Alternative Masks for Nanolithography

The use of masks coming from research field as different as colloids, polymers or nanomaterials is a recently emerging field. Recent advances in this area have developed a variety of practical routes which have a great potential to overcome or at least complete the high-cost lithographic techniques. This review focuses on three techniques that try to reduce to the nanometer range, the size of the mask. The major difference between these procedures is related to the type of mask used. The first technique is called colloidal lithography, the mask is a monodispersed-beads template. The second is the block copolymer lithography and the third technique is the nanocrystal lithography, the mask used is a nano-object. For these three parts, the synthetic routes, the improvements and the applications as well as the limitations will be pre- sented. I. INTRODUCTION Nanotechnology research is led by the demand for ever smaller device features that are required to improve perform- ance and decrease costs in microelectronics, communication and data storage. Lithographic methods are in the centre of this nanotechnology. Rapid replication of features below 50nm is currently a significantly unsolved problem. Generat- ing patterned surfaces at the nanoscale is beyond the limits of standard photolithography techniques. The wavelengths of photons limit the minimum feature sizes in the sub-100nm scale. Thus it appears that whatever technologies are used for fabrication in this region it does not represent simple engi- neering extensions of conventional photolithography. There exists an opportunity to introduce news ideas in replication of meso and nanostructures. Indeed, very few methods pro- vide the ability to work in the sub-50-nm scale (1-3). The well-established methods for the fabrication of nanometer- scale structures (2), such as scanning probe, electron and ion beam lithography provide resolution and integration density required for the industrials demands but the level of throughput rate is far below the requirement. There is a need for alternative simple techniques in order to save processing time and costs (4,5). This need has given rise to different kinds of approaches like, for example, nanoimprint lithogra- phy and soft lithography with the use of self-assembly monolayers (SAMs) (3,6,7). However, for both the mold fabrication problems remain unsolved. In this review we will focus on three techniques that try to reduce to the nanometer range, the size of the mask keep- ing a large surface coverage. The major difference between these procedures is related to the type of mask used while the standard techniques to pattern the substrate like metal depo- sition or ion-plasma etching are kept similar. The main re- quirements present in an efficient method for replication are flexibility in the engraved substrate, fidelity in the replica- tion, low density of defects and large patterned surface. Be- cause all of these characteristics are never present in a single method, it is necessary to combine techniques. Thus we present these three techniques and their extensions combined with others. The first technique is called colloidal lithogra- phy, the mask is a monodispersed-spheres template (typi- cally polystyrene beads with an average diameter of 200 nm). The second is the block copolymer lithography, block copolymers are made of two chemically different chains bonded covalently. When the blocks are incompatible, they spontaneously self-assemble into micro domains, this leads, at equilibrium, to an ordered structure which is utilized as lithographic mask. The third technique is the nanocrystal lithography, the mask used is a nano-object.