2 keV Ga+ FIB Milling for Reducing Amorphous Damage in Silicon

Abstract

Focused ion beam (FIB) specimen preparation techniques have been successfully used for nearly all types of (scanning) transmission electron microscopy (S)TEM methods. However, the milling process from high energy Ga + FIB columns (e.g. 30 keV) can impart sufficient surface damage (i.e., ~ 20 nm per specimen side for Si) to impede quantitative high resolution STEM and TEM imaging. Previous results have shown that amorphization damage in Si and GaN can be removed by chemical wet polishing after FIB milling [1,2]. However, chemical polishing methods are material dependent and are difficult or impossible to use for complex multi-phase or multilayered specimens. Previous results have shown that better high resolution TEM images can be obtained by FIB polishing a 30 keV FIB prepared specimen with a 10 keV ion beam [1,3]. Low energy broad beam Ar + ion milling has also been used to reduce FIB damage for HRTEM [1,4]. In this paper, we discuss specimen surface quality improvements using a FIB column whose ion beam energy can be reduced to 2 keV, resulting in the lowest amount of FIB damage reported to date.A blanket wafer Si substrate specimen was prepared for TEM analysis using conventional 30 keV FIB milling via the in-situ lift-out and Omniprobe total release technique [5] using an FEI Strata 400S DualBeam instrument equipped with a Sidewinder FIB column. Lines were FIB milled into the specimen using beam energies of 30 keV, 5 keV, and 2 keV, and were used to assess the sidewall damage at these energies. Then the surface of the specimen was FIB polished with the Ga + ion beam at +/-85 o incident angle at 5 keV and then at 2 keV. The sidewall damage and surface quality of the specimen was observed using HRTEM imaging with an FEI Tecnai F20 equipped with a spherical aberration coefficient (Cs) image corrector operating at 200 keV. The specimen was plasma cleaned using a Fischione plasma cleaner after each FIB milling procedure prior to TEM observation. Ion-solid interactions predict that lighter incident ions will produce more surface damage than heavier incident ions [see chap. 2 in ref. 6]. This implies that Ga + ions will yield less specimen damage compared to similar energy beams from conventional Ar + ion mills, and thus, may negate the need for broad beam Ar + ion milling of FIB prepared specimens.

Trench formation and lateral damage induced by gallium milling of silicon

“…Although the majority of the energy for 30-keV gallium is being deposited below the sample's surface, the extensive lateral damage leads to an ejection mechanism in which material is removed from the near surface region throughout the entire sample. From this information it appears that using lower energies allows for a cleaner milling process and is a more efficient use of the incident energy [19,20].…”

Analytical Transmission Electron Microscopy and Scanning Transmission Electron Microscopy Techniques for the Characterization of Nanomaterial Composition, Phase and Crystallinity

“…16 The ion-milling process can be monitored and controlled with the SEM, which provides a better precision of sputtering the target material. This is especially important when the sputtering rate by the electrons is very low, for example, during the final milling process of thin TEM foils.…”

Resolving the CaP-bone interface

“…34 Experience, a carefully tuned instrument and, more recently, the use of low energy ion cleaning steps may help reduce the likelihood of artifact occurrences. 37,38 Although not used for the study of bioceramics, Giannuzzi et al have demonstrated the ability to use FIB sequential milling and imaging to produce three dimensional reconstructions of biomaterial-bone interfaces. 39 Such three dimensional interfacial information was useful for determining the extent of bone ingrowth into implanted titanium samples, and could easily be applied to the bioceramic interface in much the same way.…”