Brief Explanation of Soft Microfabrication


Until recently the most favorable materials for photolithography were crystalline silicon, amorphous silicon, glass, quartz and metals which have several disadvanatges, such as the fact they are expensive, can be brittle and opaque. Soft materials include PDMS; PMMA; Polyimide; Hydrogel etc. Soft materials tend to be cheap, flexible, transparent to the visible light and UV and in most of the cases show improved biocompatibility. The main advantage of the soft materials is that they do not necessarily need a clean room for fabrication process. A cleanroom is the term that stands for a room environment where the concentrations of the different particles on the air is highly controlled and they are not able to change the parameters that need to stay constant such as temperature of the room, pressure and humidity. The source of particles that need to be controlled are most of the times: skin, scales, hair, clothing lint, paper, boxes, bare wood products, abrading actions. The person that needs to work on such rooms, is obligated to wear head cover, safety glasses, mask, shoe cover, sticky mat and gloves.


Soft lithography includes soft lithography, micromolding, 3-D Photopolymerization, 3-D Printing and Laser Prototyping. Soft term stands for the absence of energetic particles , such as electrons, ions or radiation particles, such as UV or X rays. Soft lithography can be differently called as molding, printing or transferring. Its prototype material is cheap and it garantuees a fast method. It is a highly applied method in microfluidics and biomedical engineering. One of the main advantages it provides is the high resolution nearly 10 nm resolution, while the biggest disadvantage is because of the complexity of the master mold. The mostly used techniques of the soft lithography include: Replica Molding (REM), Micro-contact printing, Micro-molding in capillaries (MIMIC) and solvent assisted micromolding (SAMIM). PDMS or polydimethylsiloxane is a silicon based elastomer with a hydrophobic surface, which results from the mixing of siloxane oligomers and siloxane cross-linkers. Some of the advantages that PDMS garantuees are its flexibility, elasticity, optical transparence, chemical inertness, its low surface energy, durability, low thermal expansion, biocompatibility, good resolution and the ability to seal to flat and clean surfaces. In the case the PDMS is treated on the oxygen plasma, PDMS becomes able to seal to itself, glass, silicon, silicon nitride or plastic materials.
PDMS Preparation
The gold ratio of the Silicon Elastomer Base to the Silicon Elastomer Curing Agent is 10:1 Those 2 materials are taken by a pipette and the mixing is done till the mixture turns to a white color from the clear apperarance it had initially. The mixture is put on the photoresist mold and afterwards the bubble air present should be removed by vacuum usage. Then, the PDMS is cured by being heated for 1 hour at a hot plate(100 degree Celcius). Sometimes it is more preferrable to cure on lower temperatures such as 60 for longer periods of time. After te curing the process the cooling process occurs . When PDMS turns on solid after being cured, it is cut by a knife and the parts are mounted on different glass slides.
Micro-Contact Printing
A PDMS stamp is used about formation of the self assembled monolayers over thte surfaces of substrates. Inking is important for this process as the PDMS stamp is coated and pressed over the surface due to an ink of molecules. Elastomers usage is important for the micropatterned surfaces to confront with over sized areas.
3-D Photopolymerization is a process where the layers are assembled over each other and is used for the products needed on modeling and prototyping.

References

1- lbms.iyte.edu.tr

2-https://en.wikipedia.org/wiki/Photolithography

 

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