Here's a reference for a 50-mm-diameter (yow!) silicon nitride membrane:

Silicon nitride single-layer X-ray mask
Sekimoto, M. a; Yoshihara, H.; Ohkubo, T.
a Musashino Electrical Communication Lab., Nippon Telegraph & Telephone Public
Corp., Tokyo, Japan
Journal of Vacuum Science and Technology
Volume 21, Issue 4, 1982, Pages 1017-1021

Abstract:
In a LP-CVD process, the preparation of a silicon nitride film with a small
tensile stress and low refractive index was investigated as a function of
deposition temperature and reactant gas ratio (SiH2Cl2/NH3). A small stress film
with low refractive index can be prepared easily by high temperature deposition.
Applying the film to an X-ray membrane, a new silicon nitride single-layer X-ray
mask with a large area window (such as 50 mm in diameter) and high transparency
to visible light is realized. Using this mask, a submicron resist pattern (0.5
m line and space) can be replicated easily by a Si-K X-ray exposure system.

        --Kirt Williams Agilent Technologies