Nanoimprint lithography (NIL) is an advanced nanofabrication technique capable of creating patterns and structures smaller than 10 nm with low cost, high throughput and high precision.
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Currently, NIL is used to fabricate components for data storage, optoelectronic devices, nanophotonics, optical components, biosensors and high-performance semiconductor devices. For device manufacturers, managing and avoiding defects are two of the most important challenges that can help improve product quality and yield.
Photolithography is the most widely used method for creating nanopatterns in the semiconductor industry. However, as the characteristic dimensions of the electronic components drop below 10 nm, the photolithography process becomes exponentially more complex and expensive. Over the past two decades, many research and development efforts have focused on exploring alternative nanolithography methods capable of creating sub-10 nm patterns in a more accessible, cheaper and faster manner.
Unlike the optical lithographic techniques that create nanostructures through the interaction of photons or electrons with a thin polymer layer (called resist), NIL relies on direct mechanical deformation of the resist. As a result, the method can achieve resolutions beyond the diffraction limits encountered in optical lithography techniques.
How does NIL achieve nanometer-level resolution?
The NIL method is based on the deformation of the resist layer using a template (made of quartz or silicon) engraved with the nanopattern being transferred. The resist material may be either thermoplastic or UV curable polymer. Depending on the resist material used, the two main NIL processes are the thermomechanical NIL (usually simply referred to as NIL) and the UV-NIL. In NIL, the resist layer is deposited on a substrate which is heated above the glass transition temperature of the resists. The stencil is brought into contact with the molten resist under some pressure and partially squeezes and deforms the resist layer. After lowering the resist temperature below the glass transition, the reticle and the substrate containing the relief resist layer are separated.
Alternatively, a liquid UV curable polymer can be used as a resist, which is exposed to UV light after the template has been contacted with the resist coated substrate. After curing of the resist, the template is detached from the substrate.
In either case, the direct contact between the template and the resist prints (or replicates) the nanopattern without the need for expensive light sources and collimating optics required by the photolithography methods. In addition, the use of mechanical contact instead of light for pattern transfer means that extremely high resolution can be achieved, overcoming the limitations of light diffraction or beam scattering encountered in photolithography. This simplifies the process and can reduce the manufacturing costs of the final product.
Main flaws in the NIL process
At the same time, the NIL process brings new challenges. The direct pattern transfer requires very high quality templates to ensure highly reliable pattern replication. The viscoelastic deformation of the resist requires careful consideration of the topography of the template and substrate and their chemical and mechanical properties. The interaction of the two materials affects the deformation behavior of the resist and the separation of the template, affecting pattern quality and throughput. While recent developments have overcome most of the challenges, NIL pattern defects remain one of the industry’s biggest barriers to wider adoption of the NIL process.
In the NIL process, the defects can be divided into randomly distributed and repeated. Randomly distributed defects are not repeatable in terms of location, quantity and occurrence. These can be due to foreign particles or air bubbles in the resist, incomplete contact between template and substrate, and non-uniform residual resist after separation. The repeated defects are usually related to imperfections of the template and substrate.
How do the defects arise?
The presence of a foreign particle that prevents contact between the template and the resist layer creates a defect area much larger than the particle itself. Such a defect includes the particle, a void surrounding the particle, and an area incompletely filled by the resist.
The size of the defect depends on the particle size and shape, the stiffness of the substrate and template, the pressure applied and the properties of the resist. Dosing the liquid resist for UV-NIL also carries the risk of gas bubbles becoming trapped between the template and the substrate. Then the bubbles can cause defects, such as those of foreign particles.
Another type of void defect occurs when the template and substrate are not perfectly flat and conforming. This can cause local excess or deficiency of the resist leading to incomplete pattern transfer. In addition, the increased adhesion between the template and the resists can result in incomplete separation of the template, compromising the quality of the transferred pattern.
Inspecting and rectifying defects
Unlike the photolithography process, where the photomask features are usually four times larger than the pattern features, NIL is a direct transfer process (features on the template are the same dimensions as the final pattern) requiring high-resolution inspection tools to assess the templates and the replicated patterns.
Defect inspection is an indispensable part of any industrial lithography process. Establishing an effective inspection methodology is critical to understanding the mechanisms of defect formation. Several inspection methods have been developed based on existing commercial deep UV inspection tools in combination with metrology tools scanning probe microscopy and high-throughput electron beam inspection systems.
The insights provided by the surface characterization inspection methods enabled the scientist to develop effective strategies for minimizing and eliminating defects. Designing novel micro- and nano-fluidic systems that minimize the dissolution of ambient gas in the resist during dispensing and embossing of the resist significantly reduces the number and size of bubble-associated defects.
Interferometric measurements during the template deformation process can optimize the contact pressure in real time to achieve near-perfect conformal contact between the template and the substrate. The development of low-viscosity resists along with low surface energy coatings for the templates optimizes the adhesion between the template and the resists, improving the quality of the transferred pattern and extending the life of the template.
Developing strategies for eliminating imprint defects paves the way for a wider use of NIL for the mass production of new nanodevices.
References and further reading
D. Li et al., (2017) A nanofluidics investigation of nanoscale gas bubble defects in dispenser-based nanoimprint lithography. IEEE 17th International Conference on Nanotechnology (IEEE-NANO)788-791, Available at: https://doi.org/10.1109/NANO.2017.8117426
Lan, H., and Ding, Y., (2010). Nano-imprint Lithography. In (ed.), Lithography. IntechOpen. Available at: https://doi.org/10.5772/8189
Chen, L. et al.(2005) Defect control in nanoimprint lithography. J. Vac. Science. technology. B: Microelectronics and Nanometer Structures Processing, Measurement and Phenomena 23, 2933-2938 (2005). Available at: https://doi.org/10.1116/1.2130352
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