Browse Prior Art Database

Automatic Alignment System

IP.com Disclosure Number: IPCOM000050562D
Original Publication Date: 1982-Nov-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Goodman, DS: AUTHOR

Abstract

A system is described for aligning a lithography proximity printing mask with respect to a microcircuit wafer. A diffraction grating on the wafer is illuminated with a coherent beam through a window in the mask. Additional windows on each side of the illumination window collect first-order diffracted light for separate detectors. The wafer and mask are in alignment when the detector signals have equal amplitude.

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Automatic Alignment System

A system is described for aligning a lithography proximity printing mask with respect to a microcircuit wafer. A diffraction grating on the wafer is illuminated with a coherent beam through a window in the mask. Additional windows on each side of the illumination window collect first-order diffracted light for separate detectors. The wafer and mask are in alignment when the detector signals have equal amplitude.

An alignment mark 10 on the wafer 12 consists of a symmetrical grating of any type, whose period should probably be no greater than twice the illumination wavelength, so that all orders above +1 and -1 are evanescent. The mask 14 uses two windows 16, 18 for alignment with a window shape that is unimportant except that there must be symmetry about the same line, and a central window 20 which needs only to be larger than the dimensions of the grating plus the dimensions of the capture range of the system. Above the mask windows 16, 18 are two identical detectors 22, 24 (which could also be fixed to the mask).

A uniform beam of light 26 is normal to the mask and wafer and comparable in cross-sectional size to the opening 20 in the mask. The light which falls outside the grating is specularly reflected back through the central mask opening 20. Light which hits the grating is diffracted into a 0 order, which leaves through the opening 20, and a +1 and -1 order, which hit the two detectors 22, 24. The grating 10 is located exactly mid...