Nitrogen Plasma Source
General Notes
Updated 5 February 2014 - SDS
SVTA nitrogen plasma source user manual
used for Dilute Nitrides
The nitrogen gas manifold is operated in a vent/run configuration to maintain purity of the nitrogen gas introduced into the plasma cell.
When using a new UHP N2 tank, open all downstream valves before opening the UHV N2 main valve to prevent contamination of the filter.
Do not pump on the filter!!
The needle valve to the growth chamber is bakeable, but the driver is not.
The Bravo Growth Chamber SRS PG2 (Check the Pirani cables!) gauge measures the pressure in the N2 lines from the Pirani gauge in front of the Varian turbo pump.
A second Pirani gauge, (Bravo Growth Chamber SRS PG1) measures the foreline pressure in front of the BMH70 pump.
The blue valves on the N2 manifold are used to isolate the filter for baking and filter replacement. The red valve is used to open a filter bypass line when baking the N2 lines.
To adjust the MFC setpoint, press the `Mode' button. The display will show `S_P_1' and then a numeric value. The active digit can be selected by pressing the Right Arrow button. Enter the value you want to set by using the Up Arrow button. Press the `Zero' button to commit the value and set the setpoint.
Starting the Nitrogen Plasma Source
The N2 plasma source is operational for growth in the Run configuration. To enter the Run configuration, the source must first transition from the Idle configuration to the Vent configuration, and finally to the Run configuration.
Put the N2 system into the Vent configuration 12 to 24 hours before using the nitrogen for growth.
For notes on baking the N2 system, please refer to the section below Baking the Nitrogen Delivery System.
Entering the Idle Configuration from the Off Configuration
FIRST
If the system is not in the idle configuration, the two Viton seal valves inline with the turbo pumps and the quarter-turn valve after the Mass Flow Controller (MFC) should be closed. Additionally, the MFC should be set to 0 SCCM. Check that these settings are in place before proceeding to put the system in Idle configuration.
Begin by turning on the BMH70 diaphragm pump. PG1 measures the pressure in front of the BMH 70 pump. It should stabilize between 4 torr and 100 torr after turning on the diaphragm pump.
After the pressure stabilizes, turn on the BMH 70 turbo pump. The fully-spun up turbo will operate at 1200 rpm. Wait for the PG1 pressure to reach ~4e-4 torr.
Once PG1 is ~4E-4 torr, open the valve separating the BMH 70 and the Varian turbo pumps. Spin up the Varian turbo pump by pressing Start on the turbo pump controller. The fully-spun up turbo will operate at 75 krpm.
After the Varian turbo spins up, open the quarter turn valve after the MFC.
PG2 measures the pressure in front of the Varian turbo, it should rise to about 10 torr after opening the quarter turn valve.
SLOWLY open the the Viton valve in front of the Varian turbo pump. The system is now in the idle configuration
Entering the Vent Configuration from the Idle Configuration
PG2 measures the nitrogen line pressure in front of the Varian turbo. Wait until the turbo is fully spun up and the Pirani gauge bottoms out, then set the MFC to 10 SCCM.
PG2 should be about 200 mTorr. The N2 station is now in Vent Configuration.
Leave the system in Vent configuration for 12 to 24 hours before using the N2 plasma source.
Entering the Run Configuration from the Vent Configuration
Before transitioning the N2 source to the run configuration, be sure to take all other fluxes for the growth first.
Turn off the growth chamber beam flux gauge.
Turn on the plasma-sensing photodiode.
Turn on the RF power supply and verify that the power level is all the way down. The frequency should be 13.56 MHz.
Set RF to on.
Over a period of 10 minutes, ramp the RF power to 100 W forward power at a rate of 10 W/min. The reflected power should read about 30 W.
Use the variable capacitor (black knob on the source flange plasma cell box) to tune the impedance of the source and minimize the reflected power every 10 W. Note: at this point the source should not be open to the chamber.
Adjust the MFC set point to 0.2-0.4 SCCM
Note: make sure to adjust the MFC as opening the needle valve with the MFC at 10 SCCM will trigger the growth chamber interlocks!
Close the valve in front of the Varian turbo valve.
You will need to quickly open the needle valve to the growth chamber to 150. The needle valve stays open to 150 during nitride growth.
While opening the needle valve to 150, simultaneously ramp the RF power to about 340 W. You should see the plasma strike up through the plasma tube viewport. There will be a orange-pink glow.
Note: Do not take the RF power above 400 W.
If you are unable to strike a plasma below 400 W, turn down the power to about 300 W and let this sit for 5 minutes. (This is called soaking/seasoning the plasma tube)
The Growth Chamber pressure when the plasma strikes will be in the low- to mid-10^-7 torr.
Once the plasma strikes, turn down the RF power to about 300 W and tune the variable capacitor to minimize reflected power.
Run the plasma at 300 W for 15 minutes to stabilize.
Set the power on the RF generator to obtain the desired AN1 voltage. The AN1 voltage is akin to BEP for the nitrogen cracker.
The goal is to obtain the lowest RF power to maintain \Bright Mode" (H-coupled mode) plasma.
AN1 is measuring the plasma intensity. It is linearly related to the amount of nitrogen that gets incorporated into growth.
Plasma turn on/turn off has hysteresis. Operate the plasma 10 to 20 W above the turn off point.
If the plasma does not light (as it might not if it has not been run for a considerable time) pulse the nitrogen flow by setting it to 0.5 SCCM to stimulate the plasma emission.
During Growth:
Turn off the plasma at the end of the nitrogen layer. Turn down the RF power to 100 W to extinguish the plasma.
Close the needle valve.
Open the top turbo valve.
The source is now back in vent mode. At idle, the RF power is 100 W.
Turning off the Nitrogen Plasma Source
Ramp down the RF power to 0 W.
Turn off the RF power.
Turn off power to the RF controller.
Turn off the plasma photodiode.
Set the MFC to zero.
Close the quarter-turn valve after the MFC.
Close the valve in front of the Varian turbo pump.
Turn off the Varian turbo.
Close the valve between the Varian and BMH 70 pumps.
Turn off the BMH 70 turbo. Wait for the turbo to spin down (0 Hz).
Turn off the BMH 70 diaphragm pump.
Baking the Nitrogen Delivery System
Main Steps for baking the nitrogen system:
Bypass the filter
Set the flow rate on the MFC to the maximum 10 SCCCM
Pump down the lines using the turbopump
First start the diaphragm pump, then the BMH 70 turbo pump, then the stand{alone Varian turbo pump.
The BHM 70 turbo pump spins up to 1200 Hz. The Varian turbo spins up to 75 krpm.
Let the Pirani gauge bottom out before starting the bake
Bake the system.
Close the Viton seal to the turbo pumps.
Open the needle valve to the growth chamber to 150. Use the UHP He to leak check the lines using the Bravo growth chamber RGA.
Close the needle valve to the growth chamber and open the Viton seal to the pumps.
Place the N2 system in Vent Configuration. Let the lines purge with UHP nitrogen for at least 24 hours.
Set the MFC flow rate to a low value, such as 0.2 SCCM.
Close the Viton seal to the turbo pumps.
Open the needle valve to the growth chamber to 150. Flow the nitrogen into the growth chamber with the RGA running. Look for peaks other than nitrogen and argon.
Argon is the main impurity in our nitrogen source and it is not removed by the filter. Since it is an inert gas we do not have to worry about it too much.
If you see water etc. start going up then you either have a leak or the bake was not sufficient.
Close the needle valve to the growth chamber
Set the MFC flow rate back to 10 SCCM.
Open the Viton seal to the pumps.
Regulate the flow of the UHP N2 bottle to the line to 26 PSI.
Close the bypass valve (red) and open the valves to the filter (blue).
Run UHP N2 for about 1 day to fill the lines with pure N2.
Test-spark the nitrogen plasma.
Grow amazing dilute nitride materials!
General notes and concerns
The needle valve does not need to be baked
Bake the line up to the out-of-vacuum side of the needle valve
At the joint between the line from the UHP N2 and the rack system, recrimp the heater wire lines back together using tools. Ask Rodolfo for a lesson.
The heater wire is made up of four equal-length sections in parallel
All four lines can be baked from one variac. Set it to about 60%
Do not open the filter to the lines until after the bake
Do not open the UHP nitrogen bottle to the lines until after the bake and leak check