Just doing a search on INSPEC for "x-ray mask" AND "silicon nitride"
I saw that several PECVD nitride masks were reported, so it is possible.
I don't know any details.

> -----Original Message-----
> From: Michael D Martin [mailto:michael.martin@louisville.edu]
> Sent: Tuesday, November 05, 2002 11:37 AM
> To: mems-talk@memsnet.org
> Subject: RE: [mems-talk] 1 micron thick low stress nitrides?
>
>
> Kirt, thanks for the quick reply. I have a follow up question:
>   Do you know if it is possible to grow simillarly thick low stress
> nitrides using PECVD?
>
> >>> kirt_williams@agilent.com 11/05/02 09:59AM >>>
> > -----Original Message-----
> > From: Michael D Martin [mailto:michael.martin@louisville.edu]
> > Sent: Monday, November 04, 2002 3:54 PM
> > To: mems-talk@memsnet.org
> > Subject: [mems-talk] 1 micron thick low stress nitrides?
> > Hi,
> >   Does anyone know whether it is possible to produce low
> > stress nitride
> > films that are a micron thick or thicker? If so, where can I
> > get them?
> >   a related question ...
> >   How thick of a silicon nitride film would be required to span 8mm
> > after release?
> >
> > Thanks in advance,
> >           Mike Martin
>
> In the Microfabrication Laboratory at U.C. Berkeley,
> silicon-rich, low-stress silicon nitride
> is deposited using a ratio of 4 SiH2Cl2 to 1 NH3 at 835 C.
> I regularly deposited a micron of this SiN without any observable
> problems
> such as cracking or peeling.
> This process should be available through the MEMS Exchange;
> see http://www.mems-exchange.org/
> and specifically http://www.mems-exchange.org/catalog/lsn_cvd/.
>
> Even lower stress is obtained with a 5:1 ratio, although this is not a
> standard process.
> I have deposited 3 um without problem using this.
> Providers might not be as eager to run high levels of SiH2Cl2 as it
> forms Si and HCl
> in the exhaust, which is hard on the pumps.
>
> For prior work on large spans, look up the topic "x-ray masks."
> Several groups have used gold patterns on ~1-cm spans of low-stress
> silicon nitride
> for x-ray masks.
>
>       --Kirt Williams Agilent Technologies
>
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