Miniaturisation springs forward in time

The production lab has outgrown the first pilot tests. On a regular basis the electroplated springs are delivered to the R&D department at a major Swiss watchmaker, where they are fitted in prototype watch mechanisms. However, there is still work to do on their accuracy and long-term stability.

"The base material is a silicon wafer like the ones used to produce computer chips and solar cells," explains Laetitia Philippe, who oversees the production of the springs. "This wafer is initially coated with a conductive gold layer and, later on, with a thin layer of light-sensitive paint. The shape of the spring is then projected onto it and the illuminated parts of the paint are etched out. Now the desired metallic alloy can be electroplated onto the conductive gold base."

As Philippe knows only too well, this crucial step in the process is tricky. “We need a good swirl in the galvanic bath, the right temperature, some organic additives and a current at just the right strength and – if it’s alternating current – in the right form.”

Eventually, the goal is to dissolve the springs out of the galvanic mould. Initially, the researchers use a light microscope to check whether the spring moulds are filled correctly with metal. Then the top side of the mould is fine-polished to ensure all springs are of a defined thickness; the result is verified via X-ray fluorescence analysis. Finally, the paint is removed with an oxygen plasma, the silicon wafer etched away using a strong alkaline solution and the gold coating dissolved. The remaining springs then need to go into a special washing machine for a few hours to remove any ridges and protruding metal remnants. These flawless springs then go into the watch lab for prototype production.

“Our goal is certainly not to compete with suppliers in the watch industry,” says Johann Michler, Head of the Laboratory of Mechanics of Materials and Nanostructures. “We are mainly interested in the process of miniaturisation itself.” Michler’s team studies the mechanical properties of the tiny parts with minuscule stamps and needles. After all, the properties of materials change tiny parts are built: ductile metals become harder; brittle ceramics, on the other hand, become ductile with very small component sizes. “The prerequisite for any examination, however, is that we are able to produce the objects we are interested in based on defined criteria.”

“Some process steps are closely intertwined,” says Michler. “If we change one parameter, such as the geometry of the electroplating moulds or the composition of the alloy, we usually have to adjust the preceding and subsequent steps, too. We want to understand these connections and the effects of miniaturisation in every aspect.”

Additive manufacturing in 3D

Besides two-dimensional structures, the researchers in Thun have already made progress in the production of 3D structures – also with the aid of electroplating. The required moulds are not produced by illuminating layers of paint on silicon wafers, but rather via what is known as two-photon polymerisation. This involves emitting a laser beam in a container with a special liquid plastic precursor. In the focal point of the beam, the liquid polymerises and solidifies.

The delicate structures are electroplated with a nickel boron coating. In strength tests, these metallised structures exhibited much more stability than the raw polymer scaffold. Meanwhile, the researchers have also managed to produce bridges and columns made of solid nickel that are merely a few micrometres in size. Stress tests reveal how the nickel alloys behave in these dimensions. Soon, these components might permit clock mechanisms with particularly fine mechanical complications.