Softlithography definitions

Desiccator vacuum PDMS degassing

Desiccator

A desiccator is a sealable enclosure connected to a vacuum line. It is mainly used for PDMS degassing.

PDMS-reticulated-poly-di-methyl-siloxane

PDMS Polydimethylsiloxane

PDMS is a material widely used for the rapid prototyping of microfluidic devices via soft lithography. It is a moderately stiff elastomer. PDMS is readily commercially available and is low cost. Moreover, it is nontoxic and optically transparent which turn to be essential features for conducting investigations in sciences’ life.

Microfluidic and PDMS soft lithography PDMS bonding short process

PDMS bonding

Once the PDMS-based device is released from its master, the microchannels must be closed. Conventionally, the sealing of the fluidic chip is achieved by bonding the PDMS to a glass slide. A plasma cleaner is often used in order to achieve a strong permanent bond.

Microfluidic and PDMS soft lithography :  PDMS-before and after mixing

PDMS degassing

When preparing the PDMS mixture, a curing agent must be added to the PDMS base monomer. Because the two components must be stirred vigorously, many air bubbles appear.

Bubbles may also appear when pouring the PDMS solution on the surface of the master. Because the microfluidic device could be useless, it is essential to try to minimize the formation of air bubbles. A common option to clear air bubbles is to place the PDMS mixture in a desiccator connected to a vacuum line.

Microfluidic and PDMS soft lithography : SU-8 photolithography process chart

Photolithography

Photolithography shares fundamental principles with film photography as well as techniques used to make printed circuit boards. Basically, photolithography aims at transferring a geometric pattern from a photomask to a light sensitive material such as SU-8 photoresist. It is required for fabricating the SU-8 master that enables the replication of PDMS-based microfluidic devices.

Microfluidic and PDMS soft lithography : Transparency photomask

Photomask

A photomask enables to transfer a specific pattern onto an underlying SU-8 photoresist layer when it is exposed to a UV light source. Depending on resolution needs, a photomask can be created with different materials and methods.

  • For a limited resolution, the photomask may be a laser printed overhead transparency (1200 dpi or 250 µm resolution)
  • A high resolution print can be obtained on thin polymer transparency (56 000 dpi or 10 µm resolution)
  • A resolution down to 600 nm (420 000 dpi) can be achieved with a laser quartz print in chromium deposited on the quartz

The mask controls the shape and lateral dimensions of the pattern transferred to the SU-8 film.

PDMS soft lithography

Soft-lithography

Soft lithography corresponds to a set of fabrication techniques that proves to be complementary to standard photolithography. An advantage of soft lithography is that it is compatible with a wide range of materials. In particular, elastomeric (i.e., mechanically soft) materials, such as PDMS, can be processed with soft lithography.

SU-8 baking

During the fabrication of the master, the SU-8 photoresist must be heated several times. The terms soft-bakingpost-exposure baking and hard baking are employed. They refer to different stages of the photolithography process and involve different heating times and temperatures.

SU-8 coating

Coating is one of the first steps of the photolithography process. It is intended to deposit a uniform film of SU-8 photoresist over the surface of a silicon wafer (i.e., a substrate). The thickness of the SU-8 layer spread on the surface of the wafer typically controls the height of the structures that will be imprinted in the microfluidic chip.

Microfluidic and PDMS soft lithography : SU8 immersion development

SU-8 development

Development is intended to dissolve the unwanted portions of the SU-8 photoresist which were hidden by the photomask and hence unexposed to UV light. A chemical developer can be sprayed onto the surface of the photoresist. A common option, however, is to simply immerse the wafer in a SU-8 developer.

SU-8 exposure

Exposure is used for the fabrication of the SU-8 master during the photolithography process. It takes place when the wafer has been uniformly coated with SU-8. A photomask is then positioned on top of the SU-8 layer. This latter is then exposed to ultraviolet (UV) radiations that travel through the photomask.

Microfluidic and PDMS soft lithography :  SU-8 master short process

SU-8 master

A master (sometimes termed master copy) is made of microstructures patterned on the surface of a substrate covered with SU-8 photoresist. The master is typically produced using photolithography. It is intended to serve as a mold for pouring PDMS. When the layer of PDMS is peeled away, it contains the inverse of the original pattern embossed on the surface of the master.

Microfluidic and PDMS soft lithography : SU8 negative photoresist

SU-8 photoresist

Generally speaking, a photoresist (or resist) is a light sensitive material. Basically, it is a mixture of organic compounds held together in a solvent solution. A photoresist can be positive or negative depending on how it responds to an ultraviolet (UV) light source:

  • With a positive photoresist, the areas of the photoresist exposed to the light source through a mask become more soluble. When developed, the exposed photoresist is dissolved.
  • On the contrary, with a negative photoresist such as SU-8 (Microchem Corp.), the regions exposed to the UV radiations harden. When developed, the exposed portions remain on the wafer.

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: thickness of a spin coated PDMS layer

The final PDMS layer thickness mainly depends of spin-coating speed and duration ...
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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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SU-8 mold lithography

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

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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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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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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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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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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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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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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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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 devices 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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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

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

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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