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Development of Microcontact Printing Process for Manufacturing Conductive Patterns on Microfluidic Substrates

Melinda Hale1, David Hardt1

1 Massachusetts Institute of Technology Cambridge, Massachusetts, USA

INTRODUCTION Of the multiple challenges associated with manufacturing microfluidic devices (forming the substrate, creating tools, demolding, bonding, metrology, packaging, etc.), the “back-end” processes require a large portion of time and expense in a production setting, yet receive comparatively little research attention. One such back-end process is creating electrodes on the microfluidic chip, which are necessary for the function of many microfluidic designs, either for directing fluid flow, component actuation, or measurement techniques.

The ideal electrode patterning process would not require a clean room or high temperatures, and would be low-cost, fast, accurate and repeatable, robust, and scalable to high volumes. Development of such a process would not only advance microfluidic manufacturing, but could also be extended into other applications – including flat panel displays, solar cells, flexible electronics, and organic semiconductors.

This work explores the promising approach of direct microcontact printing (µCP) of conductive ink. Microcontact printing (a derivative process of soft lithography) involves first creating a stamp with the desired features, applying a conductive material to the stamp (through sputtering, dipping, spin coating, etc.), aligning the stamp to the substrate, then transferring the material from the stamp to the substrate (either with pressure, surface tension, chemical reaction, or some combination thereof). Direct transfer of silver ink onto polymer and glass (called either µCP or flexographic printing) has been proven feasible [1-8], even down to 2 micron feature sizes and over large areas [9].

EXPERIMENTAL The transfer process of ink from a stamp to a substrate is driven by the surface energies of the stamp, substrate, and ink. The materials’ surface energies can be used to calculate the work of adhesion between the stamp (or mold) and the ink film, Wmf, and between the substrate and ink film, Wsf. Two possible adhesion regimes exist (shown in Figure 1), driven by the ratios of work of adhesion.

FIGURE 1: ADHESION REGIMENS FOR INK TRANSFER

The transfer process is also affected by the ink properties. Even if the surface energies are suitable for transfer, the edge dewetting, q, of the ink is further controlled by the pressure applied, ΔP, the thickness of the ink layer, h, the viscosity of the ink, µ, and the feature length, L [10].

Incorrect manipulation of surface energies and ink properties can lead to defects such as incomplete transfer (Figure 2).

A) INCOMPLETE TRANSFER B) COMPLETE TRANSFER FIGURE 2: EXAMPLE OF INK TRANSFER DEFECT MODE

To develop a manufacturing process that is robust to defects, we need to understand the effect of ink and printing parameters, and use that understanding to define a process envelope for microcontact printing. The first step in

TABLE 1: INK PROPERTIES. SURFACE TENSION MEASURED WITH A GONIOMETER, VISCOSITY WITH A RHEOMETER, DENSITY WITH A GAS PYCOMETER.

CCI-300 Aldrich CSD-66 Cyclohexane Toluene Xylene MES 30 MES 40 MES 50 PD-054 Viscosity, cP 13.0 12.3 75.0 14.4 2.1 2.2 6.2 5.4 22.6 800 Solids Loading, wt%

20 20 60 45 45 45 30 40 50 100

Density, g/mL 1.23 1.22 2.25 1.38 1.14 1.35 1.20 1.44 1.66 2.67 Particle Size, nm 50 150 60 10 10 10 10 10 10 7000

Surface Tension, mN/m

31.5 29.5 47.5 21.5 28.2 25.7 26.48 27.5 26.9 -

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establishing this envelope was to characterize the physical properties of a variety of inks available commercially (Table 1).

These physical ink properties control the resulting thickness of a coating of ink on glass under different spin coating speeds. Flat coatings of ink with varying thicknesses were measured with a four-point probe to determine sheet resistance (Figure 3). The sheet resistance is an important functional requirement of the finished electrode pattern.

FIGURE 3: THICKNESS VS. RESISTANCE OF INK LAYERS ON GLASS

Using this understanding of the ink properties, a protocol for successful printing was identified. The printing process is illustrated in Figure 4.

FIGURE 4: MICROCONTACT PRINTING PROCESS

A layer of Ag nanoparticle ink with 40% solids loading in mesitylene carrier was applied by spincoating onto a plasma treated glass slide. A plasma treated PDMS stamp with a hexagonal pattern (with 5µm line widths) was wrapped around a roller, and rolled across this uniform layer to pick up the ink. Then the loaded stamp was rolled across a plasma-treated glass slide under a constant force of 3 N over the 76mm width of the slide to transfer the ink to the substrate. The slide was annealed at 150C for 30 minutes. The conductive silver pattern fabricated using this µCP process is shown in Figure 5.

FIGURE 5: PRINTED HEXAGONAL PATTERN OF AG INK ON GLASS, LINE WIDTH 5UM

CONCLUSIONS The chemical and electrical properties of a wide variety of inks have been characterized, and successful printing of silver patterns has been demonstrated with a resolution of 5µm. The next steps in this project are to characterize the ink transfer process over the entire parameter space of ink properties, and thus define a manufacturing process envelope. A validated manufacturing model for the process of direct microcontact printing of conductive ink will allow µCP to be implemented in the “back-end” electrode manufacturing process for microfluidic chips, as well as extended into other industries.

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