Dear Yagmur Demircan Yalcin , apparently the fabrication process is well stablished from a 1977 AT&T patent:

https://patents.google.com/patent/US4037111A/en

A slightly more elaborate procedure is described also in some papers:

Article Cost-effective mask fabrication on Kapton membrane for deep ...

The basic idea would be the same used for stretching the membrane (leader) of a drum.A Kapton film is glued to a ring with an epoxy glue, and once the glue is cured the ring is stretched by means of screws or by pushing another ring (or a conical ring) inside the first one. Because this operation can leave some deformations on the glued area of the Kapton film, a new ring or frame with smaller diameter (size) usually made of Pyrex glass is bonded to the stretched Kapton, (in the article they use a thermal curing of this epoxy to take advance of the mismatch between the thermal coefficents of aluminum ring and the Pyrex ring, to further increase the strech over the Kapton film). Then the last step would be to cut the film around the Pyrex frame and the Kapton film would be ready to be used as substrate for X-ray lithographic mask.

The rings or frames used must be machined to be flat in the contact area with the Kapton.

Some companies applied some variations of this method, such as the one described in the following work:

Article Polyimide-based X-ray masks with advanced performance of pat...

But after a quick search, unfortunately, it looks like the company is no longer working. Hopefully some other could still provide this kind of product.

Hope this helps. Good luck with your research work.

thanks a lot :)

You´re welcome Yagmur Demircan Yalcin , I was looking for some company selling these kind of Kapton blanks, but without any real success.

Good luck with your research.

Regards.