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Based on the well-known fact that thermoplastic polymers feature a decrease of viscosity at increased frequency we propose a novel tool for frequency assisted thermal nanoimprint. The system is equipped with a stepper motor to drive into contact and to apply a static loading. In addition a piezo-unit is available that allows the superposition of the static load with a dynamic excitation. Detailed analysis of the frequency response of the overall system makes obvious that the frequency range available is limited not only by the frequency cut-off of the piezo amplifier, but also by its power or rather by the limited output current available. As a consequence the maximum frequency at full displacement is only 10 Hz. Nonetheless this should be enough to reduce the viscosity of typical imprint polymers at a low imprint temperature. The measurement system is sensitive enough to detect the small changes induced by the polymeric layer in the imprint stack, when the temperature is raised to typical imprint temperatures. Decay times for the residual force during imprint with a conventional imprint stack could be obtained from a relaxation experiment, where the piezos are used as step-displacement sources. The data are in excellent agreement with values calculated from dynamic rheological characterization experiments.

In order to utilise both techniques in the best and easiest way, thermal nanoimprint (T-NIL) and optical/UV lithography (UVL) were combined in a hybrid process to pattern the same resist layer. Typically, T-NIL as the first step defines the small, submicron patterns, UVL as the second step the larger patterns, well above 1 μm. The hybrid process is performed with a conventional Si stamp for imprint and a conventional photomask for UVL and does not require any special infrastructure. Conventional, commercially available positive and negative tone resists were used, SU-8 and AR-P 3510. In the case of SU-8, a chemically amplified negative resist with low glass temperature, the imprint is uncritical and does not result in any optical degradation. However, the post exposure bake required to finalise the crosslink reaction has to be adapted and performed as a temperature ramp. In the case of AR-P 3510 the imprint step is critical, as the photoactive component is degenerated at suitable imprint temperatures. We found that at temperatures up to 130°C this degradation can be compensated by an increase of the exposure dose. Though more critical in processing, the positive tone resist features a better definition of the transition region between imprinted patterns and UV patterns.

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.

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.

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.

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.

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.

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.

In order to examine the suitability of nanoimprinting for wafer scale pattern definition, a commercially available hot embossing system, the EV520HE of EVGroup, Austria, has been used to imprint 4 inch substrates. The EV520HE is based on a production-proven wafer bonding system which guarantees compatibility with semiconductor fabrication conditions. A 4 inch silicon wafer fully patterned with structures from 400 nm to 100 micrometers size was used as a stamp. The patterns, having a nominal height of 260 nm were defined in poly-Si over SiO2 by reactive ion etching. Different anti- sticking layers were applied to the stamps by monolayer self-assembling, among them (1,1,2,2 H perfluoroctyl)- trichlorosilane. Two different polymers, polymethylmethacrylate (PMMA) and a commercially available nanoimprint resist were used to spin-coat the substrates. Imprints were performed with temperatures of up to 225 degree(s)C, forces between 10 bar and 55 bar and holding times of 5 and 15 minutes. After separation of stamp and sample the imprints were characterized by a surface profiler and inspected by an optical microscope as well as a scanning electron microscope. Different qualities of pattern transfer according to the used process parameters were achieved, but patterning of the whole sample surface was always observed. In contrast to radiation-based lithography, the difficulties are based in imprinting of larger features whereas structures of 400 nm size were reproduced with high quality. Therefore the largest patterns of the stamp, 100 micrometers square bond pads, were used for imprint quality assessment, judged by the degree of stamp cavity filling around the pads. High quality was achieved by embossing at 225 degree(s)C with a hold time of 5 minutes at a pressure of 55 bar. For full wafer imprint only a small degradation of imprint quality from the center towards the periphery was observed. Further optimization of the process is required to minimize residual layer thickness for the hot embossing lithography step, taking into account the visco-elastic properties of the polymer material.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We summarize flow and filling effects detected during hot embossing lithography (HEL), which reflect the visco-elastic properties of the polymers as well as the specific processing conditions and stamp pattern configurations. In particular a delayed elastic recovery is reported, which is found in larger elevated stamp regions. The consequences of these effects for mask definition by HEL are discussed, and strategies for wafer-scale imprint are de-duced. Successful imprint of 4' wafer is demonstrated under adequate processing conditions.

Mask definition was performed by use of nanoimprint lithography and subsequent reactive ion etching in an oxygen plasma. Polystyrene was chosen as a polymer mask material. Different features ranging from 400 nm up to 4 micrometers were produced in the polymer layer by nanoimprint. Optimization of the residual layer removal process in oxygen RIE was performed at different pressures and self-bias voltages. Low pressure and high bias voltage are required for high quality mask definition.

We have studied anti-sticking layers for nanoimprint lithography using different types of polymers, thermoplastic and thermosetting ones. Typically thermosetting polymers have higher mask selectivity in a fluorocarbon dry etch process than the thermoplastic polymers, but replication into these materials is much more complicated. We observe a high tendency for the polymer to adhere to the stamp. To minimize the sticking problem a fluorocarbon coating of the stamp was tested. It was deposited in a radio frequency plasma reactor with C4F8 as a feed gas. The thickness of the coating was several nm. It was characterized by contact angle measurement, XPS and FTIR analysis. Such coatings could successfully reduce sticking effects during an imprint process.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Femtosecond processes in allophycocyanin, C-phycocyanin and phycoerythrocyanin trimers and monomers have been examined by means of polarization pump-probe technique. No femtosecond kinetics were observed in monomeric preparations. The isotropic absorption recovery kinetics with (tau) equals 440 +/- 50 fs which is not accompanied by anisotropy decay kinetics was obtained in allophycocyanin trimers at 612 nm. The conclusion about energy transfer between neighboring (alpha) 84 and (beta) 84 chromophores with different absorption spectra was made. The proposed model takes into account a stabilizing role of the linker peptide. Spectral and kinetic measurements were made in the 635 - 690 nm spectral region where the proposed acceptor should absorb. The bleaching of the 650-nm band occurs with a delay relative to the bleaching at 615 nm. Only a rise term was observed at 658 nm in consistence with the proposed model. Anisotropy values calculated around 650 nm at 3 ps after excitation are in the range 0.1 - 0.25 corresponding to an angle of 30 degree(s) - 45 degree(s) between the donor and acceptor transition dipole moments. A 500-fs absorption recovery and anisotropy decay process was obtained for C-phycocyanin trimers and explained by Forster energy transfer over 20.8 angstroms between neighboring (alpha) 84 and (beta) 84 chromophores of different monomeric subunits having similar absorption spectra and with a 65 degree(s) angle between their orientations. Energy transfer between violobilin ((alpha) 84) and phycocyanobilin ((beta) 84) chromophores was examined in donor and acceptor spectral regions of phycoerythrocyanin trimers, and was found to take 400 fs.