Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Electron beam imaging
Reexamination Certificate
1999-10-14
2001-07-10
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Electron beam imaging
C430S942000
Reexamination Certificate
active
06258511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charged particle beam exposure method used in the lithography process of semiconductor integration circuits and the associated mask. In particular, the present invention relates to a charged particle beam exposure method employing an electron beam and an ion beam according to the divided pattern transfer method to improve the accuracy of stitching of the patterns.
2. Description of the Related Art
The exposure method employing the charged particle beam will be explained as related art. The current charged particle beam exposure method has a high accuracy but a low throughput. To solve this problem, a variety of techniques have been developed including a partial pattern projection exposure method e.g. a cell projection, a character projection and a block exposure which is utilized at present. According to the partial pattern projection exposure method, a small pattern, e.g., squares of 5 microns, is projected as an exposure unit. If a small repetitive pattern of the circuit require exposure, such as squares of 5 micron, then each small pattern is exposed one after another using a mask on which different kinds of small repetitive patterns are formed. However, if some of the patterns are not repetitive, they are exposed according to the variable shaped beam exposure method.
A new charged particle beam reduction projection device is proposed according to the charged particle beam exposure method that enables a much higher throughput than that of the conventional pattern projection exposure method. This is possible due to the following process: a mask is prepared on which the entire pattern of a semiconductor chip is formed; following this the charged particle beam is irradiated onto a given area of the mask, and the image of a pattern laid within the area is transferred in reduction by a projection lens.
However, irradiating the charged particle beam simultaneously to the whole area of the mask does not enable accurate transfer of the pattern, due to aberrations. In addition, it is difficult to prepare the original plate of the mask. As a result of these problems, a method has recently been studied where a die (a chip on a wafer) or a plurality of dies are not exposed simultaneously; instead, small areas divided from the pattern (we call “subfield”) are exposed using an optical system with an optical field which does not, however, cover the whole chip. This method will be referred to as the divided pattern transfer method. The exposure is carried out by adjusting the focus of the image of a subfield to be projected onto the exposure surface thus minimizing aberrations such as distortion of the subfield for each individual subfield. This enables an exposure to have an excellent resolution and accuracy within a large area compared with a simultaneous transfer of the entire die.
Using the charged particle beam exposure method, the areas that can be simultaneously exposed, called the unit exposure areas, are smaller than that of the optical exposure method employing visible or ultraviolet light. Accordingly, since it is difficult to simultaneously expose the entire pattern of a semiconductor chip, it is necessary to stitch exposed portions divided from the pattern. The stitching might be accompanied by a gap or displacement, which may be reduced by improving the accuracy of pattern transfer.
Meanwhile, the mask, which is the original plate used for the wafer exposure, is also prepared by the charged particle beam exposure. To fabricate the mask, adjusting, e.g., moving and overlapping, the boundary of the pattern is possible.
This method improves the accuracy of the stitching, but reduces the throughput owing to the repeated exposures necessary. The reduction in the throughput does not affect the mask production seriously, but it affects the wafer production. Accordingly, this method is not available for the wafer exposure.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a charged particle beam exposure method employing the electron beam and the ion beam according to the divided pattern transfer method to improve the stitching accuracy of the patterns.
According to an aspect of the present invention, a charged particle beam exposure method is provided using a charged particle beam exposure device. This includes an illumination optical system for illuminating a mask with a charged particle beam and a projection optical system for projecting the charged particle beam passing through the mask onto a sensitive substrate. The method also includes the steps of dividing a pattern to be transferred onto an exposure surface of the sensitive substrate into a plurality of subfields; classifying the plurality of subfields into a plurality of stripes having widths smaller than the deflection field of the optical system which is covered by deflection of the charged particle beam; arranging the plurality of stripes comprising a plurality of subfields on the mask; upon completion of exposure of a stripe, placing the next stripe in the deflection field by uniformly moving the mask and the substrate; illuminating the mask by deflecting the illumination beam for each subfield; and transferring the pattern to the exposure surface by stitching the images of the subfields on the exposure surface. The charged particle beam exposure method comprises forming a subfield having the same pattern at the ends of adjacent two stripes, and exposing each subfield by approximately half of the exposure amount in such a fashion to ensure that the images of the subfields substantially overlap on the exposure surface when the images are stitched.
When the chip pattern is divided into a plurality of subfields having a divided pattern and the subfields are classified into a plurality of stripes, the subfield pattern laid around the boundary is formed in both adjacent stripes on the mask. That is, the subfields laid around the boundary have the same pattern. When exposed on the sensitive substrate (e.g. a wafer), the patterns in such subfields overlap at substantially the same position on the wafer for exposure. The exposure is provided by approximately half the exposure amount. Consequently, undertaking two exposures using half of the exposure amount is equal to the regular exposure amount. This averages error in positioning for stripe exposure, thus smoothing the stitching of the stripes on the wafer.
In the above explanation, the substantially same position represents the position with acceptable error for stitching. Approximately half the exposure amount means that the first exposure amount is almost equivalent to half of the regular exposure amount, a second exposure amount is also almost equivalent thereto, and the sum of the first and second exposure amounts is equivalent to the regular exposure amount. That is, it is not necessary that both the first and second exposure amounts are precisely equivalent to half the regular exposure amount; however, it is necessary that the sum of the first and second exposure amounts is roughly equivalent to the regular exposure amount. To obtain the effect of the present invention, it is necessary to keep both the first and the second exposure amounts at approximately half of the regular exposure amount.
According to another aspect of the present invention, a charged particle beam exposure method is provided using a charged particle beam exposure device comprising an illumination optical system for illuminating a mask with a charged particle beam and a projection optical system for projecting the charged particle beam passing through the mask onto a sensitive substrate. The process includes: dividing a pattern to be transferred to an exposure surface of the sensitive substrate into a plurality of subfields; classifying the plurality of subfields into a plurality of stripes having widths smaller than the deflection field of the optical system; arranging the plurality of stripes comprising a plurality of subfields on the mask; upon completion of exposing a st
Chapman and Cutler
Nikon Corporation
Young Christopher G.
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