micromachines Article
Design and Modeling of Polysilicon Electrothermal Actuators for a MEMS Mirror with Low Power Consumption Miguel Lara-Castro 1 , Adrian Herrera-Amaya 2 , Marco A. Escarola-Rosas 1 , Moisés Vázquez-Toledo 3 , Francisco López-Huerta 4, *, Luz A. Aguilera-Cortés 2 and Agustín L. Herrera-May 1 1
2
3 4
*
Micro and Nanotechnology Research Center, Universidad Veracruzana, Calzada Ruiz Cortines 455, Boca del Río, VER 94294, Mexico;
[email protected] (M.L.-C.);
[email protected] (M.A.E.-R.);
[email protected] (A.L.H.-M.) Depto, Ingeniería Mecánica, Campus Irapuato-Salamanca, Universidad de Guanajuato/Carretera Salamanca-Valle de Santiago Km. 3.5 + 1.8 km, Salamanca, GTO 36885, Mexico;
[email protected] (A.H.-A.);
[email protected] (L.A.A.-C.) Sistemas Automatizados, Centro de Ingeniería y Desarrollo Industrial/Av. Pie de la Cuesta No. 702, Desarrollo San Pablo, Querétaro 76125 México;
[email protected] Engineering Faculty, Universidad Veracruzana, Calzada Ruiz Cortines 455, Boca del Río, Veracruz 94294, Mexico Correspondence:
[email protected]; Tel.: +52-229-775-2000
Received: 14 January 2017; Accepted: 20 June 2017; Published: 25 June 2017
Abstract: Endoscopic optical-coherence tomography (OCT) systems require low cost mirrors with small footprint size, out-of-plane deflections and low bias voltage. These requirements can be achieved with electrothermal actuators based on microelectromechanical systems (MEMS). We present the design and modeling of polysilicon electrothermal actuators for a MEMS mirror (100 µm × 100 µm × 2.25 µm). These actuators are composed by two beam types (2.25 µm thickness) with different cross-section area, which are separated by 2 µm gap. The mirror and actuators are designed through the Sandia Ultra-planar Multi-level MEMS Technology V (SUMMiT V® ) process, obtaining a small footprint size (1028 µm × 1028 µm) for actuators of 550 µm length. The actuators have out-of-plane displacements caused by low dc voltages and without use material layers with distinct thermal expansion coefficients. The temperature behavior along the actuators is calculated through analytical models that include terms of heat energy generation, heat conduction and heat energy loss. The force method is used to predict the maximum out-of-plane displacements in the actuator tip as function of supplied voltage. Both analytical models, under steady-state conditions, employ the polysilicon resistivity as function of the temperature. The electrothermal-and structural behavior of the actuators is studied considering different beams dimensions (length and width) and dc bias voltages from 0.5 to 2.5 V. For 2.5 V, the actuator of 550 µm length reaches a maximum temperature, displacement and electrical power of 115 ◦ C, 10.3 µm and 6.3 mW, respectively. The designed actuation mechanism can be useful for MEMS mirrors of different sizes with potential application in endoscopic OCT systems that require low power consumption. Keywords: electrothermal actuators; endoscopic optical-coherence tomography; microelectromechanical systems (MEMS) mirror; polysilicon; SUMMiT V
1. Introduction Microelectromechanical systems (MEMS) have allowed the develop of devices with advantages such as low cost, small size, high reliability, fast response and easy integration with electronic Micromachines 2017, 8, 203; doi:10.3390/mi8070203
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circuits [1–3]. Among these devices, MEMS mirrors have potential applications such as projection displays [4], tunable optical filter [5], tunable laser [6], Fourier transform spectrometer system [7], confocal scanning microendoscope [8], optical bio-imaging [9] and optical coherence tomography [10]. For 3D endoscopic optical-coherence tomography (OCT) systems are necessary low cost MEMS mirrors composed by compact structures that have large out-of-plane deflections, minimum bias voltage and orthogonal scanning capacity [11,12]. These systems are minimally invasive and can have high resolution and reliability [12]. For this, the mirrors need high precision actuators that allow the variation of their tilting angles with low power consumption [13]. To adjust and control the mirror motion can use different actuators types, including the electromagnetic [14,15], electrostatic [16], electrothermal [17,18] or piezoelectric [19,20] actuators. Mirrors with electrostatic actuators have a fast speed, a small mechanical scanning range at non-resonance (generally 2◦ –3◦ ) and a large actuator footprint, which can be increased at resonance [21,22]. This actuation mechanism requires complex fabrication and high drive voltages about 100 V [23], which constraints its application in endoscopic OCT systems. Other actuators are the electromagnetics that generate large displacements with small driving voltage and have fast response time as well as high resonance frequency [24–26]. Although electromagnetic mirrors register problems with electromagnetic interference (EMI) and need precise assembly techniques of magnetic materials and metallic coils, limiting they use in endoscopic imaging [26]. On the other hand, piezoelectric actuators offer a large motion range combined with high speed and low electric energy [27]. Nevertheless, there are several challenges of the MEMS mirrors to develop endoscopic imaging such as charge leakage, coupling nonuniformity and hysteresis [28]. Other option is a MEMS mirror with an electrothermal actuation mechanism, which has large deflections caused by low bias voltage and does not present EMI and electrostatic discharging problems [28–32]. However, these mirrors require to decrease their footprint size, operation temperature and bias voltage as well as simplify their mechanical structure and performance. To overcome several of these challenges, we propone the design of polysilicon electrothermal actuators for MEMS mirrors based on the Sandia Ultra-planar Multi-level MEMS Technology V (SUMMiT V® ) process from Sandia National Laboratories. This electrothermal actuation mechanism has a simple structural configuration composed by an array of four polysilicon actuators, which can achieve out-of-plane displacements with low dc voltages. These actuators do not require materials with different thermal expansion coefficients due to that employ polysilicon layers with distinct wide, which are separated by 2 µm gap. This device has a small footprint size (1028 µm × 1028 µm), compact structure and simple performance with reduced temperatures. The proposed design includes the modeling of temperature behavior and maximum displacements of the actuators under steady-state conditions. Our actuation mechanism can be used for the rotation of MEMS mirrors of different sizes. The rotation orientation of the mirror can be adjusted through the selective biasing of the four actuators. Thus, the proposed design could be considered for potential applications in endoscopic OCT systems. This paper is organized as follows. Section 2 contains the design and modeling of the proposed actuation mechanism, which includes its electrothermal and structural behavior. Section 3 shows the results and discussions of temperature and out-of-plane displacements of the actuators using analytical models. Finally, the paper ends with the conclusion and future researches. 2. Design and Modeling This section presents the design and modeling of the electrothermal actuators for a MEMS mirror. It considers the temperature distribution and out-of-plane displacements of the actuators generated by different dc biasing voltages under steady-state conditions. 2.1. Structural Configuration Figure 1 shows the design of a MEMS mirror with an array of four polysilicon electrothermal actuators and springs, which are based on the SUMMiT V process [33]. The surface of the silicon
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substrate below of the actuators and mirror must be etched to allow the free motion of the actuators and mirror, mirror, as as shown shown in in Figure Figure 2. 2. Each Each actuator actuator has has two two polysilicon polysilicon structural structural layers layers (i.e., (i.e., poly3 poly3 and and and and mirror, as SUMMiT shown inVFigure 2. of Each actuator has two polysilicon structural layers (i.e., poly3by and poly4 of the process) 2.25 μm thickness with different cross-section area, separated poly4 of the SUMMiT V process) of 2.25 μm thickness with different cross-section area, separated by poly4 ofgap. the SUMMiT process)resistances of 2.25 µmofthickness with cross-section area, separated by μm Thus, the the V electrical these layers layers aredifferent not equal, equal, which allow allow temperature 22 μm gap. Thus, electrical resistances of these are not which aa temperature 2 µm gap. Thus, the electrical resistances of these layers are not equal, which allow a temperature change along along the the actuator actuator when when an an electrical electrical current current is is applied. applied. It It generates generates out-of-plate out-of-plate change change along the actuator when an electrical current is applied. It generates out-of-plate displacements displacements of of the the actuator actuator due due to to Joule Joule effect, effect, whose whose amplitudes amplitudes can can be be controlled controlled varying varying the the displacements of current the actuator due to Joule effect, whose amplitudes can be controlled varying the current values. current values. Thus, this actuator does not need materials layers with different thermal expansion values. Thus, this actuator does not need materials layers with different thermal expansion Thus, this actuator does not its need materialsprocess. layers with thermal expansion coefficients that coefficients that simplify simplify its fabrication process. Thisdifferent design includes includes actuators with with inverted coefficients that fabrication This design actuators inverted simplify its layers fabrication process. This design includes actuators with inverted structural layers structural layers to achieve achieve out-of-plane motions with opposite opposite directions, as shown shown in Figure Figure 3a,b. to structural to out-of-plane motions with directions, as in 3a,b. Thereby, the mirror mirror is connected connected to two two pair actuators actuators with inverted layers that the canmirror have is achieve out-of-plane motions with opposite directions, as shown in Figure 3a,b. Thereby, Thereby, the is to pair with inverted layers that can have displacements in opposite directions, increasing the tilting angle of the the mirror. mirror. In addition, four connected to two in pair actuators with inverted layersthe thattilting can have displacements in In opposite directions, displacements opposite directions, increasing angle of addition, four polysilicon springs (508 μm length, 5 μm width and 2.25 μm thickness) with low stiffness are increasing thesprings tilting angle of the mirror. In addition, four polysilicon springs (508 µmstiffness length, are 5 µm polysilicon (508 μm length, 5 μm width and 2.25 μm thickness) with low employed to connect the actuators with the mirror. Due to the small cross-section area and large employed to µm connect the actuators with the mirror. Due to the small cross-section and width and 2.25 thickness) with low stiffness are employed to connect the actuatorsarea with thelarge mirror. length ofsmall each spring, spring, the four four springs havelength high electrical electrical resistance that constraint the current current flow length of each the springs have high resistance the flow Due to the cross-section area and large of each spring, thethat fourconstraint springs have high electrical through them. In this work, the effect of the thermal energy through the springs and mirror is not through them. In this work, the effect of the thermal energy through the springs and mirror is not resistance that constraint the current flow through them. In this work, effect of the thermal energy considered. considered. through the springs and mirror is not considered.
Figure 1. Design Design of an an electrothermal electrothermal actuation for mechanism for the rotation rotation of of aa Figure 1. Design of an electrothermal actuation mechanism the rotationfor of a microelectromechanical Figure 1. of actuation mechanism the microelectromechanical systems (MEMS) mirror. systems (MEMS) mirror. microelectromechanical systems (MEMS) mirror.
Figure mirror design designin inaaasilicon silicondie. die. Figure2.2. 2.View Viewof of the the MEMS mirror design in silicon die. Figure View of the MEMS mirror
In the design stage, the temperature temperature and out-of-plane out-of-plane displacements of the the considering actuators design stage, the and displacements of actuators InIn thethe design stage, the temperature and out-of-plane displacements of the actuators considering different dimensions of length (L i ) and width (ω h and ω c ) of the upper (hot) and bottom considering different of length i) and width (ωh and ωc) of the upper (hot) and bottom different dimensions ofdimensions length (Li ) and width(L(ω h and ω c ) of the upper (hot) and bottom (cold) beams (cold) beams are studied. The first structural layer is formed by a polysilicon beam (ω c ) and the beams arefirst studied. The first structural layer by abeam polysilicon beam c) and the are(cold) studied. The structural layer is formed by isa formed polysilicon (ω c ) and the(ωsecond layer
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is composed by three polysilicon beams of width ω h each one, in which ω h
