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 favoured material particularly in the manufacturing of escapements and balance springs.

The elasticity of silicon is comparable to steel, with Young’s modulus between 130 and 185GPa. It 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 heat treatment (such as quenching or annealing) is necessary. It perfectly fulfils the functions of the escapement, and the friction contacts (such as the escapement wheel and pallets) do not require lubrication.

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

1. 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 lightened to the maximum without weakening its rigidity. It is therefore crucial to fully understand this technology to integrate its benefits from the component design stage.

2. Creation of the Photomask
The contours of the desired component profile are printed at a 1:1 scale on a glass plate. It 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.

3. Wafer Fabrication
The wafer consists of a silicon disk of the thickness of the components to be manufactured. A layer of photosensitive resin is applied to the surface of this disk.

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

5. Development
After exposure to UV rays, the wafer is immersed in a bath that precisely dissolves the surfaces of the wafer that were not exposed to 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. At this stage, the wafer and the lithographic part of the process are completed.

6. DRIE Etching of the Wafer
The wafer is now ready to be etched. This operation requires a clean environment and is performed in a clean room. The wafer is placed in a vacuum chamber where the etching will be carried out. Various fluorine-based plasma gases successively etch and protect the surfaces and sides of the cut silicon through passivation. This chemical attack allows for cutting components with thicknesses ranging from a few microns to several millimetres with micron-level precision.

7. Recovery of Components
Once etching is complete, the components are cut out and can be collected. Free from any machining marks, the surfaces generally require no further treatment.

DRIE technology and silicon offer numerous advantages over traditional methods of component production and previously used materials, allowing for two-level etchings. The design and technical sophistication of components have immediately benefited. Due to the precision level offered by this method and the absence of any mechanical stress during component fabrication, the limits in terms of technicality and design are impressively extended.

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, surface states 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.