The imprint process requires both imprint materials, planarization materials, surface
treatments for adhesion, and surface treatments for anti-adhesion to the mold.
Imprint is a molding technique that displaces liquids and then uses several options to “set”
the pattern including cooling below Tg, UV or thermal crosslinking, solvent evaporation.
Imprint materials must;
- Wet and adhere to the underlying layer
- Flow and fill the pattern at the imprint temperature
- Set quickly
- Survive separation
- Either resist the pattern transfer processes
- Or meet the performance requirements of a specific functional application.
One of the most attractive features of imprint is that in principle any material that can be made
to flow can be imprinted; many successful demonstrations have been made of thermal
imprint on polymers, optical glass, and silicon. However, in order to obtain the highest
performance the imprint material must be carefully engineered.
Glassy materials produce the best replication. A glass is an amorphous solid with no
internal order, as opposed to a crystalline solid with ordered crystal structure. On heating
through the glass transition (a second order thermodynamic transition) the solid starts to flow
without a change molecular order or in volume, just a change in the expansion coefficient as
shown in the example on the right. In contrast when a crystalline material is heated through a
melting transition (a first order thermodynamic transition), the order is removed and the
material flows along with a volume change.
Organic polymers make the best universal “resists”. In resist applications, the imprint
material must be resistant to the many different etches used to etch the many different
materials that appear in electronic devices. Organic polymers almost never appear in
devices, so organic polymers make excellent resists. The etch resistance of organic
polymers is a simple function of atomic composition. Specific species can be added to
customize etch resistance. The best known is adding Si atoms to enhance O2 etch resistance
To survive separation requires high cohesive strength in the imprint material and is favored
by high Tg, high molecular weight or high crosslink density. Many of the factors that improve
imprintability such as low molecular weight, can cause problems in separation. A team from
MII have shown that an effort to improve the mechanical properties of monomer mixture
materials resulted in a 100 x improvement in process life (Xu 2004). A team from Microresist
has also published mechanical property evaluations.
Low adhesion to the mold or adhesion prevention is a complicated surface effect – plenty of
witchcraft. Generally, low energy non wetting surfaces (Teflon strategy) on the mold has been
favored, for more see Molds. In injection molding, mold release agents that are low
molecular weight incompatible materials that are believed to migrate to the surface forming a
weak boundary layer. Incompatible additives have been reported as being effective in imprint
Thermal set materials
Thermal imprint requires the same material to both flow and survive separation. Low
molecular weight polymers start to flow much closer to Tg than high molecular weight
polymers. High molecular weight polymers get entangled (like a plate of spaghetti) and flow
at much higher temperatures as shown on the right where the line identified as “Melt”
corresponds to a high molecular weight polymer. Entanglement starts at molecular weights
greater then 10,000. Shrinkage in thermal imprint is a result of the temperature difference (T
imprint – Tg). The volume change vs temp shows that molding should occur as close as
possible to Tg as possible, offset by the requirements for separation.
Polymethylmethacrylate is the most common thermal imprint material. Already used as a
electron beam and UV resist. It is compatible with many processing operations. It is also a
tough hard glass making a useful structural material such needed in fluidic parts.
Shear thinning has been reported in PMMA ( Yoshikawa 2003).
Commercial materials include i8030 from Micoresist GmBh, an aromatic methacrylate with a
MW of 45K and a Tg of 115 C.
There are 2 well known chain reaction UV initiated polymerization systems; acrylate and
epoxy. Acrylates are the most common UV cured imprint materials. These are based on
mixtures on mono and di fucntional species that crosslinks to forma 3D network. The
species (R group) can be either monomers or prepolymers that have been end capped or
“functionalized” with acrylate groups. In the case of prepolymers. This is a very widely used
trick, in for example UV cured organo silicate materials such as Ormocer. Acrylates also
readily copolymerize allowing formulation of many components (Colburn 1999).
In the acrylate polymerization shown on the right; in step N a di-functional vinyl group moves
to the reactive head, the double bond breaks forming a new reactive head. In step N+1, the
next monomer reacts with the reactive head. In step N+2 a second reactive head reacts with
the other vinyl di functional group forming a crosslink between chains, Finally on step N+3,
more monomer continues the growth of the second reactive head. The end result after many
similar steps is a crosslinked network of polymer chains. The unit cell can be repeated
anywhere from 20 – 100,000 times from a single reactive head.
UV cure, room temperature crosslinking imprinting requires materials with a Tg around RT,
so that the polymer network can be formed. The starting material is a low Mw species that
must be a liquid. As a result either the layer after coating on the wafer is a liquid that must be
handled with care, or it must be drop dispensed. A liquid spun on layer is different from
photoresists which are solids after coating.
During exposure, a UV initiator, absorbs a photon and starts the polymerization. To react, the
monomer must move to the reacting polymer head . As the crosslink linking reaction
proceeds, the molecular weight increases and Tg increases as shown below. When the Tg =
imprint temperature, the monomer stops moving and the reaction stops. Post imprint
processing can be used to increase the Tg of the final material.
Oxygen poisons free radical polymerizations so either vacuum or an inert atmosphere is
needed to ensure complete reaction at the edges of the imprint.
Shrinkage in crosslinking imprint is a result of the crosslinking polymerization reaction.
Vinyl chain reactions produce shrinkage. Acrylate monomers shrink 5-7%. epoxy ring
opening polymerization have much less shrinkage (Colburn 1999). Low shrinkage epoxies
have been used to replicate diffraction gratings for many years.
Crosslinking produces an insoluble network, that can only be removed by lift off or
degradation in wet or dry oxidizing system. Dry oxidation must be used when the underlying
layer is oxidation sensitive such as metals.
Commercial materials include spin on materials PAC 10 from Toyo Goshie, and ??? from
Ormocers from Microresist can be drop dispensed materials or spun on.
MII has developed proprietary drop dispensed imprint materials, based on acrylate mixtures
A polymer can be converted to a liquid by dissolving in a solvent. The solution can then be
used to fill the mold and the heated above the Tg of the polymer to completely remove the
solvent. A wide of polymers have been demonstrated, particularly in transfer imprint
applications, including; PMMA, PDMS, and polycaprolactone.
Planarization using polymers has been widely studied, and can be effective for features
smaller than 50 um in width. Planarization is favored by using the highest possible solids
solutions, which in turn means using the lowest possible molecular weight. Low Tg also
helps because this minimizes the trapped solvent at the end of the coat process, and hence
shrinkage during bake.
The planarization layer must also be insoluble in the monomer based imprint material or the
casting solvent of spun on imprint material. Seeing as the monomer and most casting
solvents are moderately polar organics, high polarity planarization materials are favored.
High Tg also slows the penetration of the solvents, however that operates in conflict with
planarization. Finally crosslinked materials are more resistant.
If the planarization layer is to be used for lift off or solvent strip then it must be a linear polymer.
Organic anti reflecting coating (ARC) materials, such as DUV 30 J from Brewer Science have
to be resistant to photoresist overcoat in their original application. These materials crosslink
during baking. They have been used in bilayer imprint by (Colburn 1999) and (Miller 2004).
Lift off materials designed for use with resist overcoat based on polygycidylmethacrylate
PMGI are available from Shipley. PMGI has a unique resistance to typical solvents.
It is very common for hexmethydisilazane (HMDS) to be used as a surface preparation for
coating quality and ensuring adhesion.