There are different ways to make a microfluidic chip and these microfluidic fabrication techniques will lead to different kind of chips made with different materials. These techniques have evolved over time to become cheaper, easy to use and to produce original microfluidic chips quickly. We can list four families of techniques. One based on etching, wet or dry, usually used to make silicon or glass microfluidic chips, the second with thermoforming techniques 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 about polymer casting using the different softlithography processes.
Etching consists in protecting some part of a substrate and attacking the other to remove a particular depth of material. Generally, the parts that we don’t want to etch 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. Etching can be isotropic and the material will be etched in the all 3D directions and will lead to an extension of the channels. 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 orientations 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.
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.
The chemical dry etching is not using liquid but gas. The gas molecule reacts with the surface and removes the atoms from it.
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 technique is thus the most diverse and most widely used, it is also faster than other dry etching techniques.
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 needs a wafer bonding that will heat, make a pressure and a current between the substrate to create a bound.
Thermoforming consists in heating a material to make it soft, so it can be made 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 technique offers a high frequency fabrication once the parameters are optimized but the expensive cost of the equipment and mold make this solution rarely used in laboratory.
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.
A turnkey offer to fabricate your su-8 mold and pdms chips
It’s possible to realize the microstructures using direct writing processes such as conventional mechanical drilling, sawing, laser machine and powder blasting. These techniques have pros and cons; they allow to make micro devices really quickly and sometimes in 3D with laser. But the resolution can be limited and most important the shape of the channels is indeed really heterogeneous, because of the mechanical ablation of the material, plenty of defects remain over the channel walls.
The last way to make a microfluidic device is to create a mold and use polymer to replicate it. There are different ways to make the mold but the principle 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 to heat or UV then be peeled off the mold to have the microfluidic device.
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Microfabrication techniques for a circular channel
In soft lithography, the fabrication of a mold, often made in SU-8, is required for replicating PDMS microfluidic structures.
Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting
How do you perform a successful SU-8 exposure? Here you will find the tips and tricks to do it.
How do you perform a successful photoresist baking? Here you will find the tips and tricks to do it.
How do you perform a successful spin coating? Here you will find the tips and tricks to do it.
The final PDMS layer thickness mainly depends of spin-coating speed and duration.
Here you can find a complete overview of a SU-8 mold fabrication process.
Here you can find a complete overview of a PDMS chip replication.
Unlike photolithography, soft lithography can process a wide range of elastomeric materials, i.e. mechanically soft materials.
A UV Lamp to expose your SU-8 photoresist. You will find here the relevant points to think about.
A plasma cleaner to bond your PDMS chip, you will find here the relevant points to think about.
A spin coater creates a thin layer of photoresist or PDMS, you will find here the relevant information about how to choose one.
You have the choice between glass or plastic photolithography mask, but how do you choose? here is some information to help you with the decision
A hot plate to bake your SU-8 photoresist, you will find here the relevant points to think about.
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