Adhesive Bonding with SU-8 Advanced microtechnology course (P) Maria Berdova

Wafer Bonding

Adhesive Bonding W. Brockmann, P. L. Geiß, J. Klingen, B. Schröder, Adhesive Bonding, Materials, Applications and Technology, 2009.

SU-8 Epoxy-based negative photoresist Thickness <1um to >300um Formulated in gamma butyrolactone (GBL) solvent (or in cyclopentanone) High chemical and thermal stability Can be used for building rigid mechanical structures HAR 40:1 Relatively cheap, easy to process The strongest adhesion to Ti and TiO2 J. Micromech. Microeng. 17 (2007) R81–R95

Drawbacks of SU-8 Difficult removal after curing High coefficient of thermal expansion (52 ppm/◦C, for silicon and glass ~ 3 ppm/◦C) High level of tensile stress Poor adhesion to Ni substrates

General bonding procedure Clean silicon and Pyrex wafers in piranha solutions then dehydrate the wafers at 200◦C for at least 40 min Deposit, pre-bake, expose, post-bake and develop the first SU-8 layer on the silicon wafer Spin-coat and pre-bake the second SU-8 layer on the Pyrex wafer Join the two wafers at different bonding temperatures then apply pressure to eliminate trapped air bubbles with a pair of tweezers After the bonded stack cools to room temperature, blanket expose the second SU-8 layer through the Pyrex wafer Post-bake the stack with temperature ramping while applying pressure Low cooling to reduce thermal mismatch stresses in the layers J. Micromech. Microeng. 13 (2003) 732–738 Sensors and Actuators A 120 (2005) 408–415

Micronozzels Thermal oxidation Litho from unpolished back side Etching of SiO2 in BHF DRIE Patterning of 10 µm SU-8 Etching of SiO2 membranes Spinning and backing SU-8 on Pyrex Contact of Si and Pyrex substrates Exposure through the Pyrex wafer and post bake J. Micromech. Microeng. 13 (2003) 732–738

Micronozzels (leakage tests) J. Micromech. Microeng. 13 (2003) 732–738

Capacitive pressure sensor Bonding with a pressure of 60N by pumping 1 h with a surrounding pressure of 10−4 Pa, then heating in ramp for 1 h to 70 ◦C and 100 ◦C, and finally cooling down to room temperature Sensors and Actuators A 147 (2008) 672–676

Calorimetric flow sensor By applying an electrical signal to the heater element, the surrounding liquid is heated up. Then, temperature distribution is measured by means of resistance change at the downstream and upstream sensors 20 µm SU-8, exposure PMMA substrate Ti/Pt, lift-off Ox treatment for cleaning 5 µm SU-8 to prevent contact Me/liquid 20 µm SU-8 to define microchannel 125 µm Kapton PMMA substrate 60 µm SU-8 to define inlet and outlet connections Bonding Removal from 0 ul/min to 25ul/min

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