TEM和STEM高分辨影像
目前許多200KV以上的TEM都具有優於0.2奈米的儀器解析度。所以在TEM機況調整非常良好和搭配適當的試片條件下，TEM倍率超過100KX後，可看到晶體的晶格影像，這類TEM影像一般稱為高分辨影像，影像對比機構為相對比。這裡的相和前面提到的相不同，前面提到的相都是材料學的相，指的是一均勻組成的固態物質。相對比的相指的是電磁波相位的相。將電子束視為電子波，通過試片後，透射電子束和繞射電子束到成像面的路徑不同，產生波程差，造成相位差，彼此互相干涉(interfere)的結果是，同相位的波干涉產生建設性的干涉，形成亮點；反相位的波干涉產生破壞性的干涉形成暗點，最後形成一組明暗點重複交錯排列的影像，稱之為高分辨(HRTEM)影像。HRTEM影像是分析材料界面和奈米級界面反應的理想分析技術，尤其是對有磊晶關係的材料系統的界面更理想。圖B-33是一典型的例子，用分子束磊晶(MBE)法在6H碳化矽單晶基板長上一層鈦磊晶層，高溫反應後，在界面生出碳化鈦和矽化鈦二層新相。
Most of current TEM operated at 200 KV or higher has instrument resolution better than 0.2 nm. When the TEM is well aligned and a suitable crystal is obtained, lattice images are visible as TEM magnification is over 100 KX. We usually call this kind of images to be high resolution TEM (HRTEM) images. The contrast mechanism of HRTEM images is phase contrast. The “phase” is different from those phases mentioned in previous paragraphs, which mean homogenous solid matters. Here, the “phase” in “phase contrast” is the phase of electromagnetic waves. Electron beams are treated be electron waves. The transmitted beam and diffracted beams travel along different path to reach the first image plane after exiting the specimen. This results in phase shifts between the transmitted beam and diffracted beams. They interfere with each other at the first image plane, and bright spots appear at where constructive interference occurs, while dark spots appear at where destructive interference does. An image of an array of spots is formed, and we call this kind of image to be a high resolution TEM (HRTEM) image. HRTEM imaging is an idea analytic technique to explore the interface structure and interfacial reaction in nano scale, especially for material systems with epitaxial relationship. Fig.B-33 shows a typical example, the HRTEM image shows an interface structure of high temperature annealed Ti/6H-SiC with two interfacial reactants, TiC and Ti5Si3.
圖B-33. 6H-SiC\TiC\Ti5Si3\Ti的HRTEM影像。
HRTEM影像是電子波的干涉影像，是為相干(coherent)影像。影像中的黑點和白點，和晶體內的原子位置並非唯一對應，會隨試片厚度變化與TEM欠焦值變化而改變，如圖B-34所示。現代的TEM都配備CCD數位影像機，可做即時性的傅立葉轉換，判斷當下的聚焦條件。最正確的拍攝聚焦條件為謝爾策欠焦(Scherzer defocus)，此時黑點代表原子位置。過多的欠焦值會造成HRTEM影像產生偏移的現象[1]，此時晶格影像仍然清晰可見，但是偏離其正確的位置。半導體業界經常使用Si/SiO2/poly HRTEM影像量測閘極氧化層(GOX)的厚度，過多的離焦值會造成矽基板的矽原子延伸入二氧化矽層內，導致最後量測的閘極二氧化矽層厚度變小。
HRTEM images are images of interference of electron waves, is a kind of coherent image. The corresponding relationship between black and white spots with atoms is not unique, it changes with specimen thickness and TEM defocus values, as shown in Fig. B-34. Now, almost all TEM used are equipped with CCD digital cameras, live fast Fourier transfer (FFT) is available to judge the defocus condition. The optimum defocus condition for HRTEM images is Scherzer defocus, black spots in these HRTEM images are corresponding to atom positions. If too large defocus is used, image delocalization occurs, and features in the image displace from their true positions [1]. HRTEM images of Si/SiO2/poly are often used to measure the thickness of the gate oxide in semiconductor industry. The measured thickness of gate oxide (GOX) will be thinner than its true value when delocalization exists in the HRTEM image.
圖B-34.鋁在[1 1 0]晶軸方向的HRTEM影像模擬地圖，TEM操作電壓= 800 KV。(Ref: M. A. O'Keefe, in Image Calculation Techniques, handout for ASU Winter School (1993)).
相對於HRTEM影像，HRSTEM影像是用聚焦的電子束掃描試片而得，是為不相干(noncoherent)影像。影像中的白點恆為晶體的原子位置，白點的亮度隨該晶格點的平均原子序增大而增強，如圖B-35所示，InGaN層最亮，GaN次之，AlGaN最暗。
Instead of formation by the interference of electron waves, HRSTEM images are formed by focused electron beams and are non-coherent images. White spots in HRSTEM images are atom positions in the crystal. The intensity of white spots increases with the average of atomic number of the corresponding lattice points, as shown in Fig. B-35, l InGaN layers have the highest intensity, then GaN layers, and AlGaN layers have the lowest intensity.
圖B-35. GaN1\AlGaN\GaN2\InGaN量子阱結構的HRSTEM影像。極軸方向= [1 1 -2 0]。
1] David B. Williams and C. Barry Carter, “Transmission Electron Microscopy, Microscopy”, 2nd edition, Plenum Press, New York (2009)
沒有留言:
張貼留言