Method for making a photomask assembly incorporating a porous frame

A method of making a porous silica frame suitable for use in a photomask assembly, comprising:

• preparing a dry gel comprising more than 99.9% silica, using a sol-gel method;
• halogenating the dry gel by heating the dry gel from about 650° C. to about 1,050° C., at a heating rate of about 25° C./hr in an atmosphere of about 33% chlorine and about 67% helium, and maintaining the dry gel at about 1,050° C., for a duration of about 1 hour in the atmosphere;
• partially densifying the halogenated dry gel by heating the halogenated dry gel from about 1,050° C. to about 1,180° C., at a heating rate of about 25° C./hr in an atmosphere of about 7% oxygen and about 93% helium, and maintaining the halogenated dry gel at about 1,180° C. for about 4 hours;
• and
• machining the partially densified dry gel into a desired frame shape having a flatness of less than 1 micrometer.