Photocopying – Projection printing and copying cameras – With temperature or foreign particle control
Reexamination Certificate
2001-07-19
2004-03-30
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
With temperature or foreign particle control
C355S053000
Reexamination Certificate
active
06714278
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an exposure apparatus used in a photolithographic process for producing, for example, semiconductor devices, liquid crystal displays and thin film magnetic heads, among others.
In a photolithographic process for producing semiconductor devices, liquid crystal displays or other products, there have been used exposure apparatuses in which an illumination light beam is utilized for transferring a pattern formed on a photomask or a reticle (a generic term “reticle” is used hereinafter to mean either), through a projection optical system, onto a silicon wafer or a glass substrate (either is meant by the term “wafer” hereinafter) which is photosensitized or coated with a photoresist layer.
In recent years, continuous reduction in linewidth of semiconductor integrated circuits requires higher and higher registration accuracy (i.e., accuracy in alignment between patterns of two layers formed overlaid one on the other on a integrated circuit chip) for exposure apparatus, which accuracy may be, for example, as high as about 50 nm (nanometers). In addition, the size of wafers is increasing from year to year in order to improve productivity, and in fact, 200 mm diameter wafers, which are widely used at present, are being replaced with 300 mm diameter wafers.
Various types of alignment optical systems are used for overlaying an image of a circuit pattern formed on a mask onto a wafer with required registration accuracy, and the practical types of alignment optical systems may be categorized generally into three kinds which are well known in the art. The first is a Through-The-Reticle (TTR) method in which alignment marks formed on a reticle and alignment marks formed on a wafer are observed (or sensed) at a time through the projection lens. The second is a Through-The-Lens (TTL) method in which only alignment marks formed on a wafer are sensed through the projection lens, without sensing alignment marks formed on a reticle. The third is an Off-Axis method in which only alignment marks formed on a wafer are sensed through a microscope system the objective lens of which is disposed at a position distant from the projection lens by a predetermined distance.
Unfortunately, TTR and TTL methods suffer from disadvantage that the alignment light beam, used to illuminate alignment marks, will suffer from chromatic aberration of the projection lens because the wavelengths of the alignment light beam are far different from those of the illumination light beam used for making exposure, and it is a severe technical challenge to correct chromatic aberration of the projection lens with respect to the alignment light beam. The reason why the alignment light beam used for either of these two methods must have such wavelengths is that the alignment light beam should not affect the photoresist layer coated on a wafer, during the alignment process. Further, TTR method suffers from additional problems. For example, TTR method will provide throughput lower than other methods. Further, TTR method has many limitations imposed thereon: for example, TTR method is required to assign wider areas on a reticle to alignment marks formed thereon than any other methods. Due to the above inconveniences associated with TTR and TTL methods, the off-axis method has been most widely used for alignment purposes in recent years.
Nevertheless, the off-axis method has its own drawbacks. For example, the distance between the projection lens (i.e., the projection optical system) and the objective lens of the microscope system (i.e., the off-axis alignment optical system), called the baseline length, may vary under the influence of heat so that the total registration accuracy may be degraded. In other words, the registration accuracy of an off-axis alignment optical system is highly influenced by thermal stability between the projection optical system and the off-axis alignment system. For this reason, the parts or members interconnecting the projection optical system and the off-axis alignment optical system are typically made of low-thermal-expansion materials.
There may be another factor in degradation of the registration accuracy, which factor is the degradation of the measurement accuracy of the laser interferometers used for measuring the position of a stage. The stage may be a stage for carrying a wafer or one for carrying a reticle. While the following description is specifically directed to laser interferometers associated with a wafer stage, it also applies to those associated with a reticle stage. The degradation of the measurement accuracy of the laser interferometers may occur due to a variation in refractive index of air in and around the optical paths of the measuring laser beams. The variation is caused by the air fluctuations arisen from temperature gradient. It is considered that a temperature gradient may occur where a wafer is heated by an illumination light beam illuminated onto the wafer for exposure, and the heated wafer produces an upward stream of relatively hot air rising from around the wafer. One known method for suppressing any variation in refractive index of the air in and around the optical paths of the measuring beams is to produce and maintain a continuous air stream flowing along the optical paths of the measuring beams. Another known method is disclosed by Japanese published patent application No. Hei-2-199814 (No. 199814/1990), in which a local chamber is provided in addition to the main chamber confining the whole exposure apparatus therein, and separate air-conditioning is effected to each of the local and main chambers. This effectively divides the space confined in the local chamber from the remainder in the main chamber, so that the space in the local chamber may be placed under more precise air-conditioning so as to prevent errors in measurements of the laser interferometers.
While it has been long known that registration accuracy of an exposure apparatus may be affected by heat generated in the exposure apparatus, and various solutions to this problem have been proposed as described above, many drawbacks remain in this regard as illustrated below.
First, an exposure apparatus may consist of many parts and members including connection members interconnecting the projection optical system and the off-axis alignment optical system described above, which parts and members are subject to thermal expansions. Using low-thermal-expansion materials to form appropriate parts and members could be one solution if it should be possible. However, in fact, such materials generally have such low rigidities that they are difficult to use for this purpose. For example, a typical low-thermal-expansion glass-ceramic material (such as “Zerodur” (trademark)) has a sufficiently low thermal expansion coefficient of 0.1 ppm/° C. (or 0.1×10
−6
/° C.) or less, but its rigidity is very low. Materials usable to form parts and members of an exposure apparatus are limited to those having sufficient rigidities, such as invert glasses and special ceramic materials. Such high-rigidity materials typically have thermal expansion coefficients of the order of 1 ppm/° C. If the required stability in size of members is such that it only allows for a variation of about 1 nm or less in a length of about 0.1 m, then the required stability amounts to within 10 ppb (or 10×10
−9
). In order to keep this stability in size with materials having thermal expansion coefficients of about 1 ppm/° C., the stability in temperature within 0.01° C. is required. When conventional heat insulators are used to meet the required stability in temperature, many layers of heat insulating materials have to be overlaid one on another to form such heat insulators. Since the space in an exposure apparatus is so limited, it has been very difficult to achieve the required stability in temperature with conventional heat insulators which can be accommodated in the limited space.
It is to be noted that the limitation on the space in an exposure apparatus of the above type is of
Nguyen Henry Hung
Nikon Corporation
Westerman Hattori Daniels & Adrian LLP
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