Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2001-02-27
2003-08-19
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S005000, C430S296000, C430S942000
Reexamination Certificate
active
06607863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure method and exposure apparatus used when producing a semiconductor integrated circuit, a liquid crystal display element, a thin film magnetic head, or another microdevice or a photomask by photolithography and to a density filter used in such an exposure method and exposure apparatus.
2. Description of the Related Art
In photolithography, one step in the production of a microdevice, use is made of an exposure apparatus for projection exposure of images of patterns of a photomask or reticle on to a substrate for exposure (semiconductor wafer or glass plate coated with a photoresist, light-transparent substrate called a “blank”, etc.) In recent years, to deal with the increasingly large size of the exposure area accompanying the increased size of substrates, a stitching type exposure apparatus which partitions the exposure area of the substrate into a plurality of unit areas (hereinafter sometimes referred to as “shots” or “shot areas”) and successively projects and exposes images of corresponding patterns on the shots has been developed.
In such an exposure apparatus, there was sometimes misalignment in stitched portions of shots due to aberration of the projection optical system, positioning error of the mask or substrate, etc. Therefore, part of the images of patterns for one shot was superposed over part of the images of the patterns for another shot adjoining it for the exposure. At overlay parts of images of patterns, the exposure becomes greater than portions other than overlay parts, so for example the line width (width of lines or spaces) at overlay parts of patterns formed on the substrate become thinner or thicker in accordance with characteristics of the photoresist.
Therefore, the distribution of exposure at the portions forming overlay parts of the shots is set at an incline so as to become smaller the further toward the outside and the exposure of overlay parts is made equal to the exposure of portions other than overlay parts by two exposures in total so as to prevent a change in line width at these overlay parts.
As art for realizing inclined distribution of exposure at overlay parts of shots, it is known to form light-attenuating parts limiting by an incline the amount of light passing through portions corresponding to overlay parts of the reticle itself. Due to the formation of the light-attenuating parts in the reticle itself, however, the steps and cost of the manufacturing process of the reticle increase and the cost of manufacturing the microdevice etc. increase.
Therefore, an exposure apparatus is being developed which is provided with a density filter formed with a light-attenuating part similar to the above on a glass plate at a position substantially conjugate with the pattern formation surface of the reticle or which is provided with a blind mechanism having a light-blocking plate (blind) able to advance into or retract from the optical path at a position substantially conjugate with the pattern formation surface of the reticle and realizes an inclined distribution of exposure by making the light-blocking plate advance or retract during the exposure processing of the substrate.
In the above exposure apparatus of the related art, however, the density filter used for equalizing the exposure of overlay parts and the exposure of portions other than overlay parts is produced as follows. That is, use is made of an exposure sensor provided on the stage carrying the substrate for measuring the exposure and the exposure sensor is made to move in the substrate plane to measure the distribution of exposure near overlay parts. Further, the angle of inclination of the distribution of the amount of transmitted light (that is, the distribution of the transmittance of the light-attenuating part) at the density filter is set so that the exposure of overlay parts becomes equal to the exposure of portions other than overlay parts based on the results of measurement of the exposure apparatus.
If such a designed density filter is used to limit at an incline the exposure of overlay parts when overlaying and exposing parts of images of patterns, if the exposure is performed by the designed exposure, the exposure at overlay parts and the exposure at portions other than overlay parts will match and the line width of patterns formed at overlay parts will become equal to the line width of patterns formed at portions other than overlay parts. When actually trying to expose patterns, however, the width of the lines at overlay parts becomes thinner and the width of the spaces becomes thicker. This phenomenon is believed to arise due to the following reasons:
First, “flare” arising due to multiple reflections in the projection optical system provided in the exposure apparatus or multiple reflections between the substrate and projection optical system, between the substrate and reticle, and between the reticle and substrate may be considered. That is, when flare occurs, the effect of flare felt at portions other than overlay parts is constant, but since overlay parts are exposed several times, the effect of the flare is received two or four times and therefore the exposure at overlay parts becomes larger than the exposure of portions other than overlay parts. Therefore, as a result, the exposure of overlay parts becomes larger than the exposure of portions other than overlay parts, so the width of the lines becomes thinner and the width of the spaces becomes thicker.
Second, once exposed, a photoresist coated on a substrate has the characteristic of a higher optical transmittance at the exposed portions. Therefore, if overlay parts are exposed two or four times, the amount of light passing through the photoresist and reflected at the surface of the substrate increases, so as a result a greater exposure is performed than the designed exposure. Accordingly, the exposure of overlay parts becomes larger than the exposure of portions other than overlay parts, so the width of the lines becomes thinner and the width of the spaces becomes thicker.
Third, this phenomenon may be due to error in the design of the density filter. As explained above, a density filter used to make the exposure of overlay parts the same as the exposure at other than overlay parts uses an exposure sensor to measure the distribution of exposure near overlay parts and sets the distribution of exposure at an incline to become smaller toward the outside. This exposure sensor is provided with a photoelectric sensor and a slit plate provided on the light receiving surface of the photoelectric sensor. The exposure is measured by measuring by this photoelectric sensor only the exposure light passing through the slits formed in the slit plate. The slit plate is generally comprised of a quartz substrate on which chrome (Cr) is deposited and then patterned. When using these slits, the exposure light is reflected at the slit plate at the time of measurement of the exposure light and the reflected light is reflected at the projection optical system or the reticle resulting in flare. In general, since the reflectance of the chrome surface is higher than the reflectance of the substrate surface coated with the photoresist in the wavelength region of the exposure light, the amount of flare arising when measuring the spatial distribution of exposure using the exposure sensor becomes larger than the amount of flare arising at the time of actual exposure. Therefore, the exposure sensor measures a greater amount of exposure light than at the time of actual exposure and the density filter is prepared based on the result of measurement. Since the thus prepared density filter is not one which actually equalizes the exposure of overlay parts and the exposure at portions other than overlay parts, it is possible that when using this density filter for exposure, the line width of patterns formed at overlay parts and the line width of patterns formed at portions other than overlay parts become different.
Further, when the line width of the patte
Nikon Corporation
Oliff & Berridg,e PLC
Young Christopher G.
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