Methods and apparatus for measurement of relative critical dimensions

An apparatus comprising:

• a source for generating an incident electron beam;
• a scanning system for controllably deflecting the incident electron beam to scan the incident electron beam over a surface such that secondary electrons are emitted therefrom;
• a detection system for detecting the secondary electrons so as to generate an image data;
• and
• a control and processing system programmed to
• obtain a reference image,
• define a region of interest in the reference image,
• obtain a target image,
• align the target and reference images,
• locate the region of interest in the target image, and
• measure a relative critical dimension within the region of interest in the target image.

A method of measuring a relative critical dimension (RCD), the method comprising:

• scanning along a scan length that is perpendicular to the RCD;
• measuring point RCDs along the scan length;
• applying a filter to the point RCDs;
• and
• averaging the point RCDs.