Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2003-04-01
2004-05-18
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S075000
Reexamination Certificate
active
06738128
ABSTRACT:
This application claims the right of priority under 35 U.S.C. §119 based on Japanese Patent Application No. 2002-100581, filed on Apr. 2, 2002, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.
BACKGROUND OF THE INVENTION
The present invention relates to exposure apparatuses used to expose a pattern on a reticle or mask (these terms are used interchangeably in this application) onto a plate, such as a wafer and a glass plate in a manufacture process of semiconductor devices, liquid crystal display devices etc. The present invention is suitable, for example, for a scan-type exposure apparatus.
The fabrication of a device, such as a semiconductor device, an LCD device, and a thin film magnetic head using the lithography process has conventionally employed a projection exposure apparatus to transfer a circuit pattern formed on a photo-mask or reticle onto a photosensitive wafer substrate and glass plate (“wafer” hereinafter). Recent semiconductor devices etc. have been required for finer patterns. This demand requires higher resolution of the projection optical system. The higher resolution is available with a shorter wavelength of exposure light and a larger numerical aperture of the projection optical system.
A chip pattern size of one semiconductor device tends to become large, and an apparatus for exposing a larger area has been demanded.
These two requirements need a projection optical system with a large exposure area and high resolution. However, it becomes difficult to maintain imaging performance, such as distortion, within permissible accuracy for a larger exposure area and higher resolution throughout the exposure area.
A scan-type exposure apparatus has currently called attentions, which transfers a reticle pattern onto a wafer by scanning a slit-shaped, e.g., arc-shaped illumination area on the reticle and wafer synchronously.
This system illuminates a slit area on the reticle, and uses only part of the projection optical system. It thus has an advantage in easily maintaining the imaging performance, such as distortion, within predetermined accuracy.
An additional advantage is that the slit-shaped illumination on the reticle makes available the maximum diameter of the effective exposure area of the projection optical system, and the scan may enlarge an exposure area in the scan direction without affected by any restriction of the optical system.
However, a currently demanded further finer pattern requires the scan-type exposure apparatus to reduce distortion of a pattern image.
A reduction of distortion naturally leads to reductions of various aberrations in the projection optical system. Therefore, a projection optical system to be loaded on a conventional stepper is optically designed on condition that various aberrations and distortions are reduced averagely in the entire projection field. In order to maintain various aberrations and distortion within permissible ranges, the projection optical system is assembled by a method that repeats complex and arduous assemblies, adjustments and inspections, and the method includes the steps of processing a lens element and optical element with high precision, actually measuring various aberrations, and adjusting an air separation between two lenses, a lens's tilt and a parallel decentering.
In particular, the above adjustment method may adjust symmetrical components or asymmetrical but regular components of the distortion among various aberrations.
However, the above adjustment method cannot adjust so-called random components disadvantageously.
Accordingly, in order to mitigate difficulties of precise manufacture of the projection optical system and maintain the random component within a designed permissible range, Japanese Laid-Open Patent Application No. 8-203805, for example, discloses a method the steps of observing a distortion characteristic of an assembled projection optical system, and inserting a polished optical correction plate or correction optical element into a projection optical path so as to partially deflect a principal ray that passes each point in the projection field so that the observed distortion characteristic at each point in the projection field may be minimized. Japanese Laid-Open Patent Application No. 8-203805 is directed to a correction method for use with a stepper using an optical correction plate.
Japanese Laid-Open Patent Application No. 11-045842 discloses a correction method using an optical correction plate in a scan-type exposure apparatus. Japanese Laid-Open Patent Application No. 11-045842 addresses that the static distortion characteristic is averaged over a width in the projection area in a scan direction and turns to a dynamic distortion characteristic, when a mask pattern is exposed onto a photosensitive substrate using a scan type projection exposure apparatus, and corrects a random component in the dynamic distortion characteristic by arranging in a projection optical path a distortion correction plate that is made through a local polishing process of a surface of a transparent parallel plate or optical correction plate.
Japanese Laid-Open Patent Application No. 11-031652 is directed to manufactures and measurements of an optical correction plate. Japanese Laid-Open Patent Application No. 11-031652 provides an optical correction plate with a plate having a wedge angle, so as to prevent interference of backlight of the optical correction plate in measuring a surface shape of an optical correction plate using an interferometer.
While the above discusses the prior art about distortion, a fluctuation of the imaging performance of an image projected by the projection optical system should be also considered in addition to distortion to create a finer pattern. The finer pattern narrows a permissible fluctuation range of the imaging performance. In order to correct a fluctuation of the imaging performance, such as a magnification and focal position, as a result of that the projection optical system absorbs the illuminated light, as disclosed in Japanese Laid-Open Patent Applications Nos. 60-78455 and 63-58349, conventional projection exposure apparatuses have included an imaging-performance correction mechanism for detecting a quantity of light incident upon the projection optical system and for correcting the fluctuation of the imaging performance of the projection optical system according to the detected quantity of light.
For example, the mechanism disclosed in Japanese Laid-Open Patent Application No. 60-78455 previously prepares a model indicative of a fluctuation characteristic in the imaging performance in a projection optical system, calculates an energy amount of light incident upon the projection optical system at certain time intervals using a photoelectric sensor, etc. on a wafer stage mounted with a wafer, and calculates a fluctuation of the imaging performance by applying an integral value of the light energy amount to the model. This method calculates the exposure time to calculate the integral value of the light energy incident upon the projection optical system by always monitoring, for example, a signal indicative of an open state of a shutter for opening and closing illumination light, obtains the fluctuation of the imaging performance of the projection optical system according to this model, and provides a correction based on this fluctuation. This appears to solve problems associated with the fluctuation of the imaging performance due to the projection optical system that has absorbed illumination light.
However, the illumination light passes through the mask, and thermally deforms the mask that absorbs the illumination light, disadvantageously deteriorating the imaging performance. In particular, the mask forms a pattern using a light-shielding film, such as a chrome film, and the light-shielding film absorbs larger heat than a glass plate that has high transmittance. More particular, the recent technology often uses the light-shielding film with low reflectance on the mask to prevent flare in the optical system, a
Maeda Kohei
Shima Shinichi
Adams Russell
Esplin D. Ben
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