Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-22
2001-06-12
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06245466
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mask pattern design method and a photomask useful for designing a fine supplemental pattern.
In recent years, the semiconductor manufacturing techniques have extremely progressed, and semiconductor devices having a minimum processible size of 0.35 &mgr;m have been manufactured in these days. This fine downsizing of devices is realized by rapid progress of a fine pattern forming technique called an optical lithography technique.
The optical lithography means a series of steps of: forming a mask from a design pattern of an LSI; irradiating light on the mask to expose a resist coated on the wafer in accordance with a pattern drawn on the mask by a projection optical system; and developing the resist based on the exposure distribution thereby to form a resist pattern on the wafer. By etching under layers with use of the resist pattern formed through the optical lithography steps, as a mask, an LSI pattern is formed on the wafer.
In a generation in which the pattern size was sufficiently large compared with the limit resolution of a projection optical system, the plan shape of the LSI pattern desired to be formed on the wafer was directly drawn as a design pattern. Further, a mask pattern having fidelity to the design pattern was prepared. The mask pattern thus obtained was transferred onto a wafer by a projection optical system, and under layers were etched. In this manner, a pattern substantially similar to the design pattern was formed on the wafer.
However, as the patterns have come to become finer and smaller, the pattern formed on the wafer differs from a design pattern due to influences from an eclipse iof diffracted light in a projection optical system (which will be hereinafter called an optical proximity effect), accompanying remarkable bad influences.
For example, in case of a gate pattern, the top end portion of the pattern becomes shorter than that of a design pattern (which will be hereinafter called “shortening”), or corner portions of a device region such as SDG or the like are rounded (which will be hereinafter called “rounding”). These shortening and rounding are factors which cause operation errors of transistors. Also, in case of forming a dense Line and Space (L/S) pattern, the line width of an isolated pattern existing in the same layer as the L/S pattern becomes thicker or narrower than that of a finished L/S pattern. This is a factor which deteriorates the operation characteristics of the device.
To eliminate these problems, there has been a proposal for a method in which correction is made by adding a fine small supplemental pattern to the portion of a pattern where the optical proximity effect appears, thereby to prepare a mask design plan different from a conventional design pattern, and a mask is prepared in accordance with the mask design plan. An example thereof is disclosed in Japanese Patent Application KOKAI Publication No. 6-242595. The method of transfer with use of a mask prepared by adding a fine small supplemental pattern to a design pattern is effective for forming a pattern with fidelity to the design pattern on a wafer.
However, the size of this supplement pattern is generally very small. More specifically, the dimensions of this supplemental pattern, as formed on a a wafer, are in the several-ten nm range. Consequently, errors in mask dimensions or rounding at corner portions, which are caused by mask drawing process, greatly influence the finished dimensions after transfer. Therefore, it is difficult to obtain an optical proximity effect stably. Also, improvements in precision of the mask drawing process require techniques which further require much labor.
Thus, in a conventional method, finished dimensions of a supplemental pattern vary due to the mask drawing process. As a result of this, it is difficult to stably attain a desired correction effect in a pattern after transfer.
BRIEF SUMMARY OF THE INVENTION
The present invention has an object of providing a mask pattern design method which is capable of stably attaining a correction effect by means of a supplemental pattern without improving the precision of mask drawing process.
Also, the present invention has another object of providing a photomask which is capable of stably attaining a correction effect by means of a supplemental pattern without improving the precision in mask drawing.
According to the present invention, there is provided a mask pattern design method for designing a pattern of a photomask used for forming a desired design pattern onto a wafer by a projection exposure device, wherein a mask pattern is prepared by adding a supplemental pattern to a main pattern corresponding to the design pattern, and a plan shape of the supplemental pattern is determined such that a change rate of a pattern plan shape on the wafer after transfer, which is defined based on a change amount of the plan shape of the supplemental pattern and a change amount of the pattern plan shape, becomes a predetermined allowable value or less.
The main pattern corresponding to a design pattern, used herein, means a pattern having the same shape as the design pattern to be formed on a wafer.
Also, according to the present invention, there is provided a photomask used for forming a desired design pattern onto a wafer by a projection exposure device, wherein a mask pattern of the photomask comprises a main pattern corresponding to the design pattern and a supplemental pattern added to the main pattern, and a plan shape of the supplemental pattern is determined such that a change rate of a pattern plan shape on the wafer after transfer, which is defined based on a change amount of the plan shape of the supplemental pattern and a change amount of the pattern plan shape on the wafer after transfer, becomes a predetermined allowable value or less.
The main pattern corresponding to a design pattern, used herein, also means a pattern having the same shape as the design pattern to be formed on a wafer.
Also, the change rate of a pattern plan shape, described herein, is a value indicating the extent of a change of the pattern plan shape which appears on a wafer after transfer, relative to a change of the plan shape of a supplemental pattern.
In the present invention, a supplemental pattern is added to a main pattern having the same shape as a design pattern to be transferred. The plan shape of the supplemental pattern is determined such that the change amount of a pattern plan shape on a wafer after transfer in relation to a change of the plan shape of the supplemental pattern becomes a predetermined allowable value or less. In this manner, variations of the dimensions of the pattern after transfer, which are caused by variations of the dimensions of the supplemental pattern, can be restricted to be small. As a result of this, a correction effect desired for the pattern after transfer can be obtained stably.
Also, in the present invention, the change amount of the pattern plan shape on the wafer is standardized by the values of the exposure wavelength &lgr; of a light source of a projection exposure device and the numerical aperture NA of a projection lens thereof. In this manner, the standardized change amount is determined as one single meaning even if &lgr; and NA differ. Therefore, it is possible to respond flexibly to changes of illumination conditions (&lgr; and NA), so enormous processing and time required for designing a mask pattern can be shortened.
In addition, the plan shape of the supplemental pattern is determined such that the change amount of the pattern plan shape on the wafer after transfer in relation to a change in the mask plane which is caused by mask drawing process becomes a predetermined value or less. In this manner, it is possible to calculate a change amount of the plan shape on the wafer, based on variations of the dimensions of the supplemental pattern which are caused in actual drawing process. Therefore, the plan shape of the supplemental pattern capable of more stably attaining a correction effect can be determined.
A
Inoue Soichi
Kotani Toshiya
Tanaka Satoshi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Rosasco S.
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