A new practical model of an objective lens with assembling clearance for transmission electron microscope

作者：Li Wen Ping^*; Gui Li Jiang; Wu Jian; Dong Quan Lin; Yao Jun En

来源：Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment , 2011, 631(1): 134-137.

DOI：10.1016/j.nima.2010.10.074

• Assembling clearance
• Second-order finite element method
• Magnetic saturation effect
• Asymmetric magnetic flux leakage

A new objective lens model including magnetic pole pieces, coil windings, magnetic circuit as well as the assembling clearance between the pole piece and magnetic circuit is developed to obtain high simulation precision. The calculation is based on the second-order finite element method (SOFEM) with the measured B-H magnetization curves of the lens materials. The magnetic pole pieces and magnetic circuit are firstly optimized to reduce the lens saturation and obtain minimum spherical aberration coefficient, then modified to release the magnetic flux leakage caused by the increased clearance. In the end, an example is given fora 200 kVTEM with the point resolution of 0.25 nm and off-axis aberration coefficients at the image plane are calculated for dynamic correction. Results show that the magnetic circuit is unsaturated and the saturated area is only 0.8 x 4.8 mm(2) around the lower pole piece with a maximum magnetic flux density of 2.537 T. This model can reduce the magnetic flux leakage and obtain the point resolution with smaller excitation. The calculation also shows that the off-axis aberrations require correcting dynamically to acquire fine image quality under low system magnification.

• 出版日期2011-3-1
• 单位北京航空航天大学