Inspection devices making use of an electron beam have been in use to detect, for example, foreign objects and circuit pattern defects on semiconductor wafers. Among such electron beam type inspection devices, those of a scanning electron beam type for scanning a sample with a point-converged electron beam have been mainstream devices.
In an electron beam type inspection device, to inspect a sample, i.e. a semiconductor wafer, with desired resolution and sensitivity without damaging the sample, the energy of an irradiation electron beam is controlled by applying a voltage to the semiconductor wafer. This voltage application is referred to as “retarding.” Japanese Unexamined Patent Publication No. Hei11(1999)-108864 discloses an inspection device called a projection-type inspection device as an observation technique in which a retarding voltage is applied to a sample and in which an electron beam is made use of.
In the observation technique, a sample is, instead of being scanned with a point beam, irradiated with an areal beam which can cover a two-dimensional area of the sample so as to make inspection faster than when a technique making use of a scanning electron beam is used. In the technique, a negative retarding voltage whose absolute value is approximately the same as that of an acceleration voltage is applied to a sample, and an electron beam emitted toward the sample turns back immediately before colliding with the sample. This turning back of the electron beam occurs in an equipotential plane formed, above the sample, by the retarding voltage, and an image reflecting a contrast generated according to the tilt of the equipotential plane is formed.
When the retarding voltage is applied to the sample, an electric field is generated above the sample. When the sample is tilted, the electric field is also tilted. Normally, the electron beam is, by adjusting an electron optical lens, made incident perpendicularly on the sample assumed to be positioned horizontally. When the sample is not horizontally positioned, the trajectory of the electron beam deviates from the optical axis to cause, for example, beam shifting and image shading, making it difficult to obtain an accurate image for observation. Thus, a device which uses a charged particle beam and a retarding voltage can be affected by a disturbed electric field.
A technique in which the optical axis of an irradiation electron beam is corrected according to the tilt of a sample surface so as to make the irradiation electron beam orthogonal to the sample surface is disclosed in Japanese Unexamined Patent Publication No. Hei09 (1997)-293474. In this technique, a correction signal is superimposed on a deflection coil for electron beam scanning, and the electron beam is tilted so that its axis is orthogonal to the sample surface.