Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-08-27
2001-08-07
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S500000
Reexamination Certificate
active
06271923
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 references 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 2vp/&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 region s of the wafer are exposed to the radiation beam.
SUMMARY OF THE INVENTION
The invention features an interferometry system that measures changes in the angular orientation of a measurement object and that also includes at least one dynamic beam steering assembly. The dynamic beam steering assembly redirects one or more beams within the interferometry system in response to a change in the angular orientation of the measurement object. In many embodiments, the presence of the dynamic beam steering assembly permits the interferometry system to measure the angular orientation of the measurement object using only a single measurement beam to contact the measurement object.
Furthermore, in many embodiments, a control signal derived from the measurement beam contacting the measurement object causes the beam steering assembly to redirect a measurement beam to contact the measurement object at normal incidence. When at such normal incidence, the interferometry system can calculate the angular orientation of the measurement object based on one or more interferometric signals derived from the measurement beam or based on the orientation of the beam steering assembly itself. In addition, many of the embodiments involve optical configurations that preserve the linear polarizations of beams incident on the optics, e.g., the use of corner cube retroreflectors are minimized.
In general, in one aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation the interferometer directs a measurement beam along a measurement path contacting a measurement object and combines each of at least two portions of the measurement beam with a corresponding reference beam to form at least two overlapping pairs of exit beams. The interferometer includes a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element, the beam steering element positioned to direct the measurement beam to the measurement object and the measurement beam contacting the beam steering element. During operation the control circuit causes the positioning system to reorient the beam steering element in response to a change in angular orientation of the measurement object and the angle measurement system which calculates the change in angular orientation of the measurement object based on at least one of interferometric signals derived from the overlapping pairs of exit beams and the reorientation of the beam steering element.
In some embodiments the interferometer directs the at least two portions of the measurement beam to contact the beam steering element before combining each of them with the corresponding reference beam. In other embodiments, the interferometer directs the measurement beam to contact the beam steering element a second time, after which the interferometer separates the measurement beam into the at least two portions. 39.
In further embodiments, the interferometer further includes a plurality of reflective surfaces oriented to direct the measurement beam, the portions of the measurement beam, a progenitor beam for the reference beams, and the reference beams. For initial linear polarizations and propagation directions for the measurement beam and the progenitor beam, the plurality of reflective surfaces can be oriented to preserve a linear polarization for the measurement beam, the portions of the measurement beam, a progenitor beam for the reference beams, and the reference beams upon their successive reflections.
In general, in another aspect, the invention features an interferometry system including an interferometer, a control circuit, and an angle measurement system. During operation the interferometer directs a measurement beam along a measurement path contacting a measurement object, separates the measurement beam into m portions, e.g., where m is one of 2 and 3, and recombines at least a part of one of the portions with each of the remaining m−1 portions to form m−1 overlapping pairs of exit beams. The interferometer includes a beam steering assembly having a beam steering element and a positioning system to orient the beam steering element. The beam steering element is positioned to direct the measurement beam to the measurement object
Fish & Richardson P.C.
Turner Samuel A.
Zygo Corporation
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