Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
2002-06-17
2004-08-17
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S487000
Reexamination Certificate
active
06778280
ABSTRACT:
BACKGROUND
This invention relates to interferometers, e.g., displacement measuring and dispersion interferometers that measure angular and linear displacements of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system, and also interferometers that monitor wavelength and determine intrinsic properties of gases.
Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam-splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer. The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/&lgr;, where v is the relative speed of the measurement and reference objects, &lgr; is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2&pgr; phase change substantially equal to a distance change in L of &lgr;/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. Many interferometers include non-linearities such as what are known as “cyclic errors.” The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on phase changes associated with changes in optical path length pnL and/or on phase changes associated with other parameters. For example, there is first harmonic cyclic error in phase that has a sinusoidal dependence on (2&pgr;pnL)/&lgr; and there is second harmonic cyclic error in phase that has a sinusoidal dependence on 2 (2&pgr;pnL)/&lgr;. Higher harmonic and fractional cyclic errors may also be present.
Cyclic errors can be produced by “beam mixing,” in which a portion of an input beam that nominally forms the reference beam propagates along the measurement path and/or a portion of an input beam that nominally forms the measurement beam propagates along the reference path. Such beam mixing can be caused by misalignment of interferometer with respect to polarization states of input beam, birefringence in the optical components of the interferometer, and other imperfections in the interferometer components, e.g., imperfections in a polarizing beam-splitter used to direct orthogonally polarized input beams along respective reference and measurement paths. Because of beam mixing and the resulting cyclic errors, there is not a strictly linear relation between changes in the phase of the measured interference signal and the relative optical path length pnL between the reference and measurement paths. If not compensated, cyclic errors caused by beam mixing can limit the accuracy of distance changes measured by an interferometer. Cyclic errors can also be produced by imperfections in transmissive surfaces that produce undesired multiple reflections within the interferometer and imperfections in components such as retroreflectors and/or phase retardation plates that produce undesired ellipticities and undesired rotations of planes of polarization in beams in the interferometer. For a general reference on the theoretical causes of cyclic errors, see, for example, C. W. Wu and R. D. Deslattes, “Analytical modelling of the periodic nonlinearity in heterodyne interferometry,”
Applied Optics,
37, 6696-6700, 1998.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure dispersion of a gas in the measurement path of the distance measuring interferometer. The dispersion measurement can be used to convert the optical path length measured by a distance measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged. In addition to the extrinsic dispersion measurement, the conversion of the optical path length to a physical length requires knowledge of an intrinsic value of the gas. The factor &Ggr; is a suitable intrinsic value and is the reciprocal dispersive power of the gas for the wavelengths used in the dispersion interferometry. The factor &Ggr; can be measured separately or based on literature values. Cyclic errors in the interferometer also contribute to dispersion measurements and measurements of the factor &Ggr;. In addition, cyclic errors can degrade interferometric measurements used to measure and/or monitor the wavelength of a beam.
SUMMARY
The invention features an interferometry system that uses a small angular difference in the propagation directions of orthogonally polarized components of an input beam to an interferometer. The orthogonally polarized components define reference and measurement beams for the interferometer. The angular difference allows one to distinguish between the reference and measurement beam components of the input beam and facilitates the suppression of at least some of the cyclic errors caused by interferometer imperfections.
For example, birefringence in the interferometer optics, an imperfect polarizing beam-splitting surface, and/or polarization rotation by reflective surfaces in the interferometer can cause a spurious portion of the reference beam to propagate along some or all of the measurement path. Similarly, they can cause a spurious portion of the measurement beam to propagate along some or all, of the reference path. In the absence of the angular difference, at least some of the spurious beams contribute to the interferometric output signal produced after the interferometer recombines the reference and measurement beams. Such contributions complicate the interferometric signal and effectively degrade the accuracy of the measurement.
The angular difference, however, encodes the desired portion of each of the reference and measurement beams with a propagation vector that differs from its spurious portion. For example, when imperfections in the interferometer
De Groot Peter J.
Hill Henry A.
Fish & Richardson P.C.
Lyons Michael A.
Turner Samuel A.
Zygo Corporation
LandOfFree
Interferometry system and method employing an angular... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interferometry system and method employing an angular..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interferometry system and method employing an angular... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3324182