C-2 X-光能量散佈能譜- 7A/7 積碳與輻射損傷(A)-積碳

 C-2-6 積碳 (Carbon contamination) 

電子顯微鏡分析中，試片進行分析的局部區域表面很容易沈積一層以碳為主的物質，此現象稱之為積碳。積碳在目前SEM和TEM使用的真空系統中是必然的現象。聚焦的電子束又比平行電子束更容易造成積碳。目前的TEM/EDS能譜攝取都在STEM模式進行，攝取的時間又比影像攝取的時間長多數十倍，因此STEM/EDS分析後必然造成明顯的積碳，嚴重影響後續的TEM分析，所以EDS分析通常放在TEM分析項目的最後。

Carbon contamination, the surface of local region analyzed being prone to deposition of a layer of carbon contained material, is common during analysis in electron microscopes. It is inevitable for carbon contamination in SEMs and TEMs, considering the current vacuum systems used. Focal electron beams make carbon contamination more serious than parallel electron beam do. Unfortunately, all current TEM/EDS analyses are performed in STEM mode and the acquisition times for EDS analysis are much longer than those used for imaging. There is always an obvious carbon contaminated region after EDS analysis and it will cause some troubles in following TEM analysis. So, EDS analysis is usually the last item to be analyzed.    

高能電子打斷碳氫氧之間的鍵結後，氣態的氫分子和氧分子被真空系統抽走；而碳原子被電子撞擊黏附在試片表面，在電子束照射下，試片的局部區域會被加熱，黏附在試片表面的碳原子獲得足夠動能沿溫度梯度線移動，最後沉積在電子束邊緣冷熱交界處。當電子束直徑大於100奈米時，積碳區形成一甜甜圈的形狀；當電子束直徑小於50奈米時，積碳區趨向形成一錐桶的形狀。如圖C-28和圖C-29所示。在成份映像圖的攝取中，映像圖左側邊緣是積碳最嚴重的地方，而且積碳基本上在試片二面有，如圖C-30所示。

Chemical bonds of C-H-O are broken by high energy electrons under the illumination of high energy electron. Molecules of hydrogen and oxygen are pumped, while C atoms are adhered on the surface of the specimen. Carbon atoms receive energy from electron bombardment, move along the traces of temperature gradient, and stop at the boundaries of the electron beam. When the beam size is large (> 100 nm), the shape of carbon contamination is like a donut. When the beam size is small (< 50 nm), the shape of carbon contamination is cone like, as shown in Figure C-28 and Figure C-29. The left side of the elemental map is where most serious carbon contamination occurs during mapping, as shown in Figure C-30. Carbon contamination happens at both surfaces of the TEM specimen.

圖C-28 甜甜圈形貌的積碳點。(a)TEM 明場像；(b)TEM EELS 碳成分影像，影像中的雙箭頭的長度代表187奈米。

圖C-29 錐桶形貌的積碳點。(a) STEM 明場像，影像中的黑色雙箭頭的長度代表69奈米；(b) SEM 二次電子影像，圖中白色物體為積碳形成的錐桶。

圖C-30 STEM/EDS成份映像圖攝取後的TEM試片積碳情形。(a) TEM明場像，黃色虛線矩形內明顯暗色區域是成份映像圖左側的積碳情形，紅色虛線矩形內略微暗色區域是成份映像圖其他區域的積碳情形。(b)SEM二次電子影像，顯示TEM試片上方(高能電子入射面)積碳情形，白色箭頭指處，是成份映像圖左側位置。(c)SEM二次電子影像，顯示TEM試片下方積碳情形，白色箭頭指處，是成份映像圖左側位置。

目前大多數的TEM工程師都習慣用電漿清洗的方式降低TEM試片的積碳速率。基本上，電漿清洗是例行TEM分析工作中降低積碳速率的好方法。如果積碳是整個TEM分析工作極重要的問題，最佳減少積碳的方法應該是使用低溫試片座(cold stage)^[1]。圖C-31顯示一典型的Gatan低溫試片座。筆者在2003年在聯華電子工作時，曾經在FEI Tecnai F20 TEM證實使用一般試片座時，將電子束聚焦在矽晶片30秒鐘，就產生明顯的黑點；相同條件下，使用低溫試片座，將電子束聚焦在矽晶片4分鐘，仍然看不到任何黑點。低溫試片座使用液態氮冷卻試片，試片表面的碳原子在液態氮冷卻下完全被凍在原位，因此不會有積碳問題。台灣半導體業界，TEM工程師不用低溫試片座的原因在於耗時，倒入液態氮後要等約90分鐘，試片座和試片之間才會達到溫度平衡，使用後移出試片座之前又要加溫約40分鐘，使試片回到室溫。一件原來只要一小時的STEM/EDS分析工作，卻要花費二小時以上的等待，顯然浪費時間。一般而言，台灣半導體業界的TEM工程師無法領略在先進TEM分析工作中，靜候至平衡穩定的價值。

Now, most of TEM engineers are used to employ plasma cleaning to knock out dangling bonds of surface atoms to reduce the carbon deposition rate. It is a good method in routine TEM jobs. The best way to prevent carbon contamination is to use a cold stage^[1] to replace the regular specimen stage when carbon contamination is seriously concerned. A typical Gatan cold state is shown in Figure C-31. I had tested it in a FEI Tecnai F20 TEM when I worked in UMC TEM Lab. in 2003. A black spot was clearly observed after focusing the electron beam on the silicon substrate for 30 seconds with a regular specimen stage. There was no mark observed after focusing the electron beam on the silicon substrate for 4 minutes when a cold stage was used. The liquid nitrogen temperature freezes all carbon atoms on the specimen surfaces, so no carbon atom is able to move and stack locally. The reason why semiconductor TEM engineers do not like to use cold stages to prevent carbon contamination is the problem of time consuming. It takes about one and half hours to reach temperature equilibrium between the holder and the specimen after pouring liquid nitrogen into the cold stage. It will take another 40 minutes to warm up the specimen to room temperature before removing the cold stage out of the microscope. When semiconductor TEM engineers think that they can finish a STEM/EDS analysis in one hour but need to wait for extra two hours, they do not use cold stages. TEM engineers in Taiwan semiconductor industry usually do not catch the value of waiting for stable condition in advanced TEM analysis.

圖C-31 Gatan TEM低溫試片座(Gatan cold stage)，右端圓槽用於盛裝液態氮。