基于电子束剪切干涉的PIE成像技术研究

提出了基于M?llenstedt电子双棱镜的电压扫描剪切干涉全场ptychographic iterative engine(PIE)显微成像技术.从低到高逐步改变电子双棱镜的电压,并同时记录所形成的剪切干涉条纹,待测样品透射电子束的强度和相位分布就可以用PIE算法得以快速重建,而且双棱镜的方向、位置和实际电场强度分布等诸多实验中不可避免地偏差都可以在迭代过程中自动得以更正.所提技术能够克服现阶段用电子束进行PIE成像的诸多技术困扰,从而有望推动PIE技术在电子显微成像领域的发展和应用.

Shearing interferometric electron beam imaging based on ptychographic iterative engine method

Ptychographic iterative engine (PIE) method can provide high-resolution amplitude and phase distributions in short-wavelength imaging,such as electron beam and X-ray imaging.Traditional PIE relies on the sub field of view (sub-FoV) scanning,and the coincidence between these adjacent sub-FoVs is required in order to ensure the high accuracy in sample information retrieval.However,in the applications of electron beam imaging,attachments or contaminants on the sample surface will be dragged with the probe light during the sub-FoV scanning due to the adsorption of charges,and the inevitable attachment and contaminant shifting will change the probe light,therefore generating inconsistent probe light,and reducing the imaging resolution and accuracy,since the deteriorated probe light destroys the PIE scanning demands.In order to maintain the high resolution and accuracy in the electron beam imaging,the attachment and contaminant shifting during the sub-FoV scanning should be avoided.Here,a shearing interference based PIE using Möllenstedt biprism is proposed in this paper.Möllenstedt biprism is widely used in the electron beam imaging,and by applying the voltage to the wire,the generated electrical field can control the deflection of the electron beam,working similarly to a biprism modulating the wavefront passing through it.In the proposed approach,setting the Möllenstedt biprism after the sample,and changing the voltage on the Möllenstedt biprism,the beam deflection angle proportional to the added voltage can generate a series of interferograms with different fringe densities.Because the traditional sub-FoV scanning is replaced by wide-field scanning by changing the voltage on the Möllenstedt biprism,the proposed method can maintain the stable probe light,avoiding the inevitable attachment and contaminant shifting,and both the amplitude and phase can be retrieved from these interferograms by using a modified PIE algorithm.In order to verify the proposed PIE method,besides the theoretical analysis,numerical calculations are provided.The biprism phase distribution is adopted to simulate the electron beam deflection caused by the Möllenstedt biprism.Additionally,by changing the voltage on the wire,different biprism phase distributions are generated to produce various interferograms.By the modified PIE method,accurate amplitude and phase distribution within error less than 0.2% can be obtained through using less than 50 iterations,indicating a rapid convergence rate.Moreover,the errors in the imaging system, such as phase deviation,position shifting,and rotation are also quantitatively analyzed.Numerical computation proves that the direction of the biprism can be precisely determined according to the frequency distribution of the fringe,and the accurate sample information can still be retrieved even with a deviation of 30% in phase deviation and 30 μm in position shifting,proving the deviations of the direction and position of the Möllenstedt biprism,as well as the phase distribution can be corrected automatically in the iterative process.Finally,the modified PIE relying on the lensfree configuration can reach the resolution of the diffraction limit in imaging similar to those PIE approaches.The proposed technique can overcome difficulties of current PIE in using electron beam,thus promoting the development and application of PIE in electron microscopy.