Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-07-24
2003-02-25
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S398000, C250S3960ML
Reexamination Certificate
active
06525328
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a cell projection type electron beam lithography system and a pattern writing method. More specifically, the invention relates to an electron beam lithography system and pattern writing method, which are suitable for pattern writing using a low-energy electron beam.
2. Related Background Art
Electron beam lithography systems for writing patterns on the surface of substrates, such as semiconductor wafers, using an electron beam have particularly excellent resolution, so that the systems are widely noticed as systems meeting the demands that semiconductor circuits be further scaled down.
It is pointed out that electron beam lithography systems, which are put to practical use at present, have disadvantages that (1) the precision of patterns is bad due to the influence of the proximity effect, (2) throughput is low, and so forth.
The proximity effect is a phenomenon that electrons being incident at a high acceleration voltage are scattered in a resist on a wafer, and back-scattered electrons cause the resist around the incident point to be exposed, so that the precision of a pattern deteriorates. In order to correct this, it is required to correct dose in accordance with the layout of the pattern, so that the construction of the system is very complicated.
One of causes for low throughput is that the sensitivity of the resist is low. The reason for this is that the sensitivity of the resist depends on the quantity of secondary electrons produced by the incidence of an electron beam, whereas if an electron beam is incident at a high acceleration voltage, the incident electrons pass through the resist, so that required secondary electrons are not sufficiently produced.
As a technique capable of solving the above described two problems, there is a low acceleration electron beam lithography system. The low acceleration herein indicates an acceleration voltage capable of ignoring the influence of the proximity effect, specifically about 5 kV or less. Because, if the acceleration voltage is low, the energy of incident electrons is low, and the influence of back scattered electrons is small, so that the proximity effect can be reduced. In addition, the energy of incident electrons is low, the scattering cross section in the resist is large, so that the production efficiency of secondary electrons is high. Since low-energy secondary electrons contribute to the sensitizing of the resist, the improvement of the production efficiency of secondary electrons directly appears as the improvement of the sensitivity of the resist. The improvement of the sensitivity of the resist directly appears as the improvement of throughput.
Thus, the low acceleration electron beam lithography system has greater advantages than the high acceleration system, and has remarkable advantages particularly in the field of the direct writing. Moreover, as another advantage obtained by adopting the low acceleration electron beam lithography system, there is an advantage in that an electrostatic column (an electron optical system using an electric-field lens) can be used if the acceleration voltage is low. The electrostatic column has the merits of being easy to be miniaturized, and of having good response characteristics since no hysteresis is caused unlike magnetic-field lenses because an electric-field lens is used therein. However, on the other hand, there is a disadvantage in that the electrostatic column has a high lens voltage when it is used as a focussing lens. For example, when the acceleration voltage is 50 kV, the lens voltage is in the range of from 70 to 100 kV. This is of no practical use. However, if the acceleration voltage is low (5 kV or less), the lens voltage is about 10 kV. This can be put to practical use in the existing high voltage power supply technique. The electrostatic lens has been put to practical use as a deflecting system requiring high speed characteristics, not as a focussing lens. However, electron beam lithography systems where all lenses are constructed of electrostatic lenses have not been put to practical use.
Thus, according to the low acceleration electron beam lithography technique, there is some possibility of realizing a direct writing having high throughput, and there is some possibility of realizing a small lithography system having high controllability by electrostatic columns. It is expected that this is very effective in the formation of a pattern scaled down in future.
However, with respect to the low acceleration electron beam lithography system, there is a problem of the space-charge effect. The space-charge effect is a phenomenon that the flow of charged particles is restricted by the space potential formed by space charges. In the electron beam lithography system, the space-charge effect appears as the restriction of the emission current from a cathode and as the broadening of the beam diameter caused during the convergence of the electron beam including crossovers (so-called beam blurring). Since the space-charge effect depends on the energy of an electron beam, the space-charge effect conspicuously appears when the energy of electron beam is low. Therefore, the space-charge effect causes serious problems in the low acceleration electron beam column.
On the other hand, a cell projection type electron beam lithography system using electrostatic columns has been proposed by H. Sunaoshietal. (Jpn. J. Appl. Phys. Vol. 34 (1995), pp. 6679-6683 (which will be hereinafter referred to as “Background Art 1”)). Another cell projection type lithography system is disclosed by K. Hattori et. Al (J. Vac. Scl. Technol. B 11(6), November/December 1993, p2346 (which will be hereinafter referred to as “Background Art 2”)).
As shown in Background Art 2, a projection optical system is a double-reduction lens system comprising a reduction lens and an objective lens. The beam blurring caused by the space-charge effect occurs in the projection optical system and on the top surface of the substrate.
With respect to an electrostatic lens, two modes can be selected: one mode is a deceleration mode and the other mode is an acceleration mode. In general, the aberration performance of the acceleration mode is superior to that of the deceleration mode. However, the acceleration mode is of no practical use since the electric field of the lens must be a high electric field. Therefore, the deceleration mode is generally used. In the deceleration mode, after electrons are once decelerated in the lens, the electrons are accelerated again to be emitted from the lens.
An Electron beam establishes a crossover by the reduction lens once to be further decelerated in the objective lens. At this time, the electron beam is greatly affected by the space-charge effect.
Moreover, the surface of the substrate is most remarkably affected by the space-charge effect. In this portion, the beam blurring caused by the space-charge effect depends on the aperture angle and the focal length (the distance where electrons travel in a field-free space). That is, the influence of the space-charge effect increases as the aperture angle decreases, and the influence of the space-charge effect increases as the focal length increases. However, if the aperture angle is increased in order to reduce the influence of the space-charge effect, the influence of aberration increases. In particular, the influence of chromatic aberration increases when operation is carried out at a low acceleration voltage. Therefore, the aperture angle can not be so great. This is a serious problem when an electron optical system for a low acceleration electron beam lithography system is designed.
The influence of the space-charge effect on the surface of a substrate has been analyzed by Y. Yamazaki and M. Miyoshi (Optik 96, No. 4 (1994), pp. 184-186 (which will be hereinafter referred to as “Background Art 3”)). This has analyzed the relationship between the increasing ratio of the beam diameter and the aperture angle ratio due to the space-charge effect of
Miyoshi Motosuke
Okumura Katsuya
Yamazaki Yuichiro
Kabushiki Kaisha Toshiba
Nguyen Kiet T.
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