Three wavelength optical measurement apparatus and method
First Claim
1. Optical metrology apparatus comprising:
- optical source means including a plurality of laser means, at least one of which is a multi-mode laser diode means, the optical source means having an output beam that simultaneously includes at least three discrete optical wavelengths;
means, coupled to an output of the optical source means, for separating the output beam into a reference beam and into a measurement beam, the measurement beam being directed to a surface of interest;
means for combining into a combined beam the reference beam and a portion of the measurement beam that reflects from the surface of interest; and
means, responsive to a relative interferometric phase at each of the three optical wavelengths within the combined beam, for detecting a difference between an optical path length of the reference beam and an optical path length of the measurement beam.
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Abstract
Optical metrology method and apparatus wherein three optical wavelengths are generated and separated into a reference beam (RB) and an object beam (OB) having substantially equal optical path lengths. After reflecting from a surface being measured OB is combined with RB and provided to sensors which measure the intensity associated with each of the wavelengths. Any difference between the intensities is indicative of a difference in the optical path lengths of OB and RB and is a function of the polarization state of each of the three returned wavelengths. Differences in optical path length are shown to be indicative of a displacement of the object being measured. Preferably, two multimode laser diodes (12,14) are provided for generating the three optical wavelengths. Two synthetic wavelengths are derived from the three optical wavelengths and are employed to improve the precision of measurement while retaining a large dynamic range made possible by the use of a large synthetic wavelength.
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Citations
18 Claims
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1. Optical metrology apparatus comprising:
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optical source means including a plurality of laser means, at least one of which is a multi-mode laser diode means, the optical source means having an output beam that simultaneously includes at least three discrete optical wavelengths; means, coupled to an output of the optical source means, for separating the output beam into a reference beam and into a measurement beam, the measurement beam being directed to a surface of interest; means for combining into a combined beam the reference beam and a portion of the measurement beam that reflects from the surface of interest; and means, responsive to a relative interferometric phase at each of the three optical wavelengths within the combined beam, for detecting a difference between an optical path length of the reference beam and an optical path length of the measurement beam. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A method for performing optical metrology, comprising the steps of:
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operating a plurality of laser means, at least one of which is a multimode laser diode means, for generating an optical output having a plurality of optical wavelengths; modifying the optical output to provide a phase modulated reference beam and a measurement beam having a fixed polarization relationship one to another, the measurement beam being directed to and reflecting from a surface of interest; combining the phase modulated reference beam and the reflected measurement beam into a combined beam; and responsive to a relative interferometric phase at each of the three optical wavelengths within the combined beam, detecting a difference between an optical path length of the reference beam and an optical path length of the measurement beam. - View Dependent Claims (14, 15, 16, 17)
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18. A method of performing optical metrology to determine an absolute distance (L), comprising the steps of:
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operating at least one multi-mode laser diode means while generating a plurality of optical wavelengths (λ
) such that λ
1 <
λ
2 <
λ
3, and deriving two corresponding synthetic wavelengths Λ
12 >
Λ
13 therefrom;for L<
Λ
12 /4, determining a first synthetic fringe order (M12) in accordance with
space="preserve" listing-type="equation">M.sub.12 =f(M.sub.12)=f(m.sub.1)-f(m.sub.2),where m1 is the optical fringe order of λ
1 and m2 is the optical fringe order of λ
2 ;employing M12 in determining M13 in accordance with
space="preserve" listing-type="equation">M.sub.13 =f(M.sub.13)+I((M.sub.12 Λ
.sub.12 /Λ
.sub.13)-f(M.sub.13)),where the function I(a) yields an integer nearest to a value of the argument (a); determining the optical fringe order m1 in accordance with
space="preserve" listing-type="equation">m.sub.1 =f(m.sub.1)+I((M.sub.13 Λ
.sub.13 /λ
.sub.1)-f(m.sub.1)); anddetermining the absolute distance L in accordance with
space="preserve" listing-type="equation">L=(M.sub.ij Λ
.sub.ij)/2.
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Specification