Flexible Interconnect and Packaging for MEMS Moveable Neural Microelectrodes

Interconnect integrity and packaging of implantable microdevices are crucial for their longevity. MEMS devices involving microactuators have special packaging challenges because of moveable parts that can fail if any debris gets trapped in the chip. Additional challenges arise when interfacing such devices with biological tissue. The device presented here is the first MEMS device with electrodes extending off the edge of the chip that are implanted in the brain to monitor neuronal activity. The devices are fabricated using Sandia’s SUMMIT-V process, and use thermal actuators to move highly doped polysilicon electrodes bi-directionally. This research presents a MEMS Microflex Interconnect (MMFI) technology approach that uses polyimide (PI-2611) as the flex circuit. Gold stud bumps are used to “rivet” bond the flexible circuit to the bond pads on the MEMS device. In our approach, we use dual stud bumps in order to allow sufficient room for the actuators and other mechanical structures to move without hindrance. The gold-to-gold bonding eliminates intermetallic formation and gives acceptable strength without the need of an underfill. A backside dry etch is performed on the flexible circuit in order to create micro-channel openings for the moveable electrodes to extend off of the chip. Reliability testing including 85%/85°C, high temperature, thermal cycling, and thermal shock are presented using the MMFI approach. Humidity testing has shown a slight increase in contact resistance from 5.84±0.19 mΩ to 6.00±0.18 mΩ after 500hrs at 85%/85°C. High temperature (300°C) has shown significant increase 6.4±0.45 mΩ to 59.97±38.94 mΩ after 250hrs.This approach offers a lightweight, flexible, biocompatible package, which can be batch fabricated, allowing SMD to be easily bonded and the connector to be mechanically isolated from stresses. MMFI technologies can be a viable approach to package any MEMS device that interfaces with biological systems. In addition, it readily allows for scaling up to high-density devices and systems.