Ammonia oxidation in a microreactor

How do you make a microreactor?

Microreactors, like integrated circuits or chips, can be made from silicon wafers using photolithography. To begin, a photoresist coating is applied over a nitrided silicon wafer. The photoresist is a polymeric solution laced with UV-sensitive additives. A photomask containing the reactor design (a simple trench in this project) is projected onto the photoresist-coated wafer, with the photomask shielding (masking) all but the desired trench area from the UV light source (called a stepper). Where the light hits the photoresist in the trench area, it weakens and loses chemical resistance, rendering it easy to remove. So, after the UV exposure, what follows are five steps of removal of material:

1. Solvent removal of degraded photoresist from the trench area
2. Plasma etch removal of silicon nitride in the trench area (makes the trench 'double deep')
3. Oxygen plasma etch removal of untouched photoresist outside the trench area
4. Potassium hydroxide etch of silicon in the trench area (makes it 'double deep' again)
5. Plasma etch removal of silicon nitride outside the trench area.

Please note this technique is also be used to make microreactors on glass wafers. There is also a recently developed process called 'fabrication using laminar flow' that may enhance the use of the photolithographic process.

A second method of making microreactors involves making a three-dimensional mirror image master of the microreactor using the photolithographic method described above. (Where the microreactor is to have trenches, this master will have raised ridges.) This first generation master is then used to mold second generation microreactors in plastics or other substrates. Use of this method can significantly reduce the unit capital cost of the microreactors, as the molding process is cheap and fast in comparison to the photolithographic process.

A third method of making microreactors has been developed recently that provides even more advantages over the second method. It resembles the process used to make compact disks. A first generation master of the microreactor is made using photolithography as described above. (This master will be identical to the desired microreactor: trenches will be trenches.) This first generation master is used to make second generation elastomeric silicone stampers that are three-dimensional mirror images of the first generation master. These stamps are then used to make third generation microreactors, by multiple means on various substrates. The flexible nature of the second generation stampers allows for the fabrication of microreactors with three-dimensional tweaks by varying the compression of the stamper when it is set up to make the microreactors. Like the second method, this third method also has the potential to be significantly faster and cheaper than the first method.

 For more information on making microreactors, please consult these references.