Apparatus and method for projecting an array of multiple charged particle beamlets on a sample

An apparatus for inspecting a sample, the apparatus comprising:

• a sample holder for holding the sample at a sample plane;
• a charged particle column for generating an array of multiple charged particle beamlets and directing said array of multiple charged particle beamlets towards the sample holder,
• wherein said charged particle column comprises an objective lens for focusing the charged particle beamlets of said array of multiple charged particle beamlets in an array of charged particle beam spots at or near the sample plane,
• wherein the objective lens comprises a magnetic lens common for all charged particle beamlets of said array of multiple charged particle beamlets;
• a light detector for detecting photons created by one or more charged particle beamlets of said array of multiple charged particle beamlets when they impinge on the sample, or when said one or more charged particle beamlets impinge onto a layer of luminescent material after transmission through the sample,
• a light optical assembly for projecting or imaging at least part of said photons along an optical beam path onto said light detector, wherein the light optical assembly comprises an optical objective lens for collecting photons from said sample and/or said layer of luminescent material,
• wherein the apparatus further comprises an optical focus sensor which is configured to provide a signal which is dependent on the distance between the sample plane and the optical objective lens, and
• a control unit which is configured to control the position of the sample holder, at least in a direction parallel to the optical axis of the charged particle column, on the basis of the signal from the optical focus sensor to keep a part of the sample that is inspected by means of the array of multiple charged particle beamlets positioned at a desired distance from the magnetic lens to keep the array of multiple charged particle beam spots at or near the sample plane in a desired orientation.