Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-04-28
2004-04-20
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C372S023000
Reexamination Certificate
active
06724486
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a laser light source suitable for displacement and dispersion measuring interferometers, which can be used to measure displacements of high-performance stages, e.g., reticle and/or wafer stages, in a lithographic scanner or stepper systems and integrated circuit (IC) test equipment.
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. The light source for many displacement measuring interferometers is a single-wavelength, frequency-stabilized laser, see, e.g., “Recent advances in displacement measuring interferometry” by N. Bobroff,
Measurement Science
&
Technology
4, 907-926 (1993). The accuracy of the displacement measurement varies directly with the wavelength stability of the light source.
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, 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&ngr;p, where &ngr; 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.
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 A 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. For example, in lithography applications air turbulence, particularly in the region surrounding a moving wafer or reticle stage, can cause changes in n. Such changes need to be determined to obtain accurate geometric displacement measurements. If not corrected, the overlay performance and yield of a lithography tool used to manufacture ICs can be seriously limited. See, e.g., “Residual errors in laser interferometry from air turbulence and non-linearity,” by N. Bobroff,
Appl. Opt.
26, 2676-2682 (1987).
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. The conversion also requires knowledge of an intrinsic value for the refractivity of the gas. A suitable value is &Ggr;, which is the reciprocal dispersive power of the gas for the wavelengths used in the dispersion interferometry. In general, the sensitivity of the dispersion measurement to the consequences of air-turbulence correction increases as &Ggr; decreases.
SUMMARY OF THE INVENTION
The invention features a displacement and dispersion measuring interferometry system having a Helium-Neon laser light source. The light source can be a Helium-Neon laser that includes an intracavity doubling crystal and an intracavity etalon to generate two harmonically related, single-frequency wavelengths at sufficient powers for interferometric dispersion measurements. Alternatively, the light source can be a single-mode Helium-Neon laser that directs a single-frequency input beam into a resonant external cavity enclosing a doubling crystal to generate two harmonically related, single-frequency wavelengths at sufficient powers for interferometric dispersion measurements. In addition to dispersion measurements, the inherent wavelength stability of the Helium-Neon source permits high-accuracy displacement measurements. Thus, the Helium-Neon laser light source is sufficient for the interferometry system to simultaneously measure displacement and dispersion, and correct the displacement measurement for air-turbulence using the dispersion measurement.
In general, in one aspect the invention features a Helium-Neon laser light source including: a Helium-Neon gain medium; a power source electrically coupled to the gain medium which during operation causes the gain medium to emit optical radiation at a first wavelength; a nonlinear optical crystal which during operation converts a portion of the optical radiation at the first wavelength into optical radiation at a second wavelength that is a harmonic of the first wavelength; an etalon; and at least two cavity mirrors enclosing the gain medium, the non-linear optical crystal, and the etalon to define a laser cavity, wherein during operation the etalon causes the cavity to lase at a single axial mode, and wherein at least one of the cavity mirrors couples the optical radiation at the first and second wavelengths into two harmonically related, single-frequency, output beams at the first and second wavelengths.
Embodiments of the laser light source can include any of the following features. A birefringent filter can be positioned within the cavity and oriented to select a particular Helium-Neon laser transition. The front and back faces of the crystal though which the optical radiation propagates can be parallel to one another to within 1 mrad. The at least two cavity mirrors can include two end mirrors and at least one fold mirror. The at least one fold mirror can have a coating that is less than 4% reflective at 3.39 microns.
Also, the laser light source can further include a detector and an intensity controller. During operation the detector measures an intensity of a portion of the output beam at the first wavelength and sends an intensity stabilization signal to the intensity controller indicative of the intensity of the output beam at the first wavelength. The intensity controller causes the power source-to adjust current flow through the gain medium based on the intensity stabilization signal.
Furthermore, the laser light source can include different embodiments for the Helium-Neon light source. For example, the Helium-Neon gain medium can include a vacuum
Shull William A.
Zanoni Carl A.
Fish & Richardson P.C.
Font Frank G.
Lee Andrew H.
Zygo Corporation
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