Design optimization of one-turn helix: a novel compliant off-chip interconnect

As the rapid advances in integrated circuit (IC) design and fabrication continue to challenge and push the electronic packaging technology, in terms of fine pitch, high performance, low cost, and good reliability, compliant interconnects show great potential for next-generation packaging. One-turn helix (OTH) interconnect, a compliant chip-to-next level substrate or off-chip interconnect, is proposed in this work, and this interconnect can facilitate wafer-level probing as well as wafer-level packaging without the need for an underfill. The interconnect has high mechanical compliance in the three orthogonal directions, and can accommodate the differential displacement induced by the coefficient of thermal expansion (CTE) mismatch between the silicon die and an organic substrate. The fabrication of the helix interconnect is similar to the standard IC fabrication, and the wafer-level packaging makes it cost effective. In this paper, we report the fabrication of an area array of helix interconnects on a silicon wafer. Also, we have studied the effect of interconnect geometry parameters on its mechanical compliance and electrical parasitics. Thinner and narrower arcuate beams with larger radius and taller post are found to have better mechanical compliance. However, it is found that structures with excellent mechanical compliance cannot have good electrical performance. Therefore, a trade off is needed for the design of OTH interconnect. An optimization technique using response surface methodology has been applied to select the optimal structure parameters. The optimal compliant OTH interconnect will have a total standoff height of about 100 μm, a radius of about 35 μm and a cross section area of about 430 μm².