Claims Analysis
Total Claims: 11
Claim 1
Independent Claim
124 words
Method for aligning two objects in conjugate planes of an imaging system, the objects having first (3a) and second (8a) optical gratings, resp., the first grating being illuminated by a light beam (2) characterized in that first symmetrical diffraction orders (u + , u- ) from the first grating (3a) are selected, that the selected diffraction orders are focussed by the imaging system (7) coincidently on the second grating (8a) to be diffracted a second time into second diffraction orders (u +1 , u. i ) colinear to the optical axis of the imaging system, that the phase relationship between the first diffracted beams is modulated periodically and that the relative phase of the second diffraction orders in the colinear output beam is evaluated.
Claim 2
Dependent Claim
124 words
Method for aligning two objects in conjugate planes of an imaging system, the objects having first (3a) and second (8a) optical gratings, resp., the first grating being illuminated by a light beam (2) characterized in that first symmetrical diffraction orders (u + , u- ) from the first grating (3a) are selected, that the selected diffraction orders are focussed by the imaging system (7) coincidently on the second grating (8a) to be diffracted a second time into second diffraction orders (u +1 , u. i ) colinear to the optical axis of the imaging system, that the phase relationship between the first diffracted beams is modulated periodically and that the relative phase of the second diffraction orders in the colinear output beam is evaluated.
Claim 3
Dependent Claim
25 words
The method of claim 1, wherein the gratings are linear and parallel to each other for alignment in a direction perpendicular to the grating bars.
Claim 4
Dependent Claim
44 words
The method of claim 1, wherein the gratings are crossed gratings for alignment in two orthogonal directions perpendicular to the bars of the crossed gratings and wherein the polarisation directions of the second diffraction orders in each orthogonal direction are perpendicular to each other.
Claim 5
Independent Claim
42 words
The method of one of the claims 1 - 3, wherein the relative phase of the super imposed second diffraction orders is determined by measuring the intensity after shifting the relative phase of the first diffraction orders by ± 2/3 7 r.
Claim 6
Independent Claim
33 words
The method of one of the claims 1 - 4, wherein the first grating is part of a photolithographic mask and the second grating is part of a wafer to be exposed photolithographically.
Claim 7
Independent Claim
28 words
The method of one of the claims 1 - 5, wherein the optical path of the diffracted beams is arranged at the periphery of the optical imaging system.
Claim 8
Dependent Claim
105 words
Apparatus for aligning two objects in conjugate planes of an imaging system, the objects having first and a second optical gratings, resp., and comprising means to illuminate the first optical with vertical incidence, for carrying out the method of one of the claims 1 - 6, characterized in that a tiltable glass plate (4) is arranged optically behind the first grating, that the second optical grating (8a) is a reflective grating, and that a semitransparent mirror (5) is arranged in the optical axis of the imaging system to direct the light diffracted at the second grating along the optical axis to a photo detector (6).
Claim 9
Dependent Claim
89 words
The apparatus of claim 6, wherein the first and second gratings are crossed gratings, each generating a pair of diffracted beams in orthogonal planes, wherein the axis of rotation of the tiltable glass plate is arranged under 45° with respect to orthogonal planes, wherein the semitransparent mirror (15) has areas covered by a X/2 layer at the intersections with one pair of diffracted beams and wherein a polarizing beam splitter (6c) is provided in the output beam to direct the pairs of diffracted beams to different photodetectors (6a, 6b).
Claim 10
Dependent Claim
18 words
The apparatus of claim 6 or 7, wherein the tiltable glass plate is oscillated by a galvanometric actuator.
Claim 11
Independent Claim
32 words
The apparatus of one of the claims 6 - 8, wherein the glass plate is tilted for fine adjustment of the objects by a constant angle in accordance with the measured misalignment.