1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and a charged particle beam writing method, and for example, relates to a writing apparatus that corrects distortion produced in a subfield (SF) and a method therefor.
2. Description of Related Arts
A lithography technique that advances microminiaturization of semiconductor devices is an extremely important process only which forms patterns in semiconductor manufacturing processes. In recent years, with high integration of large-scale integrated circuits (LSI), a circuit critical dimension required for semiconductor devices becomes minuter year by year. In order to form a desired circuit pattern on semiconductor devices, there is required a master pattern (also called a mask or a reticle) of high precision. The electron beam writing technique intrinsically has excellent resolution and is used for manufacturing a highly precise master pattern.
FIG. 6 shows a schematic diagram for describing operations of a variable-shaped type electron beam (EB) writing apparatus. As shown in the figure, the variable-shaped electron beam writing apparatus includes two aperture plates and operates as follows: A first or “upper” aperture plate 410 has a rectangular opening or “hole” 411 for shaping an electron beam 330. This shape of the rectangular opening may also be a square, a rhombus, a rhomboid, etc. A second or “lower” aperture plate 420 has a variable-shaped opening 421 for shaping the electron beam 330 that passed through the opening 411 into a desired rectangular shape. The electron beam 330 being emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to penetrate a part of the variable-shaped opening 421 and thereby to irradiate a target workpiece or “sample” 340 mounted on a stage continuously moving in one predetermined direction (e.g. x direction) during the writing or “drawing”. In other words, a rectangular shape capable of passing through both the opening 411 and the variable-shaped opening 421 is written in the writing region of the target workpiece 340 on the stage. This method of writing or “forming” a given shape by letting beams pass through both the opening 411 and the variable-shaped opening 421 is referred to as a “variable shaping” method.
In the electron beam pattern writing apparatus, there exist some factors which degrade the accuracy of the position of a pattern. For example, such factors are position accuracy degradation generated in the electron optical system, inclination or distortion of a mirror which is set at the stage to measure the stage position, and flexure or distortion of a mask serving as a target workpiece. In particular, inclination or distortion of the mirror, or flexure or distortion of the mask changes gradually over the whole mask surface. Therefore, a gradual position error (global position error) arises over the whole mask surface because of the distortion or flexure. In electron beam pattern writing apparatuses, a writing region is divided into small regions (SF: subfield), and then writing is performed for each SF. Conventionally, the global position error mentioned above is corrected by correcting a reference position of an SF when writing (e.g., refer to Japanese Patent No. 3197024).
Furthermore, when exposing a pattern of a mask onto a silicon wafer by using an exposure apparatus and the mask which is written by the electron beam pattern writing apparatus, distortion occurs in setting the mask in the exposure apparatus. For example, in an exposure/transfer apparatus using EUV (Extreme UltraViolet) lights, which is studied in recent years, distortion occurs in the mask when fixing the mask by an electrostatic chuck. Then, it is necessary to correct the distortion in advance on the mask. Hereinafter, a distorted state in the electron beam writing apparatus and distortion produced in the exposure/transfer apparatus will be called “distortion”, and correcting the distortion will be called a “distortion correction.”
FIGS. 7A and 7B show schematic diagrams for explaining a conventional distortion correction. When distortion or flexure occurs in a writing region 20 as shown in FIG. 7A, the position of a center 24 of an SF 22 is corrected so that it may be located on a straight line as shown in FIG. 7B after writing. FIG. 7B shows an example of position relation after the correction. That is, only the coordinates of the center 24 of each SF 22 are corrected without correcting the distortion of the SF 22 itself. In this case, a part of SFs, namely the SFs 22a to 22d and the centers 24a to 24d of them, are shown. According to this method, however, the correction is equally added to the shot position in the SF 22. Therefore, there is a problem that errors resulting from the distortion or flexure in the SF still remain after writing as shown in FIG. 7B. Thus, it has been impossible to perform a sufficiently precise correction.
In recent years, with the miniaturization of patterns, it is requested to highly accurately correct errors caused by the distortion or flexure in the SF. Accordingly, there exists a problem that to correct only the SF position as mentioned above is insufficient as correction precision.