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Ti-Pd-Au as a sacrificial layer, buried interconnect and etch mask for fabrication of polyimide surface micromachined micropumps
Keywords: microfabrication, microfluidics, polyimide
We describe the use of Pd as a diffusion barrier between Ti and Au for multilayer polyimide surface micromachining, demonstrated in the fabrication of micropumps. Microfluidic pumps are used in applications such as drug delivery, Lab on Chip, microTAS, chemical and environmental sensing, and electronics cooling. Most micropumps are made by stacking and bonding multiple bulk- micromachined wafers, each with microfabricated features which together define the pump chamber, diaphragm, inlet/outlet ports, and valves or nozzles [1]. We recently demonstrated a valveless diffuser micropump which, unlike most conventional micropumps, was fabricated via surface micromachining [2]. The various pump components were built up monolithically from layers of polyimide on a silicon wafer. Each chips diaphragm was then epoxy-bonded to a commercial PZT stack actuator with 9 microns displacement for a total package size of 5 mm x 5 mm x 12 mm [2]. The micropump chip consisted of 3 structural layers of polyimide with thicknesses of 10 um, 40 um and 80 um. In between the 2nd and 3rd polyimide layers, atomic layer deposited TiO2 and electroplated copper were used as sacrificial layers and were eventually stripped to release the diaphragm. To take advantage of etch selectively, electron-beam deposited Au was used as buried electrical traces for copper electroplating, and as a reactive ion etch (RIE) etch mask and sacrificial layer to create optional check valves. One of the challenges in fabricating multiple layers of polyimide alternating with Au, Cu, and ALD TiO2 arose from the need to subject the buried metal and sacrificial layers to the high temperatures of polyimide curing repeatedly. Initially, we observed that an organic residue formed on the surface of the 2nd Au layer on the 1st polyimide, immediately following deposition of the Au. We think this was caused by outgassing of the polyimide inside the metal deposition chamber, as the polyimide had been previously cured at 285 oC. Curing all polyimide layers at 350 oC prevented the formation of such organic residue; however, this higher temperature also resulted in significant intermetallic diffusion between the Au and its Ti adhesion layer, which increased the resistance of the 2nd Au layer electrical traces by 20X, and consumed the buried 1st Au completely such that it was no longer suitable as a RIE etch mask and sacrificial layer for subsequent fabrication steps. To reduce the Ti-Au interdiffusion during polyimide curing we introduced Pd as a diffusion barrier, following reports in the literature on Ti/Pd/Au interconnects for semiconductor and optical applications (such as in Ref [3]) as well as on polyimide flexible substrates [4,5] and with polyimide encapsulation [6]. We report the increase in the Au electrical resistance, changes in the Au film morphology, and its etching characteristics in commercial etchant, with and without the Pd diffusion barrier, following annealing at 350 oC. Through significant reduction in interdiffusion between Ti and Au, we find the Ti/Pd/Au combination to be suitable as an etch mask, sacrificial layer, and buried electrical interconnect for multilayer polyimide- based surface micromachining processes.
Li-Anne Liew, senior research associate
National Institute of Standards and Technology and University of Colorado
Boulder, CO

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