Biomedical Engineering Reference
In-Depth Information
are based on different physical effects, it is not possible to use one all-embracing
simulation for the layout of these structures. Consequentially the PIMMS-elements
are simulated in parallel and optimized, with the interfaces adapted to each other.
The results are verified via the characterization of the complete system.
The PIMMS-elements can be divided in the fields of fluidic, plasma physics,
electrostatic, and high frequency technology. The fluidic part covers the supply
with plasma and sample gas, their distribution inside, and their evacuation out of
the system. By means of plasma physics both the ionization of the plasma gas by
the microwave field and the ionization of the sample gas by impact ionization are
described. Electrostatic theory governs to design the elements for acceleration,
focusing, and deflection of the plasma electrons and the sample gas ions.
Electromagnetic wave theory describes the creation of a microwave-field inside the
plasma chamber.
3.2
Plasma Chamber: Electron Source
The electron source of the PIMMS is an argon plasma. Inside the plasma chamber
the gas is ionized by a 2.45 GHz microwave field, ignited by an electric spark. In
the plasma chamber free electrons are created, that are accelerated by a static
electric field for impact ionization of the sample gas atoms. The layout of the
plasma chamber has to incorporate both the fluidic and the electrostatic require-
ments. On the one hand the gas apertures of the chamber must have the appropri-
ate dimensions to assure that the gas flow out of the chamber is low. On the other
hand the geometry must be such that most of the electrons are generated close to
the outlet of the chamber and can be extracted through this small aperture.
Electrons should be generated close to the acceleration field, which intrudes the
chamber only to a small depth.
Furthermore, the geometry must be such that two electrodes can be placed at a
distance that allows a spark generation in the available pressure range inside the
chamber to ignite the plasma [ 14 ] .
The layout of the plasma chamber is optimized for a high electron yield at a low
gas flow. Special attention was paid to obtain a low reflected power under fluctuating
pressure conditions. This is important to protect the RF-generator and to ensure a
stable plasma, which is crucial for a reliable ionization rate of the sample gas.
Figure 3 shows the resulting plasma chamber. Its diameter is 600 mm and its
outlet aperture is 40 m m wide.
Besides the silicon microwave electrodes two metal electrodes on the bottom
substrate for spark ignition of the plasma are depicted.
As previously mentioned the electrode geometry in particular affects the electron
beam, since the extracting field intrudes the chamber only to a limited depth. During
operation of the RF-plasma the electrodes are biased due to the different mobility of
ions and electrons. This biasing depletes electrons close to the electrodes, i.e., the
chamber walls. In this “dark space” or “sheath” no secondary electrons are created.
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