DIFFERENT MICROFLUIDIC FABRICATION TECHNICS

Soft-Lithography-Microfluidic-Chip

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

Silicon-etching-tutorial

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.

Wet etching

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.

Dry etching

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

Termoforming-tutorial

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

CO2-laser-direct-micro-writing

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

PDMS-soft-lithography

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.

soft-lithography-SU-8-PDMS-microfluidic-chips

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Tutorials : Introduction about soft lithography

Definition Microfluidic

Soft-lithography definitions

When you talk about microfluidic some words can be new, we gather here some common and relevant definitions ...
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Microfluidic-fabrication-technics-short

Introduction about soft-lithography for microfluidics

Unlike photolithography, soft lithography can process a wide range of elastomeric materials, i.e. mechanically soft materials ...
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PDMS-membrane-short

PDMS membrane: thickness of a spin coated PDMS layer

The final PDMS layer thickness mainly depends of spin-coating speed and duration ...
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Photolithography-mask-short

Introduction about photomask in microfluidics

A photolithography mask is an opaque plate or film with transparent areas that allow light to shine through a defined pattern ...
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Microfluidic Wafer SU8 Mold

SU-8 mold lithography

Here you can find a complete over view of a SU-8 mold fabrication process ...
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PDMS-chip-short

PDMS softlithography

Here you can find a complete over view of a PDMS chip replication ...
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Tutorials : How to choose your soft lithography instrument

Microfluidic 3D Printer

Microfluidic 3D printer

Every following technology are based on the same system of additive process, every object is build layer by layer after being sliced by an informatics systems ...
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How-to-choose-plasma-pdms

PDMS Soft lithography : Plasma cleaner

A plasma cleaner to bond your PDMS chip, you will find here the relevant points to think about ...
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UVLamp-short-HowToChoose

SU-8 photolithography : UV sources

An UV Lamp to exposed your SU-8 photoresist, you will find here the relevant points to think about ...
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Spin-coater-short-HowToChoose

SU-8 photolithography : Spin coater

A spin coater to create thin layer of photoresist or PDMS, you will find here the relevant points to think about ...
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Hot-plate-short-HowToChoose

SU-8 photolithography : Hot plates

A hot plate to bake your SU-8 photoresist, you will find here the relevant points to think about ...
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Photolithography-mask-HowToChoose

SU-8 photolithography: photomask

Basically you have the choice between glass or plastic photolithography mask, but how to do choice? you will find here some information to help you in the decision ...
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Tutorials : How to get the best soft lithography  process

PDMS microfluidic light

Soft lithography SU-8 Coating

In soft lithography, the fabrication of a mold, often made in SU-8, is required for replicating PDMS microfluidic structures ...
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Soft Lithography SU-8 baking

Soft Lithography: SU-8 baking

Replicating PDMS-based structures first requires the fabrication of a SU-8 master mold that will serve as a patterned template for PDMS casting ...
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Spin-coater-short-HowToChoose

SU-8 photolithography: Spin-coating

How do you do to have a successful spin coating? Here you will find the tips and tricks to do it ...
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Hot-plate-short-HowToChoose

SU-8 photolithography: Baking

How do you do to have a successful photoresist baking? Here you will find the tips and tricks to do it ...
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UVLamp-short-HowToChoose

SU-8 photolithography: UV exposure

How do you do to have a successful SU-8 exposure? Here you will find the tips and tricks to do it ...
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How-to-choose-plasma-pdms

Soft lithography: Glass/PDMS bonding

How do you do to have a successful PDMS bonding with a plasma cleaner? Here you will find the tips and tricks to do it ...
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Tutorials : Microfluidic device fabrication

Photolithography-mask-short

Fabrication of glass and film photomasks

The photolithography mask is an important tool in soft photolithography processes, we explain here how they are made ...
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Influence of your microfluidic laboratory environmental parameters on your photomask

PDMS chips, soft lithography ... Do it yourself ! More information about the SoftLithoBox®   Environmental and mechanical conditions can affect the ...
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Microfluidic-foundries-short

Microfluidic Foundries

If you don’t fabricate your microfluidic device by yourself, it is important to choose the right manufacturer to fabricate your microfluidic chips. Here is a list of microfluidic foundries ...
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Microfluidic-fabrication-technics-short

Microfluidic fabrication technics

It exist different technics to fabricate microfluidic devices but the main can be resume by Etching, Thermoforming, Polymer ablation and Polymer casting ...
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