Failure-Guided Process Control of Ti/AlCu Silicon Fan-Out Submounts Enabling Reliable Au-Wire Bonding in Photonic Packaging

As programmable photonic integrated circuits scale toward higher port counts and system-level reconfigurability, high-density electrical fan-out is becoming a major packaging bottleneck. Reliable Au-wire bonding to AlCu redistribution structures depends strongly on upstream wafer fabrication, although optimization often focuses on final bonding parameters. This study presents a failure-guided process-integration strategy for Ti/AlCu silicon fan-out submounts. A root-cause analysis of non-stick-on-pad (NSOP) failures was used to establish two reworkable in-line acceptance gates before irreversible assembly, covering the AlCu pattern-transfer fidelity, residual metal clearance, passivation-window integrity, and bond-pad exposure. With thermosonic bonding parameters held constant, the implementation of these gates and associated corrective actions increased the mean wire-pull force from 3.18 gf (31.2 mN) to 10.46 gf (102.6 mN), a 3.29-fold increase. All 30 measurements from the monitored-process batch exceeded the project-specific acceptance criterion of 4.3 gf (42.2 mN), and the dominant failure mode shifted from bond-pad interfacial separation to wire-neck or wire-body fracture. The optimized route enabled the high-pad-count assembly and functional verification of a programmable photonic device. Because the comparison comprised one fabrication batch per route with 30 bond-level pull tests per batch, the observed improvement is interpreted as a batch-specific process–response association rather than evidence of wafer-to-wafer or lot-to-lot reproducibility.