Method and apparatus for controlling topographical variation on a milled cross-section of a structure

A method of measuring a dimension of a cross-section of a structure composed of a first material, the method comprising:

• depositing a layer of a second material on said structure, said second material having mill rates at higher incidence angles that matches the mill rates of the first material at those incidence angles;
• directing an ion beam at the structure in order to expose a planar cross-section of the structure and said layer of a second material;
• directing an electron beam at the planar cross section;
• determining the edge positions for the desired dimensions of the cross-section;
• and
• determining the distance between said edge positions.

A method of choosing a first material to be deposited as a protective layer on a structure composed of a second material prior to milling in order to control topological variation of a cross-section of said structure, the method comprising:

• determining whether a planar or non-planar cross-section topography is desired;
• determining the approximate mill rate of the second material at higher incidence angles;
• if a planar cross-section is desired, selecting a first material from known materials having a mill rate at higher incidence angles that match the mill rates of the first material at those incidence angles;
• and
• if a non-planar cross-section is desired, selecting a first material from known materials having a mill rate at higher incidence angles that does not match the mill rates of the first material at those incidence angles.