Ion sources, systems and methods

A method, comprising:

• generating an ion beam by interacting a gas with a gas field ion source;
• interacting the ion beam with a sample including a feature, the ion beam having a spot size of 50 nm or less on a surface of a sample;
• and
• determining the size of the feature wherein the sample comprises a semiconductor article, the feature comprises a line, and the method measures line edge roughness or line width roughness.