Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-09-27
2003-01-21
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000
Reexamination Certificate
active
06509127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of electron-beam exposure that is mainly employed to manufacture a semiconductor device, and a mask as well as an electron-beam exposure system used therein, and more particularly to a mask for electron-beam exposure that is especially suited for the proximity effect correction in the scattering-angle limiting type electron-beam exposure method.
2. Description of the Related Art
In the electron-beam exposure, the proximity effect that may be caused by scattered electrons in a resist layer and within a substrate strongly affects the linewidth accuracy of patterns, which makes the proximity effect correction one of the essential techniques in the art.
In the cell projection lithography which is the most widely used method of electron-beam exposure, in order to obtain the amount of the correction dose, the dose compensation method which requires complicated calculations by the self-consistent method using the exposure intensity distribution (EID) function or the pattern density method has been currently used.
Meanwhile, in the scattering-angle limiting type (referred to as “SAL type”, hereinafter) electron-beam exposure method which has been, in recent years, attracting much interest as a next-generation electron-beam exposure technique, the correction of the proximity effect is conducted by a compensation method based on the GHOST method. The SAL type electron-beam exposure method employs the segmented transcription method in which patterns for the whole chip to be exposed. are set in a mask and, through scanning this mask, patterns are transcribed to a wafer. An exposure system used in this electron-beam exposure method is described in a later section, together with the GHOST method, in detail.
With respect to a mask for this SAL type electron-beam exposure method, there is used a mask (referred to as a “scattering membrane mask”, hereinafter) in which patterns made of an electron-beam scatterer, for example, tungsten with a thickness of 50 nm or so, is formed on an electron-beam transmittable membrane (referred to simply as a “membrane”, hereinafter) with a relatively small electron-beam scattering power, for example, a silicon nitride film with a thickness of 100 nm or so. The exposure is carried out by an electron beam consisting of electrons which are not scattered or scattered only with relatively small scattering angles, having transmitted the membrane, and the image contrast is formed on the wafer due to the difference of the electron-beam scattering between the membrane region and the scatterer region.
In the SAL type electron-beam exposure method, the proximity effect correction is performed as follows. Firstly, some of electrons that are scattered by the scatterer placed on this scattering membrane mask are selectively allowed to pass through an annular opening which is set in a limiting aperture section disposed at the position or in the vicinity of the crossover, and then these scattered electrons allowed to pass are defocused to about the back-scattering range by spherical and chromatic aberrations of an object lens and used as a correction beam to irradiate the wafer. In contrast with a conventional GHOST method wherein the weak correction exposure is performed separately from the primary exposure, with the beam formed by defocusing the inverse pattern of the pattern intended for the primary exposure over the back-scattering range, this technique is characterized in that the proximity effect correction is achieved by carrying out the correction exposure simultaneously with the pattern exposure. Such proximity effect correction as can be implemented by performing the correction exposure simultaneously with the pattern exposure may well contribute to improvement in a throughput. The method of proximity effect correction of this sort has been already reported by G. P. Watson et al. in J. Vac. Sci. Technol., B 13(6), pp.2504-2507 (1995).
As against this, as a mask (referred to as a “stencil mask”, hereinafter) utilized in the cell projection lithography or employed in an apparatus (a cell projection lithography type electron-beam exposure system) for this method, there is generally used a mask in which an opening pattern is formed in a substrate that does not allow the electron beam to transmit, for instance, a silicon substrate with a thickness of not less than 20 &mgr;m for 50 keV electron beam.
Accompanied with achievement of higher integration of a semiconductor device, as the miniaturization of patterns proceeds, however, a stencil mask made of a thick substrate as described above has started to have the following problems. Namely, when manufacturing a mask, it is difficult to form an opening pattern in a silicon substrate as thick as 20 &mgr;m with accuracy so that the variation in size is produced. Further, in respect of the electron-beam exposure, because the mask absorbs the electron-beam and generates heat, problems of lowered durability and variation of the mask position through thermal expansion may arise. Moreover, since there is a requirement to increase the accelerating voltage still further so as to reduce the aberration of the electron optical system and improve the resolution, the mask substrate tends to become thicker causing the above problems more significant.
With the stencil mask, if the mask substrate is made thinner, although the linewidth accuracy of the opening pattern is raised and the amount of heat generation is lowered, the electron beam which should be blocked in the first instance may be allowed to transmit the mask substrate section (non-opening section). As a result, a region of the resist on the wafer that should not be exposed may be exposed, which leads to a poor contrast and a low resolution.
Accordingly, with the object of solving these problems of the stencil mask in the cell projection lithography, in Japanese Patent Application Laid-open No. 97055/1998, there is disclosed a mask for electron-beam exposure, wherein an opening pattern is formed in a relatively thin mask substrate, and, in addition, an electron-beam scattering layer is formed on the backside of the mask for the purpose of scattering the electron-beam that has transmitted said mask. This electron-beam scattering layer may be a layer made of polycrystal such as polysilicon, tungsten silicide, molybdenum silicide, titanium silicide or the like or a uneven layer. It is described, therein, through formation of such an electron-beam scattering layer, the electrons that may transmit the pattern layer of the mask (the non-opening section of the substrate) can be successfully prevented from entering into the wafer.
Further, Japanese Patent Application Laid-open No. 163371/1994 discloses an electron-beam writing apparatus in which an opening is set in a substrate having a thickness less than an electron penetration depth to provide an electron-beam shaping aperture that is used as a mask and further, on the downstream side from the above-mentioned shaping aperture in the electron optical system, a mechanism is set to cut off scattered electrons which have transmitted the substrate section of the shaping aperture (mask). In this invention, there is provided a mechanism to cut off the scattered electrons which have transmitted the substrate section of the shaping aperture by setting a limiting diaphragm of a small diameter in a crossover plane so that only electrons which have passed through the opening section of the mask may be allowed to pass through, while the scattered electrons by the mask substrate section are removed by the limiting diaphragm plate. In addition, another cutting-off mechanism is described therein. Namely, an energy filter is set, and thereby decelerated electrons which have lost a part of their energy by penetrating the mask substrate section are deflected further and then removed by the limiting diaphragm.
As described above, some of stencil masks used in the cell projection lithography, by becoming thinner, may produce scattered electrons. Irradiation o
NEC Corporation
Young Christopher G.
Young & Thompson
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