Contact region fidelity, sensitivity, and control in roll-based soft lithography

Abstract

Soft lithography is a printing process that uses small features on an elastomeric stamp to transfer micron and sub-micron patterns to a substrate. Translating this lab scale process to a roll-based manufacturing platform allows precise control of the stamp contact region and the potential for large area, high rate surface patterning. In this manner, emerging devices can be produced economically, including flexible displays, distributed sensor networks, transparent conductors, and bio-inspired surfaces. Achieving and maintaining collapse-free contact of the soft stamp features is a necessary condition for printing. In the first part of the thesis, stamp behavior is examined at two length scales. First, microfeature collapse is examined across a range of dimensionless aspect ratios and pattern ratios to determine the collapse mode and the feature stiffness. Second, behavior of roll-mounted stamps is investigated on the macroscopic scale. The results of these analyses, simulations, and experiments show that contact is prohibitively sensitive as the feature scale shrinks to single microns or below. In the second part of the thesis, methods are developed to reduce the contact sensitivity. A compliant stamp architecture is introduced to tune the mechanical response of the stamp. Next, a new process for manufacturing cylindrical stamps is developed that removes limitations of planar stamp templates. The third part of the thesis addresses process control. A parallel kinematic stage is designed to manipulate the height and pitch of a roll over a substrate with submicron precision. A hybrid state-space / classical feedback control approach is used to achieve high bandwidth servo control in the presence of coupling and unmodeled dynamics. Using optical instrumentation, the stamp contact pattern is monitored and can be controlled using camera images as a control variable. Ultimately, a practical method of impedance control is implemented that demonstrates excellent disturbance rejection. The results of this thesis provide models for stamp behavior at the local microscale and the roll-based macroscale. These results illustrate the high sensitivity of contact to displacement disturbances in roll-based lithography, but also provide valuable design insight towards designing stamps and processing machinery that are robust to these inherent disturbances.