Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Statutory Invention Registration
1998-10-06
2001-07-03
Pihulic, Daniel T. (Department: 3662)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S329000
Statutory Invention Registration
active
H0001972
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to interferometry and more particularly to an autofocus system using a common path interferometer.
2. Description of the Prior Art
Autofocus systems are well known, for instance in the photography field. Typically an autofocus system includes a range finding device to determine the distance from the lens system to the object to be imaged. For instance, some autofocus systems use infrared range finding or ultrasonic range finding. In addition to the photography field, autofocus is also used in the lithography field where the goal is to form an image on a resist coated semiconductor wafer. In this case typically the focus adjustment is the distance between the projection lens system of a photolithography tool and the image plane which is the surface of the resist formed on a wafer. Photolithography as used in the semiconductor field is extremely precise and the autofocus system must correspondingly be extremely precise since the feature sizes being imaged are typically very small, for instance 0.25 micrometers. It is known to use an oblique incident optical system for lithography auto focusing, where displacement of a light beam is measured by a sensor. It is to be understood that “in focus” in this context means that the image plane is at a particular distance from the projection optical system.
However, it has been found that in general vibrations and random air fluctuations are still a problem in an autofocus system using oblique incident optics. In general, compensation for this vibration is not possible, leading to a degradation of the accuracy of the autofocusing.
SUMMARY
In accordance with this invention a common path interferometer is used in, e.g., an autofocus system and includes oblique incident optics which transform a defocus of the reflective surface to a lateral shear of the interferometer beam. A common path interferometry measures the lateral shear. The common path interferometer is a triangular path interferometer in one embodiment. Since both the signal (first) light beam and the reference (second) light beam (e.g. a clockwise and counterclockwise directional beam) pass through almost the same optical path in this type of interferometer, the system is thereby rendered independent of adverse affects from mechanical vibrations and also random air fluctuations.
Hence this common path interferometer allows measuring of any deviation between the actual position of the reflected surface and its intended position, for purposes e.g. of autofocusing, by directing a first beam and a second beam onto the reflective surface (for instance the surface of a semiconductor wafer), the reference beam and the signal beam being parallel and spaced apart. At the detector end, the first beam and the second beam are detected after being reflected from the wafer or the reflective surface. Then the deviation between these two beams is determined as a function of a distance in a plane e.g. orthogonal to that of the reflective surface, between axes defined by respectively the first beam, the reflected first beam and the second signal beam. Thus any change in focus (the position of the reflective surface) is transformed into a lateral shear between a clockwise path and a counterclockwise path of the triangular path interferometer. The lateral shear is transformed to a spatial frequency of the fringe for purposes of detection in one embodiment.
In one embodiment, the presence of a layer of heated air (atmosphere) immediately over the surface of the wafer provides some distortion to the interferometry which is compensated for in accordance with the invention.
A typical application of the present invention is for use in an exposure apparatus as used in photolithography, for example in semiconductor fabrication, as described above. See e.g. Sakakibara et al., U.S. Pat. No. 5,448,332, incorporated by reference herein in its entirety for a description of such an exposure apparatus.
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Nikon Corporation
Pihulic Daniel T.
Skjerven, Morrill, MacPherson, Franklin & Friel LLP
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