Browse Prior Art Database

Interferometric Method of Checking the Overlay Accuracy in Photolitho Graphic Exposure Processes

IP.com Disclosure Number: IPCOM000100212D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 125K

Publishing Venue

IBM

Related People

Frank-Schmidt, U: AUTHOR [+2]

Abstract

This method serves to determine the relative position of two grating patterns transferred to a silicon wafer in two successive steps. (Image Omitted)

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Interferometric Method of Checking the Overlay Accuracy in Photolitho Graphic Exposure Processes

       This method serves to determine the relative position of
two grating patterns transferred to a silicon wafer in two successive
steps.

                            (Image Omitted)

      In a previously developed method, measuring is carried out such
that the grating patterns are scanned by two laser beams focussed
thereon.  By diffraction reflection, the light propagates from the
gratings in discrete directions referred to as diffraction orders.
The phase difference of two symmetric diffraction orders is a linear
function of the displacement of the gratings relative to the laser
scanning spot.  By successively measuring this phase difference on
the two grating patterns, the relative position of the two patterns
can be determined.  For this purpose, either the wafer or the laser
spot is moved between two measurements.  For avoiding an additional
phase error, the laser spot must be moved at maximum precision
between grating patterns.

      Similar to the state of the art, the new method uses two cross
gratings as an overlay pattern.  However, these gratings have
different pitches in the direction of the overlay error to be
measured.  In the direction perpendicular thereto, the grating
pitches are identical. In two successive steps, the two gratings on
the wafer are produced interlaced (Fig. 1) rather than closely
adjacent to each other, as previously.  The ratio of the two grating
pitches should be about 1 : 2 for practical reasons.

      The optical system for measuring the overlay error is
schematically illustrated in Fig. 2 and differs from that previously
employed in that the overlay pattern to be checked on the wafer is
simultaneously illuminated with four rather than two laser beams.  On
the wafer surface, the four laser beams are recombined in pairs at
different angles of incidence symmetrically to the optical axis.

      The optical system operates as follows.  Upon passing an
electrooptical phase modulator, the beam of a laser is split into two
partial beams which are polarized perpendicularly to each other.  Two
Wollaston prisms W1 and W2 with identical splitting angles split the
two laser beams spatially into two parallel beams which are then
focussed by lens L1 in a third Wollaston prism W3.  The angle between
the two beams may be set very accurately by adjusting the height of
W2.  In front of Wollaston prism W3, the polarization directions of
the tw...