1. Introduction

Metasurface-based optical elements, consisting of two-dimensional functional structures with micro/nanoscale feature, have become a rapidly growing field of research recently due to their ability to locally manipulate the amplitude, phase and polarization of light with high spatial resolution [1–4]. It has potential applications such as quarter-wave plate, planar lens, optical vortex converter, etc [5–8]. Generally, the metasurface-based optical element consists of a transparent glass and a transparent thin film with micro/nanostructures (e.g. silicon and TiO₂) [6, 9], and the micro/nanostructures are inscribed on the surface of transparent thin films. The feature size of micro/nanostructures is required to be smaller than the light wavelength. The fabrication of micro/nanostructures on transparent thin films is critical for metasurface-based optical elements in visible or infrared range.

There are several critical characteristics and requirements for metasurface-based optical elements, 1) the feature size of minimum structures is submicrometer or even nanoscale; 2) the structures are inscribed on the surface of the transparent thin films; 3) the diameter of metasurface-based optical elements is on the order of centimeters or even meters.

Femtosecond laser direct writing is widely utilized to fabricate micro/nanostructures on transparent materials owing to its simple, flexible, versatile, and relatively low-cost route [10–12]. It is a multi-photon absorption process in transparent materials. Moreover, in machining, thermal diffusion effect is ignored since the duration time is femtosecond scale [12]. However, both the surface roughness of structures and the fabrication speed need to be further improved. So far, electron/ion beam lithography has been widely used for the fabrication of nanoscale structures due to its high accuracy and high resolution. Unfortunately, the need for high-vacuum environment and low speed restricts the techniques to small area patterning. Maskless direct laser writing technique is a good method for micro/nanofabrication due to its operation in air, relatively low cost, large patterning area and high patterning speed [13–16]. It has been widely used to fabricate arbitrary structures on photoresist thin films [15, 17–19]. In the patterning, the photoresist thin film absorbs laser spot energy and generates structural changes. The structural difference between written and unwritten regions leads to the etching selectivity in etching solutions. Therefore, micro/nanostructures are inscribed on the photoresist thin film. However, on one hand, the feature size of micro/nanostructures is determined by the laser spot, and it is difficult to reduce to submicrometer (or even nanoscale) due to optical diffraction limit. On the other hand, it is also difficult to directly fabricate micro/nanostructures on the transparent thin films because the light absorption coefficient of transparent thin films is close to zero and the laser spot energy cannot be absorbed, accordingly. Thus, for the metasurface-based optical elements, the key is how to fast fabricate large-area micro/nanostructures on the transparent thin films.

Phase-transition lithography based on direct laser writing on phase-change thin films can obtain submicrometer (or even nanoscale) micro/nanostructures due to the thermal threshold effect of phase-transition process, accompanying with the release of phase-transition latent-heat. Inspired from this case, we propose micro/nanolithography of transparent thin films through laser-induced release of phase-transition latent-heat. Although phase-change thin films can absorb laser energy and be heated, the heat cannot lead to the structural change of transparent thin film. Fortunately, the released phase-transition latent-heat further heats the transparent thin films and sufficiently leads to the local structural change, and thus the micro/nanopatterns on transparent thin films are obtained. The micro/nanopatterns are further etched in acid (or alkali) solution and the micro/nanostructure-based metasurfaces are finally realized.

2. Physical picture of micro/nanolithography

Laser-induced thermal phase-transition can obtain submicrometer (or even nanoscale) phase-transition region due to the thermal threshold effect of phase-transition process, which has been used as laser thermal lithography and applied to the fabrication of Blu-ray disk of optical data storage. Figure 1(a) gives the physical picture of laser-induced thermal phase-transition. It is worth mentioning that the laser wavelength is 405 nm in laser writing procedure. A collimated laser beam is focused onto the light absorption thin films, where the light absorption thin film is deposited on the glass substrate. The intensity of focused spot is typical Gaussian profile, as shown in left inset of Fig. 1(a). The light absorption thin film absorbs the laser spot energy and is heated to phase-transition threshold temperature. Based on the phase-transition threshold effect, the phase-transition region can be finely tuned through changing laser power and irradiation time. The submicrometer (or even nanoscale) phase-transition region can be realized when one utilizes proper peak intensity to heats the light absorption thin film to phase-transition threshold temperature. Thus submicrometer (or even nanoscale) phase-transition region is obtained at the center of laser spot. Accompanied with nanoscale phase-transition, the phase-transition latent-heat is released. For example, in chalcogenide phase change materials, the phase-transition from amorphous state to crystalline state can release structural heat, since the energy of stable crystalline phase is lower than that of metastable amorphous phase. Thus, the remained energy is released in phase-transition process, as shown in right inset of Fig. 1(a).

 

Fig. 1 Physical picture of micro/nanolithography of transparent thin films through laser-induced release of phase-transition latent-heat. (a) Schematic of laser-induced local phase-transition of the light absorption thin film. The left inset is the spot intensity profile. The right is the change of structural networks, accompanied with the release of latent-heat. (b) The transparent thin film is locally heated via phase-transition latent-heat released by light absorption thin film. (c) Generation of micro/nanopatterns on transparent thin film by spot scanning the sample. (d) The micro/nanopatterns are further changed into micro/nanostructures after wet-etching.

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Inspired from this case, we propose micro/nanolithography of transparent thin films through laser-induced release of phase-transition latent-heat of light absorption layer. The basic physical picture is so follows. Firstly, the transparent thin film is deposited onto the light absorption layer, as shown in Fig. 1(b). Then the laser beam is focused into a diffraction-limited spot (the writing spot) on the light absorption thin film. The writing spot energy is absorbed and heats the light absorption thin film. Since the laser intensity presents a Gaussian profile shown in the left inset of Fig. 1(a), the intensity at spot center is the largest and heats the light absorption thin film to phase-transition threshold temperature. Thus, the region above the threshold temperature generates evident structural changes. The phase-transition latent-heat is released and subsequently released latent-heat heats upper transparent thin film, making its structure changed. Thus, micro/nanopatterns can be written on the transparent thin film as shown in Fig. 1(c), where the feature size of micro/nanopatterns can be tuned to be smaller than the writing spot due to the phase-transition threshold effect of light absorption thin film. Micro/nanopatterns are further etched due to the etching selectivity between written and unwritten regions at acid/ alkali solution. Finally, the large-area micro/nanostructures can be inscribed on the transparent thin films, as shown in Fig. 1(d), where the feature size of micro/nanostructures is smaller than the writing spot due to laser-induced below diffraction-limited phase-transition region.

3. Selection of light absorption layer and pattern materials

In order to realize micro/nanolithography of transparent thin film, a proper light absorption material is required. Chalcogenide phase change material, such as AgInSbTe (AIST), is a promising candidate as the light absorption layer since it possesses a high absorption coefficient of about 10⁷ /m in a wide visible spectral range [20] and evident phase-transition thermal threshold effect [21]. Moreover, chalcogenide phase change material can generate latent-heat after phase-transition from amorphous state to crystalline state and its phase-transition temperature is moderate. Here, the AIST material is selected as a light absorption layer. The absorption coefficient of AIST thin film is 6.32 × 10⁷ /m at 405 nm wavelength and AIST thin film presents single photon absorption characteristics since the band gap of amorphous AIST is 1.42 eV [22]. The thermal analysis of AIST film was performed by differential scanning calorimetry (DSC) measurements (DSC 214 Polyma). Figure 2(a) gives its DSC result to confirm the latent-heat release and thermal threshold effect of phase-transition process. One can see that a sharp exothermal phase-transition (crystallization) peak occurs at about $T_p=206°\text{C}$, indicating the phase transition from amorphous to crystalline state. The area, also as crystallization latent-heat (ΔH), is 45.67 kJ/kg. Therefore, AIST thin film possesses evident thermal threshold effect and can release latent-heat after heating to phase-transition temperature.

 

Fig. 2 DSC curves of (a) AIST thin film and (b) ZnS-SiO2 thin film.

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For the patterning material, transparent ZnS-SiO₂ composite is used as an example due to high transparency in visible and near-infrared light range and low thermal conductivity of 0.67 W/(m·K). The low thermal conductivity can efficiently suppress the thermal diffusion in radial direction and is beneficial for the reduction of pattern linewidth. Another reason is that the etching selectivity of ZnS-SiO₂ thin film decreases as temperature decreases, which was found in our previous experiments. This may be because the structural change of ZnS-SiO₂ thin film is a function of temperature, and does not show any threshold effect. The DSC analysis of the as-deposited ZnS-SiO₂ thin film in Fig. 2(b) also indicates that there is an initial transformation point at 100°C, then the structural change goes on with temperature rise, and a maximum exothermal peak (T_ex) occurs at 220°C. This indicates that the ZnS-SiO₂ thin film has no obvious thermal threshold effect and significant structural rearrangement occurs at $T_{ex}=220°\text{C}$. This also tells us that the ZnS-SiO₂ thin film allows for maximum etching selectivity when the temperature exceeds to 220 °C.

4. Temperature estimation due to latent-heat release

The sample structure is designed as “ZnS-SiO₂/AIST/glass substrate”. In order to understand the effect of the phase-transition latent-heat of AIST thin film on the temperature of ZnS-SiO₂ thin film, the temperature distribution of AIST thin film is firstly determined by heat transfer equation as shown in Fig. 3. Figure 3(a) shows the three-dimensional (3-D) temperature profile. It is noted that the laser power is fixed at 0.66 mW and the irradiation time is 100 ns. Here, AIST layer will undergo thermal diffusion. The thermal diffusion coefficient (D) of AIST films can be determined by,

 

Fig. 3 Temperature profile of AIST thin film. (a) Three-dimensional (3-D) profile, and (b) two-dimensional (2-D) profile.

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Assuming that the phase-transition latent-heat release of AIST thin film is not considered, the temperature of the ZnS-SiO₂ thin film is mainly from the thermal diffusion of AIST layer. Figure 4(a) give the temperature profile of ZnS-SiO₂ layer, where the highest temperature ($T_{max}$) is only 204°C, which is lower than the critical temperature ($T_{ex}=220°\text{C}$). This cannot lead to the sufficient structural change, and it is difficult that the micro/nanopatterns on ZnS-SiO₂ thin film layer are formed.

 

Fig. 4 Temperature profile of ZnS-SiO₂ thin film. (a) Without considering the latent-heat release of AIST, and (b) with considering latent-heat release of AIST.

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Fortunately, the phase-transition latent-heat of AIST thin film can be released when the temperature exceeds to $T_p$. The released latent-heat will further heat the ZnS-SiO₂ thin film and result in the temperature rise of ZnS-SiO₂ thin film and the temperature rise ($\Delta T$) can be calculated by,

The $h_{AIST}$ and $h_{ZnS-SiO2}$ are the thickness of AIST and ZnS-SiO₂ thin film, respectively. In our work, $h_{AIST}=20 nm$ and $h_{ZnS-SiO2}=40 nm$. The $d_{AIST}$ is the diameter of phase-transition region of AIST thin film. $d_{AIST}(=100 nm)$ can be determined by finely tuning the laser power and irradiation time according to Fig. 3b, where the laser power is 0.66 mW and irradiation time is 100 ns. It is noted that the phase-transition region is considered as a cylinder due to the thickness of AIST thin films is only 20 nm. The $d_{ZnS-SiO2}$ are the diameter of structural change region of ZnS-SiO₂ thin film, and $d_{ZnS-SiO2}=d_{AIST}$ according to Fig. 1(b), where the structural change regions is also considered to be cylinder due to the thickness of ZnS-SiO₂ thin film is only 40 nm.

According to formulas (3)-(5), the temperature rise of ZnS-SiO₂ induced by the phase-transition latent-heat of AIST thin film can be roughly estimated to be $\Delta T=59°\text{C}$. Thus, combined with the temperature from thermal diffusion of AIST layer, as shown in Fig. 4(a), the temperature profile of ZnS-SiO₂ thin film is roughly estimated, and Fig. 4(b) gives the results. One can see that the peak temperature of ZnS-SiO₂ thin film can reach up to 262°C, which exceeds to the maximum exothermal peak temperature of $T_{ex}=220℃$. Thus, significant structural rearrangement can occur in ZnS-SiO₂ thin film. The structural rearrangement region is also about 100 nm, which is determined by the phase-transition threshold effect of AIST thin film. That is, the release of phase-transition latent-heat of AIST thin film is sufficient to make ZnS-SiO₂ generate evident local structural change. The micro/nanopatterns on ZnS-SiO₂ thin films can be further etched into micro/nanostructures after wet-etching.

5. Experimental results and analysis of micro/nanolithography

According to above calculation, the phase-transition latent-heat of AIST is crucial for micro/nanolithography of ZnS-SiO₂ thin film. In order to demonstrate that the micro/nanolithography of the ZnS-SiO₂ thin film results from the phase transition latent-heat release of AIST thin film, the samples with the two-layer of “ZnS-SiO₂/AIST/glass substrate” are prepared through magnetron-controlled sputtering method. The thicknesses of ZnS-SiO₂ and AIST thin films are 40 nm and 20 nm, respectively. The direct laser writing lithography onto the samples is conducted through a GaN-diode-based laser writing lithography system [13], where the GaN-diode-based laser device with 405 nm visible light wavelength is used due to its low cost and the laser wavelength of 405 nm is close to the limit of visible light. The writing operation is conducted in air. The converging lens with a numerical aperture (NA) of 0.80 is used to focus the laser beam into a diffraction-limited spot (writing spot) on the sample. The writing spot size is theoretically estimated to be 620 nm based on the diffraction limit formula of D = 1.22λ/NA, where λ is laser wavelength. Actually, the real writing spot size is about 800 nm because the optical system of laser writing lithography system is not perfect. According to our direct laser writing system, the sample size can be as large as 120 mm in diameter. One can achieve large-area patterns by modifying the material using a laser shot-by-shot mode and it takes short fabrication time owing to high-speed movement of the sample. It is worth mentioning that LND Kallepalli et.al utilized large beams and operated laser in single-shot mode. Thus they also could obtain large-area micro-structuring in addition to sub micro-meter feature size as they used microspheres as lenses [25].

In order to further verify the phase-transition process, the crystal structures of written samples are characterized by X-ray diffraction (XRD) data recorded via 18KW-D/ MAX2500V type equipment and optical imaging methods before the micro/nanopatterns are etched in hydrofluoric acid solution. XRD results of AIST thin films are revealed in Fig. 5(a), where the laser power is about 1.15 mW. The XRD result of as-deposited AIST thin film is also shown for comparison and presents an obvious amorphous characteristic. However, the written sample indicates that a sharp diffraction peak occurs and assigns to crystalline phase structure [26], which indicates that the AIST thin film is crystallized due to laser-induced heating. The optical image of the written sample is given in Fig. 5(b). The optical image also reveals obvious crystallization lines after writing. Therefore, the phase-transition process of AIST thin film happens after laser writing.

 

Fig. 5 (a) XRD curves of AIST thin film of as-deposited and laser-written samples, and (b) optical image of the written sample. The writing laser power is 1.15 mW.

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The written samples were all etched for 15 s at 0.55 mol/L hydrofluoric acid solution. Figure 6(a) presents the AFM image with rapidly tuning laser power. The writing speed is 4 m/s and the interval among trench lines is about 1 µm. There are four traces and each of the traces is written with a laser power. From left to right, the laser power is gradually increased and the linewidth changes from 120 nm to 250 nm accordingly. When the laser power is 0.66 mW, the minimum lithographic linewidth can be as low as 120 nm which is a little larger than calculated results of Fig. 4(b). This may be due to radial thermal diffusion effect ignored in the calculation. The linewidth of 120 nm is smaller than the writing spot and only about 1/7 the writing spot size. Figure 6(b) gives the 3-D AFM image, one can see that the pattern edge is steep and the surface is smooth.

 

Fig. 6 (a) 2-D AFM image, (b) 3-D AFM image of lithographic structures with rapidly tuning laser power and writing speed of 4 m/s,. (c) Influence of laser power on lithographic linewidth, where the writing speed is fixed at 1 m/s.

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In order to investigate the influence of laser power on lithographic linewidth, the theoretical calculation is firstly conducted and the results are displayed in Fig. 6(c). It can be seen that the lithographic linewidth increases gradually with the increasement of laser power. It is noted that the writing speed is fixed at 1 m/s. In parallel, experimental results at different laser powers are also given in Fig. 6(c). It can be found that the experimental values are in good agreement with the calculated results.

In order to further observe the surface morphology of micro/nanostructures obtained at different laser powers, Fig. 7 shows the AFM images, where the writing speed is fixed at 1 m/s, and the micro/nanopatterns are all etched for 15s at 0.55 mol/L hydrofluoric acid solution. Figures 7(a) and 7(b) are the 2-D and 3-D images with a laser power of 1.10 mW, respectively. In Fig. 7(a), the structures are uniform and the surface is smooth, the normal pattern linewidth is 200 nm. The 3-D image in Fig. 7(b) indicates that the structure edge is steep. Increasing the laser power to 1.18 mW, Figs. 7(d) and 7(e) display the AFM results, where the surface is smooth and the edge is steep. The line structures are uniform and the linewidth is 280 nm. Further increasing the laser power to 1.2 mW, the structure linewidth reaches to 320 nm, as shown in Figs. 7(e) and 7(f). The line structures still remain uniform and the surface is smooth with a steep edge.

 

Fig. 7 AFM images of micro/nano-structures obtained under different laser powers. Laser power of 1.10 mW (a) 2-D and (b) 3-D morphologies; laser power of 1.18 mW (c) 2-D and (d) 3-D morphologies; laser power of 1.20 mW (e) 2-D and (f) 3-D morphologies.

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The arbitrary micro/nanostructures are also written on ZnS-SiO₂ transparent thin films. For example, the nanorod antennas can be used to achieve anomalous reflection with the same polarization while split-ring structures behave as an effective medium with negative values of permittivity (ɛ) and permeability (µ) at the frequency of interest, i.e., negative refraction [27–30]. Therefore, these structures (shown in Fig. 8) are fabricated, where the laser power is fixed to 2.0 mW and the etching time is 30 s in 0.55 mol/L hydrofluoric acid solution. The structures of patterns were observed using multi-mode atomic force microscopy (AFM, VECEEO Corporation) and scanning electron microscopy (SEM, Zeiss Auriga with focusing ion beam milling system, Carl Zeiss, Germany). Figure 8(a) gives scanning electronic microscopy (SEM) image of nanorod antenna arrays. One can see clear and uniform nanorod antenna structures. The structure period is 8.0 µm. The inset is the AFM image of nanorod antenna unit cell and the feature size reaches 800 nm. Figure 8(b) shows the SEM image of split-ring arrays. The split-ring structures are uniform and the structure period is 25.0 µm. The inset of Fig. 8(b) is the AFM image of split-ring unit cell. The feature linewidth is 1.4 µm.

 

Fig. 8 SEM images of (a) nanorod antennas and (b) split-ring structures fabricated on transparent ZnS-SiO₂ thin films. The inset of (a) is AFM image of nanorod antenna unit cell and inset of (b) is AFM image of split-ring unit cell.

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7. Conclusion

Micro/nanolithography of transparent thin films is realized via laser-induced release of phase-transition latent-heat of light absorption layer. As an example, AIST and ZnS-SiO₂ are chosen as light absorption layer and transparent thin film, respectively. DSC results indicate that phase-transition latent-heat of AIST can be released after heating to. $T_p$ The calculation reveals that the released phase-transition latent-heat can heat the ZnS-SiO₂ thin film and lead to the significant structural change of ZnS-SiO₂ thin film. Moreover, the thermal threshold effect of AIST can reduce the lithographic linewidth of ZnS-SiO₂ thin film to submicrometer or even nanoscale. Using a GaN-diode-based direct laser writing lithography system, the minimum lithography linewidth obtained on the transparent ZnS-SiO₂ thin film can experimentally be as low as 120 nm, which is only about 1/7 the writing spot size. The edge of obtained structure is steep and the surface is smooth. Arbitrary structures have also been fabricated on the ZnS-SiO₂ transparent thin films. The laser-induced release of phase-transition latent-heat is a promising pathway to micro/nanolithography of transparent thin films, and has potential application in the fabrication of metasurface-based optical element.