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Although these approaches address most of the problems associated with glue-based interconnects, they all lack an important feature: reversibility.14 The need for reversible interconnects has led many to develop customized formats.22–25 Lately, a standardized reversible PEEK fitting that is glued onto the chip face has been commercialized.26–30 However, these fittings are costly and glue is again required for their attachment. As an alternative, others have developed docking stations that house the chip and that function as a socket through which all fluidic and electrical connections are made.14,31
This article describes the selection of a cyclic olefin copolymer (COC) material for the fabrication of chips with he desired optical transparency and its use in the development of micro-injection molding using mold inserts that enable he rapid and high-volume production of chip parts with ntegrated reversible ports, and finally the optimization of bonding by thermal fusion. Application of the chip is then illustrated with the preparation of a porous monolith and a chemically reactive interfacial skin directly within the channel of the chip via UV initiated polymerization.
Experimental
Optical properties
Substrates in the form of plaques were generously provided by Ticona (1.7 mm thick) and Zeon Chemicals (2.0 mm thick). A fluorimeter (HORIBA Jobin Yvon, Inc., Edison, NJ) was used to measure fluorescence from the front aperture using an excitation wavelength of 405 nm. A UV-vis spectrophotometer Varian, Walnut Creek, CA) was used to measure transmission in the UV and deep UV (DUV) range. To facilitate the comparison of transparency for plaques of different thickness, the percent transparency T2 of the material with a desired hickness l2 was calculated using eqn (1)
where T1 and l1 are the corresponding experimentally determined values.
Mold insert fabrication
A mold insert with positive raised features defining the cross sectional geometry and layout of the microfluidic channels was fabricated using a variation of the LIGA process.32–35
Substrate preparation. The 500 um thick nickel sheet (UNSN02200, National Electronic Alloy, Inc., Santa Ana, CA) was cut into 100 mm diameter discs using wire electrical discharge machining (EDM). Next, the discs were thermally annealed at a reduced pressure of 94.5 kPa and oxygen-free atmosphere consisting of 20% hydrogen and 80% argon. Annealing began by heating the substrates from 200℃to 1100℃at 2 ℃ min-1. After 2 h at 1100℃, the disks were cooled to 200℃at 2℃.min-1. The annealed substrates were then inpidually flattened by placement between two ground parallel steel plates in a Baldwin 400 kip universal testing machine. Flattening the discs requires straining them slightly beyond he yield stress, which is 185 MPa for annealed Ni 200. Next he disc surface was polished to a mirror finish by chemical mechanical polishing (CMP) performed with a donated slurry Cabot Microelectronics, Aurora, IL) at 480 g .cm-2 for 15 minutes using a pad and wafer rotation rate of 50 rpm on a conventional rotary tool. The final roughness of the surface after CMP was 10 nm.
Lithography. Lithography was carried out in the Berkeley Microfabrication Laboratory. Adsorbed water was removed from the substrates by drying at 120 ℃for 15 min and organic contaminants were removed via an oxygen plasma (300 W, 48 Pa O2, 15 min). A negative-tone photoresist (SU-8 2075, MicroChem Corporation, Newton, MA) was spin-coated on the substrate using the static dispense method. After allowing the puddle of resist (4 mL) to settle for 20 s, the resist was spread (500 rpm, 20 s, 1 krpm -1) to achieve a continuous resist coating over the substrate prior to the final spin step (1500 rpm, 20 s, 1 krpm s-1) which yields a final film thickness of 150 mm. Following the pre-exposure bake (70 ℃, 5 min 95℃ , 20 min) the substrate was allowed to cool for 10 min by natural convection. Flood exposure at 365 nm with a dose of 254 mJ cm-2 (SUSS MicroTec Inc., Waterbury Center, VT) was followed by a post-exposure bake at 70℃for 1 min and at 95℃for 10 min followed by 10 min cooling. Development was carried out at room temperature using SU-8 developer for 10 min. Finally, the substrates were thoroughly rinsed with 2-propanol and water, dried and plasma cleaned as above. Contact hot plates were used for all heating steps in order to ensure reproducibility. The 2-stage heating and 10 min cooling steps are important for minimizing resist cracking and delamination from the substrate.