Plasma etching, or reactive ion etching (RIE), is an important unit operation that is used to remove thin films. RIE is advantageous over wet etching in that it produces good anisotropy (or selectivity where one species is etched much faster than another). A drawback is that for some chemistries the selectivity is not as good and ion bombardment can cause damage. Several gases are use including O2 for photoresist (ashing), CHF3 for silicon oxide, and SF6 for silicon. Programs which specify the parameters for the various etch processes, known as recipes, can be loaded into the system and then modified as necessary.
SAFETY:
The etch gases available are SF6, CHF3, He, O2, Ar, and N2. The MSDS sheets for all of these chemicals can be found in the yellow MSDS binder outside of Meyer 425.
PROCEDURE:
I. Base Pressure and Leak Rate Check
This step is used to verify the integrity of the vacuum and to ensure optimum performance and repeatability of this system.
A. To begin, turn on the cooling water supply and return located on the wall behind the
system.
B. Open the N2 cylinder between the RIE system and the furnaces. This enables
the pneumatic valves that control gas flow throught the RIE.
C. Depress the MAN OP switch to select manual mode.
D. Depress the VAC ON switch to begin pumping down the chamber.
E. Depress PRESS switch to monitor chamber pressure.
F. Observe the base pressure; it should fall below 300 mTorr in less than 5
minutes (note that the gauge has been damaged by pump oil, and reads about 150mTorr
too high. You might want to make a note of this.) After 10 minutes note the
base pressure and record it in the log book. If the value is significantly higher
than normal contact the supervisor.
G. Turn off the VAC ON button. The main chamber is now closed off from
the pump and the pressure will begin to rise. Record the pressure increase after
1 minute, calculate the leak rate, and record it in the log book. If the value
> 50 mTorr/min or significantly higher than normal contact the supervisor.
II. Manual Operation
Always go through these steps and test the plasma at your desired conditions BEFORE you put your wafer in the reactor. Once you have verified that your conditions are suitable and a stable plasma can be obtained, then introduce the wafer for plasma processing.
A. Ensure that the cooling water is turned on (valves are directly behind the
etcher on the support column). Verify that the proper gases cylinders are connected
to the chamber lines (1) and (2). Open the valves for the gases that you wish
to use. In the log book record which gases you are using and which substrates
you are processing.
B. Turn on the RF generator.
C. Press the MAN OP button to enter the manual mode. The LED should now be illuminated and
you should be able to sequence through the process event buttons located to the right of
the MAN OP button.
D. Set up the parameters for the manual process. Toggle the L DISP button until
AUX( Base Pressure) is illuminated. Press the SET button for the left
display and note that the SET LED is lit. Now increase or decrease the left
display value until it is equal to the value you want, 30 mtorr > than your
base pressure recorded during the lead test, using the INCR and DECR buttons
below the display. Press the SET button again, this time extinguishing the set
LED. The base pressure is set. Toggle the L DISP button so POWER is illuminated
and press the SET button to enter the set mode as above. Increase or decrease
to the desired the value (reading is in Watts). Now push the SET button so that
you are once again in the read mode. Toggle the L DISP button until PRESSURE
is illuminated and change to the desired value (this is the process pressure
of your etching gas during the etching procedure). If you were going to operate
in automatic mode, you would also set the time by toggling the L DISP button
to TIME. Switch to the set mode for time and enter the desired process
time in seconds.
*Note* - In manual operation, the timer begins counting up when the RF power
is turned on and will continue to run until the RF power is turned off. In automatic
mode, the timer counts down from the time preset by the operator in the process
controller instead of counting up from zero.
E. Change the gas settings for the various gas channels. Depress the SET button
on the right display and toggle the R DISP button through the different channels.
Increase or decrease the gas setting using the INCR or DECR buttons below the
display. Press the SET button again, it is in the read mode again. *Note* -
Channel one has full scale of 100 sccm. Channel two has full scale of 250 sccm.
The calculation of gas flow is based on its gas flow conversion factor. See
attached samples and table of gas flow conversion factors.
F. Depress the VAC ON button to evacuate the chamber.
G. After about 15 seconds depress the PRESS button. The chamber pressure will be displayed. Note: if the PRESS button has not been depressed, the pressure reading is not accurate since the gauge is not exposed to the chamber.
H. When the pressure approaches the base value, turn on the gases by depressing the the GAS ON button. The pressure should approach the desired setpoint. Sometimes, however, the control system does not reach the desired pressure easily. If this happens, you can use another approach to determine the proper flow and pressure settings to get a desired process pressure. First set the process pressure to 0 mTorr. Introduce process gas into the chamber when the pressure is close to base pressure by depressing the GAS ON button. If the pressure is higher than the desired process value, decrease the gas flow setting accordingly. If the pressure is less than the desired value, increase the pressure setting as needed. Repeat this procedure until the proper pressure is obtained.
I. When the pressure is within ~10 mTorr of the desired setpoint, verify the toggle switch for the automatic tuner on the RF power is in the AUTO position (up position), and turn on the plasma by depressing the RF ON button.
IMPORTANT: The plasma should turn on instantly. Observe the forward/reflected power on the rf generator. If you have a stable plasma there should be very little reflected power (< 5 W). If it is unstable you will be able to tell because the reflected power is high immediately and does not decrease in a few seconds, or it flickers. If this happens, turn the plasma off(toggle the RF ON) and contact your supervisor. Running the plasma for extended time with high reflected power can permanently damage the system.
IMPORTANT: The plasma match network should always run in the automatic mode.The AUTO/MAN togle switched should always be left in the 'up' position. If problems exist in getting a stable plasma, it may be necessary to go to manual mode and tune C1 and C2, but this must only be done with assistance from the instructor..
J. Toggle from pressure to time so you can monitor the time. There should never be a process that needs to operate longer than 2-3 minutes.
K. To end the process, press the BLEED button. This will automatically turn off the RF power and gas flow (their LED lights will extinguish). The unit will pump down to a preset base pressure and the BLEED LED will blink while the VAC ON and PRESS button LEDs remain steady. Once it attains the desired base pressure the valve will close the system and the chamber is backfilled with nitrogen. The BLEED LED extinguishes and it is safe to open the chamber to remove/change your sample. You should notice that the chamber lid will separate slightly from the base as the pressure between inside and outside equalizes.
III. Automatic Operation
Always go through these steps and test the plasma at your desired conditions BEFORE you put your wafer in the reactor. Once you have verified that your conditions are suitable and a stable plasma can be obtained, then introduce the wafer for plasma processing.
a). Toggle the R DISP switch to read the desired gas channel.
b). Toggle the gas SET/READ switch to the SET position (LED illuminated).
c). Adjust the gas set point by using the INCR/DECR buttons.
d). Toggle the gas SET/READ switch to the READ position ( LED extinguished).
a). Toggle the L DISP switch to read PRESS . Toggle the SET/READ switch to the SET position( LED illuminated). Adjust PRESS to one selected value using INCR/DECR. ( See Independent Pressure Control, page 73 for instructions on how to set this parameter).
b). Toggle the L DISP switch to the PWR position. Adjust the RF power to the desired level in the same manner as manual operation ( make sure remote control and remote signal on the RF power generator are activated if using the RFX 600).
c). Toggle the L DISP switch to the ENDPT position. If you are desire to set and endpoint, refer to Endpoint Detection, page 36 for guidelines. Otherwise, ensure ENDPT value is set at 100 ( default setting to deactivate this option).
d). Toggle the L DISP switch to the TIME position. Adjust the process time to desired length in seconds using the DECR/INCR switch.
e). Toggle the L DISP to BP/RP. Adjust this setting for the desired chamber evacuation pressure level ( Base Pressure) that must be reached before gas flow can be activated ( typically 50-150 m Torr).
The amount of time remaining and the actual conditions of power, pressure, reflected power, and gas flow in the chamber can be monitored at any time during the course of the process:
Note: Before the programs, make sure that you can get normal plasma (The meter reading on the generator should be less than 5% in the reflected power mode) under the conditions you set. It is better to use manual operation mode to find proper parameters for your etching system before programs begin, referring to the procedures in manual operation.
IV. Shut Down
How to leave the system after you are finished processing.
A. Close the gas valves on the column behind the etcher (which you were using)
and the gas cylinder valves in gas room if you were the user of record. Be careful
not to turn off someone elses gas! Close the valves in the copper gas lines
just behind the reactor. Pump remaining gas out of the line.
B. Turn off the RF generator.
C. Turn off the water cooling..
D. In manual mode achieve base pressure. Record value in logbook. If significantly
different than at the start contact supervisor.
E. Close the N2 cylinder between the RIE system and the furnaces.