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 (T<sub>g</sub>) of the resist system before curing and roughly controls
T<sub>g</sub> 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 T<sub>g</sub> 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 (T<sub>g</sub>) 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 T<sub>g</sub>. Recipes for polymer modifications have been found out, which result in thermally stable imprints under the specified processing conditions.
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 (<i>micro resist technology </i>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.