Patent ID: 6933512

Claim:
A charged particle beam instrument comprising: a charged particle beam source for producing a charged particle beam having a probe current; a first focusing means for focusing the charged particle beam and varying the probe current of the charged particle beam impinging on a specimen; a second focusing means for varying the degree of focus of said charged particle beam impinging on the specimen; a first control portion for controlling said first focusing means; a second control portion for controlling said second focusing means; a detection aperture for intercepting a part of the current of the charged particle beam at the outer portion of the charged particle beam, the remainder of particle beam passing the aperture detector impinging the specimen; and means associated with said detection aperture for producing a detected signal, such that in response to said detected signal the first control portion for said first focusing means maintains constant the probe current of the charged particle beam impinging on the specimen and the second control portion for said second focusing means adjusts the focus of said charged particle beam.