Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
1999-03-08
2001-11-06
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S030000
Reexamination Certificate
active
06312859
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a projection exposure apparatus used for exposing a pattern on a mask onto a photosensitive substrate in a photolithographic process for manufacturing a semiconductor device, liquid crystal display device, image pick-up device (CCD), thin-film magnetic head, and the like, and more particularly to a projection exposure apparatus having a mechanism for correcting the imagery characteristic of the projection lens system.
For manufacturing a semiconductor device or the like, a projection exposure apparatus is generally used, which transfers a pattern image formed on a reticle (serving as a mask) onto a wafer (or glass plate) as a photosensitive substrate through a projection lens system. Conventionally, an exposure apparatus of a collective exposure type, such as a stepper, has been used. However, a so-called step-and-scan type projection exposure apparatus has been recently substituted for the collective exposure type projection exposure apparatus. The step-and-scan type exposure apparatus exposes a pattern image onto a shot area on the wafer, while scanning both the reticle and wafer with respect to the projection lens system.
The projection lens system used in a projection exposure apparatus requires high resolution over substantially the entire exposure area, because the circuit pattern of the reticle must be precisely transferred onto the wafer. To this end, measures have been proposed to correct aberration in the projection lens system in every stage of the design and manufacturing processes. However, because the imagery characteristic of the projection lens system easily varies in response to the changes in atmospheric pressure, environmental temperature, absorption of illumination, etc., merely satisfying a certain imagery characteristic under a specific environmental condition is insufficient in practical use.
Recent projection exposure apparatus are equipped with an imagery characteristic correction mechanism, which measures the fluctuation in parameters of environmental conditions to calculate the changing amount of the imagery characteristic, or alternatively, directly measures the changing amount of the imagery characteristic and corrects the imagery characteristic of the projection lens system. The imagery characteristic of the projection lens system may be intentionally changed so that the apparatus matches with the characteristics of other projection exposure apparatus or photosensitizers.
Examples of the technique for correcting the imagery characteristic using an imagery characteristic correction mechanism include a method for driving the optical elements (lenses) in the projection lens system or the reticle along the optical axis of the projection lens system to correct the projection magnification, isotropic distortion (barrel distortion), spherical distortion, image plane distortion, etc. Another technique is to tilt the lens element of the projection lens system or the reticle with respect to a plane perpendicular to the optical axis of the projection lens system to correct anisotropic distortion (trapezoidal distortion) and image plane inclination. Still another technique is to seal the gap between certain lenses of the projection lens system and change the internal pressure of the sealed space to adjust the refractive index of the internal air to correct the projection magnification, isotropic distortion (barrel distortion), spherical aberration and image plane distortion.
When using such a conventional imagery characteristic correction mechanism, the imagery characteristic of the projection optical system is appropriately corrected; however, unintended positional shifts of the image-forming position may occur. This is because when driving the driven object (particularly lenses within the projection lens system, or the reticle) along the optical axis of the projection lens system, the driven object often slightly slips out of the optical axis and advances obliquely because it is difficult to drive the lens or reticle strictly parallel to the optical axis. Generally, when driving the lens or reticle in the optical-axis direction, the position of the lens or the reticle is strictly controlled by, for example, a position sensor. On the contrary, with respect to the direction perpendicular to the optical axis, the movement of the driven object is simply guided by a guide mechanism because it is typically not necessary to control the movement of the lens in the direction perpendicular to the optical axis. However, slackness (vibration) or elastic deformation of the guide mechanism may cause the driven object to move slightly out of line with the optical axis, which may result in displacement of the image-forming position of the pattern image off the optical axis.
During a semiconductor manufacturing process, multiple layers of different circuit patterns are exposed onto the wafer. Each pattern must be precisely superimposed on the previous pattern formed through the previous exposure. The projection exposure apparatus generally has an alignment sensor for detecting a registration mark formed on the previous pattern to determine a proper exposure position. Examples of the alignment sensor include a TTR (through-the-reticle) sensor, which monitors both the alignment mark on the reticle (referred to as a reticle mark) and the alignment mark on the wafer (referred to as a wafer mark) simultaneously. While a TTR sensor is very precise, there are several limitations in its operation because of the simultaneous measurement of the reticle and the wafer. To this end, an off-axis method is often used, in which only the wafer mark is detected using an alignment sensor fixed to the side of the projection lens system. In the off-axis method, the positional relation (base-line amount) between the reticle mark (more precisely, the center of the projected pattern image of the reticle) and the detection center of the alignment sensor is obtained and stored in advance. When the alignment sensor detects the position of the wafer mark, displacement of the wafer during exposure is then determined based on the detection result of the sensor and the prestored positional relation.
When exposing multi-layers of circuit patterns on the photosensitive substrate, the mark formed on the photosensitive substrate is aligned with the mark-detection optical system. The substrate stage is then moved from this position by the base-line amount to execute exposure. In this manner, the reticle pattern image is aligned with the circuit pattern, which has already been formed on the photosensitive substrate.
In a conventional projection exposure apparatus, the positional relationship between the image position of the reticle mark projected on the substrate stage and the detection center of the mark-detection optical system must be accurately detected. The absolute position of the reticle in a projection exposure apparatus is not so strictly regulated, as long as the reticle position relative to the photosensitive substrate is precisely controlled. For example, the reticle position relative to the projection optical system is not strictly controlled as long as the reticle is positioned within the exposure area and the precision of the projection optical system is assured in that area. A projection optical system is generally composed of a plurality of (e.g., twenty or more) lens elements. The optical axis of the projection optical system is defined by a composition of offset components of the center axes of the respective lens elements. It is not defined by the outer diameter of the projection optical system or the positions of the lens elements. If the area of reticle positions relative to the projection optical system is too large, the exposure area of the projection optical system must also be set large, which increases the cost.
In view of the circumstance described above, the position of the reticle is conventionally adjusted with reference to the outer diameter of the lens barrel of the projection optical system at a mechanical precision of ab
Nikon Corporation
Nixon & Vanderhye P.C.
Young Christopher G.
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