慣性的迷失(Lost in inertial) - III

 慣性的迷失(Lost in inertial) - III

現在矽基半導體元件的TEM分析主要分成二個主要階段：FIB試片製備和TEM分析。由於絕大多數的矽基半導體元件使用[001]晶圓製作，電晶體元件沿{110}方向排列。因此，在第一階段，FIB工程師習慣上沿某一[110]方向切出TEM試片。在第二階段，TEM工程師也習慣上，一定是先將TEM試片傾轉，使試片的[110]方向和電子束一致，然後再進行後續的分析。

Currently, TEM analyses of silicon based semiconductor devices are divided into two main steps, FIB sample preparation and TEM analysis. Since most of Si semiconductor devices are made by [001] Si wafers and transistors patterns are aligned along {110} directions. So, in the first step, FIB engineers are used to cutting chips along one of {110} directions. In the second step, TEM engineers are also used to tilt the specimen to [110] exact zone first, then proceed following analysis.

圖1(a)顯示一張FIB的二次電子影像，圖中紅色箭頭所指之處，第一層金屬(M1)和鎢栓(W)非常接近。由於解析度的問題，此影像無法確認第一層金屬和鎢栓是否已經接觸?因此必須進行TEM分析。這個特殊案子的TEM試片製作，理想上應該是照著如圖1(b)中虛線矩形的輪廓切挖成TEM試片，才能看清楚第一層金屬和鎢栓是否已經接觸。可是由於慣性思考使然，FIB工程師仍然照慣例，沿矽基板的[110]方向切挖，按照如圖1(c)中虛線矩形的輪廓做出TEM試片。

Figure 1(a) is an FIB SE image, the position pointed by the red arrow is a place where metal 1(M1) and a tungsten plug (W) close to each. However, it is hard to say whether they are touched or not due to the resolution limit of FIB. So, a cross-section TEM specimen has to be made for following TEM analysis. In this non-routine case, the TEM specimen should be cut as indicated by the dashed-line rectangle in Figure 1(b). But, following the inertial thinking, this TEM specimen was still cut along the [110] direction, as shown by the dashed-line rectangle in Figure 1(c) routine cases.

圖1.  FIB 二次電子影像。(a)紅色箭頭指處，第一層金屬(M1)和鎢栓(W)很接近；(b)虛線矩形代表理想TEM薄片輪廓示意圖；(c)虛線矩形代表實際TEM薄片輪廓示意圖。

如果TEM工程師也是照慣例，先傾轉TEM試片，使試片的[110]正極軸方向和電子束一致，然後拍攝影像，得到如圖2(a)的影像，得到的結論將是第一層金屬的腰部和鎢栓是接觸的。可是將TEM試片做適當的傾轉後，使觀察方向接近沿圖1(a)中的紅色點線AB方向時，可以發現第一層金屬的腰部和鎢栓雖然很接近，但是仍有幾奈米的間隙，如圖2(b)所示。從圖1(a)和圖2(b)得到的訊息，對於設計工程師或製程工程師而言，修正的工作有相當程度的差異。

If the TEM engineer did this case in conventional way, tilted the specimen to [110] exact zone condition, then took pictures, one of results is shown in Figure 2(a). The conclusion drawn from this image would be that the waist of metal 1 touched with the tungsten plug. But, if the specimen is tilted to a certain degree, and the viewing direction is nearly along the red dotted line AB in Figure 1(a). We will find that the waist of metal 1 does not touch the tungsten plug. There is a gap of several nano meters between them, as shown in Figure 2(b) by tilting the specimen correctly. What needed to be tuned in design or process would be significantly different from the information of Figure 2(a) or Figure 2(b).

圖2. 橫截面型TEM明場像。(a)沿[110]正極軸方向拍攝的TEM明場像，顯示第一層金屬(M1)和鎢栓(W)接觸；(b)沿[001]軸傾轉20幾度後拍攝的TEM明場像，顯示第一層金屬和鎢栓未接觸。

在歐美日已開發國家的科技業界，FIB和TEM操作人員的教育程度至少是碩士以上。在台灣，製造業的精神延伸到材料分析領域，專科生都可以被訓練操作FIB和TEM，人力成本大幅降低，形成一材料分析量產的產業。對於例行性的、大量的半導體元件分析，也不失為一降低成本的良策。但對於非例行性的案子，制式化的慣性操作，只是得到大量排列整齊的影像和若干組彩色的成份映像而已，TEM強大的分析功能並未被充分發揮，真正的材料訊息仍遺留在試片內。

Engineers in charge of FIB and TEM are usually masters or doctors in TEM laboratories in developed countries, such as European, USA, and Japan. In Taiwan, the spirit of cost down in manufacture has been extended to the field of materials analysis. Many college students were trained to operate FIB and TEM. TEM analysis becomes an emerging industry in Taiwan due to the reduction in the labor costs. It is not bad for routine and mass TEM analysis of specified patterns in semiconductor devices. But, for non-routine cases, only mass aligned images and color composition maps were obtained from inertial standard operation. Useful information of materials was still not explored and powerful capability of TEM was not fully used.

慣性的迷失(lost in inertial)

     在日常例行性的交通中，從A處到B處，已經習慣某條行車路線後，如果臨時想去C處買個小東西，在前往途中，一不留神就會在必須轉彎的街口繼續沿舊路線前進。個人將此種行為稱之為慣性的迷失。對於許多人來說，慣性的迷失在日常生活中經常發生，大多無傷大雅。但是如果發生在工程案件和商業案件中，則會造成相當程度的損失，小則浪費幾天的人力物力，大則危及企業根本。

    After having driven from place A to place B daily for years, most people are gradually used to going to and fro along one or two routes. Occasionally, we may forget to make a turn at the right intersection when we want to buy something at the middle way, place C. I call this mistake to be “lost in inertial” that happens in daily life from time to time for many people. Most of them are harmless. However, when it happens in engineering cases and/or business cases, it will result in damages to some extent. The loss may be only a few of manpower and material resources, may be a huge damage to the enterprise.

廿幾年前在工研院材料所微結構分析實驗室工作。某天接到一個半導體元件良率的案子，客戶送來二組表面長有磊晶層的矽晶圓，經過相同的製程，用A組晶圓製作的元件的良率都低於50%，而用B組晶圓製作的元件的良率則高於95%。客戶要鑑定A組晶圓內的磊晶層是否有造成低良率的缺陷。利用TEM進行樣品的橫截面分析。第一天，完成A組晶圓的TEM試片製作，在TEM中，觀察到矽磊晶層和矽基板的界面有幾個數奈米大小的空孔。第二天，完成B組晶圓的TEM試片製作，但是在TEM中找不到矽磊晶層和矽基板的界面。估計沒有磨對位置，第三天，再製作第二個B組晶圓的TEM試片，仍然找不到矽磊晶層和矽基板的界面。

I received a case of yield issue when I worked at microanalysis laboratory in MRL, ITRI more than twenty years ago. Semiconductor devices made by two sets of epi Si wafers had big different yield under same processes. Set A had yield less than 50%, while set B had yield more than 95%. My mission was to characterize what kind of defect(s) in the epi layer. I used TEM to analyze the cross-section structure of these samples. A TEM specimen of A was made at the first working day, and several nano voids were observed at the interface of epi layer and Si substrate. A TEM specimen of B was made at the second day, but nothing except Si substrate was observed. With the assumption of grinding and polishing wrong place, a second TEM specimen of B was made at the third day. Unfortunately, there was still only one single crystal observed in TEM. The interface of epi Si layer and the Si substrate as observed in sample A was not found.

第四天，預定交付資料的前一天，完成第三個B組晶圓的TEM試片，在TEM中搜尋一遍後，仍然找不到矽磊晶層和矽基板的界面。此時已經晚上11點。疲乏的我到室外走走散心，順便思考今晚是否要研磨第四個B試片，以及如何向客戶提出延期的說詞。沈思中，忽然靈光一閃，摻雜濃度等級的矽同質磊晶層和矽基板之間，正常狀況下，本來就看不到界面。一直想要看到磊晶層和矽基板的界面是因為A試片和以往分析異質磊晶層樣品的經驗造成的慣性思維。

    The third TEM specimen of sample B was prepared and checked in the TEM at the fourth day, one day before the deadline. It was around 11:00pm, and there was still no solid data of sample B to deliver. Walked out TEM laboratory to take a rest and thought what was wrong with the TEM sample preparation of sample B as well as how to ask the customer to delay one or two days. During meditation, something came to my mind – nothing is correct. The interface of homogeneous epitaxial layer and the substrate is invisible when the epitaxial layer is well grown. I had been trapped in the inertial of previous knowledge from analyzing heterogenous epitaxial layers and sample A for all these days.

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。

EDS與EELS的競爭與比較

 C-3-7 EDS與EELS的競爭與比較

由於幾何位置的關係，傳統的TEM/EDS系統中，從試片發出的特性X-光，其中只有4%被EDS偵測器接收。也就是說EDS的偵測效率很差；另外EDS能量解析度約為130電子伏特(eV)。相對地，EELS的偵測效率可以高達90%以上，而且在使用場效電子鎗穿透式電鏡的條件下，能量解析度優於1.0電子伏特(eV)。因此在1980年代中期，EELS由SEELS演進到PEELS後，許多TEM使用者預測EDS在TEM應用領域將會逐漸被EELS取代[1]。三十幾年過去，EDS不但沒有被EELS取代，反而隨著超薄窗型和無窗型的偵測器問世，逐漸蠶食EELS分析碳、氮、氧的領域。當然，EELS能譜儀最大製作公司- Gatan也不會坐視TEM成份分析的生意大餅被吞食，近幾年開發出新型的EELS能譜儀，大幅提升在重元素的偵測能力，也就是能量損失大於1000電子伏特能區的收集能力。

Due to the geometric relationship of signals and the detector, only about 4% of characteristic X-rays emitted from the specimen can enter the traditional EDS detector. It means that the collection efficiency of the EDS detector is quite low. Besides, the energy resolution of EDS is about 130 eV. On the contrary, the collection efficiency of EELS can be over 90%, and its energy resolution can be better than 1.0 eV when a FEG TEM is used. Thus, in the mid 1980s, many TEM users predicted that EDS was going to be phased out of the TEM applications [1]. However, more than 30 years passed, EDS is not replaced by EELS in the TEM market, but blooms more and more. Currently, most of TEM users have used STEM/EDS of ultra-thin-window type detectors or windowless type detectors to map C, N, O contained phases widely. Gatan, the main manufacture of EELS spectrometers, of course, is not able to stand for the loss in the market of TEM composition analysis. The developed new generation EELS spectrometers which are more efficiently in collecting electrons suffering large energy loss, more than 1000 eV, by striking atoms of heavy elements.

圖C3-15比較三組同一個MOS結構的氮和氧的成份映像圖。(a)使用單一超薄窗EDS偵測器STEM/EDS系統，訊號收集時間90分鐘；(b)使用四無窗EDS偵測器STEM/EDS系統，訊號收集時間30分鐘；(c)使用TEM/GIF系統，攝像時間約3分鐘。明顯地，從訊號收集時間和訊號強度來說，EELS對輕元素的成份映像能力還是遠優於EDS。但是EDS的價格優勢遠大於EELS，加上EELS的操作程序遠比EDS複雜。因此，目前台灣裝設在TEM上的EDS的數目遠大於EELS。

Three sets of N and O elemental maps are shown in Figure C3-15, (a)an STEM/EDS system with mono ultra-thin-window EDS detector, collection time is 90 minutes, (b)an STEM/EDS system with four windowless EDS detectors, collection time is 30 minutes, (c) a TEM/GIF system, total imaging time is about 3 minutes. Obviously, EELS is better than EDS when collection time and the intensity of signal are considered only. However, the price of EDS is much lower than that of EELS, and the operation of EELS is much complicate compared with EDS. Therefore, the number of TEM equipped with EDS overwhelms that of TEM equipped with EELS. 

圖C3-15 MOS的EDS和EELS氮和氧成分映像圖。(a)單一超薄窗EDS偵測器 STEM/EDS系統；(b) 四無窗EDS偵測器 STEM/EDS系統；(c)TEM/GIF系統。訊號收集時間分別為90分鐘，60分鐘，3分鐘。

參考文獻

1] E. Van Cappellen, “Energy Dispersive X-ray Microanalysis in Scanning and Conventional Transmission Electron Microscopy”, in the book “X-ray Spectrometry: Recent Technological Advances”, edited by Kouichi Ysuji, Jasna Injuk, and René Van Grieken, published by John Wiley & Sons Ltd. (2004).

C-2 X-光能量散佈能譜- STEM/EDS分析上的假訊 (Artifacts in STEM/EDS Analysis)

 C-2-8  STEM/EDS分析上的假訊 (Artifacts in STEM/EDS Analysis)

由於STEM/EDS硬體的性能與軟體的功能大幅提升，近幾年來，在半導體元件的TEM分析中，EDS分析已成不可或缺的資料。目前所有的EDS分佈都在STEM模式下，用能譜影像(spectrum image)技術分析，先做出一組成份映像圖(elemental maps)，如圖C2-34所示。成份映像圖顯示元素的二維分佈狀況，每一元素映像圖內的訊號明暗度，可以顯示元素濃度的相對高低，但無法告知絕對的濃度值，而不同元素映像圖內的明暗度不能代表濃度的相對高低。元素絕對的濃度值必須選取局部區域運算，或從直線成份分佈圖，才能讀出線上各點的濃度值。

Due to significant improvements in the performance of hardware and functions of software, EDS analysis has become a necessary and routine item in TEM analysis of semiconductor devices recently. An EDS analysis is performed by spectrum image technique in STEM mode. A set of elemental maps are then extracted from this spectrum image, as shown in Figure C2-34. EDS elemental maps tell the distribution of elements in two-dimension, and relative concentration by brightness in the same elemental map, but no values of concentration, and the difference in brightness in different elemental map does not means high or low in concentration. Values of concentration of elements are available by extracting EDS spectra from local regions, or from EDS line profiles from this spectrum image. 

  

圖C2-34 STEM HAADF image and elemental maps of nano particles extracted from a spectrum image 

製程工程師往往希望拿到的EDS成份直線分佈圖，圖中元素曲線非常平滑好看。但是由於EDS訊號收集效率和能譜影像像素數目的因素，能譜影像中每一像素內的EDS訊號強度不高，從奈米層次的相拉出的EDS原子百分比成份直線分佈圖中的元素曲線通常是上下震盪的。為了滿足客戶的要求，TEM分析實驗室常對原始數據做一些後續的平滑處理，如下面各圖所示。圖(a) 是EDS原始強度line profiles，在標示 I 的區域內，各元素的訊號強度為背景值，物理上是沒有試片的區域。但是將其換算成圖(b)的EDS at%成份直線分佈圖後，雖然氮和氧的曲線震盪幅度很大，代表氮和氧的濃度變化很大，因此圖(b)圖面上的意義顯示區域I 為一含氮和氧的區域。雖然數學上，7/10和700/1000值相等，但是物理上(或統計學上或統計學上)，二者的意義完全不同。將類似區域I的訊號放入EDS at% line profiles或EDS wt% line profiles運算中，除了造成可能誤導的訊息外，物理上完全沒有意義的是。將圖(b)平滑處理成圖(c)或圖(d)之後，這些曲線平滑處理造成的假象，更容易誤導許多不知整個分析歷史的看圖者，導致工程師製程調整方向錯誤。

Process engineers always like elemental curves in EDS line profiles are smooth. However, due to the limit of EDS collection efficiency and large pixel number in spectrum images, the signal intensity is low in each pixel in spectrum images. EDS at% line profiles of phases in nano scale are usually fuzzy. To meet requirements of customers, many TEM service laboratories do some off-line data process to make EDS line profiles smooth. Some artifacts in these processed EDS line profiles may mislead engineers in process tuning, as plots shown below. Region I in EDS intensity line profiles, Figure (a), is a region with no physical specimen. But, when the y axis is transferred to at% as shown in Figure (b), it looks region I is a region consisting of N and O with large variation. Both 7/10 and 700/1000 are identical mathematically, but significantly different in physics (or statistically). It is meaningless to put region I into EDS at% line profiles or EDS wt% line profiles calculation physically, except to result in misleading information. Those fuzzy elemental curves, especially N and O in region I, in Figure (b) can be improved by data smoothing process. They are changed to Figure (c) and Figure (d) by 3-points and 5-points smoothing respectively. Those elemental curves look better compared with them in Figure (b). This will mislead people who do not know the history of these data in detail, and even guide process engineers to tune processes in wrong directions.

圖C2-24 EDS line profiles. (a) EDS intensity line profiles, (b) EDS at% line profiles, (c) EDS at% line profiles, 3-points smoothing, (c) EDS at% line profiles, 5-points smoothing.

除了引起前述的錯誤外，數據平滑處理對於只有幾奈米厚度薄膜的濃度也頗有影響。從圖(b)量得的鉭/氮化鉭薄膜約為9.2奈米，最高濃度約為71.4 at%；經三點平滑處理後，鉭訊號曲線從非對稱變成高斯對稱，半高寬為8.3奈米，最高濃度降為58.4 at%；經五點平滑處理後，鉭訊號曲線也是成高斯對稱，半高寬為10奈米，最高濃度降為48.6 at%。而IV區的銅的最高濃度始終保持在77 at%左右，平均濃度也保持在70 ~ 75 at% 之間。顯見平滑處理對於大尺寸的物體，元素曲線確實可以變得較好看，真實濃度也變化不大。但是對奈米尺寸的物體，其組成的濃度和濃度分佈都會明顯失真。

Besides errors induced as stated before, smoothing process will affect the concentration of thin films of several nano meters in thickness. The thickness of Ta/TaN is about 9.2 nm, maximum Ta concentration is 71.4 at% in Figure (b). The distribution of Ta becomes Gaussian symmetry after 3-points smoothing, the full width of half maximum is about 8.3 nm, and its maximum concentration drops to 58.4 at%. The maximum Ta concentration becomes to 48.6 at% and the FWHM is about 10 nm if a 5-points smoothing is processed. Obviously, smoothing EDS line profiles is good for objects of large scale but may cause misleading errors in objects of several nano meters. 

C-3 電子能量損失能譜(EELS) – 同素異形體與試片厚度效應

C-3-6 成份映像 – 同素異形體與試片厚度效應

C-3-4節中，圖C-41顯示EELS能譜可以區分矽的矽元素和二氧化矽的矽元素，二者L邊刃的起始能量和近邊刃微細結構明顯不同。在半導體元件的顯微結構中有多處純矽與二氧化矽相鄰的結構，EELS的成份映像技術是否可以區分它們? 圖C-46顯示純元素矽和二氧化矽的矽的分佈在EELS成份映像是可以分離的，只要三個攝像的能窗位置和寬度設置適當。

Figure C-41 in paragraph C-3-4 shows that the Si L_2,3 edges of element Si and the oxidated Si can be clearly distinguished in EELS spectra. They are different in both threshold energy and near edge fine structure. There are many sites where Si and SiO₂ are next to each other in semiconductor devices. Can they be distinguished by EELS mapping? Figure C-46 states that the distribution of Si/Si and Si/SiO₂ can be mapped separately by adequately setting those three energy windows.

圖C-46 矽的EELS成分映像圖。(a)明場像；(b)氧元素成份映像圖；(c)單晶矽和多晶矽的矽元素成份映像圖；(d)二氧化矽的矽元素成份映像圖。Ref[1]

做EELS分析的TEM試片相對上要偏薄，盡量避免入射電子產生多重散射，造成近邊刃微細結構失真。多重散射也會影響EELS成分映像圖的品質，如圖C-47所示。圖C-47(b)明場像中的鎢栓中間的縫清晰可見，而圖C-47(a)明場中的鎢栓中間的縫則只隱約可見，經驗上得知，對應圖C-47(a)的試片比對應圖C-47(b)的試片厚。因此在鈦元素成份映像圖中，薄試片(圖C-47(d))中的TiN層和Ti層的對比清晰許多。

The thickness of TEM specimen for EELS analysis needs to be thin enough to avoid multi scattering for incident electrons. Multi scattering will smear out near edge fine structure of characteristic edges of elements. Quality of EELS elemental maps will be decreased too when multi scattering occurs, as shown in Figure C-47. The seam in Figure C-47(a) is not as clear as that in Figure C-47(b). This indicates that the specimen thickness of Figure C-47(b) is thinner than that of Figure C-47(a) by experience. The contrast between the Ti layer and the TiN layer is higher in the elemental map of the thin specimen (Figure C-47(d)) than those in the thick specimen (Figure C-47(c)).

圖C-47 EELS成分映像圖的試片厚度效應。(a)厚試片的明場像；(b)薄試片的明場像；(c)厚試片的鈦元素成份映像圖；(d)薄試片的鈦元素成份映像圖。

參考文獻

1] J. S. Bow, W. T. Chang, Y. M. Tsou, H. S. Chou, and C. Chiou, Proc. ISTFA, 101-105 (2002).

 

C-3 電子能量損失能譜(EELS) - 成份映像

 C-3-5 成份映像

成份映像圖顯示某特定區域內組成元素的二維分佈情形。在TEM/STEM的分析技術中，主要獲取成份映像圖的技術有EDS成份映像圖和EELS成份映像圖二種。EDS能譜中，元素能峰和背景的強度相差甚多，亦即P/B很高，所以在EDS元素映像圖中，只要設定包含能峰的適當能窗，即使不扣除背景，就能清楚地顯示某特定元素的分布。但是在EELS能譜中，元素特性邊刃座落在一高強度的背景上，如果只設定單一能窗，則獲取的成份映像圖中可能有超過一半的訊號是背景訊號，而不是真正的元素訊號，如圖C-44的邊刃後能窗(能窗III)所蘊含的訊號。所以獲取EELS成份映像圖，除了設定包含特性邊刃的邊刃後能窗外，必須同時設定二個邊刃前的能窗，如圖C-44中的能窗I和能窗II。由二個邊刃前能窗的訊號，推算出邊刃後能窗的訊號中的背景訊號(能窗B)，扣除後，才是真正的元素訊號(能窗IV)。

Elemental maps display two dimensional distributions of elements in local interested regions. EDS mapping and EELS mapping are two main analysis techniques in TEM/STEM systems. Peaks of elements, especially major elements, in EDS spectra have high P/B ratio, elemental maps show the distributions of elements clearly once proper energy windows are set, with or without background subtraction. On the contrary, all characteristic edges fall on high background in EELS spectra, the map obtained from an energy window including part of characteristic edge will include background noise as well as true element signal, as shown in the post edge window (window III) in Figure C-44. It is necessary to set two more pre-edge windows to calculate out the background (window B) in the post edge window to obtain the true signal (window IV). 

圖C-44 EELS能譜示意圖顯示運算成份映像圖需要設定三能窗，二個邊刃前能窗: 能窗I和能窗II，和一個邊刃後能窗(能窗III)。能窗III包含背景訊號(能窗B)和真正元素訊號(能窗IV)。

圖C-45顯示一組典型TEM模式下拍攝的EELS三能窗影像和演算後的成份映像圖。圖C-45(a)為TEM明場像顯示一分析區域，此區域的組成元素包含Ti, Ni, Zr, Sb等四元素。圖C-45(b)相當於圖44中能窗IV的影像，顯示富鈦相呈一近橫躺的T字形。圖C-45(e)的邊刃後能窗影像相當於圖44中能窗III的影像，和圖C-45(c)和圖C-45(d)相比，雖然隱約顯示富鈦相的區域比周圍他相稍亮，但影像的亮度變化，會被誤認為鈦的分佈幾乎涵蓋整個區域，而且都有相當的濃度。透過二個邊刃前能窗影像算出背景影像(能窗B)，再從能窗III影像中扣除後，才能得到圖C45(b)的鈦成份映像圖。

A typical set of energy-selected images acquired in TEM mode and the final processed elemental map are shown in Figure C-45. Figure 45(a) is a TEM BF image showing an interested region being consisted of Ti, Ni, Zr, and Sb. Figure C-45(b) is the elemental map corresponding the image of energy IV in Figure 44 and indicates that the shape of the Ti-rich phase look like a lying down T. Figure C-45(e) is the Ti post-edge image corresponding to the image of window III in Figure C-44. The region of Ti-rich phase in this image is a little brighter compared with corresponding regions in Figure C-45(c) and Figure C-45(d), but is not distinguishable. A true Ti map, as shown in Figure C-45, is only obtained after background subtracted from the post-edge image.

圖C-45 EELS成分映像圖的演算。(a)明場像；(b)鈦元素成份映像圖；(c) pre-edge 1影像；(d) pre-edge 2影像；(e) post-edge影像。