Development of Photonic Integrated Microchip-Based

Abstract

<p>Microchip-based flow cytometry holds promise in replacing conventional flow cytometers, thus providing less expensive point of care alternatives. Although final products are far off, a strong step towards these goals involves developing easy, reliable processes and improved functionality. This study is part of a larger project to develop a photonic integrated microchip-based flow cytometer using optical designs and simulations to imposed stringent requirements in device fabrication. Thus the goal of this work was to perform material selection, process development and device fabrication to meet the stringent optical requirements with an overview of device testing to demonstrate the achievements.<br /> Materials needed careful selection and include an SU-8 2025 structural layer, Pyrex substrate and a polydimethylsiloxane (PDMS) sealing layer. Mismatched properties of SU-8 and Pyrex have previously provided poor bonding, namely due to surface chemistry and thermal expansion differences. To overcome this, a thin intermediate layer of polymer was introduced relaxing stresses and allowing for chemical linkages. A rough range of 186-600nm was effective and limited optical deterioration.<br /> Sealing SU-8 devices with PDMS was previously accomplished using mechanical means or low pressure reversible bonding. Strong irreversible bonding was achieved by coating oxygen plasma treated PDMS with 3-aminopropyltrimethoxysilane (APTMS) and bonding the amino groups to residual epoxy molecules on SU-8 surfaces via polycondensation reactions. Bonding could not be broken through rigorous pressure testing, with devices withstanding on average O.6-0.7MPa and up to 2.2MPa before failing at the inlet fluidic connection.<br /> Post processing procedures required a rough dicing saw compromising the SU-8 structural layer. Reversibly sealed PDMS helped reduce chipping and protect against debris. An intermediate layer thickness of 186nm was efficient and 600nm provided no further improvement.<br /> These developed processes met the optical constraints imposed and quality devices were fabricated, capable of coupling high power light through on-chip waveguides, exciting fluorescence in microchannels and providing beam shaping.</p>