[labnetwork] Heat trace issues on DCS gas lines

[John Shott]

Steve:

Thanks for your comments about use of sub-atmospheric regulators to 
avoid the problems associated with heat tape on low vapor-pressure gases.

As I've never used these regulators, I've got a few followup questions.  
In particular, my guess is that you are balancing the pressure set point 
so that you have a low enough line pressure that you don't have 
liquification problems but high enough pressure that you maintain enough 
differential pressure across your MFCs. Correct?

In LPCVD systems, what is your recommended pressure set point for DCS 
and BCl3?  Do you have an anhydrous HF vapor etch tool and, if so, do 
you also use a sub-atmospheric regulator on it?

Do you find that some MFCs are better than others when they have less 
than 15 PSI differential pressure across them?

If you have a considerable line length between your gas cylinder and the 
tool, would you place your sub-atmospheric regulator closer to the tool 
or closer to the cylinder?  Does that choice depend on the magnitude of 
temperature variation at the cylinder and/or along the line, assuming 
that temperature stability is pretty good at the tool ... but maybe not 
so good along the line or at the cylinder?

If you had a VMB between your cylinder and multiple tools, would you use 
a single sub-atmospheric regulator upstream of the VMB or individual 
regulators on each tool?

Do you expect that this approach would work equally well for a higher 
pressure process like epi where deposition pressures might be as high as 
100 Torr (~2 PSIA)?

I, for one, wish I didn't have so much experience with heat tape ... and 
thank you for your insights on an alternative approach.

Thanks again,

John

On 1/21/2015 1:13 PM, Paolini, Steven wrote:
>
> Hmmm,
>
> Not knowing the watt/foot rating of the trace, my first guess would be 
> that it got too hot under all that insulation. I have never heard of 
> straight vs. spiral advantages or disadvantages but I would be willing 
> to bet that at the fastener locations, the trace would have /cool 
> /spots since the heat is being transferred to the tubing more 
> efficiently.
>
> I quit using heat trace for low vapor pressure materials into vacuum 
> systems years ago, I’m assuming your DCS is being used for LPCVD and 
> that would be a perfect candidate for running the gas at 
> sub-atmospheric pressures to lower the boiling point. I use this 
> method with all my prone-to-liquify-high-vapor pressure gases and have 
> never experienced a “flooded” MFC since doing so.
>
> Best of luck
>
> Steve Paolini
>
> Equipment Dood
>
> Harvard University Center for Nanoscale Systems
>
> *From:*labnetwork-bounces at mtl.mit.edu 
> [mailto:labnetwork-bounces at mtl.mit.edu] *On Behalf Of *Vito Logiudice
> *Sent:* Wednesday, January 21, 2015 12:23 PM
> *To:* Labnetwork
> *Subject:* [labnetwork] Heat trace issues on DCS gas lines
>
> Dear Colleagues,
>
> We are experiencing an issue with the heat trace on our Dichlorosilane 
> gas line. The all-welded 1/4" SS line is encapsulated with a 1/2" SS 
> outer containment  line which is itself heat traced with a single 
> strand of heat trace that runs the entire length of the coax assembly. 
> The 120 foot line is insulated as shown in the attached photo. A 
> portion of the heat-trace appears to have failed prematurely (it was 
> installed less than one year ago) and we are wondering if the method 
> of installation may be the cause.
>
> The heat trace was not installed in a spiral fashion around the outer 
> 1/2" tube. Rather it was installed in a straight fashion along its 
> entire length with "heat trace fastening tape" located every four feet 
> or so. A member of my team has suggested that such a straight rather 
> than spiral installation may have caused hot spots (at the fastening 
> locations) which may have in turn caused the failure.
>
> I would appreciate hearing from the community on this point: Are the 
> heat traces around your low pressure gas lines spiral-wound around the 
> lines or are they installed in a straight fashion and somehow fastened 
> along the entire length?
>
> Other insights/suggestions on the proper heat tracing of gas lines by 
> experts in the field as well as comments on possible causes of 
> premature heat trace failure are very much welcome and appreciated. 
> Thank you.
>
> Regards,
>
> Vito
>
> --
>
> Vito Logiudice  P.Eng.
>
> Director of Operations, Quantum NanoFab
>
> University of Waterloo
>
> Lazaridis QNC 1207
>
> 200 University Avenue West
>
> Waterloo, ON           Canada N2L 3G1
>
> Tel.: (519) 888-4567  ext. 38703
>
> Email: vito.logiudice at uwaterloo.ca <mailto:vito.logiudice at uwaterloo.ca>
>
> Website: https://fab.qnc.uwaterloo.ca
>
>
>
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