**5. Conclusion**

**Figure 22.** Schematic of ESD damage: polysilicon comb finger

114 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Another large optical MEMS reliability concern is vibration [16, 17]. Due to the sensitivity and fragile nature of many MEMS, external vibrations can have disastrous implications. They may cause failure through inducing surface adhesion or through fracturing device support structures. Long-term vibration can also contribute to fatigue. Another issue can be shock. Shock is a single mechanical impact instead of a rhythmic event. A shock creates a direct transfer of mechanical energy across the device. Shocks can lead to both adhesion and fracture. Although optical MEMS devices seem fragile due to their small size, their size proved to be one of their greatest assets. Small size enables their robustness. They proved to be able to sustain low-frequency vibrations and mechanical shock without damage. However, besides being an asset, size may be related to another type of failure mechanism. Dimensions of MEMS devices are so small that the presence of the smallest particle during fabrication may cause non-functionality of one or more devices (Figure 23.). For that reason the source of each contaminating particle should be detected and eliminated, especially during packaging, because particles sealed in the package may affect operation of the device during its lifetime. Hermetic packaging can provide adequate protection, electronic contacts and, if necessary, interaction with the environment through the window transparent to light. Also, vacuum packaged devices eliminate effects of capillary stiction. Failure due to contaminations intro‐

duced during packaging is the most common failure mode of optical MEMS devices.

**Figure 23.** Schematic presentation of micromirror failure caused by particle contamination

Optical MEMS devices are still relatively unproven in telecommunications applications and the most optical MEMS devices are not yet fully qualified. A brief insight in reliability of optical MEMS devices for telecommunications applications has been presented in this chapter. Several major reliability issues have been disused: stiction, friction, fatigue, creep, etc. However, developing reliable optical MEMS component is non-trivial. Production of reliable optical MEMS device requires sophisticated design considerations and better control of microfabri‐ cation processes that are used in realization of MEMS device. One of the challenges is providing temperature insensitive, particle free, mechanically stable environment. Usually submicron alignment tolerances are required and high port count optical MEMS require handling and packaging of large numbers of optical fibers, micromirrors, lenses and electrical control leads. Light collimation and focusing, wavelength separation, precisely controlled, large, flat and highly reflective microstructures, significant control electronics are just some of the issues. Reliable packaging is an imperative. Reliable package must not prevent mechanical action of moving parts of the structure, but it should prevent transfer of heat, moisture, outgassing, etc [18, 19]. Another issue is the need for credible testing techniques applicable during fabrication, assembly and packaging, as well as during operational lifetime of the device. As the number of ports grow testing requirements become challenging since multiple, expensive laser sources and flexible test architectures are required. Besides all that, competing technologies pose significant threat to optical MEMS applications (Table 3.) Micromotors, LCD devices, planar waveguides, solid state technologies such as Lithium Niobate and Semiconductor Optical Amplifiers (SOA) can realize various wavelength and fiber management component functions although many coincide that 3D optical MEMS is the only all optical technology that can integrate such complex switching functions in a small package. The key to successful future of optical MEMS in telecommunications market lies in improvement of device structure, materials and processing. Lower losses are required that can be obtained through flatter micromirrors and better quality lenses. More ports are required to handle the expansion of the traffic and reliable and cost-effective packaging is needed to house thousands of tiny fragile MEMS structures. It should be pointed out that industrial standardization of MEMS technol‐ ogy is at least several years away [17, 20] and till then optical MEMS devices will be custom made according to customer requirements. The lack of information flow, as well as reluctance in sharing experience will keep optical MEMS devices from full commercialization although there are several commercially successful applications.


**Table 3.** Comparison of the Technology Alternatives for Wavelength Management/Fibre Management components [21]
