edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 4 32 the device resistance lies in the dielectric layer.[9,15,16] (If a larger resistance step were outside the field of view, the signal would go from bright to slightly less bright, rather than bright to dark.) Under bias (Fig. 2b plot) large resistance (≥ 200 GΩ) is measured across the circuit, further indicating minimal surface leakage resulting from milling and mounting. Current values in the Fig. 2 plot are measured from STEM EBIC images acquired at each indicated bias value. The electron beam is blanked for the first few lines of each image in the series (not shown), and measuring the current in the beam-blanked region of the image gives a measure of device current (which, being strictly beam-independent, can be subtracted as background in current-calibrated EBIC images). Two EBIC images from this STEM EBIC/bias series are shown in Fig. 2b, with EBIC being measured from the top electrode. In these images, the zero-bias image (Fig. 2b, middle image) is subtracted to remove bias-independent, conductivity related contrast. Under negative (positive) bias, beam-induced holes (electrons) are attracted to the TE, and bright (dark) signal is measured in the dielectric from EHP separation. Here STEM EBIC provides feedback on connection/isolation across the sample as well as visualization of bias-induced electronic changes, both of which are invisible to standard TEM imaging techniques. The simple device in Fig. 2 serves as a base case for preparation of a low-leakage FIB cross section. Similar techniques can be applied to more complicated devices, including off-the-shelf semiconductor components. For example, the Fig. 3 device was extracted from a commercially available GaAs HEMT using similar PFIB preparation as the Figs. 1-2 device (see Hubbard[17] for processing details). Here, one substrate electrode is connected to the gate electrode and the other to the drain (source is left floating), and each device terminal is connected to an EBIC amplifier. EBIC images in Fig. 3 show there are two distinct signals: a positive (bright) current signal in the upper metal layers, and more intense signal below the gate electrode (of opposite sign in each EBIC channel). These EBIC signals arise from SEEBIC and EHP separation, respectively, with the SEEBIC signal being comparatively much smaller, as is typical of the two EBIC modes. In the metal layers, bright SEEBIC contrast appears at device regions that are conducting and electrically connected to each amplifier. The signal from the gate-side shows continuity through the beam-deposited metal down through the gate electrode, but with a sharp boundary to the bright contrast below and to the right of it. Signal from the drain contact also stops sharply along its left edge. As with the Fig. 1 device, here SEEBIC shows continuity (bright regions) and isolation (sharp boundaries) within the lamella. Higher resolution images of the vertical boundary between the two metal regions (Fig. 4a) shows that the component’s passivating SiN coating is isolating the two electrodes (similar to the SiO2 in Fig. 2). Instead of milling down through the lamella, as is the case in Fig. 1, here the two electrodes were successfully isolated from each other just by the gradual milling of the metal cap during thinning and the cleaning steps. The direct feedback provided by STEM EBIC substantially accelerates the development of lamella device architectures (i.e., how and where to connect or cut away), where electrical testing alone would lead to blind trial-and-error from sample to sample. The region below the gate of Fig. 3 is shown at higher magnification in Fig. 4b. In the STEM EBIC image, the bright signal (measured at the gate) is consistent with Fig. 3 ADF STEM and STEM EBIC images of an off-the-shelf GaAs HEMT. The EBIC in the upper metal regions is dominated by SEEBIC, showing conductivity-related contrast, and the EBIC below the gate is dominated by EHP separation EBIC, showing contrast related to electric field and carrier mobility. The regions in yellow and below the gate are both shown at higher magnification in Fig. 4. See also Hubbard.[17]
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