Nanoimprint lithography (NIL) is an advanced nanotechnology technology that can create patterns and structures smaller than 10 nm with low cost, high throughput and high precision.
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NIL is currently used to manufacture data storage components, optoelectronic devices, nanophotonics, optical components, biosensors and advanced semiconductor devices. For device manufacturers, defect control and avoidance are two of the most important challenges that can help improve product quality and yield.
Photolithography is the most widely used nano-pattern method in the semiconductor industry. However, since the characteristic dimensions of the electronic components fall 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 that can create patterns below 10 nm in a more accessible, cheaper and faster way.
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 found in the optical lithographic techniques.
How does NIL achieve resolution at the nanometer level?
The NIL method is based on the deformation of the resist layer using a template (made of quartz or silicon) engraved with nanometrical patterns that are transferred. The resist material can be either thermoplastic or UV-curable polymer. Depending on the resist material used, the two main NIL processes are the thermomechanical NIL (commonly called NIL) and UV-NIL. In NIL, the resist layer is deposited on a substrate that is heated above the glass transition temperature of the resistor. The template is brought into contact with the molten resistor under a certain pressure and partially presses and deforms the resist layer. After lowering the resist temperature during its glass transition, the template and the substrate are separated with the embossed resist layer.
Alternatively, a liquid UV curable polymer can be used as a resist, which is exposed to UV light after the template is brought into contact with the resist coated substrate. After curing of the resist, the template is released from the substrate.
In both cases, the direct contact between the template and the resist nanopattern prints (or replicates) without the need for expensive light sources and collimating optics required by the photolithographic methods. Using mechanical contact instead of light for pattern transmission also means that extremely high resolution can be achieved, which overcomes the limitations set by light diffraction or radiation scattering found in photolithography. This simplifies the process and can reduce the end product's manufacturing cost.
Important defects in the NIL process
At the same time, the NIL process poses new challenges. The direct pattern transfer requires very high quality templates to ensure high quality pattern replication. The viscoelastic deformation of the resistor requires careful consideration of the topography of the template and the substrate and their chemical and mechanical properties. The interaction between the two materials affects the behavior of the resistance deformation and its separation from the template, which affects the pattern quality and throughput. Although recent developments have overcome most of the challenges, NIL pattern defects are still one of the industry's biggest obstacles to a wider use of the NIL process.
In the NIL process, the defects can be divided into randomly distributed and repeated. Randomly distributed defects cannot be repeated in terms of location, quantity and occurrence. These may be due to foreign particles or air bubbles in the resistor, incomplete contact between template and substrate and uneven residual resistance after separation. The repeated defects are usually related to defects in the template and the substrate.
How are the defects created?
The presence of a foreign particle which prevents the contact between the template and the resist layer creates a defect area which is much larger than the particle itself. Such defects include the particle, a cavity surrounding the particle and an area incompletely filled with the resist.
The dimension of the defect depends on the particle size and shape, the stiffness of the substrate and the template, the applied pressure and the properties of the resistance. Dispensing the liquid resistance of UV-NIL also involves risks of gas bubbles getting stuck between the template and the substrate. Thereafter, the bubbles can create defects such as those originating from foreign particles.
Another type of void defect occurs when the template and substrate are not completely flat and cone-shaped. This can cause local excess or lack of resistance, leading to an incomplete pattern transfer. In addition, the increased adhesion between the template and the resistors can result in incomplete separation of the template, thus affecting the quality of the transferred pattern.
Inspect and eliminate defects
Unlike the photolithographic process, where the photomask functions are usually four times larger than the patterned functions, NIL is a direct transfer process (functions on the template have the same dimensions as the final pattern) that require high-resolution inspection tools to assess the templates and replicated patterns.
Defect inspection is an indispensable part of all industrial lithographic processes. Establishing an effective inspection methodology is crucial to understanding the mechanisms for 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-throughput electron beam inspection systems.
The insights from the inspection methods for surface characterization enabled the researcher to develop effective strategies for minimizing and eliminating defects. Design of new micro- and nanofluidic systems that minimize ambient gas dissolution in the resist during dispensing and embossing of the resist significantly reduces the number and size of the bubble-related defects.
Interferometric measurements of the deformation of the template during the process can optimize the contact pressure in real time to achieve almost perfect conformal contact between the template and the substrate. The development of low viscosity resist together with low surface energy coatings for the templates optimizes the adhesion between the template and the resistors, which improves the quality of the transferred pattern and increases the life of the template.
Developing strategies to eliminate imprint defects paves the way for a broader use of NIL for mass production of new nano-devices.
References and further reading
D. Li et al., (2017) A nanofluidics study on nanoscale gas bubble defects in dispensation-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). Nanoimprint lithograph. I (Ed.), Lithography. IntechOpen. Available on: https://doi.org/10.5772/8189
Chen, L. et al.(2005) Defect control in nanoimprint lithography. J. Vac. Sci. Technol. B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 23, 2933-2938 (2005). Available on: https://doi.org/10.1116/1.2130352
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