DTIC ADA523557: Encapsulated Ball Bearings for Rotary Micro Machines

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by  Defense Technical Information Center

IOPPUBLISHING JOURNALOFMICROMECHANICSANDMICROENGINEERING
J.Micromech.Microeng.17(2007)S224–S229 doi:10.1088/0960-1317/17/9/S03
Encapsulated ball bearings for rotary
micro machines
CMikeWaits1,2,BruceGeil2andRezaGhodssi1
1MEMSSensorsandActuatorsLaboratory(MSAL), DepartmentofElectricaland
ComputerEngineering,InstituteforSystemsResearch,CollegePark,MD20742,USA
2USArmyResearchLaboratory,2800PowderMillRoad,Adelphi,MD20783,USA
E-mail: [email protected]
Received26February2007,infinalform8August2007
Published31August2007
Onlineatstacks.iop.org/JMM/17/S224
Abstract
Wereportonthefirstencapsulatedrotaryballbearingmechanismusing
siliconmicrofabricationandstainlesssteelballs. Themethodofcapturing
stainlesssteelballswithinasiliconracetosupportasiliconrotorboth
axiallyandradiallyisdevelopedforrotarymicromachinesandMEMSball
bearingtribologystudies. Initialdemonstrationsshowspeedsupto6.8krpm
withoutlubrication,whilespeedsupto15.6krpmwithlubricationare
possible. Qualitativeanalysisisusedtoexplainstart-upbehaviorand
investigatethewearofthestainlesssteelballandsiliconrace.
(Somefiguresinthisarticleareincolouronlyintheelectronicversion)
1. Introduction nature. The frictional properties of ball bearings allow the
possibility for higher speeds and better reliability than other
Microelectromechanicalsystems(MEMS)fabricatedsilicon contactbearingsrelyingonslidingmotionwhilemaintaining
rotary elements for micro-motors, micro-generators and fabrication simplicity and stability. Although ball bearings
micro-turbomachinery have received growing attention with havebeendemonstratedindevicessuchaslinearmicromotors
applications in power conversion and actuation. Within [6, 7] and rotary micromotors [8], they have yet to be
these technologies, the bearing mechanism is the primary integratedintothemicrofabricationprocesstofullyconstrain
determinantofdeviceperformanceandreliability. Bothactive the dynamic element. In the cases of both Modafe et al
andpassivebearingshavebeeninvestigatedforrotarymotion; [6] and Ghalichechian et al [7, 8] the dynamic element,
however, no known successful commercial implementations whether it is a rotor or a linear slider, is held onto the
areknownduetopoorreliabilityandshortlifetimes. Active balls by an electrostatic force. Their approach requires an
bearings,suchasmagneticorelectrostatichavetheadvantage additional force to maintain contact between the dynamic
of being controlled during the operation, but at the cost of element and the stator. This imposes restrictions in the
the accompanying circuitry [1]. Passive bearings have been manufacturing process and assembly with the surrounding
investigated heavily and span a large range of velocities systemcomponents. Furthermore,atestingplatformcapable
that include center-pin bushings with low revolution rates ofinvestigatingspeedsgreaterthan100’srpmforballbearings
possible [2, 3] and hydrostatic or hydrodynamic bearings hasyettobedeveloped.
with high revolution rates possible [4]. Contact passive Hence,arotaryballbearingmechanismisreportedhere
bearing mechanisms have poor reliability characteristics and whereintherollingelementsareencapsulatedattheperiphery
are limited to low speeds due to the high frictional forces oftherotortoenablehighspeedrotationwithoutrelyingonany
of sliding motion [2]. In contrast, non-contact bearings attractiveforcebetweentherotorandstator. Anexperimental
based on active elements, or pressurized gas, have been test stand is implemented to demonstrate the encapsulated
demonstrated to achieve high speeds but require complex bearing operation at rotational speeds up to 16 krpm and
fabricationprocedureswithtighttolerances[3]. serveasastepping-stoneforfuturehighspeedandhighload
Tanetal[5]haveinvestigatedthetribologyofalinearball tribology experiments. Results for both dry and lubricated,
bearingmechanismandshowedthatthefrictionalproperties full compliment bearings are also reported leading toward
between 440C stainless steel balls and silicon are quite low the development of micro turbomachinery. A qualitative
when compared to pure sliding motion due to the rolling analysisisgivenforthewearseenonthesiliconraceandthe
0960-1317/07/090224+06$30.00 ©2007IOPPublishingLtd PrintedintheUK S224
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1. REPORT DATE 3. DATES COVERED
2007 2. REPORT TYPE 00-00-2007 to 00-00-2007
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Encapsulated ball bearings for rotary micro machines
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
MEMS Sensors and Actuators Laboratory (MSAL),,Department of REPORT NUMBER
Electrical and Computer Engineering, Institute for Systems Rsch,College
Park,MD,20742
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
14. ABSTRACT
We report on the first encapsulated rotary ball bearing mechanism using silicon microfabrication and
stainless steel balls. The method of capturing stainless steel balls within a silicon race to support a silicon
rotor both axially and radially is developed for rotary micro machines and MEMS ball bearing tribology
studies. Initial demonstrations show speeds up to 6.8 krpm without lubrication, while speeds up to 15.6
krpm with lubrication are possible. Qualitative analysis is used to explain start-up behavior and investigate
the wear of the stainless steel ball and silicon race.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 6
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
Encapsulatedballbearingsforrotarymicromachines
r A
p
Stator Rotor
h (a)
w
2
w A’
1
(b)
Figure1. Schematicdrawingshowingthedesignoftheballbearing
mechanism. TherotationalaxisoftherotorisshownbytheA–A(cid:1)
dashedlineontheright-handsideofthedrawing.
Table1. Dimensionsoftheballbearingmechanism.
Parameter Dimension (c)
d 285µm
ball Silicon Dioxide
rp √ 4mm Silicon
h=d / 2+δh 205µm Photoresist
ball
ww1=d /√2+δw 320005µµmm Figure2. Simplifiedfabricationflowforthesiliconraceusinga
2 ball 2 nestedmaskinglayerandtwoDRIEsteps.
stainlesssteelballandcorrelatedtothestart-upbehaviorofthe ofh/w2canbevariedtochangetheincidentangleoftheload
bearings. on the corners of the race where the wear rate of the silicon
willbeatamaximum.
2. Racedesign
3. Encapsulatedballbearingfabrication
Thedesignandfabricationoftherotaryballbearingisbased
on commercially available 440C stainless steel balls with a The bearing mechanism is fabricated in three major steps:
diameter, d , of 285 µm and a lot diameter variation of (1)siliconracesarefabricatedonthewaferlevel;(2)ballsare
ball
0.254 µm (see figure 1). The design features balls housed placedintotheraceandanidenticalraceisbondedontopto
attheperipheryoftherotortoenableatwo-layerfabrication encapsulatethem;and(3)siliconDRIEisusedtoreleasethe
sequence for encapsulation via bonding. At the same time, rotor.
this scheme of encapsulation allows features to be patterned Thestepsusedtofabricatethesiliconracesareillustrated
oneithersideoftherotorwhilehavingminimalinfluencefrom in figure 2. A double-sided polished, (100) silicon wafer
thebearings. with a thickness of 440 µm was used with a 1.1 µm silicon
A square groove race was designed to encase the micro dioxide layer thermally grown on the surfaces. The silicon
ballbearings. Alternativedesigns,nevertheless,maybeable dioxide on the top-side of the wafer was patterned with the
toimprovetheperformanceandfatiguecharacteristicsofthe outerringofwidthw1andapitchradiusof4mm. Thesilicon
bearings. One can easily think of methods to incorporate dioxideonthebottom-sideofthewaferwaspatternedwitha
racedesignswhichbettermimicthosefromtheirmacroscopic ringofwidthw2. Figure2(a)showsthesestepswithboththe
counterparts, such as a rounded groove race or an angular topandbottomsilicondioxidespatterned. Aconcentricring
contact race; however, these may require more stringent ofwidth,w2,wasthenalignedandpatternedintophotoresist
fabricationprocesses. Housingtheballsusingasquaregroove onthetop-sideofthewafer, showninfigure2(b). Together,
racefabricatedbyadryanisotropicetchprocess,suchasdeep thepatternedphotoresistandpatternedsilicondioxideforma
reactive ion etching (DRIE), allows control of the contact nestedmaskinglayer. TwoDRIEstepsusingaUnaxisVLR
points and better repeatability when compared to other race 700systemwereemployedtoformthesiliconraceusingthe
designsandfabricationmethods. photoresist as the first masking layer and the silicon dioxide
Thedimensionsoftheballbearingmechanismarelisted asthesecondmaskinglayer. Theresultofthenestedmasking
intable1. Theballracewasdesignedtohaveapitchradius, layeretchingisillustratedinfigure2(c). Thisformsasilicon
r ,of4mmfromtherotationalaxistothecenteroftherace. race in which two identical dies on a wafer can be bonded
p
Thewidthoftheouterring,w ,is300µmtoensuretheballs togetherandencapsulatetheballs.
1
fitintotheracewiththefabricationtol√erancesinvolved. The A 1.15µm Cr /Au/AuSn/Au metal adhesion layer is
width of the nested race, w , is d / 2 +δw , so that the employed to encapsulate the balls and the device is released
2 ball 2
ballscontactatthe90◦cornersor205µmwiththetolerances, using DRIE. The metal layers were chosen from in-house
δw ,addedin. Thetotalheightoftheracebetweenthecontact device packaging experience as well as its low softening
2
points,h,isthesameasthewidth,w . Bothδhandδw are temperaturebelow300Ctolaterde-bondforinspection. This
2 2
made 4.5 µm to include fabrication tolerances and thermal process is shown in figure 3. The metal adhesion layer is
expansiondifferencesduringbonding. Furthermore,theratio deposited using electron beam evaporation through a silicon
S225
CMWaitsetal
Shadow mask Alignment Pits Silicon Race
(a)
Cr/Au/AnSn/An Layers
Figure4. Opticalpictureofadieshowingthepatternedrace,metal
(b) andalignmentpits.
Freed rotor supported
by stainless steel balls
Released Rotor
Silicone
Standoff
Encapsulated Balls
(c)
Silicon Dioxide Figure5. Opticalpictureofareleasedsiliconrotorsupportedby
Silicon
Cr/Au/AuSn/Au stainlesssteelballsattheperiphery. Afullcompliment
configurationwasusedwithmorethan60balls.
Figure3. Fabricationflowshowingencapsulationusinga
Cr/Au/AuSn/AuadhesionlayerandreleaseusingDRIE.
theracepreventingtherotorfromspinningtherebylowering
fabricationyield.
shadow mask, shown in figure 3(a). After metal deposition, Otherbondingtechniquessuchassiliconfusionbonding
thewaferisdicedintoindividualdieforbonding. are better suited when fabricating final devices for use in
Ballsaremanuallyplacedintothesiliconraceofonedie, actual systems; however, they typically produce permanent
figure3(b),andanidenticalsiliconraceisalignedusingballs bonds which are not conducive for investigating purposes.
strategically placed into square alignment pits at the corners To investigate the silicon race and stainless steel balls after
ofthedie. Thenumberoftheballsplacedintotheracemust operationatemporarybondingmethodisbettersuited. Metal
bemorethannecessarytofillhalfofthesiliconrace. Above adhesionallowsthebearingmechanismtobeseparatedona
thisnumber,theballswerechosenarbitrarily. Thealignment hotplateallowingeasyinspection.
pitsareetchedintothesiliconatthesametimeastheraceand,
therefore, are etched to a specified depth in which only half
4. Experimentalsetupandresults
of the ball protrudes out of the pit. By doing so, a 290 µm
widepitanda285µmdiameterballwillresultinamaximum
Demonstration of the encapsulated ball bearing was
misalignment of 5 µm. The aligned stack is then bonded accomplished using the setup shown in figure 6. Standoffs
togetherusingaKarlSussSB6waferbonderwitha1kTorr are placed on both the top and bottom of the rotor and the
appliedpressureatatemperatureof285◦Cusinga1.33kTorr stack is held in place using a mechanical vice. A nitrogen
H2N2forminggasatmosphere. Afterbonding,DRIEisusedto line is placed within 2 mm of the die corner using a second
etchthroughthesiliconusingthepreviouslypatternedsilicon mechanicalvice. Theflowfromthenitrogenlinecausesthe
dioxidetoreleasetherotorshowninfigure3(c). outer portion of the die to spin about the clamped center. A
Figure4showsaphotographofasiliconracediebefore PhiltecD6Fiberopticdisplacementsensorisusedtomeasure
bondingwiththemetaladhesionlayerdeposited. Thesquare theangularvelocityofthespinningsquaredie.
pitsusedforalignmentofthediebeforebondingcanbeseen Initial movement of the square die about the center
in the corners of the die. Figure 5 shows a photograph of a required a line pressure greater than 5 psi for most of the
completely released bearing with the silicon dioxide mask devices tested. Lower pressures were not able to produce
remaining. In most cases, the silicon rotor is completely enoughforceontheedgeofthissquaredietocounteractstatic
releasedaftertheDRIEsteps; however, insomecasesmetal friction,whichismuchhigherthanthedynamicfrictiononceit
particles form in the bearing race due to reflow of the metal isrotating. Insomecases,bearingshavetobemanuallystarted
adhesion layer during the bonding process. After the final toovercomethisinitialfriction. Oncetheyarespinning,line
releasesomeoftheparticlesaretoolargetoberemovedfrom pressuresaslowas1psihavebeenusedtomaintainoperation.
S226
Encapsulatedballbearingsforrotarymicromachines
Ball jamming
Spinning Device
Direction of
Motion
Nitrogen
i
Optical Displacement Sensor
Figure6. Opticalpictureofthetestset-upusedtomeasurethe Figure9. Opticalpictureofballjammingoccurringwithintheball
speedoftherotatingbearingmechanism. bearingrace.
10
y
cit 8 Bearing seized
eloM) 6
VP 4
ular (kR 2
ng 0
A 0 10 20 30 40
Time (minutes)
Figure7. Angularvelocityversustimeshowingtheinitialstart-up
transientandsettlespeedforaconstantl5.5psilinepressure.
(a)
1166
1122 Unlubricated
y
cit Lubricated
VeloM) 88
ar RP
gul(k
An 44
00
0 5 1100 1155 2200 2255 3300
Pressure (psi)
(b)
Figure8. Measuredspeedofabearingversusnitrogenlinepressure
Figure10. SEMimagesshowing(a)freshstainlesssteelballand
withandwithoutJP-8lubrication.
(b)stainlesssteelafter39minofanaverage3.2krpmrotationrate.
Figure 7 shows the angular velocity versus time using
stopped due to ball jamming. Slight agitation was used to
a nitrogen line pressure of 5.5 psi and a device which had release the balls allowing the device to spin freely until the
70stainlesssteelballs. Astart-uptransientisclearlyseenin race wear became too large where jamming occurred before
whichthedevicerotatesatarategreaterthan9krpm,butthen spinning.
dropstoanaveragerateof3.2krpm. Thesamedevicewasrestartedwith50µLofJP-8diesel
Figure 8 shows the results of angular velocity versus fueladdedintothebearingopening. JP-8isreadilyavailable
an increasing nitrogen line pressure for two cases: (1) dry andhasalubricityadditivemakingitagoodlubricant[9]. The
bearings and (2) lubricated bearings with 82 balls. Such a devicewasturnedmanuallytoallowuniformcoatingaround
demonstration shows the potential velocity benefit by only the silicon race and over the stainless steel balls. Again the
adding lubrication to increase the speed of the device. The pressurewasincreasedfrom10psito25psiandmeasurements
pressure was started at 10 psi and increased by 5 psi after takenat5psiincrementsoveraperiodof10min. TheJP-8
each measurement over a period of 12 min. The maximum evaporatedsoonthereafterandthebearingseizedagaindueto
open pressure of the nitrogen line used was 25 psi. Ball balljamming. Anincreaseof8–9krpmcanbeseenbetween
jammingoccurredafter12minofoperationcausingthedevice thedryconditionandthelubricatedconditionforeachpressure
toseize. Figure9isanopticalpictureofabearingwhichwas setting.
S227
CMWaitsetal
A
Stator Rotor
A’
(a)
A
Stator Rotor
A’
(b)
Figure11.SEMimageofasiliconrace(a)beforeand(b)aftertestingandaschematicdrawingillustratingwheretheSEMimagewastaken.
5. Discussion wearratebyordersofmagnitude. Inthecaseoffigure7,after
the initial 3min, the compressivestress becamelowenough
The testing method employed induces a high load onto the forthewearratetoshrinkbelowlevelsnoticeableduringshort
bearingdevice. Thisaswellasthesquaregrooveracedesign runs; therefore, arelatively constantangularvelocityis seen
and the brittle silicon material lead to high wear. Scanning fortheremaining39min.
electron microscopy (SEM) images were used to investigate Balljammingwhichhasoccurredforbothbearingdevices
thewearofthesiliconraceandofthestainlesssteelballs. The reported here is an inherent problem when full compliment
devicewhichoperatedfor39mininfigure7beforejamming typebearingsareused. Theracedimensionsincreaseaswear
wasseparatedonahotplateat400◦C.Figures10and11are ensues giving the balls more play and a greater chance to
SEMimagesshowingthewearofthestainlesssteelballand seize. Implementation of a low wear material on top of the
thesiliconrace,respectively. siliconracealongwithtighterfabricationtolerancescanreduce
The start-up behavior exhibited in figure 7 is believed the probability of jamming, but not eliminate it completely.
to arise from wearing of the race and ball. As the surfaces Instead, a retainer ring (or cage) could be used to isolate
roughenmorefrictionisassumedtooccur,slowingtheangular the balls from one another. In addition to eliminating ball
velocityofthedeviceforaconstantappliedforce. Thewear jamming, the use of a retainer ring can greatly reduce the
rateisafunctionofthemaximumstressincurredonthesilicon frictionsinceamuchsmallernumberofballscanbeused. A
race by the loaded ball. The maximum stress occurs at the retainerringmaintainsaseparationdistancebetweentheballs
centralpointofthecontactareaandcanbeestimatedas[10]
allowingforaminimumnumberofballstobeused. Reduction
3Q inthefrictionbydecreasingthenumberofballswillsimilarly
σ = (1)
max 2πab leadtomuchhigherspeedsaswellasbetterreliability.
inwhichQdenotestheloadexertednormaltotheracefromthe The current method of testing, that is, blowing nitrogen
ball,aisthesemi-majoraxisradiusofanellipticalcontactarea, on the face of the bearing die causes a torque about the
and b is the semi-minor axis radius for an elliptical contact. longitudinal axis. This unwanted torque arises due to a
Asthesiliconraceandtheballwearaway,boththeloadand non-uniform nitrogen flow across the top and bottom of the
thecontactareachange. Theloadchangesduetothecontact device as well as asymmetric loads on the spinning edge.
angle, which in this case did not vary significantly. For the Additionally, the device is asymmetrically loaded parallel to
wear exhibited in figure 10, the contact area changes from the nitrogen flow causing more wear on the sides opposing
a sharp semi-infinite point to a large flat area. This in turn the flow and less on the sides facing away from the flow.
significantlyreducesthecompressivestressanddecreasesthe Boththeundesiredtorqueandthenon-uniformloadingcould
S228
Encapsulatedballbearingsforrotarymicromachines
contribute to ball jamming. An improved testing method is implementation of an encapsulated rotary ball bearing will
being developed to eliminate this effect and greatly increase lead to the development of micro turbomachinery for Power
theoperationtimeofthebearingbeforejammingoccurs. MEMSaswellasothermicrosystemapplications.
There are a number of contributing factors leading to
increased ball jamming occurrence as well as the overall Acknowledgments
tribologicalpropertiesofthebearingmechanism. Firstly,the
number of stainless steel balls influences not only the load TheauthorsthanktheArmyResearchLaboratorycleanroom
distributionaffectingtheinducedstressshowninequation(1), personnel for their support in the fabrication of the devices
but will also affect the amount of friction from surface aswellasthemembersoftheMEMSSensorsandActuators
indentationandball-to-ballcollisions. Secondly,thegeometry LaboratoryattheUniversityofMaryland.
ofthesiliconracealsohasalargeinfluenceontheloadingand
thewearthatwilloccur. Moresophisticatedracegeometries,
References
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S229