳分辨率(解析度)達到0.16 nm，同時有STEM模式，而且STEM的影像解析度可達0.2 nm。二種模式的切換只是按鈕動作，加上一些微調即可。圖B-23顯示現代TEM機台擁有的功能，其中新型空錐影像最早於1992年出現在Zeiss 912 TEM，Zeiss退出TEM市場後，Philip的Tecnai系列繼承此成像功能，而後FEI-Philip到目前的Thermo-FEI的所有TEM/STEM都有此成像功能。另外上有些TEM使用特別設計的物鏡和試片承載台，可進行臨場(in-situ)實驗。而加上球面像差矯正器的TEM/STEM，影像分辨率和解析度都可優於0.1 nm，只是價格相對高許多。

TEMs were used to divided into three groups: conventional TEM (CTEM), high-resolution TEM (HRTEM), and analytic TEM (AEM). The gap of the pole piece of CTEMs is large enough to tilt the specimen up to 45 degree, so a special crystal can be analyzed from several low index zone axes. The image resolution of CTEMs is a little larger than 0.2 nm. HRTEMs had a smaller pole-piece gap, tilting angle is usually limited to be 15 degree, and has an image resolution approaching to 0.18 nm. AEMs were used for composition analysis by EDS or EELS or both in STEM mode. Its probe size was about 2 nm for a LaB6 emitter and 1 nm for FEG type. After 1995, most of TEMs are FEG type with image resolution better than 0.2 nm and coupled with STEM mode with probe size smaller than 0.2 nm due to the progress in precision machining and the aid of personal computer in operation system. Now, one TEM/STEM has functions shown in Fig. B-23. New type hollow cone showed up in Zeiss 912 in 1992, and Philips Tecnai series (then FEI-Philips and Thermo-FEI TEM/STEM) after Zeiss withdrawing from the TEM market. Some TEM/STEM can do in-situ experiment with special a designed objective chamber and specimen holders. The image resolution as well as electron probe size can be improved to beyond 0.1 nm when an objective and a C2 spherical correctors are equipped. However, the price goes up very much.

 

 

 

圖B-22. 傳統TEM機型的分類，與其主要的操作模式。

 

 

 

圖B-23. 現代TEM/STEM經常性操作模式。

 

聚焦離子束(focus ion beam, FIB)原來用於半導體元件的線路修補，利用鎵離子束轟擊功能有誤的半導體元件的局部線路，將其切割，再利用鎵離子束加強化學氣相沈積法(ion beam enhanced CVD)，在該局部區域沈積介電層(通常使用二氧化矽)，最後仍用鎵離子束加強化學氣相沈積法跨接金屬線形成新迴路。

Focus ion beam (FIB) was designed to do circuit repair. Parts of metal lines in a semiconductor device with improper function are cut by FIB with its Ga⁺ ion bombardment. These removed parts are then filled with dielectric material (SiO₂ usually) by using ion beam enhanced CVD (chemical vapor deposition). A new circuit is then re-built by connecting some metal lines by ion beam enhanced CVD metal.

 

由於FIB具有精準缺割的功能，半導體元件的橫截面分析可直接臨場進行，不用取出研磨拋光後再放回SEM進行分析，節省許多樣品製備的時間。雖然鎵離子掃描樣品時可產生二次電子，形成二次電子影像供分析樣品結構，但是由於高能離子的轟擊作用，二次電子影像看到的顯微結構已經是過去式的顯微結構。為了降低輻射損傷，盡量看到並同時保留樣品的現時真實結構，新一代的FIB加裝電子鎗，用電子束掃描成像。這種同時有離子鎗和電子鎗的FIB稱為雙束聚焦離子束(dual beam FIB)，其結構示意圖如圖B-16所示。離子束和試片表面的法線平行，電子束則和法線傾斜一個角度，目前通常是52度。

Due to its accurate cutting function, FIB can be used to analyze the cross-section structure of semiconductor devices by collecting secondary electrons excited by Ga⁺ ions to form SEIs. However, the structure is damaged after ion beam scanning, and the secondary electron image shows the past structure instead of current structure. An electron gun emitting electron beam to scan the sample was then added to the new generation FIB. This type of FIB is called dual beam FIB, schematically shown in Fig. B-16. Ion beam is parallel to the normal of the specimen, and electron beam usually tilts 52 degree away from the normal.

 

 

圖B-16. 雙束聚焦離子束結構示意圖。(Ref : DM of FEI)

現在新型的雙束FIB電子束分析，除了電子束不垂直試片表面外，其他和影像和SEM幾乎完全一樣，也有二次電子和反射電子二種模式。在FIB的SEIs或BEIs上水平距離(x)的量測也可以用影像上的標尺比對，但是垂直距離(y)則必須在FIB內，直接用影像軟體量測，或者要經過角度投影換算，如圖B-17所示。典型半導體元件的雙束FIB二次電子影像如圖B-18所示。

Current dual beam FIB can perform materials analysis by using SEI and BEI as SEMs do. The only difference is that the electron is parallel to the normal of the specimen surface in SEM but it tilts 52 degree away from the normal in FIB. Under such condition, only the horizontal distance can be measured by using the micro bar in the image, any vertical distance has to be measured by the built-in program or multiply a factor as illustrated in Fig. B-17. A typical FIB secondary electron image of a semiconductor device is shown in Fig. B-18.

 

 

圖B-17. 雙束聚焦離子束中電子影像y方向距離的修正。

 

 

 

圖B-18. 典型半導體元件的雙束FIB二次電子影像。

 

 

 

 

雙束FIB除可用二次電子和反射電子做影像分析外，也可加裝EDS做成份分析，加上本身具有臨場切割的能力，已取代一部分的SEM市場。雙束FIB另一大市場是半導體元件的TEM試片製備，在未來的章節再詳細討論。

Besides image analysis by using SE and BE, FIB is also able to perform composition analysis by attaching an EDS system. SEM has been replaced by FIB in some laboratories in many ways, because the latter has the ability of in-situ cutting as well as materials analysis. Another major application of FIB is to make TEM specimens of semiconductor devices. It will be discussed in detail later.

 

FIB 切割實心樣品做橫截面分析時，除了因離子轟擊造成的表面損傷外，沒有其他明顯的問體。但是如果切有中空的樣品時，經常會因再沈積的作用，在中空處上緣多出一層材料出來，如圖B-19示意圖所示。

There is no obvious problem in cutting solid samples, excluding surface damage due to ion beam bombardment. But there will be a significant extra layer at top and sides for a hollow structure as shown in Fig. B-19.

 

圖B-19. 中空結構的元件在FIB切割過程中，會在空孔的上緣和側邊多出一層”薄膜”。(a)without FIB cutting，(b) with FIB cutting。

 

 

二次電子和反射電子穿入試片的深度(圖B-9中的h_S和h_B)和SEM的加速電壓與試片的組成元素有關。同樣的材質，加速電壓愈大，訊號來自愈深層，如圖B-14所示；同樣的加速電壓，試片組成元素的原子序愈大，電子束穿透能力愈低，訊號來自較淺層的區域。圖B-15顯示一組不同加速電壓的雲母片二次電子影像。圖B-15 (a)的加速電壓為15 KV，圖B-15 (b)的加速電壓為3 KV。在15 KV的加速電壓下，雲母片呈半透明狀態；而在3 KV的加速電壓下，雲母片表面的微粒清晰可見。改變加速電壓，探索試片表面至深層的不同結構是SEM操作者必備的進階技術。

The real depth of SE and BE emitting from the specimen depends on the accelerated voltage and the composition of the specimen. Signals are from deeper regions when higher acceleration voltage is used for a specified specimen, as shown in Fig. B-14. For a specimen consisted of heavy elements (high atomic number atoms), the ability of penetration of the high energy electrons becomes smaller and signals emit from shallower regions. Fig. B-15 shows two SEM SEI of mica slices by 15 KV and 3 KV acceleration voltages respectively. Mica slices look semitransparent in the SEI of 15 KV, and particles on mica surfaces are clearly visible in the SEI of 3 KV. A skill SEM operator knows how to acquire the structure of different level by using different acceleration voltage.

 

 

 

圖B-14. 加速電壓對電子束穿透力與訊號產生範圍的影響。(Ref: Practical Scanning Electron Microscopy, edited by J. I. Goldstein, etc., 3rd edition, New York  (1977).)

 

 

 

圖B-15. 雲母片SEM二次電子影像。(a)V_acc = 15 KV；(b) V_acc = 3 KV。(感謝工研院工材所陳世昌先生提供，1996)

 

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). 

 

 

圖B-11.二次電子在不同試片表面的產生率的示意圖，藍色區域代表試片中產生二次電子的體積。(a)平面；(b)斜面；(c)90度轉角。

 

 

 

圖B-12. 二次電子影像中，九十度轉角結構處都會呈現白線，如紅色箭頭所指的位置。

 

 

 

圖B-13. 反射電子影像中，組成元素的原子序(z)愈大的區域，亮度愈高。白色區域的主元素為鉍(Bi, z = 83)；標示sp的灰色區域組成為Sb₂O₃ (Sb, z = 51)；標示ZG的深灰色區域組成為ZnO (Zn, z = 30)；