Nanoimprint lithography (NIL) is an advanced nanoscale patterning technology known for its high resolution and throughput at significantly lower cost.1
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Initially, NIL was used for flash-curing polymer precursors and embossing thermoplastic resists. However, as the demand for high-density material architectures in the semiconductor industry grew, new materials and methods for NIL were developed. Today, NIL influences various applications through unique elastic and plastic deformations of diverse materials.2
Process Steps and Workflow
NIL is a top-down nanofabrication method beginning with spin-coating a resin onto a substrate and pressing a rigid mold/stamp against it. The pattern transfers to the resin upon curing by heating or another method specific to the NIL technique. Once the resin hardens, the mold is separated from the substrate after cooling.3
The fundamental steps and elements remain consistent across all NIL processes. Molds are created by replicating an original template using e-beam lithography or photolithography and can be planar or in roll form.1,3 The choice of substrate, resin, and mold varies per the target application, as each material and method offers different functionalities.3
NIL Methods
Thermal Nanoimprint Lithography (T-NIL)
T-NIL uses thermoset or thermoplastic resins cured or melted using a heating-cooling cycle. For thermoplastic resins, the heating temperature is kept slightly above the glass transition temperature during pressing. The mold and substrate are separated after cooling.1 T-NIL generally uses Si, SiO2/Si, and Ni molds and is highly suitable for compostable and recyclable materials.3
Ultraviolet Nanoimprint Lithography (UV-NIL)
UV-NIL utilizes UV-curable resins and UV-transmissive mold materials (such as hard quartz and soft polymer molds). After pressing the mold onto a resin-coated substrate, 365 nm UV light is radiated through it. The mold and substrate can be separated immediately without a cooling period.1
UV-NIL is used for the rapid, low-cost mass production of nanostructured surfaces due to the resin’s low curing time and viscosity.3
Soft Lithography Techniques
Soft lithography techniques, including replica molding, micro-transfer molding, and microcontact printing, produce polymeric structures.4 These techniques rely on surface wetting to pattern substrates using materials in solution form. The materials aggregate and solidify into a film under imprinting conditions.2
Soft lithography can form hierarchical microstructures, overhanging features, and 3D architectures from new-generation polymers. 4
Material Requirements for NIL
Substrate Materials
Depending on the application, NIL employs various substrates such as Si wafers, metal plates, glass, flexible films, and curved surfaces.1 For example, transparent substrates are used for touch sensor panels.
For UV-NIL, the substrate must tolerate UV exposure. In double-sided NIL, an additional mold replaces the substrate.3
Non-planar substrates like soft elastomers are popular in soft lithography.4
Imprint Materials
The properties of imprint materials (resins and polymers) significantly impact the NIL process.
For instance, the resin’s refractive index is crucial in optical device fabrication, the glass transition temperature is important in T-NIL, the linear expansion coefficient governs the molding/demolding process, and the elastic modulus affects mold and resin separation.1 Common thermoplastic resins include polycarbonate (PC), polymethylmethacrylate (PMMA), and polyethylene terephthalate (PET). 3
Alternatively, UV-NIL uses monomers like acryl type, oxetane group, glycidyl and alicyclic epoxy groups, and vinyl ether group.1
Template Materials
Various materials can be used to prepare NIL molds. Silicon is a standard template material due to its scalability. However, the heating process can fracture its crystal structure. Hence, Ni molds are standard in T-NIL, while glass (quartz) molds are common in UV-NIL.
Metals like aluminum are also used for templates due to their mechanical durability. However, they have a short lifetime due to hole clogging by resins.1
Poly(dimethylsiloxane) (PDMS) is the most used mold for large-area NIL. This silicon-based rubber has high hydrophobicity, transparency, toughness, and gas permeability. Its soft nature helps effectively reduce the noncontact area between the mold and substrate. It is used in T-NIL, UV-NIL, as well as soft lithography.1
Soft lithography also employs novel molds like self-assembled liquid droplets, microscale balloons, nature-derived templates, and hierarchically micro-structured surfaces.4
Challenges and Considerations
Different NIL methods and materials face unique challenges. The efficiency and accuracy of the process depend on mold-resin and resin-substrate interactions.
For example, bubble trapping is a common problem due to high surface tension between the mold and substrate. Also, differing linear expansion coefficients of the mold and resin can cause thermal stress, leading to mold bending and sticking during separation.1
In UV-NIL, interference and diffraction from UV sources can induce imprinting errors and pattern defects. These errors can also arise from particulate contaminants between the mold and substrate.1
Overall, UV-NIL is expensive due to the need for specially developed resins, UV-tolerant substrates, and an ultraclean processing environment.3
While soft lithography offers promising microstructures, scaling up production remains challenging. The final product must also be refined for geometric sophistication, fidelity, and heterogeneous/hierarchical composition.4
Recent Advances and Innovations
Recent advances in NIL have developed new methods and materials to address challenges such as high throughput, cost-effectiveness, alignment, defectivity, and mask replication.1
One recent innovation is electrochemical NIL, proposed at the 16th Annual IEEE Conference. This method works directly on a semiconductor wafer without using thermoplastic or photocuring resins. It utilizes spatially confined electrochemical corrosion at metal/semiconductor phase boundaries in the electrolyte solution, fabricating functional micro/nano-structures on Si and GaAs substrates.5
Another recent development, highlighted in PhotoniX, involved using a water-soluble mold made of polyvinyl alcohol (PVA) to fabricate high-aspect-ratio nanostructures. This low-cost, flexible mold allows the direct printing of high-resolution meta-lenses for the visible wavelength regime.6
Applications in Nanotechnology and Beyond
Applications of NIL have become synonymous with the use of micro-and nanopatterns, which perform unique functions in various fields like optical devices and components, electronics, energy (electrolytic membranes in fuel cells and light path control in solar cells), microfluidics, sensors, and medical (bio-process controls and regenerative medicine).3
NIL can also perform innovative tasks like smart material actuation, performance enhancement of filtration membranes, and fabrication of soft plastic memory arrays.2 Polymeric structures made using soft lithography are utilized in technical applications such as sensing, soft robotics, and energy storage.4
Future Directions and Emerging Trends
NIL is replacing conventional photolithography in industrial manufacturing due to its ability to modify materials for adhesion, wetting, optical behavior, cellular development, and ferroelectricity. This expands NIL application to new fields involving fundamental process-structure-property relationships in materials and future electrical devices like memristors.1,2
Using novel methods and materials, the role of NIL in accelerating technological developments will continue to expand. Emerging combinations of different NIL methods promise advancements in optical materials for meta-surfaces, biological materials for cell culture, functional actuation for robotics, microdevices for molecular transport and analytics, and adaptive materials for smart systems.4
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References and Further Reading
1. Miyauchi, A. (2019). Nanoimprinting and its Applications. Jenny Stanford Publishing. ISBN: 9789814800372
2. Cox, LM., Martinez, AM., Blevins, AK., Sowan, N., Ding, Y., Bowman, CN. (2020). Nanoimprint lithography: Emergent materials and methods of actuation. Nano Today. doi.org/10.1016/j.nantod.2019.100838
3. Unno, N., Mäkelä, T. (2023). Thermal Nanoimprint Lithography—A Review of the Process, Mold Fabrication, and Material. Nanomaterials. doi.org/10.3390/nano13142031
4. Rose, MA., Bowen, JJ., Morin, S. A. (2019). Emergent Soft Lithographic Tools for the Fabrication of Functional Polymeric Microstructures. ChemPhysChem. doi.org/10.1002/cphc.201801140
5. Xu, H., Han, L., Du, B., Wang, Y., Ma, Z., Tian, ZQ., Tian, Z. W., Zhan, D. (2021). Electrochemical Nanoimprint Lithography. IEEE 16th International Conference on Nano/Micro Engineered and Molecular Systems (NEMS). doi.org/10.1109/NEMS51815.2021.9451284
6. Choi, H., Kim, J., Kim, W.-J., Seong, J., Park, C.-W., Choi, M., Kim, N.-H., Ha, J., Qiu, C., Rho, J., Lee, H. (2023). Realization of high aspect ratio metalenses by facile nanoimprint lithography using water-soluble stamps. PhotoniX. doi.org/10.1186/s43074-023-00096-
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