An Antireflective Nanostructure Array Fabricated by Nanosilver Colloidal Lithography on a Silicon Substrate
© The Author(s) 2010
Received: 29 March 2010
Accepted: 29 June 2010
Published: 14 July 2010
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© The Author(s) 2010
Received: 29 March 2010
Accepted: 29 June 2010
Published: 14 July 2010
An alternative method is presented for fabricating an antireflective nanostructure array using nanosilver colloidal lithography. Spin coating was used to produce the multilayered silver nanoparticles, which grew by self-assembly and were transformed into randomly distributed nanosilver islands through the thermodynamic action of dewetting and Oswald ripening. The average size and coverage rate of the islands increased with concentration in the range of 50–90 nm and 40–65%, respectively. The nanosilver islands were critically affected by concentration and spin speed. The effects of these two parameters were investigated, after etching and wet removal of nanosilver residues. The reflection nearly disappeared in the ultraviolet wavelength range and was 17% of the reflection of a bare silicon wafer in the visible range.
The demand for an effective fabrication method for a large-area nanostructure array has recently stimulated increased interest and research activities in the fields of optics and optoelectronics, including photovoltaic cells, light-emitting devices, and photo-detectors. Two-dimensional arrays of nanostructures have been reported to enable modulation of both the energy and the path of photons to increase efficiency and sensitivity [1–3] while also providing antireflective properties. The antireflective property improves the visibility of the transparent window, as well as the light extraction or absorption efficiency, by reducing the reflection of incident light and increasing its transmission accordingly. In fact, the reflectivity can be greatly suppressed for a wide spectral bandwidth when a nanostructure array with a subwavelength pitch can make a continuous and monotonous change in the effective refractive index from air to the solid surface [4–6]. Stability problems due to the thermal mismatch in the conventional methods applying multilayered thin films [5, 7] can be improved.
The previous fabrication strategy relied on costly e-beam lithography , whereas molding technologies, such as nanoimprinting, have emerged due to high throughput and cost-effective process capabilities [5, 8–16]. Master patterns required for molding can be fabricated by e-beam , interference lithography [8, 9], anodizing aluminum oxide [10, 11], colloidal nanolithography using polystyrene [12–14], and electron cyclotron resonance (ECR) plasma etching [15, 16]. Although molding technology is favorable for mass production, the high cost of mask preparation and the limited resources for large-area mask patterning frequently restrict its practical applications.
Simpler bottom-up fabrication using a molding stamp with a subwavelength nanostructure array can be achieved through thermal dewetting of a metal film. Sputter-coated metal film is transformed into an isolated random array of metal dots when thermally annealed, which can be used as the etch shadow mask for the following substrate etching to make an antireflective nanostructure array. Thermal dewetting of Pt/Pd alloy film on a Si wafer was previously studied for this purpose . Other previous attempts include thermal dewetting of Ni film on a SiO2 surface , Ag sputtering on a heated surface , and Ag sputtering followed by thermal dewetting on a curved surface . The transformation results from the increased surface energy of the metal film and its subsequent move to a minimum energy state, similar to the principle of Ostwald ripening [21, 22]. This approach has more promise for large-area process applications in view of enhanced uniformity and fewer defects compared to the colloidal lithography commonly with polystyrene nanoparticles [23, 24], which yields more defects and pattern irregularities as the substrate size increases. However, the vacuum deposition and high process temperatures (greater than 300 °C) used to increase the surface energy of the metal can often limit its applications and are unfavorable for polymer-coated substrates.
In this paper, we report an alternative method for fabricating an etch mask with a subwavelength nanostructure array for antireflective applications using nanosilver colloids and relatively low annealing temperatures. As-received nanosilver colloids with diameters of 20–30 nm were agglomerated into isolated nanosilver islands on a wafer-scale silicon substrate in the range of 50–80 nm via a combined mechanism of solvent dewetting and Ostwald ripening using spin coating and substrate heating. Several variables were identified, including nanosilver density, spin coating speed, nanosilver colloid size, annealing temperature, and time, to vary the size and coverage rate of the nanosilver islands. Due to the sufficiently high etch selectivity of silver to silicon, various configurations and aspect ratios of nanostructures could be easily achieved. Pillar-like nanostructures resulted, and their heights varied proportionally with etch time. The reflection rate was reduced below 5%, which is much lower than the 40% of bare silicon in the visible zone. In particular, the reduction effect of reflection was maximized in the ultraviolet (UV) region of ~300 nm with a rate of over 95%.
The antireflective nanostructure array was fabricated via reactive ion etching (Multiplex ICP—STS, Oxford Systems) with the optimized condition of C4F8 (40 sccm) and SF6 (45 sccm), following a wet removal process of nanosilver residues to form a pattern array of nanosilver islands as an etch mask. Various structural configurations of the nanopillar array were flexibly achievable due to good etch selectivity of silver to silicon. Allowing slight isotropic etching condition, the etching condition was optimized to fit into the aspect ratio of one at the present approach, if etching is done for 60 s under the given conditions. Finally, the reflectance from the ultraviolet to the visible region was measured by UV spectroscopy (Model: Varian Cary 5000) at the incidence angle of 7°, located in the OLED center of Seoul National University in Korea. The light source was tungsten–halogen for the visible region and deuterium for the ultraviolet region. The antireflective effects for the two most dominant process conditions, concentration and spin speed, were investigated for nanopillar structures with an aspect ratio close to one.
Quantitative analysis of nanosilver islands in different zone of the deposited wafer
Spin speed (rpm)
Nanosilver concentration (wt%)
The highest antireflection occurred at the higher concentration (5 wt%) and for the extended etch time in Fig. 9b, which indicates that the larger size and higher nanopillar arrays were more effective at reducing the reflection. The minimum reflection rate in Fig. 9b decreased to about 0.7% around a wavelength of 300 nm in the UV region, and 5–6% in the visible region. The antireflection rate, compared to the bare silicon wafer, was approximately 98 and 83% in the UV and visible regions, respectively for the given condition. Hence, the antireflection turned out to be more dominant in the UV region, probably because the achieved average size of the nanosilver islands (50–90 nm in the present experiments) was more suitable to interact with the UV than in the visible light rays. This agrees with the result that the reflection reduction rate varied with the nanosilver concentration and spin speed in Fig. 9. The second primary factor to affect the reflection is the nanostructure height shown in comparison of Fig. 9a, b. The minimum reflection rate in Fig. 9a, where the etched depth is around 50 nm, is about 10% in the visible region for 5 wt% concentration with the doubled antireflection efficiency from in Fig. 9b. Furthermore, the reflection data band for the given conditions apparently gets larger along with the etched depth in Fig. 9b, which is assumingly due to increasing occurrence of lift-off during etching especially for ‘3 wt%-4,000 rpm’, the coating process condition of the smallest size of nanosilver island array.
An additional minor factor to influence the reflection of silicon substrate may be the coverage rate (nanopillar density). In comparison of ‘Ag 5 wt%-4,000 rpm’ with ‘Ag 3 wt%-2,000 rpm’ in Fig. 9b, the anti-reflection effect is greater for ‘Ag 5 wt%-4,000 rpm’ although its average size of nanosilver islands is smaller than ‘Ag 3 wt%-2,000 rpm’. This probably results from the higher coverage rate in ‘Ag 5 wt%-4,000 rpm’ than in ‘Ag 3 wt%-2,000 rpm’. It represents that the density of nanopillar array is higher for ‘Ag 5 wt%-4,000 rpm’, which leads to the larger effect of anti-reflection than ‘Ag 3 wt%-2,000 rpm’. The reversed result in Fig. 9a seems to be due to the unexpected data deviation all within the limited range of tolerable inaccuracy, which always comes up for random self-assembly. Further study would be required to more clearly identify and improve the antireflection efficiency.
We investigated an alternative method to fabricate a wafer-level antireflective nanostructure array using nanosilver colloidal lithography. The combined action of dewetting and Oswald ripening contributed to the transformation of the spin-coated multilayer of nanosilver colloids into randomly distributed nanosilver islands that could be used as an etch mask for antireflective nanostructures. Accordingly, the spin-coated nanosilver colloidal layer began to be dewetted and agglomerate as it gradually solidified from its wet state under various annealing conditions. Finally, the nanosilver islands grew and sintered at their sintering temperature. From five identified process parameters, the average size and coverage rate of the nanosilver islands were affected most critically by the concentration and the spin speed. The as-received colloid size, temperature, and annealing time were less critical parameters. The average size of the resulting nanosilver islands was in the range of 50–100 nm, with a coverage rate of 40–60% and the standard deviation of 20–50%. It may not be at the acceptable level in many applications other than nanostructured optics such as antireflection window in which such randomness is even ideal to cover a wide range of wavelength spectrum.
Compared with sputter-coated metal film, reported previously [17–20], the colloidal form is more readily viscous at relatively lower temperatures. Additionally, the solvent wet state at the beginning of the temperature increase can facilitate dewetting and agglomerating of metal colloids into the isolated islands. Hence, the present approach may provide an improved method for a more effective self-assembled transformation. The wet process and low temperature annealing are advantages of the present process for extended process applications.
Etching through silicon substrates produced various structural profiles and nanopillar heights as a function of etch time. Anisotropic etching was performed to generate a nanopillar profile with nearly right-angled edges. The reflection measurements revealed that the antireflection effect was substantially large in general, and depends on the nanopillar height. The reflection nearly disappeared in the UV wavelength range and was only 17% of that of a bare silicon wafer in the visible range for the condition of the extended etch time and largest nanosilver concentration that turned out to be ideal process theme for this approach. The present data level for the antireflection rate is comparable with those reported in previous publications [17, 19].
The present fabrication method is expected to draw extensive industrial interest for producing large-area nanotemplates in a cost-effective and more accessible manner. Nanosilver island arrays could also be used for other optoelectronic applications to improve performance. For example, the array could be used as a metal dot layer to derive localized surface plasmon resonance (LSPR)-coupled light emission [25, 26]. More in-depth understanding and further investigation of the nanosilver island transformation would improve the uniformity, process stability, and throughput.
This research was supported by a grant (08K1401-00511) from the Center for Nanoscale Mechatronics and Manufacturing, one of the 21st Century Frontier Research Programs, and a Platform Project grant (10033636-2009-11) supported by the Ministry of the Knowledge Economy of Korea.
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