Inspection method and apparatus, lithographic apparatus, lithographic processing cell and device manufacturing method
View Patent ↗The simultaneous measurement of four separately polarized beams upon diffraction from a substrate is used to determine properties of the substrate. Circularly or elliptically polarized light sources are passed via up to three polarizing elements. This polarizes the light sources by 0, 45, 90 and 135°. The plurality of polarizing beamsplitters replaces the use of a phase modulator, but enables the measurement of the intensity of all four beams and thus the measurement of the phase modulation and amplitude of the combined beams to give the features of the substrate.
1. An inspection apparatus configured to measure a property of a substrate, the inspection apparatus comprising:
a light source configured to supply a radiation beam;
an optical element configured to focus the radiation beam onto a surface of the substrate;
a beamsplitter configured to separate the radiation beam once reflected from the surface of the substrate into first and second orthogonally polarized sub-beams;
a first polarizing beamsplitter configured to split the first sub-beam into two sub-sub-beams with about 0 and 90 degree polarization respectively;
a second polarizing beamsplitter rotated by about 45 degrees with respect to the first polarizing beamsplitter and configured to split the second sub-beam into two sub-sub-beams with about 45 and 135 degree polarization respectively; and
a detector system configured to detect simultaneously an angle-resolved spectrum of all four sub-sub-beams reflected from the surface of the substrate.
2. The inspection apparatus according to claim 1 , wherein the radiation beam reflected from the substrate is elliptically polarized.
3. The inspection apparatus according to claim 1 , further comprising an optical wedge configured to deflect the sub-sub-beams onto the detector system.
4. The inspection apparatus according to claim 1 , further comprising a deflector configured to deflect the sub-sub-beams onto the detector system.
5. The inspection apparatus according to claim 1 , wherein the beamsplitter and the first and second polarizing beamsplitters comprise polarizing prisms.
6. The inspection apparatus according to claim 1 , wherein the beamsplitter and the first and second polarizing beamsplitters comprise Wollaston prisms.
7. The inspection apparatus according to claim 1 , wherein the sub-beams comprise a TE-beam and a TM-beam.
8. The inspection apparatus according to claim 1 , further comprising a focussing system and an optical wedge placed in the image plane of the focussing system configured to redirect the polarized sub-sub-beams in different directions such that the sub-sub-beams arereceived at different positions on the detector.