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

 

Layer Addition during Photolithography

A brief review

Methods for layer addition include thermal oxidation, vapor deposition and electroplating. Layers needed to be adjoined after photolithography are the dielectric layer, electrical insulator, patterning mask and layer for metallization contracts. Thermal Oxidation intends to grow the oxide layers under the thickness of 1 micrometer. Usually the silicon dioxide layer is the layer where this process is done. Dry oxidation or wet oxidation can be used with this process where the dry method includes the usage of the oxygen under a high range of temperature such as 90-12000C. Furthermore the wet oxidation includes the process with the assistance of water. Film Deposition is another methof for layer addition and can be either assisted by chemicals or by physical means. The Physical Vapor Deposition can occur either by evaporation, sputtering or reactive sputtering method. The Chemical Vapor Deposition can occur in the low pressure mode or plasma enhanced. Step coverage is one of the biggest obstacles in the process of physical evaporation. This obstacle can be prevented by rotating the wafer, broadening the area of source’s deposition or elevating the temperature. On the other hand sputtering method usage eliminates this obstacle. This method uses he principle of plasma, the forth state of matter. In addition reactive sputtering uses non-inert gases such as oxygen, nitrogen, hydrogen. Radio Frequency Sputtering is a technique used by alternation of the electrical potential of the current at the radio frequencies in a vacuum and it disables the charge formation over the sputtering materials. On the other hand the chemical methods include the reaction of chemicals either thermally, electrically or optically. Atmospheric pressure vapor deposition method is perfect for depositing high rates, it is simple and enables high throughput. Its main disadvantage is that it has a poor uniformity and that is why it is more preferred for thick oxides. Low pressure CVD garantuees high rate of purity, but its rate of depositions are low and that is why it is mostly preferred for polysilicon or dielectric deposition. Plasma Enhance CVD on the other hand uses low temperature normally used for backend process but its disadvantage is because of plasma damage occuring usually in that process. Its main usage is in dielectrics coatings[1].

The most important concepts:

Planar Growth

Kinetics of Growth

Thermal Oxide

Oxidation Charts

Water Surface Colors

Film Deposition

Evaporation

Step Coverage

Sputtering

Plasma

Paschen Curve

Reactive Sputtering

Radio Frequency Sputtering

Comparison of PVD methods

Chemical Vapor Deposition

CVD mechanism

PolySi Deposition

Growth Rate Dependance

Laser Assisted CVD

Electroplating

LIGA Process

Wafer Bonding Techniques

Reference

1-http://lbms.iyte.edu.tr/ retrieved on 23.013.2018