A practical guide for the fabrication of microfluidic devices using glass and silicon

Abstract

This paper describes the main protocols that are used for fabricating microfluidic devices from glass and silicon. Methods for micropatterning glass and silicon are surveyed, and their limitations are discussed. Bonding methods that can be used for joining these materials are summarized and key process parameters are indicated. The paper also outlines techniques for forming electrical connections between microfluidic devices and external circuits. A framework is proposed for the synthesis of a complete glass/silicon device fabrication flow. I. WHY USE SILICON AND GLASS IN MICRO-AND NANO-FLUIDIC APPLICATIONS?Micro-and nano-fluidic technology continues to be embraced by biologists, 1-3 chemists, 4 and engineers throughout academia and industry. As its applications have expanded, there has been an explosion in the range of materials and processes used to fabricate these devices. The earliest microfluidic devices (e.g., gas 5 and liquid 6 chromatography devices) were made from silicon and glass and borrowed processes directly from semiconductor and microelectromechanical systems (MEMS) manufacturing. 7 Now, however, many researchers have moved away from silicon and glass, and instead use cast elastomers such as polydimethylsiloxane (PDMS)-which is ideal for swift prototyping-or thermoplastic polymers, which can be hot-embossed or injection-molded and are well suited to inexpensive manufacturing. Yet, there remain many applications where glass and silicon offer advantages over polymeric materials. In this paper, we highlight these advantages and offer a framework for selecting fabrication processes when using silicon and glass. A. The dominance of soft lithographyOver the last decade, PDMS has become virtually the default material for forming microfluidic devices, 8 because of the sheer ease with which it can be cast on to a micro-scale mould and then strongly bonded to glass. 9 The elastomeric nature of the material has been exploited to integrate fluidic valves and pumps on-chip and has simplified the production of multi-layer devices because the soft layers readily conform to each other. 10,11 Yet, the low stiffness (usually <1 MPa) of PDMS relative to amorphous thermoplastics, silicon, and glass has its own a) Authors to whom correspondence should be addressed. Electronic addresses: ciliescu@ibn.a-star.edu.sg and hkt@ntu.edu.sg. 6, 016505 (2012) drawbacks. High aspect-ratio channels are notoriously difficult to fabricate in PDMS because of their propensity to collapse. 12 Moreover, difficulties in automating the handling of such a soft material have hampered efforts to scale up PDMS device manufacturing. Meanwhile, PDMS's high oxygen and water permeabilities have proved both a blessing and a curse in different applications.The hydrophobic nature of PDMS can be an important consideration for some biological applications. In drug screening applications, for example, hydrophobic drugs as well as metabolites (urea or albumin) can be absorbed into the device's material due to hydrophobic--hydrophobic interaction...

A versatile technology platform for microfluidic handling systems, part I: fabrication and functionalization

“…Mark et al (2010), Iliescu et al (2012), Nge et al (2013), Temiz et al (2015), Kim and Meng (2015) The platform presented here is our attempt at creating a platform that can be used as widely as possible, but it is not possible to offer all advantages of other platforms, without any of their drawbacks. One of the advantages of this platform, a very thin channel wall, also is one of its drawbacks.…”

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“…The specific etching solutions depends on the chemical composition of the glass, but typically wet etching of glass is carried out using hydrofluoric acid (HF) as the chemical etchant. 44 Chemical etching dimensions ranging from 1-300 µm are possible depending on the specific process and resolution requirements, with the etching rate being a parabolic function of HF concentration (typically 0.5-10 µm.min -1 ). The use of wet etching has several limitations, most notably difficulties in controlling the etch profile which can severely limit the possible resolution of channels.…”

“…Silicon microreactors also demonstrate excellent thermal conductivity, and are often selected when a uniform reaction temperature distribution is required. 44 Well established wet and dry etching manufacturing processes enable fabrication of micro channels with controlled sidewall shape and channel dimensions from nm to mm. A wide range of micron sized flow, pressure, and temperature sensors, have been integrated into these devices.…”

“…However, unlike with glass reactor manufacture, dry etching is often the process of choice due to its increased manufacturing rates. 44 Figure 7: Example of a silicon microreactor. 47 …”

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“…The earliest microfluidic devices were fabricated with silicon or glass by using modified semiconductor manufacturing process and microelectromechanical systems (MEMS) (Lliescu et al, 2012). Presently, a silicone elastomer, namely polydimethylsiloxane (PDMS) is one of the most frequently used substrates for microfluidics as it is inexpensive and it has good optical transparency and biocompatibility.…”