1. Field of the Invention
The invention relates to a charged particle beam irradiation system which irradiates a specimen with charged particles such as electron, and particularly to a charged particle beam irradiation system which analyzes the aspect on the surface of a specimen using a secondary discharged signal.
2. Description of the Related Art
In the semiconductor manufacturing industry, a scanning electron microscope (SEM), a CD-SEM for performing critical dimension scanning, and an inspection SEM for inspecting the pattern shape and the like are widely used as an inspection device of a semiconductor device.
In these days, a highly integrated semiconductor device has been developed rapidly in the semiconductor manufacturing industry. According to this, various kinds of electrically-insulating thin films are formed on a semiconductor device. When electron beam is irradiated on an insulating material, electric charges are accumulated through secondary electron discharge and a specimen becomes charged. When an electric field is produced by the charge of the specimen, the electron beam incident on the specimen is bent and the irradiation position of the electron beam is deflected.
The amount of electric charges on the specimen is determined by the amount of an incident electron beam, the physical property of an insulating material, the presence or absence of a path having diffusion on electric charges and its resistance, and the like.
In the electron beam scan, a scanning area usually rectangular is scanned sequentially from one end to the other. The accumulated electric charges within the scanning area are spatially asymmetrical. The electric charges are diffused through a conductive material existing within the scanning area and its vicinity. The accumulated electric charges are accompanied by a time fluctuation of a large time constant. According to the spatial asymmetry and the temporal fluctuation of the accumulated electric charges, the electric field spatially and temporally fluctuates. According to this, the irradiation position of the electron beam fluctuates spatially and temporally.
In the case of a scanning electron microscope, an image is shifted according to the charge of the specimen. Hereinafter, this phenomenon is referred to as “image drift”. Generally, in the scanning electron microscope, the same area is scanned a plurality of times (each scan is referred to as “frame”) and the obtained images are averaged, to improve the S/N ratio. Therefore when the positions of the images are deflected from each other in the respective frames, the image obtained as the averaging result deteriorates remarkably in resolution. A charged particle beam irradiation system which irradiates a charged particle beam also has the same problem.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-142038,
Non-Patent Document 1: “Charge Control During Photomask Critical Dimension (CD) Metrology” written by David Joy, International SEMATEC Technology Transfer #3114452B-ENG (2004),
Non-Patent Document 2: John C. Russ: Computer-Assisted Microscopy; Plenum Publishing Corp. New York 1990: pp 40-41