Patent ID: 8178849

Claim:
An objective lens, comprising: a magnetic lens comprising an inner pole piece and an outer pole piece, wherein a gap is formed between a lower end of the inner pole piece and an inner end of the outer pole piece; an electrostatic lens comprising an electrode arrangement configured to generate an electrostatic field, the electrode arrangement comprising a first electrode and a second electrode offset from each other in a direction of traversal of charged particles during use of the objective lens; a first actuator configured to change a position of the first and second electrodes relative to a position of the magnetic lens; a first sealing member configured to prevent pressure balancing via an interspace between a lower portion of the first electrode and the inner end of the outer pole piece; and a second sealing member configured to prevent pressure balancing via an interspace between an upper end of the first electrode and the inner pole piece, wherein the magnetic lens and the electrostatic lens are adapted to be displaceable relative to each other.