10.1117/2.1201603.006382

Nanosphere photolithographyfor sub-100nm featuresHooman Mohseni, Travis Hamilton, and Alireza Bonakdar

Conventional microprocessing materials and tools are used in a novelhigh-throughput fabrication technique.

Microfabrication is commonly achieved via interactions of lightand a photoresist (an optically sensitive material). In thisprocess, the light is shaped into the desired pattern by a maskandprojected onto the photoresist. The entire photoresist sampleis then submerged within a developer, which dissolves theexposed photoresist and leaves the unexposed photoresistintact. Mask patterns can thus be imaged on a substrate. Thisstep—known as photolithography—is crucial in the microfabri-cation process. It allows selective deposition and etching of met-als and semiconductor materials, which are the building blocksof optoelectronics and microelectronics. There is currently anongoing drive to improve photolithography techniques for theproduction of increasingly small geometries and thus meet thedemand for smaller devices in everyday electronics equipment(e.g., phones, laptops, and other wireless devices). Semiconduc-tor manufacturers have responded to this demand by reducingthe minimum feature size in their photolithography systems, butthis has caused an increase in manufacturing and ownershipcosts (see Figure 1).1 Although electron-beam (e-beam) andfocused ion beam lithography are used for sequential fabricationof features at sub-micron sizes in academic settings, both thesemethods are time-consuming, costly, and unsuitable for large ar-eas.

In recent work, nanosphere photolithography (NSP) has beenintroduced as a technique to tackle the pitfalls of e-beam and ionbeam lithography in research environments.2 Moreover, NSPoffers a cost-effective choice for industry-level photonicapplications. In NSP, low-cost microspheres with unusual opti-cal properties are used. Numerical simulations of microspheresthat produce ‘photonic jets’ (regions of pencil-like focused lightthat are significantly more intense than their surroundings) werefirst reported in 2005.3 Much theoretical work has since beenconducted to identify the salient properties of these photonic

Figure 1. Comparison of common lithography techniques. The tech-niques shown include krypton fluoride (KrF), argon fluoride (ArF),ArF immersion (ArFi), extreme UV (EUV), imprint, electron-beam(e-beam) lithography, as well as nanosphere photolithography (NSP).The arrows projecting from nanosphere photolithography indicate thepotential for industry-level throughput and resolution at shorterwavelengths of light. DVD: Digital video disc.

jets. For instance, it has been shown that when the maximumintensity—or focal spot—is positioned near the sphere’s sur-face, it produces a beam waist that is smaller than one-half ofthe wavelength of the incident light.3 When a microsphere isplaced on a photoresist and illuminated, the photonic jet pro-duces a small sliver of exposed photoresist immediately belowthe sphere. By tilting the angle of incidence of the incoming light,multiple independent exposed regions can be produced fromone sphere. The sphere’s total exposed pattern is known as aunit cell. When spheres are applied in a closely packed forma-tion over a photoresist region, identical unit cells are lithograph-ically produced by each sphere. These properties are thus usedin NSP to produce sub-micron feature sizes.

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Figure 2. Scanning electron microscope image of a periodic array of unit cells. Each unit cell consists of different-sized nanopillars, with a minimumsize of about 55nm (see inset). This pattern was produced with a single NSP exposure.

In our work,4 we have demonstrated patterning with NSPby producing periodic unit cells on a photoresist. Each of ourunit cells contains nanopillars or nanoholes (depending on thephotoresist polarity), which can be symmetrically or asymmetri-cally distributed (see Figure 2). With nanopillars, we are able toachieve feature widths that are smaller than 100nm. Our workthus indicates that NSP has the potential to produce sub-100nmfeature sizes at short wavelengths.

The high throughput and low cost of NSP make it an idealtechnique for energy applications with low profit margins (e.g.,the fabrication of solar cells and LED light bulbs). Periodic arraysof nanostructures fabricated on a device surface can increasethe efficiency of both energy-harvesting and energy-transferringdevices. Companies compete to produce highly efficient prod-ucts with periodic nanostructures at scale, but they struggle toachieve this at reasonable manufacturing cost. It is thereforeunsurprising that many research groups are now beginning touse NSP processing to fabricate several types of devices. Forexample, NSP has been used to generate phosphorus-freewhite-light-emitting diodes,5 cadmium telluride nanopillars for

power enhancement in solar cells,6 and metal–insulator–metaldisks that are used to excited dark plasma modes.7

We have also recently used NSP to produce perfect absorbersand emitters in the IR region.8 Our fabricated devices included acircular meta-atom, a full-ring meta-atom, and a crescent(broken ring) meta-atom. We conducted Fourier transform IRexperiments (at incidence angles of 10, 20, and 30◦) on thesedevices and measured their absorption spectra. We also com-pared these experimental results with corresponding simulatedfinite-difference time-domain spectra. The results of this work—illustrated in Figure 3—indicate that the capture and emissionof thermal radiation could have a profound impact on futureenergy-harvesting systems.

According to the Shockley-Queisser limit, single-junctionphotovoltaic cells have a maximum efficiency of 41%.9 Solarthermophotovoltaics first convert the Sun’s energy to heat. Thephotovoltaic then produces electricity from the thermal radia-tion. With our NSP-produced perfect absorbers, we can enable

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Figure 3. Absorption spectra for three NSP-produced devices: (a) a circular meta-atom, (b) a full-ring meta-atom, and (c) a crescent (broken ring)meta-atom. Experimentally derived Fourier transform IR (exprt) and simulated finite-difference time-domain (FDTD) spectra are shown for eachdevice at three different angles of incidence (AOI).

solar thermophotovoltaics to selectively absorb sunlight and toemit this thermal radiation to achieve a much higher efficiency.In addition, as NSP is less expensive than e-beam lithography(see Figure 1), it is an ideal choice for further exploration of nano-structures and their effects on perfect absorbers.

In summary, we have demonstrated that nanosphere pho-tolithography can be used for patterning purposes, by using thistechnique to produce periodic unit cells on a photoresist. In ourwork we have been able to achieve feature sizes smaller than100nm. We have also used NSP to produce devices that are per-fect absorbers and emitters at IR wavelengths. NSP thus has thepotential to shape basic research in photonics and nanotechnol-ogy. This novel fabrication technique can also be used to reduce

the cost of manufacturing future devices for cost-sensitiveapplications, such as energy harvesting and bio-sensing. In ourongoing work, we are using novel methods to further reducethe feature sizes, while increasing the field of exposure of NSP,and we are pushing the exposure wavelength to the deep-UV re-gion. In addition, recent studies have shown that photonic jetscan be produced with different, yet simple, geometries byenhancing the surface modes of curved surfaces. These newdesigns could allow better uniformity and a wider field ofexposure per unit cell. We are also pursuing NSP-based methodsthat could produce 3D nanostructures. All these complementary

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developments provide a highly parallel process that continuesto be pushed to produce smaller and more versatile feature sizesat reasonable cost.

This work is partially supported by National Science Foundation(EECS-1206155 and EECS-1310620) and Army Research Office(W911NF-11-1-0390) awards. Northwestern University sharedfacilities and the Northwestern high-performance computingcenter were used for this work.

Author Information

Hooman Mohseni, Travis Hamilton, and Alireza BonakdarBio-Inspired Sensors and Optoelectronics LaboratoryNorthwestern UniversityEvanston, IL

Hooman Mohseni has been the recipient of numerous awards,including the National Science Foundation Faculty Early CareerDevelopment Program, the Defense Advanced Research ProjectsAgency’s Young Faculty Award, and the Northwestern FacultyHonor Roll. He also serves on the editorial board of severaljournals, has published more than 120 peer-reviewed articlesin major journals, and is a Fellow of SPIE and the OpticalSociety.

References

1. S. V. Sreenivasan, C. Willson, N. Schumaker, and D. J. Resnick, Low-cost nano-structure patterning using step and flash imprint lithography, Proc. SPIE 4608, pp. 187–194, 2002. doi:10.1117/12.4378042. W. Wu, A. Katnelson, O. G. Memis, and H. Mohseni, A deep sub-wavelength processfor the formation of highly uniform arrays of nanoholes and nanopillars, Nanotechnology18, p. 485302, 2007. doi:10.1088/0957-4484/18/48/4853023. X. Li, Z. Chen, A. Taflove, and V. Backman, Optical analysis of nanoparticles viaenhanced backscattering facilitated by 3-D photonic nanojets, Opt. Express 13, pp. 526–533, 2005.4. A. Bonakdar, M. Rezaei, R. L. Brown, V. Fathipour, E. Dexheimer, S. J. Jang, andH. Mohseni, Deep-UV microsphere projection lithography, Opt. Lett. 40, pp. 2537–2540,2015.5. K. Wu, T. Wei, D. Lan, X. Wei, H. Zheng, Y. Chen, H. Lu, et al., Phosphor-freenanopyramid white light-emitting diodes grown on f1011g planes using nanospherical-lens photolithography, Appl. Phys. Lett. 103, p. 241107, 2013. doi:10.1063/1.48401376. Y.-C. Chang, S.-M. Wang, H.-C. Chung, C.-B. Tseng, and S.-H. Chang, Observationof absorption-dominated bonding dark plasmon mode from metal–insulator–metal nanodiskarrays fabricated by nanospherical-lens lithography, ACS Nano 6, pp. 3390–3396, 2012.7. W. P. R. Liyanage, J. S. Wilson, E. C. Kinzel, B. K. Durant, and M. Nath, Fabricationof CdTe nanorod arrays over large area through patterned electrodeposition for efficient solarenergy conversion, Solar Energy Mater. Solar Cells 133, pp. 260–267, 2015.8. A. Bonakdar, M. Rezaei, E. Dexheimer, and H. Mohseni, High-throughput realiza-tion of an infrared selective absorber/emitter by DUV microsphere projection lithography,Nanotechnology 27, p. 035301, 2016. doi:10.1088/0957-4484/27/3/0353019. P. Wurfel, Physics of Solar Cells: From Basic Principles to Advanced Concepts,p. 256, Wiley, 2009.

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