Specific Process Knowledge/Etch/DRIE-Pegasus/System-description: Difference between revisions
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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 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. | ||
<!-- 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 | <!-- 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 so-called matching network as illustrated above.--> | ||
The matching unit consists of two tunable capacitors and an impedance matching network. The network measures the impedance of the circuit and automatically changes the capacitances of the capacitors (called Load and Tune) in order to match the 50 ohm impedance. The changing of the active area inside the capacitor is done by rotating the shaft as illustrated above. This is, however, by no means a simple matter and the setpoints of Load and Tune (always measured as percentages of full capacitance) must be chosen wisely or the RF matching will fail. Therefore, if the setpoints for Load and Tune in a recipe are wrong - the process will fail and most likely abort. | |||
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 may cause reflected power | |||
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. | ||