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Masks for Laser Ablation Using Phase Grating Approach

IP.com Disclosure Number: IPCOM000117682D
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 132K

Publishing Venue

IBM

Related People

Doany, F: AUTHOR [+4]

Abstract

Laser ablation is a powerful tool for the formation of penetrations through thin layers of polymers and is of considerable interest as a manufacturing tool in the electronic packaging industry. Typically, a high power excimer laser is allowed to be incident upon a mask that contains a pattern consisting of transparent regions that are imaged upon the surface of a polymer, or other material including metals, and regions that are highly reflective so as to exclude or mask the laser beam. In this way, the pattern of transparent regions of the mask can be transferred to the polymer. If the radiation is of sufficient intensity and duration, the incident radiation ablates away the polymer, or other chosen material, so forming a pattern of cuts or depressions in the form of vias, lines or other shapes.

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Masks for Laser Ablation Using Phase Grating Approach

      Laser ablation is a powerful tool for the formation of
penetrations through thin layers of polymers and is of considerable
interest as a manufacturing tool in the electronic packaging
industry.  Typically, a high power excimer laser is allowed to be
incident upon a mask that contains a pattern consisting of
transparent regions that are imaged upon the surface of a polymer, or
other material including metals, and regions that are highly
reflective so as to exclude or mask the laser beam.  In this way, the
pattern of transparent regions of the mask can be transferred to the
polymer.  If the radiation is of sufficient intensity and duration,
the incident radiation ablates away the polymer, or other chosen
material, so forming a pattern of cuts or depressions in the form of
vias, lines or other shapes.

      Due to the high power levels required, simple metal masks are
degraded by the laser and are not satisfactory.  Instead, today an
expensive high reflectivity mask is fabricated by using multilayer
stack interference filters containing 10 or more layers of
alternating high and low refractive index.  These layers must be
highly transparent and resistant to the uv radiation of the excimer
laser.  In addition, film thickness must be closely controlled to
achieve high reflectivity at the laser wavelength.  The layers are
subsequently etched to provide the desired transparent regions.  It
is clear that the ablation mask has a high degree of complexity both
in fabrication and in processing of the desired pattern on the mask.

      This disclosure describes an alternate approach that utilizes a
phase grating structure etched into a plate of an ideal material such
as silica.  In particular, exclusion of the laser radiation is
achieved by patterning the mask with an array of closely spaced
features that scatter the beam through an angle large enough that it
escapes capture by the imaging lens.  These features are typically
chosen so that the optical path length through adjacent features
differs by 1/2 wavelength so as to provide efficient scattering and
strong attenuation of the zero order beam.  The thickness difference
required is of the order of only several hundred nanometers and is
easily achieved by deposited film or etching techniques.  Light
passing through nonfeatured areas is imaged by a lens in the usual
way.  The phase shift mask approach for abalation masks, therefore,
offers a...