DEEP REACTIVE ION ETCHING (DRIE)

Silicon and its implementation using deep reactive ion etching (DRIE) were introduced into watchmaking in 2001. Silicon’s properties provide numerous advantages, making it a particularly favoured material in the manufacturing of escapements and balance springs.

The elasticity of silicon is comparable to steel, with Young’s modulus (which measures mechanical stiffness) between 130 and 185GPa. Silicon is non-magnetic and three times lighter than steel. These properties are particularly beneficial for the design of escapements, balance springs, and tourbillon cages. Due to its high hardness, no thermic treatment (such as quenching/hardening or annealing) is necessary. It wonderfully fulfils the needs of a perfectly functioning escapement, and the friction contacts (like the escape wheel and pallets) do not require lubrication.

Similar to electroforming technology (UV-LIGA), the DRIE etching method starts with a photolithographic step that offers the same design freedoms, particularly allowing for extremely light components without affecting their rigidity. The manufacture of silicon components thus involves a sequenced process in various stages:

Component Design

DRIE technology allows for component profiles that cannot be achieved by other manufacturing methods. For example, an escape wheel can be structured to be maximally light without weakening its rigidity. It is therefore crucial to fully understand this technology to integrate its benefits from the component design stage.

Creation of the Photomask

The contours of the desired component profile are printed at a 1:1 scale on a glass plate, which is reproduced as many times as possible on the surface of the photomask. Thus, UV light will only pass outside the contours of the components to be produced.

Wafer Fabrication

The wafer consists of a silicon disk as thick as the components to be manufactured. A layer of photosensitive resin is applied to the surface of this disk.

Irradiation of the Wafer

This step involves overlaying the photomask on the wafer and irradiating the surface with UV rays. Naturally, only the contours of the components to be produced are shielded from the UV rays thanks to the filter of the photomask. The resin exposed to the UV rays hardens by polymerization and cannot be dissolved in the subsequent step.

Development

After exposure, the wafer is immersed in a bath that precisely dissolves the surfaces of the wafer that were not exposed to the UV rays, specifically the exact contours of the components to be produced. This operation creates as many profiles as the surface of the wafer can accommodate. Now, the wafer and the lithographic part of the process are complete.

DRIE Etching of the Wafer

The wafer is now ready to be etched. This operation requires a clean environment and is therefore performed in a clean room. The wafer is then placed in a vacuum chamber where the etching takes place. Various fluorine-based plasma gases successively etch and protect the surfaces and sides of the silicon by passivation (the spontaneous formation of a hard, non-reactive, oxide surface film that inhibits further corrosion). This chemical etching enables components with a thickness between a few microns and several millimetres to be cut with micron-level precision.

Surface oxidation

Once the wafer has been etched, the silicon is thermally oxidised at 1,200°C.  A layer of transparent silicon dioxide is then “grown.” Its thickness (up to 3 mm) will influence the colour of the component thanks to diffraction. This surface treatment protects the component evenly and improves its hardness and tribological properties (roughness, coefficient of friction, etc.).

Recovery of Components

Once etching is complete, the resin is dissolved and the cut components simply need to be collected. The surfaces are free of all traces of machining and generally require no further treatment.

DRIE technology and silicon offer numerous advantages over traditional methods of component production and previously used materials, allowing for etching on two levels. The design and technical sophistication of watch components immediately benefited from this new method due to the precision level offered and the absence of any mechanical stress during component fabrication. Limits in terms of technology and design are extensive.

Although the process involves several steps, the implementation is rapid and cost-effective. Components produced by this process are always perfectly identical and conform to the original plan. Precision is at the micron level, and in the total absence of tooling, very tight tolerances can be achieved.

From this precision, and due to the absence of cutting tools, surfaces most often require no treatment, which is a major advantage, especially given the importance of escapement efficiency.

These numerous advantages have quickly made it an essential and widely used technology, particularly for the manufacturing of escapements and balance springs. It has truly freed up the creativity of designers and enhanced the performance of watches. Thus, DRIE technology and the emergence of silicon certainly represent one of the major advancements of the last century.