A micro corona motor

Sensors and Actuators A 118 (2005) 226–232

A micro corona motor

Sanghoon Leea,∗, Daejong Kimb, Michael D. Bryantb, Frederick F. Lingc

a Samsung Electronics Co. Ltd, 416 Metan-3Dong, Yeongtong-Gu, Suwon-City 443-742, Gyeonggi-Do, Republic of Koreab The University of Texas at Austin, Mechanical Engineering, 1 University Station, Austin, Texas, TX 78712, USA

c 250 South End Avenue, Suite 3E, New York, NY 10280, USA

Received 29 September 2003; accepted 10 August 2004Available online 5 October 2004

Abstract

A micro corona motor was fabricated and tested. Intense electric fields at sharp stator electrodes ionize air molecules, and ionized charges aretransferred onto the rotor surface. Coulomb repulsion between rotor and electrodes rotates and levitates the rotor. Important design parameters,such as electrode spacing, air gap, and electrode-tilt angle, were studied using an electrostatic field simulator. For good performance, the stator’selectrodes should be axially thick and have sharp tips. Therefore, X-ray lithography was adopted for precise, high aspect ratio characteristics.T ed SU-8p redD or showedg©

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o avoid the fabrication difficulty of a membrane X-ray mask, a built-on X-ray mask technique was employed with negative-tonhotoresist. Speed and torque of a fabricated micro motor were measured. High rotating speed (>104 rpm) was obtained with several hundC volts applied. Using the spin-down method, the torque was estimated in the order of 10 nN m. For over 20 million cycles, the motood stability with no additional control action.2004 Elsevier B.V. All rights reserved.

eywords:Micro motor; Corona; Built-on X-ray mask; LIGA; Microfabrication

. Introduction

A corona motor, a unique type of electrostatic motor[1,2],ses repulsive Coulomb forces between stator electrodes and

he rotor to create rotating motion. Voltages applied to theharp edges of the stator’s electrodes generate intense andighly non-uniform fields; the tips of these electrodes arelose to the surface of the rotor. Since strong electric fieldsan ionize air molecules resulting in a corona, the surfacef the rotor can be charged by means of a partial coronaischarge from the stator electrodes. Here, charged particlesepelled by the stator electrode drift to the rotor (purposelyade an insulator) and deposit charge onto the surface. In mo-

or operation, the stator and charged rotor interact to produceoth torque and bearing lift.Fig. 1 shows a stator and rotoronfiguration. Because of a continuous discharge betweenhe electrodes and the rotor, almost the entire surface of the

∗ Corresponding author. Tel.: +82 331 277 8648; fax: +82 331 200 4649.E-mail address:[email protected] (S. Lee).

rotor is sprayed with charges. Around the stator’s circumence, alternate electrodes have different polarities. Thsults in alternately charged sectors on the rotor, whichthe neighboring like-charged electrode. AC or even DC etation applied to the stator electrodes will drive this motorthe rotor turns, the charge residing on any section of thtor approaches the next electrode. Since alternate elecpossess opposite charge, the rotor experiences an addattractive force between the section and the next electThis augments the drive. Finally, an opposite charge isposited on the sector by the next electrode and the rotprocess continues.

Our aim is to design and fabricate a micro corona mutilizing repulsive electrostatic force. Krein[3] analyzed amicro corona motor based on the conductivity in the moair gap. However, corona motors have not been constrin the micro domain because of lack of information oncro discharges and fabrication difficulty. There have benumber of research studies on a variety of micro dischain MEMS community since the end of 1990s[4–9]. Recen

924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2004.08.017

S. Lee et al. / Sensors and Actuators A 118 (2005) 226–232 227

Fig. 1. Rotor and stator configuration and stator electrode’s tilt angle.

investigations showed similarities between macro and microdischarges, if the air gap was greater than 6�m [4,5]. Designrequirements of a corona motor, such as sharp and tall elec-trode tips can be satisfied by X-ray lithography fabricationmethods.

Driving forces of past electrostatic micro motors (e.g.,variable capacitance and induction motors) arose fromCoulomb attraction between opposite charges[10,11].Coulomb’s electrostatic force increases as distance betweencharges decreases. Tolerance between a bearing and rotor(an unbalanced asymmetric air gap) produces unwanted ra-dial motions of the rotor, which further changes the eccentricair gap and disturbs the forces. A small disturbance can un-balance the rotor and the resulting radial motions of the rotorincrease rotor–stator contact, friction, and wear, which limitsmotor life. For the corona motor, repulsion between statorelectrodes and like-charged sectors of the rotor surface levi-tate the rotor, augmenting bearing action and avoiding stictionand friction.

2. Design of micro corona motor

Important design parameters for the corona motor includenumber of corona electrodes, air-gap spacing, and electrode-ti areI -i t-w gridp rrorl wereg heo des.I s thed argeda ow-e eena rther,w trodet the

Table 1Geometric design parameters

Parameter Size

Electrode spacing 360�mAir gap 50�mRotor OD 1300�mRotor ID 500�mStator tip <1�mElectrode-tilt angle 30◦Shaft OD 500�mShaft length 5–8 mm

size of the air gap between electrodes and rotor should beminimized because Coulomb forces decrease rapidly withdistance. However, a very small air-gap size reduces the in-homogeneity of the electric field, decreasing effective coronageneration and rotor surface charging. Tolerances imposedby the bearing’s clearance also limit the minimum gap dis-tance. To enhance the field intensity at the electrode tip,we seek to increase the radial tilt angle of the electrode,seeFig. 1(b). However, to obtain a unidirectional tangentialforce (Ftangentin Fig. 1(b)), electrode-tilt angles must be lessthan 90◦ because charged particles drift to the rotor surfacewith electrode-tilt angle. Materials selected for this motor de-sign must satisfy functional requirements (insulating rotor,conductive erosion-resistant electrodes, and dielectric statorspacing) and be fabrication friendly. Materials for electrodesand insulating layer were selected for compatibility withcommon LIGA (Lithography, Gavanoformumg, and Abfor-mung[13]) processes, e.g. nickel and polymethymethacrylate(PMMA). Table 1shows geometric parameters selected bythe considerations mentioned above, andTable 2shows se-lected materials.

Table 2Materials selection

Item Material

EERS

ilt angle. Electrostatic finite element simulations[12] us-ng Lorenz 2D software (Integrated Engineering Softwnc., Manitoba, Canada) suggested 360�m electrode spacng, 50�m air gap, and 30◦ electrode-tilt angle. This sofare allows simple model construction with preciseoints in the micro domain and adjustable iteration e

imits. The details of dimensions and electrode shapesiven in Table 1. A number of design trade-offs exist. Tutput torque is proportional to the number of electro

ncreasing the number of electrodes not only reduceistance between electrodes but also increases the chrea, rendering more stable and uniform rotor motion. Hver, avoiding electrical breakdown of dielectrics betwdjacent electrodes requires wide electrode spacing. Fuidely spaced electrodes concentrate fields at the elec

ip greatly. To maximize output torque of the motor,

lectrode Nickellectrode spacers PMMAotor PMMAhaft Nickel alloy

228 S. Lee et al. / Sensors and Actuators A 118 (2005) 226–232

3. Fabrication of micro corona motor

The design of this motor demands small dimensions, hightolerances, large aspect ratios, and very sharp electrodes.X-ray lithography was adopted for fabrication because short-wavelength X-rays can satisfy the following geometric spec-ifications. Firstly, the smallest possible tip radius of a sharpelectrode is determined by the wavelength of the light source[13]. As tip radius decreases, the field intensity around thetip is enhanced allowing lower working voltage to achievea given electric field. Secondly, the high penetration energyof X-rays allows high aspect ratio structures. As the statorbecomes longer, the charged area on the rotor increases ulti-mately enhancing torques.

Fabrication of X-ray masks is important for LIGA anddeep X-ray lithographic processes. In the past, membranesmade of titanium, silicon carbide, silicon nitride, glass, andrelatively thicker substrates made of beryllium, graphite, andsilicon were used to fabricate X-ray masks. However, themask membrane is always an issue because of the fabri-cation difficulties, X-ray transparency, optical transparencyfor optical alignment, and deformation during the process.Thin membranes such as titanium and silicon nitride offervery high X-ray transmission; however, these thin membranesare very fragile. Beryllium, another high-performance mem-b ndle;h y andp ow at

con-ft y re-s uiredc golda tionalt reso-l d byp fabri-c andg

MMA ode.

Fig. 2. Fabrication process of a corona motor by two built-on X-ray masks:(a) optical lithography on SU-8 layer, (b) patterned SU-8 layer after devel-oping, (c) first X-ray exposure, (d) PMMA developing in G–G solution, (e)nickel electroplating and polishing, (f) optical lithography with a second UVmask, (g) second X-ray exposure, (h) PMMA development.

Fig. 2 shows the fabrication processes for the microcorona motor. Membraneless built-on X-ray masks werefabricated on top of the polymethylmethacrylate (PMMA)substrate not only to accelerate the fabrication process fora prototype but also to eliminate the printing gap, whichlimits the resolution of lithography. Since fabrication of abuilt-on mask is on the PMMA substrate photoresist, se-lection of photoresist should satisfy the following require-ments: (a) the baking temperature should be lower than thePMMA glass transition temperature; (b) adhesion on theCr/Au layer should be good; and (c) the developer shouldnot attack the PMMA layer. Negative-tone epoxy-basedEPONTM SU-8 (MicroChem Corp., Newton, MA, USA) wasselected for this process because of good X-ray transparency

rane is toxic. Graphite and Kapton masks are easy to haowever, graphite masks usually have high defect densitroduce high sidewall roughness, and Kapton masks sh

hermo-mechanical instability[14].To eliminate the need for an X-ray mask membrane, ‘

ormal’ or ‘built-on’ masks were developed[15,16]. Thisechnique builds absorber patterns directly onto the X-raist surface through an optical mask. To satisfy the reqontrast for high aspect ratio structures, e.g. LIGA, thebsorber should be thick because the contrast is propor

o the absorber thickness. Depending on the requiredution of applications, an optical mask can be fabricateattern generator or e-beam lithography. Besides theation issues, a built-on mask eliminates printing gapsives better resolution.

Fig. 3. (a) Prototype of a corona stator with Ni electrodes and P
(insulator) electrode separations, and (b) magnified view of an electr


Fig. 4. (a) Prototype of a PMMA corona rotor (R= 650�m) and (b) an assembly with a shaft.

(transmission coefficient of 0.95 for 6 keV with 20�m SU-8[17]), good optical transparency, various available thicknessranges (2–500�m) with excellent sidewall slope, and ease ofprocess.

Fabricating a built-on X-ray mask was initiated with aplating base on the PMMA substrate by evaporating 100A ofchromium followed by 500A of gold, using an electron beamevaporator. A 20-�m thick layer of SU-8 was coated over theplating base. The SU-8 layer was exposed to make an X-raymask pattern in contact mode through an optical mask using aUV-exposure station. After development, an 8-�m thick Au-electroplated layer was deposited at the rate of 4�m/h. Sincethe residue-free removal of SU-8 is problematic, the resistwas left in place on the X-ray masks without significantlyaffecting the exposure processes. To reduce stresses inducedby the mismatch of thermal expansion coefficients betweendifferent layers, the specimen was annealed at 80◦C for onehour and then slowly cooled.

Fig. 3(a) shows the fabricated stator of a corona motor,which consists of nickel electrodes and PMMA insulatorsbetween each electrode. A magnified view of an electrodeis shown inFig. 3(b). Several 300-�m thick PMMA rotors(Fig. 4(a)) were also fabricated using the same techniques.This micro motor’s rotor was assembled by placing the rotor(Fig. 4(a)) over an 8-mm long, 500�m diameter nickel alloys o thes t ofP ys

4

4

ea-s tion,v pre-c areao ea-

sured rotating speed by dividing the count of rotations by thetime of the count.

The actual tip radius was much larger (3–5�m) thanthe photomask pattern (1–2�m), which is larger than thedesigned value (<1�m) due to the limitation and error offabrication. Larger tip radius caused high working voltages(>700 V), and therefore only up to four electrodes couldbe used for the stable performance-avoiding arc betweenactivated electrodes.Fig. 6 shows rotor speed (rpm) ver-sus voltage applied to the electrodes. Different curves per-tain to different number of stator electrodes active. Forthe two-electrode system, a corona discharge began around700–800 V but due to the relatively large static friction atthe vee-jewel bearings, rotation started from 1100 V. Afterstartup, the applied voltage can be reduced to 850 V be-cause dynamic friction at the bearings is generally smallerthan static friction. In the four-electrode system, corona dis-charge began around 800 V and rotation started at 950 V be-cause the driving torque is proportional to the number ofelectrodes. In both cases, rotation speed increased with in-creased applied voltage. Data was recorded only when therotating motion was stable. With much higher voltage, un-stable sparking starts between electrodes, or between elec-trodes and rotor. Because the four-electrode system hadcloser spacing between activated electrodes, the arc volt-as highv

4

auset -m er ex-p ric-t statest agt own

haft. This rotor was thermally expanded and placed onthaft at 50◦C using the large thermal expansion coefficienMMA (70 × 10−6 K−1). An assembled rotor (nickel allohaft and PMMA rotor) is shown inFig. 4(b).

. Test of micro corona motor

.1. Speed measurement

Fig. 5 shows the experimental setup for the speed murement. For easy alignment and reduced bearing fricee-jewel bearings were installed at both shaft ends withiseXYZstages. A photonic sensor detected the shadedn the shaft inFig. 5, once each rotation. The sensor m

ge was lower than that for the two-electrode system.Fig. 7hows a melted rotor due to excessive current at veryoltage.

.2. Torque measurement

Precise torque measurements are very difficult beche micro motor’s torque (pN m–�N m) is too small for comercial torque meters (�N m–mN m). To estimate torqu

oughly, we used the spin-down method. Spin-downeriments allow quantifying the torques from bearing f

ion and viscous drag. When the motor runs at steady-peed, the friction and drag torquesTd equal the drivingorquesTf , i.e. Tf = Td. If we assume the friction and drorques proportional to rotation speed, during spin-d


Fig. 5. Experimental setup for rotating speed measurements with vee-jewel bearings at both ends; the alignment of stator and bearings is precisely controlledbyXYZstage. (a) Experimental setup for the speed measurement. (b) Schematic of experimental setup. (c) Assembled motor (upper bearing was removed forthe photography).

(with Td = 0):

Jdω

dt= −Kω (1)

Integration of Eq.(1)gives the time to stop (ts) from the initialsteady-state speed (ω0):

ts = J

Klog (ω0) (2)

Fig. 8(a) shows a sample signal of speed decay after turn-ing off the electrode-voltage excitation.Fig. 8(b) plots thetime to stop versus rotation speeds; this shows logarithmictendency. Error bars on the data indicate the uncertainty ofthe time measurements. The low speed data have larger un-

certainty; since at lower speed, a single revolution representsa larger percentage of the deceleration time. FromFig. 8(b),the constant of Eqs.(1) and (2),K≈ 2.5E− 11 (N m s) and forthe given rotor inertia,J= 1E− 12 (Kg m2). At steady-state,Tf = Td implies:

Td = Kω (3)

Fig. 9(a and b) plots the estimated torquesTd andTf versusspeed and voltage, withK = 2.5E − 11 (N m s).Fig. 9(b)compares the measured (using proportional constantK) andsimulated torque, described elsewhere[6], using estimatedcharge distributions on the stator and the rotor.


Fig. 6. Measurements of corona motor speed versus applied electrode volt-age with vee-jewel bearings at both ends.

Fig. 7. Rotor failure due to the excessive heat generated by high current athigh voltage.

4.3. Reliability test

The motor ran reliably for several days without failure.Fig. 10shows results of a 24-h long test with a two-electrodesystem. There was a small variation in speed but stability athigh speed (15 000 rpm) was maintained with no additionalcontrol action, as shown inFig. 10. The motor could haverun indefinitely. Even though vee-jewel bearings were usedin this test, stable rotation at high speeds suggests that repul-sive forces between the stator and the rotor make balancedrotation.

5. Discussion

Corona motors’ repulsive forces overcome friction/weardifficulties of past micro motors and generate larger torques.This motor, which required sharp electrodes and tall struc-tures for good performance, was fabricated by X-ray lithog-raphy. The higher than expected working voltage was causedby a larger than expected electrode-tip radius. The targeted

Fig. 8. (a) Sample photonic sensor signal versus time to measure the timeto stop from the initial steady-state speed (ω0 = 3500 (rpm)). (b) The timeto stop (ts) versus initial rotation speed.

electrode-tip radius was less than 1�m but due to resolutionlimits of the optical mask and fabrication errors, the fabricatedelectrodes had radii of 3–5�m. A corona discharge beginswhen the field intensity at the tip exceeds the gas breakdownstrength. A larger tip radius reduces the field intensity. Tocompensate, the working voltage was increased. However,this large working voltage encountered inter-electrode dis-charges, so only two to four electrodes could be used forstable operation. With six electrodes, unexpected intermit-tent arcing between neighboring electrodes produced unsta-ble motion. A small tip radius also allows a smaller air gapfor stable corona discharges resulting in greater torques, be-cause Coulomb’s force is inversely proportional to the dis-tance. Improved fabrication should result in a much lowerworking voltage with larger torque.

Another reduction of working voltage involves environ-mental gas species. Nitrogen or helium gases have lower ion-ization potential (N2: 251 V, He: 156 V[18]) than air (327 V).Therefore, we can expect a 20–50% reduction of workingvoltage with these gases.

Reliability and performance issues of corona motors in-volve reduction of working voltage and possible sputteringon the tip. Because the working pressure of this motor is veryhigh (atmospheric pressure) and corona current is usuallyless than�A, sputtering may not significantly change the tip


Fig. 9. Torque estimation: (a) torqueTf estimated usingK = 2.5E − 11(N m s) from the spin-down test. (b) comparison between measured andsimulatedTd (=Tf ).

radius. However, in terms of reliability, sputtering should beas small as possible. Low-mass gases, e.g. helium, or othermaterials with high resistance to ion bombardments can beused.

Although the micro corona motor worked successfully,optimization of design parameters based on the application,optimization of fabrication process, investigations of corona

Fig. 10. Reliability test of a corona motor over 24 h with two electrodes andapplied voltage of 1300 V.

discharge phenomena with a moving surface, and creation ofsensing devices are still to be done.

Acknowledgements

We thank the National Science Foundation, Grant DMI-0115527, Manufacturing Machines and Equipment Program,Division of Design, Manufacture and Industrial Innovation,and Atoz CompuNet Ltd for support for this work. We alsothank Dr Jost Gottert, Yoonyoung Jin, and staff of Centerfor Advanced Microstructure and Devices at Louisiana StateUniversity for support and equipment for fabrication.

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A micro corona motor

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