microwave oven plasma asher

This is a project I devised with my colleagues M. Flores, M. Tumelero, A. Pasa and A. D. C. Viegas at LFFS, UFSC, Brasil, and inspired by other projects (see links). It consists of a plasma asher, a device that generates a very reactive oxygen plasma to clean organic contaminants on samples.
This project requires knowledge of high voltage, electromagnetic waves, and high vacuum science. A commercial microwave oven is internally modified, so be conscious of the risks associated to it. A microwave oven is composed by a closed chamber were stationary microwaves generated by a magnetron are used to heat food. Very high power can be delivered to materials that absorb microwaves. The microwaves heat can be sensed by the body, but some parts like the eyes crystalline can be damaged without notice, so never allow microwave radiation to escape the oven by, for example, using a broken window. The magnetron is powered by a high power high voltage transformer. The AC voltage of the transformer is doubled and rectified with a diode and a high voltage capacitor, getting around 4-5KV DC, so be aware that lethal voltages and currents are possible if this capacitor is discharged, or any electrical component touched while turned on. A schematic of the system is shown in the picture.

With A) Microwave oven B) Pyrex baking tray C) Seal D) Aluminum base E) Copper hoses and conections F) Baratron (0-100 torr range) G) Flux controller H) To rotatory vaccum pump J) To gas regulator in tank The metallic case of the oven was disassembled to allow better machining. Holes were drilled, some parts cut with a dremmel. Copper pipes and brass parts soldered with tin were used for the vacuum connections, rubber and vacuum grease for the seals. For the vacuum chamber a pyrex baking tray was used. The border of the tray was flattened with the dremmel and a diamond disc. To measure the pressure, a baratron gauge was used. To save costs, the controller of the baratron was also home made. The vacuum chamber is composed by the pyrex tray and an aluminum plate. The vacuum is made with an old rotatory pump. The oxygen flux is controlled with a flux meter, and the tank pressure regulator.  Sparks are possible so avoid sharp surfaces. The microwave is designed such that dimensions are more or less multiples of half the wavelength of the microwaves, and to focus the waves in the center of the oven. The aluminum plate distorts the fields of the original design and gets hot. The concave left wall was covered with an aluminum foil to avoid sparks in the plate (just an experimental fact). The pyrex seems to get hot because of the plasma itself, so no continuous operation is desirable for more than 1 minute. The best results for cleaning are obtained at the highest possible flux, typically 4 lit/min with a pressure gradient of about half bar, and pressure in the vacuum chamber of about 1 torr or less.

Refs:
- http://www.helgiskuli.com
- Guvench, M.G., “Design and Fabrication of Impact (Acceleration) Sensors as Class Projects in a MEMS Course," Proc. A.S.E.E, AC 2009-2191, 2009.