PlasmaEtch

Plasma etch test and dose testing for e-beam resist PMMA A3

Plasma etch

The chip is placed in a plasma to remove a thin layer of organic material on the chip i.e. resist. This technique can be used to clean the metal pits and make better gates. Normally the plasma etch is followed by a quick acid dip in 10% HCl and a rinse in DI water.

Mid-september 2009 I did a plasma etch test for PMMA A3 e-beam resist. I wanted to find out if it is possible to enhance the quality of the metal gates by introducing a plasma etch step and an acid dip into the process. Because when I evaporated the metal onto the chip without doing a plasma etch and acid dip I still had resist stuck underneath the metal which resulted in very rough and uneven gates.

The test was done for PMMA A3 e-beam resist. We plasma etched chips for different times to find the rate at which the resist is removed by the asher. The outcome of this test was that we have to etch for 30 sec to remove a layer that is 50A thick.

The report for this plasma etch and dose test can be found here: Report_plasma_etching_test_dose_testing_SF_021009_corrected.doc

Dose testing

For every e-beam write you have to do a dose test to find the right dose at which to write the patterns you want to put down. The time for development will be fixed before the actual writing step and then you have to adjust the dose for the patterns to get nice features and smoothly put down metal.

The report for this plasma etch and dose test can be found here: Report_plasma_etching_test_dose_testing_SF_021009_corrected.doc

How to use the plasma asher

1. Turn on the N2 vent (even though the chamber is not under vacuum you have to switch on the vent before you start the process because then it will automatically switch to N2 vent as soon as the process is finished)
2. Undo the chamber latch and open the chamber door
3. Load chips onto the glass tray
4. Close the chamber door and close the latch
5. Turn on the mechanical pump while pressing the door
6. Once the pressure is dropping you can let go of the door, now you have to wait until the pressure reaches 70 mtorr
7. When the pressure reached 70 mtorr you switch the O2 switch on and wait for the pressure to rise to 340 mtorr (if the O2 pressure does not reach 340 mtorr check that there is still enough oxygen in the supply bottle in the grey area or if the pressure is more than 340 mtorr you can VERY CAREFULLY adjust the needle valve for the O2 flow. This valve does not respond immidiately so adjust it a bit and then wait for the response. Turn clockwise: O2 pressure falls; turn anti-clockwise: O2 pressure rises)
8. Turn on the "line on" switch, it will light up yellow
9. Make sure that "local enable" is set to "on" and "power control" is set to "internal"
10. Set the analogue timer on the asher (you can also use the digital timer but when the time is up you have to stop the plasma manually. If you use the digital timer make sure the analogue timer on the asher is set to a time that is longer than the time you set on the digital timer so that your ashing process is not turning itself off while the time on the digital timer is not up)
11. Press the "RF on" button, it will light up red. This button starts the plasma. The plasma should have a slightly blue colour. If you can see any pink in it there is probably a leak in the system and it gets contaminated with N2. The standard settings of the asher are: incident power 50 Watts (check the plasma asher log-book for the settings the last user has used. If you want to use 50 Watts the knob adjusting the power should be set to 3.06), O2 pressure 340 mtorr. When the plasma is running you can check the incident power and the reflected power on the meter readings in the bottom part of the asher. You can also change the incident power by turning the knob near the "line in" button. But you have to make sure the plasma stays stable (watch the reflected power) by adjusting the knobs "Tuning" and "Loading"
12. When the time is up the plasma will be automatically turned off (if you use the analogue timer on the asher). Otherwise you have to turn it off manually by pressing the "RF off" button. Then the "RF on" switch will turn off and the "line in" knob will light yellow. Press the "line in" button as well to switch it off. It will stay lit for some time after you pressed it but then it will fade off
13. Turn off the O2 gas switch and let the pressure drop down to 70 mtorr again
14. Once the asher reached 70 mtorr you can switch off the mechanical pump (the N2 vent will switch on automatically). Undo the latch of the chamber door
15. Oncethe chamber reachesnormal pressure the door will open by itself and you can hear the gas flow of N2
16. Remove the chips from the chamber
17. Close the door and do up the latch
18. Turn off the N2 vent
19. Set the asher back to its standard settings: power 50 Watt

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