edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 2 6 be challenging to apply to small samples, an alternative solution was needed. While there is the possibility to further reduce the imaging wavelength from 405 nm into the ultraviolet, this approach is expected to introduce additional optical complexities that limit practical applicability. As a result, improving the NA of the imaging system is considered a more viable route to enhancing imaging resolution, although the associated working‑distance limitation must be addressed. One solution to this problem is illustrated in Fig. 2b. It involves fabricating extension electrodes directly on the chip during the sample-preparation stage, prior to electrical fault isolation. The purpose of these extension electrodes is straightforward: They route the probe pads from the active area toward the chip’s edge, thereby creating sufficient space for the objective lens to be positioned closer to the chip surface. This enables the use of a high‑NA objective with a much-reduced working distance, resulting in improved imaging resolution. Using this approach, theoretical imaging resolutions of 275 nm and 176 nm are predicted for air‑gapped and oil‑immersion objectives with NA values of 0.9 and 1.4, respectively. Figure 2c shows a section of a nanoTSV via chain imaged using an oil‑immersion objective with a NA of 1.4, in the configuration shown in Fig. 2b. The metal segments interconnecting the nanoTSVs measure 630 nm and 210 nm in the horizontal and vertical directions, respectively, at a vertical pitch of 420 nm. Among liquid‑immersion objectives, oil‑immersion objectives provide the higher NA value, typically up to approximately 1.4. For this application, oil is also preferred over water as the immersion medium due to its insulating property. However, achieving this level of imaging quality requires careful fabrication of the extension electrodes, as the oil‑immersion objective used in Fig. 2c has a working distance of only 100 µm. TECHNIQUES FOR THE FABRICATION OF EXTENSION ELECTRODES The fabrication of the extension electrodes must meet several key requirements: (1) the electrodes must be sufficiently thin to fit within the objective’s working distance; (2) they must remain electrically isolated from the surrounding chip circuitry to prevent short circuits; and (3) they should ideally exhibit low electrical resistance. The probe pads to which the electrodes must make contact are also typically small (e.g., 80 µm × 60 µm, with a pitch of 100 µm), which introduces additional challenges for the fabrication process. Two fabrication methods have been successfully developed over time to meet these requirements, in chronological order: (1) the stencil‑transfer method and (2) the metal inkjet printing method. STENCIL TRANSFER METHOD The stencil‑transfer method, schematically illustrated in Fig. 3a, involves sputtering a metal layer through a stencil with apertures that match the desired metallization pattern. The stencil is directly placed and aligned on the chip under test. Laser‑cut stainless‑steel stencils have proven to be a cost‑effective and accessible solution, owing to their widespread use as solder‑paste stencils in printed circuit board manufacturing. The metal deposition is performed using a table‑top sputter coater, commonly used for scanning electron microscopy sample preparation. Figure 3b shows an optical micrograph of Fig. 2 (a) OBIRCH imaging and probing performed from the same side of the chip require the use of a large‑working‑distance objective, at the expense of reduced NA. (b) Extending the contact pads from the region of interest to the edge of the chip using low‑profile electrodes enables the use of high‑NA air or liquid‑immersion objectives. (c) Optical microscope image of a nanoTSV via chain acquired using an oil‑immersion objective and a 405 nm imaging wavelength, following the configuration shown in (b). (a) (b) (c)
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