edfas.org 7 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 2 the metallization structure after stencil removal, where two electrodes are deposited on the chip and contact a W2W via chain. In this case, a gold sputtering target was used. Additional details of the stencil‑transfer method are reported elsewhere.[11] While the stencil‑transfer method is attractive due to its simplicity and reliance on standard laboratory equipment, it has some limitations. A key limitation is the minimum achievable line width, which is typically on the order of 100 µm and largely depends on the laser‑cutting process used to fabricate the stencil. Furthermore, the method is challenged by samples that have significant surface topography, as the sputtering process is largely directional. Finally, the approach offers limited patterning flexibility, since each unique electrode layout requires a dedicated stencil. METAL INKJET PRINTING To overcome the limitations associated with the stencil‑transfer method, a metal inkjet printing approach was developed, as shown schematically in Fig. 3c.[12] This method enables the fabrication of conductive patterns using a drop‑on‑demand printing scheme, in which a conductive ink is precisely ejected from the printhead when actuated by a pulse signal applied to a piezoelectric transducer. A silver‑based ink was selected due to its high electrical conductivity, low tendency toward oxidation, and good chemical stability. The inkjet printing method offers high flexibility, enabling the rapid fabrication of customized electrodes and probe pads. The electrical resistance of the printed electrodes was measured to be approximately 8 Ω/mm, resulting in almost negligible impact on OBIRCH/LICA measurements. For applications requiring lower interconnect resistance and higher current‑carrying capacity, printing multiple lines in parallel and employing a multi‑pass printing strategy to increase the line width and thickness can be considered. Notably, the inkjet printing method enables the formation of significantly smaller features than the stencil‑ transfer approach, with minimum feature sizes below 70 µm. This capability can be further extended toward the micrometer-scale by using more advanced printing techniques, including aerosol printing. Figure 3d shows an optical micrograph of the metallization structure Fig. 3 (a) Stencil-based metal deposition method; (b) optical micrograph of the resulting metallization structure after stencil removal; (c) metal inkjet printing method; and (d) optical micrograph of the resulting metallization structure after inkjet printing. (a) (c) (b) (d)
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