UV-Curved Nano Imprint Lithography

Lithography Imprint-To produce (a mark or pattern) on a surface by pressure. Nanoscale Uv- Method

Introduction This was first invented by Prof. Stephen Chou and his students in 1995. This is a method of fabricating nanometer scale patterns. It creates patterns by mechanical deformation of imprint resist and subsequent processes. This is the road map for the 32-nm semiconductor technology

Requirements The materials required for the Imprint technique is (i) a mold with predefined surface relief nanostructures, and (ii) a suitable resist material that can be deformed and hardened to preserve the shape of the impression.

Mold Fabrication Thermal expansion co-efficient is an important factor to be considered. Si, SiO2, SiC, silicon nitride, metals,sapphire, and diamond film.

A resist material is spin-coated onto the mold surface. Steps in fabrication:- A resist material is spin-coated onto the mold surface. Followed by lithography to define the desired mold pattern. A hard masking layer, such as a metal can be deposited over the patterned resist template. Followed by a lift-off process that removes the resist template and the material on top, leaving a patterned mask layer on the Si substrate. An anisotropic RIE process is used to selectively etch away the Si material in the unmasked region, producing the surface relief structures required for NIL

Types Thermoplastic Nanoimprint lithography Uv-curved NanoImprint Lithographhy Roll imprint process, Laser-assisted direct imprint, Reverse imprint lithography, Substrate conformal imprint lithography, Ultrasonic NIL, etc.

Uv-Curved Method The UV curved Nanoimprint Lithography is normally carried out in room temperature and at low pressure. The mold material used is of transparent type fused with silica. Here we do use the soft UV for the lithographic process. The resist material used is the liquid polymer but in thermal method they use thermal plastic.

Principle:- The transparent mold is pressed over the resist material. Now the cavities are fully filled by the resist. Now the resist is curved by the UV-rays so that the resist will become solid and it is etched till the base. After demolding, a similar pattern transfer process can be used to transfer the pattern in resist onto the underneath material

Crucial process issues for NIL Thickness and uniformity of residual layer.. Pattern fidelity(Precision) Defect control Filling process

Key concern for NIL templates Template surface treatment Defect inspection. Lifetime.

Advantages Nanoimprint lithography has experimentally been shown to achieve 25nm feature size . The promise of repeatability and durability consequently leads this to low costs and ease of fabrication.

DrawBacks The surface sticking problem has not been perfected . The molding conditions haven’t been optimized . The effect of thermal expansion through the process is not fully understood.

Applications Nanoimprint lithography has been used to fabricate devices for electrical, optical, photonic and biological application. For electronics devices, NIL has been used to fabricate MOSFET, O-TFT, single electron memory. For optics and photonics, intensive study has been conducted in fabrication of subwavelength resonant grating filter, polarizers, waveplate, anti-reflective structures, integrated photonics circuit and plasmontic devices by NIL.

The future of nanoimprint Nanoimprint lithography is a simple pattern transfer process that is neither limited by diffraction nor scattering effects nor secondary electrons, and does not require any sophisticated radiation chemistry. It is also a potentially simple and inexpensive technique. However, a lingering barrier to nanometer-scale patterning is the current reliance on other lithography techniques to generate the template. As of October 2007, Toshiba is the only company to have validated nanoimprint lithography for 22 nm and beyond.

Reference S. Chou, P. Krauss, P. Renstrom, 1996, “Nanoimprint lithography,” J. Vac. Sci. Technol. B, Volume 14(6), pp. 4129-4133. S. Chou, P. Krauss, P. Renstrom, 1996, “Imprint Lithography with 25-Nanometer Resolution,” SCIENCE, Volume 272, pp. 85-87. www.wikipedeia.org