Nov 2024_EDFA_Digital

edfas.org 31 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 Fig. 2 STEM and STEM EBIC images of the Fig. 1 device without (a) and with (b) bias applied, along with a current-voltage plot measured from the device. For the EBIC images in (b), the indicated bias is applied to the TE and the zero-bias image (middle image in (a)) has been subtracted. Current values in the plot are measured from EBIC images, acquired at each bias value, by measuring the average signal in the first few lines of each scan during which the electron beam is blanked (not shown). (a) (b) the need for beam-deposited metal, preventing leakage between substrate electrodes. Metal-insulator-metal cross sections prepared by this method exhibit very low leakage, and SEEBIC conductivity mapping confirms continuity between device terminals and the substrate electrodes (precluding low leakage being the result of poor contact). The most common FIB ion is Ga+, which contaminates the milled surface of the sample with a layer of conducting Ga. In the technique described above, careful low-energy cleaning with the Ga+ beam sufficiently reduces Ga surface leakage. Plasma FIB (PFIB) systems that make use of inert ions, such as Xe+, are now commercially available and can eliminate this source of conductive contamination. In the method described below, PFIB preparation of low-leakage samples can be routinely achieved via (mostly) standard cross-section procedures. PFIB PREPARATION OF SAMPLES FOR STEM EBIC This section describes preparation of cross-section devices for STEM EBIC imaging, as well as in situ biasing, using a Xe+ PFIB. Samples are prepared using a Thermo Fisher Scientific Helios G4 PFIB UXe DualBeam FIB/SEM. The process generally begins with standard cross-section milling and lamella extraction via a probe in the FIB/SEM chamber. The lamella is then mounted to a silicon substrate patterned with electrodes surrounding a trench (see Hubbard[2]) by welding either side of the lamella to an electrode via ion beamdeposited Pt. Selective cuts are made in the mounted lamella to pre- vent shorting between device terminals, and all surfaces between the terminals are cleaned via gentle Xe+ beam milling (12 kV, 100 pA). All STEM EBIC images are acquired with a two-channel STEM EBIC system from NanoElectronic Imaging. The device in Fig. 1 is a simple Si/SiO2/Pt capacitor created in the PFIB by electron beam deposition of Pt, followed by ion beam deposition of a thick W cap, on a highly doped Si wafer coated with 90 nm of SiO2. After mounting, cuts are made in the lamella up through the substrate, and down through metal layers, to isolate the top electrode (TE) and substrate contacts (Fig. 1 diagram). The annular dark field (ADF) STEM and STEM EBIC images in Fig. 1 show the device at low magnification. EBIC is measured independently from both the top electrode and Si substrate, and with no intrinsic or applied electric field SEEBIC is the dominant EBIC mode. For each EBIC channel, positive current is measured on the regions to which each EBIC amplifier is connected, here indicating good contact between each device terminal and the substrate electrodes. The bright contrast in each channel ends sharply at the dielectric, indicating good isolation across the insulator. The region around the dielectric layer is shown at higher magnification in Fig. 2a for the same two EBIC channels. In each channel, the bright contrast is largely uniform starting from one end of the image until it reaches the dielectric, at which point the signal gradually decreases across the dielectric until the (dark) opposing electrode. This SEEBIC resistance contrast indicates that most of

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