Method and apparatus for inspecting a sample having a height measurement ahead of a focal area

Apparatus for optical inspection of a sample, comprising:

• a detector assembly, which is configured to receive radiation from a focal area on the sample;
• first and second height sensors, positioned in known locations relative to the detector assembly so as to measure respective first and second heights of the first and second height sensors relative to respective first and second points on the sample on opposing sides and outside of the focal area;
• and
• a controller, which is adapted to determine an estimated height of the detector assembly, responsively to the first and second heights measured by the first and second height sensors.

A method for optical inspection of a sample, comprising:

• receiving radiation at a detector assembly from a focal area on the sample;
• translating the focal area over the sample along a translation path;
• receiving, from a height sensor positioned in a known position relative to the detector assembly, signals so as to measure a height of the height sensor relative to a point on the sample that is ahead of the focal area by a predetermined distance along the translation path;
• and
• determining an estimated height of the detector assembly responsively to the height measured by the height sensor along the at least some of the scan lines of the raster pattern.

A method for optical inspection of a sample, comprising:

• receiving radiation at a detector assembly from a focal area on the sample;
• receiving first and second signals, respectively, from first and second height sensors positioned in known locations relative to the detector assembly so as to measure respective first and second heights of the first and second height sensors relative to respective first and second points on the sample on opposing sides of and outside the focal area;
• and
• determining an estimated height of the detector assembly responsively to the first and second heights measured by the first and second height sensors.