Helmut Schift

Nanoimprint lithography: An old story in modern times? A review

Nanoimprint lithography (NIL) is a high throughput, high-resolution parallel patterning method in which a surface pattern of a stamp is replicated into a material by mechanical contact and three dimensional material displacement. This can be done by shaping a liquid followed by a curing process for hardening, by variation of the thermomechanical properties of a film by heating and cooling, or by any other kind of shaping process using the difference in hardness of a mold and a moldable material. The local thickness contrast of the resulting thin molded film can be used as a means to pattern an underlying substrate on wafer level by standard pattern transfer methods, but also directly in applications where a bulk modified functional layer is needed. Therefore it is mainly aimed toward fields in which electron beam and high-end photolithography are costly and do not provide sufficient resolution at reasonable throughput. The aim of this review is to play between two poles: the need to establish standard pro...

Flow behaviour of thin polymer films used for hot embossing lithography

Abstract The viscous flow of thin PMMA films into microcavities during hot embossing has been investigated in order to optimise the moulding process for nanostructured surfaces. The fastest embossing times were obtained at temperatures >100°C above Tg with viscosities in the range 300 to 3000 Pa s. Two fill mechanisms have been observed: simple flow of the PMMA from the borders and formation of polymer mounds. A simple theory was used to estimate the embossing time required to fill a given stamp geometry. Thin polymer ridges with aspect ratios up to 6:1 were moulded and plastic deformation during demoulding was observed.

Nanoreplication in polymers using hot embossing and injection molding

With polymer molding techniques, it is possible to fabricate nanostructures with a replication fidelity of 25 nanometers. Both hot embossing and injection molding can be used, and cycle times of down to 4 sec can be achieved in a CD injection molding process. The resolution is far below the structure size found today in compact disc memory media. The master structures are produced by electron beam lithography and subsequent dry etching.

Controlled co-evaporation of silanes for nanoimprint stamps

A new chemical vapour deposition setup for the generation of anti-adhesive coatings on Si stamps used in nanoimprint lithography has been developed. This is suitable for controlled co-evaporation of more than one type of silane by directly injecting a premixed silane into an evacuated deposition reactor through a septum. This process was found to be very flexible and resulted in reproducible coatings. A surface coated with a mixture of mono- and trichlorosilanes shows a higher water contact angle than those of individual coatings, which is attributed to the interaction between the two types of silane molecules. In addition, the influence of process parameters, e.g. water content, temperature and number of imprints, on the coating quality will be discussed.

Pattern formation in hot embossing of thin polymer films

We have investigated pattern formation of thin PMMA films during both hot embossing and demoulding of micro- and nanostructures. During filling of the stamp cavities, compressive and capillary effects were observed, and under certain conditions periodic patterns with characteristic length scales formed. In unstructured stamp regions the form of these patterns is affected by local differences in the pressure. For structured stamps, the self-assembly is strongly influenced by the size and the shape of the stamp cavities. Rapid expansion of trapped air resulted in viscous fingering patterns and a dewetting behaviour of the polymer could be observed.

The deposition of anti-adhesive ultra-thin teflon-like films and their interaction with polymers during hot embossing

The chemical and physical interactions of ultra-thin teflon-like films at interfaces are a surface science problem with many technological implications. Such films are the material of choice for protective layers and anti-adhesive coatings. During the replication of microstructures in polymers by hot embossing, interfacial forces between the master and the replica need to be reduced by an anti-adhesive layer, in order to ensure a clean demolding process. In this work, we investigated two different teflon-like films, one obtained by ion sputtering, and the other by plasma polymerization. Using both deposition methods, we deposited thin fluorinated films on nickel substrates and conducted depth-resolved X-ray Photoelectron Spectroscopy (XPS) measurements for a detailed comparison. In a subsequent step, nickel surfaces covered by both anti-adhesive coatings were hot embossed into two different polymers. The chemical composition of both the anti-adhesive film and the polymer replicas was monitored, as a function of the number of embossings made with the same Polytetrafluoroethylene (PTFE)-treated nickel stamp. During the embossing process, a transfer of material was found to occur from the teflon-like film to the embossed polymer, consisting of fluorinated entities or small polymer chains. The influence of the operating parameters on these phenomena was also investigated and resulted in a better understanding of the film/polymer interactions under pressure and at high temperature.

Hot embossing in polymers as a direct way to pattern resist

Abstract In this work, we investigated the possiblities of Hot Embossing Lithography as a new nanoreplication technique. Different structures with feature sizes down to 50 nm were successfully replicated into a resist over an area of up to 10 cm 2 . These polymer structures were then further transferred into a hard material by means of two different pattern transfer techniques. The aspect ratio and the shape of the various replicated nanostructures were monitored throughout the process by means of scanning electron and force microscopics.

Nanostructuring of polymers and fabrication of interdigitated electrodes by hot embossing lithography

We investigated the possibilities of Hot Embossing Lithography (HEL) as a novel technique for the replication of nanostructures. A replication fidelity in polymers of less than 10 nm was achieved using anisotropically etched Si-substrates as masters. We demonstrated that nanoimprinting combined with the lift-off technique can be used to fabricate arrays of interdigitated metal electrodes. The electrodes have a length of 100 @mm and a spacing between the electrodes of 200 nm, and are defect-free over an area of 10 @mm x 100 @mm.

Nanofabrication using hot embossing lithography and electroforming

Abstract We demonstrate various nano- and microstructuring possibilities of electroforming in combination with hot embossing lithography. Periodic structures down to 120 nm and feature sizes

Fabrication of 3D nanoimprint stamps with continuous reliefs using dose-modulated electron beam lithography and thermal reflow

3D electron beam lithography and thermal reflow were combined to fabricate structures with multilevel and continuous profiles. New shapes, smooth surfaces and sharp corners were achieved. By using exposure with variable doses, up to 20 steps were fabricated in a 500 nm thick resist with a lateral resolution of 200 nm. Steps were reflowed into continuous slopes by thermal post-processing, and were transferred into silicon substrates by proportional plasma etching. The method can be used for the fabrication of 3D nanoimprint stamps with both sharp features and continuous profiles.