This paper discusses a hybrid process of thermal nanoimprint and UV lithography, carried out within UV curable SU-8
and mr-L 6000.5, respectively, on the basis of experiments and calculations of the light intensity distribution within the
photoresist. For the non-transparent Si stamps with mixed pattern sizes, ranging from approx. 100nm to 150μm, the
experiments revealed an optimum imprint temperature of 80°C for SU-8 and 70°C to 100°C for mr-L 6000.5. UV
stabilization of the imprinted patterns after stamp removal is possible with negligible loss of the patterns when a
broadband UV exposure is applied and the post exposure bake is performed as a temperature ramp, starting in the range
of the glass transition temperature of the photoresists. UV stabilized patterns of mr-L 6000.5 can be replicated
themselves (working-stamp), after applying a conventional anti-adhesive coating. A successful hybrid lithography of
thermal nanoimprint lithography and UV lithography depends on the exposure conditions as well as on the layout of the
stamp patterns with respect to the photomask pattern. Superimposing the patterns of a photomask is only uncritical for
imprinted patterns in the range of the exposure wavelength and larger, where a trench may develop underneath the edges
of imprinted patterns of ≥1μm width. This is due to a shadowing effect caused by exposure over topography. A sufficient
stabilization of smaller imprinted patterns in the range of 100nm within a hybrid lithography approach is only feasible
when the photomask patterns are not in direct vicinity of the pre-patterned photoresist.
When minimal residual layers are aimed at in the field of thermal imprint lithography (T-NIL) 'partial cavity filling'
offers a promising method to succeed. There, a filling of any of the cavities a stamp provides is avoided by a reduction of
the initial layer thickness. Due to this unusual procedure, some challenges evolve during the imprint of complex types of
structures. We report about effects that go along with partial cavity filling like physical self assembly and recovery.
Furthermore we will give some suggestions how they may be reduced or even can be avoided.
Imprint processes into thin layers can be described by the equations of squeezed flow. In case they account, in a modified form, for the filling of the stamp cavities during the imprint process, they predict significant non-uniformity of the residual layer in areas with different pattern densities. This phenomenon is even intensified by the different imprint velocities of structures with different pattern sizes. The residual layers achieved and their uniformity strongly depend on the pattern sizes and cavity sizes of the stamp. Non-uniformities are lower under cavity filling on the cost of the absolute value of the residual height. In some cases the layout of a device can in parts be adapted to the imprint process by segmentation of larger structures without changing the device functionality. When the segmentation is designed in a way, that its pattern density is similar to the major pattern density within the functional area of the device thin and uniform residual layers are achieved after very short imprint times.
The idea of subdividing the conventional hot embossing process into subsequent process steps is introduced as a modular hot embossing concept. Based on the EVG 520HE the process is separated into modules so that typical temperature cycle times can be circumvented because the imprint is carried out as an isothermal embossing process where the applied pressure is released at a temperature above Tg. Before detachment of stamp and imprinted substrate cooling is applied in a separate module. The full benefit of such a modular concept is achieved when every module is equipped with its own embossing tool and therefore with its own template. We have investigated a new way of preparing working-stamps, which are cost-efficient namely the polymeric replications of Si templates. We have chosen mr-L 6000, an UV curable imprint resist as a working-stamp material. It was possible to obtain a complete cross-link without PEB, which is beneficial with respect to pattern fidelity conservation. Furthermore XPS measurements have shown that a silanization of the cured resist is possible to serve as an anti-adhesion layer. IR alignment as a part of the modular concept and its issues have been discussed on the basis of the EVG450 bond aligner by using Si templates and Si substrates. It is shown that an embossing tool enabling alignment may affect pattern transfer.
Nanoimprint was performed in very thin layers of polystyrene (PS) in order to define a mask with minimum CD loss for a subsequent etch process at minimum etching time for opening of the mask windows after imprinting. The initial polymer layer thickness was chosen as to fill the stamp cavities with nearly no surplus of polymer material. The residual layers after imprint were in the range of 50 nm and could be cleared at 50% overetch within 90 s in an oxygen RIE step. As there was not enough polymer material available for a complete filling of the cavities when a residual layer remains, filling defects occurred. High imprint temperature and thus low viscosity led to formation of deep defects in the imprint and thus the mask to be formed by embossing. Lift off experiments revealed that within the defective regions the remaining polymer layer thickness was smaller than the imprinted residual layer. In order to avoid such mask defects the imprint temperature had to be reduced.
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
While researchers of ever more advanced NGL systems are still struggling to demonstrate the feasibility to manufacture
features well below 100 nm at an affordable cost and a reasonable throughput, nanoimprint technologies are emerging as
a possible answer to these challenges. 100 nm patterns are imprinted with a fully patterned 4 inch diameter stamp in a
low-temperature embossing process. In low temperature imprinting processes with polymers having very low glass
transition temperatures heating and cooling cycles are minimized. This enables to increase the throughput of a hot
embossing process, which is important for potential industrial applications.
Sidewall lithography was used as a low cost fabrication process for high aspect ratio stamps for hot embossing. The stamps, featuring up to 1 μm pattern height and an aspect ratio of about 5:1 were tested in a typical hot embossing process at 50 bar. Mechanical stability of the crystalline stamp structures was achieved after optimization of the respective dry etch process. At imprint temperatures of 140°C PMMA was imprinted over areas of four inch diameter. After residual layer removal in the trenches Al electrodes of 500 nm height and 200 nm width could be defined by evaporation and lift off.
One of the key questions concerning the concept of a system for hot embossing lithography is whether or not it should provide for imprinting under vacuum. We have performed experiments comparing the embossing in vacuum and in atmospheric pressure in a semi-automated imprint system. The stamps used were fully patterned, 10 cm diameter with pattern sizes ranging from 400 nm to 100 μm. It turned out that vacuum enhances the large area uniformity of the imprint by avoiding an air cushion remaining between stamp and sample during automated contact after a non-contact assembly and alignment step. Lower molecular weight polymers turned out to be more sensitive to uniformity deviation than higher molecular weight materials. Detailed analysis showed that defects typically found for relatively high processing temperatures, caused by overheated compressed air, remaining solvent in the polymer layer or even beginning polymer decomposition could be reduced substantially under vacuum embossing conditions, where the excess volume of the polymer is evacuated and free to accommodate gaseous constituents. The best result with complete cavity filling and negligible defects was obtained for imprint of a 99 kg/mol polymer at 200°C and 50 bar under vacuum. Residual layers measured across the diameter of the sample were 44.5 nm ± 9.8 nm. The non-uniformity of the residual layer is a result of the locally different pattern sizes and pattern densities of the stamp, typical for all mechanical patterning processes.
The combination of nanoimprint and UV-lithography has been demonstrated. For this purpose a UV-sensitive epoxy based resin with a low glass temperature was prepared by adding low molecular weight components, in particular by increasing the monomer content. The suitability of our approach to minimize process temperatures was tested by embossing and VU-lithography. Mix and match of both techniques was used to demonstrate that the embossing step did not degrade the UV sensitivity of the material. UV processing provided in addition a simple means for stabilization of this low T<SUB>g</SUB> material. Resists like mr-L 6000-1 xp may close the gap between 'hot embossing' 'UV-molding' and UV-lithography.