M. Wissen

Thermal imprint with negligibly low residual layer

Thermal imprint into polymer layers that are thin compared to the pattern height allows for imprints with extremely low and uniform residual layers where lift-off works without previous dry etching. The authors report about possible restrictions of this method such as unintended self-assembling and recovery of polymer underneath the imprinted structures. Both can be attributed to structure size properties and temperature influence and hinder a subsequent successful lift-off. Taking the example of two different temperatures and two different initial layer thicknesses the authors investigate their impact on the resulting structure shape and potential defects. In addition, to qualitatively estimate the residual layer thickness the authors apply lift-off and discuss the results with focus on the usability of this approach as a lithography technique.

Local mass transport and its effect on global pattern replication during hot embossing

Thickness and uniformity of the residual layer are major global quality criteria for nanoimprint lithography. When the total area of a stamp used in a mechanical replication process like hot embossing is composed of sections with different pattern density, the specific local stamp layout strongly affects residual layer uniformity. Taking the example of a positive stamp with sections of well-defined pattern density the effect of initial layer thickness on uniformity of the residual layer was investigated on the centimetre scale. Long-range uniformity is shown to be a result of local polymer flow in particular between sections of different pattern density. We found the effect to be independent of pattern size. It turned out, that initial layer thickness has to be chosen with regard to the polymer volume locally required for filling of the stamp cavities. Whereas overfilling should be avoided, underfilling may be beneficial for specific applications.

Polymer time constants during low temperature nanoimprint lithography

We investigate low temperature nanoimprint into polymers, where adequate pressure choice helps to increase the imprint velocity of larger patterns und thus decreases pattern size effects by reduction of the effective viscosity. In order to make use of shear rate effects, the imprint has to be performed within the viscoelastic plateau region and not in the viscous flow regime of the polymer. Therefore elastic effects play an important role and may lead to shape recovery of the polymer after imprint. We address the counter play of elastic effects and viscous flow by conducting experiments very near to the glass transition temperature. The elastic behavior at the beginning of the imprint is simulated for different pattern sizes and thus different aspect ratios of the stamp. The investigations show, that a reduction of temperature has to be compensated by an increased imprint time and this time increase has to consider the reduction of viscosity on the one hand and the extension of the polymer flow time const...

Impact of vacuum environment on the hot embossing process

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.

Impact of glass temperature for thermal nanoimprint

To address the importance of the parameter glass transition temperature Tg for the choice of an adequate process temperature in a thermal nanoimprint process, three polymers with different values of Tg were investigated with respect to their imprint behaviors (poly)vinyl-chloride, (poly)styrene, and (poly)methyl-methacrylate. In order to evaluate the imprint results obtained at different temperatures, the volume of the polymer squeezed into the cavities was observed, and the temperature margins for the appearance of typical pattern size dependent failure types were derived. It turned out that despite the comparable molecular weight of these materials, the imprint temperature has to be chosen at different levels above Tg. Thus, Tg is not sufficient to characterize imprint polymers, and, in addition to the molecular weight, the viscosity at a definite temperature should be known. Tg alone can only indicate a lower limit for the process temperature.

Multiple replication of three dimensional structures with undercutsa)

Three dimensional linear structures with definite undercuts of different sizes were replicated in the first step in an elastomeric material and, in the second replication step, in a negative tone resist. Undercut masters for the first step were prepared from Si by laser ablation and anisotropic wet etching. For the second step the elastomeric replica, featuring the inverse undercut, served as a master. The first replication was performed by molding or imprinting, where the prepolymer of the elastomer was cross-linked within the master in an oven or in a thermal press, respectively. The second replication of the undercut elastomeric structures into the negative tone resist was performed successfully by molding. Comparison of the Si masters with the second replication revealed shrinkage-induced differences. Undercuts of up to 60% of the overall pattern width were successfully replicated.

Dynamic mask defects in hot embossing lithography

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.

Impact of residual layer uniformity on UV stabilization after embossing

A low-Tg resist material, mr-L 6000XP, was investigated for low-temperature imprint. Its stabilization requires UV exposure and a postexposure bake. To approach the UV-induced crosslinking process, the local distribution of light intensity within the imprinted pattern was simulated. To account for a nonuniformity of the residual layer caused by pattern size and pattern density effects during imprint, different residual layer heights were adopted. The simulations show that not only the residual layer height but also the pattern size influences the resulting local intensity distribution. Experiments performed for selected residual layer heights (50, 100, 150, and 200nm) document that in fact the residual layer height affects the pattern quality obtained after stabilization. The effects identified are different for differing pattern sizes. Beyond stabilization, the results have consequences for mix and match of nanoimprint with UV lithography.

Strategies for hybrid techniques of UV lithography and thermal nanoimprint

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.

Silanized polymeric working stamps for hot embossing lithography

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.