Browse Prior Art Database

Overlay Measurement with Increased Accuracy

IP.com Disclosure Number: IPCOM000118159D
Original Publication Date: 1996-Oct-01
Included in the Prior Art Database: 2005-Apr-01
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Wagner, D: AUTHOR

Abstract

Disclosed is an overlay measuring method which uses two measurements with different focus to significantly reduce the measuring error.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 68% of the total text.

Overlay Measurement with Increased Accuracy

      Disclosed is an overlay measuring method which uses two
measurements with different focus to significantly reduce the
measuring error.

      Today's overlay measuring tools for the 350 nm linewidth chip
generation have a measuring accuracy of about 7.5 nm.  An analysis of
the overlay measuring accuracy reveals that the displacement induced
by the measuring tool is about 5 nm.

      The measuring tools mostly used are TV microscopes with white
light illumination and with objectives having a high numerical
aperture N.A..  The depth of focus of these objectives is
T=lambda./N.A.. With  typical values of lambda.=0.5 microsecond m and
N.A.=0.95, T is about 0.5 microsecondm.  The height differences of
the overlay marks may, however, be up to about 1 microsecond m.

      The overlay marks lay within the depth of focus of the
objective but significantly away from the ideal focus.  These small
distances lead to a displacement induced by the measuring tool if the
illumination is oblique or not symmetrical.

      There are two possibilities to avoid the tool induced
displacement: using symmetrical illumination or eliminating the
height difference of the overlay marks.

      The illumination is almost symmetrical in today's measuring
tools.  The solution is shown in the drawing.  A real image is
projected onto one TV camera and a virtual image is projected onto
another TV camera via a beam splitter. ...