[labnetwork] PECVD dilute gas question(s)

[Demis D. John]

Some process data regarding N2 vs He dilution:
https://wiki.nanofab.ucsb.edu/wiki/PECVD_Recipes#Low_Stress_Si3N4_(PECVD%231)
[image: image.png]

As Dr. Codreanu mentioned - you can see the Refractive Index (a proxy for
stoichiometry or a high-level "quality") is the same regardless of He or N2
%, while allowing for tuning of film stress.
So at least from a processing standpoint, either will work.

(FYI, I made this plot pretty early in grad school, so plotting N2 vs He,
where the He came from the 98% He dilution of SiH4, and N2 was the actual
MFC gas flow I varied, may or may not be the "correct" plot to have
generated...)

-- Demis (contact info <https://wiki.nanotech.ucsb.edu/wiki/Demis_D._John>)
*Reminder*: The NanoFab has a publications policy
<https://wiki.nanotech.ucsb.edu/wiki/Frequently_Asked_Questions#Publications_acknowledging_the_Nanofab>


On Mon, Apr 7, 2025 at 4:48 PM Paolini, Steven <spaolini at cns.fas.harvard.edu>
wrote:

> SiH4 diluted in Helium poses pumping challenges. Helium is notoriously
> hard to pump with a turbo and the large amount of mixture needed to grow Si
> films leads to problems with pressure control and deposition rates. I know
> I will attract some arguments against using 100% SiH4 but the main reason
> to use a pure mixture is to reduce the amount of cylinder changes that are
> well documented as being the main source of accidents.
> Equipment Dood.
> Steve Paolini
> ------------------------------
> *From:* labnetwork <labnetwork-bounces at mtl.mit.edu> on behalf of Nathan
> Aultman <naultman at creol.ucf.edu>
> *Sent:* Monday, April 7, 2025 10:49 AM
> *To:* labnetwork at mtl.mit.edu <labnetwork at mtl.mit.edu>
> *Subject:* [labnetwork] PECVD dilute gas question(s)
>
>
> Hello Labnetwork,
>
>
>
> We have a PECVD system using dilute silane (2%)/N2.  Some labs with
> similar systems use dilute silane (5%)/helium.  We are nearing the end of
> our current cylinder and will need a replacement soon.  What are the
> advantages and disadvantages when choosing the dilute gas?  Does choosing a
> helium mixture produce more desirable films and/or quality?  In other
> words, is it worth it to purchase a cylinder with helium?  Also, what would
> be the reason(s) for using helium as an additional process gas?  E.g., a
> separate MFC and gas line.
>
>
>
> Thank you all in advance.
>
>
>
> Nathan C. Aultman
>
> University of Central Florida
>
> 407-823-6852
>
> naultman at creol.ucf.edu
>
>
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