Phase-shifting point diffraction interferometer focus-aid enhanced mask
OwnerPatrick Naulleauinventor
Life of this patent
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Abstract
A phase-shifting point diffraction interferometer system (PS/PDI) employing a PS/PDI mask that includes a PDI focus aid is provided. The PDI focus aid mask includes a large or secondary reference pinhole that is slightly displaced from the true or primary reference pinhole. The secondary pinhole provides a larger capture tolerance for interferometrically performing fine focus. With the focus-aid enhanced mask, conventional methods such as the knife-edge test can be used to perform an initial (or rough) focus and the secondary (large) pinhole is used to perform interferometric fine focus. Once the system is well focused, high accuracy interferometry can be performed using the primary (small) pinhole.
Claims30
A method of focusing a phase-shifting point diffraction interferometer that defines an optical path that includes (i) a source of electromagnetic energy in the optical path, (ii) an optical element under test in the optical path, and (iii) an irradiance detector in the optical path downstream from the optical element under test, comprising the steps of:
- (a) positioning a mask in the image plane of the optical system under test positioned upstream of the irradiance detector wherein the mask defines a test-beam window and at least one reference pinhole and a focus-aid pinhole having a diameter of at least about 20% larger than that of the at least one reference pinhole;
- (b) directing a beam focused by the optical element under test through the test-beam window;
- (c) rough-focusing the beam onto the image plane;
- (d) positioning a beam divider in the optical path for dividing the electromagnetic radiation from the source into a test-beam and a reference-beam;
- (e) aligning the mask so that the test-beam passes through the test-beam window and the reference-beam passes through the focus-aid pinhole;
- (f) fine-focusing the beams on the image plane by detecting interference patterns with the irradiance detector and recovering the defocus term from the wavefront;
- and
- (g) maneuvering the mask so that the reference-beam passes through the reference-beam pinhole.
A point diffraction interferometer system that defines an optical path, said system comprising:
- (a) a source of electromagnetic energy in the optical path;
- (b) a beam divider that can be positioned in the optical path for dividing electromagnetic radiation from the source into a reference-beam and a test-beam;
- (c) an irradiance detector in the optical path downstream from the source of electromagnetic energy;
- (d) a mask that is positioned in the image plane of an optical system under test which is positioned upstream of the irradiance detector wherein the mask defines a test-beam window and at least one reference-beam pinhole and a focus-aid pinhole having a diameter of at least about 20% larger than that of the at least one reference pinhole;
- (e) means for directing a beam focused by the optical system under test through the test-beam window;
- (f) means for rough-focusing the beam onto the image plane;
- (g) means for positioning the beam divider in the optical path for dividing the electromagnetic radiation from the source into a reference-beam and a test-beam;
- (h) means for aligning the mask so that the test-beam passes through the test-beam window and the reference-beam passes through the focus-aid pinhole,
- (i) means for fine-focusing the test and reference-beams on the image plane by detecting interference patterns with the irradiance detector, and
- (j) means for maneuvering mask so that the reference-beam passes through the reference pinhole.
Dependencies are inferred from the claim text.