Scalable electrothermal MEMS actuator for optical fibre alignment

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INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 14 (2004) 1633–1639

PII: S0960-1317(04)80016-0

Scalable electrothermal MEMS actuator for optical fibre alignment R R A Syms1, H Zou1, J Yao2, D Uttamchandani2 and J Stagg1 1 Optical and Semiconductor Devices Group, Electrical and Electronic Engineering Department, Imperial College London, Exhibition Road, London, SW7 2BT, UK 2 Department of Electronic and Electrical Engineering, University of Strathclyde, 204 George Street, Glasgow, G1 1XW, UK

E-mail: [email protected]

Received 29 April 2004, in final form 7 June 2004 Published 26 August 2004 Online at stacks.iop.org/JMM/14/1633 doi:10.1088/0960-1317/14/12/006

Abstract A new geometry of high-force electrothermal actuator is demonstrated using microelectromechanical systems technology. The actuator consists of two sets of inclined, parallel, suspended beams, which are tethered at one end and linked together at the other. The first set is divided into two halves, which are connected in series and heated electrically. The second set acts as a tether for the first, so that differential thermal expansion gives rise to a lateral deflection. The design can be scaled easily to increase the actuation force, which is sufficient to deflect a cantilevered optical fibre. A bi-directional fibre alignment device is formed from two opposed actuators, which are designed to grip the fibre, and a set of fixed mounting features. Prototype devices are demonstrated by deep reactive ion etching of bonded silicon-on-insulator, and in-plane alignment of single-mode fibre is demonstrated.

1. Introduction The controlled coupling of light between a fibre and an integrated optic waveguide or a semiconductor laser remains an important function in the packaging of optoelectronic components. Current methods of achieving the very tight positional tolerances required for highefficiency coupling use expensive and bulky vibration isolated micropositioning equipment incorporating high-precision mechanical translation stages and piezoelectric actuators. Alignment is slow, serial and expensive. Alternative approaches have involved the replacement of conventional positioning equipment by a silicon substrate, which can not only accommodate optical fibres and integrated devices, but also ‘on-chip’ microactuators for alignment. Multiple actuators then allow many fibre-device couplings to be achieved simultaneously, speeding up the assembly and reducing the cost of complex optical packages. A key enabler is an integrated actuator that can generate sufficient force to deflect a standard optical fibre over a distance of around 20 µm across the aperture of a second fibre or another device. 0960-1317/04/121633+07$30.00

Microelectromechanical systems (MEMS) technology allows a variety of approaches to the construction of suitable actuators. The earliest devices used anisotropic etching to form V-shaped alignment grooves, within which the fibre was deflected by shape-memory-alloy material [1]. An alternative electrostatic actuator was demonstrated at the same time. Similar devices, but with electromagnetic actuation, have been developed for use as fibre-based switches [2]. More recently, efforts have concentrated on electrothermal actuation, which allows a high force to be obtained without complex materials processing, but only if high-aspect ratio structures are used. For example, two separated nickel electroplated structures arranged in U-shaped electrothermal bimorph geometries [3] have been used for in-plane actuation, as shown in figure 1(a) [4]. A more efficient arrangement of two series-connected bulk-micromachined actuators, in which the current flow can be directed through different parts of the structure as shown in figure 1(b), was then developed [5–7]; this has been combined with additional out-of-plane actuation to allow latching of optical fibre switches. In each case the overall device length is very large (often, over 50 mm),

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Figure 1. Approaches to electrothermally driven fibre alignment based on (a) shape bimorph actuators, (b) current bimorph actuators and (c) buckling actuators (adapted from [4, 7, 11]).

and relatively complex patterning and etching are required to fabricate the structure. Electrostatically driven integrated optic waveguide switches (which require a lower force, due to the reduced dimensions of the guiding structure, and may therefore be more compact) have also been constructed [8, 9]. Efforts have also been devoted to fibre alignment. For example, piezoelectrically driven fibre actuators with optical feedback have been used for two-axis alignment [10]. The most sophisticated alignment device to date is the 3-axis in-package MEMS aligner (IPMA) developed by Boeing [11], which uses buckling mode actuators [12–14] as shown in figure 1(c). However, an expensive fabrication technique (synchrotron exposure of thick photoresist [15]) was required to form an electroplating mould for the actuators. Silicon is a less attractive material than electroplated nickel for electrothermal actuators, because of its reduced expansion coefficient and higher thermal conductivity, but it has a potential advantage that a greater range of features may be incorporated, and that high-aspect-ratio structures may be fabricated relatively simply. There is therefore a requirement for actuator designs that may be fabricated in silicon, using a scalable geometry to overcome the reduced force. In this paper, we demonstrate an extremely simple and compact actuator design, which is fabricated by single-layer patterning and deep reactive ion etching of bonded siliconon-insulator [16]. The design—effectively a folded, multiple element buckling mode actuator—is presented in section 2, together with performance predictions obtained from a finite element model. Details of fabrication and measurements of performance are given in section 3, and controllable coupling of light between two single-mode fibres in a variable optical attenuator arrangement is demonstrated. Conclusions are presented in section 4. 1634

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Figure 2. (a) Bidirectional fibre alignment device based on a pair of folded buckling mode actuators; (b) mechanical equivalent circuit, (c) limiting states of mechanical circuit at extrema of stiffness ratios.

2. Design and modelling Figure 2(a) shows a schematic of the device, which consists of two main parts, fixed and moving. The fixed part is a trench of length L0, containing a symmetric arrangement of short, stiff spring clips, into which a short length of singlemode optical fibre may be inserted and held as a cantilever. An axial alignment marker allows the free end of the fibre to be correctly positioned, at a distance just greater than L away from the fixed end. The moving part consists of two electrothermal actuators, which are sprung against and grip the free end of the fibre. Each actuator consists of an array of 2N current-carrying beams (referred to as ‘hot arms’) that are of width w1 and separation d and lie at an angle θ 1 to the fibre axis. The hot arms are fixed at one end and mechanically coupled to the other by a connecting bar of width w3 , which is orthogonal to the fibre axis. One end of this bar makes direct contact with the fibre, and in this region an additional force is exerted laterally against the fibre when the actuator is driven. The hot arms are grouped into two sets of N beams, which are attached to separate contact pads. The two sets are series-connected by the link bar, and heated by passing a current between the pads.

Scalable electrothermal MEMS actuator for optical fibre alignment

Two more cantilevered members, that are also attached to the link bar but located outside the hot arms, complete the actuator. These two beams are of similar length to the hot arms, but are of width w2 and inclined at an angle θ 2. They carry no electrical current, and are referred to as ‘cold arms’. Their function is to restrain the axial expansion of the hot arms, thereby producing a force component in the lateral direction. At first sight, the geometry actuator is similar to the wellknown double hot arm electrothermal bimorph [17]. However, the use of two cold arms eliminates rotational motion of the actuator tip, leaving only translation. The use of two entire arrays of hot arms then introduces scalability, so that the force generated may be increased simply by increasing N. The geometry is therefore effectively a folded buckling mode actuator array, in which the cold arms provide the constraint needed to achieve lateral displacement. The single-ended design thus generated allows clear access to the fibre tip. The overall arrangement is also clearly similar to figure 1(a) in that the driven actuator must deflect both the fibre and the undriven actuator. Assuming that the fibre has lateral stiffness kF and the actuator has a corresponding stiffness kA, the equivalent mechanical circuit is as shown in figure 2(b). Here, U represents the unloaded displacement obtained for one actuator, while L is the value for the same actuator, now loaded by the fibre and undriven actuator. The displacements are related by L /U = 1/ {2 + kF /kA } .

(1)

When kA is small compared with kF the loaded deflection will be small compared with the unloaded value, since the actuator will be unable to deflect the relatively stiff optical fibre (as shown in the left-hand diagram in figure 2(c)). As kA rises, the loaded deflection will gradually tend to one half of the unloaded value, since the driven actuator will then act mainly against the undriven actuator (as in the right-hand diagram in figure 2(c)). This limit may be approached in the scalable geometry given here, either by taking N to be sufficiently large or by adjusting other geometric parameters. However, the use of an arbitrarily stiff actuator is likely to result in low power efficiency, and optimum performance is likely to be obtained when kA is comparable to kF. In fact, the electromechanical performance is governed by Young’s modulus EA, Poisson’s ratio ν A, linear expansion coefficient α A, thermal conductivity KA and resistivity ρ A of the actuator material, Young’s modulus EF and diameter dF of the fibre and the geometric parameters N, L, w1 , w2 , w3 , θ 1 and θ 2. Clearly, many parameter variations are possible, as are other layout possibilities (for example, the use of different hot and cold arm lengths, or of different fibre and actuator lengths). To investigate the likely effects, we have constructed a simple finite element model of an isolated actuator using the commercial software package Intellisuite [18]. This software allows coupling of a thermal model (which describes electrical heating and conduction and convection cooling) to a structural model (which describes the distortion caused by constrained thermal expansion). Additional thermal effects (such as radiation cooling and conduction cooling by the fibre) are ignored, and additional structural effects (such as mechanical loading by the fibre and second actuator) are estimated separately.

We illustrate the likely behaviour with a simple example. Figure 3(a) shows the model layout of a single actuator with N = 9, L = 4 mm, d = 50 µm, w1 = 20 µm, w2 = 20 µm and w3 = 100 µm. The structural depth is D = 85 µm. The material parameters have been taken as EA = 1.3 × 1011 N m−2, ν A = 0.26, α A = 2.6 × 10−6 C−1, KA = 150 W m−1 C−1 and ρ A = 0.003  cm. The resistivity has been taken to be relatively low, compared with most conventional silicon wafers, to achieve a circuit resistance comparable to that obtained experimentally in the following section. In the simulations that follow, the cold arm angle is held constant at θ 2 = 1◦ , while the hot arm angle θ 1 is variable. The model was first used to estimate the transverse spring constant kA of an isolated actuator, by applying a transverse point load to the actuator tip and measuring the resulting displacement. Figure 4(a) shows the variation of kA with θ 1; the variation is approximately parabolic, increasing from a minimum of ≈28 µN µm−1 at θ 1 = 1◦ to ≈840 µN µm−1 at θ 1 = 10◦ . Assuming the free end of the fibre can rotate, due to the weak clamping force provided by the two actuators, the spring constant of the optical  fibre can be estimated as kF = 3EF IF /L3 . Here IF = πdF4 64 is the second moment of area and dF = 125 µm is the diameter of a single-mode fibre. Assuming also that EF = 0.717 × 1011 N m−2 to model a silica fibre, we obtain kF ≈ 40.28 µN µm−1. Thus, the stiffness of the actuator becomes comparable to that of the fibre at around θ 1 = 1.75◦ . Using these two results, the displacement ratio L/U may be found from equation (1), and these data are superimposed on figure 4(a). The displacement ratio gradually rises towards 1/2 as θ 1 increases. The model was then used to calculate the power sensitivity of deflection for the unloaded actuator (defined as the lateral deflection obtained per unit of electrical power, or dU/dP, at low powers). Figure 4(b) shows the predicted variation of dU/dP with θ 1. For low θ 1, the power sensitivity is close to zero, since the hot arms have almost no component perpendicular to the cold arms. As θ 1 rises, the power sensitivity increases rapidly to a maximum when θ 1 is approximately 2◦ and then gradually falls. This behaviour is similar to conventional buckling mode actuators, which require a small central lateral offset to obtain motion in a preferred direction, but which yield lower and lower deflections as the offset increases. Equation (1) and the data of figure 4(a) were then used to estimate the sensitivity dL/dP obtained in the loaded state, and the results are shown superimposed on figure 4(b). A maximum sensitivity (of ≈1.5 µm W−1) is again obtained when θ 1 is approximately 2◦ , but the reduction in deflection obtained at larger values of θ 1 is now less pronounced. Useful performance is therefore obtained over a range of angles θ 1, from (say) 1.5◦ to 4◦ . In practice, the maximum achievable deflection will be limited by thermal damage to the heated parts of the device. The hottest region was identified by simulation as being located at the centre of the link bar. The power sensitivity of the maximum temperature is (to first order) independent of θ 1, and was estimated numerically as dTmax/dP = 35 ◦ C W−1. Loaded deflections of (say) 15 µm should therefore be obtainable with powers of 10 W and peak temperatures of 350 ◦ C. Larger deflections would be obtained at lower powers 1635

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Figure 3. (a) Layout of finite element model of a single actuator constructed with Intellisuite software; (b) SEM photograph of fibre alignment device fabricated by deep reactive ion etching of bonded silicon-on-insulator material.

by increasing L, which reduces the stiffness of both the fibre cantilever and the actuators, and also reduces the thermal drain, and there is clearly scope for further optimization.

3. Fabrication and testing Prototype devices were fabricated in commercially available bonded silicon-on-insulator (BSOI) material using an extremely simple process involving single-layer patterning and etching. The BSOI was obtained commercially as 100 mm 1636

diameter (100) orientated wafers with a device layer thickness of 85 µm and a buried oxide thickness of 2 µm. Structural features were formed by deep reactive ion etching, using a Surface Technology Systems single-chamber multiplex inductively coupled plasma etcher, operating a variant of the cyclic etching process developed by Robert Bosch GmbH [19, 20]. The hard mask was a 2 µm thick layer of photoresist. After etching, the resist was stripped using a combination of wet and dry etchants, and the oxide interlayer was removed

Scalable electrothermal MEMS actuator for optical fibre alignment

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Figure 4. (a) Theoretical variation of the actuator stiffness kA and displacement ratio L/U with the hot arm angle θ 1; (b) theoretical variation of the power sensitivity of deflection with θ 1, in both the unloaded and loaded states.

from beneath the movable parts by wet etching in buffered HF. Care was needed to ensure that the widest parts of the movable structure (the link bar) were released without detaching the cold arm anchors, which were of similar dimensions. The devices were then freeze-dried in a water/methanol mixture, and sputter-coated with 100 nm Cr and 300 nm Au to allow electrical connection. Electrical isolation between the different suspended parts is provided by a large lateral undercut, which prevents metal tracking between the pads. As a result of the metal coating, the electrical behaviour of the device is somewhat different from the previous simulation, since the majority of the current actually flows through the metal, and the circuit resistance is lower than what would be obtained using silicon alone. Devices were fabricated with similar parameters to those in the previous section, namely N = 9, L = 4 mm, d = 50 µm, w1 = 20 µm, w2 = 20 µm and w3 = 100 µm. A fixed angle of θ 2 = 1◦ was chosen, and the angle θ 1 was varied from 1◦ to 5◦ , the most promising range in figure 4(b). Figure 3(b) shows a SEM image of a completed optical fibre alignment device, before fibre insertion. The fixed end of the fibre is located by eight pairs of springs, each 20 µm wide and 400 µm long, distributed evenly over a length of L0 = 4 mm, so that the overall device length is L0 + L = 8 mm. The first pair of springs may be seen on the left of the figure. At the right, the link beam may be seen; since this beam is the widest

Figure 5. (a) Experimental variation of displacement with electrical drive power, obtained with and without a loading fibre, for a device with θ 1 = 2◦ ; (b) experimental variation of the power sensitivity of deflection with θ 1, for unloaded devices with w2 = 20 and 10 µm.

part of the structure, it has been perforated to allow penetration by the undercut etch. After preparing the end of the optical fibre by cleaving, the fibre was inserted by placing it on top of the device, and applying a small pressure to displace the mounting springs and the actuators laterally. The axial position of the fibre was then adjusted. Although there is no movable support beneath the free end of the fibre, unrestricted in-plane motion could be achieved, suggesting that there is little friction arising from contact with the substrate. Measurements of deflection were obtained using an optical microscope equipped with a ×20 objective, a video camera and a calibrated on-screen cursor measurement system. Measurements were performed with and without an inserted fibre. For example, figure 5(a) shows the variation of tip displacement with electrical drive power for an actuator with θ 1 = 2◦ , for both the loaded and unloaded cases. In each case, the variation is quasi-linear, and maximum displacements of 26 µm (unloaded) and 12 µm (loaded) were obtained at the maximum power used (≈6 W). No hysteresis was observed. Because the loaded displacement is approaching one half of the unloaded value, we may deduce from previous results that the stiffness of the actuator dominates over that of the fibre. Over this power range, the circuit resistance rose gradually from ≈15  to ≈17.5 ; the maximum voltage was ≈10 V, while the maximum current was ≈0.6 A. Above this power, thermal damage occurred to the metal coating of the link bar, 1637

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Figure 6. (a) Arrangement for fibre–fibre coupling, and (b) optical microscope views of the coupling region, without (left) and with (right) deflection of the right-hand fibre.

although larger deflections could be obtained for short periods. This result suggests that an increase in link bar dimension will result in increased power handling capacity and travel range. Figure 5(b) shows the measured variation of the power sensitivity with angle θ 1, for a complete set of actuators. This variation should be compared with figure 4(b). The device with θ 1 = θ 2 = 1◦ has almost no sensitivity; high sensitivity is obtained for θ 1 = 2◦ , and the sensitivity then gradually falls as θ 1 increases. There is reasonable qualitative agreement with the predicted values, although the experimental values are slightly higher. Also shown is a comparable set of data for devices with narrower and more flexible cold arms (w2 = 10 µm). Similar behaviour is again displayed, but, as might be expected, the power sensitivity is higher still. Using two complete fibre alignment devices arranged back-to-back as shown in figure 6(a), adjustable fibre-to-fibre coupling could be achieved. Here, the total device length is 16 mm. Figure 6(b) shows optical microscope views of the coupling region, with and without applied power. In the former case, the fibre ends are in good alignment, and in the latter there is a lateral deflection of one fibre by approximately 15 µm. The transmission between the fibres was measured using a broad-band source (an Agilent 83438A erbium ASE source) and an Agilent 86140B optical spectrum analyser, as shown in figure 7(a). All power measurements were referenced to the maximum initial throughput of light between the fibres (−1.0 dB), which had a ≈40 µm axial separation. No index matching oil was used. The variation of transmission with electrical power at 1550 nm wavelength obtained with one actuator driven is shown in figure 7(b). The maximum attenuation that could be consistently achieved was ≈21 dB; this device was thermally damaged at 24 dB attenuation, when the applied power was ≈7 W. These results demonstrate controllable alignment of single-mode fibre. 1638

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Figure 7. (a) Experimental arrangement for optical transmission measurement; (b) variation of transmission at λ = 1550 nm with power, for a device with θ 1 = 2◦ ; (c) variation of transmission with wavelength at different attenuation states.

Further optical tests involved wavelength-dependent loss (WDL) measurements. The measurements again used the ASE source and optical spectrum analyser and were referenced against the initial spectral variation of transmission. Figure 7(c) shows the variation of normalized transmission with wavelength, from 1520 to 1580 nm, obtained using different electrical drive powers. The transmission spectrum is relatively flat, even at high attenuation levels. The ripple in the spectrum is generally caused by interference between reflections from the two fibre ends, which behave as a lowfinesse Fabry-Perot cavity; however, at high attenuation there are some additional artefacts at the extremities of the spectral range caused by the finite source bandwidth. This performance is sufficient for variable optical attenuator (VOA) operation, and the low value of wavelengthdependent loss is more comparable to an image translation VOA [21] than a shutter insertion device [22]. In fact, the approach described here has a number of advantages over both types, including a reduction in the number of surfaces from which undesired optical reflections and scatter may occur. The results above represent the first demonstration that a bonded silicon device may be sufficiently powerful to deflect an optical fibre through a significant distance. The deflection demonstrated here is clearly insufficient for optical switching,

Scalable electrothermal MEMS actuator for optical fibre alignment

which would require a minimum lateral displacement of ≈62.5 µm using bi-directional motion between two standard single-mode fibres [5–7]. However, it is likely that simple scaling of the fibre and actuator length (e.g., a five-fold increase in L from 4 mm to 20 mm) would allow sufficient deflection. In the same way, a five-fold increase in L would allow similar displacements to those obtained here to be achieved with a five-fold reduction in power. This approach would also reduce the peak temperature, avoiding uncontrolled glue curing in operations involving both alignment and fixing. However, it might involve excessive chip area. More effective methods of reducing power consumption are therefore required. For VOA applications, a single actuator could be used; following previous arguments, this alteration would roughly halve power consumption. However, the device would be less useful in applications requiring bi-directional alignment. Further improvements could be obtained by altering the fabrication process to avoid metal-coating the entire structure. In this case, the current flow would be diverted through the silicon, heating the important parts more effectively. Cooling by gas conduction to the substrate could also be reduced, by etching beneath the actuator. Further work is in progress in this area.

4. Conclusions An electrothermal microactuator has been fabricated and evaluated for application in in-plane optical fibre alignment or fibre-based variable optical attenuators. The overall device includes spring alignment features for optical fibres, and a pair of actuators for bi-directional movement. Each actuator is a folded buckling mode device, and the design is scalable in a simple manner to provide an increased actuation force. Preliminary performance simulations have been carried out using a finite element model, and further modelling is ongoing to minimize electrical power consumption. Prototype devices have been fabricated as high-aspect ratio structures by single-layer patterning and etching of bonded silicon-on-insulator material, and their electrothermal performance has been characterized. Fibre deflections of over 10 µm have been obtained from a modest actuator length (4 mm), and a variable optical attenuator function has been demonstrated using a pair of devices arranged back-to-back. The method of construction employed would allow a number of enhancements to be incorporated relatively simply. For example, a clamp mechanism could be fabricated in the device layer, or an additional actuator for out-of-plane motion could be incorporated in the substrate using double-sided processing. The geometry is also suitable for realization in electroplated Ni by the LIGA process [15]. The incorporation of such ‘on-chip’ actuators on a silicon substrate, which can also accommodate optical devices, opens up the possibility for rapid and low-cost fibre–IO or fibre–semiconductor device alignment, leading to cost reduction of high functionality optical modules.

Acknowledgments The support of the EPSRC under grant GR/R07844/01 is gratefully acknowledged. The authors are also grateful for discussions with Dr David Moore of Cambridge University.

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