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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 4 6 decreasing energies—from 700 to 500 eV at 10°. Table 2 summarizes the post-FIB cleaning steps performed by Ar + milling. Iterative milling and TEM imaging were initially implemented at 700 eV to determine the layers removed, i.e., quantification of milling rates per layer. ELECTRON MICROSCOPY IMAGING AND ANALYSIS TEM and STEM imaging were performed using a TEM operated at 300 kV. The thickness of the specimens was determined through energy-filtered TEM (EFTEM) imaging using electron energy loss spectroscopy (EELS) with the spectrometer attached to the TEM. Atomic resolution STEM imageswere acquiredusing anaberration-corrected TEM (from JEOL USA) operated at 200 kV. The elemental distribution of the specimen was verified using energy dispersive x-ray spectroscopy (EDS) with the spectrometer attached to the aberration-corrected TEM microscope. RESULTS AND DISCUSSION DETERMINATION OF MILLING RATES Figure 2 shows the STEM images of the specimen before and after Ar + milling. The observed metal (M) and fin (F) layers before milling, specifically metal/fin/metal/ fin/metal/fin (labeled as M/F/M/F/M/F in Fig. 2a), were used to estimate the initial and final specimen thickness from which the milling rates were derived. The F layer comprised the gate oxide over the fin, which is 20 nm (based on the reported gate length for FinFET structures in the 14 nm node [12] ). The repetitive layers of F/M/F are two gate oxide layers with an intermetallic in between. This is identified as the gate pitch and is measured as 70 nm for the 14 nm node FinFETs. [12] Given these values, the initial specimen thickness is approximately 140 nm, based on the observed M/F/M/F/M/F in Fig. 2a. The removal of the metal (M) and fin (F) layers after the 700 eV step was observed with M/F/M/F remaining (Fig. 2b). Similarly, milling from the same side of the specimenat500eVresultedintheremovalofoneMandone F layer (Fig. 2c). With the observed single F and multiple M layers remaining after Ar + milling (Fig. 2c), the specimen thickness can be estimated to be equivalent to a single gate length of about 20 nm. The specimen thickness was later quantified using EELS measurements. Table 2 Ar + milling parameters of the FinFET specimen in Fig. 2 Parameter Step 1 (Fig. 2b) Step 2 (Fig. 2c) Step 3 (not shown) Energy [eV] 700 500 500 Beam current [pA] 100 100 100 Raster area [µm 2 ] 10 x 10 10 x 10 10 x 10 Specimen angle [°] +10 +10 -8 Milling time [mins] 15 20 5 Fig. 2 Dark field STEM images of the specimen tilted +27° before Ar + milling (a), after 700 eV Ar + milling (b), and after 500 eV Ar + milling (c). Specimen layers are identified asmetal (M) and fin (F). Figure reproduced from Reference 11. (a) (b) (c) (continued on page 8)