**4. Reliability of optical MEMS**

length-selectable switches (WSS) is being often used. The simplest WSS is a channel blocker, with a single input and output fibre, having the capability to power equalize or completely attenuate the WDM channels. The more capable 1×K WSS has a single input and K output fibres, adding the capability to independently route the individual WDM channels among the K fibres (Figure 16.). WSS with higher K requires a large micromirror tilting angle (>8°) and devices using vertical comb drive or double hinged angle amplification [10]. Gentler anglebias response at large angles can be obtained by using alternative design that uses fringe

**Figure 15.** Schematic of MARS DGE using continuous membrane and finger electrodes [3]

There are several other MEMS devices for optical networking applications such as polariza‐ tion-mode dispersion (PMD) compensators, tunable laser, etc [11]. New developments in optical MEMS are based on materials technology and cost-effective processing. Optical MEMS are also benefiting from developments of IC industry such as BSOI technology that provided realization of low stress micromirrors, as well as production of other MEMS devices with reproducible mechanical properties and excellent planarity. Continuing progress results in products with better performances such as large-scale switches, variable attenuators, tunable

electrical fields.

**Figure 14.** Tilting micromirrors - schematic of operation

108 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Reliability of optical MEMS for telecommunications is identified as the next manufacturers challenge for the forthcoming years due to a growing market and stricter requirements. Because of the vast diversity of device designs, materials and functions it is necessary to understand both technologies related variables as well as external variables such as environ‐ mental and operational conditions. MEMS reliability analysis is extremely important to identify and understand the different failure mechanisms that can be electrical or/and mechanical. Optical MEMS failure mechanisms are more complicated than those in micro‐ electronics for several reasons:


Design for test is important as well as performing parametric testing, testing during assembly, burn-in and final testing, testing during use, etc. Testing during assembly is of utmost importance for optical MEMS devices. It has two purposes. The first is to determine which devices are ready for the packaging process and the other is monitoring the yield of the packaging process. After the assembly devices are subjected to "burn-in" tests because packaged device may fail to perform due to the invasion of unwanted foreign substances such as dust particles and moisture. The main purpose of this test is to induce "infant mortality" failure on the manufacturing premises but not during operational lifetime (Figure 18). During the useful lifetime of the device the failure rate is relatively low. Failures are usually caused by external events such as vibration, shock, ESD, etc. Testing during use ensures proper functioning of the device for the intended application. Finally, device deteriorates due to intrinsic problems caused by material fatigue, frictional wear, and creep.

**Figure 18.** Failure rate as a function of time

One of the potential failure mechanisms of optical MEMS is stiction. Stiction occurs when surface adhesion at the contacting interface exceeds the restoring force. Adhesion may be driven by either capillary condensation or van der Waals forces [12]. Capillary condensation is affected by moisture and surface contamination, while van der Waals forces are affected by surface roughness. Since device dimensions are minute, gravity and other body forces do not play a significant role. Van der Waals forces are short range forces which cause materials to be attracted at the molecular level. The vulnerability to stiction can be significantly reduced by surface passivation coatings, the use of critical point (CO2) drying of MEMS devices and moisture free packaging [10]. Enclosure in a controlled atmosphere and robust hermetic packaging greatly reduce the presence of moisture. Also, anti-stick layers are commonly being used to lower the surface interaction energy and prevent stiction. These layers provide hydrophobic surfaces on which water cannot condense and capillary stiction will not occur. However, the reliability and reproducibility of these layers is an important issue because of the high temperatures required in MEMS packaging process steps. The best way to avoid stiction failures is to eliminate presence of contacting surfaces by using adequate design or to enhance restoring force. In case of MEMS micromirrors, excessive adhesive force between the landing tip and its lending site may lead to stiction failure of the device. When the electronic reset sequence is applied, sufficiently high adhesive force may obstruct the movement of the mirror. Capillary water condensation causes the landing tip of the mirror and adequate landing site to become stuck. A partial vacuum is produced at the interface due to the surface tension and great forces are required to pull the tip and the landing site apart. For this reason, the usually used method for MEMS micromirror stiction elimination are implementation of springs on the landing tips of the mirror (Figure 19.) [13]. When the mirror landing tip lands on its landing site the spring bends and stores energy that will assist the mirror in taking off the surface when the reset pulse is being applied and bias voltage is being removed.

**Figure 19.** Stiction elimination: schematic of the spring tip and its landing site

mental and operational conditions. MEMS reliability analysis is extremely important to identify and understand the different failure mechanisms that can be electrical or/and mechanical. Optical MEMS failure mechanisms are more complicated than those in micro‐

**•** MEMS devices are designed to interact with environment at various environmental

**•** they are often hermetically sealed and they are expected to have long-term performances,

**•** reliability testing for MEMS devices is not standardized unlike IC and microelectronics,

Design for test is important as well as performing parametric testing, testing during assembly, burn-in and final testing, testing during use, etc. Testing during assembly is of utmost importance for optical MEMS devices. It has two purposes. The first is to determine which devices are ready for the packaging process and the other is monitoring the yield of the packaging process. After the assembly devices are subjected to "burn-in" tests because packaged device may fail to perform due to the invasion of unwanted foreign substances such as dust particles and moisture. The main purpose of this test is to induce "infant mortality" failure on the manufacturing premises but not during operational lifetime (Figure 18). During the useful lifetime of the device the failure rate is relatively low. Failures are usually caused by external events such as vibration, shock, ESD, etc. Testing during use ensures proper functioning of the device for the intended application. Finally, device deteriorates due to

**•** for every new device new testing procedures need to be developed.

intrinsic problems caused by material fatigue, frictional wear, and creep.

Useful time Infant

One of the potential failure mechanisms of optical MEMS is stiction. Stiction occurs when surface adhesion at the contacting interface exceeds the restoring force. Adhesion may be driven by either capillary condensation or van der Waals forces [12]. Capillary condensation is affected by moisture and surface contamination, while van der Waals forces are affected by surface roughness. Since device dimensions are minute, gravity and other body forces do not

Wear-out **Time**

Mortality

**F**

**Figure 18.** Failure rate as a function of time

**ailure R**

**ate**

electronics for several reasons:

conditions (e.g., temperatures),

**•** some of the failures is impossible to predict,

110 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Friction is another mechanism that impacts the lifetime of MEMS device. It is of interest when sliding/rotating optical MEMS are in question and it sets the upper limit of MEMS device lifetime. Friction occurs when two contacting surfaces move against each other. Repeated formation and breaking of contact lead to increase of the contacting stress. When the stress exceeds the material yield strength, material loss occurs. Significant wear finally causes mechanical failure. Frictional wear can be reduced by application of certain coatings (e.g. tungsten). Also, humidity can reduce wear by forming surface hydroxide but it can lead to increased stiction. However, elimination of rubbing surfaces during optical MEMS design phase is the best way to avoid friction [14]. Figure 20. shows an example of friction when optical MEMS devices are in question. A microengine in combination with the microtransmission is often used to drive a pop-up micromirror up, out of the plane. Sets of microgears provide linear motion with high degree of force. Intimately contacting surfaces repeatedly move against each other causing the augmentation of asperities that may lead to accumulation of debris and, finally, mechanical failure.

**Figure 20.** Schematic of the microengine affected by friction and wear

Lifetime of optical MEMS devices can also be affected by fatigue. Repeated motions can cause stress that even significantly below the crack strength, leads to crack growth and eventual failure. Crack growth can be facilitated by stress corrosion and for that reason is highly sensitive to humidity. Both silicon and polysilicon are not immune to fatigue. Stress engineer‐ ing during design phase and materials selection can reduce the problem, but humidity control is the key factor to fatigue elimination. Micromirrors are often affected by fatigue. Each micromirror is hinged so it can rotate. Having in mind that each mirror will be switched thousands of times per second, hinge fatigue should be taken into consideration. In order to avoid fatigue, micromirror hinges are usually realized using thin-film technology. The fatigue properties of thin-film layers are different from those of bulk materials. Metal thin films exhibit much less fatigue than do their macroscopic counterparts since they do not have internal crystal structure because they are just a few grains thick [14]. Thin-films have less stiffness and therefore are less prone to breaking. Fatigue causes movement of dislocations to the surface of the material forming fatigue crack after enough damage has been accumulated. For that reason, not enough damage will accumulate on the thin film surface to form fatigue cracks. However, having in mind that the fatigue properties of thin films are often not known and that fatigue predictions are error prone, hinge structural materials should have material strength that far exceeds the maximum stress expected.

When strain varies with time under the constant stress, creep occurs. Movement of dislocations and diffusion of atoms trigger the deformation. It depends on the material in question, grain size, temperature and initial stress. Over the time surface flatness becomes affected by creep as well as parameters of mechanical parts. Metals are known to creep under stress, while silicon and polysilicon are more robust against creep as brittle materials. For optical MEMS devices silicon is often coated with a thin metal film. Reflective metal coatings on micromirrors are required for the desired optical performance. However, micromirors can become deformed during annealing. After cooling, micromirrors can have significant curvature due to the CTE mismatch between silicon and metal. When single sided metallization is in question, the curvature will slowly decrease as the metal creeps and not the underlying silicon. When symmetrical mirrors are in question, where both sides are metalized using two metal films deposited under different conditions, an uncontrolled increase in mirror curvature can be expected. By increasing silicon thickness flatter micromirrors can be obtained. However, that would affect the resonant frequency, response time and susceptibility to mechanical shock. It can also lead to very high drive voltages, with associated dielectric breakdown and dielectric charging issues [15]. When micromirror hinges are in question, unlike hinge fatigue, creep induced hinge memory poses a significant threat to MEMS micromirror device reliability. It is very significant life limiting failure mode that occurs when a micromirror operates in the same direction for a long period of time. When the bias voltage is removed the mirrors should return to a flat state. Their return to a non-flat state is known as a hinge memory effect (Figure 21.). The angle between the flat and non-flat state is called residual torque angle. As this angle increases, at one point the mirror will not be able to land to the other side. Main contributors to hinge memory failure are duty cycle and operating temperature, but the main cause of this type of failure is the creep [12]. As structural mirror beam materials high melting point compounds are being used such as Al3Ti, AlTi, AlN because high melting point metal often has low creep. Since it is obvious that temperature affects the lifetime of the micromirror device, thermal management is very important. In order to keep temperature in the device within the acceptable range, heatsinks are being used. Adequate thermal management significantly influences lifetime of the device allowing the mirrors to be efficiently controlled over a longer period of time.

**Figure 21.** Schematic presentation of the hinge memory failure mode

**Figure 20.** Schematic of the microengine affected by friction and wear

112 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

strength that far exceeds the maximum stress expected.

Lifetime of optical MEMS devices can also be affected by fatigue. Repeated motions can cause stress that even significantly below the crack strength, leads to crack growth and eventual failure. Crack growth can be facilitated by stress corrosion and for that reason is highly sensitive to humidity. Both silicon and polysilicon are not immune to fatigue. Stress engineer‐ ing during design phase and materials selection can reduce the problem, but humidity control is the key factor to fatigue elimination. Micromirrors are often affected by fatigue. Each micromirror is hinged so it can rotate. Having in mind that each mirror will be switched thousands of times per second, hinge fatigue should be taken into consideration. In order to avoid fatigue, micromirror hinges are usually realized using thin-film technology. The fatigue properties of thin-film layers are different from those of bulk materials. Metal thin films exhibit much less fatigue than do their macroscopic counterparts since they do not have internal crystal structure because they are just a few grains thick [14]. Thin-films have less stiffness and therefore are less prone to breaking. Fatigue causes movement of dislocations to the surface of the material forming fatigue crack after enough damage has been accumulated. For that reason, not enough damage will accumulate on the thin film surface to form fatigue cracks. However, having in mind that the fatigue properties of thin films are often not known and that fatigue predictions are error prone, hinge structural materials should have material

When strain varies with time under the constant stress, creep occurs. Movement of dislocations and diffusion of atoms trigger the deformation. It depends on the material in question, grain size, temperature and initial stress. Over the time surface flatness becomes affected by creep as well as parameters of mechanical parts. Metals are known to creep under stress, while silicon and polysilicon are more robust against creep as brittle materials. For optical MEMS devices silicon is often coated with a thin metal film. Reflective metal coatings on micromirrors are required for the desired optical performance. However, micromirors can become deformed during annealing. After cooling, micromirrors can have significant curvature due to the CTE mismatch between silicon and metal. When single sided metallization is in question, the curvature will slowly decrease as the metal creeps and not the underlying silicon. When symmetrical mirrors are in question, where both sides are metalized using two metal films deposited under different conditions, an uncontrolled increase in mirror curvature can be

Common cause of electrical failure when MEMS devices are in question is anodic oxidation on unpassivated silicon wiring and electrodes. Positively biased electrode oxidizes under the high humidity. Negatively biased electrode remains unaffected. In order to eliminate anodic oxidation the primary goal is moisture elimination by using hermetically sealed packages. Also, for any silicon used as conductors, passivation should be provided.

Environmental robustness is a great reliability concern for all MEMS devices. Examination of micromirror environmental robustness is based on standard semiconductor test requirements such as temperature cycling, thermal shock, moisture resistance, ESD, cold and hot storage life, etc. Similar to ICs, MEMS devices are also susceptible to ESD damage. Sudden transfer of charge that occurs between MEMS device and person or piece of equipment causes ESD damage when on-chip protection circuits are not available because of the incompatibility to IC processing or design complexity (Figure 22.). ESD proof clothing and tools are obligatory when elimination of ESD in MEMS fabrication is in question.

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

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
