Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2000-03-31
2001-11-20
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S243100, C250S252100, C065S397000, C501S905000
Reexamination Certificate
active
06320661
ABSTRACT:
This application claims the benefit of Japanese Application No. 11-095264, filed in Japan on Apr. 1, 1999, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for evaluating the transmittance of photolithographic optical members used in optical systems, such as lenses and mirrors, which are to be used in a specific wavelength region of about 400 nm or less, preferably about 300 nm or less, in photolithographic techniques.
The present invention also relates to a synthetic silica glass member for use in optical systems, such as lenses and mirrors, which are to be used in a photolithographic apparatus in a specific wavelength region of about 400 nm or less, preferably about 300 nm or less, in a photolithographic technique, and to a photolithographic apparatus that is constructed using such optical members.
2. Discussion of the Related Art
Conventionally, an exposure apparatus called “stepper” has been used in photolithographic techniques in which fine patterns of integrated circuits are exposed and transferred onto wafers made of silicon, etc. In the light source used in such steppers, there has been a progressive shortening of the wavelength of the light source from the g-line (436 nm) to the i-line (365 nm), and further to KrF (248.3 nm) and ArF (193.4 nm) excimer lasers as the degree of integration in LSI has increased in recent years. Thus, general purposes-use optical glass cannot be used in illumination systems or projection lenses of such excimer laser steppers, and materials of silica glass or fluorite, etc., which show high transmittances even in the ultraviolet region—e.g., at the wavelengths of excimer lasers, are required.
Furthermore, the optical system mounted in a stepper is constructed from a combination of numerous lenses. Since the transmittance drop in each lens is multiplied by the number of lenses used, it is necessary to minimize the transmittance drop in each lens in order to prevent a drop in the light power on the wafer and a drop in the throughput. For this reason, it is necessary that the silica glass or fluorite used in the illumination systems or projection lenses of excimer laser steppers have a bulk transmittance of 99.8%/cm or greater, and development aiming at an increase in the transmittance of optical elements used in the ultraviolet region has been pursued.
As the wavelengths of exposure light sources have become progressively shorter, it has become technically difficult to measure the bulk transmittance values of optical elements with high accuracy. Accordingly, a technique for the accurate measurement and evaluation of the bulk transmittance values of synthetic silica glass and crystal materials, etc., which have only a very small absorption (bulk absorption coefficient=approximately 0.001/cm), becomes indispensable for achieving a high transmittance in optical materials. Various types of wet and dry cleaning constitute one technique for the precise measurement and evaluation of the bulk transmittance values of optical materials. Especially in the vacuum ultraviolet region, such as the ArF excimer laser wavelength region (193 nm), the effects of contaminants adhering to the surfaces of evaluation samples on the measured value of the transmittance cannot be ignored. Accordingly, in the transmittance evaluation process, it is essential that the samples to be evaluated (evaluation sample) be cleaned prior to the measurement of the transmittance, so that the surface contaminants are removed.
However, after the surfaces of the evaluation samples have been cleaned by various cleaning methods, contamination by organic substances, etc. (re-contamination), proceeds during the time between the completion of cleaning and the measurement/evaluation of the transmittance, even if this time is short. The reason for this is that trace amounts of organic substances are constantly released from the constituent members of the clean room and the holder or transporting jig used to store the samples into the ambient atmosphere that surrounds the evaluation samples following cleaning. Thus, the surfaces of the evaluation samples that have been cleaned by the above-mentioned cleaning method are exposed to these organic substances. In particular, since there is nothing to prevent the re-adhesion of organic matter to the cleaned surfaces immediately after cleaning, re-contamination of the surfaces by organic matter, which is present in the ambient atmosphere, proceeds quickly.
Furthermore, the quantity of organic matter re-adhering to the surfaces of the samples following cleaning depends on the time from the completion of cleaning to the performance of the transmittance measurement. This means that even if the same samples are cleaned under the same conditions, the transmittance will differ if the time from the completion of cleaning to the measurement of the transmittance differs. This is because the amount of adhering organic matter will be different. Accordingly, in cases where the transmittance values of different samples are to be compared, in addition to cleaning the samples under the same conditions, it is necessary to perform the transmittance measurements at a fixed time following the completion of cleaning. However, in order to ensure that the transmittance is always measured at a fixed time from the completion of cleaning, only one sample must be treated in a single cleaning at a time. Accordingly, in cases where there are a plurality of samples, and especially in cases where the cleaning process requires a long processing time as in wet cleaning, etc., the cleaning process is time-consuming and inefficient since the overall evaluation process takes time.
Furthermore, in order to exclude as far as possible the effects of re-contamination of the surfaces, it is necessary that the measurement of the transmittance of the evaluation samples be performed as quickly as possible following the completion of cleaning. For this, a commercially marketed spectrophotometer, which allows measurements to be performed by a simple method, is suitable. However, in the case of such a spectrophotometer, it is necessary to increase the light power used for measurement as much as possible in order to improve the measurement precision, and it has been impossible to achieve sufficient collimation of the measurement light for this purpose at the position of the object of measurement (i.e., the light has an angle of divergence). As a result, the light path of the measurement light (transmitted light) passing through the object of measurement varies by refraction, so that it has been difficult to make an accurate determination of the transmittance.
Another aspect of the conventional photolithography technologies is described next. In general, the optical systems of such steppers are constructed from an illumination optical system, which uniformly illuminates the surface of a reticle with light from a light source, and a projection optical system, which reduces an integrated circuit pattern on the reticle with a reduction ratio of 1/5, for example, and projects this pattern onto the wafer.
As stated above, in the light source used in photolithographic apparatus, there has been a progressive shortening of the wavelength of the light source from the g-line (436 nm) to the i-line (365 nm), and further to KrF (248.3 nm) and ArF (193.4 nm) excimer lasers as the degree of integration of LSI has increased in recent years. In correspondence with this development, there has been a rising demand for photolithographic apparatus that can expose patterns having a finer minimum finished line width. However, in cases where the wavelength of the light source is in the ultraviolet region, and especially in the region of about 250 nm or less, if the lens materials used in the illumination optical system and projection optical system are materials suited for light with a wavelength longer than the i-line, the resultant light transmittance is poor and such materials are inadequate for practical use. Accordingl
Jinbo Hiroki
Komine Norio
Yoshida Akiko
Morgan & Lewis & Bockius, LLP
Nikon Corporation
Pham Hoa Q.
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