Microfabricated instrumented composite stamps for transfer printing

Transfer printing is an emerging process that enables micro- and nano-scale heterogeneous materials integration for applications such as flexible displays, biocompatible sensors, stretchable electronics, and others. It transfers prefabricated micro- and nano-scale functional structures, referred to as "ink," from growth or fabrication donor substrates to functional receiver substrates using a soft polymeric "stamp," typically made from polydimethylsiloxane (PDMS) with patterned posts for selectively engaging the ink. In high throughput implementations of the process, where several structures or inks are transferred in a single cycle, the ability to detect contact and monitor localized forces at each post during critical events in the printing process allows for the development of a robust and reliable manufacturing process. It also provides a unique vantage point from which to study fundamental issues and phenomena associated with adhesion and delamination of thin films from a variety of substrate materials. In this paper, we present a new composite stamp design consisting of SU-8 cantilevers instrumented with strain gauges, embedded in a thin film of PDMS patterned with posts, and supported by a backing layer. The fabrication of such a stamp, its testing and calibration are discussed. The use of the instrumented stamp in measuring adhesion forces between silicon and PDMS is demonstrated. New modes of programming the print cycle that monitor forces to control the stamp-substrate interaction are also demonstrated. Finally, a classifier-based approach to detecting failed pick-up or release of the ink is developed and demonstrated to work within a transfer printing cycle.