SU-8 epoxy was applied in fabricating microfluidic systems through a low-cost and robust process. The complete fabrication and packaging process is introduced in this work and further highlighted by the presentation of two application examples: a static fluidic structure design with electrodes to function as thermal bubble valves and a released elastic structure design to operate as flapper check valves.
Integration with circuitry was achieved by building up SU-8 fluidic structures over microelectronic features. The coating of liquid pre-polymer accommodates the underlying topography, while curing temperatures below 120 "C ensures CMOS compatibility.
Sealing of the fluidic structure was realized through wafer-scale bonding to glass with an additional coating of SU-8 as adhesive, while the need for precision alignment during bonding was eliminated through design. Fluidic feedthrough ports and access to electrical bond pads were formed by a low-cost, batch KOH-etch process from the backside of the silicon substrate. Fluidic and electrical interconnections were completed by the attachment of tubing and ultrasonic wirebonding.
Demonstrating the versatility of the SU-8 process, a thermal bubble valve was implemented which featured a silicon nitride membrane for thermal isolation. N-doped polysilicon electrodes were designed as thermistors and resistive heaters for generating thermal bubbles. The nitride membrane allows power savings in generating thermal bubbles and enables the monolithic integration of thermal flow rate measurement designs by reducing thermal cross talk.
Extending the basic process further, a streamlined package-and-release design was explored to produce SU-8 flexural structure as fluidic check valve. By releasing the moving parts after packaging, the process becomes much more robust and would be more suitable for large-scale production.
In addition to fabricating the basic fluidic structures through CMOS-compatible processes, packaging issues include sealing and interconnection requirements for both fluidic and electrical input must be resolved for the proper operation of microfluidic MEMS. On the other hand, low-cost design and large-scale manufacturing are central to a successful product. The application of SU-8 epoxy in fluidic microsystems is discussed in this context.
March 30, 2001
Kuan, N. (2001). Fluidic Microsystems Fabricated in Epoxy. United States: University of California, Berkeley.