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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 micromirror utilizing inverted-series-connected bimorph actuators Shane T Todd1 , Ankur Jain, Hongwei Qu and Huikai Xie Department of Electrical and Computer Engineering, University of Florida, 136 Larsen Building, PO Box 116200, Gainesville, FL 32611-6200, USA E-mail: [email protected]

Received 5 October 2005, accepted for publication 24 January 2006 Published 1 June 2006 Online at stacks.iop.org/JOptA/8/S352 Abstract A novel multi-degree-of-freedom electrothermal micromirror design that uses thermal inverted-series-connected (ISC) bimorph actuators is presented. The ISC bimorph actuators eliminate problems observed in previous designs that use single bimorph actuators. The micromirror can operate in one-dimensional (1D) piston-mode and two-dimensional (2D) tilt-mode. Analytical models for the piston-mode and tilt-mode actuations are shown and are compared to FEM simulation results. The device was fabricated using the AMI 1.5 µm CMOS (complimentary metal-oxide semiconductor) process followed by a post-CMOS micromachining process for device release. The displacement versus temperature of the micromirror was measured experimentally over a range of temperatures and compared to analytical and FEM simulation results. Experimental results showed that the micromirror displaced by 56 µm at an applied temperature 150 ◦ C. The integrated polysilicon heaters were open-circuited during the post-CMOS microfabrication. The failure was caused by pinholes in the metal layers that allowed etchants to attack the polysilicon layer during post-CMOS microfabrication. Keywords: MEMS device, micromirror, bimorph actuator

1. Introduction Micromirrors with multiple degrees of freedom (MDOF) have many applications which include optical displays, optical switches, and imaging systems. MDOF micromirrors have been demonstrated to have either strictly two rotational DOFs (2D) [1] or both two rotational DOFs and one translational DOF (3D) [2–7]. Such types of micromirrors have been designed using electrostatic, electromagnetic, piezoelectric, and electrothermal actuation mechanisms. Although any of the mentioned actuation mechanisms can be used, most previously reported MDOF micromirror designs have been based on electrostatic actuation. Electrostatic designs can achieve fast scanning speeds and good performance repeatability with low power consumption, 1 Current address: Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106, USA.

1464-4258/06/070352+08$30.00

but often require high actuation voltages, large areas, and high cost fabrication methods. Much research has been dedicated to improving the limitations of electrostatic designs. A recent example of an electrostatic MDOF micromirror design was shown by Milanovi´c et al [2], where a static rotation angle of 10◦ for an applied voltage of 80 V was achieved. In this design 3D actuation was achieved by using multiple hinge/comb drive structures to actuate a mirror plate. Electromagnetic MDOF micromirror designs can be used to achieve rotation angles at lower applied voltages. Bernstein et al demonstrated a 2D micromirror design that used orthogonally positioned quadrupole magnetic actuators to rotate a mirror plate over two axes [1]. A static rotation angle of 10◦ was achieved at only 62 mV, but the mechanical resonance frequency was an order of magnitude lower than those of typical electrostatic designs. Electromagnetic designs also require permanent magnets to be assembled below the

© 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 CMOS electronics. Furthermore, only 2D electromagnetic actuation has been demonstrated so far. Piezoelectric MDOF micromirror designs have also been investigated. Tsaur et al demonstrated a 3D micromirror design that used constrained double layered PZT single bimorph actuators [3]. A rotation angle of 10◦ was reported when operating the device at resonance using a voltage amplitude of 4 V. However, reasonable static rotation is difficult to achieve due to the mechanical constraints that exist at the connections of the beams to the mirror plate. Other difficulties associated with piezoelectric designs include hysteresis effects and exotic materials and methods needed for fabrication. Electrothermal designs, on the other hand, can achieve large 3D actuation at low voltages. Most MDOF electrothermal micromirrors have utilized single thermal bimorphs as the actuation element. The problems encountered with using single thermal bimorphs are in principle the same as the problems associated with single piezoelectric bimorphs. We will define a single bimorph beam in the next section. Singh et al showed a similar design to that in [3] except that constrained thermal bimorphs were attached to four sides of a mirror plate, where a 10◦ rotation angle was obtained at 10 V [4]. However, it is difficult to achieve large translational piston motion with such a design. Alternative MDOF micromirror designs using electrothermal actuation have been shown by Jain et al [5, 6]. These designs used free-end (unconstrained) single bimorphs to allow for large displacement without mechanical constraints. The first design allowed for 2D actuation and was based on a bimorph/mirror attached inside and orthogonal to a bimorph/frame structure [5]. This design achieved rotation angle of 13.8◦ at 10 V, but exhibited an initial tilt angle and a non-fixed reflection point (which leads to optical path shifting). The second design used compensated thermal bimorphs to eliminate the initial tilt angle problem and extended the mirror capability to 3D actuation where large piston motion was achieved [6]. This design still exhibited a non-fixed point of reflection. It is evident from the above discussion that two fundamental problems exist in current MDOF micromirror designs that use single bimorph actuators. These include mechanical constraints for designs using constrained single bimorphs and non-fixed point of reflection for designs using free-end single bimorphs. Another problem with electrothermal bimorph designs is thermal coupling between actuators. The objective of this paper is to show a novel design concept of a 3D electrothermal micromirror that eliminates the problems associated with single bimorph actuators and reduces the thermal coupling between actuators. In the next section we will discuss the single bimorph problem in more depth, and show how inverted-series-connected (ISC) bimorph actuators can be implemented to eliminate these problems. The ISC bimorph actuator design is presented in section 3. The modelling and simulation are described in section 4. The fabrication and experimental results are given in section 5.

2. Actuation principle As was mentioned previously, most piezoelectric and electrothermal MDOF micromirror designs have used single

Room Temperature

Temperature Raised

Residual Stress Initial Elevation

z x

Al

Vertical/Horizontal Displacement and Rotation

SiO2

Figure 1. Diagrams of a thermal single bimorph actuator showing the initial elevation and displacement in response to a temperature change.

bimorph actuators. A single bimorph actuator is a simple bimorph beam composed of two material layers that exhibit a strain difference when actuated. A thermal single bimorph actuator is composed of two material layers with different coefficients of thermal expansion (CTEs) as shown in figure 1. Typically the material layers of a thermal bimorph actuator are composed of one material with a high CTE (such as a metal like Al) and another material with a low CTE (such as a dielectric like SiO2 ). A thermal bimorph is usually actuated by applying a voltage to an electrical resistor that is either embedded in the bimorph or externally attached to the fixed end of a bimorph. The electrical resistor dissipates power and generates Joule heat, causing the bimorph temperature to rise, which induces a strain change in the bimorph materials and causes the bimorph to deflect as shown in figure 1. Figure 1 demonstrates that a temperature change causes the tip of a single bimorph to translate in two directions and rotate in one direction. This multi-dimensional motion at the tip of the bimorph results in problems associated with MDOF micromirror designs. MDOF micromirrors that use constrained single bimorphs exhibit limited displacement because the attachment of the single bimorph to the mirror plate prevents the bimorph from displacing to its unconstrained potential. MDOF micromirrors that use free-end single bimorph actuators do not have the mechanical constraint limitation, but exhibit a non-fixed point of reflection due to the out-of-plane actuation of the single bimorphs. Therefore it would be beneficial to replace single bimorph actuators by actuators that move purely in one direction to eliminate the described problems. The multi-dimensional motion of single bimorph actuators can be eliminated by using inverted-series-connected (ISC) bimorph actuators [7]. The ISC bimorph actuator consists of two S-shape bimorph sections attached end-to-end such that both S-shape sections point in opposite directions, as shown in figure 2. An individual S-shape section consists of two single bimorph sections attached in series where one section has a high-CTE metal (Al) top layer and a low-CTE dielectric (SiO2 ) bottom layer, and the adjacent section has opposite layer composition. This alternating construction of the material layers and the double S-shape construction allows each adjacent single bimorph section to have equal and opposite curvature upon actuation so that the entire beam deforms with no rotation angle and displaces purely in one direction at the tip. Figure 2 shows how an ISC bimorph displaces in response to a rise in temperature. Note that each S-shape section has a lateral displacement when actuated, S353

S T Todd et al Room Temperature

Temperature Raised

B

Pure Vertical Displacement

S2 B

S2

Residual Stress Initial Elevation

A

S1

Al

S1

Vertical and Horizontal Displacement

A

SiO2

z x

Figure 2. Diagrams of a thermal ISC bimorph actuator showing the initial elevation and displacement in response to a temperature change.

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 devices reported in the literature. Ervin and Brei demonstrated the operation and modelling of macroscale piezoelectric ISC bimorphs [8]. Oz and Fedder reported a thermal ISC bimorph used for lateral motion in an RF MEMS device [9]. Lim et al showed a thermal ISC bimorph that was used in a MEMS IR detector [10]. In the next section we will present a 3D electrothermal micromirror design that uses thermal ISC bimorph actuators.

3. Design and operation The 3D electrothermal micromirror design is composed of four main components which include the heaters, the ISC bimorph actuators, the mirror connections, and the mirror plate as shown in figure 3. The mirror plate is 500 µm × 500 µm in area and is composed of an Al reflective surface, an SiO2 middle layer, and a 40 µm thick bottom layer single crystal silicon membrane which ensures mirror flatness. The measured radius of curvature of a 40 µm thick mirror plate is about 0.5 m (using Vyko). A larger mirror area can steer a larger light beam, which yields higher imaging resolution, but the increased mass of a larger mirror area reduces the mechanical resonance frequency of the mirror. The geometry of the mirror area was designed to accommodate this trade-off. The heaters are each composed of a serpentine polysilicon resistor embedded in ten SiO2 beams with each beam having a length and width of 40 and 8 µm respectively. The base of each heater is attached to the bulk substrate. The ISC bimorphs are composed of alternating layers of Al and SiO2 with each single bimorph section having a length and width of 100 and 10 µm respectively. The mirror connections are composed of 11 SiO2 beams each having a length and width of 20 and 8 µm respectively. The heaters are composed of low thermal conductivity materials to maximize the temperature change for a given input power. The heater dimensions were chosen to generate temperatures in the range 25–300 ◦ C for voltages in the range 0–10 V. Each ISC bimorph is attached between the end of a heater and the base of a mirror connection. The ISC bimorph dimensions were designed to efficiently use most of S354

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

the area around the mirror and produce mechanical resonance frequencies of the order of 1 kHz. The thermal resistance of a mirror connection is designed to be very high so that only a minimum amount of heat flows from the ISC bimorphs through the mirror connections upon actuation. The mirror connection dimensions were chosen such that the thermal resistance of the mirror connections was approximately an order of magnitude higher than the thermal resistance of the heaters. The high thermal resistance of the mirror connections allows for an approximately uniform temperature to exist across the ISC bimorphs as well as a minimum amount of thermal coupling between heaters. The uniform temperature across the ISC bimorph is needed to provide equal curvature changes to the single bimorph sections that comprise the ISC bimorph. The minimization of heat flow between the ISC bimorphs to the mirror plate also reduces the thermal coupling between heaters, which improves linear performance. Figure 4 demonstrates the operation of the micromirror. After the device is released, residual stress in the ISC bimorph material layers causes the mirror plate to elevate upwards, as shown in figure 4(a). The released micromirror can function in two operation modes: piston-mode actuation and tilt-mode actuation. Piston-mode actuation is achieved by applying an equal voltage (VP ) to all heaters. In piston-mode actuation all of the heaters create an equal temperature change, which results in an equal displacement of all actuators, and a pure downward displacement of the mirror plate, as shown in figure 4(b). Tilt-mode actuation is achieved when all heaters are given a piston-mode voltage (VP ) and two heaters on opposite sides of the mirror plate are given equal and opposite tilt-mode voltages (±VT ) in addition to the piston-mode voltage, as shown in figure 4(c). The increase in voltage applied to one

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

x

y Mirror Plate

Heater

ISC Bimorph

Figure 5. Geometric representation of the ISC bimorph actuator.

Initial Elevation

(a)

VP

+ -

+ -

VP

Downward Displacement

Temperature Increases

VP

+ -

+ -

VP

(b) Temperature Increases

VP

+ -

+ -

VP + -

VT-y

Rotation Temperature Decreases

VP

+ -

+ -

+ -

-VT-y

VP

(c)

Figure 4. Illustrations of a released micromirror showing (a) initial elevation at room temperature, (b) piston-mode actuation, and (c) tilt-mode actuation. The base of each heater is attached to the bulk substrate. (This figure is in colour only in the electronic version)

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

4. Modelling and simulation To model the electrothermomechanical behaviour of the micromirror we need to model both the temperature rise of a

heater due to an input voltage and the deflection of an ISC bimorph due to a temperature rise of the heater. Since the ISC bimorph is connected to the end of the heater we need to know what the temperature is at that point. Assuming that negligible heat is lost to convection and radiation and that a negligible amount of heat flows from the ISC into the mirror plate, the temperature of the ISC bimorph will be uniform about its length and equal to the temperature at the end of the heater. Furthermore, using the assumption that convection and radiation are negligible, the maximum temperature will exist at the end of the heater and throughout the ISC bimorph. Using these assumptions, the maximum temperature change at the end of the heater due to an input voltage can be expressed as [11] 
 3 4ξ RTA 2 ˆ
T (V ) = V +1−1 (1) 4ξ 3 R0 where ξ is the thermal coefficient of electrical resistance (TCR) of the electrical resistor, R0 is the initial total electrical resistance at the ambient temperature, and RTA is the conduction thermal resistance of the heater given by RTA = L h /κwh th , where L h is the length of the heater, κ is the average thermal conductivity of the heater, wh is the sum of the beam widths of the heater, and th is the total thickness of the heater. This model can be easily extended to include convection as was shown in [11], but the presence of convection is ignored in the present model to keep the model simple for demonstration purposes. The temperature change at the end of the heater is approximately linear with voltage for voltages greater than half the critical voltage shown as [11]  3 RTA
Tˆ (V  VC /2) ≈ V (2) 4ξ R0 √ where VC is the critical voltage given by VC = 3 R0 /ξ RTA . This linear relationship is essential for proper performance of the tilt-mode actuation of the device. The vertical tip displacement of the ISC bimorph can be obtained by analysing the geometric representation shown in figure 5. Since the curvature and dimensions of all the single bimorphs that comprise the ISC bimorph are equal, the tip displacement of the ISC bimorph is simply four times the displacement of a single bimorph section. Assuming that the arc angle, φ , of a single bimorph section is small enough to make a second-order approximation of cosine, it can be shown that the ISC bimorph tip displacement is given by [12]   2β L 2 ρ b δ
Tˆ ≈
αT
Tˆ (3) tb S355

S T Todd et al

Table 1. Material properties used for analytical modelling and FEM simulation.

Fixed Reflection Point

δT

Parameters

θT

Symbol

Value

ρ0

13.2 × 10−6 m

ξ

5.85 × 10−3 K−1

κAl

237 W K−1 m−1

SiO2 thermal conductivity

κOx

1.1 W K−1 m−1

Silicon thermal conductivity

κSi

170 W K−1 m−1

κpoly

29 W K−1 m−1

E Al

65 GPa

νAl αAl E Ox νOx αOx

0.33 23 × 10−6 K−1 70 GPa 0.17

Electrical: δT

Polysilicon electrical resistivity Polysilicon TCR Thermal: Aluminium thermal conductivity

Lm

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

where L b is the length of a single bimorph section, tb is the total thickness of the bimorph,
αT is the difference of the CTEs of the material layers, and βρ is a parameter called the curvature coefficient. The curvature coefficient is a unit-less parameter that varies from 0 to 1.5 and depends on the relative layer thicknesses and elastic moduli. Note that equation (3) represents the displacement of an unloaded bimorph actuator. For piston-mode actuation, the displacement of the mirror plate in response to an applied voltage can be found by simply multiplying equations (1) and (3), which yields 
 3βρ L 2b
αT 4ξ RTA 2 δP (VP ) ≈ V +1−1 . (4) 2 tb ξ 3 R0 P As was mentioned previously, tilt-mode actuation requires that a common piston-mode voltage of VP be applied to all heaters. Tilt-mode actuation also requires that the heaters are operated in the linear temperature versus voltage range such that the common piston-mode voltage is greater than half the critical voltage (i.e., VP  VC /2). This requirement enables heaters on opposite sides of the mirror plate to increase and decrease in temperature equally when additional tilt-mode voltages of ±VT are added to the piston-mode voltage of each heater. The equal and opposite temperature change of the heaters causes the oppositely positioned ISC bimorphs to deflect by equal and opposite amounts. This deflection of the ISC bimorphs rotates the mirror plate while keeping the centre of the mirror surface fixed in space (as shown in figure 6). Thus the reflection point of the mirror stays fixed in space and the optical path of a light beam reflected off of the centre of the mirror plate does not shift. Assuming that the torsional stiffness of the ISC bimorphs positioned on the rotation axis is negligible and that the lateral and rotational stiffness of the oppositely positioned ISC bimorphs causing tilt motion is negligible, the tilt-mode rotation angle of the mirror plate can be found by analysing the geometry of figure 6, and is given by

θT (δT ) = sin−1 (2δT /L m ) ≈ 2δT /L m

(5)

where L m is the length of the mirror plate plus the length of two mirror connections. Assuming that θT is small enough to make a first-order approximation of sine, the rotation angle of the mirror plate in response to a tilt-mode voltage is given by  2βρ L 2b
αT 3 RTA θT (VT ) ≈ VT . (6) tb L m ξ R0 S356

Polysilicon thermal conductivity Mechanical: Aluminium elastic modulus Aluminium Poisson ratio Aluminium CTE SiO2 elastic modulus SiO2 Poisson ratio SiO2 CTE

0.7 × 10−6 K−1

Thus the micromirror can be linearly actuated in both piston-mode and tilt-mode when operated in a certain range. The models described above were based on the assumptions that convection is negligible, heat flow from the actuators to the mirror plate is negligible, the torsional stiffness of the actuators along the rotation axis is negligible, and that the lateral and rotational stiffness of the oppositely positioned ISC bimorphs causing tilt motion is negligible. In practice, these assumptions might not be representative of the true behaviour of the micromirror. Electrothermomechanical and mechanical mode FEM simulations were conducted using Coventorware [13] to model these effects on the behaviour of the mirror. A convection coefficient of 30 W K−1 m−2 was applied on the surface of the device in the FEM simulations. In practice the convection coefficient on the device is difficult to predict without experimental data, so the convection coefficient used in simulation was arbitrarily chosen to be reasonably close to free convection values reported in literature [14]. The material properties of the device used in modelling are shown in table 1. Most material properties were found from published data. The polysilicon TCR was set to 5.85 × 10−3 K−1 , which was experimentally measured from a previous device [11]. Analytical model and FEM simulation results for pistonmode and tilt-mode actuation are compared in figures 7(a) and (b). The analytical model and FEM simulation showed displacements of 106 and 103 µm respectively for 7 V. Figure 7(a) shows that the analytical model and FEM simulations agree within 7% for voltages greater than 4 V. The piston-mode FEM simulation results are lower than the analytical model results due to the presence of convection in the FEM simulation. The half critical voltage for piston-mode is approximately 2.4 V. The linear range of actuation past half

A multi-DOF micromirror utilizing inverted-series-connected bimorph actuators (a) 120 Linear Range 100

Displacement ( µm)

Fixed Rotation Axis and Reflection Point 80

Temperature Increased – Downward Displacement

Common Voltage for Tilt–Mode Actuation

60

40

Half Critical Voltage

20 Analytical FEM Simulation 0

0

1

2

3

4

5

6

7

Temperature Decreased – Upward Displacement

Piston–Mode Voltage (V) (b)

8 6

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

Rotation Angle ( °)

4 2

Piston–mode Voltage = 5 V

the thermal resistance of the mirror connections. This is a simple design change that could be implemented in future designs 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 tiltmode mechanical resonance frequencies were 1.6 and 3.1 kHz respectively.

0 –2 –4 –6 –8 –2

Analytical FEM Simulation –1.5

–1

–0.5

0

0.5

1

1.5

2

Tilt–Mode Voltage (V)

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 is marked in figure 7(a) to show the common piston-mode voltage to be used in the tilt-mode analysis. The tilt-mode simulation was conducted using a common piston-mode voltage of 5 V and tilt-mode voltages in the range of ±2 V. This means that all heaters are given a voltage of 5 V and heaters on opposite sides of the mirror plate are given additional voltages in the range of ±2 V. For example, if the tilt-mode voltage applied is ±0.5 V, then the total voltages on heaters on opposite sides of the mirror plate are 5.5 and 4.5 V respectively. Since the device is being operated in the linear temperature versus voltage range, the equal and opposite voltage changes cause equal and opposite temperature and displacement changes. Both the analytical and FEM simulation results for tilt-mode actuation are shown in figure 7(b). The analytical model and FEM simulation showed rotation angles of ±7.7◦ and ±3◦ respectively for ±2 V. Notice that the FEM simulation predicts a significantly lower rotation angle compared to the analytical prediction. This could be due to the torsional stiffness of the ISC bimorphs along the rotation axis, thermal coupling between heaters, and lateral and rotational stiffness of the oppositely positioned ISC bimorphs causing tilt motion. The attenuation of the rotation angle due to the torsional stiffness and thermal coupling between heaters could be mitigated if the width of the mirror connections was made shorter. A shorter mirror connection width would reduce the torque applied to actuators along the rotation axis and increase

5. Fabrication and results The device was fabricated using the AMI 1.5 µm two-metal conformal CMOS process through MOSIS [15] followed by a post-CMOS micromachining process for device release. The conformal deposition of the process is utilized to provide the alternating layer construction of the ISC bimorphs. The postCMOS micromachining process (shown in figure 9) starts with a backside silicon etch to define an Si membrane thickness under the mirror plate which ensures mirror flatness. A partial SiO2 etch follows to expose metal 2 but leaves metal 1 with SiO2 protection. Next a partial Al wet etch reduces the thickness of metal 2. Metal 2 provides protection of the top layer SiO2 of the ISC bimorphs during the following anisotropic SiO2 RIE etch that exposes the Si substrate. Ionmilling of Al in the SiO2 RIE etch removes the remaining metal 2. Finally, anisotropic DRIE and isotropic Si etches release the structure. Residual stresses in the Al and SiO2 layers cause the device to have an initial elevation of 84 µm at 21 ◦ C. Figure 10 shows an SEM photograph of a released device. The device was released successfully using the postCMOS micromachining process; however, the electrical connections to the heaters were lost during the post-CMOS micromachining process. Ideally, a conformal CMOS process with one metal layer and a different material (such as photoresist) for protection of the heater and mirror connections should be used. Unfortunately it was difficult to find a CMOS foundry process that was ideal for the device fabrication because of the limited flexibility of the available standard processes. Thus the AMI 1.5 µm process was used because it provided most of the needed fabrication steps, but we had to S357

S T Todd et al Device after CMOS Fabrication

Step 4: SiO2 RIE

Step 1: Backside Si DRIE

Step 5: Si DRIE

Step 2: Partial SiO2 RIE

Step 6: Isotropic Si Etch

Si Under Mirror Released Device

Step 3: Partial Al Wet Etch

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

(a) Al

SiO2

Si

Polysilicon

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

(b)

Figure 10. SEM photograph of a released device.

compromise by using Al as both the structural material of the ISC bimorphs (metal 1) and the protection layer of the heaters and mirror connections (metal 2). Pinholes with diameter of ∼0.5 µm (shown in figure 11) were already observed in the metal 2 layer before the wet etching in step 3. The Al wet etch in step 3 greatly enlarged the pinholes already existing in 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 pinholes were transferred to the SiO2 layer underneath metal 2 during step 4 and then to the polysilicon during step 5, resulting in the breakdown of the polysilicon in the heaters. Since the electrical connections were lost to the heaters, no electrical characterization data have been collected on the device. However, piston-mode displacement was experimentally measured in a temperature range 21–150 ◦ C. S358

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

This experiment was conducted by placing the device on a thin-film heater and observing the surface of the device under an optical microscope. The initial elevation of the mirror surface with reference to the substrate plane was measured at the ambient temperature (21 ◦ C) using the micron-scaled focus of the microscope. Subsequently voltages were applied to the heater to change the temperature of the device and displace the mirror surface. At a given heater voltage the temperature of the device was measured using a thermocouple and the new elevation of the mirror surface over the substrate plane was

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

50

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

40

Acknowledgments

Displacement ( µm)

60

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

30

20

10

0

Analytical FEM Simulation Experimental 0

20

40

60

80

100

Temperature Change (°C)

120

140

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

measured. The difference between the initial and actuated elevations of the mirror surface over the substrate plane yielded the displacement for a given temperature change. Although no electrical characterization data are present, the proper piston-mode actuation of the device was verified by this experiment. The experimental results showed a displacement of 56 µm for a temperature change of 129 ◦ C (total temperature of 150 ◦ C). The optical microscope resolution limited the accuracy of experimental displacement Figure 13 measurements to approximately ±3 µm. shows a comparison of the experimental, analytical, and FEM simulation results for thermomechanical piston-mode actuation.

6. Conclusion A novel 3D electrothermal micromirror design concept has been described. The design eliminates the problems associated with previous electrothermal designs by replacing single bimorph actuators with inverted-series-connected bimorph actuators. Analytical models and FEM simulations showed that the device can be linearly actuated with respect to applied voltage in both piston-mode and tilt-mode. The device was fabricated using the AMI 1.5 µm CMOS process followed by a post-CMOS micromachining process for device release. The device was successfully released to produce an initial elevation of 84 µm at 21 ◦ C, but the electrical connection to the polysilicon heaters was lost in the post-CMOS micromachining process. The electrical connection loss was due to the presence of pinholes in the metal layer that allowed etchants to attack the polysilicon. Thermomechanical pistonmode actuation was verified experimentally by measuring the displacement of the mirror surface over a range of temperatures. Future work includes determining an alternate

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