Abstract
The low fabrication cost of SU-8-based devices has opened the fields of point-of-care devices (POC), µTAS and Lab-on-Chip technologies, which call for cheap and disposable devices. Often this translates to free-standing, suspended devices and a reusable carrier wafer. This necessitates a sacrificial layer to release the devices from the substrates. Both inorganic (metals and oxides) and organic materials (polymers) have been used as sacrificial materials, but they fall short for fabrication and releasing multilayer SU-8 devices. We propose photoresist AZ 15nXT (MicroChemicals GmbH, Ulm, Germany) to be used as a sacrificial layer. AZ 15nXT is stable during SU-8 processing, making it suitable for fabricating free-standing multilayer devices. We show two methods for cross-linking AZ 15nXT for stable sacrificial layers and three routes for sacrificial release of the multilayer SU-8 devices. We demonstrate the capability of our release processes by fabrication of a three-layer free-standing microfluidic electrospray ionization (ESI) chip and a free-standing multilayer device with electrodes in a microchannel.
Highlights
SU-8 has gained wide acceptance in micro- and nanofabrication [1] due to its superior mechanical [2] and thermal stability [3] and excellent bio-chemical compatibility [4]
AZ 15nXT crosslinks into highly stable state, which makes it resistant against most common developers and solvents used in microfabrication
It does not dissolve or swell in solvents of SU-8 process or mr-DEV-600 a SU-8 developer. This is different from other common resists which have been used as sacrificial layers before [15,25]
Summary
SU-8 has gained wide acceptance in micro- and nanofabrication [1] due to its superior mechanical [2] and thermal stability [3] and excellent bio-chemical compatibility [4]. SU-8 excels over most other polymers due to its mechanical properties, e.g., modulus of elasticity of 5 GPa [2] and the ability to fabricate high aspect ratio structures (up to 50:1) [5,6,7,8]. When sharp tips must be made, as in nozzle and spraying devices, the substrate must be removed. A silicon substrate is a drawback in applications where high electric fields are used, like capillary electrophoresis. Two main routes exist for release: sacrificial wafer process, and sacrificial layer processes [12]. Carrier wafer sacrifice is costly and slow, and we do not consider it in this work
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