[labnetwork] Heat trace issues on DCS gas lines

[Paolini, Steven]

John,
  I'll try to answer your questions to the best of my knowledge in a different color below.

From: John Shott [mailto:shott at stanford.edu]
Sent: Thursday, January 22, 2015 1:18 PM
To: Paolini, Steven; Vito Logiudice; Labnetwork
Subject: Re: [labnetwork] Heat trace issues on DCS gas lines

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?
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?
Tricky question to answer, it is dependent of your flow requirements. My rule of thumb is to call for full flow in the MFC and slowly back of the regulator until the flow begins to suffer. I then add about 10%, this works for me. I do not have any experience with an HF vapor etch tool yet.


Do you find that some MFCs are better than others when they have less than 15 PSI differential pressure across them?
Brand wise- no. Orientation-yes! Vertical MFC installations have a constant convective flow across the sample tube that helps minimize "flooding". You must specify orientation when ordering them. Some of the newfangled digital units claim to not be orientation sensitive. I refrain from mentioning brand names since the moderator of this awesome site has asked us not to.


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?
Given the choice, I would place the regulator right at the gas source to keep the material in gaseous form for the entire circuit. Liquid condensing in any part of the circuit will have a behavior effect on components like regulators. Better to keep gas a gas.


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?
I presently run a sub-atmospheric both at the gas cabinet and VMB. The gas cabinet target pressure is 0 PSI and we reduce it down to around 5"hg at the VMB for both gases. Please keep in mind that a non sub-atmospheric regulator MIGHT appear to control at first but you will have problems with it hanging up and choking the delivery from time to time.


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 would use the same approach to setting the regulator on these higher pressure processes. There will, undoubtedly, be a needed increase in pressure to provide the differential to obtain higher flows at a higher pressure.


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
I'm always happy to help,
  Steve
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> [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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