Browse Prior Art Database

Mask Alignment

IP.com Disclosure Number: IPCOM000090537D
Original Publication Date: 1969-May-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Brunner, RH: AUTHOR

Abstract

The system shown automatically performs mask alignment and photoresist exposure operations. The detection method employed in the system is based on light reflection from etched steps in the oxide coating on a wafer. A four quadrant photodetector suitable for light beam centering such as the CD-1 Perkin Elmer centering detector is used. As in A, the target is generated on wafer 3 at diametric ends 1 and 2 of the wafer diameter. Each target is made up of a pair of circular patterns such as 1A and 1B. The patterns generally comprise two circular dots 1A and 1B of approximately 6 - 10 mils in diameter placed in a kerf area of the wafer. Normally, the target dot is made up of many fine lines etched into the oxide, with the target dots e.g., 1B, alternately spaced in operation.

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Mask Alignment

The system shown automatically performs mask alignment and photoresist exposure operations. The detection method employed in the system is based on light reflection from etched steps in the oxide coating on a wafer. A four quadrant photodetector suitable for light beam centering such as the CD-1 Perkin Elmer centering detector is used. As in A, the target is generated on wafer 3 at diametric ends 1 and 2 of the wafer diameter. Each target is made up of a pair of circular patterns such as 1A and 1B. The patterns generally comprise two circular dots 1A and 1B of approximately 6 - 10 mils in diameter placed in a kerf area of the wafer. Normally, the target dot is made up of many fine lines etched into the oxide, with the target dots e.g., 1B, alternately spaced in operation. For example, the first exposure mask contains the pattern as in B-1. One pattern of the pair is a circle containing many fine lines as 4 and the other is a framed circle as 5. After the first exposure and oxide etch, the pattern appears on the wafer as in B-2.

By illumination of the circle of fine lines, as in B-2, at an oblique angle, an energy density pattern is generated which appears as a bright dot. In the event a coated wafer is sensitive to white light, a red light, HeNe laser, can be used for illumination. In a subsequent operation, the second mask then has the pattern pair reversed as in B-3 so that the bright dot of reflected light appears beneath the frame circle 6 of the second mask. The use of a four quadrant photodetector provides for accurately centering a light pattern by determining the energy center. This technique can be applied for first aligning the mask to the system and, secondly, to align the wafer to the mask.

The sequence of operation is shown in drawing C. After an incoming wafer 7 is identified as to type and last photoresist operation and the proper mask 8 is selected, the alignment sequence begins. The mask is indexed beneath the optical system such that the target pairs 9 and 1...