If you could process a free-standing silicon membrane (possible??) with a known (large ~cm^2)) area and density, then you could weigh it using an Cahn electrobalance model G-2 and divide by area*density to obtain the thickness .. the Cahn has a mass resolution of 50 ng! I've done this with sputtered aluminum films of 100-nm thickness on glass slides to verify a profilometer measurement. I measured the glass slide before and after a simple NaOH etch removal of the aluminum, and got excellent agreement. Interestingly, the Cahn manual describes measuring the masses of 'membrane filters' in Section 5 under special techniques. Good luck! Thomas E. Wilson Professor of Physics Marshall University Science 154 One John Marshall Drive Huntington, WV 25755-2570 Tel: 304.696.2752 FAX: 304.696.2494