Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1999-12-17
2001-10-30
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S071000
Reexamination Certificate
active
06310680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a method of adjusting a scanning exposure apparatus for transferring a pattern formed on a mask onto a photosensitive substrate during a photolithography process. The invention also relates to a scanning exposure apparatus using the above adjustment method. More particularly, the invention relates to a method suitable for adjusting the image focusing characteristics of the projecting optical system of the scanning exposure apparatus.
2. Description of Related Art
A one-shot type projection exposure apparatus (stepper using the step-and-repeat process) is widely used to produce a semiconductor device or the like. With this apparatus, an image of a pattern formed on a reticle serving as a mask is transferred, via a projecting optical system onto a photosensitive substrate. With the expansion of the pattern area of semiconductor chips, it becomes necessary to further expand the exposure area of the projection exposure apparatus used to produce these chips. However, a technical difficulty arises in simply expanding the exposure area while trying to keep good image focusing characteristics of the projecting optical system. Even if the expansion is possible, the resultant projection exposure apparatus would have a great size and would be prohibitively expensive.
One know technique of effectively expanding the exposure area without expanding the effective physical exposure field of the projecting optical system is to employ a step-and-scan projection exposure apparatus. In this technique, after stepping a wafer to a scanning start position, the wafer and a reticle are scanned synchronously relative to a projecting optical system. The reticle is illuminated by exposure light via a slit with a long and narrow rectangular shape or a circular arc shape. Thereby, the image of a reticle pattern is transferred onto a wafer from one shot area to another.
In the case of a scanning projection exposure apparatus, such as a step-and-scan exposure apparatus, the size of a circuit pattern formed on a wafer along the scanning direction is not limited by the size of the projecting optical system. On the other hand, the maximum pattern size (an area in which an image can be formed) in the direction perpendicular to the scanning direction is nearly equal to the diameter of the effective exposure field of the projecting optical system. Therefore, unlike the conventional one-shot projection exposure apparatus in which the available area is limited to a square or rectangle inscribed in the effective field of its projecting optical system, the scanning exposure apparatus has the ability of exposing a wider area to form a pattern.
Furthermore, in the scanning exposure apparatus, only a part of the effective exposure field of the projecting optical system is used. That is, in the slit-shaped exposure area, the image focusing characteristics, such as the distortion and the magnification error, of the projecting optical system can be kept within allowable ranges. As a result, it is possible to achieve high-accuracy image focusing characteristics associated with the transferred image over the entire shot area. In the conventional technique, when a scanning exposure apparatus is assembled and adjusted, a test pattern image is projected at each measurement point within the slit-shaped exposure area of the projecting optical system and the positions and contrast of the projected images are measured. In accordance with the result of the measurement, the image focusing characteristics of the projecting optical system are adjusted.
In the conventional scanning exposure apparatus, among various image focusing characteristics of the projecting optical system, random components independent of the distance from the optical axis are easy to adjust since the adjustment can be performed within a narrow area. On the other hand, for systematically varying components of the image focusing characteristics, such as those varying in proportion to the distance from the optical axis, the narrow measurement area results in less amount of information available for use in the adjustment. This makes it difficult for the scanning exposure apparatus to make a high-precision adjustment compared to the one-shot type (step-and-repeat) projection exposure apparatus.
Until recently, adjustment errors which occur in the conventional techniques were within allowable ranges. However, as the size of semiconductor devices becomes increasingly smaller, the image focusing characteristics of the projecting optical system become more critical. Thus, the conventional techniques of adjusting the image focusing characteristics of the projecting optical system have difficulty in achieving required characteristics.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the present invention to provide a method for precisely adjusting a scanning exposure apparatus so that its projecting optical system has desired image focusing characteristics. It is another object of the present invention to provide a scanning exposure apparatus having the capability of making an adjustment according to the above method.
A first aspect of the present invention provides a method of adjusting a scanning exposure apparatus for transferring, via a projecting optical system PL, an image of a pattern formed on a mask (reticle) R onto a substrate W while synchronously scanning both the mask R and the substrate W with respect to illumination light IL. The method includes: (1) changing the illumination area of the illumination light IL to an area different from that used in a scanning exposure; (2) after changing the illumination area, projecting an image of a predetermined mask pattern via the projecting optical system PL and measuring the image focusing characteristics of the projecting optical system PL; and (3) adjusting the projecting optical system PL in accordance with the result of the measurement.
In the adjustment method according to the present invention, an area (second illumination area) used in the process of measuring the image focusing characteristics of the projecting optical system is set such that it is different from a mask illumination area employed in a scanning exposure process (first illumination area). The first illumination area of the scanning projection exposure apparatus is a limited area on the mask, such as a slit-shaped area. The image focusing characteristics measured in the limited area tend to include errors in components which vary systematically across the entire area of the effective exposure field of the projecting optical system. To avoid this problem, a larger area containing an area outside the above limited area is employed as the second area. With this enlarged area, it becomes possible to precisely make two-dimensional evaluations on the systematic components (e.g., distortion, curvature of field) of the image focusing characteristics of the projecting optical system PL. Thus, it becomes possible to precisely adjust the image focusing characteristics of the projecting optical system into a desired state based on the evaluation results.
The illumination area used in the scanning exposure process is, for example, a slit-shaped illumination area whose short sides extend in the scanning direction. When the image focusing characteristics of the projecting optical system PL are to be measured, the illumination area is switched to the area containing an area outside the slit-shaped illumination area thereby expanding the illumination area used to measure the image focusing characteristics of the projecting optical system PL. This ensures that the systematic components of the image focusing characteristics, such as those which vary depending on the distance from the optical axis, can be measured with high accuracy.
After the adjustment of the image focusing characteristics of the projecting optical system PL, the illumination area of the illumination light IL is returned to the normal illumination area used in the scanning exposure process. The image
Adams Russell
Nguyen Hung Henry
Nikon Corporation
Oliff & Berridg,e PLC
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