1- General notice

1. Tungsten hexafluoride WF6 is a  toxic, corrosive, nonflammable liquefied compressed gas with a 2.4 psig vapor pressure at room temperature (884 Torr).  The gas is colorless but fumes white in moist air. It hydrolyses to very corrosive hydrofluoric HF on contact with moisture. You must review very carefully the WF6 Materials Safety Data Sheet before doing any work involving this gas.
    

2. WF6 is used in CVD processes to deposit tungsten films, either from the H2 or SiH4 - silane - reduction processes. In contact with moisture, it hydrolyses:
                         WF6+H2O => WOF4+2HF
   
   The WOF4 tungsten oxide is a white solid at room temperature. Further hydrolysis of WOF4 can produce yellow WO2F2, and WO2F2 can itself hydrolyses to WO3. WOF4 and WO2F2 have a high vapor pressure and will easily sublimate if appropriate heating is provided. However if WO3 is formed,  mechanical cleaning of the line is necessary. A thorough downstream N2 purge should be used. Heating the gas lines will allow to keep WOF4 and WO2F2 in the vapor phase and therefore to keep the lines cleaner.
    

3. WF6 used in LAMP is Semiconductor grade (99.9%) and comes from a 15 lbs D size cylinder from Air Products. A 20 gallon Novapure S407  gas scrubber resin is used downstream to the  process pumps in order to convert process gases and byproducts into non-volatile solids. Always check that the scrubber is on before using WF6 and that the pressure to the scrubber pneumatic valves are at least 60 psig (otherwise the valves will not open).
    

4. Breathing protection (full mask respirator) is required for changing WF6 or when assembling/disassembling gas delivery lines. An HF Lifeline sensor should also be available. Tagouts must be used to notify users of potential risks if the system is under construction/modification. Gas lines that are susceptible to house WF6 (pure or diluted) must be clearly marked. .
    

5. At least 2 knowledgeable users must be present whenever working with WF6, either during regular process, construction phase or any maintenance activity such as pump oil change
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2 - Process gas handling procedures

2.1        Gas cylinder receiving

Delivery of SiH₄ and WF₆ cylinders will be coordinated with the supplier (Air Products) and received at the rear entrance of the JM Patterson Building.  Before entering the building the cylinder will be inspected for signs of a leak (discoloring or residue) and then will be transported to the LAMP facility and placed in a cabinet until installation.

2.2        First time installation of new cylinder

This procedure is for installaion of a new cylinder on to gas system which has not been previously used with process gases.

2.2.1       Cycle purge and leak check process line

Pump out the line upstream of the regulator using through the process tool.

a)     Pump the chamber thru the turbo and monitored by the ion gauge.
b)     Close VS4
c)     Open the process valves (PVs) and set the MFC to full open.
d)     Pump on the reactor chamber using the turbo pump.
e)     After reaching a base pressure (pump for at least 15 minutes) at the ion gauge, close the process valve immediately down stream of the MFC.
f)      After approximately 60 sec open this valve and monitor the effect at the ion gauge. 

·       A significant pressure jump MAY signify be a leak in the process plumbing.  If this is observed, repeat the test after pumping on the chamber for another 15 minutes.

2.2.2       Installing and activation new cylinder

Step 1: Connect the cylinder to the gas panel

a)     Secure cylinder in place in the gas cabinet
b)     Remove cylinder cap
c)   Install the pig tail and make sure all the valves are closed

Step 2: Cycle purge the gas panel

Starting with all valves closed.

a)     Open VS5 to start N2 flow through the venturi element to generate a vacuum at VS2.
b)     Open VS3 and set the regulator open to near full pressure delivery.  c)     Open VS1 fill the panel with purge N2 and then close VS1.
d)     Open VS2 to evacuate the panel, checking for a pressure drop to near 30" Hg at the regulator low P side.
e)     Close VS2.
f)      Repeat steps c and d AT LEAST 15 times.
g)     Complete this procedure with step d.
h)     Leave the system for overnight noting the pressure at the regulator gauge.
i)      If the next day the pressure has not rised, close VS3 and proceed to the next step.

Step 3: Cycle purge the process line. 

Deliver N2 to the process line from the purge cylinder. 

a)     Open VS1 to apply pressure to the panel.
b)     Open VS3 and set pressure at regulator to ~ 10 psi.
c)     Open VS4 to pressurize the process line with the purge N2.
d)     Close VS4.
e)     Put the WF6 MFC in full open and open the upstream PV.
f)      Open the By-Pass PV to evacuate process line back to the cylinder valve VS4.
g)     Close the By-Pass PV.
h)     Open VS4 to fill the process line, then close VS4
i)      Repeat steps f through h at least 10 times.
j)      End the process with step g, evacuating the process line.
k)     Cycle Purge the panel (see above Step 2) and leave in evacuated state.
 

Step 4:  Open the cylinder

Start with all valves closed.  Verify that downstream process valves are closed.

a)     Verify that the electronic switch for the main cylinder valve is OFF.
b)     Open the WF6 main cylinder valve  at the remote panel.
c)     Open VS3 and check pressure at the panel regulator.
d)     Open VS4.

2.3        Switching cylinders

2.3.1  Removal of used cylinder

Verify that the gas cylinder main valve is closed.

Disconnect the air line to the pneumatically operated main cylinder valve.

Step 1 Cycle purge the panel.

Starting with all valves closed.

a)     Open VS5 to start N2 flow through the venturi element to generate a vacuum at VS2.
b)     Set the regulator open to near full pressure delivery.
c)     Open VS2 to evacuate the panel, checking for a pressure drop to near 30" Hg at the regulator low P side.
d)     Close VS2.
e)     Open VS1 to fill the panel with purge N2.
f)      Close VS1.
g)     Repeat steps c and d AT LEAST 10 times.
h)     Open VS1 and pressurize the panel with purge gas to provide outward purge flow when the pigtail connection is disconnected.

Step 2 Disconnect and remove the used cylinder

a)     VERIFY that the pneumatic air line to the main cylinder valve is disconnected or that the valve is closed (if manual valve).
b)     Slowly loosen CGA cylinder fitting and disconnect.
c)     Place cap over cylinder valve.
d)     Remove cylinder and store for pick-up.

2.3.2  Installation of new cylinder

Step 1: Connect the cylinder to the gas panel

e)     Secure cylinder in place in the gas cabinet
f)      Remove cylinder cap
g)     Attach the pigtail fitting to the cylinder.

Step 2: Cycle purge the gas panel

Start with VS1 open, all other valves closed.

a)     Open VS5 to start N2 flow through the venturi element to generate a vacuum at VS2.
b)     Open VS3 and set the regulator open to near full pressure delivery. 
c)     Close VS1
d)     Open VS2 to evacuate the panel, checking for a pressure drop to near 30" Hg at the regulator low P side.
e)     Close VS2.
f)      Open VS1
g)     Repeat steps c and d AT LEAST 10 times.
h)     Complete this procedure with step e (panel evacuated).
i)      Leave the system overnight noting the pressure at the regulator gauge. [A pressure rise is associated with a leak in the panel or the CGA connection.]
j)      If no signs of a leak is detected, proceed to step 3.

Step 3:  Open the cylinder

Start with all valves closed.  Verify that dowstream process valves are closed.

a)     Verify that the electronic switch for the main cylinder valve is OFF.
b)     Open the WF6 main cylinder valve  at the remote panel.
c)     Open VS3 and check pressure at the panel regulator.
d)     Open VS4.