Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-02-23
2002-03-12
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000, C430S942000, C250S492200, C250S492220, C250S492300
Reexamination Certificate
active
06355383
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electron-beam exposure system, a mask for electron-beam exposure and a method for electron-beam exposure, which are employed for manufacturing a semiconductor device. In particular, it relates to an electron-beam exposure system, a mask for electron-beam exposure and a method for electron-beam exposure, which are suitable for proximity effect correction.
2. Description of the Related Art
In electron-beam exposure, proximity effect due to scattered electrons in a resist layer and a substrate considerably affects a pattern linewidth accuracy. Proximity effect correction is, therefore, one of the most important technical elements.
In cell projection lithography which is the most popular electron-beam exposure process, a dose compensation method has been employed, which requires a complicated calculation by the self-consistent method using the exposure intensity distribution (EID) function or the pattern density method.
On the other hand, in scattering with angular limitation in projection electron-beam lithography which has attracted attention as a next-generation electron-beam exposure technique, proximity effect has been corrected by a compensation method using a part of scattered electrons as a correction beam to which GHOST technique is applied (i.e., SCALPEL
®
GHOST technique).
A scattering-angle limiting type of electron-beam exposure process employs a segmented transfer technique where a given pattern of a whole chip to be formed or one of its several regions are divided into a plurality of segments; a mask is made with a partial pattern for each segment; and using the masks, exposure is conducted for individual segments to transfer the partial patterns and finally to transfer the given pattern on a wafer.
A mask used for the scattering-angle limiting type of electron-beam exposure process is one in which a pattern consisting of an electron-beam scatterer is formed on an electron-beam transmittable membrane which does not significantly scatter electrons (hereinafter, referred to as a “scattering membrane mask”). A wafer is exposed to an electron beam which is not scattered or scattered with a relatively small angle after passing through the electron-beam transmittable membrane. Thus, the difference of electron-beam scattering between the membrane and the scattering regions permits a figural contrast to be formed on the wafer.
The proximity effect in the scattering-angle limiting type of electron-beam exposure process is corrected by selectively transmitting a part of electrons significantly scattered by the scatterer on the scattering membrane mask into an annular opening formed in a limiting aperture disposed at a crossover plane; defocusing the transmitted scattered electrons approximately to a back-scattering range by spherical and chromatic aberration of an object lens; and then irradiating the wafer with the electrons as a correction beam. Such a proximity effect correction technique has been reported in G. P. Watson et al., J. Vac. Sci. Technol., B, 13(6), 2504-2507 (1995). This method is characterized in that a proximity effect is corrected by performing correction exposure simultaneously with the pattern exposure, whereas in a conventional GHOST technique, a wafer is separately exposed to a defocused beam of a reverse pattern for an original exposure pattern. Thus, such proximity effect correction by simultaneous correction exposure with the pattern exposure may considerably contribute to improvement in a throughput.
The conventional proximity effect correction technique in a scattering-angle limiting type of electron-beam exposure process, however, has the following problem.
The extent of a proximity effect depends on a substrate type and a mask pattern. Therefore, when performing exposure using a substrate consisting of a different material or a mask having a different pattern, a correction dose must be readjusted for proximity effect correction suitable for the substrate or the mask. When using a mask having a different electron-scatterer thickness, a limiting aperture must be replaced to one with a different opening size. The correction dose is, however, adjusted by changing dimensions such as the size and the width of the annular opening formed in the limiting aperture. To optimize the correction dose, it is, therefore, necessary to prepare another limiting aperture, which must be then placed after stopping electron-beam exposure and breaking vacuum by opening the chamber in the air. Thus, there has been the problem that according to the conventional process, attempting to optimal proximity effect correction involves a significantly reduced throughput.
Furthermore, the above scattering membrane mask used in the conventional scattering-angle limiting type of electron-beam exposure process has the following problems.
First, since transmitted electrons are also scattered in an electron-beam transmittable membrane, the energy distribution of the image-forming electrons spreads, which causes chromatic aberration, leading to beam blur. For minimizing the beam blur, a beam convergent semi-angle must be reduced. Reduction in a beam convergent semi-angle, however, makes Coulomb effect significant, resulting in a reduced resolution. The Coulomb effect may be minimized by reducing a beam current. It, however, leads to a longer exposure and therefore a reduced throughput. Thus, a scattering membrane mask has not provide adequate electron exposure properties.
Second, the scattering membrane mask is prepared by forming, on a thin (about 100 nm) silicon nitride film, a thinner (about 50 nm) patterned heavy-metal film such as tungsten. Thus, its preparation is very difficult and is of a poor yield.
Besides the above problems, the above proximity effect correction technique has the following problem.
When an underlying pattern such as an interconnection consisting of a heavy metal such as tungsten is formed on a base layer in a resist layer on a wafer surface, image-forming electrons are reflected or back-scattered by the underlying pattern. As a result, there generates a difference of an extent in proximity effect between the resist region over the region without the underlying pattern and that over the region with the pattern. In the conventional proximity effect correction method, it has been difficult to adjust a correction dose for each region in response to the underlying pattern, and furthermore, no such attempts have been conducted.
A mask used in a conventional cell projection or a system used therein (a cell projection type of electron-beam exposure system) will be described.
A conventional cell projection (or exposure system) generally uses a mask which is prepared by forming an opening pattern on a substrate which blocks an electron beam, such as a silicon substrate having a thickness of at least 20 &mgr;m (hereinafter, referred to as a “stencil mask”).
As a pattern has become finer in response to a more integrated semiconductor device, a stencil mask consisting of a thick substrate has become suffering from the following problem. In preparing a mask, it is difficult to accurately form an opening pattern on a silicon substrate as thick as at least 20 &mgr;m, leading to dimensional variation. Furthermore, in electron-beam exposure, the mask absorbs the electron beam and is thus heated, leading to its reduced durability, and is thermally expanded adequately to vary the mask position. In addition, it is required to increase an acceleration voltage for improving a resolution by reducing an aberration in the electron optical system, and therefore, the mask substrate has become thicker, causing these problems more significant.
A thinner mask may improve linewidth accuracy in the opening pattern and reduce heating while electrons to be blocked pass through the mask substrate region (non-opening region). As a result, a region in a wafer not to be exposed is exposed, leading to poor contrast and a reduced resolution.
For solving the problems, JP-A 10-97055 has disclosed a mask for electron-
NEC Corporation
Young Christopher G.
Young & Thompson
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