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THE MANUFACTURING OF SILICON COMPONENTS BY DEEP REACTIVE ION ETCHING (DRIE)

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1. Introduction

Silicon and its implementation technique by deep etching (DRIE (Deep Reactive Ion Etching)) appeared in watchmaking as early as 2001. The properties of Silicon offer many advantages and make it a valued Material, particularly for the manufacture of escapements and hairsprings.

The Elasticity of Silicon is close to that of Steel with a Young’s modulus between 130 and 185 GPa. It is Non-magnetic and three times lighter than steel. These are particularly advantageous properties for the design of Escapements, hairsprings and Tourbillon cagesTourbillon. Due to its high Hardness, no heat treatment (hardening, annealing, etc.) is necessary. It allows the Escapement’s functions and friction contacts to be fully performed (Escapement wheel – pallets in particular) without requiring Lubrication.

Like the Electroforming technology (UV-LIGA), the DRIE (Deep Reactive Ion Etching) method begins with a photolithographic step that offers the same design freedom, notably allowing components to be made extremely light without affecting their rigidity. The manufacturing of Silicon components thus consists of a process sequenced into different steps:

2. Component design

DRIE (Deep Reactive Ion Etching) technology makes it possible to achieve component profiles that are impossible to produce using other manufacturing methods. For example, an Escapement wheel can be perforated according to a defined structure in order to reduce its weight as much as possible without weakening its rigidity. It is therefore important to have a thorough understanding of this technology so that its benefits can be incorporated from the component design stage onward.

3. Creating the photomask

The outlines of the profile of the component to be produced are printed at a 1:1 scale on a glass plate. The component to be manufactured is reproduced as many times as possible on the surface of the photomask. In this way, Light (UV) can only pass through outside the outline of the components to be produced.

4. Manufacturing the wafer

The wafer consists of a Silicon Disc with a thickness corresponding to that of the components to be manufactured. A layer of photosensitive resin is applied to the surface of this disc.

5. Irradiating the wafer

This step consists of superimposing the photomask onto the wafer and irradiating its surface with UV rays. Of course, thanks to the filter formed by the photomask, only the outlines of the components to be produced will be protected from UV rays. The resist exposed to UV rays will harden through polymerization and cannot be dissolved during the next step.

6. Development

After being exposed to UV rays, the wafer is immersed in a bath that dissolves, with extreme precision, the surfaces of the wafer that were not exposed to UV rays, meaning the precise outline of the components to be produced. Through this operation, as many profiles of the component to be produced are obtained as the wafer surface can accommodate. At this stage, the wafer is finished, as is the lithographic part of the process.

7. Engraving (DRIE (Deep Reactive Ion Etching)) of the wafer

The wafer is now ready to be etched. This operation requires a clean environment and is carried out in a cleanroom. The wafer is placed in a vacuum chamber where the Engraving will be performed. Various fluorine-based gases in plasma form successively etch and protect the surfaces and sidewalls of the Silicon thus cut through passivation. This chemical attack allows the cutting of components with a thickness ranging from a few microns to several millimeters, with a precision of a few microns.

8. Surface oxidation

Once the wafer has been etched, a thermal oxidation of the Silicon is carried out at 1200°C. A layer of silicon dioxide then forms through growth. Silicon dioxide is transparent. Its thickness (which can reach up to 3 mm) will influence, through diffraction, the color of the component. This surface treatment homogeneously protects the component and improves its Hardness as well as its tribological properties (roughness, coefficient of friction, etc.).

9. Component recovery

Once the engraving is complete, the resin is dissolved and the cut components simply need to be recovered. Free from any trace of machining, the surfaces generally require no further treatment.

DRIE (Deep Reactive Ion Etching) technology and silicon offer numerous advantages over traditional component production methods and previously common materials, and allow for two-level engravings. The design and technicality of components have immediately benefited from this. Thanks to the level of precision offered by this method and the absence of mechanical stress during component manufacturing, the limits in terms of technicality and design are masterfully pushed back.

Although the process requires several steps, implementation is fast and its cost is controlled. Components made using this process are always perfectly identical and compliant with the original design. Precision is within a few microns, and, in the total absence of tooling, extremely tight tolerances can be achieved.

Thanks to this precision and the absence of cutting tools, surface finishes generally require no further treatment. This is a major advantage, particularly for gears, since friction is minimized thanks to the quality of the surfaces obtained with this technology.

These numerous advantages have quickly made it an essential technology, widely used, particularly for the manufacture of escapements and balance springs. It has truly freed designers’ creativity and improved watch performance. Thus, DRIE (Deep Reactive Ion Etching) technology and the emergence of silicon certainly constitute one of the major developments of the last century.