CTE (coefficient of thermal expansion) mismatch between two wafers has potential for brittle failure when large areas are bonded on top of one another (wafer to wafer or wafer to die bonds). To address this type of failure, we proposed patterning a polymer around metallic interconnects. For this project, utilized benzo cyclobutene (BCB) to form the bond and accommodate stress. For the metal interconnects, we used indium. To determine the benefits of utilizing BCB, mechanical shear testing of die bonding with just BCB were compared to die bonded just with oxide. These tests demonstrated that BCB, when cured for only 30 minutes and bonded at 200°C, the BCB was able to withstand shear forces similar to oxide. Furthermore, when the BCB did fail, it experienced a more ductile failure, allowing the silicon to crack, rather than shatter. To demonstrate the feasibility of using BCB between indium interconnects, wafers were pattered with layers of BCB with vias for indium or ENEPIG (electroless nickel, electroless palladium, immersion gold). Subsequently, these wafers were pattered with a variety of indium or ENEPIG interconnect pitches, diameters, and heights. These dies were bonded under a variety of conditions, and those that held a bond, were cross-sectioned and imaged. Images revealed that certain bonding conditions allow for interconnects and BCB to achieve a void-less bond and thus demonstrate that utilizing polymers in place of oxide is a feasible way to reduce CTE stress.
Plasmas formed in microscale gaps at DC and plasmas formed at radiofrequency (RF) both deviate in behavior compared to the classical Paschen curve, requiring lower voltage to achieve breakdown due to unique processes and dynamics, such as field emission and controlled rates of electron/ion interactions. Both regimes have been investigated independently, using high precision electrode positioning systems for microscale gaps or large, bulky emitters for RF. However, no comprehensive study of the synergistic phenomenon between the two exists. The behavior in such a combined system has the potential to reach sub-10 V breakdown, which combined with the unique electrical properties of microscale plasmas could enable a new class of RF switches, limiters and tuners.This work describes the design and fabrication of novel on-wafer microplasma devices with gaps as small as 100 nm to be operated at GHz frequencies. We used a dual-sacrificial layer process to create devices with microplasma gaps integrated into RF compatible 50 Ω coplanar waveguide transmission lines, which will allow this coupled behaviour to be studied for the first time. These devices are modelled using conventional RF simulations as well as the Sandia code, EMPIRE, which is capable of modelling the breakdown and formation of plasma in microscale gaps driven by high frequencies. Synchronous evaluation of the modelled electrical and breakdown behaviour is used to define device structures, predict behaviour and corroborate results. We further report preliminary independent testing of the microscale gap and RF behaviour. DC testing shows modified-Paschen curve behaviour for plasma gaps at and below four microns, demonstrating decreased breakdown voltage with reduced gap size. Additionally, preliminary S-parameter measurements of as-prepared and connectorized devices have elucidated RF device behaviour. Together, these results provide baseline data that enables future experiments as well as discussion of projected performance and applications for these unique devices.