Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-07-09
2003-03-18
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C356S400000, C356S490000
Reexamination Certificate
active
06535272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a position transducer and an exposure apparatus with the same and, more particularly, to a position transducer and an exposure apparatus with the same, a position transducer being suitable for use with an alignment sensor for an exposure apparatus for exposing a pattern on a mask to a photosensitizable substrate in a photolithographic step for manufacturing semiconductor elements, liquid crystal display elements, image pick-up tubes (CCDs etc.), thin layer magnetic heads and so on.
2. Description of the Related Art
In a photolithographic step for manufacturing semiconductor elements and so on, there has hitherto been frequently employed a so-called stepper for exposing and transcribing a pattern of a reticle as a mask on each shot area on a photosensitizable substrate, such as a wafer, a glass plate or the like, by a step-and-repeat exposure system as an exposure apparatus to be employed for exposing and transcribing a pattern of a reticle acting as a mask onto such a photosensitizable substrate with a photoresist layer coated thereon.
Recently, there is being employed an exposure apparatus of a scanning exposure type, for example, of a step-and-scan system comprising exposing a reticle to a wafer while scanning the reticle and the wafer in synchronization with a projection optical system. Such exposure apparatuses require a particularly high level of precision in alignment of the reticle with each shot area on the wafer due to the fact that multiple layers of circuit patterns are superimposed on the wafer in manufacturing semiconductor elements and so on. Therefore, alignment sensors of various types and systems are employed for such exposure apparatuses.
Among conventional alignment sensors, an alignment sensor of a so-called grating alignment method uses laser beams as a source of alignment and a wafer mark in the form of a grating with its bars or dots arranged periodically. This grating alignment method may be classified by the structure of an alignment optical system or a detection system, a number of alignment beams of light and so on. The grating alignment method may further be broken down into the following types.
A first type of the grating alignment method is of the type comprising allowing one laser beam to strike the whole area of a wafer mark on a wafer, causing two rays of diffraction light generated from the wafer mark to form an image on a reference grating, scanning the wafer mark relative to the reference grating, and detecting the position of the wafer mark on the basis of a variation with a quantity of light transmitted through the reference grating or reflected therefrom.
A second type of the grating alignment method is of the type comprising allowing two laser beams to strike the whole area of a wafer mark on a wafer from the particular yet mutually different directions of the diffraction order and detecting the position of the wafer mark on the basis of the phase of interference light generating in the identical direction from the wafer mark. This is called as an LIA (Laser Interferometric Alignment) type.
The LIA type may be classified into two groups, one group being of a homodyne interference type transferring a wafer mark relative to a static interference fringe formed by two laser beams having no frequency difference and the other being of a heterodyne interference type measuring a phase difference between a signal photoelectrically detecting an interference light (beat light) of two diffraction light components generated from the wafer mark by two laser beams having a slight difference of frequencies therebetween and a reference signal having the same frequency as the frequency difference between the two laser beams and detecting the phase difference as an amount of a pitch-directional deviation of the position of the wafer mark of the grating form from the predetermined reference point.
Where the diffraction light generated from the grating-shaped wafer mark is detected as a signal using a source of monochromatic light in the manner as described hereinabove, the shape of the grating wafer mark may become non-symmetric as multiple thin layers are superimposed on a wafer substrate more and more, or no diffraction light to be detected may be generated for laser beams of certain wavelengths striking the wafer mark due to interference of a thin layer of the surface photoresist coating or for other reasons, or errors in detecting the diffraction light may be caused to occur due to a very faint intensity of the diffraction light generated therefrom. In order to solve those problems and to enable a more accurate detection of the position of the wafer mark, there has been developed an alignment sensor of a heterodyne interference type using a source of polychromatic light having multiple wavelengths.
An alignment sensor of a heterodyne interference type using a light flux with multiple wavelengths is constructed so as to allow two laser beams having different wavelengths to strike the wafer mark of a wafer from a direction of a particular order after the two laser beams with different wavelengths are modified to provide a slight difference in frequency therebetween and to photoelectrically detect the interference light consisting of the multiple wavelength components generated therefrom. The diffraction light of each wavelength component is photoelectrically detected in the form in which it is summed up altogether on a light recipient surface of a photoelectrical detection element, as an example, so that the detection of the position of the wafer mark may be less affected by an influence of interference of the thin layer on the photoresist coating or a deviation of the diffraction light due to an influence of non-symmetric shape between the sectional shapes of the wafer mark.
Further, there is another method of detecting diffraction light using an alignment sensor of a system referred to as an LSA (Laser Step Alignment) system, like the grating alignment system, which comprises forming a laser spot on the wafer by converging one laser beam thereonto, scanning the laser spot relative to the wafer with a wafer mark with dots arranged linearly thereon through a wafer stage, and detecting the position of the wafer mark on the basis of the intensity of diffraction light generated upon passage through the wafer mark beneath the laser spot.
For such conventional alignment sensors, a photomultiplier is employed as a photodetector when a sensitivity of light for detection from the wafer mark is required to be enhanced. Such a photomultiplier, however, may become a cause to induce a variation in temperature or a temperature gradient in the atmosphere surrounding it because it generates heat upon operation in progress. On the other hand, hitherto, the alignment sensor has been provided in the vicinity of an exposing main body portion of an exposure apparatus with the object of making the exposure apparatus compact. There is a risk, accordingly, that the exposing main body portion thereof and the wafer as a recipient that is exposed to light undergo thermal expansion or other transformation, causing faults and irregularities in accuracy of alignment and of exposure to light (accuracy of superimposition). In addition, as a photomultiplier is usually large in size, it is difficult in terms of making the exposure apparatus compact in size as a whole to locate such a photomultiplier nearby the exposing main body portion thereof.
Furthermore, even when there are some cases where a photodiode or the like is employed as a photodetector, a preamplifier and other means provided on such a photodetector may also become a source of generating heat. Where the photodetector generates heat, the heat may cause turbulence of the air surrounding it, resulting in disturbance of the light to be employed for the detection of alignment. This of course may adversely affect the accuracy in the detection of the wafer mark on the wafer.
Particularly, when a source of light having multiple wavelengths is em
Inoue Jiro
Ota Kazuya
Adams Russell
Armstrong Westerman & Hattori, LLP
Kim Peter B.
Nikon Corporation
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