Microstructuring of Bulk Metallic Glass by Hot Mold Quenching — Preparation of Tools for Microinjection Molding of Polymers

Bulk metallic glasses show characteristic properties different from those of conventionally used crystalline metals. This fact bases on a different arrangement of atoms on the nanometer scale. A bulk metallic glass is an alloy quenched from the melt at a cooling rate high enough to suppress nucleation and/or growth of crystals. The resulting solid amorphous metal shows no crystalline structure and, therefore, whether dislocations nor crystal plane gliding. On a macroscopic scale, the material exhibits properties such as a high elastic limit of 2%, high strength of 1800 MPa for Zr-based glasses and a superplastic regime at temperatures above the glass transition [1-2].

Figure 1. Details of test structures replicated into bulk metallic glass (alloy: Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ [9]). a) silicon mold and b) its replication in bulk metallic glass with c) a detail. Pins of 10 µm in diameter and 20 µm height are completely replicated. A protective coating of 1 µm SiO₂ is still present on the metallic glass after removal of the Si wafer by etching. d) and e) show rims with f) a detail of 1 µm spaced rims. The mold did not melt and connect the rims, but the silicon mold between the rims is still present due to a decreased etch rate in narrow channels.

Injection molding of polymers including micro- and submicrometer features is a field which experienced a boom in the last decade with mass applications such as compact discs. Some current efforts aim to extend the field into microfluidics with applications such as the "labCD" [3], a microanalytical tool with a complex system of high aspect ratio microchannels. The large scale fabrication of these and other disposable analytic dishes asks for tools out of a material with a high strength and high elastic limit to withstand forces during pressure molding and demolding of microchannels. Additionally, a high corrosion resistance for the contact with hot polymer melts and the ability of microforming are needed. All these requirements are suitably met by Zr-based bulk metallic glasses.

High aspect ratio surface microstructures can be prepared by deep reactive ion etching of materials such as silicon or silicon oxide [4]. Typical dimensions are heights of 10 to 100 µm, widths of 1 to 100 µm and aspect ratios of 1 to 10. A first approach for the replication of these structures into metallic glass by superplastic forging (at temperatures above the glass transition) gives good results for low aspect ratio structures [5-6]. For high aspect ratios, however, the thermal stability of commercially attractive glass forming alloys is still too low and the glass starts to crystallize at temperatures above the glass transition before the structures are completely filled [7].

Therefore, we have developed a new replication technique for surface microstructures from a silicon wafer into a bulk metallic glass by hot mold quenching. At a temperature above the liquidus temperature of the alloy, high enough to allow wetting of the protected mold by the Zr-based melt, the mold is filled completely by the melt. Subsequently, the hot mold and the melt are quenched together, rapidly enough to ensure glass formation in the alloy. The high quenching rate is made possible by the high surface to volume ratio of the wafer with a thin layer of Zr-based metallic glass spread on it. Finally, the wafer and the protective interface layer are removed, uncovering a foil of bulk metallic glass with high aspect ratio surface microstructures [8].

It remains to introduce this process into a commercial equipment for large scale production of the tools and to complete the network of partners for the value chain from the design of structures, preparation of silicon molds and bulk glass forming alloy, replication of structures into metallic glass and sale of the tools for microinjection molding of polymers.

Authors

Andreas A. Kündig^a, Peter J. Uggowitzer^a, Alex Dommann^b

^a Institute or Metallurgy, Department of Materials, ETH Zentrum, Sonneggstrasse 3, CH-8092 Zurich, Switzerland
^b Institute for Microsystems, Interstate University of Applied Sciences of Technology Buchs NTB, Werdenbergstrasse 4, CH-9471 Buchs, Switzerland

References

1. A. Inoue, Bulk Amorphous Alloys, Practical Characteristics and Applications, Trans Tech Publications, Uetikon-Zürich, 1999.
2. W.L. Johnson. MRS Bulletin 10 (1999) 42.
3. Tecan Instruments LabCD is a product of the Switzerland based company Tecan.
4. F. Laermer, A. Schilp, German Patent DE4241045.
5. Y. Saotome, T. Zhang, A. Inoue, Mater. Res. Soc. Symp. Proc. 544 (1999) 385.
6. T. Zumkley, S. Suzuki, M. Seidel, S. Mechler, M.P. Macht, Mater. Sci. Forum 386-3 (2002) 541.
7. A.A. Kündig, “Bulk Metallic Glasses for Microsystems”, PhD Thesis No. 14804, ETH- Zürich (2002).
8. A.A. Kündig, M. Cucinelli, P.J. Uggowitzer, A. Dommann, “Preparation of High Aspect Ratio Surface Microstructures out of a Zr-Based Bulk Metallic Glass”, Microelect. Eng., accepted.
9. X.H. Lin, W.L. Johnson, and W.K. Rhim, Mater. Trans., JIM 38 (1997) 473.