The relationship between the resist layer thickness, the masking layer thickness and x-rays wavelength was investigated to optimize submicron structures formation in 10 - 100 micrometer resist layers. It was shown that the optimal wavelength for 50 - 100 micrometer resist layer exposure is 0.4 nm. A specially designed medium-power soft x-ray tube with a water-cooled exchangeable anode was built for experiments in a high aspect ratio structures formation. Experimental investigations were carried out for 0.417 and 1.33 nm wavelength radiation. Test x-ray masks with 0.1 micrometer Si3N4 membrane and a gold absorption layer with 0.2 - 0.7 micrometer thickness were used for high aspect ratio structures formation in ERP-9 and UVIII resists with thickness from 2 to 10 micron. The 3 micrometer thick gold mesh with 15 micrometer pitch was used as a mask for structures formation in 100 micrometer thick SU8 resist layer with 0.417 nm x-rays.