样品倾转角度对透射电镜表征纳米薄膜的影响

Effect of Sample Tilting Angle on the Characterization of Nanofilms by Transmission Electron Microscopy

以沉积在Si[100]基底的Mo/Si多层膜为例,通过透射电镜(TEM)测量了多层膜在不同倾转角度下的界面结构,并提取了多层膜的周期厚度以及单周期中Mo层和Si层的厚度。结果表明:样品沿α方向倾转时,Mo层和Si层的测量厚度几乎没有变化,但界面粗糙度增大,这是由于旋转时薄膜的厚度方向始终与电子束垂直,而电子束穿过的TEM样品厚度Z增大;样品沿β方向倾转时,由于倾转时样品截面与电子束不垂直,造成伪影严重,无法区分Mo层和Si层,多层膜的测量总厚度随倾转角的增大先增大后减小。此外,提出了样品沿β方向倾转后测量薄膜厚度的计算公式。对于较薄的薄膜,随着倾转角β的增大,测量厚度增大;对于较厚的薄膜,随着倾转角β的增大,测量厚度先增大后减小。薄膜厚度t0越小,沿β方向倾转后测量厚度的相对误差越大。当TEM样品厚度Z为10 nm时,沿β方向倾转后测量厚度的相对误差较小。

Objective Accurate characterization of Mo/Si multilayer film thickness is important for process iteration and analysis.As one of the visualization methods,transmission electron microscopy(TEM)can characterize the thickness of nanofilm deposited on a single crystal Si wafer.It can be calibrated internally through the Si substrate lattice parameters,which is very accurate.However,if we do not pay attention to the crystal orientation of the Si substrate during TEM characterization or we use amorphous substrate materials such as fused quartz,it is difficult to ensure that the cross-section of the sample is exactly perpendicular to the electron beam.As a result,the two-dimensional projection imaging of threedimensional samples produces artifacts,resulting in unknown measurement errors.Therefore,it is of great significance to study the influence of sample tilting angle on the TEM characterization of nanofilms.Methods Mo/Si multilayer films are deposited by pulsed direct current sputtering.Cross-section samples for TEM characterization are prepared by ion milling.TEM images and high-resolution TEM images of the multilayer films are obtained by TEM.The TEM cross-section samples are tilted inαandβdirections by a double tilting holder.Combined with the profile curves of the images,we obtain the thickness of the multilayer film at different tilting angles,the roughness of the interface,and the thickness of the Mo and Si layers in a single period.Results and Discussions As the Mo/Si multilayer film sample tilting in theαdirection,the thickness direction of the film is always perpendicular to the electron beam direction(Z axis),so the thickness does not change.The roughness increases,because the thickness Z of the TEM sample which the electron beam passes increases as tilling in theαdirection.It implies more projective superposition at the interface layer(Fig.4).As tilting in theβdirection,the sample cross-section is not perpendicular to the electron beam direction(Z axis),resulting in artifacts during projection imaging and a large deviation(Fig.7).A formula for measuring the thickness of thin films after the sample tilting in theβdirection is proposed.For thin films,the measured thickness increases with the increase of the tilting angleβ.For thicker films,the measured thickness first increases and then decreases with the increase of tilting angleβ.A thinner film thickness t0 causes a greater relative error of the measured film thickness after tilting in theβdirection(Fig.8).Conclusions As the sample tilting in theαdirection,the measured thickness of the Mo and Si layers is almost unchanged while the interface roughness increases.This is because the thickness direction of the film is always perpendicular to the electron beam during rotation,and the thickness Z of the TEM sample which the electron beam through increases.The artifacts caused by the sample cross-section are not perpendicular to the electron beam during tilting,which is too severe to distinguish the Mo layer and the Si layer.The measured total thickness of the multilayer film first increases and then decreases with the increasing tilting angle.The formula for calculating the thickness of the film after the sample tilting in theβdirection is presented.For thin films,the measured thickness increases with the increasing tilting angle.For thicker films,the measured thickness first increases and then decreases with the increasing tilting angle.As the film thickness t0 becomes thinner,the relative error is greater after tilting in theβdirection.When the TEM sample thickness Z is 10 nm,the relative error of measuring thickness is small after tilting in theβdirection.Therefore,when characterizing the structure and thickness of nanofilm by TEM,Si wafers should be cut in a specific direction[1-10]from the beginning of sample preparation.Then samples should be observed from the crystal band axis[110].Only in this way,it can ensure that the cross sections of Si wafers and films are exactly perpendicular to the electron beam.Photograph and analysis in the thin area of the TEM sample show that the result obtained by this method is more accurate.