Nanoimprint lithography (NIL) is a nonconventional lithographic technique for high-throughput patterning of polymer nanostructures at great precision and at low costs. Unlike traditional lithographic approaches, which achieve pattern definition through the use of photons or electrons to modify the chemical and physical properties of the resist, NIL relies on direct mechanical deformation of the resist material and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional techniques. This Review covers the basic principles of nanoimprinting, with an emphasis on the requirements on materials for the imprinting mold, surface properties, and resist materials for successful and reliable nanostructure replication.

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Nanoimprint Lithography: Methods and Material Requirements**

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.

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Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures

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 processes and tools for research and industry, and the issues that make NIL a scientific endeavor. It is not the author’s intention to duplicate the content of the reviews already published, but to look on the NIL process as a whole. The author will also address some issues, which are not covered by the other reviews, e.g., the origin of NIL and the misconceptions, which sometimes dominate the debate about problems of NIL, and guide the reader to issues, which are often forgotten or overlooked.

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

Advances in top-down and bottom-up surface nanofabrication: techniques, applications & future prospects.

Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field.

Anti-reflecting and photonic nanostructures

In this article we report on the fabrication of subwavelength antireflection structures on silicon substrates using a trilayer resist nanoimprint lithography and liftoff process. We have fabricated cone-shaped nanoscale silicon pillars with a continuous effective index gradient, which greatly enhances its antireflective performances. Our measurements show that the two-dimensional subwavelength structure effectively suppresses surface reflection over a wide spectral bandwidth and a large field of view. A reflectivity of 0.3% was measured at 632.8 nm wavelength, which is less than 1% of the flat silicon surface reflectivity.

Fabrication of large area subwavelength antireflection structures on Si using trilayer resist nanoimprint lithography and liftoff

Imprint pressure uniformity is crucial to the pattern uniformity and yield of nanoimprint lithography (NIL) and, hence, its applications. We studied a novel imprint method, air cushion press (ACP), in which the mold and substrate are pressed against each other by gas pressure rather than solid plates, and compared it with a common method, solid parallel-plate press (SPP). We found that (a) under normal imprinting conditions the measured pressure distribution across a 100-mm-diameter single imprint field in ACP is nearly an order of magnitude more uniform; (b) ACP is immune to any dust and topology variations on the backside of the mold or substrate; (c) when a dust particle is between the mold and substrate, ACP reduces the damage area by orders of magnitude; (d) ACP causes much less mold damage because of significantly less lateral shift between the mold and substrate; and (e) ACP has much smaller thermal mass and therefore significantly faster speed for thermal imprinting.

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Air cushion press for excellent uniformity, high yield, and fast nanoimprint across a 100 mm field.

To optimize nanoimprint lithography (NIL), it is essential to be able to characterize and control the NIL process in situ and in real time. Here, we present a method for in situ real-time NIL process characterization using time-resolved diffractive scatterometry (TRDS). A surface relief diffraction grating is used as the imprint mold, and the diffracted light intensity is monitored continuously during the imprint process. We use a scalar diffraction model to calculate the diffraction intensity as a function of the mold penetration ratio. Simulations show good agreement with the experimental results. Our results indicate that TRDS offers a powerful characterization tool that can be used for in situ, real-time NIL process control.

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In situ real time process characterization in nanoimprint lithography using time-resolved diffractive scatterometry

In imprint lithography, the mold is coated with a surface release layer for a non-sticking separation. Bonding strength of the release layer to the mold depends on the cleanness of the surface and the process of release layer deposition. In accordance with the invention, the mold is disposed in an evacuable chamber, cleaned to remove surface organic contamination and coated with the surface release layer in a chamber, all without relocation or undesired time delay. The chamber encloses a support chuck for the mold or substrate, a surface cleaner unit adjacent the support, a heating source adjacent the support, and advantageously, sensors of measuring chamber pressure, vapor partial pressure and moisture concentration. A vapor source connected to the chamber supplies release surfactant vapor. The mold is cleaned, and the cleaning is followed by vapor phase deposition of the surfactant. The mold is advantageously heated. Typical ways of cleaning include exposure to ozone or plasma ion etch. Surfactant vapor may be generated by liquid surface vaporization, liquid injection or spray vaporization. A surface adhesion promoter can be coated on the substrate by a similar method with the same apparatus.

Method And Apparatus To Apply Surface Release Coating For Imprint Mold

We use a novel technique, self-perfection by liquefaction (SPEL), to smooth the rough sidewalls of Si waveguides. An XeCl excimer laser with 308 nm wavelength and 20 ns pulse duration is used to selectively melt the surface layer of the waveguide. This molten layer flows under surface tension and this results in smooth sidewalls upon resolidification. Our experimental results show that this technique reduces the average sidewall roughness (1σ) from 13 to 3 nm. Our calculations show that the waveguide transmission loss due to sidewall roughness in these waveguides would be reduced from 53 to 3 dB cm−1, an improvement with light transmission five orders of magnitude greater. Due to a low viscosity of molten Si (below water), SPEL can be achieved on a Si surface within ~100 ns. This short time, together with SPEL's material selectivity, makes it possible to repair defective components on a chip without damaging surrounding components and materials, making SPEL a promising candidate for defect repair in integrated optics and nanophotonics.

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He Gao

Papers

Fabrication of large area subwavelength antireflection structures on Si using trilayer resist nanoimprint lithography and liftoff

Air cushion press for excellent uniformity, high yield, and fast nanoimprint across a 100 mm field.

In situ real time process characterization in nanoimprint lithography using time-resolved diffractive scatterometry

Method And Apparatus To Apply Surface Release Coating For Imprint Mold

Ultrafast and selective reduction of sidewall roughness in silicon waveguides using self-perfection by liquefaction