Specific Process Knowledge/Etch/DRIE-Pegasus/System-description: Difference between revisions
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=== A few words about RF matching - why it is important === | === A few words about RF matching - why it is important === | ||
(This section contains material from the Dry Etch TPT lecture) | |||
Connecting a RF generator to an AC circuit in order to drive a plasma in a dry etch tool is not as trivial as one might expect. One cannot just connect an RF generator to a coil, put a certain power through and expect a plasma generated absorb all energy. It is not possible: At RF frequencies even cables become inductors so circuits must be carefully constructed. | |||
[[File:RF matching 1.jpg|500px]] | [[File:RF matching 1.jpg|500px]] | ||
Most power RF generators are designed to transfer power into a 50 ohm load. This means that the circuit, in addition to the coil that has a certain inductance and resistance will also require additional electrical components if the total 50 ohm impedance criteria is to be met (unless the inductance and resistance of the coil are carefully chosen and constant). If the impedance is off, part of the power will be dissipated as Reflected Power for instance localized heating of RF cables due to standing waves. One must therefore have a way of controlling the circuit impedance. | |||
Most power RF generators are designed to transfer power into a 50 ohm load. This means that the circuit, in addition to the coil | <-- The fact that the plasma generated in the chamber by the coil induces a electronic current flowing in a loop (with an associate inductance) within itself that couples to the inductance of the coil making it vary over time - this is another reason for introducing the socalled matching network as illustrated above.--> | ||
The matching unit consists of two tunable capacitors and a | |||
Therefore, the capacitances of the capacitors Load and Tune must be carefully chosen to minimize the reflected power. | |||
Effects of the plasma: | Effects of the plasma: | ||
Once a plasma has been ignited in the process chamber by the coil, an electron current will be induced in the conductive part of the plasma so as to oppose the RF magnetic field. As a result of the coupling between the coil and the plasma, the inductance of the coil changes. This, in turn, changes the impedance of the generator circuit and hence causes reflected power | Once a plasma has been ignited in the process chamber by the coil, an electron current will be induced in the conductive part of the plasma so as to oppose the RF magnetic field. As a result of the coupling between the coil and the plasma, the inductance of the coil changes. This, in turn, changes the impedance of the generator circuit and hence causes reflected power | ||
RF matching network: | RF matching network: | ||
Therefore, in order to maintain the impedance matching, the two capacitances of the two capacitors, Load and Tune, need to change. This is taken care of by the RF matching network that will automatically detect changes in impedance and adjust the Load and Tune capacitors to ensure impedance matching. | Therefore, in order to maintain the impedance matching, the two capacitances of the two capacitors, Load and Tune, need to change. This is taken care of by the RF matching network that will automatically detect changes in impedance and adjust the Load and Tune capacitors to ensure impedance matching. | ||