Soft lithography
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| Soft lithography | |
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| Technology Details | |
| Technology | Lithography |
| Equipment |
List of soft lithography equipment Solvent Bench 94 |
| Materials | SU-8, PDMS |
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Soft lithography is a technique used to create micro devices or three dimensional structures by means of molding and embossing an elastomer on a mold.The most common devices fabricated with this technique are microfluidcs which are widely used in cell biology. The most common material used in this technique is PDMS (polydimethylsiloxane) a soft bio-compatible elastomer that has high thermal stability, high chemical stability, low toxicity, chemically inert, insulating, is transparent to UV and visible light, low cost, easy to mold and mechanically flexible and durable. This is why the term “soft” is used. Soft lithography is also well suited for polymers, gels, and organic monolayers. Soft lithography is widely use because it is and easy, reliable and low cost process that can create three dimensional structures (ranging from cm to microns) that can be easily replicated.
Contents
Method of operation
Mold fabrication
The first step in the process is to fabricate a mold to be replicated. The most commonly used material is a photopatternable epoxy commonly known as SU-8, this photoresist comes in a wide variety of viscosities, producing a wide variety of film thicknesses from less than to hundreds microns that are stable. SU-8 molds are fabricated with standard lithography methods.
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Release agent coating
It is necessary to coat the SU-8 mold with a monolayer of a release agent, some fluorinated compound, to facilitate the removal of the PDMS mold after cured. This is essential to extend the life of the mold. Especially necessary in the case of fine features.
PDMS mixing and pouring
 | This section may stray from the topic of the article into the topic of another article, PDMS processing. |
PDMS is fabricated by mixing two compounds XXX and XXX in a 10:1 ratio.
- It is recommended to mix the amount necessary to obtain a PDMS mold of a desired thickness. This thickness will depend on the use of the mold
- Weigh the two amounts of materials in a ratio 10:1 w/w
- Stir until thoroughly mix
- Stirring produce bubbles - remove the bubbles before pouring onto the mold
- Place the SU-8 mold, coated with the release agent, in a container where the mold can be easily removed
- Let it cure overnight at a temperature between 60 and 100 C
- When cured, remove from the mold and cut the mold in the necessary pieces
- If the final objective is a microfluidic sealed with glass, place both PDMS mold and glass in an oxygen plasma for surface activation
- Remove both pieces from the plasma, avoid touching the surfaces, align both pieces and press them together.
Applications
Soft lithography is widely used to fabricate easily replicated, low cost devices. The main application is in the fabrication of micro-fluidics for biological applications. Usually the molds are made to fit on microscope glass slides which lowers the cost of fabrication.
Parameters
In soft lithography the most important step is the creation of the mold. It is crucial, then, that the lithography or quality of the mold is optimum.
Materials
SU-8
Optional description of materials that can be processed by technology. I thinknk the best example of where this comes in handy would be with LPCVD describing the difference between HTO and LTO.
The most commonly used materials used as molds are SU-8 photoresist and PDMS. SU-8 is a commonly used epoxy-based negative photoresist. Negative refers to a photoresist whereby the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development. SU-8 polymer its name from the presence of 8 epoxy groups.
The SU-8 molecule
The epoxy groups make SU-8 molds durable and resistant, once cured they can be re-used many times.
SU-8 Developer
This is a chemical, not a material
PDMS
Polydimethylsiloxane belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones, the chemical structure can be represented as
PDMS molecule
Equipment
The equipment necessary for photolithography is a spinner and mask aligner for the mold fabrication and later an oxygen plasma source to bond PDMS molds to glass in order to seal the cahnnels
Solvent bench 94
This bench houses SU-8 developer and is interlocked with the spinner and the hot plates that will be used to pattern the SU-8. In this bench will also take place the release agent coating and the PDMS mixing.
MJB45 Mask Aligner
SU-8 lithography
Glen Asher
Oxygen plasma activation of the glass and PDMS for bonding (recipe 20)
See also
Slides from Workshop
- SU-8 Photoresist Processing [1][Glen 1000P Plasma Cleaner]]
