The motivation for the presented research was the known issue of very expansive UV transparent stamps and moulds,
which are necessary tools for UV-based patterning methods such as UV-based nanoimprint lithography, which has been
developing as an attractive alternative lithography approach in recent 10 years. Low priced polymer working stamps
could be an alternative to quartz as stamp material. UV transparent nanoimprint stamps were fabricated from sol-gel
process-derived hybrid polymer, which has the benefit of high thermal, chemical and UV radiation stability. Tailored
surface treatment and release agents were applied to ease the de-moulding process and secure the accuracy and fidelity of
the transferred patterns. To increase the life time of the hybrid polymer nanoimprint stamps some adhesives between
stamp substrate and the stamp material were used. The hybrid polymer stamps are compatible with NIL polymers and
long-term stable even at elevated temperatures of thermal imprint processes.
This paper reports on the evaluation of XP SU-8 4000NPG for potential use in nanoimprint applications using hot UV imprint lithography. The use of this material is advantageous in that it can be imprinted, exposed and sufficiently cured all at the same temperature without any temperature cycling providing an isothermal process leading to short cycle times. Uncured XP SU-8 4000NPG has a Tg below 10ºC, yet its films are sufficiently robust to be handled at temperatures from 40 to 70ºC. This resist exhibits excellent flow properties in this temperature range, which is also a range where the post exposure bake of the exposed areas is sufficient to lock in the imprinted patterns and allow easy stamp removal. Wafers can be spin coated with the 4000NPG to provide films of less than 100nm thickness to more than 500 nm and subsequently baked to remove the residual coating solvent. Precoated wafers are introduced into the imprint tool and placed on a pre-heated chuck for a few seconds to reach the set temperature, and then the imprint stamp is applied under pressure for 30-60 sec to allow adequate time to properly fill the mold. While still in the mold, the resist is exposed through the transparent stamp and simultaneously cured for as little as 10 sec in order to remove the stamp without tearing or pattern deformation. The wafer can then be immediately removed from the imprint tool. The optimal temperature is a balance between resist flow, cure rate and the thermal stresses imparted into the cured film at the higher operating temperatures.
We report the new epoxy-based curing resist mr-NIL 6000 designed for thermal NIL, where the curing reaction is
initiated by UV exposure and concurrently occurs at elevated temperature, both preferably in the imprint machine.
Especially for the application in NIL the requirements to a resist differ much from those in radiation-based lithography
where epoxy-based resists have been used for many years. High sensitivity is vital for a short cycle time. The imprint
temperature is determined by the glass transition temperature (Tg) of the resist system before curing and roughly controls
Tg of the cured polymer, which on its part, affects the mold release temperature and the thermal stability of the imprints.
An epoxy resin with low Tg was chosen allowing imprinting at 100 °C or lower temperature. UV-coupled differential
scanning calorimetry (Photo DSC) was applied to assist establishing the conditions of the resist processing. Optimum
processing conditions were finally elaborated in imprinting tests. Flow tests were performed with the imprinted and
cured resist patterns. The optimum imprint temperature was determined to be 100 °C. The imprinted patterns exhibited
good dimensional stability up to at least 120 °C. This allows releasing the mold at the imprint temperature and running
an isothermal process. The thermal stability is sufficient for subsequent processes, such as etching or metallization. The
curing reaction during imprinting enables excellent pattern transfer fidelity and a high aspect ratio of the imprinted
features. A short cycle time of ~ 1 min could be achieved so that the resist is promising for industrial applications.
Prepolymers formed from multifunctional allyl monomers can beneficially used in nanoimprint lithography (NIL), since they cure as a consequence of heating during the imprint process. Thus they have the potential to enable NIL at comparatively low temperatures while the imprinted patterns concurrently show high thermal stability, in contrast to thermoplastic polymers, where the thermal behaviour of the imprinted patterns is closely related to the glass transition temperature (Tg) of the polymers.
The use of allyl prepolymers for NIL was previously described, but only very few experimental data are known. In recent investigations on the application of allyl prepolymers for NIL a displacement of the patterns on the wafer has been observed after cooling down the imprinted polymer in the press. This could be avoided by detaching the stamp at the imprint temperature, i.e. without cooling down the press, which requires the polymer to be crosslinked to a great extent in this stage. Since high temperatures are necessary (150 °C - 190 °C), and the imprint time is still long, allyl prepolymers to be reported here have been modified aiming at a reduction of imprint temperature and time.
The admixture of free-radical initiators increases the polymerization rate and allows the polymerization to start at lower temperatures. A reduced imprint temperature (100 °C) and shorter imprint time (10 min) are achieved. Additional polymer modification by plasticizers improves the material flow during the imprint due to a lower Tg. Recipes for polymer modifications have been found out, which result in thermally stable imprints under the specified processing conditions.
Pattern replication into curable imprint resists by hot embossing offers the opportunity to use the replication after crosslinking
as a working-stamp. The replications of a 4” Si stamp into the thermoset imprint polymer mr-I 9000R-XP with a
commercial hot embossing system (EVGroup) have been coated with an anti-adhesive layer (trichlorosilane) from the
gas-phase at ambient pressure. The investigation of the quality and durability of these anti-adhesive coatings reveals
that the contact angles and the replication performance of the working-stamps were independent from the fact whether
the polymer was already cured or still thermoplastic during the anti-adhesive coating. The prepared 4” working-stamps
themselves have been successfully replicated into a low glass transition temperature resist (mr-L 6000.5 XP) and into
PMMA.
To utilize the potential of nanoimprint lithography (NIL) you need polymers, which give relief patterns with good thermal and etch resistance, a necessity for subsequent process steps. Thermoplastic polymers with high thermal stability require high imprint temperatures. Such temperatures can cause polymer degradation and problems with pattern transfer due to the different coefficients of thermal expansion of substrate, polymer and stamp. The characteristics and benefits of two types of cross-linking prepolymers with low glass transition temperature (Tg) for nanoimprinting are described. They are soluble in organic solvents and their solutions can be processed like those of poly (methyl methacrylate) (PMMA). The imprinted patterns receive high thermal and mechanical stability through cross-linking polymerization and exhibit high plasma etch resistance. The course of the polymerization was investigated to determine the appropriate conditions for the imprint process. In thermally cross-linking allyl polymers, the cross-linking occurs during imprinting. Process time and temperature depend on the polymerization rate. Volume shrinkage during the polymerization does not adversely affect imprinting. Photochemically cross-linking epoxy-based polymers permit imprint temperatures below 100°C and short imprint times. Tg of the prepolymer determines the imprint temperature. The cross-linking reaction and structural stabilization is performed after imprinting. SEM images demonstrate the realization of the cross-linking polymer approach. Isolated lines down to 50 nm width confirm the successful application of the polymers.
High viscous photoresists are required for the MEMS and MOEMS technology. Processing of thick and ultra-thick resist films is a challenging task. In this paper, procedures are presented to attain improved patterning results. Baking by infra-red radiation (IR baking) is described as an effective approach for effectively drying thick and ultra-thick resist layers. Patterning results are shown to confirm the performance and benefits of IR baking. Examples of up to 60μm thick layers of two positive tone resists, ma-P 100 and ma-P 1275 (micro resist technology GmbH, Germany), and up to 500 μm layers of chemically amplified negative tone photoresist SU-8 (MicroChem Newton, MA) are presented. IR baking allows reduced process time and lower bake temperature enabling high aspect ratio and low stress SU-8 layers.
Extensive work has been done in recent years to establish nanoimprint lithography (NIL) as a low-cost technology for nanometer-scaled resist pattern definition. Investigations have revealed that creating of small and periodic nanometer-scaled patterns adjacent to larger patterns is difficult due to the flow behavior of the resist material. In this paper mix and match of NIL and UV lithography is proposed to solve this problem and moreover to attain profiled structures within one coating. A chemically amplified broadband negative tone photoresist based on SU-8 is used to realize this idea.
Synthesis and structure of novolaks continue to be investigated due to their significant influence on the performance of photoresists. Previously a two-step procedure was reported which enables the manufacturer to vary the monomer composition and order in novolak molecules in a wide range which then allows for the tailoring of the resin properties. Recent reinvestigation of the molecular structure of these novolaks applying gel permeation chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry has given very interesting new insight into the molecular structure of the two-step synthesis novolaks. Coupling of the two analytical methods provides a new powerful tool to get detailed information on the molecular architecture which cannot be deduced by other methods. Three species of novolak chains have been found. This analytical approach can generally be applied to explore the structure of novolaks made of monomers differing in their molar masses. Using the two- step novolaks high resolution positive and negative tone photoresists for MEMS applications and lift-off processes, respectively, with high alkaline and etch resistance can be produced.
Stable polymer chromophor systems for electro-optical or other nonlinear optical applications can be obtained from dye-containing polyallylic ester polymers by corona poling combined with a simultaneous, thermal or photochemical crosslinking process. Molecular dynamic properties of these materials were observed by thermally stimulated discharge current measurements in order to estimate long-term stability of the orientational state reached after poling and cross-linking. Near glass transition several relaxation processes of the material were detected which influence the over-all relaxation time of the chromophores. The number, temperature position and relaxation strength of the different processes depend on the chemical constitution of the investigated allylic ester polymers. For certain polymers a solid state transformation was observed which can be attributed to a cyclic isomerization reaction. After poling and cross-linking at temperatures between 140 degrees Celsius and 180 degrees Celsius the investigated materials show stable orientation for years below 80 degrees Celsius. This can be concluded from isothermal and thermally stimulated relaxation experiments. In addition, these polymers are excellent film forming materials so that low- loss waveguide structures can be made from these materials either by photolithographic techniques or by molding.
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