tool.
MOLD
The mold is the master copy of the pattern, it must displace the imprint material, allow
setting the material and then release during separation.
The most common molds for thermal imprint are Nickel films, with silicon wafers and fused
silica as alternatives. The UV imprint molds must be transparent and are mostly made from
fused silica, although polydimethylsiloxane (PDMS) molds have been used as low cost ,
disposable, working molds. PDMS molds are also used for transfer imprinting of molecular
layers where a low modulus rubber is needed to make molecular contact with the substrate.
The form factor of most molds are based on either wafer or lithography mask standards.
Most masters are created by the same pattern generators used to manufacture optical
masks. Electron beams are used for features < 300 nm or laser beam for features
>300nm. These tools accept the large data files describing the pattern and scan the beam
across a resist material to expose the pattern. The resist pattern is converted into a
physically robust copy by etching into fused silica or silicon.
The most common process is shown on the right, the patterned resist is used to mask the
etch of a thin hard mask film which in turn is used to mask the etch of fused silica or
silicon. The final master is created when the chrome is stripped to produce a relief pattern
in a transparent substrate for UV imprinting. The same process is used to produce “phase
shift masks” for advanced optical lithography.
Replicates of the master are used by many groups as"working plates" to reduce the mold
costs. As shown on the right, the etched master, in fused quartz or silicon, can be used to
create multiple copies for thermal imprinting by electro-less Nickel plating. Multiple copies
for UV imprinting or transfer imprint are made by solvent cast imprinting using PDMS, or
other polymers.
The mold surface assists in release of the imprint material by miminizing and weakening
interactions between mold and imprint material. There are many different release
strategies, relatively few systematic comparisons have been reported. These strategies are
mostly designed to make “non stick” very low energy mold surface such as; deposited floro-
layers, treatment with self assembly reactive fluorocarbon-chloro silanes (Otto 2004),
deposited diamond life carbon, making the mold from a floro-polymer (Kawaguchi 2005).
Additives that migrate to the surface of imprint materials have been tried in combination
with mold treatments by AMO (Fuchs 2002).
Houle at IBM has shown that low energy, non wetting surfaces can still interact with the
imprint polymerization process and increase adhesion. She suggests that this explains
many of the conflicting published results, and opportunities exists for improved release
systems (Houle 2006)
For more on different types of mold and the fabrication techniques go to Molds.
MATERIAL
The imprint material must flow during molding and then, after setting, it must allow
separation.
Thermal imprinting uses polymers with a glass transition temperature above 50 C. There
is no change in molecular structure during the set. The material must have as low a
viscosity as possible at the molding temperature, and at the same time must have enough
mechanical strength that allows separation below Tg. So the lowest possible molecular
weight is used that still delivers adequate strength. A wide range of different polymers have
been thermally imprinted, the most popular are either methyl or aromatic substitutes
polymethacrylates. Commercial materials are supplied by Microresist Gmbh and Nanonex..
Stephen Chou has shown that single crystal silicon can be imprinted using a laser to melt
the surface.
UV imprinting starts with lower viscosity liquids that just form stable films at room
temperature. This liquid can be molded at low pressure and then is UV crosslinked to
create a polymer network with sufficient mechanical strength to allow separation, as shown
on the right. Commercial materials are supplied by Microresist Gmbh, Nanonex and Toshio
Gosie. UV crosslinked materials for functional optical applications are supplied by
Microresist Gmbh.
The lowest possible viscosity is produced by monomer mixtures, that do not form stable
films. This material must be dispensed as a series of drops (Colburn 1999)., and has
been implemented by Molecular Imprints (Xu 2004). The most common class of UV
crosslinking systems are acrylates which are available as a wide range of both monomers
and acrylate end capped prepolymers.
Imprint materials customized for a bilayer process have developed around organo-silicon
acylates by Willson's team at U. Texas and Molecular Imprints (Xu 2004).
Vinyl ether suitable for drop dispense have been reported by the IBM resist research group
(Ito 2006)
Transfer Imprint materials can be any polymer that can be solvent cast, which gives
exceptional material flexibility in functional applications. Materials designed for transfer are
sold by Transfer Devices ( ) and Liquidia ( 2006)
For more on imprint and planarization materials got to Materials.
TOOLS
The tool must mold imprint material and align the pattern to the substrate, set the material,
and then separate it from the mold. All the commercial tools operate on wafer mechanical
format substrates.
There are a large number of both in-house and commercial imprint tools. The first
imprinters were thermal set tools designed for small (<4”) wafers, or for roller imprinting of
films. They represent greatest variety of imprint tools and largest installed base.
Commercial high pressure high temperature imprinters were first supplied by Nanonex
and Obducat. Commercial tools based on bonders are supplied by EVG and Suss. There
are several other suppliers that can be found on the web. All the systems use high
pressure to force the mold and substrate together and rely on the mold and wafer flexing
into conformal contact. Research thermal systems support align by prealigning and
vacuum clamping the mold to the wafer.
For more on thermal imprint tools got to Whole Wafer Tools.
Whole wafer (up to 8” wafers) UV imprinters for spin on films are supplied by EVG and
Suss based on contact printers. Contact printers are designed for hard contact in which a
vacuum is pulled between the mask and wafer, and the wafer backside vacuum is released
so the wafer will conform to the mask under 1 atmosphere pressure. The same technique
is used for imprinting.
Several other vendors are supplying combined UV and thermal tools, including Nanonex
and Obducat. Brewer Science supply a UV based planarization imprinter.
For more on UV imprint tools got to Whole Wafer Tools.
Step and Repeat tools (S&R) have two advantages; the small field size allows fine overlay
(< 1 um), and very large substrates can be imprinted in steps whose size is determined by
the maximum automatically separable field size.
Commercial S&R UV imprint tools are supplied by Molecular Imprints, and have been
designed to maximize overlay performance by using a room temperature UV imprint
process with drop dispensed monomer resulting in the lowest possible pressure. The
latest version comes with magnification correction and sophisticated environmental
controls (Resnick 2005-1)
EVG and Suss have both announced S&R systems.
Hiroshima has built a S&R system with a unique conformality strategy Hiroshima 2005).
Very large area S&R thermal imprint systems have been reported by OMRON. There must
be a minimum distance between imprint fields so that the neighboring imprint is not
distorted by the heating of the imprint field. A scanning system (Wave Imprinter) has been
developed at Phillips.
For more go to S&R tools.
Roll to Roll tools have developed by a number of companies as in house systems for roll
to roll imprint on sheet film.
- Reflexite has thermal imprint capacity www.reflexite.com
- Spectratech has thermal imprint for holograms on film. www.spectratech.com
- Autotech, in the UK, has UV imprint capacity www.autotech.co
For more go to Roll to Roll tools.
NEXT
Use the links in the text above to go to more detail in each imprint section.
For a comparison of process capability of the different technologies go to Process
Performance
For insite into the steps before and after imprint go to the Imprint Cell Process.
Or use the tool bars.
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