techniques for modifying surfaces by imprint: - molding and “transfer printing”.
In this section we will explain;
- the different imprint processes.
- how imprint works given that all substrates are locally uneven at the molecular
level, and often microns out of flat across a substrate.
The variety of imprint processes, illustrated on the right, include;
1) Thermal imprint - starts with a spun on polymer layer, then heating the polymer
above its Tg, pushing the template into the polymer, cooling the polymer below its Tg,
and then separating to create a textured surface (Chuo 1995). This requires high
imprint pressures (~50 atmospheres).
2) UV imprint or "UV Nanoimprint Lithography (UV NIL)" - uses UV light to crosslink
an imprinted, spun on, polymer layer to create a textured surface (Haisma 1996). The
imprint pattern is the same as thermal but is produced at lower pressures.
3) UV imprint of a drop dispensed material or "Step and Flash Imprint Lithography
(S-FIL™") - uses UV light to crosslink low viscosity monomers that are dispensed as
droplets, rather than as a spun on film (Colburn 1999). This process requires the
lowest imprint pressures (<0.05 atmosphere). S-FIL offers the opportunity to modify
the material chemistry drop by drop, as well as creating surface texture.
4) Transfer imprint of a mold deposited material or "Reverse Imprint" - Solvent
casting of a polymer on to the mold followed by thermal evaporation of solvent to
create a copy of a textured surface that is then transferred to the substrate. This has
been called “reverse imprinting” (Bao 2002). Also, any of the thermal or UV cure
process can be used attach the coat and cure imprint pattern to the mold and then
transfer to a substrate. If the substrate has a pattern, this will result in bridge
structures.
5) Transfer imprint or "Microcontact printing" - transfers a chemical pattern onto a
surface using a soft template that conforms to the substrate. The raised surface of
the template contacts an “ink pad” and then contacts the substrate to transfer the
chemical (Whitesides 2001). This technique produces a flat nano-patterned chemical
surface.
6) Self assembly imprint in which objects are first self assembled on the template
and then transferred to the substrate (Kraus 2005).
7) Combined lithographies
There several examples of people using combinations of lithographies to create
unique patterns. One of the most successful is to leave chrome on the mold so that
there is both imprint of small features and contact print of large features (Cheng
2004).
Residual layer variations are produced by a combination of non flatness in molds,
substrates, devices on the substrate and pattern density variations in the imprint
as illustrated on the right.
The continuous film, or residual layer, is a direct result of the inability to squeeze out
material completely from between the mold and substrate. The imprint material “fills
the gaps” between the rigid mold and substrate caused by imperfections in the
surface of the mold and substrates, or due to devices on the surface of the
substrates. Residual layer thickness can also vary depending on imprint pattern
density when high viscosity imprint material does not flow far enough to ensure
uniform residual layers (Schultz 2003-1 and 2003-2)
Most processes requires some form of planarization and patten transfer after
imprint as illustrated on the right.
There are two planarization strategies;
1) Spin on planarization of a liquid film – a classic technique that works for features
less than 20 um. It relies on surface tension to pull the film flat.
2) Imprint planarization – using a flat mold to ensure planarization of large features,
either before patterned imprint, or as part of the patterning imprint in SFIL-R
developed by Molecular Imprints.
After planarization, the lines must be transferred by etching. In order to maintain line
width control, a large difference in etch rate is required between the planarization
layer and the imprint layer. One approach is to use organo – silicon materials in the
imprint layer (Colburn 2000).
All wafers are wedged so the mold and substrate must conform over lengths
scales of 1 -200 mm, and the mold must be separated at the end of the process.
These represent the fundamental challenges of imprinting. For more go to
Conformality, Planarization and Separation.
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For more on the Imprint step got to Imprint Mold, Material and Tool
Or use those handy tool bars.
