1. Field of the Invention
This invention relates to a pattern writing method with charged particle beam, and for example, to a writing method of a writing apparatus which varies a beam shape by letting the beam pass through two shaping apertures.
2. Description of Related Art
The lithography technique that drives advancement of micro-scaling of semiconductor devices is extremely important being the only process to form patterns in semiconductor manufacturing processes. In recent years, with high integration of large-scale integrated circuits (LSI), critical dimensions required for semiconductor device circuits are shrinking year by year. In order to form a desired circuit pattern on semiconductor devices, a master pattern (called a mask or a reticle) of high precision is required. The electron beam intrinsically has excellent resolution and is used for generating such highly precise master patterns.
FIG. 13 is a schematic diagram to illustrate operations of a variable-shaped electron beam (EB) type pattern writing apparatus. As shown in the figure, the variable-shaped electron beam writing apparatus, including two aperture plates, operates as follows: A first shaping 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 shaping 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 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through the variable-shaped opening 421 and thereby to irradiate a target workpiece or “sample” mounted on a stage which is continuously moved in one predetermined direction (e.g. X direction) during the writing or “drawing.” In other words, a rectangular shape formed as a result of passing through both the opening 411 and the variable-shaped opening 421 is written or “drawn” in the writing region of a target workpiece 340 on the stage. This method of forming a given shape by letting beams pass through both the opening 411 and the variable-shaped opening 421 is called a “variable shaped” method.
As mentioned above, each side of a formed rectangle is shaped by either of the opening 411 or the variable-shaped opening 421. In the variable-shaped type electron beam pattern writing apparatus, beam resolution degradation is induced by space charge effect. The space charge effect is proportional to multiplication result of (or product of multiplying) current density, shot area, and beam travel distance. Concretely, the space charge effect is incurred as follows: In FIG. 13, assuming the current density is constant and common over the area in question, the area of the opening 411 is denoted S0, the area of the beam which passes through the variable-shaped opening 421 after passing through the opening 411 is denoted S2, the distance from the opening 411 to the variable-shaped opening 421 is denoted L1, and the distance from the variable-shaped opening 421 to the target workpiece 340 is denoted L2. In that case, the space charge effect on the side of the beam shaped by the opening 411 is proportional to L1×S0+L2×S2. On the other hand, the space charge effect on the side of the beam shaped by the variable-shaped opening 421 is proportional to L2×S2. Since the space charge effect on the side of the beam shaped by the opening 411 is larger, resolution thereon deteriorates more than the other side.
FIG. 14 shows an example of beam profile of a beam formed by the first and second shaping apertures. It is known from FIG. 15 that the resolution deteriorates more along the side formed by the first shaping aperture (opening 411) than the other side formed by the second shaping aperture (variable-shaped opening 421).
For the charged beam (electron beam) exposure system with low acceleration voltage for directly forming a pattern onto a wafer, a technique to reduce the influence of the space charge effect induced between the first shaping aperture and the second shaping aperture, is already invented and disclosed. Specifically, in the beam exposure apparatus, plural rectangular openings with either different shape or area size are added to the first aperture, and deflection scheme is arranged so that the smallest area opening on the first aperture can be selected to enable irradiation on the desired region on the second shaping aperture. (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2006-128564).
As can be seen from FIG. 14 the resolution deteriorates more along the side formed by the first shaping aperture, which is further away from the target plate, than the other side formed by the second shaping aperture, closer to the target.
FIG. 15 shows an example of profile, dimension, and center of a beam formed by the first and second shaping apertures. It can also be seen from FIG. 15 that dimensional differences are induced among pattern 94 developed under proper condition in terms of time (threshold +/−0), pattern 92 developed in shorter developing time than the proper setting (threshold −), and pattern 96 developed in longer developing time than the proper setting (threshold +). Thus, variations of development time cause deterioration of pattern dimension accuracy. Furthermore, center positions 93, 95 and 97 of the patterns 92, 94, and 96 respectively do not match due to deteriorated and variable resolution mentioned above.
While the example mentioned above refers to a case of forming a pattern with one shot, there are, in reality, many cases where one pattern is formed by combining multiple shots. FIG. 16 shows an example of a pattern formed by combining multiple patterns. In this case, one pattern 31 is formed by stitching combining pattern 33 and pattern 35. FIG. 17 shows the two shots and their respective profiles for forming the pattern shown in FIG. 16. Combining formed beams 85 and 87 results in a new beam with the synthesized beam profile 89. Even in this case, the same problems as shown in FIG. 14 are induced. In addition, there are such error factors as; positioning error of datum position 86 of shot 85, positioning error of datum position 88 of shot 87, and a error in shot dimension L3 of shot 85 with a side resolution of which is deteriorated. It is not easy to cope with such three error factors.