**3.2.2 3D MEMS switch**

Telecommunication switches with large port count have been the main driver for the twoaxis scanner in the past few years. With increasing number of wavelengths and bandwidth in dense wavelength-division-multiplexed (DWDM) networks, there is a need for optical crossconnect (OXC) with large port count [86,87,88]. The dual axis analog scanning capability is the key for these applications since each mirror associated with the input fiber array can point to any mirror associated with the output fiber array. Implementation of NxN OXC using two arrays of N analog scanners is illustrated in Figure 3-28. Even though the implementations may vary, we can always conceptually refer to this illustration. This switch is often called a 3D MEMS switch because the optical beams propagate in three-dimensional space. 3D switch is a better choice for larger port-count NxN OXC compared to 2D switch as the number of mirrors for a 2D switch is N2. Since the optical path length is independent of the switch configuration, uniform optical insertion loss (2 to 3 dB) can be achieved. The port count of the 3D MEMS switch is limited by the size and flatness of the micromirrors, as well as their scan angle and fill factor. For a complete discussion of scaling laws for MEMS free space optical switches, see Syms [88]. For large port count (approaching 1000 x 1000), single crystal micromirror scanners are necessary to achieve large mirror size with required flatness.

During the telecom boom, several companies have invested heavily to develop 3D MEMS OXC's. These companies include (but not limited to) Lucent Technologies [86,88,89,90,91,92,93,94], Corning [95,96,97], NTT Corp. [98,99], Fujitsu Laboratories, Ltd. [100,101], Tellium, Inc. [102,103,104], and Calient Networks [105]. They have demonstrated various designs of two-axis scanners, which are the key components of these 3D MEMS switches.

Neuchatel has perfected the mirror etching technology by DRIE [85]. A surface roughness of 36 nm has been achieved. The switch has excellent optical performance: 0.3-0.5 dB optical insertion loss and 500 sec switching time, and very low polarization dependence [84].

Fig. 3.27. Schematic and SEM of 2x2 switch fabricated by SOI MEMS (Picture courtesy of

Telecommunication switches with large port count have been the main driver for the twoaxis scanner in the past few years. With increasing number of wavelengths and bandwidth in dense wavelength-division-multiplexed (DWDM) networks, there is a need for optical crossconnect (OXC) with large port count [86,87,88]. The dual axis analog scanning capability is the key for these applications since each mirror associated with the input fiber array can point to any mirror associated with the output fiber array. Implementation of NxN OXC using two arrays of N analog scanners is illustrated in Figure 3-28. Even though the implementations may vary, we can always conceptually refer to this illustration. This switch is often called a 3D MEMS switch because the optical beams propagate in three-dimensional space. 3D switch is a better choice for larger port-count NxN OXC compared to 2D switch as the number of mirrors for a 2D switch is N2. Since the optical path length is independent of the switch configuration, uniform optical insertion loss (2 to 3 dB) can be achieved. The port count of the 3D MEMS switch is limited by the size and flatness of the micromirrors, as well as their scan angle and fill factor. For a complete discussion of scaling laws for MEMS free space optical switches, see Syms [88]. For large port count (approaching 1000 x 1000), single crystal micromirror scanners are necessary to achieve large mirror size with required

During the telecom boom, several companies have invested heavily to develop 3D MEMS OXC's. These companies include (but not limited to) Lucent Technologies [86,88,89,90,91,92,93,94], Corning [95,96,97], NTT Corp. [98,99], Fujitsu Laboratories, Ltd. [100,101], Tellium, Inc. [102,103,104], and Calient Networks [105]. They have demonstrated various designs of two-axis scanners, which are the key components of these 3D MEMS

Nico de Rooij. Reprinted from [83] with permission).

**3.2.2 3D MEMS switch** 

flatness.

switches.

Fig. 3.28. Configuration for 3-D optical switch (NxN) with 2N analog scanning mirrors.

Lucent technologies employed a self-assembly technique which was driven by the residual stress in deposited thin films (Cr/Au on polysilicon) to raise two-axis polysilicon scanners (500 m mirror diameter) to a fixed position 50 m above the substrate [106,107]. The scanning electron micrograph (SEM) is shown in Figure 3-29. Two-axis scanning is achieved

Fig. 3.29. SEM of surface-micromachined 2-axis scanners Lucent Technologies (Courtesy of [109] Lucent Technologies Inc. © 2003 Lucent Technologies Inc. All rights reserved.)

Optical MEMS 323

Fig. 3.31. Comparison of switching under open-loop and closed-loop operation (Picture

Fig. 3.32. DC transfer characteristics of the two-axis scanner of Tellium, Inc., with and without sidewall driving (Picture courtesy of C. Pu. Reprinted from [104] with permission)

In this Chapter, we present the history, common actuators, and fabrications of various Optical MEMS devices for display, imaging, and telecom applications. Each actuation mechanism has its own advantage that can be optimally deployed in each application. For instance, electrostatic actuation requires very low current and achieves short range in motion suiting

Reprinted from [102] with permission)

**4. Conclusion** 

by electrostatic force between the mirror and the quadrant electrodes on the substrate. SCS two-axis scanner with long-term stability and high shock resistance has also been developed by Lucent Technologies for 3D MEMS switches [108, 109]. SCS is used to improve the mirror flatness. The long-term stability is achieved by the removal of exposed dielectric to avoid electrostatic charge-up effect (Figure 3-30). The scanning angle is 7 degrees.

Fig. 3.30. Cross section of the SCS two-axis mirror developed by Lucent Technolgies (Picture courtesy of A. Gasparyan. Reprinted from [108] with permission)

NTT Corp. has reported a two-axis micromirror array driven by terraced electrodes. The mirrors and the electrodes are fabricated on separate chips and then bonded together. The use of terraced electrodes reduces the applied voltage by half, compared to regular parallelplate-driven mirrors. The mirror is tilted by 5.4 degree at a maximum of 75 V. The resonant frequency of the fabricated mirror is approximately 1 kHz [98].

The two-axis scanner developed by Fujitsu Laboratories, Ltd. is based on vertical combdrive actuators. SOI with 100-m top and bottom (substrate) silicon layers has been used to fabricate the device [100]. The top silicon is for the moving comb fingers and mirror while the fixed fingers are made of the bottom silicon. V-shape torsion springs are adopted to improve the lateral stability which is a critical issue in comb-drive actuators. Rotation angle of +/- 5 degrees has been achieved with 60-V driving voltage.

Tellium, Inc. has demonstrated an electrostatic parallel-plate actuated two-axis scanner, featuring nonlinear servo closed-loop control [102]. The nonlinear servo closed-loop control enables the mirror to operate beyond the pull-in angle. Figure 3-31 shows the comparison of switching under open-loop and closed-loop operation. The closed-loop angular trajectory can exceed the pull-in (snap-down) angle and shows no overshoot. They have also developed a two-axis micromirror driven by both sidewall and bottom electrodes [104]. The addition of sidewall electrodes improves the linearity of the DC transfer characteristic. The mirror with sidewall electrodes also exhibits a larger scan angle than that driven by merely bottom electrodes (Figure 3-32).

by electrostatic force between the mirror and the quadrant electrodes on the substrate. SCS two-axis scanner with long-term stability and high shock resistance has also been developed by Lucent Technologies for 3D MEMS switches [108, 109]. SCS is used to improve the mirror flatness. The long-term stability is achieved by the removal of exposed dielectric to avoid

Fig. 3.30. Cross section of the SCS two-axis mirror developed by Lucent Technolgies (Picture

NTT Corp. has reported a two-axis micromirror array driven by terraced electrodes. The mirrors and the electrodes are fabricated on separate chips and then bonded together. The use of terraced electrodes reduces the applied voltage by half, compared to regular parallelplate-driven mirrors. The mirror is tilted by 5.4 degree at a maximum of 75 V. The resonant

The two-axis scanner developed by Fujitsu Laboratories, Ltd. is based on vertical combdrive actuators. SOI with 100-m top and bottom (substrate) silicon layers has been used to fabricate the device [100]. The top silicon is for the moving comb fingers and mirror while the fixed fingers are made of the bottom silicon. V-shape torsion springs are adopted to improve the lateral stability which is a critical issue in comb-drive actuators. Rotation angle

Tellium, Inc. has demonstrated an electrostatic parallel-plate actuated two-axis scanner, featuring nonlinear servo closed-loop control [102]. The nonlinear servo closed-loop control enables the mirror to operate beyond the pull-in angle. Figure 3-31 shows the comparison of switching under open-loop and closed-loop operation. The closed-loop angular trajectory can exceed the pull-in (snap-down) angle and shows no overshoot. They have also developed a two-axis micromirror driven by both sidewall and bottom electrodes [104]. The addition of sidewall electrodes improves the linearity of the DC transfer characteristic. The mirror with sidewall electrodes also exhibits a larger scan angle than that driven by merely

courtesy of A. Gasparyan. Reprinted from [108] with permission)

frequency of the fabricated mirror is approximately 1 kHz [98].

of +/- 5 degrees has been achieved with 60-V driving voltage.

bottom electrodes (Figure 3-32).

electrostatic charge-up effect (Figure 3-30). The scanning angle is 7 degrees.

Fig. 3.31. Comparison of switching under open-loop and closed-loop operation (Picture Reprinted from [102] with permission)

Fig. 3.32. DC transfer characteristics of the two-axis scanner of Tellium, Inc., with and without sidewall driving (Picture courtesy of C. Pu. Reprinted from [104] with permission)

### **4. Conclusion**

In this Chapter, we present the history, common actuators, and fabrications of various Optical MEMS devices for display, imaging, and telecom applications. Each actuation mechanism has its own advantage that can be optimally deployed in each application. For instance, electrostatic actuation requires very low current and achieves short range in motion suiting

Optical MEMS 325

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The Optical MEMS technology promises to revolutionize nearly every product category with the ability to directly manipulate light or optical signal. With the integration of microelectronics and microoptical components, it is the possible realization of complete systems on a chip.

#### **5. References**


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**13** 

*USA* 

**Optical-Thermal Phenomena in Polycrystalline** 

Many microelectromechanical systems (MEMS) applications utilize laser irradiation as an integral part of the system functionality, including projection displays, optical switches, adaptive optics (Andrews et al., 2011; Andrews et al., 2008), optical cross-connects (Knoernschild et al., 2009), and laser powered thermal actuators (Serrano & Phinney, 2008; Serrano et al., 2005). When laser irradiation is incident on small-scale systems, such as these MEMS applications, the propensity for exceeding the thermal handling capability of the devices dramatically increases, often leading to overheating, and subsequent deformation and permanent damage to the devices. In most instances, this damage is a direct consequence of the device geometry and the material thermal properties, which hinder the transport of heat out of any locally heated area. Such thermally-driven failures are common in electrically-powered systems (Baker et al., 2004; Plass et al., 2004). However, for laserirradiated MEMS, particularly those fabricated of surface-micromachined polycrystalline silicon (polysilicon), the optical properties can also affect the thermal response of the devices by altering how the laser energy is deposited within the material. Even more concerning in these types of devices is the fact that the thermal, optical, and mechanical response can be intimately coupled such that predicting device performance becomes difficult. In this chapter, we focus on understanding some of the basics of optical interactions in laserirradiated MEMS. We will first look at how the optical properties of the materials affect the laser energy deposition within a device. We will then expand upon this by looking at the coupling that exists between the optical and thermal properties, paying particular attention to the implications that transient temperature changes have in the optical response, ultimately leading to device failure. Finally, we will look at various cases of laser-induced damage in polysilicon MEMS where the device geometry and design and optical-thermal

Understanding the coupling that exists between the thermal and optical behavior in laserirradiated MEMS must begin by looking at the optical properties of the irradiated materials and at how the laser light interacts with each material. The primary factor that affects the magnitude of this interaction is the material's complex refractive index, *n n ik ˆ* . A wave

**1. Introduction** 

coupling have led to device failure.

**2. Optical interactions in MEMS** 

**Silicon MEMS During Laser Irradiation** 

Justin R. Serrano and Leslie M. Phinney

*Albuquerque, New Mexico,* 

*Engineering Sciences Center, Sandia National Laboratories* 

International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS 2003, 8-12 June 2003), vol. 1, pp.360 – 363.

