Heterogeneous Bonding of PMMA and Double-Sided Polished Silicon Wafers through H2O Plasma Treatment for Microfluidic Devices

Chao-Ching Chiang,Philip Nathaniel Immanuel,Song-Jeng Huang,Yi-Hsiung Chiu

doi:10.3390/coatings11050580

Chao-Ching Chiang, Philip Nathaniel Immanuel + Show 2 more

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https://doi.org/10.3390/coatings11050580

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Journal: Coatings	Publication Date: May 17, 2021
Citations: 8	License type: CC BY 4.0

Affiliation: National Taiwan University of Science and Technology

Abstract

In this work we report on a rapid, easy-to-operate, lossless, room temperature heterogeneous H2O plasma treatment process for the bonding of poly(methyl methacrylate) (PMMA) and double-sided polished (DSP) silicon substrates by for utilization in sandwich structured microfluidic devices. The heterogeneous bonding of the sandwich structure produced by the H2O plasma is analyzed, and the effect of heterogeneous bonding of free radicals and high charge electrons (e−) in the formed plasma which causes a passivation phenomenon during the bonding process investigated. The PMMA and silicon surface treatments were performed at a constant radio frequency (RF) power and H2O flow rate. Changing plasma treatment time and powers for both processes were investigated during the experiments. The gas flow rate was controlled to cause ionization of plasma and the dissociation of water vapor from hydrogen (H) atoms and hydroxyl (OH) bonds, as confirmed by optical emission spectroscopy (OES). The OES results show the relative intensity peaks emitted by the OH radicals, H and oxygen (O). The free energy is proportional to the plasma treatment power and gas flow rate with H bonds forming between the adsorbed H2O and OH groups. The gas density generated saturated bonds at the interface, and the discharge energy that strengthened the OH-e− bonds. This method provides an ideal heterogeneous bonding technique which can be used to manufacture new types of microfluidic devices.

Highlights

Accepted: 14 May 2021poly(methyl methacrylate) (PMMA) is a common material often used to fabricate microfluidic devices/chips, due to its good optic transparency, chemical properties, high thermal stability, low price, and biocompatibility [1]
The chemical nature of PMMA (C5 O2 H8 )n allows the preparation of microchannel patterns by hot pressing, softening, or melting upon heating; see Figure 3a
Propyl radicals occur during the reactions for the formation of dimethyl ketene (DMK) and formaldehyde [48]

Summary

Introduction

PMMA is a common material often used to fabricate microfluidic devices/chips, due to its good optic transparency, chemical properties, high thermal stability, low price, and biocompatibility [1]. This transparent polymer is available commercially in pellet, small granule, and sheet forms, which can be processed by thermoplastic methods (including injection molding, compression molding, and extrusion) for the production of microfluidic devices and low-cost disposable devices for biological applications. Polymer materials with low molecular weights and repeated units can be fabricated using a variety of physical processes and chemical etching techniques, such as coining, photolithography, electrochemical etching [3,4], and pressure-based low/room temperature bonding [5].

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