1. Introduction

Periodic fine structures generate several optical functions such as phase control [1], antireflection [2], and strong plasmon coupling [3]. These components are fabricated using a combination of lithography and dry etching. Recent outstanding progress of nanoimprinting technology is applicable for the formation of fine structures on resin surfaces [4–6]. However, the thermal and chemical reliabilities of resin are sometimes insufficiently fine for precision optics. Several reports of the relevant literature have described the direct fabrication of fine structures upon highly reliable oxide glasses using nanoimprinting [7–12]. However, severe imprinting conditions at high temperatures and pressures reduce the life expectancy of the mold, and consequently increase production costs. One solution for such problems is the DC voltage application to the electrically conductive mold during the imprinting process, which is designated as electrical nanoimprinting (ENI) [13,14]. A similar process was used for the formation of the optically active surface causing second harmonic generation [15–17]. Two benefits of ENI are low temperature and low pressure. Fine pattern transfer proceeds at temperatures lower than the glass transition temperature (T_g) if the electrical contact between glass and mold is guaranteed under appropriate pressure and DC voltage, which is expected to extend the mold lifetime. To date, several fine pattern formations were reported on some oxide glasses containing alkali ions [18,19] or silver ions [20]. Furthermore, transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDS) revealed the deficient pattern formation of network modifier cations on the soda-lime silicate glass after the ENI [21]. Such a deficient area was removed preferentially in the KOH solution, resulting in the enhancement of aspect ratio of the fine pattern. Therefore, the pattern shape after etching is closely dependent on the deficient area of network modifier cations, i.e., the local glass composition.

Conventional nanoimprinting processes including ENI are used for the rigorous transfer of the mold fine pattern to the glass surface. Recently, however, an extraordinary formation of fine patterns was found on a soda-aluminosilicate glass after ENI and the subsequent chemical etching, which was significantly different from the mold pattern. The formation could not be explained based on the migration of network modifier cation in the anode side surface to the cathode direction. This paper reports the mechanism of fine pattern formation on the electrically imprinted soda-aluminosilicate glass.

2. Experimental

A commercially available glass molding machine (GMP-211(V); Toshiba Machine Co. Ltd.) was modified for the application of DC voltage to the mold. Figure 1 shows a setup image. Using photolithography and dry etching, a one-dimensional grating mold of 700 nm period was fabricated on a SiO₂ plate of 25 × 25 × 2 mm. After platinum coating of 500 nm thickness, the mold was fixed by the upper WC mold holder. A glass plate of 10 × 10 × 1 mm was placed on the lower cathode stage. Glasses used for this study were a soda-aluminosilicate glass (NAS) and a soda-lime silicate glass (NCS) of 10 × 10 × 1 mm, with respective compositions of 23Na₂O-23Al₂O₃-54SiO₂ (T_g = 780°C) and 12Na₂O-1K₂O-6MgO-9CaO-1Al₂O₃-72SiO₂ (T_g = 550°C) in mol%. The mold was contacted to the glass plate under pressure of 3 MPa at 450°C in a N₂ atmosphere. During the pressurization period, DC voltage of 100–400 V was applied to the upper mold for 60–90 s using a DC power supply (Series EH; Glassman High Voltage Inc.). The imprinted glass surface was observed using a scanning probe microscope (Nanocute; SII Nano Technology Inc.) before and after etching in a 55% KOH solution at 70°C.

 

Fig. 1 Schematic of electrical nanoimprinting setup.

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The compositional distribution below the anode side glass surface before etching was analyzed using a TEM (JEM-2010F; JEOL Ltd.) with an EDS (Vantage, Silicon (Li) Detector, 30 mm²; Thermo Fisher Scientific Inc.) and a low-temperature holder (636 Double Tilt Liquid Nitrogen Cooling Holder; Gatan Inc.). The carbon-coated specimen was observed at 113 K or 298 K under 200 kV acceleration voltage with spot size of 1.5 nm diameter and current density of 150 pA/nm² under conditions of 1500 nm² acquisition area, dwell time of 0.1 ms, and accumulation number of 300 times.

3. Results

Figure 2 presents surface relief patterns before and after chemical etching for NCS and NAS after ENI under identical conditions. The mold pattern was imprinted faithfully on both glasses. The groove of the grating on the as-imprinted NAS was twice as deep as that of NCS. The formation of deeper dimples on NAS is attributable to the higher concentration and higher mobility of Na⁺ than those of NCS. Furthermore, the non-existence of less-mobile alkali-earth cations is a favorable factor for easy pattern transfer on NAS. The groove depth of NCS increased by selective etching of the deficient area of network modifier cations, which agreed well with our earlier reported result on elemental mapping using TEM-EDS [21]. Selective etching proceeded in the deficient area of network modifier cations. However, the etching behavior of NAS grating differed from that of NCS. No groove depth enhancement was observed from prolonged etching. Then, as shown in Fig. 3, the cross-sectional composition of the NAS grating was analyzed using TEM-EDS. The Na⁺ deficient layer of 350-nm thick was formed below the anode side surface. However, no compositional periodicity was recognized, even though clear periodicity was found in the soda-lime silicate glass after ENI [21]. Therefore, one must consider a different formation mechanism for the NAS grating.

 

Fig. 2 AFM views before and after chemical etching for (a) NCS and (b) NAS after ENI at 450°C and 100 V in applied voltage for 90 s and 3 MPa in pressure.

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Fig. 3 TEM-EDS analysis of NAS grating cross section after ENI: (a) STEM image, (b)−(d) EDS mappings of Na, Al, and Si, respectively.

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Figure 4 presents changes of cross-sectional AFM profiles for the NCS and NAS gratings depending on the etching time. Both gratings were fabricated at 450°C and 100 V in applied voltage for 90 s and 3 MPa in pressure. As the wet etching advances, the different surface profiles gradually appeared between the NCS and NAS. The NCS grating groove depth increased monotonously, maintaining a concavo-convex shape. However, it is noteworthy that the top area of the NAS grating ridge was etched preferentially. By continuing the etching, the ridge top disappeared. Finally the concavo-convex shape was reversed. As shown in Fig. 3, such a reverse pattern was formed in the uniformly alkali-ion deficient layer.

 

Fig. 4 Etching time dependence of cross-sectional AFM profiles for (a) NCS grating and (b) NAS grating.

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4. Discussion

The etching rates both of pristine and electrically imprinted glass surfaces in the KOH solution are indispensable to ascertain the depth direction information of the grating formation. Figure 5 presents the etching time dependence of NCS and NAS glass surfaces before and after ENI using a flat Pt coated SiO₂ mold. Half of the glass was immersed in the KOH solution for a definite time period. The step profile appearing at the boundary region was measured using a microfigure measuring instrument (Surfcorder ET200; Kosaka Laboratory Ltd.). The etching rates before and after ENI are presented in the figure. The etching rates of the deficient layers of network modifier ions were higher than the pristine glasses by 4.8 times for NCS and 3.3 times for NAS. The grating position from the glass surface can be estimated using these values. Figure 6 presents schematic depth information of the gratings that appeared by etching. The sinusoidal NCS grating is located approximately 100 nm below the surface after etching for 50 min. For this study, we assumed that the etching rate of the ridge surface of NCS grating is comparable to that of the pristine glass surface. In contrast, as for the NAS, the rectangular grating opposite to the initial concavo–convex shape was formed in the bottom area of the Na⁺ deficient layer of 300-nm thickness after the same etching condition. A distinctive point during the etching of NAS grating is the formation of wedgewise grooves at the center portion of the grating ridge. The depth and width of wedgewise grooves increased rapidly over the etching time between 10 and 30 min. Finally, the grating with the flat top ridge and reversed concavo-convex shape was formed after 50 min etching. Further etching gradually decreased the aspect ratio of the grating, which suggests progress of isotropic etching. Therefore, anisotropic etching occurred only in the compositionally homogeneous Na⁺ deficient layer.

 

Fig. 5 Etching rates of glass surfaces before and after ENI at 450°C, 100 V in applied voltage for 90 s and 3 MPa pressure using a Pt-coated flat mold without a fine pattern: (a) NCS and (b) NAS.

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Fig. 6 Schematics of depth position for gratings appeared by etching for (a) NCS and (b) NAS after ENI. The NCS concentration profile was referred from Ref [21].

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A plausible origin of the selective etching observed on the imprinted NAS is the intensive tension accumulated in the grating ridge. Figure 7 portrays the anode side surface profile of NAS before and after ENI using a Pt-coated flat mold without a fine pattern. The surface profile was measured using a stylus method (Surfcorder ET200; Kosaka Laboratory Ltd.). After ENI, a concave curvature of 25-nm depth was observed on the anode side surface of NAS. Such curvature suggests tensile stress generated in the Na⁺ deficient layer. In the case of the periodic pattern formation using ENI, the tension is expected to be localized at the grating ridge. Figure 8 presents an exemplary formation image of tension localized in the grating ridge. At the initial stage, the Na⁺ migration occurs below the mold contacted area. Secondly, the Na⁺ deficient layer is collapsed by the mold ridge under imprinting pressure ( = 3 MPa). Finally the Na⁺ in the non-contacted area migrates to the cathode side. The Na⁺ migration in the non-contacted area might occur by the radial electric field distribution originated on the mold ridge. Evidence of such electric field distribution is apparent at the bottom of Na⁺ deficient layer observed using TEM-EDS mapping (see Fig. 3(c)). The Na⁺ concentration profile is waved slightly, identically to the mold ridge period, which suggests a delay of Na⁺ migration caused at the NAS grating ridge center. The Na⁺ migration and subsequent collapse of Al₂O₃-SiO₂ layer proceed at 450°C, which is 330°C lower than T_g. Therefore, the permanent tension remained in the center line portion of the NAS grating ridge because of ENI. Stress corrosion is well known to occur preferentially at crack tips because a tension is localized around the crack tip [22]. Similary, the localized tension, which is induced by ENI, is probably the origin of stress corrosion. Therefore, we infer that the wedgewise groove that appeared at the beginning stage of etching is attributable to stress corrosion, which is formed not only in the grating ridge of 40-nm height, but also in the Na⁺ deficient layer of 350-nm thickness. No wedgewise groove was observed when the grating period was greater than 3 μm, which means that the smaller period is preferred for the fabrication of grating on NAS using the ENI. Similar tension might be generated in the NCS grating ridge. However, no wedgewise groove was observed because the NCS grating ridge is formed by a chemically durable non-alkali oxide glass phase, which protects selective etching accelerated by stress corrosion.

 

Fig. 7 Anode side surface profile of NAS before and after ENI at 450°C, with 100 V applied voltage for 90 s and 3 MPa pressure using a Pt-coated flat mold without a fine pattern.

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Fig. 8 Formation image of localized tension at grating ridge during ENI: (a) initial stage of Na⁺ migration, (b) collapse of mold ridge to NAS followed by second stage of Na⁺ migration, (c) generation of tension in grating ridge center. The red arrow and red pasted area respectively show schematic stress vector and localized tension induced by Na⁺ migration and collapse.

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Finally, we examined the ENI to NCS and NAS using a two-dimensional grating mold of 700-nm period. Figure 9 portrays AFM views of the mold and the NCS and NAS surfaces. The ENI and etching conditions are shown in the figure. The mold pattern was imprinted faithfully on both glasses accompanied with the migration of network modifier cations. The bottom of the dimple for NCS was etched preferentially in the KOH solution. Finally, a Moth Eye structure of 150-nm height appeared after etching for 150 min. In contrast, a complicated surface relief pattern was obtained for NAS in the early etching stage. The structure height was as high as 55 nm, which decreased gradually by continuing the etching. The formation of this complicated pattern is explainable using the same mechanism as that in the case of one-dimensional grating. A model of the pattern formation is presented in Fig. 9(d). After the ENI, the tension is localized around the two-dimensionally arrayed dimples. Especially, the highest tension is expected to be located in the center area surrounded by four dimples and the area between two dimples, resulting in the formation of a wedgewise dimple in both areas by rapid etching affected by stress corrosion. Therefore, our tension model well explains the extraordinary formation not only of one-dimensional but also of two-dimensional patterns on NAS by the ENI. Other possibility will be a restructuring of the glasses after the Na⁺ migration. Furhter investigation will be required to discuss the selective etching mechanism.

 

Fig. 9 Surface images of (a) two-dimensional mold, the imprinted surfaces of (b) NCS and (c) NAS before and after etching for 150 min and 30 min, respectively, and (d) formation image of two-dimensional stress on the imprinted NAS surface.

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5. Conclusion

One-dimensional surface relief gratings were fabricated on a soda-aluminosilicate glass (NAS) using electrical nanoimprinting (ENI), followed by chemical etching using a KOH solution. The mold grating pattern was transferred faithfully onto the NAS after ENI. However, the concavo-convex periodicity of the grating was reversed during chemical etching. The TEM-EDS analysis exemplified the formation of a 350-nm-thick homogeneous Al₂O₃-SiO₂ layer beneath the anode side surface after Na⁺ migration to the cathode direction. The formation of a wedgewise groove in the center of the grating ridge during etching, which reached the bottom of the Na⁺ deficient layer, suggests stress corrosion along tension generated during Na⁺ migration to the cathode direction. Such stress corrosion also explains the complicated surface relief pattern by etching of two-dimensional gratings formed on NAS using ENI.

Fabrication of a surface relief grating with a rectangular ridge is difficult on oxide glasses using the thermal nanoimprinting process because of the high glass viscosity at the process temperature. The alkali metal ion migration induces tension in the center of the grating ridge during electrical imprinting far below the glass transition temperature, which will open another fabrication mode of periodic structure optics in the subwavelength and resonance domains.