High Density Flex and Thin Chip Embedding Technology for Polymeric Interposer and Sensor Packaging Applications

The paper introduces a fabrication technology for high-density flex circuits. The technology is based on wafer level packaging processes such as electro chemical metal deposition as well as deposition and structuring of polymeric interlayer dielectrics to create multi-layer wiring at rigid carrier wafers. The flex circuits are created subsequently by laser assisted detach of the multi-layer stack from the carrier wafers, which can be performed at full area or partially to create flex or rigid-flex configurations. The base technology enables line pitches down to $24\ \mu\mathrm{m}$ and staggered via configurations in a stack-up of up to three internal routing layers as well as front and back side contacts with minimum pitch of $55\ \mu \mathrm{m}$ . Depending on the layer stack-up total flex thicknesses between 20 and $50\ \mu \mathrm{m}$ are possible. The advanced technology enables line pitches of $14\ \mu\mathrm{m}$ and stacked via configurations with minimum via diameters of $10\ \mu\mathrm{m}$ in $10\ \mu\mathrm{m}$ thick polymer layers and a minimum pitch of the front and back side IOs of $27\ \mu \mathrm{m}$ . A further extension of the base technology allows the embedding of ultra-thin ICs into the multi-layer stack-up. The dices are $20\ \mu\mathrm{m}$ thin and placed in face-up configuration onto a die bonding adhesive. The thin ICs are then over coated with the next polymer layer and thus embedded into the thin film multilayer stack. As one demonstrator an acceleration and vibration sensor system was build. The demonstrator includes 2 ultra-thin sensor chips which are embedded in a $50\ \mu \mathrm{m}$ thick RDL layer stack on a rigid carrier. The vibration detection is enabled by a stress sensor chip, which is embedded in a partially freestanding only $50\ \mu\mathrm{m}$ thin flex layer. As second demonstrator a 24 GHz radar sensor was build. The sensor includes 2 embedded ultra-thin 24 GHz transceiver ICs.