Specific Process Knowledge/Etch/DRIE-Pegasus/picoscope: Difference between revisions
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Changing the pressure in the plasma has a great impact on the RF matching conditions as illustrated for [[Specific_Process_Knowledge/Etch/DRIE-Pegasus/System-description#Why_RF_matching_is_extremely_important_in_the_Bosch_process | an oxygen plasma]] elsewhere. The obvious place to start is therefore the pressure. Also, it barely makes sense to claim that the pressure is 20 mtorr in the dep phase if it oscillates as shown for the Process D4 above. | Changing the pressure in the plasma has a great impact on the RF matching conditions as illustrated for [[Specific_Process_Knowledge/Etch/DRIE-Pegasus/System-description#Why_RF_matching_is_extremely_important_in_the_Bosch_process | an oxygen plasma]] elsewhere. The obvious place to start is therefore the pressure. Also, it barely makes sense to claim that the pressure is 20 mtorr in the dep phase if it oscillates as shown for the Process D4 above. | ||
Compared to many other vacuum systems the pegasus tools require a massive turbo pump | Compared to many other vacuum systems the pegasus tools require a massive turbo pump to cope with the large gas flows and to be able to provide fast pump-down times. Since turbo pumps cannot change speed, the pumping speed is constant. The pressure in the process chamber is therefore regulated by the throttle valve or APC (Automatic Pressure Valve) that sits in front of the turbo pump. It is capable of adjusting the opening towards the turbo pump - and does so very fast and vere precisely. If a process if running with some setting of gas flows, RF powers etc., the pressure is then adjusted by opening/closing the APC. | ||
The pressure in the process chamber is regulated by the throttle valve or APC (Automatic Pressure Valve) that sits in front of the turbo pump. | |||
The processes A and D4 have fixed pressure settings. This means that when transitioning from one phase (with a certain gas flow, RF powers and pressure) to another phase at, say, the same total gas flow and RF powers but with lower pressure, the APC will open up more than what is required to reach the new pressure in order to arrive there faster. When the pressure reading passes the new setting, the valve will close. This feedback loop produces the pressure oscillations seen in plots above. | |||
To get rid of the oscillations we must change from the Fixed pressure mode must to Fixed APC mode. Here, this is no feedback loop: In the transition between phases, the different gas flows, RF powers and APC setting will produce a different pressure that reached a little bit slower. | |||