Characterization and simulation of electron/ion beam damage on soft materials in FIB-SEM microscopes

Abstract

The versatility and effectiveness of focused ion beam – scanning electron microscopy (FIB-SEM) instrumentation has advanced the field of electron microscopy, with such applications as high precision ultrathin site-specific TEM sample preparation, direct-writing lithography in semiconductor field, and FIB tomography for complex biological structures. However, the beam damage, including the beam induced heating, knock-on damage, and radiolysis damage from electron beam fundamentally alters the chemistry and bonding in sample materials. Although experience-based strategies, such as lowering the beam voltage and current or optimized milling strategies, have been developed to mitigate the damage, the lack of fundamental understanding of the underlying damage mechanisms inhibit the development of FIB-SEM strategies for imaging/milling new material systems. Previous work has shown that the main damage mechanisms for soft materials system are beam heating and electron radiolysis damage. In this research, I study the electron beam damage on the EMbed 812 epoxy resin in a systematic way by studying the relationship of the degree of damage and the common beam parameters, including beam voltage, current, and total irradiated electron dose. All the damaged samples were characterized by scanning transmission X-ray microscopy (STXM) which is a unique and suitable tool for this research. Our results show that the electron damage is a main cause for the chemical alteration in a plasma-FIB lift-out resin sample. Since there is no direct way to assess the beam induced heating problem during the milling process, we developed an effective heat transfer model by COMSOL for predicting the temperature rise during the FIB milling process. In this thesis, I included a detailed study by FIB-SEM, STXM, simulation work of polyvinylidene fluoride (PVDF), poly(methyl methacrylate) (PMMA) and the Embed812 epoxy resin.