Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1987-04-13
1988-10-18
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
356401, G01V 904
Patent
active
047790016
ABSTRACT:
To align a grating on a mask with respect to an equivalent grating of a wafer in a photolithographic system where the mask is imaged by an imaging system onto the wafer, symmetrical diffraction orders (u.degree..sub.+1, u.degree..sub.-1) are focussed on the wafer grating and diffracted a second time to return colinear with the optical axis and to be deflected by a beam splitter to a photo detector. The intensity of the superimposed outbeams depends on the relative phase differences of the diffracted beams, and, hence, on the displacements of the mask and wafer gratings. The phase of the electrical output signal is determined by introducing periodic phase differences in the diffracted beams of the mask grating by a wobbling parallel glass plate. For simultaneous X-, Y- alignment, crossed gratings are used that operate on two pairs of diffracted beams. The polarization direction of one of these pairs is rotated by 90.degree. before impinging on the second grating so that each pair of diffracted beams can be fed to a separate photodetector. In a step-and-repeat photolithographic system, optical fine alignment can be performed by adjusting a fixed tilt angle of the glass plate after having determined the amount of misalignment.
REFERENCES:
patent: 4402610 (1983-09-01), Lacombat
patent: 4577968 (1986-03-01), Makosch
patent: 4631416 (1986-12-01), Trutna, Jr.
patent: 4664524 (1987-05-01), Hattori et al.
patent: 4710026 (1987-12-01), Magome et al.
H. I. Smith et al., "Alignment of X-Ray Lithography Masks Using a New Interferometric Technique-Experimental Results," J. Vac. Sci. Technol., May/Jun. 78, pp. 984-986.
G. Makosch et al., "Interferometric Method for Checking the Mask Alignment Precision in the Lithographic Process," Applied Optics, vol. 23, No. 4, Feb. 84, pp. 628-632.
H. P. Kleinknecht, "Diffraction Gratings as Keys for Automatic Alignment in Proximity and Projection Printing," Society of Photo-Optical Instru. Engrs., vol. 174, 1969, pp. 63-69.
International Business Machines - Corporation
Limanek Stephen J.
Nelms David C.
Ruoff Jessie
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