Design, Processing, and Characterization of High Frequency Flip Chip Interconnects
Doctoral thesis, 2008
The demands for high frequency interconnect techniques for microwave integrated circuits (ICs) are growing with increasing operating frequencies of wireless communication systems. Interconnects have significant effect and impact on the overall system performance at high frequencies. To provide good performance in high frequency packaging, flip chip interconnect is one of the most attractive candidates compared to other schemes with low reflection and low insertion loss due to the lower parasitics involved. The widely used bond-wire interconnect suffers from serious parasitics when operating frequency reaches the gigahertz range. The tolerances such as the wire length and loop are very tight to enable an acceptable transition. At high frequencies, however, it still encounters stronger parasitics no matter how well it is controlled.
This thesis deals with the design, processing, and characterization of flip chip interconnects at high frequencies. The main issues of the flip chip interconnect are described before the design criteria of the conventional flip chip interconnect are reviewed. In the following, the work of the hot-via transition is presented. It is a solution to the detuning effect of the microstrip (MS) flip chip assembly. The designs of the hot-via transition for the MS-to-CPW (coplanar waveguide) are presented; the results presented are currently world record for this technique to our knowledge. Another part of work in this thesis is the coaxial transition developed for the CPW-to-CPW flip chip interconnects. The coaxial-type transition was successfully fabricated in-house and demonstrated excellent transition performance up to 60 GHz. The entire fabrication processes for all demonstrated flip chip interconnect structures have been in-house developed and are described in details. All the design rules regarding to the different architectures for the flip chip interconnects are described and verified with the measured results. The main contributions of this thesis work are the innovative designs and the developments of both the hot-via transition and coaxial transition for the flip chip interconnects.