INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS

J. Opt. A: Pure Appl. Opt. 8 (2006) S352–S359 doi:10.1088/1464-4258/8/7/S10

A multi-degree-of-freedom micromirrorutilizing inverted-series-connectedbimorph actuatorsShane T Todd1, Ankur Jain, Hongwei Qu and Huikai Xie

Department of Electrical and Computer Engineering, University of Florida, 136 LarsenBuilding, PO Box 116200, Gainesville, FL 32611-6200, USA

E-mail: hkx@ufl.edu

Received 5 October 2005, accepted for publication 24 January 2006Published 1 June 2006Online at stacks.iop.org/JOptA/8/S352

AbstractA novel multi-degree-of-freedom electrothermal micromirror design thatuses thermal inverted-series-connected (ISC) bimorph actuators is presented.The ISC bimorph actuators eliminate problems observed in previous designsthat use single bimorph actuators. The micromirror can operate inone-dimensional (1D) piston-mode and two-dimensional (2D) tilt-mode.Analytical models for the piston-mode and tilt-mode actuations are shownand are compared to FEM simulation results. The device was fabricatedusing the AMI 1.5 µm CMOS (complimentary metal-oxide semiconductor)process followed by a post-CMOS micromachining process for devicerelease. The displacement versus temperature of the micromirror wasmeasured experimentally over a range of temperatures and compared toanalytical and FEM simulation results. Experimental results showed that themicromirror displaced by 56 µm at an applied temperature 150 ◦C. Theintegrated polysilicon heaters were open-circuited during the post-CMOSmicrofabrication. The failure was caused by pinholes in the metal layers thatallowed etchants to attack the polysilicon layer during post-CMOSmicrofabrication.

Keywords: MEMS device, micromirror, bimorph actuator

1. Introduction

Micromirrors with multiple degrees of freedom (MDOF) havemany applications which include optical displays, opticalswitches, and imaging systems. MDOF micromirrors havebeen demonstrated to have either strictly two rotational DOFs(2D) [1] or both two rotational DOFs and one translationalDOF (3D) [2–7]. Such types of micromirrors have beendesigned using electrostatic, electromagnetic, piezoelectric,and electrothermal actuation mechanisms. Although any of thementioned actuation mechanisms can be used, most previouslyreported MDOF micromirror designs have been based onelectrostatic actuation.

Electrostatic designs can achieve fast scanning speeds andgood performance repeatability with low power consumption,

1 Current address: Department of Electrical and Computer Engineering,University of California, Santa Barbara, CA 93106, USA.

but often require high actuation voltages, large areas, and highcost fabrication methods. Much research has been dedicatedto improving the limitations of electrostatic designs. A recentexample of an electrostatic MDOF micromirror design wasshown by Milanovic et al [2], where a static rotation angle of10◦ for an applied voltage of 80 V was achieved. In this design3D actuation was achieved by using multiple hinge/comb drivestructures to actuate a mirror plate.

Electromagnetic MDOF micromirror designs can beused to achieve rotation angles at lower applied voltages.Bernstein et al demonstrated a 2D micromirror design thatused orthogonally positioned quadrupole magnetic actuatorsto rotate a mirror plate over two axes [1]. A static rotationangle of 10◦ was achieved at only 62 mV, but the mechanicalresonance frequency was an order of magnitude lower thanthose of typical electrostatic designs. Electromagnetic designsalso require permanent magnets to be assembled below the

1464-4258/06/070352+08$30.00 © 2006 IOP Publishing Ltd Printed in the UK S352

A multi-DOF micromirror utilizing inverted-series-connected bimorph actuators

micromirror which could be difficult to integrate with CMOSelectronics. Furthermore, only 2D electromagnetic actuationhas been demonstrated so far.

Piezoelectric MDOF micromirror designs have also beeninvestigated. Tsaur et al demonstrated a 3D micromirror designthat used constrained double layered PZT single bimorphactuators [3]. A rotation angle of 10◦ was reported whenoperating the device at resonance using a voltage amplitude of4 V. However, reasonable static rotation is difficult to achievedue to the mechanical constraints that exist at the connectionsof the beams to the mirror plate. Other difficulties associatedwith piezoelectric designs include hysteresis effects and exoticmaterials and methods needed for fabrication.

Electrothermal designs, on the other hand, can achievelarge 3D actuation at low voltages. Most MDOF electrothermalmicromirrors have utilized single thermal bimorphs as theactuation element. The problems encountered with usingsingle thermal bimorphs are in principle the same as theproblems associated with single piezoelectric bimorphs. Wewill define a single bimorph beam in the next section. Singhet al showed a similar design to that in [3] except thatconstrained thermal bimorphs were attached to four sides ofa mirror plate, where a 10◦ rotation angle was obtained at10 V [4]. However, it is difficult to achieve large translationalpiston motion with such a design.

Alternative MDOF micromirror designs using electrother-mal actuation have been shown by Jain et al [5, 6]. Thesedesigns used free-end (unconstrained) single bimorphs to al-low for large displacement without mechanical constraints.The first design allowed for 2D actuation and was basedon a bimorph/mirror attached inside and orthogonal to a bi-morph/frame structure [5]. This design achieved rotation an-gle of 13.8◦ at 10 V, but exhibited an initial tilt angle and anon-fixed reflection point (which leads to optical path shift-ing). The second design used compensated thermal bimorphsto eliminate the initial tilt angle problem and extended the mir-ror capability to 3D actuation where large piston motion wasachieved [6]. This design still exhibited a non-fixed point ofreflection.

It is evident from the above discussion that twofundamental problems exist in current MDOF micromirrordesigns that use single bimorph actuators. These includemechanical constraints for designs using constrained singlebimorphs and non-fixed point of reflection for designsusing free-end single bimorphs. Another problem withelectrothermal bimorph designs is thermal coupling betweenactuators. The objective of this paper is to show a novel designconcept of a 3D electrothermal micromirror that eliminatesthe problems associated with single bimorph actuators andreduces the thermal coupling between actuators. In the nextsection we will discuss the single bimorph problem in moredepth, and show how inverted-series-connected (ISC) bimorphactuators can be implemented to eliminate these problems. TheISC bimorph actuator design is presented in section 3. Themodelling and simulation are described in section 4. Thefabrication and experimental results are given in section 5.

2. Actuation principle

As was mentioned previously, most piezoelectric andelectrothermal MDOF micromirror designs have used single

Room Temperature Temperature Raised

Residual Stress -Initial Elevation

Vertical/Horizontal Displacement and Rotation

x

z

Al SiO2

Figure 1. Diagrams of a thermal single bimorph actuator showingthe initial elevation and displacement in response to a temperaturechange.

bimorph actuators. A single bimorph actuator is a simplebimorph beam composed of two material layers that exhibita strain difference when actuated. A thermal single bimorphactuator is composed of two material layers with differentcoefficients of thermal expansion (CTEs) as shown in figure 1.Typically the material layers of a thermal bimorph actuator arecomposed of one material with a high CTE (such as a metal likeAl) and another material with a low CTE (such as a dielectriclike SiO2). A thermal bimorph is usually actuated by applyinga voltage to an electrical resistor that is either embedded in thebimorph or externally attached to the fixed end of a bimorph.The electrical resistor dissipates power and generates Jouleheat, causing the bimorph temperature to rise, which induces astrain change in the bimorph materials and causes the bimorphto deflect as shown in figure 1.

Figure 1 demonstrates that a temperature change causesthe tip of a single bimorph to translate in two directionsand rotate in one direction. This multi-dimensional motionat the tip of the bimorph results in problems associatedwith MDOF micromirror designs. MDOF micromirrors thatuse constrained single bimorphs exhibit limited displacementbecause the attachment of the single bimorph to the mirrorplate prevents the bimorph from displacing to its unconstrainedpotential. MDOF micromirrors that use free-end singlebimorph actuators do not have the mechanical constraintlimitation, but exhibit a non-fixed point of reflection due tothe out-of-plane actuation of the single bimorphs. Thereforeit would be beneficial to replace single bimorph actuators byactuators that move purely in one direction to eliminate thedescribed problems.

The multi-dimensional motion of single bimorph actuatorscan be eliminated by using inverted-series-connected (ISC)bimorph actuators [7]. The ISC bimorph actuator consistsof two S-shape bimorph sections attached end-to-end suchthat both S-shape sections point in opposite directions, asshown in figure 2. An individual S-shape section consistsof two single bimorph sections attached in series where onesection has a high-CTE metal (Al) top layer and a low-CTEdielectric (SiO2) bottom layer, and the adjacent section hasopposite layer composition. This alternating constructionof the material layers and the double S-shape constructionallows each adjacent single bimorph section to have equaland opposite curvature upon actuation so that the entire beamdeforms with no rotation angle and displaces purely in onedirection at the tip. Figure 2 shows how an ISC bimorphdisplaces in response to a rise in temperature. Note thateach S-shape section has a lateral displacement when actuated,

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Al

SiO2

Pure Vertical Displacement

Room Temperature Temperature Raised

Residual Stress -Initial Elevation

S1

S2

A

B

A

B

S1

S2

Vertical and Horizontal

Displacement

x

z

Figure 2. Diagrams of a thermal ISC bimorph actuator showing theinitial elevation and displacement in response to a temperaturechange.

e.g. point A in figure 2 moves in both x and z directions.However, the lateral shifts of S1 and S2 offset each other,resulting in a pure z-displacement at point B.

The ISC bimorph has been used in other types of devicesreported in the literature. Ervin and Brei demonstratedthe operation and modelling of macroscale piezoelectric ISCbimorphs [8]. Oz and Fedder reported a thermal ISC bimorphused for lateral motion in an RF MEMS device [9]. Limet al showed a thermal ISC bimorph that was used in aMEMS IR detector [10]. In the next section we will presenta 3D electrothermal micromirror design that uses thermal ISCbimorph actuators.

3. Design and operation

The 3D electrothermal micromirror design is composed offour main components which include the heaters, the ISCbimorph actuators, the mirror connections, and the mirror plateas shown in figure 3. The mirror plate is 500 µm × 500 µmin area and is composed of an Al reflective surface, an SiO2

middle layer, and a 40 µm thick bottom layer single crystalsilicon membrane which ensures mirror flatness. The measuredradius of curvature of a 40 µm thick mirror plate is about0.5 m (using Vyko). A larger mirror area can steer a largerlight beam, which yields higher imaging resolution, but theincreased mass of a larger mirror area reduces the mechanicalresonance frequency of the mirror. The geometry of themirror area was designed to accommodate this trade-off. Theheaters are each composed of a serpentine polysilicon resistorembedded in ten SiO2 beams with each beam having a lengthand width of 40 and 8 µm respectively. The base of eachheater is attached to the bulk substrate. The ISC bimorphsare composed of alternating layers of Al and SiO2 with eachsingle bimorph section having a length and width of 100 and10 µm respectively. The mirror connections are composed of11 SiO2 beams each having a length and width of 20 and 8 µmrespectively.

The heaters are composed of low thermal conductivitymaterials to maximize the temperature change for a giveninput power. The heater dimensions were chosen to generatetemperatures in the range 25–300 ◦C for voltages in the range0–10 V. Each ISC bimorph is attached between the end ofa heater and the base of a mirror connection. The ISCbimorph dimensions were designed to efficiently use most of

Figure 3. Top-view schematic diagram of the 3D electrothermalmicromirror design.

the area around the mirror and produce mechanical resonancefrequencies of the order of 1 kHz. The thermal resistance ofa mirror connection is designed to be very high so that only aminimum amount of heat flows from the ISC bimorphs throughthe mirror connections upon actuation.

The mirror connection dimensions were chosen suchthat the thermal resistance of the mirror connections wasapproximately an order of magnitude higher than the thermalresistance of the heaters. The high thermal resistance ofthe mirror connections allows for an approximately uniformtemperature to exist across the ISC bimorphs as well as aminimum amount of thermal coupling between heaters. Theuniform temperature across the ISC bimorph is needed toprovide equal curvature changes to the single bimorph sectionsthat comprise the ISC bimorph. The minimization of heat flowbetween the ISC bimorphs to the mirror plate also reducesthe thermal coupling between heaters, which improves linearperformance.

Figure 4 demonstrates the operation of the micromirror.After the device is released, residual stress in the ISC bimorphmaterial layers causes the mirror plate to elevate upwards, asshown in figure 4(a). The released micromirror can functionin two operation modes: piston-mode actuation and tilt-modeactuation. Piston-mode actuation is achieved by applying anequal voltage (VP) to all heaters. In piston-mode actuationall of the heaters create an equal temperature change, whichresults in an equal displacement of all actuators, and a puredownward displacement of the mirror plate, as shown infigure 4(b).

Tilt-mode actuation is achieved when all heaters are givena piston-mode voltage (VP) and two heaters on opposite sidesof the mirror plate are given equal and opposite tilt-modevoltages (±VT) in addition to the piston-mode voltage, asshown in figure 4(c). The increase in voltage applied to one

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HeaterISC Bimorph

Initial Elevation

Mirror Plate

x y

z

(a)

+-

+-

+-

+-

Temperature Increases

VP VP

VPVP

Downward Displacement

(b)

+-

+-

+-+

- +-

+-

Temperature Decreases

VPVP

VPVP

Rotation

(c)

Temperature Increases

VT-y

-VT-y

Figure 4. Illustrations of a released micromirror showing (a) initialelevation at room temperature, (b) piston-mode actuation, and(c) tilt-mode actuation. The base of each heater is attached to thebulk substrate.

(This figure is in colour only in the electronic version)

heater causes its temperature to increase and the decrease involtage applied to the heater on the opposite side causes itstemperature to decrease. If the heaters are operated in thelinear temperature versus voltage range as described in [11],the heaters on both sides of the mirror plate will increase anddecrease in temperature by equal amounts. This causes theISC bimorphs to displace by equal and opposite amounts onboth sides of the mirror plate, resulting in a rotation of themirror plate without a shift in the point of reflection. In thenext section we will show analytical models for piston-modeand tilt-mode actuation.

4. Modelling and simulation

To model the electrothermomechanical behaviour of themicromirror we need to model both the temperature rise of a

Figure 5. Geometric representation of the ISC bimorph actuator.

heater due to an input voltage and the deflection of an ISCbimorph due to a temperature rise of the heater. Since theISC bimorph is connected to the end of the heater we needto know what the temperature is at that point. Assuming thatnegligible heat is lost to convection and radiation and that anegligible amount of heat flows from the ISC into the mirrorplate, the temperature of the ISC bimorph will be uniformabout its length and equal to the temperature at the end of theheater. Furthermore, using the assumption that convection andradiation are negligible, the maximum temperature will exist atthe end of the heater and throughout the ISC bimorph. Usingthese assumptions, the maximum temperature change at theend of the heater due to an input voltage can be expressedas [11]

�T (V ) = 3

4ξ

(√4ξ RTA

3R0V 2 + 1 − 1

)(1)

where ξ is the thermal coefficient of electrical resistance(TCR) of the electrical resistor, R0 is the initial totalelectrical resistance at the ambient temperature, and RTA is theconduction thermal resistance of the heater given by RTA =Lh/κwhth, where Lh is the length of the heater, κ is the averagethermal conductivity of the heater, wh is the sum of the beamwidths of the heater, and th is the total thickness of the heater.This model can be easily extended to include convection aswas shown in [11], but the presence of convection is ignored inthe present model to keep the model simple for demonstrationpurposes. The temperature change at the end of the heater isapproximately linear with voltage for voltages greater than halfthe critical voltage shown as [11]

�T (V � VC/2) ≈√

3RTA

4ξ R0V (2)

where VC is the critical voltage given by VC = √3R0/ξ RTA.

This linear relationship is essential for proper performance ofthe tilt-mode actuation of the device.

The vertical tip displacement of the ISC bimorph can beobtained by analysing the geometric representation shown infigure 5. Since the curvature and dimensions of all the singlebimorphs that comprise the ISC bimorph are equal, the tipdisplacement of the ISC bimorph is simply four times thedisplacement of a single bimorph section. Assuming that thearc angle, φ, of a single bimorph section is small enough tomake a second-order approximation of cosine, it can be shownthat the ISC bimorph tip displacement is given by [12]

δ(�T

)≈ 2βρ L2

b

tb�αT�T (3)

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Lm

T

T

T

Fixed Reflection Point

δ

δ

θ

Figure 6. Side-view geometric diagram of tilt-mode actuation.

where Lb is the length of a single bimorph section, tb is thetotal thickness of the bimorph, �αT is the difference of theCTEs of the material layers, and βρ is a parameter called thecurvature coefficient. The curvature coefficient is a unit-lessparameter that varies from 0 to 1.5 and depends on the relativelayer thicknesses and elastic moduli. Note that equation (3)represents the displacement of an unloaded bimorph actuator.

For piston-mode actuation, the displacement of the mirrorplate in response to an applied voltage can be found by simplymultiplying equations (1) and (3), which yields

δP (VP) ≈ 3βρ L2b�αT

2tbξ

(√4ξ RTA

3R0V 2

P + 1 − 1

). (4)

As was mentioned previously, tilt-mode actuation requiresthat a common piston-mode voltage of VP be applied to allheaters. Tilt-mode actuation also requires that the heatersare operated in the linear temperature versus voltage rangesuch that the common piston-mode voltage is greater thanhalf the critical voltage (i.e., VP � VC/2). This requirementenables heaters on opposite sides of the mirror plate to increaseand decrease in temperature equally when additional tilt-modevoltages of ±VT are added to the piston-mode voltage ofeach heater. The equal and opposite temperature change ofthe heaters causes the oppositely positioned ISC bimorphs todeflect by equal and opposite amounts. This deflection of theISC bimorphs rotates the mirror plate while keeping the centreof the mirror surface fixed in space (as shown in figure 6). Thusthe reflection point of the mirror stays fixed in space and theoptical path of a light beam reflected off of the centre of themirror plate does not shift.

Assuming that the torsional stiffness of the ISC bimorphspositioned on the rotation axis is negligible and that thelateral and rotational stiffness of the oppositely positionedISC bimorphs causing tilt motion is negligible, the tilt-moderotation angle of the mirror plate can be found by analysing thegeometry of figure 6, and is given by

θT (δT) = sin−1 (2δT/Lm) ≈ 2δT/Lm (5)

where Lm is the length of the mirror plate plus the length oftwo mirror connections. Assuming that θT is small enough tomake a first-order approximation of sine, the rotation angle ofthe mirror plate in response to a tilt-mode voltage is given by

θT (VT) ≈ 2βρ L2b�αT

tbLm

√3RTA

ξ R0VT. (6)

Table 1. Material properties used for analytical modelling and FEMsimulation.

Parameters Symbol Value

Electrical:

Polysilicon electrical ρ0 13.2 × 10−6 mresistivity

Polysilicon TCR ξ 5.85 × 10−3 K−1

Thermal:

Aluminium thermal κAl 237 W K−1 m−1

conductivity

SiO2 thermal κOx 1.1 W K−1 m−1

conductivity

Silicon thermal κSi 170 W K−1 m−1

conductivity

Polysilicon thermal κpoly 29 W K−1 m−1

conductivity

Mechanical:

Aluminium elastic EAl 65 GPamodulus

Aluminium Poisson ratio νAl 0.33

Aluminium CTE αAl 23 × 10−6 K−1

SiO2 elastic modulus EOx 70 GPa

SiO2 Poisson ratio νOx 0.17

SiO2 CTE αOx 0.7 × 10−6 K−1

Thus the micromirror can be linearly actuated in bothpiston-mode and tilt-mode when operated in a certain range.The models described above were based on the assumptionsthat convection is negligible, heat flow from the actuators tothe mirror plate is negligible, the torsional stiffness of theactuators along the rotation axis is negligible, and that thelateral and rotational stiffness of the oppositely positioned ISCbimorphs causing tilt motion is negligible. In practice, theseassumptions might not be representative of the true behaviourof the micromirror.

Electrothermomechanical and mechanical mode FEMsimulations were conducted using Coventorware [13] to modelthese effects on the behaviour of the mirror. A convectioncoefficient of 30 W K−1m−2 was applied on the surfaceof the device in the FEM simulations. In practice theconvection coefficient on the device is difficult to predictwithout experimental data, so the convection coefficient usedin simulation was arbitrarily chosen to be reasonably close tofree convection values reported in literature [14]. The materialproperties of the device used in modelling are shown in table 1.Most material properties were found from published data. Thepolysilicon TCR was set to 5.85 × 10−3 K−1, which wasexperimentally measured from a previous device [11].

Analytical model and FEM simulation results for piston-mode and tilt-mode actuation are compared in figures 7(a)and (b). The analytical model and FEM simulation showeddisplacements of 106 and 103 µm respectively for 7 V.Figure 7(a) shows that the analytical model and FEMsimulations agree within 7% for voltages greater than 4 V.The piston-mode FEM simulation results are lower than theanalytical model results due to the presence of convection inthe FEM simulation. The half critical voltage for piston-modeis approximately 2.4 V. The linear range of actuation past half

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0 1 2 3 4 5 6 70

20

40

60

80

100

120

Piston–Mode Voltage (V)

Dis

plac

emen

t ( µm

)

AnalyticalFEM Simulation

Common Voltage for Tilt–Mode Actuation

Half Critical Voltage

Linear Range

–2 –1.5 –1 –0.5 0 0.5 1 1.5 2–8

–6

–4

–2

0

2

4

6

8

Tilt–Mode Voltage (V)

Rot

atio

n A

ngle

( °)

AnalyticalFEM Simulation

Piston–mode Voltage = 5 V

(a)

(b)

Figure 7. Plots showing analytical and FEM simulation results of(a) piston-mode actuation and (b) tilt-mode actuation.

the critical voltage is shown in figure 7(a). The voltage of 5 V ismarked in figure 7(a) to show the common piston-mode voltageto be used in the tilt-mode analysis.

The tilt-mode simulation was conducted using a commonpiston-mode voltage of 5 V and tilt-mode voltages in the rangeof ±2 V. This means that all heaters are given a voltage of5 V and heaters on opposite sides of the mirror plate are givenadditional voltages in the range of ±2 V. For example, if thetilt-mode voltage applied is ±0.5 V, then the total voltageson heaters on opposite sides of the mirror plate are 5.5 and4.5 V respectively. Since the device is being operated in thelinear temperature versus voltage range, the equal and oppositevoltage changes cause equal and opposite temperature anddisplacement changes. Both the analytical and FEM simulationresults for tilt-mode actuation are shown in figure 7(b). Theanalytical model and FEM simulation showed rotation anglesof ±7.7◦ and ±3◦ respectively for ±2 V. Notice that theFEM simulation predicts a significantly lower rotation anglecompared to the analytical prediction. This could be due to thetorsional stiffness of the ISC bimorphs along the rotation axis,thermal coupling between heaters, and lateral and rotationalstiffness of the oppositely positioned ISC bimorphs causingtilt motion. The attenuation of the rotation angle due to thetorsional stiffness and thermal coupling between heaters couldbe mitigated if the width of the mirror connections was madeshorter. A shorter mirror connection width would reduce thetorque applied to actuators along the rotation axis and increase

Fixed Rotation Axis and Reflection Point

Temperature Decreased –Upward Displacement

Temperature Increased –Downward Displacement

Figure 8. FEM simulation image of tilt-mode actuation at 5 ± 2 V.

the thermal resistance of the mirror connections. This is asimple design change that could be implemented in futuredesigns to increase the rotation angle in tilt-mode actuation.Figure 8 shows an image of the tilt-mode FEM simulation at±2 V. FEM simulations showed that the piston-mode and tilt-mode mechanical resonance frequencies were 1.6 and 3.1 kHzrespectively.

5. Fabrication and results

The device was fabricated using the AMI 1.5 µm two-metalconformal CMOS process through MOSIS [15] followed by apost-CMOS micromachining process for device release. Theconformal deposition of the process is utilized to provide thealternating layer construction of the ISC bimorphs. The post-CMOS micromachining process (shown in figure 9) starts witha backside silicon etch to define an Si membrane thicknessunder the mirror plate which ensures mirror flatness. Apartial SiO2 etch follows to expose metal 2 but leaves metal 1with SiO2 protection. Next a partial Al wet etch reducesthe thickness of metal 2. Metal 2 provides protection ofthe top layer SiO2 of the ISC bimorphs during the followinganisotropic SiO2 RIE etch that exposes the Si substrate. Ion-milling of Al in the SiO2 RIE etch removes the remainingmetal 2. Finally, anisotropic DRIE and isotropic Si etchesrelease the structure. Residual stresses in the Al and SiO2

layers cause the device to have an initial elevation of 84 µmat 21 ◦C. Figure 10 shows an SEM photograph of a releaseddevice.

The device was released successfully using the post-CMOS micromachining process; however, the electricalconnections to the heaters were lost during the post-CMOSmicromachining process. Ideally, a conformal CMOS processwith one metal layer and a different material (such asphotoresist) for protection of the heater and mirror connectionsshould be used. Unfortunately it was difficult to find a CMOSfoundry process that was ideal for the device fabricationbecause of the limited flexibility of the available standardprocesses. Thus the AMI 1.5 µm process was used becauseit provided most of the needed fabrication steps, but we had to

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Device after CMOS Fabrication Step 4: SiO2 RIE

Step 5: Si DRIE

Step 6: Isotropic Si Etch

Si Under Mirror

Released Device

Step 1: Backside Si DRIE

Step 2: Partial SiO2 RIE

Step 3: Partial Al Wet Etch

Al PolysiliconSiO2 Si

Figure 9. Diagram of the post-CMOS micromachining process.

Figure 10. SEM photograph of a released device.

compromise by using Al as both the structural material of theISC bimorphs (metal 1) and the protection layer of the heatersand mirror connections (metal 2). Pinholes with diameter of∼0.5 µm (shown in figure 11) were already observed in themetal 2 layer before the wet etching in step 3. The Al wetetch in step 3 greatly enlarged the pinholes already existingin the metal 2 layer. The SEM pictures in figures 12(a) and(b) show that the pinholes in the metal 2 layer were roughly 1–2 µm in diameter after post-CMOS processing. These pinholeswere transferred to the SiO2 layer underneath metal 2 duringstep 4 and then to the polysilicon during step 5, resulting in thebreakdown of the polysilicon in the heaters.

Since the electrical connections were lost to the heaters,no electrical characterization data have been collected onthe device. However, piston-mode displacement wasexperimentally measured in a temperature range 21–150 ◦C.

Figure 11. SEM photograph of pinholes in metal 2 prior topost-CMOS processing.

(a)

(b)

Figure 12. SEM photographs of pinholes in heater after post-CMOSprocessing showing (a) the entire heater and (b) a close-up of apinhole on the heater.

This experiment was conducted by placing the device on athin-film heater and observing the surface of the device underan optical microscope. The initial elevation of the mirrorsurface with reference to the substrate plane was measured atthe ambient temperature (21 ◦C) using the micron-scaled focusof the microscope. Subsequently voltages were applied to theheater to change the temperature of the device and displace themirror surface. At a given heater voltage the temperature ofthe device was measured using a thermocouple and the newelevation of the mirror surface over the substrate plane was

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0 20 40 60 80 100 120 1400

10

20

30

40

50

60

Temperature Change ( °C)

Dis

plac

emen

t ( µm

)

AnalyticalFEM SimulationExperimental

Figure 13. Analytical, FEM simulation, and experimental plots ofthermomechanical piston-mode actuation.

measured. The difference between the initial and actuatedelevations of the mirror surface over the substrate plane yieldedthe displacement for a given temperature change.

Although no electrical characterization data are present,the proper piston-mode actuation of the device was verifiedby this experiment. The experimental results showed adisplacement of 56 µm for a temperature change of 129 ◦C(total temperature of 150 ◦C). The optical microscoperesolution limited the accuracy of experimental displacementmeasurements to approximately ±3 µm. Figure 13shows a comparison of the experimental, analytical, andFEM simulation results for thermomechanical piston-modeactuation.

6. Conclusion

A novel 3D electrothermal micromirror design concept hasbeen described. The design eliminates the problems associatedwith previous electrothermal designs by replacing singlebimorph actuators with inverted-series-connected bimorphactuators. Analytical models and FEM simulations showedthat the device can be linearly actuated with respect to appliedvoltage in both piston-mode and tilt-mode. The device wasfabricated using the AMI 1.5 µm CMOS process followedby a post-CMOS micromachining process for device release.The device was successfully released to produce an initialelevation of 84 µm at 21 ◦C, but the electrical connectionto the polysilicon heaters was lost in the post-CMOSmicromachining process. The electrical connection loss wasdue to the presence of pinholes in the metal layer that allowedetchants to attack the polysilicon. Thermomechanical piston-mode actuation was verified experimentally by measuringthe displacement of the mirror surface over a range oftemperatures. Future work includes determining an alternate

fabrication method and/or design that eliminates the problemsassociated with pinholes. The feasibility of using a non-CMOSfoundry process will also be investigated.

Acknowledgments

The authors would like to thank Tanya Reidhammer for takingthe SEM photographs. This work is supported in part by theNational Science Foundation under award #BES-0423557 andthe Florida Photonics Center of Excellence.

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