Design, Fabrication, and Characterization of Freestanding Mechanically Flexible Interconnects Using Curved Sacrificial Layer

Technologies that enable integration of microelectromechanical systems (MEMS) (and sensor) chip with a complementary metal-oxide semiconductor (CMOS) integrated circuit (IC) are becoming increasingly more important as the semiconductor industry explores and tries to encompass “More-than-Moore” solutions for future electronic systems. 3-D integration, stacking of dissimilar chips that have been independently fabricated and optimized, is a promising method to integrate MEMS and its signal processing electronics into a single miniaturized package. Unlike conventional methods of integration, 3-D integration can simultaneously provide high density and high performance interconnections without the added process complexity resulting from the CMOS and MEMS process reconciliation. Vital to the success of such integration are many novel interconnect technologies such as through-silicon vias, mechanically flexible interconnect (MFI) technology is another vital interconnect technology providing low stress, area array interconnections between a MEMS chip and a CMOS IC. MFIs are batch fabricated freestanding interconnect structures fabricated using two photolithography steps. The stand-off height of MFIs are 20 μm and the dimension can be as small as 100 by 50 μm. In this paper, design, fabrication, and characterization of MFIs are presented, this paper also demonstrates SU-8 ring structures that allow fabrication and confinement of solder ball, so that MFIs can be assembled using a conventional flipchip bonder. Critical technology that enables fabrication of MFIs is the technology that uses reflowed photoresist as a sacrificial layer, development of such process is also discussed.