Solder bump array formation is a key step in micro-array flip-chip fabrication process. Fabrication of solder bumps with small diameter and high aspect ratio geometries is necessary for high pixel density and low noise detectors. Indium is an attractive material for low temperature sensing application due to its cryogenic stability, good thermal and electrical conductivity, and ductile nature. In this paper, fill quality of micro-patterned arrays with thermally evaporated indium thin film is studied. Impact of indium evaporation rate, substrate temperature, bump aspect ratio and bump array pitch on bump formation in the via trenches is investigated. The indium bump formation dynamics are discussed in detail. State-of-art indium bump size with high aspect ratio and small array pitch is achieved, along with high uniformity and low defect density.
We investigated the fabrication of small neuroelectronic device consisting of four shanks with 16 electrodes per shank for simultaneous neurochemical and brain activity monitoring. The 16 electrodes on each shank have a separation distance of 100 microns (μm). Each shank has a width of 40 μm with separation distance of 7750 μm. This design eliminates single-site recording with limited individual conductors and permits rapid characterization of multiple neurons simultaneously at multiple brain depth/sites, consequently providing ground-breaking capabilities for parsing neurochemical release and brain activity. The device is fabricated on (100) silicon substrate and is fully integrated with electrode, interconnect and bond pad fabricated on one chip. Gold rectangular pyramid electrodes are selected as the recording electrodes to enhance the non-invasiveness associated with heating and minimizing surrounding biological tissue damage. The gold electrodes are deposited on the etched silicon substrate with 600 nanometer (nm) low temperature oxide (LTO) sacrificial layer. Each electrode has top area of 6 μm x 60 μm and depth of 750 μm. The interconnects provide electrical connection between electrodes and bond pads and are sandwiched between thin polyimide layers to prevent them from breaking while maintaining the flexibility. Final bond pads and electrodes are all passivated with polyimide to provide mechanical support. Upon device release, the recording electrodes are exposed to directly contact brain structure, and the exposed bond pads are soldered on the circuit board to transport signals to the measurement instrument. The entire process involves five photomasks. Process development and integration challenges will be reviewed and discussed in the paper.