Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-11-16
2001-11-27
Anderson, Bruce (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492220, C250S492200, C250S492100, C250S398000
Reexamination Certificate
active
06323500
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electron-beam exposure systems that form fine patterns of integrated circuits on substrates such as semiconductor wafers, which are exposed to electron beams. Particularly, this invention relates to electron-beam exposure systems which are suitable for cell projection to obtain high throughputs.
This application is based on Patent Application No. Hei 10-331790 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
As compared with the conventional reduction projection techniques using ultraviolet radiation, electron-beam exposure techniques are advantageous in that very fine patterns can be formed on semiconductor wafers with high resolution. Recently, it is demanded that semiconductor devices such as memories should be manufactured with high throughputs. To cope with such a demand, the electron-beam exposure systems employ the cell projection using masks specifically designed for electron beams.
According to the cell projection, a part of a LSI pattern is repeatedly imaged on a substrate in accordance with an opening shape of an aperture. This system is advantageous particularly in manufacture of memory devices such as DRAMs (i.e., dynamic random-access memories), which contain plenty of repeated patterns, because a number of times to perform exposure can be remarkably reduced. So, as compared with the conventional variably shaped electron-beam exposure which exposes patterns in such a way that a picture is drawn with a single rectangular, it is possible to remarkably reduce a writing time.
An example of the conventional electron-beam exposure system will be described with reference to FIG.
9
. Herein, an electron source
1
radiates an electron beam, a square shape of which is formed by a first aperture
2
. The electron beam is converged by first and second electromagnetic lenses
3
,
4
. Then, the electron beam is irradiated onto an electron-beam mask
5
having an opening pattern
5
a
. The electron beam transmitted through the opening pattern
5
a
of the mask
5
is subjected to reduction and convergence by a third electromagnetic lens
6
. Then, it is transmitted through an object aperture
7
and is subjected to convergence by an objective electromagnetic lens
8
. Thus, the electron beam is finally irradiated on a surface of a semiconductor wafer W.
Incidentally, the electron source
1
of the conventional type is a square surface electron source as shown in
FIGS. 10A and 10B
, in which a radiation surface
1
a
of the electron beam is formed like a flat plane. Or, the conventional electron source employs another type of the radiation surface
1
a
whose a center portion is projected. In
FIG. 9
, opening portions of prescribed sizes are formed at center portions of the aforementioned apertures
2
and
7
to allow transmission of the electron beam. That is, the first aperture
2
has an opening portion
2
a
, while the object aperture
7
has an opening portion
7
a.
The aforementioned electron-beam exposure technique suffers from problems, as follows:
In the cell projection, a pattern of a large area is imaged on the substrate at once. This increases a beam current being transmitted through the lenses and apertures. Increase of the beam current causes a so-called Coulomb effect to become large, so that an accuracy for formation of a beam shape is deteriorated due to repulsion being effected between electrons in response to an amount of the beam current. There is a disadvantage in which the beam is broadened in size so that resolution is reduced when the Coulomb effect becomes large. For this reason, it is impossible to increase the beam current so much. Therefore, it is difficult to improve the throughput. Conventionally, the electron beams radiated from the electron source have Gaussian distribution in distribution of electron-beam intensities which are distributed in connection with beam incident semi-angles of the electron beams being irradiated on the substrate. So, electrons are concentrated at a center portion of the section of the electron beam. Therefore, the Coulomb effect caused by increasing the beam current influences the conventional electron-beam exposure technique more badly as the beam incident semi-angle becomes smaller.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an electron-beam exposure system which is capable of reducing the Coulomb effect and which is capable of improving resolution and throughput.
An electron-beam exposure system is constructed such that electromagnetic lenses, apertures and an electron-beam mask (or EB mask) are arranged and aligned in line between an electron source and a substrate such as a semiconductor wafer. Herein, an electron beam corresponding to electrons radiated from the electron source is subjected to convergence and reduction by the electromagnetic lenses while it is subjected to pattern formation by the EB mask having an opening pattern, thereafter, the electron beam is irradiated onto the substrate, so that prescribed patterns are imaged on the substrate to produce integrated circuits. Within a trajectory of the electron beam which progresses from the electron source to the substrate, there are formed three crossover planes of the electrons of the electron beam, for example. The EB mask is arranged at an object plane, while the aperture is arranged at a crossover plane. In addition, one aperture is designed as an object aperture that narrows down the electron beam in sectional shape and size.
The electron-beam exposure system of this invention is characterized by providing a specific electron-beam intensity distribution in which electrons corresponding to periphery of the section of the electron beam is made higher in intensity than electrons corresponding to a center portion of the section of the electron beam. So, the electron source is designed such that a zonal-annular projection portion is formed to surround a center portion of a radiation surface. Or, it is designed such that multiple unit projections are formed to surround the center portion of the radiation surface.
To perform oblique illumination in which the electron beam is incident on the substrate in an oblique manner, the system is equipped with a modified object aperture, at the crossover plane, which is constructed by a center shield member for cutting off electrons having small beam incident semi-angles and a ring opening portion for selectively transmitting electrons having large beam incident semi-angles.
Thus, it is possible to reduce Coulomb effect by increasing an average distance between the electrons within the electron beam. In addition, it is possible to improve resolution as well as throughput in manufacture of integrated circuits on the semiconductor wafer.
REFERENCES:
patent: 4438336 (1984-03-01), Riecke
patent: 5973333 (1999-10-01), Nakasuji et al.
patent: 6020950 (2000-02-01), Shiraishi
patent: 6166387 (2000-12-01), Muraki
patent: 2000-048762 (2000-02-01), None
patent: 09-034103 (1997-02-01), None
patent: 08-017714 (1996-01-01), None
patent: 08-162389 (1996-06-01), None
patent: 2000-012454 (2000-01-01), None
Anderson Bruce
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
NEC Corporation
Quash Anthony
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