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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.

Replication of the e-beam master into thermosetting polymers was investigated for low cost working stamp fabrication for nanoimprint lithography (NIL). Negative as well as positive replicas from a master were fabricated in a thermal replication step from duroplastic material. Without any further treatment these working stamps were able to imprint thermoplastic polymers. To demonstrate the procedure we used the custom polymers mr-I-8130 and mr-I-9030 as well as PMMA for comparison. All replications were done in a NIL process. The replication quality is excellent when a suitable imprint procedure is used.

Within the last few years a number of new techniques have emerged for definition of nm-scale patterns. They may be summarised under concepts like bottom-up or top-down, addressing the basic direction of pattern assembly. Bottom-up techniques based on the self assembly offer a very high innovation potential. At the other hand, top-down techniques are somewhat nearer to state of the art technology. These latter may be optimally suited for bridging the gap between conventional techniques and the high end self assembly.

We have investigated new aromatic polymers for nanoimprint and subsequent dry etching, namely thermoset and thermoplastic compounds. They were tested in a SiO2 patterning process under low pressure and high plasma density conditions and feature a selectivity about twice as high as poly(methylmethacrylate) (PMMA). The imprint behavior is comparable to PMMA and, in particular, the thermoplastic polymers show excellent imprint quality. This was demonstrated by replication of large arrays of lines down to 50 nm width. The thermoset polymers showed excellent dry etch stability and Teflon-like antisticking layers helped to imprint them without sticking.

A new class of copolymers potentially suitable for an imprint process is investigated. They are composed of methyl-methacrylate and a methacrylic acid, where the acid component increases the glass temperature and thus the thermal stability. The results obtained are compared to previous experiments with a methyl/butyl acrylate and the suitability for an imprint process is discussed. A modified processing sequence is proposed, where pressure is applied during the heating phase, resulting in superior imprint quality of, for example, periodic patterns.

Although the direction of flow during imprint for amorphous polymers is accessible via simulations, a direct observation in the experiment is difficult. The authors address an indirect method, the detection of imprint-induced ordering in the residual layer by optical inspection of the imprint result. The method relies on imprint materials featuring optical anisotropy when existing in an ordered state, ordering being induced by the imprint process proceeding under shearing flows. Organic polymers with π-electrons along the backbone were chosen to test the approach; similar materials with regular and random chemical chain structure were compared, where only the polymer with regular chemical structure exhibits optical anisotropy after imprint, due to its semicrystalline nature. A clear correlation of the anticipated flow direction in the residual layer with optical anisotropy in transmission was possible. Beyond visualization of the direction of flow, the experiments allowed to attribute the local ordering observed in the residual layer with poly 3-hexylthiophene-2.5-diyl to a flow-induced stretching of the polymeric network occurring at the beginning of the squeeze process.

Hybrid lithography is applied to the negative tone resist ma-N 405, successfully combining thermal nanoimprint lithography (to define submicron patterns) with conventional photolithography (for micrometer-pattern definition). When employing such a hybrid technology scheme, one often finds it difficult to achieve a good pattern fidelity in the overlapping areas due to uneven exposure relating to the formation of standing waves during exposure. To avoid such standing waves, one has to infer an antireflective layer. Alternatively, one can make a short heat treatment. Such a heat treatment levels the uneven exposure but may lead to a reflow of the preimprinted pattern. In order to minimize reflow during this postexposure bake, flood exposure is integrated into the processing sequence whereby the dose required must be optimized. The results show that with an adequate choice of dose for flood exposure and masked exposure, the reflow can be avoided, and the standing waves are reduced to an acceptable state. Particularly, these conditions make it possible to remove residual layers of up to 30 nm simply by development. The mixed patterns defined by hybrid lithography in this way can be successfully transferred into a metal pattern.

Self-assembly defects limit the suitability of partial cavity filling in thermal nanoimprint for residual layer minimization. As they represent thermodynamically stable structures already the formation of such defects has to be avoided—because once developed they cannot be removed. One impact parameter for the formation of self-assembly defects is the quality of the stamp surface, in particular its roughness. The authors compared stamps with flat surfaces obtained by potassium hydroxide etching of (110)-Si with stamps prepared by dry-etching, featuring vertical grooves or horizontal scallops (typical of deep-etching in a Bosch process). Under comparable conditions, the stamps with the vertical grooves feature the highest amount of self-assembly defects. In contrast, horizontal scallops avoid defect formation. Thus, the surface quality of the stamp sidewalls is important for a high quality imprint.

Hybrid lithography combining thermal nanoimprint with optical lithography is an effective means to prepare structures with differing geometries (nanometer, micrometer) over large areas, e.g., for sensor applications. For the realization of this technique, the suitability of photoresists for this process has to be investigated. In case of negative tone resists, the cross-linking induced or provided by exposure may be amplified in the imprint step due to heating times in the minute range. As cross-linking may impede the imprint, a subtle adjustment of the exposure dose with respect to the imprint temperature envisaged is required for successful processing. With an imprint temperature of 100 °C, good results for hybrid lithography could be obtained with two commercially available negative tone resists: ma-N 405 and ARN 4410. The latter could successfully be used for a subsequent liftoff. The best results were obtained when the resists were exposed before the imprint step.

Dynamic agitation during imprint is investigated as a means to reduce the imprint temperature of thermoplastic materials. The imprint system used consists of a stepping motor and a piezo unit as the main components. The motor controls the working point of the system, and a sinusoidal motion of the piezo unit (3 piezos at 120° with respect to each other) superimposes an additional dynamic excitation. The system was characterized with respect to its mechanical behavior by exploiting analogies between mechanical and electrical quantities. The results show that the system can be operated at a frequency of up to about 60 Hz and that the piezos act as a dynamic motion source in the system. First imprint experiments with polystyrene at 100 °C indicate the potential of this frequency-assisted imprint technique.

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.

Two low-cost techniques suitable for large area processing are combined to define submicrometer metal structures, thermal nanoimprint lithography, and sputtering. With very low residual layers after thermal nanoimprint, as obtained under partial cavity filling conditions, it is possible to skip the etching step for residual layer removal before sputtering. As imprinted structures typically feature a positive sidewall angle, the authors investigate the morphology of sputtered layers on sloped sidewalls. Independent from inclination, the sputtered layers exhibit a columnar structure with only a slight taper, where the crystallites extend perpendicular to the substrate through the whole layer thickness. This structure allows penetration of the solvent, swelling of the polymer, and lifting of the top layer as long as the adhesion to the substrate within the contact area is sufficiently high, thus enabling successful lift-off.

Thermal nanoimprint (T-NIL) is uniquely suitable for combinational lithography because it proceeds without cross-linking and does not rely on changes in properties of the imprint material. Most typical are combinations with other lithography techniques. Combinations with optical lithography may help to circumvent the pattern size dependence of nanoimprint and help to remove the residual layer by a simple development step. This work focuses on a hybrid processing combining T-NIL with optical lithography by using a single resist layer. Critical issues are the imprintability of typical photoresists and the fact that lithography has to be performed over a prepatterned topography. A well-known positive tone photoresist, AZ 1500, is compared with a previously investigated negative tone resist, SU-8. Gel permeation chromatography reveals typical differences of these photoresists compared to typical imprint polymers. The results demonstrate that molecular mobility is a more important indicator for viscosity of photoresists than molecular weight. Additionally, a simple construction method was found to allow prediction of features typically encountered during hybrid patterning.

In order to investigate the nonuniformity occurring below wider patterns during thermal nanoimprint lithography, the pressure situation is analyzed in detail. A balance of vertical forces shows that the gas pressure within the cavities is negligible, whereas the effective pressure acting on the stamp structures in contact to the polymer is increased compared to the externally applied pressure. This effective pressure has a high hydrostatic component, which may result in local stamp deformation. Imprint with a low molecular weight polymer provides evidence that the stamp structures become compressed under high pressure, resulting in a nonuniform residual layer beneath the imprinted patterns. In contrast, bending deformation of single stamp structures is found to be negligible. Pressure reduction is effective to reduce stamp compression, improving the uniformity of the residual layer. With typical imprint polymers of medium molecular weight, however, pressure reduction reduces the overall imprint depth.

Imprints into thin thermoplastic layers in thermal imprint often go along with phenomena such as physical self-assembly and recovery, which impair the resulting replication quality. To avoid recovery, in particular within larger structures, a polymer with extremely low Mw (polystyrene with a molecular weight of 1kg∕mol) is investigated. In contrast to expectations, the polymer develops a strong recovery. This occurs at such low viscosities that the imprint is also impaired by strong physical self-assembly effects. The imprint is further hampered by the fact that, due to the very low glass-transition temperature, a layer preparation without prebake is required. In addition the suitable temperature window for imprint is extremely small (T≈40°C) so that process control is hardly feasible. Polystyrene with such low Mw is not suitable for thermal imprint.

To improve control during the filling of the channels of a microfluidic device with the fluid under investigation, a new concept is presented: channels with a diamond-shaped cross section. These channels easily allow one to distinguish an unfilled channel from a filled channel by simple optical microscopy. The idea is based on the fact that incident light is totally reflected with unfilled channels, whereas it is mostly transmitted with filled channels. Preparation of such channels in thin SU-8 layers on a glass was performed via a double replication of a Si template with undercut trenches. Functionality tests with filling materials spanning the range of refractive indices of typical organic media provided an excellent contrast between filled and unfilled channels, thus demonstrating the capacity of this concept.

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.

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.

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.

Details of a mix and match process of thermal nanoimprint lithography and UV lithography have been investigated, where the smaller patterns were defined by imprint, whereas the larger patterns were prepared by UV lithography within the same layer of negative tone resist. Stabilization of the prepatterned resist after imprint by UV curing within the open windows of a mask is different from stabilization under flood exposure. In addition to reflection at the substrate, which leads to a height dependent, vertically modulated exposure dose pattern, the pattern edges, due to diffraction effects, result in an additional lateral modulation of the exposure dose. Distinct local intensity minima and maxima develop within the resist. The intensity differences are particularly strong in the case of pattern widths in the wavelength dimension. Furthermore the direction of light polarization affects the exposure dose pattern. In the case of pattern widths well above the wavelength mainly an underexposure along the edges...

A technical thermoplastic polystyrene with broad molecular weight distribution was characterised with respect to its thermo mechanical behaviour. In comparison to two other polystyrenes with narrow molecular weight distribution its temperature behaviour was investigated and could be generalised making use of well-known basic common principles of polymer rheology. The role of the temperature and the molecular weight choice with respect to an imprint process was contrasted to the role of the imprint pressure, which is a free parameter without impact on polymer characteristics. Pressure determines the initial shear rate in the process. Correspondingly simulations of the course of an imprint were performed, where shear thinning was assumed and the viscosity changed throughout the process according to the measured polymer behaviour. Two different filling situations were investigated, bottom up filling of the stamp cavities and lateral filling of the stamp cavities. In both cases the simulations show a quick decrease of the polymer height within the first seconds of the imprint. Shear rate effects suggest being effective to end up with very short processing times during thermal imprint under appropriate pressure with medium molecular weight thermoplastics. This is an interesting approach towards high throughput nanoimprint processes.

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...

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.