Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1998-10-06
2001-01-30
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S329000, C356S340000
Reexamination Certificate
active
06181420
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to interferometers, e.g., displacement measuring interferometers for measuring displacements of a measurement object such as a mask stage or a wafer stage in a lithography scanner or stepper system.
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, which 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 2 &ngr;p/&lgr;, where &ngr; is the relative velocity of the measurement and reference objects, &lgr; is the average wavelength of the measurement and reference beams, 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 L of &lgr;/(2 np), where n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a one-way distance change, e.g., the change in position of a stage that includes the measurement object.
Unfortunately, this equality is not always exact. Many interferometers include what are known as “cyclic errors,” which are contributions to the phase of the measured interference signal and have a sinusoidal dependence on the change in optical path length 2 pnL. In particular, the first order cyclic error has a sinusoidal dependence on (4 &pgr;pnL)/&lgr; and the second order cyclic error has a sinusoidal dependence on 2(4 &pgr;pnL)/&lgr;. Higher order cyclic errors can 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 ellipticity in the polarizations of the input beams and 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 2 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.
SUMMARY OF THE INVENTION
The invention features an interferometry system that minimizes cyclic errors. A phase measurement portion of the system forms two output beams from exit reference and measurement beams of an interferometer, passes one output beam through a first polarizer to form a first mixed beam, passes the other output beam through a quarter wave plate followed by a second polarizer to form a second mixed beam, and measures the phases of the first and second mixed beams. Proper alignment of the two polarizers minimizes the first order cyclic error and proper alignment of the quarter wave minimizes the second order cyclic error by averaging the measured phases of the two mixed beams.
In general, in one aspect, the invention features a phase measurement system for minimizing cyclic errors in a relative phase between exit reference and measurement beams emerging from an interferometer. The phase measurement system includes a first optical processing channel, a second optical processing channel, and a signal processor. The first optical processing channel includes a first polarizer and a first detector. During operation, a first portion of the exit reference and measurement beams pass through the first polarizer to produce a first mixed beam and wherein the first detector measures an intensity of the first mixed beam. The second optical processing channel includes a quarter wave retarder, a second polarizer, and a second detector. During operation, a second portion of the exit reference and measurement beams passes through the retarder and then the second polarizer to produce a second mixed beam and the second detector measures an intensity of the second mixed beam. The signal processor receives signals from the first and second detectors indicative of the intensities of the first and second mixed beams and processes the signals to determine, and minimize cyclic errors in, the relative phase between the exit reference and measurement beams.
Embodiments of the phase measurement system can include any of the following features.
The signal processor can include first and second phase interpolators, which during operation receive the first and second signals, respectively, and determine phases of the first and second signals, respectively. The exit reference and measurement beams can have different frequencies that define a heterodyne signal in each of the first and second signals and the phases determined by the phase interpolators can correspond to the heterodyne signals. The signal processor can further include an operational circuit connected to the first and second phase interpolators, wherein during operation the operational circuit processes the phases determined by the phase interpolators to determine the relative phase between the exit reference and measurement beams. For example, the operational circuit can average the phases determined by the phase interpolators to determine the relative phase between the exit reference and measurement beams.
Alternatively, the signal processor can include a phase-shifter that phase shifts one of the signals relative to the other signal and an operational circuit that processes the phase-shifted signal and the other signal to produce a processed signal indicative of an average intensity. For example, the processed signal can be indicative of a weighted arithmetic average of the intensities of the mixed beams. The signal processor can further include a phase interpolator connected to the operational circuit, which, during operation, determines a phase of the processed signal to give relative phase between the exit reference and measurement beams.
The exit reference and measurement beams can have substantially orthogonal linear polarizations. The fast axis of the q
Badami Vivek G.
Patterson Steven R.
Zanoni Carl A.
Fish & Richardson P.C.
Font Frank G.
Lee Andrew H.
Zygo Corporation
LandOfFree
Interferometry system having reduced cyclic errors 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 having reduced cyclic errors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interferometry system having reduced cyclic errors will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2541992