1. Introduction

Imprint lithography has an important role in pattern formation on glass surfaces. The most important imprinting techniques, including hot-embossing, nano-imprint lithography (NIL) and micro-contact printing, have been reviewed elsewhere [1–6]. NIL is a technique based on surface patterning with micro- or nano-featured templates in order to obtain new or enhanced functionalities. NIL can be realised using hard or soft moulds. Hard-mould materials include metals (e.g. Ni), silicon, fused silica, metal-coated hard polymers. The moulds themselves are difficult to make, often involving e-beam lithography and wet-etching processes. Imprinting is usually done on small areas at high loads. In contrast, soft moulds permit multiple and large-scale uniform patterning, and the imprinting is more tolerant to tilt misalignment of mould and sample [7]. Soft moulds can easily be obtained over large areas with good reproducibility and accuracy, for example by casting a liquid monomer precursor onto a master mould (e.g. Si) followed by curing. Currently, a widely used soft-mould material is poly-dimethylsiloxane (PDMS). PDMS moulds have a low surface energy, facilitating easy separation from the silicon master mould after polymerization. PDMS moulds are sufficiently durable to permit many pattern replications. The lateral resolution of soft-imprint lithography is reported to be ~50 nm [8] using commercially available nano-imprinters to apply a PDMS mould to a polymer film.

Chalcogenide glasses are composed of one or more chalcogen elements S, Se and Te that, together with other elements such as Ge, Ga, As and Sb, form stable glasses [9], e.g. binary As-S, As-Se, ternary Ga-La-S, quaternary Ge-Ga-(Sb,As)-S(Se), Ge-As-Se-Te, etc. In recent decades the doping of chalcogenide glasses by rare-earth ions has become important, the main interest lying in the very low phonon energies (300 – 350 cm⁻¹) and high quantum efficiencies of photoluminescence [10] of the doped glasses. Chalcogenide glasses are transparent to longer infrared (IR) wavelengths (up to ~20 μm for telluride glasses) than other glassy systems, e.g. silica (~3 μm) [11], tellurite (~6 μm) [12] or fluoride (~8 μm) [13] glasses (though IR transmittance of all glasses can often be enhanced by ~1 μm using costly purification treatments) (Fig. 1 ). The high IR transparency together with rare-earth doping of chalcogenide glasses have been exploited in many applications including optical lenses, gratings, waveguides or fibres for infrared light delivery, or active devices such as photonic crystals, fibre lasers, amplifiers, light sources and a variety of devices based on non-linear effects (Fig. 2 ) [9,12,14].

 

Fig. 1 Transmittance and long-wavelength absorption edge of commonly used IR materials. Materials groups are denoted by colours, i.e., red – chalcogenide glasses, black – different glassy systems and blue – crystalline IR materials. Thicknesses are 1 – 3 mm. The data were taken from Refs [15–17].

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Fig. 2 Log-log plot of non-linear refractive index n ₂ of different materials demonstrating chalcogenide glasses as highly non-linear optical materials. Redrawn from [18].

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This paper describes a simple, reliable and large-area micro-patterning of As₃S₇ chalcogenide bulk glass using soft-imprint lithography. The work follows previous reports on nano-imprint lithography on chalcogenide glasses realised by hard imprinting, e.g. by hot-embossing [19]. To the best of the authors’ knowledge, the present work shows the first successful soft imprinting of a bulk chalcogenide glass. We have not attempted to probe the limit of lateral resolution of this technique. Several studies have been published on soft patterning applied specifically to chalcogenide thin films. These works focused on the fabrication of devices (e.g. channel and planar waveguides) exploiting chalcogenide film/substrate optical contrast. In contrast, the focus of the present work is on the imprinting of bulk glasses, which have several advantages over thin films: a) their fabrication is easy, with no deposition involved; b) the physico-chemical properties of thin films are very dependent on the deposition technique; c) bulk glasses are more stable, mechanically and thermally; d) a bulk glass serves as its own substrate, eliminating the film−substrate interface; and e) thin films can deteriorate under external stimuli (e.g. intense laser pulses).

2. Experimental

A silicon master mould was fabricated on an n-type silicon wafer using standard photolithography. PDMS was made from an elastomer kit (Sylgard 184, Dow Corning) using a 10:1 mix of base oligomer to curing agent. The mixture was poured over the silicon master mould and degassed in a vacuum chamber for 30 minutes. After curing at room temperature for 12 hours and 60°C for more than 3 hours, the PDMS replica was peeled from the silicon. The As₃S₇ chalcogenide bulk glass was prepared by direct synthesis from 5N pure elements using a standard melt-quenching technique. The bulk glass was cut into slices with parallel faces and polished to optical quality.

Imprinting of the chalcogenide was achieved simply by heating a glass sample laid on the surface of a PDMS mould (Fig. 3 ), without using any nano-imprint facility, alignment, or pressure control. The imprinting pressure arose solely from the sample weight (~1 g) acting on the contact area (~1 cm × 1 cm). The heat treatment had three stages: (i) heating from room temperature to 225°C at 10 K/min; (ii) an isothermal hold for 40 minutes, and (iii) cooling down at 5 K/min. Three patterns were imprinted in a single batch under the same experimental conditions: ribs, channels (in a honeycomb pattern), and an array of holes. This last is referred to as a ‘lens-array’, as an inverse PDMS mould would indeed yield an imprinted lens array. Such an array, with tuning of the refractive index, could focus light in the near infra-red and has considerable potential for exploitation. The surface patterns resulting from this soft imprint lithography were observed by optical (Nikon OPTIPHOT-2 with CCD camera DS-Fi1) and atomic-force (Veeco 3100, tapping mode Si tips with max. radius 12 nm; software WSxM 5.0 develop 1.3 [20]) microscopy.

 

Fig. 3 a) Photographs of the PDMS mould and of its surface patterns: ‘lens-array’, ribs and channels (details of patterns dimensions are given in Table 1). Cross-sectional sketches and photographs of: b) a PDMS mould and a sample of polished As₃S₇ bulk glass; c) the imprinting process, and d) an imprinted pattern (of ribs) on the glass surface.

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3. Results and discussion

We did not observe any increase in a near-IR and a middle-IR absorption in patterned glasses due to enhanced concentration of O−H, S−H or CO₂ after imprinting. In accordance with Ref [14] no contamination of chalcogenide glass due to direct contact with PDMS polymer was observed.

The imprinting was at 225°C, i.e. ~80°C above glass transition temperature T _g = 142°C; T _g /T ≈0.83. The chalcogenide As₃S₇ glass has been chosen for its good rheological properties in the liquid state and good mechanical properties in the glassy state. Its glass transition temperature, ~142°C, is much lower than that of the better known As₂S₃ glass (T _g = 206°C [21]), which does have a good shaping ability since it can easily be drawn into fibres below 300°C [9]. The As₂S₃ liquid has a fragility of m ≈35, similar for example to Na₂O.4SiO₂ in the well known Angell plot, and lies at the transition between brittleness and plasticity while still having very good glass-forming ability [22,23]. For As₃S₇ the dependence of viscosity on temperature above T _g has not been reported, but its greater S content must make it more fluid than As₂S₃. Indeed the viscosity of As₃S₇ is low enough for imprinting at 225°C, which is significant because the PDMS mould itself is thermally stable up to at least 250°C. Chalcogenide glasses generally have a very flexible layered network (strong covalent bonds within the layer and weak van der Waals forces between layers) not found in traditional oxide glasses or other rigid network systems. The network flexibility has been described in several papers on photo-induced plasticity of chalcogenide glasses [24]. It is of interested that imprinting of chalcogenide good-glass formers has been done at ~0.8 T _g/T by either using PDMS mould [this study and 14] or metal masters at modest loads [25]. This shows similar trends in viscosity behaviour of the traditional chalcogenide glasses and their universal capability to be shaped if proper temperature treatment is chosen.

The ‘lens-array’ imprinted on the As₃S₇ surface can be observed by low-magnification optical microscopy (Fig. 4 ), confirming the possibility of large-area imprinting on a bulk chalcogenide glass.

 

Fig. 4 (a) Transmission and (b) dark-field optical micrographs of the ‘lens-array’ imprinted into the surface of the As₃S₇ bulk glass.

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Soft imprinting of the micro-scale features from patterns on the PDMS mould to the chalcogenide glass surface can easily be realised even by applying very low pressures, as shown by AFM images of the ‘lens-array’, ribs and channels (Fig. 5 ). The root-mean-square roughness of the patterned surface is R _q = 3 − 10 nm. Such a smooth surface, with roughness in the nanometre range, is required for the design and fabrication of optical and photonic devices with minimal scattering losses. Very low surface roughness is an inherently important requirement for future sub-micrometre-scale soft imprinting.

 

Fig. 5 AFM topography showing imprinted patterns: (a) ‘lens-array’ (b) rib and (c) channels. In (d), (e) and (f), the coloured lines show the areas where the pattern profiles were measured (Table 1). The width of a rib (e) was taken to be at FWHM. The widths of ‘lens-array’ elements and channels were read below the pile-ups (dashed lines) caused by plastic flow as the glass/liquid is indented.

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The pitches of the imprinted patterns on the As₃S₇ glass surface are, as expected, identical with those on the PDMS mould. The observed depths are, however, significantly lower (Table 1 ), reflecting the limited flow obtained at the low pressure arising only from the sample’s own weight. The ‘lens-array’ and ribs have depths ~0.7 – 0.8 μm, while the channels have a depth of 2 μm. The features which were pushed-up by imprinting, i.e. the ribs, have a smaller width, while the indented ‘lens-array’ and channels have a larger width than the original patterns on the mould. In the present work, somewhat irreproducible depths and widths arise from the lack of external pressure applied to the sample during imprinting. Importantly, however, the width and depth of individual patterns are uniform over the entire patterned area. This is very useful for simple, straightforward imprinting of micro-patterns over a large surface area. Future work on imprinting of nano-patterns will require optimization of applied pressure, heat treatment, and mould/sample alignment. The mechanical properties of the mould itself can be optimized by with suitable additives to the monomer liquid to tune the PDMS stiffness. The combination of PDMS and a chalcogenide glass, however, offers good options for further improvement of the current results and refining of the patterned surface features down to sub-micrometre levels.

Table 1. AFM Measurements (rms error ± 1%) of Patterns Imprinted into the As₃S₇ Bulk Glass

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Although the depths of the imprinted patterns are less than in the original mould, they are entirely adequate to change the effective surface energy of As₃S₇ bulk glass. The surface changed from mildly hydrophilic (polished) to mildly hydrophobic for the imprinted ‘lens-array’ (Fig. 6 ). The value of the contact angle on the polished glass without patterning is similar to that in Ref [26]. The change in value with patterning is consistent with Cassie-Baxter wetting (Fig. 6c) [27]. A surface fraction of air f _air can be calculated using semi-empirical formula cosΘ _{m(patterned surface)} = f _air.cosΘ _air + f _glass.cosΘ_glass, where Θ represents contact angles on different surfaces and f surface fraction of given material. The calculated value of f _air is ≈0.32, which corresponds to AFM image (Fig. 5a) f _air ≈0.34.

 

Fig. 6 Optical images show water on (a) the hydrophilic surface of polished As₃S₇ bulk glass (contact angle, Θ _m ≈62°), (b) the hydrophobic surface of the same glass with an imprinted ‘lens-array’ pattern (Θ _m ≈92°) and (c) schematic drawing of Cassie-Baxter wetting regime; σ _LS is the hard core diameter for liquid-solid interaction [29].

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In this work we have chosen As₃S₇ to demonstrate the suitability of PDMS soft imprinting for micro-patterning of a “compliant” chalcogenide bulk glass. Although As-S glasses have the advantages of a good glass-forming ability and long-term stability against crystallization, it is obvious that they are not the only suitable chalcogenides for soft imprint lithography. Many chalcogenide systems form stable glasses over a broad composition range, and thus it is relatively easy to tailor their softening temperature to the required range. Glasses from other systems such as As-S-Se, As-Se, As-Se-Te, Ge-Se, Ge-Se-Te, Ge-As-Se-Te are also expected to be suitable for soft imprinting. Some of these compositions (e.g. As₄₀Se₆₀, Ge₂₀As₂₀Se₁₄Te₄₆) have been already reported as suitable for hard-imprint hot-embossing [19,28]. In general, some other soft glassy materials of different composition could be micro-patterned by soft imprint lithography, e.g. tellurite or ZBLAN glasses having a variety of rare-earth doped analogues with visible and near-infrared luminescence.

4. Conclusion

Soft imprint lithography using a PDMS mould has been shown as an effective and straightforward method for micro-patterning of the surfaces of bulk chalcogenide glasses. Using commercially available nano-imprinters, soft imprinting of bulk chalcogenide glasses shows great promise for the reproducible fabrication of optical and photonic devices over large surface areas. The high refractive index of chalcogenide glasses could be exploited in the design and fabrication of diffractive optical elements for the infrared region, e.g. relief diffraction gratings for filtering light of selected spectral range or patterning of anti-reflection surfaces as lens arrays with optical properties tuned at any particular wavelength. Further refinement of the patterns down to nanometre scale is expected to be useful in other applications; examples include 2D-photonic crystals or devices based on a photonic crystal architecture, e.g. sensors operating in the visible and infrared spectral range and elements based on an optical non-linearity, surface-enhanced effects, evanescent light generation or plasmonic edge tuning. Soft imprint lithography should enable micro-patterning of chalcogenide glass films leading to optical and photonic waveguide devices where a variety of non-linear effects can be realised. Finally, imprinting of strongly luminescent, rare-earth doped chalcogenide soft glasses could be used in light amplification and conversion.