[labnetwork] Titanium hazard

[Pramod C Karulkar]

Fine particles of titanium ignite very easily. Aluminum  and many other 
metal particles do the same but little less vigorously. Vacuum chamber 
surfaces coated with titanium oxidize rapidly giving rise to "popping" 
sound if the vacuum system is opened in room air immediately after 
deposition.  That is why Ti deposition SOPs require a delay in opening 
the chamber to room air after venting it using N2.  Cleaning vacuum 
system that is used for Ti deposition creates hazardous situation, 
particularly if  very powdery matter is present.

Speculation on what happened:  (1) Static electricity could be a culprit 
but then the ignition would occur during other non-titanium cleaning 
operations at other similar fab stations.  (2) Wipes heavily soaked in 
isopropyl alcohol and the waste basket overwhelmed with alcohol vapor 
might have denied titanium particles access to oxygen. This is OK as 
long as the wipes with Ti particles sit in the basket and slowly 
dry/oxidize.   Turbulence created by dropping more wipes in the basket  
might have brought some dry fine particles of titanium in contact with 
oxygen in the turbulence resulting in ignition.

Allowing  system surfaces to oxidize slowly for an extended time may 
reduce chances of this happening again.    Typically this is what you 
would do: Vent the chamber using dry nitrogen at the end of the day but 
DO NOT EXPOSE IT TO ROOM AIR.  Thus, do not raise the bell jar chamber 
or open the chamber door/window that opens to room air. Allow the system 
to remain at atmospheric pressure overnight. Residual oxygen in the 
chamber will oxidize titanium deposits slowly.  Chamber can be wiped 
next day using IPA/Water mixture and vacuumed.  Some extra pumping, 
baking, pre-coating may be required to attain previous levels of vacuum 
when the system is reused for processes.

Allow an old fashioned chamber to "wait" similarly at atmospheric 
pressure before cleaning  if opening  it to air immediately after 
deposition  is necessary to  take out the samples/wafers.

You may consider using thin aluminum foil shields if possible that 
minimize need for cleaning the particle matter.  Some change in cleaning 
interval protocol (cleaning more often) may be needed to avoid build up 
of excess loose titanium.   Exposing the chamber to residual oxygen in 
between deposition runs might help to reduce the formation of atomically 
clean  Ti particles.

Hope this helps.
Pramod

Pramod C Karulkar Ph. D.
Home 2*5*3* 3*0*3 0*4*1*8
6024 33rd Street Ct NW
Gig Harbor WA 98335




On 5/6/2013 6:49 AM, Morrison, Richard H., Jr. wrote
>
> Hi Everyone,
>
> Last week one of the staff was wiping off some evaporator parts that 
> were covered with Titanium metal. He was using texwipes soaked with 
> IPA. He tossed the texwipes in the trash then accidentally dropped 
> some wipes saturated with IPA on top of them and a fire started. It 
> self extinguished after 1 minute but it melted the bottom of a plastic 
> trash can.
>
> Have any of you had that happen or something similar? Needless to say 
> this is very alarming and I need to understand what could have caused 
> this and develop a corrective action plan.
>
> Rick
>
> Draper Laboratory
>
> Group Leader Microfabrication Operations
>
> 555 Technology Square
>
> Cambridge Ma, 02139-3563
>
> www.draper.com
>
> rmorrison at draper.com
>
> W 617-258-3420
>
> C 508-930-3461
>
>
>
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