Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1998-09-18
2001-06-26
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S500000, C356S508000
Reexamination Certificate
active
06252667
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-optic 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 equal to a distance change of &lgr;/(2p).
Such interferometers are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct. a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beams with its reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
In many lithography systems and other applications, the measurement object includes one or more plane mirrors to reflect the measurement beam from each interferometer. Small changes in the angular orientation of the measurement object, e.g., pitch and yaw of a stage, can alter the direction of each measurement beam reflected from the plane mirrors. If left uncompensated, the altered measurement beams reduce the overlap of the exit measurement and reference beams in each corresponding interferometer. Furthermore, these exit measurement and reference beams will not be propagating parallel to one another nor will their wave fronts be aligned when forming the mixed beam. As a result, the interference between the exit measurement and reference beams will vary across the transverse profile of the mixed beam, thereby corrupting the interference information encoded in the optical intensity measured by the detector.
To address this problem, many conventional interferometers include a retroreflector that redirects the measurement beam back to the plane mirror so that the measurement beam “double passes” the path between the interferometer and the measurement object. The presence of the retroreflector insures that the direction of the exit measurement is insensitive to changes in the angular orientation of the measurement object. However, even with the retroreflector, the lateral position of the exit measurement beam remains sensitive to changes in the angular orientation of the measurement object. Furthermore, the path of the measurement beam through optics within the interferometer also remains sensitive to changes in the angular orientation of the measurement object.
In addition, translations of the measurement object can also impart changes to the lateral position and path of the measurement beam. For example, in applications in which the measurement object includes a mounted cube prism retroreflector to reflect the measurement beam, a translation of the measurement object transverse to the measurement beam imparts a lateral displacement to the reflected measurement beam that is twice the translation of the measurement object.
SUMMARY OF THE INVENTION
The invention features an interferometry system which includes at least one dynamic beam steering assembly for redirecting one or more beams within the interferometry system in response to changes in the angular orientation or position of the measurement object.
In general, in one aspect, the invention features an interferometry system for measuring changes in distance to a measurement object. The system includes an interferometer which during operation directs a reference beam along a reference path and a measurement beam along a measurement path contacting the measurement object and combines the reference and measurement beams to produce overlapping exit reference and measurement beams. The overlapping exit reference and measurement beams are indicative of changes in a relative optical path length between the reference and measurement paths. The interferometer includes a beam positioning system to orient the beam steering element. The beam steering element is positioned to direct at least one of the reference and measurement beams and the at least one of the reference and measurement beams contact the beam steering element. The system also includes a control circuit which during operation causes the positioning system to reorient the beam steering element in response to changes in at least one of angular orientation and position of the measurement object.
The system can include any of the following features.
The interferometer can separate an input beam into a pair of spatially separated beams to define the reference and measurement beams. Alternatively, during operation the interferometer receives a pair of spatially separated input beams to define the reference and measurement beams.
The interferometer can be a single-pass interferometer in which the measurement beam contacts the measurement object only a single time. Also, in some embodiments, the interferometer can include the measurement object, and in other embodiments, the measurement object can be separate from the interferometer. During operation the control circuit can cause the positioning system to reorient the beam steering element based on a signal generated from the exit measurement beam.
Also, during operation, the control circuit can cause the positioning system to reorient the beam steering element to do any of the following: 1) maintain the
de Groot Peter
Hill Henry A.
Fish & Richardson P.C.
Turner Samuel A.
Zygo Corporation
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