Making Microscopic Machines Using Metallic Glass

Elsevier – A new manufacturing technology allows researchers to mass produce components for use in next-generation computer storage devices and disposable medical and chemical test kits.

Researchers in Ireland have developed a new technology using materials called bulk metallic glasses to produce high-precision molds for making tiny plastic components. The components, with detailed microscopically patterned surfaces could be used in the next generation of computer memory devices and microscale testing kits and chemical reactors.

In their article published in the latest edition of the open access journal Materials Today, Michael Gilchrist, David Browne and colleagues at University College Dublin explain how bulk metallic glasses (BMGs) were discovered about thirty years ago. These materials are a type of metal alloy, but instead of having a regular, crystalline structure like an everyday metal such as iron or an alloy like bronze, the material’s atoms are arranged haphazardly. This disordered, or amorphous atomic structure is similar to the amorphous structure of the silicon and oxygen atoms in the glass we use for windows and drinking vessels.

(a) Illustrative biological length scales. Using this technology, features from cm scale (dog-bone in pane (c)) down to virus’ diameters can be replicated on polymers at high-speed. (b) Ion-beam machining of common fine-grained crystalline tool steel with features below the length scale of the grains results in an ill defined pattern on the tool. Using amorphous metal, which has no limiting micro-structural length scale, a sharply defined pattern can be produced. (c) A series of grooves and channels and the UCD crest machined into a BMG dog-bone shaped tool. The maximum channel width is 2 μm and smallest width is 250 nm. The width of the harp strings on the crest is 150 nm. The tool is used to injection mold 20.9 mm long dog-bone shaped parts. The FIB machined features, including the 150 nm wide harp strings illustrated, are well replicated without much optimization of the molding process.

Materials Today – Towards nano-injection molding

Bulk metallic glasses (BMGs), having no limiting microstructure, can be machined or thermoplastically-formed with sub-micron precision while still retaining often-desirable metallic properties such as high compressive strength. These novel materials thus have enormous potential for use as multi-scale tools for high-volume manufacturing of polymeric MEMS and information storage devices. Here we show the manufacture of a prototype BMG injection molding tool capable of producing centimeter long polymeric components, with sub-micron surface features.

The haphazard arrangement of atoms in BMGs means that they have some very different mechanical properties from conventional metals. They can be heated and molded like plastics and they can be machined with microscopic precision below the grain size of conventional metals. BMGs also retain the strength and durability of normal metals.

Gilchrist and his colleagues have now exploited the haphazard nature of the atoms in BMGs to allow them to machine microscopic features on to the surface of a BMG. This is not possible with conventional metals such as tool steel used in molds which cannot typically be machined with better than 10 micrometer precision because of its crystalline grain structure. They have then used the resulting strong and durable metallic devices to carry out injection molding of plastic components with microscopic surface patterns using a straightforward tool production route.

“Our technology is a new process for mass producing high-value polymer components, on the micrometer and nanometer-scale,” explains Gilchrist. “This is a process by which high-volume quantities of plastic components can be mass produced with one hundred times more precision, for costs that are at least ten times cheaper than currently possible.”

The research team explains that with BMG injection molding equipment it is now possible to create millimeter-sized polymer components that have surface features of a similar size to mammalian cells at 10 micrometers or even the smallest viruses at less than 100 nanometers. The new manufacturing process could thus allow ‘lab-on-a-chip’ devices to be constructed that could handle and test samples containing single cells and viruses or large biomolecules including DNA and proteins.

“These precision plastic parts are the high value components of microfluidic devices, lab-on-chip diagnostic devices, micro implantable components and MEMS sensors,” Gilchrist adds.

Once the technology is extended to the tens of nanometers length scale, the team suggests that it could be used to make high-volume, low-cost, information storage systems. The team is currently optimizing their technology with this goal in mind.

The research team concludes, “The worldwide trend of miniaturization means that these devices and components are getting progressively smaller and smaller; the problem faced by today’s technologies is that they will soon be unable to manufacture at these smaller dimensions at competitive prices. If you just consider the microfluidic devices market without the biological content: this is forecast to reach $5 billion by 2016.”

(a) ~400 nm square cavity arrays with different depths, which are machined with different FIB milling process settings: 30 kV beam voltage, 0.3 nA beam current, with out-of-plane dimensions (i.e., depth) of 100 nm, 50 nm and 200 nm from left to right. The area of each 8 × 8 array is 6 μm × 6 μm. All these features are located near a ~10 μm wide micro channels, which was machined with a 30 kV beam voltage and 5.0 nA beam current. (b) Sub-micron array of BMG pillars (7 × 7) inside a 3 μm × 3 μm micro cavity machined with 30 kV beam voltage and 1nA beam current. (c) Plan view of an 8 × 8 square cavity array with an area of 3 μm × 3 μm with length and width of the sides of each square ~200 nm. The squares were sharply patterned with FIB milling on BMG. (d) ~100 nm array of holes made by FIB milling with a 30 kV beam voltage and a 30 pA beam current. (e) Overview of replicated sub-micron scale features (c.f. Fig. 3a) using the injection molding process. (f) Replicated 3 μm × 3 μm pillar with sub-micron holes on its top (c.f. Fig. 3b). (g) Replicated 8 × 8 square pillar array (c.f. Fig. 3c); width of each pillar ~200 nm (h) Replicated sub-micron cone array (c.f. Fig. 3d).

Microinjection molding with this BMG tool successfully exhibited its capability for multi-scale replication and all the defined patterns on BMG were well replicated without process optimization even with the smallest ~100 nm features, as seen in Fig. 3e-h. The replication fidelity could be enhanced with process optimization and by using auxiliary equipment. All the sub-micron patterns can be maintained over 20 000 molding cycles.

Future prospects

This new metallic-glass based sub-micron injection molding technique offers an exploitable opportunity for high-volume, low-cost, micro and nano-fabrication of polymers. Given the span in length scale that can be produced on a single part (10-2 – 10-7 m), it is anticipated that this technology will find applications in the field of medical diagnostics. Additionally, if the length scales can be reduced to 10-8 m, a platform for high-volume, low-cost, information storage systems are within reach. Optimization of the processing conditions will assist in this endeavor.

Future challenges from a materials perspective will center on rational choices of alloys for specific applications such that properties (including strength, fatigue resistance, toughness, thermal conductivity and machinability) can be tailored to specific tooling. Although we have described BMG tools that we have used for in excess of 20,000 injection molding cycles, the lifetime of these tools under such conditions is not obvious and warrants further study. The correct choice of alloy will undoubtedly allow the useful tool-life to be optimized. Such optimization will be helped by the current rapid expansion in knowledge surrounding amorphous metals.

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