Nanomoulding with amorphous metals

Vol 457 | 12 February 2009 | doi:10.1038/nature07718

LETTERS Nanomoulding with amorphous metals Golden Kumar1, Hong X. Tang1 & Jan Schroers1

1

producing metal nanostructures13. However, electroplating of metals suffers from limited material selection and stringent process conditions and results in non-uniform mechanical properties imposed by the grain size, non-uniform deposition at sharp edges and recessed areas, surface roughness14 and undesirable residual stresses15. Metallic glasses, prepared by vitrifying molten alloys, have properties that are unusual when compared to conventional metals. Bulk metallic glasses (BMGs) are a subset of glass forming alloys that can be easily vitrified and formed into relatively large (.1 mm) amorphous sections5,6. The absence of crystallites, grain boundaries and dislocations in the amorphous structure of bulk metallic glass results in a homogeneous and isotropic material down to the atomic scale, which displays very high strength, hardness, elastic strain limit and corrosion resistance16. In addition, the temperature-dependent mechanical behaviour of BMGs is unique among metals17 (Fig. 1). Recently developed BMGs with high formability, such as Pt57.5Cu14.7Ni5.3P22.5 (ref. 18; Pt-BMG), Au49Ag5.5Pd2.3Cu26.9Si16.3 (ref. 19; Au-BMG) and Zr44Ti11Cu10Ni10Be25 (ref. 20; Zr-BMG) now offer strengths spanning Silicon, quartz, alumina... 1,000

Ideal mould materials

nc-Nickel

100 Strength (MPa)

Nanoimprinting promises low-cost fabrication of micro- and nanodevices by embossing features from a hard mould onto thermoplastic materials, typically polymers with low glass transition temperature1. The success and proliferation of such methods critically rely on the manufacturing of robust and durable master moulds2. Silicon-based moulds are brittle3 and have limited longevity4. Metal moulds are stronger than semiconductors, but patterning of metals on the nanometre scale is limited by their finite grain size. Amorphous metals (metallic glasses) exhibit superior mechanical properties and are intrinsically free from grain size limitations. Here we demonstrate direct nanopatterning of metallic glasses by hot embossing, generating feature sizes as small as 13 nm. After subsequently crystallizing the as-formed metallic glass mould, we show that another amorphous sample of the same alloy can be formed on the crystallized mould. In addition, metallic glass replicas can also be used as moulds for polymers or other metallic glasses with lower softening temperatures. Using this ‘spawning’ process, we can massively replicate patterned surfaces through direct moulding without using conventional lithography. We anticipate that our findings will catalyse the development of micro- and nanoscale metallic glass applications that capitalize on the outstanding mechanical properties, microstructural homogeneity and isotropy, and ease of thermoplastic forming exhibited by these materials5–7. Imprinting, embossing and moulding on the submicrometre scale are well-established techniques for manufacturing CDs (compact disks), DVDs (digital versatile disks), diffraction gratings, and polymer parts8,9. However, tools for imprinting on the nanometre scale (,100 nm) have only recently entered commercial production10, and such efforts hinge on the availability of durable low-cost moulds2 and suitable imprint resists. Candidate mould materials include essentially any type of solid material that can be precisely patterned and which exhibits sufficient strength and stability under embossing conditions. In conventional approaches, materials used for moulds and imprints exhibit separate and distinct temperature-dependent mechanical properties. The most commonly used mould materials are silicon and quartz2,11, which retain their high strength over a wide temperature range and can be lithographically patterned. Thermoplastic polymers comprise the majority of current imprint materials. They can be easily deformed when heated above their glass transition temperature, Tg, and readily conform to a nanoscale pattern on the master mould2. Figure 1 compares the range of mould and imprint materials, organized by the temperature-dependent strength of each. The commonly used thermoplastic, PMMA (poly(methylmethacrylate)), decreases significantly in strength from 55 MPa at 21 uC to 0.01 MPa above 160 uC (ref. 12), becoming mouldable at low pressures under typical deformation rates used for embossing. Metals, despite their attractive mechanical properties, have not been widely used for micro- and nanoscale moulding because of difficulties associated with patterning them on a length scale smaller than their grain size. Metal patterning processes, such as micromachining, self-assembly, microcutting and etching, have not yet advanced to the level of silicon lithographic techniques. Electroforming into patterned moulds is capable of

10

1

0.1

PMMA

Zr-BMG Au-BMG Pt-BMG

0.01 Ideal imprint materials 0

100

200

300

400

500

Temperature (°C)

Figure 1 | Temperature-dependent strength of materials used for moulds and imprints for micro/nanoimprinting. Silicon, quartz and alumina exhibit strengths above 1,000 MPa, making them suitable for use as mould materials. The strength of nanocrystalline (nc)-nickel decreases from 1,100 MPa at room temperature to 100 MPa at 350 uC and hence limits its use to low embossing temperatures30. PMMA is a common imprint material due to its low forming pressure at temperatures above its Tg. The strengths of bulk metallic glasses (Au-BMG, Pt-BMG, Zr-BMG) cover a wide range from ideal mould to imprint material. They are high strength materials below their respective Tg, yet soft and mouldable in the supercooled liquid region. The strength (s) of BMGs above Tg is calculated from their viscosity values by assuming Newtonian behaviour (s~3g_e) and a strain rate (_e) of 1022 s21. The strength of BMGs increases rapidly upon crystallization, as marked by the red curves.

Mechanical Engineering, Yale University, New Haven, Connecticut 06511, USA.

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LETTERS

NATURE | Vol 457 | 12 February 2009

  32g l 2 4c cos h P~ { t d d

a

(θ = 180°)

500

l/d = 3

400

t = 60 s 300 200 100

η = 107 Pa s θ = 90°

Anti-wetting θ > 90°

–100

Wetting θ < 90°

Desired moulding region

–200 –300

(θ = 0°)

–400 –500 10

20

30

50 60 40 Diameter, d (nm)

b

c

d = 13 nm

d = 35 nm

70

80

90

100

500 nm

d

e

d = 35 nm

d = 55 nm

50 µm 500 nm

2 mm

γ = 1 N m–1

0

500 nm

500 µm

ð1Þ

Here g is the viscosity, c is the metallic glass–vacuum interfacial energy (,1 N m21), h is the dynamic contact angle between the supercooled liquid and the mould, t is the filling time and P is the required pressure. The second term arises from the capillary pressure, which becomes comparable to viscous pressure for d , 1 mm and becomes the controlling factor when d approaches 100 nm. Figure 3a shows the pressure required to fill nanometre-sized (d , 100 nm) channels with aspect ratios of three for anti-wetting

Pressure, P (MPa)

the whole range (Fig. 1) from thermoplastics to hard moulds; some BMGs even exhibit yield strengths exceeding 5,000 MPa (ref. 21). Hence, metallic glasses can be considered high-strength materials that have the processibility of plastics in the supercooled liquid region above Tg for strain rates of the order of 1022 s21 (ref. 7). This is remarkably different from conventional metals, which must be heated above their melting temperatures to achieve similar processing capability. The unique softening behaviour of metallic glasses is a consequence of the crossover between the intrinsic relaxation time for flow, given by the Maxwell relaxation time, and the experimental timescale, given by the applied strain rate. The softening of metallic glasses was originally observed22 in 1968, and first used23 to reshape a metallic glass in 1978. Since then, this unique feature has been used for a wide range of processes, including net-shape fabrication24,25, surface embossing26, blow moulding27 and writing–erasing28. As shown in Fig. 2, recent advances in the microforming of metallic glasses have led to a successful fabrication of parts ranging in size from several millimetres to tens of micrometres. These free standing three-dimensional components (tweezers, scalpels, a gear and a membrane)—fabricated by hot embossing the PtBMG on a silicon mould, followed by a subsequent hot cutting process— show the capability of metallic glass microforming to precisely replicate features with flat edges and acute angles over a wide range of length scales. With polymer resists, the ultimate feature size is fundamentally limited by the length of the resist molecule, often about 2 nm (ref. 29). Metallic glasses are free from such internal size limiting factors, suggesting the ability to replicate atomic-level features. To date, however, moulding of metallic glasses has only demonstrated the replication of low-aspect-ratio structures (,1–3) when the lateral dimensions become smaller than 1 mm (ref. 26). The direct moulding of high-aspect-ratio structures below 100 nm has been challenging, even for BMGs with low supercooled liquid viscosities26. We recognized that this moulding regime is controlled by the capillary forces, not by the viscosity. The strong capillary forces can result in high moulding pressures that prevent the moulding of metallic glasses when the mould dimensions approach 100 nm or smaller. Here we have modified the Hagen–Poiseuille’s law by combining the viscous and capillary contributions for an accurate description of the moulding process on nanometre length scales. This determines the required pressure to flow a supercooled liquid bulk metallic glass into a channel of diameter d and length l:

500 µm

Figure 2 | Optical and scanning electron microscope images of threedimensional microparts, including tweezers (top left), scalpels (bottom left), a gear (top right) and a membrane (bottom right). These parts were fabricated by hot embossing the Pt-BMG on an etched silicon wafer, followed by hot cutting. The precise replication of sharp edges indicates that the supercooled liquid of metallic glass readily flows into constricted areas of the mould.

500 nm

Figure 3 | Controlling the metallic glass moulding on scales smaller than 100 nm. a, Moulding pressure required to fill a channel of diameter d and an aspect ratio of 3 with a supercooled liquid for both anti-wetting (h . 90) and wetting (h , 90) conditions. The values used for viscosity, surface tension and filling time are 107 Pa s, 1 N m21 and 60 s, respectively. For complete antiwetting (h 5 180), the required pressure becomes enormously high due to opposing capillary pressures. A completely wetting (h 5 0) supercooled liquid spontaneously fills the mould features by capillary action. However, the partially wetting supercooled liquid permits better control over the process, and the optimal range of forming parameters is represented by the section labelled ‘desired moulding region’. b–e, SEM images of Pt-BMG rods formed by embossing on porous alumina. The glassy rods of diameters 13 nm (b) and 35 nm (c) with aspect ratio exceeding 50 are formed when the Pt-BMG is heated through its supercooled liquid region (230–290 uC) under an applied pressure of 130 MPa. The process can be well controlled; glassy rods with aspect ratios of 5 fabricated by isothermal embossing at 275 uC for 60 s are depicted in d and e. 869

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NATURE | Vol 457 | 12 February 2009

(h . 90u) and wetting (h , 90u) conditions between the supercooled liquid and the mould. For anti-wetting, the applied pressure must overcome the capillary pressure, which alone can be as high as 400 MPa for a channel of 10 nm diameter and complete anti-wetting (h 5 180u) conditions. In the case of complete wetting (h 5 0u), the supercooled liquid spontaneously fills the mould cavities, making the moulding process uncontrollable. As marked in Fig. 3a, the desired region for a controllable moulding process requires partially wetting behaviour. Therefore, the ideal properties of a metallic glass used for nanomoulding include: the ability of the supercooled liquid to partially wet the mould, access to low viscosity in the supercooled liquid region, and sluggish crystallization at the moulding temperature. Inability to meet these stringent requirements held back the development of nanomoulding of metallic glasses in the past. Our recently developed Pt-based and Au-based metallic glasses exhibit the unique ability to partially wet typical mould materials like silicon, alumina and nickel, in addition to their low viscosities in the supercooled liquid. As we demonstrate here, this ideal combination of desirable wetting and access to low viscosity makes metallic glass nanomoulding a feasible and precisely controllable process. To demonstrate nanomoulding of metallic glasses with favourable wetting properties, the Pt-BMG was heated through its supercooled liquid region (230–290 uC) under an applied pressure of 130 MPa on porous alumina with pore diameters of 13 nm and 35 nm. Figure 3b and c shows scanning electron microscope (SEM) images of metallic glass nanorods that remained after dissolving the alumina in a KOH

solution. During the drying process, the glassy rods settle into bundles, making it difficult to examine individual rods. However, the aspect ratio of each glassy rod exceeds 50, which would require an infeasible pressure of more than 4,000 MPa for an anti-wetting supercooled liquid with the same viscosity of 107 Pa s. It is important to understand from equation (1) that the moulding process benefits from lower viscosities only in terms of aspect ratios, whereas the smallest mouldable size is dictated by the capillary forces. The formation of high-aspect-ratio structures with sub-50-nm lateral dimensions shown in Fig. 3 are enabled by the ability of Pt-BMG to wet the mould combined with its low viscosity (,106 Pa s) in the supercooled liquid state. The smallest moulded feature size is 13 nm, limited by the minimum alumina pore size, but extrapolation of our data using equation (1) suggests that features below 10 nm could be replicated with a suitable mould. The length of the moulded features can be controlled by varying the experimental parameters according to equation (1). Figure 3d and e shows SEM images of glassy nanorods with diameters of 35 nm and 55 nm fabricated by isothermal processing of Pt-based BMG on porous alumina at 275 uC for 60 s. The uniform dimensions of the moulded features suggest that nanomoulding of metallic glasses is an eminently controllable process. The ability to replicate high-aspect-ratio nanostructures with a high strength material by direct embossing makes BMGs unique materials for nanoimprinting. Metallic glass compositions exhibit a wide variation in glass transition temperature: 128 uC for Au49Ag5.5Pd2.3Cu26.9Si16.3 (ref. 19), 230 uC for Pt57.5Cu14.7Ni5.3P22.5 Au-BMG

c

Using the BMG1 as a mould for BMG2 with lower Tg 200 µm

BMG1 Embossing BMG on a mould at T > Tg

Releasing BMG

BMG1

BMG1

BMG2

BMG2

Crystallized BMG1

BMG1

Ni, Si or Al2O3 Ni

a

Pt-BMG

200 µm

b

200 µm

e

500 nm

Figure 4 | Schematic and experimental illustration of a processing technique based on the unique softening behaviour of BMG. First, BMG1 is embossed on a mould fabricated by conventional techniques. The mould can be any suitable material such as Ni, Si or alumina, and an SEM image of a nickel mould used here is shown in a (note that only images are labelled a, b and so on). The mould and BMG1 are separated, leaving a negative pattern of the mould imprinted on BMG1. b, An example of pattern transfer on Pt-BMG after embossing on the Ni mould. The patterned BMG1 can be used as a mould to imprint on a lower-Tg metallic glass, BMG2. This step is demonstrated by using patterned Pt-BMG to imprint on Au-BMG

Pt-BMG BMG1 Crystallizing the BMG1 and using it as a mould for the same BMG1

d

200 µm

f

g

500 nm

500 nm

(c). Alternatively, the crystallized BMG1 pattern can even be used as a mould for another amorphous sample of BMG1. The SEM image of a pattern transferred on Pt-BMG (d) from the crystallized Pt-BMG validates this concept. The process described here demonstrates the replication of 250mm-sized features from a Ni mould into Pt-BMG and Au-BMG moulds. However, it can be extended to nanometre scale. e, SEM image of a Pt-BMG mould with 55-nm-diameter rods fabricated by embossing on porous alumina. f, Holes imprinted into PMMA using the Pt-BMG mould. g, The Pt-BMG mould, after crystallization, imprints holes into the same Pt-BMG, as shown by the SEM image.

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NATURE | Vol 457 | 12 February 2009

(ref. 18), 350 uC for Zr44Ti11Cu10Ni10Be25 (ref. 20), and 637 uC for Co43Fe20Ta5.5B31.5 (ref. 21). The broad range of softening temperatures exhibited by metallic glasses can serve as a unique toolbox for nanomoulding by employing a metallic glass with higher Tg as a mould to imprint another metallic glass with lower Tg. This sequential use of alloys with different softening temperatures can be used to replicate multiple moulds, effectively extending the original mould lifetime. The procedure is schematically illustrated along with SEM images in Fig. 4. First, a BMG (described henceforth as ‘BMG1’ for clarity) is hot embossed on a patterned mould (Si, Ni or alumina) prepared via lithographic or non-lithographic methods. The SEM image of the electrodeposited Ni mould used in the present example is shown in Fig. 4a. After being released from the mould, BMG1 displays inverted replicas of the mould features, shown in an SEM image of Pt-BMG separated from the Ni mould after embossing at 270 uC (Fig. 4b). Subsequently, the patterned BMG1 can be used as a mould to imprint features on polymers or another BMG (BMG2) with lower Tg. This is demonstrated by using the patterned Pt-BMG to imprint features on Au-BMG (Fig. 4c) at 160 uC. Because the strength of Pt-BMG is significantly higher than Au-BMG at 160 uC (Fig. 1), the former can be used as a mould for the latter. Alternatively, the patterned BMG1 can be crystallized and used as a mould to imprint features on a separate sample of amorphous BMG1. Figure 4d shows an SEM micrograph of the pattern transferred onto Pt-BMG using a crystallized mould of the same alloy. The key advantage of using a crystallized mould to imprint on the same metallic glass is that the mould and the imprint are essentially of the same material with different structures, which ensures desired wetting behaviour. This allows the moulding processes to be further extended to the nanometre length scales, which otherwise were restricted to the viscosity controlled regime (d . 100 nm) owing to mounting capillary forces. Figure 4e shows an SEM image of Pt-BMG nanorods with 55 nm diameters fabricated by embossing on porous alumina as described earlier. The Pt-BMG substrate with nanorods was pressed into a PMMA sheet at 160 uC. Figure 4f shows the result: PMMA imprinted with 55-nm holes. Note that the image quality in Fig. 4f is limited by electron beam damage to the PMMA during SEM scanning. Finally, the Pt-BMG substrate with nanorods was crystallized and subsequently used as a mould for an amorphous sample of the same alloy. Figure 4g shows an SEM image of 55-nm-diameter holes imprinted in Pt-BMG using the crystallized Pt-BMG mould. The results summarized in Figs 3 and 4 demonstrate the ability of metallic glasses to precisely replicate mould features ranging from 250 mm to 13 nm. The two attractive features of metallic glass moulding—the ability to replicate features from tens of micrometres down to 13 nm and the possibility of using BMGs as nanotemplates—stem directly from the unique softening behaviour and homogeneous structure of metallic glasses. The processes described here establish the capability of metallic glasses for moulding on the micrometre to nanometre scale through the use of a simple embossing technique that takes advantage of the low viscosity and favourable wetting properties of the supercooled liquid. The superior properties of metallic glasses on the nanoscale, together with the ability to form high-aspect-ratio nanostructures, suggest wide-ranging uses in different areas. Besides direct use in nanoimprinting, another potential application for nanomoulding of metallic glasses lies in the field of re-writable data storage. Our recently developed metallic glasses exhibit low viscosity in the supercooled liquid region, and thus surface features can be ‘erased’ under the action of surface tension alone when annealed above Tg (ref. 28). The thermal stability of the alloy is sufficient to allow thousands of write and erase cycles before the metallic glass crystallizes. Such a process also presents a unique opportunity to repair a damaged mould28. The nanomoulding process can be extended to metallic glass thin films, which can be directly patterned to serve as masks for photolithography. This could evade several complex steps, such as electron-beam

writing and etching, involved in conventional chrome masks. Furthermore, the biocompatibility of metallic glasses and their unique ability to control structure on multiple length scales can be used to control cellular responses by fabricating medical implants with patterned surfaces. Finally, experimental access to extremely small structures should enable the study of fundamental aspects of plastic deformation in metallic glasses, such as size effects and shear transformation zones. METHODS SUMMARY The preparation of metallic glasses used here is described in detail in previous papers18,19. The parameters for thermoplastic forming experiments were selected on the basis of the crystallization and viscosity data of the metallic glasses. Three different types of moulds (Si, Ni and Al2O3) were used in this work. Si-based moulds were fabricated by conventional photolithographic and etching techniques. Ni-based moulds were fabricated by electroplating using the LIGA technique. Commercially available nano-porous Al2O3 was used as template for nanomoulding. For the moulding of very high aspect ratio nanostructures, the metallic glass was thermoplastically formed from Tg to Tx 2 10 uC (where Tx is the onset of crystallization) under a constant load. Controlled metallic glass nanostructures for sequential nanoimprinting were generated by isothermal moulding. The metallic glasses were released from the moulds either by etching with KOH solution (for Si and Al2O3 moulds) or mechanical separation (for Ni moulds). The hot-cutting process was used for making free-standing threedimensional BMG parts. Full Methods and any associated references are available in the online version of the paper at www.nature.com/nature. Received 1 June; accepted 27 November 2008. 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20.

21.

22.

Chou, S. Y., Krauss, P. R. & Renstrom, P. J. Imprint lithography with 25-nanometer resolution. Science 272, 85–87 (1996). Guo, L. J. Nanoimprint lithography: Methods and material requirements. Adv. Mater. 19, 495–513 (2007). Barbero, D. R. et al. High resolution nanoimprinting with a robust and reusable polymer mold. Adv. Funct. Mater. 17, 2419–2425 (2007). Zankovych, S., Hoffmann, T., Seekamp, J., Bruch, J. U. & Torres, C. M. S. Nanoimprint lithography: Challenges and prospects. Nanotechnology 12, 91–95 (2001). Greer, A. L. Metallic glasses. Science 267, 1947–1953 (1995). Johnson, W. L. Bulk glass-forming metallic alloys: Science and technology. MRS Bull. 24, 42–56 (1999). Schroers, J. & Paton, N. Amorphous metal alloys form like plastics. Adv. Mater. Process. 164, 61–63 (2006). Gates, B. D. et al. New approaches to nanofabrication: Molding, printing, and other techniques. Chem. Rev. 105, 1171–1196 (2005). Heckele, M. & Schomburg, W. K. Review on micro molding of thermoplastic polymers. J. Micromech. Microeng. 14, R1–R14 (2004). Chou, S. Y., Krauss, P. R. & Zhang, W. Sub-10 nm imprint lithography and applications. J. Vac. Sci. Technol. B 15, 2897–2904 (1997). Chou, S. Y., Keimel, C. & Gu, J. Ultrafast and direct imprint of nanostructures in silicon. Nature 417, 835–837 (2002). Palm, G., Dupaix, R. B. & Castro, J. Large strain mechanical behavior of poly(methyl methacrylate) (PMMA) near the glass transition temperature. Trans. Am. Soc. Mech. Eng. 128, 559–563 (2006). Hirai, Y., Harada, S. & Isaka, S. Nano-imprint lithography using replicated mold by Ni electroforming. Jpn. J. Appl. Phys. 1 41, 4186–4189 (2002). Haatainen, T., Majander, P., Riekkinen, T. & Ahopelto, J. Nickel stamp fabrication using step & stamp imprint lithography. Microelectron. Eng. 83, 948–950 (2006). Basrour, S. & Robert, L. X-ray characterization of residual stresses in electroplated nickel used in LIGA technique. Mater. Sci. Eng. A 288, 270–274 (2000). Schuh, C. A., Hufnagel, T. C. & Ramamurty, U. Overview No.144 – Mechanical behavior of amorphous alloys. Acta Mater. 55, 4067–4109 (2007). Spaepen, F. Microscopic mechanism for steady-state inhomogeneous flow in metallic glasses. Acta Metall. 25, 407–415 (1977). Schroers, J. & Johnson, W. L. Ductile bulk metallic glass. Phys. Rev. Lett. 93, 255506 (2004). Schroers, J., Lohwongwatana, B., Johnson, W. L. & Peker, A. Gold based bulk metallic glass. Appl. Phys. Lett. 87, 061912 (2005). Waniuk, T., Schroers, J. & Johnson, W. L. Timescales of crystallization and viscous flow of the bulk glass-forming Zr-Ti-Ni-Cu-Be alloys. Phys. Rev. B 67, 184003 (2003). Inoue, A., Shen, B. L., Koshiba, H., Kato, H. & Yavari, A. R. Cobalt-based bulk glassy alloy with ultrahigh strength and soft magnetic properties. Nature Mater. 2, 661–663 (2003). Chen, H. S. & Turnbull, D. Evidence of a glass-liquid transition in a goldgermanium-silicon alloy. J. Chem. Phys. 48, 2560–2571 (1968).

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23. Patterson, J. P. & Jones, D. R. H. Molding of a metallic glass. Mater. Res. Bull. 13, 583–585 (1978). 24. Saotome, Y. & Inoue, A. in 7th IEEE Workshop on Micro Electro Mechanical Systems 343–348 (IEEE, 1994). 25. Schroers, J. The superplastic forming of bulk metallic glasses. J. Miner. Met. Mater. 57, 35–39 (2005). 26. Saotome, Y., Okaniwa, S., Kimura, H. & Inoue, A. Superplastic nanoforging of Ptbased metallic glass with dies of Zr-BMG and glassy carbon fabricated by focussed ion beam. Mater. Sci. Forum 539, 2088–2093 (2007). 27. Schroers, J., Pham, Q., Peker, A., Paton, N. & Curtis, R. V. Blow molding of bulk metallic glass. Scr. Mater. 57, 341–344 (2007). 28. Kumar, G. & Schroers, J. Write and erase mechanisms for bulk metallic glass. Appl. Phys. Lett. 92, 031901 (2008).

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Acknowledgements We thank T. A. Waniuk and C. S. O’Hern for reading of the manuscript and E. R. Dufresne for discussion. This work was supported by the US National Science Foundation under the Material Processing and Manufacturing programme. Author Information Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to J.S. ([email protected]).

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doi:10.1038/nature07718

METHODS Custom heating plates were installed on a load cell of an Instron mechanical testing machine to allow a precise control of temperature and applied pressure during moulding experiments. The moulds (Si, Ni or Al2O3) were heated to 270 uC (for Pt-BMG) on the bottom heating plate of the modified Instron machine set-up. A piece of Pt-BMG was placed on the heated mould and after allowing 30 s for the temperature to equilibrate, the applied load was increased to attain a preset pressure value. The applied pressure was kept constant for varying time intervals depending on the mould type and features. For the moulding of high-aspect-ratio nanostructure, the Pt-BMG was formed on porous Al2O3 from 230 uC to 290 uC under a constant pressure of 130 MPa. The samples were quenched in water after removing from the load cell. Low-aspect-ratio structures separated following water quenching, owing to different thermal expansion coefficients of the mould and metallic glasses. High-aspect-ratio structures were separated by dissolving the mould in a KOH (40 wt%) solution heated to 80 uC. The isothermal moulding resulted in uniform Pt-BMG structures that were subsequently used as moulds to imprint on Au-BMG and PMMA. The Au-BMG or PMMA was thermoplastically formed on patterned Pt-BMG at 160 uC. The Pt-BMG exhibits sufficient strength at 160 uC and there was no deterioration in the Pt-BMG following its use as a mould. In a separate experiment, the Pt-BMG was thermoplastically formed on porous Al2O3 at 280 uC for 2 min to crystallize the BMG. The crystallized Pt-BMG then served as a mould to imprint on the PtBMG at 270 uC. The partial crystallization was done in order to avoid the embrittlement of the BMG mould upon complete crystallization. For hot-cutting process, the BMGs formed on moulds were reheated in the supercooled liquid region and the BMG reservoir was scraped off the mould as described previously in detail31. The patterned BMGs were characterized by DSC (differential scanning calorimetry) and SEM. 31. Schroers, J., Pham, Q. & Desai, A. Thermoplastic forming of bulk metallic glass-A technology for MEMS and microstructure fabrication. J. Microelectromech. Syst. 16, 240–247 (2007).

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