Review: Fine Embossing of Novel Glasses for Photonic Integrated Circuits

A B Seddon, D Furniss, W J Pan, P Sewell, A Loni, Y Zhang and T M Benson

Novel Photonic Glasses Research Group, George Green Institute for Electromagnetic Research

University of Nottingham, Nottingham NG7 2RD, UK e-mail: angela.seddon@nottingham.ac.uk

ABSTRACT Hot embossing of novel inorganic-compound glasses is a new fabrication technology for guided wave devices and circuitry. A patterned mould is pressed into the glass above its glass transition temperature (Tg) and replicated; cooling below Tg freezes-in the required pattern. The state-of-the-art is reviewed. Better than 0.1 µm -scale replication is shown for chalcogenide glasses and fabrication of a hot embossed monomode waveguide demonstrated. Keywords: planar waveguides; chalcogenide glasses; embossing; monomode waveguide; rib waveguide; moulding.

1. INTRODUCTION High-silica glasses are known for their excellent linear optical properties and are, for instance, the material of choice for long haul optical fibres. Novel inorganic-compound glasses such as heavy metal halides, oxides and oxyhalides, and chalcogenides, are based on more weakly chemically bonded lattices than silica. These novel glasses are therefore rather less chemically and mechanically robust than silica glass. On the other hand, they offer far more optical versatility than silica glass including: windows spanning the near-ultraviolet to far-infrared; large solubility of active rare earth dopants; low phonon energy for greater efficiency of radiative transitions; large linear and nonlinear refractive indices; high third order nonlinear optical susceptibilities; nano-glass-ceramic formation; great photosensitivity and a large acousto-optic effect. Being glasses means that compositions are not fixed to a single stoichiometry but almost infinitely variable, allowing the possibility of tailoring optical properties to suit, such as for all-optical switching, amplification, sources, sensors, memory, interconnects and electronic integration. Also inorganic compound glasses are generally isotropic hence optical orientation is not a problem compared to crystalline materials. Photonic and electronic integration on a single chip demands innovative, flexible fabrication technologies. Over the last few years we have demonstrated for the first time that hot embossing (Fig. 1) of novel inorganic-compound glasses successfully produces waveguides and other features1-4 to better than 0.1 µm -scale definition3.

Optical Components and Materials V, edited by Michel J. F. Digonnet, Shibin Jiang, John W. Glesener, J. Christopher DriesProc. of SPIE Vol. 6890, 689007, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.760888

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Figure 1: Schematic process of hot embossing of novel inorganic-compound glasses: (a) glass sample and patterned mould; (b) heating the glass sample above the glass transition temperature (Tg) and applying pressure for a prescribed time; (c) removal of pressure and cooling sample to below Tg and (d) finished glass sample with accurate replication of mould pattern in relief. Hot embossing, also called hot-nano-imprinting, -moulding or -micro-contact printing, takes advantage of accessing the supercooled-liquid temperature range above the glass transition temperature (Tg) of the glass to be embossed. A mould with the surface features required for transfer, is pressed into the glass heated above Tg (Fig. 1(b)). The mould pattern is replicated (Fig. 1(c)) and cooling below Tg freezes-in the required pattern (Fig. 1(d)). The control parameters are sample temperature, the pressure applied, and time of application, and the environment in which the process is carried out. Typical mould materials (e.g. stainless steel, silicon) should be harder than the sample and stable at the elevated temperatures required. This paper reviews achievements to date and indicates future directions. 2. HOT EMBOSSING SUB-MICRON SURFACE FEATURES ON BULK GLASSES Chalcogenide glasses have been used to demonstrate the principle of hot embossing inorganic-compound glasses. Ge17As18Se65 and Ge15As15Se17Te53 glasses were prepared from elements of ≥ 5x9s purity. Initial embossing trials were carried out under He using a Perkin Elmer Thermomechanical Analyser (TMA)2. Moulds of uncoated GaAs, silica-coated GaAs or silicon (Fig. 2(a)), were trialled; each comprised a series of etched channels 10 mm long, 1 µm to 10 µm wide in steps of 0.5 µm with a centre-line separation of adjacent channels of 50 µm. Typical replication was better than 0.2 µm3. Limitations of using the TMA were sample diameter (≤7 mm) and load (≤10 N). More accurate mould replication was achieved with larger loads (≥ 100 N) using a purpose-built press that allowed sample chamber evacuation. Figure 2(c) shows scanning electron micrographs (SEMs) of a rib replicated in the surface of Ge15As15Se17Te53 glass using the 5 µm channel of the Si mould (Fig. 2(a)). The top surface of the embossed rib was smooth and flat and the rib walls were almost vertical. The small ridge defect seen running along the rib wall (Fig. 2(c)) was replicated faithfully from a similar defect inside the silicon mould channel. Figure 4(a) shows SEMs of hemispherical holes of 300 nm diameter in a silicon-on-insulator mould, reproduced in diameter to better than 0.05 µm in relief as regular surface hemispherical protrusions (Fig. 4(b)) in the embossed Ge15As15Se17Te53 glass.

Glass

Mould

Release

Embossed sample

a) b)

c) d)

Pressure & heat

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(a) 5 µm bar (b) 50 µm bar

(c) 5 µm bar Figure 2. Scanning electron micrographs of: (a) one channel of the silicon mould; (b) the series of embossed ribs in Ge15As15Se17Te53 (at%) glass at low magnification and (c) the embossed rib, formed by (a), at higher magnification achieved in the purpose-built press. (a) 500 nm bar

(b) 500 nm bar

Figure 3. Scanning electron micrographs of: a) pattern of holes in the silicon-on-insulator mould and b) embossed surface of Ge15As15Se17Te53 (at%) glass, with accurate replication of the mould pattern (relief hole diameters are within 50 nm) achieved in the purpose-built press. 3. HOT EMBOSSING OF SPUTTERED GLASS FILMS Chalcogenide glass films of a few microns’ depth have been sputtered onto porous-Si-on-Si and GaAs wafer substrates2. A series of ribs 1 µm to 10 µm diameter has been embossed in the surface sputtered layer using the hot embossing technique. For a Ge17As18Se65 glass sputtered layer on porous-Si substrate, the refractive index difference between the glass and substrate was ~ 0.3 to enable waveguiding of the embossed ribs. However, after embossing, 100 µm-scale cracking of the glass layer was observed, thought due to thermal expansion mismatch, which prevented rib waveguiding. A 4 µm film of Ge15As15Se17Te53 sputtered onto an uncoated GaAs wafer substrate was successfully embossed using a SiO2-coated GaAs mould. However, the substrate refractive index was greater than that of the film, thus precluding normal rib waveguiding. We are at present investigating embossing sputtered glass films, for film/substrate pairs which are expansion-matched and designed to permit rib waveguiding, including double sputtered films of novel inorganic-compound glasses.

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(a)

(b)

Figure 4: Reflection optical micrographs of: (a) a silica-coated GaAs mould comprising channels 10 mm long, 1 µm to 10 µm wide in steps of 0.5 µm with centre-line separation of adjacent channels: 50 µm and (b) a 4 µm film of Ge15As15Se17Te53 (at%) sputtered onto an uncoated GaAs wafer substrate then successfully embossed using the silica-coated GaAs mould shown in (a). 4. ONE-STEP HOT EMBOSSING OF MONOMODE WAVEGUIDES A new one-step method of fabricating a monomode waveguide via embossing has been demonstrated4. As40Se60 glass fibre (50 µm diameter) drawn in-house was placed on top of a Si mould (comprising channels: 10 mm long, 1-10 µm width, in 0.5 µm increments, whose centre-lines were separated by 50 µm (Fig. 5)). The fibre was aligned along a mould channel. A Ge17As18Se65 substrate was carefully placed flat face down on top of the fibre-mould assembly. Pressing was carried out using the purpose-built press at a temperature selected to be above Tg of the As40Se60 glass, yet below Tg of the Ge17As18Se65 glass. During embossing the As40Se60 fibre spread to infill proximate mould channels, forming ribs on the flat surface of the Ge17As18Se65 substrate (Fig. 6). Several of the ribs exhibited monomode propagation at 1.55 µm wavelength (Fig. 7) as predicted by wave simulation5.

Figure 5. Scanning electron micrograph of the silicon mould comprising a series of channels 1-10 µm diameter, increasing in 0.5 µm increments. The centre-lines of adjacent channels were separated by 50 µm.

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Figure 6. Scanning electron micrograph of top view of As40Se60 (at%) ribs on the Ge17As18Se65 substrate together with a cross-sectional view at higher magnification. Sample prepared with silicon mould in Fig. 5.

Figure 7. Near-field intensity profile observed at 1.55 µm for a As40Se60 (at%) rib on the Ge17As18Se65 substrate (see fig. 6). 5. CONCLUSIONS Hot embossing of novel inorganic-compound glasses has been reviewed. Better than 0.1 µm -scale replication has been achieved and a proof-of-concept monomode optical waveguide successfully produced. Due to its relative simplicity it is likely that hot embossing of glass-based matrices offers an extremely promising route for producing high-resolution, guided-wave optical components and circuitry at low-cost, high-volume, and for a wide wavelength range. Acknowledgments: We gratefully acknowledge funding from the University of Nottingham in the form of an Overseas Research Student Award and funding from the Photonics and Electronics Interdisciplinary Centre for WeiJian Pan. 6. REFERENCES 1. Benson TM, Vukovic A, Sewell P, Loni A, Zhang Y, Pan WJ, Zhang D, O’Donnell MD, Lousteau, J, Furniss D, Seddon AB, ‘Novel glass compositions and fabrication technologies for photonic integrated circuits’, Proceedings 7th International Conference on Transparent Optical Networks,

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volume 1, pp296-299, Paper Tu B2.2, Barcelona, Catalonia, Spain, July 3-7 2005. IEEE Catalogue number of the Proceedings: 05EX1100. 2. Pan WJ, Furniss D, Rowe H, Miller CA, Loni A, Sewell P, Benson TM and Seddon AB,., ‘Fine embossing of chalcogenide glasses - A new fabrication route for photonic integrated circuits’, J. Non-Cryst. Solids 352 2515-2520 (2006). 3. Pan WJ, Furniss D, Rowe H, Miller CA, Loni A, Sewell P, Benson TM and Seddon AB. ‘Fine embossing of chalcogenide glasses – First time submicron definition of surface embossed features’, J Non-Cryst Solids 353 (13-15) 1302 – 1306 (2007).

4. Pan WJ, Rowe H, Zhang D, Zhang Y, Loni A, Furniss D, Benson TM and Seddon AB. ‘One-step hot embossing of optical rib waveguides in chalcogenide glasses’, Submitted to Electronic Letters 2007. 5. Soref R, Schmidtchen J, and Petermann K, ‘Large single-mode rib waveguides in GeSi-Si and Si-on SiO ’, J. Quantum. Electron 27 1971-1974 (1991).

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