Hybrid shearing and phase-shifting point diffraction...

Details Hybrid shearing and phase-shifting point diffraction... Hybrid shearing and phase-shifting point diffraction...

2000-07-17

2003-06-03

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Having wavefront division

C356S515000

Reexamination Certificate

active

06573997

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques for measuring optical systems offering both high dynamic range and verifiable high-accuracy.
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 an optical system, 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 imaging systems, 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 can reveal 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 measured aberrations, it is usually impossible to know that a subtraction produces a truly clean result.
To address this problem, “point diffraction interferometers” have been developed. An example of a point diffraction interferometer is the phase-shifting point diffraction interferometer (PS/PDI) 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. Pat. No. 5,835,217 “Phase-Shifting Point Diffraction Interferometer”, Inventor Hector Medecki, which are both incorporated herein by reference. Referring to
FIG. 3
, in this prior art phase-shifting point diffraction interferometer
2
a focused illumination source
4
is spatially filtered using a pinhole
6
placed in the object plane of the optical system
8
(hereafter called optic) under test. The pinhole diffracts a spherical illuminating wavefront that is focused in the image-plane of the test optic. A coarse grating beamsplitter
10
(e.g. a transmission grating) placed either before or following the test optic divides the beam into multiple overlapping diffraction orders. The series of diffracted orders each containing the aberrations of the optical system, are focused in the vicinity of the image-plane with a small lateral separation. The separation is determined by the wavelength, and the grating's pitch and position. A patterned opaque and transparent mask, called the PS/PDI mask
12
, is placed in the image-plane where it allows only two of the orders to be transmitted. One of those beams, called the test beam, is selected to pass through a relatively large window and propagate on to reach a detector
14
placed significantly beyond the image plane. A second beam, called the reference beam, is spatially filtered by a pinhole, called the reference pinhole, in the PS/PDI mask. Via pinhole diffraction, the spatially filtered reference beam, becomes a spherical reference. The reference beam combines with the unfiltered test beam creating a pattern of light and dark fringes (an interferogram) that reveals the aberrations in the test optic. Although the PS/PDI has been proven to have high-accuracy, broad applicability of the device is severely limited by its small dynamic range.
In contrast to the PS/PDI, lateral shearing interferometry (LSI) is a well-known and reliable method of optics testing that is characterized by a larger dynamic range. The LSI
20
shown in
FIG. 4
requires one less optical component than the PS/PDI but operates in nearly the same geometry. An incident focused beam
24
is spatially filtered in the object plane by a pinhole in object mask
26
, thus illuminating the test optic
28
with a spherical wave. A grating beamsplitter
30
is placed near the image-plane, where the illuminating beam is focused. The diffracted orders propagate unfiltered to the detector where they overlap. While the central zeroth-order propagates to the detector in the same manner as it would if the grating were not present, the diffracted orders propagate with an angular shear, leading to a (typically) small lateral displacement at the detector
34
. In this way, the test beam is compared with multiple sheared copies of itself. Analysis of the resultant interference pattern reveals an approximation to the gradient of the test wavefront, or the derivative in the direction of the shear. The original wavefront can be recovered by combining gradients from two (or more) directions. While the LSI has simplified geometry and operation relative to the PS/PDI, analysis of the PS/PDI data is more straightforward.
One primary difficulty encountered in the operation of the PS/PDI is alignment. For EUV optical systems with resolution on the order of 100 nm, the alignment of the focused beam onto the similarly sized reference pinhole poses a significant challenge. The alignment can be judged based on the visibility or contrast of the observed interference pattern: peak contrast indicates optimal alignment and will yield maximum signal-to-noise ratio in the measurement. With the test beam passing through a relatively large window (compared to the size of the focused beam), the fine alignment mainly affects the reference beam; its intensity rises and falls as its position on the reference pinhole changes.
For a similar reason, the quality (aberration magnitude) of the test optic also affects the achievable fringe contrast. When aberrations limit an optics ability to form a small focused beam, the light intensity transmitted through the reference pinhole suffers. With only about one-wave of rms aberration magnitude (wavefront error), the focused intensity can drop low enough to make interferogram analysis very difficult if not impossible. Furthermore, the alignment becomes significantly more difficult to perform. In this way, we say the dynamic range of the PS/PDI is limited. Its useful measurement range includes only optical systems in nearly perfect alignment.
In contrast to the PS/PDI, operation of the LSI has more relaxed alignment and dynamic range requirements. A large transmission grating is placed near the image plane of the test optic. In typical operation the grating is placed far enough from focus that

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