Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
2002-12-11
2004-03-02
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S030000
Reexamination Certificate
active
06699628
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aligning method for a scanning projection exposure apparatus for exposing, in a process for manufacturing a semiconductor integrated circuit or liquid display element, the pattern of a mask (photomask or reticle) as a master onto a substrate (wafer or glass plate) and, more particularly, to an aligning method for a scanning projection exposure apparatus for aligning the master and substrate at high precision when deformation such as deflection occurs in the master.
BACKGROUND OF THE INVENTION
In a photolithography process for manufacturing a semiconductor integrated circuit or the like, a projection exposure apparatus is used, which exposes a pattern image of a master (photomask or reticle) onto a substrate (wafer or glass plate) coated with a photoresist or the like. In this projection exposure apparatus, as the feature size of the circuit pattern as the transfer of the target decreases, the allowable range of the fluctuation amount of the image-forming characteristics of a projection image formed by a projection optical system narrows. To solve this problem, conventionally, in order to correct the fluctuation amount of the image-forming characteristics (e.g., magnification, focal position, and the like) occurring upon absorption of illumination light with a projection optical system, the projection exposure apparatus has an image-forming characteristic correcting mechanism, as disclosed in Japanese Patent Laid-Open No. 60-78455 or No. 63-58349, which detects the quantity of light incident on the projection optical system and corrects the fluctuation amount of the image-forming characteristics of the projection optical system in accordance with the detected quantity of light.
For example, a mechanism disclosed in Japanese Patent Laid-Open No. 60-78455 will be briefly described. A model corresponding to the fluctuation characteristics of the image-forming characteristics of the projection optical system is formed in advance. The quantity of light energy which becomes incident on the projection optical system at a predetermined time interval is detected by a photosensor or the like on a wafer stage upon which a wafer as the substrate is placed. The integral value of the quantity of light energy is applied to this model, and the fluctuation amount of the image-forming characteristics is calculated. In this case, an exposure time for which the integral value of the light energy incident on the projection optical system is to be obtained is calculated by, e.g., constantly monitoring a signal indicating that a shutter for opening/closing illumination light is in the open state. Hence, the current fluctuation amount of the image-forming characteristics of the projection optical system can be calculated in accordance with this model, and correction is performed on the basis of the fluctuation amount. For the time being, this solves the problem of fluctuation of the image-forming characteristics, which is caused upon absorption of the illumination light with the projection optical system.
As the illumination light also passed through a mask serving as a master, the mask thermally deforms upon absorption of the illumination light, and consequently, the image-forming characteristics are changed. Particularly, since a pattern is drawn on the mask with a light-shielding film such as a chromium film, heat absorption of the light shielding film is large, unlike in a glass substrate portion with a high transmittance. In recent years, for the purpose of preventing flare of the optical system, a technique that decreases reflection of the light-shielding film on the mask as been introduced. This further increases heat absorption with the light-shielding film.
A circuit pattern formed with the light-shielding film on the mask is not always distributed uniformly over the entire mask, but may sometimes be distributed nonuniformly. In this case, the temperature of the mask increases locally to likely cause anisotropic distortion. When a variable field stop (reticle blind) or the like is used to expose the pattern on the mask only partially, anisotropic distortion similarly occurs. This distortion in the mask leads to anisotropic distortion in the projected image.
Concerning the thermal deformation of the mask, since the thermal deformation amount, and moreover, the change amount of the image-forming characteristics, change depending on the type of the mask employed, they are difficult to correct uniformly. In other words, the amount of fluctuation, occurring upon thermal deformation, of the image-forming characteristics of a mask used for adjusting the image-forming characteristics of a projection exposure apparatus before shipping may be recognized as the fluctuation characteristics of the image-forming characteristics of this projection exposure apparatus, and may be corrected accordingly. When another mask is used, its thermal deformation amount differs, and accurate correction cannot be performed. Particularly, when exposure is to be performed by successively changing masks, unless the thermal deformation amounts of the respective masks are considered, the fluctuation amounts of the image-forming characteristics are accumulated to likely cause a large error.
As a countermeasure for this, Japanese Patent Laid-Open No. 4-192317 discloses a projection exposure apparatus which corrects a change in optical characteristics that occurs due to thermal deformation of a mask, while including the heat absorption ratio of chromium which makes up the mask pattern and the content of chromium in the pattern in the parameters.
In this prior art, correction is performed merely based on calculation, and many errors exist with respect to the actual expansion amount. For example, while heat absorbed by the mask is diffused into air by radiation and convection, it is very difficult to describe this phenomenon with a mathematical expression precisely. However, unless heat absorbed by the mask and heat emitted from the mask are estimated accurately, the expansion amount of the mask cannot be calculated.
In recent years, a projection exposure apparatus employing a so-called step-and-scan exposure method or slit-scan exposure method (to be referred to as a “scan exposure method” hereinafter) has been developed, which illuminates a mask pattern region in a slit manner, scans a mask with respect to the slit-like illumination region, and scans a wafer with respect to an exposure area conjugate to the slit-like illumination region in synchronism with scanning of the mask, thereby sequentially projecting and exposing the pattern of the mask onto the respective shot regions of the wafer. According to this scan exposure method (scanning type), a large area can be exposed without being limited by the field size of the projection optical system in the scanning direction.
In this scan exposure method, during exposure, the mask is scanned with respect to the illumination region. Accordingly, factors that should be considered regarding the mask (e.g., the cooling effect of the mask accompanying mask scanning) increase, and calculation of the thermal deformation amount of the mask becomes more complicated than in cell projection exposure. Considering the foregoing, to cope with deformation of the mask, it is more effective to measure the deformation amount of the mask directly rather than to obtain the deformation amount by calculation as in the prior art.
As prior art in consideration of this aspect, Japanese Patent Laid-Open No. 4-192317 discloses a method of measuring the deformation amount of a mask. According to the method disclosed in this application, the temperature distribution of a reticle is detected by a non-contact temperature sensor such as an infrared camera, thereby obtaining the deformation amount, or a mark is formed in the periphery of a reticle, and a displacement of this mark is detected by a detection system arranged above the reticle, thereby obtaining deformation. According to this method, in detection of the mark on the reticle with the detection s
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