Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2000-09-29
2003-01-07
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
06503671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electron beam writing method, an electron beam lithography apparatus which is called a SCALPEL apparatus, and a mask used in such an electron beam writing method and an electron beam lithography apparatus.
2. Description of the Related Art
An electron beam lithography system makes it possible to write a pattern of 0.2 &mgr;m or smaller which pattern could not be written by a light-exposure system. Hence, an electron beam lithography system had drawn attention as a technology for accomplishing a smaller size and higher performance of a semiconductor device.
However, an exposure system for exposing a plurality of wafers to an electron beam at a time has not been established so far in a field of an electron beam lithography system. Hence, though an electron beam lithography system makes it possible to write a minute pattern, it was not possible to fabricate a mass of semiconductor devices at a time.
In recent years, an electron beam lithography system in which an apparatus for exposing a plurality of wafers to an electron beam at a time is used has been suggested and put into practice for the purpose of enhancement in throughput, as a semiconductor device has been designed to have a greater area and fabricated in higher integration.
Such an electron beam lithography system makes it possible to enhance fabrication yield in a large-scale integration circuit (LSI) such as a dynamic random access memory (DRAM) including a plurality of memory cells, by preparing a mask having a pattern of memory cells, and irradiating an electron beam through the mask.
The electron beam lithography system is contemplated as a substitution of a conventional light-exposure system.
However, there occurs the proximity effect in the electron beam lithography system like in the conventional light-exposure system. Herein, the proximity effect is defined as an effect wherein a pattern is influenced with respect to a size and a shape by neighboring patterns.
For instance, when a pattern of a couple of micrometers is to be formed by the electron beam lithography system, an electron beam irradiated onto a pattern is backwardly scattered due to molecules constituting a resist and a substrate, and hence, reaches an area of a resist remote from a location at which the electron beam was irradiated onto the resist, resulting in that it is not possible to form a pattern as designed. Herein, backward scattering is generally indicated as a characteristic length &bgr;b.
Hence, many attempts have been made to compensate for the proximity effect by controlling electron beam irradiation such that electron energy directed to an edge of a pattern is kept constant.
One of the methods of compensating for the proximity effect includes the steps of observing neighboring patterns around patterns to be written, defining a rule table for deformation of a mask pattern in dependence on the neighboring patterns, and sizing the patterns to be written, with reference to the rule table, based on information about arrangement of a pattern. This method can deal with wide patterns of a semiconductor device.
However, this method is accompanied with a problem as follows. In this method, the mask is scanned at a constant rate as high as possible, and all patterns are exposed to a light for enhancing fabrication yield. Namely, the same amount of electron beam irradiation is applied to all patterns. Hence, even if the patterns are accurately sized, there is a limitation in writing a minute pattern with high accuracy, only by sizing the patterns.
The scanning rate may be set smaller to avoid the problem. However, there is caused another problem that fabrication yield is much deteriorated.
As mentioned so far, the conventional electron beam lithography system is accompanied with a problem that the proximity effect cannot be compensated for, even if electron beam irradiation or a scanning rate is varied. Hence, even if a minute pattern has to be formed in a local area, since the proximity effect cannot be compensated for, it would not be possible to form a pattern with high accuracy.
Apart from the above-mentioned method, various attempts have been made to compensate for the proximity effect.
For instance, Japanese Unexamined Patent Publication No. 7-106216 has suggested a method of compensating for the proximity effect, including the step of varying a scanning speed of electron beams.
In the suggested method, a deflection rate of electron beams is varied to thereby compensate for the proximity effect. However, the method is accompanied with a problem that it is unavoidable for an electron beam lithography system to have a too complicated structure.
Japanese Unexamined Patent Publication No. 8-264411 has suggested an electron beam lithography system and a method of compensating for the proximity effect in electron beam lithography.
The suggested system is designed to include a plurality of electron beam sources, an aperture, and means for overlapping electron beams relative to each other such that a current distribution of the electron beams on the aperture has a predetermined uniformity or profile.
However, this system is accompanied with a problem that it is quite difficult or almost impossible to make the current distribution to have a desired uniformity.
Japanese Unexamined Patent Publication No. 9-180978 has suggested a method of compensating for the proximity effect in electron beam lithography.
In the suggested method, there is used a mask partitioned into a plurality of small areas each having a width sufficiently smaller than a width of a profile of backwardly scattered electrons. Each of the small areas is formed with an aperture through which an electron beam passes, and is designed to have a size defined by subtracting a predetermined aperture area from an area of a non-exposed part of a wafer in a region associated with each of the small areas of the mask.
However, according to experiments conducted by the inventor, it was impossible to write a minute pattern with high accuracy by the above-mentioned method.
SUMMARY OF THE INVENTION
In view of the problems of the conventional electron beam lithography, it is an object of the present invention to provide an electron beam writing method which makes it possible to write a minute pattern with high accuracy and enhance a fabrication yield.
It is also an object of the present invention to provide an electron beam lithography apparatus which can do the same.
It is another object of the present invention to provide a mask suitable for the above-mentioned method and apparatus.
In one aspect of the present invention, there is provided an electron beam writing method including the steps of (a) preparing a mask having areas in each of which a divisional pattern is formed, the divisional pattern being obtained by dividing a pattern to be written, in accordance with an area density, and (b) irradiating an electron beam onto a wafer through the mask such that a different amount of irradiation of electron beam is applied through each of the areas of the mask.
In accordance with the above-mentioned method, it is possible to optimize electron beam irradiation for each of the divisional patterns formed in each of the areas of the mask. Hence, it would be possible to compensate for the proximity effect in each of the areas, ensuring that a minute pattern can be written with high accuracy.
In addition, by selecting an optimal scanning rate, a scanning rate might be increased in some of the areas. As a result, a fabrication yield can be enhanced in those areas.
Furthermore, the method can be carried out at low cost, and can be readily carried out in practical use.
For instance, the step (b) may be carried out by varying a rate at which a stage on which the mask is mounted is moved, or by varying a rate at which a stage on which the wafer is mounted is moved.
It is preferable that the areas are arranged in a line in the mask, and the wafer is reciprocating across each of the areas in the step (b).
Since the wafer can be exposed
Hayes & Soloway PC
NEC Corporation
Young Christopher G.
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