Browse Prior Art Database

Reverse Dark Field Lens

IP.com Disclosure Number: IPCOM000087548D
Original Publication Date: 1977-Feb-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 84K

Publishing Venue

IBM

Related People

Abraham, G: AUTHOR [+4]

Abstract

The alignment of a pattern mask to pre-existing patterns on a light-sensitive substrate, such as a photoresist-coated semiconductor wafer, for projection printing of the mask pattern onto the substrate is accomplished using a projection lens which is constructed to extract and detect scattered light from the edges of alignment patterns on the substrate.

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Reverse Dark Field Lens

The alignment of a pattern mask to pre-existing patterns on a light-sensitive substrate, such as a photoresist-coated semiconductor wafer, for projection printing of the mask pattern onto the substrate is accomplished using a projection lens which is constructed to extract and detect scattered light from the edges of alignment patterns on the substrate.

The system is shown schematically in Fig. 1 in which mask 2 has a window 1, which contains alignment aperture patterns. The patterns are illuminated and imaged by lens 3 onto alignment target area 4 on wafer 5. For X, Y and theta alignment, a second target area is also illuminated through a second mask window in the same manner. The illumination cone 6, which is incident on wafer 5, is subtended by the diaphragm 7 of lens 3 and completely fills the numerical aperture.

The incident light is scattered (arrows 8) out of illumination cone 6 because of the physical structure of the edge 9 of the alignment pattern formed in the process layers 10, beneath photoresist layer 11, on wafer 5, as shown in Fig. 2. Most of the scattered light is contained within a cone extending from 20 degrees to 35 degrees from optical axis 12 of cone 6. The path of the deflected rays can be traced back into lens 3. The rays hit diaphragm 7 because it defines a numerical aperture cone of plus or minus 15 degrees from the optical axis, and the rays are just outside of this cone.

As shown in Fig. 3, an auxiliary optical...