MEMS | ARTICLE
The introduction of silicon parts into commercial watches began over a decade ago. Together with its industrial partners, CSEM has developed unique watch microparts, demonstrating the improvement and innovation potential of silicon technology in watches. Currently, the principal components made of silicon are the escapement, the hairspring (‘spiral’), the plate, the balance, the escapement wheel and the pallet. Using hybridisation technology, CSEM has demonstrated the feasibility of fabricating in batch assembly a spiral-plate and spiral-plate-balance components. The corresponding patented fabrication technology is compatible with the Silinvar process that enables high thermal stability of silicon micro structures. Likewise, gold-silicon hybridisation has been used to fabricate a pinion-wheel module. CSEM has been working for many years in close collaboration with Patek Philippe and has developed several watch parts made entirely of silicon including a hairspring (Spiromax) and an escapement (Pulsomax). The latter device is a balance, called Gyromax, made of silicon and gold (see figure 3). Introduced in the limited-edition Patek Philippe perpetual calendar, it significantly improved the watch’s power reserve. Recently, CSEM successfully produced a revolutionary escapement based on a bi- stable spring designed by Girard-Perregaux (see figure 4). This innovative design, coupled with state-of-the-art micro fabrication technology, resulted in outstanding watch accuracy remaining constant over the whole power reserve. Such an amazing design could not have been realised with any other material or manufacturing technique due to the need for a particularly thin structure of 14 µm — about six times thinner than a human hair.
<< Figure 3: Patek-Philippe perpetual calendar using a Gyromax balance (insert). The silicon device is made visible as the back of the watch is transparent (lower part of the rear view of the watch). >>
Outlook
CSEM is pursuing its effort to further optimise Si technology and maintain its leading position in the field. Silicon has become a material of choice for key watch components and there is significant interest in new materials and in the new manufacturing processes that can improve the performance of these parts (higher mechanical strength, lower friction coefficients). In the constant quest to improve the performance of watch movements, hard materials like ceramics and silicon carbide (SiC) as well as various coatings and surface treatments are being investigated for their potential to increase mechanical stability and tribological properties. While SiC exhibits a high Young modulus (about 2.5 times higher than that of silicon) and a high mechanical strength, making it an excellent candidate for watch components, it is chemically inert which makes it difficult to structure and new deep reactive ion etching processes thus need to be developed. CSEM is also performing research on new surface treatments including high- temperature treatments in a hydrogen atmosphere which allow a re-arrangement of the silicon atoms, a smoothing of the etched surfaces, and a rounding of the sharp edges. The goal of such treatments is to reduce the roughness of etched surfaces and potentially increase the mechanical strength of the silicon.
30 | commercial micro manufacturing international Vol 7 No.1
<< Figure 2: A Si wheel- Au pinion sub-assembly pressfitted on an axis. >>
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