Interconnect Technology Development for 180GHz Wireless mm-Wave System-in-Foil Transceivers

In this work, a polyimide (PI) foil-based wireless transceiver, which can be placed on the top of each node chip stack, is proposed. The transceivers with Butler matrix (BM) steered antenna arrays enable directed links from each node on one PCB towards any other node on the neighboring board in the rack. These passive components can be integrated into the foil whereas the active components (mm-wave ICs – MMICs) fabricated in SiGe-technology have to be connected with low parasitic, matched (wave impedance) interconnects. First the development of fabrication of low-loss transmission line structures on PI-foils will be described. The technology is based on foils with 50µm PI-thickness with Cr/Cu seed metallization and galvanic thickened Au layer. This allows for precise definition of coplanar transmission lines with low roughness (RMS roughness of 20-530nm). The measurements of characteristic parameters show good agreement with simulated data – the deviation of parasitic components (L and C) is below than 10%. A transmission loss of about 0.5 dB/cm at 60 GHz and about 1 dB/cm at 200 GHz has been measured. These substrates have been used for flip-chip assembly of chip components in order to characterize the performance of FC-interconnect at frequencies up to 220 GHz. For this analysis test-chips with transmission lines fabricated in a 130 nm SiGe-BiCMOS technology have been used. In order to mount these chips with Al pad finish on the PI-foil substrates Au studbumps with reduced size (50µm diameter on foot and 30µm height) and thermosonic flip-chip bonding have been used. From the measurements of FC bonded test chips with µ-strip lines on the PI-foils a FC-interconnect loss of about 0.28 ± 0.05 dB per bump at 60 GHz and of about 0.73 ± 0.14 dB per bump at 200 GHz could be derived.