edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO.4 18 Examination using transmission electron microscopy (TEM)—an analytical technique in which a high‑energy electron beam is transmitted through an ultrathin sam- ple to obtain nanometer‑scale structural and material information—revealed abnormal thin layers along both the left and right sidewalls of the via. Through energy- dispersive x-ray spectroscopy (EDS) analysis, a thin carbon- rich (C-rich) layer within the titanium/titanium nitride (TiN) barrier layer, specifically between the Ti and TiN layers was discovered. Additionally, the analysis showed an oxygen-rich presence across the Ti barrier layer (Fig. 8). ACCELERATING VOLTAGE IN SEM IMAGING Accelerating voltage refers to the potential difference that accelerates electrons from the electron gun down the column and toward the sample in an SEM. Adjusting the accelerating voltage is a critical consideration in SEM imaging as it influences the electron beam interaction volume within the sample. The higher the accelerating voltage, the larger the interaction volume, and the greater amount of signal generation. Higher signal levels improve the signal-to-noise ratio in an image, which allows higher quality images or elemental data to be acquired faster through EDS. However, working at these high accelerating voltages has consequences that can obscure the results of SEM analysis. Increasing the size of the interaction volume means that more signals will be generated from within the subsurface of the sample compared to signals confined to the surface. This can be problematic when trying to analyze small or shallow features. A high-kV image may result in these shallow features becoming transpar- ent or completely invisible due to the stronger signal from the subsurface dominating the image. Using lower accel- erating voltages when imaging the samples can help to prevent these outcomes. Lower accelerating voltages limit electron penetration to the near-surface region, which is advantageous for observing fine surface details and delicate samples that may be damaged by higher energies. Sampling a shallower volume of interaction can increase the visibility of surface features and improve the spatial resolution of the SEM image. The nanogaps on the sidewalls of the anomalous vias were initially not observed using the higher acceleration Fig. 7 FIB cross-section images showing the nanogaps with respect to a correlation unit. UNVEILING VIA SIDEWALL NANOGAPS: A CASE STUDY (continued from page 15)
RkJQdWJsaXNoZXIy MTYyMzk3NQ==