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A novel vertical comb-drive electrostatic actuator using a one layer process

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2014 J. Micromech. Microeng. 24 115016

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1. Introduction

Micro electrostatic actuators have found applications as driving units in MEMS components such as tunable capaci-tors, inductors and switches [1–3]. Comb drives are widely used micro actuators [4–8]. Conventional vertical comb-drive actuators require two structural layers, i.e. one for the moving fingers and one for the fixed fingers [9–13]. A number of pro-cess steps are necessary to fabricate the two layers [14–21]. Silicon on insulator (SOI) technology is often used to fabri-cate vertical comb-drive actuators [14, 15]. Trench cutouts and trench refilling of polysilicon are also used to develop these actuators [16]. Bonding of two SOI wafers plus deep reactive ion etching is used to fabricate the fingers [17, 18]. These processes are complicated and costly. In addition, the misalignment between the moving and fixed fingers is a chal-lenge [18–21].

A new design of a vertical comb-drive actuator is proposed which requires only a one structural layer process such as the

MetalMUMPs [22] and uses the residual stress gradient defor-mation to raise the moving fingers above the fixed fingers. A Finite Element (FE) analysis and experimental verification of the actuator are presented here. The paper is organized as fol-lows: section 2 describes the design principles of the actuator. In section 3, FE simulation results and analysis of the actuator are presented. Prototypes and measurement results are pre-sented in section 4. Conclusions are listed in section 5.

2. Principle of the vertical comb-drive actuator

2.1. Residual stress gradient in MetalMUMPs

The MetalMUMPs process has a 20 µm thick electroplated nickel [22, 23] layer which is often used for fabricating RF MEMS tunable capacitors [24], inductors [25, 26] and switches [27–29] due to its high electric conductivity and large thickness. After release, the structure made of nickel film bend due to residual stress gradients along the film thickness. As

Journal of Micromechanics and Microengineering

A novel vertical comb-drive electrostatic actuator using a one layer process

Zewdu Hailu1, Siyuan He1 and Ridha Ben Mrad2

1 Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario, Canada2 Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario, Canada

E-mail: s2he@ryerson.ca

Received 15 July 2014, revised Accepted for publication 5 August 2014Published 24 October 2014

AbstractThis paper presents the design, fabrication and testing of a new residual stress gradient based vertical comb-drive actuator. Conventional vertical comb-drive actuators need two structural layers, i.e. one for the moving fingers and a second for the fixed fingers. A vertical comb-drive actuator based on a single structural layer micromachining process, using the residual stress gradient along the thickness of the nickel of the MetalMUMPs (Metal Multi-User MEMS process) fabrication process, is developed. The MetalMUMPs provides a 20 μm thick nickel film and is subject to residual stress gradients along its thickness. Two curve-up beams are devised to curve out of plane after release. The curve-up beams raise a plate with comb fingers above the substrate to form the moving fingers. The fixed comb fingers are connected to the substrate via anchors. When a voltage is applied across the moving and the fixed fingers, the moving fingers move down towards the fixed fingers. Experimental measurements on prototypes have verified the design principle. A vertical displacement of 4.81 µm at 150 V was measured.

Keywords: vertical comb-drive electrostatic actuators, single structural layer micromachining process, residual stress gradient, MetalMUMPs

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shown in figure 1, a straight clamped-clamped beam curves upward due to the negative residual stress gradient along the nickel film thickness [30] after the sacrificial layer is removed.

The actuator presented in this paper uses the residual stress gradient to deform the clamped-clamped beams to lift the moving fingers of a comb-drive actuator above the substrate and the fixed fingers.

2.2. Structure of the vertical comb-drive actuator

Figure 2(a) shows the vertical comb-drive actuator before release. Two clamped-clamped beams anchored to the sub-strate are used to curve out of plane due to residual stress gradients. A moving plate is attached to the middle of the

straight clamped-clamped beams through serpentine springs. Arrays of fingers are connected to the opposite sides of the moving plate to form the moving fingers. A set of fingers are connected to the substrate through anchors to form the fixed fingers. The moving fingers are raised above the fixed fingers and the vertical comb-drive actuator is obtained as shown in figure 2(b).

3. Simulations of the actuator

3.1. Static performance

CoventorWare [23] is used to simulate the deformation of the actuator with an average residual stress gradient value

Figure 1. Clamped-clamped beam (a) straight beam before release and (b) curve-up beam after release.

Figure 2. Residual stress gradient based comb-drive actuator. (a) Before release. (b) After release (substrate is omitted).

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of −4.72 MPa μm−1 along the thickness and a Young’s mod-ulus of 159 GPa for nickel [30]. Table 1 lists the dimensions of the actuator structure. The dimensions of the actuator are labeled in figure 3 using the quarter top view of the actuator at unreleased state.

The moving fingers and the fixed fingers have identical length and width with an overlap length of 300 μm. The width of the fingers is chosen to be 10 μm, which is 2 μm higher than the minimum allowed width according to the MetalMUMPs process rules (8 μm) for fabrication safety. The nominal thickness of the fabricated actuator is 20.5 μm including the thickness of the nickel (20 μm) and the coating gold (0.5 μm). Only the nickel layer is used in the deformation analysis since it is the layer that undergoes significant bending due to the stress gradient. The horizontal gap between a neighboring moving finger and a fixed finger is 12 μm and the release holes have a dimension of 20 μm × 20 μm in the moving plate and the curve-up beams.

Figure 4 shows the simulated deformation of the actuator using the residual stress gradient. Color maps are used to show the deformation in this figure. After release, the moving plate moves above the substrate. Consequently, the moving fingers attached on each side of the moving plate are raised above the fixed fingers and the substrate.

Various simulated profile plots of the actuator parts are shown in figures 5(a)–(f), to illustrate the surface profiles along the lines AA, BB, CC, DD, and EE as indicated in figure 4. All

the solid lines in figure 5 show the profile plots just after the actuator is released without any driving voltage. The curve-up beam achieved a deflection of 20.58 μm at its mid-point as shown in figure 5(a). The moving plate and the moving finger roots are 22.10 μm above the substrate after release as shown in figure 5(b) (marked by line EE). Figure 5(c) shows profiles of the moving plate and the curve-up beams along line BB. These illustrations verify that the moving plate and moving fingers are raised by the curve-up beams. Figure 5(e) shows a cross section view of the moving and the fixed fingers after release.

The fixed fingers as cantilever beams bend downward due to the residual stress gradient with the tip bending down by 2.15 μm as shown in figure 5(d).

The moving fingers bent down by 3.05 µm relative to the centre point of the moving plate. The top surfaces of the fixed finger tips are 24 µm lower than the top surfaces of the moving fingers at the same location along the finger longitudinal direc-tion as shown in figure 4(b). The top surfaces of the moving finger tips are 20.35 μm above the top surface of the fixed fin-gers at the same longitudinal location as show in figure 4(b).

A dc voltage is applied across the moving and fixed fin-gers of the actuator. The corresponding surface profiles are plotted using dashed curves in figures 5(a)–(f) when a 150 V is applied. Figure  5(f) shows that the moving fingers trav-elled 3.79 µm towards the fixed fingers. The displacements of the moving fingers and the moving plate are almost the

Table 1. Actuator specifications.

Structure Length (µm) Width (µm) Number Thickness (µm)

Moving fingers (LMF) 400 10 2  ×  15 20.5Fixed fingers (LFF) 400 10 2  ×  14 20.5Curve-up beam (2 L) 2558 150 2 20.5Box spring (LX/LY) 402 410 4 20.5Serpentine spring(LX/LY) 400/200 9 4 20.5Moving plate (2LP/WP) 670 300 1 20.5Lateral gap between moving and fixed fingers (d) — 12 µm — —Nickel Young’s modulus 159 Gpa Trench depth 25 µmNickel residual stress gradient −4.72 MPa µm−1

Figure 3. Top view of a quarter of the actuator.

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same within the driving voltage of 150 V. For example, the displacement difference at 150 V between the center point of the moving plate and the moving fingers tips is 0.01 µm. Hence the displacement of the moving fingers when a voltage is applied is represented by the displacement of the central point of the moving plate. The simulated displacement of the moving plate and the moving fingers at 150 V is shown in figure 5 (b). Figure 6 shows the displacement of the actuator at 150 V. The displacement can be seen from the color map by comparing figures 6 and 4.

In practical prototypes, the fixed fingers tips are blocked by the substrate when they bend for more than the distance between the fixed fingers and the substrate, which is the thick-ness of the oxide layer under the fixed fingers with a nominal value of 1.1 μm. The simulation does not include the substrate since the software cannot simulate the surface contact between the substrate and the fixed fingers bottom surface. The inaccu-racy is 1.05 μm, i.e. 2.15 μm (simulation)-1.1 μm (nominal gap between fixed fingers and substrate). This error is the tip dis-placement inaccuracy along the total length of 400 μm of fixed fingers. This inaccuracy in fixed finger tip bending does not affect the simulation of the moving fingers. When applying a voltage, this inaccuracy causes a 3% error in the simulated

force considering a vertical gap between moving and fixed fin-gers of about 20 μm.

3.2. Dynamic performance

The natural frequencies of the actuator are simulated using CoventorWare. Figure 7 shows the first two vibration modes. The double sided arrow shows the direction of motion of the moving plate and the moving fingers. The frequencies for the first two modes are 2583.15 Hz and 3204.35 Hz. The actuator could work at a frequency as high as two thousand hertz without inducing any resonances.

4. Prototypes and measurements

4.1. No-voltage measurements

Prototypes were fabricated using the MetalMUMPs. SEM images of a prototype are shown in figure  8. As shown in figures  8(a) and (b), the curve-up beams are deformed and raise the moving plate and the moving fingers above the fixed fingers. Zygo New view 6 K is used for experimental mea-surement. Figure 9 shows surface profiles of the actuator after

Figure 4. Simulation of actuator (a) deformed shape with labeled lines for profile extraction. (b) Top surfaces of the fixed and moving fingers.

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fabrication without applying a voltage. The trench bottom surface is taken as a reference to measure the deflections of the moving fingers and the curve up beams. The trench has a nominal depth of 25 µm.

Figure 9(a) illustrates the surface map of the moving and the fixed fingers. Figure 9(b) shows the profile plot along the slice line AA as indicated in figure 9(a). This measurement verified that the vertical separation between the moving and the fixed fingers is achieved. The maximum deflections of both the curve-up beams and the moving plate are shown in figure 9(c) along the line BB with its profile plot in (d). The surface map and profile of the curve-up beam is also shown in figure 9(e) and (f), respectively.

The center of the moving plate is raised by 23.74 µm due to the residual stress gradient from its unreleased position. The middle of the curve-up beam curves out of plane by 21.27 μm relative to its anchors. The moving plate is raised

slightly more than the middle of the curve-up beam because the parts connecting the moving plate and the curve-up beams also slightly bend out-of-plane. The fixed fingers bend down by 1.02 μm from root to tip, which is slightly less than the nominal distance between the lower surface of the fixed finger and the substrate, i.e. 1.1 μm. The moving fingers, like canti-lever beams, bent down by 2.56 μm from root to tip. The top surface of the tips of the moving fingers is 21.22 μm above the top surface of the fixed fingers at the same longitudinal location. The top surface of the fixed fingers tips is 23.04 μm lower than the top surface of the moving fingers at the same longitudinal location.

The vertical separation between the moving and fixed fin-gers for the current prototype is more than 20 µm leading to a vertical comb-drive actuator using the one structural layer (nickel) process MetalMUMPs. The difference between the simulation result and the prototype measurement of the

Figure 5. Simulated surface profiles of structures of the actuator without driving voltage and a driving voltage of 150 V respectively. (a) Profile of curve-up beam along line AA. (b) Profile across the moving fingers and moving plate along line EE. (c) Profile of curve-up beams and the moving plate along line BB. (d) Profile of fixed fingers along line DD. (e) Cross section of the moving and the fixed fingers after release without driving voltage along line CC. (f) Cross section of the moving and fixed fingers under 150 V actuation along line CC.

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deformation of the central point of the moving plate is about 7%. Other prototypes with different physical sizes such as curve-up beam length and width were also successfully fabricated.

4.2. Measurements with driving voltages

A lens of 20 ×  magnification was used to capture the images of figure 9(a). When dc probes were connected to the actuator pads, it was necessary to leave enough gap between the lens and the sample for the probes to avoid any mechanical con-tact. Hence, a 2 ×  magnification lens was used while applying voltages. The 2  ×  lens enables to keep a vertical space of 20 mm between the lens and the sample while the 20 ×  lends can only provide a space of less than 5 mm. Figures 9(c) and (e) are images using 20 ×  and 2 ×  lenses, respectively. When applying voltages using 2 ×  lens, not enough details of the moving fingers can be seen for measuring the displacement. The displacement of the center point of the moving plate,

which can be seen clearly under the scope when using 2  ×  lens, is measured to represent the displacement of the actuator under driving voltages. The displacement of the moving fin-gers at driving voltages is the same as the displacement of center point of the moving plate as explained in section 3. The center point of the moving plate is indicated in figures 2 and 4.

The measured displacement at the applied actuation voltages is plotted in figure  10. The highest displacement measured at 150 V is 4.81 µm while the simulation result showed 3.79 μm.

Figure 11 shows the measured surface profile plots of the moving plate and the curve-up beam at 0 V and 150 V. The spikes are removed from the raw data by sampling only the points on the top surfaces of the moving plate and the curve-up beams.

The highest driving voltage used is 150 V with very low current as is the case for an electrostatic MEMS actuator. An improved design based on this actuator with a fixed polysil-icon electrode added under the moving plate using the same

Figure 6. Simulated deformations at 150 V.

Figure 7. First two mode frequencies. (a) Mode 1: translation. (b) Mode 2: horizontal motion.

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process (as show in figure 13) can lower the driving voltage to 125 V for the same displacement. The actuator presented in this paper does not suffer from the ‘pull-in’ effect along the vertical direction. But each end finger is under unbal-anced force in the direction perpendicular to the finger length, which causes lateral ‘pull-in’ effect between the side walls of the end finger and the neighboring fixed finger. When the driving voltage is above 160 V the end finger snaps to the neighbouring fixed finger and then the whole moving part of the actuator moves. Under the driving voltage of 150 V the actuator is stable.

5. Analysis

The residual stress has been widely used in micro devices [31–39]. The actuator presented in this paper uses the residual stress gradient to construct the moving comb layer. High precision positioning, operation in temperature changing envi-ronments and long-time reliability and stability are concerns of MEMS devices using residual stress [40]. The remainder of this section discusses the following with regards to the device

presented in this paper: (1) Comparison with other micro devices that are also using residual stress; (2) Application to on/off capacitive switching; (3) Displacement variability due to temperature changes; and (4) Application to high precision positioning.

5.1. Comparison with other micro devices using residual stress

The difference between the present actuator and other reported micro devices [31–39] using residual stress lies in: (1) The present actuator uses a mature and commercial micro-fabrication process, i.e. MetalMUMPs and the residual stress gradient is in the nickel layer fabricated using this process. Other reported micro devices use lab-designed microfabrica-tion processes and the residual stress is in polysilicon, silicon nitride or a thin metal layer. A commercial microfabrication process has advantages of high yield, high maturity, high con-sistency and low cost over lab-designed processes; (2) The MetalMUMPs provides a thick metal layer (20 µm nickel) which can lead to a high quality factor for RF applications.

Figure 8. SEM images of the actuator. (a) Overall view of the actuator. (b) Image showing the moving and fixed fingers.

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5.2. Applications to on/off capacitive switching

MetalMUMPs is commercially available for more than 15 years. The residual stress of this process is consistent. The device presented here can be used as capacitive switch such as for redundancy switch matrices in satellite transceivers and space telecommunication. These applications require high RF power handling capability (several tens of watts) and high quality factors. The present actuator offers the following benefits: (1) High RF power handling capability because the comb drive actuation makes it not suffer from the ‘pull-in’ effect even at a high driving voltage. Conventional RF MEMS

capacitors can only handle RF power of 1~10 watts [41–45]; (2) High quality factor due to the thick metal layer used; and (3) Non-contact switching therefore high operation reliability. Vertical comb-drive actuators have the advantage of requiring a smaller area than lateral comb-drive actuator for a large capacitance tuning range [15].

5.3. Displacement variation due to temperature changes

The simulation of the actuator in an environment with tempera-tures varying from −60 °C to 60 °C shows that the displacement

Figure 9. Prototype measurement without driving voltage. (a) Top view of the moving fingers and fixed fingers. (b) Surface profile plot of the fixed and moving fingers. (c) Top view of the moving plate and curve-up beams. (d) Surface profile plot of the moving plate and curve-up beams. (e) Top view of the curve-up beam. (f) Profile plot of the curve-up beam.

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variations due to the temperature change is 0.466 μm as shown in figure 12. If the actuator is used as a tunable capacitor or capacitive switch, the corresponding capacitance variation due to the temperature change from −60 °C to 60 °C is 3.1% of the capacitance at room temperature, which is of the same size as the temperature change induced capacitance variation reported in [46].

5.4. Application to high precision positioning

The proposed actuator cannot be directly, i.e. in an open-loop mode, used in high precision positioning due to the stability concerns associated to residual stress. However, the proposed actuator can easily integrate sensing in conjunction with actuation by adding a fixed electrode under the moving plate.

Figure 10. Measured and simulated displacements versus actuation voltage.

Figure 11. Measured displacement of the moving plate at 150 V.

Figure 12. Simulations of the actuator at low and high temperatures. (a) Simulated actuator at −60 °C. (b) Simulated actuator at 60 °C.

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This can be done without making any change to the microfab-rication process nor to the design of the actuator. The added fixed electrode is made of polysilicon layer which is available in the MetalMUMPs process (see figure  13(a)) and can be added under the moving plate (see figure 13(b). Figure 13(a) shows the layers available in MetalMUMPs. The polysilicon layer has a thickness of 0.7 µm and is embedded between two silicon nitride layers of thickness 0.35 μm each. The moving plate and the fixed electrode are separated from each other by the sacrificial layer (Oxide 2) of thickness 1.1 μm before release. After release, a large gap (>20 µm) between them is created due to the residual stress gradient in the nickel layer. The top nitride layer creates insulation to avoid electrical contact between the moving plate and the fixed electrode.

6. Conclusion

A novel vertical comb-drive electrostatic actuator was pre-sented. The design of the actuator uses residual stress gradients and requires microfabrication of one structural layer such as MetalMUMPs to obtain vertically separated moving and fixed fingers. The method employed also avoids misalignments between the moving finger layer and fixed finger layer associ-ated with conventional vertical comb-drive actuators fabrication processes. Prototypes were fabricated using the MetalMUMPs process. Experimental tests have verified that the novel actuator achieved 4.81 µm vertical displacement at 150 V.

Acknowledgement

We would like to acknowledge CMC Microsystems Inc. for their support to fabricate the prototypes (MM1102, 2011 and MM1201, 2012) and supply of equipment for testing.

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