Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-02-16
2003-05-13
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S071000
Reexamination Certificate
active
06563565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and a method for an exposure technique in the photolithographic process, such as projecting a pattern through a projection optical system and transferring the pattern on a substrate, to manufacture advanced solid state devices such as semiconductor elements, image pick-up devices (e.g. CCD), liquid crystal display elements and thin film magnetic heads.
2. Background Art
Photographic exposure technology constitutes a nucleus of semiconductor device fabrication, and as the device circuits become more highly integrated, it becomes crucial to improve resolution capability and transfer fidelity of images produced in the optical systems. Typical photolithographic process involves such steps as application of a resist film on a substrate, exposure and development of the resist patterns. To improve image resolution and transfer fidelity, it is critical to provide a precise control over the exposure dose for an optimal degree of exposure of the photo-resist film applied to a wafer as the substrate.
Conventionally, a general practice in semiconductor fabrication plants has been to use, as exposure light, i-line of wavelength at about 365 nm produced by a mercury lamp, in a so-called step-and-repeat exposure arrangement including a stepper for image projection at an image reduction ratio of ⅕ from the reticle to the wafer. In recent years, as larger size wafers have become more common, the size of the projected pattern image have also increased, but in order to avoid excessively large projected image size, a different technique of exposure is receiving attention. This technique is called step-and-scan method, which involves a movable reticle within the field of view of the object plane in a certain direction at a constant speed, and a corresponding movable wafer within the field of view of the image plane to be moved at the same relative speed as the reduction ratio so that the overall image of a circuit pattern on the reticle can be transferred onto each region of the wafer surface.
In the conventional methods of exposure dose control, it is assumed that the transmission coefficient of a projection optical system with respect to an illumination light does not vary in the short period involved in an exposure process, so that a light meter on the wafer stage is considered to be adequate for exposure control purposes. For example, exposure on the wafer surface is determined by measuring the transmission coefficient of the projection optical system just prior to the exposure event, measuring the quantity of illumination light in a branched path of the illumination optical system, and computing the exposure necessary according to the measured values of transmission coefficient and illumination dose. In the stepper system which is static, exposure duration is controlled so that the accumulated value of the calculated amount of exposure dose will reach a certain value, and in a scanning system which is dynamic, the output power or scanning speed is controlled so as to maintain the calculated quantity of exposure dose (for a given illuminance) at a constant value.
In recent years, there has been a trend towards using shorter wavelengths for the exposure light to improve the optical resolution, such that ultraviolet light from an excimer laser source is used in some step-and-repeat and step-and-scan apparatus. There is a serious move in some production lines to use a KrF excimer laser emitting at 248 nm, and there has been promising developments for wavelengths shorter than 200 nm, such as ArF excimer lasers generating ultraviolet pulses at 193 nm.
However, ultraviolet pulses produced from ArF excimer laser contain several oxygen absorption bands within the wavelength band in their natural oscillation states, such that, to use the laser as an exposure pulse source, it is desirable to restrict the light spectrum to a wavelength band that avoids such absorption bands. Furthermore, it is desirable that the illumination path (between light source and reticle) and the projection path (between reticle and wafer) be as free of oxygen gas as possible, in other words, it is desirable that most of the illumination path and projection path be surrounded by an inert gas environment (nitrogen or helium gas). Examples of projection exposure apparatuses based on such an ArF excimer laser light source have been disclosed in Japanese Patent Applications, First Publications, Hei 6-260385 and Hei 6-260386 (corresponding to U.S. Pat. No. 5,559,584).
Presently, there are only two optical grade materials available commercially that are known to produce a relatively high transmission for such ultraviolet pulses generated by excimer lasers (especially for wavelength shorter than 200 nm); they are quartz (SiO
2
) and fluoro spar or fluorite (CaF
2
). Other such materials include magnesium fluoride and lithium fluoride, but for use as optical material for projection exposure apparatus, there are still unresolved problems of fabricability and durability.
Other associated concerns are optical projection systems for use in projection exposure apparatus. There are dioptric system (refractive) and cata-dioptric systems which combine refractive elements (lenses) with reflective elements (specially concave mirror). Regardless of which type of projection system is used, if refractive elements (transmissive elements) are involved in some parts of the system, one is forced to use at least one of either quartz or fluorite, at the present time. Furthermore, whether refractive or reflective, these optical components are used with multilayers of vapor-deposited surface coatings such as anti-reflection or protective coatings, and they are manufactured to fulfill custom specifications of certain optical components. The particular property of interest in this case is the magnitude of absolute transmissivity of lenses or absolute value of reflectivity of the optical elements made from these optical materials.
For example, for a single lens element, incident surface and exiting surface are generally both coated with anti-reflection coating so as to increase transmissivity as much as possible. In precision imaging systems such as projection exposure apparatus, there are twenty to thirty lenses to provide compensation for various aberration effects, such that, even if the transmissivity of each lens element is only slightly less than 100%, the overall transmissivity of the projection system is decreased considerably. Same is true for a projection exposure apparatus using some reflective elements so that if each reflective element has low reflection, the overall transmission of the projection system is decreased greatly.
For example, if there are twenty-five lens elements in the focusing path of the projection system, if each lens has a transmission coefficient of 96%, the overall transmission coefficient &egr; becomes fairly small at about 36% (≈0.96
25
×100). When the transmission coefficient of the projection system is low, it is necessary to either increase the luminocity (energy) of exposure light for projecting the circuit pattern on the reticle on the wafer, or use a ultraviolet-sensitive photoresist of higher sensitivity, otherwise the longer exposure required would reduce productivity. Therefore, practical solution for the projection exposure apparatus is to use an excimer laser of higher output power.
However, as trials progressed using such excimer lasers having relatively larger projected field size, a new phenomenon was discovered that, in relatively short time, the transmission coefficient of coating materials on the optical elements (e.g., anti-reflection coating) underwent dynamic changes due to the use of ultraviolet pulses (from KrF or ArF excimer lasers). It has been discovered since that the same phenomenon can occur not only on optical elements in the projection path but also on those in the illumination path for reticle (quartz plate) as well as on the reticle itself.
Such a phenomenon is considered to occ
Nguyen Henry Hung
Nikon Corporation
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