Optimized Micro-Via Technology for High Density and High Frequency (<40GHz) HermeticThrough-Wafer Connections in Silicon

Abstract

We present the design, fabrication technology, and experimental evaluation of the high frequency performance of a new type of hermetically sealed through-wafer interconnects (µ-vias) in silicon substrates.The application of these µ-vias for wafer-scale hermetic packaging of receiver and transmitter optical subassemblies at 10Gbit/s, and for packaging of micro electro mechanical devices (MEMS) is discussed. These examples illustrate the potential of the technology to simplify the design of e.g. ball grid array packages (BGAs) in a cost effective way without sacrificing RF performance even at very high frequencies.Bandwidth measurements of the µ-via structures show reflections below -25dB up to 35GHz in a coplanar configuration even with multiple µ-vias in the path of a 50 Ω coplanar line. Additional losses due to the µ-vias are very low and below the detection limit of a 2.5mm long path. The waveguide losses were about 0.13dB/mm at 10GHz and about 0.28dB/mm at 40GHz.Excellent performance of the µ-vias is achieved by reducing their effective depth. On a 350µm or 500µm thick substrate the effective via depth can e.g. be reduced to only 20µm. The remaining depth is covered by impedance controlled coplanar lines that run down the slanted side wall of cavities in the silicon substrate. The concept thus combines the mechanical stability of substrates that are a few hundreds of microns thick with the ease to fabricate µ-vias in a membrane that is only a few tenths of microns thick. The pitch of these vias can be below 100 µm allowing for very high density interconnects as e.g. required in packaging of multi channel optical modules.The coplanar lines on the cavity side walls are realized by 3D photolithography using an electro-deposited photoresist and proximity exposure. The cavities with angled side walls are wet etched in aqueous KOH solution from one side of the substrate leaving a thin membrane in the bottom of the cavity (e.g. 20µm). This membrane is then opened up from the back side at the locations of the µ-vias in an additional KOH etching step. After structuring the metal lines the openings in the membranes are hermetically sealed by metal plating. The metallization scheme on which the vias and the electrical leads are based is compatible with reflow soldering and wire bonding.As an additional advantage the cavities can be enlarged and used as head room for discrete electro/optical components that are assembled on a lid wafer, or the components can be directly assembled in the cavity. Coplanar metal lines in combination with the proposed via technology allow the impedance matched connection of these high speed components to a Ball Grid Array (BGA) on the back side of the hermetic enclosure which in turn can be soldered to a rigid circuit board or to a flexible circuit board.Due to the tight control of tolerances and the dense via pitch it is easily possible to route multiple RF ports in and out of the package even in differential configurations and with additional DC control signals while still maintaining a ver...