Nano-printing lithography (NIL) is an advanced nanofabrication technique capable of creating patterns and structures smaller than 10 nm with low cost, high performance and high precision.
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Currently, NIL is used to manufacture components for data storage, optoelectronic devices, nanophotonics, optical components, biosensors, and high-end semiconductor devices. For device manufacturers, defect control and avoidance are two of the key challenges that can help improve product quality and performance.
Photolithography is the most widely used nanopattern method in the semiconductor industry. However, as the characteristic dimensions of electronic components drop below 10 nm, the photolithography process becomes exponentially more complex and expensive. In the last two decades, much research and development efforts have been directed at exploring alternative methods of nanolithography capable of creating patterns of less than 10 nm in a more accessible, cheaper, and faster way.
Unlike optical lithographic techniques that create nanostructures by the interaction of photons or electrons with a thin layer of polymer (called resist), NIL is based on the direct mechanical deformation of resistance. As a result, the method can achieve resolutions beyond the diffraction limits found in optical lithographic techniques.
How does NIL achieve a resolution at the nanometer level?
The NIL method is based on the deformation of the resistance layer using a template (quartz or silicon) etched with the nanopatron being transferred. The resistance material can be thermoplastic or UV curable polymer. Depending on the strength material used, the two main NIL processes are thermomechanical NIL (commonly known as NIL) and UV NIL. In NIL, the resistance layer is deposited on a substrate that is heated above the glass transition temperature of the resistors. The mold comes into contact with the melted resistance under a certain pressure and partially squeezes and deforms the resistance layer. After lowering the resistance temperature below its glass transition, the template and the substrate with the embossed resistance layer separate.
Alternatively, a UV-curable liquid polymer can be used as a resistance, which is exposed to UV light after the template comes into contact with the resistance-coated substrate. After curing the resistance, the template is released from the substrate.
In both cases, direct contact between the template and the resistor prints (or replicates) the nanopatron without the need for expensive light sources and collimation optics required by photolithographic methods. In addition, using mechanical contact instead of light for the transfer of patterns means that an extremely high resolution can be achieved, thus overcoming the limitations established by the diffraction of light or the scattering of the beam found in photolithography. . This simplifies the process and can reduce the manufacturing cost of the final product.
Key defects in the NIL process
At the same time, the NIL process poses new challenges. Direct pattern transfer requires very high quality templates to ensure high fidelity pattern replication. The viscoelastic deformation of the strength requires careful consideration of the topography of the template and the substrate and its chemical and mechanical properties. The interaction of the two materials affects the deformation behavior of the resistance and its separation from the template, affecting the quality and performance of the pattern. Although recent developments have overcome most challenges, NIL pattern defects remain one of the industry’s biggest barriers to wider adoption of the NIL process.
In the NIL process, defects can be divided into randomly distributed and repeated. Randomly distributed defects cannot be repeated in terms of location, quantity, and appearance. These can result from foreign particles or air bubbles at resistance, incomplete template-substrate contact, and non-uniform residual resistance after separation. Repeated defects are usually related to imperfections in the template and substrate.
How are defects created?
The presence of a foreign particle that prevents contact between the template and the resistance layer creates a much larger defect area than the particle itself. This defect includes the particle, some void surrounding the particle, and an area incompletely filled by the resistance.
The size of the defect depends on the size and shape of the particle, the stiffness of the substrate and the template, the pressure applied and the properties of the resistance. Dispensing the liquid resistance by UV-NIL also carries the risk of trapping gas bubbles between the template and the substrate. Subsequently, bubbles can create defects such as those resulting from foreign particles.
Another type of hollow defect occurs when the template and substrate are not perfectly flat and conformable. This can lead to a local excess or shortage of resistance, resulting in an incomplete pattern transfer. In addition, increased adhesion between the template and the resistors can lead to incomplete separation of the template, thus affecting the quality of the transferred pattern.
Inspection and elimination of defects
Unlike the photolithographic process, where the characteristics of the photomask are usually four times larger than the characteristics of the pattern, NIL is a direct transfer process (the characteristics of the template have the same dimensions as the final pattern) that requires tools to high-resolution inspection for the evaluation of replicated templates and patterns.
Defect inspection is an indispensable part of any industrial lithographic process. Establishing an effective inspection methodology is critical to understanding the mechanisms of defect formation. Various inspection methods have been developed based on existing commercial deep UV inspection tools combined with metrological tools, scanning probe microscopy, and high-performance electron beam inspection systems.
The knowledge provided by surface characterization inspection methods allowed the scientist to develop effective strategies for minimizing and eliminating defects. The design of new micro and nanofluidic systems that minimize the dissolution of the ambient gas to the resistance during dispensing and the relief relief greatly reduces the number and size of defects associated with the bubble.
In-process interferometric measurements of template deformation can optimize real-time contact pressure to achieve near-perfect conformal contact between the template and the substrate. The development of low viscosity resistors together with low surface energy coatings for insoles optimizes the adhesion between the insole and the resistors, thus improving the quality of the transferred pattern and increasing the shelf life of the insole.
The development of strategies to eliminate fingerprint defects paves the way for the wider use of NIL for the mass manufacture of new nanodevices.
Continue reading: The Challenges Behind the Climbing of Nanomaterials
References and additional reading
D. Li et al., (2017) A nanofluidics study on gas bubble defects at the nanometer scale in dispensing-based nanoprinting lithography. IEEE 17th International Conference on Nanotechnology (IEEE-NANO), 788-791, available at:
Lan, H. and Ding, Y., (2010). Lithographic printing. A (Ed.), Lithography. IntechOpen. Available at:
Chen, L. et al., (2005) Control of defects in the lithography of nanoprints. J. Vac. Science. Tecnol. B: Processing, measurement and phenomena of microelectronic and nanometric structures 23, 2933-2938 (2005). Available at: https://doi.org/10.1116/1.2130352
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