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   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 , anodizing aluminum oxide , colloidal nanolithography using polystyrene [ 12 - 14 ] , and electron cyclotron resonance ( ecr ) plasma etching .
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 .
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 , 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 2030 nm
were agglomerated into isolated nanosilver islands on a wafer - scale silicon substrate in the range of 5080 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 .
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% .
korea ) , which include nanosilver particles in the range of 1030 nm dispersed in a non - polar solvent blend ( xylene , ethylene glycol , and others ; the exact information was confidential to inktech inc . ) , was diluted to 110 wt% using the product - customized thinner .
1 ) . the deposition and transformation process into nanosilver islands was presumed to proceed according to diagram shown in fig .
2 . nanosilver colloidal ink was spin - coated onto a full wafer - scale silicon substrate to form a multilayered nanosilver film ( fig .
2a ) and transformed to an isolated random array of nanosilver islands by thermal annealing during the following steps .
the continuous film is broken off and grows with increasing space between neighboring islands ( fig .
this process is affected by several parameters , including the as - received particle size , type of solvent , concentration , spin speed , annealing temperature , and time . in this work
, the average size and coverage rate ( density ) of the nanosilver islands were analyzed using concentrations from 3 to 7 wt% , and spin speeds in the range of 2,0004,000 rpm with other fixed conditions of annealing temperature ( 250 c ) and time ( 15 min ) .
the initial annealing temperature was set at 180 c , lower than nanosilver sintering temperature ( 250 c ) , to prevent instantaneous sintering of nanosilver before sufficient dewetting occurred .
tem image of as - received silver nanoparticles fabrication schematic of nanosilver islands for an antireflective etch mask 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 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 .
the transformation of multilayered nanosilver film into an isolated random array of nanosilver islands was most likely thermodynamically driven by the combined self - assembly of dewetting and ostwald ripening .
heating the plate made the surface energy of the solvent and nanosilver increase , causing microscopic dewetting of the solvent with tiny nanosilver particles captured inside a dewet droplet . further increasing the plate temperature to 250 c , increased the agglomeration of nanosilver islands due to self - assembly .
finally , the islands were sintered to create a randomly distributed array of nanosilver islands during the extended heating stage . in this process , the surface energy of the applied solvent in the nanosilver ink is the most critical factor controlling the transformation .
3 shows the transformation is completed with the use of a non - polar solvent blend at 3 wt% of nanosilver ( fig .
3a ) , while silver colloids dispersed in isopropyl alcohol ( ipa ) did not complete the transformation ( fig .
3b , c ) since the polar solvent was quite volatile due to the relatively low surface energy compared to non - polar counterparts .
effect of the nanosilver colloidal solvent on the nanosilver island transformation : a 3 wt% in non - polar solvent including xylene , etc .
, b 5 wt% in ipa ( polar solvent ) , c 5 wt% in ipa the effects of nanosilver concentration and spin speed on the formation of nanosilver islands are presented in figs . 4 and 5 , respectively . as the nanosilver concentration increased from 3 to 7 wt% at a spin speed of 4,000 rpm
the nanosilver islands began to form networks with the neighboring nanosilver islands above 7 wt% . increasing
the spin speed reduced the average size of the islands , as shown in fig .
these results imply that the thickness of the as - coated nanosilver multilayer determines the average size of the nanosilver islands proportionally .
nanosilver island size and configuration in terms of ag concentration ( 4,000 rpm applied ) : a 3 wt% , b 5 wt% , c 7 wt% effect of spin speed on the nanosilver deposition for 5 wt% : a 2,000 rpm , b 4,000 rpm a quantitative analysis was performed to more clearly define the effects of the nanosilver islands on the average size and coverage rate in fig . 6 and table 1 .
the average size increased from 58.1 nm for 3 wt% to 62.6 nm for 5 wt% by a rate of 7.7% at a spin speed of 4,000 rpm , whereas the average size increased from 69.5 nm to 79.3 nm by a rate of 14.1% at 2,000 rpm in the central region .
increased rate grows up to be 37.7 , 45.9% at 4,000 and 2,000 rpm , respectively at the outer region as shown in table 1 .
these results suggest the effect of nanosilver concentration gets more critical at lower spin speed , and outer side along with the increased standard deviation , which implies the areal uniformity is limited in the spin - coated substrate . in comparison , on the other hand , the spin speed seems to be more effective to grow up the average size of nanosilver islands .
the increase rates of 19.6 and 26.7% were driven when the spin speed reduces from 4,0002,000 rpm for 3 and 5 wt% , respectively .
for all cases , the standard deviation shows similar tendency to the average size of nanosilver islands with respect to nanosilver concentration and spin speed . it should be noted that in the present investigation , the smaller islands , less than roughly 30 nm , were not counted because they were lifted off during the following substrate etching and did not remain in the final etched nanostructure array . under the given analysis conditions ,
the coverage rate increased in proportion to the nanosilver concentration and spin speed as well .
it was expected that further grown nanosilver islands would expose more open spaces to the substrate as the nanosilver concentration increased .
however , the coverage rate is also directly related to the standard deviation of the island size , which is likely to reduce with increased spin speed because the higher spin speed improves the uniformity of the film .
therefore , the lower spin speed creates a wider spectrum of the island size with an even greater number of smaller islands than were counted in this analysis , even though the lower speed generates a larger average size .
quantitative analysis of nanosilver islands in terms of the concentration and spin speed : a average size , b coverage rate quantitative analysis of nanosilver islands in different zone of the deposited wafer figure 7 shows photographs of both the nanosilver - coated etch mask ( top ) and the etched substrate ( bottom ) . the etch mask colors were nearly the same as the bare wafer , whereas the etched substrate showed a color change to dark with considerably diminished reflection .
figure 8 shows sem images of the etched nanostructure array as a function of etch time ( 40 and 60 s ) for the samples processed at a spin speed of 2,000 rpm and nanosilver concentration of 5 wt% .
the higher nanostructure pillars ( ~110 nm ) were surely obtained as the etch time was extended to 60 s ( fig .
these sem images validate that the smaller islands , less than roughly 30 nm , tends to be lift - off during etching . hence
, the etching under the given condition creates cone - shaped nanopillars for the etch mask of nanosilver islands in the range of roughly 3060 nm , the depth of which is in proportion to the size of nanosilver islands .
this can provide the chances of a continuous and monotonous change in the effective refractive index from air to the solid surface for the reduced light reflections .
full wafer - scale view of the a nanosilver island deposited wafer and b etched wafer antireflective nanostructure array after etching and removal of nanosilver residues for the samples processed at 2,000 rpm and 5 wt% : etch time of a 40 s , b 60 s the reflective properties of the finalized nanostructure arrays on the silicon substrate were measured in the uv  visible region ( fig .
9 ) for the etched samples ( 40 and 60 s ) to investigate the effect of etched depth . compared to the bare silicon wafer , the reflections of the array were greatly reduced depending on the process conditions ( spin speed and concentration ) of the nanosilver island fabrication and etched depth .
reflection rates for the nanostructured samples with a bare silicon wafer as a reference : a the etched for 40 s , b the etched for 60 s 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 56% 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 ( 5090 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 .
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
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 .
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 .
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 50100 nm , with a coverage rate of 4060% and the standard deviation of 2050% .
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 .
the present fabrication method is expected to draw extensive industrial interest for producing large - area nanotemplates in a cost - effective and more accessible manner .
for example , the array could be used as a metal dot layer to derive localized surface plasmon resonance ( lspr)-coupled light emission .
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 .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
this article is distributed under the terms of the creative commons attribution noncommercial license which permits any noncommercial use , distribution , and reproduction in any medium , provided the original author(s ) and source are credited .
 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 5090 nm and 4065% , 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 . 
Introduction
Experimental
Results and Discussion
Conclusion
Acknowledgments
Open Access