Sagnik Pal

An Electrothermal Tip–Tilt–Piston Micromirror Based on Folded Dual S-Shaped Bimorphs

This paper presents the design, optimization, fabrication, and test results of an electrothermally actuated tip-tilt-piston micromirror with a large optical aperture of 1 mm. The fabrication of the device is a combination of thin-film surface micromachining and bulk silicon micromachining based on silicon-on-insulator wafers. The device has 3-DOF of actuations, including rotations around two axes in the mirror plane, and out-of-plane piston actuation. The micromirror shows an optical scan range of plusmn30deg about both x- and y-axes and displaces 480 mum in the z-axis, all at dc voltages that are less than 8 V. Dynamic testing of the micromirror shows that the thermal response time of each actuator is about 10 ms. Resonant frequencies of the piston and rotation motion are 336 and 488 Hz, respectively. The unique structural design of the device ensures that there is no lateral shift for the piston motion and no rotation-axis shift for the rotation scanning. With the large tip-tilt-piston scan ranges and low driving voltage, this type of device is very suitable for biomedical imaging and laser beam steering applications.

A parametric dynamic compact thermal model of an electrothermally actuated micromirror

A compact thermal model of a one-dimensional (1D) electrothermally actuated micromirror device is reported. Thermal bimorphs with integrated platinum resistors are used for generating angular rotation. A finite element thermal model is built and validated using thermal imaging data. Using a Krylov subspace-based algorithm, a reduced-order model of order 2 is extracted from the finite element model. The reduced model agrees well with finite element results. A lumped element circuit model is used to explain the second-order thermal response.

Repeatability study of an electrothermally actuated micromirror

With their large scan range and low drive voltages, electrothermally-actuated micromirrors have great potential in optical biomedical imaging applications, but the repeatability and reliability of such micromirrors are not well understood. This paper reports the conditions for achieving repeatability of the embedded resistive heater and the mirror tilt angle of an electrothermal bimorph micromirror. The upper limit of the actuation voltage that does not degrade the embedded heater performance has been established.

Analysis and Fabrication of Curved Multimorph Transducers That Undergo Bending and Twisting

Difference in strains in the layers of a multimorph beam causes it to curl, thereby leading to transduction. Straight multimorph beams have been widely used for thermal, piezoelectric, and shape-memory-alloy-based transducers that undergo out-of-plane bending, but curved multimorph beams have not been explored much. In this paper, we report thermal transducers based on curved multimorphs that have a nonzero curvature in the plane of the substrate. The distinguishing feature of curved multimorphs is that they undergo both out-of-plane bending and twisting deformations. We report the small-deformation analysis of curved multimorphs. The analytical expressions greatly expand the design space for microelectromechanical systems engineers and can lead to novel devices, including micromirrors and infrared sensors. The closed-form expressions were validated against finite-element (FE) simulation and experimental results. Experimental verification was done by monitoring a curved electrothermal multimorph beam at different temperatures. Good agreement between the analysis and experiments has been observed in the small-deformation range. At large deformations, in-plane displacement becomes significant. FE simulations and experiments are used to study large deformations.

Distributed and lumped element models for a bimorph-actuated micromirror

A procedure to model electrothermally actuated devices is developed and demonstrated using a 1D scanning micromirror. The micromirror is actuated by thermal bimorphs and an embedded platinum (Pt) resistor is used for generating Joule heating. Electrothermal, thermal and thermomechanical models are developed and integrated to generate a compact electrothermomechanical model. The electrothermal model relates the thermal power generated in the device to the applied voltage. The thermomechanical model evaluates the mirror rotation angle. The thermal model is developed by drawing analogy between heat flow in the device and current flow through an electrical transmission line. It provides the temperature of the embedded heater and the bimorph actuators. The heat loss coefficient to the surrounding atmosphere is obtained from finite element (FE) simulations. The distributed thermal resistances are represented by an equivalent circuit model with a few elements. A simplification of the circuit model is proposed when small length scales are involved. Rotation angle per unit power input predicted by the circuit model has an error of less than 8% compared to experimental results.

MEMS mirrors based on curved concentric electrothermal actuators with very small lateral shift and tilt

We present a curved concentric electrothermal actuator design that has very mall lateral shift and tilting. The actuator is made up of folded curved Al/W bimorphs with fast thermal response and high sensitivity. Two MEMS mirror designs based on this actuator design have been fabricated. The first MEMS mirror is a tip-tilt-piston scanning mirror capable of scanning up to ± 11° at voltage less than 0.6 V, and generating up to 227 µm piston displacement at less than 0.8 V. Through the entire piston motion, the lateral shift and the tilt angle of the mirror plate are less than 7 µm and 0.7°, respectively. The second MEMS mirror is a piston scanning mirror with enhanced symmetry, and less than 3 µm lateral shift and less than 0.4° tilt through the entire piston scan range have been achieved. This mirror gives large vertical displacement of about 200 µm at only 0.9 V.

Pre-Shaped Open Loop Drive of Electrothermal Micromirror by Continuous and Pulse Width Modulated Waveforms

Open loop drive is simple but it often leads to large nonlinearity. Closed loop drive can compensate nonlinearity but it increases system complexity and cost. This paper reports open loop drive methods that minimize nonlinearity by using special input waveforms. A procedure for open loop control of electrothermal micro-electro-mechanical systems is established and demonstrated by using a thermal bimorph based 1-D electrothermal micromirror. Constant linear velocity scan profile is achieved over 90% of the full scan range. Constant angular velocity scan of an electrothermal micromirror is achieved by continuous wave actuation over a ±4.5° range. A pulse width modulated (PWM) waveform that is equivalent to the continuous waveform is derived and experimentally verified. PWM based open loop control can significantly reduce hardware cost and size in case of micromirror array actuation.

An Electrothermal Micromirror with High Linear Scanning Efficiency

An electrothermal micromirror with an aperture size of 1 mm times 1.2 mm has been developed. The mirror can tilt as much as 21deg with a 15 V DC voltage. High linear scanning efficiency is obtained by using a special drive voltage waveform. The waveform is characterized by two parameters which are determined experimentally.

Fabrication of robust electrothermal MEMS devices using aluminum–tungsten bimorphs and polyimide thermal isolation

We report a novel fabrication process for bimorph based electrothermal devices that involves surface and bulk micromachining on an SOI (silicon-on-insulator) wafer. The bimorph transducers consist of aluminum (Al)-–tungsten (W) beams with a thin layer of SiO2 encapsulating the W-layer and electrically isolating it from the Al-layer. The widely different coefficients of thermal expansion (CTE) of Al and W ensure high bimorph sensitivity. The W-layer acts as an active layer of the bimorph and also as a resistive heater for actuation by Joule heating. Proof mass and rigid beams can be incorporated into the design by utilizing the device layer of the wafer. Thermal isolation between the bimorphs and the substrate; and between bimorphs and the proof mass or rigid beams is achieved by a spin-coated polyimide layer. The process ensures improved robustness compared to comparable designs that typically utilize SiO2 thin-film for thermal isolation. The process can be adapted to a wide range of electrothermal devices, and enables design and layout engineers to trade off speed for achieving lower power consumption and vice versa. We report fabrication and test results on 1D (one-dimensional) electrothermal scanning micromirrors. The fabricated devices have significantly better robustness compared to previously reported mirrors with Al-SiO2 bimorphs and SiO2 thermal isolation.

Dental optical coherence tomography employing miniaturized MEMS-based imaging probe

This paper presents the design, implementation and experimental result of a prototype MEMS-based probe for chair-side real time dental optical coherence tomography (OCT) imaging application.