November_EDFA_Digital

edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 4 “...TEM IS CRUCIAL FOR THE DEVELOPMENT AND PRODUCTION OF ADVANCED SEMICONDUCTOR DEVICES GIVEN THE DECREASING DEVICE SIZE. ” as 20-30 nm [9] and 2.5 nm [7] thick using 30 kV and 5 kV Ga FIB energy, respectively) and ion-implanted layers, which subsequently limits analytical and high-resolution elec- tronmicroscopy. This article presents concentrated, small spot (<1 µm), low energy (<1 kV) Ar + milling as a post-FIB cleaning step for reproducible specimen preparation of advanced devices with specimen thicknesses of less than 20 nm. MATERIALS AND METHODS FIB SPECIMEN PREPARATION A TEM cross section specimen was created from a de- packaged Intel Broadwell M-core processor with 14 nm FinFET structure. The FIBwas operated at 30 kV following the inverted FIB preparation described by Alvis et al. [10] However, the flip stage was omitted and additional steps were added to create a curtain-free specimen. Figure 1 shows themodified inverted FIB preparation necessary to target the FinFET structure. Using this method, removal of the top metal layers is essential because of the differ- ential milling rates of the layers, which are the source of curtaining artifacts. The added steps were performed to maintain the specimen integrity and to remove trimming artifacts that were Ga-rich post-FIBmilling. Subsequently, conventional FIB final thinning steps (30 and 5 kV) were performed. The experimental conditions of the modified inverted FIB preparation are summarized in Table 1. With the FinFET structure identified as the region of interest, low kV imaging (2 kV) of the front and back of Table 1 Experimental conditions for the modified inverted FIB preparation shown in Fig. 1. All steps were performed at 30 kV accelerating voltage in the FIB Figure Description Stage tilt Stage rotation 1a Lamella after bulk milling in typical TEM lamella preparation* 7° 0° 1b 1. Milling of top metal layers 2. Typical J or U cut and attachment to the nano manipulator* 3. Lift out the lamella* 7° 0° 1c 1. Rotate nano manipulator to 180° (not shown) 2. Specimen reattached to the bulk 52° 0° 1d Preparation to orient the lamella with the Si substrate and metal layers at the top and bottom, respectively, by rotating the stage to 180° 52° 180° 1e 1. Reattach the nano manipulator to the lamella 2. Cut the lamella free from the bulk specimen 3. Lift out the lamella and then rotate the nano manipulator to 180° 52° 180° 1f Attach the lamella to the grid* 52° 0° 1g Mill of Si substrate with the trimmings from the bulk milling 52° 0° 1h Carbon or platinum cap deposition 52° 0° *Steps are not described because they are from the manufacturer’s recommended TEM lamella preparation procedure, which is described in the FIB system documentation. the TEM specimen was performed during the final FIB polishing steps. Specimen thickness of 50 to 80 nm was achieved after the 5 kV FIB polishing step. POST FIB ARGON ION MILLING A low-energy (50 eV to 2.0 keV), concentrated argon ion milling system was used for final polishing and thinning of specimens to thickness of less than 20 nm. Similar to the FIB, the system includes a LaB 6 electron source and electron detectors (secondary electron detector [SED] and scanning transmission electron microscope [STEM] detector) that provide in situ imaging during ion milling. The FIB specimens were mounted on a specimen holder compatiblewith the ionmill and TEM, which enabled TEM characterization in between milling steps. Before ionmilling of the FinFET specimen, the number of alternating layers of fin and intermetallic layers was identified by tilting the specimen to +27° and imaging in STEM mode in the TEM. Subsequently, ion milling of the side of the specimen without the fin structure, which was identified during low kV FIB imaging, was performed at