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Linear Transport Degas, Pre-Clean, and PVD Processes for RDL Barrier/Seed Formation in Fan-Out Packaging
Keywords: RDL, PVD, Fan-Out
Linear transport sputter deposition systems are routinely used in industries as varied as architectural glass production, food processing, and silicon photovoltaic (PV) cell fabrication owing to the demonstrated productivity of such systems in High Volume Manufacturing (HVM). Crystalline silicon PV cell processes based on the Physical Vapor Deposition (PVD) of metal films share common ground with fan-out packaging processing, in that the metals being sputter deposited for silicon PV cells are typically some combination of Ti, TiW, Al, and Cu - the same materials that the Advanced Packaging industry uses for barrier/seed formation in Cu Redistribution Layers (RDL). The Process-of-Record (POR) configuration for RDL barrier and seed deposition today in fan-out packaging is typically a sputter deposition system built on a cluster tool architecture. In these PVD cluster tools, wafers are handled or handed-off many times as they make their way from the entrance loadlock to the exit. For fan-out packaging applications, reconstituted mold compound wafers will see at minimum a degas step, a pre-clean step, a Ti PVD step, and a Cu PVD step; the wafer will be touched by a transfer robot at least seven times, and the wafer will be touched by lift pins another six or more times as the wafers are raised or lowered in the various process modules. Each of these mechanical transfer operations has a fixed and dedicated time budget for the transfer, generally a function of the central robotic handler speed, and each of these mechanical transfer operations can also have a "lost opportunity" budget, as a robot occupied with a transfer from the Ti PVD module to the Cu PVD module has no opportunity to be doing anything else but that specific transfer. In contrast to cluster tools, wafers processed in linear transport systems are handled only a few times as the carrier-borne wafers make their way from the entrance loadlock, through the active process modules, and out to the exit. As a result, PVD cluster tool throughputs are on the order of 50-100 wafers per hour, while linear transport PVD systems in PV cell lines can have throughputs of 3000 wafers per hour, or more. We present here details about barrier/seed layer processes for fan-out RDL developed on a carrier-based linear transport Physical Vapor Deposition (PVD) system, including metal film uniformity results, sheet resistance results, and film adhesion results. For degas, we designed our system mindful that "Polymers lose their moisture content when they are exposed to dry environments at high temperatures," [1] and we equipped our degas module with a significant amount of vacuum pumping capacity, including Meissner coils for dedicated pumping of the water vapor evolving from epoxy mold compound substrates during degas. For pre-clean, we implemented a gridded ion beam source that produces a net electrical neutral impingement of well-controlled energetic Argon ions on the wafer or panel to be cleaned, thus avoiding Plasma process-Induced Damage (PID), which is believed to degrade device reliability. [2] For Ti and Cu PVD, we are using a Linear Scanning Magnet Array (LSMA) magnetron, which combines the basic simplicity of static magnetron geometries with a planar (back-and-forth linear) motion of the magnetic array, thus achieving much higher target utilizations than can be had with a completely static source. [3] The uniformity, sheet resistance, and adhesion results from our in-line linear transport system are comparable to current industry Processes-of-Record (POR), while the Cost of Ownership (COO) results of the in-line system are considerably more favorable than today's cluster tool POR for fan-out RDL, both for fan-out wafer level packaging (FOWLP) and also for fan-out panel level packaging (FOPLP).
Paul Werbaneth, Global Product Marketing Director
Intevac, Inc.
Santa Clara, CA

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