(Invited) Design and Characterization of a New Low Cost Thick Film Copper Metallization Transfer Process Onto PDMS Enabling Stretchable Electronics
Authors: Hilbich Daniel , Yu Gary , Gray Bonnie L. , Shannon Lesley 
Published: February 2020

We present a new low cost microfabrication technology that utilizes a sacrificial conductive paint transfer method to realize thick film copper microstructures that are embedded in polydimethylsiloxane (PDMS). Several example structures are fabricated and characterized that demonstrate the potential application of this process in flexible electronics, wearable electronics, and novel microsensor and actuator designs. This process has reduced fabrication complexity and cost compared to existing metal-on-PDMS techniques, and enables large scale rapid prototyping of designs using minimal laboratory equipment. This technology differs from others in its use of a conductive copper paint seed layer and a unique transfer process that results in copper microstuctures embedded in PDMS rather than on top of the PDMS surface. The fabrication process begins with the deposition of the seed layer onto a flexible substrate via airbrushing. A dry film photoresist layer is laminated on top and patterned using standard techniques. Electroplated copper is grown on the seed layer through the photoresist mask and transferred to PDMS through a unique baking procedure. This baking transfer process releases the electroplated copper from the seed layer, permanently embedding it into the cured PDMS. The characterization of the copper microstructures is given in terms of feature size, film thickness, surface roughness, and resistivity. The resistivity is measured under static conditions as well as under conditions of flexing and stretching using various linear and 1-dimensional Peano curve structures [1]. To quantify the stability of a structure’s conductivity under flexing, linear structures are bent over curves having various radii and the response of the resistivity is measured against the number of iterations. To measure the response under stretching, 1-dimensional Peano curve structures are fabricated and stretched until failure, while the resistivity is measured against the strain. Results show that we can achieve films 25-75 micrometers in thickness, with reliable feature sizes down to 100 micrometers and a film resistivity of approximately 7.15 micro-Ω-cm [2]. Results from current experiments will be presented that characterize the resistivity response under flexing and stretching. Process variants and future work are discussed, as well as large scale adaptations and rapid prototyping. Finally, we outline the potential uses of this technology in flexible electronics and novel sensor and actuator designs.
[1] J.A. Fan, W.-H. Yeo, Y. Su, Y. Hattori, W. Lee, S.-Y. Jung, Y. Zhang, Z. Liu, H. Cheng, L. Falgout, M. Bajema, T. Coleman, D. Gregoire, R. J. Larsen, Y. Huang, J. A. Rogers, “Fractal design concepts for stretchable electronics,” Nature Communications 5 (3266), 2014.
[2] D. Hilbich, A. Khosla, L. Shannon, B. L. Gray, “A new low-cost, thick-film metallization transfer process onto PDMS using a sacrificial copper seed,” SPIE 9060, Nanosensors, Biosensors, and Info-Tech Sensors and Systems (906007), 2014