Hella-Christin Scheer

A contribution to the flow behaviour of thin polymer films during hot embossing lithography

Using different imprint systems and process conditions, the impact of the visco-elastic properties of polymers on the fidelity of pattern transfer in hot embossing lithography has been studied. The flow behaviour of the polymer, which is a highly viscous liquid at the investigated temperatures, has been shown to be the limiting factor in the replication of micrometer sized features, but does not adversely affect nanometer structuring. This is explained using hydrodynamic considerations. The effects during filling of the cavities formed between stamp and substrate are reported, and explained in accordance with recent publications, where two different types of filling mechanisms have been discussed. Large recessed stamp areas turn out to be much more critical for the process, as they may act as mask defects. They will not restrict the applicability of hot embossing lithography in practice when appropriate imprint conditions are chosen, which have to be enabled by the equipment.

Problems of the nanoimprinting technique for nanometer scale pattern definition

We have tested nanoimprint lithography, a new and promising technique for nanometer-scale pattern definition. Preliminary experiments reveal that, besides severe sticking and adhesion problems, the problem of material transport is one inherent to this technique. There are clear indications that most of the effects found may be understood in terms of material transport. We performed experiments within a well defined pressure and temperature window which ranged from 60 to 100 bar and from 50 to 90 °C above the glass transition temperature of the poly(methylmethacrylate)-like polymer used. As a result, the quality of imprint is evaluated with respect to full area pattern transfer, based on a qualitative scanning electron microscope investigation of the fully imprinted area of 2 cm × 2 cm patterned with features of different size and shape. Optimum conditions for imprint quality are found around 100 bar and 90 °C above Tg for the specific polymer used. Although material transport will limit nanoimprint perfor...

Nanoimprint lithography with a commercial 4-in. bond system for hot embossing

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.

New polymer materials for nanoimprinting

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.

Master replication into thermosetting polymers for nanoimprinting

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.

Polymer issues in nanoimprinting technique

We have studied the suitability of poly(methylmethacrylate) (PMMA) of different molecular weight for large area pattern transfer by embossing structures above the glass transition temperature (Tg) of the specific polymer. The molecular weight covers a range of one order of magnitude (Mw∼5.0×104–8.1×105). This range was chosen in order to obtain information regarding the flow properties that we expect to depend strongly on the molecular weight at a specific temperature. Large area pattern transfer were tested by applying a stamp with both densely packed and isolated features. The feature size ranged from 100 μm down to 450 nm. At a processing temperature of 90°C above Tg we found clear indications that flow is sufficient to transfer large, isolated features even into the polymer with the highest Mw. Problems of incomplete material transport can be related to local inhomogeneities of the imprint due to a lack of parallelism between the stamp and the sample. At 50°C above Tg incomplete flow effects were observed over the whole area and for all molecular weights. This was observed only with large, isolated structures whereas small, periodic features showed a well defined transfer.

Multistep profiles by mix and match of nanoimprint and UV lithography

Driven by the steadily increasing scale of integration in microelectronics much effort has been made in recent years to establish the technique of nanoimprint lithography (NIL) as a promising approach to time and cost-effective fabrication of nanometer scale patterns. NIL enables high throughput for mass fabrication in a simple way. Experiments have revealed that nanoimprint of small and periodic patterns in the nm range is much easier obtained than for μm scale patterns due to polymer transport phenomena. Thus a combination of NIL with optical lithography is considered to be advantageous for the definition of small patterns adjacent to large patterns. The multifunctionality of the material within one layer offers new routes in device design, in particular as multistep profiles are obtainable. In this contribution a combination of NIL and UV lithography is reported using polymers which are both imprintable and photosensitive. Based on an epoxy type negative tone chemically amplified resist system patterns were easily obtained using both techniques. In the first step a pattern relief was produced through imprinting. In the second step this relief was UV exposed in a contact printer and then developed. In this way three-step profiles could be generated within one single polymer layer.

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...