Phase-shifting point diffraction interferometer mask designs

Details Phase-shifting point diffraction interferometer mask designs Phase-shifting point diffraction interferometer mask designs

1998-10-21

2001-10-23

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

Having wavefront division

C356S458000, C356S520000

Reexamination Certificate

active

06307635

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to interferometers for making highly accurate measurements of wavefront aberrations, particularly to phase-shifting point diffraction interferometers.
2. State of the Art
Optical metrology is the study of optical measurements. An area of optical metrology relevant to the present invention is the use of an interferometer to measure the quality of a test optic, such as a mirror or a lens.
One important recent application of optical metrology is the testing of projection optics for photolithography systems. Modern photolithography systems used to fabricate integrated circuits must continually image smaller features. To do so, systems are confronted with the diffraction limit of the light employed to image a pattern provided in a reticle. To meet this challenge, photolithographic systems must employ successively shorter wavelengths. Over the history of integrated circuit fabrication technology, photolithography systems have moved from visible to ultraviolet and may eventually move to x-ray radiation.
Because of the increasing difficulties posed by directly imaging a reticle pattern onto a wafer, it is desirable to use projection optics in lithography systems. Such systems include lenses or other optical elements that reduce the reticle images and project them onto the wafer surface. This allows reticles to retain larger feature sizes, thus reducing the expense of generating the reticle itself.
As with all optical elements, various aberrations such as spherical aberration, astigmatism, and coma may be present. These aberrations must be identified during the fabrication and/or testing of the projection optics, or the projection optics would introduce substantial blurring in the image projected onto the wafer.
In order to test the projection optics for various aberrations, interferometers may be employed. Conventional interferometers, based upon the Michelson design for example, employ a single coherent light source (at an object plane) which is split into a test wave and a reference wave. The test wave passes through the optic under test and the reference wave avoids that optic. The test and reference waves are recombined to generate an interference pattern or interferogram. Analysis of the interferogram and the resultant wavefront with, for example, Zernike polynomials, indicates the presence of aberrations.
The reference wave of the interferometer should be “perfect”; that is, it should be simple and well characterized, such as a plane or spherical wave. Unfortunately, beam splitters and other optics through which the reference beam passes introduce some deviations from perfection. Thus, the interferogram never solely represents the condition of the test optic. It always contains some artifacts from the optical system through which the reference wave passes. While these artifacts, in theory, can be separated from the interferogram, it is usually impossible to know that a subtraction produces a truly clean interferogram.
To address this problem, “point diffraction interferometers” have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer described in the article H. Medecki et al., “Phase-Shifting Point Diffraction Interferometer”,
Optics Letters
, 21(19), 1526-28 (1996), and in the U.S. Patent Application “Phase-Shifting Point Diffraction Interferometer”, Inventor Hector Medecki, Ser. No. 08/808,081, filed Feb. 28, 1997 now U.S. Pat. No. 5,835,217, which are both incorporated herein by reference. Referring to
FIG. 1
, in this prior art phase-shifting point diffraction interferometer
20
, electromagnetic radiation is sent to a pinhole
22
. The radiation is then sent through the test optic
24
to a grating
26
. The grating
26
produces two beams with a small angular separation. An opaque mask placed near the focal point of the test optic, contains a tiny reference pinhole, and a larger window centered on the respective foci of the two beams. The reference pinhole produces a reference wavefront by diffraction, while the window transmits the test wave without significant spatial filtering or attenuation. In effect, the beam going through the reference pinhole is filtered to remove the aberrations so that this filtered beam can interfere with the test beam that passes through the window without significant spatial filtering. An interference pattern is detected at a detector
30
. The light in the interferometer will typically be of a single wavelength. The grating
26
will transmit the zeroth-order beam straight through, but will produce a small angular change to the first-order diffractions. In the image plane, the zeroth-order, and the first-order diffractions will be in different positions, as indicated by the reference pinhole and the test window in the mask
28
. Phase-shifting is provided by moving the grating
26
perpendicular to the rulings of the grating. Phase-shifting improves the efficiency and accuracy of the system.
It is desired to have an improved phase-shifting point diffraction interferometer.
SUMMARY OF THE PRESENT INVENTION
The present invention, generally speaking, concerns improved mask designs for use with a phase-shifting point diffraction interferometer. In one aspect, a mask is provided with a reference pinhole and a test window where the separation between the center of the test beam window and the center of the reference pinhole is greater than the width of the test beam window. This embodiment prevents some of the reference beam energy from passing through the test beam window. In practice, the reference beam and the test beam each contains a distribution of energy extending laterally away from its focal point. The quality of the test optic determines the energy profile of the beams, and in the presence of scatter, mid-spatial-frequency roughness, or inadequate beam separation, high- frequency components of the reference wave will “leak” through the test beam window. In this case, the window behaves as a bandpass filter for the reference beam, transmitting a narrow range of spatial frequencies along the direction of beam separation. These unfiltered components complicate the phase-shifting data analysis considerably. By selecting a test beam window that is smaller than the beam separation distance, the magnitude of the reference beam overlap will be decreased to improve the signal-to-noise ratio in the measured interference patterns.
In a second aspect, the mask has at least two reference beam pinholes where the distance between the first reference beam pinhole to the test beam window is not the same as the distance between the second reference beam pinhole and the test beam window. In this configuration only a single, one-dimensional grating and translation stage are required. In some instances, the requirement of either a pair of orthogonal translation stages, or a single translation stage which allows two directions of motion, can be prohibitively expensive. In this embodiment, the at least two reference beam pinholes can be positioned on the same side of the test beam window.
In a third aspect, the inventive mask includes an elongated test beam window. The test beam window can be elongated in the direction that is perpendicular to the beam separation direction to produce an improved spatial frequency response in the measurement direction away from the beam separation direction.
In a fourth aspect, the inventive mask employs gratings whose rulings have been rotated 45° with respect to a single grating translation stage. With this orientation, two different orientations of the grating can be phase-shifted using a single translation stage. In this configuration, the orientation of the reference pinhole and test beam window on the mask are rotated as well.
In a fifth aspect, a two-dimensional grating can be used. The two-dimensional grating will produce a square lattice pattern of diffracted orders. The pinholes can be positioned about the square lattice pattern so that by displacing the pinholes of

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