The reflected power is defined as the portion of the power that is not coupled into the process and heads back to the generator. In microwave processes this is an important parameter to show the efficiency. This article shall describe what the value means, which parameters influence the reflection and how to minimize the reflection.
The advantage of microwave plasma systems is that, unlike comparable RF systems, reflected power does not cause any damage but can be used for process optimization.
Explanation of a microwave (-plasma) system
In the following a classical microwave plasma system is described using the example of an atmospheric burner.
- The microwave generator driven by a high voltage power supply produces microwave power in a magnetron (in the case shown here at a frequency of 2.45GHz)
- The isolator is a non-reciprocal component to protect the microwave source. It conducts the power generated in the microwave generator to the application with low loss. To protect the magnetron, the power reflected from the application is not returned to the source but diverted into a perfectly matched water load where the power is dissipated as heat in the cooling water.
- The three-stub tuner is an optional component that allows a wide process window of the system by matching the impedance of the source system to the impedance of the load. For very dynamic processes it is useful to run the tuner automatically. On the other hand, if a process is stable for microwave coupling, the impedances can be permanently matched to each other and the matching is done by individual, fixed tuning elements.
- Load represented by an Atmospheric Plasma Source (APS)
Reflection due to impedance mismatch
Root cause
Reflection always occurs when the components to be connected (e.g. microwave generator and plasma source) are not properly matched. In microwave processes this occurs e.g. due to changes of geometry in the waveguide, the change of line geometries (waveguide, coaxial conductor, stripline etc.), but also the change of the dielectric load in the process chamber. Thereby the coupling of the microwave can change due to changes of the process material, evaporating water, plasma ignition, temperature or pressure changes etc., which makes an adjustment necessary. The more abruptly these changes occur in the system, the more difficult it becomes to compensate for the mismatch with a single matching element.
All these effects can be explained by impedance matching and visualized by means of a Smith chart. The generator and the load can be assigned (frequency-dependent) complex impedances, which are matched by a complex matching network. The matching network is built by ideally resistive free inductors and capacitors. In the case of the example system in figure 1, this corresponds to the three tuning stubs of the tuner, which project into the waveguide at different depths.
Significance for MW processes
Due to the existence of the isolator as a protective element, reflections are no problem for the hardware used in microwave applications: Reflected power is converted into heat in the water load of the isolator, even total reflection is not critical. The characteristic value of the isolator indicates how much the reflection is attenuated by the application. In general, a microwave process should be operated with the lowest possible reflection in order to run the application at maximum efficiency.
Significance for RF processes
For RF plasma processes the reflection is very critical, because no isolators are available due to lower frequency of RF compared to microwave. Therefore reflected power components in the process chamber can cause unwanted discharges by arcing, in the worst case the reflected power can destroy the generator.
Impedance matching
Manual/Automatic
For standard systems, various adjustment elements can be used. Besides the already mentioned 3-stub tuner, plate tuners or E-H tuners can also be used. These can be operated manually or react automatically to changes in reflection. All these tuning elements change the transmission behavior from generator to load to optimize the matching.
Fixed Tuning
If a system is very well characterized, the transmission properties can be predicted. In this case the geometry of the transition can be fixed and does not need additional adjustment. The changeable tuning can be eliminated.
An example of such a system is shown in figure 2. The special sequence of the tuned components (magnetron, coupling stage, circulator, transition into the source, power output to the plasma) allows a perfect matching of the impedances, which ensures an almost reflection-free system.
Outro
The reflection is a process parameter observed in all plasma processes. In microwave processes the reflection is, in contrast to RF processes a relatively uncritical parameter. The reflection should be as low as possible, to optimize the efficiency of the process.
Reflection is a process parameter that is considered in all plasma processes. In microwave processes, reflection is a rather uncritical parameter in contrast to RF processes. If even small reflections in RF processes can lead to unwanted discharges (arcing) or even to destruction of the generator, in microwave systems it is a measure for the efficiency of the process. The lower the reflection, the higher the power available to the process. In microwave processes the reflected power is converted into a dummy load without endangering any parts of the overall system.
