Basha Adham, Levi George, Amrani Tamir, Li Yang, Ankonina Guy, Shekhter Pini, Kornblum Lior, Goldfarb Ilan, Kohn Amit
Department of Materials Science and Engineering, The Iby and Aladar Fleischman Faculty of Engineering, Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel.
Andrew and Erna Viterbi Faculty of Electrical and Computer Engineering, Technion - Israel Institute of Technology, Haifa 3200003, Israel.
Ultramicroscopy. 2022 Oct;240:113570. doi: 10.1016/j.ultramic.2022.113570. Epub 2022 Jun 8.
Quantitative transmission electron microscopy (TEM) often requires accurate knowledge of sample thickness for determining defect density, structure factors, sample dimensions, electron beam and X-ray photons signal broadening. The most common thickness measurement is by Electron Energy Loss Spectroscopy which can be applied effectively to crystalline and amorphous materials. The drawback is that sample thickness is measured in units of Inelastic Mean Free Path (MFP) which depends on the material, the electron energy and the collection angle of the spectrometer. Furthermore, the Elastic MFP is an essential parameter for selecting optimal sample thickness to reduce dynamical scatterings, such as for short-range-order characterization of amorphous materials. Finally, the Inelastic to Elastic MFP ratio can predict the dominant mechanism for radiation damage due to the electron beam. We implement a fast and precise method for the extraction of inelastic and elastic MFP values in technologically important oxide thin films. The method relies on the crystalline Si substrate for calibration. The Inelastic MFP of Si was measured as a function of collection semi-angle (β) by combining Energy-Filtered TEM thickness maps followed by perpendicular cross-sectioning of the sample by Focused-Ion-Beam. For example, we measured a total Inelastic MFP (β∼157 mrad) in Si of 145 ± 10 nm for 200 keV electrons. The MFP of the thin oxide films is determined by their ratio at their interface with Si or SiO. The validity of this method was verified by direct TEM observation of cross-to-cross sectioning of TEM samples. The high precision of this method was enabled mainly by implementing a wedge preparation technique, which provides large sampling areas with uniform thickness. We measured the Elastic and Inelastic Mean Free Paths for 200 keV and 80 keV electrons as a function of collection angle for: SiO (Thermal, CVD), low-κ SiOCH, AlO, TiO, ZnO, TaO and HfO. The measured MFP values were compared to calculations based on models of Wenzel, Malis and Iakoubovskii. These models deviate from measurements by up to 30%, especially for 80 keV electrons. Hence, we propose functional relations for the Elastic MFP and Inelastic MFP in oxides with respect to the mass density and effective atomic number, which reduce deviations by a factor of 2-3. In addition, the effects of sample cooling on the measurements and sample stability are examined.
定量透射电子显微镜（TEM）通常需要准确了解样品厚度，以便确定缺陷密度、结构因子、样品尺寸、电子束和X射线光子信号展宽。最常见的厚度测量方法是电子能量损失谱，它可有效地应用于晶体和非晶材料。缺点是样品厚度以非弹性平均自由程（MFP）为单位进行测量，这取决于材料、电子能量和光谱仪的收集角度。此外，弹性MFP是选择最佳样品厚度以减少动态散射的关键参数，例如用于非晶材料的短程有序表征。最后，非弹性与弹性MFP之比可以预测电子束引起的辐射损伤的主要机制。我们实现了一种快速精确的方法来提取技术上重要的氧化物薄膜中的非弹性和弹性MFP值。该方法依赖于晶体硅衬底进行校准。通过结合能量过滤TEM厚度图，然后用聚焦离子束对样品进行垂直横截面分析，测量了硅的非弹性MFP作为收集半角（β）的函数。例如，对于200 keV电子，我们测量了硅中的总非弹性MFP（β∼157 mrad）为145±10 nm。氧化物薄膜的MFP由其与硅或SiO界面处的比率确定。通过对TEM样品的交叉到横截面的直接TEM观察验证了该方法的有效性。该方法的高精度主要通过采用楔形制备技术实现，该技术提供了具有均匀厚度的大采样区域。我们测量了200 keV和80 keV电子的弹性和非弹性平均自由程作为收集角度的函数，对象包括：SiO（热生长、化学气相沉积）、低κSiOCH、AlO、TiO、ZnO、TaO和HfO。将测量的MFP值与基于Wenzel、Malis和Iakoubovskii模型的计算结果进行了比较。这些模型与测量值的偏差高达30%，特别是对于80 keV电子。因此，我们提出了氧化物中弹性MFP和非弹性MFP相对于质量密度和有效原子序数的函数关系，将偏差降低了2至3倍。此外，还研究了样品冷却对测量和样品稳定性的影响。
