Apparatus and method for inspecting a surface of a sample
The invention relates to an apparatus for inspecting a surface of a sample, wherein the apparatus comprises: at least one charged particle source for generating an array of primary charged particle beams, a condenser lens for directing all charged particle beams to a common cross-over, a lens system for directing the primary charged particle beams from the common cross-over towards the sample surface and for focusing all primary charged particle beams into an array of individual spots on the sample surface, and a position sensitive secondary electron detector positioned at least substantially in or near a plane comprising said common cross-over.
1. An apparatus for inspecting a surface of a sample, wherein the apparatus comprises
a multi beam charged particle generator for generating an array of primary charged particle beams,
a condenser lens for directing all charged particle beams to a common cross-over,
a lens system for directing the primary charged particle beams from the common cross-over towards the sample surface and for focusing all primary charged particle beams into an array of individual spots on the sample surface, wherein the primary charged particle beam path comprises an optical axis,
wherein the apparatus comprises a position sensitive secondary electron detector with a detection plane positioned at least substantially in a plane which is substantially perpendicular to the optical axis and which comprises said common cross-over, wherein the lens system is arranged for focusing secondary electron beams in the detection plane.
2. The apparatus according to claim 1 , wherein the lens system is arranged for projecting the secondary electrons from the individual spots on the sample surface to individual spots on the secondary electron detector.
3. The apparatus according to claim 2 , wherein the lens system is arranged for imaging the sample surface onto the secondary electron detector using the secondary electrons.
4. The apparatus according to claim 3 , wherein the lens system is arranged for imaging the sample surface onto the secondary electron detector with an optical magnification in a range from 5 to 400 times.
5. The apparatus according to claim 1 , wherein said secondary electron detector comprises a hole for passing said primary charged particle beams there-through.
6. The apparatus according to claim 5 , wherein said secondary electron detector is arranged such that the common cross-over is at least substantially located in the hole.
7. The apparatus according to claim 1 , wherein the lens system comprises a magnetic objective lens.
8. The apparatus according to claim 7 , wherein the apparatus comprises a sample holder which is arranged for positioning the sample surface to be immersed in the magnetic field of the objective lens.
9. The apparatus according to claim 1 , wherein the lens system comprises a single objective lens.
10. The apparatus according to claim 1 , wherein the lens system comprises a objective lens array having an array of small objective lenses.
11. The apparatus according to claim 1 , wherein the apparatus comprises a field generator for providing an electrostatic field that accelerates the secondary electrons from the sample surface towards the secondary electron detector.
12. The apparatus according to claim 11 , wherein the lens system comprises a magnetic objective lens, wherein the field generator is arranged for providing an electrostatic field between the sample and a pole piece of the magnetic objective lens.
13. The apparatus according to claim 1 , wherein the pitch between the spots on the sample surface is between 0.3 and 30 micrometers.
14. The apparatus according to claim 1 , wherein the detector is a CCD camera, a CMOS camera, an array of avalanche photo diodes, photo multipliers or PN junction semiconductor detector which gets signal from secondary electrons directly.
15. The apparatus according to claim 1 , wherein the detector comprises a fluorescent screen arranged at least substantially in or near the plane comprising said common cross-over and an optical arrangement for conveying photons from the fluorescent screen to a CCD camera, a CMOS camera, an array of avalanche photo diodes or photo multiplier.
16. A method for inspecting a surface of a sample comprising the steps of:
generating an array of primary charged particle beams using a multi beam charged particle generator;
directing all charged particle beams to a common cross-over using a condenser lens, wherein the primary charged particle beam path comprises an optical axis;
directing the primary charged particle beams from the common cross-over towards the sample surface and focusing all primary charged particle beams into an array of individual spots on the sample surface using a lens system;
wherein the lens system is arranged for focusing secondary electron beams in the detection plane; and
detecting secondary electrons which originate from the individual spots on the sample surface using a position sensitive secondary electron detector with a detection plane positioned at least substantially in a plane which is substantially perpendicular to the optical axis and which comprises said common cross-over.
17. The method according to claim 16 , in which the sample surface is moved at constant speed in a first direction while the primary charged particle beams are scanned repeatedly in a second direction at least substantially perpendicular to the first direction.
18. The method according to claim 17 , in which the lens system comprises a scanner for scanning the primary charged particle beams, wherein the method comprises the step of selecting pixels for detecting one specific secondary electron beam in dependence on the settings of the scanning of the primary charged particle beams by the scanner.