Plasma-Activated Inter-Layer Bonding of Thermoplastics for Micro- and Nanofluidic Manufacturing

Plasma-activated polymer-polymer bonding is a promising way of encapsulating micro- and nano-fluidic channels across large substrate areas, without the substantial distortion of channel geometries that can plague thermally- and solvent-assisted bonding. The process involves treating the surfaces to be bonded with an oxygen or air plasma, and then pressing the surfaces together to allow an irreversible chemical bond to form. A convenient method is desired for measuring the toughness of such a bonded interface. Simple crack-opening tests (whereby a blade prizes apart the two bonded layers and the length of the inter-layer crack determines the bond toughness) are clumsy and hard to automate. We propose that built-in microscopic crack-opening test sites be distributed across manufactured substrates. At each test site, a polymeric film bonded over a step in the substrate would peel back from the step after bonding, by a distance depending on the toughness of the bond. The presence of a wedge-shaped air gap between the covering film and the substrate leads to visible interference fringes, the spacing of which can be used to extract the bond strength. Arrays of these in situ cracks might be imaged without removing the substrate from a production line and would allow us to monitor both substrate-to-substrate and cross-substrate bond toughness variation.

Bond toughness and polymer layers' surface energies are of particular relevance in planning the fabrication of very shallow fluidic channels whose widths, w, are much larger than their depths, h. The risk of channels' collapsing during fabrication must be controlled. For channels with h ~ 1 _m or less that are fabricated with thermoplastics, we expect collapsing to occur through local deformation of the surrounding material rather than through plate-like bending of the cover plate. The analysis suggests that the pressure applied during bonding, together with the polymer-polymer interface energies that exist before and after plasma-activated bonding, will delineate, on a w/h against h plot, regions in which collapsing will and will not occur. We have demonstrated nanochannels fabricated from polymethylmethacrylate (PMMA) that are 80 nm deep and 10 _m wide and other channels that are 110 nm deep and 20 µm wide.