Laser-driven micro-transfer printing for MEMS/NEMS integration

by Al-Okaily, Ala'a

Institution: University of Illinois – Urbana-Champaign
Department: 0133
Degree: PhD
Year: 2015
Keywords: Micro-Transfer Printing
Record ID: 2062641
Full text PDF: http://hdl.handle.net/2142/72946


Heterogeneous materials integration, motivated by material transfer processes, has evolved to address the technology gap between the conventional micro-fabrication processes and multi-layer functional device integration. In its basic embodiment, micro-transfer printing is used to deterministically transfer and micro-assemble prefabricated microstructures/devices, referred to as ???ink,??? from donor substrates to receiving substrates using a viscoelastic elastomer stamp, usually made out of polydimethylsiloxane (PDMS). Thin-film release is, in general, difficult to achieve at the micro-scale (surface effects dominate). However, it becomes dependent on the receiving substrate???s properties and preparation. Laser Micro-Transfer Printing (LMTP) is a laser-driven version of the micro-transfer printing process that enables non-contact release of the microstructure by inducing a mismatch thermal strain at the ink-stamp interface; making the transfer printing process independent from the properties or preparation of the receiving substrate. In this work, extensive studies are conducted to characterize, model, predict, and improve the capabilities of the LMTP process in developing a robust non-contact pattern transfer process. Using micro-fabricated square silicon inks and varying the lateral dimensions and thickness of the ink, the laser pulse duration required to drive the delamination, referred to as ???delamination time,??? is experimentally observed using high-speed camera recordings of the delamination process for different laser beam powers. The power absorbed by the ink is measured to estimate the total energy stored in the ink-stamp system and available to initiate and propagate the delamination crack at the interface. These experiments are used as inputs for an opto-thermo-mechanical model to understand how the laser energy is converted to thermally-induced stresses at the ink-stamp interface to release the inks. The modeling approach is based on first developing an analytical optical absorption model, based on Beer-Lambert law, under the assumption that optical absorption during the LMTP process is decoupled from thermo-mechanical physics. The optical absorption model is used to estimate the heating rate of the ink-stamp system during the LMTP process that, in turn, is used as an input to the coupled thermo-mechanical Finite Element Analysis (FEA) model. Fracture mechanics quantities such as the Energy Release Rate (ERR) and the Stress Intensity Factors (SIFs) are estimated using the model. Then, the thermal stresses at the crack tip, evaluated by the SIFs, are decomposed into two components based on originating causes: CTE mismatch between the ink and the stamp, and thermal gradient within the PDMS stamp. Both the delamination time from the high-speed camera experiments and thermo-mechanical FEA model predictions are used to understand and improve the process???s performance under different printing conditions. Several studies are conducted to understand the effect of other process parameters such as the…