Fabrication of metallic patterns by microstencil lithography on polymer surfaces suitable as microelectrodes in integrated microfluidic systems

Microstencil lithography, i.e. local deposition of micrometer scale patterns through small shadow masks, is a promising method for metal micropattern definition on polymer substrates that cannot be structured using organic-solvent-based photoresist technology. We propose to apply microstencil lithography to fabricate microelectrodes on flat and 3D polymer substrates, such as PMMA or SU-8, which form parts of microfluidic systems with integrated microelectrodes. Microstencil lithography is accompanied by two main issues when considered for application as a low-cost, reproducible alternative to standard photolithography on polymer substrates. In this paper we assess in detail (i) the reduction of aperture size (clogging) after several metal evaporation steps and corresponding change of deposited pattern size and (ii) loss in the resolution (blurring) of the deposited microstructures when there is a several micrometers large gap between the stencil membrane and the substrate. The clogging of stencil apertures induced by titanium and copper evaporation was checked after each evaporation step, and it was determined that approximately 50% of the thickness of the evaporated metals was deposited on the side walls of the stencil apertures. The influence of a gap on the deposited structures was analyzed by using 18 um thick SU-8 spacers placed between the microstencil and the substrate. The presence of an 18 um gapmade the deposited structures notably blurred. The blurring mechanism of deposited structures is discussed based on a simplified geometrical model. The results obtained in this paper allow assessing the feasibility of using stencil-based lithography for unconventional surface patterning, which shows the limits of the proposed method, but also provides a guideline on a possible implementation for combined polymer-electrode microsystems, where standard photoresist technology fails.

Application of Microstencil Lithography on Polymer Surfaces for Microfluidic Systems with Integrated Microelectrodes

Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart

Microfluidic systems with incorporated microelectrodes are adopted in a variety of applications in life sciences. However, most of micro-electro-fluidic devices are fabricated based on silicon and silicon related materials micromachining processes, which are still expensive for disposable or semi-disposable sensing devices. Plastics and other polymer materials are advantageous to be used as substrates of microfluidic systems in order to realize a low-cost production process of devices, since these materials can be easily structured by e.g. hot-embossing, injection molding or thermal forming with excellent reproducibility. However, the lithographical process of microelectrode fabrication on plastic substrates is one of the critical issues, because many of these materials cannot withstand organic solvents which are unavoidable during conventional photolithography processes. Microstencil lithography, i.e., local deposition of micrometer scale metal patterns through small shadow masks, is a promising method for metal micropattern definition on plastic substrates that are not feasible for solvent-based photoresist technology. Since microstencil lithography is a resistless, single-step direct vacuum patterning method, it enables us to simplify the device fabrication process as well. Thus, we propose to apply microstencil lithography to fabricate microelectrodes onto plastic substrates as part of a low-cost production process of microfluidic devices. However, microstencil lithography is faced with several possible drawbacks. Therefore, we focused on two specific issues accompanying microstencil lithography, which are expected to affect on micro electro fluidic device fabrication process when considered as low-cost, reproducible alternative to standard lithography on plastic substrates: clogging and blurring. One of the main limitations of microstencil lithography is that the method is associated with gradual aperture clogging. To suppress the clogging phenomenon, coating stencils with self-assembled monolayers (SAM) is highly helpful, by which the useful life time of stencils can be extended. But in this study, we traced the clogging of micro-apertures without SAM coating by an observation of the stencil apertures and deposited microstructures after each evaporation step in order to asses the basic feasibility of microstencil lithography. From the series of observation, it was determined that approximately 50 % of the thickness of the evaporated metals (Ti, Cu) was deposited at the side walls of the stencil apertures, i. e., when 200 nm of Ti was evaporated through the microstencil, the thickness of deposited Ti at the side wall of the aperture was 0.1 um. Similar values were observed when Cu was used as a deposited material. The progress of clogging is clearly visible in an SEM image, in which the corresponding microstencil aperture was observed from the side having faced the substrate during the evaporation. These results suggest that the microstencils can be used repeatedly even without a cleaning treatment when the device specifications allow some micrometer tolerance of the electrode size. The other drawback of stencil lithography is related to the gap presence between the stencil and the substrate. We will also present quantitative values of gap induced clogging and discuss the blurring mechanism of deposited structures based on a simplified geometrical model.

2006

Formation of Nanowires by Self-Assembly on Highly Oriented Pyrolytic Graphite (HOPG)

Jürgen Brugger, Marc Antonius Friedrich van den Boogaart

Electrodeposition on Highly Oriented Pyrolytic Graphite (HOPG) steps has been proven to be an easy method for the preparation of metal (Pd, Cu, Ag etc) and metal oxide or sulfide (MoO2, MnO2, MoS2) nanowires in recent years. These wires can be transferred on to other insulating substrates like polymers (eg, PDMS) and epoxy or cyano acrylate glues, to be used as sensors, for example palladium for the detection of hydrogen. The main problem during their conductivity measurements is to make macroscopic contacts to these wires. Normally, silver paints are used to form contact between transferred wires, which however provides only poor control of size and reproducibility. In order to improve this, we attempt to connect the wires, by evaporating micro pads of 200-300 nm thick film of Al and 200- 400 nm thick Au through a nano stencil. The stencil used in these experiments is made of a 200 nm SiN membrane with apertures ranging from 250 nm to hundreds of micrometers. We studied several implementations of the micro-nano fabrication techniques such as follows: The structures were deposited on bare HOPG surface (before nanowire formation), HOPG surface after nano wire formation, and on glass wafer spin coated with PDMS. We also evaporated metal contacts over nanowires, which were transferred from HOPG to PDMS coated glass. Wires used in these experiments have average diameters of 50-60 nm. The distance between the sample and stencil plays a key role in the sharpness of the structures formed by shadow evaporation. The current limitation during the deposition of micro pads through stencil is the spreading of metal at the sides of the structures. This was most pronounced on graphite surface after nanowire formation and on PDMS surface with wires. This may be because after deposition of wires, the surface becomes rougher and is no more flat as bare HOPG or spin coated PDMS, so that this prevents close enough contact between substrate and stencil. The distance between sample and the stencil is kept as small as possible- in the micrometer range- to avoid spreading of the deposited structures. When stencil evaporation of 200 nm thick Au was done over wires transferred to PDMS, cracks in the gold film were observed on the polymer. This could be a result of the expansion and further contraction of PDMS during thermal evaporation and/or mechanical manipulation. In the presentation, we will show details of microelectrode formation on HOPG using the nanostencil and nanowire formation by electrochemical method.

2004

Application of Microstencil Lithography on Polymer Surfaces for Microfluidic Systems with Integrated Microelectrodes

Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart

2006

Formation of Nanowires by Self-Assembly on Highly Oriented Pyrolytic Graphite (HOPG)

Jürgen Brugger, Marc Antonius Friedrich van den Boogaart

2004