X-ray lithography mask

In a method for making a mask of varying thickness for use with X-ray lithography to provide multilevel devices using a single exposure of X-rays comprising the steps of:

• a. depositing a thin metal layer on the surface of a substrate, said layer and said substrate both being highly transparent to X-rays,
• b. adherently applying a uniformly thick layer of electron-sensitive resist on the surface of said thin metal layer,
• c. exposing an electron beam on preselected regions of said resist, in a single exposure step to a plurality of different selected areas of exposure having predetermined variable lengths of time the electron beam dwells on a selected area yielding variable densities of exposure to said electron beam,
• d. developing all such exposed regions for the same time period until only the region of resist having had the greatest density of exposure is completely removed to provide a first opened region exposing a first portion of said surface of said thin metal layer but the other exposed regions of resist are only partially removed according to their densities of exposure, leaving the surface of said thin metal layer thereunder covered,
• e. depositing a first layer of X-ray absorbing material into said first opened region onto said first portion and not where said resist remains,
• f. developing said resist again until that region of said resist having had the second greatest density of exposure is removed to provide a second opening region exposing a second portion of said surface of said thin metal layer, but the other exposed regions of resist that have had a lesser density of exposure remain partially, leaving the portions of said thin metal layer thereunder covered by resist,
• g. depositing a second layer of X-ray absorbing material into both said first and second opened regions onto said first and second portions to add an equal thickness of material on both said portions yielding a greater thickness of X-ray absorbing material on said first portion than on said second portion, and
• h. repeating steps (d) and (e) in consecutive order until all exposed resist regions are opened and a mask is produced that includes deposits of X-ray absorbing material of varying thickness on each of said portions and is accordingly differentially absorbent of an X-ray beam at said different regions of said mask.