Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-11-28
2002-05-07
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S077000
Reexamination Certificate
active
06384898
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection exposure apparatus used in a photo-lithographic process to manufacture a semi-conductor device or a liquid crystal display device, and apparatus and method for detecting a mark position.
In a photo-lithographic process of a projection exposure apparatus for manufacturing a semi-conductor device or a liquid crystal display device, there has been used an exposure apparatus in which an image of a photo-mask or a reticle (referred to as “reticle” hereinafter) on which a transfer pattern is formed is transferred onto a semiconductor wafer on which photosensitive material such as photoresist is coated or a photosensitive substrate such as a glass substrate (referred to as “wafer” hereinafter) by using a projection exposure method or a proximity exposure method. In such an exposure apparatus, since an image of a circuit pattern on the reticle must be overlapped with a circuit pattern already formed on the wafer with high accuracy, prior to exposure, positional alignment between the reticle and the wafer (referred to as “alignment” hereinafter) should be performed. The exposure apparatus performs the alignment by correctly detecting a position of each positioning mark (referred to as “alignment mark” hereinafter) formed on the wafer as well as a position of the circuit pattern in the previous process.
There are the following alignment systems. In an LSA (laser-step-alignment) system, as shown in U.S. Pat. No. 5,151,750, laser light flux is illuminated on a grid-shaped alignment mark and the position of the alignment mark is detected on the basis of change in intensity of reflected light. In an LIA (laser interference alignment) system, as shown in U.S. Pat. No. 5,151,750, coherent light fluxes are incident on a grid-shaped alignment mark from two symmetrical order directions (for example, +first-order diffraction light direction and −first-order diffraction light direction) and the position and positional deviation (in a pitch direction) of the grid mark by interfering two diffraction light components generated from the grid mark along the same direction. Further, in an FIA (field-image-alignment) system, as shown in U.S. Pat. No. 5,493,403, in a condition that a wafer stage is stopped, white light is illuminated onto an alignment mark on the wafer, and an image of the alignment mark so obtained is detected by an imaging element, the position of the alignment mark is detected by effecting image treatment process.
There are the following alignment optical systems used with such alignment systems. In a TTL (through-the lens) system, a projection optical system is used as a detection optical system and the alignment mark on the wafer is detected by the projection optical system. In a TTR (through-the-reticle) system, a reticle alignment mark provided on the reticle and the alignment mark on the wafer are simultaneously observed (detected) through a projection optical system. In an off-axis system, there is provided an exclusive optical system having an optical axis spaced apart from an optical axis of a projection optical system by a predetermined distance, the alignment mark on the wafer is observed (detected) while illuminating white light from an exclusive light source on the alignment mark.
In the past, since a wafer stage on which the wafer to be moved is rested has a predetermined positional relation to a wafer stage guide through roller bearings and is position-controlled by a rotating motor and an associated ball screw, so long as the motor is stopped, the wafer stage could easily be maintained in the stopped condition. Recently, as a so-called scan type projection exposure apparatus in which a reticle stage on which the reticle is rested and the wafer stage are scanned to perform exposure in a synchronous manner has been developed, since high speed driving and minute control of the wafer stage are required, a so-called air stage of complete non-contact type in which the stage is floated by air pressure and is driven by a linear motor of non-contact type has been used. Although the air stage is superior to the conventional stages in various points such as a moving speed, dynamic control and the like, since it is a complete non-contact type, when observed microscopically, it is difficult to maintain the stage in the stopped condition.
In the projection exposure apparatus in which the air stage of complete non-contact type which is always moved microscopically is used as the wafer stage, for example, if the alignment is effected by using the off-axis alignment optical system in the FIA system, the following problems will occur.
In the image treatment alignment using the conventional white light source, since a predetermined integrated exposure amount must be provided to an imaging element used for picking-up the image, emission of the white light illuminated onto the alignment mark on the wafer must be continued for a predetermined time period. Accordingly, during the emission of the white light, the wafer stage on which the wafer is rested must be kept stationary. If the position of the wafer stage is shifted during the emission of the light for giving the integrated exposure amount required for the imaging element, accuracy of detection of the position of the alignment mark will be worsened.
However, in the air stage of complete non-contact type as mentioned above, since it is difficult to keep the stage stationary, the position of the air stage may be minutely changed while the imaging element of the alignment optical system is picking-up the image of the alignment mark, with the result that the accuracy of detection of the position of the alignment mark will be worsened, thereby making the high accurate alignment impossible.
Although it is considered that the air stage is mechanically stopped during the detection of the alignment mark by using any fixing mechanism, this attempt is not preferable because mechanical stress is applied to the air stage whenever the air stage is forcibly stopped and an excessive time period is required for driving the fixing mechanism to fix the air stage, resulting in reduction of through-put. If a white light source of continuous emission type having illuminance capable of obtaining the required integrated exposure amount of the imaging element for so short time period so that the shift of the position of the air stage is negligible, a heat amount generating from the white light source would be considerably increased, with the result that heat shield of the exposure apparatus is made difficult.
As another related technique, in a mark position detecting apparatus of image focusing type used in such an exposure apparatus, both an image of pinch marks disposed side by side at a predetermined interval (i.e., index pattern) formed on the reticle and an image of an alignment mark (for example, line-and-space mark) on the photosensitive substrate such as a wafer or a glass substrate are focused onto an imaging element through a projection optical system a focusing optical system, whereby a position of the photosensitive substrate is detected on the basis of image data from the imaging element. Further, it is also known to provide a mark position detecting apparatus in which an index plate having a substantially conjugate relation to a wafer is provided within an alignment optical system via an objective optical system different from a projection optical system, and both an image of an index pattern on the index plate and an image of the wafer alignment mark are focused onto an imaging element through a focusing optical system, and a position of the photosensitive substrate is detected on the basis of image data from the imaging element. In such position detecting apparatuses, the index pattern and the alignment mark are imaged (or photo-taken) by several times to obtain a plurality of images, and these plural images are averaged to improve measuring accuracy.
However, When the image data from the imaging element is obtained, in order to improve SN of the image, image ac
Inoue Jiro
Suzuki Kazuaki
Adams Russell
Armstrong Westerman & Hattori, LLP
Nguyen Hung Henry
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