Specific Process Knowledge/Etch/DRIE-Pegasus/System-description: Difference between revisions
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(This section contains material from the Dry Etch TPT lecture) | (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 | 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 that a plasma generated will automatically absorb all energy. It is not possible: At RF frequencies cables become inductors so circuits must be carefully constructed. | ||
[[File:RF matching 1.jpg|500px]] | [[File:RF matching 1.jpg|500px]] | ||
Most power | Most RF power 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 elsewhere in the circuit as reflected power - for instance as 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 so-called matching network as illustrated above.--> | <!-- 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 | The matching unit consists of two tunable capacitors and an impedance matching network. The network measures the impedance of the circuit and automatically adjusts the capacitances of the two capacitors (called Load and Tune, one connected in serial, the other in parallel) in order to match the 50 ohm impedance. The adjustment of the capacitance is accomplished by changing of the active area inside the capacitor - the shaft rotates one part of the capacitor as illustrated above. In all recipes, one has to provide setpoints for the Load and Tune. If chosen correctly, then once the plasma ignites the matching network will then adjust automatically to achieve RF matching. | ||
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. | |||
It is unrealistic to achieve perfect matching (zero reflected power) so some minimum value (always less than 10 % of the input power) is found. 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 the matching network ensures that the impedance matching is still preserved. | 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 the matching network ensures that the impedance matching is still preserved. | ||