Adhesive wafer bonding of micro-actuators with SU-8 photoresist
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TECHNICAL PAPER
Adhesive wafer bonding of micro-actuators with SU-8 photoresist Deginet Admassu1
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Tejumade Durowade1 • Wei Gao1 • Silviu Velicu2 • Sivalingam Sivananthan1
Received: 22 October 2020 / Accepted: 1 November 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Adhesive wafer bonding of micro-actuators using SU-8 photoresist is reported. It was investigated in this study that the bonding temperature, bonding pressure, baking time and thickness of the cross-linked SU-8 photoresist layer affects the bonding strength of the assembled micro-actuators. The adhesive wafer bonding was performed by applying different pressures while baking the wafers at different temperatures. In this work, we have demonstrated that decreasing the exposure dose and post-exposure baking temperature play significant roles in affecting the bonding strength of assembled micro-actuators. We believe that lowering the exposure dose and post-exposure temperature will inhibit hardening of the intermediate negative photo-resist layer, and it enhances softening of SU-8 thin film, which can facilitate the bonding strength between the micro-chips.
1 Introduction SU-8 a negative photoresist that has many important features like high aspect ratio, reasonable film thickness and near UV processing, was chosen and spin-coated on the stationary membrane of micro-actuators (Bilenberg et al. 2004). The fabrication of the spacer layer involves three main steps: surface preparation, patterning the spacer layer with photolithography, and wafer bonding. The starting silicon wafers were prepared by cleaning with 25% concentrated piranha solution with 1:4 H2SO4:H2O2 to remove organic residues. The substrates were then dehydrated at 140 °C for * 10 min to promote their adhesion with a photoresist (Xiao et al. 1999). After treating the surface of
& Deginet Admassu [email protected] Tejumade Durowade [email protected]
the substrate, the photolithography process to fabricate the SU-8 spacer involved spin-coating the photoresist on the substrate, and pre-bake the photoresist to remove the solvents. Next comes exposing with ultraviolet light at various exposure doses, which depend on the requirements of the film thickness and bond strength. SU-8 photoresist needs post-exposure baking after exposure, which affects its cross-linking. The exposed wafers were then developed using SU-8 developer (Bilenberg et al. 2004; Xiao et al. 1999). Various wafers were processed under different conditions by changing the processing parameters like spinning speed and time, pre-baking conditions, exposure dose and post-exposure bake conditions to achieve the target photoresist thin film thickness. The adhesive wafer bonding experiment, wafer bonding setup, and characterization of the bonding strength of the assembled micro-actuators in this work was performed at the Microphysics Laboratory of the University of Illinois at Chicago.
Wei Gao [email protected] Silviu Velicu [email protected]
2 Adhesive wafer bonding
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