edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 2 open defect blocks signal propagation, leading to an observable change in contrast in the LICA image. In Fig. 1e, a pronounced contrast is visible within the dashed rectangle (i.e., the via chain region), transitioning from light red to dark red color, with the defect site marked by an arrow at the point of the observed contrast change. The cross-sectional image in Fig. 1f shows an example of an open via that has been localized using the LICA technique. Following the schematic diagrams shown in Figs. 1a and d, the silicon substrate of the top wafer is selectively removed above the via‑chain area prior to OBIRCH and LICA analysis. This enables the use of 405 nm laser excitation and already prepares the sample for subsequent physical analysis. Due to the shallow absorption depth of 405 nm light in silicon (less than 1 µm), the interaction volume is confined near the surface, which is advantageous for the LICA technique. In addition, shorter excitation wavelengths enable higher spatial resolution. Localized silicon removal has previously been demonstrated at the die level using wet etching and at the wafer level using dry etching.[7,8] EXTENSION ELECTRODES FOR HIGHRESOLUTION OBIRCH/LICA ANALYSIS The OBIRCH and LICA analyses in Fig. 1 were conducted at imec from the wafer frontside, where the probe pads are also located. Because electrical contact between the probe pads and the OBIRCH/LICA instruments is required during these analyses, either by direct metal probing or by wire bonding, it is critical that the objective of the laser scanning microscope (LSM) does not interfere with the probes or wire bonds, as depicted in Fig. 2a. An objective with a working distance (WD) of several millimeters is typically required for this purpose. While objectives with a lower numerical aperture (NA) meet this requirement, it is well known that the imaging resolution is also limited for such objectives. For example, an objective with an NA of 0.35 and a wavelength of 405 nm has a diffraction‑limited resolution of approximately 700 nm, according to Rayleigh’s criterion (0.61 λ/NA). It is evident that further improvements in imaging resolution are required to keep pace with the semiconductor scaling roadmaps. At the 2025 Symposium on VLSI Technology and Circuits, imec demonstrated the feasibility of extending the wafer‑to‑wafer hybrid bonding roadmap to a pitch of 250 nm.[9] Furthermore, imec’s nanoTSV roadmap now targets backside vias with diameters as small as 20 nm at a pitch of 120 nm. One well-established method to enhance imaging resolution is to use a backside solid immersion lens (SIL) in combination with an infrared laser. Using this approach, resolutions of 170 nm and 210 nm have been reported in the literature for GaAs SIL + 1064 nm and Si SIL + 1300 nm combinations, respectively.[10] Because the home‑built LSM‑based FA tool at imec does not presently support SIL integration, and the SIL method can (b) (a) (e) (c) (d) (f) Fig. 1 (a–c) Application of OBIRCH to a W2W via chain containing a soft open defect, and (d–f) LICA to a W2W via chain containing a hard open defect. The dotted rectangles in (b) and (e) outline the area where the via chain, which runs in a serpentinelike pattern, is located.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==