Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
2001-10-02
2003-10-07
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S500000
Reexamination Certificate
active
06631004
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to interferometers, e.g., displacement, angle, and dispersion measuring interferometers for measuring the position 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 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 the 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, 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.
For high performance applications such as IC manufacturing the quantity of interest is the geometrical length L and not the optical path length nL, which is what is measured by the displacement measuring interferometer. In particular, changes in nL can be caused by changes in the refractive index n rather than by geometric changes in the relative position of the measurement object. Techniques based on dispersion interferometry have been used to compensate displacement measurements for air turbulence. In particular, interferometric displacement measurements are made at multiple optical wavelengths to determine the dispersion of the gas in the measurement path. The dispersion measurement can be used to convert an optical path length measured by a distance measuring interferometer into a geometric length.
Such distance, angle, and dispersion interferometers are 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 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. The dynamic beam steering assembly can be incorporated into interferometry systems that measure distances, angle, and/or dispersion. The dynamic beam steering assembly improves the performance of the interferometry system by minimizing negative consequences of changes in the angular orientation or position of the measurement object.
In general, in one aspect, the invention features an interferometry system including: an interferometer which during operation directs a measurement beam along a measurement path contacting a measurement object and combines at least a portion of the measurement beam with another beam to form an overlapping pair of exit beams, the interferometer including 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, the measurement beam contacting the beam steering element; and a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object. The interferometer can direct the measurement beam to contact the measurement object a single time (single-pass), two times (double pass), or an even or odd numbers of times.
During operation the control circuit can cause the positioning system to reorient the beam steering element in response to a change in angular orientation of the measurement object. In one such embodiment, the interferometry system can further include a signal processor. During operation the interferometer directs a second measurement beam along a second measurement path contacting the measurement object and combines at least a portion of the second measurement beam with an additional beam to form a second overlapping pair of exit beams. The measurement beams contact the measurement object at separate locations, and during operation the signal processor determines the change in angular orientation of the measurement object based on signals derived from the overlapping pairs of exit beams. In the latter embodiment, the beam steering element can be positioned to direct both of the measurement beams, both of the measurement beams contacting the beam steering element.
In general, in another aspect, the invention features an interferometry system including: an interferometer which during operation receives an input beam, splits the input beam into a measurement beam and at least one other beam, directs the measurement beam along a measurement path contacting a measurement object, and combines at least a portion of the measurement beam with the other beam to form an overlapping pair of exit beams; 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 input beam and the overlapping pair of exit beams, the input beam and the overlapping pair of exit beams contacting the beam steering element; a control circuit which during operation causes the positioning system to reorient the beam steering element in response to a change in at least one of angular orientation and position of the measurement object. The
de Groot Peter
Hill Henry A.
Fish & Richardson P.C.
Turner Samuel A.
Zygo Corporation
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