Plasma Etching Procedure

INTRODUCTION:

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 and good anisotropy. A drawback is that for some chemistries teh selectivity is not as good and ion bombardment can cause damage. Several gases are using 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 130 mTorr in less than 1 minute. After 3 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 minutes, 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. 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, 20 mtorr > than your base pressure,  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. 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. Toggle 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 began counting up when the RF power was 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 to different channel. 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. Depress the PRESS button. The chamber pressure will be displayed.

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

I. When the pressure is within 10 mTorr of the desired setpoint 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, the reflected power is high immediately turn the plasma off(toggle the RF ON) button and contact your supervisor. Running the plasma for extended time with high reflected power can permantly 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.

J. Toggle form pressure to time so you can see monitor the time. There should never be a process that nees to operate longer than 2 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 LED’s 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.

 

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.

  1. Toggle the PROGRAM button to select the program step in which you wish to enter your desired process parameters ( for example, program 1). There are 9 program channels available.
  2. Depress the MAN OP switch to select Automatic mode of operation(LED extinguished).
  3. To program the gas setting, use the R DISP switch:

    4. 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).

  4. To program all other process parameters, use the L DISP switch:

    5. 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).

  5. Select the next program step, if any, you wish to program into the Process Controller by toggling the PROGRAM button.
  6. Repeat the above steps until all programs that you intend to use sequentially to complete a process have been entered.
  7. When your final process step has been entered, advance to the next program number and toggle the L DISP switch to the TIME position. Use the INCR/DECR switches to adjust this program’s TIME value to zero.
  8. Depress the SET/READ to READ ( the LED will be extinguished). This stores parameters in the Process Controller memory.
  9. Toggle PROGRAM to the beginning program number in the process sequence.
  10. Push START.

 

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:

  1. Toggle the SET/READ to READ position (LED extinguished).
  2. Toggle the L DISP for PRESS, POWER, TIME and RP parameter readings as desired.
  3. Toggle the R DISP for GAS1-GAS6 percent flow readings.

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 cylinders. Close the valves 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.