Design and Fabrication of a Rotary Electrostatic Micromotor Supported on Microball Bearings

N. Ghalichechian1, A. Modafe1, M. Beyaz1, C. M. Waits1,2, and R. Ghodssi1

1MEMS Sensors and Actuators Lab (MSAL), Department of Electrical and Computer Engineering, Institute for Systems Research, University of Maryland, College Park, MD 20742, USA

2Army Research Laboratory, Adelphi, MD 20783, USA Tel: 301-405-8158; Fax: 301-314-9281; Email ghodssi@eng.umd.edu

ABSTRACT

We report, for the first time, the design and fabrication of a bottom-drive, rotary, variable-capacitance micromotor supported on microball bearings. The rotor and stator of the 6-phase micromotor were designed based on 3D finite element analysis. The rotary micromotor is expected to produce 3 μNm of torque at 100 V. Different designs were successfully fabricated using a 9-level-mask process. The rotary micromotor is a platform for developing centrifugal micropumps for fuel delivery applications.

Keywords: Rotary Micromotor, Microball Bearings, Variable-Capacitance

1 – INTRODUCTION

Micromotors have great potential as the key driving mechanism for the next-generation micropumps used for fuel delivery and cooling [1-4]. Other applications of micromotors include microassembly [5], micro-propulsion, microactuation [6, 7], and microsurgery [8-10]. Microball-bearing technology in silicon provides a reliable and robust support mechanism for the rotor of micromotors and microgenerators. Design and fabrication of the first microball-bearing-supported micromachine, a bottom-drive, linear, variable-capacitance micromotor (B-LVCM) was previously demonstrated by our group [11]. The primary application of the B-LVCM is long-range, high-speed, linear micropositioning. Microball-bearing design provides less friction and wear compared to center-pin or flange designs used in conventional side drive VCMs [12, 13]. Furthermore, the fabrication process of the microball-bearing-supported machine is less complicated than gas-lubricated machines [14]. The development of the rotary machine is based on our previous work on the design, fabrication, and characterization of the linear micromotors in 3-phase [11] and 6-phase [15, 16] operation modes. Recently, the fabrication process of the device was revised and the 2nd generation of 6-phase, linear micromotors was fabricated and tested [17]. The slider displacement, the instantaneous and average velocities, acceleration, and the net force were obtained using a high-speed camera system. The average linear speed of 7.3 mm/s was achieved at 120 V and 40 Hz excitation and was in agreement with the predicated value. A

modeling scheme based on a mass-dashpot-spring system was also reported [18]. Rotary micromotors can be used in wider variety of applications compared to their linear counterparts. The rotary micromachine, presented in this paper, was designed based on the recent characterization results and fabrication process developments performed for the linear machine [17]. In this paper, we report the design and finite element modeling of the micromachine in Section 2. The fabrication process is discussed in Section 3 followed by conclusions and future work.

2 – DESGIN

The bottom-drive, rotary, variable-capacitance micromotor (B-RVCM), shown in Figure 1, is composed of three major components: rotor, stator, and microballs. The rotor is a silicon plate supported on stainless steel

Figure 1: Simplified 3D schematic of the micromotor (Dimensions are not to scale).

Stator

Rotor

Electrodes

Contact Pads

Microballs Microball Housing

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Figure 2: Schematic (a) radial cross section of the mechanical (bearing) and electrical (active) components of the machine, (b) azimuthal cross section of the stator and rotor active parts. Dimensions are not to scale. microballs. The stator is also a silicon plate that includes the active electrical components of the micromachine. Figure 2 shows the cross-section schematic of the stator and rotor. Two levels of conductors on the stator form the electrode and interconnection layers. Three layers of low-dielectric-constant (k=2.65) benzocyclobutene (BCB) polymer film are used for electrical isolation of the stator electrodes, interconnections, and substrate. The low-k dielectric film, compared to silicon dioxide insulating films, reduces the parasitic capacitance between electrodes and the stator substrate; therefore, the electrical efficiency of the micromotor is increased. In addition, the residual stress of the BCB film is lower than that of silicon dioxide. This results in less wafer curvature and better gap uniformity. The stator also has a circular housing (trench) for the microballs. The rotor has fewer components than the stator and is composed of silicon salient poles and circular microball housings. In a variable-capacitance motor, the torque is proportional to the angular gradient of the capacitance and the square of the applied voltage magnitude. Therefore, it is desirable to design the geometry of the micromachine, such that at a given voltage, the angular gradient of capacitance is maximized. The mechanical angular velocity of the rotor, when synchronized with the electrical excitation, is directly proportional to the excitation frequency and inversely proportional to the number of salient poles on the rotor. Due to the bottom-drive design, B-RVCM takes advantage of a larger active area compared to the conventional side-drive VCMs.

Table 1: Geometry specification (primary design) of the B-RVCM.

Specification Value Active area inner radius(mm) 1 Active area outer radius (mm) 5 Microball housing inner radius (mm) 5.5 Outer motor radius (mm) 7 Total device area (mm2) 153 Electrode thickness (nm) 270 Electrode width at the midpoint (μm) 90 Spacing between adjacent electrodes (μm) 30 Number of electrodes 156 Number of poles 104 Pole/trench depth (μm) 130-133 Microball diameter (μm) 284.5 Microball housing width (μm) 290 Air gap (μm) 10

The geometry of the micromotor was designed based on the 3D finite element (FE) simulation results of the machine capacitance using FEMLAB (Comsol) software. Different geometries were compared based on the torque values derived from the capacitance simulations. Table 1 shows the geometry specifications of the primary design. The simulated capacitance C(θ) and its angular derivative dC/dθ are shown in Figure 3. The torque values for four machines with different electrode densities and air gaps are calculated from the simulation data and are shown in Figure 4. As shown in the graph, smaller air gaps are desirable to obtain higher torques; however, fabrication limitations and imperfections make small gap achievement challenging. Based on the simulation as well as fabrication development results, a target air gap of 10 μm was chosen.

3 – FABRICATION

The fabrication processes of the stator and rotor of the micromotor use 7 and 2 mask levels, respectively. The stator fabrication starts with deposition of a 3 μm thick photosensitive BCB (Cyclotene 4022-35) film to isolate the active parts of the B-RVCM from the silicon substrate. This film is partially cured (at 210 °C for 40 min) to facilitate a strong metal-polymer adhesion [19]. A 20/250 nm thick chromium (Cr)/gold (Au) layer is then deposited using a DC magnetron sputtering system. This layer forms the interconnection level that connects every sixth electrode of the 6-phase machine. A BCB layer with a thickness of 2.6 μm is then spun, patterned, and partially cured to form an interlayer dielectric layer (ILD) with open vias for electrical connection between the two metal layers. The thickness of this layer was chosen such that conformal coverage of the film prevents formation of sharp edges in the second metal layer (electrodes) that could lead to early dielectric breakdown. The second metal layer is then sputtered (20/250 nm thick Cr/Au) to form the electrode layer. The

Interconnection

Electrode

Passivation BCB

Insulation BCB

Via ILD BCB

Salient Poles

Microball housing

(a)

(b)

Stator

Rotor

Microball

Silicon

Silicon

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Rotor position (degrees)0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Cap

acita

nce

(pF)

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Cap

acita

nce

deriv

ativ

e (p

F/de

gree

)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.30

0.35

0.40

0.45

0.50

0.55

0.60

Figure 3: Simulation results for the capacitance and its derivative as a function of rotor angular position.

Packing factor and air gap0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Torq

ue (μ

Nm

)

0

2

4

6

8

Gap=5 μmGap=10 μmGap=15 μm

Figure 4: Electromechanical torque as a function of the air gap and packing factor (base-design). The packing factor is defined as the ratio of the electrode width to the spacing between electrodes. electrical resistance between the electrode and interconnection layers and the sheet resistance of each layer were measured to be 23-47 Ω and 416 mΩ/sq, respectively. The third BCB film was then deposited, pattered, and partially cured to form a passivation layer. The thickness of this layer was measured to be 2 μm. Microball housings were then etched 132.4 ±0.4 μm deep (measured at the die level) into the substrate. The depth of this etch step, together with rotor housing depth, define the size of the micromotor gap. In this process the most critical step is the alignment of the microball housing with electrodes. Tight alignment (<2μm) ensures that the mechanical and electrical centers of rotation are identical. The polymer films were then fully cured at 250 °C for 1 hour. Figure 5 shows different sections of the stator. The rotor fabrication process is composed of a DRIE step to form salient poles and microball housing followed by DRIE on the back side to release the rotor, etch the center cavity as well as marks for position/speed detection. The housing depth was measured to be 129.5±0.4 μm. Figures 6.a-c show the different sections

Figure 5: Optical micrographs (top-view) of the fabricated stator (a) contact pad area, (b) outer area, (c) close-up view of the vias, and (d) stator with microballs ready for test.

Figure 6: (a) Optical micrograph (top-view) of the rotor (b) SEM of rotor showing salient poles, (c) SEM of rotor showing microball housing as well as etched pits (marks) for displacement sensing, and (d) optical micrograph of the rotor with 24 poles (high-speed design). of the rotor. Figure 6.d shows the top view of a rotor with 24 poles designed to have about 4 times higher synchronous speed compared to the primary design. The air gap of the micromotor (when using 284.5 μm-diameter microballs) was calculated to be 14-15 μm. The final step before testing the micromotor is the manual assembly of the stator, rotor, and microballs. The fabrication process presented here is the result of process development carried out to address the (a) adhesion of metal-polymer films, (b) process yield, (c) eccentricity due to misalignment of electrodes and microball housing, (d) interconnection resistance, and (e) microball housing etch uniformity.

(a) (b)

(c) (d)

Poles

Microball Housing

Displacement sensing marks 24 Poles

104 Poles

(a) (b)

(c) (d)

Microballs

Via Interconnection

Electrodes Microball Housing

Microball Housing

Pads

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4 – CONCLUSION

The design and fabrication of the first rotary micromotor supported on microball bearings were reported in this paper. The micromotor is a 6-phase, variable-capacitance machine fabricated from silicon substrates, metal conductor films, and low-k dielectric layers. Unlike conventional micromotors, a reliable and robust mechanical support of the rotor is provided by microball bearings. 3D finite element analysis was used to identify the optimum geometry to achieve a high torque. The stator and rotor of the micromotor were fabricated separately using 7 and 2-level mask processes, respectively. The successful fabrication of the micromotor was verified by initial static testing of the stator. Future work involves dynamic characterization of the micromotor and measurement of angular velocity and extraction of torque. ACKNOWLEDGEMENTS

This work was supported by the Army Research Lab under Grant No. CA#W911NF-05-2-0026, Army Research Office through the MURI Program under Grant No. ARMY-W911NF0410176 with Dr. Tom Doligalski as the technical monitor, and the National Science Foundation under Grant No. ECS-0224361. The authors would like to thank Mr. Thomas Loughran, Jonathan Hummel, and James O’Connor from the University of Maryland (UMD) clean room facilities (FabLab) for their help with the fabrication of devices. REFERENCES

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