DIFFERENT MICROFLUIDIC FABRICATION TECHNICS
Different ways exist to make a microfluidic chip and these technics will lead to different kind of chips made with different materials.
These technics have evolved over time to be cheaper, easy to use and to produce original microfluidic chips quickly. We can denote four families of technics. One based on etching, wet or dry, usually used to make silicon or glass microfluidic chips, the second with thermoforming technics such as hot/roll embossing and injection molding. The third family is the direct writing thanks to micro machines or laser. At last, the fourth family is around polymer casting using namely the different softlithography processes.
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Microfluidic chip fabrication techniques : Wet and dry etching
The etching consists in protecting some part of a substrate and attacking the other to remove a particular depth of material. Generally, the parts that don’t want to be etched are protected thanks to photoresist by photolithography process.
In the wet etching option, the material is removed using liquid chemicals or etchants. In other words, the substrate is dropped into a corrosive solution for the substrate. The etching can be isotropic and the material will be etched in the all 3D directions and will lead to an extension of the channels. The etching can also follow more preferentially some crystal plans and lead to more or less complete anisotropic etching. Indeed, the liquid etchants etch crystalline materials at different rates depending upon which crystal face is exposed to the etchant. Thanks to these phenomena a silicon wafer with a 100 orientation etched by a KOH solution will lead to walls with an angle at 54.7°.
The wet etch process can be resumed in three steps, first the diffusion of the liquid etchant to the surface, second the reaction between the liquid etchant and the material, usually a reduction-oxidation reaction, and third the diffusion of the byproducts in the reaction from the reacted surface to the bulk.
In the dry etching option, plasmas or etchant gasses remove the material. The ablation reaction can be physical (with high kinetic energy), chemical or a combination of both.
Physical dry etching:
A physical dry etching requires high kinetic energy given by a beam of ion, electron or photon. There is no chemical reaction; it is only the particle energy that knocks out the atoms from the surface.
Chemical dry etching:
The chemical dry etching is not using liquid but gas. The gas molecule reacts with the surface and removes the atoms from it.
Reactive Ion Etching (RIE):
This last method combines physical and chemical etching. Indeed the high energy collision from the ionization helps to dissociate the etchant molecules into more reactive species. This technic is thus the most diverse and most widely used, it is also faster than other dry etching technics.
The etching is commonly used with silicon or glass substrates. It is thus possible to do or mold for polymer casting or directly the microfluidic chips. In the case of the micro devices the bounding of silicon/silicon, glass/glass or silicon/glass is difficult and need a wafer bonding that will heat, make a pressure and a current between the substrate to create a bound.
Microfluidic chip fabrication techniques : thermoforming
The thermoforming, basically, consists in heating a material to make it soft so it can be put in a particular form. It is commonly used for many plastic products. It can be made by injection molding or hot embossing.
The injection molding introduces thermoplastic pellets into a heated mold to create the chip. This technic offers a high frequency fabrication once the parameters optimized but the expensive cost of the equipment and mold make this solution slightly used in laboratory.
The hot embossing is a method that consists in pressing a heated silicon or metal mold against a thermoplastic sheet. Such materials as PMMA or COC are commonly used for embossing process.
Microfluidic chip fabrication techniques :Polymer ablation
It’s possible to realize the microstructures using direct write processes such as conventional mechanical drilling, sawing, laser machine and powder blasting. These technics have poor and cons; they enable to do micro devices really quickly and sometimes in 3D with laser for example. But the resolution can be limited and most important the shape of the channels is indeed really inhomogeneous because of the mechanical ablation of the material it remains plenty of defects over the channel walls.
Microfluidic chip fabrication techniques : polymer casting
The last way to do a microfluidic device is to create a mold and use polymer to replicate it. Different ways exist to do the mold but the scheme is always the same. You need to create the negative of your design with a hard material and then pour a liquid polymer on it. The polymer can be cured thanks heat or UV then be peeled off the mold to have the microfluidic device.
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Tutorials : Introduction about soft lithography
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Tutorials : Microfluidic device fabrication