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Etchless Alignment Mark Strategy in Photolithography

IP.com Disclosure Number: IPCOM000121448D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 4 page(s) / 153K

Publishing Venue

IBM

Related People

OGrady, D: AUTHOR [+3]

Abstract

Lithographic patterns aligned and exposed using die by die step and repeat systems require matching of a reticle alignment mark to a wafer alignment mark at each die on the wafer, for all process levels. This allows for no flexibility in the use of the reticle set or process modification without costly redesign and rebuild of the reticles, or extensive wafer processing. The etchless alignment mark strategy uses a simple "cross-over" reticle which can be built in advance, and requires no additional etching or wet processing of the wafers.

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Etchless Alignment Mark Strategy in Photolithography

      Lithographic patterns aligned and exposed using die by
die step and repeat systems require matching of a reticle alignment
mark to a wafer alignment mark at each die on the wafer, for all
process levels.  This allows for no flexibility in the use of the
reticle set or process modification without costly redesign and
rebuild of the reticles, or extensive wafer processing.  The etchless
alignment mark strategy uses a simple "cross-over" reticle which can
be built in advance, and requires no additional etching or wet
processing of the wafers.

      Lithographic patterns are printed on a wafer by passing light
through a mask.  This image is projected and focused by a lens onto
the wafer which is coated by a photosensitive film (called
"photoresist").  Chemical developing of the wafer removes the
photoresist in the pattern exposed.  The remaining pattern in the
photoresist is transferred into the underlaying thin films by plasma
or chemical etching.  This process is repeated for each process
level.  Each new process level must be aligned to the previous level,
or common level, by the step and repeat exposure system (align,
focus, expose, step).  Alignment is performed for each die on the
wafer (called die-by-die alignment).  The alignment is done by
matching a mark on the reticle to a mark on the wafer.  Thus, the
reticle mark and wafer mark must be designed into product scribe line
and built with the product reticle set.

      A typical alignment sequence for four process levels (A, B, C,
D) is shown in Fig. 1.  In this example, each level aligns directly
to the previous level, as indicated by the arrows.  Thus, each
reticle must be designed to contain the reticle mark for alignment to
the previous level, and printing of a new wafer mark for use in
aligning the next level.  Shown are the reticle designs for level B
and level C.  The wafer marks are indicated by "X", and the reticle
marks are indicated by "O".  As can be seen, the reticle design must
be such that the "X" from level B and the "O" from level C must
be in the same position.
Alternate Alignment Mark Strategy

      As can be seen in Fig. 1, each level requires the wafer mark of
the previous level for alignment.  This allows no flexibility in the
manufacturing process or use of the reticle set.  If an engineering
change is deemed necessary which reverses the order of levels B and
C, for example (new sequence A, C, B, D), or a change that eliminates
level B from the process (new sequence A, C, D), a new set of
reticles must be designed and manufactured.  Such changes are costly
and time- consuming.

      A low-cost alternative is the use of a "crossover reticle".
Such a reticle contains no product pattern (thus simple to design and
inexpensive to manufacture).  The only design on the crossover is the
necessary alignment marks in the scribe line.  This reticle mimics
the actual product...