Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1999-06-22
2001-10-23
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S055000, C355S067000
Reexamination Certificate
active
06307618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure apparatus and a device manufacturing method both of which are suitable for use in the lithography process of exposing a wafer surface to an electronic circuit pattern formed on the surface of a mask or reticle (hereinafter referred to as a “reticle”) by projection exposure or scanning exposure via a projection optical system, thereby providing a device of a high integration level, during the manufacture of semiconductor devices such as ICs or LSIs, image pickup devices such as CCDs, display devices such as liquid crystal panels, or devices such as magnetic heads.
More particularly, the present invention relates to a projection exposure apparatus and a device manufacturing method both of which are suitable for use in a case in which a reticle and a wafer are aligned with high accuracy and the electronic circuit pattern on the surface of the reticle is exposure-projected on the surface of the silicon wafer at a high integration level.
2. Description of the Related Art
Conventionally, when semiconductor devices, liquid crystal panels or the like are to be manufactured with photolithography techniques, a projection exposure apparatus (stepper) is used which transfers patterns from a reticle surface to a photosensitive substrate such as a wafer or a glass plate coated with a photoresist or the like, by exposure via a projection optical system.
The recent semiconductor technology, in particular, has been proceeding into a far larger scaling stage, and the resolution of, for example, from a linewidth of 0.25 &mgr;m to a far finer pattern, has been becoming a matter of argument. Techniques which play the central role in this situation are optical exposure techniques represented by steppers. The performance of a projection lens (projection optical system) which is a performance index of the optical exposure techniques is being improved in three major directions, i.e., shorter wavelength, larger field size and higher NA (numerical aperture). In view of the shorter wavelength, lithography using light from an ArF excimer laser is vigorously being developed as the next-generation techniques.
One important item of the optical exposure techniques is to maintain the same optical performance at all times. In lithography using an ArF excimer laser, it is known that a usable glass material has the characteristic of absorption of light and the optical performance of a projection optical system varies due to the absorption of light. The absorption of light has also already been known in the field of lithography using light such as i-line. In a stepper using an ArF excimer laser, since its depth of focus becomes smaller, it is necessary to control its optical performance with higher accuracy than with conventional accuracy. The optical performance to be controlled includes various aberrations of the projection optical system. Among these aberrations, longitudinal astigmatism, curvature of field and the third-order distortion as well as higher-order distortion are most difficult to correct. Furthermore, how to correct the rotationally asymmetrical magnification of a target itself becomes a new problem.
Longitudinal astigmatism will be described below with reference to an example. A scan type of a projection optical system called a scanner has recently vigorously been developed to enlarge the exposure area of a wafer surface. In the scan type of a projection optical system, its illumination area has a slit-like shape and a reticle and a wafer are synchronously scanned with respect to the slit-like shape. If this slit-like shape of exposure light is used, since the shape of the slit is not rotationally symmetrical, heat distribution asymmetrical about the optical axis of the projection optical system occurs due to the absorption of light into glass material, and longitudinal astigmatism (longitudinal astigmatism) occurs in the projection optical system.
However, projection optical systems of the type used in conventional projection exposure apparatus are not provided with means for correcting various asymmetrical aberrations due to the absorption of exposure light by varying the optical performance of the projection optical system, so that it has been impossible to effectively correct asymmetrical longitudinal astigmatism due to the influence of the absorption of exposure light. For this reason, passive measures have conventionally merely been taken, such as a method of limiting the amount of exposure light to be made incident on a projection system and restraining the occurrence of various aberrations.
Curvature of field will be described below. Since curvature of field relates to the Petzval sum, a method of varying the power (refractive power) of an optical element such as a lens or a mirror is known as a method of correcting a variation in curvature of field due to the absorption of exposure light into a projection optical system. However, conventionally, no optical elements having variable refractive power have been applied to the projection optical systems of projection exposure apparatus, and it has been impossible to effectively correct the influence of absorption of exposure light. For this reason, passive measures have conventionally merely been taken, such as a method of limiting the amount of exposure light to be made incident on a projection system and restraining the occurrence of various aberrations.
Rotationally asymmetrical magnification will be described below. In addition to an improvement in resolution, the alignment accuracy of patterns which are stacked in layers is a factor required for optical exposure techniques for manufacturing semiconductor devices.
A technique called global alignment is generally widely used as an alignment method. The error of global alignment is mainly divided into an intershot component which is the error between individual printed shots and an intrashot component which is the error inside each individual shot. Recently as the field size has become larger, how to reduce the error of the intrashot component has become a greater problem. In an actual wafer, asymmetrical distortion occurs during the process of the wafer.
For example, if an error of 2 ppm in magnification is present as an asymmetrical uncorrectable component for a field size of 22 mm, the following error occurs:
22 mm×2 ppm=44 nm,
and the error has a value close to ⅕ times the resolution performance of a linewidth of 0.25 &mgr;m. As viewed from an overlay budget, it is apparent that this value is not within allowable limits, and how to reduce the intrashot component has become a great problem to the projection exposure apparatus.
With regard to distortion, known means is available as a method of controlling magnification and the third-order distortion. For example, there are a method of moving a plurality of optical elements with respect to each other in a projection optical system along the optical axis thereof, and a method of controlling the pressure in the gas sealed between optical elements. Magnification is a basic quantity of an optical system and can be varied without causing variations in other aberrations, but as to the third-order distortion, an optical system needs to be designed from the beginning in view of the correction of the third-order distortion because the correction of the third-order distortion offers problems such as variations in other aberrations and a narrow range of adjustment. In particular, if various image-forming methods which are generically called image enhancement such as off-axis illumination techniques or phase-shifting masks are adopted, the matching of distortion between image-forming methods becomes a problem. In this case, it is a great problem to establish the techniques of arbitrarily controlling the third-order distortion without imposing a load on design.
Higher-order distortion will be described below. In a scan type of projection exposure apparatus, it is possible to correct through scanning an asymmetrical magnifica
Sekine Yoshiyuki
Suzuki Akiyoshi
Adams Russell
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Nguyen Hung Henry
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