Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-06-20
2003-04-01
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S216000, C356S477000, C356S487000
Reexamination Certificate
active
06541759
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to interferometers, e.g., interferometers for measuring the position and orientation 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 beam components that enter the interferometer have orthogonal polarizations and frequencies separated by a heterodyne, split-frequency. The split-frequency can be produced, e.g., by Zeeman splitting, by acousto-optical modulation, or by positioning a birefringent element internal to the laser. A polarizing beam splitter directs the measurement beam along a measurement path contacting a reflective measurement object (e.g., a stage mirror), directs the reference beam along a reference path, and thereafter recombines the beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that passes through a polarizer that 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 at a split-frequency. When the measurement object is moving, e.g., by translating a reflective stage, the heterodyne signal is at a frequency equal to the split frequency plus a Doppler shift. The Doppler shift equals 2 vp/&lgr;, where v is the relative velocity of the measurement and reference objects, &lgr; is the wavelength of the measurement and reference beams, and p is the number of passes to the reference and measurement objects.
Changes in the optical path length to the measurement object correspond to changes in the phase of the measured interference signal at the split-frequency of the measurement and reference beam components that enter the interferometer, with a 2&pgr; phase change substantially equal to an optical path length change nL of &lgr;/p, where n is the average refractive index of the medium through which the light beams travel, e.g., air or vacuum, and where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object. Similarly, multiple interferometers can be used to measure changes in distance to multiple points on the measurement object from which changes in the angular orientation of the measurement object can be determined.
Such interferometers are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. The lithography systems typically include: at least one movable stage to support, orient, 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 to 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.
SUMMARY OF THE INVENTION
The invention features an interferometry system for use in measuring changes in any of an angular orientation of a measurement object, a position of a measurement object, an optical distance to the measurement object, a physical distance to the measurement object, and a dispersion in the path to the measurement object. The interferometry system includes at least one dynamic beam-steering assembly and one or more optical fibers to couple beams produced by the interferometry optics to remote photodetectors and associated electronics.
The photodetectors and associated electronics can generate significant heat during operation, which can adversely affect the interferometer optics and degrade interferometric measurements. Using optical fibers to remotely position such detection electronics reduces the heat load on the environment of the interferometer. In addition, the remote positioning decreases the sensitivity of the detector to any electrostatic discharge (ESD) and radio frequency interference (RFI) present in the interferometer environment. For example, ESD and RFI can arise when the interferometer is coupled to a microlithography stage that rapidly repositions a wafer stage or reticle.
But, problems can arise when coupling an interferometric output beam into an optical fiber. For example, in many conventional interferometers, changes in the angular orientation or position of the measurement object typically introduce a lateral displacement (or “shear”) between the interfering components of the output beam. Such lateral displacement reduces coupling efficiency into the fiber, and thereby reduces the interference signal measured by a subsequent detector. Furthermore, when using a multimode fiber, the lateral displacement can cause the interfering components of the output beam to couple into different spatial modes of the multimode fiber. As the components propagate to the end of the fiber, modal dispersion introduces a phase shift between them. Because the phase shift varies with the lateral displacement (which in turn varies with the angular orientation or position of the measurement object), it corrupts the interferometric measurement associated with the measurement object.
Fortunately, the interferometry systems disclosed herein, which include one or more dynamic beam steering assemblies, greatly reduce the lateral displacement between the interfering components of the interferometric output beam, and thereby mitigate the problems of fiber optic coupling efficiency and modal dispersion.
The dynamic beam-steering assembly redirects one or more beams within the interferometry system in response to a change in the angular orientation or position of the measurement object. Interferometry systems employing a dynamic beam steering assembly are also disclosed in the following commonly owned applications, all of which are incorporated herein by reference: U.S. application Ser. No. 09/157,131, filed Sep. 18, 1998 entitled INTERFEROMETER HAVING A DYNAMIC BEAM STEERING ASSEMBLY by inventors Henry A. Hill and Peter de Groot; U.S. application Ser. No. 09/305,876, filed May 5, 1999 entitled SINGLE-PASS AND MULTI-PASS INTERFEROMETRY SYSTEMS HAVING A DYNAMIC BEAM-STEERING ASSEMBLY FOR MEASURING DISTANCE, ANGLE, AND DISPERSION by inventors Henry A. Hill and Peter de Groot; U.S. application Ser. No. 09/305,828, filed May 5, 1999 entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill; U.S. application Ser. No. 09/384,851, filed Aug. 27, 1999 entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill; and International Application Serial No. PCT/US00/12097, filed May 5, 2000, entitled INTERFEROMETRY SYSTEM HAVING A DYNAMIC BEAM-STEERING ASSEMBLY FOR MEASURING ANGLE AND DISTANCE by inventor Henry A. Hill. In addition to the interferometry systems explicitly disclosed herein, any of the interferometry systems disclosed in the applications referenced above can modified to include one or more optical fibers to provide remote photoelectric detection.
In general, in one aspect, the invention features an interferometry system including: an interferometer which during op
Fish & Richardson P.C.
Kiknadze Irakli
Kim Robert H.
Zygo Corporation
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